A Quantitative Model of Charge Injection by Ruthenium Chromophores Connecting Femtosecond to Continuous Irradiance Conditions

TL;DR

This paper develops a comprehensive kinetic model for charge injection by ruthenium dyes into semiconductors, accurately matching experimental data across femtosecond to nanosecond timescales and predicting high charge transfer rates under solar conditions.

Contribution

It introduces a quantitative kinetic framework that incorporates multiple physical effects, providing new insights into charge injection rates and dynamics in dye-sensitized systems.

Findings

Achieves quantitative agreement with transient absorption data

Estimates charge injection rate coefficients of 10^11 to 10^12 s^-1

Predicts up to 68 electron transfer events per dye per second under sunlight

Abstract

A kinetic framework for the ultrafast photophysics of tris(2,2-bipyridine)ruthenium(II) phosphonated and methyl-phosphonated derivatives is used as a basis for modeling charge injection by ruthenium dyes into a semiconductor substrate. By including the effects of light scattering, dye diffusion and adsorption kinetics during sample preparation, and the optical response of oxidized dyes, quantitative agreement with multiple transient absorption datasets is achieved on timescales spanning femtoseconds to nanoseconds. In particular, quantitative agreement with important spectroscopic handles, decay of an excited state absorption signal component associated with charge injection in the UV region of the spectrum and the dynamical redshift of an approximately 500 nm isosbestic point, validates our kinetic model. Pseudo-first-order rate coefficients for charge injection are estimated in this work, with an order of magnitude ranging 1011 s-1 to 1012 s-1. The model makes the minimalist assumption that all excited states of a particular dye have the same charge injection coefficient, an assumption that would benefit from additional theoretical and experimental exploration. We have adapted this kinetic model to predict charge injection under continuous solar irradiation, and find that as many as 68 electron transfer events per dye per second take place, significantly more than prior estimates in the literature.