Diamine Surface Passivation and Postannealing Enhance the Performance of Silicon-Perovskite Tandem Solar Cells

We show that the use of 1,3-diaminopropane (DAP) as a chemical modifier at the perovskite/electron-transport layer (ETL) interface enhances the power conversion efficiency (PCE) of 1.7 eV band gap mixed-halide perovskite containing formamidinium and Cs single-junction cells, primarily by increasing the open-circuit voltage (V_OC) from 1.06 to 1.15 V. We find that adding a postprocessing annealing step after C60 evaporation further improves device performance. Specifically, the fill factor (FF) increases by 20% in the DAP + postannealing devices compared to the control. Using hyperspectral photoluminescence microscopy, we demonstrate that annealing helps improve compositional homogeneity at the electron-transport layer (ETL) and hole-transport layer (HTL) interfaces of the solar cell, which prevents detrimental band gap pinning in the devices and improves C₆₀ adhesion. Using time-of-flight secondary ion mass spectrometry, we show that DAP reacts with formamidinium (FA⁺) present at the surface of the perovskite structure to form a larger molecular cation, 1,4,5,6-tetrahydropyrimidinium (THP⁺), which remains at the interface. Combining the use of DAP and annealing the C₆₀ interface, we fabricate Si-perovskite tandems with a PCE of 25.29%, compared to 23.26% for control devices. Our study underscores the critical role of the chemical reactivity of diamines at the surface and the thermal postprocessing of the C₆₀/Lewis-base passivator interface in minimizing device losses and enhancing solar-cell performance of wide-band-gap mixed-cation mixed-halide perovskites for tandem applications.