Environmental Engineering Reference
In-Depth Information
light-emitting diodes, organic thin-film transistors, and organic photodiodes have
been demonstrated for practical applications in various fields [
1
-
4
]. More recently,
organic photovoltaic devices (OPVs) have been recognized as promising tech-
nologies for the utilization of solar energy because of their light weight, flexibility,
and cost-effective production with simple processability [
5
-
9
]. Furthermore, the
energy payback time of OPVs is amazingly short, relative to that of photovoltaic
cells based on silicon or other inorganic semiconductors, because they can be
fabricated at low temperatures [
9
]. In addition, OPV technologies employ non-
toxic, earth-abundant materials in the manufacturing process. At present, OPVs
based on conjugated polymers are exhibiting power conversion efficiencies (PCEs)
of up to approximately 10 % [
8
]. Although such efficiency remains low relative to
that of commercial inorganic cells, recent rapid improvements in efficiency have
generated considerable interest in OPVs for practical and truly low-cost energy
production.
Most of the high-efficiency OPVs reported to date have been fabricated based
on the bulk heterojunction (BHJ) concept, where a conjugated polymer (donor)
and a soluble fullerene (acceptor) form an interpenetrating network possessing a
large donor-acceptor interfacial area. Various approaches have been proposed to
achieve high efficiencies from BHJ devices. In this chapter, we provide an
introduction to the development of BHJ OPVs, especially for those made of
conjugated-polymer materials. We first discuss the basic fundamental principles
behind OPVs and then review some recent progress in the field, focusing on
methods for optimizing the morphologies of the photoactive layers through various
annealing treatments. We also review the engineering of the interface and optical
effects in the devices. We then outline the two most common optical methods for
improving the light absorption efficiency of OPVs: the use of optical spacers and
the triggering of surface plasmons. Finally, we examine the development of low-
band-gap polymers for the absorption of long-wavelength photons from solar
irradiation. Because our main purpose for this chapter is to introduce the basic
concepts and recent development of OPVs, rather than completely reviewing all of
the literature in this field, by necessity we have had to overlook many outstanding
contributions.
1.2 Basic Principles
Figure
1
a illustrates the typical device architecture of an OPV. The photoactive
layer, consisting of donor and acceptor materials, is sandwiched between the anode
and cathode. The working principle of an OPV can be divided into six processes:
(i) photon absorption, (ii) exciton generation, (iii) exciton diffusion, (iv) exciton
dissociation, (v) charge transport, and (vi) charge collection (Fig.
1
b) [
10
]. Upon
the absorption of photons in an organic material, excitons are generated. The
exciton binding energy in an organic material is, however, usually very large
relative to that of an inorganic semiconductor. Therefore, excitons in organic
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