Impact of CsF Post-deposition Treatment on the Electronic Structure and Transport Properties of The Cu(In,Ga)Se2 Surface and Its Interface with CdS

Heavy-alkali post-deposition treatments (PDTs) has become an indispensable step in producing high performance Cu(In,Ga)Se2 (CIGS) solar cells. The world record efficiency of CIGS solar cells has been broken in 2019 due to the post deposition treatment of CsF (CsF-PDT), which can not only increase the conversion efficiencies of CIGS solar cell with conventional Ga content, but also allows a shift in the CIGS absorber composition towards higher gallium content. It is beneficial for CIGS absorber with wide band-gap to serve as the bottom solar cell of tandem device. However, a full understanding of the mechanism behind the improvements of device performance by heavy alkali treatments has not been reached yet. Here we take Cs elements as an example to analyze the change of electrical structure on the absorber surface with different GGI induced by heavy Alkali-PDT. After CsF-PDT, we find a downward shift of the valence band edge of the absorber surface, which is consistent with the literature reports about heavy Alkali-PDT. However, the position of the conduction band in the absorber surface after CsF-PDT is more complicated. As the Ga content goes up, the trend of conduction band change gradually from upward shift to downward shift. Such bending could decrease the carrier transportation barrier which deteriorated the device performance, and reduce the interfacial recombination. Therefore, the heavy Alkali-PDT should work better for the devices with high Ga content in the absorber surface.