Study on ALD In2S3/Cu(In,Ga)Se2 interface formation

The formation of the interface between In2S3 grown by atomic layer deposition (ALD) and co‐evaporated Cu(In,Ga)Se2 (CIGS) has been studied by X‐ray and UV photoelectron spectroscopy. The valence band offset at 160°C ALD substrate temperature was determined as −1·2±0·2 eV for CIGS deposited on soda‐lime glass substrates and −1·4±0·2 eV when a Na barrier substrate was used. Wavelength dependent complex refractive index of In2S3 grown directly on glass was determined from inversion of reflectance and transmittance spectra. From these data, an indirect optical bandgap of 2·08±0·05 eV was deduced, independent of film thickness, of substrate temperature and of Na content. CIGS solar cells with ALD In2S3 buffer layers were fabricated. Highest device efficiency of 12·1% was obtained at a substrate temperature of 120°C. Using the bandgap obtained for In2S3 on glass and a 1·15±0·05 eV bandgap determined for the bulk of the CIGS absorber, the conduction band offset at the buffer interface was estimated as −0·25±0·2 eV (−0·45±0·2 eV) for Na‐containing (Na‐free) CIGS. Copyright © 2005 John Wiley & Sons, Ltd.