First-principles study of intrinsic and hydrogen point defects in the earth-abundant photovoltaic absorber Zn3P2

Abstract

Zinc phosphide (Zn₃P₂) has had a long history of scientific interest largely because of its potential for earth-abundant photovoltaics. To realize high-efficiency Zn₃P₂ solar cells, it is critical to understand and control point defects in this material. Using hybrid functional calculations, we assess the energetics and electronic behavior of intrinsic point defects and hydrogen impurities in Zn₃P₂. All intrinsic defects are found to act as compensating centers in p-type Zn₃P₂ and have deep levels in the band gap, except for zinc vacancies which are shallow acceptors and can act as a source of doping. Our work highlights that zinc vacancies rather than phosphorus interstitials are likely to be the main source of p-type doping in as-grown Zn₃P₂. We also show that Zn-poor and P-rich growth conditions, which are usually used for enhancing p-type conductivity of Zn₃P₂, will facilitate the formation of certain deep-level defects (P_Zn and P_i) which might be detrimental to solar cell efficiency. For hydrogen impurities, which are frequently present in the growth environment of Zn₃P₂, we study interstitial hydrogen and hydrogen complexes with vacancies. The results suggest small but beneficial effects of hydrogen on the electrical properties of Zn₃P₂.