DC I-V characteristics and steady-state photoconductivity of Au/Pb2CrO5/SnO2 sandwich-structure films under illumination in the visible region

The room temperature d.c. current-voltage (I-V) characteristics of an Au/Pb2CrO5/SnO2 sandwich-structure 1.39 mu m thick film have been measured for d.c. voltages, V-d.c., in the range 0.25 V less than or equal to V-d.c. less than or equal to 5.0 V. These measurements were carried out under both dark and visible-light illumination conditions. For V-d.c. < 2.5 V, the I-V curves of the sample in both dark and light environments were found to be non-linear and conform to space-charge-limited (SCL) current governed by traps uniformly distributed in energy. At higher d.c. voltages, a nearly Mott-Gurney V-2 behaviour of the dark current has been observed, whereas the I-V behaviour of the illuminated specimen was a combination of an ohmic conduction and a V-2 dependence at low illumination levels and became highly ohmic at large light intensities. This behaviour can be understood in terms of a reduction in the SCL dark current in favour of a larger ohmic d.c. photocurrent as a result of neutralization of the majority-carrier space charge by the photogenerated minority carriers of the electron-hole pairs produced under the illumination with visible light of energy <(h)over bar omega> congruent to E(G)(similar to 2.1-2.3 eV for the Pb2CrO5 material). The d.c. photocurrent, I-phot, at a fixed d.c. voltage, was found to follow a power-law dependence on light intensity, F, of the form I-phot proportional to F-Y, With the exponent gamma being dependent on the applied d.c. voltage. At the low-voltage side (V-d.c. (<1.5 V), gamma,similar to 0.5, a value usually obtained when the photoconductivity behaviour is governed by bimolecular recombination mechanisms. As the d.c. voltage is increased further, gamma increases monotonically until it saturates at a value of about 0.9 for d.c. voltages beyond 3.5 V, where monomolecular recombination processes seem to be more operative with increasing d.c. voltage.