We study external quantum efficiency (eta(EQE)) roll-off in organic light-emitting diodes (OLEDs) using thermally-activated delayed fluorescence (TADF) of 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN). Using 2CzPN intramolecular rate constants from optical analyses, we construct an exciton quenching model incorporating intersystem crossing and reverse intersystem crossing. The model indicates that singlet-triplet annihilation and triplet-triplet annihilation dominate eta(EQE) roll-off because of the relatively long 2CzPN triplet lifetime of 273 mu s. This work yields a method to relax the exciton quenching process in TADF based OLEDs. (C) 2013 Elsevier B. V. All rights reserved.
We investigate the chemical and structural properties of solution-processed thin films of P3HT blended with p-type dopant F4TCNQ. The maximum in-plane electrical conductivity of doped films is observed at a molar doping fraction of 0.17, in agreement with the binding mechanism of F4TCNQ: P3HT complexes. Through the use of X-ray diffraction, a previously unreported crystalline phase is observed for P3HT films doped above a critical threshold concentration. This crystalline phase involves the incorporation of F4TCNQ molecules into ordered polymer regions and ultimately improves charge dissociation, leading to higher carrier density in thin film. Finally, optical absorption and X-ray diffraction reveal that the chemical state of P3HT in solution has a dramatic impact on the electrical and structural properties of the blended films. (C) 2013 Published by Elsevier B.V.
Polymer solar cell modules were prepared directly on thin flexible barrier polyethylene terephthalate foil. The performance of the modules was found to be scalable from a single cell with an area of 6 cm(2) to modules with a total area of up to 186 cm(2). The substrate thickness was also explored and the performance was found to be independent of thickness in the range of 20-130 lm. The thinner substrates were found to present some challenge regarding handling but were not limited in performance. Large area modules on a substrate thickness of 45 mu m were finally prepared by full roll-to-roll processing employing P3HT:PCBM as the active material and were found to exhibit a total area efficiency of >1% (1000 W/m(-2); AM1.5G) with a typical active-area efficiency in the 1.5-1.6% for total module area of >110 cm(2) due to high fill factors in excess of 50%. The modules were also found to have an active-area efficiency of >1% under low light levels (similar to 100Wm(-2)). The modules were then subjected to extensive stability testing for a minimum of 1000 h employing several ISOS protocols. The modules presented higher than 80% of the initial performance (T80) in the dark (ISOS-D-1), in dark under elevated temperature of 65 degrees C (ISOS-D-2), under low light (ISOS-LL), under full sunlight (ISOS-L-2), and under outdoor testing (ISOS-O), which was conducted in two locations in India and Denmark. We estimate maximum T80 for those tests to be 2800, 5000, 1300, 1000, and 3500 h respectively. The modules showed significant sensitivity to high humidity and had low values for T80 for dark storage tests at 50 degrees C/85%RH (ISOS-D-3) and accelerated operation conditions with 0.7 sun/65 oC/50%RH (ISOS-L-3). We found the modules to be particularly suited for information and communications technology (ICT) and mobile applications where low humidity (< 50%) and lower temperatures (< 65 degrees C) can be anticipated and we estimate operational lifetimes in excess of 1 year. (C) 2013 Elsevier B.V. All rights reserved.
Conductive nanocomposite membranes of polypyrrole/bacterial cellulose (PPy/BC) were fabricated in situ by oxidative polymerization of pyrrole with iron (III) chloride as an oxidant and BC as a template. The morphology of the PPy/BC membrane indicated that PPy nanoparticles deposited on the BC surface connected to form a continuous nanosheath structure by taking along the BC nanofiber. The flexible PPy/BC membrane obtained with the optimized reaction condition exhibited a high electrical conductivity of 3.9 S cm(-1), which was hardly affected by bending stress. The PPy/BC membrane could be directly used as flexible supercapacitor electrodes, with a maximum discharge capacity of 101.9 mA h g(-1) (459.5 F g(-1)) at 0.16 A g(-1) current density. The capacity decreased with charge/discharge cycling, which is attributed to mechanical degradation of PPy as evidenced by scanning electron microscopy (SEM). (C) 2013 Elsevier B. V. All rights reserved.
The stability and the degradation processes of two highly efficient blue-emitting phosphorescent materials, iridium(III) bis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazolyl) borate (FIr6) and bis(2-(4,6-difluorophenyl) pyridyl-N,C2')iridium(III)picolinate (FIrpic), which are commonly used as emitters in organic light emitting diodes (OLEDs), are investigated. Using single layers devices, the optical response and the half-lifetime behavior of the materials are investigated. Layers of FIr6 exposed to UV-light show the formation of a red emitting degradation product. We analyze the chemical reactions of the materials using laser desorption/ionization time-of-flight mass spectrometry. Several products related to the chemical dissociation of the FIr6 molecule as well as charge complex formation between the emitter and the emitter dissociation products are detected. FIr6 and FIrpic are also compared by lifetime studies on commonly used OLED structures. We show that single layers and OLEDs based on FIrpic exhibit higher stability than those based on FIr6. An explanation for this behavior can be found by considering the chemical structure of the molecules. (C) 2012 Elsevier B.V. All rights reserved.
We describe a potentiometric sensor based on Electrolyte-Gated Organic Field-Effect Transistor (EGOFET) for "in vitro'' detection of dopamine. The sensing element of this device resides at the Au gate-aqueous solution interface by means of a self-assembled monolayer (SAM) composed by cysteamine and 4-formylphenyl boronic acid. The covalent and selective adsorption of dopamine induces a surface dipole potential which shifts the electrode work function and modulates the double layer capacitance. As a result, our device is capable to detect dopamine up to pico-molar concentration showing higher sensitivity with respect to other approaches. For this reason the interface engineering of our EGOFET gate is a promising route for diagnostic applications. (c) 2012 Elsevier B.V. All rights reserved.
Homoleptic triscyclometalated iridium(III) complex Ir(dbi)(3) was used as a dopant for sky blue phosphorescent organic light-emitting diodes (PHOLEDs). Its photophysical, thermal, electrochemical properties as well as the device performances were investigated. Ir(dbi)(3) exhibited high quantum yield of 0.52 in solution at room temperature. A maximum current efficiency and external quantum efficiency (EQE) of 61.5 cd A (1) and 23.1% were obtained, which are the highest ever reported for blue homoleptic iridium complexes. High efficiencies of 53.5 cd A (1) and 20.1% EQE were achieved even at the luminance of 1000 cd m (2). (C) 2013 Elsevier B. V. All rights reserved.
The spin transport properties of molecular devices constructed by hydrogen-phthalocyanine and transition metal (TM)-phthalocyanine molecule with zigzag graphene nanoribbon electrodes are investigated by the Keldysh nonequilibrium Green's function method in combination with the density functional theory. The results show that there exists giant magnetoresistance in both the hydrogen-phthalocyanine and the TM-phthalocyanine systems. The magnetoresistance ratio is much bigger than that found by Schmaus et al. [S. Schmaus, A. Bagrets, Y. Nahas, T. K. Yamada, A. Bork, M. Bowen, E. Beaurepaire, F. Evers, W. Wulfhekel, Nature Nanotechnology 6 (2011) 185-9] in single hydrogen-phthalocyanine-Co electrodes system. Moreover, it is found that the chromium-phthalocyanine molecular device is a good spin filtering device with nearly 100% spin filtering efficiency at a wide bias voltage region. The mechanisms are proposed for these phenomena. (C) 2013 Elsevier B. V. All rights reserved.
The effects of gold (Au) nanoparticles (NPs) with different morphologies (star, rod, sphere) incorporated into buffer layer, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS), of polymer-based organic solar cells (OSCs) were investigated. Solar cells having gold nanoparticles exhibited significant improvement in device efficiency relative to the reference device. The observed improvement is most likely due to the surface plasmon and enhanced light reflection and scattering properties of the Au NPs. The power conversion efficiency (PCE) is increased ca. 29% with Au nanostars, ca. 14% with Au nanorods and 11% with Au nanospheres compared to the device with no Au NP (reference device). Au nanostars provide the strongest contribution to the efficiency among all NP morphologies studied as they have large size, sharp features, and strongest localized surface plasmon resonance effect associate with their morphology. (C) 2013 Elsevier B.V. All rights reserved.
ITO-free organic solar cells with inkjet printed current collecting grids are demonstrated. For sintering those grids, thermal treatment and its faster alternative, photonic flash sintering, are applied and the characteristics of the resulting metal structures are compared with each other. The electrical potentials and resulting currents in the devices with different sintering conditions are calculated. The flash sintered current collecting grids exhibit clear advantages over thermally sintered grids in terms of geometry and conductivity. Similar conductivities are obtained after 5 s of flash sintering and 6 h of thermal sintering. This finding demonstrates the great potential of flash sintering for the roll-to-roll manufacturing of printed organic solar cells on flexible substrates. (c) 2012 Elsevier B.V. All rights reserved.
The small-molecule organic semiconductor 2,9-di-decyl-dinaphtho-[2,3-b: 2',3'-f]-thieno[3,2-b]-thiophene (C-10-DNTT) was used to fabricate bottom-gate, top-contact thin-film transistors (TFTs) in which the semiconductor layer was prepared either by vacuum deposition or by solution shearing. The maximum effective charge-carrier mobility of TFTs with vacuum-deposited C-10-DNTT is 8.5 cm(2)/V s for a nominal semiconductor thickness of 10 nm and a substrate temperature during the semiconductor deposition of 80 degrees C. Scanning electron microscopy analysis reveals the growth of small, isolated islands that begin to coalesce into a flat conducting layer when the nominal thickness exceeds 4 nm. The morphology of the vacuum-deposited semiconductor layers is dominated by tall lamellae that are formed during the deposition, except at very high substrate temperatures. Atomic force microscopy and X-ray diffraction measurements indicate that the C-10-DNTT molecules stand approximately upright with respect to the substrate surface, both in the flat conducting layer near the surface and within the lamellae. Using the transmission line method on TFTs with channel lengths ranging from 10 to 100 mu m, a relatively small contact resistance of 0.33 k Omega cm was determined. TFTs with the C-10-DNTT layer prepared by solution shearing exhibit a pronounced anisotropy of the electrical performance: TFTs with the channel oriented parallel to the shearing direction have an average carrier mobility of (2.8 +/- 0.3) cm(2)/V s, while TFTs with the channel oriented perpendicular to the shearing direction have a somewhat smaller average mobility of (1.3 +/- 0.1) cm(2)/V s. (C) 2013 Elsevier B.V. All rights reserved.
Digital printing technologies are promising as future manufacturing approaches due to their capabilities of highly flexible and additive material deposition on various substrates. In this contribution, all inkjet-printed piezoelectric polymer actuators are presented based on polyvinylidene fluoride trifluoroethylene (P(VDF-TrFE)) and electrodes printed from silver nanoparticle dispersions. The target application for the actuators described here are membrane pumps for microfluidic lab-on-a-chip (LOC) systems. For the first time, all-ink-jet-printed P(VDF-TrFE) actuators are reported and the corresponding piezoelectric d(31) coefficient is measured. For manufacturing the actuators, a low-cost procedure is employed that consists of only three inkjet printing and post-processing steps where moderate thermal treatments (T-max = 130 degrees C) are combined with plasma sintering. The processing is therefore compatible with a wide range of temperature sensitive polymer substrates, completely additive and highly flexible. A sandwich-like structure of a piezoelectric P(VDF-TrFE) layer between two silver electrodes is inkjet-printed onto a polyethylene terephthalate (PET) substrate. When a voltage is applied across the piezoelectric layer, the reverse piezoelectric effect will lead to a bending deflection of this unimorph structure. The piezoelectric d(31) coefficients are found to be approximately 7 to 9 pm V-1, which allows the generation of significant actuator deflections. For the application in a micropump, flow rates of several 100 mu L min(-1) are anticipated, which is promising for LOC applications. Most current micropumps are based on actuator elements that are fabricated separately and mounted on a passive membrane. By using all inkjet-printed actuators, as presented here, the joining step is avoided and the benefits of low-cost printed devices are added to the well-developed processing approaches for microfluidic chips. (C) 2013 Elsevier B. V. All rights reserved.
We demonstrated a high performance flexible multi-barrier containing a silica nanoparticle-embedded organic-inorganic hybrid (S-H) nanocomposite and Al2O3. The multi-barrier was prepared by low-temperature Al2O3 atomic layer deposition and with a spin-coated S-H nanocomposite. The moisture barrier properties were investigated with a water vapor transmission rate (WVTR), estimated by a Ca test at 30 degrees C, 90% R.H.. Moisture diffusion was effectively suppressed by the sub-700 nm thick multi-barrier incorporating well-dispersed silica nanoparticles in the organic layer. A low WVTR of 1.14 x 10 (5) g/m(2) day and average transmittance of 85.8% in the visible region were obtained for the multi-barrier. After bending under tensile stress mode, the moisture barrier property of the multi-barriers was retained. The multi-barrier was successfully applied to thin-film encapsulation of OLEDs. The thin-film encapsulated OLEDs showed practicable current-voltage-luminance (I-V-L) characteristics and stable real operation over 700 h under ambient conditions. (C) 2013 Elsevier B. V. All rights reserved.
Bottom-gate, top-contact (inverted staggered) organic thin-film transistors with a channel length of 1 mu m have been fabricated on flexible plastic substrates using the vacuum-deposited small-molecule semiconductor 2,9-didecyl-dinaphtho[2,3-b:2',3'-f] thieno[3,2-b] thiophene (C-10-DNTT). The transistors have an effective field-effect mobility of 1.2 cm(2)/V s, an on/off ratio of 107, a width-normalized transconductance of 1.2 S/m (with a standard deviation of 6%), and a signal propagation delay (measured in 11-stage ring oscillators) of 420 ns per stage at a supply voltage of 3 V. To our knowledge, this is the first time that megahertz operation has been achieved in flexible organic transistors at supply voltages of less than 10 V. (C) 2013 Elsevier B. V. All rights reserved.
Ammonia (NH3) gas sensors based on pentacene organic field-effect transistors (OFETs) are fabricated using polymers as the dielectric. Compared with those incorporating poly(vinyl alcohol), poly(4-vinylphenol) or poly(methyl methacrylate) dielectric, a low detect limitation of 1 ppm and enhanced recovery property are obtained for OFETs with polystyrene (PS) as gate dielectric. By analyzing the morphologies of pentacene and electrical characteristics of the OFETs under various concentrations of NH3, the variations of the sensing properties of different dielectrics based OFET-sensors are proved to be mainly caused by the diversities of dielectric/pentacene interfacial properties. Furthermore, low surface trap density and the absence of polar groups in PS dielectric are ascribed to be responsible for the high performance of NH3 sensors. (C) 2013 Elsevier B. V. All rights reserved.
The geometries and electronic structures of organic dye sensitizers, CCT1A, CCT2A, CCT3A, CCT1PA, and CCT2PA comprising double-donor groups, pi-spacer, and acceptor group forming D-D-pi-A system, were studied using DFT and TDDFT. The calculated results have shown that TDDFT calculation using a newly-designed functional which takes into long-range interaction, CAM-B3LYP, was reasonably capable of predicting the excitation energies and the absorption spectra of the molecules. The adsorption of these dyes on the TiO2 anatase (101) surface and the electron injection mechanism were also investigated using a dye-(TiO2)(38) cluster model, employing PBE and TD-CAM-B3LYP calculations, respectively. The adsorption energy (E-ads) of CCTnA (n = 1-3) was calculated to be -15.26, -18.93, and -20.12 kcal/mol respectively, indicating strong adsorption of dye to a TiO2 surface by carboxylate groups. These calculated results suggested that the CCT3A is a promising candidate for highly efficient DSSCs. It was shown that the electron injection mechanism occurs by direct charge-transfer transition in a dye-TiO2 interacting system, resulted in the stronger electronic coupling strengths of the anchoring group of the dyes and the TiO2 surface which corresponded to higher observed J(SC) as expected in CCT3A dye. Through a combined theoretical and experimental investigation we have shown that the trend of charge-injection efficiency in dye-sensitized solar cells constituted from dyes is determined by the adsorption energy of dye-(TiO2)(38) complexes. (C) 2012 Elsevier B.V. All rights reserved.
The commonly studied architecture of organic light-emitting diodes (OLEDs) faces the obstacle of limited light outcoupling from the planar glass substrate, which greatly affects the device external efficiency. Introduction of microstructures is considered as an effective approach to extract photons trapped within the device. To reach this end, a two-steps fabrication technique based on breath figure patterns and replica molding is here presented. Through this approach elastomeric microlens arrays having different morphological features are obtainable in a fast and simple way. We show how the mere application of these patches on the external face of a conventional OLED leads to a neat efficiency enhancement up to 34%. An increase of light intensity at viewing angle between 30 and 60 degrees is also demonstrated. These results are a proof of principle that improving the performance of an OLED by a non-lithographic surface modification strategy and without altering its functioning, is feasible. (C) 2012 Elsevier B.V. All rights reserved.
Flexible displays are attracting considerable attention as a visual interface for applications such as in electronic papers and paper electronics. Passive or active matrix addressing of individual pixels require display elements that include proper signal addressability, which is typically provided by non-linear device characteristics or by incorporating transistors into each pixel. Including such additional devices into each pixel element make manufacturing of flexible displays using adequate printing techniques very hard and complicated. Here, we report all-printed passive matrix addressed electrochromic displays (PMAD), built up from a very robust three-layer architecture, which can be manufactured using standard printing tools. Poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) serves as the conducting and electrochromic pixel electrodes and carbon paste is used as the pixel counter electrodes. These electrodes sandwich self-assembled layers of a polyelectrolyte that are confined to desired pixel areas via surface energy patterning. The particular choice of materials results in a desired current vs. voltage threshold that enables addressability in electronic cross-point matrices. The resulting PMAD operates at less than 3 V, exhibits high colour switch contrast without cross-talk and promises for high-volume and low-cost production of flexible displays using reel-to-reel printing tools on paper or plastic foils. (C) 2013 Elsevier B. V. All rights reserved.
We demonstrated an organic/inorganic multi-barrier and encapsulation for flexible OLED devices. The multi-barrier consisted of a silica nanoparticle-embedded hybrid nanocomposite, in short, S-H nanocomposite, and MgO, which were used as organic and inorganic materials, respectively. The S-H nanocomposite was spin-coated followed by UV curing. The thickness of the S-H nanocomposite was 200 nm, and 40 nm of MgO was deposited by atomic layer deposition (ALD) using Mg(CpEt)(2) and H2O at 70 degrees C. The results of a Ca test showed that the 4.5 dyads of the MgO/S-H nanocomposite had a low water vapor transmission rate (WVTR) of 4.33 x 10(-6) g/m(2)/day and an optical transmittance of 84%. The normalized luminance degradation of the thin film encapsulated OLED was also identical to that of glass-lid encapsulation after 1000 h of the real operation time. We proposed low temperature ALD as a deposition method to create relatively thin film for OLED passivation without degradation, such as creation of dark spots. The results confirmed that it may be feasible for our multi-barrier to passivate flexible OLEDs devices. (C) 2013 Elsevier B.V. All rights reserved.
Percolation networks from silver nanowires can be used as a transparent electrode and promising alternative to the commonly used ITO. Here, the deposition of such a nanowire based network by dip coating, a versatile and scalable method, is described and characterized in detail. After identifying appropriate processing parameters, nanowire grids with conductivity and transmittance values rivaling ITO have been achieved. As a main issue, the roughness of the film was addressed and the influence on the device in terms of shunt paths was investigated. Using this network as anode, small molecule organic solar cells with varying stack structure were fabricated and yield efficiencies comparable to cells on ITO. (C) 2012 Elsevier B.V. All rights reserved.