Tandem design sets new efficiency record for plastic solar cells

24 Jul 2007

Linking two organic solar cells with different absorption characteristics allows a wider range of the spectrum to be used.

A US-Korea research team has developed organic solar cells with a power-conversion efficiency of 6.5% at illuminations of 200 milliwatts/cm², compared with the 5% or less they claim to have previously been attained by polymer cell structures (Science, 317, 222).

"This is the highest level achieved for solar cells made from organic materials," commented Alan Heeger of the University of California. "I am confident that we can make additional improvements that will yield efficiencies sufficiently high for commercial products."

The device exploits a novel tandem design in which two multilayered organic solar cells are essentially connected together in series. As well as producing more voltage, this arrangement can enable absorption over a broader range of the solar spectrum than one cell can provide on its own.

The cell's design uses a wide band-gap semiconductor for the first cell and a smaller band-gap semiconductor for the second cell, an architecture that improves the light harvesting without increasing internal resistance. Each layer is based on conjugated semiconducting polymers and fullerene derivatives, and is processed from solution.

The two cells are separated and connected by a transparent layer of TiO_x, which serves as an electron transport and collecting layer for the first cell, and as a stable foundation for the fabrication of the second cell. Previous tandem cells have used semi-transparent metal layers, but the transparency of the TiO_x ensures no loss of light intensity at the cell junction and contributes to the improved efficiency. Depositing the connecting layer from solution via sol-gel chemistry helps avoid any interlayer mixing, and means that the whole cell is fabricated by solution processing.

Storage under nitrogen for 3500 hours resulted in a drop in efficiency from 6.5% to 5.5%, a relatively small decrease after nearly half a year. After 40 hours of continuous irradiation under an AM1.5G filter, the recognized solar equivalent for testing photovoltaic cells, the tandem cell retained 70% efficiency, and over 60% after 100 hours. These tests indicate that the cells should be robust enough to survive in real-world applications.

When commercialized, the tandem cell technology could contribute to the use of lap top computers and other devices in third world countries or other areas that are off the electricity grid. "The cells offer special opportunities for as renewable energy sources," said Heeger. "They can be fabricated to extend over large areas by means of low-cost printing and coating technologies that can simultaneously pattern the active materials on lightweight flexible substrates."