Synergistic effects of bulk selenization and interface tailoring on the performance of CuGaSe2 solar cells

With its wide-bandgap, CuGaSe2 (CGS) is a prime candidate for top-cell junctions in tandem photovoltaics; yet its performance is severely limited by an open-circuit voltage deficit driven by native bulk defects and sub-optimal heterointerfaces. This study highlights that overcoming the intrinsic limitations of CGS-based solar cells requires a synergistic approach coupling bulk defect mitigation with precise interface engineering. Here, a novel in-situ protocol using pulsed electron deposition—a physical vapor deposition technique based on high-energy electron beam ablation of a target—is established, integrating growth and selenization to heal native selenium vacancies, yielding stoichiometric CGS absorbers with improved crystallinity. To address interfacial losses, we investigate a tunable hybrid CdS/Zn1−xSnxO (ZTO) buffer layer, revealing a strict physical trade-off governing electrostatic integrity. Whereas a thick CdS baseline guarantees robust passivation and prevents leakage, yielding a power conversion efficiency of 3.75% and a Voc of 705 mV, it also introduces short-wavelength parasitic absorption. Conversely, the complete elimination of the CdS layer causes a severe electrical degradation, collapsing the VOC to 335 mV. Advanced electrical characterizations are consistent with sputter-induced irreversible damage occurring during ZTO deposition as the origin of this failure: the unpassivated interface shrinks the depletion width from 550 nm to 300 nm and introduces a dense continuum of shallow interfacial traps (160–190 MeV) that completely overwhelms the intrinsic bulk defect response (320 MeV). Ultimately, we show that systematically thinning the CdS film effectively mitigates parasitic absorption to maximize the short-circuit current, while revealing that even an ultra-thin CdS layer acts as a critical physical shield against sputter damage, successfully recovering the fill factor. These findings prove that high-efficiency, Cd-reduced CGS solar cells demand a carefully balanced hybrid architecture to ensure conformal coverage and preserve a passivated heterojunction.