Which potential for Kesterite absorbers in tandem solar cells: a quantitative modelling approach

The potential of Kesterite absorbers used both as top or bottom cell, in combination with crystalline silicon bottom cell and a Perovskite top cell respectively, is investigated using a combination of optical and electrical modelling. Using a transfer matrix approach to determine the transmission of a given top cell, the electrical behavior of the bottom cell in tandem condition is evaluated. Unlike past studies on a related topic, the results reported here are deemed close to quantitative, relying on a consistent set of experimental data for both the optical and electrical model. After demonstrating the closeness of a simulated CZTSe baseline solar cell with its experimental counterpart, an incremental set of experimentally realistic optimizations are investigated to further enhance the PV performance. A combination of a 21%-Perovskite subcell with a 17%-CZTSe subcell is found sufficient to overcome the single junction detailed balance limit and approach the 30% efficiency threshold. Following a similar approach, a wide bandgap CZG(S,Se) top cell is evaluated in combination with a state-of-the-art c-Si bottom cell. Such design is found markedly more challenging for the Kesterite top cell with the necessary use of innovative selective contacts and a reduction of the bulk defect density by two orders of magnitude to approach the 30% efficiency threshold. Each specific optimization will be discussed in the context of current experimental trends in Kesterite solar cells, and this work will conclude by offering perspectives for full Kesterite tandem solar cells as well as multijunction devices with 3 subcells or more. This work offers, for the first time, a reliably quantified overview of the potential of Kesterite absorbers in multijunction devices, and will help experimentalists identifying and focusing their efforts toward the current bottlenecks of this technology.