Solar cells crank up efficiency
Team led by ORNL's Jun Xu has fabricated an improved nanocone PV cell
Illustration of light-to-power conversion efficiency of the ORNL nanocone PV cell showing the nanocone shape generating a high electric field in the vicinity of the tip junction.
A team led by Oak Ridge National Laboratory's Jun Xu has fabricated a nanojunction photovoltaic cell in which conical zinc oxide nanostructures are partially surrounded by a layer of cadmium telluride. The light-to-power conversion efficiency of the ORNL nanocone PV cell is much greater than that of a cell fabricated from the same materials in the conventional planar configuration and is among the highest observed for a nanojunction PV cell.
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 Xu, a member of ORNL's Chemical Sciences Division.
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 to 1.8 percent efficiency of conventional planar structure of the same materials.
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 nextgeneration 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.—Ron Walli