Efficiency improvement for post-sulfurized CIGS solar cells enabled by in situ Na doping

Despite sulfurization offers the advantage of improving the photovoltaic performance in preparing Cu(In,Ga)Se₂ (CIGS) absorbers, deep level defects in the absorber and poor energy level alignment on the front surface are still main obstacles limiting the improvement of power conversion efficiency (PCE) in sulfided CIGS solar cells. Herein, an in-situ Na doping strategy is proposed, in which the tailing effect of crystal growth is used to promote the sulfurization of CIGS absorbers. It is found that the grain growth is supported by Na incorporating due to the enrichment of NaSe_x near the upper surface. The high soluble Na during grain growth can not only suppress intrinsic In_Cu donor defects in the absorber, but also tailor S distribution in bulk and the band alignment at the heterojunction, which are both beneficial for the effective electron carriers. Meanwhile, the Na aggregation near the bottom of the absorber also contributes to the crystalline quality increasing and favorable ultra-thin MoSe₂ formation at back contact, resulting in a reduced barrier height conducive to hole transport. PCE of the champion device is as high as 16.76% with a 28% increase. This research offers new insights into synthesizing CIGS solar cells and other chalcogenide solar cells with superior cell performance when using an intense sulfurization process.