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Most of today's commercially available solar cells are made from inorganic
materials such as highly-purified silicon, which makes them expensive and less
competitive with other sources of energy such as coal. The next generation of
solar cells will be light, flexible, attractive and most importantly, cheap,
because they will be made from organic (plastic) materials. Their flexible
lightweight properties will enable them to be deployed over a wide range of new
applications for example furnishings, building components etc. enabling energy
to be generated where it is used.
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Solar cells, also called photovoltaic
(PV) cells by scientists, convert sunlight directly into electricity. PV gets
its name from the process of converting light (photons) to electricity
(voltage), which is called the PV effect. |
Plastic solar cells produced from organic semiconductors offer the potential
to deliver efficient solar energy conversion with low-cost fabrication. A major
challenge to overcome is to improve efficiency, there is a need to develop
light-absorbing materials with efficient charge separation and charge transport
properties and fabricate them into active layers of solar cells with a
controlled nanomorphology.
Similar to devices made from silicon, the light-absorbing layer of organic
photovoltaic (OPV) cells consist of a p-type (electron donor, D) and an n-type
(electron acceptor, A) material, Figure 1. Commonly used p-type organic
semiconductors include polymers based on thiophene building blocks such as P3HT,
PBTTT, PCPDTBT.1 The best n-type materials so far
have been fullerene derivatives such as PC61BM and PDI. This active
layer composite is sandwiched between a transparent anode (e.g. indium tin
oxide, ITO) and a metallic (e. g. aluminium, Al) cathode.
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Figure. 1 Energy
diagram of an organic solar cell showing the processes involved in generating a
photocurrent. Also shown are examples of typical p- and n-type organic
semiconductors. |
In order to generate a charge, the incident light excites an electron in the
donor material from its ground (or highest occupied molecular orbital - HOMO) to
the excited state (or lowest unoccupied molecular orbital - LUMO) and leaves
behind a hole or a positive charge (Step 1 in Fig. 1). This exciton then travels
to the D-A interface, where it undergoes a charge transfer to the LUMO level of
the acceptor (Step 2). Transport of the electron to the electrode and
recombination with the hole through the external circuit produces a photocurrent
(Step 3).
One of the key challenges in organic materials is their inherently low
dielectric constant, resulting in relatively short electron diffusion lengths
compared to inorganic semiconductors.2 In order to
achieve an efficient electron transfer between the donor and the acceptor it is
necessary that the two materials are within 10 nm proximity (Fig. 2A). However,
despite the usual high absorption coefficient of organic dyes, a minimum
thickness of 100 nm is required to maximize the light absorption.
This problem can be overcome by optimizing the interface between the donor
and the acceptor by arranging the two materials in a dispersed or bulk
heterojunction (BHJ) morphology during the fabrication process (Fig. 2B). As a
result, an interpenetrating network is formed consisting of donor- and
acceptor-rich domains with a high interfacial area while providing channels for
charge transport to the electrodes (Fig. 2C).
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Figure. 2 Scheme of
(A) bilayer OPV, structurally similar to traditional inorganic solar cells, (B)
ideal and (C) real BHJ. The arrows indicate pathways for the charge transport to
the electrodes. |
While most prototype solar cells developed in the laboratory are fabricated
by either spin-coating or sublimation, the real potential for future low cost
power generation comes from the fact that those organic semiconductors are able
to be manufactured at high volume through existing commercial reel-to-reel
printing process on large area flexible substrates.3
Scientists and engineers around the world have begun to develop processes to
achieve this goal.
In Australia, researchers from CSIRO's
(Commonwealth Scientific and Industrial Research Organisation) Future Manufacturing Flagship, partnering with national and
international groups, have recently reported successful printing trials in
collaboration with Securency International, a banknote printing company.4 Operating at full speed, the production could be ramped up
to print 200 meters per min or a total of 100 kms per day. Assuming a targeted
10% efficiency, enough solar cells could be printed in five months to generate
the equivalent electricity output of one gigawatt powerstation (Fig. 3).
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Figure. 3 Flexible
organic solar cells on printing press |
Despite all the advantages and enormous potential for low cost power
generation, organic solar cells will need continuing investment and further
development in order to enter the consumer market. In particular, questions
related to morphology and long-term stability of the organic materials have to
be answered. Nevertheless, the great prospects offered by organic solar cells
will lead us on an exciting journey towards environmentally-friendly and
sustainable power generation in the future.
References
1. A.J. Moule and K. Meerholz "Morphology control in
solution-processed bulk-heterojunction solar cell mixtures" Advanced Functional
Materials 19 (2009) 3028-3036
2. B.A. Gregg and M.C. Hanna
"Comparing organic to inorganic photovoltaic cells: Theory, experiment, and
simulation" Journal of Applied Physics 93 (2003) 3605-3614
3.
F.C. Krebs "Fabrication and processing of polymer solar cells: A review of
printing and coating techniques" Solar Energy Materials & Solar Cells 93
(2009) 394-412
4. Media release: http://www.csiro.au/news/Trials-for-printable-plastic-solar-cells.html
(last accessed October 2009)
Copyright AZoNano.com, Dr. Matthias Haeussler (CSIRO)