Organic Electronics 2008 Volume 9 №01

Elsevier. — 153 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.1-13
Dong Ouk Kim, Min Hye Lee, Jun Ho Lee, Tae-Woo Lee, Kwang Jin Kim, Young Kwan Lee, Taesung Kim, Hyouk Ryeol Choi, Ja Choon Koo, Jae-Do Nam. Transparent flexible conductor of poly(methylmethacrylate) containing highly-dispersed multiwalled carbon nanotube
Abstract:
Multiwalled carbon nanotubes (MWCNTs) were incorporated into a poly(methyl methacrylate) (PMMA) solution to develop a transparent and flexible composite conductive film. Monitoring the MWCNT-granule size in the PMMA solutions as well as after the film casting, the self-aggregation of MWCNTs was thoroughly investigated to provide a highlydispersed polymeric conductor. In addition to the degree of acid treatment of MWCNTs, the dipole moment of solvent and the random-coil length of polymer were considered to be the key factors for the MWCNTs to retain the highly-dispersed state in the polymer matrix after solidification. Investigating several solvent systems, dimethylformamide was found to have the best dispersing capability for the MWCNT/PMMA system to give a surface electrical conductivity up to 10^-2 S/cm at ca. 3,0 wt% of MWCNT, which was considered to be well above what had been reported for such a low level of MWCNT loading, with a light transmittance over 95%. Finally, the polymer-rich layer, which is usually formed on the coating surface due to the surface tension and wetting characteristics of the MWCNT/PMMA mixture, was mechanically peeled off to give an increase in electrical conductivity of nearly two orders of magnitude.14-20
Qin Jia Cai, Mary B. Chan-Park, Jun Zhang, Ye Gan, Chang Ming Li, Tu Pei Chen, Beng S. Ong. Bottom-contact poly(3,3'''-didodecylquaterthiophene) thin-film transistors with reduced contact resistance
Abstract
A dramatic, ~20-fold, reduction in the contact resistance of the bottom-contact poly(3,3'''-didodecylquaterthiophene) (PQT-12) thin-film transistors was achieved through a simple treatment of gold (Au) source and drain electrodes. The Au electrode treatment involved simply immersing the Au electrodes into Piranha solution prior to the deposition of the organic semiconductor. This treatment led to significant improvement of device performance. Channel length scaling analysis indicates that the contact resistance is reduced by about one order of magnitude, resulting in enhancement of estimated field-effect mobility by about a factor of five. Transport characteristic analysis suggests that the improved efficiency of charge carrier injection is probably due to increased dopant density of PQT-12 at the electrode/PQT-12 interface.21-29
Kipyo Hong, Jong Won Lee, Sang Yoon Yang, Kwonwoo Shin, Hayoung Jeon, Se Hyun Kim, Chanwoo Yang, Chan Eon Park. Lower hole-injection barrier between pentacene and a 1-hexadecanethiol-modified gold substrate with a lowered work function
Abstract:
We used ultraviolet photoemission spectroscopy (UPS) to study the hole injection barrier at the interface between pentacene and a gold surface treated with 1-hexadecanethiol (HDT). Through these UPS in-situ experiments, we found that the energy barrier between HDT-modified gold and pentacene was 0,74 eV. This energy barrier was 0,11 eV smaller than that between bare gold and pentacene, despite the work function of HDT-modified gold being 1,08 eV lower than that of bare gold. This result does not follow the typical trend, whereby decreasing the work function of a metal increases the energy barrier. The observed behavior can be explained by two factors. First, the bare gold substrate exhibited a large interface dipole, whereas the HDT-modified gold did not. And second, pentacene on the HDT-modified gold substrate had a lower ionization energy than pentacene on bare gold. This finding can be explained in terms of the polarization energy related to the more crystalline structure of pentacene on the HDT-modified gold substrate, which was established by X-ray diffraction analysis. For comparison, we also measured the injection barrier between the amorphous organic semiconductor, N,N'-diphenyl-N,N'-bis(1-naphthyl-1,1'-biphenyl-4,4'-diamine (α-NPD)), and HDT-modified gold.30-38
Kyul Han, Yeonjin Yi, Won Jun Song, Sang Wan Cho, Pyung Eun Jeon, Hyunbok Lee, Chung-Nam Whang, Kwangho Jeong. Dual enhancing properties of LiF with varying positions inside organic light-emitting devices
Abstract:
A multilayer organic light-emitting device (OLED) has been fabricated with a thin (0,3 nm) lithium fluoride (LiF) layer inserted inside an electron transport layer (ETL), aluminum tris(8-hydroxyquinoline) (Alq₃). The LiF electron injection layer (EIL) has not been used at an Al/Alq₃ interface in the device on purpose to observe properties of LiF. The electron injection-limited OLED with the LiF layer inside 50 nm Alq₃ at a one forth, a half or a three forth position assures two different enhancing properties of LiF. When the LiF layer is positioned closer to the Al cathode, the injection-limited OLED shows enhanced injection by Al interdiffusion. The Al interdiffusion at least up to 12,5 nm inside Alq₃ rules out the possible insulating buffer model in a small molecule bottom-emission (BE) OLED with a thin, less than one nanometer, electron injection layer (EIL). If the position is further away from the Al cathode, the Al diffusion reaches the LiF layer no longer and the device shows the electroluminescence (EL) enhancement without an enhanced injection. The suggested mechanism of LiF EL efficiency enhancer is that the thin LiF layer induces carrier trap sites and the trapped charges alters the distribution of the field inside the OLED and, consequently, gives a better recombination of the device. By substituting the Alq₃ ETL region with copper phthalocyanine (CuPc), all of the electron injection from the cathode of Al/CuPc interface, the induced recombination at the Alq₃ emitting layer (EML) by the LiF EL efficiency enhancer, and the operating voltage reduction from high conductive CuPc can be achieved. The enhanced property reaches 100 mA/cm² of current density and 1000 cd/m² of luminance at 5 V with its turn-on slightly larger than 2 V. The enhanced device is as good as our previously reported non-injection limited LiF EIL device [Yeonjin Yi, Seong Jun Kang, Kwanghee Cho, Jong Mo Koo, Kyul Han, Kyongjin Park, Myungkeun Noh, Chung Nam Whang, Kwangho Jeong, Appl. Phys. Lett. 86 (2005) 213502].39-44
Bikas C. Das, Amlan J. Pal. Switching between different conformers of a molecule: Multilevel memory elements
Abstract:
We report voltage-driven electrical bistability in an organic semiconductor, namely Ponceau SS. Conductance switching to different levels or ‘‘multilevel switching’’ in devices based on thin-films is due to different density of high-conducting molecules. In a monolayer of Ponceau SS, we have observed one low-conducting and two high-conducting states. This is due to three configurable planes of the molecule exhibiting at least two stable high-conducting conformers. Apart from establishing conductance switching to be a molecular phenomenon, the multilevel conductance in a monolayer shows that a single molecule can exhibit multilevel memory application.45-50
Kun-Ming Yeh, Yun Chen. Improved performance of polymer light-emitting devices based on blend of MEH–PPV and vinyl copolymer with 1,3,4-oxadiazole chromophores
Abstract:
We have developed a simple method to overcome the intrinsic defect of well-known poly[2-methoxy-5-(2'-ethylhexoxy)-p-phenylenevinylene] (MEH–PPV), i.e. rampant inter-chain interaction and imbalanced hole and electron fluxes, by blending with copolymer of polystyrene containing pendant aromatic 1,3,4-oxadiazole (PSOXD12). The addition of PSOXD12 reduces the inter-chain interaction and balances charge carrier transport simultaneously. Photoluminescence (PL), PL excitation and electroluminescence (EL) spectra of the blends reveal that the inter-chain interactions, such as aggregation and excimer/exciplex, are reduced markedly due to the presence of PSOXD
12. Enhanced EL device performance has been achieved (16,261 cd/m², 4,79 cd/A) as a result of both reduced inter-chain interaction and balanced charge transport.51-62
Z.H. Huang, X.T. Zeng, X.Y. Sun, E.T. Kang, Jerry Y.H. Fuh, L. Lu. Influence of plasma treatment of ITO surface on the growth and properties of hole transport layer and the device performance of OLEDs
Abstract:
Surface energy of indium tin oxide (ITO) surfaces treated by different plasmas, including argon (Ar-P), hydrogen (H₂-P), carbon tetrafluoride (CF₄-P), and oxygen (O₂-P), was measured and analyzed. The initial growth mode of hole transport layers (HTLs) was investigated by atomic force microscope observation of thermally deposited 2 nm thick N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) on the plasma treated ITO surfaces. The results show that different plasma treatments of ITO influence the growth of HTLs in significantly different ways through the modification of surface energy, especially the polar component. The O₂-P and CF₄-P were found to be most effective in enhancing surface polarity through decontamination and increased dipoles, leading to more uniform and denser nucleation of NPB on the treated ITO surfaces. It was further found that increased density of nucleation sites resulted in a decreased driving voltage of OLEDs. Under the same fabricating conditions, a lowest driving voltage of 4,1 V was measured at a luminance of 200 cd/m² for the samples treated in CF₄-P, followed by the samples treated in O₂-P (5,6 V), Ar-P (6,4 V), as-clean (7,0 V) and H₂-P (7,2 V) plasma, respectively. The mechanisms behind the improved performance were proposed and discussed.63-69
Tatsuo Mori, Shunsuke Nishino, Takao Nishikawa, Satoshi Ogawa. Suppressive effect of a self-assembled monolayer on the polycrystallization of naphthyl-substituted diamine derivative thin film
Abstract:
We observed the polycrystallization process of a naphthyl-substituted diamine derivative (NPD) thin film on a selfassembled monolayer (SAM)-modified indium–tin–oxide (ITO) substrate. Fluorine-substituted SAM suppressed the growth of the polycrystalline region. After estimating the surface morphology of the polycrystalline region using AFM measurement, correlation between the molecular migration rate on the substrate, the rearrangement rate at the growth point, and the provision rate from the amorphous region clarified the polycrystallization growth mechanism for the NPD thin film. The formation of a channel around the polycrystalline region and the molecular migration rate on the substrate play important roles in polycrystalline region growth.70-76
F.A. Yildirim, R.R. Schliewe, W. Bauhofer, R.M. Meixner, H. Goebel, W. Krautschneider. Gate insulators and interface effects in organic thin-film transistors
Abstract:
This paper presents a detailed characterization of different thermosetting polymers to be used as gate dielectrics in organic thin-film transistors. Selected materials yield smooth films with good insulation properties and offer attractive processing conditions. Bottom-gate transistors were prepared using these dielectrics and compared to hybrid transistors with surface-treated SiO₂ as the dielectric. Gate bias induced leakage and solvent effects were investigated by preparing metal/insulator/semiconductor devices. Poly(3-hexylthiophene) (P3HT) transistors with organic dielectrics exhibited higher channel conductivity and lower mobility values with respect to P3HT-hybrid transistors and pentacene transistors. The importance of dielectric/semiconductor interface was discussed by comparing the performances of pentacene and P3HT transistors produced on different dielectrics.77-84
Musubu Ichikawa, Soichi Fujimoto, Yuta Miyazawa, Toshiki Koyama, Norimasa Yokoyama, Tetsuzo Miki, Yoshio Taniguchi. Bipyridyl substituted triazoles as hole-blocking and electron-transporting materials for organic light-emitting devices
Abstract:
We have demonstrated bipyridyl substituted triazole derivatives (Bpy-TAZs) as an electron transporting material for organic light-emitting devices (OLEDs). Substitution of triazole with bipyridyl is a good way to improve electron-transporting ability of triazoles with keeping good hole-blocking ability, which is a useful property of triazole derivatives. A Bpy-TAZ has high electron mobility of above 10^-4 cm²/V s. Moreover, by employing one of Bpy-TAZs as a hole-blocking and electron-transporting material for phosphorescent OLEDs, lower operation voltage was achieved with keeping the same external quantum efficiency of electroluminescence (almost 10%) as compared with the conventional hole-blocking and electron-transporting bilayer consisting of bathocuproine and tris (8-hydroxyquinolinato) aluminum.85-94
Bin Fan, Roland Hany, Jacques-Edouard Moser, Frank Nuesch. Enhanced cyanine solar cell performance upon oxygen doping
Abstract:
The effect of exposing cyanine–fullerene C₆₀ bilayer solar cells to ambient atmosphere is investigated. For exposure times of a few hours and concomitant light soaking, the device performance experiences a drastic power efficiency increase going from 0,14% to 1,2% measured at 30 mW/cm² simulated solar irradiation. The 10-fold enhancement is attributed to the photoinduced doping involving oxygen and water leading to the formation of reactive superoxide anions and mobile holes in the cyanine layer. The influence of water and dry oxygen are investigated separately. While water deteriorates the device performance, dry oxygen leads only to a partial increase of efficiency. Annealing does not ameliorate the performance of doped devices. Although then the cyanine layer features more crystallinity, the considerable morphological changes cause diffusional loss in charge carrier collection. Doping of not annealed devices brings a sizeable efficiency enhancement that highlights the importance of charge carrier transport in cyanine dye based solar cells.95-100
Qiaoli Niu, Yuxuan Shao, Wei Xu, Lei Wang, Shaohu Han, Nanliu Liu, Junbiao Peng, Yong Cao, Jian Wang. Full color and monochrome passive-matrix polymer light-emitting diodes flat panel displays made with solution processes
Abstract:
1,5 in. diagonal red, green, and blue monochrome passive-matrix (PM) polymer light-emitting diodes (PLED) flat panel displays (FPDs) with format 96 x 64 were fabricated by spin-coating technology with device structure of ITO/PEDOT/ Emissive layer/Ba/Al. During spin-coating process, by rearranging the location and the direction of the panel with respect to the center of the spinner, the piling of organic materials under the cathode separator was significantly reduced, resulting in a more uniform light emission. The final display showed neither dead pixels nor dead lines. Current efficiencies of 1,37, 9,5 and 1,44 cd/A, and CIE color coordinates of (0,62, 0,37), (0,37, 0,60) and (0,15, 0,13), for red, green, and blue monochrome displays, respectively, have been achieved. Further, 1,5 in. full color PM PLED FPDs with format 96 x RGB x 64 was successfully fabricated by inkjet printing technology. The current efficiency was about 0,75 cd/A at full screen white with color coordinates located at (0,34, 0,35). A color gamut of 50% NTSC was obtained. For all the displays, the 5-point
uniformity was more than 80%.101-110
Meng-Huan Ho, Chia-Ming Chang, Ta-Ya Chu, Teng-Ming Chen, Chin H. Chen. Iminodibenzyl substituted distyrylarylenes as dopants for blue and white organic light-emitting devices
Abstract:
A series of highly efficient blue materials based on iminodibenzyl-substituted distyrylarylene (IDB-series) fluorescent dyes using the concept of steric-compression have been designed and synthesized by means of a rigidized and over-sized ring. The steric-compression effect can shorten the effective conjugation length (chromophore) of the molecule and the added phenyl moiety in the core can alleviate the propensity for molecular aggregation. These materials also possess high glass transition temperature over 100 °C. The blue IDB-Ph device achieved a maximum external quantum efficiency of 4,8% with a Commission Internationale de l’Eclairage (CIEx,y) coordinate of (0,16, 0,28). When applied in two-element white OLED system, the IDB-Ph doped device achieved a luminance efficiency of 11.0 cd/A with a CIEx,y color coordinate of (0,29, 0,36).111-118
S. Duhm, A. Gerlach, I. Salzmann, B. Broker, R.L. Johnson, F. Schreiber, N. Koch. PTCDA on Au(111), Ag(111) and Cu(111): Correlation of interface charge transfer to bonding distance
Abstract:
The electronic structure at the interfaces of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and the metal surfaces Au(111), Ag(111) and Cu(111) was investigated using ultraviolet photoelectron spectroscopy (UPS). By combining these results with recent X-ray standing wave data from PTCDA on the same substrates clear correlation between the electronic properties and the interface geometry is found. The charge transfer between the molecule and the metal increases with decreasing average bonding distance along the sequence Au-Ag-Cu. Clear signatures of charge-transfer-induced occupied molecular states were found for PTCDA on Ag(111) and Cu(111). As reported previously by Zou et al. [Y. Zou et al., Surf. Sci. 600 (2006) 1240] a new hybrid state was found at the Fermi-level (EF) for PTCDA/Ag(111), rendering the monolayer metallic. In contrast, the hybrid state for PTCDA/Cu(111) was observed well below EF, indicating even stronger charge transfer and thus a semiconducting chemisorbed molecular monolayer. The hybridisation of molecular and Au electronic states could not be evidenced by UPS.119-128
H.L. Gomes, A.R.V. Benvenho, D.M. de Leeuw, M. Colle, P. Stallinga, F. Verbakel, D.M. Taylor. Switching in polymeric resistance random-access memories (RRAMS)
Abstract:
Resistive switching in aluminum-polymer-based diodes has been investigated using small signal impedance measurements. It is shown that switching is a two-step process. In the first step, the device remains highly resistive but the low frequency capacitance increases by orders of magnitude. In the second step, resistive switching takes place. A tentative model is presented that can account for the observed behavior. The impedance analysis shows that the device does not behave homogenously over the entire electrode area and only a fraction of the device area gives rise to switching.129-135
E.V. Emelianova, M. van der Auweraer, G.J. Adriaenssens, A. Stesmans. Carrier mobility in two-dimensional disordered hopping systems
Abstract:
A model of the equilibrium 2D hopping mobility in a disordered organic semiconductor is formulated for arbitrary charge carrier densities and arbitrary temperatures. The calculated dependence of the 2D mobility upon inverse temperature is compared with experimental data obtained on 2D carrier transport in poly(3-hexylthiophene) thin film field-effect transistors.136-142
Y. Divayana, X.W. Sun. Sequentially doped blue electrofluorescent organic light-emitting diodes
Abstract:
We report on a blue fluorescent [4,4'-bis(9-ethyl-3-carbazovinylene)-1-1'-bisphenyl] organic light-emitting diode with a sequentially doped device architecture introduced recently. The emission layer consists of a few repeating cells, similar to a multiple quantum well structure, which are made of sequentially evaporated host and guest layers. An external quantum efficiency as high as 2,8% photons/electron was obtained, comparable with that fabricated using the conventional doping method. Without degrading the efficiency, devices with varied emission spectra (peak wavelength shifted from 472 to 488 nm) were produced by simply varying the guest layer thickness. The spectrum shift is due to changes in local order or aggregate state, or both.143-148
M. Mas-Torrent, S. Masirek, P. Hadley, N. Crivillers, N.S. Oxtoby, P. Reuter, J. Veciana, C. Rovira, A. Tracz. Organic field-effect transistors (OFETs) of highly oriented films of dithiophene-tetrathiafulvalene prepared by zone casting
Abstract:
We report on the preparation of large area coverage of well-oriented films of dithiophene tetrathiafulvalene (DT-TTF) from solution by using the zone casting technique. The X-ray analysis shows that the molecules are highly ordered in the films with the stacking direction parallel to the substrate. We further demonstrate that it is possible to prepare organic field-effect transistors (OFETs) employing these films. The devices reveal a remarkable OFET mobility with a maximum value of 0,17 cm²/V s. The fact that the films are prepared from solution makes these devices eminently suitable for lowcost electronics.149-154
Alexander Alexeev, Joachim Loos. Conductive atomic force microscopy (C-AFM) analysis of photoactive layers in inert atmosphere
Abstract:
Conductive atomic force microscopy (C-AFM) measurements have been performed on the MDMO-PPV/PCBM system which is potentially interesting for applications as active layer in polymer solar cells (PSCs). It is demonstrated that C-AFM analysis performed in air for ambient conditions provides only inadequate information about the local electrical properties. The main reason is that the samples chemically degrade when in contact with air. Moreover, we speculate that also the adsorbed water layer interferes with reliable nanoscale electrical measurements. In contrast, when performed in inert atmosphere C-AFM analysis offers consistent results of e.g. the I–V characteristics with lateral resolution better than 50 nm, and is able to detect local heterogeneities of these I–V characteristics at the sample surface.