Abstract
Triplet–triplet annihilation upconversion (TTA-UC) has great potential to significantly improve the light harvesting capabilities of photovoltaic cells and is also sought after for biomedical applications. Many factors combine to influence the overall efficiency of TTA-UC, the most fundamental of which is the spin statistical factor, η, that gives the probability that a bright singlet state is formed from a pair of annihilating triplet states. The value of η is also critical in determining the contribution of TTA to the overall efficiency of organic light-emitting diodes. Using solid rubrene as a model system, we reiterate why experimentally measured magnetic field effects prove that annihilating triplets first form weakly exchange-coupled triplet-pair states. This is contrary to conventional discussions of TTA-UC that implicitly assume strong exchange coupling, and we show that it has profound implications for the spin statistical factor η. For example, variations in intermolecular orientation tune η from to through spin mixing of the triplet-pair wave functions. Because the fate of spin-1 triplet-pair states is particularly crucial in determining η, we investigate it in rubrene using pump–push–probe spectroscopy and find additional evidence for the recently reported high-level reverse intersystem crossing channel. We incorporate all of these factors into an updated model framework with which to understand the spin statistics of TTA-UC and use it to rationalize the differences in reported values of η among different common annihilator systems. We suggest that harnessing high-level reverse intersystem crossing channels in new annihilator molecules may be a highly promising strategy to exceed any spin statistical limit.
Highlights
Bright, emissive singlet excitons can be created from the fusion of two dark triplet excitons through the photophysical process of triplet−triplet annihilation (TTA).[1]
We have shown how factors rarely considered in discussions of the spin statistics of TTA can have a profound effect on the efficiency
When the triplet-pairs are weakly exchange-coupled, our simulations show that varying the intermolecular orientation tunes the spin statistical factor from 2 for parallel chromophores to 2 for perpendicular chromophores, through variations in the spin mixing of the triplet-pair wave functions
Summary
Emissive singlet excitons can be created from the fusion of two dark triplet excitons through the photophysical process of triplet−triplet annihilation (TTA).[1]. The probability that a pair of annihilating spin-1 triplet excitons results in a spin-0 singlet exciton is given by the spin statistical factor, η, with 0 ≤ η ≤ 1. For OLEDs and TTAmediated photon upconverters, materials systems with a high value of η would result in very efficient device performance.[2,3,7,8,23,24] despite its fundamental importance, the triplet−triplet interactions that govern the value of η are not, in general, fully understood or appreciated. Several potential strategies for designing materials with a high value of η have been largely overlooked to date
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