|Physics Department | Center For Optical Technologies | Lehigh University|
Excitons, Light, and Electric Current in Organic Molecular Crystals
Organic molecular crystals operate in a relatively unexplored region in the landscape of solid-state physics. This region is characterized by narrow bands with transport at the boundary between hopping and band transport, and by excitons with high binding energy that are an important hurdle towards free carrier photoexcitation. As a consequence, the transport of energy in the form of excitons in these materials is a fundamental physical process with several interesting and peculiar properties.
A small rubrene crystal.
Tightly bound excitons are the dominant result of photon absorption in these materials, and dissociation of excitons into free charge carriers can occurs only via interaction with defects and heterointerfaces, which is one important bottleneck towards using cheap and lightweigh plastic components for harvesting solar light in organic photovoltaics.
There are other interesting effects. Exchange symmetry has a strong effect on Coulomb energy in organic molecules and it can be that the triplet state of an excited molecule (a triplet exciton in a crystal) has a much lower energy (by a factor of 2 or more) than the singlet state. Because of this, the zero-spin singlet excitons can undergo a (spin conserving!) fission process into two spin-1 triplet excitons. And when the energy of the triplet excitons is about one half the energy of a singlet, then these triplet excitons can — upon interaction with each other — fuse, pooling their energies to re-create a singlet exciton.
It follows that one must ask about diffusion length of excitons in organic molecular mateirals. In organic photovoltaics diffusion of excitons is tantamount to energy diffusion towards an appropriate harvester. But what limits exciton diffusion length? Molecular ordering is certainly one aspect, but what other material properties can be modified to favor it? Could one develop a molecular material where exciton diffusion lengths can reach distances of the order of micrometers? In fact, we have found that excitons in rubrene single cryrstals efficiently transform into triplet excitons that then have a long lifetime and a long diffusion length, of about 4 micrometers in one particular direction in the crystal.
In our research group, we do fundamental research that targets the physical mechanisms of exciton dynamics, diffusion, and dissociation in organic molecular crystals. Newly available high quality crystals like rubrene offer the opportunity to investigate these processes in a well-defined and controlled system. As an example to to study the fundamental limits to the exciton diffusion length. We have recently demonstrated that the rubrene single crystal is an ideal system to establish a better fundamental understanding of exciton dynamics. This is because of several specific properties that are all found simultaneously in rubrene, such as highly efficient singlet-to-triplet and triplet-to-singlet conversion that allow the direct optical observation of triplet exciton diffusion (by imaging the light emitted by a triplet exciton gas because of triplet-triplet interactions), and the fact that triplet excitons can easily dissociate into free carriers by an as yet unidentified interaction with defects (leading to a photocurrent that, after pulsed excitation, keeps growing all the time while triplet excitons are still alive).
Research on exciton dynamics in molecular crystals dates back to just after the invention of the laser, but many open questions remain today, not least because of modern potential optoelectronic applications (organic photovoltaics) and the fact that we are now in a position to apply new techniques to obtain new relevant experimental data.
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