Particularly in view of the increasingly rapid increase in global warming, renewable energies will play an ever-greater role in the future. So far, however, the proportion of such energies in Germany is still quite low. For example, the proportion of photovoltaics in gross electricity generation last year was a mere 7.4 %. A figure that is partly due to the relatively low yield of electricity generated by these types of systems.
Although the sun can provide a solar energy output of around 1,000 watts per square meter under cloudless skies in our geographical climes, the monocrystalline silicon solar cells commonly available on the market only convert up to around 20 % of this into electricity.
Pros and cons
Organic solar cells perform even worse, with an efficiency of only 12.6 % at most. This world record for organic photovoltaic (OPV) has been held since September 2019 by the research group led by Prof. Christoph Brabec. He is Director of the Institute for Materials in Electronics and Energy Technology (i-MEET) at the Chair of Materials Science at Friedrich Alexander University Erlangen-Nürnburg (FAU) in Germany.
The multicellular module with a surface area of 26 cm² was developed at the Energy Campus Nuremberg (EnCN). “If we get it up to more than 20 % in the laboratory, we will achieve perhaps 15 % in practical applications. That way it can really compete with the silicon solar cell,” Prof. Brabec states.
What’s more, organic solar cells have also been less stable so far, but on the other hand, they are superior to those made of crystalline silicon in some respects. They are cheaper to manufacture and can be used more flexibly. Prof. Brabec and his group have therefore been working for years on ways to minimize these disadvantages. One of the students, FAU junior scientist Andrej Classen, has now been able to show in his doctoral thesis that efficiency can be increased through the use of luminescent acceptor molecules.
Windows as power generators
The advantages of organic solar cells are self-evident, the scientists underline. They are thin and flexible like a film and can be easily adapted to a variety of substrates. “The wavelength at which sunlight is absorbed can be ‘adjusted’ via the macromolecule that is used. For example, an office window coated with organic solar cells, which absorbs light in the red and infrared spectral range, would not only shield against heat radiation but also generate electricity at the same time. “A factor that is becoming increasingly important in light of the climate crisis is the operating time a solar cell needs to generates more energy than its manufacture has expended. This so-called energy payback time (EPBT) is highly dependent on the technology used and the location of the photovoltaic (PV) system.”
According to the latest estimates by the Fraunhofer Institute for Solar Energy Systems (ISE), the energy payback time of silicon-based PV modules in Switzerland is 2.5 – 2.8 years. With organic solar cells, the energy payback time would only be a few months, says Dr. Thomas Heumüller, a research associate at the chair of Prof. Brabec.
Energy loss for charge separation
Furthermore, the organic solar cell has another decisive disadvantage compared to the ‘classic’ silicon solar cell: Sunlight does not immediately generate free charge carriers, explains Dr. Heumüller, but rather so-called excitons, where the positive and negative charges are still bound. “To get free charge carriers capable of generating electricity, an acceptor is needed that only attracts the negative charge and hence leads to charge separation”.
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In order to separate the charges, a specific force is required, which is referred to as the driving force. This in turn is dependent on the molecular structure of the polymers in use. Up until now, particular molecules from the substance class of fullerenes have primarily been used as electron acceptors in organic solar cells because they have a powerful driving force. In the meantime, however, the researchers have figured out that “a great driving force comes at the expense of electrical voltage.” This means that the output of the solar cell decreases, “according to the formula that applies to direct current, power equals voltage times amperage.”
Specific molecules enhance performance
In order to find out how small the driving force can get to be able to accomplish a complete charge separation of the exciton, Andrej Classen compared combinations of four donor and five acceptor polymers. These had already proven their potential for organic solar cells and Classen then fabricated 20 solar cells under exactly the same conditions with a driving force of almost zero to 0.6 electron volts.
Based on the measurement results, he provided the first evidence of a Boltzmann equilibrium between excitons and separated charges, known as Charge Transfer (CT) states, which had already been assumed in research. “The closer the driving force is to zero, the further the state of equilibrium is towards the exciton side,” says Dr. Larry Lüer, a specialist in photophysics in the Brabec research group. In future, research must consequently concentrate on preventing the exciton from decaying, i.e. on increasing its excitation ‘lifetime’. To date, the focus had only been on the lifetime of the CT state.
The exciton can be attenuated by emitting light (luminescence) or generating heat. By skilfully modifying the polymer, heat generation could be reduced to a minimum, so that mainly the luminescence would remain. “The increase in efficiency of organic solar cells, therefore, results from highly luminescent acceptor molecules,” Classen emphasizes.
Andrei Klassen’s work has been published in the scientific journal Nature Energy.