BERKELEY, Calif., March 28 -- A new generation of solar cells that combines nanotechnology with plastic electronics has been launched with the development of a semiconductor-polymer photovoltaic device by researchers with the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley (UCB). Such hybrid solar cells will be cheaper and easier to make than their semiconductor counterparts, and could be made in the same nearly infinite variety of shapes as pure polymers.
Paul Alivisatos -- a chemist who with Berkeley Lab's Materials Science Division (MSD) and UCB's Chemistry Department -- led the research team, who reported its hybrid solar cell development in the March 29, 2002 issue of the journal Science. Other members of the team were Wendy Huynh, a graduate student with UCB's Chemistry Department, and Janke Dittmer, an MSD staff scientist.
"We have demonstrated that semiconductor nanorods can be used to fabricate readily processed and energy-efficient hybrid solar cells together with polymers," saids Alivisatos, a leading authority on the production of nanosized semiconductor crystals and director of The Molecular Foundry, a center for nanoscience now being established at Berkeley Lab.
The use of solar, or photovoltaic, cells -- devices that can absorb and convert light into electrical power -- has been limited to date because production costs are so high. Even the fabrication of the simplest semiconductor cell is a complex process that has to take place under exactly controlled conditions, such as high vacuum and temperatures between 400 and 1,400 degrees Celsius.
Ever since the discovery in 1977 of conducting plastics (polymers that feature conjugated double chemical bonds, which enable electrons to move through them), there has been interest in using these materials in the fabrication of solar cells. Plastic solar cells can be made in bulk quantities for a few cents each; however, the efficiency with which they convert light into electricity has been quite poor compared to the power-conversion efficiencies of semiconductor cells.
"The advantage of hybrid materials consisting of inorganic semiconductors and organic polymers is that potentially you get the best of both worlds," said Dittmer. "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."
At the heart of all photovoltaic devices are two separate layers of materials, one with an abundance of electrons that functions as a "negative pole," and one with an abundance of electron holes (vacant, positively-charged energy spaces) that functions as a "positive pole." When photons from the sun or some other light source are absorbed, their energy is transferred to the extra electrons in the negative pole, causing them to flow to the positive pole and creating new holes that start flowing to the negative pole. This electrical current can then be used to power other devices, such as a pocket calculator.
In a typical semiconductor solar cell, the two poles are made from n-type and p-type semiconductors. In a plastic solar cell, they're made from hole-acceptor and electron-acceptor polymers. In their new hybrid solar cell, Alivisatos, Huynh and Dittmer used the semicrystalline polymer known as poly(3-hexylthiophene), or P3HT, for the hole acceptor or negative pole, and nanometer-sized cadmium selenide (CdSe) rods as the positive pole.
"We chose P3HT because it can be processed in solution and has been used by many research groups around the world who are working on plastic transistors," said Huynh. "Also, it is the conjugated polymer with the highest hole mobility found so far. Higher hole (and electron) mobility means that charges are transported more quickly, which reduces current losses."
The cadmium selenide rods measured 7 nanometers in diameter and 60 nanometers in length (a nanometer is one billionth of a meter, less than one-hundred millionth of an inch). Alivisatos led an earlier study in which the technique for growing semiconductor nanocrystals into two-dimensional rods was first developed. Prior to that work, nanocrystals had always been grown as one-dimensional spheres. Using rod-shaped nanocrystals rather than spheres provided a directed path for electron transport to help improve solar cell performance.
"With CdSe rods measuring 7 nanometers by 60 nanometers, our hybrid solar cells achieved a monochromatic power conversion efficiency of 6.9 percent, one of the highest ever reported for a plastic photovoltaic device," said Alivisatos. Monochromatic power efficiency measures the ability to convert room light (which is mostly visible light) into electricity.
The Berkeley researchers prepared their solar cells by codissolving the nanorods with the P3HT and spin-casting the hybrid solution onto a glass substrate. They found that by keeping the length of the rods constant while modifying the diameter enabled them to tune the absorption spectrum of the cells so that it overlapped with the solar emission spectrum. This not only enables their hybrid cells to collect more light than typical plastic solar cells, but it also opens the door for high-efficiency devices in the future, such as tandem solar cells.
Although the efficiency of the Berkeley hybrid cells for converting sunlight into electricity was only 1.7 percent at A.M. 1.5 (when the sun is at a 41.8-degree angle to the horizon), which is far off the mark of the best semiconductor solar cells, Dittmer said there is ample opportunity for improvement.
"The most important step is to increase the amount of sunlight absorbed in the red part of the spectrum, which we can do by going to other semiconductor materials such as cadmium telluride. Also, our published hybrid solar cells have a very simple structure, in order to investigate the science behind them. In the future, many engineering tricks can be applied to make the cells more efficient." The Berkeley researchers have already been approached by companies that are interested in commercializing this technology.