Transparent Electrode and Buffer Layer Combination for Reducing Carrier Recombination and Optical Loss Realizing over...

The 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 (J_SC) is enhanced (decreased optical loss), when ZnS(O,OH), (Cd,Zn)S, and Zn_0.8Mg_0.2O buffers with a large band gap energy (E_g) are applied as a replacement of CdS buffer. The J_SC is further increased, reducing the optical loss more, when Zn_0.9Mg_0.1O:B is used as the transparent conductive oxide (TCO) with a larger E_g 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 (V_OC,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 greater reduction of the V_OC,def and the high conversion efficiency (η) of about 21%. Under the trade-off between V_OC,def and optical loss, the highest η of 22.6% is attained with the lowest power loss (or the highest V_OC × J_SC) in the Cs-treated Cd-free CIGSSe solar cell with an optimized structure of glass/Mo/CIGSSe/Zn_0.8Mg_0.2O/Zn_0.9Mg_0.1O:B, fabricated by the all-dry process, where the Zn_0.8Mg_0.2O buffer is prepared by the sputtering method. This occurs because the J_SC is the highest attributable to the larger E_g of Zn_0.8Mg_0.2O buffer than those of the CdS and (Cd,Zn)S.