Impact of microstructure on the electron-hole interaction in lead halide perovskites

Despite the remarkable progress in the performance of devices based on the lead halide perovskite semiconductor family, there is still a lack of consensus on their fundamental photophysical properties. Here, using magneto-optical transmission spectroscopy we elucidate the impact of the microstructure on the Coulomb interaction between photo-created electron-hole pairs in methylammonium lead triiodide (MAPbI₃) and the triple-cation lead mixed-halide composition, Cs₀.₀₅(MA₀.₁₇ FA₀.₈₃)₀.₉₅Pb(I₀.₈₃Br₀.₁₇)₃ (Cs: cesium, MA: methylammonium, FA: formamidinium) by investigating thin films with a wide range of grain sizes from tens of nanometers to microns. At low temperatures, in which thermal fluctuations of the interactions are frozen and the rotational disorder of the organic cation is negligible, the exciton binding energy and reduced effective mass of carriers remain effectively unchanged with grain size. We conclude that the microstructure plays a negligible role in the Coulomb interaction of the photo-created electron-hole pairs, in contrast to previous reports. This renewed understanding of the relationship between these fundamental electronic properties and the microstructure is critical for future fundamental studies and improving device design.The authors acknowledge support from the Australian Government through the Australian Renewable Energy Agency (ARENA) and the Australian Centre for Advanced Photovoltaics (ACAP). The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for any information or advice contained herein. S. D. S. acknowledges funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement number PIOF-GA-2013-622630. This work was partially supported by ANR JCJC project milliPICS, the Région Midi-Pyrénées under contract MESR 13053031, BLAPHENE project under IDEX program Emergence and Programme des Investissements d'Avenir under the program ANR-11-IDEX-0002-02, reference ANR-10-LABX-0037-NEXT. Part of the work has been supported by TERASPEC grant within IDEX Emergence program of University of Toulouse. Zhuo Yang and Nan Zhang hold a fellowship from the Chinese Scholarship Council (CSC). This work was supported by EPSRC (UK) via its membership to the EMFL (grant no. EP/N01085X/1). M. A. J. gratefully acknowledges Cambridge Materials Limited for a PhD scholarship

Nucleation and Growth Control of HC(NH₂)₂PbI₃ for Planar Perovskite Solar Cell

HC­(NH₂)₂PbI₃ perovskite solar cells have emerged as a promising alternative to CH₃NH₃PbI₃ perovskite solar cells due to their better thermal stability and lower bandgap. In this work, we have demonstrated a reliable fabrication technique for HC­(NH₂)₂PbI₃ planar perovskite solar cells by controlling nucleation and crystallization processes of the perovskite layer through a combination of gas-assisted spin coating and the addition of HI additive in the perovskite precursor. A narrow distribution of power conversion efficiencies (PCEs) can be achieved with an average of 13% with negligible hysteresis when measured at a scanning rate of 0.1 V/s. The best performance device has a PCE of 16.0%. It is shown that by using optimized conditions we can consistently form dense, uniform, pinhole-free good crystalline, lead-iodide-impurities-free HC­(NH₂)₂PbI₃ film that has been comprehensively characterized by scanning electron microscopy, X-ray diffraction, Kelvin probe force microscopy, photoluminescence, and electroluminescence in this work