M. Wohlgenannt

Large magnetoresistance in nonmagnetic π-conjugated semiconductor thin film devices

Following the recent observation of large magnetoresistance at room temperature in polyfluorene 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 study includes a range of materials that show greatly different chemical structure, mobility, and spin-orbit coupling strength. We study both hole and electron transporters at temperatures ranging from 10 K to 300 K. We observe large negative or positive magnetoresistance (up to 10% at 300 K and 10 mT) depending on material and device operating conditions. We discuss our results in the framework of known magnetoresistance mechanisms and find that none of the existing models can explain our results.

Hyperfine interaction and magnetoresistance in organic semiconductors

We explore the possibility that hyperfine interaction causes the recently discovered organic magnetoresistance (OMAR) effect. We deduce a simple fitting formula from the hyperfine Hamiltonian that relates the saturation field of the OMAR traces to the hyperfine coupling constant. We compare the fitting results to literature values for this parameter. Furthermore, we apply an excitonic pair mechanism model based on hyperfine interaction, previously suggested by others to explain various magnetic-field effects in organics, to the OMAR data. Whereas this model can explain a few key aspects of the experimental data, we 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.

Conjugation-length dependence of spin-dependent exciton formation rates in pi-conjugated oligomers and polymers

We have measured the ratio, r = sigma(S)/sigma(T) of the formation cross section, sigma of singlet and triplet excitons from polarons in pi-conjugated oligomer and polymer films, using a spectroscopic technique we developed recently. We discovered a universal relation between r and the conjugation length (CL): r(-1) depends linearly on CL-1, irrespective of the chain structure. Since r is directly related to the maximum possible electroluminescence quantum efficiency in organic light emitting diodes (OLED), our results indicate that polymers have an advantage over small molecules in OLED applications.

Excitons, polarons, and laser action in poly(p-phenylene vinylene) films

We have used a multitude of linear and nonlinear cw optical spectroscopies to study the optical properties of water precursor poly(p-phenylene vinylene) (PPV) thin films. These spectroscopies include absorption, photoluminescence, photoinduced absorption and their respective optically detected magnetic resonance, and electroabsorption spectroscopy. We have studied singlet and triplet excitons, polarons, and laser action in PPV films. We found that the lowest-lying absorption band is excitonic in origin. It consists of two absorption components due to a bimodal distribution of the polymer chain conjugation lengths. Electroabsorption spectroscopy unambiguously shows the positions of the lowest-lying odd parity exciton 1Bu at 2.59 eV and two of the higher-lying even-parity excitons, namely, mAg at 3.4 eV and kAg at 3.7 eV. From these exciton energies we obtained a lower bound for the exciton binding energy in PPV, Eb(min)=E(mAg)−E(1Bu)=0.8 eV. The quantum efficiency spectrum for triplet exciton photogenerati...

A two-site bipolaron model for organic magnetoresistance

The recently proposed bipolaron model for large “organic magnetoresistance” (OMAR) at room temperature is extended to an analytically solvable two-site scheme. It is shown that even this extremely simplified approach reproduces some of the key features of OMAR, viz., the possibility to have both positive and negative magnetoresistance, as well as its universal line shapes. Specific behavior and limiting cases are discussed. Extensions of the model, to guide future experiments and numerical Monte Carlo studies, are suggested.

Magnetoresistance in π-conjugated organic sandwich devices with varying hyperfine and spin–orbit coupling strengths, and varying dopant concentrations

We study a recently discovered organic magnetoresistive (OMAR) effect whose underlying mechanism is currently not known with certainty. We examine the hypothesis that OMAR is caused by spin-dynamics by varying the strength of the hyperfine and spin–orbit coupling in the organic semiconductor material. We show that unlike most other materials, C60 devices do not exhibit the OMAR effect. Therefore hydrogen atoms and the resulting strong nuclear hyperfine coupling is a necessary prerequisite for the observation of OMAR. We investigate the dependence of OMAR on the strength of the spin-coupling introduced by heavy atoms in two widely used organic semiconductors, and find the characteristic magnetic field scale shifts to much larger fields. We also study OMAR sandwich devices made from the conducting polymer PEDOT. We show that in PEDOT the observed effect is caused by interface resistance, distinct from the case of intrinsic devices where OMAR is related to the bulk resistance of the (undoped) organic semiconductor.

Photophysics of excitation energy transfer in highly fluorescent polymers

Abstract We report the optical properties of highly fluorescent guest host systems of two conjugated polymers. The blue emitter laddertype poly(para-phenylene) (LPPP) is blended as a host with the red emitter poly(perylene-co-diethynylbenzene) (PPDB) as a guest at sub-percent and percent level concentrations. We use transient and steady-state photoluminescence as well as near-steady-state photoinduced absorption to show that an efficient excitation energy transfer of Forster type occurs between the blue emitting host and the red emitting guest. The spectral signatures of emissive and absorptive photogenerated species in both polymers are presented. In addition, we describe analytical relations to determine the lifetime of these species from photomodulation spectroscopy.

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We present the design principles for a pen-input organic light-emitting diode (OLED) display based on the recently discovered organic magnetoresistance effect (OMAR). In the prototypical OLED material Alq3, OMAR is as large as 10% for small magnetic fields, B=10 mT at room temperature. We construct a pen-input screen consisting of an 8 times 8 pixel OMAR array made from Alq3, together with a magnetic pen that emits an ac magnetic field. We describe a multiplexed detection scheme that uses a single filter/amplifier circuit to sequentially scan the individual pixels for the presence of the magnetic pen. For this scheme to work efficiently, it requires using frequencies on the order of 100 kHz. We demonstrate that our OMAR devices can indeed follow such high frequencies under certain operating conditions

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We have studied the interplay between the morphology and photophysics of poly(9,9-dioctyl fluorene) (PFO) films. Spin-casting from solution produces a glassy film with a broad absorption band and a large Stokes shift due to exciton migration. Upon slowly warming a sample from 80 to 300 K, a fraction of the sample converts into a new phase with extended conjugation, which we refer to as the β-phase. Its absorption band exhibits a clear vibronic structure, and the fluorescence red-shifts by approximately 0.1 eV. The photoinduced absorption (PA) spectrum of the glassy phase is dominated by triplet excitons, whereas, both charged polarons and triplet excitons are seen in the PA spectrum of a sample containing both phases.

8 Pixel Array Pen-Input OLED Screen Based on Organic Magnetoresistance

We report on the experimental characterization of a recently discovered large magnetoresistive effect in polyfluorene and in Alq3 organic light-emitting diodes. We also observe similar magnetic field effects (MFEs) of comparable magnitude in electroluminescence and photocurrent measurements. We provide a comprehensive overview of all these three types of MFE. To the best of our knowledge, the mechanism causing these MFEs is not currently known with certainty. Moreover, we show that experiments in bipolar, electroluminescent devices do not allow determination of whether the MFE acts on the carrier density or carrier mobility, making any attempt at explaining it ambiguous. As a remedy, we perform magnetoresistance measurements in hole-only polyfluorene devices and show that the MFE acts on the carrier mobility rather than carrier recombination.