Highly Efficient Hole Transport Layer-Free Low Bandgap Mixed Pb–Sn Perovskite Solar Cells Enabled by a Binary Additive System

The development of high-performance hole transport layer (HTL)-free perovskite solar cells (PSCs) with a simplified device structure has been a major goal in the commercialization of PSCs due to the economic advantage of low manufacturing cost. Unfortunately, low bandgap (E_g) mixed Pb–Sn perovskites, which have promising utility for constructing efficient all-perovskite tandem solar cells, have rarely been explored in simplified HTL-free device configurations. In this study, efficient band bending and defect engineering at the interface between perovskite and indium tin oxide (ITO) are realized via a binary additive system using copper thiocyanate (CuSCN) and glycine hydrochloride (GlyHCl). Using mixed Pb–Sn perovskites decorated with crystalline p-type CuSCN, the energy level alignment at the hole extractive interface is modulated in favor of hole extraction, simultaneously increasing hole mobility. Suppressed nonradiative carrier recombination in the perovskite bulk, or across the charge extractive interface, is further achieved by GlyHCl without disturbing the efficient hole transfer characteristics. Notably, a more optimized band alignment is achieved at the hole extractive interface with the addition of GlyHCl. The HTL-free mixed Pb–Sn PSC shows an efficiency up to 20.1% under forward bias with negligible hysteresis, comparable to state-of-the-art high-performance full-structured mixed Pb–Sn PSCs.