We present a mechanism for the recently discovered magnetoresistance in disordered pi-conjugated materials, based on hopping of polarons and bipolaron formation, in the presence of the random hyperfine fields of the hydrogen nuclei and an external magnetic field. Within a simple model we describe the magnetic field dependence of the bipolaron density. Monte Carlo simulations including on-site and longer-range Coulomb repulsion show how this leads to positive and negative magnetoresistance. Depending on the branching ratio between bipolaron formation or dissociation and hopping rates, two different line shapes in excellent agreement with experiment are obtained.

No abstract

Electroluminescence in organic light-emitting diodes arises from a charge-transfer reaction between the injected positive and negative charges by which they combine to form singlet excitons that subsequently decay radiatively. The quantum yield of this process (the number of photons generated per electron or hole injected) is often thought to have a statistical upper limit of 25 per cent. This is based on the assumption that the formation cross-section of singlet excitons, sigmaS, is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, sigmaT; that is, sigmaS/sigmaT approximately 1. However, recent experimental and theoretical work suggests that sigmaS/sigmaT may be greater than 1. Here we report direct measurements of sigmaS/sigmaT for a large number of pi-conjugated polymers and oligomers. We have found that there exists a strong systematic, but not monotonic, dependence of sigmaS/sigmaT on the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated pi-electrons, and quantify this process using exact valence bond calculations. The calculated sigmaS/sigmaT reproduces the experimentally observed trend. The calculations also show that the strong dependence of sigmaS/sigmaT on the optical gap is a signature of the discrete excitonic energy spectrum, in which higher energy excitonic levels participate in the charge recombination process.

Following the recent discovery of large magnetoresistance at room temperature in polyfluorence sandwich devices, we have performed a comprehensive magnetoresistance study on a set of organic semiconductor sandwich devices made from different pi-conjugated polymers and small molecules. The measurements were performed at different temperatures, ranging from 10K to 300K, and at magnetic fields, B < 100mT . We observed large negative or positive magnetoresistance (up to 10% at 300K and 10mT) depending on material and device operating conditions. We compare the results obtained in devices made from different materials with the goal of providing a comprehensive picture of the experimental data. We discuss our results in the framework of known magnetoresistance mechanisms and find that none of the existing models can explain our results.

We report on the discovery of a large, room temperature magnetoresistance (MR) effect in polyfluorene sandwich devices in weak magnetic fields. We characterize this effect and discuss its dependence on voltage, temperature, film thickness, electrode materials, and (unintentional) impurity concentration. We usually observed negative MR, but positive MR can also be achieved under high applied electric fields. The MR effect reaches up to 10% at fields of 10mT at room temperature. The effect shows only a weak temperature dependence and is independent of the sign and direction of the magnetic field. We find that the effect is related to the hole current in the devices.

We explore the possibility that hyperfine interaction causes the recently discovered organic magnetoresistance (OMAR) effect. Our study employs both experiment and theoretical modelling. An excitonic pair mechanism model based on hyperfine interaction, previously suggested by others to explain magnetic field effects in organics, is examined. Whereas this model can explain a few key aspects of the experimental data, we, however, uncover several fundamental contradictions as well. By varying the injection efficiency for minority carriers in the devices, we show experimentally that OMAR is only weakly dependent on the ratio between excitons formed and carriers injected, likely excluding any excitonic effect as the origin of OMAR.Comment: 10 pages, 7 figures, 1 tabl

We present a theory for spin diffusion in disordered organic semiconductors, based on incoherent hopping of a charge carrier and coherent precession of its spin in an effective magnetic field, composed of the random hyperfine field of hydrogen nuclei and an applied magnetic field. From Monte Carlo simulations and an analysis of the waiting-time distribution of the carrier we predict a surprisingly weak temperature dependence, but a considerable magnetic-field dependence of the spin-diffusion length. We show that both predictions are in agreement with experiments on organic spin valves.

We have measured the ratio, r = σS/σT of the formation cross section, σ of singlet (σS) and triplet (σT ) excitons from oppositely charged polarons in a large variety of π-conjugated oligomer and polymer films, using the photoinduced absorption and optically detected magnetic resonance spectroscopies. The ratio r is directly related to the singlet exciton yield, which in turn determines the maximum electroluminescence quantum efficiency in organic light emitting diodes (OLED). We discovered that r increases with the conjugation length, CL; in fact a universal dependence exists in which r −1 depends linearly on CL −1 , irrespective of the chain backbone structure. These results indicate that π-conjugated polymers have a clear advantage over small molecules in OLED applications.The efficiency of fluorescence-based organic light emitting diodes (OLED) is determined by the fraction of injected electrons (e) and holes (h) that recombine to form emissive spin-singlet excitons, rather than the nonemissive spin-triplet excitons. If the process by which these excitons form were spin-independent, then the maximum quantum efficiency, η max of OLEDs would be limited to 25% [1], which is the statistical limit. The reason for the 25% statistical limit is that the combination of two spin-1/2 particles gives four possible total spin states; three of which have total spin 1, only one is a singlet state. But recent reports have indicated that η max in π-conjugated OLEDs ranges between 22% to 63% [2-6], and the reason for this variation is under investigation. In particular the dependence of η max on the conjugation length (CL) was recently tested by measuring η max in a monomer and related polymer, and the possibility that η max increases with the CL in π-conjugated materials was advanced [6].For systems which are light emitting the quantum efficiency, η EL for electro-luminescence (EL), is η EL = η 1 η 2 η 3 , where η 1 is the singlet emission quantum efficiency, η 2 is the fraction of the total number of excitons that are singlets, and η 3 is the probability that the injected e and h find each other to form e-h pairs [7]. Since both η 1 and η 3 < 1, it follows that η EL < η 2 = η max . We have recently developed [5] a spectroscopic technique based on photoinduced absorption (PA) and PA-detected magnetic resonance (PADMR) spectroscopies, which allows direct measurement of the ratio r = σ S /σ T of the formation cross-section, σ of singlet (σ S ) and triplet (σ T ) excitons from oppositely charged polarons in films of π-conjugated materials. In the limiting case that the spin relaxation rates are much faster than the exciton formation rates from e-h polaron pairs we showed that η max = (1 + 3r. Thus the study of the ratio r in organic materials also provides information about η max in OLEDs. Using this spectroscopic technique we could infer r [5], and consequently η max in a variety of π-conjugated materials without the need to fabricate OLEDs based on the particular material as the active layer. We found that r depends on the ma...