Simultaneous improvement of efficiency and stability of inverted organic solar cell via composite hole transport layer

Abstract

It is generally believed that the inverted structure is more beneficial for constructing highly stable organic solar cells (OSCs), but the power conversion efficiency (PCE) of current inverted OSCs lags significantly behind that of conventional-structure ones. Herein, a novel composite hole transport layer (HTL) is developed by combining a small molecule, [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) with molybdenum oxide (MoO₃) to simultaneously optimize efficiency and stability. Benefiting from the favorable surface morphology, enhanced built-in potential, and suppressed recombination of this composite HTL, an impressive PCE enhancement from 17.46% for the control device based on MoO₃ alone to 18.49% for the optimal device with the composite HTL is achieved with the PM6:L8-BO-F:Y6-BO active layer, which represents the best PCE for inverted OSCs to date. In addition to achieving a high PCE, adopting this 2PACz/MoO₃ HTL also improves the device stability. The unencapsulated device maintains 96.4% of its initial PCE after being stored in nitrogen for 8000 h, while that of the MoO₃-based device degrades to 86.3%. Additionally, after 220 h of continuous illumination under a white light emitting diode light source at 100 mW cm⁻², the 2PACz/MoO₃-based device maintains 75.5% of its initial PCE value, while that of the MoO₃-based device degrades to 65.5%. These results demonstrate the superiority of this composite HTL in simultaneously enhancing the efficiency and stability of the inverted device and providing an efficient strategy for fabricating high-efficiency inverted OSCs.