Optical- and photocurrent-detected magnetic resonance studies on conjugated polymer/fullerene composites

Scharber, Markus C.; Schultz, N. A.; Sariciftci, Niyazi Serdar; Brabec, Christoph J.

doi:10.1103/physrevb.67.085202

Cited by 58 publications

(46 citation statements)

References 32 publications

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“…It is important to note that the term bipolaron here refers to the double occupation of electron states and not to the charge states, bipolarons are in fact electrostatically unipolar. Other studies have concluded that these pairs are always of bipolar nature in the true sense of bipolarity in that they are pairs of charge carriers with opposite charge, namely electrons and holes, forming so-called polaron pairs [32][33][34][35][36][37][38][39]. This ambiguity with regards to charge polarity has remained unresolved.…”

Section: Introductionmentioning

confidence: 99%

Discrimination between spin-dependent charge transport and spin-dependent recombination in π-conjugated polymers by correlated current and electroluminescence-detected magnetic resonance

et al. 2016

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Spin-dependent processes play a crucial role in organic electronic devices. Spin coherence can give rise to spin mixing due to a number of processes such as hyperfine coupling, and leads to a range of magnetic field effects. However, it is not straightforward to differentiate between pure single-carrier spin-dependent transport processes which control the current and therefore the electroluminescence, and spin-dependent electron-hole recombination which determines the electroluminescence yield and in turn modulates the current. We therefore investigate the correlation between the dynamics of spin-dependent electric current and spin-dependent electroluminescence in two derivatives of the conjugated polymer poly(phenylene-vinylene) using simultaneously measured pulsed electrically detected (pEDMR) and optically detected (pODMR) magnetic resonance spectroscopy. This experimental approach requires careful analysis of the transient response functions under optical and electrical detection. At room temperature and under bipolar charge-carrier injection conditions, a correlation of the pEDMR and the pODMR signals is observed, consistent with the hypothesis that the recombination currents involve spin-dependent electronic transitions. This observation is inconsistent with the hypothesis that these signals are caused by spin-dependent charge carrier transport. These results therefore provide no evidence that supports earlier claims that spindependent transport plays a role for room temperature magnetoresistance effects. At low temperatures, however, the correlation between pEDMR and pODMR is weakened, demonstrating that more than one spin-dependent process influences the optoelectronic materials properties. This conclusion is consistent with prior studies of half-field resonances that were attributed to spin-dependent triplet exciton recombination which becomes significant at low temperatures when the triplet lifetime increases. PACS

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Section: Introductionmentioning

confidence: 99%

Discrimination between spin-dependent charge transport and spin-dependent recombination in π-conjugated polymers by correlated current and electroluminescence-detected magnetic resonance

et al. 2016

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“…[30, Such photogeneration of carriers is particularly important in blend systems for photovoltaic applications. [84][85][86] However, mere absorption measurements are usually less sensitive than, for example, fluorescence-based investigations. In the former case differential changes over a large background are measured, whereas in the latter case the background can, in principle, be set to zero.…”

Section: Electron Spin Resonance Spectroscopy Of Organic Semiconductorsmentioning

confidence: 99%

Coherent Spin Manipulation in Molecular Semiconductors: Getting a Handle on Organic Spintronics

Lupton¹,

McCamey²,

Boehme³

2010

ChemPhysChem

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Organic semiconductors offer expansive grounds to explore fundamental questions of spin physics in condensed matter systems. With the emergence of organic spintronics and renewed interest in magnetoresistive effects, which exploit the electron spin degree of freedom to encode and transmit information, there is much need to illuminate the underlying properties of spins in molecular electronic materials. For example, one may wish to identify over what length of time a spin maintains its orientation with respect to an external reference field. In addition, it is crucial to understand how adjacent spins arising, for example, in electrostatically coupled charge‐carrier pairs, interact with each other. A periodic perturbation of the field may cause the spins to precess or oscillate, akin to a spinning top experiencing a torque. The quantum mechanical characteristic of the spin is then defined as the coherence time, the time over which an oscillating spin, or spin pair, maintains a fixed phase with respect to the driving field. Electron spins in organic semiconductors provide a remarkable route to performing “hands‐on” quantum mechanics since permutation symmetries are controlled directly. Herein, we review some of the recent advances in organic spintronics and organic magnetoresistance, and offer an introductory description of the concept of pulsed, electrically detected magnetic resonance as a technique to manipulate and thus characterize the fundamental properties of electron spins. Spin‐dependent dissociation and recombination allow the observation of coherent spin motion in a working device, such as an organic light‐emitting diode. Remarkably, it is possible to distinguish between electron and hole spin resonances. The ubiquitous presence of hydrogen nuclei gives rise to strong hyperfine interactions, which appear to provide the basis for many of the magnetoresistive effects observed in these materials. Since hyperfine coupling causes quantum spin beating in electron–hole pairs, an extraordinarily sensitive probe for hyperfine fields in such pairs is given.

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“…A variety of spin-dependent models have been developed based on the observed signs of cwODMR and cwEDMR signals as well as experimental parameters, like pair generation rates, temperature, MW power, and modulation frequency. Examples for such studies exist for a-si 2,6,7,10,[12][13][14]16,53,60 , InP nanocrystals 20 , II-IV semiconductors 19,61 , PbI 2 films 48 , nanoparticles 18 , and organic semiconductors 30,31,34,35,37,39,42,44,49,50 . For instance, it has been generally accepted that radiative and non-radiative recombination results in enhancement and quenching of cwODMR signal respectively 2,7,18,48,62 , and all recombination processes and all de-trapping processes result in quenching and enhancement of cwEDMR signals, respectively 2,62 .…”

Section: Introductionmentioning

confidence: 99%

“…Because cwODMR and cwEDMR can be used to distinguish overlapping recombination bands and their dynamics in disordered materials, they have also been used to investigate (usually amorphous) organic semiconductors: cwODMR and cwEDMR have provided information about spin-pairs dominating electronic processes and their transitions in conducting polymers [28][29][30][31][32][33][34][35][36][37][38][39] , small molecules [40][41][42] , and polymer or small molecule/fullerene blends 43,44 . The effect of isotopic modification on magnetic field effects in organic semiconductors also has been observed by ODMR 45 , and the intersystem-crossing time has been extracted from the modulation frequency dependence 46 .…”

Section: Introductionmentioning

confidence: 99%

Modulation frequency dependence of continuous-wave optically/electrically detected magnetic resonance

et al. 2012

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Continuous wave optically and electrically detected magnetic resonance spectroscopy (cwODMR/cwEDMR) allow the investigation of paramagnetic states involved in spin-dependent transitions, like recombination and transport. Although experimentally similar to conventional electron spin resonance (ESR), there exist limitations when applying models originally developed for ESR to observables (luminescence and electric current) of cwODMR and cwEDMR. Here we present closed-form solutions for the modulation frequency dependence of cwODMR and cwEDMR based on an intermediate pair recombination model and discuss ambiguities which arise when attempting to distinguish the dominant spin-dependent processes underlying experimental data. These include: 1) a large number of quantitatively different models cannot be differentiated, 2) signs of signals are determined not only by recombination, but also by other processes like dissociation, intersystem-crossing, pair generation, and even experimental parameter such as, modulation frequency, microwave power, and temperature, 3) radiative and non-radiative recombination cannot be distinguished due to the observed signs of cwODMR and cwEDMR experiments.

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Optical- and photocurrent-detected magnetic resonance studies on conjugated polymer/fullerene composites

Cited by 58 publications

References 32 publications

Discrimination between spin-dependent charge transport and spin-dependent recombination in π-conjugated polymers by correlated current and electroluminescence-detected magnetic resonance

Discrimination between spin-dependent charge transport and spin-dependent recombination in π-conjugated polymers by correlated current and electroluminescence-detected magnetic resonance

Coherent Spin Manipulation in Molecular Semiconductors: Getting a Handle on Organic Spintronics

Modulation frequency dependence of continuous-wave optically/electrically detected magnetic resonance

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