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

Abstract 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‐9 H ‐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/C 60 interface.