Bradforth Research Group
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Singlet Fission
Laura Estergreen
and Jimmy Joy


Conventional solar cells which consist of a single p-n junction have a maximum theoretical efficiency of 34% (the Shockley-Queisser limit). This limit can be surpassed by using the phenomenon of Singlet Fission (SF) - wherein two excitons are produced for every photon that is absorbed. This process involves the excitation of a ground state chromophore to an excited singlet state. This chromophore couples to a ground singlet state chromophore such that two triplets are formed, hence, Singlet Fission.

In collaboration with the Mark Thompson group at USC, we have studied a variety of organic materials that demonstrate singlet fission in thin films. We have used time correlated single photon counting (TCSPC) and transient absorption (TA) to follow and quantify the excited state processes. There is still much to understand about this phenomena! The role of chromophore coupling with respect to the orientation and distance between chromophores still needs to be elucidated. In addition, the singlet fission process must involve at least one intermediate state in which the electrons are redistributed between two chromophores such that the overall spin is conserved. The spins of two chromophore are correlated. And various theoretical models have implicated charge transfer or charge resonance states as additional intermediates.

In order to understand such intermediate states and the nature of the chromophore coupling, bichromophores of conjugated organic molecules are being probed spectroscopically – in these systems the triplets cannot further separate. We are also looking at polyacene systems that can be suspended in solution as nanoparticles - these particles resemble smaller domains present in amorphous films but have significant solvent exposure. Depending on the solvent polarity or polarizability a charge transfer intermediate state should be stabilized and can be revealed by time-resolved spectroscopies.