Multiscale modeling of lateral hole hopping across TiO2 surfaces.
Valerie Vaissier a
International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Ecublens, Switzerland, 2014 May 11th - 14th
Organizers: Michael Graetzel and Mohammad Nazeeruddin
Poster, Valerie Vaissier, 162
Publication date: 1st March 2014

At the border between biology and chemistry dye molecules are subject to an increasing interest in the field of renewable energies. Initially tailored for Dye Sensitized Solar Cells (DSSC), these molecules are now widely used in many emerging technologies including solid state DSSC, photo-catalytic systems for water-splitting or hydrogen production. One of their key roles within these devices is the transfer of oxidation power: charges can be transferred from one molecule to another across a monolayer. It is the molecular monolayer properties at the nanoscale that govern the conductivity of a full film. Experiments can provide us with the characteristics of a device on a case-to-case basis but a clear link between the scales with good understanding of the dye structure to function relationship is missing.

In this work we introduce a multi-scale method to model hole diffusion dynamics through a monolayer of dye molecules anchored to a nanocrystalline film. We treat the intermolecular charge transfer step as a non-adiabatic hopping process and calculate the hopping rate as a function of the electronic coupling (J) and the reorganization energy (λ). First we propose a numerical method1 based on DFT to calculate the inner- and outer-sphere reorganization energies and show that the nature of the surrounding medium dominates λ, consistent with the high values of λ observed experimentally (λ ~ 1 V)2. Next, we combine molecular dynamics and DFT to study the influence of the dyes dynamical physical arrangement on J. Finally we incorporate these parameters into a continuous time random walk, accounting for disorder, to estimate the hole diffusion coefficient at the scale of the dyes monolayer. The results show reasonable agreement with experimental measurements on a range of dye sensitized films.3

 


Superimposition of 10 frames of ab initio MD trajectory of a pair of indolene dyes anchored on a slab of anatase surface. It illustrates the space spanned by the various conformations explored by the dyes over 15 ps. J is calculated for every frame to explicitely account for configurational disorder in the transport simulation.
[1] Vaissier, V. Barnes, P. Kirkpatrick, J. and Nelson, J. PCCP, 2013, 15, 4804-4814. [2] Moia, D. Vaissier, V. et al. Chem. Sci.,2013 ,5, 281-290. [3] Vaissier, V. Mosconi, E., Pastore M, Barnes, P. De Angelis F. and Nelson, manuscript in preparation
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