Study on ALD In 2 S 3 /Cu(In,Ga)Se 2 interface formation

The formation of the interface between In 2 S 3 grown by atomic layer deposition (ALD) and co‐evaporated Cu(In,Ga)Se 2 (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 In 2 S 3 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 In 2 S 3 buffer layers were fabricated. Highest device efficiency of 12·1% was obtained at a substrate temperature of 120°C. Using the bandgap obtained for In 2 S 3 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.