Page 229 - Organic Electronics in Sensors and Biotechnology

206 Cha pte r S i x

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polymer and molecular donors. Sariciftci et al. demonstrated the first
such device in 1993, using soluble MEH-PPV (Fig. 6.7e) as the donor
material, and they reported a 20-fold improvement in photocurrent rela-
tive to devices without C . Although C is the most studied of the
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molecular acceptor materials, alternative acceptors have also been inves-
tigated in the context of heterojunction OPV devices. Perylenes (Fig. 6.7b)
have relatively high electron affinities and are known for their photocon-
ductive properties, having been widely exploited in xerographic appli-
cations. Indeed, in many xerographic devices a bilayer photoconductor
is used, with the perylene in contact with a hole transport layer. In
contrast to most organic dyes, perylenes are very stable and are com-
monly used as colorants for paints and plastics. Moderately efficient
double-layer cells with peak external quantum efficiencies greater than
10% have been fabricated using bilayers of pentacene (Fig. 6.7d) and
perylenes. 20
Applying the heterojunction approach to solution-processable poly-
mers is more of a challenge, since the process of depositing the sec-
ond layer is liable to dissolve and wash away the first layer. One
solution is to find materials that are soluble in different ‘‘orthogonal’’
solvents, which allows the acceptor layer to be deposited without dis-
turbing the predeposited donor layer. An alternative strategy is to
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use a donor material that is prepared via a thermal conversion route,
such as poly(p-phenylenevinylene) (PPV), which is subsequently
rendered insoluble by curing. There is increasing interest in the ability
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of certain materials to self-organize into relatively complex struc-
tures, a phenomenon with the potential to significantly reduce manu-
facturing costs by in effect allowing devices to “build” themselves.
For instance, if two polymers with differing polarities are dissolved
in a common solvent, they may sometimes stratify into discrete layers
when deposited onto a suitably treated substrate, allowing a bilayer
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structure to be formed in a single deposition step. Alternatively, het-
erojunction OPV devices may be fabricated by “lamination” of two
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organic layers. In this approach, the thin films of the donor and
acceptor layers are deposited onto separate anode- and cathode-
coated substrates, respectively, and the layers are subsequently fused
together under heat and pressure. Adhesion between the organic lay-
ers may be promoted by mixing a small quantity of the electron
acceptor into the hole acceptor and vice versa before deposition.
The above techniques notwithstanding, the vast majority of solution-
processed solar cells and photodetectors are based around the bulk
heterojunction architecture, in which a composite layer of the two
materials is cast directly from solution. 25–27 For polymer systems, the
donor and acceptor polymers can be simply mixed together in the
same solvent and deposited by, e.g., spin-coating or printing as with
a simple single-layer device. For small-molecule devices, the distrib-
uted architecture can be achieved through codeposition of donor and
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acceptor materials in vacuum, although some solution-processable