Optical properties of Cu(In,Ga)3Se5, Cu(In,Ga)5Se8 and Cu(In,Ga)(Se,S)2
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StatusThe thesis was presented on the 4 July, 2008Approved by NCAA on the 18 September, 2008 Abstract – 1.39 Mb / in romanian – 0.45 Mb / in russianThesisCZU 621.315.592
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Summary
The work is dedicated to the complex investigation of the optical properties of the ordered defect compounds Cu(In,Ga)3Se5, Cu(In,Ga)5Se8 and the chalcopyrite Cu(In,Ga)(Se,S)2.
Optical absorption spectra of CuIn5Se8, CuGa3Se5 and CuGa5Se8 single crystals grown by chemical vapour transport have been investigated. The energy gap Eg for CuIn5Se8, CuGa3Se5 and CuGa5Se8 is equal to 1.27 (1.21), 1.80 (1.71) and 1.92 (1.79) eV at 10 (300) K, respectively. The temperature dependence of Eg was studied by means of the Vina model and the Pässler model taking into account electron–phonon interaction as the main mechanism of the temperature variation of the band gap energy. The effective energy of phonon modes that participate in the shift of Eg with T is equal to 19, 23 and 15 meV for CuIn5Se8, CuGa3Se5 and CuGa5Se8, respectively. It was also found that the major contribution of phonons to the shift of Eg versus temperature in CuIn5Se8 and CuGa3Se5 was mainly from optical phonons.
Room temperature pseudodielectric function spectra ε(ω) = ε1(ω) + iε2(ω) of the ordered defects compounds Cu(In,Ga)3Se5, Cu(In,Ga)5Se8 with slightly different Cu contents grown by Bridgman method, have been measured by spectroscopic ellipsometry in the 0.8–4.5 eV. The structures observed in ε(ω) spectra have been analyzed using different methods, including fitting the numerically differentiated experimental spectrum (second derivative) to analytical line shapes and modeling using the simulated annealing algorithm. As a result, the energies corresponding to the fundamental gap (E0) and higher critical points (E1A, E1B) have been determined. It is found that Cu-poor samples of ordered defects compounds show an increase in band gap and show a depression of the absorption coefficient in the spectral region of 1–3 eV. The results presented offer a valuable set of optical-constant data for Cu(In,Ga)3Se5, Cu(In,Ga)5Se8 that can be useful for the design of solar cells based on these compounds.
The reflection spectroscopy of chalcopyrite CuGaSe2, CuGaS2 and CuInS2 single crystals grown by chemical vapour transport has been applied for light polarized perpendicular (E ⊥ c) and parallel (E || c) to the optical axis in the photon energy range between 2 and 6 eV at 77 K. The spectral dependences of the complex dielectric function are calculated using the Kramers–Kronig relations. As a result, the energy band structure of these crystals at photon energies higher than the fundamental band gap is derived from the analysis of the structures observed in the ε(ω) and in the reflectivity spectra. Additionally, the spectral dependences of the refractive index, extinction coefficient and absorption coefficient of CuGaSe2, CuGaS2 and CuInS2 crystals are determined. Photoreflectivity of CuGaSe2 at 10 K at photon energies lower than the fundamental band gap was used to determine the exciton and band parameters of CuGaSe2.
The obtained results were published in 19 scientific papers.
The thesis is written in Russian and contains 118 text pages, 47 figures and 101 references.
Oficial Reviewers
- Ion Tighineanu
doctor habilitat, professor, Institute of Mathematics and Computer Science - Dmitrii Nedeoglo
doctor habilitat, professor, Moldova State University
Theses
