Graphene for Transparent Electrodes and Organic Electronic Devices - Organic Solar Cells: Materials and Device Physics - page 86

incorporate a conductive material into them. Chen et al. have reported a hybrid
material prepared from graphene and poly(3,4-ethyldioxythiophene) (PEDOT)
[ 37 ], which showed good transparency and electrical conductivity flexibility
together with high thermal stability and was easily processed in both water and
organic solvents (Fig. 4.3 a, b). Conductivities of 0.2 S/cm and light transmittance
of greater than 80 % in the 400-1800 nm wavelength range were observed for
films with thickness of tens of nm. In the view of the vacancies and topological
defects on the rGO sheet owing to the release of oxygen-containing functional
groups, another strategy to improve the conductivity is to repair the defects. Liu
et al. reported a method for real-time repair of the newborn vacancies by intro-
ducing carbon radicals in the thermal annealing process via a rapid-heating process
(Fig. 4.3 c-f) [ 38 ]. The sheet conductivity of thus-obtained single-layer graphene
was raised more than sixfold to 350-410 S/cm (while retaining [96 % transpar-
ency). This method provides a simple and efficient process for obtaining highly
conductive transparent graphene films. Considering the high conductivity of car-
bon nanotubes, Tung et al. reported a hybrid nanocomposite comprised rGO and
carbon nanotubes (G-CNT) (Fig. 4.3 g) [ 39 ]. By introducing CNTs, the conduc-
tivity of the hybrid material was enhanced, while sacrificing little in transparency.
G-CNT film by spin coating with sheet resistance of 240 X/sq and 86 % trans-
mittance was obtained. In addition, G-CNT hybrid film exhibited better mechan-
ical stabilities than ITO. In a comparison experiment, after bending to 60 more
than 10 times, the resistance of the brittle ITO film increased by three orders of
magnitude, while the G-CNT electrode remained nearly unchanged. P3HT:PCBM
BHJ device using G-CNT hybrid material as the transparent electrode exhibited a
PCE of 0.85 %. The J sc , V oc , and FF were 3.47 mA/cm 2 , 0.583 V, and 0.42,
respectively. The low J sc and FF are likely due to poor contact at the interface
between the G-CNT and the polymer blend.
4.2.4 Tuning the Work Function of Graphene-Based
Transparent Electrode
In organic electronic devices, work functions of electrodes play an important role
and have to be tuned to minimize the energy barriers for charge injection/extraction
and improve the device performance. For example, in the common OPV device,
the work function of ITO anode should be improved to match with the HOMO of
the donor. UV-ozone treatment can improved the work function of ITO, however,
a PEDOT:PSS layer with high work function (5.2 eV) are still need to facilitate the
hole transfer as well as improve the surface quality. For graphene, its work function
is presumed to be 4.5 eV [ 40 ]. However, the work function of graphene is variable
according to the different sizes, layer structures, functionalizations, doping, and
surface modification. Recently, Hang et al. reported to use alkali carbonates to
dope the rGO-SWCNT composites and modify their work functions [ 41 ]. The
doping and work functions were characterized by XPS (Fig. 4.4 a). A clear trend