Investigations of charge extraction and trap-assisted recombination in polymer solar cells via hole transport layer doped with NiO nanoparticles

Polymer solar cells (PSCs) are limited in their power conversion efficiency (PCE) by trap-assisted recombination caused by localized deep trap states. Therefore, developing efficient hole transport layers (HTLs) for PSCs is essential for facilitating the hole extraction stage from the photoactive layers. As part of this study, an organic/inorganic hybrid HTL was prepared by mixing synthesized nickel oxide (NiO) nanoparticles (NPs) of average size ranging from 16 to 21 nm with poly(3-hexylthiophene) (P3HT). Based on this hybrid HTL, multilayer PSCs with an ITO/ZnO/P3HT:PC61BM/P3HT:NiO NPs/MoO3/Ag structure were fabricated and characterized. As a result of the incorporation of the optimum ratio of 5 wt% NiO NPs in the P3HT layer, the PCE obtained 3.73% compared to pristine devices (2.70%) fabricated without the P3HT layer. A charge extraction by increasing voltage (CELIV) measurement confirmed enhanced hole extraction at the active layer/HTL interface. The low fluorescence emission of the P3HT/NiO NPs film enabled efficient hole extraction at the active layer/hybrid HTL interface. Adding NiO NPs beyond 5 wt% to the P3HT layer causes deep trap states to form, which facilitated trap-assisted recombination and slowed down the mobility of the holes. Consequently, we can gain insight into how to improve PSC performance with hybrid HTL.