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Angewandte Reviews QDs can also function similarly to the dye photosensi- tizers in DSSCs,[55] whereby QDs possess several advantages, such as the ability to tune their optical properties by changing their dimensions, better junction formation, and more sig- nificantly the larger-than-unity quantum efficiency due to efficient MEG effects.[53] Structures consisting of interpene- trating junctions formed between QDs and organic semi- conductor polymers also exhibit the potential to convert light into electricity effectively.[56] The recently demonstrated doping of QDs with metal impurities enables even more sophisticated control of the properties of the QDs in addition to simple variation of their dimensions; record high efficiency of 5.4% was reported when CdS QDs were doped with Mn2+.[57] The emerging field of plasmonics has made it possible to guide and localize light at the nanoscale for the development of novel photonic/optoelectronic applications.[58] Design approaches based on metallic nanostructures that support surface plasmons can also be used to improve absorption in PV devices and thus enable more efficient photon manage- ment and a significant reduction in the physical thickness of PV devices.[59] Metallic nanostructures enhance the perfor- mance of PV devices by trapping and coupling freely propagating plane waves from the sun into the absorbing semiconductor thin film, and by coupling sunlight into surface-plasmon modes supported at the metal/semiconduc- tor interface as well as guided modes in the semiconductor slab. These plasmonic couplings are beneficial in their enhancement of PV performance, as enabled by recently developed inexpensive and scalable techniques for fabricating patterned metallic nanostructures with sophisticated control over the nanoscale dimensions. However, owing to the resonant nature of the plasmonic effect, absorption can only be enhanced at certain wavelengths. In a recent study it was shown that broadband absorption in plasmonic solar cells could be enhanced and a conversion efficiency of 8.1% attained by the controlled engineering of plasmonic nano- structures to strongly scatter the incident light in a large angular range with the minimization of detrimental particle absorption.[60] 3.1.4. Solar Cells Based on Low-Dimensional Nanostructures As evident from the above discussions, improvement of the performance of PV devices largely depends on the optimization of photon absorption and photoinduced-carrier collection.[61] However, the simultaneous optimization of both processes is nontrivial for solar cells based on conventional structures owing to the dilemma between the improvement of optical absorption and the minimization of recombination. Novel structural configurations, such as ordered arrays of one-dimensional nanostructures, are expected to not only improve light absorption and assist interface/junction forma- tion, but also facilitate the transport and collection of electrons.[61, 62] The use of ordered arrays of low-dimensional nano- structures to improve the photon absorption of PV devices has been investigated both theoretically and experimen- tally:[63] strong broadband optical absorption superior to that Z. L. Wang and W. Wu &&&& www.angewandte.org 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2012, 51, 2 – 24 These are not the final page numbers! found for thin-film structures was observed. The ability to manage photon absorption efficiently through the use of intentionally engineered nanostructures enables significant reduction of the total amount of active materials required for the fabrication of solar cells. The composition, shape, morphology, and dimensions of the nanowires (NWs) and the periodicity of the array can all be manipulated to tailor the absorption spectrum of the as-fabricated solar cell.[64] In a conventional planar thin-film solar cell, the high-quality crystalline film must be thick enough for carriers to be collected and incident photons absorbed along the same axis. Solar cells built on an array of vertically aligned one- dimensional nanostructures, however, can improve the col- lection of the photoinduced electrons by orthogonalizing and separating the paths of light propagation and electron collection, as shown schematically in Figure 4 a. Conse- Figure 4. Schematic illustration of a) charge separation in planar thin- film solar cells and nanowire/nanopillar-based solar cells (from Ref. [63b], Copyright 2009 Nature Publishing Group) and b) an optical- fiber-based three-dimensional solar cell (from Ref. [39b]). ITO = indium tin oxide. quently, the diffusion path for photoexcited minority carriers can be decreased, and the efficient collection of carriers can be made possible even in materials of low crystalline quality.[37a, 63b, 65] This helps relax the stringent requirements in materials manufacturing and thus reduces the cost sub- stantially. One-dimensional nanostructures have been investigated intensively and utilized in all types of PV devices mentioned above.[66] For example, energy-conversion efficiencies of about 10 and 3% have been reported for DSSCs based on TiO2 NWs and ZnO NWs, respectively.[37a,67] A solar cell based on an array of Si NWs with a projected efficiency of Ü ÜPDF Image | Self-Powered Nanosystems
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