Editorial pubs.acs.org/JPCL
Excitons Racing Against the Clock mong the Perspective articles in this issue of the Journal of Physical Chemistry Letters is a comprehensive report on advances in studies and applications of exciton diffusion in conjugated polymers. Tamai et al. (Tamai, Y.; Ohkita, H.; Benten, H.; Ito, S. Exciton Diffusion in Conjugated Polymers: From Fundamental Understanding to Improvement on Photovoltaic Conversion Efficiency. J. Phys. Chem. Lett. 2015, DOI: 10.1021/acs.jpclett.5b01147.) explain the role of exciton diffusion in organic solar cells, where electronic excitation is transported to interfaces where electron transfer is initiated. They emphasize the challenge that active layer thickness and morphology is often dictated by the limits of how far excitons can diffuse in disordered organic films. Understanding the mechanisms of exciton diffusionthat is, electronic energy transferand how it is used and optimized in natural photosynthetic systems as well as chemical materials is a topic of multidisciplinary interest.1 How far can an exciton diffuse? Quantitatively, it is usually thought that the diffusion length is around 10 nm, but is that the limit? The issue deciding diffusion length is quite simple, it is the competition between a random walk of the excitation among the molecules of the material with a rate controlled by the energy transfer rate, and a time limit for the migration controlled by the excited state lifetime. Because the lifetime of singlet excited states is of the order 1 ns, energy migration is a race against a fast clock. The clock runs according to the rate of decay of the excited state, given by a sum of the radiative krad and nonradiative rates knonrad
A
k flu = k rad + k rad
that orientations are optimized and electronic coupling is reasonably large. Owing to the way the rate expression changes as a function of electronic coupling, this optimization is not as straightforward as one might imagine.3 (c) Engineer the environment to reduce screening effects (i.e., the refractive index n in the Förster equation). This is often the opposite optimization required for effective exciton dissociation and stabilization of mobile carriers. (d) Maximize the spectral overlap with narrow spectral line shapes (i.e., small reorganization energies) and minimal vibronic structure. This is the opposite requirement for light harvesting, where a broad spectral cross-section is desirable. (e) Use directed transport, typically with an energy gradient, in order to unravel and direct the diffusive walk. Energy gradients are very effective but necessarily lead to voltage losses in a device. All in all, optimizing exciton diffusion lengths involves challenges from the most fundamental levels of dynamical theories for condensed-phase dynamics up to engineering longrange transport processes in chemical materials. There are many length and time scales that must be considered and compromises among desirable properties of the system as a whole, making this a fascinating project in physical chemistry.
Gregory D. Scholes*
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(1)
2π κ2 |μd |2 |μa |2 4 6 J ℏ nR
*E-mail:
[email protected]. Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
(1) Scholes, G. D.; Fleming, G. R.; Olaya-Castro, A.; van Grondelle, R. Lessons from Nature about Solar Light Harvesting. Nat. Chem. 2011, 3, 763−74. (2) Olaya-Castro, A.; Scholes, G. D. Energy Transfer from FörsterDexter Theory to Quantum Coherent Light-Harvesting. Int. Rev. Phys. Chem. 2011, 30, 49−77. (3) Ishizaki, A.; Fleming, G. R. Unified Treatment of Quantum Coherent and Incoherent Hopping Dynamics in Electronic Energy Transfer: Reduced Hierarchy Equation Approach. J. Chem. Phys. 2009, 130, 234111.
(2)
Although Forster theory is an approximate theory, it does capture the essence of the problemthe energy transfer rate is tied to the “clock” because both krad and kEET scale with |μ|2. In other words, as we ensure energy transfer jumps are faster by engineering more strongly absorbing chromophores, we concomitantly decrease the time available for energy migration. Nevertheless, there are several tuning knobs in eqs 1 and 2 that can be used to optimize exciton diffusion lengths, and their interplay and optimizations are often not intuitive. (a) Ensure knonrad is negligible (i.e., the fluorescence quantum yield of the chromophores is close to unity). This includes making sure that additional nonradiative relaxation pathways are not introduced by interchromophore interactions, so-called concentration quenching. (b) Chromophores can be optimally organized so © 2015 American Chemical Society
AUTHOR INFORMATION
Corresponding Author
where krad is proportional to the molecule’s transition strength (transition dipole moment squared). According to Förster theory, the energy transfer rate is decided by the transition dipole strengths of the donor and acceptor (μd and μa), the orientation κ and separation R of those molecules, the refractive index of the medium n, and the integrated overlap J of the donor fluorescence spectrum and the acceptor absorption spectrum2 kEET ≈
Department of Chemistry, Princeton University, Washington Rd, Princeton, New Jersey 08544, United States
Published: September 3, 2015 3390
DOI: 10.1021/acs.jpclett.5b01763 J. Phys. Chem. Lett. 2015, 6, 3390−3390
