Improving DFT Modeling of EPR Data for Small Molecule Organic Photovoltaics

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PROJECT SUMMARY The movement of an electron from one redox active site to another is central to the function of many biological systems including respiration and photosynthesis. In natural photosynthesis, Photosystem II (PSII) catalyzes the water splitting reaction and is seen as a template for designing new solar devices. A critical part of this design process is to fully understand electron transfer processes in smaller model systems. We have recently investigated electron transfer in polymer-fullerene solar devices (bulk heterojunction solar cells) by combining electron paramagnetic resonance (EPR) experiments with density functional theory (DFT) calculations. However, we found that comparing small oligomers to experimental results of the full polymer system made interpretation of the results difficult. We hypothesize that comparing our DFT calculations directly to the results for small oligomers will allow a better assessment of the agreement between calculation and experiment. It is important to determine what functionals are suitable if DFT calculations are to be used to guide solar cell design. In Aim 1, we will determine which functionals adequately model oligomers that contain silicon and germanium as central atoms (rather than the more common carbon). In Aim 2, we will determine the true extent of charge delocalization by comparing results for oligomers that span the range suggested for delocalization lengths. At the conclusion of this work, we will have expanded knowledge of how to model electron donating systems and provided tools that can be used to model newly developed solar devices which will make such devices more economically viable and reduce our reliance on fossil fuels.