Visible-Transparent Electron-Selective Self-Assembled Monolayer for Electron-Transport-Layer-Free High-Efficiency and Flexible Organic Solar Cells

Compared to hole-selective ones, the availability of electron-selective self-assembled monolayers ( es -SAMs) remains limited, with little knowledge regarding their autonomous application without metal-oxide-based electron-transport layers (ETLs) in organic solar cells (OSCs). We demonstrate an “ETL-free” inverted OSC that exhibits a power conversion efficiency (PCE) of 17.35% by utilizing BPD-3PA among newly developed visible-transparent es -SAMs. This PCE exceeds that of the ZnO-based control device, representing among the highest PCEs for ETL-free inverted OSCs. The enhanced performance of the BPD-3PA-based device is attributed to improved charge-transport/-extraction properties and reduced recombination loss, as originated from interfacial engineering-driven improvements in dipoles and surface wettability/contact, in addition to favorable energy-level alignment and work function. Furthermore, the BPD-3PA-based device on a flexible substrate demonstrates exceptional 47% PCE retention after a 5000-cycle bending test, far exceeding 1.5% PCE retention of the ZnO-based one. Our results highlight the great potential of BPD-3PA for high-efficiency, flexible ETL-free devices in forthcoming SC technologies.