Browsing by Author "List, E. J. W."

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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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    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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    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.