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

Abstract

The open-circuit voltage (VOC) deficit in wide-bandgap (Ag,Cu) (In,Ga)Se2 (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 VOC 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 VOC. An ammonia-based absorber rinsing step reduces VOC at growth temperature 415 °C and low Rb doses, whereas DI-water rinsing better preserves VOC and boosts short-circuit current density (JSC). VOC-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 VOC-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.