M. Schubert, R. Steyrleuthner, S. Bange, A. Sellinger, D. Neher, “Charge transport and recombination in bulk heterojunction solar cells containing a dicyanoimidazole-based molecular acceptor”, physica status solidi (a)206 (2009) 2743-2749 DOI
Carrier transport and recombination have been studied in single component layers and blends of the soluble PPV-derivative poly[2,5-dimethoxy-1,4-phenylenevinylene-2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] (M3EH-PPV) and the small molecule acceptor 4,7-bis(2-(1-hexyl-4,5-dicyanoimidazole-2-yl)vinyl) benzo[c][1,2,5]-thiadiazole (HV-BT). Measurements on single carrier devices show significantly smaller electron mobility in the blend compared to the pure HV-BT layer, which is suggestive of the formation of isolated clusters of the acceptor in a continuous polymer matrix. The significant change in fill factor (FF) with increasing illumination intensity is consistently explained by a model taking into account bimolecular recombination and space charge effects. The decay of the carrier density after photoexcitation has been studied by performing photo-CELIV measurements on pure and blend layers. It is found that the decay at long delay times follows a power-law dependence, which is, however, not consistent with a Langevin-type bimolecular recombination of free charges. A good description of the data is obtained by assuming trimolecular recombination to govern the charge carrier dynamics in these systems.
M. Castellani, I. Salzmann, P. Bugnon, S. Yu, M. Oehzelt, N. Koch, “Structural and electronic implications for carrier injection into organic semiconductors”, Applied Physics A97 (2009) 1-9 DOI
We report on the structural and electronic interface formation between ITO (indium-tin-oxide) and prototypical organic small molecular semiconductors, i.e., CuPc (copper phthalocyanine) and α-NPD (N,N′-di(naphtalen-1-yl)-N,N′-diphenyl-benzidine). In particular, the effects of in situ oxygen plasma pretreatment of the ITO surface on interface properties are examined in detail: Organic layer-thickness dependent Kelvin probe measurements revealed a good alignment of the ITO work function and the highest occupied electronic level of the organic material in all samples. In contrast, the electrical properties of hole-only and bipolar organic diodes depend strongly on the treatment of ITO prior to organic deposition. This dependence is more pronounced for diodes made of polycrystalline CuPc than for those of amorphous α-NPD layers. X-ray diffraction and atomic force microscopic (AFM) investigations of CuPc nucleation and growth evidenced a more pronounced texture of the polycrystalline film structure on the ITO substrate that was oxygen plasma treated prior to organic layer deposition. These findings suggest that the anisotropic electrical properties of CuPc crystallites, and their orientation with respect to the substrate, strongly affect the charge carrier injection and transport properties at the anode interface.
S. Inal, M. Castellani, A. Sellinger, D. Neher, “Relationship of Photophysical Properties and the Device Performance of Novel Hybrid Small-Molecular/Polymeric Solar Cells”, Macromolecular Rapid Communications30 (2009) 1263 DOI
We investigate solar cells comprised of a vinazene derivative (HV-BT) as the electron acceptor and the well-known polymer poly(3-hexylthiophene) as the electron donor. In the as-prepared blend, most of the excited state species, including the excimers on HV-BT, are quenched at the heterojunction. Although the photophysical properties of the blends change upon annealing, the blend solar cells largely remain uninfluenced by such treatments. A significant improvement is, however, observed when inducing phase separation at a longer length scale, for example, in solution-processed bilayer devices. Hereby, both the fill factor (FF) and the open circuit voltage are considerably increased, pointing to the importance of the heterojunction topology and the layer composition at the charge extracting contacts. An optimized device exhibits a power conversion efficiency of close to 1%.
S. Joshi, P. Pingel, S. Grigorian, T. Panzner, U. Pietsch, D. Neher, M. Forster, U. Scherf, “Bimodal Temperature Behavior of Structure and Mobility in High Molecular Weight P3HT Thin Films”, Macromolecules42 (2009) 4651-4660 DOI
We report a temperature dependent crystalline structure of spin-coated thin films of high molecular weight regioregular poly(3-hexylthiophene) (P3HT) (Mn ~ 30000 g/mol) and its correlation with charge carrier mobility. These investigations show a reversible change of the crystalline structure, where the interlayer lattice spacing (100) along the alkyl side chains continuously increases up to a temperature of about 220 °C; in contrast, the in-plane π−π distance reduces with increasing temperature. These changes in structure are reversible and can be repeated several times. The temperature-induced structural properties differ for thick and thin films, pointing to a surface/interface role in stabilization of the layer morphology. In contrast to the structural changes, the carrier mobility is rather constant in the temperature range from room temperature up to 100–120 °C, followed by a continuous decrease. For thick layers this drop is significant and the transistor performance almost vanishes at high temperature, however, it completely recovers upon cooling back to room temperature. The drop of the charge carrier mobility at higher temperatures is in contrast with expectations from the structural studies, considering the increase of crystalline fraction of the polycrystalline layer. Our electrical measurements underscore that the reduction of the macroscopic mobility is mostly caused by a pronounced decrease of the intergrain transport. The thermally induced crystallization along (100) direction and the creation of numerous small crystallites at the film–substrate interface reduce the number of long polymer chains bridging crystalline domains, which ultimately limits the macroscopic charge transport.
R. Steyrleuthner, S. Bange, D. Neher, “Reliable electron-only devices and electron transport in n-type polymers”, Journal of Applied Physics105 (2009) 064509 DOI
Current-voltage analysis of single-carrier transport is a popular method for the determination of charge carrier mobilities in organic semiconductors. Although in widespread use for the analysis of hole transport, only a few reports can be found where the method was applied to electron transport. Here, we summarize the experimental difficulties related to the metal electrode leakage currents and nonlinear differential resistance (NDR) effects and explain their origin. We present a modified preparation technique for the metal electrodes and show that it significantly increases the reliability of such measurements. It allows to produce test devices with low leakage currents and without NDR even for thin organic layers. Metal oxides were often discussed as a possible cause of NDR. Our measurements on forcibly oxidized metal electrodes demonstrate that oxide layers are not exclusively responsible for NDR effects. We present electron transport data for two electron-conducting polymers often applied in all-polymer solar cells for a large variety of layer thicknesses and temperatures. The results can be explained by established exponential trapping models.
M. Schubert, C. Yin, M. Castellani, S. Bange, T.L. Tam, A. Sellinger, H.-H. Hörhold, T. Kietzke, D. Neher, “Heterojunction topology versus fill factor correlations in novel hybrid small-molecular/polymeric solar cells”, The Journal of Chemical Physics130 (2009) 094703 DOI
The authors present organic photovoltaic (OPV) devices comprising a small molecule electron acceptor based on 2-vinyl-4,5-dicyanoimidazole (Vinazene™) and a soluble poly(p-phenylenevinylene) derivative as the electron donor. A strong dependence of the fill factor (FF) and the external quantum efficiency [incident photons converted to electrons (IPCE)] on the heterojunction topology is observed. As-prepared blends provided relatively low FF and IPCE values of 26% and 4.5%, respectively, which are attributed to significant recombination of geminate pairs and free carriers in a highly intermixed blend morphology. Going to an all-solution processed bilayer device, the FF and IPCE dramatically increased to 43% and 27%, respectively. The FF increases further to 57% in devices comprising thermally deposited Vinazene layers where there is virtually no interpenetration at the donor/acceptor interface. This very high FF is comparable to values reported for OPV using fullerenes as the electron acceptor. Furthermore, the rather low electron affinity of Vinazene compound near 3.5 eV enabled a technologically important open circuit voltage (Voc) of 1.0 V.
P. Pingel, A. Zen, D. Neher, I. Lieberwirth, G. Wegner, S. Allard, U. Scherf, "Unexpectedly high field-effect mobility of a soluble, low molecular weight oligothiophene fraction with low polydispersity", Applied Physics A95 (2009) 67 DOI
Layers made from soluble low molecular weight polythiophene PQT-12 with low polydispersity exhibit a highly ordered structure and charge-carrier mobilities of the order of 10-3 cm2/(Vs), which we attribute to its proximity to monodispersity. We propose that polydispersity is a decisive factor with regard to structure formation and transport properties of soluble low molecular weight polythiophenes.
J. M. Ilnytskyi, D. Neher, M. Saphiannikova, M. R. Wilson, L. M. Stimson, “Molecular Dynamics Simulations of Various Branched Polymeric Liquid Crystals”, Molecular Crystals and Liquid Crystals496 (2008) 186 DOI
We discuss molecular dynamics simulations for several types of polymeric liquid crystals. Dendrimers with a variety of mesogenic attachments are studied in isotropic, nematic and smectic A solvents, with an emphasize on the coupling between molecular shape and the structure of the phase. The structure and dynamics of liquid crystal side-chain polymers are studied within different bulk phases. Here, again a strong coupling is noted between molecular shape and molecular organisation within each phase. We also consider photo-induced deformation in azobenzene-containing polymers and demonstrate that the “opposite sign” of these deformations, observed experimentally in liquid crystalline and amorphous systems, can be explained solely by the reorientation of trans-isomers of azobenzene.
S. Bange, A. Kuksov, D. Neher, A. Vollmer, N. Koch, A. Ludemann, S. Heun, "The role of poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate) as a hole injection layer in a blue-emitting light-emitting diode", Journal of Applied Physics104 (2008) 104506 DOI
The authors study the role of the conducting polymer poly(3,4-ethylenedioxythiophene:poly(styrenesulphonate) (PEDOT:PSS) in determining the transient and steady-state operation of a blue-emitting polymer light-emitting diode. Combining the results from photoemission spectroscopy, time-of-flight photocurrent measurements, and studies on hole-only devices reveals a significant barrier for the injection of holes into the polymer. Simulations with a numerical drift-diffusion model, however, show that the injection currents determined from single-carrier devices cannot account for the rapid transient luminance onset and the efficient steady-state luminance output of the corresponding bipolar light-emitting devices. It is shown that the transient electroluminescence traces measured at different external bias can be well reproduced when assuming the presence of a weak barrier for electron extraction at this interface, which is attributed to electron accumulation at a thin phase-segregated PSS-rich layer at the surface of PEDOT:PSS. In addition, interface conditioning, presumably due to electron trapping near PEDOT:PSS, renders the anode-polymer interface nearly Ohmic. This conditioning, however, occurs on intermediate time scales normally not addressed by either transient or steady-state measurements.
S. Joshi, S. Grigorian, U. Pietsch, P. Pingel, A. Zen, D. Neher & U. Scherf, “Thickness Dependence of the Crystalline Structure and Hole Mobility in Thin Films of Low Molecular Weight Poly(3-hexylthiophene)”, Macromolecules41 (2008) 6800 DOI
The morphology of thin films at the polymer-to-insulator interface is of great importance for OFET applications. In order to find a relation between the thickness dependence of structural order and the electrical parameters in low molecular weight (Mw ~ 2.5 kDa) poly(3-hexylthiophene) (P3HT), we have performed grazing-incidence X-ray diffraction and field effect mobility measurements. The samples were prepared from solutions with different concentrations by spin-coating mainly onto HMDS-pretreated Si/SiO2 substrates, resulting in film thicknesses that vary between 10 and 200 nm. The X-ray diffraction curves display Bragg peaks of nanocrystallites diluted into an amorphous matrix where the orientational distribution of the crystallites changes significantly as a function of film thickness. The orientation of nanocrystals was found to be random for the thickest films. Reducing the film thickness, we found an increase in the alignment of the stacking direction of molecules along the surface normal. At same time the mean crystal size along the film normal decreases less than the decrease of film thickness. This is interpreted by a preferential pinning of nanocrystals at the film-to-insulator interface when the crystal size becomes in the order of the film thickness, i.e., below 25 nm. The model of pinning effect is supported by temperature-resolved X-ray measurements performed between room temperature and melting temperature. For films thicker than 25 nm the phase transition appears rather continuously with temperature, but it becomes sharp for thinner films. In contrast to X-ray measurements the field effect mobility is found to be constant within the whole investigated range. Our findings give evidence that the charge transport in low molecular weight P3HT is dominated by the ultrathin layer stabilized at the film-to-insulator interface. Despite the very uniform orientation of the crystallites within this layer, the field effect mobility remains low for all thicknesses. This is attributed to the presence of amorphous regions between highly crystalline domains, which ultimately limits the charge transport in the layer plane.
F. Jaiser, D. Neher, A. Meisel, H.-G. Nothofer, T. Miteva, A. Herrmann, K. Müllen & U. Scherf, “Energy and charge transfer in blends of dendronized perylenes with polyfluorene”, The Journal of Chemical Physics129 (2008) 114901 DOI
Two generations of polyphenylene dendrimers with a perylene diimide core are compared with a nondendronized tetraphenoxyperylene diimide model compound regarding their application in organic light-emitting diodes (OLEDs). Single layer devices with blends of the first and second generation dendrimers in polyfluorene are investigated as active layers in OLEDs, and the effect of dendronization on the emission color and electroluminescence intensity is studied. In photoluminescence, higher degrees of dendronization lead to a reduction in Förster transfer from the polyfluorene host to the perylene, resulting in a larger contribution of the blue host emission in the photoluminescence spectra. In electroluminescence, the dopants appear to act as active traps for electrons, resulting in a predominant generation of excitons on the dye. This gives rise to a remarkably stronger contribution of red emission in electroluminescence than in photoluminescence where energy is exchanged exclusively via Förster transfer. The pronounced color change from red to blue with higher degrees of dendronization and larger driving voltages is explained by the competition of the recombination of free electrons with holes and trapping of electrons by the emitting guest.
C. Yin, M. Schubert, S. Bange, B. Stiller, M. Castellani, D. Neher, M. Kumke & H.-H. Hörhold, “Tuning of the Excited-State Properties and Photovoltaic Performance in PPV-Based Polymer Blends”, The Journal of Physical Chemistry C112 (2008) 14607 DOI
The authors use solvents with different boiling points and a mixture of these solvents to tune the morphology of blends formed from poly[2,5-dimethoxy-1,4-phenylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (M3EH-PPV) and poly[oxa-1,4-phenylene-1,2-(1-cyano)-ethylene-2,5-dioctyloxy-1,4-phenylene-1,2-(2-cyano)-ethylene-1,4-phenylene] (CN-ether-PPV). In photoluminescence (PL), the emission of as-prepared films spin-coated from chloroform (CF) is entirely dominated by an exciplex, with no evidence for the radiative decay of either the M3EH-PPV or the CN-ether-PPV exciton. Evidently these intrachain excited species dissociate rapidly by intermolecular charge transfer, pointing to a highly intermixed blend morphology. On the other hand, the PL of films deposited from 1,2,4-trichlorobenzene (TCB) exhibits predominant emission from the M3EH-PPV exciton, indicating the presence of rather pure M3EH-PPV domains in the phase-separated polymers layers. The blend morphology is shown to have a large influence on the solar cell properties and particularly on the fill factor. For an annealed layer coated from a 1:4 TCB:CF mixture, a fill factor of 44% was achieved, which is among the highest values reported for polymer polymer blends. For all blends the photocurrent rises linearly with light intensity, implying that bimolecular recombination and the formation of space charge from the photogenerated carriers is of minor importance. Charge carrier mobilities and bimolecular recombination coefficients were measured on the very same polymer blends used for solar cell fabrication, utilizing the method of charge carrier extraction by linearly increasing voltage technique (photo-CELIV). These studies did not reveal a significant effect of the blend morphology on the bulk carrier transport and recombination. The authors conclude that the photovoltaic properties are mainly determined by processes on the very local scale, namely the competition between the field-induced dissociation and recombination of the initially formed polaron pairs. It is proposed that the nanomorphology has a profound effect on the initial separation of these Coulombically bound electron hole pairs and on the probability that they recombine via the formation of interfacial exciplexes or, possibly, via intrachain triplet excitons.
X.H. Yang, F.-I. Wu, D. Neher, C.-H. Chien, C.-F. Shu, “Efficient Red-Emitting Electrophosphorescent Polymers”, Chemistry of Materials20 (2008) 1629-1635 DOI
We report the synthesis and characterization of a series of electrophosphorescent polymers with a polyfluorene backbone, containing a red-emitting iridium complex and carrier-transporting units as the substitutes of the C-9 position of fluorene. Different specimens and contents of the hole-transporting moieties were found to influence the electroluminescent properties of the polymers. Incorporation of a hole-transporting/electron-blocking interfacial layer and utilization of an efficient electron injection cathode led to the saturated red-emitting electrophosphorescent polymer devices with the peak luminance efficiency and power conversion efficiency of 9.3 cd A−1 and 10.5 lm W−1, respectively. The efficiencies were 8 cd A−1 / 8 lm W−1 and 5.9 cd A−1 / 4.6 lm W−1, respectively, at 100 cd m−2 and 1000 cd m−2.
A. Zen, P. Pingel, D. Neher, U. Scherf, “Organic transistors utilising highly soluble swivel-cruciform oligothiophenes”, physica status solidi (a)205 (2008) 440-448 DOI
A series of soluble swivel-cruciform oligothiophene dimers with increasing chain length - bis(terthiophene) (BT3), bis(pentathiophene) (BT5) and bis(heptathiophene) (BT7) - was used as semiconductor in organic field-effect transistors. A field-effect mobility of 3.7×10-5 cm2/Vs and a current on/off ratio of >103 was obtained with dimer BT5. The attachment of terminal n-hexyls to BT5 provides a swivel-cruciform α,α′-dihexylpentathiophene dimer (DHPT-SC) with increased solubility. XRD and AFM analysis of the resulting thin films reveals that they are highly crystalline. Using the DHPT-SC swivel cruciform in solution-processed organic field-effect transistors, we obtained a field-effect mobility of 0.012 cm2/Vs and a current on/off ratio of >105, which is among the highest values reported to date for wetprocessed OFETs utilizing oligothiophenes.
A. Tsami, X.-H. Yang, F. Galbrecht, T. Farrell, H. Li, S. Adamczyk, R. Heiderhoff, L. J. Balk, D. Neher, E. Holder, “Random fluorene copolymers with on-chain quinoxaline acceptor units”, Journal of Polymer Science Part A: Polymer Chemistry45 (2007) 4773-4785 DOI
Two series of novel random polyfluorene copolymers containing quinoxaline units were prepared by stressing the coupling according to Yamamoto. The first series contains 2,3-bis-(4'-tert-butyl-biphenyl-4-yl)benzo[g]quinoxaline and the second series 2,3-bis-(4'-tert-butyl-biphenyl-4-yl)quinoxaline as energy accepting unit. The copolymers were identified by gel permeation chromatography, infrared spectroscopy, and nuclear magnetic resonance spectroscopy. Thermal properties were analyzed by thermal gravimetric analysis and differential scanning calorimetry revealing amorphous copolymers that are stable up to 430 °C. The morphology was investigated using atomic force microscopy. The optical properties in solutions and thin films were analyzed. Furthermore, the thin film electro-optical properties were determined in monolayer polymer light-emitting devices. Single layer devices were built with efficiencies ranging from 0.15 to 2.0 cd/A. For the random copolymers with 5 mol % benzo[g]quinoxazoline in the polyfluorene backbone some threefold efficiency enhancement from 1.1 to 3.0 cd/A was achieved by utilizing an ultra thin interlayer of poly(9,9-di-n-octylfluorene-2,7-diyl)-alt-[1,4-phenylene-(4-sec-butylphenylimino)-1,4-phenylene] between PEDOT:PSS and the emissive random copolymer layer.
E. F. Aziz, A. Vollmer, S. Eisebitt, W. Eberhardt, P. Pingel, D. Neher, N. Koch, “Localized Charge Transfer in a Molecularly Doped Conducting Polymer”, Advanced Materials19 (2007) 3257-3260 DOI
Molecular-acceptor doping of polythiophene results in high conductivity, ca. 1 S cm-1, in solution-processed thin films. Charge-transfer complex formation leads to new hybrid orbitals, which are derived from the highest occupied level of the donor (polythiophene) and the lowest unoccupied level of the acceptor (tetrafluorotetracyanoquinodimethane).
S. Bange, A. Kuksov, D. Neher, “Sensing electron transport in a blue-emitting copolymer by transient electroluminescence”, Applied Physics Letters91 (2007) 143516 DOI
A variation of the transient electroluminescence technique is introduced which allows us to selectively study the electron transport in a thin polymer layer. It relies on the formation of an insoluble interlayer from a formerly solvable polymer and enables probing of unipolar electron transport despite of injection barriers. It opens up possibilities to gain insight into the operation of light-emitting diodes. Applicability to a blue-emitting spirobifluorene-based copolymer is shown by comparison to time-of-flight results for electron and hole transport and evidence supplied for an intermixing of electron and hole dynamics through blocking of electrons at the polymer/anode interface.
T. Kietzke, D. Neher, M. Kumke, O. Ghazy, U. Ziener, K. Landfester, “Phase Separation of Binary Blends in Polymer Nanoparticles”, Small3 (2007) 1041 DOI
Polymer blends in nanoparticles have been studied by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. The TEM studies show that blend particles formed from two immiscible polymers by the miniemulsion process exhibit biphasic morphologies. The fact that no core-shell type but Janus-like structures were found indicates that the surface free energies between both polymers and the solution-water interface (including the surfactant molecules) are similar; therefore, the blend morphology and composition of the individual phases are mainly determined by the interaction between the two polymer components. Both the TEM studies and the PL experiments provide strong evidence that phase separation in these particles strictly follows the Flory-Huggins theory. This highlights the applicability of the nanoparticle approach to fabricate blend systems with well-controllable properties and to study structure-property relationships under well-defined conditions.
J. M. Ilnytskyi, D. Neher, "Structure and internal dynamics of a side chain liquid crystalline polymer in various phases by molecular dynamics simulations: A step towards coarse graining", The Journal of Chemical Physics126 (2007) 174905 DOI
Side chain liquid crystalline polymer with relatively long spacer was modeled on a semiatomistic level and studied in different liquid crystalline phases with the aid of molecular dynamics simulations. Well equilibrated isotropic, polydomain smectic and monodomain smectic phases were studied for their structural and dynamic properties. Particular emphasis was given to the analysis on a coarse-grained level, where backbones, side chains, and mesogens were considered in terms of their equivalent ellipsoids. The authors found that the liquid crystalline phase had a minor influence on the metrics of these objects but affected essentially their translational and orientational order. In the monodomain smectic phase, mesogens, backbones, and side chains are confined spatially. Their diffusion and shape dynamics are frozen along the mesogen director (the one-dimensional solidification) and the reorientation times increase by one to one-and-half orders of magnitude. In this phase, besides obvious orientational order of mesogens and side chains, a stable detectable order of the backbones was also observed. The backbone director is confined in the plane perpendicular to the mesogen director and constantly changes its orientation within this plane. The backbone diffusion in these planes is of the same range as in the polydomain smectic phase at the same temperature. A detailed analysis of the process of field-induced growth of the smectic phase was performed. The study revealed properties of liquid crystalline polymers that may enable their future fully coarse-grained modeling.
C. Yin, B. Pieper, B. Stiller, T. Kietzke, D. Neher, “Charge carrier generation and electron blocking at interlayers in polymer solar cells”, Applied Physics Letters90 (2007) 133502 DOI
The authors show that an electron-donating polymer interlayer and a spin coated layer of an electron-accepting polymer form a defined polymer-polymer heterojunction. Directional photoinduced charge transfer and efficient electron blocking at this heterojunction is clearly seen in Kelvin probe measurements. The photocurrent characteristics of this well-defined bilayer structure as well as of the respective blend device can be consistently fitted by models taking into account only the field dependence of charge carrier generation. Apparently, the efficiency to form free carriers is the determining process in both types of polymer-polymer solar cell structures.
F.-I. Wu, X.-H. Yang, D. Neher, R. Dodda, Y.-H. Tseng, C.-F. Shu, “Efficient White-Electrophosphorescent Devices Based on a Single Polyfluorene Copolymer”, Advanced Functional Materials17 (2007) 1085 DOI
An efficient white-light-emitting polymer (W3) is realized by covalently attaching a green fluorophore and a red phosphor into the backbone and the side chains, respectively, of polyfluorene at a concentration of 0.04 mol %. In addition, charge-transporting pendant units are included to improve carrier injection and transport. White-electrophosphorescent devices with the structure ITO/PEDOT:PSS/W3/CsF/Al (ITO: indium tin oxide; PEDOT:PSS: poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene)) exhibit a low turn-on voltage of 2.8 V and a luminance of ca. 103 cd m-2 at below 6 V. The peak luminance and power-conversion efficiencies are 8.2 cd A-1 and 7.2 lm W-1, respectively. Furthermore, the device shows relatively stable white emission: the Commission Internationale d'Éclairage (CIE) chromaticity coordinates of the devices change only slightly from (0.35,0.38) at 10 mA cm-2 to (0.33,0.36) at 100 mA cm-2, with an almost constant color render index (CRI) value of 82 at all measured current densities.
A. Zen, P. Pingel, F. Jaiser, D. Neher, J. Grenzer, W. Zhuang, J.P. Rabe, A. Bilge, F. Galbrecht, B.S. Nehls, T. Farrell, U. Scherf, R.D. Abellon, F.C. Grozema, L.D.A. Siebbeles, “Organic Field-Effect Transistors Utilizing Solution-Deposited Oligothiophene-Based Swivel Cruciforms”, Chemistry of Materials19 (2007) 1267 DOI
Two types of highly soluble oligothiophene-based swivel cruciforms are presented as semiconducting materials in OFETs. Incorporation of these penta-aryl oligomers into a swivel-cruciform type architecture greatly enhances the solubility of the oligomers, which lends itself to the fluidic preparation of thin films of the materials. XRD and AFM analysis of the resulting thin films reveals that they are highly crystalline. OFETs with pentathiophene-based swivel cruciforms exhibit a maximum field-effect mobility of 0.012 cm2/Vs, which is among the highest values reported to date for wet-processed OFETs utilizing oligothiophenes. In contrast, transistors made from bithiophene-phenyl-based cruciforms give overall lower mobilities, although the layers appear more crystalline in the AFM images. Pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) experiments of the swivel cruciforms suggest that charge transport between crystalline domains may ultimately be the determining factor.
C. Yin, T. Kietzke, D. Neher, H.-H. Hörhold, “Photovoltaic properties and exciplex emission of polyphenylenevinylene-based blend solar cells”, Applied Physics Letters90, 092117 (2007) DOI
By studying the photoluminescence emission and photovoltaic properties of blends of polyphenylenevinylene (PPV)-based electron donating and accepting polymers, the authors observed a strict anticorrelation between relative exciplex emission in the solid state and photovoltaic efficiency of corresponding blend devices. Comparative studies on defined bilayer geometries showed that the overall shape of the current-voltage characteristics under illumination is independent of device geometry and layer thickness. Consequently, they conclude that the photocurrent in PPV-based blends is mainly determined by the efficiency to form free carriers rather than by free carrier recombination.
C. Yin, T. Kietzke, M. Kumke, D. Neher, H.-H. Hörhold, “Relation between exciplex formation and photovoltaic properties of PPV polymer-based blends”, Solar Energy Materials and Solar Cells91, 411 (2007) DOI
As a new record for pure polymer-blend solar cells, an energy conversion efficiency (ECE) of 1.7% was recently achieved for M3EH-PPV:CN–ether–PPV (Poly[oxa-1,4-phenylene-1,2-(1-cyano)-ethylene-2,5-dioctyloxy-1,4-phenylene-1,2-(2-cyano)-ethylene-1,4-phenylene]) based devices [T. Kietzke, H.-H. Hörhold, D. Neher, Chem. Mater.17 (2005) 6532]. Even though that photoluminescence experiments indicated that 95% of the photogenerated excitions were dissociated in the blend, the external quantum efficiency reached only 31%. Thus more than 2/3 of the dissociated excitons were lost for the energy conversion. In order to identify the processes which limit the photovoltaic efficiency of polymer-blend solar cells, studies on the steady state and time-resolved photoluminescence of the individual polymer and polymer blend were performed. In the polymer-blend layer, we observed a considerable long-wavelength emission due to exciplex formation. The exciplex emission can be reduced by thermal annealing. At the same time the IPCE of the blend-based device increased, indicating a more efficient generation of free-charge carriers. These findings lead to the conclusion that charge-carrier recombination via exciplex formation constitutes one of the loss channels which limits the efficiency of polymer solar cells.
- N. Mechau, M. Saphiannikova, D. Neher, “Molecular tracer diffusion in thin azobenzene polymer layers”, Applied Physics Letters89 (2006) 251902 DOI
Translational diffusion of fluorescent tracer molecules in azobenzene polymer layers is studied at different temperatures and under illumination using the method of fluorescence recovery after photobleaching. Diffusion is clearly observed in the dark above the glass transition temperature, while homogeneous illumination at 488 nm and 100 mW/cm2 does not cause any detectable diffusion of the dye molecules within azobenzene layers. This implies that the viscosity of azobenzene layers remains nearly unchanged under illumination with visible light in the absence of internal or external forces.
X. H. Yang, F. Jaiser, B. Stiller, D. Neher, F. Galbrecht, U. Scherf, “Efficient Polymer Electrophosphorescent Devices with Interfacial Layers”, Advanced Functional Materials16 (2006) 2156 DOI
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 (external quantum efficiency = 6 %) and a maximum power-conversion efficiency of 5 lm/W can be realized.
A. Bilge, A. Zen, M. Forster, H.B. Li, F. Galbrecht, B.S. Nehls, T. Farrell, D. Neher, U. Scherf, “Swivel-cruciform oligothiophene dimers”, Journal of Materials Chemistry16 (2006) 3177 DOI
Synthesis and electronic properties of three swivel-cruciform oligothiophene dimers—bis(terthiophene) (BT3), bis(pentathiophene) (BT5) and bis(heptathiophene) (BT7)—with increased solubility in organic solvents are reported. We obtained a field-effect mobility of 3.7 × 10–5 cm2 V–1 s–1 and a current on/off ratio of >103 for a solution-processed OFET device with dimer BT5 as p-type semiconductor.
T. Kietzke, D. A. M. Egbe, H.-H. Hörhold, D. Neher, “Comparative Study of M3EH-PPV-Based Bilayer Photovoltaic Devices”, Macromolecules39 (2006) 4018 DOI
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.
X.H. Yang, D.C. Müller, D. Neher, K. Meerholz, “Highly efficient polymeric electrophosphorescent diodes”, Advanced Materials18 (2006) 948 DOI
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. They improve hole injection and act as electron-blocking layers, without the need to include exciton-or hole-blocking layers.
A. Zen, A. Bilge, F. Galbrecht, R. Alle, K. Meerholz, J. Grenzer, D. Neher, U. Scherf, T. Farrell, “Solution processable organic field-effect transistors utilizing an alpha,alpha'-dihexylpentathiophene-based swivel cruciform”, Jornal of the American Chemical Society128 (2006) 3914 DOI
A pentathiophene-based swivel cruciform, which allows rotation between the cruciform arms, was synthesized. Homogeneous microcrystalline films were processed from solution, and field-effect transistors utilizing this dimer gave hole mobilities up to 0.012 cm2/Vs.
A. Zen, M. Saphiannikova, D. Neher, J. Grenzer, S. Grigorian, U. Pietsch, U. Asawapirom, S. Janietz, U. Scherf, I. Lieberwirth, G. Wegner, “Effect of Molecular Weight on the Structure and Crystallinity of Poly(3-hexylthiophene)”, Macromolecules39 (2006) 2162 DOI
Recently, two different groups have reported independently that the mobility of field-effect transistors made from regioregular poly(3-hexylthiophene) (P3HT) increases strongly with molecular weight. Two different models were presented: one proposing carrier trapping at grain boundaries and the second putting emphasis on the conformation and packing of the polymer chains in the thin layers for different molecular weights. Here, we present the results of detailed investigations of powders and thin films of deuterated P3HT fractions with different molecular weight. For powder samples, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate the structure and crystallization behavior of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements reveal a strong decrease of the crystallization temperature and, most important, a significant decrease of the degree of crystallinity with decreasing molecular weight. To study the structure of thin layers in lateral and vertical directions, both transmission electron microscopy (TEM) and X-ray grazing incidence diffraction (GID) were utilized. These methods show that thin layers of the low molecular weight fraction consist of well-defined crystalline domains embedded in a disordered matrix. We propose that the transport properties of layers prepared from fractions of poly(3-hexylthiophene) with different molecular weight are largely determined by the crystallinity of the samples and not by the perfection of the packing of the chains in the individual crystallites.
J. Ilnytskyi, M. Saphiannikova, D. Neher, “Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study”, Condensed Matter Physics9 (2006) 87 DOI
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 perpendicular to the field.
B. Luszczynska, E. Dobruchowska, I. Glowacki, J. Ulanski, F. Jaiser, X.H. Yang, D. Neher, A. Danel, “Poly(N-vinylcarbazole) doped with a pyrazoloquinoline dye: A deep blue light-emitting composite for light-emitting diode applications”, Journal of Applied Physics99 (2006) 024505 DOI
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.
X.H. Yang, F. Jaiser, S. Klinger, D. Neher, “Blue polymer electrophosphorescent devices with different electron-transporting oxadiazoles”, Applied Physics Letters88 (2006) 021107 DOI
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.
X.H. Yang, D. Neher, “Electrophosphorescent Devices”, in “Organic Light-Emitting Devices” (eds. K. Müllen & U. Scherf), Wiley-VCH, Weinheim (2006) 333
T. Kietzke, H.H. Hörhold, D. Neher, “Efficient polymer solar cells based on M3EH-PPV”, Chemistry of Materials17 (2005) 6532 DOI
We report on polymer blend solar cells with an external quantum efficiency of more than 30% and a high overall energy conversion efficiency (ECE) under white light illumination (100 mW/cm2) of up 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-phenylene-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)ethenylene-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.
D.A.M. Egbe, C. Ulricht, T. Orgis, B. Carbonnier, T. Kietzke, T. Peip, M. Metzner, M. Gericke, E. Birckner, T. Pakula, D. Neher, U.W. Grumm, “Odd-even-effects and the influence of length and specific positioningof alkoxy side chains on the optical properties of PPE-PPV polymers”, Chemistry of Materials17 (2005) 6022 DOI
This contribution reports the combined influences of odd-even effects and the specific positioning of alkoxy side chains OR1 = (OCn+10H2(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≡C–Ph–C≡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 sp3-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 (λf = 556 nm) and P18/8 (λf = 614 nm), was red-shifted relative to that of their even counterparts (λf = 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 Φf-value of 24%. The strong π-π 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 electroluminescence, 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.
M. Saphiannikova, D. Neher, “Thermodynamic theory of light-induced material transport in amorphous azobenzene polymer films”, Journal of Physical Chemistry B109 (2005) 19428 DOI
It was discovered 10 years ago that the exposure of an initially flat layer of an azobenzene-containing polymer to an inhomogeneous light pattern leads to the formation of surface relief structures, accompanied by a mass transport over several micrometers. However, the driving force of this process is still unclear. We propose a new thermodynamic approach that explains a number of experimental findings including the light-induced deformation of free-standing films and the formation of surface relief gratings for main inscription geometries. Our basic assumption is that under homogeneous illumination, an initially isotropic sample should stretch itself along the polarization direction to compensate the entropy decrease produced by the photoinduced reorientation of azobenzene chromophores. The magnitude of the elastic stress, estimated by taking the derivative of the free energy over the sample deformation, is shown to be sufficient to induce plastic deformation of the polymer film. Orientational distributions of chromophores predicted by our model are compared with those deduced from Raman intensity measurements.
T. Kietzke, B. Stiller, K. Landfester, R. Montenegro, D. Neher, “Probing the local optical properties of layers prepared from polymer nanoparticles”, Synthetic Metals152 (2005) 101 DOI
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.
P. Karageorgiev, D. Neher, B. Schulz, B. Stiller, U. Pietsch, M. Giersig, L. Brehmer, “From anisotropic photo-fluidity towards nanomanipulation in optical near field”, Nature Materials4 (2005) 699 DOI
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.
D.A.M. Egbe, B. Carbonnier, E.L. Paul, D. Mühlbacher, T. Kietzke, E. Birckner, D. Neher, U.W. Grummt, T. Pakula, “Diyne-containing PPVs: Solid-state properties and comparison of their photophysical and electrochemical properties with those of their Yne-containing counterparts”, Macromolecules38 (2005) 6269 DOI
Diyne-containing poly(p-phenylene-vinylene)s, of general chemical structure –[Ph–C≡C–C≡C–Ph–CH≡CH–Ph–CH≡CH]–n, obtained through polycondensation reactions of 1,4-bis(4-formyl-2,5-dioctyloxyphenyl)-buta-1,3-diyne with various 2,5-dialkoxy-p-xylylenebis(diethylphosphonates) 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 –[Ph–C≡C–Ph–CH≡CH–Ph–CH≡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 lowers the HOMO and LUMO levels, thereby enhancing the electron affinity of diynes compared to ynes. The "wider opening" introduced by the second yne unit facilitates moreover the movement of charges during the electrochemical processes leading to minimal discrepancy, ΔEg between the optical and electrochemical band gap energies. Ynes, in contrast, show significant side-chain-dependent ΔEg 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/diyne/Ca/Al.
A. Zen, D. Neher, K. Simly, A. Holländer, U. Asawapirom, U. Scherf, “Improving the performance of organic field effect transitors by optimizing the gate insulator surface”, Japanese Journal of Applied Physics Part 144 (2005) 3721 DOI
The effect of oxygen plasma treatment and/or silanization with hexamethyldisilazane (HMDS) on the surface chemistry and the morphology of the SiO2-gate insulator were studied with respect to the performance of organic field effect transistors. Using X-ray photoelectron spectroscopy (XPS), it is shown that silanization leads to the growth of a polysiloxane interfacial layer and that longer silanization times increase the thickness of this layer. Most important, silanization reduces the signal from surface contaminations such as oxidized hydrocarbon molecules. In fact, the lowest concentration of these contaminations was found after a combined oxygen plasma/silanization treatment. The results of these investigations were correlated with the characteristic device parameters of polymer field effect transistors with poly(3-hexylthiophene)s as the semiconducting layer. We found that the field effect mobility correlates with the concentration of contaminations as measured by XPS. We, finally, demonstrate that silanization significantly improves the operational stability of the device in air compared to the untreated devices.
N. Mechau, M. Saphiannikova, D. Neher, “Dielectric and mechanical properties of azobenzene polymer layers under visible and ultraviolet irradiation”, Macromolecules38 (2005) 3894 DOI
Photoinduced changes in the mechanical and dielectric properties of azobenzene polymer films were measured utilizing the method of electromechanical spectroscopy. The measurements revealed a strong correlation between the time-dependent behavior of the plate compliance and the dielectric constant under irradiation. Actinic light causes a light softening of the film that also manifests itself in the increase of the dielectric constant, whereas ultraviolet irradiation results in an initial plasticization of the film followed by its hardening. The latter is accompanied by decrease of the dielectric constant. A semiquantitative model based on the kinetics of the photoisomerization process in azobenzene polymers is proposed. We assume that both visible and ultraviolet irradiation increase the free volume in the layer due to photoisomerization. Additionally, ultraviolet light increases the modulus of the polymer matrix due to the presence of a high density of azobenzene moieties in the cis state. These assumptions allowed us to reproduce the time-dependent behavior of the bulk compliance as well as the dielectric constant at different irradiation intensities, for both visible and ultraviolet light, with only two adjustable parameters.
S. A. Bagnich, H. Bässler, D. Neher, “Exciton dynamics in ladder-type methyl-poly(para-phenylene) doped with phosphorescent dyes”, Journal of Luminescence112 (2005) 377 DOI
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.
F. Galbrecht, X. H. Yang, B. S. Nehls, D. Neher, T. Farrell, U. Scherf, “Semiconducting polyfluorenes with electrophosphorescent on-chain platinum-salen chromophores”, Chemical Communications (2005) 2378 DOI
The synthesis of statistical fluorene-type copolymers with on-chain Pt-salen phosphorescent units and their use in electrophosphorescent OLEDs is reported.
A. Zen, M. Saphiannikova, D. Neher, U. Asawapirom, U. Scherf, “Comparative study of the field-effect mobility of a copolymer and a binary blend based on poly(3-alkylthiophene)s”, Chemistry of Materials17 (2005) 781 DOI
The performance of highly soluble regioregular poly[(3-hexylthiophene)-co-(3-octylthiophene)] (P3HTOT) as a semiconducting material in organic field-effect transistors (OFETs) is presented in comparison to that of the corresponding homopolymers. Transistors made from as-prepared layers of P3HTOT exhibit a mobility of ca. 7 x 10-3 cm2/Vs, which is comparable to the performance of transistors made from as-prepared poly(3-hexylthiophene) (P3HT) and almost 6 times larger than the mobility of transistors prepared with poly(3-octylthiophene) (P3OT). On the other hand, the solubility parameter δp of P3HTOT is close to that of the highly soluble P3OT. Moreover, compared to a physical blend of poly(3-hexylthiophene) and poly(3-octylthiophene), the mobility of P3HTOT devices is almost twice as large and the performance does not degrade upon annealing at elevated temperatures. Therefore, the copolymer approach outlined here may be one promising step toward an optimum balance between a Sufficient processability of the polymers from common organic solvents, a high solid state order, and applicable OFET performances.