Photoconductivity of CdS-CdSe granular films: influence of microstructure

We study experimentally the photoconductivity of CdS-CdSe sintered granular films obtained by the screen printing method. We mostly focus on the dependences of photoconductivity on film's microstructure, which varies with changing heat-treatment conditions. The maximum photoconductivity is found for samples with compact packing of individual grains, which nevertheless are separated by gaps. Such a microstructure is typical for films heat-treated during an intermediate (optimal) time. In order to understand whether the dominant mechanism of charge transfer is identical with the one in monocrystals, we perform temperature measurements of photoresistance. Corresponding curves have the same peculiar nonmonotonic shape as in CdSe monocrystals, from which we conclude that the basic mechanism is also the same. It is suggested that the optimal heat-treatment time appears as a result of a competition between two mechanisms: improvement of film's connectivity and its oxidation. Photoresistance is also measured in vacuum and in helium atmosphere, which suppress oxygen and water absorption/chemisorption at intergrain boundaries. We demonstrate that this suppression increases photoconductivity, especially at high temperatures.Comment: 12 pages, 8 figures, final versio

Electronic defects in CdSe nanocrystals embedded in GeS2 amorphous matrix

Electronic defects in CdSe nanocrystals of a-GeS2/nc-CdSe superlattices and composite films are investigated and compared with results obtained for similar SiOx/CdSe films. A wide band of localized states centred at 0.55 eV below the conduction band edge is seen in both groups of samples and identified with defects in the nanocrystal bulk. A band at similar to 0.7 eV below the conduction band is well resolved in SiOx/CdSe samples but not seen in GeS2/CdSe films. As this feature is ascribed to defects at the CdSe-CdSe interface, a lower density of such defects is assumed in the latter case. In GeS2/CdSe samples a new band located at 0.50 eV below the conduction band appears. It is attributed to defects at the GeS2-CdSe interface. Optical absorption measurements reveal that defect concentration above the valence band of CdSe nanocrystals in GeS2/CdSe samples is lower than in SiOx/CdSe ones. Steady-state photoconductivity of GeS2/CdSe samples shows that at low temperatures the mobility-lifetime product in CdSe nanocrystals decreases with decreasing nanocrystal size. This observation is related to deep defects at the interface of CdSe nanocrystals and reflects the increasing surface to volume ratio

Effects of thermal annealing and long-term ageing on electronic defects in CdSe thin films

Defect distributions in CdSe thin films, 'as deposited', following thermal annealing, and after 10 years' storage under room conditions are investigated. Steady-state photoconductivity measurements at low temperatures suggest a decrease in the density of 'slow' recombination centres following annealing or storage. Transient photocurrent and thermally stimulated current spectroscopics reveal a peak in the density of states at 0.65 eV below the conduction band edge in the as-deposited film. This broadens and shifts towards the conduction band edge on annealing. Stored films exhibit an almost flat defect distribution, which may result from a combination of both types of defect. Raman scattering measurements suggest that both storage and annealing result in increased structural order

Electronic defects in CdSe nanocrystals embedded in GeS2 amorphous matrix

Electronic defects in CdSe nanocrystals of a-GeS2/nc-CdSe superlattices and composite films are investigated and compared with results obtained for similar SiOx/CdSe films. A wide band of localized states centred at 0.55 eV below the conduction band edge is seen in both groups of samples and identified with defects in the nanocrystal bulk. A band at ~ 0.7 eV below the conduction band is well resolved in SiOx/CdSe samples but not seen in GeS2/CdSe films. As this feature is ascribed to defects at the CdSe-CdSe interface, a lower density of such defects is assumed in the latter case. In GeS2/CdSe samples a new band located at 0.50 eV below the conduction band appears. It is attributed to defects at the GeS2-CdSe interface. Optical absorption measurements reveal that defect concentration above the valence band of CdSe nanocrystals in GeS2/CdSe samples is lower than in SiOx/CdSe ones. Steady-state photoconductivity of GeS2/CdSe samples shows that at low temperatures the mobility-lifetime product in CdSe nanocrystals decreases with decreasing nanocrystal size. This observation is related to deep defects at the interface of CdSe nanocrystals and reflects the increasing surface to volume ratio

Effects of thermal annealing and long-term ageing on electronic defects in CdSe thin films

Defect distributions in CdSe thin films, ‘as deposited’, following thermal annealing, and after 10 years ’ storage under room conditions are investigated. Steady-state photoconductivity measurements at low temperatures suggest a decrease in the density of ‘slow ’ recombination centres following annealing or storage. Transient photocurrent and thermally stimulated current spectroscopies reveal a peak in the density of states at 0.65 eV below the conduction band edge in the as-deposited film. This broadens and shifts towards the conduction band edge on annealing. Stored films exhibit an almost flat defect distribution, which may result from a combination of both types of defect. Raman scattering measurements suggest that both storage and annealing result in increased structural order