Solar power promises an endless supply of clean, renewable energy. However, to make solar power feasible, solar cells need to be made from inexpensive materials using a low-cost, low-energy-intensive manufacturing process. Moreover, these cells need to efficiently and cost-effectively do their job: convert sunlight into electricity. A team of researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab) has begun to rethink the solar cell by using semiconductor nanowires instead of crystal silicon wafers. So far, they report, the results have been promising.
Today, most conventional solar cells are made from ultra-pure, single crystal silicon wafers. These wafers are extremely expensive and fabricating them is a complex, energy-intensive and costly process. Semiconductor nanowires are 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 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.
Berkley Lab scientists have formulated a process to create these light-collecting nanowires using a solution-based technique. Cadmium sulfide nanowires are dipped into a solution of copper chloride. The reaction converts the surface layer of the cadmium sulfide into a copper sulfide shell.
These inexpensive and easy-to-make nanowire solar cells boosted 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. However, for this technology to be commercially viable, nanowires need to be about 10 percent efficient. The team’s scientists believe this is an attainable goal.