Electromodulated photoinduced absorption : A new spectroscopy in π- conjugated polymer/C60 blends

Optical Science, Engineering and Instrumentation '97, 1997, San Diego, CA, United StatesMartin Liess, Paul A. Lane, Zakya H. Kafafi, M. Hamaguchi, Masanori Ozaki, Katsumi Yoshino, and Z. Valy Vardeny "Electromodulated photoinduced absorption: a new spectroscopy in π-conjugated polymer/C60 blends", Proc. SPIE 3142, Fullerenes and Photonics IV, (1 November 1997). DOI: https://doi.org/10.1117/12.29335

Optical absorption spectra in fullerenes C60 and C70: Effects of Coulomb interactions, lattice fluctuations, and anisotropy

Effects of Coulomb interactions and lattice fluctuations in the optical absorption spectra of C60 and C70 are theoretically investigated by using a tight binding model with long-range Coulomb interaction and bond disorder. Anisotropy effects in C70 are also considered. Optical spectra are calculated by using the Hartree-Fock approximation followed by the configuration interaction method. The main conclusions are as follows: (1) The broad peaks at excitation energies, 3.7eV, 4.7eV, and 5.7eV, observed in experiments of C60 molecules in a solution are reasonably described by the present theory. Peak positions and relative oscillator strengths are in overall agreement with the experiments. The broadening of peaks by lattice fluctuations is well simulated by the bond disorder model. (2) The optical gap of C70 is larger when the electric field of light is parallel to the long axis of the molecule. The shape of the frequency dispersion also depends on the orientation of the molecule. These properties are common in the free electron model and the model with Coulomb interactions. (3) The spectrum of C70 averaged over bond disorder and random orientations is compared with experiments in a solution. There is an overall agreement about the spectral shape. Differences in the spectra of C60 and C70 are discussed in connection with the symmetry reduction from a soccerball to a rugbyball.Comment: PACS numbers: 78.66.Qn, 78.20.Dj, 71.35.+z, 31.20.Tz; LaTeX, 15 pages, 5 figures (Physical Review B); Note: Please request figures to Authors. They will be sent via snail mai

Optical absorption spectra of A6C60 and A6C70: Reduction of effective Coulomb interactions in Frenkel excitons

We theoretically investigate optical absorption spectra of \soc^{6-} and \rug^{6-}, and discuss relations with the optical properties of alkali metal doped fullerides A_6\soc and A_6\rug. This is a valid approach for systems where Frenkel exciton effects are dominant. We use a tight binding model with long ranged Coulomb interactions and bond disorder. Optical spectra are obtained by the Hartree-Fock approximation and the configuration interaction method. We find that the Coulomb interaction parameters, which are relevant to the optical spectra of A_6\soc (A_6\rug) in order to explain the excitation energies and relative oscillator strengths of absorption peaks, are almost the half of those of the neutral \soc (\rug). The reduction of the effective Coulomb interactions is concluded for the heavily doped case of \soc and \rug. This finding is closely related with the experimental fact that dielectric constants of fullerides which are maximumly doped with alkali metals become about twice as large as those of the neutral systems.Comment: Note: A full preprint with figures should be requested to the author. It will be sent by air-mail.; E-mail: [email protected]

Electronic properties of silole-based organic semiconductors

© 2004 American Institute of Physics. The electronic version of this article is the complete one and can be found at: http://dx.doi.org/10.1063/1.1804155DOI: 10.1063/1.1804155Erratum: The third author’s name was misspelled in the original article. The correct spelling is Z. H. Kafafi.We report on a detailed quantum-chemical study of the geometric structure and electronic properties of 2,5-bis(6′-(2′,2″-bipyridyl))-1,1-dimethyl-3,4-diphenylsilole (PyPySPyPy) and 2,5-di- (3-biphenyl)-1,1-dimethyl-3,4-diphenylsilole (PPSPP). These molecular systems are attractive candidates for application as electron-transport materials in organic light-emitting devices. Density Functional Theory (DFT), time-dependent DFT, and correlated semiempirical (ZINDO/CIS) calculations are carried out in order to evaluate parameters determining electron-transport and optical characteristics. Experimental data show that PyPySPyPy possesses an electron-transport mobility that is significantly greater than PPSPP, while PPSPP has a significantly larger photoluminescence quantum yield; however, the theoretical results indicate that the two systems undergo similar geometric transformations upon reduction and have comparable molecular orbital structures and energies. This suggests that intermolecular interactions (solid-state packing, electronic coupling) play significant roles in the contrasting performance of these two molecular systems

Organic light emitting devices with saturated red emission using 6,13-diphenyl pentacene

Organic electroluminescent devices with saturated red emission were developed using 6,13-diphenylpentacene (DPP) doped into tris(8-hydroxyquinolinato) aluminum III (Alq_3). DPP exhibits a narrow emission spectrum giving rise to a saturated red peak, centered around 625 nm, with excellent chromaticity coordinates (x = 0.63 and y = 0.34) in accordance with the Commission Internationale de l'Eclairage. An absolute photoluminescence (PL) quantum yield (Φ_PL) of ～30% was measured for a composite film of 0.55 mol % of DPP doped into Alq_3. An electroluminescence (EL) quantum efficiency of 1.3% at 100 A/m^2, close to the estimated theoretical limit (1.5%), was measured for an unoptimized device structure that consists of an active emissive layer sandwiched between hole- and electron-transport layers. In addition, the EL quantum efficiency is constant or stable over a wide range of current densities (1-1000 A/m^2) or luminance values (1-1000 cd/m^2)