Tuesday, July 9, 2013

Elucidating singlet fission

Singlet fission is the process where a excited spin singlet state of an organic molecular complex can decay into two spin triplet states on spatially separated molecules. This process has received considerable attention recently because of the possibility of using it to increase the efficiency of organic photovoltaic cells. Several previous posts considered some of the fascinating science involved, including the reverse process of triplet-triplet annihilation.

There are a beautiful series of papers by Timothy Berkelbach, Mark Hybertsen, and David Reichman

Microscopic theory of singlet exciton fission. I. General formulation

Microscopic theory of singlet exciton fission. II. Application to pentacene dimers and the role of super exchange

To me these papers provide a quality benchmark for theoretical work in the field and highlight the limitations and confusion of some of the previous work on the subject.
Why do I like the papers? The authors
  • use diabatic states as a starting point. This connects to chemical intuition about the mixed character of the excited states  (see the Figure above). 
  • combine quantum chemistry calculations with effective Hamiltonians.
  • highlight the importance of the environment in determining the excited state dynamics.
  • acknowledge the limitations of quantum chemical calculations for providing accurate parameters for effective Hamiltonians and the excited states. This leads them to consider how the efficiency of singlet fission can vary significantly with the excited state energies and the extent of their charge transfer character (see the Figure below).
  • show how the effect of the environment can be treated reasonably reliably in an [relatively low cost] approximation schemes [e.g., Redfield and Polaronic Quantum Master Equation=PQME].
  • answer an important question. The singlet fission does occur via an "intermediate" charge transfer state, even when that state is at high energy.
Hopefully this approach will be applied to other important processes of relevance to organic photovoltaic cells, including charge separation in bulk heterojunction cells and in the Gratzel cell.

Seogjoo Jang, Timothy Berkelbach, and David Reichman have a preprint where they have applied the PQME formalism to a donor--bridge-acceptor system.

An outstanding challenge remains to understand how the coherent triplet pair decoheres into uncorrelated triplet states on the two molecules.

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