TY - JOUR A1 - Hahn, Tobias A1 - Tscheuschner, Steffen A1 - Saller, Christina A1 - Strohriegl, Peter A1 - Boregowda, Puttaraju A1 - Mukhopadhyay, Tushita A1 - Patil, Satish A1 - Neher, Dieter A1 - Bässler, Heinz A1 - Köhler, Anna T1 - Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C-60 and PCBM JF - The journal of physical chemistry : C, Nanomaterials and interfaces Y1 - 2016 U6 - https://doi.org/10.1021/acs.jpcc.6b08471 SN - 1932-7447 VL - 120 SP - 25083 EP - 25091 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Parkinson, P. A1 - Aharon, E. A1 - Chang, M. H. A1 - Dosche, Carsten A1 - Frey, G. L. A1 - Köhler, Anna A1 - Herz, L. M. T1 - Dimensionality-dependent energy transfer in polymer-intercalated SnS2 nanocomposites N2 - We have investigated the influence of dimensionality on the excitation-transfer dynamics in a conjugated polymer blend. Using time-resolved photoluminescence spectroscopy, we have measured the transfer transients for both a three-dimensional blend film and for quasi-two-dimensional monolayers formed through intercalation of the polymer blend between the crystal planes of an inorganic SnS2 matrix. We compare the experimental data with a simple, dimensionality- dependent model based on electronic coupling between electronic transition moments taken to be point dipoles. Within this approximation, the energy-transfer dynamics is found to adopt a three-dimensional character in the solid film and a two-dimensional nature in the monolayers present in the SnS2-polymer nanocomposite. Y1 - 2007 UR - http://prb.aps.org/toc/PRB/v75/i16 U6 - https://doi.org/10.1103/Physrevb.75.165206 ER - TY - JOUR A1 - Devi, Lekshmi Sudha A1 - Al-Suti, Mohammed K. A1 - Dosche, Carsten A1 - Khan, Muhammad S. A1 - Friend, Richard H. A1 - Köhler, Anna T1 - Triplet energy transfer in conjugated polymers : I. Experimental investigation of a weakly disordered compound N2 - Efficient triplet exciton emission has allowed improved operation of organic light-emitting diodes (LEDs). To enhance the device performance, it is necessary to understand what governs the motion of triplet excitons through the organic semiconductor. Here, we have investigated triplet diffusion using a model compound that has weak energetic disorder. The Dexter-type triplet energy transfer is found to be thermally activated down to a transition temperature T- T, below which the transfer rate is only weakly temperature dependent. We show that above the transition temperature, Dexter energy transfer can be described within the framework of Marcus theory. We suggest that below T-T, the nature of the transfer changes from phonon-assisted hopping to quantum-mechanical tunneling. The lower electron-phonon coupling and higher electronic coupling in the polymer compared to the monomer results in an enhanced triplet diffusion rate. Y1 - 2008 ER - TY - JOUR A1 - Yang, C. A1 - Vardeny, Z. V. A1 - Köhler, Anna A1 - Wohlgenannt, M. A1 - Al-Suti, Mohammed K. A1 - Khan, Muhammad S. T1 - Spectroscopic study of spin-dependent exciton formation rates in pi-conjugated semiconductors : Comparison with electroluminescence techniques N2 - It has been found in recent measurements that the singlet-to-triplet exciton ratio in organic light-emitting diodes (OLEDs) is larger than expected from spin degeneracy, and that singlet excitons form at a larger rate than triplets. We employed the technique of optically detected magnetic resonance to measure the spin-dependent exciton formation rates in films of a polymer and corresponding monomer, and explore the relation between the formation rates and the actual singlet-to-triplet ratio measured previously in OLEDs. We found that the spin-dependent exciton formation rates can indeed quantitatively explain the observed exciton yields, and that singlet formation rates and yields are significantly enhanced only in polymer OLEDs, but not in OLEDs made from the corresponding monomer Y1 - 2004 ER - TY - JOUR A1 - Mak, Chris S. K. A1 - Hayer, Anna A1 - Pascu, S. I. A1 - Watkins, Scott E. A1 - Holmes, Andrew B. A1 - Köhler, Anna A1 - Friend, Richard H. T1 - Blue-to-green electrophosphorescence of iridium-based cyclometallated materials. Y1 - 2005 UR - http://www.rsc.org/ej/CC/2005/b508695g.pdf U6 - https://doi.org/10.1039/b508695gb70 SN - 0022-4936 ER - TY - JOUR A1 - Khan, Amena L. T. A1 - Sreearunothai, Paiboon A1 - Herz, Laura M. A1 - Banach, Michael J. A1 - Köhler, Anna T1 - Morphology-dependent energy transfer within polyfluorene thin films Y1 - 2004 ER - TY - JOUR A1 - Hayer, Anna A1 - Köhler, Anna A1 - Arisi, E. A1 - Bergenti, I. A1 - Dediu, A. A1 - Taliani, C. A1 - Al-Suti, Mohammed K. A1 - Khan, Muhammad S. T1 - Polymer light-emitting diodes with spin-polarised charge injection. Y1 - 2004 SN - 0379-6779 ER - TY - JOUR A1 - Khan, Muhammad S. A1 - Al-Mandhary, Muna R. A. A1 - Al-Suti, Mohammed K. A1 - Al-Battashi, Fathiya R. A1 - Al-Saadi, Sultan A1 - Bjernemose, Jens K. A1 - Mahon, Mary F. A1 - Chawdhury, Nazia A1 - Yonus, Muhammad A1 - Köhler, Anna A1 - Teat, Simon J. A1 - Marseglia, Elizabeth A. A1 - Tedesco, Emilio A1 - Feeder, Neil A1 - Ahrens, Birte A1 - Raithby, Paul R. T1 - Synthesis, characterisation and optical spectroscopy of platinum(II) di-ynes and poly-ynes incorporating condensed aromatic spacers in the backbone Y1 - 2004 U6 - https://doi.org/10.1039/B405070C SN - 1477-9226 ER - TY - JOUR A1 - Sandee, A. J. A1 - Williams, Charlotte K. A1 - Evans, N. R. A1 - Davies, J. E. A1 - Boothby, C. E. A1 - Köhler, Anna A1 - Friend, Richard H. A1 - Holmes, Andrew B. T1 - Solution-processible conjugated electrophosphorescent polymers Y1 - 2004 ER - TY - JOUR A1 - Köhler, Anna A1 - Beljonne, David T1 - The singlet-triplet exchange energy in conjugated polymers Y1 - 2004 SN - 1616-3028 SN - 1616-301X ER - TY - JOUR A1 - Mermer, Ö. A1 - Veeraraghavan, G. A1 - Francis, T. L. A1 - Sheng, Y. A1 - Nguyen, D. T. A1 - Wohlgenannt, M. A1 - Köhler, Anna A1 - Al-Suti, Mohammed K. T1 - Large magnetoresistance in nonmagnetic pi-conjugated semiconductor thin film devices N2 - 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. Y1 - 2005 ER - TY - JOUR A1 - Hayer, Anna A1 - Khan, A. L. T. A1 - Friend, Richard H. A1 - Köhler, Anna T1 - Morphology dependence of the triplet excited state formation and absorption in polyfluorene Y1 - 2005 ER - TY - JOUR A1 - Zhang, Ning A1 - Hayer, Anna A1 - Al-Suti, Mohammed K. A1 - Al-Belushi, Rayya A. A1 - Khan, Muhammad S. A1 - Köhler, Anna T1 - The effect of delocalization on the exchange energy in meta- and para-linked Pt-containing carbazole polymers and monomers N2 - A series of novel platinum-containing carbazole monomers and polymers was synthesized and fully characterized by UV-VIS absorption, luminescence, and photoinduced absorption studies. In these compounds, a carbazole unit is incorporated into the main chain via either a para- or a meta-linkage. We discuss the effects of linkage and polymerization on the energy levels of S-1, T-1, and T-n. The S-1-T-1 splitting observed for the meta-linked monomer (0.4 eV) is only half of that in the para-linked monomer (0.8 eV). Upon polymerization, the exchange energy in the para- linked compound reduces, yet still remains larger than in the meta-linked polymer. We attribute the difference in exchange energy to the difference in wave function overlap between electron and hole in these compounds. (c) 2006 American Institute of Physics Y1 - 2006 UR - http://scitation.aip.org/getpdf/servlet/ GetPDFServlet?filetype=pdf&id=JCPSA6000124000024244701000001&idtype=cvips&doi=10.1063/1.2200351&prog=normal U6 - https://doi.org/10.1063/1.2200351 ER - TY - JOUR A1 - Hayer, Anna A1 - de Halleux, Veronique A1 - Köhler, Anna A1 - El-Garoughy, Abdel A1 - Meijer, E. W. A1 - Barbera, Joaquin A1 - Tant, Julien A1 - Levin, Jeremy A1 - Lehmann, Matthias A1 - Gierschner, Johannes A1 - Cornil, Jerome A1 - Geerts, Yves Henri T1 - Highly fluorescent crystalline and liquid crystalline columnar phases of pyrene-based structures N2 - A concept for highly ordered solid-state structures with bright fluorescence is proposed: liquid crystals based on tetraethynylpyrene chromophores, where the rigid core is functionalized with flexible, promesogenic alkoxy chains. The synthesis of this novel material is presented. The therniotropic properties are studied by means of differential scanning calorimetry (DSC), cross-polarized optical microscopy (POM), and X-ray diffraction. The mesogen possesses an enantiotropic Col(h) phase over a large temperature range before clearing. The material is highly fluorescent in solution and, most remarkably, in the condensed state, with a broad, strongly red shifted emission. Fluorescence quantum yields (Phi(F)) have been determined to be 70% in dichloromethane solution and 62% in the solid state. Concentration- and temperature-dependent absorption and emission studies as well as quantum-chemical calculations on isolated molecules and dimers are used to clarify the type of intermolecular interactions present as well as their influence on the fluorescence quantum yield and spectral properties of the material. The high luminescence efficiency in the solid state is ascribed to rotated chromophores, leading to an optically allowed lowest optical transition Y1 - 2006 UR - http://pubs.acs.org/journal/jpcbfk U6 - https://doi.org/10.1021/Jp0573689 SN - 1520-6106 ER - TY - JOUR A1 - Evans, Nicholas R. A1 - Devi, Lekshmi Sudha A1 - Mak, Chris S. K. A1 - Watkins, Scott E. A1 - Pascu, Sofia I. A1 - Köhler, Anna A1 - Friend, Richard H. A1 - Williams, Charlotte K. A1 - Holmes, Andrew B. T1 - Triplet energy back transfer in conjugated polymers with pendant phosphorescent iridium complexes N2 - The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent iridium complex 13 and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescent organic light-emitting diodes. Red- emitting phosphorescent iridium complexes based on the [Ir(btp)2(acac)]fragment (where btp is 2-(2 '- benzo[b]thienyl)pyridinato and acac is acetylacetonate) have been attached either directly (spacerless) or through a - (CH2)(8)-chain (octamethylene-tethered) at the 9-position of a 9-octylfluorene host. The resulting dibromo- functionalized spacerless (8) or octamethylene-tethered (12) fluorene monomers were chain extended by Suzuki polycondensations using the bis(boronate)-terminated fluorene macromonomers 16 in the presence of end-capping chlorobenzene solvent to produce the statistical spacerless (17) and octamethylene-tethered ( 18) copolymers containing an even dispersion of the pendant phosphorescent fragments. The spacerless monomer 12 adopts a face-to-face conformation with a separation of only 3.6 angstrom between the iridium complex and fluorenyl group, as shown by X-ray analysis of a single crystal, and this facilitates intramolecular triplet energy transfer in the spacerless copolymers 17. The photo- and electroluminescence efficiencies of the octamethylene-tethered copolymers 18 are double those of the spacerless copolymers 17, and this is consistent with suppression of the back transfer of triplets from the red phosphorescent iridium complex to the polyfluorene backbone in 18. The incorporation of a -(CH2)(8)- chain between the polymer host and phosphorescent guest is thus an important design principle for achieving higher efficiencies in those electrophosphorescent organic light-emitting diodes for which the triplet energy levels of the host and guest are similar Y1 - 2006 UR - http://pubs.acs.org/journal/jacsat U6 - https://doi.org/10.1021/Ja0584267 SN - 0002-7863 ER -