Exciton-polaron complexes in pulsed electrically-detected magnetic resonance

Several microscopic pathways have been proposed to explain the large magnetic effects observed in organic semiconductors, but identifying and characterising which microscopic process actually influences the overall magnetic field response is challenging. Pulsed electrically-detected magnetic resonance provides an ideal platform for this task as it intrinsically monitors the charge carriers of interest and provides dynamical information which is inaccessible through conventional magnetoconductance measurements. Here we develop a general time domain theory to describe the spin-dependent reaction of exciton-charge complexes following the coherent manipulation of paramagnetic centers through electron spin resonance. A general Hamiltonian is treated, and it is shown that the transition frequencies and resonance positions of the exciton-polaron complex can be used to estimate inter-species coupling. This work also provides a general formalism for analysing multi-pulse experiments which can be used to extract relaxation and transport rates

Using coherent dynamics to quantify spin-coupling within triplet-exciton/polaron complexes in organic diodes

Quantifying the spin-spin interactions which influence electronic transitions in organic semiconductors is crucial for understanding their magneto-optoelectronic properties. By combining a theoretical model for three spin interactions in the coherent regime with pulsed electrically detected magnetic resonance experiments on MEH-PPV diodes, we quantify the spin-coupling within complexes comprising three spin-half particles. We determine that these particles form triplet-exciton:polaron pairs, where the polaron:exciton exchange is over 5 orders of magnitude weaker (less than 170 MHz) than that within the exciton. This approach providing a direct spectroscopic approach for distinguishing between coupling regimens, such as strongly bound trions, which have been proposed to occur in organic devices.Comment: 5 pages, 4 figure

Slow Hopping and Spin Dephasing of Coulombically Bound Polaron Pairs in an Organic Semiconductor at Room Temperature

Polaron pairs are intermediate electronic states that are integral to the optoelectronic conversion process in organic semiconductors. Here, we report on electrically detected spin echoes arising from direct quantum control of polaron pair spins in an organic light-emitting diode at room temperature. This approach reveals phase coherence on a microsecond time scale, and offers a direct way to probe charge recombination and dissociation processes in organic devices, revealing temperature-independent intermolecular carrier hopping on slow time scales. In addition, the long spin phase coherence time at room temperature is of potential interest for developing quantum-enhanced sensors and information processing systems which operate at room temperature

Strongly exchange-coupled triplet pairs in an organic semiconductor

From biological complexes to devices based on organic semiconductors, spin interactions play a key role in the function of molecular systems. For instance, triplet-pair reactions impact operation of organic light-emitting diodes as well as photovoltaic devices. Conventional models for triplet pairs assume they interact only weakly. Here, using electron spin resonance, we observe long-lived, strongly-interacting triplet pairs in an organic semiconductor, generated via singlet fission. Using coherent spin-manipulation of these two-triplet states, we identify exchange-coupled (spin-2) quintet complexes co-existing with weakly coupled (spin-1) triplets. We measure strongly coupled pairs with a lifetime approaching 3 µs and a spin coherence time approaching 1 µs, at 10 K. Our results pave the way for the utilization of high-spin systems in organic semiconductors.Gates-Cambridge Trust, Winton Programme for the Physics of Sustainability, Freie Universität Berlin within the Excellence Initiative of the German Research Foundation, Engineering and Physical Sciences Research Council (Grant ID: EP/G060738/1)This is the author accepted manuscript. The final version is available from Nature Publishing Group at http://dx.doi.org/10.1038/nphys3908

Accounting for Impact? The Journal Impact Factor and the Making of Biomedical Research in the Netherlands

The range and types of performance metrics has recently proliferated in academic settings, with bibliometric indicators being particularly visible examples. One field that has traditionally been hospitable towards such indicators is biomedicine. Here the relative merits of bibliometrics are widely discussed, with debates often portraying them as heroes or villains. Despite a plethora of controversies, one of the most widely used indicators in this field is said to be the Journal Impact Factor (JIF). In this article we argue that much of the current debates around researchers’ uses of the JIF in biomedicine can be classed as ‘folk theories’: explanatory accounts told among a community that seldom (if ever) get systematically checked. Such accounts rarely disclose how knowledge production itself becomes more-or-less consolidated around the JIF. Using ethnographic materials from different research sites in Dutch University Medical Centers, this article sheds new empirical and theoretical light on how performance metrics variously shape biomedical research on the ‘shop floor.’ Our detailed analysis underscores a need for further research into the constitutive effects of evaluative metrics

Theory of triplet-triplet annihilation in optically detected magnetic resonance

Triplet-triplet annihilation allows two low-energy photons to be upconverted into a single high-energy photon. By essentially engineering the solar spectrum, this allows solar cells to be made more efficient and even exceed the Shockley-Quiesser limit. Unfortunately, optimizing the reaction pathway is difficult, especially with limited access to the microscopic time scales and states involved in the process. Optical measurements can provide detailed information: triplet-triplet annihilation is intrinsically spin dependent and exhibits substantial magnetoluminescence in the presence of a static magnetic field. Pulsed optically detected magnetic resonance is especially suitable, since it combines high spin sensitivity with coherent manipulation. In this paper, we develop a time-domain theory of triplet-triplet annihilation for complexes with arbitrary spin-spin coupling. We identify unique “Rabi fingerprints” for each coupling regime and show that this can be used to characterize the microscopic Hamiltonian