Quantum Dot Solar Cells Based on CdSe-Assembled Titania Semiconductors
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1102-LL04-05
Quantum Dot Solar Cells Based on CdSe-Assembled Titania Semiconductors Chien-Tsung Wang*, and Chung-Hsiao Yeh Chemical and Materials Engineering Department, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliou, Yunlin, 640, Taiwan
ABSTRACT In the study, semiconductor quantum dots (QDs) of CdSe nanocrystals have been demonstrated effective as the light energy harvesting assemblies in solar cells. Colloidal CdSe QDs were synthesized by the one-pot approach and linked through a surface modifier onto titania (TiO2) nanoparticles. The TiO2-CdSe composite, while employed as the photoanode in a photoelectrochemical apparatus, exhibited a higher photon-to-electron conversion efficiency (3-fold) than the TiO2 alone, and also a higher stability for photocurrent generation, according to photocurrent transient responses. The optical absorbance and photoluminescence of the CdSe colloid showed a blue shift in the absorption edge with decreasing the particle size (band energy gap shifts from 2.0 to 2.19 eV), suggesting a quantum size effect. The CdSe particle size was determined up to 5 nm by a transmission electron microscopy. A scheme describing the charge carrier rectification for the coupled semiconductor system is proposed. INTRODUCTION Ceramic oxide semiconductors have attracted a greater deal of attention in many photoelectrochemical applications. For example, titanium dioxide (TiO2) is most frequently used as a photo-anode in solar energy cells, due to its high corrosion-resistance. This n-type semiconductor generates photoelectrons upon it absorbs light energy greater its band energy gap between Ti(3d) and O(2p) orbitals (e.g., 3.2 eV). However, the large band gap limits the energy absorption in the visible light region. Great progress has been made by Grätzel and O’Reagan [1] with invention of the so-called dye-sensitized solar cells (DSSC), fabricated through coating an organo-ruthenium dye onto a nanostructured TiO2 electrode. The Grätzel cell presents a high solar energy conversion efficiency, up to 10%, and good long-term stability, but the dye is too expensive to be favorably applied for cell commercialization. Another material used as the light harvester in TiO2-based DSSC is the nanocrystalline narrow-band-gap semiconductor, such as quantum dots (QDs) CdSe [2,3]. For example, in a QDs-TiO2 cell system under illumination, electrons that are photo-generated in the smaller band
gap QDs can flow to the conduction band of the TiO2 semiconductor while the holes remains in the QDs [4], prior to charge transfer to the electrolyte solution. In addition to the capability to rectify electron flow, the semiconductor quantum dots provide the ability to match the solar spectrum better because their energy absorption can be tuned with particle size [4]. Kamat and coworkers [5] reported a photo-to-charge carrier generation efficiency of 12% in a composite electrode of CdSe nanocrystals linked to mesoporous TiO2 films and found significant loss of electrons due to scatterin
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