Stepwise energy level regulation via bilayer self-assembled hole-transport materials for efficient and stable inverted perovskite solar cells

The optimization of hole transport layer (HTL) is crucial for achieving high efficiency and stability in inverted perovskite solar cells (PSCs) due to its role in facilitating hole transport and passivating the perovskite bottom interface. While self-assembled monolayers (SAMs) are commonly used for this purpose, the inherent limitations of a single SAM, such as fixed energy levels and rigid structure, restrict their adaptability for different perovskite components and further efficiency enhancement. Here, we demonstrate a stepwise deposition method for SAM-based HTLs to address this issue. We regulated the energy level gradient by depositing two SAMs with distinct energy levels, while the interactions between the phosphate groups in the SAMs and perovskite effectively reduce defect density at the bottom interface of the perovskite film. The as-fabricated PSCs achieved enhanced efficiency and stability with PCEs of 25.7% and 24.0% for rigid and flexible PSCs, respectively; these devices maintain 90% of their initial PCE after 500 h of maximum power point tracking, and retain 98% of their initial PCE after 4,000 bending cycles, representing one of the most stable flexible PSCs reported to date.