In addition, so-called luminescent solar converters (LSC) employ spectrum modification as well ( Goetzberger 2008, Goetzberger and Greubel 1977). 1, as a relatively easy and cost-effective means to enhance conversion efficiency. Modification of the spectrum by means of so-called down- and/or upconversion or -shifting is presently being pursued for single junction cells ( Richards 2006a), as illustrated in Fig. However, as these cells only contain a thin absorber layer, the optimum spectrum response occurs at about 550 nm ( Schropp and Zeman 1998, Van Sark 2002 ).

For (multi)crystalline silicon ((m)c-Si) solar cells О» opt= 1100 nm (with E g= 1.12 eV) for amorphous silicon (a-Si:H) the optimum wavelength is О» opt= 700 nm (with E g= 1.77 eV). Such a quasi-monochromatic solar cell could in principle reach efficiencies over 80%, which is slightly dependent on band gap ( Luque and MartГ 2003 ). 2011), although this is reached at a concentration of 418 times.ДЄs single-junction solar cells optimally perform under monochromatic light at wavelength О» opt~ 1240/ E g(with О» optin nm and E gin eV), an approach вЂњsqueezingвЂќ the wide solar spectrum (300-2500 nm) to a single small band spectrum without too many losses would greatly enhance solar cell conversion efficiency. Being the most mature approach, it is not surprising that the current world record conversion efficiency is 43.5% for a GaInP/GaAs/GaInNAs solar cell ( Green et al. Nanotechnology is essential in realizing most of these concepts ( Soga 2006, Tsakalakos 2008), and semiconductor nanocrystals have been recognized as вЂbuilding blocksвЂ™ of nanotechnology for use in next generation solar cells ( Kamat 2008). In general they are referred to as Third or Next Generation photovoltaics (PV) ( Green 2003, Luque et al. 2003a) and down- and up-converters ( Trupke et al. 2007), quantum dot concentrators ( Chatten et al.

2010), intermediate band gaps ( Luque and Marti 1997), multiple exciton generation ( Klimov 2006, Klimov et al. Several routes have been proposed to address spectral losses, and all of these methods or concepts obviously concentrate on a better exploitation of the solar spectrum, e.g., multiple stacked cells ( Law et al. These fundamental spectral losses in a single-junction silicon solar cell can be as large as 50% ( Wolf 1971), while the detailed balance limit of conversion efficiency for such a cell was determined to be 31% ( Shockley and Queisser 1961). Photons with energy E phlarger than the band gap are absorbed, but the excess energy E phвЂ“ E gis lost due to thermalization of the generated electrons. Photons with an energy E phsmaller than the band gap are not absorbed and their energy is not used for carrier generation.

Conventional single-junction semiconductor solar cells only effectively convert photons of energy close to the semiconductor band gap ( E g) as a result of the mismatch between the incident solar spectrum and the spectral absorption properties of the material ( Green 1982, Luque and Hegedus 2003). One of the prominent research areas of nanomaterials for photovoltaics involves spectral conversion. The possibility to tune chemical and physical properties in nanosized materials has a strong impact on a variety of technologies, including photovoltaics.

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