Publication date: 31st March 2013
In recent years, multiple exciton generation (MEG) in semiconductor quantum dots (QDs) has received much interest, because MEG has a potential to produce an appreciable improvement in an energy conversion efficiency of solar cells and solar fuels through increased photocurrent [1]. MEG in some QDs such as PbSe, PbS, CdSe, PbTe, and Si QDs has been observed at threshold photon energies of 2-3 times the HOMO-LUMO transition energy (Eg) using transient absorption spectroscopy and time-resolved photoluminescence [1,2]. However, several recent reports have questioned the experimental results on the quantum yields of MEG in QDs and even its existence [3]. Further theoretical and experimental studies to better understand the mechanism and occurring conditions of MEG in QDs are necessary and important. In this study, we apply an improved transient grating (TG) technique [4] to characterize hot carrier cooling and MEG in PbS colloidal QDs. The improved TG technique is one kind of pump-probe methods and transient refractive index changes in the sample due to photoexcited carriers can be measured. Thus, ultrafast photoexcited carrier dynamics can be monitored by using this technique. We have characterized pump light intensity and photon energy dependences of the TG responses in PbS colloidal QDs [4]. We found that besides a peak existing at about 300 fs in the TG responses, a new peak appeared at about 3 ps when the photon energy of the pump light is larger than 2.7Eg. The new peak intensity decreased as the photon energy of the pump light decreased and the peak disappeared for the photon energies smaller than 2.7Eg. In addition, a fast Auger recombination decay with a decay time of about 100 ps was observed when the photon energy is larger than 2.7Eg. We think that the first peak at about 300 fs resulted from photoexcited hot carriers and the second peak at about 3 ps resulted from MEG in the PbS QDs. We succeeded in separate detection of hot carrier and MEG in semiconductor QDs using the transient grating technique [4].
[1] A. Nozik, Chem. Phys. Lett., 457, 3 (2008).
[2] R. D. Schaller and V. I. Klimov, Phys. Rev. Lett., 92, 186601 (2004).
[3] G. Nair, S. M. Geyer, L. –Y. Chang and M. G. Bawendi, Phys. Rev. B, 78, 125325 (2008).
[4] Q. Shen, K. Katayama, T. Sawada, S. Hachiya, and T. Toyoda, Chem. Phys. Lett., 542, 89 (2012).
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