@article{YangJaiserStilleretal.2006, author = {Yang, Xiao Hui and Jaiser, Frank and Stiller, Burkhard and Neher, Dieter and Galbrecht, Frank and Scherf, Ullrich}, title = {Efficient polymer electrophosphoreseent devices with interfacial layers}, series = {Advanced functional materials}, volume = {16}, journal = {Advanced functional materials}, number = {16}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.200500834}, pages = {2156 -- 2162}, year = {2006}, abstract = {It is shown that several polymers can form insoluble interfacial layers on a poly (ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer after annealing of the double-layer structure. The thickness of the interlayer is dependent on the characteristics of the underlying PEDOT.PSS and the molecular weight of the polymers. It is further shown that the electronic structures of the interlayer polymers have a significant effect on the properties of red-light-emitting polymer-based electrophosphorescent devices. Upon increasing the highest occupied molecular orbital and lowest unoccupied molecular orbital positions, a significant increase in current density and device efficiency is observed. This is attributed to efficient blocking of electrons in combination with direct injection of holes from the interlayer to the phosphorescent dye. Upon proper choice of the interlayer polymer, efficient red, polymer-based electrophosphorescent devices with a peak luminance efficiency of 5.5 cd A(-1) (external quantum efficiency = 6 \%) and a maximum power-conversion efficiency of 5 Im W-1 can be realized.}, language = {en} } @article{LuszczynskaDobruchowskaGlowackietal.2006, author = {Luszczynska, Beata and Dobruchowska, Ewa and Glowacki, Ireneusz and Ulanski, Jacek and Jaiser, Frank and Yang, Xiaohui and Neher, Dieter and Danel, Andrzej}, title = {Poly(N-vinylcarbazole) doped with a pyrazoloquinoline dye : a deep blue light-emitting composite for light- emitting diode applications}, issn = {0021-8979}, doi = {10.1063/1.2162268}, year = {2006}, abstract = {We investigated the spectral properties of light-emitting diodes based on a deep blue-emitting pyrazoloquinoline dye doped into a poly(N-vinylcarbazole)-based matrix. Even though the electroluminescence (EL) of the host is redshifted and broadened with respect to the emission of the dye, the EL spectrum becomes fully dominated by the dye emission at concentrations of ca. 2 wt \%. This is attributed to a competition of exciplex formation on the matrix and exciton formation on the dye.}, language = {en} } @article{YangJaiserKlingeretal.2006, author = {Yang, X. H. and Jaiser, Frank and Klinger, S and Neher, Dieter}, title = {Blue polymer electrophosphorescent devices with different electron-transporting oxadiazoles}, doi = {10.1063/1.2162693}, year = {2006}, abstract = {We report that the performances of blue polymer electrophosphorescent devices are crucially depending on the choice of the electron transporting material incorporated into the emissive layer. Devices with 1,3-bis[(4-tert- butylphenyl)-1,3,4-oxidiazolyl]phenylene (OXD-7) doped at similar to 40 wt\% into a poly(vinylcarbazole) matrix exhibited significantly higher efficiencies than those with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), yielding maximum luminous and power efficiency values of 18.2 Cd/A and 8.8 lm/W, respectively. Time resolved photoluminescence measurements revealed a long lifetime phosphorescence component in layers with PBD, which we assign to significant triplet harvesting by this electron-transporting component. (c) 2006 American Institute of Physics}, language = {en} } @article{QuZhangGrimsdaleetal.2004, author = {Qu, J. Q. and Zhang, J. Y. and Grimsdale, A. C. and Mullen, K. and Jaiser, Frank and Yang, X. H. and Neher, Dieter}, title = {Dendronized perylene diimide emitters : Synthesis, luminescence, and electron and energy transfer studies}, issn = {0024-9297}, year = {2004}, abstract = {Aggregation of chromophores in the solid state commonly causes undesirable red shifts in the emission spectra and/or emission quenching. To overcome this problem, we have prepared soluble perylenetetracarboxidiimide dyes in which the chromophores are effectively shielded by polyphenylene dendrimers attached in the bay positions. Models show that attachment of the shielding units in the bay position should provide more efficient shielding than attaching them via the imide moieties. The dendrimers possess excellent film-forming properties due to alkyl substituents on their peripheries. The lack of a red shift in emission upon going from solution to the solid state indicates the dendrons suppress interaction of the emissive cores, leading to pure red-orange emission. Single-layer LEDs produce red-orange emission with relatively low efficiency especially for the higher generation dendrons, which is attributed to poor charge conduction. LEDs using blends of the dendrimers and the undendronized dye as a model compound in PVK have been investigated, and a model to extract relative charge injection rates through the dendritic scaffold from the spectral contributions in the EL spectra is developed}, language = {en} } @article{LiBenduhnQiaoetal.2019, author = {Li, Tian-yi and Benduhn, Johannes and Qiao, Zhi and Liu, Yuan and Li, Yue and Shivhare, Rishi and Jaiser, Frank and Wang, Pei and Ma, Jie and Zeika, Olaf and Neher, Dieter and Mannsfeld, Stefan C. B. and Ma, Zaifei and Vandewal, Koen and Leo, Karl}, title = {Effect of H- and J-Aggregation on the Photophysical and Voltage Loss of Boron Dipyrromethene Small Molecules in Vacuum-Deposited Organic Solar Cells}, series = {The journal of physical chemistry letters}, volume = {10}, journal = {The journal of physical chemistry letters}, number = {11}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.9b01222}, pages = {2684 -- 2691}, year = {2019}, abstract = {An understanding of the factors limiting the open-circuit voltage (V-oc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5\% is achieved with an external quantum efficiency (EQE) maximum of 68\% at 700 nm.}, language = {en} } @article{SteyrleuthnerSchubertJaiseretal.2010, author = {Steyrleuthner, Robert and Schubert, Marcel and Jaiser, Frank and Blakesley, James C. and Chen, Zhihua and Facchetti, Antonio and Neher, Dieter}, title = {Bulk electron transport and charge injection in a high mobility n-type semiconducting polymer}, issn = {0935-9648}, doi = {10.1002/adma.201000232}, year = {2010}, abstract = {Bulk electron transport in a high mobility n-type polymer is studied by time-of-flight photocurrent measurements and electron-only devices. Bulk electron mobilities of similar to 5 x 10(-3) cm(2)/Vs are obtained. The analysis of the electron currents suggests the presence of an injection barrier for all conventionally used low workfunction cathodes.}, language = {en} } @article{SalertKruegerBagnichetal.2013, author = {Salert, Beatrice Ch. D. and Krueger, Hartmut and Bagnich, Sergey A. and Unger, Thomas and Jaiser, Frank and Al-Sa'di, Mahmoud and Neher, Dieter and Hayer, Anna and Eberle, Thomas}, title = {New polymer matrix system for phosphorescent organic light-emitting diodes and the role of the small molecular co-host}, series = {Journal of polymer science : A, Polymer chemistry}, volume = {51}, journal = {Journal of polymer science : A, Polymer chemistry}, number = {3}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0887-624X}, doi = {10.1002/pola.26409}, pages = {601 -- 613}, year = {2013}, abstract = {A new matrix system for phosphorescent organic light-emitting diodes (OLEDs) based on an electron transporting component attached to an inert polymer backbone, an electronically neutral co-host, and a phosphorescent dye that serves as both emitter and hole conductor are presented. The inert co-host is used either as small molecules or covalently connected to the same chain as the electron-transporting host. The use of a small molecular inert co-host in the active layer is shown to be highly advantageous in comparison to a purely polymeric matrix bearing the same functionalities. Analysis of the dye phosphorescence decay in pure polymer, small molecular co-host film, and their blend lets to conclude that dye molecules distribute mostly in the small molecular co-host phase, where the co-host prevents agglomeration and self-quenching of the phosphorescence as well as energy transfer to the electron transporting units. In addition, the co-host accumulates at the anode interface where it acts as electron blocking layer and improves hole injection. This favorable phase separation between polymeric and small molecular components results in devices with efficiencies of about 47 cd/A at a luminance of 1000 cd/m(2). Investigation of OLED degradation demonstrates the presence of two time regimes: one fast component that leads to a strong decrease at short times followed by a slower decrease at longer times. Unlike the long time degradation, the efficiency loss that occurs at short times is reversible and can be recovered by annealing of the device at 180 degrees C. We also show that the long-time degradation must be related to a change of the optical and electrical bulk properties.}, language = {en} } @article{HoffmannJaiserHayeretal.2013, author = {Hoffmann, Sebastian T. and Jaiser, Frank and Hayer, Anna and Baessler, Heinz and Unger, Thomas and Athanasopoulos, Stavros and Neher, Dieter and Koehler, Anna}, title = {How Do Disorder, Reorganization, and Localization Influence the Hole Mobility in Conjugated Copolymers?}, series = {JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, volume = {135}, journal = {JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, number = {5}, publisher = {AMER CHEMICAL SOC}, address = {WASHINGTON}, issn = {0002-7863}, doi = {10.1021/ja308820j}, pages = {1772 -- 1782}, year = {2013}, abstract = {In order to unravel the intricate interplay between disorder effects, molecular reorganization, and charge carrier localization, a comprehensive study was conducted on hole transport in a series of conjugated alternating phenanthrene indenofluorene copolymers. Each polymer in the series contained one further comonomer comprising monoamines, diamines, or amine-free structures, whose influence on the electronic, optical, and charge transport properties was studied. The series covered a wide range of highest occupied molecular orbital (HOMO) energies as determined by cyclovoltammetry. The mobility, inferred from time-of-flight (ToF) experiments as a function of temperature and electric field, was found to depend exponentially on the HOMO energy. Since possible origins for this effect include energetic disorder, polaronic effects, and wave function localization, the relevant parameters were determined using a range of methods. Disorder and molecular reorganization were established first by an analysis of absorption and emission measurements and second by an analysis of the ToF measurements. In addition, density functional theory calculations were carried out to determine how localized or delocalized holes on a polymer chain are and to compare calculated reorganization energies with those that have been inferred from optical spectra. In summary, we conclude that molecular reorganization has little effect on the hole mobility in this system while both disorder effects and hole localization in systems with low-lying HOMOs are predominant. In particular, as the energetic disorder is comparable for the copolymers, the absolute value of the hole mobility at room temperature is determined by the hole localization associated with the triarylamine moieties.}, language = {en} } @article{AlSa'diJaiserBagnichetal.2012, author = {Al-Sa'di, Mahmoud and Jaiser, Frank and Bagnich, Sergey A. and Unger, Thomas and Blakesley, James C. and Wilke, Andreas and Neher, Dieter}, title = {Electrical and optical simulations of a polymer-based phosphorescent organic light-emitting diode with high efficiency}, series = {Journal of polymer science : B, Polymer physics}, volume = {50}, journal = {Journal of polymer science : B, Polymer physics}, number = {22}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0887-6266}, doi = {10.1002/polb.23158}, pages = {1567 -- 1576}, year = {2012}, abstract = {A comprehensive numerical device simulation of the electrical and optical characteristics accompanied with experimental measurements of a new highly efficient system for polymer-based light-emitting diodes doped with phosphorescent dyes is presented. The system under investigation comprises an electron transporter attached to a polymer backbone blended with an electronically inert small molecule and an iridium-based green phosphorescent dye which serves as both emitter and hole transporter. The device simulation combines an electrical and an optical model. Based on the known highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of all components as well as the measured electrical and optical characteristics of the devices, we model the emissive layer as an effective medium using the dye's HOMO as hole transport level and the polymer LUMO as electron transport level. By fine-tuning the injection barriers at the electron and hole-injecting contact, respectively, in simulated devices, unipolar device characteristics were fitted to the experimental data. Simulations using the so-obtained set of parameters yielded very good agreement to the measured currentvoltage, luminancevoltage characteristics, and the emission profile of entire bipolar light-emitting diodes, without additional fitting parameters. The simulation was used to gain insight into the physical processes and the mechanisms governing the efficiency of the organic light-emitting diode, including the position and extent of the recombination zone, carrier concentration profiles, and field distribution inside the device. The simulations show that the device is severely limited by hole injection, and that a reduction of the hole-injection barrier would improve the device efficiency by almost 50\%.}, language = {en} } @article{ZenPflaumHirschmannetal.2004, author = {Zen, Achmad and Pflaum, J. and Hirschmann, S. and Zhuang, W. and Jaiser, Frank and Asawapirom, Udom and Rabe, J. P. and Scherf, Ullrich and Neher, Dieter}, title = {Effect of molecular weight and annealing of poly (3-hexylthiophene)s on the performance of organic field-effect transistors}, year = {2004}, abstract = {The optical, structural, and electrical properties of thin layers made from poly(3-hexylthiophene) (P3HT) samples of different molecular weights are presented. As reported in a previous paper by Kline et al., Adv. Mater 2003, 15, 1519, the mobilities of these layers are a strong function of the molecular weight, with the largest mobility found for the largest molecular weight. Atomic force microscopy studies reveal a complex polycrystalline morphology which changes considerably upon annealing. X-ray studies show the occurrence of a layered phase for all P3HT fractions, especially after annealing at 1.50 degreesC . However, there is no clear correlation between the differences in the transport properties and the data from structural investigations. In order to reveal the processes limiting the mobility in these layers, the transistor properties were investigated as a function of temperature. The mobility decreases continuously with increasing temperatures; with the same trend pronounced thermochromic effects of the P3HT films occur. Apparently, the polymer chains adopt a more twisted, disordered conformation at higher temperatures, leading to interchain transport barriers. We conclude that the backbone conformation of the majority of the bulk material rather than the crystallinity of the layer is the most crucial parameter controlling the charge transport in these P3HT layers. This interpretation is supported by the significant blue-shift of the solid-state absorption spectra with decreasing molecular weight, which is indicative of a larger distortion of the P3HT backbone in the low-molecular weight P3HT layers}, language = {en} }