Synergistic optimization of interfacial energy-level alignment and defect passivation toward efficient annealing-free inverted polymer solar cells

Abstract

A highly efficient inverted polymer solar cell (PSC) has been demonstrated by incorporating an insulating inorganic cerium fluoride (CeF₃) layer between the annealing-free zinc oxide (ZnO) electron transport layer (ETL) and the photoactive layer. The CeF₃-embedded solar cell fabricated by a full annealing-free process presents an overwhelming power conversion efficiency (PCE) of up to 10.53% for the PTB7-Th:PC₇₁BM-based blend system, which far surpasses that of the reference device with a bare ZnO interlayer (8.48%). Ultraviolet photoelectron spectroscopy revealed that the interfacial energy level alignment has been intentionally improved by forming an interfacial dipole layer at the interface between the ZnO ETL and the ultrathin CeF₃ interlayer, leading to a reduced energy barrier for electron transport as well as depressed photogenerated carrier recombination. Concurrently, the addition of CeF₃ also passivates the interfacial defect states of the ZnO film and ameliorates the inherent incompatibility with the hydrophobic photoactive layer. This protocol demonstrates that the utilization of the ZnO/CeF₃ multifunctional interlayer affords a facile but effective strategy for achieving high-performance inverted PSCs, which does not require postprocessing thermal annealing treatments, laying a good foundation for the future developments of organic photovoltaic devices.