A study of photophysics, photoelectrical properties, and photoconductivity relaxation dynamics in the case of nanocrystalline tin(II) selenide thin films

Charge-carrier transport properties in chemically deposited thin films composed of close-packed tin(II) selenide nanocrystals were studied both in the dark and under conditions when internal photoelectric effect is manifested in the nanostructured material. Stationary and time-resolved experiments were performed to characterize the photophysics of quantum-confined nanocrystals synthesized in thin film form. The charge-carrier transport through the films was found to occur predominantly by the thermionic emission mechanism in temperature range from 300 to 585 K. The corresponding crystal boundary barrier height in the case of as-deposited films is : 160 meV, and the trap states which cause a partial charge-carrier depletion of the nanocrystals are situated below the Fermi level. Thermal band gap energy was found to be 0.90 eV, which is somewhat lower than the value calculated from optical spectroscopic data (1.10 eV). A physical basis for these differences is proposed. Both the internal photoelectric effect and the crystal boundary barrier height modulation upon illumination were found to contribute to the photoresponse of nanocrystalline films, the surface and bulk recombination velocities being comparable. Indirect and direct band gap energy values of 1.23 and 1.54 eV were calculated for the nanocrystalline films on the basis of photoresponse data. The nonequilibrium conductivity was found to relax exponentially with a time constant of 1.78 ms, which corresponds to the average lifetime of minority charge carriers (electrons). Lux-ampere characteristics of the films further support the linear photoconductivity relaxation mechanism, contrary to what is usually found in the case of amorphous materials.