Hysteresis in perovskite solar cells: experimental and theoretical evidence of its defect-related origin

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

Oral, Simone Meloni, presentation 271
Publication date: 5th February 2015

Organic inorganic lead halide perovskite solar cells have recently reached an efficiency of more than 20%¹, and their fabrication is relatively inexpensive. A phenomenon that was observed, and still need to be understood and addressed, is the hysteresis of the J-V curves. First mentioned by Dualeh et al.², this phenomenon was investigated in more detail in two recent articles^3,4, by Snaith et al. and Tress et al. The former concludes that the hysteresis is most likely due to phenomena taking place at the interface between the perovskite absorbing layer and the hole/electron blocking material, even though they do not exclude that it is related to the bulk of the perovskite. The latter shows that the origin in the hysteresis is a compensatinged field by ionic movement. In this work, we combined experiments and simulations to gain insights into the atomistic origin of the J-V hysteresis in perovskite solar cells. We measured the J-V curves of MAPbI₃ and MAPbBr₃ perovskites (MA = Methylammonium), and from these obtained the difference in photocurrent density (DJ) due to  hysteresis at a given forward voltage e.g. 600 mV. Then, DJ was determined at various temperatures and fitted with an Arrhenius-like formula. From this we concluded that hysteresis is due to a thermally activated process, with an activation barrier of ~0.15-0.3 eV. This barrier is lower for Br- and higher for I-perovskites. Using rare event simulations, namely performing string calculations⁵, we have computed the activation energy of the migration of a series of defects, and found very good agreement between the experimental and theoretical results indicating that the observed hysteresis is most likely due to the migration of halide vacancy defects. We also investigated the effect of the monovalent cation on the activation barrier, and found that its effect is indirect via the stretching or squeezing of the lattice. We conclude that the application of an external field produces a polarization of the ionic charges, namely halide vacancies, present in the perovskite absorbing layer. This polarization produces a counter field that, in turn, results in the hysteresis observed in the J-V curves.

[1] http://www.nrel.gov/ncpv/images/efficiency_chart.jpg [2] Dualeh, A.; Moehl, T.; Tetreault, N.; Teuscher, J.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M., “Impedance Spectroscopic Analysis of Lead Iodide Perovskite-Sensitized Solid-State Solar Cells. “, Acs Nano 2014, 8, 362-373. [3] Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Mohammad K, N.; Gratzel, M., “Understanding the rate-dependent J-V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field”, Energy & Environ. Science, 2015. 10.1039/c4ee03664f [4],Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Tse-Wei Wang, J.; Wojciechowski, K.; Zhang, W.; “Anomalous Hysteresis in Perovskite Solar Cells”, J. Chem. Phys. Lett. 2014, 5, 1511-1515 [5] E, W.; Ren, W.; E. Vanden-Eijnden, E.; “Simplified and improved string method for computing the minimum energy paths in barrier-crossing events”, J. Chem. Phys. 2007, 126, 164103;

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