Organic Electronics 2008 Volume 9 №05

Elsevier. — 379 p. — ISSN: 1566-1199.551-556
Dong-Jin Yun, Shi-Woo Rhee. Effect of molybdenum electrode annealing on the interface properties between metal and pentacene
Abstract:
Contact resistance between molybdenum (Mo) electrode and pentacene was studied with transmission line method (TLM). The Mo electrodes were annealed at 200 °C, 400 °C, 600 °C and 800 °C for 1 h and pentacene layer of 300 Е thickness was vacuum deposited on patterned Mo to form Mo-pentacene contact. Current–voltage measurement for Mo–pentacene contact showed linear relationship and it was confirmed that ohmic contact was formed. XRD and AFM measurements showed that Mo could be crystallized at annealing temperatures above 600 °C. 800 °C annealed Mo showed larger grains and work function was increased from 4,60 eV to 4,80 eV due to the decrease in defect density. The contact resistance was reduced down to 11,2 MΩ cm from 37,8 MΩ cm of as-deposited Mo. Also the pentacene film deposited on annealed Mo was denser with better crystallinity. Bottom contact organic field-effect transistor made with 800 °C annealed Mo showed better performance than as deposited Mo.557-568
Wei-Sheng Huang, Jiann T. Lin, Hong-Cheu Lin. Green phosphorescent iridium dendrimers containing dendronized benzoimidazole-based ligands for OLEDs
Abstract:
A series of novel non-conjugated functionalized benzoimidazole-based dendrimers containing peripheral benzyl ether type dendrons have been synthesized and characterized. These compounds undergo cyclometalation with iridium trichloride to form iridium(III) complexes. The emission wavelengths of these dendrimers are in the range from 510 to 530 nm, and the photoluminescence quantum yields (PLQYs) in the range from 0,45 to 0,
80. Dendrimers (Gn)₂Ir(acac) and (Gn)₃Ir exhibit a reversible one-electron oxidation wave at ~0,55 V and ~0,37 V (vs. Ag/AgNO₃), respectively. With a device configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid)/4,4’-bis(N-carbarzolyl)biphenyl:(G2)₃Ir 20 wt% dopant/1,3,5-tris(2-N-phenyl-benzoimidazolyl) benzene/LiF/Al has a maximum external quantum efficiency (EQE) of 17,6% and a maximum current efficiency of 61,5 cd/A.569-574
L. Mattias Andersson, Wojciech Osikowicz, Fredrik L.E. Jakobsson, Magnus Berggren, Lars Lindgren, Mats R. Andersson, Olle Inganäs. Intrinsic and extrinsic influences on the temperature dependence of mobility in conjugated polymers
Abstract:
The temperature dependence of charge carrier mobility in conjugated polymers and their blends with fullerenes is investigated with different electrical methods, through field effect transistor (FET), space charge limited current (SCLC) and charge extraction (CELIV) measurements. Simple models, such as the Gaussian disorder model (GDM), are shown to accurately predict the temperature behavior, and a good correlation between the different measurement methods is obtained. Inconsistent charge carrier concentrations in the modeling are explained through intrinsic non-equilibrium effects, and are responsible for the limited applicability of existing numerical models. A severe extrinsic influence from water in FETs with a hydrophilic insulator interface is also demonstrated. The presence of water leads to a significant overestimate of the disorder in the materials from measurements close to room temperature and erratic behavior in the 150–350 K range. To circumvent this problem it is shown to be necessary to measure under ultra high vacuum (UHV) conditions.575-581
Calvin K. Chan, Wei Zhao, Stephen Barlow, Seth Marder, Antoine Kahn. Decamethylcobaltocene as an efficient n-dopant in organic electronic materials and devices
Abstract:
n-Doping of copper phthalocyanine (CuPc), which has an electron affinity (EA) of 3,52 eV, by decamethylcobaltocene (DMC) is demonstrated. DMC has a remarkably low solid-state ionization energy (IE) of 3,3 eV, as measured by ultra-violet photoemission spectroscopy (UPS). Further UPS measurements show a large 1,4 eV upward shift of the Fermi-level within the single particle gap of CuPc between the p- and n-doped films. n-Doping is also confirmed by current–voltage (I-V) measurements, which show a 106-fold increase in current density due to improved electron injection and enhanced conductivity of the bulk film. An organic p-i-n CuPc homojunction is also fabricated using F4-TCNQ and DMC as p- and n-dopants, respectively. Current–voltage characteristics demonstrate excellent rectification with a turn on voltage of approximately 1,3 eV, which is consistent with the built-in voltage measured by UPS and capacitance–voltage (C–V) measurements.582-590
Jun Sakai, Tetsuya Taima, Kazuhiro Saito. Efficient oligothiophene:fullerene bulk heterojunction organic photovoltaic cells
Abstract:
Novel small-molecule bulk heterojunction photovoltaic (PV) cells consisting of oligothiophen (alpha-sexithiophen: 6T) and fullerene (C60) have been developed. Oligothiophen is well known as a good hole-transport material, and by changing the number of thiophen rings and making chemical modifications or substitutions, its characteristics relevant to PV applications (such as carrier mobility, energy level, packing, and ordered structure) can be controlled. Thus far it has been difficult to fabricate films of oligothiophene-fullerene blends with suitable morphology by using the common co-evaporation method, because oligothiophene crystallizes easily during film deposition. The present study found that the morphology of 6T:C₆₀ blends strongly depending on the composition of 6T:C₆₀. Suitable morphology was obtained only for films deposited with the co-evaporation of excess C₆₀. It is likely that excess C60 prevents the crystallization of 6T. By successfully controlling the film morphology, we were able to demonstrate good PV performance in oligothiophene:fullerene bulk heterojunction PV cells for the first time. Moreover, it was found that PV performance could be further improved by inserting a C₆₀ layer between the blend layer and exciton blocking layer.591-601
C. Videlot-Ackermann, H. Brisset, J. Ackermann, J. Zhang, P. Raynal, F. Fages, G.H. Mehl, T. Tnanisawa, N. Yoshimoto. Effect of end-substitutions of distyryl-oligothiophenes by hexyl chains on environmental stability in organic thin film transistors
Abstract:
In this study, solution and solid-state properties as well as the organic thin film transistor (OTFT) behaviour of two a,x-hexyl-distyryl-oligothiophenes (DH-DSnT, n = 2, 4) are presented. The optical and electrochemical properties of the oligomers in solution were investigated by UV–vis absorption and photoluminescence spectroscopies, and cyclic voltammetry. Liquid crystal properties were studied by differential scanning calorimetry (DSC) and optical polarising microscopy (OPM). High-vacuum evaporated thin films were studied by optical absorption, X-ray diffraction and atomic force microscopy (AFM), and implemented as p-type semiconducting layers into organic thin film transistors (OTFTs). The results are investigated to study the influence of alkyl chains on the a,x-end positions of the distyryl-oligothiophene skeleton (DSnT). Furthermore, detailed shelf-live tests of hole mobility (l), threshold voltage (Vt), on current (ION) and off current (IOFF) under atmospheric conditions (air, temperature) of OTFTs based on DS4T and DH-DS4T show that the presence of alkyl chains in a,x-end positions of distyryl-oligothiophenes is detrimental to the environmental stability of OTFTs over time.602-608
D.S. Sutar, S. Lenfant, D. Vuillaume, J.V. Yakhmi. Electronic structure of highly crystalline polyaniline by study of tunneling conduction in n⁺-Si/self-assembled monolayer/polyaniline heterostructures
Abstract:
Highly crystalline polyaniline (PANI) films were deposited on degenerated silicon (n⁺-Si) substrates covered with its native oxide (SiO₂), surface modified with amino-silane selfassembled monolayers (SAM). Scanning electron microscopy studies reveal formation of single crystal domains scattered all over the surface of film. Height and current images obtained using current-sensing AFM (C-AFM) exhibit pyramidal topography of crystallites, and inhomogeneous conductivity. As the native oxide and SAM acts as a very thin insulating layer ( 2 nm) between the metal-like substrate (degenerated Si) and the PANI film, it forms n⁺-Si/SiO₂/SAM/PANI metal-insulator-semiconductor (MIS) heterostructure. C-AFM probe was used for I–V measurements on the MIS structures and study the tunneling conduction across it. The conductance spectra derived from I–V characteristics corroborates well with the polaron-lattice band structure of doped PANI with presence of polaron bands between the HOMO-LUMO energy gap. These polaron bands are well-resolved from our CAFM measurements and they are located about 0.25 eV below the LUMO and above the HOMO.609-616
Yang Liu, Xutang Tao, Fuzhi Wang, Xiangnan Dang, Dechun Zou, Yan Ren, Minhua Jiang. Efficient non-doped pure blue organic light-emitting diodes based on an asymmetric phenylenevinylene with cis double bond
Abstract:
We reported an asymmetric phenylenevinylene with a cis double bond 2-(4-(p-tolyl)styryl)-1,4-dip-tolylbenzene (cis-TSDTB) and its use as efficient deep-blue emitter for organic light-emitting diodes (OLEDs) applications. The crystal structure of cis-TSDTB showed torsion configuration and asymmetric geometry, which make it packing in a reduced intermolecular interaction arrangement. And its single crystals showed excellent fluorescence owing to this unique molecular configuration. Typical OLEDs using cis-TSDTB as non-doped emitters exhibited saturated blue light with the CIE 1931 coordinates of (0,15, 0,10), which is quite close to the National Television Standards Committee (NTSC) blue standard. High luminescence efficiency (3,4 cd A^-1) and high brightness (9855 cd m^-2) have been realized in the device. All of these outstanding results indicated that cis-phenylenevinylene will be a promising candidate as blue light-emitting materials.617-624
A. Hadipour, B. de Boer, P.W.M. Blom. Device operation of organic tandem solar cells
Abstract:
A generalized methodology is developed to obtain the current–voltage characteristic of polymer tandem solar cells by knowing the electrical performance of both sub cells. We demonstrate that the electrical characteristics of polymer tandem solar cells are correctly predicted for both the series and parallel connection of the sub cells. The agreement with experiments allows us to investigate the effect of a reduced open-circuit voltage, short-circuit current or fill factor in one of the sub cells on the performance of the tandem cell. A low fill factor in one of the sub cells leads to a stronger reduction of the efficiency in a series configuration as compared to the parallel tandem device.625-634
Ricardo Vivas-Reyes, Francisco Núñez-Zarur, Emiliano Martınez. Electronic structure and reactivity analysis for a set of Zn-chelates with substituted 8-hydroxyquinoline ligands and their application in OLED
Abstract:
A set of four Zn organometallic complexes with aromatic substituents in the 5-position of quinoline skeleton connected through an amido linkage to the ligands were studied and compared, by using electronic descriptors such as the energy gap between the highest occupied molecular orbital (HOMO) and the energy of the lowest unoccupied molecular orbital (LUMO), molecular orbitals’ surfaces, atomic charges, global hardness, local softness, Fukui function, and global and local electrophilicity indexes. All the geometries were optimized in the scheme of density functional theory (DFT) with the hybrid functional B3LYP and the 6-31G(d) basis set. The results show that the geometries of the molecules were all distorted tetrahedral, which are unaffected by substitution when they are compared with the prototypical OLED material Znq
2. The frontier molecular orbitals HOMO and LUMO are delocalized over both ligands and are confined within a specific region. The reactivity analysis shows that the specific atoms prone to receive electrons are those associated with the lowest unoccupied molecular orbital (LUMO).635-640
Caili Wang, Fuhe Wang, Xiaodi Yang, Qikai Li, Zhigang Shuai. Theoretical comparative studies of charge mobilities for molecular materials: Pet versus bnpery
Abstract:
Charge mobility is the most important issue for organic semiconductors. We calculate the electron and hole mobilities for prototypical polycyclic hydrocarbon molecules, perylothiophene (pet) and benzo(g,h,i)-perylene (bnpery) using Marcus electron transfer theory coupled with a diabatic model and a homogeneous diffusion assumption to obtain the charge mobility. The first-principles DFT calculations show that the hole mobility is about an order of magnitude higher than the electron mobility in pet. However, we find that for bnpery, the electron and hole transports are balanced, namely, very close in mobility, indicating the possible application in light-emitting field-effect transistor. The crystal packing effects on the frontier orbital coupling are found to be essential to understand such differences in transport behaviors.641-648
Reinhard Bauer, Walter J. Finkenzeller, Udo Bogner, Mark E. Thompson, Hartmut Yersin. Matrix influence on the OLED emitter Ir(btp)2(acac) in polymeric host materials – Studies by persistent spectral hole burning
Abstract:
Fundamental photophysical properties of the phosphorescent organometallic complex Ir(btp)₂(acac) doped in the polymeric matrices PVK, PFO, and PVB, respectively, are investigated. PVK and PFO are frequently used as host materials in organic light emitting diodes (OLEDs). By application of the laser spectroscopic techniques of phosphorescence line narrowing and persistent spectral hole burning – improved by a synchronous scan technique – we studied the zero-field splitting (ZFS) of the T1 state into the substates I, II, and III. Thus, we were able to probe the effects of the local environment of the emitter molecules in the different amorphous matrices. The magnitude of ZFS is determined by the extent of spin-orbit coupling (SOC) of the T1 state to metal-to-ligand charge transfer (MLCT) states. Only by mixings of MLCT singlets, a short-lived and intense emission of the triplet state to the singlet ground state becomes possible. Thus, sufficiently large ZFS is crucial for favorable luminescence properties of emitter complexes for OLED applications. The analysis of the spectral hole structure resulting from burning provides information about the ZFS values and their statistical (inhomogeneous) distribution in the amorphous matrices. For Ir(btp)₂(acac), we found a significant value of ≈18 cm^-1 for the splitting between the substates II and III for all three matrices. Interestingly, for PVK the width of the ZFS distribution is found to be ≈14 cm^-1 – almost twice as large as for PFO and PVB. Consequently, for a considerable fraction of Ir(btp)2(acac) molecules in PVK, the ZFS is relatively small and thus, the effective SOC is weak. Therefore, it is indicated that a part of the emitter molecules shows a limited OLED performance.649-655
Li Zhao, Jian-hua Zou, Ju Huang, Chun Li, Yong Zhang, Chang Sun, Xu-hui Zhu, Junbiao Peng, Yong Cao, Jean Roncali. Asymmetrically 9,10-disubstituted anthracenes as soluble and stable blue electroluminescent molecular glasses
Abstract:
Asymmetrically 9,10-disubstituted anthracene derivatives 1a/1b have been synthesized and characterized. The new compounds exhibit a high solubility and can be easily purified by chromatographic methods. Thin solid films based on these compounds combine intrinsic amorphous morphology with pure blue emission and high solid-state photoluminescent efficiencies. These materials have been used as solution-processed active emitters in electroluminescent devices leading to interesting device performances. The effect of partial fluorination of a sub-unit of one of the compounds on the properties of the material is discussed.656-660
P. Sullivan, T.S. Jones. Pentacene/fullerene (C₆₀) heterojunction solar cells: Device performance and degradation mechanisms
Abstract:
We demonstrate power conversion efficiencies of 1,5% from molecular photovoltaic devices based on bilayer pentacene/fullerene heterojunctions under 1 sun AM1,5G simulated irradiation. Importantly, we demonstrate the independence of the device performance on active area in the range 0,05-0,65 cm², a critical consideration for future scaling up to manufacture. A degradation study under constant solar illumination shows two parallel mechanisms for degradation; a photooxidation resulting in a drop of the generated photocurrent, and a UV annealing effect reducing the fill factor, both of which can be eliminated by careful choice of analysis conditions.661-666
Pardhasaradhi Vemulamada, Gong Hao, Thomas Kietzke, Alan Sellinger. Efficient bulk heterojunction solar cells from regio-regularpoly(3,3'''-didodecyl quaterthiophene)/PC₇₀BM blends
Abstract:
Blends of regio-regular poly(3,3'''-didodecyl quaterthiophene) (PQT-12) with (6,6)-phenyl-C70-butyric acid methyl ester (PC₇₀BM) were investigated as active layers for application in organic photovoltaics (OPV). By optimizing the PQT-12:PC₇₀BM composition ratio and annealing conditions, power conversion efficiencies (g) of 1,4% could be obtained. Effects of different preparation parameters on the incident photon conversion efficiency (IPCE), short-circuit (Jsc), fill factor (FF), open-circuit voltage (Voc) and g are discussed.667-672
Chih-Yu Chang, Feng-Yu Tsai, Syue-Jhao Jhuo, Miin-Jang Chen. Enhanced OLED performance upon photolithographic patterning by using an atomic-layer-deposited buffer layer
Abstract:
This study addresses the problem of patterning-induced degradations to organic lightemitting diodes (OLEDs) by using a thin (10 Å) atomic-layer-deposited (ALD) Al₂O₃ film as both an electron-injection layer and a protecting layer for the electroluminescent material, poly[1-methoxy-4-(20-ethyl-hexyloxy)-2,5-phenylene vinylene] (MEH-PPV). With the ALD Al₂O₃ film, the OLEDs not only withstood an aggressive photolithographic patterning process without any degradation but unprecedentedly showed increased luminous efficiency (by 100%) and lowered turn-on voltage (by 19%) afterward. Although the ALD precursor, trimethylaluminum (TMA), was found to damage the MEH-PPV layer through electrophilic addition to the vinylene groups of MEH-PPV during the deposition of the Al₂O₃ film, its damaging effect was eliminated by pre-treating the MEH-PPV surface with isopropyl alcohol (IPA), whose hydroxyl groups scavenged TMA throughout the ALD process. The performance of the photo-patterned OLEDs was further improved by using a high-conductivity hole-injection layer, which increased accumulation of holes at the EL–buffer interface to enhance electron injection. The method reported herein improves the applicability of photolithography to OLED fabrication, promising to resolve the issue of patterning that has in part impeded OLED’s commercialization.673-677
Se Hyun Kim, Hoichang Yang, Sang Yoon Yang, Kipyo Hong, Danbi Choi, Chanwoo Yang, Dae Sung Chung, Chan Eon Park. Effect of water in ambient air on hysteresis in pentacene field-effect transistors containing gate dielectrics coated with polymers with different functional groups
Abstract:
We investigated the effect of water in ambient air on the hysteresis phenomena of pentacene field-effect transistors with polymer/SiO₂ gate dielectrics. The polarity of the polymer surface was controlled by using three different polymers: poly(styrene), poly(4-hydroxy styrene), and poly(4-vinyl pyridine). Water diffusion into the interface between the pentacene and the gate dielectric was driven by the polarity of the polymer surface, resulting in considerable hysteresis and degradation of device performances. The observed hysteresis behaviors can be explained in terms of donor- and acceptor-like trap formation by water molecules that have diffused between the pentacene and the gate dielectric. The different effects of these traps on hysteresis depending on the functional groups at the polymer surface were also investigated.678-686
Soo Young Kim, Jong-Lam Lee. Effect of interfacial layer thickness on the formation of interface dipole in metal/tris(8-hydroxyquinoline) aluminum interface
Abstract:
We determined the interface dipole energies between interfacial layers with different thicknesses coated on indium tin oxides (ITOs) and tris(8-hydroxyquinoline) aluminum using ultraviolet and synchrotron radiation photoemission spectroscopy. After O₂ plasma treatment on 20-nm thick metal coated ITO, the work function and interface dipole energy increased. In 2-nm thick metal coated ITO, no change in the interface dipole energy was found though the work function increased. According to the valence band spectra, 2-nm thick metals are fully oxidized, but 20-nm thick metals are partially oxidized after O₂ plasma treatment. Therefore, it is considered that the contribution of the surface dipole by the deposition of Alq₃ on 2-nm thick metal is lower, resulting in a lower interface dipole. Thus, the thickness of interfacial layer has a great impact on the formation of interface dipole.687-691
Yubin Zhang, Junfeng Ren, Guichao Hua, Shijie Xie. Effects of polarons and bipolarons on spin polarized transport in an organic device
Abstract:
A polaron–polaron interaction model is suggested to study the spin injection and transport in an organic semiconductor (OSC) device. The evolutions of spin polarons and spinless bipolarons are calculated from the drift-diffusion equations, in which both the polaron-bipolaron transition and the spin-flipping of a spin polaron are included. Then the spin polarized current is obtained. It is found that the polarons are responsible for the spin polarized transport in an OSC. Different from the case in a normal inorganic semiconductor, spinless bipolarons will affect the spin polarization of the OSC device. Finally, effects of the spin-flip time and the mobility of the carriers on the spin polarization in an organic device are discussed.692-698
Yu Wanga, Yulin Hua, Xiaoming Wub, Lijuan Zhang, Qingchuan Hou, Feng Guan, Nan Zhang, Shougen Yin, Xiaoman Cheng. High-efficiency and multi-function blue fluorescent material for organic electroluminescent devices
Abstract:
We demonstrate one high-efficiency blue fluorescent material, N-(4-((E)-2-(6-((E)-4-(diphenylamino)styryl)naphthalen-2-yl)vinyl)phenyl)-N-phenylbenzenamine, with an emissive peak of 472 nm and the hole-transporting property speculated from different devices. It can function either as the single emissive layer or as the dye doped in N,N'-dicarbazolyl-4-4'-biphenyl (CBP). The former shows a maximum current efficiency and luminance of 7,06 cd/A (0,04 mA/cm²) and 16930 cd/m², in contrast to 11,5 cd/A (4,35 mA/cm²) and 25690 cd/m² for the latter. The better performance of the latter can be attributed to the bipolar carrier transport property of CBP and the hole-blocking and electron-transporting characteristic of 4,7-diphenyl-1,1'-phenanthroline (BPhen), which resulting in a good balance of holes and electrons. Moreover, the Commission Internationale De L’Eclairage coordinates of the latter change slightly from (0,162, 0,3) to (0,148, 0,268) upon increasing the voltage from 3 V to 14 V.699-710
Piotr Matyba, Mats R. Andersson, Ludvig Edman. On the desired properties of a conjugated polymer-electrolyte blend in a light-emitting electrochemical cell
Abstract:
We present results from a systematic study on the influence of the conjugated polymer (CP) on the performance of planar light-emitting electrochemical cells (LECs) with a device structure of Au/{CP + poly(ethylene oxide) (PEO) + KCF₃SO₃}/Au. We have employed six different CPs, and we demonstrate that in order to attain a fast turn-on time and a strong light emission intensity, it is critical that the p-type doping (oxidation) potential of the CP is situated within the electrochemical stability window of the {PEO + KCF₃SO₃} electrolyte. We also find that a high ionic conductivity of the active material correlates with a small phase separation between the CP and the {PEO + KCF₃SO₃} electrolyte, and that a doping concentration of ~0,1 dopants/CP repeat unit is a generic feature of the progressing doping fronts in all investigated devices. Finally we report the first observation of a light emission zone positioned in close proximity to the positive anode in a CP-based LEC.711-720
Taek Ahn, Jin Woo Kim, Yoojeong Choi, Mi Hye Yi. Hybridization of a low-temperature processable polyimide gate insulator for high performance pentacene thin-film transistors
Abstract:
We have synthesized a novel fully soluble and low-temperature processable polyimide gate insulator (KSPI) through the one-step condensation polymerization of the monomers 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride and 4,4-diaminodiphenylmethane. Fully imidized KSPI was found to be completely soluble in organic solvents such as N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), c-butyrolactone, dimethylsulfoxide (DMSO), and 2-butoxyethanol. Thin films of KSPI can be fabricated at only 150 C and a pentacene OTFT with KSPI as a gate dielectric was found to exhibit a field effect mobility of 0,22 cm²/V s. To obtain a high performance organic thinfilm transistor (OTFT), the KSPI surface was modified in our new technique by hybridization with a non-polar side chain containing a polyimide insulator (PI). The carrier mobility of a pentacene OTFT with a hybridized polyimide gate insulator (BPI-3) was found to be 0,92 cm²/V s. Our new low-temperature processable polyimides show promise as gate dielectrics for OTFTs.721-726
Sun Hee Lee, Min Hee Choia, Seung Hoon Hana, Dong Joon Chooa, Jin Jang, Soon Ki Kwonb. High-performance thin-film transistor with 6,13-bis(triisopropylsilylethynyl) pentacene by inkjet printing
Abstract:
We have studied the performance improvement of organic thin-film transistor (OTFT) with a solution based TIPS pentacene (6,13-bis(triisopropylsilylethynyl)pentacene) by inkjet printing. The TIPS pentacene with 1,0 wt.% solution in 1,2-dichlorobenzene was used for printing of an active layer of OTFT. The OTFT printed at room temperature shows a shoulder-like behavior but it disappears for the OTFT printed at the substrate temperature of 60 °C. The OTFT on plastic exhibited an on/off current ratio of ~10⁷, a threshold voltage of -2.0 V, a gate voltage swing of 0,6 V/decade and a field-effect mobility of 0,24 cm²/Vs in the saturation region.727-734
A.M. Nardes, M. Kemerink, M.M. de Kok, E. Vinken, K. Maturova, R.A.J. Janssen. Conductivity, work function, and environmental stability of PEDOT:PSS thin films treated with sorbitol
Abstract:
The electrical properties of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) thin films deposited from aqueous dispersion using different concentrations of sorbitol have been studied in detail. Although it is well known that sorbitol enhances the conductivity of PEDOT:PSS thin films by three orders of magnitude, the origin and consequences of sorbitol treatment are only partly understood and subject of further study. By thermal annealing of spin coated PEDOT:PSS/sorbitol films and simultaneously monitoring the conductivity, we demonstrate that the strong increase in conductivity coincides with evaporation of sorbitol from the film. Hence, sorbitol is a processing additive rather than a (secondary) dopant. Scanning Kelvin probe microscopy reveals that sorbitol treatment causes a reduction of the work function from 5,1 eV to 4,8–4,9 eV. Sorbitol also influences the environmental stability of the films. While the conductivity of the pristine PEDOT:PSS films increases by about one order of magnitude at 50% RH due to an ionic contribution to the overall conductivity, films prepared using sorbitol exhibit an increased environmental stability with an almost constant conductivity up to 45% RH and a slight decrease at 50% RH. The higher stability results from a reduced tendency to take up water from the air, which is attributed to a denser packing of the PEDOT:PSS after sorbitol treatment.735-739
P. Stallinga, A.R.V. Benvenho, E.C.P. Smits, S.G.J. Mathijssen, M. Cölle, H.L. Gomes, D.M. de Leeuw. Determining carrier mobility with a metal–insulator–semiconductor structure
Abstract:
The electron and hole mobility of nickel-bis(dithiolene) (NiDT) are determined in a metal–insulator–semiconductor (MIS) structure using admittance spectroscopy. The relaxation times found in the admittance spectra are attributed to the diffusion time of carriers to reach the insulator interface and via Einstein’s relation this yields the mobility values. In this way, an electron mobility of 1:9 x 10^-4 cm²/Vs and a hole mobility of 3:9 x 10^-6 cm²/Vs were found. It is argued that the low mobility is caused by an amphoteric mid-gap trap level. The activation energy for electrons and holes from these traps is found to be 0,46 eV and 0,40 eV, respectively.740-746
Claudio Girotto, David Cheyns, Tom Aernouts, Fateme Banishoeib, Laurence Lutsen, Thomas J. Cleij, Dirk Vanderzande, Jan Genoe, Jef Poortmans, Paul Heremans. Bulk heterojunction organic solar cells based on soluble poly(thienylene vinylene) derivatives
Abstract:
We report on the comparison of photophysical and photovoltaic properties of three different soluble alkyl derivatives of the low bandgap poly(2,5-thienylene vinylene) (PTV), synthesized using the dithiocarbamate precursor route. The solubility of the precursor material in dichlorobenzene is enhanced by the addition of hexyl, dihexyl and dodecyl sidegroups to the polymer chain. The materials were characterized in solid state by means of absorption, ellipsometry and atomic force microscopy of films made from both the pristine alkyl-PTVs and alkyl-PTVs:([6,6]-phenyl C61-butyric acid methyl ester) (PCBM) mixtures in a 1:1 ratio. The materials showed an optical bandgap below 1,7 eV, derived from the absorption spectrum of the polymers. Field-effect transistors made of these materials showed hole mobilities in the range of 10^-7 to 10^-6 cm²/Vs. Bulk heterojunction solar cells made with the polymer:PCBM blend reached efficiencies above 0,6%.747-752
Leonidas C. Palilis, Paul A. Lane, Gary P. Kushto, Balaji Purushothaman, John E. Anthony, Zakya H. Kafafi. Organic photovoltaic cells with high open circuit voltages based on pentacene derivatives
Abstract:
Heterojunction organic photovoltaic devices were fabricated using C₆₀ as the electron acceptor and several pentacene derivatives with triisopropylsilylethynyl functional groups as the electron donor. The open circuit voltage (V_oc) of functionalized pentacene-based cells is significantly higher (0,57-0,90 V) than for cells based on unsubstituted pentacene (0,24 V), due to the higher oxidation potentials of these pentacene derivatives. The performance of pentacene derivative cells is limited by lower current densities than the reference pentacene/C₆₀ cell. The absorption spectra of films and solutions of pentacene derivatives closely resemble one another, leading us to conclude that these films are amorphous in nature. Weak intermolecular coupling in the derivative films results in lower charge mobility and shorter exciton diffusion lengths relative to pentacene.753-756
Mayumi Uno, Y. Tominari, J. Takeya. Fabrication of high-mobility organic single-crystal field-effect transistors with amorphous fluoropolymer gate insulators
Abstract:
High-mobility rubrene single-crystal field-effect transistors are built on highly water- and oil-repellent fluoropolymer gate insulators. Roughness is introduced at the surface once to provide good adhesion to metal films and photoresist polymers for stable electrodes. Before constructing interfaces to crystals, smoothness of the fluoropolymer surface is recovered by annealing at a moderate temperature to maximize carrier mobility. Mobility values estimated in the saturation region reproducibly exceeded 15 cm²/V s for all the 10 devices, reaching 30 cm²/V s for the best two devices. The results demonstrate that the water-repellency and smoothness of the dielectric polymers are favorable for the excellent transistor performance.757-766
Mahmut Kus, Özgül Hakli, Ceylan Zafer, Canan Varlikli, Serafettin Demic, Serdar Özçelik, Siddik Icli. Optical and electrochemical properties of polyether derivatives of perylenediimides adsorbed on nanocrystalline metal oxide films
Abstract:
We report optical and electrochemical properties of polyether derivatives of perylenediimides (PDIs) thin films formed in various materials (semiconductor, insulator, amorphous and self-assembly). Perylenediimides adsorbed on nanocrystalline TiO₂ (NT) nanocrystalline alumina (NA), amorphous silicon (PS) and neat self-assemblied (SA) films were prepared and characterized based on spectroscopic, electrochemical, spectro-electrochemical techniques. The absorption and fluorescence spectra of PDIs in chloroform exhibit vibronic features. The fluorescence quantum yields (Uf) of PDIs with end amino substituents in chloroform solutions are over 0,95, while the quantum yield of triethoxyphenyl substituted PDI Uf value is 0,024 in solution. Optical spectroscopy proves that PDIs in metal oxide thin films form aggregated type complexes. An electrochromism, a color change from red to blue/violet, is observed on metal oxide films, that indicates existence of mono and dianion forms of PDIs. Reversibility of electrochemical reductions in NT film depends on the scanning rate. However, electrochromism in NA films is stable and reversibility is independent from scanning rate. Stable mono and diaionic species are formed on NA films. SA films show broad absorption peaks during the voltammetric scan. On the other hand, the first reduction onset potentials of PDIs are almost equal to the onset potential of capacitive current of TiO₂ which lead to low efficiency in dye-sensitized solar cells.767-774
M. Grobosch, M. Knupfer. Electronic properties of the interface between the organic semiconductor α-sexithiophene and polycrystalline palladium
Abstract:
The interface properties of α-sexithiophene (α-6T) and polycrystalline, atomically clean and contaminated palladium have been studied by combined core level X-ray photoemission spectroscopy and valence band ultraviolet photoemission spectroscopy. Our data indicate for the atomically clean palladium substrate a chemical reaction between the a-sexithiophene molecules and the substrate during the formation of a monolayer of flat lying α-6T molecules. We find an interface dipole of -1,2 eV and an injection barrier for holes of about 0,7 eV. In the case of ex-situ treated, contaminated palladium as metal electrode material we find a reduced interface dipole -0.4 eV and hole injection barrier 0,5 eV. By the comparison to the results of α-sexithiophene on gold we demonstrate the importance of the strength of chemical reactions at the interface.775-782
Almantas Pivrikas, Philipp Stadler, Helmut Neugebauer, Niyazi Serdar Sariciftci. Substituting the postproduction treatment for bulk-heterojunction solar cells using chemical additives
Abstract:
Postproduction treatment of poly (3-alkylthiophene) based bulk-heterojunction solar cells is an important and required procedure in order to fabricate organic solar cells with highest power conversion efficiencies. Postproduction treatment, by means of annealing solar cells at elevated temperatures during which an external voltage is simultaneously applied, is not very suitable for large scale production schemes due to the need for well controlled environment and involvement of flexible substrates. A faster and easier method for improving the efficiency, like a simple addition of chemicals, would be highly desirable. We have studied the effect of alkyl thiol addition into bulk-heterojunction solar cells, based on poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) mixtures as reported first by Santa Barbara group [J. Peet, C. Soci, R.C. Coffin, T.Q. Nguyen, A. Mikhailovsky, D. Moses, G. C. Bazan, Appl. Phys. Lett. 89 (2006) 252105, J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, Nat. Mater. 6 (2007) 497], which gives the same final results in even higher performance of organic solar cells through increased power conversion efficiency and thus substitutes the postproduction treatment. Red-shift in optical absorption is seen in the films with alkyl thiol, resembling the absorption of thermally annealed films. Based on steady-state current–voltage characteristics and transient charge carrier extraction by linearly increasing voltage (CELIV) measurements, the conductivity of thermally annealed and films with alkyl thiol is found to be an order of magnitude higher than in films spun from pristine blends of P3HT and PCBM. Charge carrier mobility measurements indicate significant increase in carrier mobility, consistent with the improved structural order and formation of interpenetrating network between the donor and acceptor in the bulk-heterojunction solar cells.783-789
Eiji Kawabe, Hiroyuki Yamane, Ryohei Sumii, Kenji Koizumi, Yukio Ouchi, Kazuhiko Seki, Kaname Kanai A role of metal d-band in the interfacial electronic structure at organic/metal interface: PTCDA on Au, Ag and Cu
Abstract:
We analyzed the vacuum level shift (Δ) induced by the dipole layer at the interfaces between perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and noble metals (Au, Ag and Cu). The variation of D observed by ultraviolet photoelectron spectroscopy does not show a simple dependence on the metal work function, which contradicts the prediction by the induced density of interface states (IDIS) model proposed by Vázquez et al. [H. Vázquez, F. Flores, R. Oszwaldowski, J. Ortega, R. Pérez and A. Kahn, Appl. Surf. Sci 234 (2004) 107]. We found that two factors, (1) the energy separation between the lowest unoccupied molecular orbital (LUMO) of PTCDA and the metal d-band states, which results in the attractive effect due to the orbital hybridization, and (2) the coupling matrix element between the adsorbate states and the metal d-band states, which result in the repulsive effect due to the orbital orthogonalization between the adsorbate states and the metal d-band states, have a clear correlation with the Δ formation. Our results indicate that the interactions between the molecular orbitals of PTCDA and the metal d-band states play an important role in determining the interfacial electronic structure, which has not been taken into account within the framework of the IDIS model.790-796
Vipul Singh, Anil K. Thakur, Shyam S. Pandey, Wataru Takashima, Keiichi Kaneto. A comparative study of Al and LiF:Al interfaces with poly (3-hexylthiophene) using bias dependent photoluminescence technique
Abstract:
The qualitative nature of the interface of poly (3-hexylthiophene) (P3HT) with Aluminum (Al) and LiF modified Aluminum (LiF:Al) has been studied using photoluminescence (PL) spectra. It was observed that coating Al on pristine P3HT film resulted in an increase in the degree of intrachain disorder. Also, a blue shift in the peak PL intensity was observed. These observations have been explained on the basis of intrachain disorder induced by the deposition of Al via thermal evaporation on P3HT film. Furthermore, bias dependence of PL spectra for ITO/P3HT/Al and ITO/P3HT/LiF:Al type Schottky cells were studied. Under the reverse bias conditions, PL quenching was found to relate directly to the depletion layer width. The increase in depletion width was found to be higher in the Al cell as compared to that of the LiF:Al cell. However, the photocurrents were higher in LiF:Al coated cells. These observed results have been explained on the basis of the difference in their respective band alignment.797-804
Kyu-Sung Kim, Young-Min Jeon, Joon-Woo Kim, Chil-Won Lee, Myoung-Seon Gong. Blue light emitting OLED using new spiro[fluorene-7,9'-benzofluorene] host and dopant materials
Abstract:
A new spiro-type compound, 2-(10-biphenylanthracene)-spiro[fluorene-7,9'-benzofluorene] (BH-3B) containing anthracene moiety was prepared for the blue host material. Also new dopant materials, 2-[4'-(phenyl-4-vinylbenzeneamine)phenyl-spiro[fluorene-7,9'-benzofluorene] (BH-3BD) and 4-[2-naphthyl-4'(phenyl-4-vinylbenzeneamine)]phenyl (BD-1N) were successfully synthesized and a blue OLEDs were made from them. The structure of the device was as follows; ITO/DNTPD/a-NPD/Host:5% dopant/Alq₃/Al-LiF. Among all of the devices, the device obtained from BH-3B host doped with 5% BH-3BD showed the best electroluminescence characteristics. The emission peak of EL is at 456 nm and the CIE value is (0,15, 0,14). The brightness of the device is up to 5407 cd/m² at 10 V with the maximum EL efficiency of 3,4 cd/A.805-808
Jae-Hyun Lee, Dong-Seok Leem, Jang-Joo Kim. High performance top-emitting organic light emitting diodes with copper iodide-doped hole injection layer
Abstract:
Efficient top-emitting organic light-emitting diodes were fabricated using copper iodide (CuI) doped 1,4-bis[N-(1-naphthyl)-N0-phenylamino]-4,4'-diamine (NPB) as a hole injection layer and Ir(ppy)₃ doped CBP as the emitting layer. CuI doped NPB layer functions as an efficient p-doped hole injection layer and significantly improves hole injection from a silver bottom electrode. The top-emitting device shows high current efficiency of 69 cd/A with Lambertian emission pattern. The enhanced hole injection is originated from the formation of the charge transfer complex between CuI and NPB.809-815
E.A. Thomsen, D.J. Keeble, B. Lochab, P.L. Burn, H. El-Mkami, I.D.W. Samuel. Photoinduced charge separation in poly(1,4-phenylenevinylene) derivatives studied by electron paramagnetic resonance
Abstract:
We report a study of photoinduced charge separation in poly(1,4-phenylenevinylene) (PPV) derivatives with enhanced exciton dissociation using light-induced electron paramagnetic resonance (EPR). Four polymers were studied; all contained alkoxy and fluorenyl side chain units, and two of them also contained a nitro group on the 7-positon of the fluorenyl moieties. Pulsed EPR detected a light induced polaron for all the polymers with the intensity significantly higher for those with nitro groups. A second center was observed for these materials and spectra could be fitted assuming a radical anion was localized on the nitro groups. The fraction of radical anion and polaron centers was comparable, consistent with intramolecular exciton dissociation for the PPV derivatives containing the nitro groups.816-820
Mohammed F. Mabrook, Christopher Pearson, Dan Kolb, Dagou A. Zeze, Michael C. Petty. Memory effects in hybrid silicon-metallic nanoparticle-organic thin film structures
Abstract:
We report on the electrical behaviour of metal–insulator–semiconductor (MIS) structures fabricated on silicon substrates and using organic thin films as the dielectric layers. These insulating thin films were produced by different methods, including spin-coating (polymethylmethacrylate), thermal evaporation (pentacene) and Langmuir–Blodgett deposition (cadmium arachidate). Gold nanoparticles, deposited at room temperature by chemical self-assembly, were used as charge storage elements. In all cases, the MIS devices containing the nanoparticles exhibited hysteresis in their capacitance versus voltage characteristics, with a memory window depending on the range of the voltage sweep. This hysteresis was attributed to the charging and discharging of the nanoparticles from the gate electrode. A maximum memory window of 2,5 V was achieved by scanning the applied voltage of an Al/pentacene/Au nanoparticle/SiO₂/p-Si structure between 9 and -9 V.821-828
G. Derue, D.A. Serban, Ph. Leclère, S. Melinte, P. Dammanb, R. Lazzaroni. Controlled nanorubbing of polythiophene thin films for field-effect transistors
Abstract:
We present results obtained by applying the nanorubbing process to improve the electrical performance of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films. Essentially, we use a scanning atomic force microscope tip to induce a controlled deformation on the surface consisting of parallel grooves with a period imposed by the scanning parameters. The optical characterization of the rubbed zones highlights an orientation of P3HT chains along the scanning direction. When the nanorubbing process is orienting the polymer chains within the channel of a field-effect transistor, we observe that the charge carrier mobility increases (decreases) when the tip scans parallel (perpendicular) to the source-drain axis. This difference likely stems from the polymer chains orientation induced by the alignment process.829-833
Frank Verbakel, Stefan C.J. Meskers, René A.J. Janssen, Henrique L. Gomes, Antonius J.M. van den Biggelaar, Dago M. de Leeuw. Switching dynamics in non-volatile polymer memories
Abstract:
The time dependence of resistive switching in metal-metal oxide-organic semiconductormetal diodes is investigated. The switching dynamics is controlled by two intrinsic time dependences. A single switching event occurs in a time scale of 400 nanoseconds, but the maximum repetitive switching between ON- and OFF-states is limited by a ‘‘dead time of a few milliseconds. The dead time is the waiting time after programming in which a next switch is inhibited. Therefore, fast repetitive pulsing prevents the observation of non-volatile switching and limits the maximum clock rate at which these memories can be used. Understanding the origin of this dead time is crucial to future memory applications. Furthermore, the occurrence of a dead time is possibly the origin of the huge variation in
the reported switching times.834-838
Naotoshi Suganuma, Noriyuki Shimoji, Yoshiaki Oku, Suguru Okuyama, Kazumi Matsushige. Organic light-emitting transistors with split-gate structure and PN-hetero-boundary carrier recombination sites
Abstract:
Novel light-emitting transistors (OLETs) with the split-gate electrode divided into two parts for independent control of electron and hole were devised, in addition to the PN-hetero-boundary combined with the electron and hole transport materials along carrier channels. With this device structure, the on/off ratio of 1000 or more in the current and the luminance were achieved. Which is 100 times or more large compared with earlier reported single-gate type PN-hetero-boundary light-emitting transistor [N. Suganuma, N. Shimoji, Y. Oku, K. Matsushige, Novel organic light-emitting transistors with PN-heteroboundary carrier recombination sites fabricated by lift-off patterning of organic semiconductor thin films, J. Mater. Res. 22 (2007) 2982; N. Suganuma, N. Shimoji, PCT Int. Appl. WO2007/010925; N. Suganuma, PCT Int. Appl. WO2007/026703]. In this device, the luminance of about 100 cd/m2 was obtained at 15 V in the source–source voltage (also known as the source–drain voltage) with the turn-on voltage of less than 10 V. The horizontal PNhetero-boundary structure was implemented for the first time by using the photolithographic patterning of the organic semiconductor thin-films. This patterning technique can be applied in fabricating not only organic light-emitting transistors reported in this article but also organic integrated circuit or organic display.