Review of Literature on Organic Light-Emitting Diode Devices

CHAPTER 7

Review of Literature on Organic Light-Emitting Diode Devices 7.1 INTRODUCTION Attracting interest around the globe, organic light-emitting diodes (OLEDs) have become the most promising displays and power-saving solid-state lighting (SSL) sources available today. This energy-efficient lighting technology plays an important role in reducing global consumption of electricity by almost 50%. OLEDs are semiconductors made from organic carbon-based materials that emit light when electricity is applied. They have the potential to far exceed the energy efficiencies of existing displays and incandescent, fluorescent lighting. The vision of SSL has largely been driven by the desire to reduce energy consumption and to create pollution-free lighting. Novel research is being carried out to stimulate the development of the science and technology needed to enable the potential of SSL through OLED devices. This chapter reviews the literature on red, blue, and green (RBG) and white OLED devices since the very first device architectures to the progress on the fabrication of different layers of ecofriendly and energy-efficient OLEDs.

7.2  DEVICE ARCHITECTURE The history of OLED device architectures reveals an increase in the complexity of the devices as shown in Fig. 7.1. Earlier devices employed a simple monolayer structure and subsequently more and more layers have been employed, which perform specialized functions as specified in Table 7.1. The first report of EL in anthracene in monolayer devices was given by Pope et  al. in 1963 [2] and later by Helfrich and Schneider in 1965 [3]. However, the electroluminensce (EL) phenomenon from organic materials remained a pure academic interest for almost two decades due to the difficulty of growing large-size single crystals and the requirement of very high voltage (~1000 V) to produce the luminance. VanSlyke and Tang in 1985 [4] and Tang et  al. in 1988 [5] demonstrated that the Principles and Applications of Organic Light Emitting Diodes (OLEDs). DOI: http://dx.doi.org/10.1016/B978-0-08-101213-0.00007-2

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Principles and Applications of Organic Light Emitting Diodes (OLEDs)

Figure 7.1  History of OLED architectures [1].

poor performance of the early monolayer device could be dramatically improved in a two-layer device, just by the addition of a hole-transport layer (HTL) with thin, amorphous film stacking in the device structure. Subsequently, the Kodak group improved the power-conversion efficiencies of organic EL devices by doping the emitting layer. Heterostructure configurations were implemented by inserting several layers such as a buffer layer between the anode and HTL [6–8], electron-transport layer (ETL), and hole-blocking layer (HBL) [9] or an interlayer between the cathode and ETL [10,11] in the device structure to improve the device performance. It has been observed that the EL efficiency of OLEDs can be increased by carrier or exciton confinement within a multilayer device. This multilayer device structure often enhances the drive voltage of OLEDs. Chemical doping with either electron donors (for electrontransport materials) or electron acceptors (for hole-transport materials) can significantly reduce the voltage drop across these films. These devices with either a HTL or ETL- doped layer show improved performance, but the operating voltages are still high. Huang and his group proposed the concept of p-type doped HTL and n-type doped ETL [12]. P-i-N (P-dopedintrinsic-N-doped) OLED structure devices show high luminance and efficiency at extremely low operating voltages. However, the narrow

Table 7.1  Requirements of different layers in OLEDs S. no. OLED layer Work function Purpose (ϕ) in eV

1.

Substrate

4.7–4.9

2.

Anode

4.5–5.1

3.

HIL

–

4.

HTL

–

5.

Emissive layer

–

6.

ETL

–

7.

Cathode

2.9–4.0

Serves as base for deposition of all layers Serves as a positive electrode Blocks the electrons for recombination

Requirement

Transparent, high work function Low roughness and high work function High mobility, electron blocking capacity, high glass-transition temperature Transport holes and blocks Good hole-transporting electrons capacity Site for emission of light High efficiency, lifetime, color due to recombination of purity holes and electrons Transport electrons and Good electron-transporting block holes capacity Serves as a negative Low work function, high electrode transparency

Materials

Glass, plastics, metal foils ITO, graphene CuPC, PtPC, MeO-TPD NPB, TPD, α-NPD Organic complexes like Alq3 Liq, TPBi, PBD Alloys of magnesium with silver LiF/Al, etc.

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thickness of the emitting layer in P-i-N OLEDs and the complex design architecture of phosphorescent OLEDs are not desirable from a manufacturing perspective. Hence, researchers are now concentrating on overcoming these challenges and improving the electricity-to-light conversion efficiency, device stability, lifetime, material selection, proper encapsulation, and novel fabrication technologies, with reduced manufacturing cost.

7.3  REVIEW OF LITERATURE ON RED OLEDs Ample research is being conducted in an effort to develop ecofriendly materials for emission of light in the red region of the visible spectrum. Dresner was the first to consider organic material for the fabrication of practical red electroluminescent devices [13] in 1969. Soon after, organic thin film for EL studies was reported by Kampas and his coworker in 1977 [14] and Kalinowski et al. in 1985 [15]; they employed these thin films for multicolor display applications. However, the work was not successful due to low stability factor of the thin-film organic EL and broad nature of the luminescent spectra. Hybrid organic materials based on Eu3+ are better known to exhibit photoluminescence with a very sharp spectral band [16]. Kido et al. in 1991 investigated the suitability of Eu(ttfa)3 complex as a red-light emitter in cathode ray tubes [17]. In 1993, organic EL devices using lanthanide complexes such as Tb (acac)3 and Eu (ttfa)3 (Tb: terbium, acac: acetylacetonato, Eu: europium, and ttfa: thenoyltrifluorouetonato) were reported by Kido et  al. [18]. In 1994, a bright-red EL was observed from tris(dibenzoylmethanato) phenanthroline Eu3+[Eu(DBM)3Phen] as red light-emitting material. The second added ligand, phenanthroline, acts to saturate the coordination number of Eu ions and to improve the fluorescence intensity, volatility, and stability of the Eu complex [19]. Again in the same year, Kido et al. developed bright-red light-emitting EL devices with highly monochromatic light using trivalent europium complexes as the emitter lanthanide complex. However, they exhibited a poor carrier transport property, which can be improved by codeposition with the carrier transport material [20]. In 1995, Sano et  al. [21] presented a report on the fabrication of multilayer EL cells with the emission of an Eu complex formed by a vacuum-vapor deposition technique. A volatile Eu-complex Eu(TTA)3(phen) was synthesized and applied to EL cells, which employed 1AZM-HEX (host material) as a emitting layer and Eu(TTA)3phen as a dopant [21]. In 1999, Miyamoto et  al. [22] synthesized a Eu(III) β-diketonate complex, Eu(DBM)3Phen. Thin film of Eu(DBM)3Phen doped with

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phosphorescent material in OLED showed excellent EL spectra at room temperature, which mainly depends on the host materials and the extent of energy transfer from the triplet states of the phosphorescent materials to the ligand triplet state of the Eu complex [22]. With two novel second ligands, 2-(2-pyridyl)benzimidazole (HPBM) and 1-ethyl-2-(2-pyridyl)benzimidazole (EPBM), two europium complexes, Eu(DBM)3HPBM and Eu(DBM)3EPBM (DBM: dibenzoylmethanato), were synthesized and used as emitting materials in organic EL devices by Huang et  al. in 2001. The anatomy of the fabricated red-light-originating device was ITO/TPD/Eu(DBM)3HPBM (or) Eu(DBM)3EPBM/Al and ITO/TPD/ Eu(DBM)3EPBM/AlQ/Al. The EL of Eu(DBM)3EPBM was found to be much higher than that of Eu(DBM)3HPBM. A maximum luminance of 180 cd/m2 in the triple-layered device of Eu(DBM)3EPBM was achieved at 18 V [23]. Chen et  al. [24] used a series of tris-(8-hydroxyquinoline) metal chelates with central metal ions (Al3+, Ga3+, In3+) as the host materials. A red fluorescent dye, 4-(dicyanomethylene)-2-t-butyl-6(8-methoxy-1,1,7,7-tetra methyljulolidyl-9-enyl) 4H-pyran (DCJMTB), was used as the emitter/guest dopant material. The doped devices with Gaq3 as the host materials produced high efficiencies and saturated redcolor chromaticity. The device with 1% DCJMTB doped in Gaq3 showed a current efficiency of 2.64 cd/A. The color coordinates of the Gaq3:1% DCJMTB device were found to be 0.63, 0.36 [24]. In the same year, Yang et  al. fabricated a red single-layer type of EL device based on a copolymer containing carbazole, Eu complex, and methyl methacrylate [25]. In 2002 Ma et  al. [26] employed N,N-bis{4-[2-(4-dicyanomethylene6-methyl-4H-pyran-2-yl) ethylene] phenyl} aniline(BDCM) with two (4-dicyanomethylene)-4H-pyran electron-acceptor moieties and a triphenylamine electron-donor moiety for application in OLEDs. The threelayered EL device with the structure ITO/CuPc/DPPhP/BDCM/Mg:Ag had a power-on voltage of <4 V, with bright luminance of 582 cd/m2 at 19 V suggesting the excellent electron-injection property of BDCM. Several devices using a europium complex Eu(TTA)3(DPPz) (TTA = 2-thenoyltrifluoroacetonate; DPPz = dipyrido[3,2-a:2′,3′-c] phenazine) as dopant emitter were fabricated [27]. The device architecture and the chemical structure of the europium complex are shown in Fig. 7.2. With the device structure TPD (50 nm)/Eu:CBP (4.5%, 30 nm)/BCP (30 nm)/ Alq (25 nm), they obtained external quantum efficiency of 2.1%, current efficiency of 4.4 cd/A, power efficiency of 2.1 lm/W, and brightness of 1670 cd/m2. In order to develop new materials for red EL devices, a

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Principles and Applications of Organic Light Emitting Diodes (OLEDs)

Figure 7.2 Device architecture and chemical structure of the europium complex employed by Ma et al. [26].

novel ligand, 2-phenyl-imidazo[4,5-f]1,10-phenanthroline (L), and an europium (III) complex with dibenzoylmethanate (DBM), Eu(DBM) L, was synthesized by Gao et  al. [28]. Single-crystal X-ray diffraction showed that Eu(DBM)3L belongs to the orthorhombic, space group Pbca with cell dimensions of a = 52.1655(6) nm, b = 52.0834(5) nm, c = 52.6469(7) nm, V = 511.942(5) nm, and Z = 58, D = 51.307 g/cm. Each europium atom six coordinated with six oxygen atoms from three bidentate DBM anions and two nitrogen atoms from one bidentate L, forming a distorted square antiprism. The complex can be easily evaporated and can be used as a red-light-emitting material. Upon improvement, the device with of ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,19biphenyl-4,49-diamine (TPD) (40 nm)/Eu(DBM)L(50 nm)/2,9-di-3-4, 7-diphenyl-1,10-phenanthroline (bathocuproine or BCP) (20 nm)/tris(8hydroxyquinoline) aluminum (Alq3) (30 nm)/cathode gave off pure red light with luminance of 42 cd/m2 at 16 V. Duan et al. in 2005 successfully prepared two substituted phenanthrolines (L = DEP: 5-diethylamino1,10-phenanthroline and PiPhen: 5-Piperidine-1,10-phenanthroline) and europium complexes based on these ligands Eu(TTA)3(L) (Eu-L) were synthesized from EuCl3, 2-thenoyltrifluoroacetone (TTA) and L in good yields [29]. These complexes emit a strong sharp red band at ~612 nm in solution and also in solid state. The HOMO levels of these complexes were at 5.6 eV, and EL devices using these two europium complexes as dopant emitters successfully emit saturated red light. In 2006 Qu et  al. [30] synthesized two novel polymers, PQP (poly(3,7-N-octyl phenothiozinylcyanoterephthalylidene)) and PQM

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Figure 7.3  (A) Chemical structure of Eu(TTA)2(N-PHA) and (B) device architecture of OLED device fabricated by Yanfei et al. [31].

(poly(3,7-N-octyl phenothiozinylcyanoisophthalylidene)), containing phenothiazine for application in red- and orange-light-emitting diodes. The single-layer EL devices of ITO/PQP (or PQM)/Mg: Ag and multilayer devices of ITO/PQP (or PQM): N,N′-diphenyl-N,N′-bis(3-methylphenyl)[1,10-biphenyl]-4,40-diamine (TPD) (44 nm)/2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP, 5  nm)/tris(8 hydroxyquinolinato) aluminum (Alq3, 20 nm)/Mg:Ag were fabricated. The EL spectra from the devices based on PQP, PQM peaked at a wavelength of 664 nm, 608 nm with maximum brightness of 60 cd/m2, 150 cd/m2, at an applied voltage of 17 V and 14 V, respectively. In 2007, novel rare earth complex Eu (TTA)2 (N-HPA) Phen(TTA = thenoyltrifluoroacetone, N-HPA = N-phenylanthranilic acid, and phen = 1,l0-phenathroline), which contains three different ligands, was synthesized by Yanfei et al. [31]. The Eu complex was blended with poly N-vinylcarbazole (PVK) in different weight ratios and spin-coated into films. The chemical structure of Eu(TTA)2(N-PHA) and device architecture of the OLED device fabricated by Yanfei et al. are shown in Fig. 7.3A and B, respectively. Multilayer structural devices consisting of ITO/PVK: Eu (TTA)2(N-HPA) phen/ BCP/Alq3/Al were fabricated with PVK: Eu (TTA)2(N-HPA) as lightemitting layer. Increasing the concentration of Eu in the PVK thin film would inhibit the emission of PVK to different degrees. Pure-red luminescence of europium (III) was observed when the doping weight ratio was approximately 1:5, indicating an effective energy transfer from PVK to

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rare earth complex at this ratio. For application in OLEDs, efficient new red phosphorescent iridium (III) complexes, bis[2,3-diphenyl-4- methylquinolinato-C2,N] iridium(III) acetylacetonate [Ir(4-Me-2,3-dpq)2(acac)] and bis[2,3-di(4-methoxy-phenyl)-4-methyl-quinolinato-C2,N] iridium(III) acetylacetonate [Ir(4-Me-2,3-dpq(OMe)2)2(acac)], were synthesized from the two-step reactions of IrCl3·xH2O with a corresponding ligand by Park et  al. [32] in 2008. Electroluminescent devices with a configuration of ITO/2-TNATA/NPB/CBP:dopant/BCP/Alq3/Liq/Al were fabricated. Ir(4-Me-2,3-dpq)2(acac) and Ir(4-Me-2,3-dpq(OMe)2)2(acac) showed a luminous efficiency of 8.10 and 9.81 cd/A at a current density of 20 mA/cm2, respectively. The CIE coordinates of Ir(4-Me-2,3-dpq)2(acac) and Ir(4-Me-2,3-dpq(OMe)2)2 (acac) were 0.644, 0.352, and 0.615, 0.375, respectively [32]. In 2009, Haq et  al. [33] fabricated efficient red organic light-emitting material with a wide bandgap, i.e., 9,10-bis(2-naphthyl) anthracene (ADN) doped with 4-(dicyano-methylene)-2-t-butyle-6-(1,1,7,7-tetramethyl-julolidyl-9-enyl)-4H-pyran (DCJTB) as a red dopant and 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10(2-benzothiazolyl) quinolizine-[9,9a,1gh] coumarin (C545T) as an assistant dopant. The C545T dopant did not emit by itself but did assist the energy transfer from the host (ADN) to the red-emitting dopant through a cascade energy-transfer mechanism. This approach significantly improved the EL efficiency of OLEDs. In the same year, Lyu et al. [34] fabricated a highly efficient phosphorescent silicon-cored spiro-bifluorene derivative (SBPTS-PSB) as a host material for red phosphorescent Ir(III) complexes. Three phosphorescent guests, (piq)3Ir, (piq)2Ir(acac), and (btp)2Ir(acac), were doped in the SBP-TS-PSB host and the device performance was investigated. The external quantum efficiency, power efficiency, and CIE color coordinates obtained from (piq)2Ir(acac), (piq)3Ir, and (btp)2Ir(acac)based devices were 14.6%, 10.3 lm/W (0.68, 0.32) at current density of 1.5 mA/cm2, 13.5%, 7.8 lm/W (0.66, 0.32) at current density of 1.3 mA/ cm2, and 9.9%, 7.0 lm/W (0.66, 0.31) at J = 0.5 mA/cm2, respectively [34]. In 2010, the performance of a red OLED device was improved by codoping 2-formyl-5,6,11,12-tetraphenylnaphthacene(2FRb) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljulolidyl-9-enyl)4H-pyran(DCJTB) in tris-(8-hydroxyquinoline) aluminum (Alq3) host as the emitting layer by Li et  al. [35]. The device with 1 wt% DCJTB and 2.4 wt% 2FRb in Alq3 host gave a saturated red emission with CIE chromaticity coordinates 0.65, 0.35 and maximum current efficiency as high

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as 6.45 cd/A, which were 2- and 2.4-fold larger than that of the device with 1 wt% DCJTB (3.28 cd/A) in Alq3 host and the device with 2.4 wt% 2FRb (2.72 cd/A) in Alq3 host at the current density of 20 mA/cm2, respectively. The improvement could be attributed to the effective utilization of host energy by both energy transfer, trapping in the EL process and the depression of concentration quenching between the dopant molecules. In 2011 Kalyani et  al. [36] designed multilayer OLED devices with the configuration of ITO/m-MTDATA (1000  Å)/α-NPD(200 Å)/TPBi: Eu0.4Y0.6(TTA)3Phen(250 Å)/Alq3(250 Å)/LiF:Al (10:1200 Å) and ITO/mMTDATA(1000 Å)/α-NPD(200 Å)/TPBi:Eu0.5Y0.5(TTA)3Phen(250 Å)/ Alq3(250  Å)/LiF:Al(10:1200  Å). Different characterization techniques such as I-V, J-V-L, V-L characteristics, CIE coordinates, and EL spectra were carried out for the fabricated devices at room temperature in ambient atmosphere. Bright and efficient EL devices with narrow luminescent emission and narrow bandwidth were obtained. Full-width at half-maximum (FWFM) was found to be <5 nm for both devices. The power-on voltage of device I and device II was found to be 13 V and 17 V, respectively, with λemi centered at 612 nm. With the host/dopant combination, maximum brightness of 185.6 and 44.72 cd/m2 was observed for devices I and II, respectively. The device anatomy and molecular structures of the materials used are shown in Figs. 7.4 and 7.5, respectively. Highly efficient single-layer OLEDs based on blended cationic iridium (Ir) complexes as emitting layer have been demonstrated using narrow bandgap Ir complex [Ir(Meppy)2(pybm)](PF6) (C1) as guest and wide bandgap cationic Ir complex [Ir(dfppy)2(tzpy-cn)](PF6) (C2) as host. As

Figure 7.4  Device stack of fabricated OLED devices. (A) Device I and (B) device II.

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Principles and Applications of Organic Light Emitting Diodes (OLEDs)

Figure 7.5  Molecular structure of the materials used in different layers of OLED devices.

compared with single cationic Ir complex-emitting layer, these host–guest systems exhibit highly enhanced efficiencies, with maximum luminous efficiency of 25.7 cd/A and external quantum efficiency of 8.6%, which are nearly three-fold of those of pure C1-based devices. Compared with a multilayer host-free device containing C1 as emitting layer and TPBI as ETL, the above single-layer devices also demonstrated two-fold enhancement efficiencies. The high efficiencies achieved in these host–guest

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Figure 7.6  Structure of EL devices, energy diagram, and molecular structures of the different materials [38].

systems are among the highest values reported for ionic Ir complex-based SSL-emitting devices. The results demonstrate that the ionic Ir complexbased host–guest system provides a new approach to achieving highly efficient OLEDs with single-layer device structure and solution-processing technique [37]. Two heteroleptic iridium (III) complexes, Ir(piq)2(dbm) and Ir(btp)2(acac), have been tested as emitters for phosphorescent OLEDs (PhOLEDs). The structure of these EL devices, the energy diagram, and the molecular structures of the different materials employed in these devices are shown in Fig. 7.6. Interestingly, device performance exhibited a marked insensitivity in the dopant concentration. In this study, a dibenzoylmethane (dbm)-based complex was also tested for OLEDs. To evaluate the emissive properties of this new emitter belonging to a family of complexes that has not been investigated yet, identical devices were prepared with the well-known red dopant Ir(btp)2(acac) for comparison. The new complex Ir(piq)2(dbm) exhibited comparable performance to that obtained with Ir(btp)2(acac) [38]. Song [39] synthesized two highly efficient red phosphorescent Ir (III) complexes, bis[2,3-diphenylquinoxalinato-N,C2′]iridium(III) pyrazinate (dpq)2Ir(prz) (1) and bis[2,3-iphenylquinoxalinato-N,C2′]iridium (III) 5-methylpyrazinate (dpq)2Ir (mprz) (2), which were both based on

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Principles and Applications of Organic Light Emitting Diodes (OLEDs)

the use of 2,3-diphenylquinoxaline as the ligand. Both Ir (III) complexes were found to be soluble in common organic solvents, and uniform thin films were readily spin-coated onto substrates. Phosphorescent OLEDs produced using a ITO/PEDOT:PSS/TCTA:TPBi:Ir (III) complex/cathode configuration had a maximum external quantum efficiency of 7.32% and a luminance efficiency of 7.15 cd/A with CIE coordinates of 0.70, 0.30 for compound 2, which remained stable on increasing current density. The efficiencies of these PhOLEDs were higher than those previously reported for solution-processed pure red PhOLEDs. Recently, Zhou et  al. attempted to enhance the EL performance of trivalent europium complex Eu(TTA)3phen (TTA: thenoyltrifluoroacetone and phen:1,10phenanthroline) by designing the device structure with stepwise energy levels. The widely used bipolar material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) was chosen as host material, while the doping concentration of Eu(TTA)3phen was optimized to be 4%. To facilitate the injection and transport of holes, the MoO3 anode modification layer and 4,4′,4″-tris(carbazole-9-yl) triphenylamine (TcTa) HTL were inserted in sequence. Efficient pure-red emission with suppressed efficiency roll-off was obtained, which can be attributed to the reduction of accumulation of holes, the broadening of the recombination zone, and the improved balance of holes and electrons on Eu(TTA)3phen molecules. Finally, the device with 3 nm MoO3 and 5 nm TcTa obtained the highest brightness of 3278 cd/m2, current efficiency of 12.45 cd/A, power efficiency of 11.50 lm/W, and external quantum efficiency of 6.60%. Such a device design strategy helps to improve the EL performance of emitters with low-lying energy levels and provides a chance to simplify device-fabrication processes [40].

7.4  REVIEW OF LITERATURE ON GREEN OLEDs Bright-green emission from aluminum tris(8-hydroxyquinolinate) (Alq3) thin-film organic layers was first demonstrated by Tang and VanSlyke in 1987 [41]. Later, in 1990, Kido et al. [20] reported a double-layer OLED containing Tb-tris-(acetylacetonato), Tb(acac)3 as green-light-emitting material, N,N′-diphenyl-N,N′-bis(3-methylphenyl)1,1′-biphenyl-4,4′ diamine(TPD) as HTL with device structure, ITO/TPD/Tb(acac)3/Al. Strong emission peak at 544 nm peak, corresponding to the 5D4→7F5 transition of the Tb3+ ion, was observed [42]. Kasim et al. [43] synthesized a new conjugated polymer, poly(2,bquinoline vinylene) (PQV), which

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Figure 7.7  Chemical structure of Tb(MTP)3Phen [44].

exhibited maximum fluorescence at 515 nm when excited at 430 nm. Polymer light emitting diodes (PLEDs) constructed with ITO/PQV/Al configuration displayed broad emission peak at 530 nm. A thermally stable Tb-tris(tetradecylphethalate) phenanthroline complex Tb(MTP)3Phen was prepared and used as green-emitting layer by Ma et al. in 1999 [44]. The chemical structure of Tb-tris(tetradecylphethalate) phenanthroline complex Tb(MTP)3(Phen) is shown in Fig. 7.7. Multilayered EL device consisting of ITO/poly (p-phenylenevinylene) (PPV)/PVK: Tb (MTP)3(Phen)/Alq3/Al has been fabricated. In 2000, Lin et  al. designed a double-device structure, ITO/PVK: PBD: Tb (MDP)3/Alq3/Al. They achieved sharp-green emission with luminance of 152 cd/m2 at 24  V and poor external quantum efficiency of 0.017% [45]. Phosphorescent dendrimers with fac-tris(2-phenyl-pyridyl) iridium (III) cores, biphenyl-based dendrons, and 2-ethylhexyloxy surface groups, which emit green light, were reported by Markham et  al. The solution-processable green phosphorescent dendrimers were used to fabricate highly efficient single-layer devices as well as bilayer OLEDs, giving efficiencies of up to 16% with 40 lm/W at 400 cd/m2 [46–48]. In 2003 Palilis et  al. [49] reported 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-diphenyl silacyclopentadiene (PPSPP), 9-silafluorene-9spiro-1′-(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 PL quantum yields of 0.85, 0.87, and 0.94, respectively, were used as emissive materials. The structures of synthesized complexes and the device structure of the fabricated OLEDs by Palilis et  al. are shown in Fig. 7.8. The high electron mobility silole derivative,

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N

N

CH3

NPB

N

Si H3C

N

N

TPD

Si CH3 N

N

H3C N

H3C

CH3 Si

PyPySPyPy

2PSP

Si H3C

CH3 Si

PPSPP ASP Mg:Ag PyPySPyPy EML/ETL NPB HTL ITO

Mg:Ag PyPySPyPy ETL Silole EML NPB (TPD) HTL ITO

Figure 7.8  Structures of synthesized complexes and device structure of the fabricated OLEDs by Palilis et al. [49].

2,5-bis(2′,2″-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadien e(PyPySPyPy), was also 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(3methylphenyl)-1,1-biphenyl-4,4′-diamine (TPD) as the hole-transport material showed low operating voltages of 4–4.5  V at a luminance of

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Figure 7.9 Structures of Alq3 and NPB, and structure of the fabricated device by Hwang et al. [51].

100 cd/m2 and high external EL quantum efficiencies of 3.4–4.1% at 100 A/m2. Mishra et  al. [50] reported a bluish-green emission from OLEDs based on aluminum complex, bis(2-methyl 8-hydroxiquinoline) aluminum hydroxide (Almq2OH), as emissive material [50]. In 2005 Hwang et al. developed a stable green OLED using an Al–Cu alloy as a cathode material. The device structure is shown in Fig. 7.9, where CFx is N,N′bis-s1-naphthyld-N,N′-diphenyl,1,1′-biphenyl-4,4′-diamine(NPB), tris(8-quinolinolato)aluminum (Alq3), and lithium acetate (CH3COOLi) were used as the hole-injection material, hole-transport material, lightemitting material, and electron-injection material, respectively [51]. Ku et  al. [52] reported highly efficient undoped green OLEDs by incorporating a novel 9,9-diarylfluorene-terminated 2,1,3-benzothiadiazole (DFBTA), which exhibited an excellent solid-state photoluminescence quantum yield of about 81%. The optimal device ITO/DPAInT2/ DPAInF/TCTA/DFBTA/Alq3/LiF/Al displayed impressive device characteristics, with maximum external quantum efficiency of 12.9 cd/A. Liu et  al. in 2009 investigated highly efficient phosphorescent OLEDs [53] based on an orange/red emission iridium complex as the guest and five green emission iridium complexes as the host material, respectively. In 2010 Cho et  al. [54] synthesized Ir(Cz-ppy)2(Cz-Fppy)1, Ir(Cz-ppy)1 (Cz-Fppy)2, Ir(Cz-Fppy)3, and Ir(Cz-ppy)3, which exhibited green emission at 515, 511, 496, and 520 nm, respectively. With cationic iridium complexes as dopants and poly (N-vinylcarbazole) as host He et  al. [55]

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fabricated highly efficient blue-green to red and white OLEDs by solution processes. Complexes with cyclometalated 2-phenylpyridine ligands showed better device performance than those containing cyclometalated 2-(2,4-difluorophenyl)pyridine ligands. With the addition of an electrontransporting and exciton-blocking layer, the devices showed improved performance, achieving peak current efficiencies of 24.3, 25.3, 20.5, and 4.2 cd/A for the blue-green, green, yellow, and red EL, respectively. In 2011, Kim et al. fabricated high-efficiency OLEDs, in which solution-processed ambipolar blends of hole- and electron-transport polymer hosts doped with a green-emitting iridium complex were sandwiched between a photo-crosslinked HTL and a vacuum-deposited ETL. The ambipolar host blends consisted of blends of bis-oxadiazole-functionalized poly(norbornene) electron-transport materials and poly(N-vinylcarbazole). For the best device examined, an external quantum efficiency of 13.6% and a maximum luminous efficiency of 44.6 cd/A at 1000 cd/m2 with a power-on voltage of 5.9 V was obtained [56]. The energy-level diagram showing the estimated ionization potential (HOMO) and electron affinity (LUMO) levels for the materials investigated by Kim et  al. are shown in Fig. 7.10. Two heteroleptic iridium (III) complexes using carbene as cyclometalated ligands and pyridine-triazole as ancillary ligands, namely (fpmi)2Ir(mtzpy) (1) and (fpmi)2Ir(phtzpy) (2) (fpmi:1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C,C20, mtzpy: 2-(5-methyl-2H-1,2,4-triazol-3-yl) pyridine,

Figure 7.10  Energy-level diagram showing estimated Ionization energy (IE) and electron affinity (EA) levels for materials investigated by Kim et al. [56].

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phtzpy: 2-(5-phenyl-2H-1,2,4-triazol-3-yl)pyridine), were synthesized by Li et  al. in 2014. Both complexes exhibited bright greenish-blue phosphorescence (λmax = 490 nm) with quantum yields of about 0.50. The comprehensive density functional theory (DFT) approach was then performed to gain insight into their photophysical and electrochemical nature. The fabrication of OLEDs, employing complexes 1 and 2 as phosphorescent dopants, was successfully achieved. Among them, the device based on complex 1 exhibited considerable power efficiency (ηp) of 11.43 lm/W and current efficiency (ηc) of 11.78 cd/A.With the merit of intrinsic characteristic of complex 1, a white OLED comprised of 1 and one orange phosphor (pbi)2Ir(biq) achieved a peak hp of 9.95 lm/W and ηc of 10.81 cd/A, together with CIE coordinates 0.34, 0.40. The results indicate that the two iridium (III) complexes reported here are promising phosphorescent dyes for OLEDs [57]. The device configuration and energy-band diagram of the greenish-blue OLEDs and the molecular structures of compounds used by Li et al. are shown in Fig. 7.11. A green OLED with an extremely high power efficiency of over 100 lm/W was realized through energy transfer from an exciplex. An optimized OLED showed a maximum external efficiency of 25.7% and a power efficiency of 79.4 lm/W at 1000 cd/m2, which is 1.6-times higher than that of state-of-the-art green thermally activated delayed fluorescence (TADF) OLEDs [58]. A new class of highly phosphorescent Pt(II) complexes (Pt1–Pt3) based on rigid symmetric tetra dentate ligands (L1–L3) were designed and synthesized by Zhang et  al. recently. L1–L3 ligands are analogous to N,N-di(2-phenylpyrid-6-yl)aniline (L) except that one coordination phenyl group in L is replaced by other motifs with different

Figure 7.11 Device configuration and energy-band diagram of the greenish-blue OLEDs, and the molecular structures of the compounds used by Li et al. [57].

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electron donating/accepting capabilities. The effect associated with the modulation of a single coordination group within each ligand on the photophysical and EL properties of Pt1–Pt3 was investigated systematically. Among Pt1-Pt3, Pt1 had the highest HOMO due to the presence of a strong electron-donating group (3-methylindole), and exhibited the narrowest bandgap; Pt2 had the lowest HOMO due to the lack of strong donor group within the structure, and showed the widest bandgap. The OLEDs based on these three complexes showed yellowish-green to greenish-yellow EL with high efficiency. Notably, the device based on Pt1 at the doping level of 10 wt% achieved a maximum efficiency of 53.0 cd/A, 35.9 lm/W, and 16.3% with CIE coordinates 0.44, 0.53 [59].

7.5  REVIEW OF LITERATURE ON BLUE OLEDs Among the three primary RGB colors, the synthetic protocols and fabrication methods of green and red phosphors meet the necessary requirements. Conversely, the design and fabrication of blue phosphors and consequent devices is still an ongoing challenge. In 1992, Grem et  al. was the first to report blue EL from OLEDs containing poly(p-phenylene) (PPP) [60]. Organic EL devices with multilayer structures were fabricated using a 1,2,4-triazole derivative as the carrier transport layer. 3-(4-Biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) was found to be electron-transporting, and a cell with a structure of glass substrate/ITO/triphenylamine derivative (TPD)/TAZ/Alq/Mg:Ag exhibited bright-blue EL from the TPD layer. A luminance of 3700 cd/m2 with an emission peak at 464 nm was achieved at a drive voltage of 16 V [61]. A wide range of oligo(p-phenylenevinylene)s with alkyl [62–64] or alkoxy [65,66] substituents have been synthesized. Oligo(p-phe-nylene)s have been used as blue emitters [67,68] in EL devices and exhibit highfluorescence quantum yields. Tao and Suzuki [69] reported the blue emitter LiB(qm)4 in the device structure of ITO/PVK:NPB (50 nm)/ LiB(qm)4 (60 nm)/Mg:Ag. Power efficiency and luminance of the device were 1.3 lm/W and 6900 cd/m2, respectively. Li and coworker prepared Y, La, and Gd ion complexes with M(acea)3(Phen) [70]. However, these complexes were used as electron-transporting materials for the organic emitter, N,N′-bis(1-naphthyl-1,1′-biphenyl-4,4′-diamine) (NPB) with OLED structure ITO/NPB/M(acea)3 (Phen)/Mg:Ag. Hong and his coworker were the first to use Tm3+ ion in OLEDs [71]. They prepared a tris(acetylacetonato)-monophenanthroline Tm complex and double-layer

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cell with anatomy ITO/PVK/Tm complex/Al. Liu and Wang in 2001 [72] reported the blue emitter Bepp2. The Bepp2 was put in two differently structured devices: (1) ITO/NPB (60  nm)/Bepp2 (50 nm)/LiF (1 nm)/Al (200 nm) and (2) ITO/CuPc (15 nm)/NPB (60 nm)/Bepp2 (50  nm)/LiG (1  nm)/Al (200  nm). Fluorene-based blue EL polymers poly[9,9′-bis(2-ethylhexyl)fluorene-2,7-diyl] end-capped with N,N′bis(4-methylphenyl)-N-phenylamine [73] and poly(9,9′-dioctylfluorene2,7-diyl) (PF8, PFO) [74] also showed blue emission. The OLEDs fabricated with FIrN4 (iridium (III) bis(4,6 difluorophenylpyridinato) (5-(pyridin-2-yl)-tetrazolate)) as dopant in mCP (1,3-bis(9-carbazolyl) benzene) exhibited near-saturated blue electro phosphorescence with CIE coordinates 0.15, 0.24 [75]. Cheng et  al. [76] fabricated pure-blue OLEDs with CIE coordinates 0.1638, 0.094 at 16  V using a derivative of oligo(phenylenvinylene), 2,5-diphenyl-1,4-distyrylbenzene with two trans-double bonds, as a light-emitting layer. By introducing perylene as a dopant in the light-emitting layer, luminance and luminous efficiency dramatically improved from 1400 cd/m2 to 5500 cd/m2 and 1.18 cd/A to 3.18 cd/A, respectively. Ding et  al. [77] fabricated blue OLEDs using undoped 9,10-di(2-naphthyl)anthracene (ADN) as the emitting layer, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,10-biphenyl-4,40-amine (TPD) as HTL, and one of tris-(8-hydroxy-quinolinato) aluminum (Alq3), 4,7-diphenyl-1,10-phenanthroline (Bphen) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) as ETL. In 2010, Zheng developed efficient OLEDs by doping fluorescent- and phosphorescent-type emitters individually into two different hosts separated by an interlayer to form a fluorescence–interlayer–phosphorescence (FIP) emission architecture. Blue OLED with FIP emission structure comprising p-bis(p-N,N-diphenylaminostyryl) benzene (DSA-Ph) and bis[(4,6-di-fluorophenyl)-pyridinate-N,C2′]picolinate (FIrpic) exhibited a peak luminance efficiency of 15.8 cd/A at 1.54 mA/cm2 and a power efficiency of 10.2 lm/W at 0.1 mA/cm2 [78]. Haq et  al. [79] synthesized blue OLEDs with 9,10-bis(2naphthyl)-2-t-butylanthracene (TBADN) doped with (3 wt%) p-bis(pN,N-diphenyl-aminostyryl) benzene (DSA-Ph) as an emitting layer; the typical device structure was ITO/HIL (5 nm)/NPB (25 nm)/EML (35  nm)/ETL(15  nm)/LiF (0.8  nm)/Al (100  nm). At the current density of 20 mA/cm2, its driving voltage and power efficiency was 5.2  V and 4.2 lm/W, which was independently reduced by 48% and improved by 44% as compared with those of the m-MTDATA/Alq3-based one,

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respectively. In 2010, Pu et  al. [80] synthesized hole-transporting arylamino-9,10-diphenyl anthracene derivatives by C–N cross-coupling with palladium catalyst. These materials showed higher glass-transition temperatures (135–177°C). Alq3-based green-light-emitting devices containing the arylamino-9,10-diphenylanthracene derivatives as HTL were fabricated. A new series of blue fluorescent emitters based on t-butylatedb is(diarylaminoaryl) anthracenes were synthesized by Lee et  al. [81]. Into these blue materials, t-butyl groups were introduced to prevent molecular aggregation between the blue emitters through steric hindrance and to reduce self-quenching. To improve efficiency, multilayered OLEDs were fabricated into a device structure of ITO/NPB(50 nm)/blue emitters doped in ADN(30 nm)/Alq3(20 nm)/Liq(2 nm)/Al(100 nm). All devices showed efficient blue emissions. Highly efficient sky blue emissions with a maximum luminance of 11,060 cd/m2 at 12 V and respective luminous and power efficiencies of 6.59 cd/A and 2.58 lm/W at 20 mA/cm2 were accomplished. The peak wavelength of the EL was observed at 468 nm with CIE coordinates 0.159, 0.198 at 12.0  V. In addition, a deep-blue device with CIE coordinates of (0.159, 0.151) at 12 V showed a luminous efficiency of 4.2 cd/A and power efficiency of 1.66 lm/W at 20 mA/cm2. RGB phosphorescent P-i-N homojunction devices by using a series of bipolar host materials including 2,6-bis(3-(carbazol-9-yl)phenyl) pyridine (2,6DCzPPy), 3,5-bis(3-(carbazol-9-yl)phenyl) pyridine (3,5DCzPPy), and 4,6-bis(3-(carbazol-9-yl)phenyl) pyrimidine (4,6DCzPPm) were demonstrated by Cai et  al. [82] in 2011. Chen et  al. [83] synthesized three anthracene derivatives featuring carbazole moieties as side groups –2-tert-butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz9PhAnt), 2-tert-butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-carbazolyl)]phenyl} anthracene(tCz9PhAnt), and 2-tert-butyl-9,10-bis{4′-[3,6-di-tert-butyl(9-carbazolyl)]biphenyl-4-yl}anthracene (tCz9Ph2Ant) for use in blue OLEDs with high glass-transition temperature of 220°C as shown in Fig. 7.12. They exhibited strong blue emissions in solution, with high quantum efficiency of 91%. Blue-light-emitting host materials with a spiro[benzo[de]anthracene-7,90-fluorene] core, 3-[10-(naphthalene-1-yl)anthracene-9-yl] spiro[benzo[de]anthracene-7,90-fluorene] (NA-SBAF), and 3-[10-(naphthalene-1-yl)anthracene-9-yl]-1-methylspiro[benzo[de]anthracene7,90-fluorene] (NA-MSBAF) were designed and synthesized via coupling reactions. Introduction of a spiro group into the anthracene moieties led

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Figure 7.12 Chemical structures of anthracene derivatives and device structure of blue OLEDs fabricated by Chen et al. [83].

to reduction in crystallization tendency and a high glass-transition temperature. Typical blue fluorescent OLEDs with the configuration of ITO/N,N′-di(1-naphthyl)-N,N′-bis[(4-diphenylamino)phenyl]-biphenyl-4,40-diamie (60 nm)/N,N,N′,N′-tetra(1-biphenyl)-biphenyl-4,40diamine(30 nm)/host: dopant (30 nm, 5%)/LG201 (ETL, 20 nm)/LiF/Al were developed using SBAF-type anthracene derivatives as host material and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph) as sky-blue dopant material. A device obtained from NA-SBAF doped with DSA-Ph was compared with that of 9,10-dinaphthalene-2-yl-anthracene and showed blue color purity of 0.150 and 0.217, a luminance efficiency of 7.57 cd/A, and an external quantum efficiency >5.15% at 5.0 V [84]. In 2015 Chang [85] demonstrated fully solution-processed blue OLEDs with n-type doped multilayer graphene as the top electrode. The work function and sheet resistance of the graphene were modified by an aqueous process that can also transfer graphene on organic devices as the top electrodes. With n-doped graphene layers used as the top cathode, all-solution processed transparent OLEDs can be fabricated without any vacuum process. Recently, Zhu et al. synthesized two blue emitters, 2,7-bis(9-benzyl9H-carbazol-2-yl)pyrene and 2,7-bis(4-(5-phenyl-1,3,4-oxadiazol-2-yl) phenyl)pyrene by a Suzuki coupling reaction. The emission peaks of the two emitters were found to be 430 and 439 nm with 87.5 and 68.6% fluorescence quantum yields in chloroform, respectively. The emitters both had good thermal stability (Tg > 160°C). Nondoped blue OLEDs with these emitters were achieved with x, y coordinates 0.17, 0.11 and 0.16,

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0.15, respectively, which are very close to the National Television System Committee standard blue [86].

7.6  REVIEW OF LITERATURE ON WHITE OLEDs White organic light-emitting diodes (WOLEDs) are becoming more popular as third-generation (3G) lighting sources due to their outstanding features such as low energy consumption, low cost, high efficiency, and flexible properties, extremely long life, high durability, and the fact that they are pollution-free. They are used in full-color displays and as backlights for LCD displays and energy-efficient traditional lighting sources [87]. Several approaches have been proposed to engender white-light with an assortment of OLED device configurations and diverse emissive materials [88–90]. As a promising candidate for lighting, white OLEDs should generate light with spectral distribution similar to that of natural sunlight covering the full visible range as much as possible. To obtain high brightness WOLEDs, blue-emitting OLEDs are mainly used in combination with yellow phosphor to partially downconvert the blue emission into light with longer wavelengths or by a combination of RGB phosphor. With intrinsically high quantum efficiency, WOLEDs incorporating phosphorescent emitters have become the most promising candidates for meeting the stringent efficiency requirements for lighting applications today [91,92]. For lighting purposes, light sources with CIE coordinates closer to the ideal white point (0.33, 0.33), CRI above 80, and CCT similar to those of the blackbody radiation between 2500K and 6500K are required for better color purity [93]. Research on WOLEDs has rapidly increased since the first demonstration of WOLED by Kido et al. by dispersing blue, red, and green fluorescent dye in polyvinylcarbazole (PVK) that together produce white light with efficiency <1 lm/W [94]. White-light emission is usually observed by a set of different luminophores with distinct emission colors, typically two (blue and orange/yellow) or three (blue, green, and red). Fig. 7.13 represents the general strategies used to develop devices that combine multiple emitters in the EML to produce white light [95]. Kido et al. [96] used Tb3+ and Eu3+ complexes to achieve multilayer white OLEDs with Eu (aca)3Phen binuclear complexes as the emitting layer. These devices emit bright-white light with more uniformity and energy efficiency than that of fluorescent lights. In 1999, Deshpande et  al. obtained white-light emission by the sequential energy transfer between different layers. The device configuration was ITO/α-NPD/α-NPD: DCM2(0.6–8 wt%)/BCP/

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Figure 7.13 General approaches to generating white light from OLEDs using multiple emitters: (A) single-EML structure, (B) multilayer EML structure, (C) stacking and tandem structure, and (D) striped structure [95].

Alq3/Mg:Ag (20:1)/Ag. 4,4′-Bis (N-(1-napthyl-N phenylamino)) biphenyl (α-NPD) was used as a hole-injection layer, α-NPD: DCM2 (2,4-(dicyanomethylene)-2-methyl-6-(2-(2,3,6,7-tetrahydro-1H,5H benzo(i,j)quinolizin-8-yl) vinyl)-4H-pyran) was used as HTL and as well as an emitting layer, 2,9-dimethyl-4,7-diphenyl-1,10- phenanthroline (BCP) was used to block holes, and Alq3 was used as ETL and Mg:Ag alloy followed by a thick layer of Ag deposited as the cathode [97]. Use of phosphorescent dopants as emitters in a segregated-layer WOLED was first demonstrated by D’Andrade et  al. in 2001 [98] with the device structure ITO/poly(ethylene-dioxythiophene):poly(styrene sulphonic acid) (PEDOT:PSS)/4,4′-bis[N-(1-napthyl)-N-phenylamino] biphenyl (α-NPD) 30 nm/4,4′-N,N′-dicarbazolebiphenyl (CBP) 20  nm:6  wt% iridium(III) (bis(4,6-di-fluorophenyl)-pyridinato-N,C2) picolinate (FIrpic)/CBP layer 8 wt%: bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3) iridium (acetylacetonate) (Btp2Ir(acac)), 2 nm/CBP 8 wt% bis(2-phenyl benzothiozolato-N,C2′) iridium (acetylacetonate) (Bt2Ir(acac))2 nm/2,9-dimethyl-4au,7-diphenyl1,10-phenanthroline (BCP) as the final organic layer, which served as both a blocking and ETL. Efficiency of about 5.2% external quantum efficiency, CIE coordinates 0.35, 0.36, CRI of 83, and peak brightness over 30,000 cd/ m2 were achieved. A white-light-emitting device was fabricated by Xiao et al.

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Figure 7.14  Schematic cross-section of fabricated OLED structure by Xiao et al. [99].

[99] in 2005, with a structure of ITO/NPB/BCP/Alq3/LiF/Al as shown in Fig. 7.14. The HBL (BCP) results in a mixture of lights from NPB molecules (blue light) and Alq3 molecules (olivine light), thereby emitting white light. A maximum brightness of 5740 cd/m2 with EL efficiency of 2.12 cd/A at the applied voltage of 18 V was achieved. The schematic cross-section of the fabricated OLED structure is shown in Fig. 7.15. In 2005 Tsou et al. fabricated white OLEDs with CIE coordinates 0.32, 0.32 by doping 1% DCM2 in the BCP layer [101]. In the same year, Gong et  al. reported high-performance multilayer white light-emitting PLEDs fabricated by using a blend of luminescent semiconducting polymers and organometallic complexes as the emission layer and water- or ethanol-soluble PVKSO3Li as the hole-injection/transport layer and t-Bu-PBD-SO3Na as the electron-injection/ETL [102].Yu et al. in 2009 reported white LEDs using a blue InGaN LED precoated conjugated copolymer/quantum dots (QDs) composite (green-emitting poly{(9,9-dioctyl-2,7-divinylenefluorenylene)alto-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)}/red-emitting CdSe QDs) as a hybrid phosphor. The white LED of the hybrid phosphor containing 20 wt% QDs had a luminous efficiency of 44.2 lm/W at 20 mA with CIE coordinates 0.3297, 0.3332, CCT 5620K, and CRI 75.3, respectively [103]. Bright WOLEDs with a single active layer were demonstrated from blue-emitting zinc complex bis(2-(2-hydroxyphenyl)benzoxazolate) zinc [Zn(hpb)2] doped with orange luminescent 4-(dicyanomethylene)2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM) dye. White EL spectrum from Zn(hpb)2 was achieved by adjusting the concentration of DCM dye. Additionally, WOLEDs with a structure of ITO/α-NPD/ Zn(hpb)2:DCM (x%)/BCP/Alq3/LiF/Al have been fabricated. The EL spectra with two peaks at 446 and 555 nm were obtained. The device

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Figure 7.15  (A) White organic light-emitting diodes configuration, (B) WOLED device, and molecular structures of (C) DCM dye and (D) Zn(hpb)2 [100].

emitted white light at 10 V with CIE coordinates 0.27, 0.31 and brightness 1083 cd/m2. The maximum current efficiency of the device was 1.23 cd/A at 9.5 V and maximum luminance reached 2210 cd/m2 at 12 V [100]. The configuration of the WOLEDs and the molecular structures of DCM dye and Zn(hpb)2 are shown in Fig. 7.15. Chang et al. reported high color-rendering pure-white phosphorescent OLEDs by iridium complex Ir(dfbppy)(fbppz)2 and a wide-band-width yellow-emitting osmium complex Os(bptz)2(dppee). They achieved a CRI of 81 and CIE coordinates 0.33, 0.33, which is close to the ideal white emission [104]. Tyagi et  al. in 2010 demonstrated a WOLED by double layers of blue Zn(hpb)2 and yellow Zn(hpb)mq emitting materials. It was

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observed that when the thickness of Zn(hpb)mq layer was increased the dominant wavelength shifted from bluish to yellowish region, and CIE coordinates 0.29, 0.38 with a low power-on voltage (5 V) was achieved [105]. Chen et  al. in 2010 [106] fabricated WOLEDs utilizing two primary color emitters without any additional blocking layer. Anthracene was deposited directly above the rubrene (Rb)-doped NPB yellow lightemitting layer with a structure of ITO/2TNATA(20 nm)/NPB(20 nm)/ NPB: rubrene (2%)(10 nm)/ADN(30 nm)/Alq3(20 nm)/LiF(1 nm)/ Al(100 nm), a white light with CIE coordinates 0.344,0.372 at a current density of 30 mA/cm2 was generated. In 2011 Hu et al. [107] reported a theoretical investigation of the white-light emission from a single polymer system with simultaneous blue polyfluorene as host and orange 2,1,3-benzothiadiazole (BTD)-based derivative as dopant emission. They employed quantum chemical approaches to study variations in electronic and optical properties as a function of the chemical composition of the backbone in BTD-based derivatives. The chemical structure of model polymers is shown in Fig. 7.16. In the same year, Seo et  al. [108] demonstrated hybrid white organic light-emitting diodes (HWOLEDs) on EL characteristics for codoped spacer ratio effect using N,N′-dicarbazolyl-3,5-benzene (mCP) and 4,7-diphenyl-1,10-phenanthroline (BPhen). They achieved external quantum efficiency of 6.01%, power efficiency of 8.12 lm/W, and CIE coordinates of 0.37, 0.41 at 1000 cd/m2. In 2011, Wang explored the relationship between the electronic properties of a host/dopant system and obtained a high-efficiency single-dopant white polymer light-emitting device with two novel blue-emitting cyclometalated iridium (III) complexes of (dfppy)2Ir(Tfl-pic) and (dfppy)2Ir(Brfl-pic) where dfppy is 2-(2,4-difluorophenyl)pyridine, Tfl-pic, and Brfl-pic are picolinic acid derivatives containing trialkylfluorene and dibromoalkylfluorene units bridged with an alkoxy chain, respectively. Both iridium (III) complexes exhibited blue emission in dichloromethane solution and their neat films and possessed good dispersibility and thermal properties. Two different devices using (dfppy)2Ir(Tfl-pic) as a single-component emitter and a blend of poly(Nvinylcarbazole) and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole as the host matrix were fabricated [109]. Han et al. fabricated flexible OLEDs by modifying the graphene anode to have high work function and low sheet resistance. They achieved extremely high luminous efficiencies of 37.2 lm/W in fluorescent OLEDs and 102.7 lm/W in phosphorescent OLEDs [110]. Semiconductor-based light sources with high energy

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Figure 7.16 (A) Chemical structure of model polymer (PF)n–(OMC-CH3)m and (B) investigated BTD-based derivatives (OMC and its derivatives) [107].

efficiencies are critical technologies for reducing the global carbon footprint. In particular, white OLEDs have received huge worldwide attention in recent years, partially due to their success in the flat-panel display market and features such as a unique thin, flat, foldable form factor. An overview on the current status of OLEDs for lighting applications is very well depicted by Chang et al. in 2013. Furthermore, a detailed overview of the state-of-the-art on white OLED design concepts including their working principles was presented [111]. A double-layered graphene/poly (3,4-eth ylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conductive film was prepared, in which the PEDOT:PSS layer was obtained by using a spray-coating technique. Flexible white phosphorescent OLEDs based on the graphene/PEDOT:PSS conductive film was fabricated.

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Phosphorescent material tris(2-phenylpyridine) iridium (Ir(ppy)3) and the fluorescent dye 5,6,11,12-tetraphenylnapthacene (Rubrene) were codoped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host. N,N′diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine(NPB) and 4,7-diphenyl-1,10-phenanthroline (Bphen) were used as HTL and ETL, respectively, and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) was used as blue-light-emitting layer. The device presented pure white-light emission with CIE coordinates 0.31, 0.33 and exhibited excellent light-emitting stability during the bending cycle test with a radius of curvature of 10 mm [112]. The device configuration of the flexible phosphorescent WOLEDs based on grapheme/PEDOT:PSS conductive film and the chemical structures of the organic materials used in this work are shown in Fig. 7.17A and B, respectively. In 2015, Sakumaet et  al. proposed a self-layered technique to form an emitting layer with a pseudo-multilayered structure by onestep coating and demonstrated the feasibility of the concept [113]. They also fabricated a highly efficient WOLED with the proposed technique. A maximum power efficiency of 70 lm/W was obtained by improving the effective radiation efficiency, carrier balance efficiency, and light-extraction efficiency. White-emissive devices with a dual-emitting layer based on the orange and blue (FIrPic) phosphor were recently fabricated by Zhang et al. They showed CIE coordinates 0.33, 0.41 and 0.31, 0.40 and maximum current efficiencies of 8.9 and 13.8 cd/A [114]. Current development of the synthesis of single-layer graphene and its future prospects has been reported in many reviews [115].

7.7 CONCLUSIONS Organic light-emitting materials have been attracting the attention of researchers from industry and academic institutions due to their applications in OLED devices, flat-panel displays, and solid-state lighting. Although OLED has the potential to redefine many present-day lighting and display solutions, there are a few hurdles to overcome. Fabrication of highly reliable, efficient, and long-life blue OLEDs is still challenging, due to the difficulty in aligning the energy levels at the layer interfaces. Thus manufacturing processes are expensive and simpler and cheaper technology has to be developed in order to commercialize them. Reliable testing standards are also needed to establish consistency and to reduce uncertainty. Researchers should concentrate on improving various factors such

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Figure 7.17  (A) Device configuration of flexible phosphorescent WOLEDs based on grapheme/PEDOT:PSS conductive film. (B) Chemical structures of the organic materials used in this work [112].

as electricity-to-light conversion efficiency, device stability and lifetime, material selection, proper encapsulation, methods to maintain uniformity over large areas, novel fabrication technologies, and reducing manufacturing costs. The literature on RBG and white light-emitting device

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architectures reveals various approaches to enhancing the efficiency and lifetime of OLEDs and handling the degradation issues of the organic materials for OLEDs. If we succeed in improving the efficiency, performance, and lifetime, current lighting systems can be replaced by ecofriendly, energy-efficient green technology called solid-state lighting, which would play a significant role in reducing global energy consumption. Cutting-edge research predicts a bright future for display devices as the next generation of light sources for general illumination, from homes to commercial applications, offering low energy consumption and reduced maintenance. As discussed in this chapter, OLEDs can pave the way for a new era of large-area lighting, which is transparent, flexible, and environmentally friendly.

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