Ultrafast transient grating characterization for PbS based quantum dot sensitized solar cells
Taro Toyoda a b, Qing Shen a b, Kenji Katayama c
a The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo, 182, Japan
b CREST, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan
c Chuo University, Tokyo 112-8551, Japan
Oral, Qing Shen, presentation 032
Publication date: 1st April 2013

Recently, semiconductor quantum dots (QDs) sensitized solar cells (QDSCs) have attracted significant interest because the QDs have some unique properties in solar cell applications [1,2]. First, the energy gaps of the QDs can be tuned by controlling their size. Secondly, semiconductor QDs have large extinction coefficients. Thirdly, semiconductor QDs have the potential to generate multiple electron-hole pairs with one single photon absorption, which would lead to incident-photon-to-current (IPCE) efficiencies of over 100% [3,4]. In addition, QDSCs can be made very cheaply using simple chemical methods, so QDSCs are expected to be a candidate of the promising next-generation cost-effective high efficiency solar cells. However, the energy conversion efficiency of QDSCs is still less than 5-6%. Therefore, fundamental studies on the preparation and characterization of QD-based solar cells are very important for improving their photovoltaic properties.

In this talk, we will focus on the characterization of carrier dynamics of PbS QDs for solar cell applications using an ultrafast transient grating (TG) technique [5]. First, we will report on the direct observation of multiple exciton generation (MEG) in PbS colloidal QDs using the TG method. We found that MEG began at 200 fs after optical absorption, and that the carrier density in LUMO of PbS QDs due to MEG increased and finally saturated after about 2 ps. Then, a fast decay due to Auger recombination of the multiple excitons with a lifetime of a few 10 ps, together with a slow decay, due to the single exciton relaxation with a lifetime longer than ns, were observed. We succeeded in directly observing the occurrence and dynamics of MEG in PbS QDs using the TG technique [6]. From these results we can know that charge separation in the QDs have to be finished within a few 10 ps for utilization of MEG in solar cells. Secondly, we will report on the ZnS surface modification effects on the properties of PbS QDSCs. Ultrafast carrier dynamics of the QDs adsorbed on TiO2, including trapping and recombination, was found to change greatly with increasing the ZnS coating numbers. These results imply that surface defects of QDs decreased greatly with ZnS surface coating. Correlations between carrier dynamics and photovoltaic property will be discussed.

Acknowledgments: This research was supported by JST PRESTO program. 

[1] I. Mora-Sero, J. Bisquert, J. Phys. Chem. Lett. 1, 3046 (2010). [2] P. V. Kamat, J. Phys. Chem. C 112, 18737 (2008). [3] R. D. Schaller and V. I. Klimov, Phys. Rev. Lett 92, 186601 (2004). [4] A. J. Nozik, M. C. Beard, J. M. Luther, M. Law, R. J. Ellingson, and J. C. Johnson, Chem. Rev. 110, 6873 (2010). [5] Q. Shen, Y. Ayuzawa, K. Katayama, T. Sawada, and T. Toyoda, Appl. Phys. Lett, 97, 263113 (2010). [6] Q. Shen, K. Katayama, T. Sawada, S. Hachiya, and T. Toyoda, Chem. Phys. Lett. 542, 89 (2012).
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