Physics of Cu(In,Ga)Se₂ microcells under ultrahigh illumination intensities

In order to develop photovoltaic devices with increased efficiency using less rare semiconductor materials, the concentrating approach was applied on Cu(In,Ga)Se₂ thin film devices. Microscale solar cells down to a few micrometers wide were fabricated. They show, at around x475, an efficiency of 21.3%, thanks to concentrated illumination (532 nm laser), compared to 16% efficiency under non-concentrated illumination. Due to the miniaturization, ultrahigh fluxes can be studied (> x1000), without damaging the device. We analyse the high concentration regime of these micro-devices. Under ultrahigh light fluxes the collection efficiency decreases on certain devices. We attribute this effect to the screening of the electric field at the junction under high illumination. Numerical simulations of p-n junctions under intense fluxes corroborate this hypothesis. We built a homemade finite element method program, solving Poisson and continuity equations without resorting to the minority carrier approximation. We study the electric field at a p-n junction as a function of illumination intensity, and highlight the screening phenomena. Cu(In,Ga)Se2 thin films prove to be appropriate for a use under concentration, leading to significant gains in terms of efficiency and material usage. On these particular devices, ultrahigh illuminations can be used and the electric regime studied.