ECE Seminar Series: Dr. Luke Daemen (UCR)

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Flyer: Singlet Fission in Organic Semiconductors: Observation of the Triplet Excitonic In Acenes with Neutron Vibrational Spectroscopy Abstract: Photovoltaics (PV) convert sunlight into electricity with zero emissions, providing technical solutions for the world’s ever-growing energy demand while simultaneously preventing climate change from carbon dioxide emissions. The Shockley−Queisser limit puts the maximum theoretical single junction PV device efficiency at ∼33%. Typical crystalline or polycrystalline silicon solar panels have an efficiency closer to 20% with higher performance -but more expensive- systems demonstrated. Organic semiconductors present a number of advantages over silicon for various applications including photovoltaics: lightweight, mechanical flexibility, tunability, and integrability with other materials and systems. Perhaps the greatest advantage is the promise of increased efficiency compared to the present Si-based technology. Singlet fission (SF) is a spin-allowed process that converts a singlet exciton into a pair of triplet excitons between molecular organic chromophores. This carrier multiplication process increases the maximum theoretical PV efficiency to ∼44%. Converting a single photon into a pair of excited charges spurred research into the use of SF molecules to improve photovoltaic device efficiency, as the ability to overcome the Shockley−Queisser limit would have profound impacts on the price and land use of photovoltaic energy generation. While SF is not fully understood, many studies have been dedicated to molecules that can undergo SF and into incorporating SF into devices for increased efficiencies including photovoltaics, photodetectors, and organic light emitting diodes. Neutron vibrational spectroscopy in conjunction with high-performance computing and time-dependent density functional theory was used to observe the formation of the triplet state in tetracene and pentacene The spectroscopic results reveal intermolecular structural relaxation due to the presence of a triplet excited state. The structural dynamics of the combined excited state molecule and surrounding tetracene molecules are further studied using time-dependent density functional theory which shows that the singlet and triplet levels shift due to the excited state geometry, reducing the uphill energy barrier for SF.