Effect of top cell absorber thickness and bifacial illumination on monolithic (Ag,Cu)(In,Ga)Se2/Si tandem solar cells

Wide bandgap (Ag,Cu)(In,Ga)Se2 (ACIGSe) is a viable candidate as top cell absorber in tandem devices. In this work, we investigate monolithic two-terminal ACIGSe-Si tandem cells with a bandgap of 1.5 eV. Our research focuses on the direct preparation of the ACIGSe top cell on the Si bottom cell, the impact of the top cell absorber thickness and the gain through bifacial illumination. Bifacial monocrystalline silicon solar cells with tunnel oxide passivated contacts (TOPCon) are used as bottom cells. Considering that the TOPCon structure allows high temperature treatments without severe device degradation, TOPCon silicon cells are in principle suitable for ACIGSe deposition at evaporation temperatures of approximately 600°C. Variation of the ACIGSe absorber thicknesses reveals an optimum for low top cell absorber thicknesses. The optimum shifts to thicker absorbers of 1 μm when operating the tandem device bifacially with back illumination reaching the highest output power of 16.2 mW cm-2 with addition rear illumination of 52% of 1-sun illumination. An electro-optical computer simulation supports the experimental outcome.