Polymer Modification on the NiOx Hole Transport Layer Boosts Open-Circuit Voltage to 1.19 V for Perovskite Solar Cells

Inverted-structure perovskite solar cells (PVSCs) applying NiO_x as the hole transport layer (HTL) have attracted increasing attention. It is still a challenge to optimize the contact between NiO_x and the perovskite layer and to suppress energy loss at the interface. In this study, interface engineering was carried out by modifying the NiO_x layer with different polymers such as polystyrene, poly(methyl methacrylate) (PMMA), or poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) to improve the surface contact between NiO_x and the perovskite, to decrease the defect states, and to make the energy level alignment better. The NiO_x/PMMA-based device presents a V_oc as high as 1.19 V because of the improved interfacial contact and the interaction of the carbonyl and methoxy group with Pb²⁺. The NiO_x/PTAA-based device with the structure ITO/NiO_x/PTAA/(MAPbI₃)_0.95(MAPbBr₂Cl)_0.05/PCBM/BCP/Ag exhibits the highest power conversion efficiency of 21.56% with a high V_oc of 1.19 V. The enhanced performance can be attributed to the deepened highest occupied molecular orbital level of NiO_x/PTAA, which matched well with that of the perovskite and suppressed interface energy loss as well. This work provides a facile approach for efficiently improving the V_oc of NiO_x-based PVSCs.