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Exciton fission in organic crystals may provide a way to enhance the efficiency of organic photovoltaics.  Exciton fission is an intrinsic property of the solid state structure, and does not depend on the size of the individual crystal.  In other words, there is no upper limit on crystal size, unlike inorganic systems where multiple exciton generation in these systems is a direct consequence of the quantum confinement..  Furthermore, exciton fission in organic systems tends to produce more robust product states.  Although the S*à2S₁ singlet-singlet fission channel (analogous to multiple exciton generation in the inorganic nanocrystals) is known, the much more common reaction is the S*à2T₁ singlet-triplet channel.  The low radiative rates of triplet excitons ensure their survival for periods orders of magnitude longer than for singlet excitons.  From measurements of delayed fluorescence in anthracene and tetracene molecular crystals, triplet lifetimes on the order of microseconds and diffusion lengths on the order of microns can be obtained in high quality samples.

The possible advantages of organic crystalline systems for MEG or exciton fission have motivated our current study of tetracene, the best-studied molecule where this phenomenon has been observed.  Our research has two main areas of inquiry:

1)  What are the dynamics of exciton fission in solid tetracene samples, and how are they affected by sample morphology and the delocalization of the singlet exciton?

2)  How can we efficiently harvest the triplets that are produced via fission as electron hole pairs?

Picosecond luminescence measurements have been used to both establish a timescale for the fission process and to measure the lifetime of the triplets in various types of solid samples.  In single crystals of tetracene, our measurements agree with those of previous workers that the initially excited singlet fluorescence decays within about 200 ps.  After this initial decay, both single crystals and polycrystalline evaporated films show a weak, long-lived delayed fluorescence component due to recombination of two triplet excitons back into a singlet.  Thus the delayed fluorescence lifetime can be used to estimate the triplet lifetime as well.  For single crystals, this lifetime ranges between 5 and 20 microseconds.  For the polycrystalline films, the delayed fluorescence lifetime is much shorter, on the order of 100-200 ns.  The origin of this shortened lifetime could be either nonradiative relaxation of the triplets back to the ground state, which would limit the amount of time they have to be converted into electron-hole pairs, or it could result from trapping or conversion into a charge-separated state.  These last two outcomes would be more desirable from the point of view of ultimately producing electron hole pairs.  The task is to determine the fate of these nonluminescent species over both short and long timescales.  Obviously, photoluminescence can only directly probe the singlet exciton.  To directly monitor the triplet and charge-separated states, we turn to transient absorption spectroscopy.  

Our approach to the transient absorption spectroscopy is two-pronged.  First, we want to establish the excited state properties of molecular tetracene in solution.  Once we understand its properties at the level of individual molecules, we can then see how those properties are modified by aggregating the molecules together into a crystal.  We have performed transient absorption on tetracene in solution with ~100 fs time resolution.  Perhaps surprisingly, there is very little literature on the spectroscopy of the polyacene molecules in solution, despite their importance in organic electronics.  We found that the transient absorption spectrum of tetracene in toluene is remarkably complicated, with at least three induced absorption features centered at 450 nm, 650 nm, and 1200 nm.  All these features can be assigned to known electronic transitions.  These features all decay at the same rate as the fluorescence, indicating that they originate from the first singlet excited state, representing transitions to higher lying excited states.  At times longer than the singlet lifetime, the bleach signal at 480 nm remains, superimposed on a broad triplet-triplet absorption centered at 460 nm.  These solution measurements have two implications for experiments on the solid state.  First, assignment of induced absorptions in the near infrared to charge separated states or triplets should be made with caution, since the singlet state also absorbs in these regions.  Second, the singlet absorption at 450 nm overlaps with that of the triplet-triplet absorption.  So any measurements of the triplet formation time must take into account the dynamics of this overlapping singlet transition as well.  

Ultrathin films of tetracene solid are grown via vacuum evaporation.  Transient absorption signals from these films are much stronger than what would be expected based on solution measurements.  This enhanced nonlinear optical response is consistent with the idea that the singlet states in the solid are delocalized, superradiant excitons, rather than excitations localized on a single molecule.  The largest signal component at 530 nm decays on a timescale of 15 ps or so, faster than the instrument response function of our earlier luminescence measurements.  The induced absorption at 650 nm is now dwarfed by the singlet exciton response.  The induced absorption at 450 nm is still present, and the dynamics in this wavelength region are particularly complicated.  We are currently trying to extract formation times for the triplet and then monitor its subsequent evolution in order to make contact with our time-resolved luminescence measurements.  

To summarize, we now have several people actively engaged in making samples and characterizing the time-resolved dynamics of tetracene in both molecular (solution) and solid state form.  Our initial results point to the complexity of the excited state spectra of this molecule, and confirm that delocalization plays a key role in its optical response in the crystalline state.