Single threshold solar cells absorb photons only above a certain energy threshold, and the photon energy in excess of this threshold is generally lost as heat. One strategy to make better use of high energy photons, near or above twice the band gap, is to split excitons in two. In organic materials, the process by which this is performed is singlet fission. We have shown that the Shockley-Queisser limit is lifted to 45.9% for an ideal singlet fission device.
Endothermic singlet fission is desired for maximum energy conversion efficiency, and such systems have been considered to form an excimer-like state with multi-excitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. In this talk we show, using TIPS-tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states, to the detriment of singlet fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet-triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet fission material must be designed that avoids excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.
The talk will summarize worldwide efforts to date in singlet fission photovoltics.
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