Solar spectrum conversion for photovoltaics using nanoparticles

The possibility to tune chemical and physical properties in nanosized materials has a strong impact on a variety of technologies, including photovoltaics. One of the prominent research areas of nanomaterials for photovoltaics involves spectral conversion. Conventional single-junction semiconductor solar cells only effectively convert photons of energy close to the semiconductor band gap (Eg) 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). Photons with an energy Eph smaller than the band gap are not absorbed and their energy is not used for carrier generation. Photons with energy Eph larger than the band gap are absorbed, but the excess energy Eph – Eg is lost due to thermalization of the generated electrons. These fundamental spectral losses in a singlejunction 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). 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. 2010), intermediate band gaps (Luque and Marti 1997), multiple exciton generation (Klimov 2006, Klimov et al. 2007), quantum dot concentrators (Chatten et al. 2003a) and down- and up-converters (Trupke et al. 2002a, b), and down-shifters (Richards 2006a, Van Sark 2005). In general they are referred to as Third or Next Generation photovoltaics (PV) (Green 2003, Luque et al. 2005, Martí and Luque 2004). Nanotechnology is essential in realizing most of these concepts (Soga 2006, Tsakalakos 2008), and semiconductor nanocrystals have been recognized as ‘building blocks’ of nanotechnology for use in next generation solar cells (Kamat 2008). 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. 2011), although this is reached at a concentration of 418 times.