Opto-electronic properties and solar cell efficiency modelling of Cu₂ZnXS₄ (X = Sn, Ge, Si) kesterites

Abstract In this work, first-principles calculations of Cu₂ZnSnS₄, Cu₂ZnGeS₄ and Cu₂ZnSiS₄ are performed to highlight the impact of the cationic substitution on the structural, electronic and optical properties of kesterite compounds. Direct bandgaps are reported with values of 1.32, 1.89 and 3.06 eV respectively for Cu₂ZnSnS₄, Cu₂ZnGeS₄ and Cu₂ZnSiS₄ and absorption coefficients of the order of 10⁴ cm⁻¹ are obtained, indicating the applicability of these materials as absorber layer for solar cell applications. In the second part of this study, ab initio results are used as input data to model the electrical power conversion efficiency of kesterite-based solar cells. In that perspective, we used an improved version of the Shockley–Queisser model including non-radiative recombination via an external parameter defined as the internal quantum efficiency. Based on predicted optimal absorber layer thicknesses, the variation of the solar cell maximal efficiency is studied as a function of the non-radiative recombination rate. Maximal efficiencies of 25.71%, 19.85% and 3.10% are reported respectively for Cu₂ZnSnS₄, Cu₂ZnGeS₄ and Cu₂ZnSiS₄ for vanishing non-radiative recombination rate. Using an internal quantum efficiency value providing experimentally comparable {V}_OC values, cell efficiencies of 15.88%, 14.98% and 2.66% are reported respectively for Cu₂ZnSnS₄, Cu₂ZnGeS₄ and Cu₂ZnSiS₄. We confirm the suitability of Cu₂ZnSnS₄ in single junction solar cells, with a possible efficiency improvement of nearly 10% enabled through the reduction of the non-radiative recombination rate. In addition, Cu₂ZnGeS₄ appears to be an interesting candidate as top cell absorber layer for tandem approaches whereas Cu₂ZnSiS₄ might be interesting for transparent photovoltaic windows.