Quantum Dots and Nanowires Demonstrate Potential for Efficient Solar Cells
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Quantum Dots and Nanowires Demonstrate Potential for Efficient Solar Cells As solar power technology becomes more widely adopted, intensive research efforts are underway to find the next generation of inexpensive but efficient photovoltaics beyond crystalline silicon. One example is quantum-dot-sensitized solar cells, which harvest sunlight using quantum dots adsorbed on a network of TiO2 nanoparticles. However, the disordered structure of these mesoscopic TiO2 networks can allow photogenerated electrons to recombine with holes, which reduces overall efficiency. K. Leschkies and colleagues at the University of Minnesota have combined quantum dots (QDs) with ordered arrays of single-crystal semiconductor nanowires to produce photovoltaic devices in which electron–hole recombination is significantly reduced. As reported in the June issue of Nano Letters (p. 1793; DOI: 10.1021/nl070430o), the research group theorized that the ordered nanowire array would more efficiently guide photogenerated electrons to the photoanode than disordered TiO2 networks. To fabricate the device, the researchers began by growing ZnO nanowires onto a transparent, conducting SnO2 substrate. The wires had lengths between 2 μm and 12 μm and diameters of 75–125 nm. Separately, the group prepared CdSe nanocrystals approximately 3 nm in diameter, which they capped with mercaptopropionic acid (MPA). By immersing the nanowires in a methanol dispersion of the QDs, the researchers were able to attach the QDs to the nanowires. Treating the nanowires with oxygen plasma before immersion significantly increased the attachment efficiency. To complete the device fabrication, the group assembled the QD-decorated nanowire photoanode face-to-face with a SnO2 photocathode, filling the 25-μm space between the electrodes with a hybrid liquid electrolyte. The resulting devices displayed a photovoltaic effect and clear evidence that photogenerated electrons in the QDs were injected into the nanowires. While the overall energy-conversion efficiency of the system was less than 0.5%, this was limited primarily by the surface area of the nanowires. The internal quantum efficiencies were as high as 58%, indicating that the nanowires efficiently collected and guided photogenerated electrons. The researchers reported that their solar cells were stable for a few hours to a few days in air. They said that liquid electrolyte is known to degrade quantum dots over time, and that optimization of the cells will require a transition to a different elec-
trolyte, which will be the subject of future work. Given these results, devices based on single-crystal nanowires sensitized with solar-tailored quantum dots may someday become the next-generation photovoltaic system of choice. COLIN MCCORMICK
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