TY  - JOUR
A1  - Salert, Beatrice Ch. D.
A1  - Krueger, Hartmut
A1  - Bagnich, Sergey A.
A1  - Unger, Thomas
A1  - Jaiser, Frank
A1  - Al-Sa'di, Mahmoud
A1  - Neher, Dieter
A1  - Hayer, Anna
A1  - Eberle, Thomas
T1  - New polymer matrix system for phosphorescent organic light-emitting diodes and the role of the small molecular co-host
JF  - Journal of polymer science : A, Polymer chemistry
N2  - 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.
KW  - charge transport
KW  - conducting polymer
KW  - degradation
KW  - host-guest systems
KW  - light-emitting diodes
KW  - random copolymer
KW  - synthesis
KW  - UV-vis spectroscopy
Y1  - 2013
U6  - https://doi.org/10.1002/pola.26409
SN  - 0887-624X
VL  - 51
IS  - 3
SP  - 601
EP  - 613
PB  - Wiley-Blackwell
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Inal, Sahika
A1  - Koelsch, Jonas D.
A1  - Chiappisi, Leonardo
A1  - Janietz, Dietmar
A1  - Gradzielski, Michael
A1  - Laschewsky, André
A1  - Neher, Dieter
T1  - Structure-related differences in the temperature-regulated fluorescence response of LCST type polymers
JF  - Journal of materials chemistry : C, Materials for optical and electronic devices
N2  - We demonstrate new fluorophore-labelled materials based on acrylamide and on oligo(ethylene glycol) (OEG) bearing thermoresponsive polymers for sensing purposes and investigate their thermally induced solubility transitions. It is found that the emission properties of the polarity-sensitive (solvatochromic) naphthalimide derivative attached to three different thermoresponsive polymers are highly specific to the exact chemical structure of the macromolecule. While the dye emits very weakly below the LCST when incorporated into poly(N-isopropylacrylamide) (pNIPAm) or into a polyacrylate backbone bearing only short OEG side chains, it is strongly emissive in polymethacrylates with longer OEG side chains. Heating of the aqueous solutions above their cloud point provokes an abrupt increase of the fluorescence intensity of the labelled pNIPAm, whereas the emission properties of the dye are rather unaffected as OEG-based polyacrylates and methacrylates undergo phase transition. Correlated with laser light scattering studies, these findings are ascribed to the different degrees of pre-aggregation of the chains at low temperatures and to the extent of dehydration that the phase transition evokes. It is concluded that although the temperature-triggered changes in the macroscopic absorption characteristics, related to large-scale alterations of the polymer chain conformation and aggregation, are well detectable and similar for these LCST-type polymers, the micro-environment provided to the dye within each polymer network differs substantially. Considering sensing applications, this finding is of great importance since the temperature-regulated fluorescence response of the polymer depends more on the macromolecular architecture than the type of reporter fluorophore.
Y1  - 2013
U6  - https://doi.org/10.1039/c3tc31304b
SN  - 2050-7526
VL  - 1
IS  - 40
SP  - 6603
EP  - 6612
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Lu, Guanghao
A1  - Blakesley, James C.
A1  - Himmelberger, Scott
A1  - Pingel, Patrick
A1  - Frisch, Johannes
A1  - Lieberwirth, Ingo
A1  - Salzmann, Ingo
A1  - Oehzelt, Martin
A1  - Di Pietro, Riccardo
A1  - Salleo, Alberto
A1  - Koch, Norbert
A1  - Neher, Dieter
T1  - Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors
JF  - Nature Communications
N2  - Polymer transistors are being intensively developed for next-generation flexible electronics. Blends comprising a small amount of semiconducting polymer mixed into an insulating polymer matrix have simultaneously shown superior performance and environmental stability in organic field-effect transistors compared with the neat semiconductor. Here we show that such blends actually perform very poorly in the undoped state, and that mobility and on/off ratio are improved dramatically upon moderate doping. Structural investigations show that these blend layers feature nanometre-scale semiconductor domains and a vertical composition gradient. This particular morphology enables a quasi three-dimensional spatial distribution of semiconductor pathways within the insulating matrix, in which charge accumulation and depletion via a gate bias is substantially different from neat semiconductor, and where high on-current and low off-current are simultaneously realized in the stable doped state. Adding only 5 wt% of a semiconducting polymer to a polystyrene matrix, we realized an environmentally stable inverter with gain up to 60.
Y1  - 2013
U6  - https://doi.org/10.1038/ncomms2587
SN  - 2041-1723
VL  - 4
IS  - 1-2
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Pingel, P.
A1  - Neher, Dieter
T1  - Comprehensive picture of p-type doping of P3HT with the molecular acceptor F(4)TCNQ
JF  - Physical review : B, Condensed matter and materials physics
N2  - By means of optical spectroscopy, Kelvin probe, and conductivity measurements, we study the p-type doping of the donor polymer poly(3-hexylthiophene), P3HT, with the molecular acceptor tetrafluorotetracyanoquin-odimethane, F(4)TCNQ, covering a broad range of molar doping ratios from the ppm to the percent regime. Thorough quantitative analysis of the specific near-infrared absorption bands of ionized F(4)TCNQ reveals that almost every F(4)TCNQ dopant undergoes integer charge transfer with a P3HT site. However, only about 5% of these charge carrier pairs are found to dissociate and contribute a free hole for electrical conduction. The nonlinear behavior of the conductivity on doping ratio is rationalized by a numerical mobility model that accounts for the broadening of the energetic distribution of transport sites by the Coulomb potentials of ionized F(4)TCNQ dopants. DOI: 10.1103/PhysRevB.87.115209
Y1  - 2013
U6  - https://doi.org/10.1103/PhysRevB.87.115209
SN  - 1098-0121
VL  - 87
IS  - 11
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Proctor, Christopher M.
A1  - Kim, Chunki
A1  - Neher, Dieter
A1  - Thuc-Quyen Nguyen, 
T1  - Nongeminate recombination and charge transport limitations in diketopyrrolopyrrole-based solution-processed small molecule solar cells
JF  - Advanced functional materials
N2  - Charge transport and nongeminate recombination are investigated in two solution-processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)-based donor molecules, mono-DPP and bis-DPP, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PCBM). While the bis-DPP system exhibits a high fill factor (62%) the mono-DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current-voltage characteristics indicate that the mono-DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono-DPP system (2 x 10(-5) cm(2) V-1 s(-1) versus 34 x 10(-5) cm(2) V-1 s(-1)). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.
KW  - charge transport
KW  - solar cells
KW  - photovoltaic devices
KW  - organic electronics
KW  - characterization tools
Y1  - 2013
U6  - https://doi.org/10.1002/adfm.201202643
SN  - 1616-301X
SN  - 1616-3028
VL  - 23
IS  - 28
SP  - 3584
EP  - 3594
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Li, Hongguang
A1  - Babu, Sukumaran Santhosh
A1  - Turner, Sarah T.
A1  - Neher, Dieter
A1  - Hollamby, Martin J.
A1  - Seki, Tomohiro
A1  - Yagai, Shiki
A1  - Deguchi, Yonekazu
A1  - Möhwald, Helmuth
A1  - Nakanishi, Takashi
T1  - Alkylated-C-60 based soft materials regulation of self-assembly and optoelectronic properties by chain branching
JF  - Journal of materials chemistry : C, Materials for optical and electronic devices
N2  - Derivatization of fullerene (C-60) with branched aliphatic chains softens C-60-based materials and enables the formation of thermotropic liquid crystals and room temperature nonvolatile liquids. This work demonstrates that by carefully tuning parameters such as type, number and substituent position of the branched chains, liquid crystalline C-60 materials with mesophase temperatures suited for photovoltaic cell fabrication and room temperature nonvolatile liquid fullerenes with tunable viscosity can be obtained. In particular, compound 1, with branched chains, exhibits a smectic liquid crystalline phase extending from 84 degrees C to room temperature. Analysis of bulk heterojunction (BHJ) organic solar cells with a ca. 100 nm active layer of compound 1 and poly(3-hexylthiophene) (P3HT) as an electron acceptor and an electron donor, respectively, reveals an improved performance (power conversion efficiency, PCE: 1.6 + 0.1%) in comparison with another compound, 10 (PCE: 0.5 + 0.1%). The latter, in contrast to 1, carries linear aliphatic chains and thus forms a highly ordered solid lamellar phase at room temperature. The solar cell performance of 1 blended with P3HT approaches that of PCBM/P3HT for the same active layer thickness. This indicates that C-60 derivatives bearing branched tails are a promising class of electron acceptors in soft (flexible) photovoltaic devices.
Y1  - 2013
U6  - https://doi.org/10.1039/c3tc00066d
SN  - 2050-7526
VL  - 1
IS  - 10
SP  - 1943
EP  - 1951
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Inal, Sahika
A1  - Chiappisi, Leonardo
A1  - Kölsch, Jonas D.
A1  - Kraft, Mario
A1  - Appavou, Marie-Sousai
A1  - Scherf, Ullrich
A1  - Wagner, Manfred
A1  - Hansen, Michael Ryan
A1  - Gradzielski, Michael
A1  - Laschewsky, André
A1  - Neher, Dieter
T1  - Temperature-regulated fluorescence and association of an Oligo(ethyleneglycol)methacrylate-based copolymer with a conjugated Polyelectrolyte-the effect of solution ionic strength
JF  - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry
N2  - Aqueous mixtures of a dye-labeled non-ionic thermoresponsive copolymer and a conjugated cationic polyelectrolyte are shown to exhibit characteristic changes in fluorescence properties in response to temperature and to the presence of salts, enabling a double-stimuli responsiveness. In such mixtures at room temperature, i.e., well below the lower critical solution temperature (LCST), the emission of the dye is strongly quenched due to energy transfer to the polycation, pointing to supramolecular interactions between the two macromolecules. Increasing the concentration of salts weakens the interpolymer interactions, the extent of which is simultaneously monitored from the change in the relative emission intensity of the components. When the mixture is heated above its LCST, the transfer efficiency is significantly reduced, signaling a structural reorganization process, however, surprisingly only if the mixture contains salt ions. To elucidate the reasons behind such thermo- and ion-sensitive fluorescence characteristics, we investigate the effect of salts of alkali chlorides, in particular of NaCl, on the association behavior of these macromolecules before and after the polymer phase transition by a combination of UV-vis, fluorescence, and H-1 NMR spectroscopy with light scattering and small-angle neutron scattering measurements.
Y1  - 2013
U6  - https://doi.org/10.1021/jp408864s
SN  - 1520-6106
VL  - 117
IS  - 46
SP  - 14576
EP  - 14587
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Schubert, Marcel
A1  - Preis, Eduard
A1  - Blakesley, James C.
A1  - Pingel, Patrick
A1  - Scherf, Ullrich
A1  - Neher, Dieter
T1  - Mobility relaxation and electron trapping in a donor/acceptor copolymer
JF  - Physical review : B, Condensed matter and materials physics
N2  - To address the nature of charge transport and the origin of severe (intrinsic) trapping in electron-transporting polymers, transient and steady-state charge transport measurements have been conducted on the prototype donor/acceptor copolymer poly[2,7-(9,9-dialkyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PFTBTT). A charge-generation layer technique is used to selectively address transport of the desired charge carrier type, to perform time-of-flight measurements on samples with < 200 nm thickness, and to combine the time-of-flight and the photocharge extraction by linearly increasing voltage (photo-CELIV) techniques to investigate charge carrier dynamics over a wide time range. Significant trapping of free electrons is observed in the bulk of dioctyl-substituted PFTBTT (alt-PF8TBTT), introducing a strong relaxation of the charge carrier mobility with time. We used Monte-Carlo simulation to simulate the measured transient data and found that all measurements can be modeled with a single parameter set, with the charge transport behavior determined by multiple trapping and detrapping of electrons in an exponential trap distribution. The influence of the concomitant mobility relaxation on the transient photocurrent characteristics in photo-CELIV experiments is discussed and shown to explain subtle features that were seen in former publications but were not yet assigned to electron trapping. Comparable studies on PFTBTT copolymers with chemical modifications of the side chains and backbone suggest that the observed electron trapping is not caused by a distinct chemical species but rather is related to interchain interactions.
Y1  - 2013
U6  - https://doi.org/10.1103/PhysRevB.87.024203
SN  - 1098-0121
VL  - 87
IS  - 2
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Lange, Ilja
A1  - Kniepert, Juliane
A1  - Pingel, Patrick
A1  - Dumsch, Ines
A1  - Allard, Sybille
A1  - Janietz, Silvia
A1  - Scherf, Ullrich
A1  - Neher, Dieter
T1  - Correlation between the open circuit voltage and the energetics of organic bulk heterojunction solar cells
JF  - The journal of physical chemistry letters
N2  - A detailed investigation of the open circuit voltage (V-OC) of organic bulk heterojunction solar cells comprising three different donor polymers and two different fullerene-based acceptors is presented. Bias amplified charge extraction (BACE) is combined with Kelvin Probe measurements to derive information on the relevant energetics in the blend. On the example of P3HT:PC70BM the influence of composition and preparation conditions on the relevant transport levels will be shown. Moderate upward shifts of the P3HT HOMO depending on crystallinity are observed, but contrarily to common believe, the dependence of V-OC on blend composition and thermal history is found to be largely determined by the change in the PCBM LUMO energy. Following this approach, we quantified the energetic contribution to the V-OC in blends with fluorinated polymers or higher adduct fullerenes.
Y1  - 2013
U6  - https://doi.org/10.1021/jz401971e
SN  - 1948-7185
VL  - 4
IS  - 22
SP  - 3865
EP  - 3871
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Inal, Sahika
A1  - Kölsch, Jonas D.
A1  - Sellrie, Frank
A1  - Schenk, Jörg A.
A1  - Wischerhoff, Erik
A1  - Laschewsky, André
A1  - Neher, Dieter
T1  - A water soluble fluorescent polymer as a dual colour sensor for temperature and a specific protein
JF  - Journal of materials chemistry : B, Materials for biology and medicine
N2  - We present two thermoresponsive water soluble copolymers prepared via free radical statistical copolymerization of N-isopropylacrylamide (NIPAm) and of oligo(ethylene glycol) methacrylates (OEGMAs), respectively, with a solvatochromic 7-(diethylamino)-3-carboxy-coumarin (DEAC)-functionalized monomer. In aqueous solutions, the NIPAm-based copolymer exhibits characteristic changes in its fluorescence profile in response to a change in solution temperature as well as to the presence of a specific protein, namely an anti-DEAC antibody. This polymer emits only weakly at low temperatures, but exhibits a marked fluorescence enhancement accompanied by a change in its emission colour when heated above its cloud point. Such drastic changes in the fluorescence and absorbance spectra are observed also upon injection of the anti-DEAC antibody, attributed to the specific binding of the antibody to DEAC moieties. Importantly, protein binding occurs exclusively when the polymer is in the well hydrated state below the cloud point, enabling a temperature control on the molecular recognition event. On the other hand, heating of the polymer-antibody complexes releases a fraction of the bound antibody. In the presence of the DEAC-functionalized monomer in this mixture, the released antibody competitively binds to the monomer and the antibody-free chains of the polymer undergo a more effective collapse and inter-aggregation. In contrast, the emission properties of the OEGMA-based analogous copolymer are rather insensitive to the thermally induced phase transition or to antibody binding. These opposite behaviours underline the need for a carefully tailored molecular design of responsive polymers aimed at specific applications, such as biosensing.
Y1  - 2013
U6  - https://doi.org/10.1039/c3tb21245a
SN  - 2050-750X
SN  - 2050-7518
VL  - 1
IS  - 46
SP  - 6373
EP  - 6381
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  -