Critical Points in the Band Structure of Absorber and Buffer Layers of CuIn $_{{\bm 1}-{\bm x}}$Ga$_{\bm x}$ Se$_{\bf 2}$ Solar Cells by Electroreflectance Spectroscopy

We explore the use of electroreflectance (ER) spectroscopy at room temperature to characterize simultaneously Cu(In,Ga)Se
₂
(CIGS) absorber and CdS/i-ZnO buffer layers of thin-film solar cells. Absorber layers are coevaporated from elemental sources. The CdS buffer layers are deposited by chemical bath deposition. We apply ER at different incidence angles to distinguish ER signals from interference fringes. Bandgaps, critical point energies, and broadening parameters that describe the band structure of absorber and buffer films are determined by fitting the ER spectra with a third-derivative functional form. Absorbers grown with a vertical gradient in the Ga/(Ga+In) (GGI) ratio are compared with cells without a GGI gradient but the same integral GGI compositions. We also measure samples with no gradient but different integral GGI ratios of 0 (CuInSe
₂
), 0.3, 0.6, and 1 (CuGaSe
₂
). The ER signal of CdS yields three excitonic transitions, consistent with theory.