Understanding Postdeposition Treatments of Hole-Transporting Self-Assembling Molecules for Perovskite/Silicon Tandem Solar Cells

The introduction of carbazole-based self-assembling molecules (SAMs) as hole transport layers (HTLs) has been a key step in the development of highly efficient perovskite-based solar cells. To this date, most SAM-related studies have focused on the optimization and understanding of SAMs by changing system parameters before or during the SAM formation process. Postdeposition treatments, like a washing step or annealing treatment, are commonly utilized but have not yet been thoroughly investigated and optimized. Here, these treatments are systematically studied for the SAM [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) as the HTL in perovskite/silicon tandem solar cells. The multimethod experimental characterization of Me-4PACz layers, which introduces cyclic voltammetry as a valuable technique for HTL characterization, gives detailed insights into how postdeposition treatments affect the SAM and is further supported by optoelectrical device simulations. While a washing step removes loosely bound and less ordered agglomerates to create a pure monolayer, an enhanced SAM annealing temperature improves the orientation of Me-4PACz monolayers. Combined, the implementation of these optimized postdeposition treatments leads to an improvement of 3.4%abs in average power conversion efficiency and thus the fabrication of a 28.2%-efficient perovskite/silicon tandem solar cell, notably without any perovskite bulk passivation or passivation at the perovskite/C60 interface.