Highly efficient electroluminescence of new wide band gap ladder-type poly(para-phenylenes)

Tasch, S.; Niko, A.; Leising, G.; Scherf, Ullrich

doi:10.1063/1.115722

Cited by 214 publications

(83 citation statements)

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“…The methyl-substituted poly͑para-phenylene͒-type ladderpolymer (m-LPPP) is highly attractive for applications in organic light-emitting diodes 1 ͑OLED's͒ and as the active medium in organic solid-state lasers 2 due to its high photoluminescence ͑PL͒ quantum yield PL ϳ30% in film and ϳ100% in solution, 3 and the high interchain order in the solid state. 4,5 From previous studies on m-LPPP a comprehensive picture of the generation of polarons [6][7][8] and triplet excitons 8,9 ͑TE's͒ from singlet excitons ͑SE's͒ was drawn.…”

Section: Introductionmentioning

confidence: 99%

Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study

et al. 2001

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The steady-state photoinduced absorption ͑PA͒, photoluminescence ͑PL͒, PL-detected magnetic resonance ͑PLDMR͒, and PA-detected magnetic resonance ͑PADMR͒ of poly-and oligo-͑para-phenylenes͒ films is described. In particular, the excitation density ͑laser power͒ N 0 dependence of the PA, PL, and PLDMR signals is analyzed by means of a rate equation model, which describes the dynamics of singlet excitons ͑SE's͒ and polarons in all three experiments quantitatively with the same set of parameters. The model is based on the observations that mobile SE's are quenched by trapped and free polarons and that the spin-1 2 magnetic resonance conditions reduce the total polaron population. Since the sublinear N 0 dependences of the positive ͑PL-enhancing͒ spin-1 2 PLDMR and the polaron PA band are essentially the same, we conclude that PLDMR is due to a reduced quenching of SE's by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. This also suggests that it is a very significant process in luminescent -conjugated materials and organic light-emitting devices. Consequently, the quenching mechanism needs to be taken into account, especially at high excitation densities, which is of great importance for the development of electrically pumped polymer laser diode structures.

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Section: Introductionmentioning

confidence: 99%

Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study

et al. 2001

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“…Furthermore, the increase of the current and the brightness for drive voltages above the onset is less steep in the LED than in the LEC (see also Ref. 16). …”

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“…This value of the EL efficiency of the LEC should be compared to that of an ITO/m-LPPP/Al LED, which is up to 1%. 16 The main advantage of the LECs based on m-LPPP compared to PLEDs based on m-LPPP is the much lower voltage required for the onset of electroluminescence. The onset of current flow and the light emission for LECs based on m-LPPP using Al and ITO as the electrodes occurs at 2.7 V in the reverse direction (when recorded from 0 V to negative voltages with ITO as the cathode) (see Fig.…”

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Blue and Green Light-Emitting Electrochemical Cells with Microsecond Response Times

Tasch

Gao

Wenzl

et al. 1999

Electrochem. Solid-State Lett.

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Blue and green light-emitting electrochemical cells (LECs) based on a ladder-type poly(paraphenylene) (m-LPPP) are described. These LECs are characterized by an extremely low onset for light emission (between 2.2 and 2.7 V in reverse direction for blue emission) yielding an external quantum efficiency of approximately 0.3% and a brightness of about 250 cd/m 2 at 10 V. The LECs based on m-LPPP show a fast pulse response below 30 µs, which is ascribed to restricted motion of the ions in the LEC under pulse operation. © 1999 The Electrochemical Society. S1099-0062(98)09-017-8. All rights reserved.Manuscript submitted September 10, 1998; revised manuscript received February 8, 1999. Available electronically March 18, 1999 Light-emitting diodes based on conjugated polymers (PLEDs) can be realized with promising efficiencies over the entire visible spectral range. [1][2][3][4][5][6] The most attractive potential application of PLEDs is their use in emissive flat panel displays. In order to realize PLEDs with operating lifetimes which fulfill the values required for industrial applications, efficient devices with low operating voltages are necessary. Low operating voltages are more easily realized with small bandgap conjugated polymers, because the mismatch between the energy bands of the polymers and the work functions of the electrodes 1 is smaller than for wide bandgap polymers. Thus, it has proved to be easier to realize red-orange and green PLEDs with low operating voltages and high efficiencies. 1,2 However, the fabrication of blue PLEDs with low turn-on voltages and sufficient lifetimes is absolutely necessary to realize multicolor flat panel displays. Moreover, one of the most promising ways to produce multicolor displays is based upon blue PLEDs, and the realization of red and green light emission through down-conversion. [7][8][9] Recently, a new type of polymer light-emitting device, the polymer light-emitting electrochemical cell (LEC) has been described and analyzed. [10][11][12] LECs have the advantage that the electroluminecsence (EL) onset occurs for all emission colors at bias voltages which are approximately equal to the single-particle energy gap E g divided by the electron charge e. [11][12][13] In LECs, electrochemical doping of the conjugated polymer takes place near both electrodes thereby creating a p-type doped region near the anode and an n-type doped region near the cathode. In between these p-and n-doped zones a region with low, intrinsic conductivity remains. Thus, a p-i-n junction is created in situ. 11 The formation of the p-i-n junction is not an instantaneous process but depends on the mobility of the ions in the three-component blend consisting of a conjugated polymer (which has to be both electrochemically active and luminescent), an ion transport polymer, and a molecular salt. We demonstrate the realization of efficient blue and green LECs based on blends of the conjugated polymer m-LPPP [methyl substituted ladder-type poly(paraphenylene), prepared according to Ref. 14] and poly(eth...

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“…1, inset. mLPPP is an attractive p-conjugated polymer for blue-light emitting devices [6] due to its high photoluminescence (PL) quantum yield; this is caused, in part, by the intrachain order in the film induced by the planarization of neighboring phenyl rings [7]. mLPPP was chosen as a model polymer because of its reduced inhomogeneity and small defect density [8].…”

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Photogeneration Action Spectroscopy of Neutral and Charged Excitations in Films of a Ladder-Type Poly(Para-Phenylene)

et al. 1999

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The photogeneration quantum efficiency action spectra of long-lived neutral and charged excitations in films of a ladder-type poly(para-phenylene) were measured. We found that both triplet and polaron action spectra show, in addition to a step function increase at the optical gap, a monotonic rise at higher energies. For triplets this rise is explained by singlet exciton fission into triplet pairs from which the triplet exciton energy in the gap was obtained; this energy was also confirmed by measuring the weak phosphorescence band. For polarons the photogeneration increase at high energies is modeled by a novel hot electron interchain tunneling process. [S0031-9007(99)08980-2] PACS numbers: 78.55. Kz, 72.20.Jv, 78.30.Jw, 78.66.Qn The photogeneration dynamics of singlet excitons and secondary photoexcitations, such as triplets and polarons, in p-conjugated polymers have been usually measured by picosecond transient spectroscopic techniques rather than cw spectroscopies, since their photogeneration processes usually occur in the subnanosecond time domain [1]. On the other hand, the excited states energy levels of these polymers are often measured by cw optical techniques including electroabsorption [2], resonant Raman scattering [3], and by two and three photon nonlinear optical spectroscopies [4]. In this Letter we employ a new version of the cw photomodulation (PM) spectroscopy technique, which is capable of measuring the photogeneration quantum efficiency, h, of long-lived secondary photoexcitations without the need of transient optical techniques. We show that when the PM signal is measured under conditions far from the steady state, then it directly correlates with h; therefore the PM action spectrum (PMAS) measured under these conditions gives the dependence of h on the excitation photon energy, E, i.e., h͑E͒.We have used the PMAS technique to obtain h͑E͒ of singlet and triplet excitons and polarons, respectively, in films of methyl-substituted ladder-type poly(paraphenylene) (mLPPP) [5] shown in Fig. 1, inset. mLPPP is an attractive p-conjugated polymer for blue-light emitting devices [6] due to its high photoluminescence (PL) quantum yield; this is caused, in part, by the intrachain order in the film induced by the planarization of neighboring phenyl rings [7]. mLPPP was chosen as a model polymer because of its reduced inhomogeneity and small defect density [8].For the excitation beam in the PMAS measurements we used either an Ar 1 laser at several discrete E, or a monochromatized xenon lamp beam to continuously vary E between 2 and 4.5 eV. The excitation beam was modulated with a chopper at a frequency, f, between 10 to 4 kHz. A combination of various incandescent lamps, diffraction gratings, optical filters, and solid state detectors was used to span the probe energy (hv) between 0.1 and 3 eV. The PM spectrum as a function ofhv was obtained by dividing the pump beam induced changes in transmission, DT ͑v͒, by the probe transmission T ͑v͒, where Da 2d 21 DT ͞T and d is the film thickness; DT was measu...

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Highly efficient electroluminescence of new wide band gap ladder-type poly(para-phenylenes)

Abstract: Blue electroluminescent devices with high quantum efficiency from alkoxysubstituted poly(paraphenylene vinylene)trimers in a polystyrene matrix

Cited by 214 publications

References 19 publications

Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study

Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study

Blue and Green Light-Emitting Electrochemical Cells with Microsecond Response Times

Photogeneration Action Spectroscopy of Neutral and Charged Excitations in Films of a Ladder-Type Poly(Para-Phenylene)

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