With the creation of a 3-D nanocone-based solar cell platform, the light-to-power conversion efficiency of photovoltaics has been boosted by nearly 80 percent. The technology substantially overcomes the problem of poor transport of charges generated by solar photons. These charges — negative electrons and positive holes — typically become trapped by defects in bulk materials and their interfaces and degrade performance. "To solve the entrapment problems that reduce solar cell efficiency, we created a nanocone-based solar cell, invented methods to synthesize these cells, and demonstrated improved charge collection efficiency," said Jun Xu, a researcher in Oak Ridge National Laboratory's chemical sciences division. Nanocone-based solar cell consisting of n-type nanocones, p-type matrix, transparent conductive oxide (TCO) and glass substrate. (Image: ORNL) The new solar structure consists of n-type nanocones surrounded by a p-type semiconductor. The n-type nanocones are made of zinc oxide and serve as the junction framework and the electron conductor. The p-type matrix is made of polycrystalline cadmium telluride and serves as the primary photon absorber medium and hole conductor. With this approach at the laboratory scale, Xu and colleagues were able to obtain a light-to-power conversion efficiency of 3.2 percent, compared with the 1.8 percent efficiency of conventional planar structures from the same materials. "We designed the three-dimensional structure to provide an intrinsic electric field distribution that promotes efficient charge transport and high efficiency in converting energy from sunlight into electricity," Xu said. Key features of the solar material include its unique electric field distribution that achieves efficient charge transport; the synthesis of nanocones using inexpensive proprietary methods; and the minimization of defects and voids in semiconductors. The latter provides enhanced electric and optical properties for conversion of solar photons to electricity. Because of efficient charge transport, the new solar cell can tolerate defective materials and reduce cost in fabricating next-generation solar cells. "The important concept behind our invention is that the nanocone shape generates a high electric field in the vicinity of the tip junction, effectively separating, injecting and collecting minority carriers, resulting in a higher efficiency than that of a conventional planar cell made with the same materials," Xu said. The research was supported by the Laboratory Directed Research and Development program and the US Department of Energy's Office of Nonproliferation Research and Engineering. Other contributors to this technology are Sang Hyun Lee, X-G Zhang, Chad Parish, Barton Smith, Yongning He, Chad Duty and Ho Nyung Lee. For more information, visit: www.ornl.gov