Over 14% Efficiency of Directly Sputtered Cu(In,Ga)Se2 Absorbers without Postselenization by Post-Treatment of Alkali Metals

Direct sputtering of a single quaternary Cu(In, Ga)Se-2 (CIGS) target without postselenization is a promising approach to fabricating CIGS absorbers. However, the device efficiency of the quaternary-sputtered CIGS is limited to 10%-11% due to the low and uncontrollable Se supply during the quaternary sputtering process. Here, an enhanced efficiency of 14.1% is reported by directly sputtering from a CIGS target without extra Se supply followed by sequential postdeposition treatments (PDT) of NaF and KF. The effects of different post-treatments of alkali metals on quaternary-sputtered CIGS thin films are discussed in detail. A Cu-depleted surface is not observed in the quaternary-sputtered CIGS thin films after KF-PDT, different from the observation in the coevaporated CIGS, in which the Cu-depleted surface layer induced by KF-PDT enhances the efficiency. On the other hand, it is found that KF-PDT reduces Se vacancies more effectively than NaF-PDT, which could be another electrically benign behavior of KF-PDT. The effective passivation of Se vacancies after KF-PDT overcomes the Se-poor nature of the quaternary sputtering process without postselenization. Therefore, KF-PDT combined with Na doping, which is known to annihilate InCu defects, significantly improves minority carrier lifetime and cell performance.