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Molecular Modeling of Vibrationally Resolved Electron Transfer for Solar Energy Conversion

Energy is the lifeblood of modern society. Solar cell technology stands out among the many forms of renewable energy due to the abundant total solar irradiance of 174,000 TeraWatt/year. According to the Shockley-Queisser (SQ) principle, the maximum solar energy conversion efficiency is 33% when a single-band solar cell is in thermal equilibrium with a heat bath at 300K. Aiming to overcome the so-called SQ thermodynamic limit, two distinct paradigms, namely (1) the hot-carrier photovoltaics; and (2) the multiple exciton generation have been proposed for the third-generation solar cell. In a hot-carrier solar cell (HCSC), the photo-excited charge carriers are driven out of thermal equilibrium by their excessive kinetic energy. As a consequence, a much higher efficiency of 66% can be reached by minimizing the threshold energy, and in turn, diminishing radiation losses. An exciting protype of HCSC is a carboxyazulene molecule adsorbed on the anatase TiO2 [101] surface (Fig. 1). Using our newly developed functional mode hot electron transfer theory, the electron injection rate exhibits a sharp increase when the vibrational quanta of the photo-excited dye molecule changes from 2 to 3, in excellent agreement with a recent femtosecond pump-probe spectroscopy experiment. More importantly, our simulations demonstrate that the average energy of injected electrons could be readily modulated by selectively populating the initial vibrational states within the exited dye molecule through controlled incident wavelength. Singlet fission is a practical implementation of multiple exciton generation in dye-sensitized solar cell. It is a spin-allowed transition that converts a singlet exciton into a correlated triplet pair, raising the solar energy conversion efficiency up to 44%. Through our functional mode singlet fission theory study, the SF in single-crystal tetracene was found to follow a direct S1S0 → T1T1 route that achieves an experimentally consistent rate of 0.02 ps-1. By contrast, in single-crystal pentacene (Fig. 2), the low-lying charge-transfer (CT) intermediate notably enhances the electronic coupling and vibrational mixing between the S1S0 and T1T1 states, leading to a much faster SF rate of 27.7 ps-1. Their distinct SF mechanisms in spite of their structural similarity suggest the importance of vibronic superposition for facile triplet pair formation.

Hanning Chen

George Washington University, Washington, D.C

Created February 12, 2018, Updated October 2, 2018