Organic Electronics 2012 Volume 13 №07

Elsevier. — 167 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.1121-1128
Yonghua Chen, Qi Wang, Jiangshan Chen, Dongge Ma, Donghang Yan, Lixiang Wang. Organic semiconductor heterojunction as charge generation layer in tandem organic light-emitting diodes for high power efficiency
Abstract:
High-performance tandem organic light-emitting diodes (OLEDs) employing a buffermodified C60/pentacene organic semiconductor heterojunction (OHJ) as a charge generation layer (CGL) are demonstrated. The unique cooperation of charge generation, transport, and extraction processes occurred in the OHJ-based CGL remarkably reduces the operational voltage. As a result, an approximately twofold enhancement in power efficiency (21,9 μmW^-1 VS 10,1 μmW^-1) can be achieved that has previously been suggested to be difficult for tandem OLEDs. When the pentacene is replaced by zinc phthalocyanine (ZnPc), copper phthalocyanine (CuPc), or phthalocyanine (H₂Pc), a similar power efficiency improvement can be also achieved. The novel design concept of the buffer-modified OHJ-based CGL is superior to that of the conventional CGLs. The investigations on the operational mechanism are performed, from which it is found that the mobile charge carriers firstly are needed to be accumulated at both sides of the heterojunction interface and then transport along the two organic semiconductors in terms of their good carrier transport characteristics under an external electrical field, and finally inject into the corresponding electroluminescent (EL) units by the interfacial layers.1129-1135
Yujie Chen, Johannes Herrnsdorf, Benoit Guilhabert, Yanfeng Zhang, Alexander L. Kanibolotsky, Peter J. Skabara, Erdan Gu, Nicolas Laurand, Martin D. Dawson. Modification of emission wavelength in organic random lasers based on photonic glass
Abstract:
Control of the emission wavelength of a random laser (RL) system over a 7-nm waveband is demonstrated using a green-emitting p-conjugated polymer infiltrated into a photonic glass formed by nano/micro-size monodisperse silica spheres. The use of a solution-based conjugated polymer enables the complete filling of the voids within the photonic glass without suffering from quenching and the gain can therefore be maximized. The emission wavelength of these structures is set by a combination of the material system spectral gain and of the transport mean free path, the latter being controlled by the mean diameter of the spheres in the nano-scale range. Transport mean free paths of photons in the RL’s active region are calculated using Mie scattering theory and corroborated with coherent backscattering measurements. Further wavelength modification is also possible by changing the pump spot size and the pump fluence.1136-1140
Takayuki Kuwabara, Tomoaki Nakashima, Takahiro Yamaguchi, Kohshin Takahashi. Flexible inverted polymer solar cells on polyethylene terephthalate substrate containing zinc oxide electron-collection-layer prepared by novel sol–gel method and low-temperature treatments
Abstract:
Flexible and air-stable polymer solar cells were fabricated on a polyethylene terephthalate (PET) substrate. The cell structure was indium tin oxide (ITO) on PET/zinc oxide (ZnO)/[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM):regioregular poly(3-hexylthiophene) (P3HT)/poly (3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS)/Au, this being called the ZnO cell. Reproducible cell performances were obtained despite the ZnO cells being fabricated in air and at low temperature, using a novel ZnO precursor solution containing zinc(II) acetylacetonate as a metal source and acetylacetone as a Zn²⁺ complexing agent. The power conversion efficiency (PCE) of the flexible ZnO cells without sealing was 2,15% under irradiating AM1,5G simulated sunlight at 100 mWcm^-2. In addition, the performance of the non-sealed ZnO cells was almost constant in ambient atmosphere under continuous light irradiation for 100 h.1141-1145
Sook Hee Jeong, Jun Yeob Lee. Dibenzofuran derivative as high triplet energy host material for high efficiency in deep blue phosphorescent organic light-emitting diodes
Abstract:
A high efficiency deep blue phosphorescent organic light-emitting diodes was developed using an weak electron transport type high triplet energy host material with dibenzofuran and phosphine oxide units. The host material showed a high triplet energy of 3,01 eV and was effective as the host material for deep blue phosphorescent organic light-emitting diodes. The device performances could be optimized by managing the doping concentration of phosphorescent dopants and a high quantum efficiency of 25,9% with a color coordinate of (0,14, 0,22) was achieved.1146-1151
Chuan Liu, Yun Li, Takeo Minari, Kazuo Takimiya, Kazuhito Tsukagoshi. Forming semiconductor/dielectric double layers by one-step spin-coating for enhancing the performance of organic field-effect transistors
Abstract:
We report one-step formation of the gate dielectric and conduction channel for enhancing the performance of organic field effect transistors (OFETs). The resulting OFET with the semiconductor/dielectric bi-layers spun in ambient conditions exhibits μ_FET up to 1,6 cm²/V s and on-off ratio higher than 10⁶, no additional treatment needed. Contact angle measurements and absorption spectra reveals that a well-defined semiconductortop and dielectric-bottom film form after spin-coating the mixture of the two components, which is due to the surface induced self-organized phase separation. Compared to the single layer semiconductor film, the staggered film exhibits over 5 times higher mobility and nearly 90% reduced hysteresis in OFET. The higher performance is attributed to the simultaneous optimization in the dielectric interface and semiconductor crystallization. The approach is significant for the fabrication of low cost, easy processed and high performance OFETs.1152-1157
Yong-Hoon Kim, John E. Anthony, Sung Kyu Park. Polymer blended small molecule organic field effect transistors with improved device-to-device uniformity and operational stability
Abstract:
We report high performance organic thin-film transistors (OTFTs) with improved deviceto- device uniformity and operational stability using polymer-blended small molecule organic semiconductor, 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT). The diF-TESADT blended with poly(α-methylstyrene) was spin-cast to form bottom-contact OTFTs, and an average carrier mobility of more than 0,16 cm²/V s with more uniform surface morphology and device-to-device uniformity compared to neat diF-TESADT devices were achieved. Additionally, the polymer-blended OTFTs have shown improved operational stability under gate bias-stress possibly due to blocking of ambient oxygen and moisture by vertically separated insulating matrix polymer.1158-1165
Arun Tej Mallajosyula, S. Sundar Kumar Iyer, Baquer Mazhari. Capacitance–voltage characteristics of P3HT:PCBM bulk heterojunction solar cells with ohmic contacts and the impact of single walled carbon nanotubes on them
Abstract:
Capacitance–voltage (C–V) characteristics of P3HT:PCBM devices of two different thicknesses are correlated with current density–voltage (J–V) characteristics. The rising portion of the C–V characteristics coincides with the exponential current density below the built-in voltage. The negative capacitance (NC) of these devices is a low frequency phenomenon and it occurs in trap-free space charge limited current (SCLC) regime. The onset frequencies of NC for devices with and without SWNTs also do not follow direct relation with effective mobility. The NC in thin devices has non-monotonic change with voltage for thin devices showing that interface state kinetics can be the reason for its occurrence. The NC of thick devices, on the other hand, increases monotonically with voltage showing that bulk properties dominate in these. Addition of SWNTs to these devices for efficiency improvement does not modify their built-in voltage. Also, the SWNTs do not affect the forward NC behaviour. However, the devices containing SWNTs show NC in reverse bias also which has different frequency dependence with voltage. The reverse bias NC is attributed to the large non-linear reverse current by charge injection into the additional energy levels introduced by SWNTs.1166-1170
Shree Prakash Tiwari, Jungbae Kim, Keith A. Knauer, Do Kyung Hwang, Lauren E. Polander, Stephen Barlow, Seth R. Marder, Bernard Kippelen. Complementary-like inverters based on an ambipolar solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative
Abstract:
We report on high-mobility top-gate organic field-effect transistors (OFETs) and complementary-like inverters fabricated with a solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative as the channel semiconductor and a CYTOP/Al₂O₃ bilayer as the gate dielectric. The OFETs showed ambipolar behavior with average electron and hole mobility values of 1,2 and 0,01 cm² V^-1 s^-1, respectively. Complementary-like inverters fabricated with two ambipolar OFETs showed hysteresis-free voltage transfer characteristics with negligible variations of switching threshold voltages and yielded very high DC gain values of more than 90 V/V (up to 122 V/V) at a supply voltage of 25 V.1171-1177
Jia Hu, Zhongwei Wu, Huaixin Wei, Tao Song, Baoquan Sun. Effects of ZnO fabricating process on the performance of inverted organic solar cells
Abstract:
The effects of zinc oxide (ZnO) fabricating process on the performance of the inverted bulk heterojunction (BHJ) solar cells were explored in this study. The ZnO layers were prepared by either sputtering or solution-processed method. These ZnO films on the indium tin oxide (ITO) substrates were used as the cathode of the inverted solar cells. It was found that the topography of the ZnO films played a leading role on the device performance. The devices based on solution-processed ZnO films displayed better electric output compared with that of sputtered ones. The measurement of capacitance against bias voltage indicated that ZnO film with certain degree of roughness exhibited high charge extraction efficiency, which resulted in improved device performance. The measurement of ultraviolet photoelectron spectroscopy revealed that a shift of work function was observed due to the fabricating process of ZnO films.1178-1184
Y.D. Wang, B. Di, Y. Meng, X.J. Liu, Z. An. The dynamic formation of trions in conjugated polymers
Abstract:
The dynamic formation process of a trion by recombination of an on-chain bipolaron with an off-chain counterion trapped in a radical molecule, under the influence of an external electric field, has been investigated using a nonadiabatic evolution method. We demonstrate that the dynamic formation of the trion depends sensitively on the strength of the applied electric field, the radical potential, and the coupling between the polymer chain and radical. The trion can be formed efficiently via the bipolaron-trapped counterion reaction under the appropriate conditions. Our simulation provides theoretical evidence for the trion formation channel proposed by Kadashchuk et al. The bipolaron-trapped counterion reaction also makes an additional contribution to electroluminescence in conjugated polymers that lies beyond the set of processes involving polaron recombination.1185-1191
Christopher Lombardo, Zi-En Ooi, Eric Danielson, Ananth Dodabalapur. Electrical characteristics of lateral organic bulk heterojunction device structures
Abstract:
Lateral structures have been used to characterize charge transport phenomena in organic bulk heterojunctions. Through the analysis of the current vs. voltage relationships and their light intensity dependence, space charge limited extraction currents and injection currents have been observed and characterized. Additionally, the drift length of charge carriers has been estimated by characterizing devices of varying lengths. These studies show that lateral structures are a promising way to study the basic physics of organic bulk heterojunction materials as they offer degrees of freedom unavailable in sandwich structures and such studies complement what can be learned from conventional sandwich structures.1192-1196
Vahid Sathi, Nasrin Ehteshami, Javad Nourinia. New frequency-reconfigurable microstrip antenna composed of organic semiconductor polymer
Abstract:
A novel frequency-reconfigurable microstrip antenna composed of organic semiconductor polymer (P3HT) is proposed. Resonance frequency of the antenna is tuned in 6,8–7,73 GHz band, by changing the light illumination intensity of a 5 W/cm² white light source. Behavior of the antenna under different light intensities is investigated and compared to a reference copper antenna. Measured radiation patterns are identical in higher and lower resonant bands. Measured radiation efficiency and gain of the proposed antenna are compared in higher and lower bands. The results indicate that, it is possible to obtain reasonable performance albeit with modest radiation efficiencies.1197-1203
Hirohiko Fukagawa, Takahisa Shimizu, Noriyuki Ohbe, Shizuo Tokito, Katsumi Tokumaru, Hideo Fujikake. Anthracene derivatives as efficient emitting hosts for blue organic light-emitting diodes utilizing triplet–triplet annihilation
Abstract:
The molecular design strategies for the host materials suitable for highly efficient, blue fluorescent organic light-emitting diodes (OLEDs) are demonstrated. The device characteristics of blue fluorescent OLEDs are compared with different host materials. Some devices exhibit a highly efficient blue electroluminescence with a high external quantum efficiency of more than 7%. The correlation between OLED efficiency and triplet–triplet annihilation is characterized by measuring the up-conversion of triplet excited states into singlet ones. The host materials require an anthracene unit and a bulky molecular structure to preventthe overlap of anthracene units between adjacent molecules in the film.1204-1212
William Kylberg, Ying Zhang, Arthur Aebersold, Fernando Araujo de Castro, Thomas Geiger, Jakob Heier, Simon Kuster, Chang-Qi Mac, Peter Bäuerle, Frank Nüesch, Jean-Nicolas Tisserant, Roland Hany. Oligothiophene dendron-decorated squaraine dyes: Synthesis, thin film formation, and performance in organic solar cells
Abstract:
Squaraine (SQ) dye-based organic semiconductor hybrids 6T-SQ and 18T-SQ functionalized with oligothiophene dendrons were synthesized via Suzuki–Miyaura coupling. The electronic coupling between the oligothiophene dendrons and the squaraine core was rather weak, as suggested from UV–vis spectra, cyclic voltammetry measurements and molecular modeling. Thin films of pure SQ were characterized by a pronounced solvent- and heatinduced crystallization tendency. The dendrons substantially hindered the squaraine core crystallization, and 18T-SQ films remained amorphous after annealing or storage for several weeks. PCBM disrupted dye crystallization in blends, and smooth and stable films could be coated. Heat treatment of blended films induced dewetting for SQ:PCBM and 6T-SQ:PCBM, but 18T-SQ:PCBM remained again stable. These morphological film features could consistently explain the performance of dye-fullerene solar cells. The best performance (η ~ 1,5%) was obtained for simple bilayer 6T-SQ:C₆₀ or 18T-SQ:C₆₀ cells without annealing. Our results demonstrate that the attachment of decorating moieties to a central light-absorbing core unit in molecular push–pull systems can be used to adjust the optoelectronic and morphological film properties of small molecular semiconductors with a strong tendency towards crystallization.1213-1222
Yu-Ai Duan, Yun Geng, Hai-Bin Li, Xiao-Dan Tang, Jun-Ling Jin, Zhong-Min Su. Theoretical study on charge transport properties of cyanovinyl-substituted oligothiophenes
Abstract:
When the oligothiophene is substituted by dicyanovinyl (DCV) or tricyanovinyl (TCV) group, how does its transport property change? Here, we will mainly focus on exploring the influence on charge transport properties of introducing a strong electron-withdrawing DCV/TCV group to the thiophene units within Marcus–Levich–Jortner formalism at the level of density functional theory. The results show that the introduction of cyanovinylsubstituents improves the molecular п-stacking, decreases the frontier molecular orbital energy levels and reorganization energies, and increases the transfer integrals and mobilities, comparing with their parent thiophene molecules. It is interesting to find the phenomenon that enriching intermolecular interactions can be favorable for controlling the transport channel and thus get high mobility, which would be shown by the angular resolution anisotropic mobilities analysis. Besides, the simulated packing motifs of dimers for 3a and 3b without crystal structures reported indicate that their packing may form the slip п–п stacking, and that 3b may be a good ambipolar material. In a word, compared with corresponding thiophene analogues and tetracyanoquinodimethane, these compounds may become the candidates for the n-type or ambipolar organic semiconductor materials.1223-1228
Z.R. Wang, J.Z. Xin, X.C. Ren, X.L. Wang, C.W. Leung, S.Q. Shi, A. Ruotolo, P.K.L. Chan. Low power flexible organic thin film transistors with amorphous Ba_0,7Sr_0,3TiO₃ gate dielectric grown by pulsed laser deposition at low temperature
Abstract:
We deposited amorphous Ba_0,7Sr_0,3TiO₃ (BST) on silicon and plastic substrate under 110 °C by pulsed laser deposition (PLD) and use it as the dielectric of the organic transistor. Depends on the thickness of BST layer, the highest mobility of the devices can achieve 1,24 cm² V^-1 s^-1 and 1,01 cm² V^-1 s^-1 on the silicon and polyethylene naphthalate (PEN) substrate, respectively. We also studied the upward and downward bending tests on the transistors and the dielectric thin films. We found that the BST dielectric pentacene transistor can maintain the mobility at 0,5 cm² V^-1 s^-1 or higher while the bending radius is around 3 mm in both upward and downward bending. Our finding demonstrates the potential application of PLD growth high-k dielectric in the large area organic electronicsdevices.1229-1236
Guangjun Nan, Zesheng Li. Phase dependence of hole mobilities in dibenzo-tetrathiafulvalene crystal: A first-principles study
Abstract:
The organic semiconductor dibenzo-tetrathiafulvalene (DBTTF) has presented different polymorphs in solid packing, but the structure–property relationship is little clarified in the literature which is important for the design of high-performance organic semiconductors. In this study, the charge transport in DBTTF crystals for the α phase and β phase is investigated from the first-principles calculations and the Marcus charge transfer theory. The one-, two- and three-dimensional mobilities are obtained simultaneously from a set of identical trajectories with an improved random walk technique. It is found that the α-phase crystal presents a smaller three-dimensional mobility than that in the β-phase crystal although the mobility is much large along the c axis in the α-phase crystal. This is attributed to that the electronic couplings are mainly confined within the c axis for the α-DBTTF while the electronic couplings are more uniform in the three-dimensional space for the β-DBTTF which thus provides more transport pathways for the charge transport. As a result, the β-DBTTF may have a larger potential for practical applications inorganic electronics.1237-1244
Diana Nanova, Sebastian Beck, Andreas Fuchs, Tobias Glaser, Christian Lennartz, Wolfgang Kowalsky, Annemarie Pucci, Michael Kroeger. Charge transfer in thin films of donor–acceptor complexes studied by infrared spectroscopy
Abstract:
The degree of charge transfer in thin films of organic charge transfer (CT)-complexes, which are deposited via thermal evaporation, is examined via infrared-spectroscopy. We demonstrate a linear relationship between the shift in the excitation energy of the CNstretching mode of CT-complexes with the acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the charge transfer. The measured correlation corresponds very well with DFT calculations. For Na-TCNQ we observe a splitting in the peak of the CN-stretching mode, which can be explained by the coupling of two modes and was confirmed by the calculations. In CT-complexes with partial charge transfer the appearance of an electronic excitation is demonstrated.1245-1249
Mounggon Kim, Jun Yeob Lee. Improved power efficiency in deep blue phosphorescent organic light-emitting diodes using an acridine core based hole transport material
Abstract:
A high triplet energy hole transport material with an acridine core was synthesized as the hole transport material for deep blue phosphorescent organic light emitting diodes. The acridine core based hole transport material showed a high triplet energy of 2,89 eV for efficient triplet exciton blocking and highest occupied molecular orbital of 5,96 eV for efficient hole injection in deep blue phosphorescent organic light-emitting diodes. The acridine core based hole transport material showed low driving voltage than common high triplet energy hole transport material and power efficiency of deep blue phosphorescent organic light-emitting diodes was improved by more than 50%.1250-1254
Ji Hoon Park, Hyunjin Lim, Hyeonsik Cheong, Kyu Min Lee, Hyun Chul Sohn, Gyubaek Lee, Seongil Im. Anisotropic mobility of small molecule-polymer blend channel in organic transistor: Characterization of channel materials and orientation
Abstract:
We report on the anisotropic properties of solution-processed thin-film transistors (TFTs) fabricated by the vertical flowing of the viscous 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and isotactic (i-) poly-(methyl methacrylate) (PMMA) blend. During film formation processes, lateral crystalline growth of TIPS-pentacene and TIPS-pentacene/i-PMMA phase-separation were observed. Such anisotropic crystallization of TIPS-pentacene channel caused anisotropic mobility with respect to the channel orientation, so that it could be used for composing of a logic inverter where two TFTs with different channel orientations were serially connected.1255-1260
Hyeok Kim, Jin-Hyuk Bae, Sin-Doo Lee, Gilles Horowitz. An effective method to minimize the leakage current in organic thin-film transistors by using blends of various molecular weights
Abstract:
We report on an effective method to minimize the leakage current in an organic thin-film transistor (OTFT) by using a polymeric gate insulator, poly(vinyl phenol) (PVP). When the molecular weight (M_w) of the PVP varies, only the leakage current is affected under constant remaining electrical parameters. More importantly, through a binary mixing between two different M_w, it is found that the leakage current can be minimized. This is attributed to a reduction of the free volume in the blended PVP layer, leading to a more vigorous cross-linking reaction, as compared to a single molecular weight PVP.1261-1265
Bin Li, Mengqi Zhou, Yu Lu, Chi-Yueh Kao, Jung-Woo Yoo, Vladimir N. Prigodin, Arthur J. Epstein. Effect of organic spacer in an organic spin valve using organic magnetic semiconductor V[TCNE]_x
Abstract:
We studied the role of organic semiconductor spacer tris (8-hydroxyquinoline) ðAlq3) in a hybrid spin valve which is comprised of V[TCNE]_x (x = 2, TCNE: tetracyanoethylene) and Fe as the ferromagnetic layers. We compare two types of devices: Fe/ V[TCNE]_x/Al and Fe/Alq₃/ V[TCNE]_x/Al, showing that organic spacer is not indispensable for the appearance of the spin valve effect. However, the device with Alq₃ spacer has magnetoresistance (MR) value one order of magnitude larger than the device without spacer. The MR of both devices diminish with increasing temperature, while only the Fe/Alq₃/ V[TCNE]_x/Al device shows room-temperature MR.1266-1276
Gitish K. Dutta, Satish Patil. Solution processable quinoxaline based molecular materials for organic field effect transistors
Abstract:
Three new solution processable quinoxaline based donor–acceptor–donor (D–A–D) type molecules have been synthesized for application in field effect transistors. These molecules were characterized by UV–visible spectroscopy, thermal gravimetric analysis, differential scanning calorimetry and cyclic voltammetry. DFT calculation gives deeper insight into the electronic structure of these molecules. The crystallinity and morphology features of thin film were investigated using X-ray diffraction. These molecules show liquid crystalline phase confirmed by DSC and optical polarizing microscopy. Investigation of their field effect transistor performance indicated that these molecules exhibited п-type mobility up to 9,7 x 10^-4 cm² V^-1 s^-1 and on/off ratio of 10⁴.1277-1288
Fuli Zhang, Lian Duan, Juan Qiao, Guifang Dong, Liduo Wang, Yong Qiu. Solution-processed blue–green organic light-emitting diodes based on cationic iridium complexes with 1-pyridyl-3-methylimidazolin-2-ylidene-C,C^2' as the ancillary ligand
Abstract:
Two new blue emitting cationic iridium complexes with N-heterocyclic carbene–pyridine as the ancillary ligand, namely, [Ir(ppy)₂(pymi)]PF₆ and [Ir(dfppy)₂(pymi)]PF₆ (pymi is 1-pyridyl-3-methylimidazolin-2-ylidene-C,C^2', ppy is 2-phenylpyridine, dfppy is 2-(2,4-difluorophenyl)pyridine and PF₆^- is hexafluorophosphate), have been prepared, and the photophysical and electrochemical properties together with X-ray crystal structures have been investigated. In CH₃CN solutions, [Ir(ppy)₂(pymi)]PF₆ and [Ir(dfppy)₂(pymi)]PF₆ exhibit blue light emission with the peaks at 472 and 451 nm, respectively. Both photophysical properties and quantum chemical calculations indicate that photoluminescences of these complexes are mainly from ppy- or dfppy-based ³п → п* states. Solutionprocessed organic light-emitting diodes (OLEDs) based on [Ir(ppy)₂(pymi)]PF₆ and [Ir(dfppy)₂(pymi)]PF₆ give blue–green electroluminescence (506 and 482 nm, respectively). At a doping concentration of 5 wt.%, the device based on [Ir(ppy)₂(pymi)]PF₆ reaches a maximum efficiency of 5б2 cd A^-1, which indicates that this complex is a promising phosphor for achieving efficient electrophosphorescence in the blue–green region.