Enhanced p-Type Conductivity of NiOx Films with Divalent Cd Ion Doping for Efficient Inverted Perovskite Solar Cells

The effect of substitutional metal dopants in NiO_x on the structural and electronic structures is of great interest, particularly for increasing the p-type conductivities as a hole transport layer (HTL) applied in perovskite solar cells (PSCs). In this paper, experimental fabrications and density functional theory calculations have been carried out on Cd-doped NiO_x films to examine the effect of divalent doping on the electronic and geometric structures of NiO_x. The results indicate that divalent Cd dopants reduced the formation energy of the Ni vacancy (V_Ni) and created more V_Ni in the films, which enhanced the p-type conductivity of the NiO_x films. In addition, Cd doping also deepened the valence band edge, reduced the monomolecular Shockley-Read-Hall (SRH) recombination losses, and promoted hole extraction and transport. Hence, the PSCs with Cd:NiO_x HTLs manifest a high efficiency of 20.47%, a high photocurrent density of 23.00 mA cm^-2, and a high fill factor of 79.62%, as well as negligible hysteresis and excellent stability. This work illustrates that divalent elements such as Cd, Zn, Co, etc. may be potential dopants to improve the p-type conductivity of the NiO_x films for applications in highly efficient and stabilized PSCs.