Heavy Alkali Treatment of Post-Sulfurized Cu(In,Ga)Se2 Layers: Effect on Absorber Properties and Solar Cell Performance

This contribution evaluates a sequential post-deposition treatment of Cu(In,Ga)Se-2(CIGS) films, consisting of 1) a post-sulfurization in elemental S-atmosphere and 2) a subsequent treatment by heavy alkali fluorides (Alk-PDT). First, the effect of the sulfurization step on the corresponding solar cell performance is investigated and optimum process parameters, leading to an efficiency improvement, are identified. Losses in carrier collection observed after S-incorporation are attributed to an increased grain boundary (GB) recombination. It is found that the corresponding reduction in short-circuit current density can be mitigated by a RbF- or KF-PDT, supposedly by depleting GBs in Cu. However, in strong contrast to non-sulfurized CIGS, the Alk-PDT results in a lower open-circuit voltage and distortions in the current-voltage (I-V) characteristics for sulfurized absorbers. Possible explanations are the absence of a wide-gap surface phase and/or air exposure between the post-treatment steps. It is further proposed that a back contact barrier may be responsible for the distortions inI-V.