Role of Ag addition on the microscopic material properties of (Ag,Cu)(In,Ga)Se2 absorbers and their effects on losses in the open-circuit voltage of corresponding devices

Abstract

Ag alloying of Cu(In,Ga)Se2 (CIGSe) absorbers in thin-film solar cells leads to improved crystallization of these absorber layers at lower substrate temperatures than for Ag-free CIGSe thin films as well as to enhanced cation interdiffusion, resulting in reduced Ga/In gradients. However, the role of Ag in the microscopic structure–property relationships in the (Ag,Cu)(In,Ga)Se2 thin-film solar cells as well as a correlation between the various microscopic properties of the polycrystalline ACIGSe absorber and open-circuit voltage of the corresponding solar cell device has not been reported earlier. In the present work, we study the effect of Ag addition by analyzing the differences in the various bulk, grain-boundary, optoelectronic, emission, and absorption-edge properties of ACIGSe absorbers with that of a reference CIGSe absorber. By comparing thin-film solar cells with similar band-gap energies ranging from about 1.1 to about 1.2 eV, we were able to correlate the differences in their absorber material properties with the differences in the device performance of the corresponding solar cells. Various microscopic origins of open-circuit voltage losses were identified, such as strong Ga/In gradients and local compositional variations within individual grains of ACIGSe layers, which are linked to absorption-edge broadening, lateral fluctuations in luminescence-energy distribution, and band tailing, thus contributing to radiative VOC losses. A correlation established between the effective electron lifetime, average grain size, and lifetime at the grain boundaries indicates that enhanced nonradiative recombination at grain boundaries is a major contributor to the overall VOC deficit in ACIGSe solar cells. Although the alloying with Ag has been effective in increasing the grain size and the effective electron lifetime, still, the Ga/In gradients and the grain-boundary recombination in the ACIGSe absorbers must be reduced further to improve the solar-cell performance.