RbGeI₃/CuZnSnSSe/CuInGaSe/CdS tandem solar cell with improved performance and lowered cost

Tandem cells, involving copper indium gallium selenide/cadmium sulphide (CIGS/CdS) layers, exhibit high performances, but CIGS involve costly metals with questionable environmental impacts. To minimize draw backs, a new tandem cell is proposed here. In the new design, the bottom sub-cell CIGS layer thickness is reduced and complemented by adding copper zinc tin sulphide-selenide (CZTSSe) as additional absorber layer. The top sub-cell involves the stable eco-friendly RbGeI₃ perovskite. The complete tandem structure isMo/CIGS/CZTSSe/CdS/ZnO(i)/ZnO(Al)/CuCrO₂(HTL)/RbGeI₃/TiO₂(ETL)/, where CuCrO₂ is hole-transport layer(HTL), TiO₂ electron-transport layer (ETL), ZnO transparent conducting oxide(TCO), ZnO(i) intrinsic layer and Mo molybdenum electrode. The TCAD Silvaco-simulation is used to optimize some parameters such as layer thicknesses, band gaps and doping concentrations. The new bottom sub-cell CZTSSe/CIGS exhibits improved performance showing the added value of inserting CZTSSe layer. The tandem-cell performance is further improved by the top sub-cell RbGeI₃ layer. To overcome difficulties in optimizing tandem-cell parameters together, two parameters have been optimized at one time to reach best cell performance. Then, other parameters are similarly optimized, and so on. Matching, between band gaps and bottom- and top-absorber layer thicknesses, is studied. The new structure exceeds the Shockley-Queisser limitations, with efficiency > 35% and open circuit potential ~ 1.7 V.