In the same furnace they submerged the nanopillars, once grown, in a thin layer of hole-rich cadmium telluride, which acted as a window to collect the light. The two materials in contact with each other form a solar cell in which the electrons flow through the nanopillars to the aluminum contact below, and the holes are conducted to thin copper-gold electrodes placed on the surface of the window above.
The efficiency of the test device was measured at six percent, which while less than the 10 to 18 percent range of mass-produced commercial cells is higher than most photovoltaic devices based on nanostructured materials even though the nontransparent copper-gold electrodes on top of the Javey group's test device cut its efficiency by 50 percent. In future, top contact transparency can easily be improved.
Other factors that greatly affect the efficiency of a 3-D nanopillar-array solar cell include its density and the exposed length of the pillars in contact with the window material. These dimensions are easily optimized in future generations of the device.
Concerned with practical applications as well as theoretical performance, the researchers made a flexible solar cell of the same design by etching away the aluminum substrate and substituting a thin layer of indium for the bottom electrode. They sheathed the whole solar cell in clear plastic (polydimethylsiloxane) to make a bendable device, which could be flexed with only marginal effect on performance and no degradation of performance after repeated bending.
"There are lots of ways to improve 3-D nanopillar photovoltaics for higher performance, and ways to simplify the fabrication process as well, but the method is already hugely promising as a way to low
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory