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

The effect of substitutional metal dopants in NiO 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 films to examine the effect of divalent doping on the electronic and geometric structures of NiO. 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 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 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 films for applications in highly efficient and stabilized PSCs.