Browsing by Author "Graupner, W."

Now showing 1 - 13 of 13
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    Charged defects in highly emissive organic wide-band-gap semiconductors
    List, E. J. W.; Kim, C. H.; Shinar, J.; Pogantsch, A.; Leising, G.; Graupner, W. (AIP Publishing, 2000-04)
    A combined photoluminescence (PL) -detected magnetic-resonance (PLDMR) and thermally stimulated current (TSC) study of defects in wide-band-gap para-phenylene-type semiconductors is described. As TSC probes the density of mobile charge carriers after detrapping and PLDMR reveals the influence of trapped charges on the PL, their combination yields the concentration of traps, their energetic position, and their contribution to PL quenching. The reported trap densities, which are 2 x 10(16) for the polymer and 1 x 10(14) cm(-3), for the oligomer, are the lowest reported for para-phenylene-type materials. (C) 2000 American Institute of Physics. [S0003-6951(00)03615-9].
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    Creation of a gradient polymer-fullerene interface in photovoltaic devices by thermally controlled interdiffusion
    Drees, M.; Premaratne, K.; Graupner, W.; Heflin, James R.; Davis, Ruth M.; Marciu, D.; Miller, M. (AIP Publishing, 2002-12-01)
    Efficient polymer-fullerene photovoltaic devices require close proximity of the two materials to ensure photoexcited electron transfer from the semiconducting polymer to the fullerene acceptor. We describe studies in which a bilayer system consisting of spin-cast 2-methoxy-5-(2(')-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and sublimed C-60 is heated above the MEH-PPV glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk, gradient heterojunction is created bringing the fullerene molecules within the exciton diffusion radius of the MEH-PPV throughout the film to achieve highly efficient charge separation. The interdiffused devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface. (C) 2002 American Institute of Physics.
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    Direct evidence for singlet-triplet exciton annihilation in pi-conjugated polymers
    List, E. J. W.; Scherf, U.; Mullen, K.; Graupner, W.; Kim, C. H.; Shinar, J. (American Physical Society, 2002-12-06)
    A blend of two conjugated polymers with different optical band gaps and different triplet exciton (TE) magnetic resonance features was studied by means of photoinduced absorption (PA) and photoluminescence detected magnetic resonance. From the latter we find that a reduction of TE's on either of the two polymers enhances the radiative singlet exciton (SE) decay on both polymers nearly identically. The PA results rule out other possible mechanisms to yield this signal, except one: a long-range SE-TE annihilation due to a dipole-dipole transfer mechanism. Based on this finding, we calculate the transfer radius and quantify the annihilation process for this system and for other conjugated polymers.
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    Geometry-dependent electronic properties of highly fluorescent conjugated molecules
    Yang, S. C.; Graupner, W.; Guha, S.; Puschnig, P.; Martin, C.; Chandrasekhar, H. R.; Chandrasekhar, M.; Leising, G.; Ambrosch-Draxl, C.; Scherf, U. (American Physical Society, 2000-09-11)
    We present a combined experimental/theoretical study of the electronic properties of conjugated para-phenylene type molecules under high pressure up to 80 kbar. Pressure is used as a tool to vary the molecular geometry and intermolecular interaction. The influence of the latter two on singlet and triplet excitons as well as polarons is monitored via optical spectroscopy. We have performed bond structure calculations for the planar poly(para-phenylene) and calculated the dielectric function. By varying the intermolecular distances and the length of the polymer repeat unit the observed pressure effects can be explained.
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    The influence of self-absorption on the photoluminescence of thin film CdS demonstrated by two-photon absorption
    Ullrich, B.; Schroeder, R.; Graupner, W.; Sakai, H. (Optical Society of America, 2001-07)
    By means of two-photon excited photoluminescence, we demonstrate the influence of self-absorption on the emission properties of thin (1.5 mum) film CdS formed by laser ablation. The excitation of the sample is performed with 200 fs pulses at 804 nm (1.54 eV). The photoluminescence spectrum takes the form of a single peak centered at 510 nm (2.43 eV) at 300 K. The spectrum is shifted about 45 meV to lower energies with respect to the photoluminescence excited by one photon absorption. By fitting the photoluminescence spectra with the Roosbroeck-Shockley relation and Urbach's rule, it is shown by Beer's law that the shift is caused by self-absorption. The results further provide evidence of low impurity concentration and excellent surface quality. They also confirm the outstanding optical properties of thin film CdS formed by pulsed-laser deposition and suggest the application of the films for effective up-conversation materials in ultra-fast experiments. (C) 2001 Optical Society of America.
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    Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study
    List, E. J. W.; Kim, C. H.; Naik, A. K.; Scherf, U.; Leising, G.; Graupner, W.; Shinar, J. (American Physical Society, 2001-10-15)
    The steady-state photoinduced absorption (PA), photoluminescence (PL), PL-detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) No 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's) 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-enhancing) 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 pi -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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    Kinetics of singlet and triplet excitons in a wide-band-gap copolymer
    Loi, M. A.; Gadermaier, C.; List, E. J. W.; Leising, G.; Graupner, W.; Bongiovanni, G.; Mura, A.; Pireaux, J. J.; Kaeriyama, K. (American Physical Society, 2000-01-15)
    Transient and photomodulation spectroscopy is used in order to determine decay times and densities of both emitting and absorbing species in the wide band-gap semiconductor poly-2,5-diheptyl-1,4-phenylene-alt-2, S-thienylene (PDHPT). The wide band gap of this material is a consequence of the large twisting of the neighboring constituents of the polymer chain. Transient spectroscopy revealed a monoexponential decay of singlet excitons in PDHPT solutions with a radiative and nonradiative decay time of 1.9 and 1.6 ns, respectively. For the solid film photoluminescence decays biexponentially. This distinct decay behavior in solid state is attributed to the migration of the excitons towards quenching sites favored by the three dimensionality in the bulk material, which hence reduces the photoluminescence quantum yield. Infrared studies suggest that upon photo-oxidation one introduces carbonyl groups in PDHPT, which shortens the decay times by introducing photoluminescence quenching centers. Photomodulation spectroscopy is exploited to determine steady-state density of triplet excitons along with their decay dynamics and we found that the steady-state density of the triplets can be as high as 10(16) cm(-3) in this material. Furthermore, we determined the generation probability of triplets to be 2 x 10(-3).
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    Localized triplet excitations and the effect of photo-oxidation in ladder-type poly(p-phenylene) and oligo(p-phenylene)
    List, E. J. W.; Partee, J.; Shinar, J.; Scherf, U.; Mullen, K.; Zojer, E.; Petritsch, K.; Leising, G.; Graupner, W. (American Physical Society, 2000-04-15)
    The photophysics of methyl-bridged poly(para-phenylene)-type ladder polymer (m-LPPP) and oligomer films and solutions is described and discussed. The spin sensitive properties, such as the formation and properties of polaron pairs and triplet excitons (TE's) were studied using X-band photoluminescence (PL) detected magnetic resonance (PLDMR). The PLDMR results and quantum chemical calculations show unambiguously that the TE wave-function extent is much smaller than that of the singlet exciton (SE). The weaker vibronic structure of the triplet photoinduced absorption (PA) band of m-LPPP relative to the singlet absorption is assigned to the small energy difference between the geometry of the lowest lying and excited triplet states. This is an additional indication of the strong localization of the triplet wave-function as compared to that of the SE. Finally, the influence of photo-oxidation on the PLDMR and PA is analyzed and discussed in relation to the photoconductivity of the materials.
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    Photogeneration action spectroscopy of neutral and charged excitations in films of a ladder-type poly(para-phenylene)
    Wohlgenannt, M.; Graupner, W.; Leising, G.; Vardeny, Z. V. (American Physical Society, 1999-04-19)
    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 tripler 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].
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    Photogeneration and recombination processes of neutral and charged excitations in films of a ladder-type poly(para-phenylene)
    Wohlgenannt, M.; Graupner, W.; Leising, G.; Vardeny, Z. V. (American Physical Society, 1999-08-15)
    We introduce a version of the cw photomodulation technique, measured far from the steady state, for obtaining the quantum efficiency, eta, of long-lived photoexcitations in pi-conjugated polymers. We apply this technique to films of a ladder-type poly(para-phenylene) [mLPPP] for studying the photogeneration action spectra, eta(E), and recombination kinetics of photogenerated neutral and charged excitations such as singlet and triplet excitons and charged polarons. Whereas the eta(E) spectrum for singlet excitons shows a step function increase at a photon energy, E, close to the optical gap (similar or equal to 2.6 eV), both triplet and polaron eta(E) spectra show, in addition, a monotonous rise at higher E. The rise for triplets is explained by singlet exciton fission into triplet pairs, and from a model fit we get the triplet exciton energy (similar or equal to 1.6 eV). For polarons this rise is modeled by an electron intersegment tunneling process. The electroabsorption spectrum is also measured and analyzed in terms of Stark shift of the lowest lying exciton, 1 B-u, and enhanced oscillator strength of the important mA(g) exciton. A consistent picture for the lowest excited state energy levels and optical transitions in the neutral (singlet and triplet) and charged manifolds is presented. From both the exciton binding energy of similar or equal to 0.6 eV and the singlet-triplet energy splitting of similar or equal to 1 eV, we conclude that the e-e interaction in mLPPP is relatively strong. Our results are in good agreement with recent ab initio band structure calculations for several pi-conjugated polymers. [S0163-1829(99)13531-8].
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    Primary optical excitations and excited-state interaction energies in sexithiophene
    Loi, M. A.; Martin, C.; Chandrasekhar, H. R.; Chandrasekhar, M.; Graupner, W.; Garnier, F.; Mura, A.; Bongiovanni, G. (American Physical Society, 2002-09-11)
    Based on a unique combination of angle-resolved transmission spectroscopy and transmission data at high pressure, we identify the primary photoexcitations and the relevant excited-state interaction energies in a sexithiophene crystal. Optical excitations include charge-transfer excitons and Davydov polaritons. By extrapolation, we predict that in sexithiophene at hydrostatic pressures above 180 kbar, intermolecular excitations are lower in energy than intramolecular ones. The results are representative for a wide class of pi-conjugated molecular semiconductors because (1) the pertinent interaction energies and lengths scales are nearly identical and (2) published data on different molecules are consistent with our interpretation.
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    Random walks with imperfect trapping in the decoupled-ring approximation
    Aspelmeier, T.; Magnin, J.; Graupner, W.; Täuber, Uwe C. (Springer-Verlag, 2002-08-01)
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    Ultrafast photogeneration mechanisms of triplet states in para-hexaphenyl
    Zenz, C.; Cerullo, G.; Lanzani, G.; Graupner, W.; Meghdadi, F.; Leising, G.; De Silvestri, S. (American Physical Society, 1999-06)
    We present femtosecond pump-probe measurements, both conventional and electric field-assisted, on organic light-emitting devices based on para-hexaphenyl. The dominant triplet excition generation mechanism is assigned to nongeminate bimolecular recombination of photogenerated, spin-1/2 polarons. This process is active within a few hundred femtoseconds after photoexcitation and involves about 20% of the initially excited states. At higher photoexcitation densities, we observe an additional triplet generation mechanism, which occurs in the 10-ps time domain, due to fusion of singlet excitons and subsequent fission into correlated triplet pairs. The latter decay on the 10(2)-ps time scale by geminate recombination.