Bifunctional Compound Induced Dual Back Surface Fields for Efficient Hole Transport Layer-Free Perovskite Solar Cells

Carbon-based hole transport layer-free (C-HTL-free) perovskite solar cells (PSCs) are promising for low-cost and stable photovoltaics, but the HTL absence deteriorates their power conversion efficiency (PCE) due to the lack of back surface field (BSF). In this work, the benefits of forming dual BSFs in improving the PCE of C-HTL-free PSCs are first investigated by simulation. Then, trityl tetrakis(pentafluorophenyl)borate (Tr⁺TPFB^-) is introduced into the C-HTL-free PSCs by post-treatment for the first time, which enables the formation of dual BSFs. TPFB^- passivates n-doping defects in perovskite and leads to the formation of perovskite n-p homojunction, while Tr⁺ extracts electrons from carbon and lowers its work function, which succeeds realizing dual BSFs. This improves the separation and extraction of photocarriers within the device, which is evidenced by the photoluminescence lifetime imaging of the device cross-section for the first time. As a result, the average open-circuit voltage increases significantly by about 70 mV, which largely contributes to the improvement of PCE with a champion value of 20.79% obtained.