On the Mitigation of Ni3+-Induced Losses in NiO Hole Transport Layers for Metal Halide Perovskite Solar Cells

NiO is widely investigated as a hole transport layer (HTL) in p–i–n metal halide perovskite solar cells. While previous studies have focused on improving device performance by lowering NiO film resistivity through increased Ni3+ content, Ni3+ states, particularly at the NiO surface, act as trap states and trigger redox reactions with the perovskite. This study investigates the performance of devices based on plasma-assisted atomic layer deposited (ALD) NiO film, NiOAlanis, characterized by a ∼2.7% higher relative average Ni valence and low resistivity (4 ± 1 Ω∙cm), compared to thermal ALD NiO film, NiOBu-MeAMD ((1.9 ± 0.3) × 104 Ω∙cm). NiOAlanis-based devices yield a poor power conversion efficiency (PCE) of ∼2.4%, compared to devices based on NiOBu-MeAMD (∼14.0%) and commonly used Me-4PACz (∼17.1%) or PTAA (~15.6%), primarily due to a low JSC (∼4.5 mA/cm2). X-ray photoelectron spectroscopy reveals that a redox reaction occurs between formamidinium iodide of the perovskite and Ni3+ states of NiOAlanis, leading to an interfacial charge-extraction barrier. To improve device performance, we report on an H2 plasma treatment of NiOAlanis (“NiOAlanis,H2 pl.”), which decreases the average Ni valence to ∼2.2%, relative to NiOBu-MeAMD, and increases the resistivity to (1.1 ± 0.1) × 104 Ω∙cm, indicating a decrease in relative Ni3+ content. Consequently, NiOAlanis,H2 pl.-based devices show improved charge collection, increasing the JSC to ∼20.5 mA/cm2 and leading to a PCE of ∼15.1%. Moreover, incorporating H2 plasma treatment in every ALD cycle (“NiOAlanis,ABC H2pl.”) further lowers the relative Ni3+ content owing to an increase in hydrogen content from 3 ± 1 to 9 ± 1 at./nm3 and increases the resistivity beyond the detection limit. Notably, NiOAlanis,ABC H2pl.-based devices recover a ∼110 mV VOC deficit typically observed in other NiO-based devices, achieving values comparable to those of PTAA- or NiO/Me-4PACz-based devices, due to suppression of interfacial redox reactions. Our results suggest that focusing on Ni3+ content suppression as well as interfacial passivation can provide a pathway towards high-performing devices.