Page 269 - Organic Electronics in Sensors and Biotechnology

246 Cha pte r S i x

The second problem is that plastics suffer from significant ther-
mal shrinkage and are therefore only suited to low-temperature
processing methods. ITO by contrast should ideally be deposited and
71
annealed above 350°C to achieve optimal film quality. The deposi-
tion of ITO on plastic substrates must be carried out at significantly
lower temperatures, resulting in porous films with lower conductiv-
ity, reduced transparency and poorer substrate adhesion, which leads
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ultimately to reduced device quality. These issues, together with the
tendency of ITO to crack when the substrate is bent, have led research-
ers to seek alternative anode materials for flexible applications. A
variety of materials systems have been proposed including other
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metal oxides, thin metallic films, polymer-metal composites, 71
73
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polymer-fullerene composites, and conducting polymers. 75–83 Con-
ducting polymers are especially appealing because they can be depos-
ited over large areas using printing methods, with obvious potential
for reel-to-reel implementation. There are still considerable issues to
be addressed before conducting polymers become a suitable replace-
ment for ITO––most notably the need for improved transparency and
conductivity––but materials such as polyaniline and PEDOT:PSS
already show considerable promise as polymeric anodes. PEDOT:
PSS is used extensively as an antistatic coating in the photographic
film industry, where it is deposited from water-based solution by roll-
to-roll coating.
In recent work, Huang et al. 81, 83 have reported efficient ITO-free
P3HT:PCBM based organic photodiodes using flexible polyethylene-
terephthalate (PET) substrates coated with PEDOT:PSS (Fig. 6.27). The
devices were fabricated by depositing a patterned layer of PEDOT:PSS
directly on a PET substrate, followed by a spin-coated film of the
donor/acceptor blend, and finally a thermally evaporated aluminum
cathode. The devices had high external quantum efficiencies of 58%,
comparable to similar devices fabricated on rigid glass substrates, indi-
cating that PEDOT:PSS is a credible replacement for ITO in organic
photodetectors. Importantly, the devices had low dark currents of
†
1.5 pA/mm under near short-circuit conditions, slightly lower than
2
comparable devices fabricated on ITO-coated glass. Moreover, they
exhibited very low dark currents under reverse bias, rising to just
0.12 nA/mm at a reverse bias of 1 V––nearly three orders of magni-
2
tude lower than a comparable device on ITO-coated glass.

† PEDOT:PSS is currently less suitable as an anode material for organic solar cells
.
84
due to its relatively high resistivity (e.g., 1 Ω cm for the Baytron P formulation ),
which causes significant internal energy dissipation in the PEDOT:PSS contact.
However, new materials systems such as vapor-phase polymerized PEDOT 85
(which eliminates the need for insulating PSS) are starting to challenge indium tin
oxide in terms of conductivity and transparency. These efforts may ultimately lead
to high-efficiency solar cells based on PEDOT or a similar conducting polymer.