Ionic Effects on Space Charge Formation at Halide Perovskite Interfaces with Contact Materials

Understanding interfacial effects, such as charge recombination and injection, is a crucial issue to improve the photo conversion efficiency of hybrid perovskite solar cells (PSC) [1,2]. The space charge equilibrium at the interface between hybrid perovskites and contact layers is commonly interpreted according to electronic equilibration only. However, it has been shown that halide perovskites are mixed ionic-electronic conductors and that ionic adsorption at their interface with oxides can have a critical role in the determination of the space charge situation[3]. The resulting ionically-induced charge carrier redistribution may have significant repercussions on device behavior. Therefore, it is important to understand whether such ionic effects are relevant to interfaces involving other materials that are commonly used in high-performance solar cell applications, and how they can be controlled.<br/>In this study, we discuss experimental investigation of ionic effects at the interface between CH₃NH₃PbI₃ (MAPI) and a number of insulating, electron or hole transporting oxide materials. We use zeta potential measurements to establish the expected ionic interaction on the surface of oxide particles. We then perform electrical measurements to quantify the ionic and electronic conductivities of composite films, where nanoparticles of the oxide material are embedded in MAPI, as well as of structures where a mesoporous oxide film is infiltrated with MAPI. We study the iodine partial pressure dependence of ionic and electronic conductivities in these films, allowing us to extract the sign of the space charge potential in MAPI when in contact with the oxide phase. Secondly, we demonstrate that molecular functionalization can successfully modify the space charge behavior at the oxide-MAPI interface in both composite and mesoporous films.<br/>With this study, we aim to provide a complete model including both ionic and electronic effects to describe the equilibrium space charge situation at the interface between halide perovskites and contact materials. Our results will help the design of photovoltaic architectures and the interpretation of experimental data accounting for the mixed conducting properties of halide perovskites.<br/><br/>[1] D. Luo, R. Su, W. Zhang, Q. Gong, R. Zhu, <i>Nature Review Materials</i> <b>2020</b>, 5, 44-60.<br/>[2] M. M. Travakoli, M. Saliba, P. Yadav, P. Holzhey, A. Hagfeldt, S. M. Zakeeruddin, M. Gratzel, <i>Adv. Energy Mater.</i> <b>2019</b>, 9, 1802646.<br/>[3] G. Y. Kim, A. Senocrate, D. Moia, J. Maier, <i>Adv. Funct. Mater.</i> <b>2020</b>, 30, 2002426.