Saturday, November 13, 2010

A grand challenge: calculate the charge mobility of a real organic material

Previously I have written several posts about charge transport in organic materials for plastic electronic and photovoltaic devices. This week I looked over two recent articles in Accounts of Chemical Research that discuss progress at the very ambitious task of using computer simulations to calculate/predict properties of real disordered molecular materials beginning with DFT calculations of specific molecules. I was pleased to see that Marcus-Hush electron transfer theory plays a central role in both papers,

Electronic Properties of Disordered Organic Semiconductors via QM/MM Simulations (from the group of Troy Van Voorhis at MIT). 

Modeling Charge Transport in Organic Photovoltaic Materials (from Jenny Nelson's group at Imperial College London).

I have several questions and concerns about the latter paper.

The authors do a rather sophisticated simulation of a time of flight experiment where they put a charge accumulation on one side of the sample, apply an electric field, and measure
the average charge velocity, and extract the mobility. Intermolecular hopping rates are given by Marcus-Hush theory with parameters extracted from DFT calculations.

1. Is this the most efficient and reliable way to calculate mobility?
Generally in solid state physics one uses the fluctuation-dissipation relation. In this case Einstein's relation the mobility can be obtained from the diffusion constant. The diffusion constant is just the intermolecular hopping rate times the square of the intermolecular distance.

2. The mobility computed depends on the thickness of the sample.

3. How was the calculation benchmarked? Does this method give reliable
results for simpler systems, e.g., a naphthalene crystal?

4. The abstract makes some very strong claims,
"these computational methods simulate experimental mobilities within an order of magnitude at high electric fields. We ... reproduce the relative values of electron and hole mobility in a conjugated small molecule... We can reproduce the trends in mobility wiht molecular weight ... we quantitatively reproduce...On the basis of these results, we conclude that all of the necessary building blocks are in place for the predictive simulation of charge transport in macromolecular electronic materials and that such methods can be used as a tool toward the future rational design of functional organic electronic materials."

But when I look at the graph above it looks to me that the method often disagrees with experiment by more than an order of magnitude and fails to capture any electric field dependence. I could not find any discussion of temperature dependence.

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