Size-dependent photoluminescence dynamics of CuInS2 quantum dots and charge injection on titanium oxide film

CuInS2 quantum dots (QDs) with different sizes are synthesized by a solvothermal process, and then linked on the FTO conducting glass and TiO2 nanoparticle films. Photoluminescence (PL) lifetimes measured by time-resolved PL decay indicate that PL lifetimes firstly increase then decrease with the size increasing of CuInS2 quantum dots, which is due to the ratio between the radiative excitons traps and nonradiative excitons traps. Temperature-independent PL spectra of the 4.9 nm CuInS2 QDs prove that there is no emission peak shift from 5 K to 200 K, which is related with the emission from the trapped states, and the linear fitting character of the temperature dependence of the full width at half maximum comes from the coupling of the carriers to the acoustic phonons modes. With the CuInS2 quantum dots linking on TiO2 nanoparticle films, the PL lifetime is obviously reduced. The gradually larger conduction band energy offset between CuInS2 quantum dots and TiO2 induces the PL lifetime reducing with the size decreasing of the CuInS2 QDs, which can induce the faster electron injection rate constant for the CuInS2 quantum dots with small size.