High-phase purity two-dimensional perovskites with 17.3% efficiency enabled by interface engineering of hole transport layer

State-of-the-art p-i-n-based 3D perovskite solar cells (PSCs) use nickel oxide (NiO_X) as an efficient hole transport layer (HTL), achieving efficiencies >22%. However, translating this to phase-pure 2D perovskites has been unsuccessful. Here, we report 2D phase-pure Ruddlesden-Popper BA₂MA₃Pb₄I₁₃ perovskites with 17.3% efficiency enabled by doping the NiO_X with Li. Our results show that progressively increasing the doping concentration transforms the photoresistor behavior to a typical diode curve, with an increase in the average efficiency from 2.53% to 16.03% with a high open-circuit voltage of 1.22 V. Analysis reveals that Li doping of NiO_X significantly improves the morphology, crystallinity, and orientation of 2D perovskite films and also affords a superior band alignment, facilitating efficient charge extraction. Finally, we demonstrate that 2D PSCs with Li-doped NiO_X exhibit excellent photostability, with T₉₉ = 400 h at 1 sun and T₉₀ of 100 h at 5 suns measured at relative humidity of 60% ± 5% without the need for external thermal management.