Chemistry Reference
In-Depth Information
Fig. 29 HBC derivatives with controlled liquid crystalline mesophases and self-assembly
[
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inevitably causes low device performance. There are two types of alignment of
columnar discs on substrates: homogeneous edge-on (column axis parallel to a
substrate) and homeotropic face-on (column axis perpendicular to a substrate).
Generally, for FETs an edge-on organization is required to allow charge carriers to
drift along the column axis direction from the source electrode to the drain electrode
under controlled gate voltage. However, for photovoltaic devices the face-on arrange-
ment of the discs is preferred that allows faster charge transport between the top and
bottom electrodes and favors the photovoltaic performance. Furthermore, the donor
columns are accompanied by homeotropically aligned acceptor columns in photo-
voltaic devices. Techniques including zone-casting, solution-casting on preoriented
and friction-deposited poly(tetrafluoroethylene) (PTFE) surfaces, and Langmuir and
Langmuir-Blodgett (LB) techniques are developed to induce uniform order in the
alignment of the discs and improve device performance.
Many examples are available of using HBC derivatives in organic electronics,
and one representative example is Schmidt-Mende's report, in which HBC 155 was
used as the donor while perylene diimide was used as the acceptor to build a bulk
heterojunction (BHJ) device that leads to an external quantum efficiency of 34% at
490 nm and a power efficiency up to 2%. This remarkable device performance for
discotic small molecules is due to efficient photo-induced charge transfer between
the HBC and PDI, as well as effective transport of charges through vertically
segregated perylene and HBC aromatic systems [
113
]. Another example is the
introduction of 9,9-dioctylfluorenyl moiety at the periphery of HBC as replacement
of alkyl chains, which always show negative effects on charge transport in the bulk
