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Accurate characterization of charge-transfer excited states

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Simulation tools to help solar cell design

An EU-funded researcher worked with experts in the Basque Region and California to develop a new method of simulating charge-transfer states, which lie at the heart of creating solar cells for renewable energy.

Energy

An EU-funded researcher has developed a model for better simulations of charge transfer (CT) processes, which play a key role in photosensitisers and photocatalytic reactions, exploited to produce solar energy. “The new method to compute charge transfer excited states has been presented in several conferences, receiving very good feedback from the scientific community,” said Dr Eduard Matito, an expert on electron correlation at the University of the Basque Country in northern Spain. He helped Dr Eloy Ramos-Cordoba develop the method in the EU AccuCT (Accurate characterization of charge-transfer excited states) project, with the support of the Marie Skłodowska-Curie programme. Dr Ramos-Cordoba also spent the first two years of his research programme at the University of California, Berkeley in the United States, working with the team of Pro. Martin Head-Gordon, which focuses on the development of new electronic structure methods and their implementation as efficient computer algorithms. “It was thanks to those new skills that I was able to propose a new family of methods to treat CT states,” said Dr Ramos-Cordoba. Razor-sharp simulation Charge transfer processes take place when electrons, negatively charged particles, are moved from one part of a molecule to another. The movement creates a hole – a positive charge region – and an excess of electrons in another part of the molecule. This movement is often the previous step to a chemical reaction or a physical process. The most common way to simulate charge transfer processes and other molecular photochemical processes is using computer programmes that operate using ‘Density Functional Theory’. However, its approximations are not so accurate for CT processes since electrons in them interact at large, rather than short, distances. “Current Density Functional Approximations (DFAs) often fail to provide an accurate and reliable answer, giving much overestimated excitation energies,” explained Dr Ramos-Cordoba. That method also struggles to simultaneously describe small and large molecules, since it does not ascribe a margin of error proportional to the size of the molecule. The AccuCT method is much more accurate. “In AccuCT we developed a new method that properly describes CT states and that can be combined with a short-range DFA to produce a long-range corrected Density Functional,” said Dr Ramos-Cordoba. The new method can simulate any kind of electronic excitation. “By correcting one of the flaws of DFAs, we expect to improve the functional and, therefore, provide better predictions of properties that were previously difficult to simulate,” said Dr Matito. Better simulations pave the way for easier development of solar cells. Molecules undergoing charge-transfer separation upon radiation by visible light – photosensitisers – are used to create solar cells, which employ the energy from the visible light radiation to create an electric current. Dr Ramos-Cordoba’s work on the project helped win him a Juan de la Cierva Incorporación grant from the Spanish government to continue his work at the University of the Basque Country. The researchers on AccuCT shared their findings in 10 conferences and in three peer-reviewed journals and are writing another three. “We are currently collaborating with two research groups that want to use our indices to develop new simulation methods,” said Dr Matito.

Keywords

AccuCT, charge transfer processes, solar cells, photosensitisers, solar energy

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