Study on ALD In_2S_3/Cu(In,Ga)Se_2 Interface Formation

摘要

The formation of the interface between In_2S_3 grown by atomic layer deposition (ALD) and co-evaporated Cu(In,Ga)Se_2 (CIGS) has been studied by X-ray and UV photo electron spectroscopy. The valence band offset at 160 deg 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_2S_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_2S_3 buffer layers were fabricated. Highest device efficiency of 12-1 percent was obtained at a substrate temperature of 120 deg C. Using the bandgap obtained for In_2S_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 ( - 045 + - 0-2 eV) for Na-containing (Na-free) CIGS.