@article{KietzkeStillerLandfesteretal.2005, author = {Kietzke, Thomas and Stiller, Burkhard and Landfester, Katharina and Montenegro, Rivelino V. D. and Neher, Dieter}, title = {Probing the local optical properties of layers prepared from polymer nanoparticles}, issn = {0379-6779}, year = {2005}, abstract = {It is well known that the performance of solar cells based on a blend of hole-accepting and electron-accepting conjugated polymers as the active material depend crucially on the length scale of the resulting phase separated morphology. However, a direct control of this morphology is difficult if the layer is prepared from an organic solvent. To circumvent this difficulty, recently a universal method to fabricate defined nano-structured blend layer using nanoparticles dispersed in water was demonstrated. These nanoparticles were prepared with the miniemulsion method, which allows for the preparation of semiconducting polymer nanospheres (SPNs) with diameters in the range of 30 to 300 nanometres. Since the process starts from the active material dissolved in a common solvent, it can be applied to the fabrication of nanoparticles of blends of polymers with oligomers or even with inorganic materials. We present here for the first time scanning near field optical microscopy (SNOM) investigations on these novel nanostructured polymer layers. We show that by spin-coating a mixture of two different dispersions a nanoparticle monolayer with a statistically distribution of the nanoparticles can be obtained. Mixing conjugated polymer nanoparticles with some inert particles like polystyrene beads may allow for the preparation of nano-sized light emitters}, language = {en} } @article{KarageorgievNeherSchulzetal.2005, author = {Karageorgiev, Peter and Neher, Dieter and Schulz, Burkhard and Stiller, Burkhard and Pietsch, Ullrich and Giersig, Michael and Brehmer, Ludwig}, title = {From anisotropic photo-fluidity towards nanomanipulation in the optical near-field}, issn = {1476-1122}, year = {2005}, abstract = {An increase in random molecular vibrations of a solid owing to heating above the melting point leads to a decrease in its long-range order and a loss of structural symmetry. Therefore conventional liquids are isotropic media. Here we report on a light-induced isothermal transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the film behaves as an isotropic viscoelastic fluid, it shows considerable fluidity only in the direction parallel to the light field vector under linear polarized light. The fluidization phenomenon is related to photoinduced motion of azobenzene-functionalized molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also describe here how the photofluidization allows nanoscopic elements of matter to be precisely manipulated}, language = {en} } @article{GalbrechtYangNehlsetal.2005, author = {Galbrecht, Frank and Yang, X. H. and Nehls, B. S. and Neher, Dieter and Farrell, Tony and Scherf, Ullrich}, title = {Semiconducting polyfluorenes with electrophosphorescent on-chain platinum-salen chromophores}, issn = {1359-7345}, year = {2005}, abstract = {The synthesis of statistical fluorene-type copolymers with on-chain Pt-salen phosphorescent units and their use in electrophosphorescent OLEDs is reported}, language = {en} } @article{EgbeUlbrichtOrgisetal.2005, author = {Egbe, D. A. M. and Ulbricht, C. and Orgis, Thomas and Carbonnier, B. and Kietzke, Thomas and Peip, M. and Metzner, M. and Gericke, M. and Birckner, Eckhard and Pakula, T. and Neher, Dieter and Grumm, U. W.}, title = {Odd-even effects and the influence of length and specific positioning of alkoxy side chains on the optical properties of PPE-PPV polymers}, issn = {0897-4756}, year = {2005}, abstract = {This contribution reports the combined influences of odd-even effects and the specific positioning of alkoxy side chains OR1 = (OCn+H-10(2(n+10)+1)) and OR2 = (OCnH2n+1) (with n = 6, 7, 8, 9) on the phenylene-ethynylene and phenylene- vinylene segments, respectively, on the optical properties of hybrid polymers P(n+10)/n of general repeating unit: -Ph-C equivalent to C-Ph-C equivalent to C-Ph-CH=CH-Ph-CH=CH-. For the polymeric materials, visual color impression varies alternatively between orange red (P16/6 and P18/8) and yellow (P17/7 and P19/9) according to the odd and even features of the alkoxy side chains, where odd or even relates to the total number of sp(3)-hybridized atoms within the side chains. This side chain related effect is ascribed to both absorptive and emissive behaviors of the polymers on the basis of photophysical investigations in the bulk. Almost identical thin film absorption spectra were obtained for all four materials; however, the photoluminescence of the odd polymers, P16/6 (lambda(f) = 556 nm) and P18/ 8 (lambda(f) = 614 nm), was red-shifted relative to that of their even counterparts (lambda(f) = 535 nm). Further, the P18/8 maximum at 614 nm can be readily assigned to excimer emission, as evidenced by the largest Stokes shift (5600 cm(- 1)), largest fwhmf-value (3700 cm(-1))(,) and the lowest Phi(f)-value of 24\%. The strong pi-pi interchain interaction in P18/8, due to loose alkoxy side chains packing, does not only favor fluorescence quenching but also enable an effective inter- as well as intra-molecular recombination of the generated positive and negative polarons in electrolurninescence, which explains the good EL properties of this polymer irrespective of the solvent used. A voltage-dependent blue shift of the EL spectra of up to 100 nm was observed for P18/8 devices prepared from aromatic solvents. This red to green EL shift as observed with increasing voltage is assigned to conformational changes of the polymer chains with increasing temperature}, language = {en} } @article{EgbeCarbonnierPauletal.2005, author = {Egbe, D. A. M. and Carbonnier, B. and Paul, E. L. and Muhlbacher, D. and Kietzke, Thomas and Birckner, Eckhard and Neher, Dieter and Grummt, U. W. and Pakula, T.}, title = {Diyne-containing PPVs : Solid-state properties and comparison of their photophysical and electrochemical properties with those of their Yne-containing counterparts}, issn = {0024-9297}, year = {2005}, abstract = {Diyne-containing poly(p-phenylene-vinylene)s, 4a-d, of general chemical structure-(Ph-C\&3bond; C-C\&3bond; C-Ph- CH\&3bond; CH-Ph-CH\&3bond; CH-)(n), obtained through polycondensation reactions of 1,4-bis(4-formyl-2,5-dioctyloxyphenyl)- buta-1,3-diyne (2) with various 2,5-dialkoxy-p-xylylenebis(diethylphosphonates), 3a-d, are the subject of this report. The polymers exhibit great disparity in their degree of polymerization, n, which might be ascribed to side-chain-related differences in reactivity of the reactive species during the polycondensation process and which led to n-dependent absorption (solution and solid state) and emission (solution) behaviors of the polymers. Polarizing optical microscopy and differential scanning calorimetry are employed to probe their thermal behavior. The structure is investigated by means of wide-angle X-ray diffraction for both isotropic and macroscopically oriented samples. Comparison of photophysical (experimental and theoretical) and electrochemical properties of the polymers with those of their yne- containing counterparts 6a-d [-(Ph-C\&3bond; C-Ph-CH\&3bond; CH-Ph-CH\&3bond; CH-)(n)] has been carried out. Similar photophysical behavior was observed for both types of polymers despite the difference in backbone conjugation pattern. The introduction of a second yne unit in 4 lowers the HOMO and LUMO levels, thereby enhancing the electron affinity of polymers 4 compared to polymers 6. The "wider opening" introduced by the second yne unit facilitates moreover the movement of charges during the electrochemical processes leading to minimal discrepancy, Delta E-g between the optical and electrochemical band gap energies. Polymers 6, in contrast, show significant side-chain-dependent Delta E-g values. Low turn-on voltages between 2 and 3 V and maximal luminous efficiencies between 0.32 and 1.25 cd/A were obtained from LED devices of configuration ITO/PEDOT:PSS/polymer 4/Ca/Al}, language = {en} } @article{BagnichBasslerNeher2005, author = {Bagnich, Sergey A. and Bassler, H. and Neher, Dieter}, title = {Exciton dynamics in ladder-type methyl-poly(para-phenylene) doped with phosphorescent dyes}, issn = {0022-2313}, year = {2005}, abstract = {The luminescence of a ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with platinum-porphyrin dye PtOEP covering the concentration 10(-3)-5\% by weight has been measured employing cw and transient techniques. Upon excitation into the range of absorption of the host, strong phosphorescence of the dopant is observed. Possible ways of populating the dopant triplet state are considered. (c) 2004 Elsevier B.V. All rights reserved}, language = {en} } @article{KietzkeHorholdNeher2005, author = {Kietzke, Thomas and Horhold, H. H. and Neher, Dieter}, title = {Efficient polymer solar cells based on M3EH-PPV}, year = {2005}, abstract = {We report on polymer blend solar cells with an external quantum efficiency of more than 30\% and a hi-h overall energy conversion efficiency (ECE) under white light illumination (100 mW/cm(2)) Of Lip to 1.7\% using a blend of M3EH- PPV (poly [2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5(2-ethylhexyloxy)-(1,4-pheiiylene-1,2-ethenylene)]) and CN-ether-PPV (poly[oxa-1,4-phenylene-1,2(1-cyano)ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-(2-cyano)ethellyiene-1,4- phenylene]). We attribute these high efficiencies to the formation of a vertically composition graded structure during spin coating Photoluminescence measurements performed on the blend layers indicated the formation of exciplexes between both types of polymers, which we propose to be one factor preventing even higher efficiencies}, language = {en} } @article{ZenBilgeGalbrechtetal.2006, author = {Zen, Achmad and Bilge, Askin and Galbrecht, Frank and Alle, Ronald and Meerholz, Klaus and Grenzer, J{\"o}rg and Neher, Dieter and Scherf, Ullrich and Farrell, Tony}, title = {Solution processable organic field-effect transistors utilizing an alpha,alpha '-dihexylpentathiophene- based swivel cruciform}, doi = {10.1021/Ja0573357}, year = {2006}, language = {en} } @article{YangMuellerNeheretal.2006, author = {Yang, Xiaohui and M{\"u}ller, David C. and Neher, Dieter and Meerholz, Klaus}, title = {Highly efficient polymeric electrophosphorescent diodes}, year = {2006}, abstract = {Polymeric electrophosphorescent LEDs with internal quantum efficiencies approaching unity have been fabricated. Such performance levels are previously unknown for OLEDs. The key to this success is redox chemically doped oxetane- crosslinkable hole-transporting layers with multilayer capability (see figure). They improve hole injection and act as electron-blocking layers, without the need to include exciton-or hole-blocking layers}, language = {en} } @article{KietzkeEgbeHoerholdetal.2006, author = {Kietzke, Thomas and Egbe, Daniel A. M. and H{\"o}rhold, Hans-Heinrich and Neher, Dieter}, title = {Comparative study of M3EH-PPV-based bilayer photovoltaic devices}, issn = {4018-4022}, doi = {10.1021/Ma0601991}, year = {2006}, abstract = {We have recently shown that efficient polymer solar cells can be fabricated by using a weakly soluble derivative of poly-p-vinylene (M3EH-PPV) as the electron donor. Here we present studies on bilayer devices using organic electron acceptors with varying LUMO levels and M3EH-PPV. It is found that the open-circuit voltage scales linearly with the LUMO level of the acceptor, reaching values as high as 1.5 V when cyano-substituted poly(p-phenyleneethynylene)-alt- poly(p-phenylenevinylene) copolymers are used. Further, we discovered that for an increasing number of triple bonds in the repeat unit of the acceptor polymer the device performance decreases with increasing thickness of the acceptor layer. Also, the quantum efficiency was smaller when using polymers with higher LUMO levels. Thus, further effort is needed to design optimum acceptor polymers for devices exhibiting large open-circuit voltage and high quantum efficiency}, language = {en} } @article{IlnytskyiSaphiannikovaNeher2006, author = {Ilnytskyi, Jaroslav and Saphiannikova, Marina and Neher, Dieter}, title = {Photo-induced deformations in azobenzene-containing side-chain polymers : molecular dynamics study}, issn = {1607-324X}, year = {2006}, abstract = {We perform molecular dynamics simulations of azobenzene containing side-chain liquid crystalline polymer subject to an external model field that mimicks the reorientations of the azobenzenes upon irradiation with polarized light. The smectic phase of the polymer is studied with the field applied parallel to the nematic director, forcing the trans isomers to reorient perpendicularly to the field (the direction of which can be assosiated with the light polarization). The coupling between the reorientation of azobenzenes and mechanical deformation of the sample is found to depend on the field strength. In a weak field the original smectic order is melted gradually with no apparent change in the simulation box shape, whereas in a strong field two regimes are observed. During the first one a rapid melting of the liquid crystalline order is accompanied by the contraction of the polymer along the field direction (the effect similar to the one observed experimentally in azopenzene containing elastomers). During the slower second regime, the smectic layers are rebuilt to accomodate the preferential direction of chromophores perperdicular to the field.}, 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{YinSchubertStilleretal.2008, author = {Yin, Chunhong and Schubert, Marcel and Stiller, Burkhard and Castellani, Mauro and Neher, Dieter and Kumke, Michael Uwe and H{\"o}rhold, Hans-Heinrich}, title = {Tuning of the excited-state properties and photovoltaic performance in PPV-based polymer blends}, doi = {10.1021/Jp803977k}, year = {2008}, 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{KurpiersNeher2016, author = {Kurpiers, Jona and Neher, Dieter}, title = {Dispersive Non-Geminate Recombination in an Amorphous Polymer:Fullerene Blend}, series = {Scientific reports}, volume = {6}, journal = {Scientific reports}, publisher = {Nature Publishing Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/srep26832}, pages = {10}, year = {2016}, abstract = {Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer:fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions.}, language = {en} } @article{NeherKniepertElimelechetal.2016, author = {Neher, Dieter and Kniepert, Juliane and Elimelech, Arik and Koster, L. Jan Anton}, title = {A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents}, series = {Scientific reports}, volume = {6}, journal = {Scientific reports}, publisher = {Nature Publishing Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/srep24861}, pages = {9}, year = {2016}, abstract = {Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit α to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.}, language = {en} }