Advantage of Zn(O,S) Over CdS Buffer for Low-Gap (Ag,Cu)(In,Ga)Se₂ in Tandem Applications

Abstract

The compositional flexibility of the compound semiconductor (Ag,Cu)(In,Ga)Se 2 ((A)CIGS) allows the fabrication of cells with band gaps close to 1.0 eV. These cells are well suited as bottom cells in tandem devices, for example, together with perovskite top cells. Using an established inline process with a standardized cell fabrication workflow, we compare the two buffer materials CdS and Zn(O,S) for their usability in low band gap (A)CIGS solar cells. Whereas CdS is the more suitable buffer choice for cells with band gaps around 1.15 eV, toward lower band gaps, Zn(O,S) is shown to be the better-performing buffer. The difference in open-circuit voltage between CdS and Zn(O,S) closes when reducing the Ga content in the absorber due to lower nonradiative losses. In addition, quantum efficiency becomes steeper around the band gap energy of Zn(O,S) cells, yielding an improvement in short-circuit current under filtered light conditions. The implications for four-terminal tandem devices with a perovskite top cell and a CIGS bottom cell, with efficiency up to 27.5% on 0.5 cm 2 cells and 22.0% on 10 cm 2 modules, are discussed. For (A)CIGS cells with low GGI, Zn(O,S) has the potential to replace CdS as the material of choice in tandem application, mostly due to reduced V OC losses.