A lithium-boron complex. lithium tetra (2-methyl 8-hydroxy quinolinato) boron (LiBqm(4)), which emits greenish-blue light has been synthesized by the reaction of LiBH4 with 2-methyl 8-hydroxyquinoline in a molecular weight ratio of 1:4. and Used for organic light emitting diodes (OLEDs) Fabrication. The complex has been characterized by optical absorption, Fourier Transform Infrared (FTIR), photoluminescence (PL) and electroluminescence (EL) spectroscopic methods. The thermo-gravimetric analysis (TGA) suggests that the material is quite thermally stable LIP to 380 degrees C. An optical band gap of 3.15 eV using the well-known Tauc relation and an exciton binding energy of 0.69 eV has been evaluated. OLEDs have been fabricated in ITO/N,N'-di-(3-methyl-phenyl)-N,N'diphenyl-4,4'diaminobiphenyl(TPD)/LiBqm(4)/Al configuration. PL and EL peaks exist at 503 nm. I-V characteristic of device reveals that at higher voltages the current follows I proportional to V-III law, where m > 1

Chandra, Subhas

Dhawan, S. K.

Kamalasanan, M. N.

Kumar, Lokendra

Kumar, Pankaj

Misra, Aparna

IR@NPL

Indian Journal of Engineering & Materials Sciences Vol. 12, August 2006, pp. 357-361       Synthesis and characterization of greenish-blue light emitting lithium-boron complex for organic light emitting diode applications  Aparna Misraa, Pankaj Kumara, Lokendra Kumarb, S K Dhawana, M N Kamalasanana* & Subhas ChandraaaPolymeric & Soft Materials Section, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110 012, India bDepartment of Physics, Allahabad University, Allahabad 211 002, India  Received 6 May 2005; accepted 22 May 2006 A lithium-boron complex, lithium tetra (2-methyl 8-hydroxy quinolinato) boron (LiBqm4), which emits greenish-blue light has been synthesized by the reaction of LiBH4 with 2-methyl 8-hydroxyquinoline in a molecular weight ratio of 1:4, and used for organic light emitting diodes (OLEDs) fabrication. The complex has been characterized by optical absorption, Fourier Transform Infrared (FTIR), photoluminescence (PL) and electroluminescence (EL) spectroscopic methods. The thermo-gravimetric analysis (TGA) suggests that the material is quite thermally stable up to 380oC. An optical band gap of 3.15 eV using the well-known Tauc relation and an exciton binding energy of 0.69 eV has been evaluated. OLEDs have been fabricated in ITO/N,N′-di-(3-methyl-phenyl)-N,N′diphenyl-4,4′diaminobiphenyl(TPD)/LiBqm4/Al configuration. PL and EL peaks exist at 503 nm. I-V characteristic of device reveals that at higher voltages the current follows I ∝ Vm law, where m > 1. IPC Code: H01L 51/50 In the recent years there has been a lot of interest among researchers towards organic electroluminescence (EL) because of its potential applications in thin and full colour flat panel display systems. Organic electroluminescence involves various operations like carrier injection at the electrodes, transport of carriers through the materials, excitons formation and their radiative decay1-4. Since the first report on a highly efficient light emitting diode based on a small molecule by Tang et al5. in 1987 and that based on a conjugated polymeric material in 1990 by Burroughes et al6., a dramatic improvement has been achieved in the efficiency and lifetime of organic electroluminescent devices7. This fast improvement has recently led to the commercialization of OLED based full colour flat panel displays. OLEDs are two electrode systems containign anode and cathode and organic materials are sandwiched between them. Generally, OLEDs based on small molecules consists of vacuum deposited emitter layers 5,8-14, while those based on polymers, consists of either spin coated or dip coated layers15-19. In both types of the devices the emitter layers are casted over a high work function optically transparent and electrically conducting indium tin oxide (ITO) coated glass or plastic substrate, which serves as the hole-injecting anode. The electron-injecting cathode is usually a low work function metal such as aluminum, magnesium, calcium or their alloys. On the application of a bias voltage, holes and electrons are injected from the anode and cathode respectively into emitter layer, where they recombine to form the singlet and triplet excitons of which singlet excitons decay radiatively to give light at a wavelength corresponding to the emitter band gap. We report the synthesis and characterization of LiBqm4 boron complex and its suitability for OLEDs applications. It has been recognized that the charge transport mechanism is an important phenomenon in the organic light emitting diodes (OLEDs) and it is being studied extensively20-22. To improve the performance of OLEDs a good understanding of basic device physics is very necessary. OLEDs have been fabricated in a triphenyl diamine derivative (TPD)/LiBqm4 bilayer configuration. The complex emits a quite bright greenish-blue light with a peak wavelength at 503 nm. The Commission Internationale I’Eclairage (CIE) co-ordinates at different voltages have been studied and have been found to be independent of the applied voltage. This independence of CIE co-ordinates on the applied voltage makes LiBqm4 a good candidate for full colour display applications. The current-voltage characteristic of the OLEDs shows that the charge transport through ___________________ ∗For correspondance (E-mail: mnkamal@mail.nplindia.ernet.in) INDIAN J ENG. MATER. SCI., AUGUST 2006   358LiBqm4 is space charge limited with a combined effect of diffusion, trapping and recombination of the charge carriers.  Experimental Procedure  Synthesis of the complex The complex has been synthesized by the reaction of lithium borohydride (2 mmol) with four times of 2-methyl 8-hydroxyquinoline (8 mmol) in ethanol solution at room temperature23. Lithium borohydride and 2-methyl 8-hydroxyquinoline were purchased from MERCK India Limited. Route of the material synthesis is shown in Scheme 1. The product was purified with a micro-soxhlet extractor for 8 h using diethyl ether as a solvent, followed by vacuum sublimation at a pressure of 5×10-5 Torr. For all the spectroscopic studies of the complex in thin films, the films were prepared by the vacuum evaporation of complex on different substrates. OLEDs fabrication OLEDs have been fabricated on the pre-patterned, pre-cleaned ITO coated glass substrates. The substrates were patterned using standard photolithography technique and then cleaned with soap solution followed by boiling in trichloroethylene and isopropyl alcohol and finally dried under vacuum at about 140oC. Prior to any material deposition cleaned ITO substrates were exposed to oxygen plasma for 5 min. The devices were fabricated in the configuration ITO/TPD/LiBqm4/Al. TPD (25 nm) was evaporated onto the ITO substrate to facilitate better transport of holes into the electron-hole recombination zone (emissive layer). Subsequently, the emissive material LiBqm4 (35 nm) was evaporated onto the TPD layer. Finally, a 150 nm thick aluminium (cathode) was evaporated on LiBqm4 in the same vacuum conditions. All the evaporations were carried out in a diffusion pumped vacuum chamber at a base pressure of 1×10-5 Torr. TPD and LiBqm4 were evaporated at the rate of 1~2 Å/s, while Al was evaporated at about 10 Å/s. The thickness of the thin films was measured using a HINDHIVAC quartz crystal thickness monitor DTM-101. The active area of the devices fabricated was 2×2 mm2. After fabrication the devices were transferred to a glove box, which resulted in a brief exposure of the devices to air. The devices were sealed in a glove box under dry nitrogen atmosphere and then were taken out. Now, all the electrical measurements were done in the ambient atmosphere. The device configuration is shown in Fig. 1. Keithley Source measure unit 2400 interfaced with a computer was used to measure the current versus voltage (I-V) characteristics of the device.  Results and Discussion Optical absorption studies were carried out on the vacuum deposited LiBqm4 thin film (300 nm) on a quartz substrate in the range 200-800 nm. A Shimadzu UV 2401 PC spectrophotometer was used to study the optical absorption of the complex at room temperature. Fig. 2 shows the absorption maximum at 275 nm with another peak at 348 nm and the cutoff wavelength at 410 nm, which is same as that reported earlier23. The optical band gap was calculated from the optical absorption and using the Tauc relation,  Scheme 1—Synthesis of LiBqm4.   Fig. 1—Schematic diagram of the organic light emitting diode fabricated.   Fig. 2—Absorbance, photoluminescence and electroluminescence spectra of LiBqm4. MISRA et al.: ORGANIC LIGHT EMITTING DIODES   359which relates the absorbance A, with the band gap24, 25 as,   ngEhAh )( −= νν  … (1)  where n is ½ for direct band gap material and 2 for indirect band gap materials, A is the absorbance, Eg is the band gap corresponding to a particular absorption occurring in the film and hν is the photon energy. The value of the direct optical band gap was obtained from extrapolation of the straight-line portion of the A2 versus hν plot, to A = 0. Fig. 3 shows the linearity of A2 with the photon energy hν. Using the above relation the band gap of the complex has been estimated to be 3.15 eV.  The PL and EL spectra of thin films (Fig. 2) were recorded using HR 2000 Ocean Optics spectrometer with an optical fiber array. The emission peak lies at 503 nm, which is 30 nm shorter than that of Alq326. This blue shift is attributed due to different co-ordination structure of boron complexes compared to Alq327. EL spectrum is similar to PL spectrum, which shows that both emissions are due to the singlet excitons decay only. It has been reported that in the Mq3 complexes (M = Al, Ga, In), the 8-hydroxyquinoline works as a bidentate ligand and form chelate complexes with central metals28. But in case of LiBqm4, the central boron atom has small size, therefore only four oxygen atoms are co-ordinated to it and 2-methyl 8-hydroxyquinoline works as a monodentate ligand. It is well known fact that as the covalent nature of the metal-nitrogen bonding in Mq3 complexes is decreased, emission shifts to shorter wavelength29-30. LiBqm4 emission spectrum is blue shifted in comparison to Mq3 complexes, because of the absence of co-ordination bonds between boron and nitrogen atoms. With the help of EL peak the exciton energy Eex was calculated to be 2.46 eV using the relation Eex = 1240/λmax (nm), where λmax is the EL peak position. The band gap Eg = 3.15 eV determined from the absorbance measurements allows us to calculate the excitons binding energy Ebind = Eg – Eex ~ 0.69 eV. The EL and the CIE coordinates of the emitted light were taken at different applied voltages, and were found to be independent of the applied voltage. The EL emission stayed at 503 nm and the CIE co-ordinates at (0.069, 0.523) (Fig. 4).  The thermogravimetric analysis (TGA) has been carried out to determine the thermal stability of the complex. The TGA curve of LiBqm4 is shown in   Fig. 3⎯Selected environmental carcinogens along with estrogen (usually, these chemicals have a structural similarity with estrogen).  Fig. 4—Electroluminescence spectra of ITO/TPD/LiBqm4/Al device at different voltages.    Fig. 5—TGA curve (solid) and its first derivative (dotted) at the rate of 10 oC/min under N2 purging.  INDIAN J ENG. MATER. SCI., AUGUST 2006   360Fig. 5. It has been recorded on a Mettler TA 3000 system at a scan rate of 10oC/min under nitrogen atmosphere. It is clear from the curve that the complex is thermally stable up to 380oC under ambient conditions and the maximum weight loss occurs at about 515oC. Fig. 6 shows the FTIR spectrum of the vacuum deposited complex on freshly cleaved KBr single crystal. It suggests the molecular contents of the material. The strong peaks below 900 cm-1 are because of out-of-plane bending of ring C-H bonds and indicate the presence of aromatic ring. The strong peak at 665  cm-1 is because of-the-out of plane ring C=C bending. The peaks at 1099 cm-1 are because of the C-O stretching. The peaks at 1277-1365 cm-1 suggest aromatic amine resonance (C–N–C stretching in pyridine ring). The bands at 1458-1560 cm-1 are because of carbon to carbon (C=C) stretching within the ring. The peaks at 2857 and 2916 cm-1 are because of the presence of aliphatic C-H stretching indicating the presence of a methyl group. The peak at 3033 cm-1 is because of aromatic C-H stretching. The C-H out-of-plane bending and aliphatic C-H stretching decide the presence of the methyl-substituted quinolines.  Fig. 6—FTIR spectrum of LiBqm4 on freshly cleaved KBr single crystal.   Fig. 7—I-V characteristic of ITO/TPD(25 nm)/LiBqm4(35 nm)/Al(150 nm) device in forward bias (active area = 4 mm2). Inset shows the log-log representation of the I-V characteristic. At higher voltages I ∝ Vm, where m > 1.  The room-temperature I-V characteristic of ITO/TPD (25 nm)/LiBqm4 (35 nm)/Al (150 nm) diode in the forward bias is shown in Fig. 7. We considered the applied voltage as the operating voltage when emitted light is firstly detected. The operating voltage of the above diode was observed to be 6 V. The current rises steeply above 6 V, and it results sudden rise in the brightness. The emission was uniform over the entire active area of the device. Inset in Fig. 7 represents the I-V charactertic on log-log scale. It is clear from the inset that initially at low voltages the slop of I-V characteristic is nearly 1 and current is ohmic up to about 1 V. This ohmic behaviour of I-V characteristic may be due to background doping carriers or thermally generated carriers21,31. As the voltage increases further, slop of I-V curve increases and current follow the Vm law where m > 1. This I ∝ Vm behaviour of current is space charge limited with a combined effect of diffusion, trapping and recombination of charge carriers31.   Conclusions An organic fluorescent lithium-boron complex has been synthesized and used as active layer in bilayer OLED devices. The complex is thermally stable up to 380oC. The position of the PL and EL peaks at the same wavelength (503 nm) show their origin due to singlet excitons decay only. The complex has an exciton binding energy of 0.69 eV. The CIE co-ordinates have been observed independent of the applied voltage. Results show that synthesized fluorescent boron complexes have great potential as greenish-blue emitters for OLED full color display applications.  Acknowledgments Authors would like to thank Prof. Vikram Kumar, Director NPL for his keen interest in this work. Thanks are also due to Dr. Anil Kumar Gupta, Dr. S S Bawa, Late Dr S C K Misra, and Mr R R Koireng from NPL, New Delhi, India and Dr V D Vankar and Dr Neeraj Khare from IIT Delhi, India, for their kind help and co-operation. Financial support by the Ministry of MISRA et al.: ORGANIC LIGHT EMITTING DIODES   361Information and Communication Technology, Government of India and CSIR (India) Network project No. CMM-0010 is gratefully acknowledged. One of us (PK) is thankful to CSIR, India for the award of Junior Research Fellowship.  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Synthesis and characterization of greenish-blue light emitting lithium-boron complex for organic light emitting-diode applications

357-361A lithium-boron
complex, lithium tetra (2-methyl 8-hydroxy quinolinato) boron (LiBqm4),
which emits greenish-blue light has been synthesized by the reaction of LiBH4
with 2-methyl 8-hydroxyquinoline in a molecular weight ratio of 1:4, and used
for organic light emitting diodes (OLEDs) fabrication. The complex has been
characterized by optical absorption, Fourier Transform Infrared (FTIR),
photoluminescence (PL) and electroluminescence (EL) spectroscopic methods. The
thermo-gravimetric analysis (TGA) suggests that the material is quite thermally
stable up to 380oC. An optical band gap of 3.15 eV using the
well-known Tauc relation and an exciton binding energy of 0.69 eV has been
evaluated. OLEDs have been fabricated in ITO/N,N′-di-(3-methyl-phenyl)-N,N′diphenyl-4,4′diaminobiphenyl(TPD)/LiBqm4/Al
configuration. PL and EL peaks exist at 503 nm. I-V characteristic of
device reveals that at higher voltages the current follows I ⍺ Vm
law, where m > 1

Kumar, Lokendra Kumar

Dhawan, S K Dhawan

Kamalasanan, M N

NOPR

Synthesis and characterization of greenish-blue light emitting lithium-boron complex for organic light emitting diode applications

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Synthesis and characterization of greenish-blue light emitting lithium-boron complex for organic light emitting-diode applications

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