Endothermic singlet fission is hindered by excimer formation

Abstract

Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices. Endothermic singlet fission is desired for a maximum energy-conversion efficiency, and such systems have been considered to form an excimer-like state with multiexcitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. Here we show, using 5,12-bis((triisopropylsilyl)ethynyl)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 to avoid excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.

Optimizing photon upconversion by decoupling excimer formation and triplet triplet annihilation

“…38 Dover et al have shown the conversion between monomers and excimers through triplet pair and free diffusion during both singlet fission and TTA-UC. 39 We have further suggested that another main reason for the surprisingly large amount of excimer emission in perylene based photon upconversion is due to a preassembly step on the triplet energy surface, already before the upconversion event. 40 Alkyl substitution offers the opportunity to manipulate the stacking interactions of perylene with minimal impact on the excited levels of the individual molecules.…”

Structure and photophysics of indigoids for singlet fission: Cibalackrot

“…The presence of excimer formation pathways has previously been implicated as deleterious in endothermic SF situations. 57,58 Excimer formation is less likely at low temperature, which is evident from the structured fluorescence and the slowing of the CT decay, from which at least a portion of the excimers arise ( Figure S7). The CT yield can be determined with the aid of the known radical cation extinction coefficients ( Figure S8) to be roughly 20% for 2a, 30% for 2á and 40% for 2â at room temperature.…”

Revealing the Nature of Singlet Fission under the Veil of Internal Conversion

“…[1] Therefore,SFcan potentially offset the thermal losses and enhance the maximum power conversion efficiency of photovoltaic devices. [1][2][3] Theenergetic requirement of E(S 1 ) % 2 E(T 1 )by an efficient SF process is not frequently met in most organic molecules.C onsequently,o nly ah andful of molecules are known to fulfil this thermodynamic condition, including tetracene, [4] pentacene, [5] some of their derivatives, [6] perylenediimide, [7] oligoenes, [8] and donor-acceptor polymers. [9] SF has been implemented in photovoltaic device and peak external quantum efficiency above 100 %h as been obtained.…”