In-line deposited Cu(In,Ga)Se2 solar cells: influence of deposition temperature and Na co-evaporation on carrier collection

Abstract

We investigated the influence of deposition temperature and sodium co-evaporation on the carrier collection properties of Cu(In,Ga)Se 2 solar cells. I – V and bias-dependent quantum efficiency measurements were performed on cells deposited at 550 and 420 °C with and without sodium co-evaporation. A reduction of deposition temperature leads to a deterioration of solar cell output parameters due to reduced sodium diffusion from the substrate as well as an increased grain boundary density. Bias-dependent quantum efficiency measurements reveal that these effects lead to a decrease of minority carrier diffusion length. When sodium is co-evaporated during absorber growth at low T dep , solar cells with V OC and FF values in the range of high-temperature-deposited devices are realised. However, j sc is not enhanced. Bias-dependent quantum efficiency measurements show that an applied bias voltage does not influence the quantum efficiency curve when sodium is co-evaporated during CIGS growth. The origin of this behaviour is not completely understood.