Organic Electronics 2011 Volume 12 №11

Elsevier. — 217 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.1755-1762
Jen-Hsien Huang, Fang-Chung Chen, Cheng-Lun Chen, Annie Tzuyu Huang, Yu-Sheng Hsiao, Chin-Min Teng, Feng-Wen Yen, Pelin Chen, Chih-Wei Chu. Molecular-weight-dependent nanoscale morphology in silole-containing cyclopentadithiophene polymer and fullerene derivative blends
Abstract:
We have investigated the effect of polymer molecular weight (MW) on the morphology and efficiency of bulk heterojunction (BHJ) solar cells comprised of poly[(4,4'-bis(2-ethylhexyl) dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(5,5'-thienyl-4,4'-dihexyl-2,2'-bithiazole)-2,6-diyl] (Si-PCPDTTBT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM). Striking morphological changes are observed in BHJ films upon the change of the polymer MW. Atomic force microscopy and transmission electron microscopy studies suggest that high MW polymer generated high degree of phase separation, leading to formation of an interpenetrating network for carrier transport. The X-ray diffraction investigation indicated that increased п–п stacking in Si-PCPDTTBT with increasing polymer MWs results in an increase in hole mobility of Si-PCPDTTBT and electron mobility of PCBM as well as the red shift absorption spectrum in BHJ films. The solar cells based on PCBM with high-MW Si-PCPDTTBT deliver power conversion efficiencies of 3,33%.1763-1767
Jeong-Hwan Lee, Po-Sheng Wang, Hyung-Dol Park, Chih-I Wu, Jang-Joo Kim. A high performance inverted organic light emitting diode using an electron transporting material with low energy barrier for electron injection
Abstract:
A high performance inverted green emission organic light emitting diode with a maximum external quantum efficiency of 20% and a maximum power efficiency of 80 lm/W was realized by properly selecting an electron transporting material to have no energy barrier for electron injection between the n-doped electron transporting layer (n-ETL) and the ETL. Based on the energy levels and the current density–voltage characteristics of electron only devices, we demonstrate that the interface between an n-ETL and an ETL even in homojunction is as important as the interface between the cathode and the n-ETL for efficient electron injection into an emitting layer.1768-1776
U. Bielecka, P. Lutsyk, K. Janus, J. Sworakowski, W. Bartkowiak. Effect of solution aging on morphology and electrical characteristics of regioregular P3HT FETs fabricated by spin coating and spray coating
Abstract:
The paper reports on a comparative study of poly(3-hexylthiophene) (P3HT) films as well as P3HT organic field effect transistors (OFETs) fabricated by the spin coating and spray coating techniques, using fresh and aged (for 2 months) chloroform solutions. The films obtained from aged solutions contain nanostructured fibers of the polymer. The presence of nanofibers in P3HT thin films affects the performance of the OFETs: charge carrier mobilities in the samples deposited from the aged solutions appeared over one order of magnitude higher than those fabricated using fresh solutions. Particularly high mobilities, exceeding 10^-2 cm² V^-1 s^-1, were found in spin coated samples deposited from aged solutions. Additionally, the latter samples exhibit a certain degree of anisotropy, associated with the action of centrifugal force during the deposition.1777-1782
Chang-Yu Lin, Chih-Hung Tsai, Heng-Tien Lin, Li-Chi Chang, Yung-Hui Yeh, Zingway Pei, Yu-Rung Peng, Chung-Chih Wu. High-frequency polymer diode rectifiers for flexible wireless power-transmission sheets
Abstract:
In this work, we successfully implemented flexible high-frequency rectifying diodes on polycarbonate substrates using the polythiophene-based semiconducting polymers. We show that the appropriate conductivity of the intermediate hole-injection layer PEDOT:PSS between the anode and the semiconducting polymer is critical to simultaneously achieve enhanced forward currents and rectification ratios. Among the semiconducting polymers studied (P3HT and PQT-12), the diodes based on carefully purified PQT-12 exhibited higher forward currents, higher rectification ratios, much better environmental stability, and much higher operation frequencies. A forward current density up to ~6 A/cm² at 5 V, a rectification ratio up to 2 x 10⁴ at ±5 V, and an operation frequency of the diode rectifier over 14 MHz were achieved with the PQT-12 rectifying diodes. To demonstrate the use of the high-speed polymer rectifying diodes in wireless power transmission, we integrate the polymer-based vertical Schottky diode rectifier, printed antenna and capacitors on plastic substrates to demonstrate a flexible wireless power-transmission sheet that can convert a 13,56 MHz electromagnetic wave into the dc power to drive load devices.1783-1787
Tai Peng, Guofang Li, Yu Liu, Yu Yang, Ling Wang, Ying Wu, Yue Wang. Highly efficient phosphorescent organic light-emitting diodes using a beryllium metal–chelate complex as electron-transporting host material
Abstract:
A classical fluorescent metal–chelate complex bis(2-(2-hydroxyphenyl)-pyridine)beryllium (Bepp2) has been used as an efficient electron-transporting host material to construct highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) with an orangeemitting phosphorescent guest bis(7,8-benzoquinolinato) iridium (III) (N,N'-diisopropylbenzamidine) ((bzq)₂Ir(dipba)). Due to the well-matched energy levels of Bepp₂ with the corresponding hole-/electron- transporting (HT/ET) materials and the high-efficiency and complete energy transfer of this host–guest system, the Bepp₂-based PHOLEDs exhibit rather low driving voltage (2,8 V) and high peak EL efficiencies of over 70 cd A^-1 for luminous efficiency, 55 μmW^-1 for power efficiency, and 23% for external quantum efficiency, a performance significantly better than that using CBP as the host.1788-1793
Salima Alem, Ta-Ya Chu, Shing C. Tse, Salem Wakim, Jianping Lu, Raluca Movileanu, Ye Tao, Francis Bélanger, Denis Désilets, Serge Beaupré, Mario Leclerc, Sheila Rodman, David Waller, Russell Gaudiana. Effect of mixed solvents on PCDTBT:PC₇₀BM based solar cells
Abstract:
We investigated the effect of solvents on the morphology, charge transport and device performance of poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₀BM)based solar cells. To carry out this investigation, chloroform and 1,2-dichlorobenzene were chosen as good solvents of the two compounds. Films prepared with chloroform exhibit larger domains than those prepared with 1,2-dichlorobenzene and their size increases with the amount of PC₇₀BM. Fine tuning of the domain size was realized by using a solvent of mixed chloroform and 1,2-dichlorobenzene. At a mixing ratio of 50%:50%, a power conversion efficiency of 6,1% was achieved on PCDTBT:PC₇₀BM (1:3) devices with an active area of 1 cm², under air mass 1,5 global (AM 1,5 G) irradiation at 100 mW/cm².1794-1799
Zi-En Ooi, Samarendra P. Singh, Serene L.G. Ng, Gregory K.L. Goh, Ananth Dodabalapur. Analysis and modelling of lateral heterostructure field-effect bipolar transistors
Abstract:
Lateral heterostructure field-effect bipolar transistors (LH-FEBTs) are thin-film transistors that have a distinct heterojunction located roughly midway between the source and drain contacts, with a p-type semiconductor on one side of the junction, and an n-type semiconductor on the other. These devices have potential in display applications but are relatively new to the research community. In this paper, we describe the fabrication of a hybrid LHFEBT using pentacene and ZnO as the p- and n-type semiconductors, respectively, and describe its unusual bell-shaped electrical transfer characteristics. Using an equivalent circuit approach, we analyse quantitatively how the main features of the current–voltage curves relate to semiconductor properties such as carrier mobility and threshold voltage – information that is essential to the design of such devices.1800-1805
J.C. Ribierre, T. Aoyama, T. Muto, P. André. Hybrid organic–inorganic liquid bistable memory devices
Abstract:
We demonstrate the realization of the first liquid nonvolatile rewritable memories relying on a blend of liquid carbazole and silver nanoparticles. When an external bias voltage is applied across this single active layer based structure, a bistable current with an ON/OFF ratio of about 2 x 10² is observed. Write–read–erase cycles and recorded information stability are also demonstrated. This study provides evidence that hybrid materials hold unexplored promises for original nonvolatile flash memories and should stimulate strong interest in the area of low-cost, large area, flexible data storage devices.1806-1814
Shuo Chai, Shu-Hao Wena, Ke-Li Han. Understanding electron-withdrawing substituent effect on structural, electronic and charge transport properties of perylene bisimide derivatives
Abstract:
A series of n-type perylene bisimide (PBI) derivatives with electron-withdrawing substituents at both bay and imide nitrogen positions were investigated. The effects of these substituents on internal energy relaxation, molecular orbitals, air stability, electronic properties and charge transport behaviors were systematically discussed with density functional theory (DFT) which has been demonstrated reliable for organic semiconductor study. The investigated derivatives with electron-withdrawing substituents show favorable performances in terms of these properties. The LUMO levels are greatly stabilized by at least 0,3 eV and these derivatives show the strong absorption from 400 to 700 nm which match with the solar spectra very well. The charge transport mainly happens between molecules in the same organic molecular layer and electronic couplings between layer-to-layer molecules are very weak, thus the mobility along layer-to-layer direction is less efficient. The variation of molecular packings and intermolecular interactions produce the highly anisotropic mobilities. The derivative with two fluorine atoms at bay positions and –CH₂C₃F₇ at imide group has broad and strong absorption in the UV-Visible region and the electron mobility could get to 0,514 cm² V^-1 s^-1 which is greatly encouraging for molecular and material design in organic solar-cell devices. These calculated results are in good agreement with the experimental data. However, not all the functionalization with halogen substituents would induce the increase of the electronic coupling and electron mobility. The derivatives with four halogen substituents at the bay positions could not show advantages in terms of electron mobility which is induced by the distorted conjugated structures. The theoretical understanding of these n-type organic semiconductors figures out the importance of tuning the molecular geometry to get high performance semiconductor materials.1815-1821
T.N.T. Nguyen, Y.G. Seol, N.-E. Lee. Organic field-effect transistor with extended indium tin oxide gate structure for selective pH sensing
Abstract:
In this study, organic field-effect transistors (OFETs) with extended gate structure were fabricated for selective pH sensing applications. Indium tin oxide (ITO) was used as extended gate electrode as well as an active layer for H⁺ sensing. The threshold voltage of the fabricated ion-selective OFET was varied by the changes in the electrochemical potential at the ITO electrode surface upon its exposure to buffer solutions with variable pH values. The sensor showed excellent linearity and a high sensitivity of 57–59 mV/pH in the pH range of 2–12 The selectivity of the ITO sensing layer to H⁺ ions was also investigated by measuring the interfering effect of Ca²⁺ and K⁺ ions in the buffer pH solutions. The results showed that the Ca²⁺ and K⁺ ions weakly interfere with the selective pH sensing of the ITO-extended gate OFET sensor device.1822-1825
Xiaoran Tong, Stephen R. Forrest. An integrated organic passive pixel sensor
Abstract:
We demonstrate an imaging passive pixel sensor circuit consisting of a bottom-gate, topcontact pentacene organic thin-film transistor (OTFT) integrated with a top-illuminated, inverted subphthalocyanine/C₆₀ organic photodetector (OPD). The vacuum-deposited OTFT utilizes parylene as the gate insulator, achieving a drain current ON/OFF ratio of 10⁵. The transistor hole mobility is 0,09 ± 0,02 cm²/V s. The inverted OPD has a dark current of 20 pA at a reverse bias of 1,5 V. By integrating the two components, a 12-bit dynamic range passive pixel sensor is achieved, with an OFF current of 31 ± 5 pA and a pixel readout time of 0,4 ± 0,05 ms, limited by the discharge time of the OTFT channel. The integrated pixel has potential for use in large-scale focal plane array imagers.1826-1834
Kuo-Jung Huang, Yu-Sheng Hsiao, Wha-Tzong Whang. Selective growth and enhanced field emission properties of micropatterned iron phthalocyanine nanofiber arrays
Abstract:
In this study, we developed a simple method for the micropatterned growth of iron phthalocyanine (FePc) nanofiber arrays using a thermal evaporation process. By controlling the surface energy and the temperature of the substrate (T_sub), we obtained FePc films featuring a grain-like (in-plane) morphology on Si surfaces (higher surface energy) and a fiberlike (out-of-plane) morphology on Ag surfaces (lower surface energy) within a certain range of values of T_sub. On the Ag surfaces, these temperature-induced FePc nanofibers featured a high aspect ratio (AR) of 30,3 ± 3,6, with a mean length of 699 ± 216 nm and a mean radius of 22,2 ± 4,3 nm, as-prepared at a value of Tsub of 240 °C. The FePc films obtained at values of T_sub of 25, 120, 180, and 240 °C all possessed α-phase crystalline structures. Because the growth structures of the FePc molecules on the Si and Ag substrates were quite different, we could control the growth of micropatterned 1D FePc nanofiber arrays on previously patterned Ag/Si substrates. From the comparison of the field emission (FE) properties in different ARs of patterned devices, higher AR (30,3 ± 3,6) of devices (FE-240-P; T_sub of 240 °C) exhibited better FE performance than lower AR (6,0 ± 2,6) of devices (FE-180-P; Tsub of 180 °C). The FE current density of devices (Tsub of 240 °C) increased from 0,13 mA/cm² for the unpatterned device (FE-240-N) to 6,77 mA/cm² for the patterned device (FE-240-P) at an applied electric field of 12 V/lm. The turn-on electric fields required to produce a current density of 10 μA/cm² were 7,7 and 10,3 V/lm for the patterned and unpatterned FePc emitters, respectively. From the slopes of Fowler–Nordheim plots, we estimated the field enhancement factors (β) of FE-240-P and FE-240-N to be 314 and 329, respectively. Studies of the emission current stability revealed that the FePc nanofibers possessed outstanding anti-degrading capability. During stability tests, the micropatterned FePc emitter (FE-240-P) displayed an efficient emission current with fluctuations of less than 20%. Because this facile platform allows control over the morphologies of films of small organic molecules merely by tuning the surface energy of the substrates, such micropatterned-FePc nanofibers might have great applicability in practical field emitters.1835-1840
Xian-Feng Dong, Xiao-Xue Li, Shi-Jie Xie. Theoretical investigation on organic magnetoresistance based on Zeeman interaction
Abstract:
Based on the magnetic field-spin Zeeman interaction, a mechanism is suggested to explain the organic magnetoresistance (OMR). It is found that a considerable magnetoresistance (MR) could be obtained in organic semiconductors (OSCs). The value of MR is sensitive to the carrier concentration and the mobility (or the charge-transfer integral). We speculate that the MR in organic materials should be phonon dependent. This investigation is consistent with the present experimental observations on OMR.1841-1845
Donghyun Kim, Jaewook Jeong, Hwarim Im, Sungmo Ahn, Heonsu Jeon, Changhee Lee, Yongtaek Hong. Holography and plasma oxidation for uniform nanoscale two dimensional channel formation of vertical organic field-effect transistors with suppressed gate leakage current
Abstract:
Vertical organic field-effect transistors (VOFETs) with nanoscale channel openings have been fabricated using pentacene as an active layer material. To achieve uniform nanoscale two-dimensional channel openings, a laser holography lithography has been introduced. Uniformly distributed and well-aligned holes with 250 nm diameter were successfully obtained with the laser holography lithography. VOFET devices with these channel openings have shown high on/off ratio of about 103 without any further treatment. Gate leakage current was also decreased with an additional insulating layer generated on the gate electrode sidewall via plasma oxidation.1846-1851
Tae-Jun Ha, David Sparrowe, Ananth Dodabalapur. Device architectures for improved amorphous polymer semiconductor thin-film transistors
Abstract:
In this letter, the performance characteristics of single-gate and dual-gate thin-film transistors (TFTs) with amorphous indenofluorene–phenanthrene copolymer semiconductor active layers are reported. Optimized single-gate devices possess mobilities up to 0,15 cm²/V-s and width-normalized contact resistance of 1275 Ωcm. These results were obtained through the combination of a recessed source/drain structure and suitable surface treatments of source/drain contact electrodes. The characteristics of dual-gate indenofluorene–phenanthrene copolymer TFTs with polymer gate insulators are also reported. This structure exhibits increased on-current, reduced threshold voltage, improved sub-threshold swing and increased on–off current ratio compared to single-gate architectures.1852-1857
Ching-Ting Lee, Hung-Chun Chen. Performance improvement mechanisms of organic thin-film transistors using MoO_x-doped pentacene as channel layer
Abstract:
Organic thin-film transistors (OTFTs) with various MoO_x-doped pentacene channel layers were fabricated and investigated. Compared the OTFTs with the 0,50 mol% MoO_x-doped pentacene to the conventional OTFTs without MoO_x dopant, the maximum output current was increased from -11,6 to -37,9 μA, the effective field-effect mobility was enhanced from 0,71 to 1,60 cm²/V-s, the threshold voltage was reduced from -21,2 to -14,8 V, and the on/off current ratio slightly decreased from 3,6 x 10⁶ to 1,2 x 10⁶. The performance improvement was attributed to the highest occupied molecular orbital (HOMO) of the MoO_x-doped pentacene gradually approached to the Au work function with increasing the doping percentage of MoO_x, which led to reduce the contact resistance and to enhance the p-type characteristics of the MoO_x-doped OTFTs by increasing the hole density and enhancing the hole-injection efficiency. However, the output current and the fieldeffect mobility decreased with an increase of the MoOx doping percentage, if the doping mole percentage of MoO_x was higher than 0,50%. This behavior was attributed to the Fermi level pinning effect, gradual increase of hole concentration and significant degradation of crystallinity.1858-1863
Qing Wang, Yan Zhou, Hua Zheng, Jian Shi, Chunzeng Li, Chanmin Q. Su, Lei Wang, Chan Luo, Diangang Hu, Jian Pei, Jian Wanga, Junbiao Peng, Yong Cao. Modifying organic/metal interface via solvent treatment to improve electron injection in organic light emitting diodes
Abstract:
By simply spin-coating the solvents, such as ethanol and methanol, on top of the organic active layer, the performance of polymer organic light-emitting diodes is significantly enhanced. The quantum efficiency is increased by as large as 58% for low work function Ba/Al cathode devices after solvent treatment. An interface dipole between the organic layer and the metal layer induced by the solvent, either from the intrinsic dipole or the interaction between the solvent and the cathode metal, is responsible for the device performance improvement. The interface dipole layer, which is confirmed by the Kelvin Probe Force Microscopy and the photovoltaic measurements, lifts the vacuum level on the metal side, thereby reducing the electron injection barrier at the organic/metal interface, and leading to better device performance.1864-1871
Hongmei Zhang, Jianyong Ouyang. High-performance inverted polymer solar cells with lead monoxide-modified indium tin oxides as the cathode
Abstract:
The photovoltaic stability of polymer solar cells (PSCs) can be greatly improved by adopting an inverted device structure. This paper reports high-performance inverted PSCs with lead monoxide (PbO)-modified indium tin oxide (ITO) as the cathodes. A thin PbO layer can effectively lower the work function of ITO from 4,5 to 3,8 eV. The optimal inverted PSCs with poly(3-hexylthiophene) (P3HT) as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor exhibited high photovoltaic performance: open-circuit voltage of 0,59 V, short-circuit current density of 10,8 mA cm^-2, fill factor of 0,632, and power conversion efficiency of 4,00% under simulated AM1,5G illumination (100mWcm^-2). The photovoltaic efficiency is significantly higher than that of the control inverted PSCs with unmodified ITO as the cathode. It is even better than that of the control PSCs with normal architecture, which have an optimal efficiency of 3,5%. The lowering in the work function by the PbO modification is attributed to the charge transfer between PbO and ITO, as evidenced by the X-ray photoelectron spectra.1872-1878
Jongwoon Park, Hyokyun Ham. Sputter-patterned ITO-based organic light-emitting diodes with leakage current cut-off layers
Abstract:
We demonstrate the cost-effective fabrication of organic light-emitting diodes (OLEDs) using a sputter-patterned indium–tin-oxide (ITO). This scheme brings in a leakage current on the slope of the sputter-patterned ITO edges due to spike-like surface. To suppress it, we place thermally evaporated organic insulating molecules right on the ITO edges for preventing hole leakage, just below the aluminum (Al) cathode for blocking electron leakage, or both on the ITO edges and below the Al cathode. It is demonstrated that blocking off both hole- and electron-leak pathways (via the spikes) is highly desired to enhance the current efficiency and lifetime of the sputter-patterned ITO-based OLEDs.1879-1885
Jian V. Li, Alexandre M. Nardes, Ziqi Liang, Sean E. Shaheen, Brian A. Gregg, Dean H. Levi. Simultaneous measurement of carrier density and mobility of organic semiconductors using capacitance techniques
Abstract:
We present a method to measure both the majority carrier density and mobility in organic semiconductors from the voltage and frequency dependence of capacitance (C–V–f). Poly(3-hexylthiophene) (P3HT) is used as the prototypical material. The carrier density, and its spatial distribution in a planar device structure, is obtained from a subset of the C–V–f data by conventional capacitance–voltage analysis. We show that the validity of the carrier density extraction depends critically on the measurement frequency. Namely, one should make sure that the measurement frequency is lower than the modified dielectric relaxation frequency, which is characteristically low in organic semiconductors due to their low carrier mobility. Our method further exploits the voltage dependence of the modified dielectric relaxation frequency to measure the conductivity and carrier mobility. This mobility extraction method requires no complex fitting or simulation. Nor does it assume any particular dispersive model of mobility a priori. The carrier density, mobility, and conductivity of P3HT all increase with temperature from 250 to 300 K. The activation energies of mobility and conductivity are 0,15 ± 0,01 and 0,24 ± 0,03 eV, respectively.1886-1892
Pascal Wolfer, Maria Laura Santarelli, Luigi Vaccaro, Liyang Yu, Thomas D. Anthopoulos, Paul Smith, Natalie Stingelin, Assunta Marrocchi. Influence of molecular architecture and processing on properties of semiconducting arylacetylene: Insulating poly(vinylidene fluoride) blends
Abstract:
Blends of chemically readily accessible, small-molecular arylacetylene derivatives with poly(vinylidene fluoride) (PVDF) are presented that allow reliable solution processing of field-effect transistor (FET) architectures with electronic characteristics comparable to those of the neat semiconductors. We demonstrate that having the chemical means and corresponding processing protocols to control solid-state microstructures by either adjusting the chemical nature of the organic semiconductor, blend composition or deposition temperature, permit straight-forward comparison between materials and allow probing if electronic characteristics are affected by the chemical structure of the organic semiconductor and/or selected processing protocols.1893-1898
Soon Ok Jeon, Jun Yeob Lee. Above 20% external quantum efficiency in green and white phosphorescent organic light-emitting diodes using an electron transport type green host material
Abstract:
Highly efficient green and white phosphorescent organic light emitting diodes were developed using a green phosphorescent host material based on phenyl substituted spirobifluorene. A high quantum efficiency of 25,3% was achieved in the green phosphorescent device and a high quantum efficiency of 21,6% was obtained in the white device with a stacked emitting structure of deep blue and red:green emitting layers.1899-1902
Yu-Chiang Chao, Yi-Sa Huang, Hsin-Ping Wang, Ssu-Ming Fu, Chien-Hao Huang, Yen-Chun Liang, Wen-Cheng Yang, Yu-Sheng Huang, Gao-Fong Chang, Hsiao-Wen Zan, Hsin-Fei Meng, Chen-Hsiung Hung, Chien-Chung Fu. An organic hydrogel film with micron-sized pillar array for real-time and indicator-free detection of Zn²⁺
Abstract:
A hydrogel sensing film for a real-time and indicator-free detection of Zn²⁺ is developed by embedding a fluorescent indicator 11,16-bis(phenyl)-6,6,21,21-tetramethyl-m-benzi-6,21-porphodimethene in a hydrogel host poly(2-hydroxyethyl methacrylate). The sensing film shows high stability and selectivity to Zn²⁺. The sensitivity of the sensing film is increased by fabricating a micron-sized pillar array on the surface of the sensing film to increase the surface area. For Zn²⁺ concentrations of 10^-4 and 10^-3 M, the response time is 30 and 3 s, respectively.1903-1908
Akarin Intaniwet, Joseph L. Keddie, Maxim Shkunov, Paul J. Sellin. High charge-carrier mobilities in blends of poly(triarylamine) and TIPS-pentacene leading to better performing X-ray sensors
Abstract:
A new class of X-ray sensor – in which there is a blend of poly(triarylamine) (PTAA) and 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene in the active layer of a diode structure – has been developed. The crystalline pentacene provides a fast route for charge carriers and leads to enhanced performance of the sensor. The first time-of-flight charge-carrier mobility measurement of this blend is reported. The mobility of PTAA and TIPS-pentacene in a 1:25 molar ratio was found to be 2,2 x 10^-5 cm² V^-1 s^-1 (averaged for field strengths between 3 x 10⁴ and 4 x 10⁵ V cm^-1), which is about 17 times higher than that obtained in PTAA over the same range of field strengths. This higher mobility is correlated with a fourfold increase in the X-ray detection sensitivity in the PTAA:TIPS-pentacene devices.1909-1913
Maarten Rockelé, Duy-Vu Pham, Arne Hoppe, Jürgen Steiger, Silviu Botnaras, Manoj Nag, Soeren Steudel, Kris Myny, Sarah Schols, Robert Müller, Bas van der Putten, Jan Genoe, Paul Heremans. Low-temperature and scalable complementary thin-film technology based on solution-processed metal oxide n-TFTs and pentacene p-TFTs
Abstract:
In this work, a completely scalable integration process is presented for organic–inorganic complementary logic, based on low-temperature spin-coated n-type metal oxide TFTs and thermally evaporated p-type pentacene TFTs. Both transistor types are photolithographically processed side-by-side, without the use of any shadow mask. High performance n-type metal oxide TFTs, post-annealed at a maximum temperature of only 250 °C, exhibit saturation mobilities exceeding 2 cm²/(V s), subthreshold swing as low as 0,19 V/decade and I_on/I_off ratios beyond 10⁷ after integration with p-type pentacene TFTs. Using this hybrid complementary technology, 5-stage and 19-stage ring-oscillators are demonstrated, operating at supply voltages as low as 2,5 V. The ring-oscillators oscillate at a frequency of more than 110 kHz, corresponding to stage delays as low as 0,74 μs, at a supply bias of 20 V.1914-1919
Tai Peng, Kaiqi Ye, Yu Liu, Ling Wang, Ying Wu, Yue Wang. Novel beryllium complex as the non-doped emitter for highly efficient deep-blue organic light-emitting diode
Abstract:
Efficient blue organic light-emitting diodes have been developed based on one novel fluorescent beryllium complex bis(2-(2-hydroxyphenyl)-4-methyl-pyridine)beryllium (Be(4-mpp)₂). The simple double-layer device based on Be(4-mpp)₂ as the EML as well as the ETL not only shows pure and stable blue emission with the CIE coordinates of (0,14, 0,09), but also presents very high EL efficiency in terms of both the peak values (5,4% for EQE and 4,2 μmW^-1 for PE) and the EQE value remaining P4,0% in very wide brightness range (10–10,000 cd m^-2) that indicates very good operational stability. They are the highest EL efficiencies ever reported for such saturated and stable OLED (CIE: x 0,15, y 0,10) to the best of our knowledge.1920-1926
Zhengang Wang, Alex Pronschinske, Daniel B. Dougherty. Scanning tunneling microscopy of a disordered Alq₃–metal interface
Abstract:
Scanning tunneling microscopy is used to observe the structure of the first monolayer of Alq₃ on Cu(1 1 0). Individual molecules can be resolved at very low coverage where there is also evidence of significant surface mobility. As Alq₃ surface coverage increases, molecules aggregate by forming pairs and chain-like structures without ever establishing long range order. Interfacial disorder is maintained even after annealing to 590 K. The tendency to form chain-like aggregates is attributed to anisotropic intermolecular interactions. Anisotropies in these interactions are implicated in chain formation by illustrative density functional theory calculations and the role of direct dipolar interactions, anisotropic van der Waals interactions, and substrate-mediated interactions is discussed.1927-1935
Yu Bai, Jing Feng, Yue-Feng Liu, Jun-Feng Song, Janne Simonen, Yu Jin, Qi-Dai Chen, Jian Zi, Hong-Bo Sun. Outcoupling of trapped optical modes in organic light-emitting devices with one-step fabricated periodic corrugation by laser ablation
Abstract:
Introduction of microstructures into an organic light-emitting device (OLED) is being considered as an effective approach to outcouple photons trapped in waveguide (WG) and surface plasmon-polariton (SPP) modes within the devices. However, the attempt has been hampered by the difficulty in applying lithographic patterning technologies on organic materials. Here, we show the end has been simply reached by one-step directly laser ablating the hole-transporting layer of the OLEDs without inducing any optical or electrical deterioration. Three times efficiency enhancement has been experimentally attained from the corrugated OLEDs, which has then been ascribed by numerical simulation to the efficient outcoupling of the SPP and WG modes to radiation.1936-1942
Björn Bräuer, Roopali Kukreja, Ajay Virkar, Hylke B. Akkerman, Andreas Fognini, Tolek Tyliszczak, Zhenan Bao. Carrier mobility in pentacene as a function of grain size and orientation derived from scanning transmission X-ray microscopy
Abstract:
Pentacene field-effect transistors were prepared on silicon nitride membranes for scanning transmission X-ray microscopy (STXM) investigations. The membranes were modified by different self-assembled monolayers (SAMs). Pentacene was deposited atop the SAM-treated membrane and the in-plane orientation of the grains were subsequently investigated by polarization dependent STXM measurements. The grain sizes were determined and compared to those obtained from atomic force microscopy (AFM) measurements. Statistical analysis of the grain orientation was correlated with the charge carrier mobility of the films, in which we observed an increase in the mobility with increasing grain size and decreasing surface roughness of the SAM.1943-1947
Xizu Wang, Jian Wei Ho, Qingyi Yang, Hoi Lam Tam, Gui Xin Li, Kok Wai Cheah, Furong Zhu. Performance enhancement in organic photovoltaic devices using plasma-polymerized fluorocarbon-modified Ag nanoparticles
Abstract:
In this work, Ag nanoparticles were modified by an ultra-thin plasma-polymerized fluorocarbon film (CF_X) to form a composite CF_X-modified Ag nanoparticles/indium tin oxide (ITO) anode for application in organic photovoltaic (OPV) devices. A CFX-modified Ag nanoparticles/ITO anode exhibited a superior surface work function of 5,4 eV suited for application in OPV devices. The performance of zinc phthalocyanine:fullerene-based OPV devices showed a significant improvement when the structural identical cells are made with the CFX-modified Ag nanoparticles/ITO. This work yielded a promising power conversion efficiency of 3,5 ± 0,1%, notably higher than that with a bare ITO anode (2,7 ± 0,1%).1948-1956
Scott A. Mauger, Adam J. Moulé. Characterization of new transparent organic electrode materials
Abstract:
We characterize thin films of a new conductive copolymer, sulfonated poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl) (S-P3MEET), with various chemical additives to determine if the films are suitable for use as a hole transport layer in organic electronic devices. Using atomic force microscopy, Kelvin probe, and contact angle measurements, we probe the surface morphology and composition of S-P3MEET thin films. We then use DC conductivity measurements and spectroscopic ellipsometry to determine how the additives change the bulk properties of the films. We find that the additives decrease conductivity, increase work function, and alter the complex refractive index. The resulting properties make some preparations of S-P3MEET advantageous for use in light emitting diodes.1957-1962
Ronggang Shangguan, Guangyuan Mu, Xianfeng Qiao, Lei Wang, Kok-Wai. Cheah, Xunjin Zhu, Chin H. Chen. Low sublimation temperature cesium pivalate complex as an efficient electron injection material for organic light-emitting diode devices
Abstract:
Cesium pivalate ((CH₃)₃CCOOCs) has been synthesized and applied as an electron injection material for organic light-emitting diodes, which showed low sublimation temperature of 180 °C. Typical bilayer structure of ITO/NPB (60 nm)/Alq₃ (50 nm)/EIL/Al was used to evaluate the electron injection efficacy of (CH₃)₃CCOOCs, the results showed (CH₃)₃CCOOCs/Al exhibits better electron injection than LiF/Al cathode and the power efficiency was improved by about 19% at current density of 50 mA/cm². More interestingly, in the typical three layer OLED structure ITO/2-TNATA (60 nm)/NPB (10 nm)/Alq₃:2% C545T (40 nm)/MADN (15 nm)/(CH₃)₃CCOOCs (2 nm)/Al, the maximum current efficiency is up to 20 cd/A with Commission Internationale d’Eclairage (CIE_x,y) color coordinates of (x = 0,30, y = 0,65) at current density of 140 mA/cm², which indicates that the non-aromatic alkali metal complex can also have good match with the chemically stable compound and exhibit good electron injection properties.1963-1972
Jong Bok Kim, Ze-Lei Guan, Stephanie Lee, Eleni Pavlopoulou, Michael F. Toney, Antoine Kahn, Yueh-Lin Loo. Modular construction of P3HT/PCBM planar-heterojunction solar cells by lamination allows elucidation of processing–structure–function relationships
Abstract:
Contrary to polymer solar cells with bulk-heterojunction active layers, devices with planarheterojunction active layers allow the decoupling of active layer phase separation from constituent crystallization, and their relative influence on device performance. We fabricated
planar-heterojunction devices by first processing the electron donor and electron acceptor in isolation; they were subsequently laminated across the donor–acceptor interface to establish electrical contact. Thermal annealing was intentionally avoided after lamination to maintain the pristine charge transfer interface. Lamination thus obviates the need for solvent orthogonality; more importantly, it provides independent process tuning of individual organic semiconductor layers, ultimately allowing control over constituent structural development. We found the short-circuit current density of planar-heterojunction solar cells comprising poly(3-hexyl thiophene), P3HT, and [6,6]-phenyl-C₆₁-butyric acid methyl ester, PCBM, as the electron donor and acceptor, respectively, to be generally independent of the annealing history of P3HT. On the contrary, thermal annealing PCBM prior to lamination mainly led to a reduction in short-circuit current density. This deterioration is correlated with the development of preferentially oriented PCBM crystals that hinders electron transport in the vertical direction.