High efficiency of solution-processed inverted organic solar cells enabled by an aluminum oxide conjunction structure†

Abstract

The lack of solution-processed hole transport layers (HTLs) has become an obstacle not only to developing all-solution-processed inverted organic solar cells (OSCs) but also to enabling their full potential for high-throughput fabrication. One of the major problems is the distinct difference in surface-free energy between most HTLs and the organic bulk heterojunction (BHJ) active layer, which causes inherent wetting behavior problems and poor film quality. Here, we unveil the interesting features of the aluminum oxide (Al₂O₃) nanocrystal-based conjunction structure in improving the interfacial properties between a solution-processed HTL (namely PIDT-F:POM) and the organic active layer. Our results showed that the structure of Al₂O₃/HTL can generally be applied to different active layers in inverted OSCs, providing high power conversion efficiency (PCE) and stability. By additional solution-processing of the Al₂O₃ conjunction structure on the hydrophobic active layer, the wettability of the hydrophilic HTL on the modified BHJ surface is significantly improved, as evidenced by the reduced water contact angles. Remarkably, the new structure of BHJ/Al₂O₃/PIDT-F:POM offers superior electrical properties, explained by the excellent hole extraction capability and reduced interfacial recombination. Consequently, Al₂O₃/PIDT-F:POM yields high PCEs, reaching 18.4% in inverted OSCs. Notably, their PCE and stability are even better than those of the control cells made from widely adopted evaporated molybdenum trioxide (MoO₃). The work demonstrates the new concept of establishing efficient solution-processed HTLs and alternative pathways for promoting inverted OSCs for practical applications.