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 to not only developing all-solution-processed inverted organic solar cells (OSCs) but also enabling their full potential of 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 causing the inherent wetting behaviors 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 show that the structure of Al2O3/HTL can generally serve in different active layers for inverted OSCs with high power conversion efficiency (PCE) and stability. By additional solution-processing the Al2O3 conjunction structure on the hydrophobic active layer, the wettability of hydrophilic HTL on the modified BHJ surface can be significantly improved, as evidenced by the reduced water contact angles. Remarkably, the new structure of BHJ/Al2O3/PIDT-F:POM offers superior electrical properties and thus endows inverted OSCs 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 that 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.