Ion-Beam Sputtering of NiO_x Hole Transporting Layers for p-i-n Halide Perovskite Solar Cells

Ion-beam sputtering offers significant benefits in terms of deposition uniformity and pinhole-free thin films without limiting the scalability of the process. In this work, the reactive ion-beam sputtering of nickel oxide has been developed for the hole transporting layer of p-i-n perovskite solar cells (PSCs). The process is carried out by the oxidation of the scattered Ni particles with additional post-treatment annealing regimes. Using a deposition rate of 1.2 nm/min allowed the growth of a very uniform NiO_x coating with the roughness below 0.5 nm on polished Si wafer (15 × 15 cm²). We performed a complex investigation of structural, optical, surface, and electrical properties of the NiO_x thin films. The post-treatment annealing (150-300 °C) was considered an essential process for the improvement of optical transparency, decrease of defect concentration, and gain of charge carrier mobility. As a result, the annealed ion-beam-sputtered NiO_x films delivered a power conversion efficiency (PCE) up to 20.14%, while the device without post-treatment reached the value of 11.84%. The improvement of the output performance originated from an increase of the short-circuit current density (J_sc), open-circuit voltage (V_oc), shunt, and contact properties in the devices. We also demonstrate that the ion-beam sputtering of NiO_x can be successfully implemented for the fabrication of large area modules (54.5 cm²) and PSCs on a flexible plastic substrate (125 μm).