Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid

Summary

A dye-sensitized solar cell was solvated by RTILs; using optimized empirical potentials, a molecular dynamics simulation was applied to compute vibrational properties. The obtained vibrational spectra were compared with experiment and ab initio molecular dynamics; various empirical potential spectra show how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction.

Abstract

The accurate molecular simulation prediction of vibrational spectra, and other structural, energetic and spectral characteristics, of photo-active metal-oxide surfaces in contact with light-absorbing dyes is an ongoing thorny and elusive challenge in physical chemistry. With this in mind, a molecular-dynamics (MD) simulation were performed by using optimized empirical potentials for a well-representative and prototypical dye-sensitized solar cell (DSC) solvated by a widely-studied room temperature ionic liquid (RTIL), in the guise of a [bmim]⁺[NTf2]^- RTIL solvating an N719-sensitizing dye adsorbed onto 101 anatase-titania. In doing so, important insights were gleaned into how using a RTIL as the electrolytic hole acceptor modulates the dynamical and vibrational properties of a N719 dye, estimating the spectra for the DSC photo-active interface via Fourier transformation of mass-weighted velocity autocorrelation functions from MD. The acquired vibrational spectra were compared with the experiment spectra and those sampled from ab initio molecular dynamics (AIMD); in particular, various empirical-potential spectra generated from MD provide insight into how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction. In any event, careful fitting of empirical force-field models has been shown to be an effective tool in handling DSC vibrational properties, when validated by AIMD and an experiment.

Introduction

In dye-sensitized solar cells (DSCs), the optical band gap of semiconductors is bridged by a light-absorbing, or sensitizing, dye. DSCs require continual recharging: therefore, a redox electrolyte is essential to foster this constant supply of charge (typically in the form of an I^-/I^3-, in an organic solvent). This facilitates the passage of holes from the sensitizing dye to the electrolyte, with injected photo-excited electrons into the metal-oxide substrate passing through to an external circuit, with eventual recombination taking place at the cathode¹. A crucial aspect underpinning DSCs' positive outlook for a wide ....

Protocol

1. Performing MD Simulation using DL_POLY

1. Construct the DSC-systems initial structure of the N719-dye adsorbed to an anatase-titania (101) surface solvated by [bmim]+[NTf2]^- taken from previous work^12,13. Draw the required structure using VESTA software.
2. Choose the N719 cis-di(thiocyanato)-bis (2,2'-bipyridl-4-carboxylate-4'-carboxylic acid)-ruthenium (II)-sensitizing dye with no counterions and ensure the presence of two surface-bound protons to provide for overall-system charge neutrality.
   NOTE: Indeed, in depth studies by De Angelis et al. have established thi....

Results

Structural Properties of Binding Motifs
Representative binding motifs for the four different partial-charge sets are depicted in Figure 2, after 15 ps of MD. In Figure 2a, for the (above-described) literature-derived charges, it can be seen that there is a prominent hydrogen-bonding interaction with a surface proton. From careful analyses of the trajectory, the hydrogen bonds are mostly surfa.......

Discussion

Ab initio simulation techniques are expensive to perform and hence to perform simulation on much longer timescales would require the use of empirical forcefields for at least some of the DSC system. Towards this end, an equivalent atomistic model was created of the [bmim]⁺[NTf2]^- solvated interface, using an empirical, classical-simulation forcefield for MD. The anatase was modeled using the Matsui-Akaogi (MA) forcefield, whilst the dye structure was handled using OPLS parameters. For the RTIL, four.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
This was a molecular simulation, so no experimental equipment was used.
The name of the software was DL-POLY (the 'Classic' version of which is available under GnuPublic Licence, via sourceforge)