Efficient monolithic perovskite-Si tandem solar cells enabled by an ultra-thin indium tin oxide interlayer

Tandem solar cells are promising for overcoming the theoretical single junction solar cell efficiency limit. Here we report an elegant approach to monolithically integrate perovskites and Si-heterojunction tandem solar cells using an ultra-thin indium tin oxide (ITO) interlayer. It is found that a 1.7 nm ITO interlayer is sufficient to provide the interfacing between the silicon and perovskite subcells with minimum parasitic resistance and efficient trap states for carrier recombination. Thinner ITO interlayers result in discontinuous films while thicker ITO interlayers with improved lateral conductivity are less effective in localising shunts especially for large (≥1 cm²) area tandem cells. Furthermore, it is found that a tandem cell with a 26 nm thick 〈p〉a-Si:H junction in the bottom cell performs better than that with a 13 nm thick junction. When combined with a 1.7 nm ultra-thin ITO layer and a front anti-reflection coating, our champion 1.0 cm² cell achieved 27.2% power conversion efficiency (PCE) and a fill factor (FF) of 82.4% under reverse scan. This FF is the highest of any perovskite-Si tandem cell. The ultra-thin ITO layer is suitable for large area cells. 24.2% efficiency was demonstrated on an 11.8 cm² cell. A 21.1% efficient 65.1 cm² (∼4 inch round) cell was also fabricated. This representative cell shows negligible efficiency loss after 600 hours of continuous 1 sun illumination under MPP tracking. This work provides insights into the role of ultra-thin interfacing ITO layers for monolithic large-area perovskite-Si tandem cells.