A new process recruits the sun not only as a source of power but also to produce the solar cells that make this possible.
The heart of the process, developed by chemical engineers at Oregon State Univ., is a continuous-flow solar microreactor (CFSMR). The CFSMR harnesses the sun's energy to provide heat for the production of copper indium diselenide (CuInSe2) nanoparticles - a promising photoactive material for thin-film solar cells.
Thin-film solar cells offer several advantages over conventional siliconbased photovoltaic (PV) cells. One of the most significant benefits is that they could be made via low-cost printing techniques, in which the PV layers are coated onto a substrate in a continuous process. Key to the development of these low-cost thin-film solar cells is the ability to produce nanoparticle inks at high throughput.
While several techniques have been developed to produce CuInSe2 nanoparticles, most of them rely on small batch reactors that require long synthesis times and high temperatures. The new approach employs a continuous-flow reactor.
"The continuous-flow system provides a convenient means to increase production rates to commercial scales," says Chih-Hung Chang, a professor of chemical engineering at Oregon State Univ. "Micro-sized reactors allow for finely tuned control of heat and mass transfer properties, vastly reduced reaction times, and the ability to easily adjust reaction parameters. Solar energy input, in this case from an artificial source, enables the possibility of a zero energy impact chemical manufacturing process for the production of nanoscale materials," Chang says.
The CFSMR system consists of stainless steel tubing wound into two separate coiled zones. A solution of copper chloride, indium chloride, and elemental selenium in oleic acid and trioctylphosphine is injected into the tubing and pumped to the first reaction zone - a region of coiled tubing that rapidly heats the precursor solution to temperatures high enough to start nuclÉation. Next, the solution flows to the second reaction zone - another section of coiled tubing that is also heated and allows the particles to grow. Once the particles reach the desired size, they are collected in a glass vial. To simulate sunlight, artificial radiation provided by 20-W halogen lamps heated the two reaction zones, and temperature was controlled by adjusting the distance between the lamps and the tubing.
Chang and his team investigated the impact of temperature and residence time in each reaction zone on the properties of the CuInSe2. Nanoparticles produced at lower temperatures (l 60°C in both zones) with a 1 -min residence time were of irregular shape and exhibited a wide size distribution. Increasing the temperature in the first zone to 190°C and keeping that of the second zone at 160°C produced a narrow size distribution of nanoparticles at two different residence times (1 min and 0.5 min).
"Our system can synthesize solar energy materials in minutes compared to other processes that might take 30 minutes to two hours," Chang says. "This gain in operation speed can^^_ lower cost." t*H3
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