Influence of indium/gallium gradients on the Cu(In, Ga)Se-2 devices deposited by the co-evaporation without recrystallisation

Abstract

In the laboratory scale, cells based on Cu(In, Ga)Se-2 grown by the 3-stage process reach the best performance because of high open-circuit voltage and short-circuit current (V-OC-J(SC)) combination. One of the reasons for that could be the V-shaped gradient of Ga to In atomic ratio throughout the Cu(In, Ga)Se-2 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)Se-2 (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 V-OC-J(SC) 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)Se-2 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)Se-2 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. (C) 2014 Elsevier B. V. All rights reserved.