Hole transporting layer engineering via a zwitterionic polysquaraine toward efficient inverted perovskite solar cells

In inverted perovskite solar cells (PSCs), perovskite crystals grow on the bottom hole transporting layer (HTL), and the HTL not only functions to extract and transport holes, but also manipulates the perovskite crystallization process and affects perovskite film quality. Poly(triarylamine) (PTAA) represents one of the most widely used and highly-efficient hole transport material in inverted PSCs. However, strong hydrophobicity makes the PTAA layer incompatible to the polar perovskite precursor solution, consequently, solvent pre-wetting or introducing an amphiphilic interlayer is always applied to PTAA layer before depositing perovskite film. Herein, we develop an efficient approach of modifying the bottom interface of perovskite layer by blending PTAA with a p-type zwitterionic polysquaraine (PASQ-IDT). The zwitterionic PASQ-IDT not only significantly improves the wettability of PTAA film toward perovskite precursor, leading to large perovskite grains with uniform sizes, but also enables more efficient carrier extraction at the perovskite/HTL interface, accompanied with efficiently reduced trap densities at the bottom surface. As a result, MAPbI3-xClx-based inverted PSCs utilizing the modified PTAA HTL show an impressive efficiency exceeding 21 %, which represents a high value for MA-based inverted PSCs.