Solar cells made of semiconducting nanowires can be flexible and easy to make. To improve the cells’ pliability and drive down costs, researchers would like to make the solar cells’ transparent electrode from graphene instead of the current preferred material, indium tin oxide (ITO). Researchers have now solved a problem that had slowed the development of such graphene-based devices: growing nanowires on graphene without degrading its properties (Nano Lett., DOI: 10.1021/nl303920b). Their solar cells made with graphene electrodes work nearly as well as similar ITO-based devices.
Today’s thin-film solar cells use ITO films as transparent electrodes. But the films are expensive and brittle. Low-cost graphene would be a much better alternative for large, bendable devices, researchers think.
Silvija Gradečak and her colleagues at Massachusetts Institute of Technology wanted to make this switch in nanowire-based solar cells that they recently developed (Nano Lett., DOI: 10.1021/nl1030166). These cells contained semiconductor nanowires and organic polymers that bridge two electrodes. One of the electrodes is transparent to let light into the device. The polymers absorb light and generate electrons, while the nanowires speed the movement of those electrons through the cell, improving its efficiency. The solar cells are flexible, lightweight, and easy to produce, Gradečak says.
The MIT team made these cells by growing vertical nanowire arrays on the surface of ITO. Unfortunately, growing such arrays on graphene has been difficult. Others have reported epitaxial methods for growing zinc oxide nanowires on graphene, but the technique’s conditions degrade graphene’s electronic and mechanical properties.
Now the team has worked out how to grow zinc oxide nanowires on graphene while preserving its properties. They start by spin-coating graphene with a conductive organic polymer This polymer layer allows them to then spin-coat a uniform zinc oxide film on top. Immersing the film in a nanowire growth solution gave them an array of 400-nm-high zinc oxide nanowires on the graphene. Finally, the researchers deposited light-absorbing organic polymer or lead sulfide quantum dots on the nanowires. Placing a metal electrode on top completed the solar cell. The quantum dot cells had light-to-electricity conversion efficiencies as high as 4.2%; the efficiency of similar ITO-based cells was 5.1%.
The researchers make the devices on a glass substrate, but there should be no barrier to making them on flexible plastic substrates because the entire fabrication process uses low temperatures that won’t melt plastic, Gradečak says. In addition to working on plastic-based devices, they are trying to increase the efficiency of the devices by decreasing the resistance of the graphene sheets.
“The advantage of this method is that it uses a low-temperature process that is relatively simple and cheap,” says Helge Weman, at the Norwegian University of Science and Technology. But compared to previous nanowire-growing methods, it might not allow the same degree of control of nanowire structure and chemical properties, he says. Nevertheless, he commends the researchers for being the first to grow nanowires on graphene to make solar cells.