Molecular modeling for the photovoltaic of the future

We’ve already explained what organic photovoltaics (OPVs) and luminescent solar concentrators (LSCs) are, however, let’s have a quick recap... The two new technologies were designed and developed at our Research Centre for Renewable Energies and Environment in Novara, in collaboration with a range of international partners. They overcome the limits of traditional solar panels. OPVs are robust, light and flexible, while LSCs are used in Eni Ray Plus intelligent windows to create transparent surfaces that produce energy. Using the two technologies along with traditional solar panels will make even large buildings energy independent. Where silicon panels on roofs are not enough to provide electricity on all floors, OPV panels on walls will come in handy, as will LPCs in windows. Unlike silicon, neither OPVs nor LSCs need to face perfectly south, and they work well under scattered light, be it at midday, dawn or dusk, and even in cloudy weather. But now let’s find out how these innovative technologies are invented.

An Eni researcher with a yellow LSC panel

The solar energy puzzle

After receiving their research target, a group of scientists and researchers from various disciplines get to work. Their first task is to study everything discovered and invented so far in the scientific world for the specific field. Back in the day, they had to bury themselves in scientific libraries for weeks in search of publications and patents –Novara is still home to one of the biggest libraries of industrial chemistry in Italy and possibly in Europe. But now everything is done online, which is much faster. Once they get an idea of the situation as it stands, they look to improve the existing technology and find new molecules or polymers (chains of molecules) with the desired properties for their research. As their aim is to turn sunlight into electricity, they have to find molecules capable of capturing a photon from the sun and modifying its molecular structure so as to remove an electron (with a negative charge) and create a lacuna (with a positive charge) from a different part of that structure. Once the electrical charges have been separated, they need to find conducting molecules to carry the electron to one electrode and send the hole to the opposite electrode. In doing so, they create a potential difference and produce electricity. Simple, right? Not quite. To find the appropriate molecules, they have to employ an army of experts in organic chemistry, polymers, characterisation and the layers that constitute the photovoltaic cells, to try all the possible molecule combinations, no two the same, some differing only by a few atoms, over whole generations.

Molecular modelers in action

But a certain type of chemist speeds up the process: molecular modellers. Part laboratory chemists, part physicists, part mathematicians, these hybrid scientists make a mathematical model of the molecules, polymers and even molecular reactions within the device. They use sophisticated algorithms with unwieldy names like “ab initio quantum mechanics simulation” and “density functional theory” to create a mathematical model of the distribution, connection and motion of nuclei in molecules under study. This model lets them calculate how clouds of electrons are spread and how all the particles interact, meaning they can also calculate the electrical properties of non-existent molecules.

Theoretical scientists working on LSCs and OPVs might then study these properties, for example, the ones that let molecules interact with photons: the rays of light produced by the sun. The models have to be extremely detailed and complex. The mathematical formulae that govern the properties and reactions involved are so intricate that an army of technicians armed with a calculator would take an infinite amount of time to get a result. This is why it’s crucial to have a very powerful supercomputer with sophisticated algorithms, like HPC5, on which Eni’s experts can do multiple calculations on multiple molecules, in feasible time frames.

The computing power of Eni's new supercomputer HPC5, offers significant help to scientific research

The research does not stop

Once they’ve identified the molecules with the most promising properties, the organic chemists find ways to create them through synthesis. The resulting molecules, unique and the first of their kind, are polymerised to get more complicated macromolecules. Then other expert researchers stack them in layers on top of OPV cells, or dissolve them into a transparent polymer in the case of LSCs, and pass them on to yet more researchers, who characterise them by analysing their properties, how they interact with light and whether they are stable or tend to break spontaneously. The process is cyclical – the experimental scientists come up with results that theoretical scientists add to their models to further improve their predictions and hand them back over to the experimental ones. At this point, the ideal molecules are finally ready for the desired technology, and it’s time for development and implementation.

Even now, at the Research Centre for Renewable Energy, New Energy and Materials Science, theoretical and experimental researchers are designing and implementing new devices, based on advanced materials, which will help to improve the performance of Eni Organic Photovoltaic and Luminescent Solar Concentrator Systems even more. Research never stops.

The author: Luca Longo

Industrial chemist specialized in theoretical chemistry. He was a researcher for 30 years before moving on to Eni's scientific communication.

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