Solar Cells Based on Cu(In, Ga)Se2 Thin-Film Layers

Abstract

This paper presents the results of experimental studies of spectral, dark current–voltage and light load characteristics of the selenide–copper–gallium–indium (Cu(In, Ga)Se₂) solar cell. Тhe main fundamental parameters of the photoactive semiconductor layer Cu(In, Ga)Se_2, such as the band gap, the resistivity of the layer, the equilibrium majority–carrier concentration, the lifetime and the product μ_nτ_n of nonequilibrium minority carriers from the spectral, photoelectric and dark current–voltage characteristics are determined. Based on an analysis of the light-load current–voltage characteristics at various solar radiation powers (50–1000 W/m²), the main parameters of the p–n junction were determined, as well as the nonideality factor and the magnitude of the reverse diode saturation current; a photogeneration mechanism was established in the studied solar radiation range, which had the character of a diffusion mechanism, where carrier recombination in the photoactive layer did not have a significant effect. We found that in conditions of real solar lighting (Р_rad = 50–1000 W/m²), the output parameters of the solar cell – short-circuit current, open circuit voltage, the maximum output power increases with Р_rad. The fill factor (FF) of the light-current–voltage characteristics has a maximum at Р_rad ≈ 200 W/m², and an efficiency has a maximum value at Р_rad ≈ 600 W/m². The observed dependences of FF and efficiency are explained by the dependence of the series (R_ser) and shunt (R_sh) resistance of a solar cell on P_rad. To maintain the efficiency of a solar cell based on thin-film layers Cu(In, Ga)Se₂, equally high in conditions of increased radiation, as well as in conditions of low solar radiation, it is necessary that R_ser decreases and R_sh does not change with P_rad.