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

In this work, first principle calculations of Cu2 ZnSnS4 (CZTS), Cu2 ZnGeS4 (CZGS) and Cu2 ZnSiS4 (CZSS) 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 CZTS, CZGS and CZSS. In addition, absorption coefficient values of the order of 10^4 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 (absorption coefficient, refractive index and reflectivity) are used as input data to model the electrical power conversion efficiency of kesterite-based solar cell. In that perspective, we used an improved version of the Shockley-Queisser theoretical 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 25.88 %, 19.94 % and 3.11 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 for vanishing non-radiative recombination rate. Using a realistic internal quantum efficiency which provides 𝑉OC values comparable to experimental measurements, solar cell efficiencies of 15.88, 14.98 and 2.66 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 (for an optimal thickness of 1.15 𝜇m). With this methodology we confirm the suitability of Cu2ZnSnS4 in single junction solar cells, with a possible efficiency improvement of 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.