Effects of ultraviolet illumination and temperature on ionic motion in single-crystalline CuIn2Te3.5

A CuIn2Te3.5 single crystal behaves as a mixed ionic and electronic conductor. Under application of a constant potential to a paint graphite/CuIn2Te3.5/paint graphite solid state device, a variation of the current intensity, I, with time is observed whereas the potential across the device, Vm, remains constant in all measurements. An equivalent electrical circuit permits to understand this behaviour assuming that mobile ions are blocked at the CuIn2Te3.5/paint graphite interface, therefore changing the interface potential, whereas the electronic conduction is possible across that interface. Two ionic diffusion coefficients have been computed from σ(1/t) curves. Consecutive measurements have shown that the current intensity takes different values under identical electrical conditions whereas Vm is constant. These changes have been explained by the formation of a residual interface potential. The ultraviolet (UV) illumination effect has been studied by comparison of measurements carried out under dark and UV illumination conditions. Lower values of I and the interface potential are observed under dark than under UV illumination conditions. The ratio between the diffusion coefficients under dark and UV illumination conditions is close to 2.2 for both diffusion coefficients. Cu bond-breaking can explain the decrease of the ionic diffusion coefficients under UV illumination. The temperature effect in the ionic motion has been analysed in the 293–383 K range. The increase of the number of free electronic carriers and mobile ions with temperature permits to compute their activation energies, 0.28 and 0.8 eV for the electronic carriers and mobile ions, respectively, whereas the two ionic diffusion coefficients do not vary. In the 338–348 K temperature range, the ionic resistance remains practically constant and the diffusion coefficients increase considerably; this behaviour would indicate that a reversible phase change is probably taking place.