NiO<i>_x</i> Hole Transport Layer for Perovskite Solar Cells with Improved Stability and Reproducibility

Abstract

In this study, highly stable, low-temperature-processed planar lead halide perovskite (MAPbI_3-<i>x</i> Cl <i>_x</i> ) solar cells with NiO <i>_x</i> interfaces have been developed. Our solar cells maintain over 85% of the initial efficiency for more than 670 h, at the maximum power point tracking (MPPT) under 1 sun illumination (no UV-light filtering) at 30 °C, and over 73% of the initial efficiency for more than 1000 h, at the accelerating aging test (85 °C) under the same MPPT condition. Storing the encapsulated devices at 85 °C in dark over 1000 h revealed no performance degradation. The key factor for the prolonged lifetime of the devices was the sputter-deposited polycrystalline NiO <i>_x</i> hole transport layer (HTL). We observed that the properties of NiO <i>_x</i> are dependent on its composition. At a higher Ni³⁺/Ni²⁺ ratio, the conductivity of NiO <i>_x</i> is higher, but at the expense of optical transmittance. We obtained the highest power conversion efficiency of 15.2% at the optimized NiO <i>_x</i> condition. The sputtered NiO <i>_x</i> films were used to fabricate solar cells without annealing or any other treatments. The device stability enhanced significantly compared to that of the devices with PEDOT:PSS HTL. We clearly demonstrated that the illumination-induced degradation depends heavily on the nature of the HTL in the inverted perovskite solar cells (PVSCs). The sputtered NiO <i>_x</i> HTL can be a good candidate to solve stability problems in the lead halide PVSCs.