Nanostructured materials for improved photoconversion
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Introduction Current photovoltaic (PV) technologies are largely based on single-crystalline or polycrystalline silicon films (firstgeneration PV). Typical conversion efficiencies of silicon-based commercial modules range between 10% and 15%, at a cost of $3–$4/Wp (Watt peak).1 Thin-film semiconductors, such as amorphous Si, CdTe, and copper-indium-gallium-selenide (CIGS) (second-generation PV) have lower cost, but also lower conversion efficiency.1 Over the last decade, the need to make solar energy more competitive with conventional energy sources has driven the investigation of a class of new photoconversion technologies, often collectively referred to as third-generation PV, that have both lower cost and improved efficiency compared to existing PV technologies. One particularly ambitious goal of third-generation PV is to exceed the Shockley-Queisser efficiency limit (31% at one-sun illumination). This upper limit was derived2 under a set of assumptions that normally are valid for conventional single-junction solar cells but may not apply to novel materials or novel architectures, such as those proposed for third-generation PV.
Among third-generation PV technologies, nanostructured materials offer a number of potentially useful properties: (1) The ability to tune the bandgap and the absorption spectrum by changing the size, shape, and surface termination of the nanostructures3 could enable the realization of multijunction solar cells made from a single semiconductor material. (2) The ability to synthesize Type II core/shell nanocrystals4 and nanowires5 (in which the core and the shell have a staggered bandgap alignment) opens the way to achieve exciton dissociation without the need for an applied external field. (3) Efficient multiple-exciton generation (MEG) upon absorption of a single high-energy photon (Figure 1) by semiconductor nanocrystals6–8 or carbon nanotubes9 could enhance the photocurrent, and thus the photoconversion efficiency, by reducing the fraction of high-energy photons whose excess energy is lost to heat. (4) The existence of a phonon bottleneck that hinders carrier relaxation (Figure 1) could enable hot-carrier transport in nanostructured solar cells, thereby potentially increasing the open-circuit voltage.10 Although many experimental studies11,12 have shown that carrier relaxation is very fast in conventional
Alberto Franceschetti, National Renewable Energy Laboratory, USA; [email protected] DOI: 10.1557/mrs.2011.35
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MRS BULLETIN • VOLUME 36 • MARCH 2011 • www.mrs.org/bulletin
© 2011 Materials Research Society
NANOSTRUCTURED MATERIALS FOR IMPROVED PHOTOCONVERSION
electronic-structure and quantum-chemistry codes. During the last decade, however, significant progress has been made in the application of both density-functional methods and semi-empirical methods to nanostructured materials. DFT has long been the method of choice for first-principles band structure and total-energy calculations of real materials systems. Recent computational and algorithmic developments, coupled with th
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