Defect states in CdSe nanocrystalline layers

Defect states and recombination processes are investigated in SiOx/nanocrystalline CdSe multilayer and composite films, and CdSe single films, using steady-state and transient photoconductivity and thermally-stimulated current measurements. A reduction in mobility-lifetime product as layer thickness is reduced is assigned to increased deep defect density at the nanocrystal surface. Transient photocurrent falls rapidly at typically 0.1 µs as carriers are trapped in deep states, and then continues to fall as a power law of index close to –1. At elevated temperatures, the transient current in multilayer films falls below the steady-state dark current. This is explained by the effect of potential barrier profiles on recombination. The density of states distribution obtained is fairly broad in multilayers and composite films while in single layer films it is noticeably peaked. Two defect bands located at 0.55 and 0.7 eV below the conduction band edge are identified, the former associated with the nanocrystal bulk and the latter with the CdSe–CdSe interface. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)