Efficient NiOx Hole Transporting Layer Obtained by the Oxidation of Metal Nickel Film for Perovskite Solar Cells

In the preparation of PSCs, NiOx is considered to be one very promising hole transport layers for its intrinsic p-type doping nature, deep-lying valence band, and high light transmittance. However, the thickness, uniformity, and composition of the NiOx film are not easy to accurately control by the solution spin-coating process. In this work, instead of the solution-processed NiOx, metal Ni is first deposited on the FTO/glass substrate by an e-beam evaporator and then converted to the NiOx by exposing it in the air with the help of annealing, which avoids the usage of a toxic solvent in the solution method. The deposition thickness and composition of the NiOx films could be well controlled. The results demonstrate that the Ni-oxidized NiOx HTL shows wonderful energy alignment, better charge extraction capability, and great efficiency in photoluminescence quenching when in contact with the perovskite film. By combining the two-step solution process method, high-performance PSC has been achieved based on the Ni-oxidized NiOx HTL, which is well above that based on the PEDOT:PSS and the solution-processed NiOx. Also, transient photocurrent (TPC) and transient photovoltage (TPV) measurements demonstrate that the device based on the Ni-oxidized NiOx HTL possesses a suppressed charge recombination and an outstanding ability of charge extraction. Furthermore, the Ni-oxidized NiOx-based devices exhibit better stability compared to the devices based on solution-processed NiOx and PEDOT:PSS HTL. All the results show that the Ni oxidation method is an efficient method to prepare the NiOx HTL in PSCs.