Enhanced Power Conversion Efficiency in Tin Halide Perovskite Solar Cells with Zinc Iodide Interlayers

Optimal alignment of the energy levels can improve the performance of perovskite solar cells. In this work, we show that energy level alignment can be enhanced by a vacuum-deposited zinc iodide (ZnI₂) layer inserted between tin perovskite and a C₆₀ electron transport layer. The open-circuit voltage of FA_0.75MA_0.25SnI₃ (FA: formamidinium; MA: methylammonium) solar cells increased by more than 0.1 V, while the power conversion efficiency increased from 10.2% to 12.8%. The shelf lifetimes of the devices are also improved. ZnI₂ interlayers are also effective with other perovskites compositions, such as FASnI₃ and PEA_0.15FA_0.85SnI₃ (PEA: phenethylammonium), with their device efficiencies increasing from 6.8% to 11.6% and from 6.2% to 9.5%, respectively. The energy level diagram, derived from Kelvin probe measurements, suggests that ZnI₂ interlayers reduce the band offset between perovskite and C₆₀. As a result, the electron accumulation width in C₆₀ is reduced, mitigating nonradiative charge carrier recombination losses and increasing the device performance.