Organic Electronics 2003 Volume 4 №02-03

Elsevier. — 153 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.45-48
Stephen R. Forrest. The road to high efficiency organic light emitting devices
Abstract:
Advances leading to very high efficiency organic light emitting devices for use in flat panel displays and solid state illumination are described. The topics addressed in the Special Issue on High Efficiency Organic Light Emitting Devices are introduced.49-59
Daisuke Mutaguchi, Kenji Okumoto, Yutaka Ohsedo, Kazuyuki Moriwaki, Yasuhiko Shirota. Development of a new class of hole-transporting and emitting vinyl polymers and their application in organic electroluminescent devices
Abstract:
A new class of hole-transporting vinyl polymers, poly{4-vinyl-40-[bis(40-tert-butylbiphenyl-4-yl)amino]biphenyl} (PVBAB) and poly{4-vinyl-40-[N,N-bis(9,9-dimethylfluoren-2-yl)amino]biphenyl} (PVFAB), and a new emitting vinyl polymer, poly(2-{4-[4-vinylphenyl(4-methylphenyl)amino]phenyl}-5-dimesitylborylthiophene) (PVPhAMB-1T), were designed and synthesized. These new vinyl polymers form smooth amorphous films with high glass-transition temperatures of ca. 200 C. PVBAB and PVFAB possess electron-donating properties, and PVPhAMB-1T possesses bipolar character with both electron-donating and accepting properties, exhibiting strong fluorescence in solution and as films. Organic electroluminescent devices using PVBAB or PVFAB as a hole-transport layer and N,N'-dimethylquinacridone-doped tris-(8 quinolinolato)aluminum as an emitting layer were thermally stable and exhibited very high performance. The use of PVPhAMB-1T as an emitting material also permitted the fabrication of a high-performance, green-emitting organic EL device.61-69
Till Spehr, Robert Pudzich, Thomas Fuhrmann, Josef Salbeck. Highly efficient light emitters based on the spiro concept
Abstract:
We present a comparison of different molecular glasses based on the spiro-concept with respect to their photoemission properties. The absorption and emission spectra as well as the photoluminescence quantum yields in solution are characterized. For thin amorphous films, prepared by vacuum vapor deposition, we examined amplified spontaneous emission (ASE) by optical pumping with picosecond pulses at 337 nm. Efficient ASE emission with thresholds of down to 1 μJ/cm² was observed.71-76
Thomas D. Anthopoulos, Jonathan P.J. Markham, Ebinazar B. Namdas, Justin R. Lawrence, Ifor D.W. Samuel, Shih-Chun Lo, Paul L. Burn. Influence of molecular structure on the properties of dendrimer light-emitting diodes
Abstract:
Iridium-based phosphorescent dendrimers have shown much promise as highly efficient light emitting materials for organic light emitting diodes (OLEDs). Here we report the effects of modifying the chemical structure on the emissive and charge transport properties of Ir(ppy)₃ based electrophosphorescent dendrimers. We investigate a novel para linked first generation (G1) iridium dendrimer. This material is compared to G1 and G2 meta linked dendrimers. We show that by blending these dendrimers into a CBP host, high external quantum efficiencies of over 10% and luminous efficiencies of 27 lm/W can be achieved.77-87
Vadim I. Adamovich, Steven R. Cordero, Peter I. Djurovich, Arnold Tamayo, Mark E. Thompson, Brian W. D 'Andrade, Stephen R. Forrest. New charge-carrier blocking materials for high efficiency OLEDs
Abstract:
Three strategies for preparing high efficiency OLEDs are demonstrated, which involve the use of hole and electron blocking layers. The first of these strategies involves the use of a cyclometallated iridium compound (bis(2-(4,6-difluorophenyl) pyridyl-N,C2')iridium(III) picolinate, FIrpic) as a hole-blocking material for green and blue emissive OLEDs. Devices which utilized FIrpic as a combined hole blocking and electron transporting layer gave external quantum efficiencies 14% (device structure: anode/HTL/EL/FIrpic/cathode, HTL=hole transport layer, EL=emissive layer). When the FIrpic layer of this device was replaced with bathocuproine (BCP), the device efficiency dropped to 12%. A host-guest approach to the formation of a hole blocking layer (HBL) has also been demonstrated. FIrpic was doped into two different wide energy band-gap organic matrix materials (i.e. octaphenyl–cyclooctatetraene, OPCOT, and 1,3,5-tris-phenyl-2-(4-biphneyl)benzene, SC5) forming a mixed HBL. Devices with doped OPCOT gave quantum efficiencies comparable to those with a BCP HBL, while the SC5 based devices gave higher efficiency than their BCP blocked counterparts. When blue electrophosphorescent devices are prepared in a conventional OLED structure (i.e. anode/HTL/EL/HBL/ETL/cathode), excessive HTL emission is often observed, resulting from electron leakage from the doped CBP layer into the HTL. This electron leakage can be eliminated by inserting an electron blocking layer (EBL) between the HTL and luminescent layers. Both fac-tris(1-phenylpyrazolato,N,C20)iridium(III) (Irppz) and Iridium(III) bis(1-phenylpyrazolato,N,C20)(2,2,6,6-tetramethyl-3,5-heptanedionato-O,O) have been used as efficient EBLs. The insertion of an EBL leads to both improved color purity and quantum efficiency, relative to devices without EBLs. For example, a white emitting device with the structure ITO/HTL/EL/HBL/ETL/LiF/Al gave an external efficiency of 1.9% and nearly exclusively HTL emission. Addition of a 100 AA Irppz layer between the HTL and EL gave a device with an external quantum efficiency of 3.3% and electroluminescence from only the EL.89-103
M. Pfeiffer, K. Leo, X. Zhou, J.S. Huang, M. Hofmann, A. Werner, J. Blochwitz-Nimoth. Doped organic semiconductors: Physics and application in light emitting diodes
Abstract:
In this paper, we discuss recent experiments which prove that evaporated organic films can be efficiently doped by coevaporation with organic dopant molecules. Key advantages for devices are the high conductivity and the formation of ohmic contacts despite large energetic barriers. For p-type doping, efficient doping is possible for a variety of polycrystalline and amorphous materials. Despite the differences in the microscopic behavior, all basic effects known from doped inorganic semiconductors are found in organics as well. However, efficient n-type doping with stable molecular dopants is still a challenge. Organic light emitting diodes (OLED) with conductivity doped transport layers show significantly improved properties: For instance, we have achieved a brightness of 100 cd/m2 already at a voltage of 2.55 V, well below previous results for undoped devices. The advantages of doping are even more pronounced for top-emitting, inverted OLED structures: Due to the ohmic contacts nearly independent of the contact properties, it is possible to realize inverted topemitting devices with parameters comparable to standard devices. Our doping technology is thus a significant advantage for active-matrix OLED displays and other displays on opaque substrate.105-111
Shizuo Tokito, Mitsunori Suzuki, Fumio Sato, Motoaki Kamachi, Kourou Shirane. High-efficiency phosphorescent polymer light-emitting devices
Abstract:
In this paper, we describe the performance of polymer light-emitting devices (PLEDs) that are based on phosphorescent polymers involving a carbazole unit and an iridium-complex unit. The PLEDs exhibit red, green or blue emission, depending on the phosphorescent polymer used in the emissive layer. We achieved highly external quantum efficiencies of 5.5%, 9% and 3.5% in respective red, green and blue PLEDs by selecting the electron transport material for the emissive layer and optimizing the content of the iridium-complex unit in the phosphorescent polymer chain. Furthermore, we demonstrated white emission in PLEDs by using blue-phosphorescent and red-phosphorescent polymers. An external quantum efficiency of 4.5% was obtained for this emission.113-121
Leonidas C. Palilis, Hideyuki Murata, Manabu Uchida, Zakya H. Kafafi. High efficiency molecular organic light-emitting diodes based on silole derivatives and their exciplexes
Abstract:
Here, we report the performance of molecular organic light-emitting diodes (MOLEDs) using novel fluorescent silole derivatives as highly efficient blue and green-emitting organic materials. Three silole derivatives, namely 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PPSPP), 9-silafluorene-9-spiro-10-(2',3',4',5'-tetraphenyl)-1'H-silacyclopentadiene (ASP) and 1,2-bis(1-methyl-2,3,4,5,-tetraphenylsilacyclopentadienyl)ethane (2PSP), with high solid-state photoluminescence (PL) quantum yields of 0.85, 0.87 and 0.94, respectively, were used as emissive materials. Another high electron mobility silole derivative, 2,5-bis(2',2''-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), was used as the electron transport material. MOLEDs using these siloles as emitters and N,N'-diphenyl-N,N'-(2-napthyl)-(1,1'-phenyl)-4,4'-diamine (NPB) or N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,40diamine (TPD) as the hole transport material show low operating voltages of 4–4.5 V at a luminance of 100 cd/m² and high external electroluminescence (EL) quantum efficiencies of 3.4–4.1% at ~ 100 A/m². MOLEDs based on PPSPP and PyPySPyPy exhibit red-shifted EL spectra which are assigned to exciplexes formed at the interface between the hole transporting layer NPB or TPD and the PPSPP or PyPySPyPy light-emitting layer, respectively.123-130
Seth Coe-Sullivan, Wing-Keung Woo, Jonathan S. Steckel , Moungi Bawendi, Vladimir Bulovi . Tuning the performance of hybrid organic/inorganic quantum dot light-emitting devices
Abstract:
The luminescence of inorganic core-shell semiconductor nanocrystal quantum dots (QDs) can be tuned across much of the visible spectrum by changing the size of the QDs while preserving a spectral full width at half maximum (FWHM) as narrow as 30 nm and photoluminescence efficiency of 50% [Journal of Physical Chemistry B 101 (46) (1997) 9463] [1]. Organic capping groups, surrounding the QD lumophores, facilitate processing in organic solvents and their incorporation into organic thin film light-emitting device (LED) structures [Nature 370 (6488) (1994) 354] [2]. A recent study has shown that hybrid organic/inorganic QD-LEDs can indeed be fabricated with high brightness and small spectral FWHM, utilizing a phase segregation process which self-assembles CdSe(ZnS) core(shell) QDs onto an organic thin film surface [Nature 420 (6917) (2002) 800] [3]. We now demonstrate that the phase segregation process can be generally applied to the fabrication of QD-LEDs containing a wide range of CdSe particle sizes and ZnS overcoating thicknesses. By varying the QD core diameter from 32 A to 58 A, we show that peak electroluminescence is tuned from 540 nm to 635 nm. Increase in the QD shell thickness to 2.5 monolayers ( 0.5 nm) improves the LED external quantum efficiency, consistent with a F€orster energy transfer mechanism of generating QD excited states. In this work we also identify the challenges in designing devices with very thin ( 5 nm thick) emissive layers [Chemical Physics Letters 178 (5–6) (1991) 488] [4], emphasizing the increased need for precise exciton confinement. In both QD-LEDs and archetypical all-organic LEDs with thin emissive layers, we show that there is an increase in the exciton recombination region width as the drive current density is increased. Overall, our study demonstrates that integration of QDs into organic LEDs has the potential to enhance the performance of thin film light emitters, and promises to be a rich field of scientific endeavor.131-141
A. van Dijken, A. Perro, E.A. Meulenkamp, K. Brunner. The influence of a PEDOT:PSS layer on the efficiency of a polymer light-emitting diode
Abstract:
The mutual influence of the constituent layers in a polymer light-emitting diode on the photoluminescence quantum efficiency of the light-emitting polymer is investigated. It is shown that a specific chemical interaction can occur between poly-(3,4-ethylenedioxythiophene):poly-(styrenesulphonic acid) (PEDOT:PSS) and poly-(p-phenylenevinylene) (PPV). It is further shown that PEDOT:PSS has a considerable effect on molecular dopants dispersed in the polymeric host. In the case of PPV the interaction with PEDOT:PSS results in the creation of defect states in an interface region between the PEDOT:PSS layer and the PPV layer. The presence of these defect states results in a considerable quenching of the PPV photoluminescence. For devices based on PPV, already at voltages below the built-in voltage the PEDOT:PSSinduced defect states can be filled with charge carriers, a process that can be monitored with field-dependent photoluminescence measurements and with electrical impedance measurements. Filling of defect states with charge carriers constitutes a dynamic photoluminescence quenching mechanism, that is present in addition to the previously mentioned static quenching mechanism due to the presence of defect states. As a result, the photoluminescence intensity of PPV in a working device under operating conditions can be quenched by at least 20% with respect to the intensity at zero applied volt. To indicate that these effects involve a specific interaction between PEDOT:PSS and PPV, it is shown that the effects are absent in devices based on poly-(2,7-spirofluorene) (PSF) instead of PPV, and that the effects are independent of the type of cathode material. Defect states in PPV can be created through an electrophilic addition reaction involving the PPV vinylene bond and protons from PEDOT:PSS.143-148
Jun Yeob Lee, Jang Hyuk Kwon, Ho Kyoon Chung. High efficiency and low power consumption in active matrix organic light emitting diodes
Abstract:
Phosphorescent full color 2.2 inch quarter common intermediate format (QCIF) active matrix organic light emitting diodes (AMOLED) have been developed for mobile phone application. Red and green phosphorescent emitters have been adopted as light emitting components and the efficiency of the AMOLED as measured was enhanced up to 10.9 cd/A at a white brightness of 100 cd/m². The power consumption of the AMOLED was reduced below 200 mW due to high efficiency and low driving voltage.149-154
Z.R. Hong, O. Lengyel, C.S. Lee, S.T. Lee. A mechanistic study of exciplex formation and efficient red light-emitting devices based on rare earth complexes
Abstract:
The mechanisms of exciplex formation between hole-transporting material N,N0-diphenyl-N,N0-bis(3 methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD) and electron-transporting materials tris(dibenzoylmethanato)-mono(bathophenanthroline)-
rare earth (RE(DBM)₃bath) in TPD/RE(DBM)₃bath bilayer electroluminescence (EL) devices were studied. The formation process was identified by using fluorescent dye as dopant. It was found that interaction between the excited states of RE(DBM)3bath and the ground state of TPD molecules resulted in the exciplex. The recombination zone of the TPD/RE(DBM)₃bath device was proved to be mainly in the RE(DBM)₃bath layer near the organic interface. On the other hand, by using dopant as efficient energy acceptor in RE-complex hosts, we found that exciplex emission was quenched thoroughly and efficient red light emission was observed, proving that RE(DBM)3bath may act as an efficient energy donor in EL devices. In the case of Eu3þ as the central ion, maximum EL efficiency and highest brightness of red light emission reached 2.6% and 2000 cd/m², respectively.155-164
Raymond C. Kwong, Michael S. Weaver, Min-Hao Michael Lu, Yeh-Jiun Tung, Anna B. Chwang, Theodore X. Zhou, Michael Hack, Julie J. Brown. Current status of electrophosphorescent device stability
Abstract:
Operational stabilities of high-efficiency green and red electrophosphorescent bottom-emission devices with various emitting dopants have been studied. Operational lifetimes of 10,000 h or more, operated at an initial brightness of 600 and 300 cd/m² for green and red, respectively, are reported. Operational stabilities of top-emission electrophosphorescent devices and electrophosphorescent devices built on barrier-coated plastic substrates have also been studied and show lifetimes 5000 and 2000 h, respectively, under display level brightness conditions.165-177
Ruidong Xia, George Heliotis, Yanbing Hou, Donal D.C. Bradley. Fluorene-based conjugated polymer optical gain media
Abstract:
We report a detailed investigation of the optical gain properties of a series of semiconducting polyfluorenes with photoluminescence emission spectra that span the entire visible spectrum. Stimulated emission was demonstrated at low pump pulse energies (> 0.1 μJ for 10 ns, 10 Hz pulses) in the blue, green and red parts of the spectrum via amplified spontaneous emission (ASE) measurements on asymmetric slab waveguides. These structures exhibit large net gains g ≤74 cm ^-1, and corresponding gain cross-sections σ > 7x10 ^-16 cm², together with very low loss coefficients, aP3:2 cm
1. The spectral location of the maximum waveguide amplification can be widely tuned (DkP30 nm) by controlling the allowed propagating modes or by blending the active emitter with a second larger optical gap polyfluorene. Our results confirm that fluorene-based polymers are attractive gain media for use in highly tuneable solid-state polymer lasers and amplifiers.179-189
Anna Kohler, Joanne Wilson. Phosphorescence and spin-dependent exciton formation in conjugated polymers
Abstract:
The mechanism for the formation of singlet and triplet states in conjugated polymer-based light emitting diodes (LEDs) is crucial in determining the overall efficiencies of these devices. If simple spin statistics apply then singlets and triplets should be formed in the ratio 25:
75. However, the non-emissive nature of triplet states in these materials, as well as other loss mechanisms within the devices, mean that this ratio is not straightforward to measure. Nevertheless, recent experimental advances have made it possible to determine many of the properties of triplet states. Here we review what is now known about triplet states and their photophysics in conjugated polymers. We place particular emphasis on measurements of the singlet generation fraction in LEDs, and discuss the experimental techniques that have been used, such as direct comparison of photoluminescence and electroluminescence efficiencies, triplet absorption cross section measurements and magnetic resonance measurements. All of these techniques give values for the singlet generation fraction in polymers that are significantly larger than the 25% expected, and many of them have also shown that in shorter oligomers this value decreases to be closer to 25%. We also give a brief overview of recent theories for the processes of singlet and triplet formation in polymer devices.191-197
M. Segal, M.A. Baldo. Reverse bias measurements of the photoluminescent efficiency of semiconducting organic thin films
Abstract:
Photoluminescent (PL) efficiency measurements are an essential component of the characterization of high efficiency organic light emitting devices. We demonstrate that a reverse bias technique for measuring PL efficiency possesses several advantages. Using a two-tone synchronous detection scheme, it is found that nonlinearities in the reverse bias photoresponse are a probe for the presence of charge transfer states that may confuse the interpretation of PL data. Compared to alternative techniques, reverse bias measurements are also relatively unaffected by photoinduced changes in material properties.