Johannes Ress, Sascha Tominschek, ST Informatik

Arbeitsgruppe Prof. Dr. Anna Köhler für organische Halbleiter am Lehrstuhl Experimentalphysik 2
Lehrstuhl Experimentalphysik II
Arbeitsgruppe Prof. Dr. Anna Köhler

Role of transport band edge variation on delocalized charge transport in high-mobility crystalline organic semiconductors

Andrey Kadashchuk, Fei Tong, Robby Janneck, Ivan I. Fishchuk, Alexander Mityashin, Egon Pavlica, Anna Köhler, Paul Heremans, Cedric Rolin, Gvido Bratina and Jan Genoe

Phys. Rev. B 96, (2017) 125202
medium4

We demonstrate that the degree of charge delocalization has a strong impact on polarization energy and thereby on the position of the transport band edge in organic semiconductors. This gives rise to long-range potential fluctuations, which govern the electronic transport through delocalized states in organic crystalline layers. This concept is employed to formulate an analytic model that explains a negative field dependence coupled with a positive temperature dependence of the charge mobility observed by a lateral time-of-flight technique in a high-mobility crystalline organic layer. This has important implications for the further understanding of the charge transport via delocalized states in organic semiconductors.

Excited State Dynamics and Conformations of a Cu(II)-Phthalocyanine-Perylenebisimide Dyad

Kevin Wilma, Thomas Unger, Sinem Tuncel Kostakoglu, Manuel Hollfelder, Christoph Hunger, Andreas Lang, Ayse Gul Gurek, Mukundan Thelakkat, Jürgen Köhler, Anna Köhler, Stephan Gekle and Richard Hildner

Phys. Chem. Chem. Phys. 19 (2017) 22169
TOC_FINAL-Dyade

We investigate the excited state dynamics and the conformation of a new molecular donor-bridge-acceptor system, a Cu(II)-phthalocyanine (CuPc) covalently linked via a flexible aliphatic spacer to a perylenebisimide (PBI). We performed time-resolved polarization anisotropy and pump-probe measurements in combination with molecular dynamics simulations. Our data suggest the existence of three conformations of the dyad: two more extended, metastable conformations with centre-of-mass distances > 1 nm between the PBI and CuPc units of the dyad, and a highly stable folded structure, in which the PBI and CuPc units are stacked on top of each other with a centre-of-mass distance of 0.4 nm. In the extended conformation the dyad shows emission predominantly from the PBI unit with a very weak contribution from the CuPc unit. In contrast, for the folded conformation the PBI emission of the dyad is strongly quenched due to fast energy transfer from the PBI to the CuPc unit (3 ps) and subsequent intersystem-crossing (300 fs) from the first excited singlet state of CuPc unit into its triplet state. Finally, the CuPc triplet state is deactivated non-radiatively with a time constant of 25 ns.

π-Conjugated Donor Polymers: Structure Formation and Morphology in Solution, Bulk and Photovoltaic Blends

Richard Hildner, Anna Köhler, Peter Müller-Buschbaum, Fabian Panzer and Mukundan Thelakkat

Adv. Energy Mater. 2017, 1700314
aenm201700314-gra-0001-m

The field of conjugated polymers has expanded in the last years considerably and impressive performance, both in field effect transistors and photovoltaic devices has been achieved. After the initial emphasis on improving the performance, more emphasis is recently given to fundamental studies on structure formation. Therefore, this review concentrates on systematic correlation studies of structure formation in solution, in bulk and thin films as well as in photovoltaic blends of donor-type π-conjugated polymers. The main focus is on the correlation of structure, morphology and molecular chain orientation as a function of macromolecular properties such as molecular weight, dispersity, non-covalent intramolecular and intermolecular interactions, solvent interactions and innovative processing techniques. The tools applied for elucidating fundamental information of structure formation and orientation mainly consist of optical spectroscopy and scattering techniques (SAXS/WAXS/GIWAXS). Since the field of conjugated polymers is very vast in terms of chemical structural diversity, only selected examples of donor polymers are covered here and the emerging class of n-type conjugated polymers are not included. The focus is not on the structural variation or their performance in solar cells or transistors in terms of record efficiencies, but on the systematic studies leading to a structure-property correlation in donor polymers.

Spectroscopic Study of Thiophene–Pyrrole-Containing S,N-Heteroheptacenes Compared to Acenes and Phenacenes

Alexander Rudnick, Christoph Wetzel, Steffen Tscheuschner, Holger Schmalz, Astrid Vogt, Andreas Greiner, Heinz Bässler, Elena Mena-Osteritz, Peter Bäuerle and Anna Köhler

J. Phys. Chem. B, Article ASAP
jp-2017-02935v_0006

In this study, we report a detailed spectroscopic study concerning the energy levels and vibrational structure of thiophene–pyrrole-containing S,N-heteroacenes. The aim of the study is first, to understand the differences in the photoluminescence (PL) efficiencies in this structurally similar series and second, to compare the electronic structure of S,N-heteroacenes to that of linear acenes and phenacenes, with a view to derive guidelines for the design of singlet fission materials. For S,N-heteroacenes comprising seven fused heterocyclic rings, we observe a higher PL quantum yield for derivatives with terminal thienothiophene units than for thienopyrrole-capped ones. This is assigned to a stronger tendency of the thienopyrrole-capped derivatives to form nonemissive associates in dilute solution, producing emissive excimers at higher concentration. By conducting time-resolved PL studies at 77 K, we further determine the lowest singlet and triplet energies for the S,N-heteroacenes with three, five, and seven fused rings. We show that their energies evolve with oligomer length analogously to those of phenacenes, yet in a fundamentally different way from that of linear acenes. This difference in evolution is attributed to the increasingly biradical character in acenes with increasing chain length in contrast to the S,N-heteroacenes and phenacenes.

Impact of Structural Dynamics on the Optical Properties of Methylammonium Lead Iodide Perovskites

Fabian Panzer, Cheng Li, Tobias Meier, Anna Köhler and Sven Huettner

Adv. Energy Mater. (2017) 1700286
Panzer2017-AdvEnergyMater

Organolead halide perovskites have attracted a lot of attention over the recent years mostly due to their bright prospective application in photovoltaic devices. For further development, characterization of their physical properties plays a seminal role in order to gain an in-depth understanding of these mid-bandgap ionic semiconductors. Their unique optical and electronic properties are a result of their characteristic electronic structure. Temperature dependent optical spectroscopy, i.e., absorption and photoluminescence (PL) characterization, gives access to their electronic nature that draws strong correlations to their structural properties. Those properties include static and dynamic disorder, and defects or phase transitions, which will be demonstrated in the first part of this research view. The second part focuses on ion migration in these hybrid semiconductors, which can strongly affect the slow dynamics of optical properties. Light-activated ions result in a number of complex processes that can lead to an increase, but also a decrease of PL intensity, or induce PL intermittency. Parameters like light intensity, crystal quality, and defect density all influence these processes, and ultimately the electronic nature of the hybrid perovskites. We will briefly summarize current achievements and point out challenges for upcoming research.

Thiophene–pyrrole containing S,N-heteroheptacenes: synthesis, and optical and electrochemical characterisation

Christoph Wetzel, Astrid Vogt, Alexander Rudnick, Elena Mena-Osteritz, Anna Köhler and Peter Bäuerle

Org. Chem. Front., (2017)
340_Wetzel_OrgChemFront_2017_accepted-2

A novel family of S,N-heteroheptacenes SN7a–d with a variable thiophene–pyrrole ratio and a heteroring fusion sequence is presented. All SN7-derivatives can be synthesized in efficient multi-step synthetic routes with good overall yields. The crucial cyclization step to the stable and soluble fused systems is achieved by multiple Pd-catalyzed Buchwald–Hartwig aminations or C–S coupling reaction in high yields. The comparison of the optoelectronic properties provides interesting structure–property relationships and gives valuable insights into the role of nitrogen atoms within the series of thiophene–pyrrole containing S,N-heteroheptacenes. Remarkable differences in emission behaviour and noticeable correlations of the oxidation potentials of the S,N-heteroheptacenes offer helpful information for the construction of other heteroacenes. Additionally, UV-Vis-NIR absorption spectra of the corresponding radical cations generated by chemical oxidation were monitored and compared.

Efficient Charge Separation of Cold Charge-Transfer States in Organic Solar Cells Through Incoherent Hopping

Stavros Athanasopoulos, Steffen Tscheuschner, Heinz Bässler and Anna Köhler

J. Phys. Chem. Lett. 8 (2017) 2093−2098
Bild1234

We demonstrate that efficient and nearly field-independent charge separation of electron–hole pairs in organic planar heterojunction solar cells can be described by an incoherent hopping mechanism. Using kinetic Monte Carlo simulations that include the effect of on-chain delocalization as well as entropic contributions, we simulate the dissociation of the charge-transfer state in polymer–fullerene bilayer solar cells. The model further explains experimental results of almost field independent charge separation in bilayers of molecular systems with fullerenes and provides important guidelines at the molecular level for maximizing the efficiencies of organic solar cells. Thus, utilizing coherent phenomena is not necessarily required for highly efficient charge separation in organic solar cells.

Crosslinked Semiconductor Polymers for Photovoltaic Applications

Frank-Julian Kahle, Christina Saller, Anna Köhler and Peter Strohriegl

Adv. Energy Mater. (2017) 1700306
TOC

Organic solar cells (OSCs) have achieved much attention and meanwhile reach efficiencies above 10%. One problem yet to be solved is the lack of long term stability. Crosslinking is presented as a tool to increase the stability of OSCs. A number of materials used for the crosslinking of bulk heterojunction cells are presented. These include the crosslinking of low bandgap polymers used as donors in bulk heterojunction cells, as well as the crosslinking of fullerene acceptors and crosslinking between donor and acceptor. External crosslinkers often based on multifunctional azides are also discussed. In the second part, some work either leading to OSCs with high efficiencies or giving insight into the chemistry and physics of crosslinking are highlighted. The diffusion of low molar mass fullerenes in a crosslinked matrix of a conjugated polymer and the influence of crosslinking on the carrier mobility is discussed. Finally, the use of crosslinking to make stable interlayers and the solution processing of multilayer OSCs are discussed in addition to presentation of a novel approach to stabilize nanoimprinted patterns for OSCs by crosslinking.

Interplay of localized pyrene chromophores and π-conjugation in novel poly(2,7-pyrene) ladder polymers

Alexander Rudnick, Kim-Julia Kass, Eduard Preis, Ullrich Scherf, Heinz Bässler and Anna Köhler

The Journal of Chemical Physics (2017) 174903
TOC Grafik-2

We present a detailed spectroscopic study, along with the synthesis, of conjugated, ladder-type 2,7-linked poly(pyrene)s. We observe a delocalization of the first singlet excited state along the polymer backbone, i.e., across the 2,7 linkage in the pyrene moiety, in contrast to earlier studies on conjugated 2,7-linked poly(pyrene)s without ladder structure. The electronic signature of the pyrene unit is, however, manifested in an increased lifetime and reduced oscillator strength as well as a modified vibronic progression in absorption of the singlet state compared to a ladder-type poly(para-phenylene) (MeLPPP). Furthermore, the reduced oscillator strength and increased lifetime slow down Förster-type energy transfer in films, where this transfer occurs to sites with increasing inter-chain coupling of H-type nature.

Interplay between hopping and band transport in high-mobility disordered semiconductors at large carrier concentrations: The case of the amorphous oxide InGaZnO

I. I. Fishchuk, A. Kadashchuk, A. Bhoolokam, A. de Jamblinne de Meux, G. Pourtois, M. M. Gavrilyuk, A. Köhler, H. Bässler, P. Heremans and J. Genoe

Phys. Rev. B 93, 195204
medium2

We suggest an analytic theory based on the effective medium approximation (EMA) which is able to describe
charge-carrier transport in a disordered semiconductor with a significant degree of degeneration realized at high
carrier concentrations, especially relevant in some thin-film transistors (TFTs), when the Fermi level is very
close to the conduction-band edge. The EMA model is based on special averaging of the Fermi-Dirac carrier
distributions using a suitably normalized cumulative density-of-state distribution that includes both delocalized
states and the localized states. The principal advantage of the present model is its ability to describe universally
effective drift and Hall mobility in heterogeneous materials as a function of disorder, temperature, and carrier
concentration within the same theoretical formalism. It also bridges a gap between hopping and bandlike
transport in an energetically heterogeneous system. The key assumption of the model is that the charge carriers
move through delocalized states and that, in addition to the tail of the localized states, the disorder can give rise
to spatial energy variation of the transport-band edge being described by a Gaussian distribution. It can explain
a puzzling observation of activated and carrier-concentration-dependent Hall mobility in a disordered system
featuring an ideal Hall effect. The present model has been successfully applied to describe experimental results
on the charge transport measured in an amorphous oxide semiconductor, In-Ga-Zn-O (a-IGZO). In particular, the
model reproduces well both the conventional Meyer-Neldel (MN) compensation behavior for the charge-carrier
mobility and inverse-MN effect for the conductivity observed in the same a-IGZO TFT. The model was further
supported by ab initio calculations revealing that the amorphization of IGZO gives rise to variation of the
conduction-band edge rather than to the creation of localized states. The obtained changes agree with the one
we used to describe the charge transport. We found that the band-edge variation dominates the charge transport
in high-quality a-IGZO TFTs in the above-threshold voltage region, whereas the localized states need not to be
invoked to account for the experimental results in this material.

Influence of Crosslinking on Charge Carrier Mobility in Crosslinkable Polyfluorene Derivatives

Frank-Julian Kahle, Irene Bauer, Peter Strohriegl and Anna Köhler

J .Polym. Sci., Part B: Polym. Phys. 55 (2017) 112–120
TOC-graphic_new_5

Carrier mobility is a key parameter for the application of conjugated polymers. In this work, a series of polyfluorenes (PF2/6) with different fractions of crosslinkable acrylate groups is investigated. Mobility measurements are carried out to assess the influence of crosslinking with different photoinitiators on the performance of the material. For the regime of low to medium charge carrier density, relevant for OLEDs and OPVs, we used a novel technique based on the injection of charge carriers from the electrodes of an optoelectronic device: MIS-CELIV (MIS: metal-insulator-semiconductor). For large charge carrier densities we performed OFET measurements. We find that using optimized conditions crosslinking does not influence the hole mobility in the investigated system. Furthermore, we demonstrate that the crosslinking process may be triggered solely by thermal activation and UV-illumination without the need of any initiator. Thus, densely crosslinked networks are obtained without the formation of undesired decomposition products from added photoinitiator.

Temperature Induced Order−Disorder Transition in Solutions of Conjugated Polymers Probed by Optical Spectroscopy

Fabian Panzer, Heinz Bässler and Anna Köhler

J. Phys. Chem. Lett. (2017) 114–125
fabi2

The aggregation of π-conjugated materials significantly impacts the photophysics and performance of optoelectronic devices. Nevertheless, little is known about the laws governing aggregate formation of π-conjugated materials from solution. In this Perspective, we compare, discuss, and summarize how aggregates form for three different types of compounds, that is, homopolymers, donor–acceptor type polymers, and low molecular weight compounds. To this end, we employ temperature-dependent optical spectroscopy, which is a simple yet powerful tool to investigate aggregate formation. We show how optical spectra can be analyzed to identify distinct conformational states. We find aggregate formation to proceed the same in all these compounds by a coil-to-globule-like first-order phase transition. Notably, the chain expands before it collapses into a highly ordered dense state. The role of side chains and the impact of changes in environmental polarization are addressed.

Monomolecular and Bimolecular Recombination of Electron– Hole Pairs at the Interface of a Bilayer Organic Solar Cell

Tobias Hahn, Steffen Tscheuschner, Frank-Julian Kahle, Markus Reichenberger, Stavros Athanasopoulos, Christina Saller, Guillermo C. Bazan, Thuc-Quyen Nguyen, Peter Strohriegl, Heinz Bässler and Anna Köhler

Adv. Funct. Mater. (2017) 27 1604906
toc-figure-6

While it has been argued that field-dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT-type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p-DTS(FBTTh2)2 layer as donor material and C60 as acceptor. The authors measure JV-characteristics as a function of intensity and charge-extraction-by-linearly-increasing-voltage-type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (Lp) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p-DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s-shape of the JV-curves and the concomitant additional decrease of the FF with increasing layer thickness.

Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C60 and PCBM

Tobias Hahn, Steffen Tscheuschner, Christina Saller, Peter Strohriegl, Puttaraju Boregowda, Tushita Mukhopadhyay, Satish Patil, Dieter Neher, Heinz Bässler and Anna Köhler

J. Phys. Chem. C (2016) 25083–25091
toc-5

In an endeavor to examine how optical excitation of C60 and PCBM contribute to the photogeneration of charge carriers in organic solar cells, we investigated stationary photogeneration in single-layer C60 and PCBM films over a broad spectrum as a function of the electric field. We find that intrinsic photogeneration starts at a photon energy of about 2.25 eV, i.e., about 0.4 eV above the first singlet excited state. It originates from charge transfer type states that can autoionize before relaxing to the lower-energy singlet S1 state, in the spirit of Onsager’s 1938 theory. We analyze the internal quantum efficiency as a function of electric field and photon energy to determine (1) the Coulombic binding and separation of the electron–hole pairs, (2) the value of the electrical gap, and (3) which fraction of photoexcitations can fully separate at a given photon energy. The latter depends on the coupling between the photogenerated charge transfer states and the eventual charge transporting states. It is by a factor of 3 lower in PCBM. Close to the threshold energy for intrinsic photoconduction (2.25 eV), the generating entity is a photogenerated electron–hole pair with roughly 2 nm separation. At higher photon energy, more expanded pairs are produced incoherently via thermalization.

Initiator-free crosslinking of oxetane functionalized low bandgap polymers: an approach towards stabilized bulk heterojunction solar cells

Philipp Knauer, Tobias Hahn, Anna Köhler and Peter Strohriegl

J. Mater. Chem. C (2016) 10347
toc-figure-2

A critical issue of bulk heterojunction (BHJ) solar cells is the instability of the morphology of the polymer:fullerene blend over long operation times. We report the synthesis of crosslinkable derivatives of the low bandgap polymer PFDTBT, poly(2,7-(9,9-dialkylfluorene)-alt-(5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole))), and the stabilization of BHJ solar cells by crosslinking. Oxetane units are attached to the polymer side chains as crosslinkable functional groups. We study the crosslinking of the polymers via cationic ring opening polymerization of the oxetanes and show that our materials rapidly form insoluble networks. Our materials also crosslink in the presence of fullerenes. We report for the first time that crosslinking takes place upon prolonged heating to 100 °C without any added initiator. The best efficiency and thermal stability are found in thermally crosslinked BHJ solar cells. After 30 hours at 100 °C, 65% of the initial efficiency are retained and no further decay is observed up to 100 hours.

Crosslinkable low bandgap polymers for organic solar cells

Peter Strohriegl, Christina Saller, Philipp Knauer, Anna Köhler, Tobias Hahn, Florian Fischer and Frank-Julian Kahle

Proc. of SPIE Vol. 9942, 99420O
spie_figure1-1

We present a number of polyfluorene based conjugated polymers with crosslinkable acrylate and oxetane units. These
polymers can be crosslinked by free radical polymerization in the case of acrylates and by cationic ring opening
polymerization for oxetanes. Upon polymerization densely crosslinked networks are formed which are completely
insoluble. We show that the diffusion coefficient of C60 in polyfluorene is reduced by a factor of 1000 by crosslinking.
MIS-CELIV measurements are used to monitor changes in the charge carrier mobility upon crosslinking. It shows that
using appropriate conditions, e.g. low initiator concentrations or thermal crosslinking, the charge carrier mobility is not
reduced by crosslinking. Solution processed three layer organic solar cells were realized with a crosslinkable fluorene
based copolymer containing acrylate groups. The efficiency is increased from 1.4% for the reference to 1.8% in the three
layer cell with a crosslinked exciton blocking layer. A critical issue of BHJ cells is the instability of the morphology of
the polymer:fullerene blend over long operation times at elevated temperature. We present a crosslinkable derivative of
the low bandgap polymer PFDTBT which contains oxetane units. BHJ cells with the crosslinked PFDTBT derivative and
PCBM were tested in accelerated aging experiments at 100 °C for times up to 100 h. Stabilization was clearly observed
in crosslinked BHJ cells compared to the non-crosslinked reference. We show for the first time that oxetane containing
polymers can be thermally crosslinked without any added initiator. Initiator free crosslinking is particularly attractive as
it avoids the formation of decomposition products, and thus potential electron traps and quenching sites from the
initiator.

Emission Enhancement and Intermittency in Polycrystalline Organolead Halide Perovskite Films

Cheng Li, Yu Zhong, Carlos Andres Melo Luna, Thomas Unger, Konstantin Deichsel, Anna Gräser, Jürgen Köhler, Anna Köhler, Richard Hildner and Sven Huettner

Molecules 21 (2016) 1081
cheng

Inorganic-organic halide organometal perovskites have demonstrated very promising performance for opto-electronic applications, such as solar cells, light-emitting diodes, lasers, single-photon sources, etc. However, the little knowledge on the underlying photophysics, especially on a microscopic scale, hampers the further improvement of devices based on this material. In this communication, correlated conventional photoluminescence (PL) characterization and wide-field PL imaging as a function of time are employed to investigate the spatially- and temporally-resolved PL in CH3NH3PbI3−xClx perovskite films. Along with a continuous increase of the PL intensity during light soaking, we also observe PL blinking or PL intermittency behavior in individual grains of these films. Combined with significant suppression of PL blinking in perovskite films coated with a phenyl-C61-butyric acid methyl ester (PCBM) layer, it suggests that this PL intermittency is attributed to Auger recombination induced by photoionized defects/traps or mobile ions within grains. These defects/traps are detrimental for light conversion and can be effectively passivated by the PCBM layer. This finding paves the way to provide a guideline on the further improvement of perovskite opto-electronic devices.

Synthesis, spectroscopic characterization, thermal and luminescent properties of new organosulfur-functionalized platinum(II) bis(alkenylarylalkynyl) complexes

Md. Mostafizur Rahman, Muhammad Younus, Masnun Naher, Mostafa Kamal Masud, Akihiro Nomoto, Akiya Ogawa, Alexander Rudnick, Anna Köhler

Journal of Organometallic Chemistry 818 (2016) 185-194
rudni

A series of organosulfur-functionalized trans-platinum(II) bis(alkenylarylalkynyl) complexes, having one
tolylthio moiety in each alkenyl backbone with general formula trans-[(PEt3)2Pt{C^C-Ar-CH]
CH(SC6H4eCH3)}2], (2ae2d), (where, Ar ¼ phenylene, biphenylene, 2,5-dimethylphenylene, and 2,5-
dimethoxyphenylene) were synthesized in good to excellent yields with good regioselectivity. As
compared to the absorption band of trans-platinum(II) bis(alkynylarylalkynyl) complexes, we found that
the position of the lowest energy absorption bands in the trans-platinum(II) bis(alkenylarylalkynyl)
complexes were red-shifted, after the functionalization of the trans-platinum(II) bis(alkynylarylalkynyl)
complexes with arylthiol. For all trans-platinum(II) complexes, the lowest energy absorption bands in the
UV/Vis spectra, in chloroform solution, at room temperature, were observed in the range 362e394 nm,
and under excitation at the wavelength of the absorption maximum exhibited the emission peak
maximum at room temperature in the range 401e426 nm. The newly synthesized complexes are not
exhibited phosphorescence at room temperature but are exhibited at low temperature, 77 K. All the new
platinum(II) complexes have been fully characterized by spectroscopic analysis as well as elemental
analysis, and the trans square-planar arrangement at the platinum centre has been confirmed by singlecrystal
X-ray diffraction study of complex 2a

Watching Paint Dry: The Impact of Diiodooctane on the Kinetics of Aggregate Formation in Thin Films of Poly(3-hexylthiophene)

Markus Reichenberger, Sebastian Baderschneider, Daniel Kroh, Steffen Grauf, Jürgen Köhler, Richard Hildner and Anna Köhler

Macromolecules 49 (2016) 6420-6430
Reichi2

We have investigated how the addition of 1,8-diiodooctane (DIO) alters the formation of disordered and ordered phases in a film of poly(3-hexyl-thiophene-2,5-diyl) (P3HT). By combining in situ time-resolved absorption spectroscopy with 60 ms time resolution, optical and transmission electron microscopy and spatially resolved photoluminescence spectroscopy, we show that, in addition to the excitonic coupling, the film formation process during spin-coating as well as the subsequent long-time film drying process differ significantly when DIO is added to a solution of P3HT. During spin-coating, the addition of DIO reduces the actual time for transformation from disordered to ordered phase, even though it increases the time until the disorder–order transition sets in. In place of a solidification front, we observe an all-over solidification throughout the entire film. The phase separation between nonaggregated and aggregated phase increases when using DIO, with compositional variation in the content of aggregated phase on a micrometer scale.

Effect of Thermal and Structural Disorder on the Electronic Structure of Hybrid Perovskite Semiconductor CH3NH3PbI3

Shivam Singh, Cheng Li, Fabian Panzer, K. L. Narasimhan, Anna Graeser, Tanaji P. Gujar, Anna Köhler, Mukundan Thelakkat, Sven Huettner and Dinesh Kabra

J. Phys. Chem. Lett. 7 (2016) 3014–3021
Unbenannt18

In this Letter, we investigate the temperature dependence of the optical properties of methylammonium lead iodide (MAPbI3 = CH3NH3PbI3) from room temperature to 6 K. In both the tetragonal (T > 163 K) and the orthorhombic (T < 163 K) phases of MAPbI3, the band gap (from both absorption and photoluminescence (PL) measurements) decreases with decrease in temperature, in contrast to what is normally seen for many inorganic semiconductors, such as Si, GaAs, GaN, etc. We show that in the perovskites reported here, the temperature coefficient of thermal expansion is large and accounts for the positive temperature coefficient of the band gap. A detailed analysis of the exciton line width allows us to distinguish between static and dynamic disorder. The low-energy tail of the exciton absorption is reminiscent of Urbach absorption. The Urbach energy is a measure of the disorder, which is modeled using thermal and static disorder for both the phases separately. The static disorder component, manifested in the exciton line width at low temperature, is small. Above 60 K, thermal disorder increases the line width. Both these features are a measure of the high crystal quality and low disorder of the perovskite films even though they are produced from solution.

The influence of torsion on excimer formation in bipolar host materials for blue phosphorescent OLEDs

Alexander A. Rudnick, Sergey Bagnich, Daniel Wagner, Stavros Athanasopoulos, Peter Strohriegl and Anna Köhler

J. Chem. Phys. 144 (2016) 214906
Figure 5_1

We present a combined detailed spectroscopic and quantum chemical study on the bipolar host materials BPTRZ and MBPTRZ in solution and in neat film. In the two compounds, the hole transporting carbazole is separated from the electron transporting triazine moiety by a fully aromatic but non-conjugated meta-linked biphenyl unit. The two materials differ by an additional steric twist at the biphenyl in MBPTRZ, which is achieved by methyl-substitution in 2- and 2′-position of the biphenyl. We find that while the twist shifts the triplet state in MBPTRZ to higher energies (3.0 eV in solution) compared to BPTRZ (2.8 eV in solution), this also localizes electron density on the carbazole moiety, leading to excimer formation in neat films.

Compact Layers of Hybrid Halide Perovskites Fabricated via the Aerosol Deposition Process—Uncoupling Material Synthesis and Layer Formation

Fabian Panzer, Dominik Hanft, Tanaji P. Gujar, Frank-Julian Kahle, Mukundan Thelakkat , Anna Köhler and Ralf Moos

Materials 9 (2016) 9040277
Bild123

We present the successful fabrication of CH3NH3PbI3 perovskite layers by the aerosol
deposition method (ADM). The layers show high structural purity and compactness, thus making
them suitable for application in perovskite-based optoelectronic devices. By using the aerosol
deposition method we are able to decouple material synthesis from layer processing. Our results
therefore allow for enhanced and easy control over the fabrication of perovskite-based devices, further
paving the way for their commercialization.

‘‘Hot or cold’’: how do charge transfer states at the donor–acceptor interface of an organic solar cell dissociate?

Heinz Bässler and Anna Köhler

Phys. Chem. Chem. Phys. 17 (2015) 28451-28462
c5cp04110d-f1_hi-res

Electron transfer from an excited donor to an acceptor in an organic solar cell (OSC) is an exothermic
process, determined by the difference in the electronegativities of donor and acceptor. It has been
suggested that the associated excess energy facilitates the escape of the initially generated electron–
hole pair from their mutual coulomb well. Recent photocurrent excitation spectroscopy on conjugated
polymer/PCBM cells challenged this view. In this perspective we shall briefly outline the strengths and
weaknesses of relevant experimental approaches and concepts. We shall enforce the notion that the
charge separating state is a vibrationally cold charge transfer (CT) state. It can easily dissociate provided
that (i) there is electrostatic screening at the interface and (ii) the charge carriers are delocalized, e.g. if
the donor is a well ordered conjugated polymer. Both effects diminish the coulomb attraction and
assure that the in-built electric field existing in the OSC under short current condition is already
sufficient to separate most the CT states. The remaining CT excitations relax towards tail states of the
disorder controlled density of states distribution, such as excimer forming states, that are more tightly
bound and have longer lifetimes.

Organic solar cells with crosslinked polymeric exciton blocking layer

T. Hahn, C. Saller, M. Weigl, I. Bauer, T. Unger, A. Köhler and P. Strohriegl

Phys. Status Solidi A 212 (2015) 2162-2168
18

We show that the performance of an organic solar cell can be increased by the introduction of an additional polymeric exciton blocking layer. In order to realize this, the novel polymer PFTPDAc with pendant acrylate groups is developed. Thin films are coated from a PFTPDAc solution and subsequently crosslinked by irradiation. Thereby, the film becomes completely insoluble and allows spincoating of a second polymer layer on top. We realize a three layer solar cell which contains a crosslinked PFTPDAc interlayer on top of the molybdenum oxide anode and layers of the low-bandgap polymer PCDTBT and C60. In comparison with a reference cell without the interlayer the EQE is significantly increased in the spectral region between 400 nm and 650 nm. From current-voltage measurements a power conversion efficiency of 1.8% is determined. PL measurements show that the increase of solar cell performance is attributed to exciton blocking by the PFTPDAc interlayer.

Revealing structure formation in PCPDTBT by optical spectroscopy

Christina Scharsich, Florian S. U. Fischer, Kevin Wilma, Richard Hildner, Sabine Ludwigs and Anna Köhler

J. Polym. Sci., Part B: Polym. Phys. 20 (2015) 1416-1430
17

The low-bandgap polymer poly{[4,4-bis(2-ethylhexyl)-cyclopenta-(2,1-b;3,4-b′)dithiophen]-2,6-diyl-alt-(2,1,3-benzo-thiadiazole)−4,7-diyl} (PCPDTBT) is widely used for organic solar cell applications. Here, we present a comprehensive study of the optical properties as a function of temperature for PCPDTBT in solution and in thin films with two distinct morphologies. Using absorption and photoluminescence spectroscopy as well as Franck-Condon analyses, we show that PCPDTBT in solution undergoes a phase transformation at 300 K from a disordered to a truly aggregated state on cooling. The saturation value of aggregates in solution is reached in PCPDTBT thin films at any temperature. In addition, we demonstrate that the photophysical properties of the aggregates in films are similar to those in solution and that a low percentage of thermally activated excimer states is present in the films at temperatures above 200 K.

A Combined Theoretical and Experimental Study of Dissociation of Charge Transfer States at the Donor − Acceptor Interface of Organic Solar Cells

Steffen Tscheuschner , Heinz Bässler , Katja Huber and Anna Köhler

J. Phys. Chem. B, 119 (2015) 10359–10371
jp-2015-051383_0010

The observation that in efficient organic solar cells almost all electron–hole pairs generated at the donor–acceptor interface escape from their mutual coulomb potential remains to be a conceptual challenge. It has been argued that it is the excess energy dissipated in the course of electron or hole transfer at the interface that assists this escape process. The current work demonstrates that this concept is unnecessary to explain the field dependence of electron–hole dissociation. It is based upon the formalism developed by Arkhipov and co-workers as well as Baranovskii and co-workers. The key idea is that the binding energy of the dissociating “cold” charge-transfer state is reduced by delocalization of the hole along the polymer chain, quantified in terms of an “effective mass”, as well as the fractional strength of dipoles existent at the interface in the dark. By covering a broad parameter space, we determine the conditions for efficient electron–hole dissociation. Spectroscopy of the charge-transfer state on bilayer solar cells as well as measurements of the field dependence of the dissociation yield over a broad temperature range support the theoretical predictions.

Triplet energies and excimer formation in meta- and para-linked carbazolebiphenyl matrix materials

Sergey A. Bagnich, Alexander Rudnick, Pamela Schroegel, Peter Strohriegl and Anna Köhler

Phil.Trans.R.Soc. A 373 (2015) 20140446
F2.medium

We present a spectroscopic investigation on the effect of changing the position where carbazole is attached to biphenyl in carbazolebiphenyl (CBP) on the triplet state energies and the propensity to excimer formation. For this, two CBP derivatives have been prepared with the carbazole moieties attached at the (para) 4- and 4′-positions (pCBP) and at the (meta) 3- and 3′-positions (mCBP) of the biphenyls. These compounds are compared to analogous mCDBP and pCDBP, i.e. two highly twisted carbazoledimethylbiphenyls, which have a high triplet energy at about 3.0 eV and tend to form triplet excimers in a neat film. This torsion in the structure is associated with localization of the excited state onto the carbazole moieties. We find that in mCBP and pCBP, excimer formation is prevented by localization of the triplet excited state onto the central moiety. As conjugation can continue from the central biphenyls into the nitrogen of the carbazole in the para-connected pCBP, emission involves mainly the benzidine. By contrast, the meta-linkage in mCBP limits conjugation to the central biphenyl. The associated shorter conjugation length is the reason for the higher triplet energy of 2.8 eV in mCBP compared with the 2.65 eV in pCBP.

Relaxation dynamics and exciton energy transfer in the low-temperature phase of MEH-PPV

Cristina Consani, Federico Koch, Fabian Panzer, Thomas Unger, Anna Köhler and Tobias Brixner

J. Chem. Phys. 142 (2015) 212429
1.4918645.figures.online.f5

Understanding the effects of aggregation on exciton relaxation and energy transfer is relevant to control photoinduced function in organic electronics and photovoltaics. Here, we explore the photoinduced dynamics in the low-temperature aggregated phase of a conjugated polymer by transient absorption and coherent electronic two-dimensional (2D) spectroscopy. Coherent 2D spectroscopy allows observing couplings among photoexcited states and discriminating band shifts from homogeneous broadening, additionally accessing the ultrafast dynamics at various excitation energies simultaneously with high spectral resolution. By combining the results of the two techniques, we differentiate between an initial exciton relaxation, which is not characterized by significant exciton mobility, and energy transport between different chromophores in the aggregate.

Origin of Meyer-Neldel type compensation behavior in organic semiconductors at large carrier concentrations: Disorder versus thermodynamic description

I. I. Fishchuk, A. Kadashchuk, A. Mityashin, M. M. Gavrilyuk, A. Köhler, H. Bässler, J. Genoe, H. Sitter and N. S. Sariciftci

Phys. Rev. 90 (2014) 245201
medium1

We have extended an effective medium approximation theory [Fishchuk, Kadashchuk, Genoe, Ullah, Sitter, Singh, Sariciftci, and Bässler, Phys. Rev. B 81, 045202 (2010)] to investigate how polaron formation affects the Meyer-Neldel (MN) compensation behavior observed for temperature-dependent charge-carrier transport in disordered organic semiconductors at large carrier concentrations, as realized in organic field-effect transistors (OFETs). We show that the compensation behavior in organic semiconductor thin films can be consistently described for both nonpolaronic and polaronic hopping transport in the framework of the disorder formalism using either Miller-Abrahams or polaron Marcus rates, respectively, provided that the polaron binding energy is small compared to the width of the density of states (DOS) distribution in the system. We argue that alternative models based on thermodynamic reasoning, like the multiexcitation entropy (MEE) model, which assumes charge transport dominated by polarons with multiphonon processes and ignores the energy disorder, are inherently not applicable to describe adequately the charge-carrier transport in disordered organic semiconductors. We have suggested and realized a test experiment based on measurements of the compensation behavior for the temperature-dependent conductivity and mobility in OFET devices to check the applicability of these models. We point out that the MN behavior observed in thin-film OFETs has nothing to do with the genuine MN rule predicted by the MEE approach, but rather it is an apparent effect arising as a consequence of the functional dependence of the partial filling of the DOS in a disordered system with hopping transport. This fact is fully supported by experimental results. The apparent MN energy was found to depend also on the shape of the DOS distribution and polaron binding energy.

Ultrafast Energy Transfer between Disordered and Highly Planarized Chains of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)

Thomas Unger, Fabian Panzer, Cristina Consani, Federico Koch, Tobias Brixner, Heinz Bässler and Anna Köhler

ACS Macro Lett 4 (2015) 412-416
Unger1

Upon cooling a solution of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), a phase transition occurs, leading to the formation of aggregates. We have studied the dynamics of singlet excitons in MEH-PPV solution below the critical temperature of the phase transition using steady-state photoluminescence measurements and pump–probe fs-spectroscopy at different temperatures. Spectral analysis indicates the coexistence of disordered chromophores with highly planarized chromophores. The high planarity is evidenced by a remarkably high 0–0/0–1 peak ratio in the spectra. By spectrally separating the contributions of either type of chromophore to the pump–probe signal we find that energy transfer takes place within less than 1 ps from disordered, unaggregated chain segments to highly planarized, aggregated chain segments. The short time scale of the energy transfer indicates intimate intermixing of the planarized and disordered polymeric chromophores.

Spectroscopic Signature of Two Distinct H-Aggregate Species in Poly(3-hexylthiophene)

Fabian Panzer, Michael Sommer, Heinz Bässler, Mukundan Thelakkat and Anna Köhler

Macromolecules 48 (2015) 1543–1553
Fabi1

In an endeavor to correlate the optoelectronic properties of π-conjugated polymers with their structural properties, we investigated the aggregation of P3HT in THF solution within a temperature range from 300 to 5 K. By detailed steady-state, site-selective, and time-resolved fluorescence spectroscopy combined with Franck–Condon analyses, we show that below a certain transition temperature (265 K) aggregates are formed that prevail in different polymorphs. At 5 K, we can spectroscopically identify two H-type aggregates with planar polymer backbones yet different degree of order regarding their side chains. Upon heating, the H-character of the aggregates becomes gradually eroded, until just below the transition temperature the prevailing “aggregate” structure is that of still phase-separated, yet disordered main and side chains. These conclusions are derived by analyzing the vibrational structure of the spectra and from comparing the solution spectra with those obtained from thin films that were cooled slowly from the melting temperature to room temperature and that had been analyzed previously by various X-ray techniques. In addition, site selectively recorded fluorescence spectra show that there is—dependent on temperature—energy transfer from higher energy to lower energy aggregates. This suggests that they must form clusters with dimensions of the exciton diffusion length, i.e., several nanometers in diameter.

Excimer Formation by Steric Twisting in Carbazole and Triphenylamine-Based Host Materials

Sergey A. Bagnich, Stavros Athanasopoulos, Alexander Rudnick, Pamela Schroegel, Irene Bauer, Neil C. Greenham, Peter Strohriegl and Anna Köhler

J. Phys. Chem. C 119 (2015) 2380-2387
Excimer Formation by Steric Twisting in Carbazole and Triphenylamine-Based Host Materials

This paper presents a detailed spectroscopic investigation of luminescence properties of 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and N,N,N′,N′-tetraphenylbenzidine (TAD) in solutions and neat films. These compounds are compared to their derivatives CDBP and TDAD that contain methyl groups in the 2 and 2′ position of the biphenyl core. We find that whereas steric twisting in CDBP and TDAD leads to a high triplet energy of about 3.0 and 3.1 eV, respectively, these compounds also tend to form triplet excimers in a neat film, in contrast to CBP and TAD. By comparison with N-phenylcarbazole (NPC) and triphenylamine (TPA), on which these compounds are based, as well as with the rigid spiro analogues to CBP and TAD we show that the reduced excimer formation in CBP and TAD can be attributed to a localization of the excitation onto the central biphenyl part of the molecule.

Does Excess Energy Assist Photogeneration in an Organic Low-Bandgap Solar Cell?

Tobias Hahn, Johannes Geiger, Xavier Blase, Ivan Duchemin, Dorota Niedzialek, Steffen Tscheuschner, David Beljonne, Heinz Bässler and Anna Köhler

Advanced Functional Materials 8 (2015) 1287-1295
Tobi1

The field dependence of the photocurrent in a bilayer assembly is measured with the aim to clarify the role of excess photon energy in an organic solar cell comprising a polymeric donor and an acceptor. Upon optical excitation of the donor an electron is transferred to the acceptor forming a Coulomb-bound electron–hole pair. Since the subsequent escape is a field assisted process it follows that photogeneration saturates at higher electric fields, the saturation field being a measure of the separation of the electron–hole pair. Using the low bandgap polymers, PCDTBT and PCPDTBT, as donors and C60 as acceptor in a bilayer assembly it is found that the saturation field decreases when the photon energy is roughly 0.5 eV above the S1–S0 0–0 transition of the donor. This translates into an increase of the size of the electron-hole-pair up to about 13 nm which is close to the Coulomb capture radius. This increase correlates with the onset of higher electronic states that have a highly delocalized character, as confirmed by quantum-chemical calculations. This demonstrates that accessing higher electronic states does favor photogeneration yet excess vibrational energy plays no role. Experiments on intrinsic photogeneration in donor photodiodes without acceptors support this reasoning.

Ground State Bleaching at Donor–Acceptor Interfaces

Christian Schwarz, Felix Milan, Tobias Hahn, Markus Reichenberger, Stephan Kümmel and Anna Köhler

Advanced Functional Materials 41 (2014) 6439–6448
Bild 51

Charge separation at the donor–acceptor interface is a key step for high efficiency in organic solar cells. If interfacial hybrid states exist already in the dark it is plausible that they can have a major impact on the dissociation of optically generated excitations. In this work we probe such interfacial states via steady state absorption spectroscopy. A substantial bleaching of the absorption spectrum is found near the absorption edge when an electron-accepting layer of either trinitrofluorenone (TNF), C60, or a perylene-diimide derivative is deposited on top of a layer of electron-donating conjugated polymers, such as MEH-PPV or various poly-phenylene. This is in part attributed to the formation of ground state complexes with low oscillator strength. The experiments bear out a correlation between the reduction of the absorbance with the energy gap between the donor-HOMO and acceptor-LUMO, the effective conjugation length of the donor, and the efficiency of exciton dissociation in the solar cell. The effect originates from mixing of the donor-HOMO and the acceptor LUMO. Calculations using density functional theory support this reasoning. Implications for efficiency of organic solar cells will be discussed.

Analytic model of hopping transport in organic semiconductors including both energetic disorder and polaronic contributions

I. I. Fishchuk, A. Kadashchuk, S. T. Hoffmann, S. Athanasopoulos, J. Genoe, H. Bässler and A. Köhler

AIP Conf. Proc. 47 (2014) 47-52
Bild 50

We developed an analytical model to describe hopping conductivity and mobility in organic semiconductors including both energetic disorder and polaronic contributions. The model is based on the Marcus jump rates with a Gaussian energetic disorder, and it is premised upon a generalized Effective Medium approach yet avoids shortcoming involved in the effective transport energy or percolation concepts. The carrier concentration dependence becomes considerably weaker when the polaron energy increases relative to the disorder energy, indicating the absence of universality that is at variance with recent publications.

The Impact of Polydispersity and Molecular Weight on the Order− Disorder Transition in Poly(3-hexylthiophene)

Fabian Panzer, Heinz Bässler, Ruth Lohwasser, Mukundan Thelakkat and Anna Köhler

J. Phys. Chem. Lett. 5 (2014) 2742−2747
Panzer

Conjugated poly(3-hexylthiophene) (P3HT) chains are known to exist at least
in two distinct conformations: a coiled phase and a better ordered aggregated phase.
Employing steady state absorption and fluorescence spectroscopy, we measure the course of
aggregation of P3HT in tetrahydrofuran (THF) solution within a temperature range of 300 K
to 170 K. We show that aggregation is a temperature controlled process, driven by a
thermodynamic order−disorder transition. The transition temperature increases with the
molecular weight of the chains and can be rationalized in the theory of Sanchez. This implies
a smearing out of the phase transition in samples with increasing polydispersity and erodes
the signature of a first order phase transition. The detection of a hysteresis when undergoing
cooling/heating cycles further substantiates this reasoning.

Measuring Reduced C 60 Diffusion in Crosslinked Polymer Films by Optical Spectroscopy

Florian Fischer , Tobias Hahn , Heinz Bässler , Irene Bauer , Peter Strohriegl and Anna Köhler

Adv. Funct. Mater. 39 (2014) 6172-6177
Fischer

The diffusion of fullerenes such as C 60 and PCBM in organic semiconductors
is a key factor in controlling the effi ciency of organic solar cells, though
it is challenging to measure and to control. A simple optical method based
on photoluminescence quenching is developed to assess the diffusion of
a quencher molecule such as C 60 through a semiconducting polymer fi lm,
in this case made with the polymer polyfl uorene. When the mobility of the
polymer chains is reduced by chemical crosslinking, the diffusion coeffi cient
of C 60 can be reduced by up to three orders of magnitude.

Influence of the Excited-State Charge-Transfer Character on the Exciton Dissociation in Donor−Acceptor Copolymers

Katharina Neumann, Christian Schwarz, Anna Köhler and Mukundan Thelakkat

J. Phys. Chem. C 118 ( 2014) 27–36
Influence of the excited state-2

We synthesize a polytriphenylamine homopolymer and two donor–acceptor copolymers (D–A-copolymers) based on triphenylamine (TPA) as donor in combination with two different acceptor moieties to study the effect of the acceptor unit on the excited-state charge-transfer characteristics (CT-characteristics) and charge separation. The two acceptor moieties are a dicyanovinyl group in the side chain and a thieno[3,4-b]thiophene carboxylate in the main chain. Absorption and photoluminescence studies show new CT-bands for both of the D–A-copolymers. Field-dependent charge extraction studies in bilayer solar cells indicate a stronger CT-character for the copolymer in which the acceptor group is less conjugated with the copolymer backbone. The D–A-copolymer carrying the acceptor unit in the main chain exhibits smaller excitonic CT-character and good conjugation leading to less-bound electron–hole pairs and a better charge separation. This fundamental study gives insight into the interdependence of conjugation, charge carrier mobility, and solar cell performance for two different D–A-copolymers.

Rod-Like Nano-Light Harvester

Jun Ling , Zhicheng Zheng , Anna Köhler , Axel H. E. Müller

Macromol. Rapid Commun. 35 (2014) 52-55
123456

Imitating the natural “energy cascade” architecture, we present a single-molecular rod-like nano-light harvester (NLH) based on a cylindrical polymer brush. Block copolymer side chains carrying (9,9-diethylfl uoren-2-yl)methyl methacrylate units as light absorbing antennae (energy donors) are tethered to a linear polymer backbone containing 9-anthracenemethyl methacrylate units as emitting groups (energy acceptors). These NLHs exhibit very effi cient energy absorption and transfer. Moreover, we manipulate the energy transfer by tuning the donor–acceptor distance.

Novel host materials for blue phosphorescent OLEDs

Peter Strohriegl, Daniel Wagner, Pamela Schrögel, Sebastian T. Hoffmann, Anna Köhler, Ute Heinemeyer, Ingo Münster

Proc. of SPIE 8829 (2013) 882906
Bild20

We present two classes of host materials for blue phosphors. The first are carbazole substituted biphenyls 1-9. In these CBP-type materials the triplets are confined to one half of the molecules by using either twisted biphenyls or by a metalinkage of the carbazoles to the biphenyl. We obtained high triplet energies of 2.95-2.98 eV and high glass transition temperatures in the range of 100-120 °C. OLEDs were fabricated using the host material 6 and the carbene emitter Ir(dbfmi) with pure blue emission at 450 nm. The devices achieved an external quantum efficiency of 8.7% at 100 cd/m2 and 6.1% at 1000 cd/m2. MBPTRZ with an electron transporting biscarbazolyltriazine that is separated from the hole transporting carbazole by a non-conjugated, meta-linked biphenyl unit is an example for a bipolar matrix material. The excellent glass forming properties and the high Tg of 132 °C ensure morphological stability in OLEDs. The meta-linkage and the additional twist at the biphenyl unit, which is achieved by two methyl groups in the 2- and 2’-position of the biphenyl in MBPTRZ leads to a decoupling of the electron accepting and electron donating part and therefore to a high triplet energy of 2.81 eV. DFT calculations show a clear separation of the electron and hole transporting moieties. A phosphorescent OLED with MBPTRZ as host and FIrpic as emitter reached a maximum external quantum efficiency of 7.0%, a current efficiency of 16.3 cd/A and a power efficiency of 6.3 lm/W.

Triazine Based Bipolar Host Materials for Blue Phosphorescent OLEDs

Daniel Wagner, Sebastian T. Hoffmann, Ute Heinemeyer, Ingo Münster, Anna Köhler and Peter Strohriegl

Chem. Mater.,25 (2013) 3758–3765
14

Two novel bipolar host materials BPTRZ and MBPTRZ were synthesized, in which the hole transporting carbazole is separated from the electron transporting triazine moiety by a fully aromatic, but nonconjugated meta-linked biphenyl unit. The additional twist at the biphenyl in MBPTRZ, which is achieved by methyl-substitution in 2- and 2 ′ -position of the biphenyl leads to a higher triplet energy of 2.81 eV compared to 2.70 eV for BPTRZ. Both materials possess high thermal stabilities and good glass forming properties. An organic light emitting diode with MBPTRZ as host for the blue phosphorescence emitter FIrpic shows a maximum luminance of 30600 cd/m 2 and a maximum external quantum e ffi ciency of 7.0%.

The red-phase of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV): A disordered HJ-aggregate

Hajime Yamagata, Nicholas J. Hestand, Frank C. Spano, Anna Köhler, Christina Scharsich, Sebastian T. Hoffmann and Heinz Bässler

J. Chem. Phys. 139,(2013) 114903
15

The ratio of the 0-0 to 0-1 peak intensities in the photoluminescence (PL) spectrum of red-phase poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], better known as MEH-PPV, is significantly enhanced relative to the disordered blue-phase and is practically temperature independent in the range from T = 5 K to 180 K. The PL lifetime is similarly temperature independent. The measured trends are accounted for by modeling red-phase MEH-PPV as disordered π-stacks of elongated chains. Using the HJ-aggregate Hamiltonian expanded to include site disorder amongst electrons and holes, the absorption and PL spectra of cofacial MEH-PPV dimers are calculated. The PL 0-0/0-1 line strength ratio directly responds to the competition between intrachain interactions which promote J-aggregate-like behavior (enhanced PL ratio) and interchain interactions which promote H-aggregate-like behavior (attenuated PL ratio). In MEH-PPV aggregates, J-like behavior is favored by a relatively large intrachain exciton bandwidth – roughly an order of magnitude greater than the interchain bandwidth – and the presence of disorder. The latter is essential for allowing 0-0 emission at low temperatures, which is otherwise symmetry forbidden. For Gaussian disorder distributions consistent with the measured (inhomogeneous) line widths of the vibronic peaks in the absorption spectrum, calculations show that the 0-0 peak maintains its dominance over the 0-1 peak, with the PL ratio and radiative lifetime practically independent of temperature, in excellent agreement with experiment. Interestingly, interchain interactions lead only to about a 30% drop in the PL ratio, suggesting that the MEH-PPV π-stacks – and strongly disordered HJ-aggregates in general – can masquerade as single (elongated) chains. Our results may have important applications to other emissive conjugated polymers such as the β-phase of polyfluorenes.

Unified description for hopping transport in organic semiconductors including both energetic disorder and polaronic contributions

I. I. Fishchuk, A. Kadashchuk, S. T. Hoffmann, S. Athanasopoulos, J. Genoe, H. Bässler and A. Köhler

Physical Review B 88,(2013) 125202
Neu2

We developed an analytical model to describe hopping transport in organic semiconductors including both
energetic disorder and polaronic contributions due to geometric relaxation. The model is based on a Marcus
jump rate in terms of the small-polaron concept with a Gaussian energetic disorder, and it is premised upon
a generalized effective medium approach yet avoids shortcomings involved in the effective transport energy
or percolation concepts. It is superior to our previous treatment [Phys. Rev. B 76, 045210 (2007)] since it is
applicable at arbitrary polaron activation energy Ea with respect to the energy disorder parameter σ. It can be
adapted to describe both charge-carrier mobility and triplet exciton diffusion. The model is compared with results
fromMonte Carlo simulations.We show (i) that the activation energy of the thermally activated hopping transport
can be decoupled into disorder and polaron contributions whose relative weight depend nonlinearly on the σ/Ea
ratio, and (ii) that the choice of the density of occupied and empty states considered in configurational averaging
has a profound effect on the results of calculations of theMarcus hopping transport. The σ/Ea ratio governs also
the carrier-concentration dependence of the charge-carrier mobility in the large-carrier-concentration transport
regime as realized in organic field-effect transistors. The carrier-concentration dependence becomes considerably
weaker when the polaron energy increases relative to the disorder energy, indicating the absence of universality.
This model bridges a gap between disorder and polaron hopping concepts.

Controlling the π-Stacking Behavior of Pyrene Derivatives: Influence of H-Bonding and Steric Effects in Different States of Aggregation

Andreas T. Haedler, Holger Misslitz, Christian Buehlmeyer, Rodrigo Q. Albuquerque, Anna Köhler and Hans-Werner Schmidt

ChemPhysChem,14, (2013) 1818 – 1829
1

The performance of opto-electronic devices built from low-molecular-weight dye molecules depends crucially on the stacking properties and the resulting coupling of the chromophoric systems. Herein we investigate the influence of H-bonding amide and bulky substituents on the π-stacking of pyrene-containing small molecules in dilute solution, as supramolecular aggregates, and in the solid state. A set of four pyrene derivatives was synthesized in which benzene or 4-tert-butyl benzene was linked to the pyrene unit either through an ester or an amide. All four molecules form supramolecular H-aggregates in THF solution at concentrations above 1×10−4 mol/l. These aggregates were transferred on a solid support and crystallized. We investigate: the excimer formation rates within supramolecular aggregates; the formation of H-bonds as well as the optical changes during the transition from the amorphous to the crystalline state; and the excimer to monomer fluorescence ratio in crystalline films at low temperatures. We reveal that in solution supramolecular aggregation depends predominantly on the pyrene chromophores. In the crystalline state, however, the pyrene stacking can be controlled gradually by H-bonding and steric effects. These results are further confirmed by molecular modeling. This work bears fundamental information for tailoring the solid state of functional optoelectronic materials.

To Hop or Not to Hop? Understanding the Temperature Dependence of Spectral Diffusion in Organic Semiconductors

Stavros Athanasopoulos , Sebastian T. Hoffmann , Heinz Bässler , Anna Köhler and David Beljonne

J. Phys. Chem. Lett., 4 , (2013) 1694–1700
Neu1

In disordered organic semiconductors, excited states and charges move by hopping in an inhomogeneously broadened density of states, thereby relaxing energetically (“spectral diffusion”). At low temperatures, transport can become kinetically frustrated and consequently dispersive. Experimentally, this is observed predominantly for triplet excitations and charges, and has not been reported for singlet excitations. We have addressed the origin of this phenomenon by simulating the temperature dependent spectral diffusion using a lattice Monte Carlo approach with either Miller–Abrahams or Förster type transfer rates. Our simulations are in agreement with recent fluorescence and phosphorescence experimental results. We show that frustrated and thus dispersive diffusion appears when the number of available hopping sites is limited. This is frequently the case for triplets that transfer by a short-range interaction, yet may also occur for singlets in restricted geometries or dilute systems. Frustration is lifted when more hopping sites become available, e.g., for triplets as a result of an increased conjugation in some amorphous polymer films.

Role of the effective mass and interfacial dipoles on exciton dissociation in organic donor-acceptor solar cells

Christian Schwarz, Steffen Tscheuschner, Johannes Frisch, Stefanie Winkler, Norbert Koch, Heinz Bässler and Anna Köhler

Phys. Rev. B 87 (2013) 155205
Christian1

Efficient exciton dissociation at a donor-acceptor interface is the crucial, yet not fully understood, step for obtaining high efficiency organic solar cells. Recent theoretical work suggested an influence of polymer conjugation length and of interfacial dipoles on the exciton dissociation yield. This necessitates experimental verification. To this end, we measured the dissociation yield of several polymer/C60 planar heterojunction solar cells up to high electric fields. The results indeed prove that the yield of exciton dissociation depends strongly on the conjugation length of the polymers. Complementary photoemission experiments were carried out to assess the importance of dipoles at the donor-acceptor interfaces. Comparison of exciton dissociation models with experimental data shows that the widely used Onsager-Braun approach is unsuitable to explain photodissociation in polymer/C60 cells. Better agreement can be obtained using “effective mass” models that incorporate conjugation length effects by considering a reduced effective mass of the hole on the polymer and that include dielectric screening effects by interfacial dipoles. However, successful modeling of the photocurrent field dependence over a broad field range, in particular for less efficient solar cell compounds, requires that the dissociation at localized acceptor sites is also taken into account.

How Do Disorder, Reorganization, and Localization Influence the Hole Mobility in Conjugated Copolymers?

Sebastian T. Hoffmann, Frank Jaiser, Anna Hayer, Heinz Bässler, Thomas Unger, Stavros Athanasopoulos, Dieter Neher and Anna Köhler

J. Am. Chem. Soc.135 (2013) 1772-1782
ja-2012-08820j_0010

In order to unravel the intricate interplay between disorder effects, molecular reorganization, and charge carrier localization, a comprehensive study was conducted on hole transport in a series of conjugated alternating phenanthrene indenofluorene copolymers. Each polymer in the series contained one further comonomer comprising monoamines, diamines, or amine-free structures, whose influence on the electronic, optical, and charge transport properties was studied. The series covered a wide range of highest occupied molecular orbital (HOMO) energies as determined by cyclovoltammetry. The mobility, inferred from time-of-flight (ToF) experiments as a function of temperature and electric field, was found to depend exponentially on the HOMO energy. Since possible origins for this effect include energetic disorder, polaronic effects, and wave function localization, the relevant parameters were determined using a range of methods. Disorder and molecular reorganization were established first by an analysis of absorption and emission measurements and second by an analysis of the ToF measurements. In addition, density functional theory calculations were carried out to determine how localized or delocalized holes on a polymer chain are and to compare calculated reorganization energies with those that have been inferred from optical spectra. In summary, we conclude that molecular reorganization has little effect on the hole mobility in this system while both disorder effects and hole localization in systems with low-lying HOMOs are predominant. In particular, as the energetic disorder is comparable for the copolymers, the absolute value of the hole mobility at room temperature is determined by the hole localization associated with the triarylamine moieties.

No more breaks for electrons

Anna Köhler

Nature Materials 11 (2012) 836–837
untitled

In semiconducting polymers, the mobility of negative charges is typically much smaller than that of positive
charges. Identification of a universal electron-trap level that is associated with water complexation now clarifies
this difference and provides guidelines for the design of improved organic semiconductors.

How do Triplets and Charges Move in Disordered Organic Semiconductors? A Monte Carlo Study Comprising the Equilibrium and Nonequilibrium Regime

Sebastian T. Hoffmann,Stavros Athanasopoulos, David Beljonne, Heinz Bässler and Anna Köhler

J. Phys. Chem. C 116 (2012) 16371–16383
untitled

We have investigated how electronic excitations that couple via short-range interaction, i.e., triplet excitations and charge carriers, move in a disordered organic semiconductor. In this systematic study, we paid special emphasis to the transition from quasi-equilibrium to nonequilibrium transport as the temperature is lowered from 300 to 10 K. As a method, we used Monte Carlo simulations employing both Marcus as well as Miller–Abrahams (MA) transition rates. The simulation parameters are the degree of static energetic disorder, the geometric reorganization energy, and the degree of electronic coupling among the hopping sites. In the case of conjugated polymers, the effects of intrachain versus interchain transport are taken into account. In the simulations, we monitor the spectral relaxation of excitations as well as their diffusivity. We find that, below a disorder controlled transition temperature, transport becomes kinetically frustrated and, concomitantly, dispersive. In this temperature regime, transport is controlled by single phonon tunneling, tractable in terms of MA rates, while in the high temperature regime multiphonon hopping, described by Marcus rates, prevails. The results also provide a quantitative assessment of dispersive excitation transport within the intermediate temperature regime in which no analytic theory is available so far. Quantitative agreement between simulation and previous experiments allows one to extract system parameters such as the minimum hopping time and to delineate the parameter range in which Marcus and MA rates should be used in transport studies.

Why Does the Electrical Conductivity in PEDOT:PSS Decrease with PSS Content? A Study Combining Thermoelectric Measurements with Impedance Spectroscopy

Thomas Stöcker, Anna Köhler, Ralf Moos

Journal of Polymer Science Part B: Polymer Physics, (2012) 976–983
pub3_small

We have investigated the electrical transport properties of poly(3,4-ethylenedioxythiophen)/poly(4-styrene-sulfonate) (PEDOT:PSS) with PEDOT-to-PSS ratios from 1:1 to 1:30. By combining impedance spectroscopy with thermoelectric measurements, we are able to independently determine the variation of electrical conductivity and charge carrier density with PSS content. We find the charge carrier density to be independent of the PSS content. Using a generalized effective media theory, we show that the electrical conductivity in PEDOT:PSS can be understood as percolation between sites of highly conducting PEDOT:PSS complexes with a conductivity of 2.3 (Xcm)1 in a matrix of excess PSS with a low conductivity of 103 (X cm)1. In addition to the transport properties, the thermoelectric power factors and Seebeck coefficients have been determined. VC 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 976–983, 2012

An Order−Disorder Transition in the Conjugated Polymer MEH-PPV

Anna Köhler, Sebastian T. Hoffmann and Heinz Bässler

J. Am. Chem. Soc. 134 (2012) 11594–11601
ja-2012-02408a_0007

The poly(p-phenylene vinylene) derivative MEH-PPV is known to exist as two morphologically distinct species, referred to as red phase and blue phase. We show here that the transition from the blue phase to the red phase is a critical phenomenon that can be quantitatively described as a second order phase transition with a critical temperature Tc of 204 K. The criticality is associated with the trade-off between the gain in the electronic stabilization energy when the π- system of a planarized chain can delocalize and the concomitant loss of entropy. We studied this transition by measuring the absorption and fluorescence in methyltetrahydrofuran (MeTHF) in two different concentrations as a function of temperature. The spectra were analyzed based upon the Kuhn exciton model to extract effective conjugation lengths. At room temperature, the chains have effective conjugation lengths of about five repeat units in the ground state (the blue phase), consistent with a disordered defect cylinder conformation. Upon cooling below the critical temperature Tc, the red phase with increased effective conjugation lengths of about 10 repeat units forms, implying a more extended and better ordered conformation. Whereas aggregation is required for the creation of the red phase, its electronic states have a predominant intrachain character.

The role of C-H and C-C stretching modes in the intrinsic non-radiative decay of triplet states in a Pt-containing conjugated phenylene ethynylene

Anna Köhler, Amena L. T. Khan, Joanne S. Wilson, Carsten Dosche, Mohammed K. Al-Suti et al.

J. Chem. Phys. 136, 094905 (2012)
Unbenannt2

The intrinsic non-radiative decay (internal conversion) from the triplet excited state in phosphorescent dyes can be described by a multi-phonon emission process. Since non-radiative decay of triplet excitons can be a significant process in organic light-emitting diodes, a detailed understanding of this decay mechanism is important if the overall device efficiency is to be controlled. We compare a deuterated Pt(II)-containing phenylene ethynylene with its non-deuterated counterpart in order to investigate which phonon modes control to the non-radiative decay path. We observe that deuteration does not decrease the non-radiative decay rate. A Franck-Condon analysis of the phosphorescence spectra shows that the electronic excitation is coupled strongly to the breathing mode of the phenyl ring and the C≡C carbon stretching modes, while high-energy C-H or C-D stretching modes play an insignificant role. We, therefore, associate the internal conversion process with the carbon-carbon stretching vibrations.

Does conjugation help exciton dissociation? A study on poly(p-phenylene)s in planar heterojunctions with C60 or TNF

Christian Schwarz, Heinz Bässler, Irene Bauer, Jan-Moritz Koenen, Eduard Preis, Ullrich Scherf and Anna Köhler

Advanced Materials 24 (2012) 922–925
Unbenannt5

Internal photocurrent quantum yields near 100% can be obtained from the separation of loosely bound geminate pairs when sufficiently large electric fields are applied to organic heterojunctions. The fields needed for complete electron–hole dissociation decrease to those prevailing in organic solar cells under operating conditions when well-conjugated polymers are employed.

Control of Aggregate Formation in Poly(3-hexylthiophene) by Solvent, Molecular Weight and Synthetic Method (Editor’s Choice Paper)

Christina Scharsich, Ruth H. Lohwasser, Michael Sommer, Udom Asawapirom, Ullrich Scherf, Mukundan Thelakkat, Dieter Neher, Anna Köhler

J. Poly. Sci. B: Polymer Physics 50 (2012) 442–453
image description

Aggregate formation in poly(3-hexylthiophene) depends on molecular weight, solvent, and synthetic method. The interplay of these parameters thus largely controls device performance. In order to obtain a quantitative understanding on how these factors control the resulting electronic properties of P3HT, we measured absorption in solution and in thin films as well as the resulting field effect mobility in transistors. By a detailed analysis of the absorption spectra, we deduce the fraction of aggregates formed, the excitonic coupling within the aggregates, and the conjugation length within the aggregates, all as a function of solvent quality for molecular weights from 5 to 19 kDa. From this, we infer in which structure the aggregated chains pack. Although the 5 kDa samples form straight chains, the 11 and 19 kDa chains are kinked or folded, with conjugation lengths that increase as the solvent quality reduces. There is a maximum fraction of aggregated chains (about 55 ± 5%) that can be obtained, even for poor solvent quality. We show that inducing aggregation in solution leads to control of aggregate properties in thin films. As expected, the field-effect mobility correlates with the propensity to aggregation. Correspondingly, we find that a well-defined synthetic approach, tailored to give a narrow molecular weight distribution, is needed to obtain high field effect mobilities of up to 0.01 cm2/Vs for low molecular weight samples (=11 kDa), while the influence of synthetic method is negligible for samples of higher molecular weight, if low molecular weight fractions are removed by extraction.

On the Formation Mechanism for Electrically Generated Exciplexes in a Carbazole-Pyridine Copolymer

Anna Hayer, Tanguy Van Regemorter, Bianca Höfer, Chris S. K. Mak, David Beljonne, Anna Köhler

J. Poly. Sci. B: Polymer 50 (2012) 361-369
mgra001

Although carbazole-containing copolymers are frequently used as hole-transporting host materials for polymer organic light-emitting diodes (OLEDs), they often suffer from the formation of undesired exciplexes when the OLED is operated. The reason why exciplexes sometimes form for electrical excitation, yet not for optical excitation is not well understood. Here, we use luminescence measurements and quantum chemical calculations to investigate the mechanism of such exciplex formation for electrical excitation (electroplex formation) in a carbazole–pyridine copolymer. Our results suggest that the exciplex is formed via a positively charged interchain precursor complex. This complex is stabilized by interactions that involve the nitrogen lone pairs on both chain segments.

Auf dem Weg zu neuen Solarzellen: Selbstorganisation von Polymeren fördert die Ladungstrennung

Christian Wißler M.A. (Hrsg.)

Mediendienst Forschung Universität Bayreuth, 2011 – Nr. 40 // 26. Oktober 2011

Lichtenergie in elektrischen Strom zu verwandeln ist die Funktion von Solarzellen, die
damit eine zentrale Bedeutung für die Gewinnung und Nutzung erneuerbarer Energien
haben. Einem Forschungsteam der Universität Bayreuth, der LMU München und der
TU München ist es jetzt gelungen, in organischen Makromolekülen erstmalig den Prozess der Stromerzeugung aus Licht von Anfang an zu beobachten und zu verstehen.
Ein weltweit einzigartiger lasertechnischer Versuchsaufbau und die Verwendung von
Silizium-basierten Strukturen machten diese Forschungserkenntnisse möglich.

Charge transport in organic semiconductors

Heinz Bässler, Anna Köhler

Top Curr Chem 312, (2012) 1-66

Charge transport in organic semiconductors is a timely subject. Today, organic semiconductors are already widely used commercially in Xerography. For display and lighting applications they are employed as light emitting diodes (LEDs or OLEDs) or transistors, and they are making progress to enter the solar cell market.[1-6] As a result, interest in the science behind this novel class of materials has risen sharply. The optoelectronic properties of organic semiconductors differ from that of conventional inorganic crystalline semiconductors in many aspects and the knowledge of organic semiconductors is imperative to further advance with the associated semiconductor applications.[7] A central problem is the understanding of the mechanisms related to charge transport

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photvoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

Daniel Herrmann, Sabrina Niesar, Christina Scharsich, Anna Köhler, Martin Stutzmann and Eberhard Riedle

J. Am. Chem. Soc. 133 (2011) 18220-18233
ja-2011-07887q_0011

Despite the central role of light absorption and the subsequent generation of free charge carriers in organic and hybrid organic-inorganic photovoltaics, the precise process of this initial photoconversion is still debated. We employ a novel broadband (UV-Vis-NIR) transient absorption spectroscopy setup to probe charge generation and recombination in the thin films of the recently suggested hybrid material combination poly(3-hexylthiophene)/silicon (P3HT/Si) with 40 fs time resolution. Our approach allows for monitoring the time evolution of the relevant transient species under various excitation intensities and excitation wavelengths. Both in regioregular (RR) and regiorandom (RRa) P3HT, we observe an instant (<40 fs) creation of singlet-excitons, which subsequently dissociate to form polarons in 140 fs. The quantum yield of polaron formation through dissociation of delocalized excitons is significantly enhanced by adding Si as an electron acceptor, revealing ultrafast electron transfer from P3HT to Si. P3HT/Si films with aggregated RR-P3HT are found to provide free charge carriers in planar as well as in bulk heterojunctions and losses are due to nongeminate recombination. In contrast for RRa-P3HT/Si, geminate recombination of bound carriers is observed as the dominant loss mechanism. Site-selective excitation by variation of pump wavelength uncovers an energy transfer from P3HT coils to aggregates with a 1/e transfer time of 3 ps and reveals a factor of 2 more efficient polaron formation using aggregated RR-P3HT compared to disordered RRa-P3HT. Therefore, we find that polymer structural order rather than excess energy is the key criterion for free charge generation in hybrid P3HT/Si solar cells.

Triplet-Triplet Annihilation in a Series of Poly(p-phenylene) Derivatives

S.T. Hoffmann, J.M. Koenen, U. Scherf, I. Bauer, P. Strohriegl, H. Bässler, A. Köhler

J. Phys. Chem. B 115 (2011) 8417-8423
jp-2011-02763e_0001

We have studied the temperature dependence of phosphorescence (Ph) and delayed fluorescence (DF) in two series of poly(p-phenylene) derivatives within a temperature range from 10 to 300 K under quasi-stationary conditions. One set of materials consists of the dimer, trimer, and polymer of ethylhexyl-substituted poly(fluorene) (PF2/6) and thus allows us to assess the effects of oligomer length. The second series addresses the influence of energetic disorder and conjugation length by being composed of the polymers alkoxy-substituted poly(p-phenylene) (DOO-PPP), poly(indenofluorene) (PIF), and ladder-type poly(p-phenylene) (MeLPPP). Under low light intensities, the DF features a maximum at a certain temperature Tmax. For the dimer and trimer, the Tmax coincides with the temperature at which the phosphorescence has decayed to 1/2 of the value at 10 K, while Tmax shifts to lower temperature values along the series DOO-PPP, PIF, and MeLPPP and approaches T = 0 K for MeLPPP. By applying conventional kinetic equations we show that the occurrence of a maximum in the DF intensity is the consequence of generalized thermally activated triplet exciton transport toward quenching sites. We find the quenching rates at 0 K to be in the range of 1 s–1 for the polymers, while they are more than an order of magnitude lower for the oligomers.

Diffusion-Limited Energy Transfer in Blends of Oligofluorenes with an Anthracene Derivative

R.Q. Albuquerque, C.C. Hofmann, J. Köhler, A. Köhler

J. Phys. Chem. B 115 (2011) 8063-8070
jp-2011-02333w_0008

Organic semiconductor devices such as light-emitting diodes and solar cells frequently comprise a blend of molecular or polymeric materials. Consequently, resonant energy transfer between the components plays a major role in determining device performance. Energy transfer may take place through either single-step donor-acceptor transfer, realized for example as Forster transfer, or as a sequence of donor-donor transfers toward the acceptor site. Here we use a well-defined model system comprising an oligofluorene trimer, pentamer, or heptamer as the donor in combination with an anthracene derivative as the acceptor in order to study the rate and mechanism of energy transfer in thin films by time-resolved photoluminescence spectroscopy. We find the transfer process to be entirely dominated by sequential donor-donor transfer. In addition, we observe a strong dependence on oligomer length with an optimum energy transfer rate for the pentamer.

A series of CBP-derivatives as host materials for blue phosphorescent organic light-emitting diodes

Pamela Schrögel, Ausra Tomkeviciene, Peter Strohriegl, Sebastian T. Hoffmann, Anna Köhler and Christian Lennartz

J. Mat. Chem. 21 (2011) 2266-2273
GA

We report a series of CBP-derivatives with superior thermal and electronic properties for the use as host materials for blue electrophosphorescent organic light emitting diodes. We applied a systematic variation of the substitution pattern in the 2- and 2´-position of the biphenyl unit and the 3- and 6-position of the carbazole moieties. In contrast to the crystalline parent compound CBP, all methyl and trifluoromethyl substituted derivatives show amorphous behaviour. Substitution in the 2- and 2´-position of the biphenyl causes a twisting of the phenyl rings. Hence, the degree of conjugation of the molecules is limited which leads to enlarged triplet energies of approximately 2.95 eV compared to 2.58 eV for CBP. The methyl substitution at the active 3- and 6-position of the pendant carbazole units yields materials with an electrochemically stable behaviour against oxidation.

Synthesis and characterization of platinum (II) di-ynes and poly-ynes incorporating ethylenedioxythiophene (EDOT) spacers in the backbone

Muhammad S. Khan, Mohammed K. Al-Suti, Hakkikulla H. Shah, Said Al-Humaimi, Fathiya R. Al-Battashi, Jens K. Bjernemose, Louise Male, Paul R. Raithby, Ning Zhang, Anna Köhler and John E. Warrend

Dalton Tans., 40 (2011) 10174
GA1

A series of trimethylsilyl-protected di-alkynes incorporating 3,4-ethylenedioxythiophene (EDOT) linker
groups Me3Si–C C–R–C C–SiMe3 (R = ethylenedioxythiophene-3,4-diyl 1a, 2,2¢-bis-3,4-
ethylenedioxythiophene-5,5¢-diyl 2a, 2,2¢,5¢,2¢¢-ter-3,4-ethylenedioxythiophene-5,5¢¢-diyl 3a) and the
corresponding terminal di-alkynes, H–C C–R–C C–H 1b–2b has been synthesized and characterized
and the single crystal X-ray structure of 1a has been determined. CuI-catalyzed dehydrohalogenation
reaction between trans-[(Ph)(Et3P)2PtCl] and the terminal di-alkynes 1b–2b in iPr2NH/CH2Cl2 (2 : 1
mole ratio) gives the Pt(II) di-ynes trans-[(Et3P)2(Ph)Pt–C C–R–C C–Pt(Ph)(Et3P)2] 1M–2M while
the dehydrohalogenation polycondensation reaction between trans-[(nBu3P)2PtCl2] and 1b–2b (1 : 1 mole
ratio) under similar reaction conditions affords the Pt(II) poly-ynes trans-[Pt(PnBu3)2–C C–R–C C-]n
1P–2P. The di-ynes and poly-ynes have been characterized spectroscopically and, for 1M and 2M, by
single-crystal X-ray which confirms the “rigid rod” di-yne backbone. The materials possess excellent
thermal stability, are soluble in common organic solvents and readily cast into thin films. Optical
absorption spectroscopic measurements reveal that the EDOT spacers create stronger donor-acceptor
interactions between the platinum(II) centres and conjugated ligands along the rigid backbone of the
organometallic polymers compared to the related non-fused and fused oligothiophene spacers.

Triplet Excimer Emission in a Series of 4,4′-Bis(N-carbazolyl)-2,2′-biphenyl Derivatives

ST Hoffmann, P Schrögel, M Rothmann, R Albuquerque, P Strohriegl and A Köhler

J. Phys. Chem. B 115 (2011) 414-421
jp-2010-07408e_0005

Carbazole-based materials such as 4,4´-bis(N -carbazolyl)-2,2´-biphenyl (CBP) and its derivatives are frequently used as matrix materials for phosphorescent emitters in organic light emitting diodes (OLED)s. An essential requirement for such matrix materials is a high energy of their first triplet excited state. Here we present a detailed spectroscopic investigation supported by density functional theory (DFT) calculations on two series of CBP derivatives, where CH3 and CF3 substituents on the 2- and 2´-position of the biphenyl introduce strong torsion into the molecular structure. We find that the resulting poor coupling between the two halves of the molecules leads to an electronic structure similar to that of N -phenyl-3,6-dimethylcarbazole, with a high triplet-state energy of 2.95 eV. However, we also observe a triplet excimer emission centered at about 2.5-2.6 eV in all compounds. We associate this triplet excimer with a sandwich geometry of neighboring carbazole moieties. For compounds with the more polar CF3 substituents, the lifetime of the intermolecular triplet excited state extends into the millisecond range for neat films at room temperature. We attribute this to an increased charge-transfer character of the intermolecular excited state for the more polar substituents.

What controls triplet exciton transfer in organic semiconductors?

A Köhler, H Bässler

J. Mater. Chem. 21 (2011) 4003-4011
GA2

Dexter-type triplet transfer is a phenomenon that is ubiquitous in the field of molecular electronics, and that takes place at the interface of chemistry, physics and biology. It may be considered as a correlated transfer of two charges, and thus, models originally developed for charge transfer may be applied to describe triplet transfer. In dilute fluid solutions, triplet transfer from a donor to an acceptor is well-understood and it has been described in terms of Marcus theory, i.e. taking into account distortions in the molecule and its surroundings. In amorphous thin films, that are used for organic semiconductor applications, the effects of energetic disorder prevail, and they need to be considered for an appropriate description of triplet energy transfer. We present here an overview on recent experimental and theoretical work concerning a unified description of triplet energy transfer.

What Determines Inhomogeneous Broadening of Electronic Transitions in Conjugated Polymers?

ST Hoffmann, H Bässler, A Köhler

J. Phys. Chem. B 114 (2010) 17037–17048
jp-2010-07357y_0011

Energetic disorder is manifested in the inhomogeneous broadening of optical transitions in π-conjugated organic materials, and it is a key parameter that controls the dynamics of charge and energy transfer in this promising class of amorphous semiconductors. In an endeavor to understand which processes cause the inhomogeneous broadening of singlet and triplet excitations in π-conjugated polymers we analyze continuous wave absorption and photoluminescence spectra within a broad range of temperatures for (i) oligomers of the phenylenevinylene family (OPVs) and MEH-PPV in solution and (ii) bulk films of MEH-PPV and members of the poly(p-phenylene) family (PPPs). We use a Franck-Condon deconvolution technique to determine the temperature dependent S1-S0 0-0 and T1-S0 0-0 transition energies and their related variances. For planar compounds, the transition energies can be related to the oligomer length, which allows us to infer the effective conjugation length for the nonplanar compounds as a function of temperature. With this information we can distinguish between intrachain contributions to the inhomogeneous line broadening that are due to thermally induced torsional displacements of the chain elements, and other contributions that are assigned largely to dielectric interactions between the chain and its environment. We find that in solution, temperature-induced torsional displacements dominate the line broadening for the alkyl derivatives of OPVs while in the alkoxy derivatives the Van der Waals contribution prevails. In films, σ is virtually temperature independent because disorder is frozen in. We also establish a criterion regarding the ratio of inhomogeneous line broadening in singlet and triplet states. The results will be compared to a recent theory by Barford and Trembath.

Triplet energy transfer in conjugated polymers. III. An experimental assessment regarding the influence of disorder on polaronic transport

ST Hoffmann, E Scheler,JM Koenen, M Forster, U Scherf, P Strohriegl,H Bässler, A Köhler

Phys.Rev. B 81 (2010) 165208

It is a general notion that, in organic semiconductors, the transport of electronic excitations, such as neutral excitons of singlet or triplet type and charge carriers, is controlled by both, polaron and disorder effects. For compounds with low energetic disorder triplet exciton diffusion can be described in the framework of Marcus-theory [see Sudha Devi et al.,Phys. Rev. B 78, 045210 (2008)], and a theoretical model for diffusion in more disordered compounds has been developed [see Fishchuk et al., Phys. Rev. B 78, 045211 (2008)]. Here we experimentally demonstrate that such a modified Marcus-type model is suitable to describe triplet exciton transport in commonly used poly(p-phenylene)-type polymers and oligomers. In particular, we provide aquantitative spectroscopic assessment of the polaronic and the disorder contribution to triplet exciton transport as a function of conjugation length. Franck-Condon analyses of the phosphorescence spectra and temperature-dependent triplet diffusion combined with analytic transport theory demonstrate that, in contrast to charge carriers, Marcus-type jump rates with dominantly polaronic activation energies control the motion of triplet excitons above a transition temperature.

Hole-transporting host-polymer series consisting of triphenylamine basic structures for phosphorescent polymer light-emitting diodes

MW Thesen, B Höfer, M Debeaux, S Janietz, A Wedel, A Köhler, HH Johannes, H Krueger

J. Pol. Sc. A 48 (2010) 3417-3430

A series of novel styrene derived monomers with triphenylamine-based units, and their polymers have been synthesized and compared with the well-known structure of polymer of N,N0-bis(3-methylphenyl)-N,N0-diphenylbenzidine with respect to their hole-transporting behavior in phosphorescent polymer light-emitting diodes (PLEDs). A vinyltriphenylamine structure was selected as a basic unit, functionalized at the para positions with the following side groups: diphenylamine, 3-methylphenyl-aniline, 1- and 2-naphthylamine, carbazole, and phenothiazine. The polymers are used in PLEDs as host polymers for blend systems with the following device configuration: glass/indium–tin–oxide/PEDOT:PSS/polymer-blend/CsF/Ca/Ag. In addition to the hole-transporting host polymer, the polymer blend includes a phosphorescent dopant [Ir(Me-ppy)3] and an electron-transporting molecule (2-(4-biphenyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole). We demonstrate that two polymers are excellent hole-transporting matrix materials for these blend systems because of their good overall electroluminescent performances and their comparatively high glass transition temperatures. For the carbazole-substituted polymer (Tg = 246°C), a luminous efficiency of 35 cd A-1 and a brightness of 6700 cd m-2 at 10 V is accessible. The phenothiazine-functionalized polymer (Tg= 220°C) shows nearly the same outstanding PLED behavior. Hence, both these polymers outperform the well-known polymer of N,N0-bis(3-methylphenyl)-N,N0- diphenylbenzidine, showing only a luminous efficiency of 7.9 cd A-1 and a brightness of 2500 cd m -2 (10 V).

Spectral diffusion in poly(para-phenylene)-type polymers with different energetic disorder

ST Hoffmann, H Bässler, JM Koenen, M Forster, U Scherf, E Scheler, P Strohriegl, A Köhler

Phys. Rev. B 81 (2010) 115103

We have employed quasicontinuous fluorescence and phosphorescence spectroscopy within a temperature range between 10 and 500 K to monitor the spectral diffusion of singlet and triplet excitons in a series of π-conjugated polymers. We investigated (i) how spectral diffusion is controlled by the degree of energetic disorder present in the amorphous film (that is reflected by the inhomogeneous broadening of the photoluminescence spectra) and (ii) how this process depends on the range of the electronic coupling (by comparing singlet exciton diffusion via long-range Förster transfer against triplet exciton diffusion by short-range Dexter transfer). For singlets, we find that the fluorescence spectra bear out a bathochromic shift upon cooling the sample down to a critical temperature below which the shift saturates. This bathochromic shift is a signature of spectral relaxation. Random-walk theory applied to excitation transport within a Gaussian density-of-states distribution is both necessary and sufficient to rationalize the experimental results in a quantitative fashion. The same behavior is observed for triplets in weakly disordered systems, such as in a polymer containing platinum in the main chain and a ladder-type polyphenylene. In contrast we observe a hypsochromic shift of the phosphorescence spectra below a characteristic temperature for triplets in systems with at least moderate energetic disorder. The hypsochromic shift proves that triplet exciton relaxation becomes frustrated because thermally activated exciton jumps that otherwise promote spectral diffusion become progressively frozen out. The frustration effect is controlled by the jump distance and thus it is specific for triplet excitations that migrate via short-range coupling among strongly localized states as compared to singlet excitons.

Triplet states in organic semiconductors

A Köhler, H Bässler

Materials Science and Engineering R, (2009) 71-109

Today’s technology is not possible without optoelectronic devices such as light-emitting diodes, transistors and solar cells. These basic units of modern electronic appliancesmay be made not only from traditional inorganic semiconductors, but also from organic semiconductors, i.e. hydrocarbonmolecules that combine semiconducting properties with some mechanical properties such as easy processability and flexibility. The weak van der Waals forces that bind the molecules to a solid imply a low dielectric constant, so that coulomb and exchange interactions between electrons are significant. As a result, photoexcitation or electrical excitation results in strongly bound electron–hole pairs, so-called excitons. Depending on the relative orientation of the electron and hole spin, the exciton may be of a overall singlet or triplet spin state. While the fluorescent singlet state has been investigated intensively since the first reports of organic electroluminescence, research into the properties of the phosphorescent triplet state has intensified mainly during the last decade. In this review we give an overview on the photophysical processes associated with the formation of triplet states and their decay, as well as the energy levels and energy transfer processes of triplet states. We aim to give a careful introduction for those new to this particular research area as well as to highlight some of the current research issues and intriguing questions for those familiar with the field. The main focus of this review is on molecular assemblies and polymer films, though relevant work on molecular crystals is also included where it assists in forming a larger picture.

Synthesis and comparison of the optical properties of platinum (II) poly-ynes with fused and non-fused oligothiophenes

L Sudha Devi, MK Al-Suti, N Yhang, SJ Teat, L Male, HA, Sparkes, RP Rainthby, MS Khan, A Köhler

Macromolecules 42 (2009) 1131-1141
ma-2008-02399a_0010

The synthesis and characterization of the thieno[3,2-b]thiophene and dithieno[3,2-b:2′,3′-d]thiophene containing platinum(II) poly-ynes and their molecular precursors is described and the electronic structure is established by absorption, luminescence and photoinduced absorption measurements. A comparison of the electronic structure of the fused and the nonfused oligothiophenes, thieno[3,2-b]thiophene, dithieno[3,2-b:2′,3′-d]thiophene, 2,2′-bithiophene, and 2,2′:5′,2” -terthiophene incorporated in platinum(II) poly-ynes is reported. We find the singlet S1 and triplet T1 and Tn excited states to be at higher energy in thin films made from the fused systems than from the nonfused systems. For ligands with the same number of rings, we attribute this to the decreased number of double bonds in the fused system and to the presence of an additional sulfur atom in spacers with the same number of double bonds.

Triplet energy transfer in conjugated polymers. I. Experimental investigation of a weakly disordered compound

L Sudha Devi, MK Al-Suti, C. Dosche, MS Khan, RH Friend, A Köhler

Phys. Rev. B 78 (2008) 045210

Efficient triplet exciton emission has allowed improved operation of organic light-emitting diodes (LEDs). To enhance the device performance, it is necessary to understand what governs the motion of triplet excitons through the organic semiconductor. Here, we have investigated triplet diffusion using a model compound that has weak energetic disorder. The Dexter-type triplet energy transfer is found to be thermally activated down to a transition temperature TT, below which the transfer rate is only weakly temperature dependent. We show that above the transition temperature, Dexter energy transfer can be described within the framework of Marcus theory. We suggest that below TT, the nature of the transfer changes from phonon-assisted hopping to quantummechanical tunneling. The lower electron-phonon coupling and higher electronic coupling in the polymer compared to the monomer results in an enhanced triplet diffusion rate.

Triplet energy transfer in conjugated polymers. II. A polaron theory description addressing the influence of disorder

II Fishchuk, A Kadashchuk, L Sudha Devi, P Heremans, H Bässler, A Köhler

Phys. Rev. B 78 (2008) 045211

Motivated by experiments monitoring motion of triplet excitations in a conjugated polymer containing Pt-atoms in the main chain (see Paper I), a theoretical formalism for electronic transport has been developed. It considers the interplay between polaronic distortion of the excited chain elements and disorder treated in terms of effective-medium theory. The essential parameters are the electronic coupling J, the polaronic binding energy λ that determines the activation energy of polaron motion Ea, and the variance σ of the density of states distribution controlling the incoherent hopping motion. It turns out that for the weak electronic coupling associated with triplet motion (J a few meV), the transfer is nonadiabatic. For a critical ratio of σ/Ea< 0.3, Marcus-type multiphonon transport prevails above a certain transition temperature. At lower temperatures, transport is disorder controlled consistent with the Miller-Abrahams formalism. Theoretical results are consistent with triplet transport in the Pt-polymer. Implications for charge and triplet motion in random organic semiconductors in general are discussed.

Effect of solvent on the conformation of isolated MEH-PPV chains intercalated into SnS2

E Aharon, S Breuer, F Jaiser, A Köhler, GL Frey

Chem. Phys. Chem. 9 (2008) 1430-1436

Photophysical processes in conjugated polymers are influenced by two competing effects: the extent of excited state delocalization along a chain, and the electronic interaction between chains. Experimentally, it is often difficult to separate the two because both are controlled by chain conformation. Here we demonstrate that it is possible to modify intra-chain delocalization without inducing inter-chain interactions by intercalating polymer monolayers between the sheets of an inorganic layered matrix. The red-emitting conjugated polymer, MEH-PPV, is confined to the interlayer space of layered SnS2. The formation of isolated polymer monolayers between the SnS2 sheets is confirmed by X-ray diffraction measurements. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the incorporated MEH-PPV chains reveal that the morphology of the incorporated chains can be varied through the choice of solvent used for chain intercalation. Incorporation from chloroform results in more extended conformations compared to intercalation from xylene. Even highly twisted conformations can be achieved when the incorporation occurs from a methanol:chloroform mixture. The PL spectra of the MEH-PPV incorporated SnS2 nanocomposites using the different solvents are in good agreement with the PL spectra of the same solutions, indicating that the conformation of the polymer chains in the solutions is retained upon intercalation into the inorganic host. Therefore, intercalation of conjugated polymer chains into layered hosts enables the study of intra-chain photophysical processes as a function of chain conformation.

Exciton dynamics in blends of phosphorescent emitters

C Schuetz, B Höfer, F Jaiser, H Krueger, M Thesen, S Janietz, A Köhler

Physica status solidi B 245 (2008) 810-813

Cascading energy transfer is a usually undesired effect in organic-host guest systems that are designed for light-emitting purposes. Here we demonstrate a chemical approach to suppressing undesired energy transfer from the blue to the red emitter in multicomponent polymer blends. We have combined a red, green and blue emitting Ir-complex each with charge transporting molecules to a side-chain copolymer. The covalent attachment of the phosphorescent emitter is found to prevent cascading energy transfer through steric shielding. This approach approximately doubles the efficiency and brightness of polymer light emitting diodes.

Dimensionality-dependent energy transfer in polymer-intercalated SnS2 nanocomposites

P Parkinson, E Aharon, MH Chang, C Dosche, G. Frey, A. Koehler, LM Herz

Phys. Rev. B 75 (2007) 165206

We have investigated the influence of dimensionality on the excitation-transfer dynamics in a conjugated polymer blend. Using time-resolved photoluminescence spectroscopy, we have measured the transfer transients for both a three-dimensional blend film and for quasi-two-dimensional monolayers formed through intercalation of the polymer blend between the crystal planes of an inorganic SnS2 matrix. We compare the experimental data with a simple, dimensionality-dependent model based on electronic coupling between electronic transition moments taken to be point dipoles. Within this approximation, the energy-transfer dynamics is found to adopt a three-dimensional character in the solid film and a two-dimensional nature in the monolayers present in the SnS2-polymer nanocomposite.

The effect of delocalization on the exchange energy in meta- and para-linked Pt-containing carbazole polymers and monomers

Ning Zhang, Anna Hayer, Mohammed K. Al-Suti, Rayya A. Al-Belushi, Muhammad S. Khan and Anna Köhler

J. Chem. Phys. 124 (2006) 244701

A series of novel platinum-containing carbazole monomers and polymers was synthesized and fully characterized by UV-VIS absorption, luminescence, and photoinduced absorption studies. In these compounds, a carbazole unit is incorporated into the main chain via either a para- or a meta-linkage. We discuss the effects of linkage and polymerization on the energy levels of S1, T1, and Tn. The S1-T1 splitting observed for the meta-linked monomer (0.4 eV) is only half of that in the para-linked monomer (0.8 eV). Upon polymerization, the exchange energy in the para-linked compound reduces, yet still remains larger than in the meta-linked polymer. We attribute the difference in exchange energy to the difference in wave function overlap between electron and hole in these compounds.

Triplet Energy Back Transfer in Conjugated Polymers with Pendant Phosphorescent Iridium Complexes

Nicholas R. Evans, Lekshmi Sudha Devi, Chris S. K. Mak, Scott E. Watkins, Sofia I. Pascu, Anna Köhler, Richard H. Friend, Charlotte K. Williams and Andrew B. Holmes

J. Am. Chem. Soc. 128 (2006) 6647-6656
ja0584267n00001

Abstract: The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent iridium complex 13 and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescent organic light-emitting diodes. Red-emitting phosphorescent iridium complexes based on the [Ir(btp)2(acac)] fragment (where btp is 2-(2¢-benzo[b]thienyl)pyridinato and acac is acetylacetonate) have been attached either directly (spacerless) or through a -(CH2)8- chain (octamethylene-tethered) at the 9-position of a 9-octylfluorene host. The resulting dibromo-functionalized spacerless (8) or octamethylene-tethered (12) fluorene monomers were chain extended by Suzuki polycondensations using the bis(boronate)-terminated fluorene macromonomers 16 in the presence of end-capping chlorobenzene solvent to produce the statistical spacerless (17) and octamethylene-tethered (18) copolymers containing an even dispersion of the pendant phosphorescent fragments. The spacerless monomer 12 adopts a face-to-face conformation with a separation of only 3.6 Å between the iridium complex and fluorenyl group, as shown by X-ray analysis of a single crystal, and this facilitates intramolecular triplet energy transfer in the spacerless copolymers 17. The photo- and electroluminescence efficiencies of the octamethylene-tethered copolymers 18 are double those of the spacerless copolymers 17, and this is consistent with suppression of the back transfer of triplets from the red phosphorescent iridium complex to the polyfluorene backbone in 18. The incorporation of a -(CH2)8- chain between the polymer host and phosphorescent guest is thus an important design principle for achieving higher efficiencies in those electrophosphorescent organic light-emitting diodes for which the triplet energy levels of the host and guest are similar.

Highly Fluorescent Crystalline and Liquid Crystalline Columnar Phases of Pyrene-Based Structures

Anna Hayer, Véronique de Halleux, Anna Ko1hler, Abdel El-Garoughy, E. W. Meijer, Joaquín Barberá, Julien Tant, Jeremy Levin, Matthias Lehmann, Johannes Gierschner, Jérôme Cornil and Yves Henri Geerts

J. Phys. Chem. B 110 (2006) , 7653-7659

A concept for highly ordered solid-state structures with bright fluorescence is proposed: liquid crystals based on tetraethynylpyrene chromophores, where the rigid core is functionalized with flexible, promesogenic alkoxy chains. The synthesis of this novel material is presented. The thermotropic properties are studied by means of differential scanning calorimetry (DSC), cross-polarized optical microscopy (POM), and X-ray diffraction. The mesogen possesses an enantiotropic Colh phase over a large temperature range before clearing. The material is highly fluorescent in solution and, most remarkably, in the condensed state, with a broad, strongly red shifted emission. Fluorescence quantum yields (¼F) have been determined to be 70% in dichloromethane solution and 62% in the solid state. Concentration- and temperature-dependent absorption and emission studies as well as quantum-chemical calculations on isolated molecules and dimers are used to clarify the type of intermolecular interactions present as well as their influence on the fluorescence quantum yield and spectral properties of the material. The high luminescence efficiency in the solid state is ascribed to rotated chromophores, leading to an optically allowed lowest optical transition.

Morphology dependence of the triplet excited state formation and absorption in polyfluorene

A Hayer, ALT Khan, RH Friend, A Köhler

Phys. Rev. B 71 (2005) 241302

Poly(9,9-dioctylfluorene) (PFO) is widely used as a highly efficient blue-emitting polymer for light-emitting diode applications. Films of PFO are known to contain a planar β-phase or a disordered glassy phase that affects the emission characteristics. We have studied the morphology dependence of the photoinduced absorption in such PFO films and find that the T1-Tn absorption signal differs between the two phases in energy, linewidth, and intensity. The lower intensity we observe in the β-phase as compared to the glassy phase is evidence for a lower T1 population in the former. We attribute this to firstly, reduced intersystem crossing and secondly, increased charge generation due to both more extended conjugation along the chains and higher interchain interactions in the planar, well-ordered β-phase. The reduced energy and smaller linewidth of the triplet absorption in the β-phase as compared to the glassy phase are indicative of a more delocalized Tn state and a higher degree of order in the β-phase, respectively.

Large magnetoresistance in nonmagnetic pi-conjugated semiconductor thin film devices

Ö Mermer, G Veeraraghavan, TL Francis, Y Sheng, DT Nguyen, M Wohlgenannt, A Köhler, MK Al-Suti, MS Khan

Phys. Rev. B. 72 (2005) 205202

Following the recent observation of large magnetoresistance at room temperature in polyfluorene sandwich devices, we have performed a comprehensive magnetoresistance study on a set of organic semiconductor sandwich devices made from different π -conjugated polymers and small molecules. The study includes a range of materials that show greatly different chemical structure, mobility, and spin-orbit coupling strength. We study both hole and electron transporters at temperatures ranging from 10 K to 300 K. We observe large negative or positive magnetoresistance (up to 10% at 300 K and 10 mT) depending on material and device operating conditions. We discuss our results in the framework of known magnetoresistance mechanisms and find that none of the existing models can explain our results.

Blue-to-green electrophosphorescence of iridium-based cyclometallated materials

CSK Mak, A Hayer, SI Pascu, SE Watkins, AB Holmes, A Köhler, RH Friend

Chem. Commun., 37 (2005) 4708-4710
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The photo- and electroluminescence properties of a series of novel, heteroleptic, mer-cyclometallated iridiumcomplexes have been fine-tuned from green to blue by changing the substituents on the pyridyl ring of the phenylpyridyl ligand. The X-ray crystal structures of two Ir-based triazolyl complexes are reported.

Spectroscopic study of spin-dependent exciton formation rates in pi-conjugated semiconductors: Comparison with electroluminescence techniques.

C. Yang, ZV Vardeny, A Köhler, M Wohlgenannt, MK Al-Suti, MS Khan

Phys. Rev. B, 70 (2004) 241202

It has been found in recent measurements that the singlet-to-triplet exciton ratio in organic light-emitting diodes (OLEDs) is larger than expected from spin degeneracy, and that singlet excitons form at a larger rate than triplets. We employed the technique of optically detected magnetic resonance to measure the spindependent exciton formation rates in films of a polymer and corresponding monomer, and explore the relation between the formation rates and the actual singlet-to-triplet ratio measured previously in OLEDs. We found that the spin-dependent exciton formation rates can indeed quantitatively explain the observed exciton yields, and that singlet formation rates and yields are significantly enhanced only in polymer OLEDs, but not in OLEDs made from the corresponding monomer.

The singlet-triplet exchange energy in conjugated polymers

A Köhler und D Beljonne

Advanced Functional Materials, (Feature article), 14 (2004),11

Electron–electron interactions in organic semiconductors split the lowest singlet and triplet states by the exchange energy, ?EST. Measurement of singlet and triplet emission spectra in a large number of conjugated polymers yield an almost constant ?EST value close to 0.7?eV. This is in contrast to the situation in molecules, where the exchange energy is found to depend on molecular size and to vary over a wide range. Quantum-chemical calculations are performed to address the origin of the constant exchange energy in phenylene-based conjugated polymers. The electron–hole separation in the lowest singlet and triplet excited states is found to be independent of the p-conjugated backbone, and saturates for chains longer than a few repeating units, resulting in a constant exchange energy. In shorter conjugated oligomers, confinement of the excitations destabilizes the singlet with respect to the triplet through exchange interactions and leads to a larger and size-dependent singlet–triplet energy separation.

Morphology-dependent energy transfer within polyfluorene thin films

ALT Khan, P Sreearunothai, LM Herz, MJ Banach, A Köhler

Phys. Rev. B., 69 (2004) 085201

We have performed a detailed study of the photoluminescence from thin films of blue-light-emitting poly(9,9-dioctylfluorene) containing different fractions of planarized (ß-phase) chains within the glassy polymer film. By choosing solvents with a range of polarities and boiling points we were able to cast films with reliable control of the relative amounts of ß-phase chains present. We analyzed the emission spectra in terms of Franck-Condon progressions and found that, at low temperatures (8 K), the luminescence can be modeled accurately by considering two distinct contributions from the two phases present in the film. The Huang-Rhys parameter for the ß phase is shown to be approximately half the value obtained for the glassy phase, in agreement with a more delocalized exciton in the ß phase. Time-resolved photoluminescence measurements on a film containing roughly 25% of ß phase reveal a fast transfer of excitations from the glassy to the ß phase, indicating that the two phases are well intermixed. Assuming the transfer dynamics to be governed by dipole-dipole coupling, we obtain a Förster radius of 8.2±0.6nm, significantly larger than the radius typically found for excitation transfer within the glassy phase. These results are consistent with the large spectral overlap between the emission of the glassy phase and the absorption of the ß phase and explain why the latter dominates the emission even from films containing only a small fraction of ß-phase chains.

Solution-processable conjugated electrophosphorescent polymers

AJ Sandee, CK Williams, N Evans, JR Davies, CE Boothby, A Köhler, RH Friend and AB Holmes

J. Am. Chem. Soc. 126 (2004) 7041-7048
ja039445on00001

We report the synthesis and photophysical study of a series of solution-processible phosphorescent iridium complexes. These comprise bis-cyclometalated iridium units [Ir(ppy)2(acac)] or [Ir(btp)2(acac)] where ppy is 2-phenylpyridinato, btp is 2-(2‘-benzo[b]thienyl)pyridinato, and acac is acetylacetonate. The iridium units are covalently attached to and in conjugation with oligo(9,9-dioctylfluorenyl-2,7-diyl) [(FO)n] to form complexes [Ir(ppy-(FO)n)2(acac)] or [Ir(btp-(FO)n)2(acac)], where the number of fluorene units, n, is 1, 2, 3, 10, 20, 30, or 40. All the complexes exhibit emission from a mixed triplet state in both photoluminescence and electroluminescence, with efficient quenching of the fluorene singlet emission. Short-chain complexes, 11-13, [Ir(ppy-(FO)n-FH)2(acac)] where n = 0, 1, or 2, show green light emission, red-shifted through the FO attachment by about 70 meV, but for longer chains there is quenching because of the lower energy triplet state associated with polyfluorene. In contrast, polymer complexes 18-21 [Ir(btp-(FO)n)2(acac)] where n is 5-40 have better triplet energy level matching and can be used to provide efficient red phosphorescent polymer light-emitting diodes, with a red shift due to the fluorene attachment of about 50 meV. We contrast this small (50-70 meV) and short-range modification of the triplet energies through extended conjugation, with the much more substantial evolution of the p-p* singlet transitions, which saturate at about n = 10. These covalently bound materials show improvements in efficiency over simple blends and will form the basis of future investigations into energy-transfer processes occurring in light-emitting diodes.

Polymer light-emitting diodes with spin-polarised charge injection

A Hayer, A Köhler, E Arisi, I Bergenti, A Dediu, C Taliani, M Al-Suti, MS Khan

Synth. Met. 147, (2004) 155-158

This study is concerned with the characterisation of the injection properties of the manganite compound La0.7Sr0.3MnO3 (LSMO) and its use as a spin-polarised anode in polymer light-emitting diodes. Charge carrier injection and electroluminescence in a polymer LED with an LSMO anode is achieved. A dependence of the current–voltage characteristics of LEDs with LSMO anodes on the temperature is observed and compared to the behaviour of LEDs with standard ITO anodes. The temperature dependence is shown to be due to improved hole injection at low temperatures, possibly originating from magnetic ordering upon cooling.

Synthesis, characterisation and optical spectroscopy of platinum(II) di-ynes and poly-ynes incorporating condensed aromatic spacers in the backbone

MS Khan, MRA Al-Mandhary, MK Al-Suti, FR Al-Battashi, S Al-Saadi, B Ahrens, JK Bjernemose, MF Mahon, PR Raithby, M Younus, N Chawdhury, A Köhler, EA Marseglia, E Tedesco, N Feeder, SJ Teat

Dalton Trans., (15): 2377-2385 2004.

A series of protected and terminal dialkynes with extended p-conjugation through a condensed aromatic linker unit in the backbone, 1,4-bis(trimethylsilylethynyl)naphthalene, 2a, 1,4-bis(ethynyl)naphthalene, 2b, 9,10-bis(trimethylsilylethynyl)anthracene 3a, 9,10-bis(ethynyl)anthracene 3b, have been synthesized and characterized spectroscopically. The solid-state structures of 2a and 3a have been confirmed by single crystal X-ray diffraction studies. Reaction of two equivalents of the complex trans-[Ph(Et3P)2PtCl] with an equivalent of the terminal dialkynes 1,4-bis(ethynyl)benzene 1b and 2b–3b, in iPr2NH–CH2Cl2, in the presence of CuI, at room temperature, afforded the platinum(II) di-ynes trans-[Ph(Et3P)2Pt–C ≡ C–R–C ≡ C–Pt(PEt3)2Ph] (R = benzene-1,4-diyl 1c; naphthalene-1,4-diyl 2c and anthracene-9,10-diyl 3c) while reactions between equimolar quantities of trans-[(nBu3P)2PtCl2] and 2b–3b under similar conditions readily afforded the platinum(II) poly-ynes trans-[–(nBu3P)2Pt–C ≡ C–R–C ≡ C–]n (R = naphthalene-1,4-diyl 2d and anthracene-9,10-diyl 3d). The Pt(II) diynes and poly-ynes have been characterized by analytical and spectroscopic methods, and the single crystal X-ray structures of 1c and 2c have been determined. These structures confirm the trans-square planar geometry at the platinum centres and the linear nature of the molecules. The di-ynes and poly-ynes are soluble in organic solvents and readily cast into thin films. Optical spectroscopic measurements reveal that the electron-rich naphthalene and anthracene spacers create strong donor–acceptor interactions between the Pt(II) centres and conjugated ligands along the rigid backbone of the organometallic polymers. Thermogravimetry shows that the di-ynes possess a somewhat higher thermal stability than the corresponding poly-ynes. Both the Pt(II) di-ynes and the poly-ynes exhibit increasing thermal stability along the series of spacers from phenylene through naphthalene to anthracene.

Phosphorescence and spin-dependent exciton formation in conjugated polymers

A Köhler and JS Wilson

Organic electronics 4 (2003) 179 – special issue

The mechanism for the formation of singlet and triplet states in conjugated polymer-based light-emitting diodes (LEDs) is crucial in determining the overall efficiencies of these devices. If simple spin statistics apply then singlets and triplets should be formed in the ratio 25:75. However, the non-emissive nature of triplet states in these materials, as well as other loss mechanisms within the devices, mean that this ratio is not straightforward to measure. Nevertheless, recent experimental advances have made it possible to determine many of the properties of triplet states. Here we review what is now known about triplet states and their photophysics in conjugated polymers. We place particular emphasis on measurements of the singlet generation fraction in LEDs, and discuss the experimental techniques that have been used, such as direct comparison of photoluminescence and electroluminescence efficiencies, triplet absorption cross section measurements and magnetic resonance measurements. All of these techniques give values for the singlet generation fraction in polymers that are significantly larger than the 25% expected, and many of them have also shown that in shorter oligomers this value decreases to be closer to 25%. We also give a brief overview of recent theories for the processes of singlet and triplet formation in polymer devices.

Low-energy vibrational modes in phenylene oligomers studied by THz time-domain spectroscopy

MB Johnston, LM Herz, ALK Khan, A Köhler, AG Davies, EH Linfield

Chem. Phys. Lett. 377 (2003), 256-262

Following the recent observation of large magnetoresistance at room temperature in polyfluorene sandwich devices, we have performed a comprehensive magnetoresistance study on a set of organic semiconductor sandwich devices made from different π -conjugated polymers and small molecules. The study includes a range of materials that show greatly different chemical structure, mobility, and spin-orbit coupling strength. We study both hole and electron transporters at temperatures ranging from 10 K to 300 K. We observe large negative or positive magnetoresistance (up to 10% at 300 K and 10 mT) depending on material and device operating conditions. We discuss our results in the framework of known magnetoresistance mechanisms and find that none of the

Control of alpha and beta phase formation in polyfluorene thin films via Franck-Condon Analysis

ALT Khan, MJ Banach and A Köhler

Synth. Met., 139 (2003), 905-907

In the blue light-emitting polymer poly(9,9-dioctylfluorene) (PFO) two distinct morphological phases have been identified, that is the glassy phase and a planar ß-phase. We show that the proportion of the formation of the two phases can be controlled by simply spinning thin films from different solvents such as chloroform and cyclopentanone. We use a Franck–Condon analysis with vibrational modes obtained from Raman spectra to analyze the emission spectra at low temperature. Films spun from cyclopentanone are dominated by from the ß-phase emission while films spun from chloroform have a high fraction of glassy phase emission. Comparison between absorption and emission spectra shows evidence for spectral diffusion from the glassy to the ß-phase. We discuss the Huang–Rhys parameters for both phases.

Polarisation of singlet and triplet excited states in a platinum-containing conjugated polymer

JS Wilson, RJ Wilson, RH Friend, MK Al-Suti, MRA Al-Mandhary, MS Khan, A Köhler

Phys. Rev. B., 67 (2003) 125206

We investigate the polarization of optical transitions associated with the singlet S1 and triplet T1 and Tn excited states in a uniaxially aligned platinum-containing conjugated polymer which contains a 2-methoxy-5-(´-ethyl)-hexyloxy (MEH)-substituted phenyl ring. For the singlet S1 state, which is extended along the polymer chain, we find the corresponding absorption and emission to be polarized parallel to the chain as seen for other conjugated polymers. However, for the triplet excited states, we find that the emission from the highly localized T1 state has components both parallel and perpendicular to the polymer chain, while the absorption from T1 into the delocalized Tn state is polarized entirely parallel to the chain. We discuss this connection between the spatial extent of the excited state and the polarization of the associated optical transitions and consider how the spin-orbit coupling mechanism can influence the polarization of emission from the T1 state.

Effect of interchain interactions on the absorption and emission of poly(3-hexylthiophene)

PJ Brown, DS Thomas, A Köhler, JS Wilson, J-S Kim, CM Ramsdale, H Sirringhaus, RH Friend

Phys. Rev. B., 67 (2003) 064203

The absorption spectrum of polythiophene and its derivative poly(3-hexylthiophene) (P3HT) is usually described in terms of an intrachain exciton coupled to a single phonon mode. We show that this model is too simplistic for highly ordered, regioregular P3HT and that, analogous to the case of charged polarons in this material, interchain interactions must be taken into account to correctly describe the absorption spectrum. We show that the lowest energy feature in the p-p* region of the absorption spectrum is associated with an interchain absorption, the intensity of which is correlated with the degree of order in the polymer. Correspondingly, we show that the emission from P3HT also exhibits contributions from both interchain and intrachain states, in a manner similar to that recently shown for poly(phenylenevinylene). Having reinterpreted the physical origin of the features in the absorption and emission spectra of P3HT, we then model these spectra and show how they evolve as the degree of order in the polymer is changed by varying several physical parameters including temperature and regioregularity of the polymer.

Synthesis and characterization of new acetylide-functionalised aromatic and hetero-aromatic ligands and their dinuclear platinum complexes

MS Khan, MK Al-Suti, MRA Al-Mandhary, B Ahrens, JK Bjernemose, MF Mahon, L Male, PR Raithby, J Wilson, RH Friend, A Köhler

J. Chem. Soc., Dalton Trans., (2003) 65-73.

A new series of rigid rod protected and terminal dialkynes with extended p-conjugation through aromatic and hetero-aromatic linker units in the backbone, 2,5-bis(trimethylsilylethynyl)-1-(2-ethylhexyloxy)-4-methoxybenzene 1a, 2,5-bis(ethynyl)-1-(2-ethylhexyloxy)-4-methoxybenzene 1b, 5,8-bis(trimethylsilylethynyl)quinoline 2a, 5,8-bis(ethynyl)quinoline 2b, 2,3-diphenyl-5,8-bis(trimethylsilylethynyl)quinoxaline 3a, 2,3-diphenyl-5,8-bis(ethynyl)quinoxaline 3b, 4,7-bis(trimethysilylethynyl)-2,1,3-benzothiadiazole 4a and 4,7-bis(ethynyl)-2,1,3-benzothiadiazole 4b, has been synthesised. Treatment of the complex trans-[Pt(Ph)(Cl)(Et3P)2] with half an equivalent of the diterminal alkynes 1b–4b in iPr2NH–CH2Cl2, in the presence of CuI, at room temperature, afforded the platinum(II) di-yne complexes trans-[(Et3P)2(Ph)Pt–C ≡ C–R–C ≡ C–Pt(Ph)(Et3P)2] [R = 1-(2-ethylhexyloxy)-4-methoxybenzene-2,5-diyl 1c, quninoline-5,8-diyl 2c, 2,3-diphenylquinoxaline-5,8-diyl 3c, 2,1,3-benzothiadiazole-4,7-diyl 4c] in good yields. The new acetylide-functionalised ligands and the platinum(II) s-acetylide complexes have been characterised by analytical and spectroscopic methods and X-ray single crystal structure determinations for 2c–4c. The absorption spectra of the complexes 2c–4c show substantial donor–acceptor interaction between the platinum(II) centres and the conjugated ligands. The photoluminescence spectra of 1c–3c show characteristic singlet (S1) and triplet (T1) emissions. Both the singlet and triplet emissions as well as the absorption decrease in energy with increasing electronegativity of the spacer groups along the series 1c–4c.

Synthesis and optical characterisation of platinum (II) poly-yne polymers incorporating substituted 1,4-diethylbenzene derivatives and and investigation of the intermolecular interactions in the diethylbenzene molecular precursors

MS Khan, MRA Al-Mandhary, MK Al-Suti, TC Corcoran, Y Al-Mahrooqi, JP Attfield, N Feeder, WIF David, K Shankland, RH Friend, A Köhler, EA Marseglia, E Tedesco, CCTang, PR Raithby, JC Collings, KP Roscoe, AS Batsanov, LM Stimson, TB Marder

N. J. Chem. 27 (2003) 140-149

A series of 1,4-diethynylbenzene (1) derivatives, H–C ≡ C–R–C ≡ C–H with R = C6H3NH2 (2), C6H3F (3), C6H2F2-2,5 (4), C6F4 (5), C6H2(OCH3)2-2,5 (6) and C6H2(OnC8H17)2-2,5 (7) has been synthesised and their crystal structures determined by single crystal (2–5) or powder (6, 7) X-ray diffraction. The C ≡ CH · · · pC ≡ C hydrogen bonds dominating structure 1 are gradually replaced by C ≡ C–H · · · F ones with the increase of fluorination (3 → 5), or completely replaced by C ≡ CH · · · N and NH · · · pC ≡ C bonds in 2, and C ≡ CH · · · O in 6 and 7. The related platinum-based polymers, trans-[–Pt(PnBu3)2–C ≡ C–R–C ≡ C–]n (R = as above and C6H4,) have been prepared and characterised by spectroscopic methods and thermogravimetry, which show that the amino- and methoxy-derivatives have lowest thermal stability while the fluorinated ones exhibit increasing thermal stability with increasing fluorination. Optical spectroscopic measurements reveal that substituents on the aromatic spacer group do not create strong donor–acceptor interactions along the rigid backbone of the organometallic polymers.

Synthesis, characterisation and optical spectroscopy of diynes and poly-ynes containing derivatised fluorenes in the backbon

MS Khan, MRA Al-Mandhary, MK Al-Suti, B Ahrens, MF Mahon, L Male, PR Raithby, CE Boothby, A Köhler

J. Chem. Soc., Dalton Trans., (2003) 74-84.

A series of trimethylsilyl-protected and terminal mono- and bis-alkynes based on 9,9-dioctylfluorene, 2-(trimethylsilylethynyl)-9,9-dioctylfluorene 1a, 2-ethynyl-9,9-dioctylfluorene 1b, 2,7-bis(trimethylsilylethynyl)-9,9-dioctylfluorene 2a, 2,7-bis(ethynyl)-9,9-dioctylfluorene 2b, have been synthesised. Reaction of trans-[(PnBu3)2PtCl2] with 2 equivalents of the terminal ethyne 1b yields the mononuclear platinum(II) diyne 3, reaction of trans-[(Ph)(Et3P)2PtCl] with 0.5 equivalents of the diterminal ethyne 2b gives the dinuclear platinum(II) diyne 4 while 1 : 1 reaction between trans-[(PnBu3)2PtCl2] and 2b gives the platinum(II) poly-yne 5. Treatment of 2,5-dioctyloxy-1,4-diiodobenzene with 1b in 1 : 2 stoichiometry produces the organic di-yne 6 while 1 : 1 reaction between 2,5-dioctyloxy-1,4-diiodobenzene and 2b, 2,7-bis(ethynyl)fluorene or 2,7-bis(ethynyl)fluoren-9-one produces the organic co-poly-ynes 7–9. All the new materials have been characterised by analytical and spectroscopic methods and the single crystal X-ray structures of 2a and 3 have been determined. The diynes and poly-ynes are soluble in organic solvents and are readily cast into thin films. Optical spectroscopic measurements reveal that the attachment of octyl side-chains on the fluorenyl spacer reduces inter-chain interaction in the poly-ynes while a fluorenonyl spacer creates a donor–acceptor interaction along the rigid backbone of the organometallic poly-ynes and organic co-poly-ynes.

Fluorescence and phosphorescence in organic materials – (Review article)

A Köhler, JS Wilson, RH Friend

Adv. Mater. 14 (2002) 701-707 and Advanced Engineering Materials 4 (2002) 453-459.

Progress reports are a new type of article in Advanced Engineering Materials, dealing with the hottest current topics, and providing readers with a critically selected overview of important progress in these fields. It is not intended that the articles be comprehensive, but rather insightful, selective, critical, opinionated, and even visionary. We have approached scientists we believe are at the very forefront of these fields to contribute the articles, which will appear on an annual basis. The article below describes the latest advances in fluorescence and phosphorescence in organic materials.

The singlet-triplet energy gap in organic and Pt-containing phenylene ethynylene polymers and monomers

A Köhler, JS Wilson, RH Friend, MK Al-Suti, MS Khan, A Gerhard, H Bässler

J. Chem. Phys. 116 (2002) 9457-9463.

We have studied the evolution of the T1 triplet excited state in an extensive series of phenylene ethynylene polymers and monomers with platinum atoms in the polymer backbone and in an analogous series of all-organic polymers with the platinum(II) tributylphosphonium complex replaced by phenylene. The inclusion of platinum increases spin–orbit coupling so T1 state emission (phosphorescence) is easier to detect. For both, the platinum-containing polymer series and for the all-organic polymers, we find the T1 state to be at a constant separation of 0.7±0.1 eV below the singlet S1 state. It is not possible to change this singlet–triplet splitting by altering the size or the charge-transfer character of the polymer repeat unit or by changing the electron delocalization along the polymer backbone. The S1–T1 gap can be increased by confining the S1 state in oligomers and monomers.

Synthesis, characterisation and electronic properties of a series of platinum(II) poly-ynes containing novel thienyl-pyridine linker groups

MS Khan, MRA Al-Mandhary, M Al-Suti, N Feeder, S Nahar, A Köhler, RH Friend, PJ Wilson, PR Raithby

J. Chem. Soc., Dalton Trans. (2002) 2441-2448.

The synthesis and characterisation of a series of novel poly-yne polymers, trans-[–(PnBu3)2Pt(C ≡ C–L–C ≡ C)-]n (L = 2-(2′-thienyl)-pyridine (L1), 2,5-bis(2′-thienyl)pyridine (L2), 6,6′-bis(2?-thienyl)-3,3′-bipyridine (L3)), and the corresponding platinum dimers trans-[(PEt3)2(Ph)Pt(C ≡ CLC ≡ C)Pt(Ph)(PEt3)2] are described. The dimers were prepared by the reaction of two equivalents of trans-[Pt(PEt3)2(Ph)Cl] with one equivalent of the diterminal diyne HC ≡ C–L–C ≡ CH, in CuI catalysed dehydrohalogenation reactions in CH2Cl2–Pri2NH, while the polymers are obtained similarly from the diyne and one equivalent of trans-[Pt(PnBu3)2Cl2]. The optical absorption and photoluminescence spectra of the polymers with L1 and L2 linker groups are reported. The optical gaps are 465 nm (2.67 eV) and 486 nm (2.55 eV), respectively. The photoluminescence spectrum for each polymer exhibits one band at room temperature. At 10 K, low-energy emission bands emerge which are attributed to emission from the triplet excited state. The results of these studies are compared with those obtained for other platinum-containing poly-yne and related organometallic polymers.

Structural characterisation of a series of acetylide-functionalised oligopyridines and the synthesis, characterisation and optical spectroscopy of platinum di-yne and poly-ynes containing oligopyridyl linker groups in the backbone

MS Khan, MRA Al-Mandhary, MK Al-Suti, AK Hisahm, PR Raithby, B Ahrens, MF Mahon, L Male, EA Marseglia, E Tedesco, RH Friend, A Köhler, N Feeder, SJ Teat

J. Chem. Soc., Dalton Trans. (2002) 1358-1368.

A series of trimethylsilyl-protected bis(ethynyl)oligopyridine derivatives Me3SiC ≡ C–R–C ≡ C–SiMe3 (R = 2,2′-bipyridine-5,5′-diyl (1a), 2,2′-bipyridine-6,6′-diyl (2a), 2,2′:6′,2?-terpyridine-6,6?-diyl (3a), 4′-phenyl-2,2′:6′,2?-terpyridine-6,6?-diyl (4a)) has been synthesised and 2a–4a have been characterised by single crystal X-ray crystallography. The corresponding terminal di-ynes H–C ≡ C–R–C ≡ CH (1b–4b) and their dinuclear platinum(II) complexes trans-[(Et3P)2(Ph)Pt–C ≡ C–R–C ≡ C–Pt(Ph)(Et3P)2] (1M–4M) have been characterised spectroscopically and by single-crystal X-ray crystallography for 2M. Novel platinum(II) poly-yne polymers trans-[Pt(PBun3)2–C ≡ C–R–C ≡ C–]n (1P–4P) containing the oligopyridyl linker groups in the backbone have been synthesised by the CuI-catalysed dehydrohalogenation polycondensation reaction of 1b–4b and trans-[(Bu3P)2PtCl2] in Pri2NH–CH2Cl2. The polymeric materials exhibit decreasing thermal stability with increasing number of pyridine units in the linker group. In the absorption and phosphorescence spectra, the platinum(II) poly-yne and di-yne complexes 1P, 1M show red shifts whereas the complexes 2P–3P, 2M–3M show blue shifts of the S1 and T1 states. At room temperature, the phosphoresence spectra indicate some excimer formation whereas at 10 K, only intra-chain emission occurs. The results of the photophysical studies are compared with those obtained for other platinum(II)-containing poly-ynes and related organometallic polymers.

Spin-dependent exciton formation in pi-conjugated compounds

JS Wilson, AS Dhoot, AJAB Seeley, MS Khan, A Köhler, RH Friend

Nature 413 (2001) 828-831.

The efficiency of light-emitting diodes (LEDs) made from organic semiconductors is determined by the fraction of injected electrons and holes that recombine to form emissive spin-singlet states rather than non-emissive spin-triplet states. If the process by which these states form is spin-independent, the maximum efficiency of organic LEDs will be limited to 25 per cent1. But recent reports have indicated fractions of emissive singlet states ranging from 22 to 63 per cent2±5, and the reason for this variation remains unclear. Here we determine the absolute fraction of singlet states generated in a platinum-containing conjugated polymer and its corresponding monomer. The spin-orbit coupling introduced by the platinum atom allows triplet-state emission, so optically and electrically generated luminescence from both singlet and triplet states can be compared directly.We find an average singlet generation fraction of 22 6 1 per cent for the monomer, but 57 6 4 per cent for the polymer. This suggests that recombination is spin-independent for the monomer, but that a spin-dependent process, favouring singlet formation, is effective in the polymer. We suggest that this process is a consequence of the exchange interaction, which will operate on overlapping electron and hole wavefunctions on the same polymer chain at their capture radius.

The energy gap law for triplet states in Pt-containing conjugated polymers and monomers

JS Wilson, N Chawdhury, MRA Al-Mandhary, M Younus, MS Khan, PR Raithby, A Köhler, RH Friend

J. Am. Chem. Soc. 123 (2001) 9412-9417.

The energy gap law established for aromatic hydrocarbons and rare earth ions relates the nonradiative decay rate to the energy gap of a transition through a multiphonon emission process. We show that this energy gap law can be applied to the phosphoresce of a series of conjugated polymers and monomers for which the radiative decay rate has been enhanced through incorporation of a heavy metal. We find that the nonradiative decay rate from the triplet state T1increases exponentially with decreasing T1-S0 gap for the polymers and monomers at 300 and 20 K. Comparison of the nonradiative decay of polymers with that of their corresponding monomers highlights the role of electron-lattice coupling.

Triplet states in a series of Pt containing ethynylenes

JS Wilson, A Köhler, RH Friend, MK Al-Suti, MRA Al-Mandhary, MS Khan, PR Raithby

J. Chem. Phys. 113 (2000) 7627-7634.

By use of optical steady state and time resolved spectroscopy, we studied the evolution of the triplet excited state in a series of six ethynylenic polymers of the structure [-Pt(PBu3n)2 -C ≡ C-R-C ≡ C-]n where the spacer unit R is systematically varied to give optical gaps from 1.7–3.0 eV. The inclusion of platinum in the polymer backbone induces a strong spin-orbit coupling such that triplet state emission (phosphorescence) associated with the conjugated system can be detected. Throughout the series we find the S1-T1 (singlet-triplet) energy splitting to be independent of the spacer R, such that the T1 state is always 0.7±0.1?eV below the S1 state. With decreasing optical gap, the intensity and lifetime of the triplet state emission were seen to reduce in accordance with the energy gap law.

Evolution of lowest singlet and triplet excited states with number of thienyl rings in platinum poly-ynes.

N Chawdhury, A Köhler, RH Friend, W-Y Wong, M Younus, PR Raithby, J Lewis, MRA Al-Mandhury, YC Corcoran, MS Khan

J. Chem. Phys. 110 (1999) 4963-4970.

Soluble, rigid-rod organometallic polymers trans-[-Pt(PBu3n)2–C ≡ C–R–C ≡ C–]8 (R ≡ bithienyl 2, terthienyl 3) have been synthesized in good yields by the CuI-catalyzed dehydrohalogenation reaction of trans-[-Pt(PBu3n)2Cl2] with one equivalent of the diterminal alkynyl oligothiophenes H–C ≡ C–R–C ≡ C–H in CH2Cl2/iPr2NH at room temperature. We report the thermal properties, and the optical absorption, photoluminescence, and photocurrent action spectra of 1 (trans-[–Pt(PBu3n)2–C ≡ C–R–C ≡ C–]8, R ≡ thienyl), 2 and 3 as a function of the number of thiophene rings within the bridging ligand. With increasing thiophene content, the optical gap is reduced and the vibronic structure of the singlet emission changes toward that typical for oligothiophenes. We also find the intersystem crossing from the singlet excited state to the triplet excited state to become reduced, while the singlet–triplet energy gap remains unaltered. The latter implies that, in these systems, the T1 triplet excited state is extended over several thiophene rings. The photoconducting properties do not depend on the size of the thiophene fragment. We discuss and compare our results with studies on oligothiophenes and related organometallic polymers.

The effects of H2O and O2 on the photocurrent spectra of MEH-PPV

N Chauwdhury, A Köhler, MG Harrison, DH Hwang, AB Holmes, RH Friend

Synth. Met. 102 (1999) 871-872.

Photocurrent (PC) and current-voltage measurements were taken on MEH-PPV [poly(2-methoxy-5-(2′-ethyl-hexyloxy)-paraphenylene vinylene] in an as-made photocell, ITO / MEH-PPV (265 nm) / Al, in the photocell under ambient air, under a mixture of oxygen + nitrogen, and in vacuum after annealing. We discuss the different spectral responses and current-voltage characteristics obtained for the different atmospheres.

Donor-acceptor interactions in organometallic and organic poly-ynes

A Köhler, M Younus, MRA Al-Mandhary, PR Raithby, MS Khan, RH Friend

Synth. Met. 101 (1999) 246-247.

We compare the spectroscopic properties of donor-acceptor type poly-ynes of the type [D-C=C-A-C=C-]n with a fluorine-substituted phenylene or a thieno-pyrazine unit as acceptor, and platinum or alkoxy-substituted phenylene as donor.

Charge separation in localised and delocalised electronic states in polymeric semiconductor

A Köhler, DAd Santos, D Beljonne, Z Shuai, J-L Brédas, RH Friend, AB Holmes, A Kraus, K Müllen

Nature 392 (1998) 903-906.

Conjugated polymers such as poly(p-phenylene vinylene)s (PPVs) allow low-cost fabrication of thin semiconducting films by solution processing onto substrates. Several polymeric optoelectronic devices have been developed in recent years, including field-effect transistors, light-emitting diodes, photocells and lasers. It is still not clear, however, whether the description of electronic excitations in these materials is most appropriately formulated within a molecular or semiconductor (band-theory) picture. In the former case, excited states are localized and are described as excitons; in the latter they are delocalized and described as free electron–hole pairs. Here we report studies of the electronic states associated with optical excitations in the visible and ultraviolet range for the conjugated polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), by means of photocurrent measurements and quantum-chemical calculations. We find several photocurrent spectral features between 3 and 5 eV which are coupled with bands in the absorption spectrum. On modelling the excited states in this energy range, we have discovered an important feature that is likely to be general for materials composed of coupled molecular units: that mixing of delocalized conduction- and valence-band states with states localized on the molecular units produces a sequence of excited states in which positive and negative charges can be separated further at higher energies. In other words, these excited states facilitate charge separation, and provide a conceptual bridge between the molecular (localized) and semiconductor (delocalized) pictures.

Synthesis, electrochemistry and spectroscopy of blue platinum(II) poly-ynes and di-ynes

M Younus, A Köhler, S Cron, N Chawdhury, MRA Al-Mandhary, MS Khan, J Lewis, NJ Long, RH Friend, PR Raithby

Angew. Chemie, Internat. Ed. 37 (1998) 3036-3039

The smallest band gap observed so far (1.77 eV) for an organometallic polymer is exhibited by the blue, rigid-rod polymer 2, which is prepared by the reaction of trans-[PtCl2(PnBu3)2] with one equivalent of 1.

Synthesis and electronic structure of platinum-containing poly-ynes with aromatic and heteroaromatic rings

N Chauwdhury, A Köhler, RH Friend, M Younus, NJ Long, PR Raithby, J Lewis

Macromolecules 31 (1998) 722-727.

We have studied the dependence of intersystem crossing and the spatial extent of singlet and triplet excitons in platinum-containing poly-yne polymers with the general formula [Pt(PR3)2C ≡ CLC ≡ C]n (R = Et, nBu; L = pyridine, phenylene, or thiophene) as a function of electron delocalization in the spacer group L. We also report the synthesis route of those compounds. The optical absorption, photoluminescence, and photoinduced absorption of the corresponding polymers and monomers have been measured. We find that conjugation is increased but intersystem crossing is reduced by the electron-rich thiophene unit, while the opposite occurs for the electron-deficient pyridine unit as compared to the phenylene unit. For all investigated systems, we find that the singlet excited state and a higher lying Tn triplet excited state extend over more than a repeat unit while the T1 triplet state remains localized to less then one repeat unit.

Electronic excitations in luminescent conjugated polymers

RH Friend, GJ Denton, JJM Halls, NT Harrisson, AB Holmes, A Köhler, A Lux, SC Moratti, K Pichler, N Tessler, K Towns, HF Wittmann

Solid State Commun. 102 (1997) 249-258.

We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved and single-layer EL diodes achieve luminous efficiencies above 2 Lumen W-1 and peak brightnesses up to 90 000 cd m-2. We discuss measurements of photoconductivity, photovoltaic response, photoluminescence excitation spectra and stimulated emission in films of PPV. We consider that the photoexcited state in these films of PPV is the intrachain singlet exciton. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments and can be used as the active lasing medium when incorporated in suitable microcavity structures.

UV photocurrent spectroscopy in poly(p-phenylene vinylene) and derivatives

A Köhler, DAd Santos, D Beljonne, Z Shuai, J-L Brédas, RH Friend, SC Moratti, AB Holmes

Synth. Met. 84 (1997) 675-676.

We present photocurrent measurements of poly(p-phenylene vinylene) (PPV) and derivatives for photon energies up to 5.0 eV. We observe photocurrent peaks related to the first, second and third absorption peaks, which we associate with the dissociation of a succession of increasingly energetic excitons. We have performed INDO/SCI calculations for the five-ring PPV oligomer, which show different distributions of the electronic charge of the exciton wavefunction over the chain at different excitation energies. We discuss the significance of these states for the photoconduction in PPV and question the applicability of a band-based model.

Electronic processes of conjugated polymers in semicondutor device structures

RH Friend, GJ Denton, JJM Halls, NT Harrisson, AB Holmes, A Köhler, A Lux, SC Moratti, K Pichler, N Tessler, K Towns, HF Wittmann

Synth. Met. 84 (1997) 463-470.

We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved, and single-layer EL diodes achieve luminous efficiencies above 2 Lumens/W and peak brightnesses up to 90,000 cd/m2. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments, and can be used as the active lasing medium when incorporated in suitable microcavity structures.

Spatial extent of the singlet and triplet excitons in transition metal-containing poly-ynes

D Beljonne, HF Wittmann, A Köhler, S Graham, M Younus, J Lewis, PR Raithby, MS Khan, RH Friend, JL Brédas

J. Chem. Phys. 105 (1996) 3868-3877.

We present a joint experimental and theoretical investigation of the electronic excitations in transition metal-containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and triplet excited states is characterized. We find that p conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain-length dependence of the singlet–singlet, S0?S1, singlet–triplet, S0?T1, and triplet–triplet, T1?Tn, transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T1, is strongly localized on a single phenylene ring while the S1 and Tn states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang–Rhys analysis of the triplet emission spectrum in model systems.

Enhanced photocurrent response in photocells made with platinum-poly-yne/C60 blends by photoinduced electron-transfer

A Köhler, HF Wittmann, RH Friend, MS Khan, J Lewis

Synth. Met. 77 (1996) 147-150.

We report photocurrent and photoluminescence measurements on blends of the organometallic polymer Pt-poly-yne and C60. Due to efficient intersystem crossing, triplet excitons with a binding energy of at least 0.8 eV are created after photoexcitation of Pt-poly-yne. We find that electrons are transferred from Pt-poly-yne onto C60 by efficient ionization of the triplet exciton. This process gives rise to a photocurrent quantum yield of up to 1–2% in photocells fabricated in a layer structure of indium-tin oxide (ITO)/Pt-poly-yne and 7 wt.% C60/aluminium. Evidence for the ionization of the strongly bound triplet excitons is provided by studies of luminescence in the blends. There is only partial quenching of the singlet emission band, but complete quenching of the triplet emission band, and the lifetime of the triplet emission at room temperature is reduced from 2 to 0.1 µs upon addition of 7 wt.% C60.

Synthesis of new conjugated thiophene polymers

R Cervini, AB Holmes, SC Moratti, A Köhler, RH Friend

Synth. Met. 76 (1996) 169-171.

Results are presented which explore the synthesis, characterization and photoluminescence of new types of conjugated copolymers which incorporate fused thiophene units. Such materials possess intense purple-red colours and some absorb in the near-infrared. The redox processes showed that the polymers were both p- and n-dopable.

Photocurrent measurements on aggregates in ladder-type poly(p- phenylene)

A Köhler, J Gruener, RH Friend, K Müllen, U Scherf

Chem. Phys. Lett. 243 (1995) 456-461.

We report measurements of the photovoltaic effect on single layer devices made with a ladder-type poly-(paraphenylene) (LPPP). Besides the photocurrent in the blue spectral range associated with p to p* intra-chain absorption we observe an additional response in the yellow part of the spectrum centered at 2.25 eV. We also observe a weak absorption feature at the same energy, and we consider that both arise from absorption into p electron states formed over several aggregated polymer chains. The photocurrent due to excitation of these states is enhanced by two orders of magnitude compared to blue excitation and vanishes under reverse bias (i.e. ITO negative). We find that the photocurrent response due to the aggregate states is more sensitive to oxygen exposure than the response due to intra-chain absorption.

The photovoltaic effect in a platinum poly-yne

A Köhler, HF Wittmann, RH Friend, MS Khan, J Lewis

Synth. Met. 67 (1994) 245-249

We have carried out measurements of the photovoltaic effect in a platinum-containing poly-yne. A photocell was fabricated with the platinum poly-yne sandwiched between indium-tin oxide (ITO) and aluminium electrodes. A strong photoresponse was seen at the threshold of the singlet exciton absorption near 3.2 eV, with a quantum yield for carrier generation of up to 0.6%. This is comparable to that found in similar devices made with poly(phenylenevinylene) (PPV). It is known that the presence of the platinum atom gives very efficient crossover from singlet to triplet exciton, and we discuss here the effect of triplet excitons on the photogeneration of charge carriers. We also analyse the spectral response of the photocurrent in terms of charge carrier ranges.

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