Abstract
Singlet fission is a process that splits collective excitations, or excitons, into two with unity efficiency. This exciton splitting process, unique to molecular photophysics, has the potential to considerably improve the efficiency of optoelectronic devices through more efficient light harvesting. While the first step of singlet fission has been characterized in great detail, subsequent steps critical to achieving overall highly-efficient singlet-to-triplet conversion are only just beginning to become well understood. One of the most elementary suggestions, which has yet to be tested, is that an appropriately balanced coupling is necessary to ensure overall highly efficient singlet fission; that is, the coupling needs to be strong enough so that the first step is fast and efficient, yet weak enough to ensure the independent behavior of the resultant triplets. In this work, we show how high overall singlet-to-triplet conversion efficiencies can be achieved in singlet fission by ensuring that the triplets comprising the triplet pair behave as independently as possible. We show that side chain sterics govern local packing in amorphous pentacene derivative nanoparticles, and that this in turn controls both the rate at which triplet pairs form and the rate at which they decay. We show how compact side chains and stronger couplings promote a triplet pair that effectively couples to the ground state, whereas bulkier side chains promote a triplet pair that appears more like two independent and long-lived triplet excitations. Our results show that the triplet pair is not emissive, that its decay is best viewed as internal conversion rather than triplet-triplet annihilation, and perhaps most critically that, in contrast to a number of recent suggestions, the triplets comprising the initially formed triplet pair cannot be considered independently. This work represents a significant step toward better understanding intermediates in singlet fission, and how molecular packing and couplings govern overall triplet yields.
Highlights
Singlet ssion is an exciton splitting process in molecular materials that has attracted considerable attention because of its fundamental signi cance and technological potential
The results of the analysis indicate that, because of the picosecond and sub-picosecond triplet pair formation timescales in nanoparticles of these pentacene derivatives, maximizing the matrix element for triplet pair formation has little effect on the initial triplet pair yield; that is, we estimate essentially unity triplet pair yields in all cases (Table 1)
These results indicate that the photoinduced absorption spectrum of short-lived triplet pairs in the near-infrared exhibits a transition clearly reminiscent of crystalline triplet excitations, whereas the photoinduced absorption of the shortlived triplet pairs in the visible spectral region more strongly resembles that of isolated-chromophore singlet excitations
Summary
Singlet ssion is an exciton splitting process in molecular materials that has attracted considerable attention because of its fundamental signi cance and technological potential. Incorporation of a singlet ssion sensitizer into a photovoltaic cell, for example, more effectively harvests the solar spectrum and can boost the maximum device efficiency by greater than 30% In order for the process to be useful, high overall singlet-to-triplet conversion efficiencies in singlet ssion are imperative. Involves the conversion of one light-absorbing, singlet exciton into an overall singlet, correlated triplet pair which subsequently decoheres to form two fully independent triplet excitations.[9,11,12] Various authors[5,13,14,15,16,17,18] have proposed a modi ed 6240 | Chem.
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