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Fast-Response Organic Light-Emitting Diode for Interactive Optical CommunicationTakeshi Fukuda1 and Yoshio Taniguchi2[1] Department of Functional Materials Science, Saitama University, Japan[2] Shinshu University, Japan1. Figure 5 shows the relative output EL intensity as a function of modulation frequency for the two OLEDs, that is, devices A and B with rubrene doped Alq3 and Alq3 as EMLs, respectively. Figure 6(a) shows the relationship between the applied pulse voltage and the rise time of output EL intensity of OLEDs with different thicknesses in the ETL. Figure 7 shows the relative EL intensity of OLEDs with different thicknesses of the ETLs when the sine wave voltage was applied to the device. Figure 9(a) shows the relationship between the relative EL intensity and the frequency of the applied sine wave voltage for the three OLEDs (devices C, D, and E).
Figure 13(a) shows the influence of PL intensity on the frequency of the violet laser diode for two organic materials, DSB and Alq3.
Figure 13(b) shows the relationship between the cutoff frequency of PL intensity and the fluorescence lifetime of the organic emitting material. Figures 14(a) and (b) show rise and decay times of the output EL intensity while applying the pulse voltage with the duration of 1 ?s. Figure 15 shows absorption spectra of organic neat thin films of guest materials (DSB, DPVBi, and BCzVBi) and the PL spectrum of the CBP neat film. Figure 16(a) shows the relationship between the frequency of an applied sine wave voltage and the output relative EL intensity of the devices L and M, which consisted of ZnS and Alq3 as ETLs, respectively. Figure 16(b) shows the influence of the sine wave voltage on the cutoff frequency for devices L and M. Figure 18 shows the input electrical signal and the output optical signal as a function of the time. 2001 Nearly 100% internal phosphorescence efficiency in an organic light emitting device, J. 2001 Electron mobility in tris(8-hydroxyquinoline)aluminum thin films determined via transient electroluminescence from single- and multilayer organic light-emitting diodes, J. 2007 Transient property of optically pumped organic film of different fluorescence lifetimes, Appl. 2007 Influence of carrier-injection efficiency on modulation rate of organic light source, Opt. 2007 Enhanced Modulation Speed of Tris(8-hydroxyquinoline)aluminium-Based Organic Light Source with Low-Work-Function Electrode, Jpn. 2003 Dynamic turn-on behavior of organic light-emitting devices with different work function cathode metals under fast pulse excitation, J. 2006 Bipyridyl oxadiazoles as efficient and durable electron-transporting and hole-blocking molecular materials, J.
2002 Application of Organic Light Emitting Diode Based on the Alq3 Emissive layer to the Electro-Optical Conversion Device, IEICE Trans.
2007 The role of magnetic fields on the transport and efficiency of aluminum tris(8 -hydroxyquinoline) based organic light emitting diodes, J. 2005 Magnetic field effects and CIDEP due to the d-type triplet mechanism in intra-molecular reactions, J. 2007 Tuning magnetoresistance between positive and negative values in organic semiconductors, Nature Mat. 2003 Magnetic field effects on emission and current in Alq3-based electroluminescent diodes, Chem. 1992 Fission of a higher excited state generated by singlet exciton fusion in an anthracene crystal, Chem. 2007 Characterization of triplet-triplet annihilation in organic light-emitting diodes based on anthracene derivatives, J. 2009 Magnetic field dependent triplet-triplet annihilation in Alq3 -based organic light emitting diodes at different temperatures, J. 2007 Tunnel magnetoresistance of C60 -Co nanocomposites and spin-dependent transport in organic semiconductors, Phys. 2007 Determination of charge-carrier transport in organic devices by admittance spectroscopy: Application to hole mobility in ?-NPD, Phys. 2006 Control of magnetic-field effect on electroluminescence in Alq3 -based organic light emitting diodes, Appl. 2006 Fractions of singlet and triplet excitons generated in organic light-emitting devices based on a polyphenylenevinylene derivative, Phys.
1995 Magnetic field effects on the photodissociation reaction of triphenylphosphine in non-viscous homogeneous solutions, Chem. 2008 The magnetic field effect on the transport and efficiency of group III tris(8 -hydroxyquinoline) organic light emitting diodes, J. 2007 Enhancement of quantum efficiency of organic light emitting devices by doping magnetic nanoparticles, Appl.
2009 Very High Magnetocurrent in Tris-(8 -hydroxyquinoline) Aluminum-Based Bipolar Charge Injection Devices- Appl.
Above photograph Organic devices emitting red, green, blue and white light, developed by the EPMM group [photograph by Nathaniel Stevenson].
High Efficiency Red Phosphorescent Organic Light-Emitting Diodes with Simple StructureRamchandra Pode11 and Jang Hyuk Kwon21[1] 1Department of Physics,, Korea1. Organic light emitting diodes (OLEDs) based on multi-layer OLED structures were started in the middle of 80th.
OLEDs realized a great success in the display market, since OLEDs have rapid response less than micro-second, full-color and high contrast ratio.
As organic semiconductors, we combine electron and hole transport layers with an emitter layer to enhance EL efficiency. Concept of the interactive visible optical communication system using the OLED and the OPD with the high response speed. Cross sectional view of the conventional OLED structure and limiting factors of the transmission speed of the OLED. Relative EL intensity while applying the sine wave voltage for (a) the device A with rubrene:Alq3 and (b) the device B with Alq3 as EMLs.
Influence of the pulse voltage on (a) rise and (b) decay times of the OLEDs with different thickness of the ETL (Alq3). Relative EL intensity while applying the sine wave voltage for OLEDs with different thicknesses of the ETLs. Influence of the pulse voltage on (a) rise and (b) decay times of the OLEDs with different thickness of the HTL (?-NPD).
Frequency dependence of relative EL intensity for devices F, G, and H with Ca, Al, and MgAg as metal cathodes, respectively. Schematic configuration of the experimental setup to estimate the influence of the relative PL intensity while irradiating the violet laser diode.
Absorption spectra of DSB, DPVBi, and BCzVBi neat films and the PL spectrum of the CBP neat film. Input electrical signal (yellow) and the output optical signal (pink) as a function of the time.
Introduction In recent years, many types of electronic equipment have come into wide use in our lives. Here, the barrier height was calculated to be the difference between the work function of the metal cathode and the LUMO level of Alq3 used as the ETL.
The response speed of the OLED also increased when the low-work function metal electrode was used for the EIL-inserted OLED.
For both organic films, PL intensity decreases with increasing frequency of the violet laser diode due to the decay time of the PL. This result is a consequence of fluorescence lifetimes without the influences of the capacitance and the carrier mobility, which are known to affect the response speed of the OLED. The PL spectrum of CBP showed the peak wavelength at 411 nm, and the PL intensity rapidly decreased in both shorter and longer wavelengths.
The relative EL intensity of the organic-inorganic hybrid device (device L) showed higher response speed compared to the OLED (device M). By comparing the devices L and M, we found that the cutoff frequency was influenced by the applied voltage for only the device L with ZnS.
The output optical signal was received using the pen-type receiver module when the Figure 18.Input electrical signal (yellow) and the output optical signal (pink) as a function of the time.
I-V and light emitting properties of (a) fluorescent and (b) phosphorescent devices without magnetic field.
IntroductionThis chapter aims at reviewing magnetic field effects (MFE) in organic light emitting diodes (OLED) with an emphasis on our study under high magnetic field up to 9 T.
It occurs when the density of the excited states created from radical pairs is high and they are migrating. In contrast to the results of the fluorescent device, it did not show the magnetic field dependence. We observed that the conductance of the minority carrier changes linearly as a function of magnetic field as shown in Fig. Since the EL intensity is determined only by the charge balance if the carrier recombination rate is proportional to the radiation (Scott et al.
However, our experiments showed that the current remains almost constant as a function of the magnetic field in the mid ~ high B range.
This result indicates that intersystem crossing occurs so fast in RPs and excited state molecules that Larmor precession in ?g mechanism does not affect the recombination kinetics. Maximum EQE characteristics of fabricated five red PHOLEDs with increasing R-EL unit from 1 to 5. I-V-L characteristics of fabricated three PHOLEDs ; (a) current-voltage characteristics (b) luminance-voltage characteristics (c) current efficiency-luminance (d) power efficiency luminance. Current density (J)-Voltage(V)-Luminance (L) and Efficiency characteristics of single layer red PHOLEDs.
Luminance-current Efficiency characteristics of various single layer devices fabricated with different locations of doped regions. Commission Internationale de l’Eclairage (CIE) color emission coordinates of red PHOLEDs described in Fig. Stern-Volmer plot showing the effect of Bebq2 fluorescence quenching by (Ir(phq)2acac) dopant. Spectral overlapping of the photoluminescence spectrum from Bebq2 and the absorption spectrum from Ir(phq)2acac.Table 8.
IntroductionAfter the first report of electroluminescence in anthracene organic materials in monolayer devices in 1963 by Pope et al.
A large effort has been made to increase the emission efficiency and improve the stability of OLEDs. Ultra thin films of organic material (typically 100nm) can be deposited on transparent electrode (ITO) substrates by various techniques. Although OLEDs using fluorescence materials achieved high reliability, EL efficiencies are low because of their limitation of the singlet exciton production efficiency (~25 %) by electrical excitation. Especially, mobile phones and personal computers have been widely used by many people, and this fact causes the drastically change of our lives. Here, the EL intensities at various modulation frequencies are normalized with respect to the EL intensity at a frequency of 100 kHz. The relative EL intensity at the high frequency region increased with decreasing the thickness of the ETL. Here, the EL intensities at various frequencies are normalized with respect to the EL intensity at a frequency of 100 kHz.
This experimental result showed that cutoff frequencies were 160 MHz and 20 MHz for DSB and Alq3, respectively. Therefore, we can estimate the direct influence of the fluorescence lifetime on the response time of the OLED.
Both rise and decay times decreased with increasing pulse voltage due to the high carrier mobility at the high electric field. This result indicates that the low electron mobility of Alq3 causes the low response speed. The low drive voltage of the OLED has been required for all the applications, such as mobile phones, flat panel displays, general lightings, and visible optical communications. The frequency of the input electrical signal was 1 Mbps.pseudo-random signals were applied to the OLED.
This subject includes organic spintronics in general, which is a hot subject attracting many researchers recently. The successful OLED technology has already been put to practical use in car-monitor, mobile phone display and organic TV.
Also, we can realize extremely low driving voltage of OLEDs by proper selection of organic material and device configuration. Since exciton production ratio under electrical pumping is 1:3 for the singlet-to-triplet exciton, phosphorescence materials are crucial for high EL efficiency. In addition, we can connect global networks using mobile phones and personal computers, and we can get much information in a short time without moving.
It was observed that the relative EL intensity of device A with the rubrene doped Alq3 EML is higher than that of device B, which has the Alq3 EML. The cutoff frequency corresponds to the responses speed of the OLED; therefore, this result indicates that the response speed of the OLED was improved with decreasing capacitance of the emitting area. This result indicates that the response speed increased with decreasing thickness of the ETL, which corresponds to the electron travelling length from the metal cathode to the EML.
The difference of the cutoff frequency can be explained by the fluorescence lifetime of the organic material. The transient characteristic of PL is strongly dependent on the fluorescence lifetime, and the response is considered to increase utilizing the short fluorescence lifetime of the organic material as a light-emitting layer of OLEDs. In addition, the rise times of devices I (DSB), J (DPVBi), and K (BCzVBi) were 58, 345, and 257 ns at the pulse voltage of 5 V, respectively.
The peak wavelengths of absorption spectra of DSB, DPVBi, and BCzVBi were 418, 354, and 372 nm, respectively. On the other word, we can realize the increased response speed utilizing the ZnS layer with high electron mobility as the ETL. Since singlet-triplet conversion in excitons is critically important in the current efficiency of OLEDs, spintronics aspects of OLEDs should be studied in detail.


The characteristic magnetic field strength which gives the inflection points in (a)-(c) greatly differs from each other as discussed in the following. It will be interesting to measure the magnetoconductance of organic semiconductors doped with phosphorescent dyes.5.
Now, OLED developments aiming for high efficiency and long lifetime have been extensively studied both in academia and industry in the worldwide. Recently OLEDs using phosphorescence materials achieved exciton production efficiency of ~100 %, resulted in 20% external EL efficiency.
Nowadays, several mobile networks are widely used, such as, Bluetooth, ultra wideband, ZigBEE, and so on. In the case of the institutive optical communication system, the large emitting area is important factor to connect between the OLED and the OPD. The electron injection time is calculated from the electron mobility, the thickness, and the applied electric field.
On the other hand, the rise time was little influenced by the thickness of the HTL ranged from 20 nm to 40 nm, as shown in Figs.
The relative EL intensity at the high frequency region corresponds to the response speed of the OLED.
The cutoff frequency increased with decreasing barrier height, which affects the efficiency of injecting electrons into the organic layer from the metal cathode. 9(a), we found that the cutoff frequency increased by inserting Liq layer for all the devices with the different cathode materials.
Based on previous researches, the energy transfer efficiency of dye-doped OLEDs depends on the overlap integral of the emission spectrum of the host material and the absorption spectrum of the guest material.
However, due to the difficulty in the fabrication of stable devices, the number of the researches has been limited.We have found two things up to now, by making very stable OLEDs and measuring them under high magnetic fields.
It is expected that the contributions can be elucidated by measuring the device properties in the wide range of the magnetic field. Magnetoconductance measurement of unipolar devicesIn order to investigate the charge balance factor which might influence the EL efficiency, we measured the magnetoresistance of the majority and minority carriers in ?-NPD and Alq by making the unijunction devices with different work function electrodes (Au and Cs).
This is reasonable because Alq layer (200nm) is much thicker than the thickness of emission region of ordinary devices. ConclusionWe reviewed recent studies on organic spintronics and MFE on chemical reactions in relation to the MFE on OLEDs. Furthermore, the global computer networks will be used unconsciously without thinking the connection in near future, and many researchers demonstrated unique concepts of intuitive interface modules. Therefore, this result indicates that device C has a higher response speed than devices D and E. Here, Li has low work function of 2.9 eV, and thus appears to be a good candidate for injecting electrons into the Alq3 layer.
In generally, the large capacitance of organic layers limits the response speed of OLEDs owing to the large emitting area and the thinness of organic layers compared to semiconductor emitting devices. The interfacial mixing and the damage caused by the electrode formation might also justify this assumption. Therefore, we can estimate the electron injection time of 450 ns, 250 ns, 150 ns, and 50 ns for OLEDs with thicknesses in 40 nm, 30 nm, 20 nm, and 10 nm, respectively. It is known that diluted Li-metal alloys can act a cathode and exhibit much better transient characteristics than a pure metal cathode. The measured spectral overlap was different from each combination of the host-guest system, and the largest spectral overlap was achieved using DSB as a guest material.
Effect of high magnetic field on organic light emitting diodes In the previous sections, we have reviewed the MFEs on charge transport in organic semiconductor devices including OLEDs and chemical reaction kinetics.
We also measured magnetoconductance of unipolar devices and observed that only minority carriers show significant magnetoconductance decreasing linearly with the magnetic field (15% at 9T in Alq).
Organic electroluminescent devices having improved power conversion efficiencies by doping the emitting layer were also realized around the same time by the Kodak group. The measurement results of the rise times were longer than the estimated electron injection times. Therefore, we can conclude that the rise time of optical response is primarily associated with the carrier dynamics between the applying voltage and the generation of light. Therefore, efficient Forster energy transfer from the host material to the guest material is considered to be realized in the case of DSB doped CBP. However, we can realize the error-free data transmission at a speed of 1 Mbps using the transceiver module with the OLED. Since some of the exciton-related effects saturate at relatively low magnetic field (~ 1T), it is expected that the contribution of the above mechanisms in MFE will be separated if high magnetic field is applied. In addition, the carrier transport time from the electrode to the EML is related with the thickness of HTL and the ETL. Figure 7.Relative EL intensity while applying the sine wave voltage for OLEDs with different thicknesses of the ETLs. Figure 9.a) Frequency dependence of relative EL intensity for devices C, D, and E with Ca, Al, amd MgAg as metal cathodes, respectively. Combination of host-guest materials in EMLIn the previous chapter, the fluorescence lifetime of the EML is important factor to realize the fast response speed of the OLED. This system consisted of the transceiver module with the OLED and the pen-type receiver module with the semiconductor photo diode at a point, as shown in Fig 16.
In principle, MFE on charge transport and recombination has similarity with chemical reaction under magnetic field, and the terminology and concept should be parallel between these subjects. In this section, we present our experimental study of MFE on OLEDs under high magnetic field (Goto et al., 2010). The voltages were chosen to give current in the range of 10~100 ?A and the results are shown after normalization at zero field. In addition, the efficient energy transfer from the host material to the guest material is also key parameter for the increased response speed. These values were estimated time-resolved PL intensity using a femtosecond pulse laser with the center wavelength of 390 nm and a streak camera. Organic-inorganic hybrid deviceIn the previous section, we showed that the low electron mobility of the ETL prevents the improved response speed of the OLED. When the point of the pen-type receiver module approaches the emitting area of the OLED, you can get information from the OLED. ConclusionIn this chapter, we demonstrated fast-response OLEDs for the intuitive visible optical communications.
We will try to combine the current knowledge of organic charge transport, OLEDs, spintronics and MFEs of chemical reaction to make a unified picture of these issues. It is easily noticed that the MFEs on the conductivity of the majority carriers (holes in ?-NPD and electrons in Alq) are negligible, whereas linear decreases in the conductivity was observed for the minority carriers (electrons in ?-NPD and holes in Alq). Usually, the operating voltage for higher brightness was much higher than the thermodynamic limit which is 2.4 eV for a green device. Such short fluorescence lifetimes were assumed to give little effect on response speed of device. Furthermore, the emitting area was 2 mm x 2mm, and the many people can touch without thinking the precious alignment between the pen-type receiver module and the OLED.The fabrication process and the experimental results are discussed as bellows.
We successfully achieved the more than 20 MHz by optimizing device parameters, such as the emitting area, the thickness of the carrier transport layer, the metal cathode, the fluorescence lifetime of the emitting material, the combination of the host-guest material, and the semiconductor ETL. Preparation of compact and stable OLEDs for the measurement in high magnetic fieldSince the sample space of the high field magnet is small, the sample must be as compact as possible, while maintaining the stability to warrant the reliable measurement.
Chemical doping with either electron donors (for electron transport materials) or electron acceptors (for hole transport materials) can significantly reduce the voltage drop across these films. This is because that the electron mobility of Alq3 is much lower than the hole mobility of ?-NPD. We deposited cupper-phthalocyanine (CuPc) as a hole injection leayr, ?-NPD as a HTL, rubrene in Alq3 as an EML, Alq3 as an ETL, and LiF as an EIL subsequently, upon the ITO-coated glass substrate. Finally, we also demonstrated the demonstrator of the institutive optical data transmission system using the OLED as the transceiver. These devices with either HTL or ETL doped layer show improved performance; but the operating voltages were still rather higher than the thermodynamic limit.
Furthermore, the emitting area of the OLED and the receiving area of the OPD can be controlled by changing the deposition areas of electrodes, which sandwiches organic layers. The ZnS layer has higher electron mobility compared to the organic electron transport material. If the pen-type receiver module is touched the emitting area of the OLED, we can get the pseudo-random signals without thinking the precious alignment between the OLED and the pen-type receiver module.7.
Spins in OLEDsThe roles of spins in OLEDs and other organic devices are discussed well but have not been clarified quantitatively in experiments.
Subsequently, Leo and his group proposed the concept of p-type doped HTL and n-type doped ETL (J.
Therefore, we have proposed that the free space optical data transmission is suitable for the next generation visible optical communication system due to the alignment-less connection. Energy gap between metal the cathode and the adjacent organic layerIn generally, holes and electrons (carriers) are injected from an anode and a cathode, respectively.
Influence of fluorescence lifetime of EML and response speed of OLEDThe fluorescence life time of organic emitting materials is important factor to determine the response speed of the OLED.
Therefore, higher response speed can be realized compared to the OLED even though the emitting area is large. Since the degradation of organic layers is caused by humidity and oxygen, the device was deposited by employing the conventional thermal evaporation at 6.0 x 10-6 Torr without breaking the vacuum. The visible light of the OLED announces the connection point, and everyone can get optical information by touching the visible light using the receiver module (OPD). The injection efficiency of carriers is defined by the energy level difference between an electrode and an adjacent organic layer (Kampen et al., 2004). Then, the fabricated device was encapsulated under nitrogen atmosphere using UV-curable adhesives and cavity glass lids.The inset of Fig. These p-i-n structure devices show high luminance and efficiency at extremely low operating voltages.
Moreover, OLEDs and OPDs can be fabricated by printing processes, resulting in the low-fabrication cost and the flexible devices.
Here, the fluorescence lifetime of BCzVBi was 0.6 ns, and it was short enough to realize the fast response speed. Since most of the organic semiconductors are used as intrinsic, the charges are transported via HOMO (highest occupied molecular orbital, in the case of holes) or LUMO (lowest unoccupied molecular orbital, in the case of electrons) of organic semiconductor molecules, usually by hopping in amorphous devices.
100nm N’,N’-Di(naphthalene-1-yl)-N,N’ dipheyl-benzidine (?-NPD) and 100nm Tris-(8-hydroxyquinolino) aluminum (Alq) were deposited successively as the hole transporting layer and emitting & electron transporting layer, respectively.
Indeed all these devices have multilayer structure with high current- and power-efficiencies, but thin emitting layer. The glass substrates were cleaned in deionized water, detergent, and isopropyl alcohol sequentially under ultrasonic waves, and then treated with 50 W oxygen plasma for 5 minutes just before use. Electrons and holes finally meet in one luminescent molecule and make excited states or excitons. Nevertheless, narrow thickness of emitting layer in p-i-n OLEDs and complex design architecture of phosphorescent OLEDs are not desirable from the manufacturing perspective.In recent years, white phosphorescent OLEDs (PHOLEDs) have received a great deal of attention owing to their potential use in high performance and brightness displays, solid state lighting, and back lighting for Liquid Crystal Displays. Finally, the following 10 species of organic materials were deposited on glass substrates, and molecular structures of these organic materials are shown in Fig. By applying the pulse voltage, the modulated optical signal generated from the emitting area of the OLED.
The important thing is that there are two kinds of excitons, namely singlet excitons and triplet excitons.
As a result, the efficiency of injecting electrons into an organic layer form a metal cathode is low, and the high drive voltage is necessary. In addition, the pen-type receiver module consisted of the photo-diode at a point, the comparator (NEC, ?PC271G2), the operational amplifier (Linear Technology, LT1192 S8), and many electric parts (resistors, capacitor, and mechanical switch).Pseudo electric signals were applied to the OLED to demonstrate the data transmission between the OLED and the photo diode. Although singlet excitons can be relaxed radiatively, triplet excitons cannot emit light in ordinary materials due to the spin selection rule. The reported cutoff frequency of the output power, which indicates the response speed, has been achieved up to 25 MHz for the OLED with a small area of 300 ?m circle. The deposition of the organic layers (?-NPD and Alq, Luminescence Technology Corporation) was performed using Knudsen-cells in a vacuum chamber with a base pressure during evaporation of ~10-7 Torr. After passing through the second harmonic generator, the center wavelength and the pulse width of the femtosecond pulse laser were 390 nm and 112 femtosecond, respectively. The emission efficiency of OLEDs are governed by this factor and it is well known that incorporation of heavy atoms (Pt, Ir etc.) in the luminescent dye molecule greatly alleviate this burden via intersystem crossing (Baldo et al. All the organic films radiated photoluminescences (PLs), when the femtosecond pulse laser was irradiated.
Consequently, the demand for the efficient true red bright color for multiple color display and lighting purposes has been significantly enhanced. Then, we investigated the organic-inorganic hybrid device using ZnS as the ETL (Fukuda et al., 2008a). The radiated PL was captured with a spectrometer and a streak camera (Hamamatsu Photonics, A5760), then time-resolved PL spectra were measured. Since the chemical synthesis of the luminescent molecules with heavy atoms is not fully developed and the heavy atoms are costly, other methods such as applying magnetic field to OLED (Kalinowski 1997) or mixing magnetic nanoparticles in the device (Hu et al.
This is because that the response speed of the OLED is limited by the low electron mobility of the organic ETL material, and ZnS has higher electron mobility than organic materials.
To investigate the effects of an inserted EIL, we fabricated a similar set of OLEDs using a thin 8-hydroxyquinolinato lithium (Liq) layer with thickness of 0.4 nm as an EIL.
Finally, Mono-exponential fitting was employed to derive the FL from the measured time-resolved PL intensity.The frequency dependence of PL intensity was measured to investigate the direct relationship between the cutoff frequency of PL intensity and the fluorescence lifetime of the organic neat film. Finally, we demonstrated the intuitive optical communication system utilizing the OLED as a transceiver. Doping of 5% Btp2Ir(acac) in CBP was performed by controlling the evaporation rate by monitoring the quartz crystal microbalances.The sample OLEDs and unipolar devices were transferred from the deposition chamber to glove box filled with dry N2 without exposing them to air.
In this system, we succeeded in the transmission of the pseudo-random signal with 1 Mbps and the movie files with 230 kbps, when the pen-type photo-diode is touched the emitting area of the OLED. The current efficiency of the OLEDs with Liq is less sensitive to a change in EIL (Liq) thickness than that of OLEDs with the conventional EIL material of LiF, resulting in their suitability for mass production. These approaches uses MFEs on carrier injection, transport and recombination, which are related with spintronics of organic semiconductors.


The current status of phosphorescent red OLEDs, multiple quantum well, two layers, and single layer configurations for red PHOLEDs are discussed in sections 2, 3, 4, and 5, respectively. The organic neat film was excited by the violet laser diode (NDHV220APAE1-E, Nichia corp.).
Ideal host and guest system for the optimum performance of the red PHOLEDs is presented in section 6. Limiting factor of the response speed of the OLEDThe conventional OLED consists of a transparent anode, several organic layers, and a metal cathode, as shown in Fig. The center wavelength of the excited violet laser diode was 405 nm, and the all the organic neat film absorbs the excited light. 20 mm x t 3 mm pyrex tube and two t 0.1mm glass plates) using photo-hardening epoxy (Threebond 3124) together with a zeolite desiccant (Shinagawa Kasei Co.
Finally, the conclusion of the present study is illustrated in the section 7 of this chapter.2. Phosphorescent OLED devicesIn recent years, phosphorescent organic light-emitting devices (PHOLEDs) are acquiring the mainstream position in the field of organic displays owing to their potential use in high brightness applications. The each organic layers are called as the hole injection layer (HIL), the HTL, the EML, the HBL, and the HTL. And then, PL intensity was observed using the avalanche photo diode (S5343, Hamamatsu Photonics), which was located perpendicular to the optical axis of the laser diode, as shown in Fig. 2007), very few experiments in relation to the MFE on organic semiconductors have been performed under high magnetic field larger than 2T (Reufer et al. Measurement under magnetic fieldMFE was measured at 300K in superconducting magnet using Physical Property Measurement System (PPMS; Quantum Design). When the voltage is applied between the transparent anode and the metal cathode, holes and electrons (carriers) are injected into the organic layers, respectively. Organic spintronicsSpintronics study is now extended to all kinds of semiconductor materials. Moreover, PL spectra were measured by the spectrophotometer (USB 2000, OceanOptics Company) also located perpendicular to the optical axis of the laser diode.
The magnetic field was increased from 0 T to 9 T and then was decreased from 9 T to 0 T in order to check the temporal changes.
Since organic semiconductors consist of light elements such as carbon, hydrogen, oxygen and nitrogen, lifetime of spin polarized carriers might be long in organic semiconductors. The emission intensity was measured with photon counter H7155-21 (Hamamatsu) in magnetic shielding made of thick iron plates and cylinders.
That is to say the response speed of OLEDs is limited by the time from the applying voltage to the generation of light caused by the carrier recombination.
2002), many papers have been published on the spin injection and transport in organic semiconductors.
The shielding of photon counter was tested and it was confirmed that there was no magnetic field dependence on its output. We examined the details of these processes and the method to improve the response speed of the OLED. Most of the researches have been focused on performance of spin valves and magnetoresistance of organic semiconductors. A spin valve is a two terminal device consisting of a non-magnetic layer sandwiched by two different magnetic electrodes. Results of fluorescent OLEDFirst we show the characteristics of the fluorescent device without applying the magnetic field. Iridium (III) and platinum (II) phosphorescent emitters are widely used as triplet dopants molecule.
Fabrication process of the OLED and the experimental setup to estimate the response speed of the OLEDThe fabrication process of the OLED is described in the following sentence. The coersive forces of two electrodes are different and spin-polarized carrier injection and scattering makes characteristic magnetic field dependence of the device characters (I-V curve). Figure 4(a) shows the emission intensity and current of the fluorescent OLED as a function of voltage, together with those of a phosphorescent device (Fig.4(b)). OLEDs were fabricated on glass substrates covered with a patterned indium tin oxide (ITO) anode. Various organic semiconductors have been attempted in the device structures, and the large difference in the device resistance (magnetoresistance; MR) are achieved depending upon the spin orientation of the magnetic electrodes. At first MR was only substantially observed at low temperatures, but recently great MR at room temperatures are frequently reported. However, since the magnetic field dependence of TTA appears only at low temperatures (Lei et al. The prepared glass substrates were cleaned in deionized water, detergent, and isopropyl alcohol sequentially under ultrasonic waves, and then treated with oxygen plasma for 5 min.
A variation of this research is MR measurement of mixture of magnetic nanoparticles and organic semiconductors. Next, several organic layers, an EIL and a metal cathode were thermally deposited successively using a conventional vacuum deposition system at a base pressure of below 5.0 x 10-6 Torr.
Some samples were prepared by codeposition of magnetic metals (cobalt etc.) and organic semiconductors. Thus, the guest molecules are thought to act as deep traps for electrons and holes in the emitting layer, causing an increase in the drive voltage of the PHOLED.
2007) can be observed and its origin has been elucidated by x-ray magnetic circular dichromism (Matsumoto et al. Further, the dopant concentration in such a host-guest system is usually as high as about 6 ~ 10 percent by weight (wt%) because injected charges move through dopant molecules in the emitting layer.
The amplitude of the sine wave voltage was controlled using an attenuator (Furuno Electric, VHF-STEP) and a high speed amplifier (ARF Japan, ARF-15237-25). Therefore, self-quenching or triplet-triplet annihilation by dopant molecules is an inevitable problem in host-guest systems with high doping concentrations. In addition, several resistances and capacitances were used to reduce the frequency dependence of the amplitude of the applied sine wave voltage, as shown in Fig.
2010).An important topic related to the subject in the following is MR of devices without magnetic (or spin polarized) materials.
Strong increase in conductance is observed in organic semiconductors when weak magnetic field (~ 100 mT) is applied. It means that the hysteresis comes from the charge injection process from the electrodes to the emission layer.
The generated light wad guided into a plastic optical fiber (Moritex, PJR-FB250) with the diameter of 250 ?m. We found that the "hysteresis" is dependent on both of the magnetic field and the time from the start of the current flow. Then, the output EL intensity was observed using an avalanche photodiode (Hamamatsu Photonics, S5343) and an oscilloscope (Yokogawa Electronic, DL-1740). The frequency dependence of EL intensity was measured by changing the modulation frequency of the sine wave voltage from 100 kHz to 10 MHz. Magnetic field effect in chemical reactionsFirst, we will follow up the current understanding of MFE in chemical reactions. In addition, the rise and decay times of output EL intensity were also measured while applying a pulse voltage with a width of 1 ?s to investigate the transient properties of the OLED.
The rise and decay times were defined as the times required for the optical response to change from 10% to 90% and from 90% to 10% of the maximum EL intensity, respectively.
In a simplified picture, those reactions proceed via intermediate state whose energy can be altered by the magnetic field. The emission efficiency (and also the emission intensity and the current) increase steeply as a function of B when it was less than 0.02 T as reported in the literature, and gradually decreases as B was further increased.
The energy difference can be due to the Zeeman effect on spin triplet state, which does not work on the spin singlet state. It should be noted that the decrease in the mid~ high B region is convex function, which cannot be explained by widely accepted behavior of hfc, ?g and TTA mechanisms which exhibit concave behavior against B.
Therefore the reaction path between the singlet to or from the triplet can change the reaction rate. We will discuss this point later.In order to see dependence on B more clearly, we re-plotted Fig.
Device area (capacitance of the organic layer)The conventional OLED consists of several organic layers with a total thickness of less than 200 nm due to the low carrier mobility of organic materials, resulting in the large capacitance of an emitting area. It must be noted that the Zeeman energy is too small to alter the reaction path in a single molecule. The capacitance of an emitting area is well known to affect pulse voltage-transient current characteristics, and the large capacitance of the organic layer causes the long decay time of the transient current while applying a pulse voltage.
Consequently, the selection of suitable host candidates is a critical issue in fabricating high efficiency PHOLEDs.
Therefore it is considered that the intermediate state to which magnetic field can affect is a “radical pair”, in which an anion radical and a cation radical are placed closely and about to transfer charges.
By now, previous papers demonstrated that the response speed of the OLED increases by reducing the capacitance of the emitting area.
A class of narrow band gap fluorescent material utilizing beryllium complexes as host and ETL for efficient red phosphorescent devices has been proposed. The energy difference between the singlet and the trpilet is very small because the spin-spin interaction is small due to the large distance belonging to different (but adjacent) atoms (or molecules or ions) and can be comparable with the Zeeman energy.
Characteristics of narrow band-gap phosphorescent hosts are: (1) Small energy band gap, (2) Small energy gap between singlet state and triplet state, and (3) Good electron transport characteristic. MFE on chemical reaction rate comes from the radical pairs.The MFE on chemical reaction rates are complicated and are classified as follows. Simple structure red PHOLED, using narrow band gap fluorescent host materials has demonstrated a high device performance and low manufacturing cost.3. While in OLEDs, only few reports about the MQW structure with good carrier confinement ability were presented till to date. In these articles, the MQW effect has been reported in the fluorescent devices, wherein real device efficiency is not so high besides the poor emission color stability. Since Ir(ppy)3 was doped in all quantum well layers, charge carriers couldn’t be effectively confined in this device as carriers move via dopant molecules.
Consequently, stable high efficiency results in such a MQW structure couldn’t be realized.In this section, we report the real MQW device structure having various triplet quantum well devices from a single to five quantum wells. Owing to confinement of the triplet energy at the emitting layers in the fabricated MQW device, the highest phosphorescent efficiency is obtained among reported tris(1-phenylisoquinoline)iridium (Ir(piq)3) dopant OLEDs (H. The MQW structure is realized using a wide band-gap hole and electron transporting layers, narrow band-gap host and dopant materials, and charge control layers (CCL). ExperimentalFigure 3(a) shows the configuration of fabricated red PHOLEDs, having a MQW structure. The 40 nm thick 4, 4’, 4”-tri(N-carbazolyl)triphenylamine (TCTA) hole transport layer (HTL) doped with WO3 (doping concentration 30 %) is deposited on an indium tin oxide (ITO) coated glass substrate.
To prevent the non-radiative quenching of triplet excitons generated at the heavily doped HTL, an electron blocking buffer layer of 12 nm thick TCTA was deposited. In order to confine and control a hole and electron in the EML, the CCL layer with a thickness of 5 nm was deposited inside of EML.
Later, Bepp2 hole and exciton blocking buffer layer was deposited and followed by a 10 % Cs2CO3-doped Bepp2 electron transport layer (ETL). As triplet energies of charge carrier layers and CCL are higher than the host molecule (Fig.
Finally, Al cathode was deposited in another deposition chamber without breaking the vacuum. The substrates were cleaned with acetone and isopropyl alcohol sequentially, rinsed in de-ionized water, and then treated in UV-ozone immediately before loading into the high vacuum chamber (~ 2 ? 10-7 Torr). Therefore, holes and electrons cannot be easily transported through the CCL, resulting in the rise of driving voltage and efficiency decrease.
However, the suitable HOMO and LUMO energy levels in Bepp2 CCL can control the carrier movement at ease. As a result, Bepp2 CCL partially confines holes and electrons at the first EML and some of holes and electrons arrive at the second EML after transporting through the Bepp2 CCL. The 5nm thickness of Bepp2 CCL shows reasonable efficiency and voltage increase values.In our double QW devices, hole barriers by CCLs are probably a dominant factor to control the current flow as hole barriers between HOMO levels of dopant and CCL are relatively high compared with electron barriers. Figure 3.a) Energy band diagrams of fabricated red PHOLEDs with multiple quantum well structures.
Here ?ln(J) was calculated from the current density differences of single QW and double QW devices at 5 V and ?? is the HOMO energy levels difference between the dopant and CCL. Due to lower hole barrier with Bepp2 compared with other CCL materials, the current flow is much easier with no hindrance. Hole carriers can over-flow in the Bepp2 CCL, creating more excitons in the double QW structure.
Therefore, the device D with the Bepp2 CCL improves the recombination efficiency of the electron-hole pairs.Indeed, the efficiency with two quantum well device structure with Bepp2 CCL is significantly improved. The operating voltages in the MQW structure were increased by increasing the number of R-EL units because any addition of QW units offers additional resistance to the conduction of current. From ?ln (J) data between single QW and double QW, we have calculated that 29% hole carriers can go the second EML through a Bepp2 CCL. The over-flowing ratio of hole carriers with repeating additional QW and Bepp2 CCL are shown in the inset of Fig. The simple calculation results for n=3 and 4 were obtained by assuming 29% hole carrier overflow result for n=2. Real experimental data obtained from the J-V characteristics at 5V well agree with the calculated results, indicating our carrier overflow assumption is reasonable. The excitons can be confined upto 71% in the first QW existing adjacent to the TCTA buffer layer and 21% excitons in to the next second QW. By increasing the number of quantum wells to n=3 and n=4, the efficiency drop is not significant (over 12%) because electrons can reach to first and second QWs due to the negligible barrier to electron transport. However, the driving voltage is enhanced with increasing the number of QW structures and eventually 5 QW structure does not work properly.



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