Modulation of oxygen in NiO:Cu films toward a physical insight of NiO:Cu/c-Si heterojunction solar cells

p-type Cu-doped NiO (NiO:Cu) thin films, a transition metal oxide as hole transport layer material, were fabricated using sputtering technology at a high substrate temperature. It is found that cooling oxygen pressures (Pc_o2), with the range of 10⁻⁴–2 × 10⁴ Pa, modulating oxygen vacancies in the films, have an important impact on the property of NiO:Cu films through Ni and Cu oxidization level. The NiO:Cu film with a proper cooling oxygen pressure, 10³ Pa in this work, reveals high hole concentration and transmittance, leading to a relatively high photovoltaic performance in the NiO:Cu/c-Si heterojunction solar cell. The lack of oxygen, e.g., cooled at oxygen pressure of 10⁻⁴ Pa, reduces Ni³⁺/Ni²⁺ and Cu¹⁺/Cu²⁺ ratio in films, leading to the low hole concentration. The hole concentration is found to be related to open circuit voltage (V_oc) and fill factor (FF) in NiO:Cu heterojunction solar cell devices. But the excess of oxygen, e.g., cooled at oxygen pressure of 10⁴ Pa, slightly increases Ni²⁺ and Cu²⁺ content, which is not helpful to the good device performance. The optical band gap of the NiO:Cu film increases with increase in Pc_o2. The external quantum efficiencies of the devices are found to be fully consistent with the variation trend of optical properties of NiO:Cu films, which explained the change in short-circuit current (J_sc) of the NiO:Cu/Si heterojunction solar cells. Therefore, this work enlightens a detailed understanding the effect of oxygen on NiO:Cu film photoelectrical properties and provides novel strategies for optimizing NiO:Cu/c-Si heterojunction solar cell performances.