Graphene for Transparent Electrodes and Organic Electronic Devices - Organic Solar Cells: Materials and Device Physics - page 81

mobility, high electrical conductivities, and low contact resistance with organic
materials, graphene has been extensively studied in the field of organic electronic
devices [ 2 ], such as organic photovoltaics (OPVs) [ 3 - 5 ], organic light-emitting
diodes (OLEDs) [ 6 , 7 ], field effect transistors (FETs) [ 8 , 9 ], and etc. There are
quite some broad reviews about graphene in many aspects [ 10 - 15 ], Herein, we
will restrict ourselves to summarize and discuss transparent electrodes based on
graphene and application of graphene in the electronic devices, especially our
works for OPVs.
4.2 Graphene for Transparent Electrode
As the essential part of optoelectronic devices, the ideal transparent electrodes
should have high transparency, low sheet resistance, proper work function, and
low preparation cost. Currently, indium tin oxide (ITO) plays a dominant role and
is the market standard for most of transparent electrode applications. However, the
use of ITO as transparent electrode may be limited for its intrinsic drawbacks such
as the costly preparative methods (sputtering, evaporation, pulsed laser deposition,
and electroplating), limited source of indium on the Earth, toxic property of
indium for environment and humans, its intrinsic brittleness property, etc. Thus,
developing new transparent conducting materials has become necessary. Graph-
ene-based transparent electrodes have drawn great attentions owing to its excellent
properties such as high mobility, high transparency, etc. Several excellent reviews
have discussed the updated progress and comprehensive applications of the
transparent electrodes based on graphene [ 16 , 17 ]. Herein, we will give a brief
summarization of the graphene-based transparent electrode for organic electronic
devices, especially for OPVs.
4.2.1 Transparent Electrode Based on rGO
In contrast with graphene based on other preparation methods such as microme-
chanical exfoliation [ 18 ], epitaxial growth [ 19 ], and chemical vapor deposition
(CVD) [ 20 ], reduced graphene oxide (rGO) [ 21 - 23 ] has been studied primarily
owing to the advantages of high throughput preparation, low cost, and the sim-
plicity of the fabrication technique.
In our initial work, we have studied spin-coated GO thin films by different
reduction treatments with hydrazine reduction and/or high-temperature annealing,
producing films with sheet resistances as low as 10 2 -10 3 X/sq with 80 %
transmittance for 550 nm light (Fig. 4.1 a) [ 24 ]. Using the neat rGO film as the
transparent anode, we have demonstrated solution-processed polymer OPVs with
poly-(3-hexylthiophene) (P3HT) as the electron donor and phenyl-C 61 -butyric
acid methyl ester (PCBM) as the acceptor (Fig. 4.1 b) [ 25 ]. After spin-coating