Revealing surface modifications of caesium indium selenium treated Cu-rich Cu(In,Ga)Se2: A study of the suppression of the interfacial recombination

Abstract

Cu(In,Ga)Se2 (CIGS) solar cell is a kind of high efficiency thin film photovoltaic device. These CIGS solar cells with high efficiency tend to be Cu-poor. Actually, the Cu-rich CIGS solar cells which have the superior electrical characteristics should have higher efficiency compared with the Cu-poor ones. However, the performance (especially the VOC) of Cu-rich devices is limited by the serious interfacial recombination. To suppress the interfacial recombination and improve the efficiency of Cu-rich devices, we co-evaporate CsF, In and Se at the surface of the Cu-rich absorber (Cs-In-Se-PDT). By this way, the compound of Cs-In-Se is formed at the absorber surface, which leads to the valence band energy, work function and the grain boundary recombination at the surface of the Cu-rich absorber decrease. This contributes to inhibiting the interfacial recombination. Especially, the tunneling assistant interfacial recombination can be suppressed significantly. Besides, the Cs-In-Se-PDT mainly changes the Cu content of the absorber surface, the advantage of Cu-rich in the interior of the absorber can still be maintained. After Cs-In-Se-PDT, the VOC of the Cu-rich device (CGI = 1.03 %) increases by about 90 mV, and the device efficiency also boosts from 12.7 % to 16.04 %.