Most of us are familiar with silicon solar cells that are found on the roofs of modern houses. These cells consist of two layers of silicon that contain different atoms such as boron and phosphorus. In combination, these layers direct the charges generated by the absorbed sunlight onto the electrodes – this (photo) current can then be used to power electronic devices.
The situation is somewhat different for organic solar cells. Here two organic materials are mixed together and not arranged in a layer structure. They are mixtures of different types of molecules. One type, the acceptor, likes to take electrons from the other, the donor. To quantify the likelihood of “electron transfer” occurring between these materials, one measures the so-called “electron affinity” and “ionization energy” of each material. These quantities indicate how easy it is to add or extract an electron from a molecule. In addition to determining the efficiency of organic solar cells, electron affinity and ionization energy also control other material properties such as color and transparency.
A solar cell is created by pairing donor and acceptor materials. In an organic solar cell, light particles (“photons”) transfer their energy to electrons. Excited electrons leave positive charges, so-called “holes”. These electron-hole pairs are then separated at the interface between the two materials, which is due to the differences in electron affinity and ionization energy.
Until now, scientists have assumed that both electron affinity and ionization energy are equally important for the functionality of solar cells. Researchers from KAUST and MPI-P have now found that in many donor-acceptor mixtures it is mainly the difference in ionization energy between the two materials that determines the efficiency of the solar cell. The combination of results from optical spectroscopy experiments carried out in the group of Frédéric Laquai at KAUST and computer simulations carried out in the group of Denis Andrienko, MPI-P, in the department headed by Kurt Kremer, enabled precise design rules Dyes to be derived for molecules to maximize solar cell efficiency.
“In the future, it would be conceivable, for example, to manufacture transparent solar cells that only absorb light outside the range that is visible to humans – but then with maximum efficiency in this area,” says Denis Andrienko, co-author of the study published in the journal Nature Materials. “With such solar cells, entire house fronts could be used as an active surface,” adds Laquai.
The authors believe that these studies will enable them to achieve 20% solar cell efficiency, a goal the industry has in mind for low-cost application of organic photovoltaics.
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