Space charges at the interfaces in optoelectronic devices significantly influence charge carrier dynamics, such as charge carrier transport and recombination, thereby affecting device efficiency.

 CH₃NH₃PbI₃ (MAPI) is a mixed-ionic electronic conductor, where iodide vacancies are the majority ionic mobile carriers, with minor contribution to ion conduction from Pb²⁺ and MA⁺ defects ^[1]. Due to its mixed conducting property, it is essential to consider both electronic and ionic interactions and their impact on space charge. However, while electronic equilibration (Fermi level alignment) is the established method to analyze equilibrium space charges in solar cell materials, including ionic interactions in the analysis of interfaces within MAPI based is yet largely overlooked.

 First, we discuss the experimental investigation of ionically-generated space charges in MAPI when it is in contact with alumina. Zeta potential measurements are performed on alumina nanoparticles immersed in electrolyte containing salts relevant to MAPI, and they indicate a strong ion interaction between alumina and positive charged ions in MAPI precursor solution. These are expected to adsorb on their surface, result in the depletion of positive charge carriers and accumulation of negative charge carriers in MAPI in proximity of its interface with alumina nanoparticles. Using conductivity measurements in composite thin films as a function of the iodine partial pressure, we demonstrate that such interfacial ionic equilibration induces space charge potentials in the order of 700 – 800 mV at equilibrium, consistent with our previous report ^[2]. We then investigate the role of surface modification using chemisorbed organic molecular monolayers on alumina in altering the space charge situation within MAPI. Zeta potential results show that the ionic interaction can be modified through the surface modification of alumina, which we assign to a reduced ionic adsorption induced by steric effects or changes in the alumina surface chemistry. The results confirm the formation of reduced ionic space charge potentials in MAPI for the films with surface modified alumina, demonstrating that molecules adsorbed on the surface of contact phases are a promising tool to tune the space charge situation at the interfaces with MAPI.

Finally, extending this strategy to solar cell devices, we discuss the role of ionically-generated space charge regions in the active layer using the investigated alumina:MAPI composites. We demonstrate the detrimental role of having distributed p-n junction in the active layer for device performance, an effect that can be mitigated by surface modification. Finally, we address the impact of space charges and their modulation in MAPI infiltrated within mesoporous TiO₂ layers, a commonly adopted electron transport layer in MAPI based solar cells. This study will help the design of perovskite devices with improved interfacial properties and the interpretation of experimental data accounting for the mixed conducting nature of halide perovskites.