Transparent Electrode and Buffer Layer Combination for Reducing Carrier Recombination and Optical Loss Realizing over a 22%-Efficient Cd-Free Alkaline-Treated Cu(In,Ga)(S,Se)2 Solar Cell by the All-Dry Process

The 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 is enhanced (decreased optical loss), when ZnS(O,OH), (Cd,Zn)S, and ZnO buffers with a large band gap energy (E) are applied as a replacement of CdS buffer. The J is further increased, reducing the optical loss more, when ZnO:B is used as the transparent conductive oxide (TCO) with a 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 the buffer/absorber interface and in the quasi neutral region, are reduced, thereby reducing open-circuit voltage deficit , 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 greater reduction of the V and the high conversion efficiency (eta) of about 21%. Under the trade-off between V and optical loss, the highest eta of 22.6% is attained with the lowest power loss (or the highest V in the Cs-treated Cd-free CIGSSe solar cell with an optimized structure of glass/Mo/CIGSSe/ZnO:B, fabricated by the all-dry process, where the ZnO buffer is prepared by the sputtering method. This occurs because the J(SC) is the highest attributable to the larger EO buffer than those of the CdS and (Cd,Zn)S.