5452 wileyonlinelibrary.com proceed on ultrafast (≈100 fs) time scales, allowing it to out-compete other decay channels and achieve high effi ciencies. [ 3 ] The essential condition for effi cient SEF is the energetic alignment of the singlet and triplet states, such that 2 E (T 1 ) ≤ E (S 1 ). A recent combined theoretical and experimental study of SEF rates in a range of acene solids has demonstrated that the rate of SEF is also greatly affected by the strength of intermolecular coupling within the fi lm. [ 4 ] In the canonical system, pentacene, triplet pair formation is exothermic and the intermolecular coupling is strong, resulting in SEF with an 80 fs time constant and nearly 200% yield. [ 5 ] Though most experimental studies of SEF have involved crystalline, polycrystalline or amorphous solids, the most basic unit capable of SEF is a pair of chromophores. Indeed, it was recently demonstrated in concentrated solutions of TIPS-pentacene that singlet fi ssion can proceed at high efficiency through bimolecular diffusional interactions. [ 6 ] However, early attempts to directly control the interaction between chromophores through the use of covalent dimers have not been as successful. The most notable systems in this regard are tetracene and 1,3-diphenylisobenzofuran. These materials are found to exhibit effi cient SEF in the solid state, but their covalent dimers achieved triplet yields of only a few percent. In both of these studies, [ 7 ] the two SEF chromophores were joined by a range of linkers to modify the strength of the electronic coupling between them, with the aim of tuning the rate and effi ciency of SEF. The impact was subtle, and it thus remains unclear why covalent dimers have proved ineffi cient to date. Current models suggest that dimers should be asymmetric or contain signifi cant cofacial interaction between chromophores to attain high triplet yields. [ 2,8 ] Interestingly, a recent study of pentacene dimers separated by a phenyl spacer unit achieved triplet yields above 100% in spite of using the same symmetric bonding motifs of the earlier tetracene dimers. [ 9 ] In this work, we report highly effi cient intramolecular SEF in a new type of covalent dimer, with triplet yields of up to 192 ± 3%. The molecule used in this study, 13,13′-bis(mesityl)-6,6′-dipentacenyl (DP-Mes, Figure 1 a), consists of two pentacenes directly bonded through a single C C bond with two bulky mesityl groups at the meso -positions. The geometry of the dimer, with two nearly orthogonal pentacene cores, is unlike Tuneable Singlet Exciton Fission and Triplet-Triplet Annihilation in an Orthogonal Pentacene DimerSteven Lukman , Andrew J. Musser , Kai Chen , Stavros Athanasopoulos , Chaw K. Yong , Zebing Zeng , Qun Ye , Chunyan Chi , Justin M. Hodgkiss , Jishan Wu , * Richard H. Friend , and Neil C. Greenham * Fast and highly effi cient intramolecular singlet exciton fi ssion in a pentacene dimer, consisting of two covalently attached, nearly orthogonal pentacene units is reported. Fission to triplet excitons from...

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We developed an analytical model to describe hopping transport in organic semiconductors including both energetic disorder and polaronic contributions due to geometric relaxation. The model is based on a Marcus jump rate in terms of the small-polaron concept with a Gaussian energetic disorder, and it is premised upon a generalized effective medium approach yet avoids shortcomings involved in the effective transport energy or percolation concepts. It is superior to our previous treatment [Phys. Rev. B 76, 045210 (2007)] since it is applicable at arbitrary polaron activation energy E a with respect to the energy disorder parameter σ . It can be adapted to describe both charge-carrier mobility and triplet exciton diffusion. The model is compared with results from Monte Carlo simulations. We show (i) that the activation energy of the thermally activated hopping transport can be decoupled into disorder and polaron contributions whose relative weight depend nonlinearly on the σ /E a ratio, and (ii) that the choice of the density of occupied and empty states considered in configurational averaging has a profound effect on the results of calculations of the Marcus hopping transport. The σ /E a ratio governs also the carrier-concentration dependence of the charge-carrier mobility in the large-carrier-concentration transport regime as realized in organic field-effect transistors. The carrier-concentration dependence becomes considerably weaker when the polaron energy increases relative to the disorder energy, indicating the absence of universality. This model bridges a gap between disorder and polaron hopping concepts.

We present a theoretical study of charge transport in disordered semiconducting polymers that relates the charge mobility to the chemical structure and the physical morphology in a novel multiscale approach. Our studies, focusing on poly(9,9-dioctylfluorene) (PFO), show that the charge mobility is dominated by pathways with the highest interchain charge-transfer rates. We also find that disorder is not always detrimental to charge transport. We find good agreement with experimental time-of-flight mobility data in highly aligned PFO films.