Relevance of Ge incorporation to control the physical behaviour of point defects in kesterite

TL;DR

This study uses first-principles calculations to analyze how germanium incorporation affects point defects in kesterite, revealing reduced non-radiative recombination and potential efficiency improvements in solar cells.

Contribution

It provides new insights into the physical behavior of intrinsic point defects in Ge-doped kesterite, highlighting defect formation energies and their impact on recombination.

Findings

Ge doping reduces lattice distortion and non-radiative recombination.

Substitutional defects XZn act as recombination centers.

Ge incorporation lowers the carrier capture cross section.

Abstract

To reduce the prominent VOC-deficit that limits kesterite-based solar cells efficiencies, Ge has been proposed over the recent years with encouraging results, as the reduction of the non-radiative recombination rate is considered as a way to improve the well-known Sn-kesterite world record efficiency. To gain further insight into this mechanism, we investigate the physical behaviour of intrinsic point defects both upon Ge doping and alloying of Cu2ZnSnS4 kesterite. Using a first-principles approach, we confirm the p-type conductivity of both Cu2ZnSnS4 and Cu2ZnGeS4, attributed to the low formation energies of the VCu and CuZn acceptor defects within the whole stable phase diagram range. Via doping of the Sn-kesterite matrix, we report the lowest formation energy for the substitutional defect GeSn. We also confirm the detrimental role of the substitutional defects XZn (X=Sn,Ge) acting as recombination centres within the Sn-based, the Ge-doped and the Ge-based kesterite. Finally, we highlight the reduction of the lattice distortion upon Ge incorporation resulting in a reduction of the carrier capture cross section and consequently a decrease of the non-radiative recombination rate within the bulk material.