Publication date: 1st July 2014
An intense effort aimed at third-generation solar cells is being undertaken. Semiconductor quantum dots (QDs) have been studied for their light harvesting capability as sensitizers [1-3]. Despite the potential advantages, a major breakthrough in conversion efficiency of QD-sensitized solar cells (QDSCs) that equals or exceeds dye-sensitized solar cells (DSSCs) has yet to be reported. Fundamental understanding of the surface chemistry of semiconductor QD adsorption is lacking, and this deficiency needs to be addressed. We describe the adsorption and growth of CdSe QDs on single crystals of rutile TiO2 with different crystal orientations. We used AFM to characterize the morphology of the QDs and photoacoustic (PA) spectroscopy for the optical absorption. Photoelectron yield (PY) spectroscopy was applied to characterize the valence band maximum (VBM) of the single crystal TiO2. The AFM images and the absorbance measurements showed that the number of CdSe QDs grown on the (111) surface was larger than those grown on the (110) and (001) surfaces. The adsorption becomes linearly proportional to the adsorption time. However, the rate of adsorption is different for each crystal orientation. The crystals grow higher on (111) surfaces than on (110) and (001) surfaces. The position of VBM for (111) surface is higher than those for the (110) and (001) surfaces. Hence, the growth of CdSe QDs on (111) surfaces is more active than on the other orientations. The increase in the average diameter of CdSe QDs with adsorption time is independent of the crystal orientation of TiO2. Although the growth rate of CdSe QDs on (001) surfaces is lower than other surfaces, the crystal quality is better on the former.
[1] L. Diguna, Q. Shen, J. Kobayashi, and T. Toyoda, Appl. Phys. Lett. 91, 023116 (2007).
[2] T. Toyoda and Q. Shen, J. Phys. Chem. Lett. 3, 1885 (2012).
[3] T. Toyoda, W. Yindeesuk, K. Kamiyama, S. Hayase, and Q. Shen, J. Phys. Chem. C 118, (2014)DOI: 10.1021/jp412657x.
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