Large and persistent photoconductivity (LPPC) in semiconductors is due to the trapping of photogenerated minority carriers at crystal defects. Theory has suggested that anion vacancies in II-VI semiconductors are responsible for LPPC due to negative-U behavior, whereby two minority carriers become kinetically trapped by lattice relaxation following photoexcitation. By performing a detailed analysis of photoconductivity in CdS, we provide experimental support for this negative-U model of LPPC. We also show that LPPC is correlated with sulfur deficiency. We use this understanding to vary the photoconductivity of CdS films over nine orders of magnitude, and vary the LPPC characteristic decay time from seconds to 10 4 s by controlling the activities of Cd 2+ and S 2− ions during chemical bath deposition. We suggest a screening method to identify other materials with long-lived, nonequilibrium, photoexcited states based on the results of ground-state calculations of atomic rearrangements following defect redox reactions, with a conceptual connection to polaron formation.

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In this paper, a system is designed and developed to measure the positive corona current in coaxial wire-cylinder gaps. The characteristic parameters of corona current pulses, such as the amplitude, rise time, half-wave time, and repetition frequency, are statistically analyzed and a new set of empirical formulas are derived by numerical fitting. The influence of space charges on corona currents is tested by using three corona cages with different radii. A numerical method is used to solve a simplified ion-flow model to explain the influence of space charges. Based on the statistical results, a stochastic model is developed to simulate the corona pulse trains. And this model is verified by comparing the simulated frequency-domain responses with the measured ones.