Slowing the Phase Transition of a Suite of Perovskites Via Passivation

The global drive for expanding renewable energy production to combat global warming demands for increasing solar energy production. Metal halide perovskite solar cells (PSCs) have gained notoriety in recent years due to their high efficiency, comparable to silicon photovoltaic (PV) cells, and low manufacturing costs. They realize their full potential in tandem solar cells with silicon which have now reached over 34% power conversion efficiency, compared with 27.3% for silicon alone. A major barrier preventing widespread commercial implementation is the instability of halide perovskites when exposed to ambient conditions. One such critical factor in MHP stability are surface defects (halide vacancies) during fabrication generating under coordinated Pb²⁺ ions at the surface and grain boundaries. These sites act as nonradiative recombination sites which reduce the PSC performance and, more importantly, can act as sites for degradation processes to begin. Therefore, surface defect engineering is a crucial component in improving the stability of PSCs and is done via surface passivation. The ligands acting as lewis bases will bind to the under coordinated Pb²⁺ ions reducing the number of surface defects. A class of passivator ligands have shown to improve both the phase and thermal stability of the perovskite with the chemical composition of Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃. In this work, we use this suite of surface passivators in a dip coating process which is broadly compatible with a suite of halide perovskite films. To determine which ligands improve thermal and phase stability, initial explorations of thermal stability and phase stability have been performed. This work extends the applicability of surface passivation to a wide range of halide perovskite films and film formation methods, and is a stepping stone towards a better understanding of surface defects in halide perovskite degradation and to improve perovskite stability in the real world environment.