The solar energy that reaches the Earth is more than one thousand times greater than the amount of energy humanity currently consumes, but catching those rays and putting them to work presents many scientific and technological challenges. Silicon photovoltaic cells, the most commonly used technology to convert solar energy into electricity, are still too expensive to compete on a large scale with fossil fuels. Recent advances in nanotechnology, however, have enabled researchers to shrink the thickness, and hence the cost, of silicon solar cells while still capturing as many photons as possible. In a review article published in the Nov./Dec. 2012 issue of the Journal of Vacuum Science and Technology A, Stanford researchers present an overview of new nanoscale methods to enhance the absorption of photons in thin silicon solar cells.
"Nanostructures present an exciting opportunity for photon management and novel ways to produce high efficiency and low-cost solar cells," says Yi Cui, a professor of materials science and engineering at Stanford and one of the authors of the review paper. "The recent developments in advanced nanophotonics simulation, nanostructure fabrication, and optical testing have shown great promise."
Silicon photovoltaic systems currently fill approximately 80-90 percent of the solar energy market. Silicon is abundant, non-toxic, and stable. Thin silicon solar cells are less expensive than traditional panels, but one of the problems with the thin cells is that traditional methods to increase light absorption, such as micrometer-sized surface patterns, are no longer as effective as the thickness shrinks. Instead, scientists have begun to explore how nanoscale structures can increase light absorption in thin solar cells.
Yi and his colleagues review many different nanostructures that have been shown to help thin silicon capture more light. For example, nanocones and nanowires can enhance absorption by minimizing reflections. The nanostructures make the light absorption less dependent on the incident angle of the light, which is important for solar cells since the angle of the sunlight changes throughout the day. Periodic nanostructures, such as nanwires, nanoholes, nanocones, nanodomes, and nanoshells, can also scatter the light in thin silicon, improving the light absorption for a given thickness.
Fabricating such nanostructures on a commercial scale still remains a challenge. "Technically, the cost-effective ways for producing those nanostructures are yet to be developed," says Cui. He and his co-authors discuss a promising technique called colloidal lithography, which can create feature sizes under 100 nanometers without using complex equipment. The technique is scalable and inexpensive, but does include etching steps, which can limit its application to certain materials.
"The direct benefit from the nanoscale photon management is to decrease the thickness of an absorber layer for cost reduction and potentially for efficiency enhancement," notes Cui. "The indirect benefit is to decrease the balance of system (BOS) cost. Approximately 60 percent of a silicon photovoltaic system cost comes from the BOS. Thin and light-weight silicon solar cells can be packaged with cheap plastic substrates, and they can be shipped and installed easily. It would be an important part of the solution."
Cui says that researchers are still investigating which nanostructures best balance excellent optical absorption and electronic functions and how best to manufacture these nanostructures. But he remains very optimistic about solar energy's future. He notes that solar energy is abundant, safe, and clean and offers important energy security benefits. "Our learning curve in the solar cell industry has reached a point [that is] very close to being competitive with fossil fuel energy generation," he says. "We should give the last extra push to the solar cell area to make it completely competitive without subsidies."