Organic Electronics 2008 Volume 9 №04

Elsevier. — 131 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.419-424
Patrick Marmont, Nicolas Battaglini, Philippe Lang, Gilles Horowitz, Jaehyung Hwang, Antoine Kahn, Claire Amato, Patrick Calas. Improving charge injection in organic thin-film transistors with thiol-based self-assembled monolayers
Abstract:
The aim of this work is to improve charge injection by interposing an appropriately oriented dipole layer between contact and semiconductor in organic thin-film transistors (OTFTs). OTFTs are fabricated with pentacene semiconductor and gold source and drain contacts. The contacts are modified with self-assembled monolayers (SAMs) made of alkane or fluorinated alkane thiols. Ultraviolet photoelectron spectroscopy (UPS) shows a respective decrease and increase of the work function of the electrodes. Consistent with these results, we observe an increase and a decrease, respectively, of the contact resistance of the OTFTs, and a further decrease when shortening the length of the fluorinated molecule.425-431
Shusuke Kanazawa, Musubu Ichikawa, Youki Fujita, Ryu Koike, Toshiki Koyama, Yoshio Taniguchi. The effect of thiophene sequence separation on air-stable organic thin-film transistor materials
Abstract:
The relationship between thiophene sequences and organic thin-film transistor (OTFT) characteristics was studied to determine their effect on ionization potential, molecular orientation, and air stability. Two types of molecular structures were used: continuous sequence and divided sequence thiophenes. The length of thiophene sequence did not affect FET characteristics but did affect ionic potential and air stability. Furthermore, materials with divided thiophene sequences showed no change in OTFT characteristics when exposed to air. These results suggest that separation of thiophene sequences can improve air stability, which is a problem of thiophene-based materials.432-438
Hoon-Seok Seo, Young-Se Jang, Ying Zhang, P. Syed Abthagir, Jong-Ho Choi. Fabrication and characterization of pentacene-based transistors with a room-temperature mobility of 1,25 cm²/Vs
Abstract:
Pentacene-based transistors produced by a novel neutral cluster beam deposition method were characterized, and the effects of the surface pretreatments were examined. Atomic force microscopy and X-ray diffraction showed that the cluster beams were quite efficient in growing high-quality, crystalline thin films on SiO2 substrates at room-temperature without any thermal post-treatment, and that an amphiphilic surfactant, octadecyltrichlorosilane (OTS), enhances the packing density and crystallinity significantly. The observed field-effect mobilities (leff) were among the best reported thus far: 0,47 and 1,25 cm²/Vs for the OTS-untreated and -pretreated devices, respectively. The device performance was found to be consistent with the estimated trap density and activation energy, which were derived from the transport characteristics for the temperature dependence of leff in the range of 10-300 K.439-444
Mingsheng Xu, Akira Ohno, Shinji Aramaki, Kazuhiro Kudo, Masakazu Nakamura. Factors influencing local potential drop in bottom-contact organic thin-film transistor using solution-processible tetrabenzoporphyrin
Abstract:
By exploiting atomic-force-microscope potentiometry, we have studied the local potential distribution in the solutionprocessible tetrabenzoporphyrin (BP) bottom-contact thin-film transistor under controlled atmospheres. It is found that abrupt and big potential drops mainly appeared at the domain boundaries and cracks in the BP film when the transistor was under operation, indicating a dominant influence of domain boundary and crack on the device performance. Exposure of the device to O₂ drastically reduced the potential drops at some boundaries, which is the main reason for the improved device performance by O₂ exposure.445-451
Jin Hyung Jun, Kyungah Cho, Junggwon Yun, Kwang S. Suh, TaeYoung Kim, Sangsig Kim. Enhancement of electrical characteristics of electrospun polyaniline nanofibers by embedding the nanofibers with Ga-doped ZnO nanoparticles
Abstract:
Polyaniline nanofibers embedded with undoped ZnO nanoparticles (NPs) or Ga-doped ZnO (ZnO:Ga) NPs were fabricated and their structural and electrical properties were investigated. The uniform distribution of the NPs inside the polyaniline nanofibers was confirmed by transmission electron microscopy analysis. Polyaniline nanofibers embedded with ZnO:Ga-NPs showed their higher conductivities, compared with polyaniline nanofibers embedded with undoped ZnONPs. Single nanofibers electrospun from a mixture of a polyaniline solution with a 30 vol% ZnO:Ga-NPs dispersed-solution showed approximately five times higher conductivity than those electrospun from the polyaniline solution alone. This observation indicates that the embedding of the ZnO:Ga-NPs significantly enhances the electrical characteristics of the polyaniline nanofibers.452-460
Jae-Wook Kang, Deug-Sang Lee, Hyung-Dol Park, Ji Whan Kim, Won-Ik Jeong, Young-Seo Park, Se-Hyung Lee, Kyungmoon Gob, Jong-Soon Lee, Jang-Joo Kim. A host material containing tetraphenylsilane for phosphorescent OLEDs with high efficiency and operational stability
Abstract:
A host material containing tetraphenylsilane moiety, 9-(4-triphenylsilanyl-(1,1',4,1'')-terphenyl-4''-yl)-9H-carbazole (TSTC), was synthesized for green phosphorescent organic light emitting diodes. The tetraphenylsilane moiety was introduced to provide high triplet energy level, thermal and chemical stability, and glassy properties leading to high efficiency and operational stability of the devices. Ir(ppy)₃ based OLEDs using the TSTC host and DTBT (2,4-diphenyl-6-(4'-triphenylsilanyl-biphenyl-4-yl)-1,3,5-triazine) hole blocking layer (HBL) resulted in the maximum external quantum efficiency of 19,8% and the power efficiency of 59,4 lm/W. High operational stability with a half lifetime of 160,000 h at an initial luminance of 100 cd/m² was achieved from an electrophosphorescent device using TSTC host and BAlq HBL.461-465
P. Kury, K.R. Roos, D. Thien, S. Mollenbeck, D. Wall, M. Horn-von Hoegen, F.-J. Meyer zu Heringdorf. Disorder-mediated ordering by self-interfactant effect in organic thin film growth of pentacene on silicon
Abstract:
We have studied the morphology of pentacene films on different silicon surfaces with orientations between Si(111) and Si(001). Pentacene molecules are immobile on the Si surface, covering and passivating reactive defect sites. Molecules deposited on top of a closed amorphous wetting layer exhibit isotropic diffusion with diffusion lengths of several tens of micrometers. Neither steps, nor the surface structure affect the film morphology. The wetting layer thus acts as an interfactant, and isolates the molecular film from the substrate, explaining the high quality of pentacene films.466-472
Yan Luo, Junshan Gao, Chuanwei Cheng, Yufeng Sun, Xiguang Dua, Guoyue Xu, Zhenling Wang. Fabrication micro-tube of substituted Zn–phthalocyanine in large scale by simple solvent evaporation method and its surface photovoltaic properties
Abstract:
Micro-tubular structure of tetra(2-isopropyl-5-methylphenoxy) substituted Zn–phthalocyanine (T2) has been obtained in large scale by simple solvent evaporation method. FE-SEM micrographs indicate that most of the micro-tubes are in hollow rectangular shape. XRD measurement demonstrated that the walls of the micro-tubes are arranged in highly ordered nanoscopic structure. Both theXRD measurement and the UV–vis absorption spectrum of the micro-tubes indicated anH-aggregate of the substituted Zn–phthalocyanine molecules (2) in the micro-tubes. Formation mechanism of micro-tube has been proposed. Field-induced surface photovoltage spectrum (EFISPS) inferred that (T2) has p-type character.473-480
X.F. Lu, L.A. Majewski, A.M. Song. Electrical characterization of mica as an insulator for organic field-effect transistors
Abstract:
Electrical properties of conjugated polymer films, including poly(3-hexylthiophene)-2,5-diyl (P3HT), poly(3,3'''-didodecylquarterthiophene) (PQT-12), and poly(triarylamine) (PTAA), on mica substrates have been studied. The test structure was similar to a standard organic field-effect transistor but with a 150-lm-thick commercially available mica gate insulator/substrate, which allowed to obtain a field-effect mobility of P3HT as high as 0,08 cm²/Vs in the linear regime in ambient air. The influence of interface treatment, thermal annealing, and measurement conditions on the electrical properties of the P3HT films has been characterized and analyzed. We also studied the time dependence of the carrier concentration and mobility before and after a thermal annealing process. The results indicate that mica is a promising insulator for organic field-effect transistors, apart from already being one of the common thin-film materials widely used in electric capacitors.481-486
Junhyuk Jang, Ji Whan Kim, Nohhwal Park, Jang-Joo Kim. Air stable C₆₀ based n-type organic field effect transistor using a perfluoropolymer insulator
Abstract:
Air stable n-type organic field effect transistors (OFETs) based on C₆₀ are realized using a perfluoropolymer as the gate dielectric layer. The devices showed the field-effect mobility of 0,049 cm²/V s in ambient air. Replacing the gate dielectric material by SiO₂ resulted in no transistor action in ambient air. Perfluorinated gate dielectric layer reduces interface traps significantly for the n-type semiconductor even in air.487-494
Zhefeng Li, Jiangbo Yu, Liang Zhou, Hongjie Zhang, Ruiping Deng, Zhiyong Guo. 1,54 μm Near-infrared photoluminescent and electroluminescent properties of a new Erbium (III) organic complex
Abstract:
The crystal structure of Er(PM)₃(TP)₂ [PM = 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone, TP = triphenyl phosphine oxide] was reported and its photoluminescence properties were studied by UV–vis absorption, excited, and emission spectra. The Judd–ofelt theory was introduced to calculate the radiative transition rate and the radiative decay time of 3,65 ms for the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ion in this complex. The antenna-effect and phonon-assisted energy-transfer were introduced to discuss the intramolecular energy transfer from ligands to Er³⁺ ion. Based on this Er(III) complex as the emitter, the multilayer phosphorescent organic light emitting diode was fabricated with the structure of ITO/NPB 20 nm/Er(PM)₃(TP)₂ 50 nm/BCP 20 nm/AlQ 40 nm/LiF 1 nm/Al 120 nm, which shows the typical 1,54 μm near-infrared (NIR) emission from Er³⁺ ion with the maximum NIR irradiance of 0,21 μW/cm².495-500
Arup K. Rath, Amlan J. Pal. Resistive switching in Rose Bengal and other Xanthene molecules is a molecular phenomenon
Abstract:
There has been a debate on the mechanism of resistive switching in Rose Bengal and other Xanthene class molecules. While some authors proposed that the switching was due to an oxide layer at the Rose Bengal/Aluminum interface, some inferred the switching as an extrinsic effect like filament formation. We show results from Rose Bengal and other Xanthene class molecules on doped Si. Conductance switching in such monolayers induced by Pt/Ir tip of a scanning tunneling microscope (STM) in a non-contact mode shows that resistive switching in these molecules, initially reported by us in 2003 (in thin films), is indeed a molecular phenomenon.501-506
Yasemin Arslan Udum, Asuman Durmus, Gorkem E. Gunbas, Levent Toppare. Both p- and n-type dopable polymer toward electrochromic applications Yasemin
Abstract:
Synthesis and electrochemical polymerization of 8,11-bis(4-hexylthiophen-2-yl)acenaphtho[1,2-b]quinoxaline (HTAQ) were performed. The polymer was characterized by cyclic voltammetry, UV–Vis-NIR spectroscopy and colorimetry. Cyclic voltammetry studies revealed that the polymer was susceptible to both p- and n-doping. The electroactive polymer has fast switching time and high optical contrast. Spectroelectrochemistry also showed that the polymer is capable of being switched from bluish-green and a highly transmissive green upon p-type doping. Highly fused nitrogen containing acceptor group was utilized in the polymer backbone which was resulted in a polymer that has a significant potential for n-type doping. The existence of a true n-type doping process was proven by the results of both cyclic voltammetry and spectroelectrochemistry studies.507-514
A.J. Tunnell, D.R. Hines, Elba Gomar-Nadal, E.D. Williams. Printing-induced improvements of organic thin-film transistors
Abstract:
To understand the observation of improved pentacene (Pn) thin-film transistor mobility in flexible printed devices, a method for performing electrical measurements of organic thin-film transistors (OTFT) during the process of transfer printing has been developed. Different sample configurations were designed to test two aspects of the printing process: (1) the formation of the source/drain contacts a Pn thin-film, and (2) the formation of the transfer printed Pn/dielectric interface. In situ measurements show that pressure-induced contacts of gold (Au) electrodes result in a factor of seven mobility improvement compared with evaporation of top Au electrodes on an otherwise identical device configuration. Annealing the laminated device up to 90 °C caused no further improvement, and heating above 90 °C degraded performance. The mobility of a transfer printed device with the rough, as-grown top surface of the Pn in contact with the dielectric was found to increase dramatically with subsequent annealing for a sample temperature up to 120 °C. This is attributed to annealing-induced structural changes in the Pn film at elevated temperatures, consistent with X-ray bulk measurements showing enhanced crystal morphology in transfer printed Pn thin-films.515-521
Lai-Wan Chong, Ten-Chin Wen, Yuh-Lang Lee, Tzung-Fang Guo. The modification of silver anode by an organic solvent (tetrahydrofuran) for top-emissive polymer light-emitting diodes
Abstract:
An organic solvent, tetrahydrofuran (THF), was employed to modify the Ag anode of a top-emissive polymer lightemitting diode (T-PLED) for improving the hole injection capability and the performance of a T-PLED device. The X-ray photoelectron spectroscope analysis shows that the THF molecules were chemically adsorbed on the Ag surface, forming oxygen-rich species by substrate-catalyzed decomposition. The THF-modification were found to enhance the hole injection on the Ag anode, decrease the threshold voltage, and increase the light intensity and luminous efficiency of a T-PLED device, attributing mainly to the increase of work function of the Ag anode.522-532
Soon-Ok Jeon, Young-Min Jeon, Joon-Woo Kim, Chil-Won Lee, Myoung-Seon Gong. Blue organic light-emitting diode with improved color purity using 5-naphthyl-spiro[fluorene-7,9'-benzofluorene]
Abstract:
Novel spiro-type blue host material, 5-naphthyl-spiro[fluorene-7,9'-benzofluorene] (BH-1SN) and dopant material, 5-diphenyl amine-spiro[fluorene-7,9'-benzofluorene] (BH-1DPA) were successfully synthesized, and a blue OLED was made from them. The structure of the blue device is ITO/DNTPD/a-NPD/BH-1SN:5% dopant/Alq₃ or ET4/Al–LiF. Here, a-NPD is used as the hole transport layer, DNTPD as the hole injection layer, BH-1DPA or BD-1 as the blue dopant materials, Alq₃ or ET4 as the transporting layer and Al as the cathode. The blue devices doped with 5% BH-1DPA and BD-1 show blue EL emissions at 444 and 448 nm at 7 V, respectively, and a high efficiency of 3,4 cd/A at 5 V for the device was obtained from BH-1SN:5% BD-1/ET4 The CIE coordinates of the blue emission are 0,15, 0,08 at an applied voltage of 7 V for the device obtained from BH-1SN/5% BH-1DPA/Alq₃.533-538
Yan Xiong, Wei Xu, Chun Li, Bo Liang, Li Zhao, Junbiao Peng, Yong Cao, Jian Wang. Utilizing white OLED for full color reproduction in flat panel display
Abstract:
Full color reproduced by white OLED (organic light emitting diodes) coupled with transmission color filters for the application in flat panel display was analyzed theoretically and experimentally. A white emitter made up of three primary emission peaks was found to be superior to a white emitter made up of complementary colors, i.e. two emission peaks. However, though the full color reproduction by white emitter possesses the merit of manufacturing simplicity, it is much worse than the color reproduction by three individual primary color sub-pixels, in terms of power efficiency and the power consumption. Adjusted to the same external quantum efficiency, the full color pixel made of three primary color sub-pixels is 10 times more efficient than that made of white emitter coupled with color filters.539-544
Mingdong Yi, Jinying Huang, Dongge Maa, Ivo A. Hummelgen. Large current gain and low operational voltage permeable metal-base organic transistors based on Au/Al double layer metal base
Abstract:
In order to realize the common-emitter characteristics of the tris(8-hydroxyquinoline) aluminium (Alq₃)-based organic transistors, we used Au/Al double metal layer as the base, thus the vertical metal-base transistors with structure of Al/n-Si/Au/Al/Alq₃/LiF/Al were constructed. It was found that the contact properties between the base and the organic semiconductors play an important role in the device performance. The utilization of Au/Al double layer metal base allows the devices to operate at high gain in the common-emitter and common-base mode at low operational voltage.545-549
Tokiyoshi Umeda, Daisuke Kumaki, Shizuo Tokito. High air stability of threshold voltage on gate bias stress in pentacene TFTs with a hydroxyl-free and amorphous fluoropolymer as gate insulators
Abstract:
We investigated the air stabilities of threshold voltages (Vth) on gate bias stress in pentacene thin-film transistors (TFTs) with a hydroxyl-free and amorphous fluoropolymer as gate insulators. The 40-nm-thick thin films of spin-coated fluoropolymer had excellent electrical insulating properties, and the pentacene TFTs exhibited negligible current hysteresis, low leakage current, a field-effect mobility of 0,45 cm²/Vs and an on/off current ratio of 3 x 10⁷ when it was operated at -20 V in ambient air. After a gate bias stress of 10⁴ s, a small Vth shift below 1,1 V was obtained despite non-passivation of the pentacene layer. We have discussed that the excellent air stability of Vth was attributed to the insulator surface without hydroxyl groups.