Browsing by Author "Lin, Hao-Wu"

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All-small-molecule efficient white organic light-emitting diodes by multi-layer blade coating
(2012-05-01)
Yeh, Han-Cheng
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Meng, Hsin-Fei
;
Lin, Hao-Wu
;
Chao, Teng-Chih
;
Tseng, Mei-Rurng
Blue and white small-molecule organic light-emitting diodes are fabricated by multi-layer blade coating on hot plate at 80 degrees C with hot wind. Uniform multi-layer structures are made without dissolution due to rapid drying. Only small molecules originally developed for vacuum deposition are used. For hole transport layer of, 4',4 ''-tris(carbazol-9-yl)triphenylamine (TCTA), electron transport layer of 2,2',2 ''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TBPI), emissive layer host of, 6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine(26DCzPPy), triplet emitters of bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic), and cathode of LiF/Al, the peak current efficiency for blue emission is 25.1 cd/A (10.8% and 9.3 lm/W). Orange emitter iridium(III) bis (4-(4-t-butylphenyl) thieno[3,2-c] pyridinato-N,C2')acetylacetonate (PO-01-TB) is added to obtain white emission with CIE coordinate of (0.39, 0.46) [1]. The current efficiency is 34.2 cd/A (11.6% and 12 lm/W) at maximum, 32.4 cd/A at 1000 cd/m(2), and 31 cd/A at 10,000 cd/m(2). (C) 2012 Elsevier B.V. All rights reserved.
Blade coating of Tris(8-hydroxyquinolinato)aluminum as the electron-transport layer for all-solution blue fluorescent organic light-emitting diodes
(2016-02-01)
Chang, Yu-Fan
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Meng, Hsin-Fei
;
Fan, Gang-Lun
;
Wong, Ken-Tsung
;
Zan, Hsiao-Wen
Organic light-emitting diodes (OLEDs) with a low driving voltage and efficient blue fluorescence were fabricated through blade coating. Tris(8-hydroxyquinolinato)aluminum (Alq(3)) is a relatively stable electron-transporting material commonly used in evaporation. However, depositing Alq(3) through a solution process is difficult because of its extremely low solubility organic solvents, a result of its symmetrical molecular structure. In this study, Alq(3) was successfully deposited through blade coating at a very low concentration below 0.1wt%. The OLEDs contained co-dopants BUBD-1 and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph), and a high-band-gap host 2-methyl-9,10-bis(naphthalen-2-yl) anthracene (MADN) as the emission layer with the following structure: ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:2% BUBD-1:1% DSA-Ph (50 nm)/TPBI (30 nm)/LiF (0.8 nm)/Al (100 nm)or ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:3% BUBD-1 (50 nm)tris(8-hydroxyquinolinato)aluminum (Alq(3); 10 nm)/LiF (0.8 nm)/Al (100 nm). 2,7-disubstituted fluorene-based triaryldiamine(VB-FNPD)is the cross-linking transporting material. The device exhibited a peak current efficiency of 5.67 cd/A for Alq(3) and 5.76 cd/A for TPBI. The device with Alq(3) has operated lifetime seven times higher than the device with TPBI. (C) 2015 Elsevier B.V. All rights reserved.
Cofacial Versus Coplanar Arrangement in Centrosymmetric Packing Dimers of Dipolar Small Molecules: Structural Effects on the Crystallization Behaviors and Optoelectronic Characteristics
(2016-07-20)
Chou, Shu-Hua
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Kang, Hao-Wei
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Chang, Shu-Ting
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Wu, Kuan-Yi
;
Bazan, Guillermo C.
Two D-pi-A-A molecules (MIDTP and TIDTP) composed of an electron-rich ditolylamino group (D) and an electron-deficient 5-dicyanovinylenylpyrimidine (A-A) fragment bridged together with indeno[1,2-b]thiophene (IDT) were synthesized. These molecules provide an opportunity to examine in-depth the impact of side-chain variations (methyl vs p-tolyl) on the crystallization behaviors, solid-state morphology, physical properties, and optoelectronic characteristics relevant for practical applications. X-ray analyses on single-crystal structures indicate that methyl-substituted MIDTP forms coplanar antiparallel dimers via C-H center dot center dot center dot S interactions and organizes into an ordered slip-staircase arrays. In contrast, p-tolyl-bearing TIDTP shows cofacial centrosymmetric dimers via pi-pi interactions and packs into a less-ordered layered structures. The X-ray diffraction analyses upon thermal treatment are consistent with a superior crystallinity of MIDTP, as compared to that of TIDTP. This difference indicates a greater propensity to organization by introduction of the smaller methyl group versus the bulkier p-tolyl group. The increased propensity for order by MIDTP facilitates the crystallization of MIDTP in both solution-processed and vacuum-deposited thin films. MIDTP forms solution-processed single-crystal arrays that deliver OFET hole mobility of 6.56 x 10(-4) cm(2) V-1 s(-1), whereas TIDTP only forms amorhpous films that gave lower hole mobility of 1.34 x 10(-5) cm(2) V-1 s(-1). MIDTP and TIDTP were utilized to serve as donors together with C-70 as acceptor in the fabrication of small-molecule organic solar cells (SMOSCs) with planar heterojunction (PHJ) or planar-mixed heterojunction (PMHJ) device architectures. OPV devices based on higher crystalline MIDTP delivered power conversion efficiencies (PCEs) of 2.5% and 4.3% for PHJ and PMHJ device, respectively, which are higher than those of TIDTP-based cells. The improved PCEs of MIDTP-based devices are attributed to better hole-transport character.
Continuous blade coating for multi-layer large-area organic light-emitting diode and solar cell
(2011-11-01)
Chen, Chun-Yu
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Chang, Hao-Wen
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Chang, Yu-Fan
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Chang, Bo-Jie
;
Lin, Yuan-Sheng
A continuous roll-to-roll compatible blade-coating method for multi-layers of general organic semiconductors is developed. Dissolution of the underlying film during coating is prevented by simultaneously applying heating from the bottom and gentle hot wind from the top. The solvent is immediately expelled and reflow inhibited. This method succeeds for polymers and small molecules. Uniformity is within 10% for 5 cm by 5 cm area with a mean value of tens of nanometers for both organic light-emitting diode (OLED) and solar cell structure with little material waste. For phosphorescent OLED 25 cd/A is achieved for green, 15 cd/A for orange, and 8 cd/A for blue. For fluorescent OLED 4.3 cd/A is achieved for blue, 9 cd/A for orange, and 6.9 cd/A for white. For OLED with 2 cm by 3 cm active area, the luminance variation is within 10%. Power conversion efficiency of 4.1% is achieved for polymer solar cell, similar to spin coating using the same materials. Very-low-cost and high-throughput fabrication of efficient organic devices is realized by the continuous blade-only method. (C) 2011 American Institute of Physics. [doi:10.1063/1.3636398]
Efficient inverted quasi-bilayer organic solar cells fabricated by using non-halogenated solvent processes
(2014-09-07)
Chang, Jung-Hao
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Wang, Hsiao-Fang
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Lin, Wei-Chieh
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Chiang, Kai-Ming
;
Chen, Kuan-Chen
Here we demonstrate the fabrication of novel, "quasi-bilayer" inverted organic photovoltaic devices using halogen-free solvents. The inferior solubility of pristine fullerene in non-halogenated solvents was used to control the interpenetration of upper polymeric donor layers with bottom fullerene layers. Notably, island-like nano-morphologies were revealed by AFM, SEM, TEM, cross-sectional TEM images and PL quenching measurement. Correlation between device performance, thin-film nano-morphology and ac impedance was observed. High efficiencies of 6.55% and 7.15% were observed for PBDTTT-C-T and PTB7 cells, respectively. These results demonstrate that this novel process not only offers an effective new method to control the morphology of solar active layers but, more importantly, could also be applied to a wide range of current material systems to produce efficient devices that comply with the non-toxic halogen-free requirement.
Efficient inverted quasi-bilayer organic solar cells fabricated by using non-halogenated solvent processes
(2014-09-07)
Chang, Jung-Hao
;
Wang, Hsiao-Fang
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Lin, Wei-Chieh
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Chiang, Kai-Ming
;
Chen, Kuan-Chen
Here we demonstrate the fabrication of novel, "quasi-bilayer" inverted organic photovoltaic devices using halogen-free solvents. The inferior solubility of pristine fullerene in non-halogenated solvents was used to control the interpenetration of upper polymeric donor layers with bottom fullerene layers. Notably, island-like nano-morphologies were revealed by AFM, SEM, TEM, cross-sectional TEM images and PL quenching measurement. Correlation between device performance, thin-film nano-morphology and ac impedance was observed. High efficiencies of 6.55% and 7.15% were observed for PBDTTT-C-T and PTB7 cells, respectively. These results demonstrate that this novel process not only offers an effective new method to control the morphology of solar active layers but, more importantly, could also be applied to a wide range of current material systems to produce efficient devices that comply with the non-toxic halogen-free requirement.
General application of blade coating to small-molecule hosts for organic light-emitting diode
(2014-10-01)
Chang, Hao-Wen
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Lee, Yi-Ting
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Tseng, Mei-Rurng
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Jang, Meng-Hao
;
Yeh, Han-Cheng
Blade coating is applied to multi-layer phosphorescent OLED with five small-molecule hosts for the emission layer, including bis[3,5-di (9H-carbazol-9-yl)phenyl]diphenylsilane (SimCP2), 2,6-bis(3-(9H-carbazol-9-yl)phenyl) pyridine (26DCzPPy), 4,4\',4 \'\'-tris-(N-carbazolyl)-triphenylamine (TCTA), 9,9-bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]fluorene (TBCPF), and 2,7-bis(diphenylphosphoryl)-9,9\'-spirobi[fluorene] (SPPO13). In general, blade coating gives low surface roughness around 0.2 nm without phase separation of the emitter and the host. In the large area of 2 cm by 3 cm the film thickness distribution is within 10% and uniform light-emission is achieved. 1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7) is added to tune the electron transport. Among all the hosts, 26DCzPPy and SimCP2 have by far the best electron hole balance and consequently show the highest efficiency. For SimCP2, the maximal efficiency is 15.8 cd/A for blue and 24.2 cd/A for white emission. The order of efficiencies for the hosts is found to be quite different from the order in vacuum evaporation for the same device structures. (C) 2014 Elsevier B.V. All rights reserved.
High-efficiency polymer solar cells by blade coating in chlorine-free solvents
(2014-04-01)
Tsai, Pei-Ting
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Tsai, Chia-Ying
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Wang, Chun-Ming
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Chang, Yu-Fan
;
Meng, Hsin-Fei
High-performance polymer cells are typically fabricated by employing toxic solvents such as dichlorobenzene and chlorobenzene. In this study, blade coating with the chlorine-free solvents toluene and xylene is applied to polymer solar cells that contained the low band-gap polymer PBDTTT-C-T blended with [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM). The highest efficiencies of the cells fabricated in toluene and xylene solutions were 6.09% and 6.11%, respectively. Atomic force microscopy images show that the films formed by blade coating using toluene and xylene were extremely smooth, with roughness of only 1 nm. This blade coating has a rapid-drying in a few seconds without the long-running thermal or solvent annealing. The possibility of high-volume environmentally-friendly fabrication of efficient polymer solar cells with minimal material waste is thus demonstrated using a combination of chlorine-free solvents and blade coating. (C) 2014 Elsevier B.V. All rights reserved.
Highly efficient inverted rapid-drying blade-coated organic solar cells
(2012-04-01)
Chang, Jung-Hao
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Chen, Yi-Hong
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Lin, Hao-Wu
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Lin, Yu-Ting
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Meng, Hsin-Fei
Efficient inverted bulk-heterojunction (BHJ) poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) organic solar cells fabricated by rapid-drying blade-coated were demonstrated. Optimized self-organization interpenetration networks and donor/acceptor domain sizes were obtained while maintaining the smooth surface morphology. By integrating with low-temperature-processed sol-gel ZnO electron extraction layer, power conversion efficiency (PCE) up to 4.4% under AM1.5G 1 sun illumination is achieved, compared to fast drying but low efficiency (1.2%) and high efficiency but with long-time solvent annealing treatment (4.3%) control cells deposited by spin coating in chlorobenzene (CB) and 1,2-dichlorobenzene (DCB) solution, respectively. The novel deposition technique reveals a promising process for highly efficient, high throughput, stable morphology organic solar cells fabrication. (c) 2012 Elsevier B.V. All rights reserved.
Highly efficient organic solar cells using a solution-processed active layer with a small molecule donor and pristine fullerene
(2014)
Lin, Hao-Wu
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Chang, Jung-Hao
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Huang, Wei-Ching
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Lin, Yu-Ting
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Lin, Li-Yen
A new strategy has been successfully established to realize high efficiency small molecule organic solar cells with a solution-processed active layer composed of a small organic molecule as the donor and pristine C-70 as the acceptor. Using 1,2-dichlorobenzene as solvent, a homogeneous donor/C-70 blend active layer can be effectively formed either by spin- or bar-coating techniques. This method delivers organic solar cells with high power conversion efficiencies up to 5.9%.
Interface and thickness tuning for blade coated small-molecule organic light-emitting diodes with high power efficiency
(2013-09-28)
Chang, Yu-Fan
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Chiu, Yu-Chian
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Chang, Hao-Wen
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Wang, Yi-Siang
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Shih, Yi-Lun
We developed a general method based on fluorescence microscopy to characterize the interface dissolution in multi-layer organic light-emitting diodes (OLEDs) by blade coating. A sharp bi-layer edge was created before blade coating, with the bottom layer being insoluble and top layer soluble. After blade coating, fluorescence images showed that the edge of the top layer shifted when the layer dissolved completely, whereas the bottom layer's edge remained in place as a positioning mark. The dissolution depth was determined to be 15-20 nm when the emissive-layer host of 2,6-bis (3-(9H-carbazol-9-yl) phenyl) pyridine (26DCzPPy) was coated on the hole-transport layer of N, N'-bis(naphthalen-1-yl)-N, N'-bis(phenyl)-benzidine(NPB), which was consistent with a sudden drop in efficiency of orange OLEDs with layer thickness below 20 nm. Thus, the layer thickness of OLEDs was optimized to stay more than 20 nm for blade coating. For a two-color white OLED with the structure TCTA/26DCzPPy:PO-01-TB:FIrpic/TPBI, efficiency was 24 cd/A and 8.5 lm/W at 1000 cd/m(2). For a three-color white OLED with Os(fptz)(2)(dhpm) added as the emitter, the efficiency was 12.3 cd/A and 3.7 lm/W at 1000 cd/m(2). For a green device with the structure TCTA/26DCzPPy: Ir(mppy)(3)/TPBI, the efficiency was 41.9 cd/A and 23.4 lm/W at 1000 cd/m(2). VC 2013 AIP Publishing LLC.
ITO-free inverted polymer solar cell on metal substrate with top-illumination
(2014-01-01)
Chen, En-Chen
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Tsai, Pei-Ting
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Tsai, Chia-Ying
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Chang, Jung-Hao
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Huang, Zheng-Yu
High-performance polymer solar cells are conventionally fabricated on the rigid and costly indium-tin-oxide (ITO) glass substrates, which are incompatible with the roll-to-roll process with low cost. This study shows that polymer solar cells can be made on ITO-free flexible stainless steel with only a moderate compromise in power conversion efficiency. A blend of poly (3-hexylthiophene) (P3HT) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) was used as the active material for the inverted structure. The bottom electrode was an alloy of Al and Au spontaneously formed from a bi-layer during thermal annealing of the active layer. This alloy had a low work function of 3.9 eV, and a high resistance to oxide formation throughout the entire process. Semi-transparent Ag was used as the top electrode. These solar cells on stainless steel had an efficiency of 2.6%. The majority of the loss came from the top electrode transmission at approximately 60%. This all-metal approach makes roll-to-roll polymer cell fabrication possible. (C) 2013 Elsevier B.V. All rights reserved.
Multilayer rapid-drying blade coating for organic solar cells by low boiling point solvents
(2014-06-01)
Chen, En-Chen
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Tsai, Pei-Ting
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Chang, Bo-Jie
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Wang, Chun-Ming
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Meng, Hsin-Fei
A bulk heterojunction organic solar cell with poly(3-hexylthiophene) (P3HT) as the donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor is deposited using blade coating on a hot plate at 80 degrees C with hot air of 70 degrees C applied from above. In contrast to the 30 min of conventional dichlorobenzene solvent annealing, the rapid-drying blade coating forms a dry film in 1 s. The fabrication throughput is substantially enhanced. The blade-coated film has a smoother surface roughness of 3.5nm compared with 10.5nm for solvent annealing; however, the desired phase separation in the 50nm scale forms despite the rapid drying. A single layer solar cell exhibits power conversion efficiency of 4.1% with blade coating in chlorobenzene, which is the same as solvent annealing device. A multilayer device with carrier blocking layers fabricated entirely of the less toxic toluene also exhibits efficiency of 4.1%. (C) 2014 The Japan Society of Applied Physics
Performance Characterization of Dye-Sensitized Photovoltaics under Indoor Lighting
(2017-04-20)
Chen, Chia-Yuan
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Jian, Zih-Hong
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Huang, Shih-Han
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Lee, Kun-Mu
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Kao, Ming-Hsuan
Indoor utilization of emerging photovoltaics is promising; however, efficiency characterization under room lighting is challenging. We report the first round-robin interlaboratory study of performance measurement for dye-sensitized photovoltaics (cells and mini-modules) and one silicon solar cell under a fluorescent dim light. Among 15 research groups, the relative deviation in power conversion efficiency (PCE) of the samples reaches an unprecedented 152%. On the basis of the comprehensive results, the gap between photometry and radiometry measurements and the response of devices to the dim illumination are identified as critical obstacles to the correct PCE. Therefore, we use an illuminometer as a prime standard with a spectroradiometer to quantify the intensity of indoor lighting and adopt the reverse-biased current voltage (I-V) characteristics as an indicator to qualify the I-V sampling time for dye-sensitized photovoltaics. The recommendations can brighten the prospects of emerging photovoltaics for indoor applications.
Single-emission-layer white organic light-emitting devices: Chromaticity and colour-rendering consideration
(2014-02-01)
Fan, Chia-Chan
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Huang, Ming-Hong
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Lin, Wei-Chieh
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Lin, Hao-Wu
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Chi, Yun
The chromaticity and colour-rendering capability of solution-processed single emission layer (EML) white organic light-emitting diodes (W-OLEDs) can be precisely tuned by manipulating the dopant compositions in the EMLs. In this work, we numerically modelled binary, ternary, and quaternary doping single EML W-OLEDs. The correlated colour temperature (CCT), colour-rendering index (CRI), and spectral designs were correlated. The simulation predicted that the quaternary doping system possesses the best chromaticity performance. The corresponding binary, ternary and quaternary doping single EML W-OLEDs were fabricated and characterised to verify the calculation. The solution-processed quaternary doping W-OLEDs were designed with CRI values up to 85, deviations from the Planckian locus (D-u'v') as low as +0.0009, an EQE of 13.7%, a power efficacy of 14.7 lm/W and current efficiency of up to 24.9 cd/A at 1000 cd/m(2). (C) 2013 Elsevier B.V. All rights reserved.
Solid-state light-emitting electrochemical cells employing phosphor-sensitized fluorescence
(2010)
Su, Hai-Ching
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Lin, You-Heng
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Chang, Chih-Hao
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Lin, Hao-Wu
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Wu, Chung-Chih
We report highly efficient phosphor-sensitized solid-state light-emitting electrochemical cells (LECs) utilizing a phosphorescent cationic iridium complex [Ir(dFppy)(2)(SB)](+)(PF(6)(-)) as the host and a fluorescent cationic dye (R6G) as the guest. Photophysical studies show that R6G retains a high photoluminescence quantum yield (PLQY) in highly polar media, revealing its suitable use as an emitting guest in an ionic host matrix. Such solid-state LECs achieve quantum efficiency (cd A(-1)) efficiency, and power efficiency up to 5.5% photon/electron, 19 cd A(-1) and 21.3 lm W(-1), respectively. The device quantum efficiency achieved is among the highest reported for fluorescent LECs and is higher than one would expect from the PLQY of the R6G fluorescent dye in the host film, thus indicating that phosphor-sensitization is useful for achieving highly efficient fluorescent LECs. Moreover, using narrow-band fluorescent emitters, such as R6G (FWHM, similar to 50 nm), is effective in improving the color saturation of solid-state LECs based on cationic complexes.
Thermal and angular dependence of next-generation photovoltaics under indoor lighting
(1970-01-01)
Chen, Chia-Yuan
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Kuo, Ting-Yang
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Huang, Chien-Wu
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Jian, Zih-Hong
;
Hsiao, Po-Tsung
Next-generation photovoltaic technologies such as dye-sensitized solar cells, organic thin-film photovoltaics and perovskite solar cells are promising to efficiently harvest ambient light energy. However, more and deeper understanding of their photovoltaic characteristics is essential to create new applications under room light illumination. In this study, for the first time, the difference in temperature coefficients and angular dependence of photovoltaic parameters for the large-area devices are investigated systematically under the compact fluorescent lamp and light-emitting diode light. These emerging photovoltaic devices, compared with the single crystalline silicon solar cells, not only have higher open-circuit voltage (up to approximate 1 V) and better power conversion efficiency (in the range of 9.2% similar to 22.6%) but also exhibit less temperature dependent voltage and output power (< -0.6% degrees C-1), as well as broader angular response (over 75 degrees). The state-of-the-art dye-sensitized and organic thin-film devices also show advantageously positive temperature coefficients of current, and the latter even has positive thermal dependence of fill factor. These features suggest the next-generation photovoltaic devices are more favorable than the conventional crystalline silicon solar cells for real-life indoor applications.
Unmodified small-molecule organic light-emitting diodes by blade coating
(2012-10-01)
Chang, Yu-Fan
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Chiu, Yu-Chian
;
Yeh, Han-Cheng
;
Chang, Hao-Wen
;
Chen, Chun-Yu
Blade coating with substrate heating and hot wind is demonstrated to be a general platform for multi-layer deposition of unmodified small-molecule organic semiconductors. Most unmodified small molecules, originally designed for vacuum evaporation, can be blade coated while the solubility is above 0.5 wt.%. High uniformity is achieved for scale over 5 cm. Orange devices by evaporation and blade coating are compared with 4,4'-bis(carbazol-9-yl)biphenyl (CBP) as the host, iridium(III) bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C-2') acetylacetonate (PO-01-TB) as the emitter. The efficiency difference is within 10%. When 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) is used as the host, the current efficiencies are 40 cd/A for orange, 32 cd/A for green, and 20 cd/A for blue. The optimized organic light-emitting diodes (OLED) structure developed for vacuum deposition can therefore be exactly copied by the low cost blade coating method in solution. (C) 2012 Elsevier B.V. All rights reserved.
Vacuum-free lamination of low work function cathode for efficient solution-processed organic light-emitting diodes
(2012-03-01)
Chang, Yu-Fan
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Chen, Chun-Yu
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Luo, Fang-Tsai
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Chao, Yu-Chiang
;
Meng, Hsin-Fei
The low work function cathode of blade-coated organic light-emitting diode is transferred from a soft polydimethylsiloxane (PDMS) mold by lamination without vacuum. The cathode is a bilayer of polyethylene glycol (PEG) (< 10 nm) and Al (100 nm). A sacrificial layer of polystyrene with low Mw 1500 and melting point of 120 degrees C is inserted between the cathode and PDMS for the subsequent mold removal at 150 degrees C by melting polystyrene. Current efficiency of 3 cd/A (1.1%) and luminance of 2500 cd/m(2) are achieved for green polyfluorene fluorescent emitter. 25 cd/A (8.2%) and 3200 cd/m(2) are achieved for green phosphorescent tris[2-(p-tolyl) pyridine] iridium(III) (Ir(mppy)(3)) emitter in polymer blend host. The efficiency is about 70% of the devices with thermally evaporated cathode. The turn-on voltage is about 5 V higher. (C) 2011 Elsevier B. V. All rights reserved.