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Energy Transfer in Supramolecular Nanostructures

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New study explores solar cell conversion mechanisms

The study of quantum dynamics of natural systems could be the key to explaining solar cell efficiency in energy conversion. Simulations have helped shed further insight into this complex mechanism.

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Photovoltaic technology represents an important route to meeting global energy demands in a sustainable way by harvesting the Sun's light for electricity production. Designers of synthetic polymers for such applications have faced a stumbling block created by the inherent difficulty of attaining high quantum yield in energy conversion. Even the best-performing systems achieve only 5 % conversion efficiency. EU-funded scientists working on the EASE (Energy transfer in supramolecular nanostructures) project looked to nature for inspiration. The team exploited state-of-the-art computational methods and computer systems to explore the mechanisms by which natural photosynthetic centres achieve high quantum yield. A great deal of work was dedicated to studying ultrafast transfer processes in carbon nanotube (CNT) systems functionalised with chromophores. With highly efficient energy transfer, porphyrin-CNT complexes are ideal for organic photovoltaics and organic light-emitting diodes. Researchers thoroughly studied the electronic structure of the porphyrin chromophore and CNT transitions using density functional theory methods. Then they determined the molecule spectral densities and reflecting electronic-vibrational (vibronic) couplings that play a crucial role in the transfer phenomena under study. To this end, researchers introduced a protocol where optimisation of excited-state geometries serves to determine vibronic couplings. Electronic couplings between the porphyrin and CNT fragments were deduced from the transition densities. In-house code was employed to obtain the relevant spectral densities from the electronic structure data. All this work led to obtaining the parametric equations of the vibronic-coupling Hamiltonian function for the porphyrin-CNT system. This function was then used in numerical calculations that were based on the multi-configuration time-dependent Hartree (MCTDH) method. Researchers compared MCTDH with numerical results from the partially linearised density matrix method. Porphyrins are a stepping stone for follow-up studies regarding the competing energy and charge transfer processes. In addition to facilitating important scientific outcomes, funding has provided numerous opportunities for training and presentation of results at international conferences and in outreach activities. EASE has made an important contribution to the field of energy transfer in supramolecular structures through its computational models and their application.

Keywords

Solar cell, quantum dynamics, energy conversion, quantum yield, energy transfer, vibronic

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