Space-charge-limited conductivity in evaporated cadmium selenide thin films

We have studied the basic d.c. electrical properties of evaporated CdSe films in the context of the development of inexpensive solar cells. Earlier measurements have shown significant variations, which depend on the preparation and previous treatment of the films. In the present work Al-CdSe-Al sandwich structures were studied, with the CdSe thickness in the range 0.1-1.0 mu m. D.c. capacitance varied inversely with film thickness, yielding a permittivity of 7.82 X 10-(11) F m(-1) (relative permittivity 8.83). Current density was proportional to applied voltage at low voltage levels, but followed a power law at higher voltages with exponent typically 2.5; the transitional voltage was directly proportional to the square of the film thickness. The results were interpreted as ohmic conduction at low voltages and space-charge-limited conductivity (SCLC) dominated by an exponential trap distribution, at higher voltage levels. Measurements of current density as a function of inverse temperature for different applied voltages in the SCLC region enabled the derivation of typical mobilities in the range of (7.65-10.15) X 10(-5) m(2) V-1 s(-1) using the results of our existing theory. This value of mobility and the derived trapping parameters were in general agreement with some earlier measurements.