Singlet fission (SF)has been regarded as the most promising molecular strategy to break the Shockley–Queisserlimit on power conversion ef#64257;ciency (PCE) of single junction solar cells, significantlyraising it from 33% to 44%. Singlet fission is a multiple exciton process in whichan organic chromophore in an excited state shares it energy with a neighboringground-state chromophore and both are converted into triplet excited states. Singletfission, like many other internal conversion processes, takes place on a picosecondor even sub-picosecond time scale, thus causing the difficulty in singletfission study. Ultrafast transient absorption spectroscopy enablesinvestigation of photophysical and photochemical reactions taking place on timescalesranging from tens of femtoseconds to a few nanoseconds in real time and thusopened a new era of research in singlet fission.
Intramolecularsinglet #64257;ssion (iSF) is a more suitable approach in practical applicationcompared with intermolecular singlet fission (xSF). The latter is prone to beaffected by crystal packing, morphology, and other factors. For example, onlywhen neighboring chromophores are electronically coupled in the solid state orby diffusive collisions in highly concentrated solutions that acenes canundergo xSF. Though iSF has been observed in crystalline solids and aggregates of oligoacenes, these two arenot the most appropriate candidates for implementing high-efficiency SF-basedsolar cells because of their high band gap and low mobility. Compared with crystalline solids andaggregates of oligoacenes, conjugated polymers have a low-bandgap, and can be easilyprocessed into thin films with good phase separation. For these reasons, wedecided to study the polymer iSF systems — tetrakis-pentaphenyl spirobifluorenesystems.
One of the mostrepresentative works on polymer iSF systems is Matthew’s work on bipentacenesand pentacene#8722;tetracene heterodimers.[2,3] Employing transient absorptionspectroscopy, ultrafast photoluminescence spectroscopy, and triplet photosensitization,he demonstrated intramolecular singlet fission with triplet yields approaching200% per absorbed photon in a series of bipentacenes, and fast and e#64259;cient intramolecularsinglet fission in a series of asymmetric pentacene#8722;tetracene heterodimers. Hispioneering works offer a well-developed method for us to study polymer iSFsystems. First, using triplet photosensitization, they identify the unique TASspectral signature of the triplet state on the bipentacenes andpentacene#8722;tetracene heterodimers molecule. Using the determined tripletspectrum, they can decompose the observed spectral kinetics into populationkinetics of the singlet and triplet states. Recently, the Xia Research Group has provided new insights into singlet fission using ultrafast transientabsorption technique, showed that polymer inter-chain interactions have greateffect on intramolecular singlet fission and must be carefully engineered. Inthis work we have synthesized a tetramer with four tetracene side chain, whichwe believe will increase the stability relative to other acenes.
The works mentioned above,pioneering as they are, have given us a clear and systematic method to theacene dimers system, however, there are few research on acene tetramerscompared with acene dimers. On the other hand, the stability of acenes is poor andhighly susceptible to degradation in the presence of light and oxygen. Manyresearchers have tried to improve their stability by introducing substituentson the molecular skeleton, or by introducing spacers to adjust their property.Here,