Large and Persistent Photoconductivity due to Hole-Hole Correlation in CdS

TL;DR

This study investigates large, persistent photoconductivity in CdS, linking it to sulfur vacancies and hole-hole interactions, and demonstrates how to control LPPC over nine orders of magnitude through chemical deposition parameters.

Contribution

It provides experimental support for the negative-U model of LPPC in CdS and introduces a method to tune photoconductivity and decay times by controlling defect chemistry during synthesis.

Findings

LPPC in CdS is correlated with sulfur deficiency.

Photoconductivity can be varied over nine orders of magnitude.

Decay time of LPPC can be tuned from seconds to 10^4 seconds.

Abstract

Large and persistent photoconductivity (LPPC) in semiconductors is due to the trapping of photo-generated 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 photo-excitation. 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 seconds, 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, non-equilibrium, photo-excited states based on the results of ground-state calculations of atomic rearrangements following defect redox reactions, with a conceptual connection to polaron formation.