Optimizing Thin-Film Tandem Solar Cells: The Impact of Bandgap Grading in ACIGS Subcell on Performance

Abstract

Two-terminal perovskite/ACIGS tandem solar cells were numerically simulated and optimized using Silvaco TCAD tools. The perovskite model was designed to match the fabricated structure, making it suitable as the top cell in tandem configurations. Similarly, the ACIGS solar cell was calibrated to align with the fabricated device, ensuring it functions effectively as the bottom cell beneath the perovskite layer. Individual simulations of these cells demonstrated efficiencies of up to 20.92% for the perovskite and 23.6% for the ACIGS cell. The two-terminal tandem model was simulated by integrating the top and bottom cells, with a transparent contact electrically connecting the subcells in series. The impact of the notch region (minimum bandgap) and Ga composition in single- and double-graded profiles in thin-film ACIGS was examined. The primary goal was to increase ${V} _{\text {oc}}$ without losing ${J} _{\text {sc}}$ of the tandem device. To achieve this, the Ga content was varied from 0.30 to 0.85 ( ${x} _{{1}}$ ) for the first layer and from 0.05 to 0.60 ( ${x} _{{2}}$ ) for the second layer of the ACIGS material for various notch positions. The optimized tandem device achieved an efficiency of 29.14%, with a ${J} _{\text {sc}}$ of 18.04 mA/cm2, a ${V} _{\text {oc}}$ of 2.037 V, and a fill factor (FF) of 79.25%. This was achieved using a V-shaped Ga content gradient (GGI profile) with ${x} _{{1}} =0.70$ , ${x} _{{2}} =0.40$ , and a notch position of 200 nm for a 2- $\mu $ m-thick ACIGS layer. These numerical simulations provide insights into the effect of Ga content and bandgap grading on tandem solar cell performance.