Optimal Design of Thin Cu2ZnSn(S1-xSex)4 Solar Cells

Modeling solar cells to determine their performance has become a fundamental tool for the optimal design of this kind of devices. Solar cell models help optimizing the fabrication parameters so that increased efficiencies can be achieved, reducing the development and production costs. With this purpose, we have developed a unified analytical model for designing thin film solar cells that takes into account the specific differences associated to thin film solar cells as compared to volumetric conventional cells. In this work, we apply this model to Cu2ZnSn(S1-xSex)(4) (CZTS) solar cells with ZnO:Al/CdS/Cu2ZnSn(S(1-x)Sex)(4)/Mo cell structure. Short circuit current density (Jsc), open circuit voltage (Voc), fill factor FF and efficiency (eta) calculations have been done for cells with Cu2ZnSnS4 (1.5 eV) and Cu2ZnSnSe4 (1.05 eV) absorber layers as a function of the absorber thickness in the range from 0.3 mu m to 2 mu m. These results indicate that it is possible to obtain high efficiencies for absorber layer thickness less than 1 mu m, whenever the recombination velocity (S) at the back contact is below 10(3) cm/s. Additionally, efficiency as a function of the CZT(S1-xSex)(4) absorber layer bandgap has also been calculated, when the thickness is varied from 0.75 mu m to 2 mu m. It was determined that the highest conversion efficiency (around 17%) can be obtained for a bandgap around 1.47 eV, thickness of 0.75 mu m and back surface recombination S = 10(2) cm/s. It is concluded that reduction of both the back surface recombination velocity (S) and the interface recombination velocity (S-i) should be fundamental for achieving very thin (less than 1 mu m) CZTS solar cells with high efficiencies.