How do organic solar cells work

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Organic solar cells are based on the same functional principle as crystalline photovoltaic modules: a semiconductor changes from a non-conductive to a conductive state by absorbing light. The efficiency of organic solar cells is currently still comparatively low, but the potential for future increases is very high.

Organic semiconductors

The term “organic” comes from chemistry and simply means that it is a matter of carbon compounds. Organic semiconductors can be manufactured in innumerable variants and the greatest problem in research in this area is to find the optimal materials. Essentially, this still works according to the “trial and error” principle, because the processes in a solar cell are very complex and the degree of efficiency can only be theoretically predicted very imprecisely. Choosing the optimal materials is so complicated because there are a number of completely different properties that determine their efficiency. First of all, it is important that the semiconductor can use as large a proportion of the optical spectrum as possible to generate electricity. In addition, it is of great importance that the free conduction electrons generated by the light do not immediately recombine with the atoms. Finally, the electrical resistance should also be as low as possible so that a large part of the electricity generated is not converted into heat. To make matters worse, the organic semiconductor must also match the connection electrodes used so that no major current losses occur at the transition area. And last but not least, the material should also have a long service life. At this point it becomes clear, as an example, that these objectives are difficult to reconcile with one another and that compromises are therefore necessary: ​​on the one hand, as much light as possible should be absorbed, on the other hand, the absorption of the particularly high-energy ultraviolet light destroys the carbon compounds, which leads to rapid decomposition of the material .

The efficiency

The efficiency of organic solar cells in mass production is currently less than ten percent, with seven to eight percent being typical. Custom-made products in research laboratories jump the ten percent hurdle, but are still so complex to manufacture that the cost advantage of this technology would be lost. How the efficiency of organic cells will develop in the next few years is difficult to predict. Conventional crystalline silicon modules, which currently achieve an efficiency of around 20 percent, are limited in their development. For physical reasons, you can only use around 30 percent of the incident solar radiation, which sets relatively narrow limits on the possible degree of efficiency. Such an upper limit does not exist for organic solar cells. This is primarily due to the fact that new materials are constantly being developed, the exact properties of which no one can know today. However, it is also due to the fact that so-called tandem solar cells are much easier and cheaper to implement. Tandem solar cells consist of several superimposed layers of semiconductors that use different parts of the light spectrum. Organic semiconductors are vapor-deposited in thin layers. Multi-layer solar cells can therefore be manufactured much more easily.

Will organic solar cells replace crystalline cells?

In the long term with certainty! Looking at developments in recent years, it is difficult to imagine that someone will still be installing heavy silicon modules on the roof 50 years from now. In the short and medium term, both technologies will exist in parallel. Organic solar cells will open up completely new fields of application for photovoltaics. These solar cells consist of thin and almost invisible foils that can be used in a variety of ways. Even clothing can be coated in this way, turning the jacket into a charger for the smartphone. For the foreseeable future, however, crystalline photovoltaic modules will still be unrivaled in the field of ground-mounted systems. Due to the currently significantly higher degree of efficiency, they make better use of the available space. The significantly longer service life also offers long-term investment security.

Conclusion

Organic solar cells are a cheap and flexible alternative to crystalline modules, but they have a significantly lower performance and a significantly shorter service life. As with many other technologies, one can take an optimistic and a pessimistic point of view when assessing future potential. The pessimistic point of view emphasizes what can be achieved with today's knowledge. With today's knowledge, it is completely unclear how the performance of crystalline modules is to be achieved, let alone significantly exceeded. The optimistic point of view asks which limits of technology are not to be overcome from a scientific point of view and otherwise trusts that all problems that can be solved in principle will actually be solved at some point. Viewed in this way, the potential of organic solar cells is immense.