Multijunction Multiphoton Devices
Multijunction nanostructured injection solar cells can be designed by appropriate choice of the
absorber to absorb and quantitatively convert incident photons to electric current in selective
spectral regions of the solar emission, while maintaining high transparency in the remaining
wavelength range (see Figure 33). Absorbers (sensitizers in the case of dye-sensitized solar cells;
other inorganic or organic compounds in the cases of nanowires, nanorods, nanocylinders, or
organic bulk heterojunctions) with appropriate excitation energies and charge injection properties
will need to be developed and characterized. Also, there will be a need to determine the
fundamental factors influencing the incident photon-to-current conversion efficiency of the
sensitized layer. The conditions for forming appropriate multilayered structures by techniques
such as screen-printing, to facilitate the fabrication and optimization of multijunction structures,
will need to be developed and studied.
Figure 33 Quadruple junction solar cell comprising sensitized mesoscopic
oxides of different color or thin-film photovoltaic cells as light-absorbing
layers. The theoretical conversion efficiency of such a device is close to 50%.Multiple Charge Carrier Generation
Calculated thermodynamic efficiency limits in single-junction solar cells (~32%) assume that
absorption of an individual photon results in the formation of a single electron-hole pair and that
all photon energy in excess of the energy gap is lost as heat. This limit, however, can be
surpassed via multiple exciton (electron-hole pair) generation (MEG) by single-photon
absorption as was predicted (Nozik 2002) and observed optically in PbSe and PbS quantum dots
(Schaller and Klimov 2004; Ellingson et al. 2005). The ability to generate multiple charge
carriers upon absorption of one photon could lead to greatly enhanced photocurrent and,
ultimately, to very high efficiency solar cells.
SCIENTIFIC CHALLENGES
To move efficiencies towards a target of 50%, it is vital to address a number of fundamental
scientific issues that include
- Control of nanoarchitecture,