Microscopic (Dis)order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites

Helen Grüninger, Menno Bokdam, Nico Leupold, Paul Tinnemans, Ralf Moos, Gilles A. De Wijs, Fabian Panzer and Arno P. M. Kentgens

J. Phys. Chem. C (2021)

Recent developments in thefield of high efficiencyperovskite solar cells are based on stabilization of the perovskitecrystal structure of FAPbI3while preserving its excellentoptoelectronic properties. Compositional engineering of, forexample, MA or Br mixed into FAPbI3results in the desiredeffects, but detailed knowledge of local structural features, such aslocal (dis)order or cation interactions of formamidinium (FA) andmethylammonium (MA), is still limited. This knowledge is,however, crucial for their further development. Here, we shed lighton the microscopic distribution of MA and FA in mixedperovskites MA1−xFAxPbI3and MA0.15FA0.85PbI2.55Br0.45bycombining high-resolution double-quantum1H solid-state nuclear magnetic resonance (NMR) spectroscopy with state-of-the-artnear-first-principles accuracy molecular dynamics (MD) simulations using machine-learning force-fields (MLFFs). We show that ona small local scale, partial MA and FA clustering takes place over the whole MA/FA compositional range. A reasonable driving forcefor the clustering might be an increase of the dynamical freedom of FA cations in FA-rich regions. While MA0.15FA0.85PbI2.55Br0.45displays similar MA and FA ordering as the MA1−xFAxPbI3systems, the average cation−cation interaction strength increasedsignificantly in this double mixed material, indicating a restriction of the space accessible to the cations or their partialimmobilization upon Br−incorporation. Our results shed light on the heterogeneities in cation composition of mixed halideperovskites, helping to exploit their full optoelectronic potential.

Recent Advances and Perspectives on Powder‐Based Halide Perovskite Film Processing

Nico Leupold and Fabian Panzer

Adv. Funct. Mater. (2021) 2007350

Halide perovskites have undergone an impressive development and could be used in a wide range of optoelectronic devices, where some of them are already at the edge of commercialization, e.g., perovskite solar cells. Recently, interest in perovskites in powder form has increased, as for example, they are found to exhibit high stability and allow for easy production of large quantities. Accordingly, also the topic of processing thin and thick films on the basis of perovskite powders is currently gaining momentum. Here, perovskite powder can form the basis for both, typical wet and solvent‐based processing approaches, as well as for dry processes. In this Progress Report, the recent developments of halide perovskites in powder form and of film processing approaches are summarized that are based on them. The advantages and opportunities of the different processing methods are highlighted, but their individual drawbacks and limitations are also discussed. Prospects are also pointed out and possible steps necessary to unlock the full potential of powder‐based processing methods for producing high quality thick and thin perovskite layers in the future are discussed.

Impact of Pressure and Temperature on the Compaction Dynamics and Layer Properties of Powder-Pressed Methylammonium Lead Halide Thick Films

Christina Witt, Andreas Schmid, Nico Leupold, Maximilian Schultz, Julian Höcker, Andreas Baumann, Ralf Moos and Fabian Panzer

ACS Appl. Electron. Mater. 8 (2020) 2619–2628

While halide perovskite X-ray detectors based on single crystals could achieve extraordinary sensitivities, detectors based on polycrystalline thick films lag behind in efficiency. This is unfortunate since the processing methods for producing polycrystalline thick films, especially by pressure treatment of powders, are suitable for upscaling. Here, we investigate in detail the pressing of readily prepared powders of methylammonium lead halide perovskites MAPbI3 and MAPbBr3 to thick layers. By time-dependent pressure measurements, we monitor the occurring compaction dynamics, identifying two relaxation processes with different timescales. When pressing at elevated temperatures from room temperature (RT) to 100 °C, the pressure relaxations change drastically. While the layer properties such as relative density and surface roughness only improve to a certain degree by increasing the pressure at RT, we observe relative densities >97%, considerable reduction in surface roughness, and a significant increase in grain size with tempered pressing. Analyses regarding time-dependent pressure relaxations of tempered pressing allow attributing the dynamics to a sintering process, where we find the sinter onset to be surprisingly low at about 30 °C, mainly independent of the applied pressure (10–100 MPa). Our results will allow for an improved and more targeted powder processing of halide perovskite thick films as they are promising candidates for efficient X-ray detectors.

High Versatility and Stability of Mechanochemically Synthesized Halide Perovskite Powders for Optoelectronic Devices

Nico Leupold, Konstantin Schötz, Stefania Cacovich, Irene Bauer, Maximilian Schultz, Monika Daubinger, Leah Kaiser, Amelle Rebai, Jean Rousset, Anna Köhler, Philip Schulz, Ralf Moos and Fabian Panzer

ACS Appl. Mater. Interfaces 11 (2019) 30259−30268

We show that mechanochemically synthesized halide perovskite powders from a ball milling approach can be employed to fabricate a variety of lead halide perovskites with exceptional intrinsic stability. Our MAPbI3 powder exhibits higher thermal stability than conventionally processed thin films, without degradation after more than two and a half years of storage and only negligible degradation after heat treatment at 220 °C for 14 h. We further show facile recovery strategies of nonphase-pure powders by simple remilling or mild heat treatment. Moreover, we demonstrate the mechanochemical synthesis of phase-pure mixed perovskite powders, such as (Cs0.05FA0.95PbI3)0.85(MAPbBr3)0.15, from either the individual metal and organic halides or from readily prepared ternary perovskites, regardless of the precursor phase purity. Adding potassium iodide (KI) to the milling process successfully passivated the powders. We also succeeded in preparing a precursor solution on the basis of the powders and obtained uniform thin films for integration into efficient perovskite solar cells from spin-coating this solution. We find the KI passivation remains in the devices, leading to improved performance and significantly reduced hysteresis. Our work thus demonstrates the potential of mechanochemically synthesized halide perovskite powders for long-time storage and upscaling, further paving the way toward commercialization of perovskite-based optoelectronic devices

Setup to Study the in Situ Evolution of Both Photoluminescence and Absorption during the Processing of Organic or Hybrid Semiconductors

Michael Buchhorn, Stefan Wedler and Fabian Panzer

J. Phys. Chem. A, 122 (2018) 9115–9122

In situ measurement techniques, applied during the solution processing of novel semiconductors such as organic semiconductors or hybrid perovskites, have become more and more important to understand their film formation. In that context, it is crucial to determine how the optical properties, namely photoluminescence (PL) and absorption, evolve during processing. However, until now PL and absorption have mostly been investigated independently, significantly reducing the potential insights into film formation dynamics. To tackle this issue we present the development of a detection system that allows simultaneous measurement of full absorption and PL spectra during solution processing of the investigated film. We also present a spin-coater system attachable to the detection system, where the temperature of the substrate on which the film is processed can be changed. We performed test measurements by spin coating the well-known conjugated polymer P3HT demonstrating the potential of this technique. By considering absorption and corresponding PL, we extract the PL quantum yield (PLQY) during processing, which decreases with substrate temperature. Furthermore, we identify a significant red shift of the PL just prior to the onset of the aggregation process, indicating the importance of chain planarization prior to solid film formation.

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

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.