A new coating for solar
panels
could lead to more efficient solar collection.
The energy from sunlight falling on only 9 percent of California’s Mojave
Desert could power all of the United States’ electricity needs if the energy
could be efficiently harvested, according to some estimates. Unfortunately,
current-generation solar
cell
technologies are too expensive and inefficient for wide-scale commercial
applications.
A team of Northwestern University researchers has developed a new anode
coating strategy that significantly enhances the efficiency of solar energy
power conversion. A paper about the work, which focuses on “engineering”
organic material-electrode interfaces in bulk-heterojunction organic solar
cells, is published online this week in the Proceedings of the National Academy
of Sciences (PNAS).
This breakthrough in solar
energy
conversion promises to bring researchers and developers worldwide closer to the
goal of producing cheaper, more manufacturable and more easily implemented solar
cells. Such technology would greatly reduce our dependence on burning fossil
fuels for electricity production as well as reduce the combustion product:
carbon dioxide, a global
warming
greenhouse gas.
Tobin J. Marks, the Vladimir N. Ipatieff Research Professor in Chemistry in
the Weinberg College of Arts and Sciences and professor of materials science and
engineering, and Robert Chang, professor of materials science and engineering in
the McCormick School of Engineering and Applied Science, led the research team.
Other Northwestern team members were researcher Bruce Buchholz and graduate
students Michael D. Irwin and Alexander W. Hains.
Of the new solar energy conversion technologies on the horizon, solar cells
fabricated from plastic-like organic materials are attractive because they could
be printed cheaply and quickly by a process similar to printing a newspaper
(roll-to-roll processing).
To date, the most successful type of plastic photovoltaic cell is called a
“bulk-heterojunction cell.” This cell utilizes a layer consisting of a mixture
of a semiconducting polymer (an electron donor) and a fullerene (an electron
acceptor) sandwiched between two electrodes -- one a transparent electrically
conducting electrode (the anode, which is usually a tin-doped indium oxide) and
a metal (the cathode), such as aluminum.
When light enters through the transparent conducting electrode and strikes
the light-absorbing polymer layer, electricity flows due to formation of pairs
of electrons and holes that separate and move to the cathode and anode,
respectively. These moving charges are the electrical current (photocurrent)
generated by the cell and are collected by the two electrodes, assuming that
each type of charge can readily traverse the interface between the
polymer-fullerene active layer and the correct electrode to carry away the
charge -- a significant challenge.
The Northwestern researchers employed a laser
deposition technique that coats the anode with a very thin (5 to 10 nanometers
thick) and smooth layer of nickel oxide. This material is an excellent conductor
for extracting holes from the irradiated cell but, equally important, is an
efficient “blocker” which prevents misdirected electrons from straying to the
“wrong” electrode (the anode), which would compromise the cell energy
conversion efficiency.
In contrast to earlier approaches for anode coating, the Northwestern nickel
oxide coating is cheap, electrically homogeneous and non-corrosive. In the case
of model bulk-heterojunction cells, the Northwestern team has increased the cell
voltage by approximately 40 percent and the power conversion efficiency from
approximately 3 to 4 percent to 5.2 to 5.6 percent.
The researchers currently are working on further tuning the anode coating
technique for increased hole extraction and electron blocking efficiency and
moving to production-scaling experiments on flexible substrates.