Researchers in China and the United States have teamed up with colleagues in Sweden to develop efficient organic solar cells with a very low driving force.
A paper published in Nature Energy describes solar cells with an energy efficiency of 9.5%, which means that 9.5% of the energy in the sunlight is converted to electricity.
Linköping University (LiU), announcing the breakthrough last week, said that the solar cells have a significantly lower driving force and faster charge separation than previous cells. This implies that the intrinsic limitations of organic solar cells are no greater than those of other photovoltaic technologies, bringing them a step closer to commercialization, the university explained.
Previously, it was believed that efficient operation of organic solar cells required a large driving force, limiting their efficiency.
As LiU explains, when photons emitted by the sun are absorbed by organic semiconductors, strongly bound excitons (rather than free carriers) are generated. The driving force is a measure of the energy that is used to split the excitons into free carriers. The driving force results in the loss of the photovoltage, a key parameter for the solar cell. The lower the driving force, the higher the photovoltage.
According to the university, efficient organic solar cells are traditionally usually composed of semiconducting polymers and carbon balls known as 'fullerene' and a large driving force over 0.3 eV is usually needed for efficient charge generation.
For the latest study, the researchers replaced fullerene with a semiconducting small molecule and found that that a low driving force is needed for efficient operation of the devices. What's more, organic solar cells based on polymer/small molecule combinations are more stable.
The research group was led by Feng Gao, lecturer at LiU's Department of Physics, Chemistry and Biology (IFM), together with He Yan at the Hong Kong University of Science and Technology, and Kenan Gundogdu at North Carolina State University.