Force field parameters for large-scale computational modeling of sensitized TiO 2 surfaces

Abstract

Force field parameters for large scale computational modeling of sensitized TiO2-anatase surfaces are developed from ab initio molecular dynamics simulations and geometry optimization based on Density Functional Theory (DFT). The resulting force field, composed of Coulomb, van der Waals and harmonic interactions, reproduces the ab initio structures and the phonon spectra density profiles of TiO2-anatase nanostructures functionalized with catechol, a prototype of an aromatic linker commonly used to sensitize TiO2 nanoparticles with Ru(II)-polypyridyl dyes. In addition, simulations of interfacial electron injection and electron-hole relaxation dynamics demonstrate the capabilities of the resulting molecular mechanics forcefield, as applied in conjunction with mixed quantum-classical methods, for modeling quantum processes that are critical for the overall efficiency of sensitized-TiO2 solar cells.Keywords: Semiconductors, solar cells, TiO2. I IntroductionThe large-scale use of photovoltaic devices for electricity generation is still prohibitively expensive. Fifteen years ago, a promising development emerged with the realization of solar cells based on dye-sensitized TiO 2 films.1 These photovoltaic devices rapidly achieved light-to-electric energy conversion efficiency as high as 10 % for simulated solar light, exploiting the high surface area of the nanoporous TiO 2 films deposited on conducting glasses. However, attempts to achieve further improvements in efficiency have been hindered by the lack of fundamental understanding of the molecular and electronic processes associated with the constituent operational steps. This problem continues to be a standing challenge, despite significant research effort reported in recent years, including experimental, 2-18 computational 19-36 and theoretical [37][38][39][40][41][42][43][44] studies. It is therefore essential to develop new experiments and theoretical studies to construct realistic models of dye-sensitized TiO 2 interfaces and gain insight on the design of surface-sensitization with optimum photon-to-current conversion efficiency. This paper reports the development of force-field parameters for atomistic modeling of dye-sensitized TiO 2 surfaces and the application of the resulting force field, in conjunction with mixed quantum-classical methods, for simulations of electron-hole pair relaxation dynamics after photoexcitation of TiO 2 -anatase sensitized with catechol. Catechol/TiO 2 -anatase nanoparticles are particularly suited for developing and testing force-field parameters for large-scale computations of dye-sensitized TiO 2 surfaces. Catechol has raised significant experimental interest as a prototype of an aromatic linker upon which a wide range of molecular structures can be attached for specific functionalities, including applications to photovoltaic devices with high photon-tocurrent conversion efficiencies. [45][46][47] As a model sensitizer, catechol lowers the TiO 2 absorption threshold

A perspective on using experiment and theory to identify design principles in dye-sensitized solar cells

“…However, although the size of the system is manageable for MD, even using modest computational resources, the major obstacle is the availability of a force field that captures the interactions inherent in both the inorganic TiO 2 and the organic dye. If there is no appropriate force field combining the inorganic and organic components, an option is to tailor-make one, but this is a lengthy and complex process involving empirical data and/or QM and ab initio MD [75]. Another option is to decrease the number of molecules by at least one order of magnitude and employ ab initio MD such as Car-Parrinello MD [76].…”