Photoacoustic Spectra of CuInSe2 Thin Films Prepared by Chemical Spray Pyrolysis

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Abstract
Photoacoustic (PA) spectra of CuInSe2 thin films prepared by chemical spray pyrolysis have been measured and compared with the Raman and X-ray diffraction results. Stoichiometric films having a sphalerite structure exhibit a PA band at ∼0.95 eV, which is considered to be due to the electronic transition from copper atom substituting indium site (CuIn) acceptor to indium atom substituting copper site (InCu) donor. In-rich films having the chalcopyrite structure with the In2Se3 second phase exhibit PA bands at ∼0.90 eV and ∼0.83 eV. The former PA band is considered to be due to the electronic transition from copper vacancy (VCu) acceptor to indium atom substituting selenium site (InSe) donor. The latter PA band is considered to be related to the defect complex involving InCu. For Cu-rich films, a PA signal below the absorption edge increases with increasing Cu/In ratio. This behavior is attributed to the increase in the content of the Cu-Se compound.