High charge carrier mobilities in thin films of organic-inorganic halide perovskites prepared by stoichiometric physical vapour deposition

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Poster, Eline M. Hutter, 288
Publication date: 5th February 2015

The reported efficiencies of devices based on organic-inorganic metal halide perovskites (OMHPs) are rapidly increasing, however a solid understanding of their photophysical properties is still lacking. In this work, we study the opto-electric properties of 200 nm thin films of CH₃NH₃PbI₃. These are prepared by physical vapour deposition (PVD) of the precursors CH₃NH₃I and PbI₂ in a ratio close to stoichiometric, followed by annealing. Atomic Force Microscopy shows homogeneous flat films in which the variation in thickness is less than 2 nm. From the X-ray diffraction pattern, showing only the (110) and (220) directions, we deduce that this PVD method yields highly crystalline films with domain sizes exceeding hundreds of nanometers. The electrodeless time-resolved microwave conductivity technique (TRMC) is used to examine the dynamics of charge carriers that are generated upon photo-excitation of OMHP films. With TRMC, the charge carrier generation yield and the electron and hole mobilities are obtained from the photo-conductance. For incident light intensities ranging from 10¹⁰ to 10¹² photons/cm², corresponding to carrier densities of 10¹⁵ to 10¹⁷ cm^-3, we consistently find maximum effective charge carrier mobilities in the order of several tens of cm²/Vs, as reported previously for this deposition method.¹ Note that this is an order of magnitude higher than in CH₃NH₃PbI₃layers prepared via different routes.^2,3 We expect that the large crystal domains contribute to these high carrier mobilities. Additionally, while for CH₃NH₃PbI₃  layers prepared by wet chemical depositiontechniques the majority of the charge carriers recombine via second order electron-hole recombination,^2,3 examination of the present TRMC decay traces show a predominant intensity independent, first order decay route. Furthermore, TRMC measurements recorded at different temperatures ranging from 195 and 355 K show relative small variations in photo-conductance and decay kinetics. Possible explanations for the present observations are presented and discussed. In short, we show that homogeneous flat films can be prepared by PVD, in which the charge carrier mobilities are substantially higher than in solution-processed OMHPs.
Typical mobilities obtained from time-resolved photo-conductance measurements on a 200 nm thin film of CH3NH3PbI3 prepared by physical vapour deposition.
(1) Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Energy Environ. Sci. 2014, 7, 2269–2275. (2) Savenije, T. J.; Ponseca, C. S.; Kunneman, L.; Abdellah, M.; Zheng, K.; Tian, Y.; Zhu, Q.; Canton, S. E.; Scheblykin, I. G.; Pullerits, T.; Yartsev, A.; Sundstro, V. J. Phys. Chem. Lett. 2014, 5, 2189–2194. (3) Ponseca, C. S.; Savenije, T. J.; Abdellah, M.; Zheng, K.; Yartsev, A.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; Wolf, J.-P.; Sundström, V. J. Am. Chem. Soc. 2014, 136, 5189–5192.

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