Photophysics, photoelectrical properties and photoconductivity relaxation dynamics of quantum-sized bismuth(III) sulfide thin films

Electrical and photoelectrical properties (including both the stationary photoresponse and the photocarriersʹ relaxation dynamics) of nanocrystalline semiconducting bismuth(III) sulfide thin films were investigated. The experimental design of photoelectrical properties was achieved by controlling the chemistry of the deposition process (varying the reagent concentration in the reaction system) and also by physical means (controlling the crystal dimensions by post-deposition annealing). The band gap energy of thin films characterized by most pronounced photoelectrical properties was calculated, on the basis of measured photoconductivity spectral response curves, by several approaches. All of the obtained values are in very good agreement with the corresponding ones obtained from optical spectroscopy data within the framework of parabolic approximation for dispersion relation. On the basis of measured temperature dependence of dark electrical resistivity of nanocrystalline bismuth(III) sulfide films, the thermal band gap energy and the ionization energy of the impurity level (of donor type) were calculated. The corresponding values are 1.50 and 0.42 eV. Dynamics of non-equilibrium charge carriersʹ relaxation processes was studied with the oscilloscopic method. By analysis of the photoconductivity decay kinetics data it is found that recombination of non-equilibrium charge carriers is carried out according to the linear mechanism. The calculated relaxation time of photoexcited charge carriers is 1.58 ms, the relaxation processes occurring via local trapping centers. Recombination processes occurring via a single-type trapping center can be described within the framework of the Schockley–Read model. The practically linear regime detected in the measured lux–ampere characteristics of the studied films (image) indicate as well a linear recombination mechanism of the photoexcited charge carriers.