by: Yasmin Ajirniar
From mid-summer to late fall of 2015, the cosmopolitan city of Milan hosted the World Expo towards the northeast of its city center. There, the sleekest designs and most novel technology was featured to the theme of “Feeding the planet, Energy for life”. Regarding the non-traditional energy concerns, a collaboration between Merck and Belectric, called the solar trees, inhabited the German Pavilion; It was an installation that was not only symbolic in its design but also suggestive in its technological feat.
The large semi-transparent arching structures that captured the spirit of the “Field of Ideas” were erected from printed organic materials that could harvest electricity from solar energy. Most excitingly, the science of organic photovoltaics (OPVs) behind the World Expo spectacle is not too far away from being more widely incorporated.
Unlike silicon based solar cells, acceptor-donor polymers and small molecules such as fullerenes comprise the carbon-based systems of OPV’s such that they create a mobile electron and an electron hole—a free charge carrier. The cascade of events that lead to the free charge carrier begins with the absorption of a photon. Highly conjugated polymers then donate the resultant exciton, the electrostatically bound electron and electron hole, to a small molecule electron acceptor. In the process of transferring the electron, an energetic difference between the two materials splits the exciton; thus arriving at the free charge carrier.
Unfortunately, because the charge mobility of these systems is limited, the efficiency of OPV’s has not yet reached that of inorganic solar cells. While inorganic materials typically achieve an efficiency of 15-22%, organic materials were once reported, at most, to be 14%. However, within the last two years, an efficiency of 17.3% was reported by using new architectures, such as tandem cells (cells which split the range of the spectrum of incoming photons).
Inherent to the nature of the material, OPV’s do have a significant advantage over their counterparts such that the low cost and variety in synthetic techniques. For example, while, on average, traditional silicon solar cells cost 10.5 cents per kilowatt-hour, producing electricity from an OPV solar cell would cost 7 cents per kilowatt-hour, according to a BBC environmental correspondant's report in 2018. Further, greater tunability and diversity can be achieved in polymer synthesis. Because thiophenes have unique photochemical properties that lend themselves useful in the construction of organic solar cells, synthetic techniques, such as the Hinsberg Thiophene Ring Synthesis and use of Lawesson’s Reagent, are particularly relevant.
In addition, the facile incorporation of organic solar cells into existing infrastructure not only enables wider implementation but also reduces installation costs. Compare rooftop solar panels: since silicon based solar panels are so heavy that they require sufficient structural support from roofs, installing these panels, and perhaps installing new framework to withstand the added weight, can be very costly. However, with dyes and inks containing the absorbers and/or polymers, thin, lightweight organic panels can be printed, manipulated, and readily installed at a lower cost and disturbance to the existing structure.
A clear, or rather semi-transparent, demonstration of innovation, design, and practicality behind organic photovoltaics, the curved, patterned solar trees of the World Expo appropriately embody the “seedlings” that would grow and disseminate. While the exposition was a feat for the 2015 World Expo, it is probable and beneficial to pursue research, production, and integration of OPV solar cells. Although efficiency of organic systems has historically been less than that of inorganic systems, new architectures have increased their efficiency. And given the low cost of harvesting electricity, tunability, low weight, and high flexibility, the future of OPV’s looks promising.