Improving VOC in wide bandgap (Ag,Cu) (In,Ga)Se2 solar cells for voltage-matched ACIGS/Si tandem modules

The open-circuit voltage (V OC ) deficit in wide-bandgap (Ag,Cu) (In,Ga)Se 2 (ACIGS) absorbers remains as the main limitation for their application in tandem solar cells. The roles of absorber growth temperature, surface treatment, and rubidium fluoride (RbF) post-deposition treatment (PDT) on the V OC of wide-bandgap ACIGS solar cells on transparent substrates and their integration into voltage-matched two-terminal ACIGS/Si tandems are investigated. Growth below 415 °C leads to poor morphology, Cu deficiency at the rear contact, and diode non-idealities, whereas higher temperatures improve film quality and V OC . An ammonia-based absorber rinsing step reduces V OC at growth temperature 415 °C and low Rb doses, whereas DI-water rinsing better preserves V OC and boosts short-circuit current density (J SC ). V OC -loss analysis based on the photoluminescence quantum yield (PLQY) is used to separate absorber bulk and interface contributions which are challenging to distinguish in complete solar cells. Quasi-Fermi-level splitting and diffusion-length estimates reveal that non-radiative recombination dominates V OC -loss, limiting the benefit of Ga back-grading. Building on these insights, we present the first experimental demonstration of a voltage- and area-matched two-terminal bifacial ACIGS/Si tandem module, implementing a fully laser-interconnected top ACIGS submodule. Voltage matching is achieved at 10 % rear illumination, with power density generation continuing to increase linearly beyond the voltage matching condition. In contrast to the more frequent series-connected tandems constrained by current matching conditions, the circuitry-2T architecture enables additional power gains under varying bifacial illumination. • Wide-bandgap ACIGS on TCO are V OC limited by non-radiative recombination. • Discrimination of bulk and front interface recombinations by PLQY-based analysis. • Investigation of growth temperature, alkali PDT and rinsing on recombination mechanisms. • First demonstration of voltage- and area-matched 2T bifacial ACIGS/Si tandem module. • Voltage-matched circuitry-2T tandems very resilient to varying bifacial illumination.