12 September 2011
The formation of well-controlled junctions tunes the performance of
organic solar cells.
A solar cell made using CFT and a
schematic diagram of the
high-quality donor-acceptor junction
Solar cells made from organic semiconductors rather than silicon are
relatively easy to fabricate but their energy conversion performance
has so far been limited due to the typically poor quality of their
The quality of the ‘donor-acceptor’ junction
between the negatively and positively charged regions is
particularly important in such devices.
Keisuke Tajima and
colleagues at the University of Tokyo in Japan and the Beijing
Institute of Technology in China have now demonstrated an
unprecedented degree of control over this donor-acceptor junction.1
The high internal electric field that arises at the donor-acceptor
junction separates the light-generated free charges in the cell and
allows them to contribute to the production of an electrical
current. If the contact between the two materials that make up the
interface is poor, however, these free charges recombine and are
Tajima and his colleagues used a technique they had previously
developed called contact film transfer (CFT) to form their junction.
CFT involves making and stacking two separate semiconducting films
without high temperatures or pressures, leading to the formation of
a flat, well-defined interface.
The researchers used CFT to fabricate a solar cell using the common
P3HT, and found that its
performance was comparable to that for a device made using
conventional methods. They then added a fluorine-containing
monolayer to one of the semiconductors before it was stacked to
create an electric dipole (above image).
This addition allowed the maximum voltage produced by
the solar cell to be tuned by varying the inserted monolayer
material without changing the combination of bulk materials.
maximum voltage of 0.95 V achieved by Tajima and his team is also
the highest recorded for a PCBM/P3HT-based solar cell.
The combination of CFT and surface monolayers therefore appears to
be an effective means for controlling the physics of charge
separation, with application not only in energy conversion but also
in more fundamental studies of charge separation processes.
A particularly appealing target, says
Tajima, is to control the mixed interfaces found inside bulk heterojunction devices, which currently have the best efficiencies
Tada, A.1, Geng, Y.1,2, Wei,
Q.3, Hashimoto, K.1,3 & Tajima, K.1,3 Tailoring organic heterojunction interfaces in bilayer polymer photovoltaic
devices. Nature Mater. 10, 450–455 (2011). |
1. Department of Applied
Chemistry, School of Engineering, University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
2. Department of Materials, School of Materials, Beijing
Institute of Technology, 5 South Zhongguancun Street,
Haidian District, Beijing 100081, China
3. Hashimoto Light Energy Conversion Project, Exploratory
Research for Advanced Technology, Japan Science and
Technology Agency, Japan