Hot Carrier Cooling and Multiple Exciton Generation in Alloy CdxHg1-xTe and PbS Quantum Dots CFharacterized Using a Transient Grating Technique

Qing Shen^a^,^b, Taro Toyoda^a^,^b, Andrey Rogach^c, Stephen Kershaw^c, Kenji Katayama^d, Shuzi Hayase^e^,^b

^aThe University of Electro-Communications, Japan, Japan

^bCREST, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan

^cDepartment of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, Hong Kong

^dFaculty of Science and Engineering, Chuo University, Tokyo 112-8551

^eKyushu Institute of Technology, Japan, 204 Hibikino Wakamatsu-ku, Kitakyushu - Fukuoka, 808, Japan

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe September Meeting 2015 (NFM15)

Santiago de Compostela, Spain, 2015 September 6th - 15th

Poster, Qing Shen, 023
Publication date: 8th June 2015

Multiple exciton generation (MEG) in semiconductor quantum dots (QDs) has received much interest, because MEG has a potential to produce an appreciable improvement in the energy conversion efficiency of solar cells 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]. We have proved that an improved transient grating (TG) technique [3,4] is very useful to characterize photoexcited carrier dynamics, including hot carrier cooling and MEG in QDs. In this study, for the first time, we studied hot carrier cooling and MEG in alloy CdxHg1-xTe colloidal QDs using the TG methods [5]. The TG measurements revealed a composition x dependent multiple exciton generation process which competes with phonon mediated carrier cooling to deplete the initial hot carrier population. The interplay between these two mechanisms is strongly dependent on the electron effective mass which in these alloys has a marked composition dependence and may be considerably lower than the hole effective mass. For a composition x = 0.52, we measured a maximum carrier multiplication quantum yield of 199% with pump photon energy 3 times the bandgap energy, Eg, whilst the threshold energy is calculated to be just 2.15Eg. These results indicate that alloy CdxHg1-xTe QDs are promising materials for solar cell application. For comparison, hot carrier cooling and MEG in PbS QDs will also be discussed.

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