Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division, led the development of a solution-based technique for fabricating core/shell nanowire solar cells using the semiconductors cadmium sulfide for the core and copper sulfide for the shell. These inexpensive and easy-to-make nanowire solar cells boasted open-circuit voltage and fill factor values superior to conventional planar solar cells. Together, the open-circuit voltage and fill factor determine the maximum energy that a solar cell can produce. In addition, the new nanowires also demonstrated an energy conversion efficiency of 5.4-percent, which is comparable to planar solar cells.
“The solution-based cation exchange reaction provides us with an easy, low-cost method to prepare high-quality hetero-epitaxial nanomaterials,” Yang says. “Furthermore, it circumvents the difficulties of high-temperature doping and deposition for typical vapor phase production methods, which suggests much lower fabrication costs and better reproducibility. All we really need are beakers and flasks for this solution-based process. There’s none of the high fabrication costs associated with gas-phase epitaxial chemical vapor deposition and molecular beam epitaxy, the techniques most used today to fabricate semiconductor nanowires.”
Yang and his colleagues believe they can improve the energy conversion efficiency of their solar cell nanowires by increasing the amount of copper sulfide shell material. For their technology to be commercially viable, they need to reach an energy conversion efficiency of at least ten-percent
Schematic shows how to make core/shell nanowire solar cell starting from left with a CdS nanowire (green) that is dipped in CuCl where cation exchange reaction creates a Cu2S shell coating (brown). Metal contacts are then deposited on the CdS core and Cu2S shell. (Image courtesy of Yang, et. al
Typical solar cells today are made from ultra-pure single crystal silicon wafers that require about 100 micrometers in thickness of this very expensive material to absorb enough solar light. Furthermore, the high-level of crystal purification required makes the fabrication of even the simplest silicon-based planar solar cell a complex, energy-intensive and costly process.
A highly promising alternative would be semiconductor nanowires – one-dimensional strips of materials whose width measures only one-thousandth that of a human hair but whose length may stretch up to the millimeter scale. Solar cells made from nanowires offer a number of advantages over conventional planar solar cells, including better charge separation and collection capabilities, plus they can be made from Earth abundant materials rather than highly processed silicon. To date, however, the lower efficiencies of nanowire-based solar cells have outweighed their benefits.