Page 230 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 207

small-molecule materials are also available. 29, 30 The performance of
bulk heterojunction devices is intimately related to the morphology
of the donor/acceptor blends. A fine-structured mixing of the donor
and acceptor materials is preferred for efficient exciton dissociation,
as this enhances the total area of the donor/acceptor interface, and
the excitons consequently have less far to diffuse to reach a dissocia-
tion site. However, the blend morphology also influences the trans-
port of charges back to the collecting electrodes. Ideally, the donor
and acceptor materials should form an interpenetrating network, in
which complete transport pathways from the dissociation site to the
31
relevant electrode exist for both electrons and holes. Breaks or
islands in the network can form trapping sites for charges (see Fig. 6.6),
which can be detrimental to device performance since they increase
the probability of electron-hole recombination.
The morphology can be influenced through a variety of processing
techniques. In the case of small-molecule devices, the morphology
depends primarily on the strength of interactions between individual
deposited molecules and their thermal energy, which together deter-
mine the ease of molecular rearrangement. The morphology may be
controlled to some extent by varying the vapor pressure, the substrate
temperature, and the deposition rates. In the case of solution-processed
polymer devices, the phase separation arises during the deposition of
the solution. 23, 32, 33 As the solvent evaporates and the interchain dis-
tances decrease, the interactions are increased, and the mixture that
exists in solution begins to de-mix to form domains rich in one mate-
rial or the other. This phase separation is eventually arrested when
the interchain interactions become sufficiently strong to prevent fur-
ther rearrangement. The solvent boiling point and polarity, as well as
the ambient conditions during deposition, can all influence the nature
of the morphology, as can substrate surface treatments. For example,
the surface can be treated to attract the donor material to the (anode-
coated) substrate during deposition of the polymer blend solution,
ensuring a proper connection of the hole accepting donor regions to
the anode.
The majority of bulk heterojunction devices comprise a small-
molecule acceptor dispersed in a polymer donor; all-polymer
systems are still relatively uncommon due to the relative scarcity
of good electron transporting polymers. The most widely used
small-molecule acceptors are based on perylenes and solubilized
fullerene derivatives such as [6,6]-phenyl-C61-butyric acid
methyl ester (PCBM, Fig. 6.7g), whereas the donors are often sub-
stituted poly(phenylene-vinylene)s (PPVs) and polythiophenes.
Initial interest in PPVs was driven mainly by their use in organic
light-emitting diodes and the considerable flexibility they pro-
vide in terms of chemical structure engineering. The two most
widely used PPV derivatives to date have been poly[2-methoxy-5-
(3¢,7¢-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV)