Influence of indium/gallium gradients on the Cu(In,Ga)Se2 devices deposited by the co-evaporation without recrystallisation

In the laboratory scale, cells based on Cu(In,Ga)Se2 grown by the 3-stage process reach the best performance because of high open-circuit voltage and short-circuit current (VOC-JSC) combination. One of the reasons for that could be the V-shaped gradient of Ga to In atomic ratio throughout the Cu(In,Ga)Se2 layer, which results from large differences in the diffusion coefficients of In and Ga. The location of the lowest Ga-content in the Cu(In,Ga)Se2 (i.e. Ga notch), also corresponds to the Cu-poor to Cu-rich transition during the 2nd stage. Since this transition is associated to a phenomenon of recrystallisation, the arising question is whether high VOC-JSC combination is effectively inherent to V-shaped gradient or to recrystallisation. In our work we attempt to eliminate the influence of recrystallisation to exclusively study the influence of Ga/In gradients. Our approach was to co-evaporate samples by the one-step process with different gradients by the continuous modification of In and Ga fluxes during the deposition and keeping constant that of Cu in a way that its ratio to group III elements was 0.9. With this method, we could obtain a set of Cu(In,Ga)Se2 layers either free of gradient, with linear gradient (i.e. no notch) or V-shaped gradient with notch at a different distance from the Cu(In,Ga)Se2 surface. We observe that depending on the presence of notch in conduction band or the position of notch it is possible to modify the impact of secondary barriers on current-voltage characteristics.