Transient absorption and time-resolved EPR spectroscopy show an ultra-fast spin-mixing CT state in a small diketopyrrolopyrrole-based molecule blended with fullerene.

The bimolecular recombination characteristics of conjugated polymer poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2- b :2′,3′- d ]silole)-2,6-diyl-alt-(2,5-bis 3-tetradecylthiophen-2-yl thiazolo 5,4- d thiazole)-2,5diyl] (PDTSiTTz) blended with the fullerene series PC60BM, ICMA, ICBA, and ICTA have been investigated using microsecond and femtosecond transient absorption spectroscopy, in conjunction with electroluminescence measurements and ambient photoemission spectroscopy. The non-Langevin polymer PDTSiTTz allows an inspection of intrinsic bimolecular recombination rates uninhibited by diffusion, while the low oscillator strengths of fullerenes allow polymer features to dominate, and we compare our results to those of the well-known polymer Si-PCPDTBT. Using μs-TAS, we have shown that the trap-limited decay dynamics of the PDTSiTTz polaron becomes progressively slower across the fullerene series, while those of Si-PCPDTBT are invariant. Electroluminescence measurements showed an unusual double peak in pristine PDTSiTTz, attributed to a low energy intragap charge transfer state, likely interchain in nature. Furthermore, while the pristine PDTSiTTz showed a broad, low-intensity density of states, the ICBA and ICTA blends presented a virtually identical DOS to Si-PCPDTBT and its blends. This has been attributed to a shift from a delocalized, interchain highest occupied molecular orbital (HOMO) in the pristine material to a dithienosilole-centered HOMO in the blends, likely a result of the bulky fullerenes increasing interchain separation. This HOMO localization had a side effect of progressively shifting the polymer HOMO to shallower energies, which was correlated with the observed decrease in bimolecular recombination rate and increased “trap” depth. However, since the density of tail states remained the same, this suggests that the traditional viewpoint of “trapping” being dominated by tail states may not encompass the full picture and that the breadth of the DOS may also have a strong influence on bimolecular recombination.

Triplet states are ubiquitous in organic electronics and their properties are increasingly being exploited to enhance device efficiencies. The difficulty in accurately probing triplet states dictates that more fundamental understanding is required of their properties. In this work a hybrid co-polymer of F8BT with 10 % by weight zinc porphyrin was synthesised and a transient absorption spectroscopy study performed. It was observed that a dual energy transfer mechanism was active, whereby the ultimate fate of each photogenerated F8BT singlet exciton depended upon its distance to a porphyrin unit. F8BT excitons generated within the bulk of the F8BT polymer showed typical F8BT photophysics, with the small proportion of F8BT triplets created able to diffuse to and undergo triplet energy transfer to the porphyrin units. In contrast, F8BT singlet excitons formed within their diffusion length to a porphyrin unit displayed singlet energy transfer, followed by ISC to create the lower energy porphyrin triplet. Intriguingly, the F8BT-HAPAPP triplets generated have a lifetime intermediate between the two pristine materials. DFT calculations suggest that this is due to orbital mixing between energetically close benzothiadiazole-and porphyrin-localised molecular orbitals, creating a mixed F8BT/porphyrin triplet state.

Efficient charge photogeneration in conjugated polymers typically requires the presence of a second component to act as electron acceptor. Here, we report a novel low band-gap conjugated polymer with a...