Refractive index extraction and thickness optimization of Cu2ZnSnSe4thin film solar cells

Cu2ZnSnSe4 (CZTSe) thin film solar cells are promising emergent photovoltaic technologies based on low-bandgap absorber layer with high absorption coefficient. To reduce optical losses in such devices and thus improve their efficiency, numerical simulations of CZTSe solar cells optical characteristics \ncan be performed based on individual optical properties \nof each layer present in the cell structure. In this contribution, we have first determined the optical coefficients of individual thin films (i.e., (n, k) of the absorber, buffer, and window layers) to build a realistic model simulating the optical behavior of the whole cell stack we propose. Optical \ncharacterization was performed using two approaches, one \nbased on ellipsometry measurements for characterizing thin \nflat cadmium sulfide (CdS) and zinc oxide (ZnO) layers and the \nother relying on reflectance and transmission (R/T) analysis for \nthe rough CZTSe absorber. Then, we performed numerical \nsimulations using as input experimental optical parameters \npredicting optimal CZTSe cell structure minimizing optical \nlosses. The impact of each layer’s thickness on the cell’s shortcircuit current has been studied. A set of optimal thicknesses of each of the active layers was proposed. Finally, the proposed optical optimization was experimented practically leading to CZTSe cells with 9.7% and 10.4% efficiencies.