Device simulation of all-perovskite four-terminal tandem solar cells: towards 33% efficiency

Inorganic–organic hybrid perovskites offer wide optical absorption, long charge carrier diffusion length, and high optical-to-electrical conversion, enabling more than 25% efficiency of single-junction perovskite solar cells. All-perovskite four-terminal (4T) tandem solar cells have gained great attention because of solution-processability and potentially high efficiency without a need for current-matching between subcells. To make the best use of a tandem architecture, the subcell bandgaps and thicknesses must be optimized. This study presents a drift-diffusion simulation model to find optimum device parameters for a 4T tandem cell exceeding 33% of efficiency. Optimized subcell bandgaps and thicknesses, contact workfunctions, charge transport layer doping and perovskite surface modification are investigated for all-perovskite 4T tandem solar cells. Also, using real material and device parameters, the impact of bulk and interface traps is investigated. It is observed that, despite high recombination losses, the 4T device can achieve very high efficiencies for a broad range of bandgap combinations. We obtained the best efficiency for top and bottom cell bandgaps close to 1.55 eV and 0.9 eV, respectively. The optimum thickness of the top and bottom cells are found to be about 250 nm and 450 nm, respectively. Furthermore, we investigated that doping in the hole transport layers in both the subcells can significantly improve tandem cell efficiency. The present study will provide the experimentalists an optimum device with optimized bandgaps, thicknesses, contact workfunctions, perovskite surface modification and doping in subcells, enabling high-efficiency all-perovskite 4T tandem solar cells.