Optical Spectroscopic Probes of Degradation and Metastability in Polycrystalline (Ag,Cu)(In,Ga)Se2 Absorbers

We employ optical spectroscopic probes, based on steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) excitation spectroscopy to spatially and energetically resolve metastable defects in polycrystalline (Ag,Cu)(In,Ga)Se2 (ACIGS) absorbers. First, we show that ACIGS has similar potential fluctuations as CIGS; that is alloying with Ag does not result in significant changes in the electronic disorder. We subsequently demonstrate that optical spectroscopy is able to identify metastable defects states in ACIGS, the nature of which is subtly modified by dark-heat degradation. These defects are converted into minority carrier lifetime-limiting recombination states upon light-soaking under 1 Sun illumination for 24 hours. We identify the activation energy for thermal emission from trap states near the front surface of the ACIGS absorber. Finally, variation of the excitation wavelength reveals the depth distribution of the shallow defects. We tentatively ascribe the defects to complexes that include Se-vacancies.