Effect of an ultra-thin 2D transport layer on eco-friendly Perovskite/CIGS tandem solar cell: A numerical study

Abstract

The major limitation of two-terminal (2T) tandem configuration is its lower operating current density, as electrically, the higher and lower bandgap cells are connected in series. In such an arrangement, the output of the tandem cell is constrained by the cell that produces the smallest photocurrent. To mitigate this issue, we have proposed a 2T lead-free Perovskite/CIGS tandem structure using ultra-thin transport materials with which high photocurrent and higher efficiency have been achieved. Cesium tin germanium tri-iodide (CsSn0.5Ge0.5I3), a lead–free perovskite with a bandgap of 1.5 eV and copper indium gallium selenide (CIGS) with a bandgap of 1.1 eV, have been utilised as an active layers for the top and bottom sub cells, respectively. These sub cells have been first evaluated in a single junction architecture under standard solar spectrum (AM 1.5 spectrum), yielding a power conversion efficiency (PCE) of 25.72% and 26.20%, respectively. They are then investigated for tandem configuration for which the current matching point is obtained by changing the thickness of the top and bottom active layers. This requirement is met when the top and bottom sub cells in the tandem cell have optimum thicknesses of 298.15 nm and 1000 nm, respectively. Finally, to evaluate tandem performance parameters, the top sub-cell is illuminated under the standard AM 1.5 spectrum, keeping its active layer thickness at 298.15 nm, while the performance parameter of bottom sub-cell are computed under the calculated filtered spectrum. With an improved open circuit voltage (VOC) of 1.94 V, the proposed tandem structure attained a high efficiency of 34.87%.