RESEARCH DIRECTIONS
Several paths exist toward the realization of photovoltaic devices with efficiency greater than
50%, including multiple junction solar cells (tandems), solar cells using optical frequency
shifting (such as up/down conversion or thermophotonics), multiple exciton generation (MEG)
from a single photon, multiple energy level solar cells (such as intermediate band solar cells),
and hot carrier solar cells (Marti and Luque 2003; Green 2004). In addition to high efficiency,
such cells must also be low in cost, made of polycrystalline thin films grown on inexpensive
substrates.
Multiple Junction Solar Cells
Multiple junction solar cells, or tandem solar cells, consist of multiple, single-junction solar cells
joined together or stacked upon each other, with each solar cell absorbing the part of the solar
spectrum closest to its band gap. Existing tandem devices have achieved efficiencies over 37%
(Green et al. 2003) at a concentration of 173 suns, and further efficiency increases can be
achieved by increasing the number of different junctions. Despite the high efficiency potential,
tandem devices experience a fundamental limitation relating to the availability of materials that
simultaneously allow high efficiency through low defect densities and the choice of optimal band
gaps. In addition to fundamental advances in understanding defects and recombination, exploring
new materials and nanostructures may also revolutionize multiple junction devices by allowing
control over band structure, growth, and defects.
Optical Frequency Shifting
Optical frequency shifting cells involve the
transformation of the solar spectrum from one
with a broad range of energies to one with the
same power density but a narrow range of photon
energies (see Figure 21). One central feature of
these approaches, which include up and down-
conversion (Trupke et al. 2002; Trupke et al.
2002a) (i.e., creating a single high-energy photon
from two lower-energy photons or creating two
lower-energy photons from a single higher-
energy photon, respectively) and
thermophotonics (Green 2004) (i.e., using the
refrigerating action of an ideal light emitting
diode to increase the emitted photon energy), is
that the transformation of the solar spectrum is
done separately with a material that is not part of
the actual solar cell, thus increasing the efficiency of an existing solar cell structure via
additional coatings or external elements. There are several fundamental challenges in these
approaches, including demonstration of cooling due to optical emissions, as well as more
efficient processes for up-conversion.
Figure 21 Schematic for optical frequency
shifting