Surface Modifications of Na and K Metal Incorporated Cu(In,Ga)Se2 Absorbers Investigated by Synchrotron‐Based Spectroscopies

Na and K metals have been evaporated on a Cu(In,Ga)Se2 (CIGSe) thin film solar cell absorber at 400 °C in order to investigate the effect of alkali metal incorporation on the very near-surface (up to 5 nm) region of the CIGSe absorber using soft X-ray spectroscopy techniques, focusing on the main compositional and electronic modifications of the absorber surface. Quantitative X-ray photoelectron spectroscopy (XPS) showed Cu deficiency and Se enrichment on the CIGSe surface after alkali treatment which may play a role in assisting Na diffusion away from the surface, leaving behind a significantly higher K content than Na along the entire range of the CIGSe surface region probed, although nominally equal amounts of Na and K metal have been evaporated onto the CIGSe surface. A [K]/([K]+[Cu]) concentration ratio of 0.99 ± 0.01 at an information depth of ≈1.7 nm from the surface may indicate the formation of a wide band gap compound like KInSe2 (Eg ∼ 2.67 eV) on the CIGSe surface as a result of alkali metal deposition. Ultra-violet photoemission spectroscopy (UPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements further confirm a 1.09 eV surface band gap increase along with a type-inversion at the surface of the alkali metal-incorporated CIGSe as compared to the untreated CIGSe with a surface band gap of 1.3 ± 0.2 eV. These changes in the surface composition and electronic structure of the modified CIGSe surface as a result of the alkaline treatment could be attributed to the increase in alkali-treated CIGSe-based thin film solar cell efficiencies seen in recent years.