Organic solar cells usually consist of two layers of semiconductors – one acts as a donor, the other as an acceptor. In contrast to conventional silicon, which is produced using a great deal of energy, the polymer layers can be transferred from a solution directly onto a carrier film. On the one hand, this means comparatively low production costs and, on the other hand, the flexible modules can be used more versatilely in urban areas than silicon cells. For a long time, fullerenes, carbon-based nanoballs, were regarded as ideal acceptors, but the intrinsic losses in fullerene-based composites limit the efficiency potential too much. The FAU has therefore made a paradigm shift: Together with their Chinese partners, the scientists found a new organic molecule that absorbs more light than fullerenes; in addition, it is very durable.
The great advances in performance and longevity have made organic, hybrid-printed photovoltaics interesting for commercial use. However, for the development of practical prototypes, the technology must be transferred from the laboratory scale of a few square millimeters to the standardized scale of one square centimeter. However, such scaling usually results in considerable losses. The scientists succeeded in significantly reducing these transfer losses. The main focus of the optimization work was on the compatibility of donor and acceptor as well as the balance of short-circuit current density and open-circuit voltage – important prerequisites for the highest possible energy yield.
The result is an energy efficiency of 12.25 percent – a new certified record for solution-processed organic single stack solar cells with an area of one square centimeter, whose acceptor does not consist of fullerenes. Interestingly, the researchers also succeeded in keeping the scaling losses so low that the highest laboratory value on a small area of just under 13 percent was only slightly undercut. At the same time, a product-relevant stability could be demonstrated under simulated operating conditions such as temperature and sunlight. In the next step, the model is scaled to module size in the solar factory of the future at the Nuremberg Energy Campus (EnCN) before the development of practical prototypes begins.
These results have been published in the scientific magazine “Nature Energy”.