Midgap states and energy alignment at interconnect are crucial for perovskite tandem solar cells

Monolithic, two-terminal stacking of wide and narrow band-gap perovskite absorbers can overcome the single-junction efficiency limitations. Empirically, two subcells need to be electrically coupled via an interconnecting layer (IL) where the majority charge carriers need to be recombined efficiently, avoiding minority recombination. This study explores how increasing the density of states (DOS) in the IL can encourage majority recombination and, hence, improve the open-circuit voltage (V oc ). By using indium oxide doped with zinc oxide (IZO) in the IL, we found that adjusting film formation techniques can tune DOS and other properties. Additionally, higher in-plane resistance at the interconnect and favorable energy alignment across ILs are crucial for improving the power conversion efficiency (PCE). Compared to commonly used Au nanoparticles, the optimized ILs improved the PCE from 22.32% to 26.39%, the V oc from 1.84 to 1.94 V, and the short circuit current density from 15.42 to 17.18 mA/cm 2 .