Page 227 - Organic Electronics in Sensors and Biotechnology

204 Cha pte r S i x

In the case of discrete heterojunction devices, only those excitons
generated within a diffusion range of the interface yield free carriers
(since the others are lost by recombination before reaching the inter-
face). Hence, to ensure high device efficiencies, it is important to
select materials in which the diffusion range is comparable to the
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active layer thickness (~100 nm). (Additional blocking layers may be
needed to prevent excitons from diffusing to the electrodes where
they are liable to be quenched.) In the case of bulk heterojunction
solar cells, donor/acceptor interfaces are formed throughout the film
thickness (hence the name), and the main requirement is to match the
length scale of the microscopic blend to the exciton diffusion range to
ensure efficient dissociation. 10, 11
Once dissociated, the electron and hole must be transported to
the cathode and anode, respectively, from where they are extracted
into the external circuit in the form of an electric current. In the case
of discrete heterojunction devices, the holes and electrons pass
through pure donor and acceptor phases, respectively, and they are
therefore channeled efficiently to the electrodes with minimal risk of
recombination (except when they are in close proximity to the donor/
acceptor interface). In the case of bulk heterojunction devices, the two
phases are intimately mixed, and there is consequently an apprecia-
ble risk of electron-hole recombination before the charges reach the
electrodes. To minimize this risk, continuous percolation pathways
are required from the point of generation to the electrodes in order to
shuttle the charge carriers rapidly to the electrodes before they have
an opportunity to recombine (see, however, the discussion in Sec. 6.2).
In spite of the apparent difficulty of avoiding recombination in such
circumstances, a suitably optimized bulk heterojunction device can
exhibit short-circuit quantum efficiencies approaching 100%.

6.3.2 Device Fabrication
The first successful demonstration of an OPV device based on the
discrete heterojunction architecture was reported by Tang in 1986.
12
The active layers consisted of successive vacuum-deposited films of
copper phthalocyanine (the donor, Fig. 6.7a) and a perylene tetracar-
boxylic di-imide (the acceptor, Fig. 6.7b). The device (which, like
many of the OPV devices reported to date, was developed as a solar
cell rather than a photodetector) exhibited a power conversion effi-
ciency of 1 percent under simulated AM2 illumination, a record value
for an organic solar cell at the time and a value that was to remain
unchallenged for several years to come.
The most extensively investigated discrete heterojunction devices
are those based on organic/fullerene bilayers. 11, 13 The first fullerene to
be tested in an OPV application was C (Fig. 6.7c)––a material in which
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the 60 electrons from the p orbitals give rise to a delocalized π system
z
14
similar to that in conjugated molecules and polymers. C has an
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