The Role of SnO₂ Processing on Ionic Migration in Multi-Halide Perovskites

Moving towards a future of efficient, accessible, and less carbon reliant energy devices has been at the forefront of energy research innovations for the past 30 years. Multi-halide perovskite (MHP) thin films have gained significant attention due to their flexibility of device applications and tunable capabilities for improving power conversion efficiency. Many behavioral aspects to MHP’s are thoroughly investigated: functionality of grain boundaries, recombination effects, ionic migration patterns, and hysteresis. Each avenue serving as gateway to optimize device performance, consideration must be given to chemical synthesis processing techniques.<br/><br/>Chemical Vapor Deposition (CVD) is a widely used technique for thin film coatings due to its ability for producing high volume batches of MHP’s with larger grain sizes, fewer defects, and fewer grain boundary formations. Additionally, nanoparticle processing has been applied to induce enlargement of grain boundaries, showcasing larger current signals than its MHP counterparts. Therefore, how does common substrate processing techniques (i.e. CVD, nanoparticles, hybrid) influence the behavior of MHP phenomenon such as ion migration and grain boundary formation? Speculated as inducing ionic recombination and driving I-V hysteresis in MHP’s, understanding how chemistry can be tuned to reduce such effects would be optimal.<br/><br/>We demonstrate how a hybrid approach of CVD and nanoparticle SnO₂ substrate processing significantly improves the performance of (FAPbI₃)_0.97(MAPbBr₃)_0.03perovskites in comparison to each technique utilized on its own. Higher performing hybrid devices exhibit fused grain boundary formations, not seen in exclusive CVD or nanoparticle devices. Conductive Atomic Force Microscopy (c-AFM) was used to track fused boundary locations and differentiate them from topographic features. Such fusing behavior has been previously observed to showcase higher counts of current and reduce defects such has halide vacancies.<br/><br/>In summary, to understand the chemistry behavior with respect to each device interface, Time of Flight Secondary Ionization Mass Spectrometry (ToF-SIMS) depth profiling was applied. Migration of K⁺, Na⁺, Ca⁺, FA⁺, MA⁺ was found in hybrid devices, in addition to Ca⁺ and Na⁺ clustering on the perovskite/air layer. Salt clustering could be correlated to the fusing effect demonstrated in the surface morphology imaged in c-AFM. Presence of K⁺ has shown to reduce defects driven by alkali iodides like NaI^-and Ca⁺ can help with enlarging the bandgap layer in studies where Ca+ was used to replace Pb⁺. Furthermore, reduced separation between positive ion such as MA⁺ and FA⁺from negative ions can decrease the potential responsible for I-V hysteresis.