Page 266 - A Comprehensive Guide to Solar Energy Systems

Chapter 12 • Organic Photovoltaics 269

absorb visible and near-Ir light [52,53]. An alternative approach to increasing the effi-
ciency of light harvesting is to use two different donors with complimentary absorption
spectra (or two acceptors with complimentary absorption spectra) in the same bulk het-
erojunction [51,54–56]. despite the additional complexity in the morphology of the bulk
heterojunction layer, this approach has proved to be remarkably effective.
even for today’s best performing OPVs, it is recognized that there is considerable scope
for reducing losses in V oc , not only by engineering the energy offset at the organic hetero-
junction to the absolute minimum required for Frenkel exciton dissociation [44], but also
by improving the degree of crystallinity in both donor and acceptor phases to minimize
the energy cost in dissociating the charge transfer exciton [42]. A different approach to
minimizing the energy cost of splitting the exciton and recombination losses is to increase
the dielectric constant (ε r ) of both the donor and acceptor materials, which reduces the
coulombic energy of attraction between the electron and hole and thus the energy need-
ed to separate them. Computational analysis predicts that a power conversion efficiency
of ∼20% is within reach if the ε r can be increased from current values of 2–4 up to ∼7.5
[42,57]. notably, increasing ε r to ∼9 would reduce the exciton binding energy to an in-
significant level such that a donor/acceptor heterojunction would no longer be required
[42]. One way to increase the dielectric constant, well matched to solution processable
organic semiconductors, is to modify the solubilizing side chains so that they include po-
lar bonds or dipolar terminating moieties that are free to rotate in response to an electric
field [57,58].
Beyond engineering the photoactive layer in OPVs, improvements in device efficiency
will also result from advances in antireflective coatings and high-performance substrates
and electrodes with refractive indices matched to that of the organic semiconductors. An-
other rapidly advancing area is in the development of universal strategies for maximiz-
ing light absorption in OPVs by light trapping using nanophotonic or plasmonic electrode
structures [59]. One approach that has received relatively little attention to date, but is
actually very well matched to OPV device architectures, is to trap the incident light as
plasmonic excitations (i.e., collective oscillations of the conduction band electrons) at the
surface of a nanostructured metal electrode [60,61] (Fig. 12.10). Similar to metal nanopar-
ticles, nano-holes in a metal film can have a very large absorption cross-section, which
enables strong coupling with incident light, while having the advantage of being confined
to the plane of the electrode and so not undermining the electrical integrity of the diode.
due to the very large absorption coefficient of organic semiconductors used to harvest
light in OPVs, light trapped as plasmonic excitations at the surface of one (or both) elec-
trodes in an OPV can be used to directly excite electrons from the HOmO to the lumO
in an adjacent organic semiconductor, before the plasmonic excitations dissipate their
energy as heat in the metal. While research in this area is at an early stage, this approach
is very attractive because it offers a universally applicable means of increasing J sc without
increasing the complexity of device fabrication. For commercial viability, the process used
to nanostructure the electrode must be low cost and easily scalable, ruling out the use of
conventional lithographic methods for the formation of a high density of nanoapertures.