Halogen doping of p-type inorganic hole transport layer: electronic nature-based dopant engineering for modulating hole selectivity in inverted planar perovskite solar cells

Abstract

Ionic doping is a widely employed strategy to improve the sub-optimal energy level alignment at the NiO_x/perovskite interface to mitigate the extensive interfacial trap-assisted recombination losses. However, although this approach is based on the idea of p-doping an inherently p-type NiO_x semiconductor, the effect of electronegative non-metal anions as dopants in p-type NiO_x is a relatively unexplored aspect in ionic doping. Thus, in this work, we correlate for the first time, the impact of electropositive alkali metal cation dopants and electronegative halide anion dopants on regulating the properties of NiO_x hole transport layer (HTL). The findings of this study validated the feasibility of employing halide dopants to instigate p-doping in p-type inorganic HTLs, while complying well with the doping asymmetry of wide band gap metal oxides. This was evidenced by the increase in Ni³⁺ defect density according to the XPS analysis and noticeable increment in work function verified from the contact potential difference studies. This indicated a Fermi level shift toward the valence band edges, similar to alkali metal dopants, but caused by a distinct electronic mechanism. The interface trap state passivation complemented their electropositive analogues, as evidenced by the lower Urbach energy, higher photoluminescence (PL) intensity, and enhanced surface photovoltage, indicating refined interfaces. Thus, this research showed the potential of halogen dopants in p-type HTLs, enabling halo-functional design methods that address interface enhancement and charge transportation concerns concurrently.