Wednesday, 05 October, 2011
Doping for solar cells
New approach enhances the formation of charge carriers
Organic solar cells are made of organic molecules that can be used to convert light energy into electricity. By exploiting sunlight, they represent one of the most promising technological approaches to meeting the rising demand for energy from environmentally benign sources. However, organic solar cells suffer from one major drawback they are much less efficient than conventional semiconductor cells made from inorganic materials. A research team at LMU Munich, led by Dr. Enrico Da Como and Professor Jochen Feldmann, Director of the Photonics and Optoelectronics Group at LMU, has now come up with a strategy for improving the electrical properties of organic semiconductors. Organic solar cells basically consist of two components, a polymer, which transfers an electron when it absorbs a quantum of light, and a so-called fullerene, which accepts the electron. This charge-transfer event constitutes the essential first step in the generation of electricity from light, which requires the separation of negatively charged electrons from positively charged holes. Da Como and Feldmann, who are also members of the Nanosystems Initiative Munich (NIM), a Cluster of Excellence at LMU, have now taken a closer look at this elementary reaction, and gained new insights into the dynamics of the underlying physical processes: In order to improve the efficiency of organic solar cells, we need to understand and optimize the process of charge transfer at the interface between polymer and fullerene, Da Como explains. This process involves the generation of an intermediate state, called an excited complex, in which the charges are facing each other at the interface between neighboring molecules. At this stage, charge recombination may occur, which results in a loss of energy and thus reduces the efficiency of the solar cell. The researchers used a unique combination of spectroscopic methods to monitor recombination, and found a way to inhibit it and so enhance charge separation. We have been able to show that the incorporation of low concentrations of an additional molecule into the polymer an approach that is known as doping inhibits the formation of excited complexes and so increases the rate of charge separation, says Da Como. This represents an entirely new concept in the physics of charge transfer in organic semiconductors, and could open a new route to increasing the efficiency of organic solar cells. The team is now investigating prototypes to test whether the concept is workable in practice, and to determine how the doping process affects the morphology and conductivity of organic solar cells.