University of California, Berkeley, chemists have found a way to make cheap plastic solar cells that are flexible enough to paint onto any surface and potentially able to provide electricity for wearable electronics or other low-power devices.
The chemists’ first crude solar cells are said to have achieved efficiencies of 1.7 percent. The best solar cells, which are very expensive semiconductor laminates, convert, at most, 35 percent of the sun’s energy into electricity.
The solar cell created at Berkeley is a hybrid, comprised of tiny nanorods dispersed in an organic polymer or plastic. A layer only 200 nanometers thick is sandwiched between electrodes, and can currently produce about 0.7 volts. The electrode layers and nanorod/polymer layers could be applied in separate coats, simplifying production. Unlike today’s semiconductor-based photovoltaic devices, plastic solar cells can be manufactured in solution in a beaker without the need for clean rooms or vacuum chambers.
‘Today’s high-efficiency solar cells require very sophisticated processing inside a clean room and complex engineering to make the semiconductor sandwiches,’ said A. Paul Alivisatos, professor of chemistry at UC Berkeley and a member of the Materials Science Division of Lawrence Berkeley National Laboratory. ‘And because they are baked inside a vacuum chamber, they have to be made relatively small.’
The team’s process for making hybrid plastic solar cells involves none of this. ‘We use a much dirtier process that makes it cheap,’ said graduate student Wendy U. Huynh.
The technology reportedly takes advantage of recent advances in nanotechnology, specifically the production of nanocrystals and nanorods pioneered by Alivisatos and his laboratory colleagues. These are chemically pure clusters of from 100 to 100,000 atoms with dimensions on the order of a nanometer. Because of their small size, they exhibit unusual and interesting properties governed by quantum mechanics, such as the absorption of different colours of light depending upon their size.
Huynh and post-doctoral fellow Janke J. Dittmer manufactured nanorods in a beaker containing cadmium selenide, aiming for rods of a diameter of 7 nanometers to absorb as much sunlight as possible. They also aimed for nanorods as long as possible – in this case 60 nanometers.
They then mixed the nanorods with a plastic semiconductor, called P3HT – poly- (3-hexylthiophene) – and coated a transparent electrode with the mixture. The thickness, 200 nanometers is a factor of 10 less than the micron-thickness of semiconductor solar cells. An aluminium coating acting as the back electrode completed the device.
The nanorods act like wires. When they absorb light of a specific wavelength, they generate an electron plus an electron hole – a vacancy in the crystal that moves around just like an electron. The electron travels the length of the rod until the aluminium electrode collects it. The hole is transferred to the plastic, which is known as a hole-carrier, and conveyed to the electrode, creating a current.
P3HT and similar plastic semiconductors currently are a hot area of research in solar cell technology, but by themselves these plastics are reportedly fortunate to achieve light-conversion efficiencies of several percent.
‘All solar cells using plastic semiconductors have been stuck at two percent efficiency, but we have that much at the beginning of our research,’ Huynh said. ‘I think we can do so much better than plastic electronics.’
‘The advantage of hybrid materials consisting of inorganic semiconductors and organic polymers is that potentially you get the best of both worlds,’ Dittmer added. ‘Inorganic semiconductors offer excellent, well-established electronic properties and they are very well suited as solar cell materials. Polymers offer the advantage of solution processing at room temperature, which is cheaper and allows for using fully flexible substrates, such as plastics.’
They also hope to tune the nanorods to absorb different colours to span the spectrum of sunlight. An eventual solar cell might have three layers; each made of nanorods that absorb at different wavelengths.