Electronic nature of the surface defect layer in Cu(In,Ga)Se2: Sensitivity analysis of carrier mobility and defect states via Muon-SCAPS study

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Cu(In,Ga)Se 2 (CIGS) solar cells achieve high light-to-power conversion efficiency but remain constrained by recombination losses associated with a surface defect layer (SDL). Muon spin spectroscopy (μSR) measurements identified a ∼50 nm near-interface region in CIGS exhibiting pronounced defect-induced lattice strain, markedly reduced after CdS buffer layer deposition. In this work, μSR findings are corroborated by SCAPS simulations that evaluate the electronic impact of the SDL by varying carrier mobility, defect density, and energy levels. Deep acceptor-type defects, particularly those with energies > Eᵥ + 0.4 eV and concentrations of 10 15 –10 16 cm −3 , significantly lower device efficiency, mainly through short-circuit current density reduction. The CdS-induced passivation observed by μSR is consistent with a decrease in both defect depth and concentration. These combined results connect μSR measurements, surface defect properties, and device behavior, highlighting SDL passivation as a key pathway toward higher-efficiency CIGS photovoltaics.