Organic Electronics 2011 Volume 12 №07

Elsevier. — 181 p. — ISSN: 1566-1199.«Organic Electronics» is a journal whose primary interdisciplinary focus is on materials and phenomena related to organic devices such as light emitting diodes, thin film transistors, photovoltaic cells, sensors, memories, etc.Papers suitable for publication in this journal cover such topics as photoconductive and electronic properties of organic materials, thin film structures and characterization in the context of organic devices, charge and exciton transport, organic electronic and optoelectronic devices.«Organic Electronics» provides the forum for applied, fundamental and interdisciplinary contributions spanning the wide range of electronic properties and applications of organic materials. A Letters section is included for rapid publication of short articles announcing significant and highly original results.1103-1107
Kwang H. Lee, Chan Ho Park, Kimoon Lee, Taewoo Ha, Jae Hoon Kim, Jungheum Yun, Gun-Hwan Lee, Seongil Im. Semi-transparent organic/inorganic hybrid photo-detector using pentacene/ZnO diode connected to pentacene transistor
Abstract:
Semi-transparent organic–inorganic hybrid photo-detector cells were fabricated on glass substrate, composed of pentacene/ZnO photodiodes and pentacene thin-film transistors (TFTs). Semi-transparent NiO_x window with a large band gap of ~4 eV was used as an ohmic electrode on 50 nm-thin pentacene channel in the TFT and simultaneously on the 150 nm-thick pentacene layer, which is deposited on ZnO for p–n diode, to absorb visible and ultraviolet photons from pentacene and ZnO, respectively. Our photo-detector exhibited a minimum temporal response of ~300 ms for photo-induced output voltage at -1 V input and -3 V supply voltage.1108-1113
D.K. Hwang, C. Fuentes-Hernandez, J.B. Kim, W.J. Potscavage Jr., B. Kippelen. Flexible and stable solution-processed organic field-effect transistors
Abstract:
Highly stable, solution-processed, small molecule-polymer blend organic field-effect transistors (OFETs) with a top-gate geometry were demonstrated on a flexible polyethersulfone (PES) substrate. The top-gate dielectric was a bi-layer comprised of CYTOP and a high-k Al₂O₃ layer grown by atomic layer deposition (ALD). A solution processed 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and poly(triarylamine) (PTAA) blend was used as the organic semiconductor. TIPS-pentacene and PTAA blend OFETs with the CYTOP/Al2O3 bi-layer top gate dielectric showed an averaged saturation mobility value of 0,24 ± 0,08 cm²/Vs at operation voltages below 8 V. A constant direct-current bias stress test was carried out to examine their operational stability for 2 h. Under bias stress, neither significant change in mobility nor shift in the threshold voltage has been observed in these OFETs. To evaluate the real potential of these OFETs towards the development of circuit components commonly used in electronic applications, a resistive-load inverter was implemented by connecting an OFET to an external load resistor. Excellent stability of the transistor led to electrically stable inverters with negligible variations of the voltage transfer characteristics before and after bias stress. After the operational stability test, these OFETs were exposed to air and then were subjected to bending experiments. Even after exposure to air for 4 months and bending for 30 min, no significant changes in performance were observed in either a single transistor device or in a resistive-load inverter.1114-1119
Daniel Ping-Kuen Tsang, Mei-Yee Chan, Anthony Yiu-Yan Tam, Vivian Wing-Wah Yam. Host engineering for improving the performance of blue phosphorescent organic light-emitting devices
Abstract:
We report an effective means to greatly improve the performance of blue phosphorescent organic light-emitting devices by doping well-known electron- or hole-transporting triplet host material, specifically, 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) or 4,4',4''-tris(N-carbazolyl)triphenylamine (TCTA), into emissive layer to form mixed host structures. A superior device performance with high current and power efficiencies of 47,4 cd/A and 37,3 μm/W, respectively, and more importantly, a theoretical maximum of external quantum efficiency of 21,9%, can be achieved for an optimized TCTA-doped device. On the other hand, doping of TAZ into hole-transporting host material would degrade device performance. It is found that, in addition to the well-known effects of charge carrier balance, both energy levels and triplet energies of mixed host materials play crucial roles in determining the device performance. As confirmed by the time-resolved phosphorescence decay studies, the lifetime of FIrpic is strongly dependent on the triplet energy of host materials and the shorter lifetime of triplet excitons with high Stern–Volmer quenching constant of 0,1564%^-1 in TAZ-doped films is the origin of the low electroluminescence efficiency of the TAZ-doped devices.1120-1125
Huai-Yuan Tseng, Balaji Purushothaman, John Anthony, Vivek Subramanian. High-speed organic transistors fabricated using a novel hybrid-printing technique
Abstract:
Printing has received attention as a technique to realize low-cost electronic systems. High performance printed transistors are desired to allow the implementation of augmented system functionality in printed systems. Unfortunately, the low resolution of conventional printing hinders printed transistors from delivering high switching speeds. In this work, we have developed and demonstrated a novel print-and-drag (PND) technique that combines inkjet printing with a dip-pen-like dragging process to create highly scaled features. Using this technique, we have demonstrated highly scaled fully printed transistors and inverters using a state-of-the-art printer. The fully printed inverters have propagation delays as low as 15 μs due to the highly-scaled device structures. The technique is compatible with multi-nozzle printing, and thus is promising for the realization of low-cost and high performance printed electronics.1126-1131
Teresa L. Chen, John Jun-An Chen, Luis Catane, Biwu Ma. Fully solution processed p-i-n organic solar cells with an industrial pigment – Quinacridone
Abstract:
We report solution processed organic solar cells with quinacridone (QA), an industrial pigment, as the electron donor. Applying simple spin casting and thermal annealing, trilayer devices with a pure donor (p) layer, a bulk heterojunction (i) layer, and a pure acceptor (n) layer have been fabricated. Tert-butoxycarbonyl quinacridone (t-BOC QA), a soluble yellow precursor of industrial red pigment of quinacridone, was synthesized by replacing the H atom of the NH group on QA with a t-BOC group. Uniform thin films were prepared by spin casting t-BOC QA solutions, which could be converted into insoluble thin films by thermal treatment to remove the solubilizing groups. This conversion allowed for the subsequent depositions of multiple layers without the use of orthogonal solvents. The p-i-n devices showed much higher device performance than their bilayer and simple bulk heterojunction
counterparts, exhibiting power conversion efficiencies (PCEs) as high as 0,83%.1132-1136
J.B. Kim, C. Fuentes-Hernandez, D.K. Hwang, S.P. Tiwari, W.J. Potscavage Jr., B. Kippelen. Vertically stacked complementary inverters with solution-processed organic semiconductors
Abstract:
We report on vertically stacked complementary inverters implemented with a solutionprocessed [6,6]-phenyl c61 butyric acid methyl ester (PCBM) n-channel thin-film transistor (TFT) fabricated on top of a 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and poly(triarylamine) (PTAA) blend p-channel TFT. With a shared common gate electrode positioned between two dielectric layers, bottom-contact p- and top-contact n-channel TFTs showed saturation mobility values of 0.25 and 0,004 cm²/V s and threshold voltages of -3,9, and 0,3 V, respectively. The inverter yielded a gain value of -24 V/V with a switching threshold voltage value of 3,3 V at a supply voltage of 7 V. This demonstration of the use of solution-processed semiconductors in a vertically stacked complementary inverter geometry is a step forward towards the development of low-cost complementary electronics.1137-1141
Wenyu Ji, Jialong Zhao, Zaicheng Sun, Wenfa Xie. High-color-rendering flexible top-emitting warm-white organic light emitting diode with a transparent multilayer cathode
Abstract:
A top-emitting warm-white organic light emitting diode (TEWOLED) was fabricated with a conductive transparent MAM cathode [MAM = MoO₃ (40 nm)/Ag (17 nm)/MoO₃ (40 nm)], which had a higher color rendering index (CRI) than that of corresponding ITO-based bottom-emitting organic light-emitting device (OLED). We measured and calculated the optical transmittance of multilayer MAM fabricated on PET substrate by vacuum thermal evaporation. The average transmittance in visible range is above 84%, which is similar to the conventional indium tin oxide (ITO). The weak microcavity effect on the device performance was also studied. MAM multilayer has the potential for using as transparent conductor electrodes for white OLEDs, especially for flexible devices due to its unique optical and electrical properties.1142-1145
K.N. Bourdakos, L.A. Cury, A.P. Monkman. The dependence of the amplified spontaneous emission on the waveguide excitation length for high quantum efficiency conjugated polymers
Abstract:
The waveguide properties of high quantum efficiency thin film of poly(9,9-dioctyl-fluorene-2,7-diyl) conjugated polymer has been studied at room temperature. The analysis of the enhancement of the amplified spontaneous emission intensity at different excitation lengths has been performed based on the theoretical method developed by Silvestre et al. (2008) [20]. The increase in the threshold intensity with decreasing excitation length has been experimentally observed. This occurs as a response to the cavity gain in order to overcome the total losses when its length decreases, which represents an important factor to support the gain dependence on the excitation length. Even though highly efficient polymers can reach thresholds at relatively lower intensities because the losses are more easily overcome, the observed intensity behavior of the amplified spontaneous emission can only be explained assuming a gain dependence on the excitation length.1146-1151
R. Capelli, J.J. Amsden, G. Generali, S. Toffanin, V. Benfenati, M. Muccini, D.L. Kaplan, F.G. Omenetto, R. Zamboni. Integration of silk protein in organic and light-emitting transistors
Abstract:
We present the integration of a natural protein into electronic and optoelectronic devices by using silk fibroin as a thin film dielectric in an organic thin film field-effect transistor (OFET) ad an organic light emitting transistor device (OLET) structures. Both n- (perylene) and ptype (thiophene) silk-based OFETs are demonstrated. The measured electrical characteristics are in agreement with high-efficiency standard organic transistors, namely charge mobility of the order of 10^-2 cm²/V s and on/off ratio of 10⁴. The silk-based optoelectronic element is an advanced unipolar n-type OLET that yields a light emission of 100 nW.1152-1158
Nicholas T. Kattamis, Neil D. McDaniel, Stefan Bernhard, Craig B. Arnold. Ambient laser direct-write printing of a patterned organo-metallic electroluminescent device
Abstract:
In this work we use laser direct-write (LDW) to fabricate patterned [Ru(dtb-bpy)₃]²⁺(PF₆^-)₂ electroluminescent devices under ambient processing conditions. Device fabrication is accomplished via laser micromachining of a transparent conducting oxide top electrode, LDW printing the active organo-metallic material, and vapor depositing the bottom electrode. Nuclear magnetic resonance spectroscopy is used to ensure the transfer of damage-free luminophore material. Devices tested in air are shown to exhibit emission spectra, luminous efficiencies, and lifetimes similar to literature values for devices fabricated in nitrogen environments. The versatility of laser direct-write printing is then demonstrated by printing multi-color luminophore patterns with diameters down to 10 μm for future use in high-resolution device fabrication. This approach is compatible with largearea organic electronics that require the fabrication of high-resolution architectures.1159-1165
Mario Petrosino, Alfredo Rubino. Effects of the PEDOT interface trap distribution in polymeric OLEDs
Abstract:
Four kinds of commercial PEDOT dispersions have been used to realize ITO/PEDOT/PF₆/Al polymeric OLEDs. The effects of the different PEDOT layers have been analyzed by the means of static current–voltage characterizations and admittance spectra. The measurements have revealed a good agreement with the Nicollian–Brews model returning the density of the interface traps; moreover the role of the trapping rate has been focused in terms of density of carriers available for the conduction.1166-1169
Eva J. Feldmeier, Christian Melzer. Multiple colour emission from an organic light-emitting transistor
Abstract:
The unique property of ambipolar organic light-emitting field-effect transistors that the position of the light emission zone within the transistor channel can be controlled by the applied biases opens novel aspects for applications. In this contribution we demonstrate a transistor with electrically tuneable emission colour by taking advantage of the controlled displacement of the light emission zone. The colour change is achieved due to a colour conversion layer covering partially the transistor channel. Depending on the position of charge carrier recombination the emitted light of the transport layer or the converted light from the colour conversion layer can be detected.1170-1176
Min Jung Kim, Hyo Weon Heo, Yong Kweon Suh, Chung Kun Song. Morphology control of TIPS-pentacene grains with inert gas injection and effects on the performance of OTFTs
Abstract:
We applied Ar gas injection while drying droplets of TIPS-pentacene in a quartz tube and controlled the growth direction of the grain boundary and the morphology. The organic thin film transistors (OTFTs) showed a high mobility of 0,53 ± 0,02 cm²/V s with a small deviation of 3,7% when the gas direction was parallel to the source–drain current, resulting in the direction of the grain boundary being parallel to the current. In contrast, OTFTs having grain boundaries perpendicular to the current flow displayed mobility of 0,06 ± 0,02 cm²/V s.1177-1183
Won Hoe Koo, SamYul Boo, Soon Moon Jeong, Suzushi Nishimura, Fumito Araoka, Ken Ishikawa, Takehiro Toyooka, Hideo Takezoe. Controlling bucking structure by UV/ozone treatment for light extraction from organic light emitting diodes
Abstract:
We describe a simple and effective method to control the periodicity of the buckling structure by thermal evaporation and ultraviolet/ozone (UVO) treatment so as to effectively extract the waveguide modes in organic light emitting diode (OLED) devices, inferring an underlying buckling mechanism. The periodicity of the buckles decreased from 460 to 300 nm with the UVO treatment time, shifting the emission wavelength showing the highest enhancement ratio toward the shorter wavelengths in the device with buckles.We exemplified the outcoupling characteristics of the waveguide modes by the buckles with different periodicities.1184-1191
Huawei Li, Roberto Termine, Nicolas Godbert, Luigi Angiolini, Loris Giorgini, Attilio Golemme. Charge photogeneration and transport in side-chain carbazole polymers and co-polymers
Abstract:
The photoconductivity, hole mobility and charge photogeneration efficiency of a series of side-chain carbazole homopolymers and copolymers (with azo side-chains) have been investigated. Cyclic voltammetry measurement of frontier orbitals energies show that the HOMO energy is determined by the nature and the position of attachment of the linker between the main chain and the carbazole, the azo-moiety being not relevant in this respect. Hole mobility is not influenced by the HOMO energy but seems to depend on the degree of conformational mobility of the side-chains, reaching values of the order of 10^-3 cm² V^-1 s^-1 in the best cases. The HOMO energy is instead extremely important when considering photogeneration efficiency, that can change by 10 orders of magnitude depending on the density of the carbazole side-chains in co-polymers and on the linker nature and attachment position.1192-1197
Michael M. Rothmann, Evelyn Fuchs, Christian Schildknecht, Nicolle Langer, Christian Lennartz, Ingo Münster, Peter Strohriegl. Designing a bipolar host material for blue phosphorescent OLEDs: Phenoxy-carbazole substituted triazine
Abstract:
A novel phenoxy-carbazole substituted triazine host material (PCTrz) was synthesised. PCTrz exhibits a high triplet bandgap of 2,91 eV and a high glass transition temperature of 148 °C. A non-conjugated ether bond separates the oxidation- and reduction-site in the molecule. Good charge carrier transport properties in single carrier devices prove the potential as host for blue phosphorescent emitters. An organic light emitting diode with PCTrz as host for FIrpic exhibited a high current efficiency of 13,5 cd/A a luminance of 1000 cd/m² and a maximum luminance of 29,000 cd/m².1198-1210
Xian-Kai Chen, Lu-Yi Zou, Shuang Huang, Chun-Gang Min, Ai-Min Ren, Ji-Kang Feng, Chia-Chung Sun. Theoretical investigation of charge injection and transport properties of novel organic semiconductor materials—cyclic oligothiophenes
Abstract:
By employing the Marcus electron transfer theory coupled with two-state model, the Brownian diffusion assumption and density functional theory (DFT), we have investigated the charge injection and transport properties for three types of the cyclic oligothiophenes (A, B and C in Fig. 1), and their corresponding linear molecules (linear-A, linear-B and linear-C in Fig. 1). By indentifying 13 distinct nearest-neighbor hopping pathways based on the crystal structures of the molecules reported (A and B), we predicted the electronic coupling matrix elements for a wide variety of charge transfer pathways using the ‘‘energy splitting in dimer’’ (ESD) method and their carrier mobility. The theoretical results indicate that they possess large hole carrier mobility, importantly, very outstanding properties of electron transport. The major reason should be that (1) the closed ring structure restricts the rotation of the thiophene rings in charge transfer process, (2) the introduction of the alkynyl and double bonds weakens the coulomb repulsion between lone electron pairs of sulfur atoms, the ring stain and the steric effect resulting from the butyls, more importantly, (3) their introduction stabilizes LUMO level, in result to decrease electronic organization energy (ke) and thereby improve their ability of electron transport. The temperature dependences of the carrier mobility were theoretically investigated for molecules A and B and analyzed within the hopping mechanism.1211-1215
Yanguang Zhang, Zhao Li, Salem Wakim, Salima Alem, Sai-Wing Tsang, Jianping Lu, Jianfu Ding, Ye Tao. Bulk heterojunction solar cells based on a new low-band-gap polymer: Morphology and performance
Abstract:
Polymer-fullerene bulk heterojunction (BHJ) solar cells have gained much attention in the past few years due to their potential as a low cost photovoltaic technology. The state-ofthe-art power conversion efficiency (PCE) has been improved to 7% regime by the recent development of thieno[3,4-b]thiophene based low-band-gap polymers. This paper reports a new low-band-gap polymer with alternating thieno[3,4-b]thiophene and benzo[1,2-b:4,5-b']dithiophene units for applications in BHJ solar cells. By optimizing the nano-scale morphology with DIO (1,8-diiodooctane) additive, the PCE of BHJ solar cells based on this new polymer has been improved dramatically from 1,4% to 4,8%. The correlation between photovoltaic performance and film morphology has been established for this new polymer/fullerene system.1216-1222
Gang Wang, Xiangkai Fu, Jing Huang, Chuanlong Wu, Liu Wu, Qiuliang Du. Synthesis of a new star-shaped 4,4'-bipyridine derivative and its multicolor solid electrochromic devices
Abstract:
A new star-shaped 4,40-bipyridine derivative (1,3,5-tri(1-methyl-4,40-bipyridinium bromide)-2,4,6-trimethyl benzene) TBTB was synthesized and characterized. The electrochromic behavior, electrochromic mechanism and electro-optical properties of the new type of monosubstituted 4,4'-bipyridine derivative were investigated by cyclic voltammetry and UV–vis absorption spectra. Moreover, a serial of environmentally friendly solid electrochromic devices (ECD) based on TBTB were constructed successfully and their electrochromic performance were further studied. The ECD presented a stable as well as multicolor electrochromic change from blue to violet–blue and then to violet–red between 0 V and +3,0 V. They exhibited a fast switching time (~0,3 s for coloring and ~0,5 s for bleaching) between the violet–blue (+2,0 V) reduced state and the transparent light blue oxidized state (-2,0 V), with a contrast ranging from 25% to 40% at 530 nm and a coloration efficiency of 279 cm² C^-1.1223-1226
Liang Shen, Yang Xu, Fanxu Meng, Fumin Li, Shengping Ruan, Weiyou Chen. Semitransparent polymer solar cells using V₂O₅/Ag/V₂O₅ as transparent anodes
Abstract:
We demonstrate semi-transparent inverted polymer solar cells with transparent anodes, made of Vanadium pentoxide (V₂O₅)/silver (Ag)/V₂O₅. The inner V₂O₅ layer was introduced as a buffer layer to improve holes collection, while the outer V₂O₅ layer served as a light coupling layer to enhance optical transmittance of the device. The transmittance and reflectance of V₂O₅ (10 nm)/Ag (13 nm)/V₂O₅ (x = 20, 40, 60, 80 nm) electrode are measured and compared, and the dependence of the device performances on the thickness of the outer V₂O₅ layer was investigated. The results show that the maximum transmittance of 90%, which appears from 400 to 700 nm, is obtained when the thickness of outer V₂O₅ layer is 40 nm.1227-1235
R. Müller, S. Smout, C. Rolin, J. Genoe, P. Heremans. High mobility short-channel p-type organic transistors with reduced gold content and completely gold-free source/drain bottom contacts
Abstract:
Pure gold layers are traditionally used for the fabrication of source/drain bottom contacts of high performance short channel pentacene transistors. In this work, these layers are successfully replaced by low cost metallic bilayers, consisting of a thick aluminum layer with a good electrical conductivity covered by a thin noble metal layer (gold or palladium) providing good charge injection into the organic semiconductor. Corresponding pentacene transistors with 5 lm channel length achieve saturation mobilities in the range of 0,4–0,5 cm²/(V s), comparable to those of reference pentacene transistors based on 30 nm thick pure gold bottom contacts, but for a metal cost down to 5% of that of the reference transistors.1236-1240
Hong Wang, Zhuoyu Ji, Liwei Shang, Yingping Chen, Maixing Han, Xin Liu, Yingquan Peng , Ming Liu. Nonvolatile nano-crystal floating gate OFET memory with light assisted program
Abstract:
In this paper, gold (Au) nanocrystals based nonvolatile floating gate OFET memory devices with light assisted program are studied. Programmable memory characteristics were observed according to the programming/erasing operations and the memory window and on/off memory ratio were dramatically improved from 18 V, 10² to 63 V, 10⁵ through light assisted program, respectively. Good reliability was confirmed by the data retention and endurances characteristics. Therefore, this method has potential applications in organic memory devices and integrated circuits.1241-1246
Eran Avnon, Nir Yaacobi-Gross, Elina Ploshnik, Roy Shenhar, Nir Tessler. Low cost, nanometer scale nanoimprinting – Application to organic solar cells optimization
Abstract:
We show that nanoimprinting can be robust and reliable enough to be compatible with device optimization procedure. This is enabled by a method for producing nanoimprint master stamps using block copolymer lithography. This method is not only robust and low cost but it also allows patterning with nanometer size features over centimeter scale area. The large number of stamps easily produced in parallel are used to fabricate organic solar cells with controlled interpenetrating heterojunction. The power conversion efficiency is shown to increase with the pattern depth reaching a two fold increase for only ~35 nm deep pattern.1247-1252
Hitoshi Sakamoto, Sho Igarashi, Kazuma Niume, Masayuki Nagai. Highly efficient all solid state dye-sensitized solar cells by the specific interaction of CuI with NCS groups
Abstract:
Highly efficient all solid state dye-sensitized solar cells (DSSC) were fabricated by the specific interaction of CuI with the NCS groups of the dye molecules and that of the counter electrodes. The counter electrode containing NCS groups was prepared by coating the PEG solution containing NCS groups. The counter electrode was connected with the NCS groups of the dye molecules by CuI. The specific interaction between the CuI with the NCS groups of the dye molecules and that of the counter electrode could lower the impedance of the interfaces, and facilitate the injection of the hole into the counter electrode to give the highly efficient all solid state DSSC. The specific interaction was investigated by Xray photoelectron spectroscopy. The performance was improved by introducing PEDOT layer between the interface of FTO and the PEG containing NCS groups. The improvements are due to the reduction of the junction resistance across the interface in the presence of interlayer, which result in improved hole injection. The DSSC were stable and exhibited high efficiency (4,2%). They could be manufactured with inexpensive and low quality materials, and the practical use is promising.1253-1257
Loig Kergoat, Nicolas Battaglini, Luciano Miozzo, Benoit Piro, Minh-Chau Pham, Abderrahim Yassar, Gilles Horowitz. Use of poly(3-hexylthiophene)/poly(methyl methacrylate) (P3HT/PMMA) blends to improve the performance of water-gated organic field-effect transistors
Abstract:
Poly(3-hexylthiophene)/poly(methyl methacrylate) (P3HT/PMMA) blends were used as the semiconducting layer in water-gated organic field-effect transistors (OFETs), which resulted in improving the electrical performance of the previously reported devices with pure P3HT. Topographic investigations by atomic force microscopy carried out on blends with various PMMA to P3HT ratio reveal a lateral phase separation of the two components. All transistors operate at very low voltage (below 1 V), with a threshold voltage ranging form 0,3 to 0,5 V. An optimum of the composition of the blend is found with 70% of PMMA, leading to a maximum on/off current ratio and a mobility comparable to that of pure P3HT.1258-1263
Summer R. Ferreira, Robert J. Davis, Yun-ju Lee, Ping Lu, Julia W.P. Hsu. Effect of device architecture on hybrid zinc oxide nanoparticle:poly (3-hexylthiophene) blend solar cell performance and stability
Abstract:
Hybrid zinc oxide nanoparticle (ZnO np):poly(3-hexylthiophene) (P3HT) photovoltaic devices with a blend morphology in the active layer show up to a ninefold improvement in device efficiency above devices with a planar donor–acceptor interface. However, blend devices in a conventional architecture have very poor stability upon white light exposure. Blend devices in an inverted architecture have not been previously achieved because the commercially available poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole transport layer etches the ZnO when PEDOT:PSS is deposited on top of the ZnO np:P3HT blend. Here we report the successful demonstration of an inverted ZnO np:P3HT blend solar cells that is made possible through the use of a pH neutralized PEDOT:PSS hole transport layer, and show how the inverted device architecture leads to greatly improved device stability under white light exposure.1264-1270
Hong Jiang, Dawei Kang, Shijie Xie, Avadh Saxena. Effect of spin-flip scattering on current polarization in an organic spin filter
Abstract:
We studied the effect of spin-flip scattering to understand the spin-dependent quantum transport properties through an organic spin filter. We found that the electronic orbitals of the organic polymer are spin-mixed when the spin-flip scattering is included. The Peierls energy gap is reduced due to the spin-flip effect, which is a peculiar property of the organic ferromagnetic polymer. An analysis based on the extended Landauer–Büttiker formula shows that the spin polarization of the current through the spin filter decreases with the spin-flip scattering. However, the device keeps the spin filter function intact even when a considerable spin-flip scattering is included. We also discuss the competing effects of spin-flip scattering and ferromagnetism of the organic interlayer on the spin filter.1271-1274
Jianpu Wang, Xiaoyang Cheng, Mario Caironi, Feng Gao, Xudong Yang, Neil C. Greenham. Entirely solution-processed write-once-read-many-times memory devices and their operation mechanism
Abstract:
We investigate the mechanism of operation of low-power write-once-read-many-times (WORM) memory devices based on injection of electrons from ZnO into PEDOT:PSS (polydioxythiophene doped with polystyrenesulfonic acid). Using Raman spectroscopy and in situ absorbance measurements, we directly observe the change of doping level of PEDOT during the device switching. Our results clearly show that the change of device conductance is due to the dedoping of p-doped PEDOT by injected electrons. Based on this understanding, we further demonstrate an entirely solution-processed low-power WORM device by inkjet printing metal electrodes onto arbitrary substrates.1275-1278
K.V. Raman, J. Chang, J.S. Moodera. New method of spin injection into organic semiconductors using spin filtering tunnel barriers
Abstract:
In an organic light emitting diode, injection of spin-polarized current can be expected to influence the light emission. For the emission to occur, the bias voltage must exceed the device turn-on voltage. However in magnetic junctions with conventional ferromagnetic electrodes, the magnetoresistance decreases as bias increases. This response limits the use of such ferromagnets for spin injection in organic light emitting diodes where high bias is needed. A way out of this limitation is shown here, by utilizing the spin-filter phenomenon to generate current spin polarization, and inject into an organic semiconductor. Europium sulfide was used as the spin-filter layer with rubrene as the barrier to form a quasi-magnetic junction. The junctions showed magnetoresistance which increased as bias voltage was increased, with a high value of 6% at 1,8 V. These results show the potential to use spin-filter systems for investigating electroluminescence in organic light emitting diodes with spin-polarized electrons. Ferrite compound spin-filters with Curie temperature much above room temperature are a potential class of materials to realize room temperature spintronics devices.1279-1284
Feng Liu, Bobbara Sanyasi Rao, Jean-Michel Nunzi. A dye functionalized silver–silica core–shell nanoparticle organic light emitting diode
Abstract:
A rhodamine dye functionalized silver–silica core–shell nanostructure was synthesized and applied into an organic light emitting device (OLED). Electroluminescence (EL) of the diode surprisingly exhibits much broader spectrum than its photoluminescence (PL), which breaks a new path for white light OLED material design and optimization. In addition, silver nanoparticles (NPs) surface plasmon renders much stable luminescence to the rhodamine dye by favoring its radiative decay channel. It makes hybrid NPs a good light source which might have potential application in single photon emitting devices. To our knowledge, this is the first report on a rhodamine based OLED.