The research interest in halide perovskites has gained momentum enormously over the last recent years, also due to the demonstration of high-efficient perovskite-based optoelectronic devices. A prerequisite for such highly efficient devices is to realize high-quality perovskite layers, which requires a deep understanding about the perovskite formation and good process control. In that context, in situ optical spectroscopy during the processing of halide perovskites has become increasingly popular. Even though it is a relatively easily accessible yet powerful tool for studying perovskite formation, there exist some technical and analytical aspects that need to be considered to unfold its full potential. In this Perspective, we give an overview of the latest developments in the field of in situ optical spectroscopy to control and better understand the film processing of halide perovskites. We highlight possibilities and pitfalls regarding the analysis of measured optical data, discuss the development of technical concepts, and address future prospects of optical in situ spectroscopy.
Konstantin Schötz and Fabian Panzer
J. Phys. Chem. A (2021)
Structural Diversity in Layered Hybrid Perovskites, A2PbBr4 or AA′PbBr4, Templated by Small Disc-Shaped Amines
Yuan-Yuan Guo, Lin-Jie Yang, Simon Biberger, Jason A. McNulty, Teng Li, Konstantin Schötz, Fabian Panzer and Philip Lightfoot
Inorg. Chem. 59 (2020) 12858–12866
We present three new hybrid layered lead(II) bromide perovskites of generic composition A2PbBr4 or AA′PbBr4 that exhibit three distinct structure types. [TzH]2PbBr4 ([TzH+] = 1,2,4-triazolium) adopts a (001)-oriented layer structure and [AaH]2PbBr4, ([AaH+] = acetamidinium) adopts a (110)-oriented type, whereas [ImH][TzH]PbBr4, ([ImH+] = imidazolium) adopts a rare (110)-oriented structure with enhanced corrugation (i.e., 3 × 3 type). The crystal structures of each are discussed in terms of the differing nature of the templating molecular species. Photoluminescent spectra for each are reported and the behaviors discussed in relation to the different structure of each composition.
Investigating the Tetragonal-to-Orthorhombic Phase Transition of Methylammonium Lead Iodide Single Crystals by Detailed Photoluminescence Analysis
Konstantin Schötz, Abdelrahman M. Askar, Anna Köhler, Karthik Shankar and Fabian Panzer
Adv. Optical Mater. (2020) 2000455
Here, the phase‐transition from tetragonal to orthorhombic crystal structure of the halide perovskite methylammonium lead iodide single crystal is investigated. Temperature dependent photoluminescence (PL) measurements in the temperature range between 165 and 100 K show complex PL spectra where in total five different PL peaks can be identified. All observed PL features can be assigned to different optical effects from the two crystal phases using detailed PL analyses. This allows to quantify the fraction of tetragonal phase that still occurs below the phase transition temperature. It is found that at 150 K, 0.015% tetragonal phase remain, and PL signatures are observed from quantum confined tetragonal domains, suggesting their size to be about 7–15 nm down to 120 K. The tetragonal inclusions also exhibit an increased Urbach Energy, implying a high degree of structural disorder. The results first illustrate how a careful analysis of the PL can serve to deduce structural information, and second, how structural deviations in halide perovskites have a significant impact on the optoelectronic properties of this promising class of semiconductors.
Investigating two-step MAPbI3 thin film formation during spin coating by simultaneous in situ absorption and photoluminescence spectroscopy
Mihirsinh Chauhan, Yu Zhong, Konstantin Schötz, Brijesh Tripathi, Anna Köhler, Sven Huettner and Fabian Panzer
J. Mater. Chem. A, 8 (2020) 5086-5094
To date, the two-step processing method represents an attractive route for the thin film formation of halide perovskites. However, a fundamental understanding of the film formation dynamics in the case of spin coating methylammonium iodide (MAI) on PbI2 has not been established yet. Here we apply in situ optical spectroscopy during the two-step film formation of the model halide perovskite MAPbI3 via spin coating. We identify and analyze in detail the optical features that occur in the photoluminescence and the corresponding absorption spectra during processing. We find that the film formation takes place in five consecutive steps, including the formation of a MAPbI3 capping layer via an interface crystallization and the occurrence of an intense dissolution–recrystallization process. Consideration of confinement and self-absorption effects in the PL spectra, together with consideration of the corresponding absorption spectra allows quantification of the growth rate of the initial interface crystallization, which is found to be 11 nm s−1 under our processing conditions. We find that the main dissolution–recrystallization process happens at a rate of 445 nm s−1, emphasizing its importance to the overall processing.
Konstantin Schötz, Abdelrahman M. Askar, Wei Peng, Dominik Seeberger, Tanaji P. Gujar, Mukundan Thelakkat, Anna Köhler, Sven Huettner, Osman M. Bakr, Karthik Shankar and Fabian Panzer
J. Mater. Chem. C 8 (2020) 2289-2300
Despite the rapidly increasing efficiencies of perovskite solar cells, the optoelectronic properties of this material class are not completely understood. Especially when measured photoluminescence (PL) spectra consist of multiple peaks, their origin is still debated. In this work, we investigate in detail double peak PL spectra of halide perovskite thin films and single crystals with different material compositions. By different optical spectroscopic approaches and quantitative models, we demonstrate that the additional PL peak results from an extensive self-absorption effect, whose impact is intensified by strong internal reflections. This self-absorption accounts for the unusual temperature dependence of the additional PL peak and it implies that absorption until far into the perovskite’s Urbach tail is important. The internal reflections entail that even for thin films self-absorption can have a significant contribution to the PL spectrum. Our results allow for a clear assignment of the PL peaks by differentiating between optical effects and electronic transitions, which is a necessary requirement for understanding the optoelectronic properties of halide perovskites.
Impact of excess PbI2 on the structure and the temperature dependent optical properties of methylammonium lead iodide perovskites
Tobias Meier, Tanaji P. Gujar, Andreas Schönleber, Selina Olthof, Klaus Meerholz, Sander van Smaalen, Fabian Panzer, Mukundan Thelakkat and Anna Köhler
J. Mater. Chem. C 6 (2018) 7512-7519
We investigate the impact of excess PbI2 in the precursor solution on the structural and optical properties of thin films of the model hybrid perovskite methylammonium lead iodide (MAPbI3). We find that excess of PbI2 in the precursor solution results in crystalline PbI2 in the final thin film that is located at the grain boundaries. From UPS we find that this crystalline PbI2 phase has no direct impact on the electronic structure of MAPbI3. In contrast to this, temperature dependent absorption measurements indicate a systematic change in the temperature dependence of the exciton binding energy in the perovskite. We also observe a decrease in the critical temperature and a concomitant smearing out of the tetragonal–orthorhombic phase transition as a function of excess PbI2. Our results thus help to better understand the exact role of PbI2 in the perovskite layer and pave the way for a more tailored design of perovskite solar cells.
Tanaji P. Gujar, Thomas Unger, Andreas Schönleber, Martina Fried, Fabian Panzer, Sander van Smaalen, Anna Köhler and Mukundan Thelakkat
Phys. Chem. Chem. Phys., 20 (2018) 605-614
We report a systematic investigation on the role of excess PbI2 content in CH3NH3PbI3 perovskite film properties, solar cell parameters and device storage stability. We used the CH3NH3I vapor assisted method for the preparation of PbI2-free CH3NH3PbI3 films under a N2 atmosphere. These pristine CH3NH3PbI3 films were annealed at 165 °C for different time intervals in a N2 atmosphere to generate additional PbI2 in these films. From XRD measurements, the excess of PbI2 was quantified. Detailed characterization using scanning electron microscopy, X-ray diffraction, UV-Visible and photoluminescence for continuous aging of CH3NH3PbI3 films under ambient condition (50% humidity) is carried out for understanding the influence of different PbI2 contents on degradation of the CH3NH3PbI3 films. We find that the rate of degradation of CH3NH3PbI3 is accelerated due to the amount of PbI2 present in the film. A comparison of solar cell parameters of devices prepared using CH3NH3PbI3 samples having different PbI2 contents reveals a strong influence on the current density–voltage hysteresis as well as storage stability. We demonstrate that CH3NH3PbI3 devices do not require any residual PbI2 for a high performance. Moreover, a small amount of excess PbI2, which improves the initial performance of the devices slightly, has undesirable effects on the CH3NH3PbI3 film stability as well as on device hysteresis and stability.
Fabian Panzer, Cheng Li, Tobias Meier, Anna Köhler and Sven Huettner
Adv. Energy Mater. 19 (2017) 1700286
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.
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
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.
Reversible Laser Induced Amplified Spontaneous Emission from Coexisting Tetragonal and Orthorhombic Phases in Hybrid Lead Halide Perovskites
Fabian Panzer, Sebastian Baderschneider, Tanaji P. Gujar, Thomas Unger, Sergey Bagnich, Marius Jakoby, Heinz Bässler, Sven Hüttner, Jürgen Köhler, Ralf Moos, Mukundan Thelakkat, Richard Hildner and Anna Köhler
Adv. Optical Mater. 6 (2016) 917-928
The photoluminescence in a lead halide perovskite is measured for different temperatures (5–300 K) and excitation fluences (21–1615 μJ cm−2). It is found that amplified spontaneous emission (ASE) is observed for an excitation density larger than about 1 × 1018 cm−3 for both the tetragonal phase above 163 K and the orthorhombic phase below about 163 K. The fluence that is required to obtain this excitation density depends on temperature and phase since the nonradiative decay of excitations is temperature activated with different activation energies of inline image and inline image for the tetragonal and orthorhombic phase, respectively. The ASE from the tetragonal phase—usually prevailing at temperatures above about 163 K—can also be observed at 5 K, in addition to the ASE from the orthorhombic phase, when the sample is previously exposed to a fluence exceeding 630 μJ cm−2 at a photon energy of 3.68 eV. This additional ASE can be removed by mild heating to 35 K or optically, by exposing the sample by typically a few seconds with a fluence around 630 μJ cm−2. The physical mechanism underlying this optically induced phase transition process is discussed. It is demonstrated that this phase change can, in principle, be used for an all-optical “write–read–erase” memory device.