Organic solar cells are cheaper to manufacture and more flexible than their crystalline silicon counterparts, but they do not offer the same level of efficiency or stability. A group of researchers led by Prof. Christoph Brabec, director of the Institute for Materials for Electronics and Energy Technology (i-MEET) at the Chair for Materials Science and Engineering of the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), has been working for several Years of improving these properties. Andrej Classen, a young researcher at FAU, showed in his doctoral thesis that efficiency can be increased with luminescent acceptor molecules. His work has now been published in the journal Nature Energy.
On a clear day in Europe, the sun can deliver around 1000 watts of radiation energy per square meter. Conventional monocrystalline silicon solar cells convert up to a fifth of this energy into electricity, which corresponds to an efficiency of around 20 percent. Prof. Brabec’s working group has held the world record for efficiency in an organic photovoltaic module of 12.6% since September 2019. The multi-cell module developed on the Energie Campus Nürnberg (EnCN) has a surface of 26 cm². “If we can achieve over 20% in the laboratory, we could possibly reach 15% in practice and become a real competition for silicon solar cells,” says Prof. Brabec.
Flexible application and high energy efficiency in production
The advantages of organic solar cells are obvious: They are thin and flexible like foil and can be adapted to different substrates. The wavelength at which the sunlight is absorbed can be “adjusted” via the macro modules used. An office window coated with organic solar cells, which absorbs the red and infrared spectrum, would not only shield off thermal radiation, but also generate electricity at the same time. One criterion that is becoming increasingly important in the face of climate change is the operating time after which a solar cell generates more energy than was required for its manufacture. This so-called energy recovery time depends heavily on the technology used and the location of the photovoltaic system. According to the latest calculations by the Fraunhofer Institute for Solar Energy Systems (ISE), the energy recovery time for silicon PV modules in Switzerland is around 2.5 to 2.8 years. According to Dr. Thomas Heumüller, research assistant at the chair of Prof. Brabec, shortens this time for organic solar cells to just a few months.
Loss of power during charge separation
Compared to a “traditional” silicon solar cell, its organic equivalent has a clear disadvantage: sunlight does not immediately generate a charge for the flow of current, but so-called excitons, in which the positive and negative charges are still bound. “An acceptor that only attracts the negative charge is required to initiate charge separation, which in turn creates free charges that can be used to generate electricity,” explains Dr. Heumüller. A certain driving force is required to separate the charges. This driving force depends on the molecular structure of the polymers used. Since certain molecules from the fullerene material class have a high driving force, they were previously the preferred choice for electron acceptors in organic solar cells. However, in the meantime, scientists have found that a high driving force has a detrimental effect on tension. This means that the power of the solar cell decreases according to the formula that applies to direct current – power is equal to voltage times current.
Andrej Classen wanted to find out how small the driving force has to be in order to achieve a complete charge separation of the exciton. To do this, he compared combinations of four donor and five acceptor polymers that have already proven their potential for use in organic solar cells. Classen used it to produce 20 solar cells under exactly the same conditions with a driving force of almost zero to 0.6 electron volts.
Increase in performance with certain molecules
The measurement results provided evidence for a theory already accepted in research – a “Boltzmann equilibrium” between excitons and separate charges, the so-called charge transfer states (CT). “The closer the driving force reaches zero, the more the equilibrium shifts in the direction of the excitons,” says Dr. Larry Lüer, specialist in photophysics in the Brabec working group. This means that future research should focus on preventing the exciton from decaying, which means that its excitation life is extended. So far, research has only focused on the life span of the CT condition. Excitons can decay through the emission of light (luminescence) or heat. By cleverly modifying the polymers, the scientists were able to reduce heat generation to a minimum and maintain luminescence as much as possible. “The efficiency of solar cells can therefore be increased with highly luminescent acceptor molecules,” predicts Andrej Classen.
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