by Professor Kian Ping Loh
Transparent and conducting electrodes are needed for applications in solar
cells and energy conversion platform like water splitting. To date, there are
not many types of transparent and conducting electrodes that can be mass
produced cheaply. Available on the market are electrode materials like Indium
Tin Oxide (ITO) and Fluorine doped tin oxide. The diminishing supply of indium
and its rising cost motivates scientist to search for an alternative electrode
material. Moreover, ITO is fragile and can neither be used in flexible
electronics nor thermally processed at high temperature.
A soft membrane type material that is mechanically tough and flexible is
needed. Graphene is a single layer of carbon sheet with the carbon atoms
interconnected in a crystalline honeycomb network. It turns out that highly
conducting, ultrathin graphene films can be a good substitute for ITO in all
carbon-based flexible electronics because of its transparency and flexible
Graphene has been extensively studied due to its unique electronic and
mechanical properties as well as its eagerly projected role in the all-carbon
post CMOS technological revolution.1-3 Its
two-dimensional (2D) aromatic sheet structure as well as its high conductivity,
transparency, mechanical strength and flexibility, imparts great advantages on
graphene as a candidate material for the development of "plastic electronics."
In principle cost should not be a major issue for the production of graphene
since it can be produced by chemical vapor deposition (CVD) using methane as the
gas feed, diluted in hydrogen or argon.4,5 Large area growth and roll to roll processing of graphene
is now entering into the first stage of commercial production. CVD-deposited
graphene films can be transferred onto glass to generate a new generation of
transparent and conducting electrodes. Due to its flexible and sensitive
characteristics, graphene membrane can be used in touch screen panels of
Recently, Professor Kian Ping Loh and his colleagues at the Department of
University of Singapore fabricated large-area, continuous, highly
transparent and conducting multilayer graphene films with sheet resistance of
200 ¦¸/square by chemical vapor deposition (CVD) method.6 The CVD grown graphene film can be readily transferred
onto glass using polydimethylsiloxane (PDMS) stamp approach and was used as the
anode for application in organic photovoltaic cells.
Figure 1. CVD
deposited graphene can be used as transparent anode in organic solar cell,
offering the advantage of flexibility, transparency and high electrical
After non-covalent modification with an organic molecule known as pyrene
buanoic acid succidymidyl ester (PBASE), the power conversion efficiency (PCE)
of the organic solar cells increased from 0.21% of the unmodified films to 1.71
%. This performance corresponds to ~55.2 % of the PCE of an identical device
made with indium tin oxide (ITO) anode, e.g., ITO/PEDOT-PSS/P3HT/PCBM/Al
(PCE=3.1%). This finding paves the way for the substitution of the ITO anode
with low cost graphene film in photovoltaic and electroluminescent devices.
Besides chemical vapor deposition of graphene, graphene derivatives can also
be solution-processed.7,8 Chemists
usually used the oxidized form of graphene, known as graphene oxide,7 or generate graphene from graphite using
intercalation/exfoliation methods. These graphene derivatives show wide ranging
solubility in a range of solvents depending on their preparation methods.
Solution processing allows graphene to be spin coated or ink-jet printed on any
substrates, this is very useful for developing flexible all-carbon electronics
circuit on flexible substrates.
Professor Kian Ping Loh and his colleagues have developed high mobility,
printable carbon circuit using solution-processed graphene recently.9 Such type of all-carbon based electronics can be thermally
processed at temperature as high as 1000¡ãC in vacuum or non-oxidizing
environments. The solution-processability of graphene derivatives allows the
fabrication of inorganic-graphene or organic-graphene composites10 to be achieved readily using wet chemistry methods.
Graphene hybrid materials, e.g. graphene coated by quantum dots or infra-red
dyes, should demonstrate enhanced performance in photovoltaics. The enhancement
of photocurrent generation arises from the efficient dissociation of exciton at
the graphene-organic dye or graphene-inorganic semiconductor interface, as well
as the increase in spectral absorption bandwidth due to the extended conjugation
present in graphene.
NRF-CRP grant "Graphene Related Materials and Devices, R-143-000-360-281
1. Geim, A. K.; Novoselov, K. S. Nat.
Mater. 6, 183 (2007).
2. Novoselov, K.
S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.;
Grigorieva, I. V.; Firsov, A. A. Science, 306, 666(2004).
3. Rycerz, A.; Tworzydlo, K. J.; Beenakker, C. W. J.; Nat.
Phys. 3, 172-175 (2007).
4. K. S. Kim, Y. Zhao, H. Jang, S. Y.
Lee, J. M. Kim, Kwang, S. Kim, J. H. Ahn, P. Kim, J. Y. Choi, B. H. Hong, Nature
457, 760 (2009).
5. Xuesong Li, Weiwei Cai, Jinho An, Seyoung
Kim, Junghyo Nah, Dongxing Yang, Richard Piner,1 Aruna Velamakanni, Inhwa Jung,
Emanuel Tutuc, Sanjay K. Banerjee, Luigi Colombo, Rodney S. Ruoff, Science,
2009, 324, 1312.
6. Yu Wang, Xiaohong Chen, Yulin Zhong,
Furong Zhu and Kian Ping Loh, Appl. Phys. Letts. 95, 063302 (2009)
7. Daniel R. Dreyer, Sungjin Park, Christopher W. Bielawski and
Rodney S. Ruoff, Chem. Soc. Rev., 39, 228 (2010)
8. Goki Eda,
Giovanni Fanchini, and Manish Chhowalla, Nature Nanotechnology 3 270-274
9. Wang SA, Ang PK, Wang ZQ, Kian Ping Loh, Nano Lett.,
10, 92 (2010).
10. Xuan Wang, Linjie Zhi, and Klaus M¨¹llen,
Nano Lett., 8, 333 (2008)
Copyright AZoNano.com, Professor Kian Ping Loh (National
University of Singapore)