Towards Highly Efficient Fully Evaporated Perovskite/Si Tandem Solar Cells

Thanks to its abundance on Earth, its suitable bandgap, and its non-toxicity, silicon dominates over 90% of the photovoltaic market. However, the efficiency of single-junction silicon cells is approaching its theoretical limit of 29%, with a current experimental record of 26.8%. In the quest for overcoming this limit and for reducing the cost of electricity, perovskite/silicon tandem cells have become the most promising contenders.<br/><br/>Pyramid texturing is a standard process in the fabrication of the best performing silicon technologies since it is known to enhance light trapping in the infrared and to reduce primary reflection due to double-bounce effects. However, most of the perovskite-on-Si work has been reported on flattened silicon substrates due to the incompatibility of the textured surfaces with the standard solution-processing of the top perovskite absorbers.<br/><br/>Here, we present the conformal vacuum deposition of wide bandgap perovskites atop fully textured silicon heterojunction cells. We have optimized vacuum deposition processes for different wide bandgap perovskite compositions, mainly FACsPbIBr and MAPbIBr. Their bandgaps range between 1.65 and 1.70 eV, making them ideal candidates as top cells in perovskite/Si tandems. As evidenced from cross-sectional SEM images, our co-evaporation processes enable a conformal coverage of the pyramidal surface of the silicon substrates avoiding shorts between the subcells. This leads to Voc values of above 1.8 V, which correspond well to the sum of the Voc of the perovskite top cell and the silicon bottom cell. Moreover, the use of QCM sensors for monitoring the sublimation of the top cell enables fine tuning of the absorber thickness. This allows to achieve current matching between the subcells as we have been able to demonstrate through EQE measurements. The J-V curves show FFs above 70% and no detectable hysteresis. All in all, we will show our latest results on perovskite/Si tandems employing textured Si and fully sublimed top cells.