Transparent electrode and buffer layer combination for reducing carrier recombination and optical loss realizing over 22%-efficient Cd-free alkaline-treated Cu(In,Ga)(S,Se)2 solar cell by all-dry process.

Structures of (K or Cs) alkaline-treated Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells are developed, and their carrier recombination rates are scrutinized. It is determined that short-circuit current density (JSC) is enhanced (decreased optical loss), when ZnS(O,OH), (Cd,Zn)S, and Zn0.8Mg0.2O buffers with large band-gap energy (Eg) are applied as replacement of CdS buffer. The JSC is further increased, more reducing the optical loss, when Zn0.9Mg0.1O:B is used as transparent conductive oxide (TCO) with larger Eg and lower free carrier absorption than those of ZnO:Al. Furthermore, all carrier recombination rates throughout the devices with K or Cs treatment, especially at buffer/absorber interface and in quasi neutral region, are reduced, thereby reducing open-circuit voltage deficit (VOC,def), well consistent with the simulated ones. The carrier recombination rate at the buffer/absorber interface is further decreased, when the CdS and (Cd,Zn)S buffers, deposited by chemical bath deposition, are applied, leading to the more reduction of the VOC,def and the high conversion efficiency (eta) of about 21%. Under the trade-off between VOC,def and optical loss, the highest eta of 22.6% is attained with the lowest power loss (or the highest VOC x JSC) in the Cs-treated Cd-free CIGSSe solar cell with an optimized structure of glass/Mo/CIGSSe/Zn0.8Mg0.2O/Zn0.9Mg0.1O:B, fabricated by all-dry process, where the Zn0.8Mg0.2O buffer is prepared by the sputtering method. This occurs because the JSC is the highest attributable to the larger Eg of Zn0.8Mg0.2O buffer than those of the CdS and (Cd,Zn)S.