Organic Electronics 2011 Volume 12 №04

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.557-561
W.C. Luk, K.M. Yeung, K.C. Tam, K.L. Ng, K.C. Kwok, C.Y. Kwong, A.M.C. Ng, A.B. Djurišic. Enhanced conversion efficiency of polymeric photovoltaic cell by nanostructured antireflection coating
Abstract:
A nanostructured antireflection coating has been fabricated on glass side of an indium tin oxide (ITO) coated glass by self-assembling a monolayer of nano-silica spheres. Significant reduction in the weighted reflectance from 10,2% to 7,2% at 5° incident angle is observed. At 60° incident angle, the weighted reflectance of ITO glass with antireflection coating is still about 3% lower than that of bare ITO glass. This nanostructured antireflection coating has been applied on the glass surface of the ITO substrate in a polymer photovoltaic cell. Improvement in the power conversion efficiency from 1,80% to 2,05% at normal incidence has been demonstrated. The relative enhancement in the conversion efficiency is almost independent of the incident angles.532-565
Yu-Hung Chen, Yu-Jen Cheng, Guan-Ru Lee, Chih-I Wu, Tun-Wen Pi. Improvements of electron injection efficiency using subphthalocyanine mixed with lithium fluoride in cathode structures of organic light emitting diodes
Abstract:
The improvement of current injection efficiency in organic light emitting diodes is demonstrated with incorporation of lithium fluoride (LiF) mixed with subphthalocyanine (SubPc) as electron injection layers. The photoemission spectra indicate that the lithium released from LiF would remove chlorine atoms from SubPc molecules and create large density of gap states at the interfaces. These gap states, resulted from the addition of SubPc in conventional aluminum/LiF/ tris-(8-Hydroxyquinoline)-aluminum (Alq₃) cathode structures, further enhance the electron injection efficiency in OLEDs. The results demonstrate a relatively unique feature of SubPc which can be used to improve both electron and hole injection efficiency.566-574
Matthieu Manceau, Dechan Angmo, Mikkel Jørgensen, Frederik C. Krebs ITO-free flexible polymer solar cells: From small model devices to roll-to-roll processed large modules
Abstract:
Manufacturing of flexible ITO-free polymer solar cell modules by roll-to-roll methods (R2R) is described. Inverted devices with top illumination were built on a Kapton foil and an Aluminum/Chromium bi-layer system was used as electron contact. The layer structure was Kapton/Al/Cr/P3HT:PCBM/PEDOT:PSS/Ag (printed) and devices were encapsulated. Small area cells (3 cm² active area) were first carefully optimized investigating the influence of a number of discrete parameters on performance. A maximum power conversion efficiency of 1,4% was achieved under 1 sun illumination (AM 1,5G, 1000Wm^-2). Optimized labscale single devices were then transferred to a full R2R process combining slot-die coating and screen printing. All the layers were processed from solution under ambient conditions. Two different concepts were explored: (i) serially connected stripe modules (to reduce the Ohmic losses) and (ii) monolithic modules (to achieve high geometric fill factor and increase the flexibility of the process). For this second concept, the only layer that needs to be patterned is the silver grid electrode and the grid pattern design can then be readily tuned. As an example, four different patterns were used and the resultant performances compared. Modules comprising 16 serially connected cells gave total area efficiencies up to 0,5% (235 cm² – 1% on the active area) while the best monolithic ones gave 0,35% (100 cm² – 0,4% on the active area). The freshly prepared devices consistently showed an inflection point in the IV curve indicative of a rather poor photovoltaic behavior. Upon light exposure and repeated IV scans the inflection point partially disappeared, and performance significantly increased.575-581
Wen-Yi Hung, Zhong-Wen Chen, Hong-Wei You, Fu-Chuan Fan, Hsiao-Fan Chen, Ken-Tsung Wong. Efficient carrier- and exciton-confining device structure that enhances blue PhOLED efficiency and reduces efficiency roll-off
Abstract:
In this study we used a known host material diphenylbis[4-(9-carbazoyl)phenyl]silane (SiCa) to realize a highly efficient bis[4,6-(difluorophenyl)-pyridinato-N,C^2']picolinate (FIrpic)–based blue phosphorescent device exhibiting high efficiencies of up to 18,3%, 41,5 cd/A, and 31 μm/W in a carrier- and exciton-confined device structure. At a practical brightness of 1000 cd/m², the device performance (gext) remained as high as 16,7%. The enhanced efficiency and reduced roll-off resulted mainly from the introduction of (a) DTAF as an HTL that efficiently blocked and confined excitons within the FIrpic-doped emissive layer, and (b) a thin layer of DPPS between the EML and TAZ, serving as an exciton- and hole-blocking layer to suppress exciton quenching by FIrpic at the EML–ETL interface. In addition, we incorporated a red phosphorescent dopant (Mpq2Iracac) into the blue device to fabricate a white phosphorescent organic light-emitting device, which exhibited satisfactory efficiencies (11,6%, 21,2 cd/A, 14,2 μm/W) with the Commission Internationale de L’éclairage (CIE) coordinates of (0,35, 0,38).582-588
Dandan Song, Qi Wanga, Suling Zhao, Hany Aziz. Dependence of carrier recombination mechanism on the thickness of the emission layer in green phosphorescent organic light emitting devices
Abstract:
Delayed EL measurements are used to elucidate carrier recombination and light emission mechanism in phosphorescent organic light emitting devices (PhOLEDs) based on 4,4'-bis(9-carbazolyl)-1,1'-biphenyl (CBP) and fac-tris(2-phenylpyridine) iridium (Ir(ppy)₃) host:guest system. The results show that changing the thickness of the emitting layer (EML) leads to marked changes in charge-trapping and host–host triplet–triplet-annihilation (TTA) patterns, suggesting that carrier transport and recombination processes change depending on EML thickness. The results suggest a change in carrier recombination and exciton formation mechanism, depending on EML thickness, from a scenario (for EMLs 20 nm) in which recombination occurs mostly on the guest rather than the host thus creating excitons directly on the guest, and hence is not strongly influenced by host-to-guest energy transfer; to another scenario (for EMLs 20 nm) where carrier recombination and exciton creation on the host is not negligible, and hence also the role of host-to-guest energy transfer.589-594
V.R. Nikitenko, A.R. Tameev, A.V. Vannikov. Mechanism of enhanced mobility and conductivity at donor–acceptor organic interfaces
Abstract:
The model of metallic type or enhanced hopping conductivity with reduced activation energy at the donor–acceptor interface formed by organic dielectrics is suggested. High density of electrons and holes at the interface arises due to the integer charge transfer from donor to acceptor molecules. Significant part of charge carriers can avoid thermal activation and tunneling in course of their transport if the interfacial energy barrier is low enough. The critical value of an interfacial energy barrier, density of free electrons, the values of two-dimensional mobility and conductivity of charge carriers are estimated by numerical modeling and compared with experimental data.595-601
Lei Wang, Zhi-Yong Wu, Wai-Yeung Wong, Kok-Wai Cheah, Hong Huang, Chin H. Chen. New blue host materials based on anthracene-containing dibenzothiophene
Abstract:
A series of novel host blue-emitting materials based on anthracene have been synthesized and investigated in which, a п-electron-rich dibenzothiophene moiety was introduced as a side group to fine adjust the HOMO/LUMO levels of the anthracene and enhance the ability of hole-transport and electron-injection. These materials exhibit good film-forming capabilities, and display strong blue fluorescence in solution. To explore the electroluminescent properties of these materials, sky blue devices of 2-TNATA (60 nm)/NPB (10 nm)/Host:BUBD-1 (40 nm)/Alq₃ (15 nm)/LiF (1 nm)/Al (150 nm) were fabricated. For the device with 9, 10-bis(dibenzothiophen-4-yl)anthracene as the host, an electroluminescence efficiency of 13,14 cd/A and 6,76 lm/W at 20 mA/cm² and 6,09 V were achieved, with Commission Internationale d’Eclairage (CIE_x,y) color coordinates of (x = 0,15, y = 0,30).602-608
Yu-Tai Tao, Kun-Yang Wu, Ko-Hui Huang, Tsong-Ping Perng. Odd–even modulation of electrode work function with self-assembled layer: Interplay of energy barrier and tunneling distance on charge injection in organic light-emitting diodes
Abstract:
In this study, a series of trifluoromethyl-terminated alkanethiols with different chain length (CF₃(CH₂)_nSH, n = 5, 6, 10, 11, 12 and 13), were used to form self-assembled monolayers(SAMs) on silver and gold surfaces. The SAM modulates the metal work function as a result of the different orientation of the terminal dipolar CF₃ group due to different chain tilt of the SAM on Au and Ag surfaces, respectively. The SAM-modified Ag and Au were used as the anode in the fabrication of hole-only devices as well as top-emitting organic lightemitting diodes (TOLEDs) with different hole-transporting layers. The correlation between current density/luminous efficiency and the SAM chain length/work function were analyzed to provide information on charge balance. Thus with NPB as the hole-transporting layer, where little difference between the HOMO energy level and the work function of the SAM-modified electrode, the charge injection is modulated by the chain length (tunneling distance-dependent) of the SAM used for modification, decreasing with increasing chain length. With BPAPF as the HTL, where a barrier is present, the charge injection is modulated more by the work function (injection barrier-dependent), which shows an odd–even alternation.609-613
Yun Cheol Han, Cheol Jang, Kuk Joo Kim, Kyung Cheol Choi, KyungHo Jung, Byeong-Soo Bae. The encapsulation of an organic light-emitting diode using organic–inorganic hybrid materials and MgO
Abstract:
The characteristics of organic–inorganic multi-barriers were investigated in this study. Polymerized cycloaliphatic epoxy hybrid materials (hybrimers), synthesized by a sol-gel method and MgO were used as an organic and inorganic material, respectively. We performed a Ca test at 30 °C and 90% R.H. A multi-barrier of 6 dyads on 100 μm thick PET resulted in a water vapor transmission rate (WVTR) of 4,9 x 10^-5 g/m² day and optical transmittance of 81,8% in the visible region (350–850 nm). We suggested relatively simple passivation method with a solution process involving an organic material coupled with low temperature deposition of MgO using an electron beam evaporator. Due to the solution process of the organic layer, the OLEDs were passivated without dark spots or an additional protective layer. After driving of 740 h with a constant current source under ambient conditions, a half-life time of 15,000 h was estimated by the stretched exponential decay (SED) model with an initial luminance value L_o = 1000 cd/m².614-622
Dan Deng, Yang Yang, Jing Zhang, Chang He, Maojie Zhang, Zhi-Guo Zhang, Zhanjun Zhang, Yongfang Li. Triphenylamine-containing linear D-A-D molecules with benzothiadiazole as acceptor unit for bulk-heterojunction organic solar cells
Abstract:
Three solution-processable triphenylamine (TPA)-containing low band gap D-A-D molecules with benzothiadiazole (BT) as acceptor unit and TPA-(4-hexyl)thiophene (TPA-HT), TPA-(4-hexyl)thieno[3,2-b]thiophene (TPA-HTT) and bis(octyloxy)TPA-(4-hexyl)thieno[3,2-b]thiophene (boTPA-HTT) as donor units, TPA-HT-BT, TPA-HTT-BT and boTPAHTT-BT, have been designed and synthesized for the application in the bulk-heterojunction organic solar cells (OSCs). The molecules show broad absorption band in the visible range and appropriate HOMO energy levels for the donor materials in OSCs. The OSC devices were fabricated with the molecules as donor and PC70BM as acceptor. The power conversion efficiencies of the OSCs based on TPA-HT-BT, TPA-HTT-BT and boTPA-HTT-BT reached 1,44%, 1,44% and 0,75%, respectively, under the illumination of AM.1,5, 100 mW/cm².623-633
Thomas M. Brown, G. Mattia Lazzerini, Lisa J. Parrott, V. Bodrozic, Lukas Bürgi, Franco Cacialli. Time dependence and freezing-in of the electrode oxygen plasma-induced work function enhancement in polymer semiconductor heterostructures
Abstract:
Indium tin oxide (ITO), Au and Pt are materials of interest as high work function contacts for organic semiconductor devices. In this paper the relative energy level line-up of these materials is investigated both as bare surfaces or part of a polymer/conductor interface. Kelvin probe (KP) measurements show that the estimated work function for Au and Pt surfaces, evaporated under normal high vacuum (HV) fabrication conditions and measured in air, can be significantly lower (by ~0,2 to 0,9 eV) than those of clean surfaces evaporated and measured in the more demanding and clean ultra high vacuum (UHV), that are often used as reference values. The work function of all surfaces increases significantly (from ~0,4 to 1 eV) after an oxygen plasma, but then decreases upon air exposure, with different rates for different materials. The effect of the plasma wears off most rapidly for Au whilst it is more resilient for ITO. Most interestingly, via KP and electroabsorption measurements of the built-in potential on polymer/conductor and conductor/polymer/conductor structures, we demonstrate that a plasma-induced enhancement of the work function is ‘‘frozen in’’ by the application of a polymer semiconductor layer over the plasma-treated surfaces and can be made to last for years by proper device encapsulation. These results have strong implications on the understanding, fabrication, design and stability of organic semiconductor devices.634-640
Kang-Jun Baeg, Dongyoon Khim, Ju-Hwan Kim, Minji Kang, In-Kyu You, Dong-Yu Kim, Yong-Young Noh. Improved performance uniformity of inkjet printed n-channel organic field-effect transistors and complementary inverters
Abstract:
In the present study, we demonstrate inkjet-printed n-type organic field-effect transistors (OFETs) and their complementary inverters with high performance uniformity, using soluble N,N0-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN₂). The device performance and uniformity were improved by ink-jet printing a PDI8-CN₂ solution onto a heated substrate (60 °C). The printed features, which were discontinuous crystalline films at RT, were uniform films when the substrate temperature was increased to 60 °C. Optimized n-channel PDI8-CN₂ FETs showed a high field-effect mobility of 0,05–0,06 cm²/Vs, a high on/off ratio of ~10⁶, and a high uniformity that was within 10% with a bottom-gate/bottom-contact device configuration. Inkjet-printed organic complementary inverters were constructed by direct inkjet-printing of n-channel (PDI8-CN₂) and p-channel (6,13-bis(triisopropyl-silylethynyl)-pentacene or poly(3-hexylthiophene)) organic semiconductors onto silicon dioxide gate dielectrics. The inkjet-printed organic complementary inverters exhibited a high voltage gain of more than 15 and small standard deviation of inverting voltage and gain of ±0,95 V and ±0,56, respectively, for measuring 12 samples from four difference batches.641-647
Shuyan Shao, Fengmin Liu, Gang Fang, Baohua Zhang, Zhiyuan Xie, Lixiang Wang. Enhanced performances of hybrid polymer solar cells with p-methoxybenzoic acid modified zinc oxide nanoparticles as an electron acceptor
Abstract:
Enhanced performances of hybrid polymer solar cells based on a blend of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and p-methoxybenzoic acid (MBA) modified zinc oxide (ZnO) nanoparticles were reported. Both the short-circuit current and open-circuit voltage of solar cells were improved by using MBA modified ZnO nanoparticles as electron acceptor. The overall power conversion efficiency was increased from 1,09% to 1,52% under AM 1,5 G 100 mW/cm² illumination after ZnO was modified with MBA at 0,1 wt.%. It was found that modification of ZnO with 0,1 wt.% of MBA did not affect electron and hole transport properties of the active layer. However, a fine phase separation between MEH-PPV donor and MBA-modified ZnO acceptor was obtained. The fine phase separation enhanced the exciton dissociation efficiency and hence the power conversion efficiency of solar cells.648-653
Junhong Zhou, Na Ai, Lei Wang, Hua Zheng, Chan Luo, Zhixiong Jiang, Shufu Yu, Yong Cao, Jian Wang. Roughening the white OLED substrate’s surface through sandblasting to improve the external quantum efficiency
Abstract:
A simple, low cost, and scalable process based on sandblasting to modify the OLED substrate’s surface, has been successfully developed. The rough surface produced by sandblasting efficiently suppressed the internally generated photons’ waveguide mode, and enhanced the white OLED’s EQE from 9% to 11,6% without changing the emission spectrum and the Lambertian emitter property. The improvement of the EQE was found to monotonically decrease with the surface roughness within the roughness range from around 1 μm to 3 μm. The white OLED’s luminous efficiency was increased from 15,5 cd A^-1 to 20 cd A^-1. Furthermore, the realization of rough surface on a 3,5 inch white lighting panel not only improved the 9-point uniformity from 80% to 90%, but also demonstrated the manufacturing scalability of the sandblasting technique.654-659
A.A. Shcherbakov, A.V. Tishchenko, D.S. Setz, B.C. Krummacher. Rigorous S-matrix approach to the modeling of the optical properties of OLEDs
Abstract:
A new approach for the simulation of OLEDs based on a plane wave field expansion is proposed. Exact general formulae are given that allow for the calculation of their optical characteristics including losses in all layers in an arbitrary OLED structure. The S-matrix approach is used which ensures numerical stability of the algorithm. A simple way is proposed to take into consideration non-coherent reflections in a thick substrate. The validity of the model is demonstrated by comparing with experimental data.660-665
Jan Gilot, Martijn M. Wienk, René A.J. Janssen. Measuring the current density – voltage characteristics of individual subcells in two-terminal polymer tandem solar cells
Abstract:
Measuring the current density to voltage (J–V) characteristics of the separate subcells in two-terminal polymer tandem solar cells enables assessing the individual contributions of the two subcells to the total performance. We present two different methods to determine the J–V characteristics of the subcells in a two-terminal tandem cell without interfering with the light incoupling of the cell. The first method employs an extra proximity metal electrode that acts as a voltage probe. The proximity electrode is in contact with the recombination layer that separates the two subcells, but is positioned outside the illuminated area of the tandem cell. The second method uses bias-dependent external quantum efficiency measurements of two-terminal tandem solar cells and subsequent integration with the solar spectrum to determine the J–V curves of the subcells. The methods show good mutual agreement. For a 4,7% efficient solution processed two-terminal polymer tandem cell, based on PFTBT:PCBM as wide band gap and pBBTDPP2:PCBM as small band gap layer, we find that the subcells contribute with efficiencies of 2,9% and 1,8%.666-676
Poopathy Kathirgamanathan, Sivagnanasundram Surendrakumar, Raghava Reddy Vanga, Seenivasagam Ravichandran, Juan Antipan-Lara, Subramaniam Ganeshamurugan, Muttulingham Kumaraverl, Gnanamoly Paramaswara, Vincent Arkley. Arylvinylene phenanthroline derivatives for electron transport in blue organic light emitting diodes
Abstract:
A series of novel arylvinylene substituted phenanthroline derivatives has been synthesised and their application as electron transporting materials in organic light emitting diodes (OLEDs) has been investigated. One particular derivative, namely, 2,9-bis-(2-thiophen-2-yl-vinyl)-[1,1']phenanthroline (C-4) shows significantly lower operating voltage (36% reduction) and increase of power efficiency of up to 83% compared to tris (8-quinolinolato) aluminium (III) (Alq₃). C4 also shows substantially longer (nearly 6-fold) life-time than bathophenanthroline (BPhen) in blue devices. Doping of C4 with electron donors such as m-MTDATA, α-NPB and lithium quinolinolate (Liq) further reduces the turn-on voltage and operating voltage.677-681
Shuming Chen, Hoi-Sing Kwok. Top-emitting white organic light-emitting diodes with a color
conversion cap layer
Abstract:
We report top-emitting white organic light-emitting diodes (WOLEDs) by thermally evaporating non-doped (4-(4-(1,2,2-triphenylvinyl)phenyl)-7-(5-(4-(1,2,2-triphenylvinyl)phenyl) thiophen-2-yl)benzo[c][1,2,5]thiadiazole) (BTPETTD) as a color conversion cap layer on top-emitting blue OLEDs. With a 240 nm cap layer, 74,5% of the blue photon energy are absorbed and converted to red emission with a conversion efficiency of 40%. By mixing the unabsorbed blue emission and red emission, the resulting top-emitting WOLEDs exhibit a broad band spectra with CIE coordinates of (0,34, 0,35), high color stability over a wide range of driving voltages, and peak efficiency of 17,7 cd/A and 8,7 μm/W.682-685
Leah Lucas Lavery, Gregory Lewis Whiting, Ana Claudia Arias. All ink-jet printed polyfluorene photosensor for high illuminance detection
Abstract:
An all-printed photosensor based on polyfluorene derivatives has been developed for high illuminance range detection ( 100 klux). The active layer is a blend of hole-accepting poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFB) and electron-accepting poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT). A top-absorbing architecture was used with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent electrode. The surface energy of the PEDOT:PSS ink was modified in order to allow printing directly onto the hydrophobic surface of the PFB:F8BT blend layer, placing the transparent electrode at the top of the light sensor structure. The all-printed photosensor shows a linear response over multiple measurements within the illuminance range of (100–400) klux.686-693
Kuo-Jung Huang, Yu-Sheng Hsiao, Wha-Tzong Whang. Low-temperature formation of self-assembled 1,5-diaminoanthraquinone nanofibers: Substrate effects and field emission characteristics
Abstract:
In this study, we used thermal evaporation with a low sublimation temperature (42 °C) to deposit 1,5-diaminoanthraquinone (DAAQ) in various morphologies—including nanorods, nanocornerstones, and nanofibers-onto various substrates. Three major factors influenced the growth of vertically aligned DAAQ nanofibers on the electrodes: a low water contact angle (WCA) for the substrate and intermolecular hydrogen bonding and п–п interactions between the DAAQ molecules. On Au and Ti substrates (low-WCA), the DAAQ nanofibers DAAQ-Au and DAAQ-Ti, respectively, possessed great verticality and high aspect ratios; they also exhibited field emission characteristics, with maximum emission current densities of 0,31 and 0,65 mA/cm², respectively, at an applied electric field of 12 V/lm. The turnon electric fields for producing a current density of 10 μA/cm² were 8,5 V/μm for DAAQ-Au and 8,25 V/lm for DAAQ-Ti. From the slopes of Fowler–Nordheim plots, we calculated the field enhancement factors (β) of DAAQ-Au and DAAQ-Ti to be 447 and 831, respectively. Field emission stability studies revealed that the DAAQ nanofibers possessed outstanding anti-degrading capability. The emission current did not decrease, but rather increased slightly, after 3000 s. Given the advantages of this simple low-temperature process and the impressive anti-degrading field emission characteristics, such DAAQ nanofibers have great potential for use in various electronics applications (e.g., as organic field emitters).694-702
Yuan-Hsiang Yu, Chien-Hsun Huang, Jui-Ming Yeh, Ping-Tsung Huang. Effect of methyl substituents on the N-diaryl rings of anthracene-9,10-diamine derivatives for OLEDs applications
Abstract:
A series of N-diaryl-anthracene-9,10-diamine derivatives with methyl substituents at meta or para position of N-diaryl rings were synthesized and used as dopants for Organic Light-Emitting Devices (OLED). The effects of substituted methyl substituents have been compared based on both materials and solid device evaluation. The performance of solid state devices in the configuration of ITO/N,N'-bis(naphthalene-1-yl)-N,N'-bis(phenyl)benzidine (NPB) (40 nm)/3% dopant: 9,10-di(2-naphthyl)anthracene (AND) (40 nm)/tris(8-hydroxyquinoline) aluminum (Alq₃) (20 nm)/LiF (1,5 nm)/Al (200 nm) have been studied. As compared to the non-substituted dopant N,N,N',N'-tetraphenyl-anthracene-9,10-diamine, current efficiency was promoted to approximately 2,9, 6,33, and 7,94 cd/A, and power efficiency was improved by 0,74, 2,26 and 3,02 μm/W, for dopants with the substituted methyl groups at meta, para and both meta/para positions, respectively. The device, prepared by using the dimethyl-substituted dopant at meta/para position, showed the best performance when the luminance of the device reached 24,991 cd/m² with CIE (x, y) = (0,38, 0,59), and the efficiency of the device reached 24,99 cd/A and 7,48 μm/W at a driving current of 100 mA/cm². As compared with a device made of a coumarin (C545T)/Alq₃-based emitting layer, the dimethyl-substituted dopant/ADN-based device developed in this study, exhibited great improvement in device performance.703-709
Zicheng Ding, Rubo Xing, Qiang Fu, Dongge Ma, Yanchun Han. Patterning of pinhole free small molecular organic light-emitting films by ink-jet printing
Abstract:
Small molecular organic light-emitting diodes (SMOLEDs) were fabricated with an ink-jet printed film of 2-(t-butyl)-9,10-bis (20-naphthyl) anthracene (TBADN) doped with 4,4'-bis[2-{4-(N,N-diphenylamino)phenyl vinyl] (DPAVBi) as emitting layer. Dewetting behavior of ink-jet printed TBADN/DPAVBi solutions were restrained by adding cyclohexylbenzene or α-chloronaphthalene to the main solvent chlorobenzene. The high boiling point and high viscosity of cyclohexylbenzene and α-chloronaphthalene has increased the thickness of liquid films and the viscosity, which restrained the dewetting from thermodynamics and kinetics aspect. Uniform TBADN/DPAVBi films obtained by ink-jet printing from chlorobenzene/cyclohexylbenzene solution have been used in the fabrication of matrix display of SMOLEDs. The OLEDs has a turn-on voltage of 5,5 V, the maximum luminance of 289 cd/m² and the maximum current efficiency of 0,71 cd/A.710-715
Takayuki Chiba, Yong-Jin Pu, Ryoichi Miyazaki, Ken-ichi Nakayama, Hisahiro Sasabe, Junji Kido. Ultra-high efficiency by multiple emission from stacked organic light-emitting devices
Abstract:
We investigated the charge generation characteristics of intermediate layer, consisting of an organic acceptor material 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN₆) and an organic donor material N,N0-di(naphthalene-1-yl)-N,N'-diphenylbenzidine (NPD), for a stacked organic light-emitting device (OLED). A stack consisting of an ultrathin 1 nm of LiF and of Al was used as an electron-injection layer (EIL) from HAT-CN₆ to an adjacent electron transporting layer (ETL). The orange-emitting fluorescent stacked OLED with the charge generation layer and the electron injection layer exhibited twice higher current efficiencies and a longer operational lifetime than those of the corresponding unstacked device under a high luminance. We also fabricated a green phosphorescent OLED showing an extremely high current efficiency of 256 cd/A.716-723
Shun-ichiro Watanabe, Hisaaki Tanaka, Hiroshi Ito, Shin-ichi Kuroda, Tatsuo Mori, Kazuhiro Marumoto, Yukihiro Shimoi. Direct determination of interfacial molecular orientations in field-effect devices of P3HT/PCBM composites by electron spin resonance
Abstract:
Field-induced electron spin resonance (FI-ESR) measurements have been performed on metal–insulator-semiconductor diode structures using regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) composites with various mixing ratios on a parylene gate insulator. For lower PCBM mixing ratios, clear FI-ESR signals of hole carriers were observed around g = 2,0031, with the external magnetic field normal to the substrate plane, indicating the edge-on orientation of P3HT lamellar structures at the device interface. On the other hand, for higher PCBM mixing ratios above 50 wt.%, a new additional peak was observed at g = 2,0022, which is characteristic of the pp-orbital axis, indicating the occurrence of flat-on P3HT domains. Ambipolar charge transport was also observed in field-effect transistors using P3HT/PCBM on parylene. With increasing PCBM fraction, from 0 to 100 wt.%, the field-effect mobility of holes decreased from 10^-3 to 10^-6 cm²/Vs, while that of electrons increased from 10^-5 to 10^-2 cm²/Vs. These results demonstrate that the flat-on orientation strongly reduces the charge carrier transport for holes because two-dimensional charge transport assisted by п–п interaction is interrupted by the presence of the flat-on structures.