Microscopic origins of performance losses in (Ag,Cu)(In,Ga)Se₂ thin-film solar cells

Ag alloying of CIGSe absorbers in thin film solar cells provides the means to fabricate absorber layers with slightly wider band-gap energies at lower substrate temperatures than for CIGSe layers without Ag. In the present study, three solar cells with ACIGSe photoabsorber of same band-gap energies of about 1.2 eV exhibit different open-circuit voltages V-OC. The influence of the microscopic properties of the ACIGSe absorber on the V-OC of the solar cell was investigated. Several characterization techniques in scanning electron microscopy were applied in a correlative manner on the identical specimen positions in addition to time-resolved photoluminescence and external quantum efficiency measurements. Differences in microstructural and optoelectronic properties such as average grain size, effective electron lifetime, absorption edge broadening by compositional gradients, and fluctuations in the spatial luminescence distribution were identified as origins of radiative and nonradiative loss mechanisms.