Impact of Internal Losses on the Efficiency of All\nPerovskite Tandem Solar Cells: Modeling and Analysis

Hybrid\nhalide perovskites are the most promising alternatives for\nphotovoltaic applications due to their superior optoelectronic properties.\nThey can also be used in tandem geometry along with other state-of-the-art\nsolar cell technologies such as Si and CIGS. Although the efficiency\nhas reached over 31% for Si/perovskite tandem solar cells, the manufacturing\ncomplexity and increased fabrication cost prohibit commercialization.\nHowever, a tunable bandgap in perovskites can be utilized to fabricate\nall-perovskite tandem solar cells with device efficiency comparable\nto that of Si/perovskite tandem solar cells. In this article, we have\ninvestigated all-perovskite tandem solar cell efficiency theoretically\nand proposed the optimum bandgap for top and bottom cells. We have\ndemonstrated that the calculated efficiency (∼32%) from the\nconventional series connection is overestimated, as we have observed\na very high fill factor of ∼85%. This is due to the underestimated\nlosses at the interfaces and individual cells. We have therefore\nsimulated the current–voltage characteristics of the tandem\nsolar cells using an equivalent circuit model. The presence of internal\nresistance in each cell plays a crucial role in determining the overall\ndevice performance. The fill factor decreases to ∼80%, while\nthe short-circuit current density and open-circuit voltages are changed\nonly marginally. Keeping the bottom cell’s bandgap at 1.2 eV,\nwe have achieved a maximum efficiency of 29.8% with the top cell bandgap\nof 1.75 eV. Further increase in the bandgap reduces the overall efficiency\ndue to a decrease in short-circuit current density in the top cell.