Researchers from Würzburg, working on the Bavarian initiative Solar Technologies Go Hybrid, report progress in using solar energy: they have developed an innovative light harvesting system. To efficiently convert sunlight into electrical energy or other forms of energy, the first step is an efficient light harvesting system. Ideally, this system should be panchromatic, meaning it absorbs the entire spectrum of visible light. Natural models for this are light-harvesting antennas in plants and bacteria. They capture a wide spectrum of light for photosynthesis but are very complex in structure and require many different colors to transfer the energy of absorbed light to the central point. Human-developed light-harvesting systems also have drawbacks: although inorganic semiconductors such as silicon absorb panchromatically, they absorb light weakly. Very thick layers of silicon in the micrometer range are required to absorb enough light energy, making solar cells relatively bulky and heavy. Organic dyes suitable for solar cells are much thinner: their layer thickness is only about 100 nanometers. However, they can barely absorb a wide spectrum of light and are therefore not particularly efficient.

Thin layer absorbs a lot of light energy
Researchers at Julius-Maximilians-Universität (JMU) in Würzburg, Germany, have presented an innovative light-harvesting system in the journal Chem that differs significantly from previous systems. "Our system has a band structure similar to that of inorganic semiconductors. This means it absorbs light panchromatically across the entire visible range and uses the high absorption coefficients of organic dyes. As a result, it can absorb a large amount of light energy in a relatively thin layer, similar to natural light-harvesting systems," says JMU chemistry professor Frank Würthner. His team from the Institute of Organic Chemistry / Center for Nanosystem Chemistry designed the light-harvesting system at JMU and investigated it together with the group of Professor Tobias Brixner from the Institute of Physical and Theoretical Chemistry.

Four colors in an ingenious arrangement
Simply put, the innovative light-harvesting antenna from Würzburg consists of four different merocyanine dyes that are folded and arranged close to each other. The complex arrangement of molecules enables ultra-fast and efficient energy transfer within the antenna. The researchers named the prototype of the new light-harvesting system URPB. The letters represent the wavelengths of light absorbed by the four dye components of the antenna: U for ultraviolet, R for red, P for purple, and B for blue.

Proven performance through fluorescence
Researchers proved that their new light-harvesting system works so well by measuring the so-called fluorescence quantum yield. This involves measuring how much energy the system emits in the form of fluorescence. This can provide conclusions about the amount of light energy the system has previously collected. Result: the system converts 38 percent of the radiated light energy into fluorescence across a broad spectral range - the four colors on their own, on the other hand, achieve less than one percent to a maximum of three percent. The right combination and skillful spatial arrangement of dye molecules in the layer make a big difference.

Source: Faculty of Chemistry and Pharmacy, Würzburg