Photoelectrical characterization of chalcopyrite Cu(In_0.6Ga_0.4)₃Se₅

The compound Cu(In_0.6Ga_0.4)₃Se₅ prepared by fusion in sealed tube is a wide band gap semiconductor; it crystallizes in the P-chalcopyrite structure with a direct transition of 1.42 eV. Its transport properties exhibit a semi-conducting behavior, which seem to be not intrinsic but rather attributed to selenium vacancies, and the conductivity is well described by small-polaron hopping: σ= σ₀exp{-29/kT}(Ohm cm)^-1 with an effective mass of ∼2m₀. The thermo-power is negative and changes little with temperature, suggesting that the conduction mechanism is mostly due to electron hopping. The analyzed material shows a chemical stability over a broad pH range; the semi-logarithmic plot in KOH solution displays an exchange current density of 27 μA cm^-2 and a corrosion potential of -0.204 V_SCE. The capacitance measurement (C^-2-V) exhibits a linear behavior, characteristic of n type conductivity, from which a flat band potential of -0.530 V_SCE and an electron density of 3.49 x 10²⁰ cm^-3 are determined. The Nyquist plot shows a semicircle due to the predominance of the bulk contribution (=127 Ohm cm²) and a low density of surface states. The centre is localized below the real axis with a depression angle of 12 ascribed to a constant phase element (CPE). The straight line in the low frequencies region is due to the Warburg diffusion. (authors)