Mapping Rb-Perovskite Photovoltaics: Photovoltage and Photocurrent at the Nanoscale

The addition of Rb⁺ cation at the A-site of the traditional perovskite solar cells has significantly helped in overcoming shortcomings in device performance. While some of these developments are well understood at the macroscopic level, a comprehensive study on the electrical characteristics (current and voltage) at the nanoscale remains incomplete. Here, we resolve the electrical performance of both dual-cation (Cs_0.17FA_0.83Pb(I_0.87Br_0.17)₃) and quad-cation (Cs_0.07Rb_0.03FA_0.76MA_0.14Pb(I_0.85Br_0.15)₃) perovskite solar cells by measuring photovoltage and photocurrent, spatially, at the nanoscale. Our results indicate reduced J-V hysteresis (90%), a decrease in voltage heterogeneity (34%), and a faster residual voltage decay post illumination (55%) in quad-cation perovskites imparted by Rb⁺ cations. We establish that quad-cation perovskite film shows half the photoinactivity seen in dual-cation perovskite. At the macroscopic level, we find that both compositions display similar photocurrent performance. Surprisingly, at the nanoscale, we unravel higher photocurrent (46%) for quadruple-cation perovskite. Further, we concluded no correlation of film morphology and grain size with photocurrent for either chemical composition.