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

To reduce the prominent V-OC-deficit that limits kesterite-based solar cell efficiencies, Ge has been proposed over the recent years with encouraging results as the reduction of the non-radiative recombination rate is considered as an approach 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 upon both 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 V-Cu and Cu-Zn acceptor defects within the whole stable phase diagram range. By doping of the Sn-kesterite matrix, we report the lowest formation energy for the substitutional defect Ge-Sn. We also confirm the detrimental role of the substitutional defects X-Zn (X = Sn, Ge) acting as recombination centres within the Sn-based, the Ge-doped and the Ge-based kesterite. Upon Ge incorporation, we highlight, along with the increase of the X-Zn (X = Sn, Ge) neutral defect formation energy, the reduction of the lattice distortion resulting in the reduction of the carrier capture cross section. Both of these elements lead to a decrease of the non-radiative recombination rate within the bulk material following Sn substitution by Ge.