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) (CIGSSe) solar cells are developed, and their carrier recombination rates are scrutinized. It is determined that short-circuit current density (J) is enhanced (decreased optical loss), when ZnS(O,OH), (Cd,Zn)S, and ZnMgO buffers with large band-gap energy (E) are applied as replacement of CdS buffer. The J is further increased, more reducing the optical loss, when ZnMgO:B is used as transparent conductive oxide (TCO) with larger E 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 (V), 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 V and the high conversion efficiency (η) of about 21%. Under the trade-off between V and optical loss, the highest η of 22.6% is attained with the lowest power loss (or the highest V × J) in the Cs-treated Cd-free CIGSSe solar cell with an optimized structure of glass/Mo/CIGSSe/ZnMgO/ZnMgO:B, fabricated by all-dry process, where the ZnMgO buffer is prepared by the sputtering method. This occurs because the J is the highest attributable to the larger E of ZnMgO buffer than those of the CdS and (Cd,Zn)S.