Self-Doping Fullerene Electrolyte-Based Electron Transport Layer for All-Room-Temperature-Processed High-Performance Flexible Polymer Solar Cells

To achieve high-performance large-area flexible polymer solar cells (PSCs), one of the challenges is to develop new interface materials that possess a thermal-annealing-free process and thickness-insensitive photovoltaic properties. Here, an n-type self-doping fullerene electrolyte, named PCBB-3N-3I, is developed as electron transporting layer (ETL) for the application in PSCs. PCBB-3N-3I ETL can be processed at room temperature, and shows excellent orthogonal solvent processability, substantially improved conductivity, and appropriate energy levels. PCBB-3N-3I ETL also functions as light-harvesting acceptor in a bilayer solar cell, contributing to the overall device performance. As a result, the PCBB-3N-3I ETL-based inverted PSCs with a PTB7-Th:PC71BM photoactive layer demonstrate an enhanced power conversion efficiency (PCE) of 10.62% for rigid and 10.04% for flexible devices. Moreover, the device avoids a thermal annealing process and the photovoltaic properties are insensitive to the thickness of PCBB-3N-3I ETL, yielding a PCE of 9.32% for the device with thick PCBB-3N-3I ETL (61 nm). To the best of one's knowledge, the above performance yields the highest efficiencies for the flexible PSCs and thick ETL-based PSCs reported so far. Importantly, the flexible PSCs with PCBB-3N-3I ETL also show robust bending durability that could pave the way for the future development of high-performance flexible solar cells.