First principles spectroscopy of CuInSe2, CuGaSe2, and CuInS2 chalcopyrite compounds

Full text : Chalcopyrite compounds as represented by CuInSe2 are known to be a strong light absorber, and their optical properties have attracted much attention both from academic and industrial viewpoints. In this study, it was theoretically calculated optical spectra of CuInSe2 CuGaSe2, and CuInS2 by using our first principles spectroscopy system PHASE/UVSOR. The system is the first principles software working as a virtual spectrometer on computer, based on the density functional pseudopotential (DFT-PP) method. Electronic structures of CuInSe2, CuGaSe2, and CuInS2, were calculated in the GGA approximation, by using ultra-soft PPs of the Vanderbilt type, and optical absorption spectra of these compounds were calculated from the electronic structures by considering the direct transition between valence to conduction bands at each k-point in the Brillouin zone. The band gap underestimation by DFT calculations, which is due to the selfinteraction of Kohn-Sham electrons, is corrected by using the scissors operator approach that introduces the rigid shift of conduction bands in energy to reproduce experimental band gaps. This article shows GGA calculated optical absorption spectra of CuInSe2 and CuGaSe2, along with experimental spectra for these compounds. The first principles calculation well reproduced experimental spectra of these compounds; the spectra of both the compounds have a plateau region in absorption strength in a lower energy region (2.6 to 3.6 eV for CuInSe2 and 3.5 to 4.5eV for CuGaSe2), and the spectra of CuGaSe2 has a blue-shifted structure of those of CuInSe2. Electronic structure analyses revealed the plateau region in the spectra is due to Se 4p-Cu3d valence bands near the band gap, which are less dispersive because of localized Cu 3d bands. Calculations showed that CuInS2 have similar optical absorption spectra having a plateau region in 3 to 4 eV. The chalcopyrite compounds investigated here have similar optical spectra though the position of plateau region depends on the compounds. The optical spectra of CuInSe2 has the plateau region in visible wavelengths so that the material is confirmed to be an effective photovoltaic material compared with the other compounds, absorbing solar light more strongly