LEDs Based on Small Molecules

CHAPTE R 7

LEDs Based on Small Molecules Lian Duan Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing, China

7.1 Introduction 7.1.1 History In the 1960s, the research on organic light-emitting materials began to develop rapidly. In 1963, Pope et al., at New York University, realized the electroluminescence (EL) of organic crystal germanium for the first time. The device was made of silver colloid and sodium chloride (0.1 M NaCl) is an electrode, and blue fluorescence is observed in the neon, but the driving voltage of the device is as high as 400 V with a film thickness of 20 μm, and only a weak blue light is emitted and the device efficiency is extremely low (Pope et al., 1963). Subsequently, in 1982, based on the work of Pope in Vinettt et al., the translucent gold was used as the anode material, the film thickness of the organic crystal crucible was further reduced by vacuum evaporation, and the film thickness was successfully prepared to be 0.6 μm. The device drives a voltage of only 30 V, but it still has less than 1% efficiency (Vincett et al., 1982). The research on organic luminescent materials was once practically nonexistent. Until 1987, Tang et al. of the Kodak Company in the United States prepared a double layer of organic semiconductor (OSC) materials of 8-hydroxyquinoline aluminum (Al) and aromatic diamine by vacuum thermal evaporation. The device uses a two-layer device structure to solve the energy-level matching of the electrode material and the organic lightemitting layer material and avoids the luminescence-quenching effect caused by the electrode being too close to the organic layer and succeeds in the organic light-emitting diodes (OLEDs) (Tang et al., 1987). Developed as a flat-panel-display technology with practical value. In 1990, the University of Cambridge, RH Friend Group first reported on the nature of the use of high-molecular-polymer material polyphenylene ethylene (PPV) EL at a lower driving voltage, creating a polymer-based new era of material flat-panel-display technology (Burroughes et al., 1990). In 1992, Heeger et al. pioneered the use of solution spin-coating Advanced Nanomaterials for Solar Cells and Light-Emitting Diodes. https://doi.org/10.1016/B978-0-12-813647-8.00007-2 © 2019 Elsevier Inc. All rights reserved.

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216  Chapter 7 with polyethylene terephthalate (PET) as the substrate for flexible electronic devices, showing the future of flexible display of OLED in the future (Gustafsson et al., 1992). In 1997, Japan Pioneer Company developed the first organic flat-panel-display product-car audio display. In the same year, Idemitsu Corporation developed a 5-in. passive drive, full-color OLED display. In 1998, the Forrest group of Princeton University adopted a method of doping phosphorescent dyes, which fully utilized the attenuating triplet excitons of phosphorescent materials due to spin-orbit coupling (SOC), and the internal quantum efficiency (IQE) of the device was based on the law of rotation is fixed, and the theoretical 25% limit is increased to 100%, thus opening up a new field of organic electrophosphorescence (Gu et al., 1997). In the same year, the white-light EL was first realized by Professor Kido et al. of Yamagata University of Japan (Hebner et al., 1998). In recent years, with the continuous development of display technology, OLED display technology has an all-solid state, is visible in the entire spectral range, is ultralight and ultrathin, has a wide viewing angle, high brightness, low voltage, and low power consumption, can be used for flexible substrates, and has a wide range of operating temperatures. High definition and fast response are widely used in flat-panel displays and solid-state lighting.

7.1.2  Basic Structures and Principles of OLED 7.1.2.1  Basic structures The single-layer structure is the simplest type of OLED device. As shown in Fig. 7.1, it is composed only of an organic luminescent material between the anode and the cathode. This is also the device structure used in early OLEDs. Under the action of the applied voltage, the holes of the anode and the electrons of the cathode overcome the injection barriers and are recombined after being phase-transferred to generate photons. Although a single-layer device has the advantages of a simple preparation method, a single-layer device based on an organic light-emitting material exists such that the light-emitting layer is close to the metal electrode, and there are many defects, resulting in a large number of nonradiative transitions and a

Fig. 7.1 Basic structure of the OLED single-layer device.

LEDs Based on Small Molecules  217

Fig. 7.2 Basic structures of the OLED devices with (A), (B) double layers, and (C) multilayers.

single transmission property of the material and a positive load. The problem of unbalanced transmission of the stream results in generally poor performance of single-layer devices. At present, the device structure commonly used in OLED devices is as shown in Fig. 7.2C, which mainly includes an anode, a hole-injection layer (HIL), a hole-transporting layer (HTL), an emission layer (EML), an electron-transporting layer (ETL), an electron-injection layer (EIL), and a cathode. Prior to this, OLED devices could be classified into single- and double-layer types. The earliest reported Pope components are the simplest, but due to the heterogeneous charge transport properties of organic light-emitting materials and the difficulty in the transmission of electrons and holes, the efficiency of the prepared single-layer devices is very low, and it is not used in large quantities. With the rapid development of organic EL technology, Tang et al. first reported a two-layer OLED device in 1987. The hole transport material (HTM) of this device is an aromatic amine compound, and the light-emitting layer uses a layer of pure Alq3. However, due to the concentration-quenching effect of the luminescent material itself, the device efficiency is very low, and the luminescent material used in the device also serves as the electron hole-transporting function, which is equivalent to the EML in the current device structure and has high material requirements. The double-layer device structure consists of electrodes, a light-emitting layer, and a carrier transport layer. As shown in Fig. 7.2A and B, the luminescent layer usually uses a luminescent material with higher fluorescence efficiency

218  Chapter 7 and a certain carrier-transporting ability. With the development of technology and people in pursuit of higher efficiency and better performance of OLED devices, a multilayer device structure has been proposed, in which EILs and HILs are widely used. At present, most highly efficient OLED devices adopt multilayer device structures (Fig. 7.2C). Compared with the previous ones, devices employing multilayer structures benefit from material selection and device structure optimization. This will effectively improve the charge injection and charge transmission, and greatly improve the device efficiency. Currently, the most commonly used OLED device is a carrier double-injection, lightemitting device (LED). In this type of device, there is no p-n junction and free carriers. Finally, under the action of an electric field, electrons and holes are injected into the organic film, transmitted, and recombined. The technology for achieving luminescence is a solid semiconductor device. The working principle of OLED is as follows: When the device is subjected to forward bias derived from direct current, the energy of the applied voltage will cause electrons and holes to be injected into the device from the cathode and the anode, respectively. The holes transport on the highest occupied molecular orbital (HOMO) and the electrons transport on the lowest unoccupied molecular orbital (LOMO). They finally combine with each other at the position of the emitting layer to form electron-hole pairs of binding energy levels. That is, the excitons that are the final carriers are radiated to attenuate and release a large number of photons. In general, the basic principles of OLED device illumination can be roughly summarized as incorporating three processes: (1) carrier injection, (2) carrier transport, and (3) carrier recombination and radiation attenuation. 7.1.2.2  Carrier injection In the carrier-injection process, under the action of a strong electric field and after electrons and holes overcome the interface barrier, they are injected from the anode and cathode of the OLED device, respectively; that is, the hole transitions from the Fermi level of the anode to the HOMO of the organic material. Similarly, electrons transition from the Fermi level of the cathode to the LUMO level of the organic material. In order to understand the injection mechanism of carriers, there are important theoretical models to use, including Schottky thermionic emission, tunneling injection, and displacement of vacuum levels. In Schottky thermal electron emission, when the electrons have sufficient kinetic energy at higher-temperature conditions, the high-energy electrons reach the electrode surface and escape. This phenomenon is called the thermal emission effect, and the hot electron injection limits the current:  eV    −Φ B  j = A∗T 2 exp   exp   − 1 (7.1)  nK BT    K BT 

LEDs Based on Small Molecules  219 where j is the current density; A∗ is the Richardson constant; ΦB is the barrier that must be overcome for hot electron injection; V is the operating voltage of the device; n is the ideal factor; KB is the Boltzmann constant; and T is the temperature. Tunneling injection refers to the tunneling injection limiting current expressed by considering the Fowler-Nordheim current under the condition that the carrier injection is related to the electric field strength, but independent of temperature: jFN

3  2 2 2 a Φ A∗  eF  B exp  − = 3 eF Φ B  ak   

(

    

(7.2)

)

1

where F is the electric field strength at the barrier; a = 4π 2 m∗ 2 / h; m∗is the mass of the effective carrier; the displacement of the vacuum level means that the vacuum-level shift effect due to the electric double-layer structure of the interface needs to be considered at the metal organic interface. 7.1.2.3  Carrier transport In the carrier-transport process, electrons are transported at the LUMO level of the EIL and the ETL, driven by an applied electric field, and finally injected into the light-emitting layer of the organic material through a transition. In the light-emitting layer of an organic material, carriers pass through a barrier or a tunneling effect across the barrier existing in the interface. Since the organic thin film has no continuous energy band and there are delocalized free electrons, electrons continuously jump in the organic layer to migrate from one molecule of the LUMO level to the other molecule's LUMO level. Depending on the type of electrical contact, the current of the OLED device can be divided into an injection-control type and a space-charge limitation by the organic layer. The various currents of the OLED device can be divided into five types: (1) Hot electron injection limiting current:  eV    −Φ B  j = A∗T 2 exp   exp   − 1  K BT   nK BT   (2) Tunneling injection limiting current: jFN

3  2 2 2 a Φ A∗  eF  B exp  − = 3 eF Φ B  ak   

(3) Space charge limiting current without traps or traps: 9 v2 j = εε 0 µ 3 8 d

    

(7.3)

(7.4)

(7.5)

220  Chapter 7 where ε0 is the vacuum dielectric constant; ε is the relative dielectric constant of the organic material used for the luminescent layer; μ is the mobility of the carrier; and d is the thickness of the device. The interface barrier is small, and the I-V curve of the device using polymers and small molecules is in good agreement with this theory. (4) Space charge limiting current with trap: 9 V2 j = εε 0 µ .Θ. 3 8 d

(7.6)

where Θ represents the ratio of free carriers to the total number of carriers. (5) Trap charge limiting current:  2m + 1  j = qµ N    m +1 

m +1

 m εε . m0   m + 1 q Nt

 v m +1  2 m +1 d

(7.7)

where N is the density of states of the charge level: 1

 2σ 2  2 m = 1 + 2 2   16 K B T 

(7.8)

At present, research has shown that generally, most OLED devices conform to the trap charge limiting mode. That is, the current and voltage of the device are in a power-off relationship. The five cases given here are the basic models often used in the study of the carrier injection and transmission of OLED devices, but there are often cases where the characteristics of the device in the operating voltage range cannot be well explained by only one model. There are several theories that need to be explained at the same time, so there are great differences in the injection and transport of carriers for different organic light-emitting-layer materials, electrode contacts, and device structures. 7.1.2.4  Carrier recombination and radiation attenuation With carrier recombination and radiation attenuation, when the electrons and holes injected from the anode and cathode transition from the Fermi level to the LUMO and the HOMO levels of the organic material, respectively, the electrons and holes recombine under the Coulomb force. Electron-hole pairs (i.e., excitons), are formed. An exciton is a bound state of electrons and holes. When the exciton radiation is attenuated, the photons are emitted after the end of life. The wavelength corresponds to the corresponding energy interval, and finally, the light of the corresponding wavelength is released, including singlet and triplet excitons. There are three times as many of the latter as the former, and the radiation transition of the singlet excitons produces fluorescence, whereas the radiation transition of the triplet excitons produces phosphorescence.

LEDs Based on Small Molecules  221

7.2  Recent Progress in OLED Materials 7.2.1  Anode Materials In 1987, Dr. Tang used indium tin oxide (ITO) as a transparent anode to prepare highperformance OLED devices (Tang et al., 1987). ITO has good electrical properties, with only 10–100 Ω/□ of sheet resistance. Besides, it has good light transmittance in the visible range. In addition, the work function of ITO can be increased to 5 eV after ultraviolet (UV)ozone treatment, which effectively reduces the hole-injection barrier of the anode portion. It is an anode material with excellent comprehensive performance (Helander et al., 2011). Up to now, ITO has become the most widely used transparent anode in OLEDs and photovoltaic (PV) devices by virtue of its excellent properties, such as high light transmittance, electrical conductivity, and work function. However, ITO is mainly prepared by magnetron-sputtering deposition and requires expensive raw materials, thereby resulting in high production cost of the transparent anode-based OLED device. Moreover, the ductility of ITO is poor and cannot meet the needs of developing flexible organic electronic devices (Li et al., 2013a, b). Therefore, exploring anode materials with excellent photoelectric properties to replace conventional ITO electrodes is crucial for further promoting the development of OLEDs and PV devices. In recent years, anode materials including transparent conductive films, graphene, metal nanowires, and conductive polymers have been gradually applied to the preparation of highperformance OLED devices and flexible devices. These new anode materials also have good development prospects due to their excellent photoelectric properties. In2O3, ZnO, SnO2, and TiO2, as typical metal oxide films, as well as a doping system formed of metal like Al or gallium (Ga), are widely used for preparing an anode material of an OLED device. In 2016, Morales-Masis et al. optimized the doping ratio of ZnO:Al and SnO2 to prepare a high-performance ZTO transparent oxide film. The sheet resistance Rsh of ZTO/Ag/ZTO anode is only 9 Ω/□. It has a higher light transmittance than ITO in the visible light range. The ZTO/metal grid is used as the anode of a small-molecule OLED device with a maximum current efficiency of 130 cd/A (Morales-Masis et al., 2016). Lee et al. used a high-refractive-index TiO2 with graphene as the anode of a OLED device to enhance the microcavity resonance of the anode (Lee et al., 2016a, b). The maximum external quantum efficiency (EQE) of the prepared green phosphorescent device reached 40.8%, and the maximum power efficiency reached 160 lm/W. Graphene is expected to be applied to transparent electrodes of flexible organic electronic devices due to its unique photoelectric and mechanical properties. By chemical doping, atomic substitution, and other processes, the conductivity of the graphene electrode can be effectively improved and the carrier injection performance and the luminous efficiency of the OLED device can be improved. Duk et al. spin-coated a bis(trifluoromethanesulfonyl)-amide

222  Chapter 7 (TFSA) solution in nitromethane on a multilayer graphene electrode to increase the work function of graphene from 4.4 to 5.1 eV by p-doping (Kim et al., 2013a, b). Also, the injection barrier from the electrode to the HTL is reduced and the sheet resistance is reduced by about 65%. The current efficiency and power efficiency of the corresponding OLED device exceed those of the ITO anode-based reference device. In 2017, Chiu et al. used borondoped graphene as the transparent anode of OLED devices, and low concentration of borondoped graphene could avoid film defects caused by chemical doping, and its transmittance was as high as 97.5%, hole mobility up to 1600 cm2/(Vs), the maximum EQE of the green OLED device based on boron-doped graphene anode can reach 24.6%, the maximum power efficiency reaches 99.7 lm/W (Wu et al., 2017). Nanowires of high-work-function metals such as copper (Cu), gold (Ag), and silver (Au) have become potential alternative anode materials for ITO. The Ag nanowire film prepared by Lim et al. has electrical conductivity and light transmittance comparable to ITO (40.2 Ω/□, 94.8%) (Sim et al., 2016). The nanomesh embedded in the electrode prepared by polydimethylsiloxane (PDMS) has good electrical conductivity and tensile properties, which can be used as an ideal flexible transparent electrode. Lin et al. modified Ag nanowires (AgNWs) with MoOx solution, and MoOx aggregated at the nodes of Ag nanowires, which significantly reduced the sheet resistance (29.8 Ω/□) of Ag transparent electrodes and maintained better light transmittance (Chang et al., 2015). The green phosphorescent OLED device based on a MoOx-modified, Ag nanowire (AgNW) anode can achieve a power efficiency of 14.2 lm/W at 1000 cd/m2 and a starting voltage of 2.9 V. The polymer transparent electrode has many advantages, such as excellent conductivity, high light transmittance, low cost, and flexibility, and is a new type of anode material suitable for flexible OLED devices. At present, polymer transparent electrodes represented by poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) have received much attention from researchers. Yong, H.K., et al. combined the theoretical simulation and device results to optimize the thickness of the HTL and the PEDOT:PSS transparent anode to prepare a high-performance, long-living OLED device (Yong et al., 2013). Wu et al. from National Taiwan University used a two-layer PEDOT:PSS film with a high refractive index of TiO2 as the anode of the OLED device, and the maximum EQE of the device was close to 39% (Jiao et al., 2016). Lee et al. found that H2SO4 treatment affected the structural rearrangement of PEDOT:PSS and improved its crystallization performance (Kim et al., 2014a, b, c, d). The polymer film after acid treatment has a conductivity of up to 4380 S/cm. In addition, composite transparent electrodes based on metals and carbon nanotubes not only have high light transmittance, but also have good electrical conductivity and tensile properties, which have been deeply studied in the field of OLED applications. The metalcomposite electrode mostly adopts a sandwich structure of dielectric and metal, which can maintain high light transmittance and conductivity of the metal and hole-injection performance of the medium. Tang et al. of Suzhou University designed a metal-dielectric

LEDs Based on Small Molecules  223 composite electrode (MDCE) based on Ca:Ag alloy film (Xu et al., 2015). The modification of MoO3/Ca:Ag/MoO3 improved the hole-injection performance of the electrode. A greenlight, flexible OLED device based on such a nanostructured metal-dielectric composite electrode (NMDCE) can achieve an EQE of 45.6% and a power efficiency of 95.1 lm/W at 1000 cd/m2. There is no significant attenuation in device performance after 800 bends. UCLA Pei et al. prepared composite electrodes of silver nanowires (AgNWs) and single-walled carbon nanotubes (SWNTs) (Li et al., 2014a, b, c). Compared with ITO, the maximum EQE of green-light devices based on nanocomposite electrodes was as high as 38.9%. The device is increased by 246%. In recent years, the rapid development of OLED transparent electrode technology has significantly improved the efficiency and lifetime of these devices. These new transparent electrodes have high permeability and electrical conductivity, and most of them adopt more energy-saving and environmentally friendly processing and preparation processes, especially showing unique development potential in flexible electronic devices.

7.2.2  Hole-Injection Materials and Anode Interface Modification The hole-injection barrier in the OLED device is mainly derived from the difference between the Fermi level of the anode material and the HOMO level of the HTL. At present, the most widely used ITO anode (WF = 4.7 eV) has a large hole-injection barrier with a common HTM (the HOMO level is about 5.4 eV), which restricts the efficiency of hole injection in the device. Therefore, the introduction of hole-injection materials, interface modification, and other design strategies can effectively reduce the hole-injection barrier, improve the carrier recombination ratio, and prepare high-performance OLED devices. The modification of the interface of ITO can effectively improve the work function of the anode material, better match the HOMO level of the HTM, reduce the hole-injection barrier, and improve the carrier injection and recombination efficiency. At present, the more common modification method is UV-ozone treatment of ITO. After modification, the work function of ITO can be increased from 4.7 to 5.1 eV. Park et al. studied the work function and surface morphology of ZnO:F anodes and found that this fluorine-doping modification strategy can reduce the on-voltage of the device and improve the current efficiency of the device (Choi et al., 2014). In addition, the introduction of a HOMO level that matches the implant-layer material can improve the surface morphology of the ITO film, reduce the hole-injection barrier, and improve the efficiency and lifetime of the device. Such materials mainly include PEDOT:PSS, 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA), and the like. Friend et al. found that the work function of PEDOT:PSS and ITO increased by 0.3 and 0.15 eV, respectively, after methanol treatment (Tan et al., 2014). This modification strategy can further improve the hole-injection performance of polythiophene materials. Bradley et al. used copper thiocyanate (CuSCN) with excellent light-transmittance and hole

224  Chapter 7 transport properties as a hole-injecting material, and the maximum current efficiency of the prepared green-light device was 50 cd/A (Perumal et al., 2015). Another type of hole-injecting material mainly includes insulating materials such as lithium fluoride (LiF), a self-assembled monomolecular layer, or a fluoropolymer. Such a thin layer of insulating material can significantly improve the roughness of the ITO surface, reduce leakage current, and improve the hole-injection efficiency of the device. Bao et al. studied the modification of the work function of Al, Ag, Au, ITO, and fluorine-doped tin oxide (FTO) by copolymers of difluoroethylene and hexafluoroethylene (PVDF-HFP) (Hinckley et al., 2016). It is found that the mechanism of the PVDF-HFP-modified electrode is similar to that of LiF and self-assembled monolayers (SAMs), and it mainly improves the work function of the electrode by forming the interface dipole with the electrode. It can reduce the hole-injection barrier, while improving the coverage of the film to improve the efficiency and working life of the OLED device. Transition metal oxides, including MoO3, WO3, NiO, V2O5 have been extensively studied as hole-injecting materials for OLED devices. Such metal oxides generally have a high work function and are preferred p-type dopant materials and hole-injection materials. In 2013, Colsmann et al. prepared WO3 thin films by the solution method and vacuum evaporation method, respectively, and applied them as HILs to OLED devices (Höfle et al., 2013). It was found that the current efficiency of an OLED device using WO3 as an HIL was increased from 8 to 14 cd/A compared to a reference device based on a polymer implant material. The MoO3 thin film prepared by He et al. by the wet method exhibits better hole-injection performance than PEDOT:PSS in both OLED and OSC (Liu et al., 2014). Lu et al. systematically studied the energy-level arrangement of the C60/metal oxide (TMO) interface and characterized and analyzed the energy level at the interface by means of in situ UV photo mission spectroscopy (Fig. 7.3) (Chai et al., 2014). Studies have shown that the energy-level alignment of the C60/TMO interface is in good agreement with the Universal Energy-Level Alignment (UNLA) rule. Theoretical derivation and experimental results based on the Fermi-level balance at the interface show that when the work function of TMO is distributed between the LUMO and HOMO levels, the distribution of energy levels obeys the Schottky-Mott rule when the work function of TMO is distributed. When the LUMO and HOMO levels are outside, the Fermi level will be pinned near the HOMO (or LUMO) level. The design strategy of using p-type doping in OLEDs helps to reduce the hole-injection barrier, increase the carrier density in the bulk phase, and reduce the driving voltage of the device while improving device efficiency. Commonly used p-type dopants mainly include C60, F4-TCNQ, MoOx, and the like. Most of these p-type doping materials are strong electron acceptors, and charge transfer (CT) complexes are formed by electron transfers between host and guest in the doping system, which facilitates carrier injection between the HTL and

LEDs Based on Small Molecules  225 0 EAorg

fsub

IEorg 1

EAorg

fsub

∆ EH (eV)

fsub

~2.3 eV

Vacuum level alignment

~0.3 eV Slope ~0.83 EF pinned to HOMO

2

EF pinned to LUMO

3

ZrO2 TiO2 V2O5 4

5 fsub (eV)

Co3O4 MoO3

NiO CuO WO3

6

7

Fig. 7.3 Energy-level arrangement at the C60/TMO interface.

the anode, while at the same time clearly improving the performance of OLEDs. Liao et al. studied a hole-injecting material based on MoO3:Ts-CuPc (Deng et al., 2015). The results of combined absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) show that they form a CT complex in solution, which reduces the hole-injection barrier by 0.27 eV and exhibits excellent hole-injection performance in OLED devices. The EQE can reach 22.5%. In 2013, Qiu et al. from Tsinghua University systematically studied the Re2O7:NPB doping system by using low-temperature, vapor-depositable Re2O7 as a high-efficiency p-type dopant (Jia et al., 2013). There is a strong CT between 25 mol% of rhenium heptoxide (Re2O7) and 4,4′-N,N-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB). The hole mobility of the doped film is increased by nearly one order of magnitude at an electric field strength of 3×105 V/cm. In addition, Lewis acids such as FeCl3 and BiF3 are also expected to be highly efficient p-type doping material (Bin et al., 2014). Schmid and others researched a series of Bi (III) carboxylate derivatives (Pecqueur et al., 2016). They found that such Lewis acids are prone to incomplete CT, with HTMs having an aromatic amine structure, increasing the mobility of the doped film. The doping system using such an insulating material as a p-type dopant has good light transmittance and high electrical conductivity in the visible light range. And it is expected to be applied to the preparation of a high-efficiency, long-living OLED device.

226  Chapter 7 In recent years, the research on the doping process of p-type doping has gradually gained wide attention. The use of a highly efficient and controllable doping process further improves the doping effect. Diao et al. applied a nanopore structure to construct a doping system (Zhang et al., 2017a, b, c, d). It was found that the introduction of a porous structure can effectively enhance the CT between C8-BTBT and F4-TCNQ, and the formation of a CT complex is also useful to fill traps. After doping, the hole mobility of C8-BTBT is increased by nearly seven times, and the switching ratio of the device is 106. This doping technique of the porous structure can effectively improve the performance of OSC devices. Kippelen et al. used phosphomolybdic acid in nitromethane solution to achieve p-type doping of thickness, which can significantly improve the work function and conductivity of PMA-doped film (Kolesov et al., 2017). In addition, its photo-oxidation stability and the corresponding life expectancy of organic photovoltaic (OPV) devices have increased significantly. Sirringhaus studied the p-type doping system constructed by PBTTT and 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4-TCNQ) based on a thiophene structure (Kang et al., 2016). It was found that the introduction of p-type dopant into the side-chain region of the polymer by solid phase diffusion has the beneficial effect of maintaining the polymerization. The highly ordered structure of the body region facilitates efficient CT and achieves excellent doping effects. In 2016, Tang et al. improved the work function of conductive polymers by bonding ion doping. The doping of various counterions has a large adjustment range of work functions (3.0–5.8 eV), which can be used for organic light-emitting diodes (Tang et al., 2016). The electrode interface modification of various OSC devices can achieve better Ohmic contact at the anode of the device by p-type doping, thereby improving the efficiency of hole injection.

7.2.3  Cathodes and Electron Injection An effective method of electron injection is n-doping, which not only provides a way to finely control the energy level of electron-transport materials, but also greatly improves the efficiency and even enhances the stability of OLEDs. To achieve an efficient electron transfer and realize an n-doping process, the ionization energy of n-dopants must be energetically lower than the LUMO energy (or EA) of electron transport material (ETM). The LUMO energy of commonly used ETMs (PCBM and NTCDA) for OTFTs and OSCs is about −4 eV, while that of ETMs (BPhen and TPBi) for OLEDs is always >−3 eV. Thus, it is intrinsically more difficult to find suitable n-dopants for ETMs of OLEDs than that of OTFTs or OSCs due to the need for n-dopants with much lower ionization energy. And if the ionization energy of n-dopants is <3.6 eV, the reduction of O2(H2O)2 complexes is possible, and the n-dopants are easily oxidized in the presence of ambient oxygen and water. Thus, with increasing LUMO energy, the difficulty to find suitable n-dopants is increased. In the previous study, various of n-dopants have been proposed, such as alkali metals, organic radicals, organometallic compounds, but most have only shown strong n-doping ability in

LEDs Based on Small Molecules  227 ETMs with lower LUMO energy, such as PCBM and NTCDA, which are commonly used in OSCs and OTFTs. As for OLEDs, the n-dopants are mainly based on highly reducing alkali metals. And the OLED industry, with multibillions of outputs, still relies on highly reactive metals to ensure desirable device performances. But the high evaporation temperature and exciton quenching by metal diffusion severely restrict the wide application of these n-dopants. And in recent studies, much effort has been put into this aspect, and some breakthroughs have been achieved. Since the first report of using alkali metals as n-dopants in organics, they have still been widely used in OLEDs due to their extremely low ionization energy, which facilitates the electron transfer with ETMs. But Parthasarathy et al. (2001) realized that the high evaporation temperature during vacuum evaporation may cause Li ion diffusion throughout the organic layer, with diffusion lengths up to 700 Å (±100), thus leading to severe exciton quenching and largely reducing device efficiency and stability. To circumvent high deposition evaporation temperatures and exciton quenching caused by reducing metals, a series of organic compounds [bis(ethylenedithio) tetrathiafulvalene, or BEDT-TTF] and organometallic compounds such as decamethylcobaltocene (Co(η5-C5Me5)2), with high-lying HOMOs also have been studied. Incorporation of these novel n-dopants has similar effects on the shift of EF and conductivity, which fabricates high-efficiency OSCs. According to the effective atomic number rule, sandwiched organometallic compounds with 19 valence electrons such as cobaltocene and rhodocene are naturally reactive n-dopants because they tend to form stable 18-electron cations via electron transfers from dopants to ETMs. Cheng et al. (2016) and Su et al. (2015) confirmed the n-doping effects of Co(η5-C5Me5)2 with low ionization energy of 3.30 eV and achieved a 106-fold increase in current density, with enhanced electron injection and increased conductivity as well. Cotton et al. first reported a class of closed-shell molecules with general formula M2(hpp)4 with extremely low ionization energy, where M is Cr, Mo, or W; and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a] pyrimidine. The low ionization energy of these molecules derives from delta-bonding orbitals of the quadruple metal-metal bond and strong interactions between orbitals on hpp, which makes them promising candidates for efficient n-doping. Menke, T., et al., 2012 investigated the application of a bimetal complex (Cr2(hpp)4 and W2(hpp)4) in OSCs, in which the n-doped C60 shows an extremely high conductivity (up to 4 S/cm) and decreased activation energy at low doping ratios. Due to the strongly reduced characteristic of the materials discussed here thus far, such as alkali metals, organic electron donors, and reactive organometallic compounds, these airsensitive n-dopants have to be performed with extra care under an inert atmosphere, which limits their extensive application in OSCs and OLEDs.

228  Chapter 7 To avoid using air-sensitive materials directly, the substractive method of using precursors to release strongly reducing n-dopants in situ is widely used in organic electronics. The precursors include inorganic salts (LiH, Li3N, Cs2CO3, Rb2CO3, CH3COONa); organic salts (crystal violet, pyronin B chloride, and o-MeO-DMBI-I); organic hydrides (leuco-crystal violte and o-MeO-DMBI-H); and dimers ((DMBI)2 and (RuCp∗Mes)2). The most fundamental class of n-dopants are precursors based on alkali metals, which shows strong n-doping ability to reduce a wide range of ETMs, even with high LUMO levels (>−3 eV). Using precursors of reactive metals could avoid using these metals directly in ambient conditions, thus largely improving the air stability, but still does not avoid the problem of atom diffusion, which results in severe exciton quenching and reduced device efficiency and stability. Moreover, inorganic precursors always decompose at temperatures that are much higher than the normal enaporation temperatures of organic ETMs. Thus, coevaporation of inorganic precursors and organic ETM in the organic chamber will result in outgassing of organic materials from the chamber wall during evaporation because of relatively high heating temperature. Using organic precursors was first reported by Yong, H.K., et al., who utilized pyronin B chloride to dope into NTCDA. This technique is still widely used in organic electronics to avoid the high evaporation temperature and the atom diffusions of inorganic precursors. And upon doping, conductivities of up to 2×10−4 S/cm are obtained, which are four orders of magnitude higher than that of undoped NTCDA film. Then the triphenylmethane dye of crystal violet and halide salt of 1,3-dimethylbenzimidazolium have been reported, realizing the efficient n-doping process in ETMs, such as C60 and NTCDA, and largely improving their conductivity. However, due to the low electron-donating ability of used organic, radical-based n-dopants to date, they have mainly shown strong n-doping ability in ETMs, with relatively low LUMO energies of below −4 eV (such as C60 and NTCDA), and have rarely been used in OLEDs, which need ETMs with a rather high LUMO energy of above −3 eV (such as BPhen and TPBi). Recently, Duan et al. carefully studied the thermal-deposition process of o-MeO-DMBI-I and revealed that it tends to lose iodine (I) and form a reactive organic radical o-MeO-DMBI upon heating in vacuum, which increases the mobility and conducticity of BPhen, a widely used ETM in OLEDs, as shown in Fig. 7.4A (Duan et al., 2015). Then, they used the following two ways to further improve the n-doping efficiency: •

•

They designed new organic radicals with higher singly occupied molecular orbitals (SOMOs), and thus stonger electron-donating ability (Fig. 7.4B) to enhance the electron transfers between organic radicals and ETMs (Duan et al., 2016). Instead of using a bulk CT process, they could optimize it by interface CT to avoid the complicated codeposition process and minimize the influence of dopants to electron transport in ETMs (Duan et al., 2017).

LEDs Based on Small Molecules  229 The n-doping mechanism and the molecular structures of precursors N+

O

N

N+

– l2

I–

N+

N+

–

I

N+

O

N

O I

N

N+

–

O O I–

N

O

R1-I

(A)

– e–

N

O

N

O

O

R2-I

R3-I

To improve the n-doping efficiency Molecular design

O– O N

SOMO energy (eV)

p–

N

–2.0

O

m–

N N

–2.2 O N

–2.4

–2.53 eV

–2.10 eV O

–2.29 eV

N

–2.6

O O

ype

n-t

ng opi

lity

abi

d

R1 R2 R3 Molecules with different n-type doping ability

(B)

Interface charge-transfer process

Bulk

Interface

– +

– – + +

Electric field + –

+ – – +

AI

Charge-transfer

– – + + AI – – + + Charge-transfer – ETMs for OLEDs + Organic radicals

Fig. 7.4 (A) Left: The depositing process of organic salts upon heating and the n-doping mechanism between newly formed organic radicals and ETMs. Right: The molecular structures of new design precursors with higher n-doping efficiency. (B) The two methods to improve the n-doping efficiency of organic radicals. Left: SOMO energies of designed organic radicals with inserted pictures of electron-density plots of SOMO. Right: The composition of bulk CT and interface CT processes.

230  Chapter 7 In this way, 2-(2,4,6-trimethoxyphenyl)-1,3dimethyl-1H-benzoimidazol-3-ium (R3), with the strongest electron-donating ability and decomposed from a well-designed precursor of 2-(2,4,6-trimethoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium (R3-I), could achieve an efficient n-doping process and largely reduce the electron-injection barrier in OLEDs using an interface CT process. In addition, it outperforms the organic radical (2-(2-methoxyphenyl)-1, 3-dimethyl-1H-benzoimidazol-3-ium, o-MeO-DMBI or R1) and the widely used inorganic electron-injection material (LiF) to boost device efficiency and enhance device stability. Recently, Ho et al. reported a very novel and general strategy to achieve solution-processed n-doped films with ultralow ϕWF and achieve Ohmic contacts at cathodes by using selfcompensated anions (Tang et al., 2016). As shown in Fig. 7.5, the self-compensated, heavily doped polymer scheme uses an electron-doped polymer OSC core with charge compensation by covalently bonded counterions. In contrast to self-doped polymers by anions, the selfcompensated doped polymers are produced by deliberate electron when doped with strong donors, following by internal ion exchange to remove the inserted counterions as soluble salts.

Fig. 7.5 (A) The molecular structures of N1 and N2. (B) J-V characteristics of bipolar diodes using a selfcompensated polymer N1 as an EIL at the cathode. (C) J-V characteristics of electron-only diodes using a self-compensated polymer N2 as an EIL at the cathode. Reproduced with permission.

LEDs Based on Small Molecules  231 Fig. 7.5 shows the current density-voltage (J-V) characteristics of bipolar diodes using a self-compensated polymer N1 as EIL. N1 shows the better ability to be an EIL than Al or Ag on DPPT2-TT, a prototype of the low-band-gap diketopyrrolopyrrole (DPP) polymer (the estimated EA is about 3.5 eV). And the self-compensated, electron-doped, benzothiazolefluorene copolymer N2 with a tethered trimethylammonium counterion has a vacuum ϕWF of 3.9 eV, which is nearly as effective as calcium (Ca) at providing Ohmic electron injection into PFOP (the estimated EA is about 2.7 eV) when used as the EIL for the electron-only device.

7.2.4  Hole Transport Materials Arylamine was heavily researched for a long period of time as an HTM. When applied as an HTM, triarylated amines with the biphenyl inner core could greatly improve the device’s EL efficiency and operational stability, and a typical example, NPB was widely used. NPB has a simple chemical structure, which is easy to synthesize and purify; however, it also has a disadvantage, as its glass-transition temperature (Tg) is as low as 98°C. Thus, scientists paid more attention to developing HTMs with high hole mobility and thermal stability. Su Shijian’s group, a team from South China University of Technology, designed and synthesized six kinds of HTMs based on carbazole and triarylamine, as shown in Fig. 7.6.

Fig. 7.6 Chemical structures of HTMs based on carbazole and triarylamine.

232  Chapter 7 The introduction of carbazole allows these materials to have better hole-transporting ability and higher triplet energy levels, while the presence of rigid groups also increases their effective molecular weight, thereby improving the thermal stability. The low ionization energy of the triarylamine group also increases its hole mobility. The photoluminescence (PL) test showed that except for the emission wavelength of compound d (Fig. 7.6) at 410 nm, the emission wavelengths of the other five compounds were all within the range of 380 nm (±2 nm). Thermogravimetric analysis (TGA) showed that the decomposition temperatures of the compounds a–f (Fig. 7.6) were 409°C, 454°C, 421°C, 440°C, 425°C, and 456°C, respectively. The Tg of the compounds a–d and f (Fig. 7.6) were determined by differential scanning calorimetry (DSC), as 133°C, 153°C, 141°C, 158°C, and 151°C, respectively, and the Tg of the compound e (Fig. 7.6) could not be measured in the DSC curve. Fluorescent OLED devices with the materials a–f (Fig. 7.6) as the HTLs and Alq3 as the light-emitting layer showed excellent performance (Xiang et al., 2017). Designing traditional low-molecular-weight HTM derivatives by cross-linking, increasing their molecular weight, and introducing rigid structures are typical strategies for designing and synthesizing novel high-thermal-stability HTMs, such as the TPD derivatives TPTE and spiro-TAD (as shown in Fig. 7.7). Meerholz et al. of the University of Cologne in Germany designed and synthesized a series of derivatives of TAPC XTAPC (Fig. 7.8). XTAPC has a wide band gap and a high triplet level, thus significantly improving its exciton-blocking ability and reducing the efficiency roll-off the device (Liaptsis et al., 2013). Krüger et al. of the Fraunhofer Institute in Germany synthesized an XL:(BuO)4TCTA copolymer, with a TCTA derivative monomer and polymer structure (as shown in Fig. 7.9) by thermal initiation polymerization. The CBP:Ir(ppy)3-comprised, green-OLED device using a polymer such as HTM has a current efficiency range from about 87 to 92 cd/A, compared to a device using a monomer as an HTM. The stability of the material is greatly improved, and cross-linked polymers can achieve long-term stability for a year (Limberg et al., 2016).

Fig. 7.7 Chemical structures of (A) TPD, (B) TPTE, and (C) spiro-TAD.

LEDs Based on Small Molecules  233

Fig. 7.8 Chemical structures of (A) TAPC and (B) XTAPC.

Fig. 7.9 Chemical structures of (A) (BuO)4-TCTA monomer and (B) XL:(BuO)4-TCTA copolymer.

7.2.5  Electron Transport Materials Most OSC materials have weaker electron transport capabilities than hole transport capabilities, and electron transport is more susceptible to water and oxygen. Scientists are committed to obtaining high mobility and high stability electron transport materials through molecular design. In 2014, Su Shijian of the South China University of Technology synthesized a series of pyridine-containing ETMs, whose structure is shown in Fig. 7.10. The Tg of the seven materials is higher than Tm3PyPB (Tg=79°C) by at least 12°C, and the highest temperature is even more than the latter for 40°C. They have suitable HOMO/LUMO energy levels, and the HOMO level of most materials is 6.3–6.7 eV, providing good hole-blocking ability. And because of their high triplet energy level, they also have excellent exciton-blocking ability. In the 26DCzPPy:FIrpic blue OLED with Tm3PyP26PyB as ETM, luminance of 1 cd/m2 and 100 cd/m2 were achieved at low driving voltages of 2.61 and 3.03 V, respectively.

234  Chapter 7 N

N

N

N

N N

N N

N

N

N N

N

N

N

Tm3PyPB

Tm3PyPPB

N

Tm3PyP24PyB

Tm3PyP42PyB

N

N

N

N

N N

N N

N

N

N

N

N

N

N

N

N

Tm3PyP35PyB

N N

N

N

N

Tm2PyP26PyB

N

N

N

N

Tm3PyP26PyB

Tm4PyP26PyB

Fig. 7.10 Chemical structures of pyridine-containing ETMs.

When 46DCzPPm with a smaller ΔEST is selected as the host material, the voltage at 100 cd/m2 brightness can be as low as 2.70 V (Ye et al., 2014). In 2015, Cheng et al. of Tsinghua University in Hsinchu, Taiwan, synthesized three terphenyl-oxadiazole derivatives, which can be used as general-purpose ETMs for red, green, and blue (RGB) OLEDs. The structures of these materials are shown in Fig. 7.11. The electron mobility of the three materials, PhOXD, 3PyOXD, and 4PyOXD, were obtained by space-charge-limited-current measurement, which were 4.2×10−6, 1.2×10−5 and 1.6×10−5 cm2/Vs, respectively, which is close to the level of traditional ETMs such as TAZ, BAlq, and BCP+Alq3. The RGB OLED devices using these three materials as ETLs achieve a maximum EQE of more than 26% and a low efficiency roll-off (Shih et al., 2015). Kang et al., from Korea University, reported four silicon-containing ETMs (structure shown in Fig. 7.12). Their Tg is in the range of 100–141°C and shows good thermal stability. The single-electron device test measured the electron mobility of the b and c (Fig. 7.12) materials under the electric field of 1 MV/cm2 to be 1.93×10−5 and 3.67×10−5 cm2/Vs, respectively. The HOMO of all four compounds is about 6.5–6.6 eV, which gives them good hole-blocking ability. The Ir(ppy)3 green OLED device prepared with compound c (Fig. 7.12) for ETMs realized the current efficiency and EQE of 62.8 cd/A and 18%, respectively (Yi et al., 2015).

LEDs Based on Small Molecules  235

Fig. 7.11 Chemical structures of terphenyl-oxadiazole derivatives.

Duan et al. of Tsinghua University designed and synthesized two kinds of novel electronic transmission materials: 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazol-1-yl) phenyl) anthracene (BPBiPA) and 9,10-bis(6-phenylpyridin-3-yl)anthracene (DPPyA). The electron mobility of BPBiPA reaches 1.55×10−3 cm2/Vs at the electric field intensity of 3×105 V/cm, which is higher than the classical ETMs 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene (TPBi) and 4,6-bis(3,5-di-3-pyridinylphenyl)-2-methyl-Pyrimidine (B3PYMPM) by one and two orders of magnitude, respectively (Fig. 7.13). It is generally believed that the triplet energy level of the ETM should be high enough to achieve a better exciton confinement effect, thereby realizing high-efficiency and low efficiency roll-off in the device. However, although the T1 level of the compound BPBiPA is only 1.82 eV, the large steric structure increases its intermolecular distance, and the anthracene group of the low T1 level is protected by phenyl benzimidazole with a high T1 level. Thus, the Dexter energy transfer from the triplet in the emitting layer to the ETL is effectively suppressed. Using BPBiPA as the ETL, CzTrz:5TCzBN as the luminescent layer of blue OLED achieved a maximum EQE of 21.3% and maintained it at 21.2% and 17.8% at 1000 cd/m2 and 5000 cd/m2, respectively. Using BPBiPA as the ETL, the DICTRZ:Ir(ppy)3 green OLED achieved a maximum EQE of 25.5% and a low turn-on voltage of

236  Chapter 7

Fig. 7.12 Chemical structures of four silicon-containing ETMs.

2.3 V. The device’s operational lifetime T90, measured at the initial brightness of 5000 cd/m2, exceeded 400 h, which was higher than the 18% maximum EQE and far higher than the 140 h T90 operational lifetime when the TPBi was the ETM (Zhang et al., 2017a, b, c, d). DPPyA also achieved high electron mobility of 1.5×10−3 cm2/Vs under the electric field of 5.5×105 V/cm due to the existence of good molecular stacking. After optimization, an extremely low operating voltage and ultrahigh power efficiency were realized in the green OLED with an emitting layer structure of DIC-TRZ:BPBPA:Ir(mppy)3. The driving voltage was only 2.95 V at the brightness of 10,000 cd/m2, and the power efficiency at this brightness reached 109 lm/W, which is the highest reported value to date (Zhang et al., 2017a, b, c, d). Based on these two materials, another ETM, 2-phenyl-1-(4-(6-phenylpyridin-3-yl)anthracen9yl)phenyl)-1H-benzo[d]imidazole (BiPyA), was also reported by the group the following year, and its structure is shown in Fig. 7.13 (Zhang et al., 2018). BiPyA combined the advantages of these two materials, having good exciton-blocking ability and a mobility at

LEDs Based on Small Molecules  237

DPPyA

(A)

BiPyA

SDD of T 1

SDD of T 1 c

8000

SDD of T 1

10,000

DPPyA BPBiPA BiPy A

2

Cu r r en t d en s i t y (A/m )

10,000

DPPyA BPBiPA BiPyA PXZ-DPS

1000

Co u n t s

6000 4000

100

10

2000

0

(B)

BPBiPA

2

4 6 Voltage (V)

8

10

2

4

6 Time (µs)

8

10

Fig. 7.13 (A) Chemical structures of ETMs. (B) Current density-voltage curves of devices and (C) transient curves of devices and films.

the same order of magnitude. As an ETM for fluorescent OLEDs with thermally activated delayed fluorescent sensitizers (TSF-OLEDs), an OLED device having a emitting layer and a PhCzTrz:PXZ-DPS:PhtBuPAD structure achieves a maximum EQE of 24.6% and a small efficiency roll-off. It can be seen that such material can be an excellent choice for the TSFOLEDs ETL.

7.2.6  Fluorescence Materials Beyond Exciton Statistics According to exciton statistics, injected electrons and holes will recombine and form singlet and triplet excitons at a ratio of 1:3 in OLEDs that employ small molecules to emit light. For most fluorescent emitters, they can only use singlets to emit light. As a result, their theoretically maximum IQEs are 25%. The question of how to utilize triplet excitons has become a key issue to enhance device efficiencies. Several tools, such as thermally activated delayed fluorescence (TADF), triplet-triplet annihilation (TTA), hybrid local charge transfer (HLCT), dual states, triplet upconversion, and phosphorescent materials, have been proposed.

238  Chapter 7 7.2.6.1  TADF (also known as E-type delayed) materials In TADF materials, the thermally activated reverse intersystem crossing (RISC) mechanism allows upconversion of low-energy triplet states to the emissive singlet level and 100% IQE can be achieved. For efficient TADF materials, several requirements need to be satisfied, such as ensuring a small energy splitting between singlet and triplet excited states (ΔEST), but maintaining a high photoluminescence quantum yield (PLQY). The core parameter of TADF is the rate of RISC (kRISC), which can be written as kRISC =

2 2π  SO T 〉 ρ FC 〈 S1 H 1 

(7.9)

 SO T 〉 is an SOC matrix element between the S and T states, and ρ is the where 〈 S1 H 1 1 FC 1 Frank-Condon-weighted density of states, which is a decreasing function of ΔEST. For the sake of fast RISC and ultimately efficient TADF, both small ΔEST and considerable SOC matrix elements have to be realized simultaneously. The energy difference between S1 and T1 is dependent on the exchange energy J between the singlet excited state and the triplet excited state, shown as follows:

∆ EST = 2 J

(7.10)

and J can be expressed as the overlap between the HOMO and LUMO of the molecule: J = ∫ ∫ HOMO (1) LUMO ( 2 )

e2 HOMO ( 2 ) LUMO (1) dr1dr2 r1 − r2

(7.11)

To decrease ΔEST, we need to separate hole and electron densities over different parts of the molecule. Thus, an electron donor-acceptor (DA) strategy is adopted as an efficient approach in which D units and A units are connected to form a strong CT excited state. To further separate the FMO, a twisted structure can be introduced to keep the donor and acceptor perpendicular to each other. On the other hand, the SOC effect in pure organic molecules has attracted increasing attention due to its potential value to facilitate RISC. El-Sayed’s rule indicates that SOC is allowed between states of different symmetry and electronic configurations. It can be concluded that the difference in the nature of the excited states (CT excited state and CT versus localized excited state, LE) increases SOC matrix elements. Because S1 displays a dominant CT character (1CT), it is the localized triplet state (3LE) that leads to a strong SOC with S1. As a result, the energy vicinity of 1CT and 3LE is favorable for achieving a fast RISC rate.

LEDs Based on Small Molecules  239 Blue materials

The earliest reported blue TADF material is the sky-blue material 2CzPN, shown in Fig. 7.14. Due to the large steric hindrance between adjacent electron donors (carbazole groups), the carbazole and the electron acceptor (dicyanobenzene) are nearly perpendicular to each other, the HOMO of the molecule is almost completely separated from the LUMO, and the ΔEST of the molecule is only 0.34 eV. The corresponding OLED device based on 2CzPN as emitter (with PPT as the host) achieved the maximum luminescence wavelength (λEL) of 470 nm and the maximum EQE of 8% (Uoyama et al., 2012). However, the efficiency roll-off phenomenon of the 2CzPN-based OLED devices is very serious. When using mCP as the host, the corresponding OLED device has an EQE of 13.6% at a low current density (J=0.01 mA/cm2). As the current density increases to 10 mA/cm2, the EQE reduces to 3.6%. Considering the intersystem crossing (ISC) and RISC processes by theoretical simulation, Adachi et al. found that the long T1 exciton lifetime of 2CzPN (273 μs) leads to severe singlet-triplet annihilation (STA) and TTA at high current density, which is the main reason for the large roll-off of the device (Masui et al., 2013). The luminescence wavelength and efficiency of the molecule can be adjusted by changing the relative position and number of electron donors and acceptors to change the strength of the intramolecular push-pull electron effect. By changing the relative positions of the two cyano

Fig. 7.14 Structure of a cyanobenzene blue TADF molecule.

240  Chapter 7 groups and the two carbazoles in 2CzPN, the lifetimes of the delayed portions of the obtained isomers CzTPN and DCzIPN were 11.3 and 1.2 μs, respectively, which were shorter than that of 2CzPN (Nishimoto et al., 2014; Cho et al., 2015a, b). Among them, adopting PzCz as the host, an OLED device doped with 3% CzTPN emits bluegreen-light (λEL is 494 nm) and EQEmax is 15%; because of the weaker ICT effect of DCzIPN, the λEL of OLED device with mCP as the host is 462 nm and the EQEmax is 16.4%. Duan et al. reduced one cyano group to weaken the effect of ICT, causing the hypsochromic molecule spectra: cyanobenzene as an electron acceptor and carbazole and 3,6-di-tert-butylcarbazole as electron donors, and they prepared four blue/sky-blue TADF molecules. With mCBP as the host, the device achieved optimal performance when the dopant concentration was 40 wt%. Among them, the 4CzBN-based device achieved λEL of 458 nm and EQEmax of 10.6%. The λEL of the 4TCzBNbased device is 463 nm and the EQEmax is 16.2%; the λEL of the devices based on 5CzBN and 5TCzBN is 490 nm, and the EQEmax of these devices is 16.7% and 21.2%, respectively. It can be concluded that introducing the tert-butyl group, 4TCzBN and 5TCzBN, improves the device efficiency. That is because the introduction of t-butyl groups reduces the ΔEST of the molecule, increases the RISC rate, and improves the PLQY of those materials (Zhang et al., 2016a, b, c, d). Adachi et al. used a comprehensive set of complementary experimental techniques, demonstrating that kRISC cannot be determined by the ΔEST alone and strongly relies on the excited states (Hosokai et al., 2017). They focused on Cz-phthalonitrile (CzPN) and CzBN derivatives: 4CzIPN, 2CzPN, 2CzBN, o-3CzBN, m-3CzBN, p-3CzBN, 4CzBN, and 5CzBN (Fig. 7.15). Unlike the CzPN derivatives, CzBN derivatives have similar ΔEST values (about 0.2±0.04 eV), but with a completely different kRISC. By looking at excited-state dynamics, they attribute the different kRISC value to the formation of delocalized excited states, requiring a linearly positioned Cz pair in a D-A-D structure, which facilitates RISC (Fig. 7.16). Recently, they modified 5CzBN by using 3,6-diphenylcarbazole (DPhCz) and 3,6-dimethylcarbazole (DMeCz) as the second type of donor (D2) (Fig. 7.17) (Hosokai et al., 2018). They found that introducing a D2 into a D-A system can tune the energy position of 3 LE1 relative to 1CT1 and 3CT1, while at the same time weakly pinning the 1CT1 and 3CT1 energy levels to those of the original D-A system, resulting in an increase of kRISC according to a strong, second-order, spin-vibronic coupling between 3LE1 and 3CT1. In addition, 3 LE1 can be adjusted by changing the number or donor ability of the D2 units (Fig. 7.18). TADF-OLEDs using the molecule as emitter exhibit not only a remarkable enhancement of operational stability, but also suppression of exciton annihilation processes with an EL EQE of more than 20%, even in the high-luminance region. In 2015, Lee et al. from South Korea used a fluorine atom (F) with a weaker electron accepting propriety than a cyano group to replace the cyano group in dicyanobenzene to weaken the ICT effect and change the number of F atoms and carbazole to adjust the luminescence spectra. At the same time, F atoms can also improve the solubility of the

LEDs Based on Small Molecules  241

Fig. 7.15 Molecular structures of CzPN and CzBN derivatives and linearly positioned Cz moieties (highlighted in blue).

Fig. 7.16 Relation between kRISC and ΔEST (LE) of TADF-inactive and TADF-active molecules in energy-level diagram.

material, which helps in the formation of film from solution. They designed and synthesized two blue molecules, 3CzFCN and 4CzFCN (Fig. 7.19). Using SiCz as the host, the λEL of 3CzFCN- and 4CzFCN-based OLEDs prepared by spin-coating are 463 nm and 471 nm, respectively, and the EQEmax is 17.8% and 20%, respectively, which is higher than the EQEmax (12.9%, 17.3%) of the corresponding devices prepared by vacuum evaporation (Cho et al., 2015a, b).

242  Chapter 7

Fig. 7.17 Chemical structure of 5CzBN and chemical structure of D-D2-A-type CzBN derivatives. The second type of substituted donor units is highlighted in color.

Interrupting the molecule conjugation structure can also make the luminescence of the molecule hypsochromic. Adachi et al. used a conjugated interrupted diphenyl sulfone group as an electron acceptor to prepare a series of TADF materials (Zhang et al., 2012; Zhang et al., 2014a, b, c, d, e; Wu et al., 2014). They first designed and synthesized three blue materials, DPA-DPS, tDPA-DPS, and DTC-DPS, with diphenylamine, 4-tertbutyldiphenylamine, and 3,6-di-tert-butylcarbazole as electron donors (Fig. 7.20). The λEL of the corresponding OLED device with DPEPO as the host is 420, 425, and 400 nm, respectively, and EQEmax is 2.9%, 5.6%, and 9.9%, respectively (Zhang et al., 2012). Due to the large ΔEST of the molecules (0.54, 0.45, and 0.32 eV), the lifetime of the T1 exciton is too long, resulting in a large roll-off of the device. Subsequently, in 2014, they synthesized DMOC-DPS such that the 3,6-site of carbazole were replaced by methoxy with stronger electrons, and the ΔEST of the molecule was reduced to 0.21 eV. Using DPEPO as the host, the corresponding OLED has a λEL of 460 nm and an EQEmax of 14.5%. At 1000 cd/m2, the EQE is 3.7%, which is an improvement over the DTC-DPS-based device (Zhang et al., 2014a, b, c, d, e). In the same year, Adachi et al. used 9,9-dimethyl acridine as an electron donor to synthesize DMAC-DPS. The OLED device with DPEPO as the host has a λEL of 470 nm, an EQEmax of 19.5%, and EQE can remain at 16% at 1000 cd/m2.

PhCz

3.10 3.05

2DMeCzPh

Energy (eV)

3.00 2.95 2.90 2.85

1

CT1 0.32 eV

CT1

0.17 eV

0.30 eV

2.80 2.75

2DPhCzPh

1

0.17 eV 3

2.70

0.16 eV

0.15 eV

CT1

4Cz1DPhCzBN

LE1

CT1

0.16 eV

0.19 eV 0.13 eV

3

CT1

3Cz2DPhCzBN

3

CT1

2Cz3DPhCzBN

Fig. 7.18 Energy-level diagram of various CzBN derivatives. The dashed red lines indicate the 3LE1 of CzPh, for reference.

LEDs Based on Small Molecules  243

3Cz2DMeCzBN

3

3

1

CT1

CT1 CT1

5CzBN

1

CT1

0.15 eV 3

2.65

1

244  Chapter 7

Fig. 7.19 Structure of blue TADF molecules containing F atoms.

The authors believe that when the local excited triplet state (3LE) of TADF molecule is close to or higher than the CT triplet state (3CT) of the molecule, it is beneficial to improve luminous efficiency (Wu et al., 2014). In 2015, Adachi et al. prepared an undoped OLED device using DMAC-DPS pure film as an emitting layer. The optimized device has a λEL of 480 nm and an EQEmax of 19.5% (Zhang et al., 2015a, b, c). Similarly, Adachi et al. also designed and synthesized a series of D-A-D molecules using benzophenone as an electron acceptor.

Fig. 7.20 Structure of various blue TADF molecules based on diphenyl sulfone and benzophenone.

LEDs Based on Small Molecules  245 Among them, Cz2BP and CC2BP are blue molecules (Fig. 7.20), and λEL of the devices with DPEPO as the host are at 446 nm and 484 nm, respectively, EQEmax was 8.1% and 14.3%, respectively (Lee et al., 2014a, b). Triazine is a class of well-stabilized electron acceptors, and many triazine-based, highefficiency blue TADF molecules have been reported (Hirata et al., 2015; Kim et al., 2016a, b; Kim et al., 2015), as shown in Fig. 7.21. By comparing the structure and photophysical properties of TADF materials based on triazine/carbazole derivatives, Adachi et al. found that expanding the delocalization of the HOMOs/LUMOs of molecules can reduce ΔEST while suppressing the reduction of kSr , simultaneously, achieving small ΔEST and high kSr at the same time. Meanwhile, the Ph3Cz-TRZ-based OLED device has a λEL of 487 nm and an EQEmax up to 20.6% (Hirata et al., 2015). In 2015, Kim et al. used a new electron donor, replaced sp3 hybridized C atoms in 9,9-diphenyl acridine with Si atoms, and used triazine as an electron acceptor to design and synthesize DTPDDA molecules. The ΔEST of DTPDDA is only 0.14 eV, and the T1 excitons can be completely upconverted to S1 excitons. The corresponding OLED device with a dual-host system of mCP and TSPO1 has a λEL of 468 nm and an EQEmax of up to 22.3% (Kim et al., 2016a, b).

Fig. 7.21 Structure of triazine-based blue TADF molecules.

246  Chapter 7 Lee et al. proposed a design strategy of twin emitters: two TADF molecules are connected by an electron donor. They used carbazole as an electron donor and found that the various positions of the two carbazoles have a great influence on the properties of the molecule. Compared to the 2,3′-position or the 3,4′-position of the linkage, the 3,3′-position connected 33TCzTTrz has a smaller ΔEST of 0.25 eV, and the corresponding OLED device has a λEL of 472 nm and an EQEmax of 25% (Fig. 7.21) (Kim et al., 2015). The TADF molecules composed of electrons with donor and acceptor luminescence are generally in the CT state; the half-width of the spectrum is wide and the color purity is poor. Hatakeyama et al. used the resonance effect of the B atom and N atom to achieve the separation of molecular HOMO/LUMO without introducing electron donors and acceptors; the ΔEST of the designed and synthesized DABNA-1 and DABNA-2 molecules is between 0.14 and 0.18 eV due to the rigid conjugate structure. Meanwhile, the oscillator’s f (S0→S1) in these two molecules is very large—0.205 and 0.415, respectively (Fig. 7.22). The corresponding OLED devices have λEL of 459 nm and 467 nm, respectively, and EQEmax of 13.5% and 20.2%, respectively. The half-width of the spectrum is only 28 nm, and the CIE coordinates are (0.12, 0.13) (Hatakeyama et al., 2016). The CT characteristics of TADF materials make it difficult to construct deep-blue materials. Therefore, the number of deep-blue materials based on TADF is small and the efficiency is low. In order to obtain a deep-blue dye, Adachi’s group proposed a simple and effective method for constructing deep-blue materials—namely, introducing a methyl group at a suitable position and effectively adjusting the ΔEST of the material without lowering the band gap and luminous efficiency of the material. The photophysical properties of the material and DFT calculations show that the lowest triplet of the D-A material is well controlled. The introduced methyl group can effectively regulate the interaction between the electron donor and the acceptor. The highest EQEs of the devices based on Cz-TRZ3 and Cz-TRZ4 were 19.2% and 18.3%, respectively, while the device CIE was (0.148, 0.098) and (0.150, 0.097), respectively. These properties are the highest performance achieved in deep-blue materials to date (Cui et al., 2017). Cheng at al. from Tsinghua University in Taiwan designed two delayed fluorescent materials based on pyridine benzophenone carbazole (namely, DCBPy and DTCBPy). Both molecules have small ΔESTs of 0.03 eV and 0.04 eV, respectively. The PL transient decay curve shows that both are TADF materials. The PL efficiencies of the two materials in the solution are 14% and 36%, respectively, while the efficiencies in the solid state are 88% and 91.4%. Devices based on DCBPy and DTCBPy as dyes give blue and green luminescence, respectively, with device efficiencies of 24% and 27.2%, while the efficiency roll-off at high brightness is small. The crystal of DTCBPy indicates significant interaction between the ortho-carbazole and the 4-pyridyl group, which plays an important role in achieving small ΔEST and high PL efficiency (Rajamalli et al., 2016).

LEDs Based on Small Molecules  247

Fig. 7.22 (A) Chemical structures of DABNA-1 and DABNA-2. (B) Chemical structures of carbazole and triazine based deep-blue materials. (C) Chemical structures of DCBPy and DTCBPy. (D) Chemical structure of Ac-OPO and Ac-OSO and related HUMO and LUMO distribution.

248  Chapter 7 Yasuda et al. from Kyushu University designed and synthesized two high-efficiency blue dyes, Ac-OPO and Ac-OSO, using a phosphorus-oxygen group and a sulfone group as the acceptor and a dimethyl acridine group as the donor. The highest device efficiencies for the two devices were 12.3% and 20.5%, respectively, and the CIE coordinates were (0.15, 014) and (0.16, 0.26), respectively. At the same time, the Ac-OSO-based blue device has a very small efficiency roll-off, and the EQE at a brightness of 1000 cd/m2 remains at 13%. In addition, due to the high RISC efficiency, the device achieves nearly 100% IQE, which far exceeds the 25% limit of traditional blue dyes (Lee et al., 2016a, b). Green materials

Currently, 4CzIPN remains one of the best green TADF materials (Fig. 7.23). It was first designed and synthesized by Adachi et al. in 2012 (Uoyama et al., 2012). Devices using 5 wt% 4CzIPN doped CBP film as the EML realized a green emission of 507 nm and an EQE of 19.3%. Subsequently, Lee, Kim, and other research groups optimized the device structures and achieved high EQEs (above 25%) by using appropriate host systems (Sun et al., 2014a, b, c; Kim et al., 2014a, b, c, d and Cho et al., 2014a, b). By adopting mCP and B3PYPM as the exciplex host, the device fabricated by Kim realized EQEs as high as 29.6%. This high performance can be attributed to the balanced hole and electron transmission in devices. Even though efficiency was high, their efficiency roll-off was serious. At a high luminance of 10,000 cd/m2, its EQE significantly dropped to 14.5%. Lee et al. designed and synthesized DDCzIPN by combining two identical TADF molecules through an electron acceptor moiety (called a dual emitter; see Fig. 7.24). Compared with DCzIPN, which possesses only a luminance core and PLQY of 67%, DDCzIPN exhibits a higher PLQY of 91% As a result, the EQE of the corresponding OLED device also increased, from 16.4% for DCzIPN to 18.9% for DDCzIPN. However, due to the enhanced degree of

Fig. 7.23 Chemical structures of 4CzIPN, m4CzIPN, and t4CzIPN.

LEDs Based on Small Molecules  249

Fig. 7.24 Chemical structures of DCzIPN and DDCzIPN.

conjugate of the molecule, the device emission shifted from 462 nm for DCzIPN to 497 nm for DDCzIPN (Cho et al., 2015a, b). Adachi et al. used a diphenyl sulfone group as an acceptor and phenoxazine as a donor to synthesize the green-light material PXZ-DPS (Fig. 7.25). It possesses low ΔEST of 0.08 eV and high PLQY of 80%. The device using 10 wt% PXZ-DPS doped CBP film as the EML realized green emissions of 520 nm, with high EQE of 17.5%. Even at a high luminance of 1000 cd/m2, the EQE could still be maintained at 16% (Lee et al., 2015a, b). Su et al. linked two sulfone groups into a closed loop and designed a new electron acceptor, thianthrene‐9,9′,10,10′‐tetraoxide. By connecting it to the donor 9,10-dimethyl acridine unit via a phenyl bridge, they designed and synthesized a green TADF emitter, ACRDSO2, that shows ΔEST of 0.058 eV and a high PLQY of 71%. The corresponding efficiency of an OLED device prepared by vacuum evaporation exhibited a high EQE of 19.2%, with an emission peak of 534 nm. As for the spin-coating device, its EQEmax was 17.5% (Xie et al., 2016). Wang et al. oxidized the S atom in 9-thioxanthone into a sulfone group and used it as a novel electron acceptor. By connecting it to a donor unit N-phenylcarbazole, they synthesized the green TADF material TXO-PhCz. It exhibits aggregation-induced O O

S

O O S O

N N

N

O

O

PXZ-DPS

S

O

O

N

O S O ACRDSO2

TXO-PhCz

Fig. 7.25 Chemical structures of various green TADF emitters using diphenyl sulfone as acceptors.

250  Chapter 7 luminescence (AIE) accompanied with a ΔEST of 0.073 eV and a PLQY of 93% in pure film. The fabricated device realized a high EQE of 21.5% (Wang et al., 2014). In 2012, Adachi et al. synthesized the green-light material PXZ-TRZ with triazine as the electron acceptor and phenoxazine as the electron donor (Fig. 7.26). The highly distorted molecular structure makes its HOMO and LUMO completely separate. Its calculated ΔEST is merely 0.070 eV, and the PLQY of 6 wt% PXZ-TRZ doped CBP film is 65.7%. The corresponding device has an emission peak of 529 nm with EQE of 12.5%, but the device roll-off was very serious (Tanaka et al., 2012). In 2015, Kaji et al. changed the electron donor to diphenylamine carbazole and synthesized the green TADF emitter DACT-II. The ΔEST of this material is almost zero (i.e., 0.0052 eV), and its T1 exciton can effectively convert to an S1 exciton through RISC. As a result, 9 wt% DACT-II doped CBP film shows an almost unity PLQY (100%). Due to these superior properties, the corresponding OLED device exhibited emissions of 522 nm with maximum EQE of 29.6%. Even at high brightness of 3000 cd/ m2, the EQE could still be maintained at 16.2%. Adoption of a light extraction structure can further cause the maximum EQE to rise to 41.5% (Kaji et al., 2015). Because B atoms have strong electron-deficient properties, B-containing compounds usually have good electron-withdrawing ability. Thus, B atoms are often introduced into TADF molecules to construct novel electron acceptors. Kitamoto et al. designed a novel electron acceptor POB. When phenoxazine is used as an electron donor, the synthesized molecule POB-PXZ (Fig. 7.27) emits green light and ΔEST of 0.028 eV. The doped film has a PLQY of 99%, and the corresponding device has a λEL of 503 nm with an EQEmax of 22.1% (Kitamoto et al., 2015). Kaji et al. used triphenylboron as the electron acceptor and phenoxazine as the electron donor to synthesize the green-light molecule PXZ-Mes3B. Its ΔEST was only 0.008 eV, and doped film in CBP has a high PLQY of 92%. The corresponding OLED device had emissions of 502 nm and an EQEmax of 22.8% (Suzuki et al., 2015).

N N

N O

N

N

N

N

N

N N

PXZ-TRZ

DACT-II

Fig. 7.26 Chemical structures of green TADF emitters using triazine as acceptors.

LEDs Based on Small Molecules  251

O

B

N

POB-PXZ

O

O

N

B

PXZ-Mes 3B

Fig. 7.27 Chemical structures of green TADF emitters containing B atoms.

Red materials

Red-emitting molecules have a lower energy of S1 and T1. According to the energy gap law, the nonradiation rate constant (knr) of the molecule increases exponentially with the decrease of the energy gap. Therefore, efficient red-light TADF molecules are less. In the molecule of the dicyanobenzene/carbazole-based twisted configuration reported by Adachi et al. in Nature, in order to increase the ICT effect to red-shift the molecule emission, they induced phenyl groups to the 3,6-site carbons of carbazole in the 4CzTPN molecule to increase the electron-donating ability of the carbazole. The synthesized 4CzTPN-Ph molecule (Fig. 7.24) has a λPL of 577 nm in the toluene solution. Using CBP as the host, the OLED devices showed EQEmax of 11.2%, but relatively large efficiency roll-off (Uoyama et al., 2012). In order to increase the kSr of molecular, Wang et al. of Jilin University designed and synthesized the molecular structure of TPA-DCPP (Fig. 7.24), in which the dihedral angle between the electron acceptor (DCPP) and the bridging group (benzene ring) is only 35 degrees. The approximately planar configuration makes the HOMO of the molecule overlap with the LUMO, giving a 0.13-eV ΔEST of the molecule. The λPL in the toluene solution is 588 nm, and the PLQY is 84%. Using TPBi as the host, the λEL of the 10 and 20 wt% TPA-DCPP doped OLED devices were 648 and 668 nm, respectively, and the EQEmax values were 9.6% and 9.8%, respectively. In addition, using the pure film of TPA-DCPP as the luminescence, the OLED device emits light into the near-infrared (NIR) region with λEL of 710 nm and EQEmax of 2.1% (Wang et al., 2015). Adachi et al. used a heptazine ring (HAP) with strong electron-withdrawing properties and a rigid planar structure as an electron acceptor, and a tert-butyl-substituted triphenylamine as an electron donor to synthesize an orange-red-light molecule HAP-3TPA (Fig. 7.28), wherein t-butyl can not only increase the electron-donating property, but also its large steric hindrance effect can increase the distance between adjacent molecules and prevent the formation of excimer. In toluene, HAP3TPA possesses a low ΔEST of 0.17 eV, a peak emission at 560 nm and a high PLQY of 95%. With 26mCPy as the host material, the λEL of the OLED device doped with 1 wt% HAP-3TPA is red-shifted to 610 nm and the EQEmax is 17.5% (Li et al., 2013a, b).

252  Chapter 7

N NC N

CN N

N

CN N

N

N

N

N

N

N N

N

N

CN N

4CzTPN-Ph

N

N

N

TPA-DCPP

Fig. 7.28 Chemical structure of 4CzTPN-Ph, TPA-DCPP, and HAP-3TPA.

HAP-3TPA

LEDs Based on Small Molecules  253 In order to increase the kSr of red-emitting TADF molecules without increasing their ΔEST, Adachi et al. designed a series of D-Ph-A-Ph-D-type molecules by using ruthenium (Ru) as an electron acceptor and a bridging-group benzene ring between the Ru and the electron donor (Fig. 7.29). Compared to the corresponding D-A-D molecule, the introduction of a bridging-group benzene ring can increase the distance between the electron donor and the acceptor, so that the molecule’s ΔEST stays substantially constant while kSr increases. On the other hand, when diphenylamine is used as the electron donor, the ΔEST of the molecule without the bridged-group benzene ring (DPA-AQ) is 0.29 eV, and the CBP doped with 1 wt% DPA-AQ film has a PLQY of 50%; meanwhile, the ΔEST of the DPA-Ph-AQ molecule became 0.24 eV after the introduction of the benzene ring, and the PLQY of the CBP-doped 1 wt% DPA-Ph-AQ film increased to 80%. When film of CBP doped with 10 wt% DPA-Ph-AQ is used as the light-emitting layer, the λEL of the corresponding OLED device is red-shifted to 624 nm and the EQEmax is 12.5%, but the roll-off is still very serious (Zhang et al., 2014a, b, c, d, e). Baldo et al. used a triptycene skeleton such that the electron donor and acceptor were on different arms, and the HOMO and LUMO of the molecule only had a small overlap on the intermediate sp2 hybrid carbon atoms (see Fig. 7.30). The ΔEST of the molecule TPAQNX(CN)2, with dicyanquinoline (QNX(CN)2) as the electron acceptor and triphenylamine (TPA) as the electron donor, is 0.11 eV, and the PLQY after oxygen removal in cyclohexane solution was 44%. Using mCP as the host, the λEL of the OLED doped with 10 wt% of TPAQNX(CN)2 was 573 nm and the EQEmax was 9.4% (Kawasumi et al., 2015). Exciplex materials

In addition to the single-molecular TADF, Adachi’s group of Kyushu University demonstrated that the exciplex systems consisting of donor and acceptor molecules are also TADF materials. Generally, the exciplex system consists of donor molecules and acceptor molecules, and it is formed by the interaction between the donor HOMO and the acceptor LUMO levels (Fig. 7.31). The LUMO of the exciplex material is distributed on the acceptor, and the HOMO is distributed on the donor. Compared to the single-molecular TADF, the HOMO and LUMO distributions of the exciplex system are more separated, so that a smaller ΔEST can be obtained. In 2015, Zhang Xiaohong from Soochow University and Lee Chun-Sing from the City University of Hong Kong designed and synthesized a new bipolar acceptor molecule DPTPCz, and the conventional HTMs NPB, TCTA, and TAPC were used as donor molecules to construct a three exciplex systems. The luminescence of the exciplex exhibits a significant red shift relative to the luminance of the constituting molecules. In particular, the PLQY of the exciplex based on TAPC and DPTPCz has reached 68%, while ΔEST of the exciplex was

254  Chapter 7

N

O O N N

O

N

O

DPA-Ph-AQ

DPA-AQ

N

O

O N N O

O

N

BBPA-Ph-AQ

BBPA-AQ

O

O

N

N N O

DMAC-AQ

N

O

DMAC-Ph-AQ

Fig. 7.29 Chemical structure of D-A-D and D-Ph-A-Ph-D molecules.

only 47 meV, which proved that the material has a high radiative decay rate and a high RISC efficiency. The highest EQE of devices based on the TAPC and DPTPCz systems reached 15.4%. The luminescence energy of the exciplex obtained by spectrometry is comparable to that obtained by electrochemical measurement, demonstrating that the energy of the corresponding exciplex can be calculated by measuring the energy levels of the donor and acceptor molecules. At the same time, the work also proved that for exciplex, high triplet donor and acceptor molecules are necessary to prevent exciton loss of exciplex molecules and promote the RISC process (Liu et al., 2015a, b).

LEDs Based on Small Molecules  255

Fig. 7.30 Chemical structure and HOMO/LUMO distribution of the triptycyl-type molecule TPA-QNX(CN)2.

O

N N

N

N

N

O

N N

DPTPCz

P

NPB

O

MAC O N

N

P

N

N N

N N

O P

N

N

N

N

PO-T2T TAPC

mCP

TCTA

Fig. 7.31 Chemical structures of common donor and acceptor molecules.

In 2016, Zhang Xiaohong from Soochow University and Lee Chun-Sing from the City University of Hong Kong proposed the introduction of single-molecular TADF materials as acceptor groups into exciplexes, so that there are two upconversion modes: exciplex and single-molecular TADF. Therefore, compared to the traditional exciplex system, this system can achieve higher upconversion efficiency and more excitons can be utilized to achieve greater efficiency. At the same time, when the energy of the donor molecule in the exciplex is higher than the energy of the TADF material as the acceptor molecule, the RISC process of the monomolecular TADF material can trap the triplet excitons of the donor molecule at high current densities, thereby alleviating the efficiency roll-off. Based on this strategy, these authors selected the TADF material MAC as the donor molecule and the PO-T2T as the acceptor molecule and constructed such an exciplex system. As a contrast, they also chose mCP as the donor and PO-T2T as the acceptor to construct the

256  Chapter 7 exciplex. Both systems exhibited similar PL efficiency and ΔEST. The highest EQE for devices based on MAC:PO-T2T is 17.8%, which is the highest device efficiency based on excimer complexes. As a comparison, the EQE of the mCP:PO-T2T system is only 8.6%. These results demonstrate that MAC:PO-T2T has higher RISC efficiency than does mCP:POT2T. In addition, the device efficiency based on MAC:PO-T2T is smaller, and the efficiency can be maintained at 12.3% at 1000 cd/m2, which proves the design strategy of this exciplex (Liu et al., 2016). Theoretically, exciplexes are capable of achieving 100% IQE, but exciplex-based devices are much less efficient than single-molecular TADF devices. Kim et al. of Seoul National University in South Korea analyzed the kinetics of quantum efficiency in the exciplex. It is found that the nonradiative transition rate of the material is the main factor influencing the efficiency of the exciplex. They chose TCTA as the donor and B4PYMPM as the acceptor molecule to construct the exciplex. At 150 K, the device has an EQE of 25.2% and an IQE of 100%. When the temperature is room temperature, the IQE of the device is only 48.3% and the EQE is 11%. The attenuation of efficiency is due to the increase of the nonradiative transition rate of the exciplex system as the temperature increases. These results show that when the nonradiative transition rate is too large, even if the upconversion rate is large, there is no guarantee that a sufficiently high device efficiency can be obtained (Kim et al., 2016a, b). Host and assisted-host materials

Because TADF materials contain both electron acceptors and electron donors, they usually have good electron and hole transport abilities, and these materials can convert T1 excitons into S1 excitons through RISC and then transfer energy to other molecules. Therefore, it can be a good host material for OLEDs (Zhang et al., 2014a, b, c, d, e). Duan Lian’s group from Tsinghua University used TADF materials as the host materials of conventional fluorescent materials. As shown in Fig. 7.32, the triplet state of the TADF host material can be upconverted to its singlet state by RISC, and the energy of the singlet excitons can be transferred to the conventional fluorescent dye through a long-range energy transfer process and emit light. This luminescence mechanism, called thermally activated sensitized fluorescence (TASF) luminescence, combines the high efficiency of TADF materials with the high luminous efficiency, high color purity, low roll-off, and long life of fluorescent dyes to create high-performance devices. Two TADF materials were selected as hosts, and the EQE of the fluorescent device constructed with a PLQY of up to 80% of the conventional yellow fluorescent dye reached 12.2%, and the power efficiency reached 44 lm/W. In the same year, Adachi’s group also proposed a similar strategy. They used traditional wide-band-gap materials as the host material and TADF materials as assistant dopants, which were then doped with conventional fluorescent materials (as shown in Fig. 7.33). The EQE of the devices from blue to red can reach 13.4%–18%. Moreover, due to the addition of the TADF assistant dopant, the recombination region of the carrier is changed and the operational

LEDs Based on Small Molecules  257

Fig. 7.32 Schematic diagram of energy transfer of TADF material as a host of fluorescent material.

Fig. 7.33 Schematic diagram of energy transfer of TADF material as an assistant dopant for fluorescent materials.

stability of the device is improved (Furukawa et al., 2015; Nakanotani et al., 2014). Kyulux, a Japanese company of which Adachi is one of the founders, called this TADF-sensitized fluorescence technology “hyperfluorescence” and considered it to have a good application prospect. In 2018, Duan Lian's group made breakthroughs in TASF to further inhibit the direct recombination of electrons and holes on fluorescent dyes and the energy loss pathway

258  Chapter 7

Fig. 7.34 Chemical structures of PAD, CH3PAD, tBuPAD, PhtBuPAD, and BiPyA.

such as Dexter energy transfer from host to fluorescent dye. On one hand, introducing electrically inert groups such as methyl, tert-butyl, and phenyl-tert-butyl to the periphery of the anthracene-based green-dye PAD to protect the electroactive core of the fluorescent dye and Dexter energy transfer from the host to the fluorescent dye can be inhibited (as shown in Fig. 7.34); The device structure is designed to achieve a multienergy transfer pathway from the interface exciplex to the TADF host to the fluorescent dye (as shown in Fig. 7.35). For the first time, a TASF device has an EQE of more than 20%, The maximum EQE and power efficiency are up to 24% and 71.4 lm/W, and the EQE and power efficiency are still as high as 22.6% and 52.3 lm/W, respectively, at a high brightness of 5000 cd/m2 (Zhang et al., 2017a, b, c, d). Based on the strategy described here, the researchers used an asymmetric anthracene-based ETM (BiPyA), which has high electron mobility and good exciton-blocking performance, further improving TASF-OLED performance. EQEmax and PEmax are 24.6% and 76 lm/W, respectively, which is the highest efficiency reported for current TASF devices (Zhang et al., 2017a, b, c, d).

h

e 75%

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Interfacial exciplex

T1

S0

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Fig. 7.35 Schematic diagram of a multienergy transfer pathway from an interface exciplex to a TADF-assisted host, and then to the fluorescent dye.

LEDs Based on Small Molecules  259 Duan Lian’s group also used TADF materials as hosts for phosphorescent OLEDs. In 2013, they used TADF materials as a host for the first time to promote the injection of charges from the transport layer to the light-emitting layer, thereby reducing the voltage of the device. The constructed green-light device achieved a minimum turn-on voltage of 2.19 V. This is the first research to use TADF materials as the host material for OLED (Zhang et al., 2013a, b). Furthermore, they also demonstrate that the triplet excitons of the TADF material can be upconverted to a singlet state, thereby transferring energy to the phosphorescent dye through long-range Förster energy transfer. This long-range Förster energy transfer enables more efficient energy transfer than the conventional phosphorescent OLEDs, which can transfer the triplet to the dye only through the short-range Dexter interaction. Therefore, complete energy transfer can be achieved with a low doping concentration (<3 wt%). However, devices using conventional host materials can achieve a full energy transfer only at higher doping concentrations (>8 wt%). Therefore, devices using TADF materials as the host can reduce the cost of the device. Because the energy transfer is more effective, the triplet exciton concentration of a phosphorescent device at high current density can be effectively reduced, thereby reducing the quenching of the triplet state and alleviate efficiency roll-off (Zhang et al., 2014a, b, c, d, e). Solution-processable materials

TADF materials have undergone significant progress because TADF-OLEDs exhibit extraordinary performance, and so they have been given priority in newly developed OLED technologies. Meanwhile, solution-processable OLEDs are attracting increasing attention due to their potential application in low-cost, large-area, solid-state lighting (Fig. 7.36). A large number of materials with TADF characteristics have been developed, including small molecules, polymers, and dendrimers, and they can be used in solution-processed devices, exhibiting good performance. Che et al. developed a series of luminescent cyclometalated Au (III) complexes with auxiliary aryl ligands. These complexes showed sky-blue to green electroluminescence with EQEs of up to 23.8%. According to the authors, the key to realize high quantum efficiency of metal complexes is taking advantage of the emissive triplet intraligand (3IL) excited state and avoiding nonradiative triplet ligand-to-ligand charge transfer (3LLCT). In this study, the TADF character of the gold (III) complexes is caused by the presence of an amino substituent on the auxiliary aryl ligand, which is twisted with respect to the C4N4C ligand plane and leads to the separation of HOMO and LUMO (To et al., 2017). Lee et al. managed to improve the solubility of 4CzIPN by introducing methyl or tert-butyl groups on it, synthesizing soluble TADF dopants m4CzIPN and t4CzIPN. The latter material showed better performance because the tert-butyl group not only increases the solubility of the dopant materials, but also stabilizes the morphology of the spin-coated film, which is revealed from atomic force microscopy (AFM) images. Devices based on the three emitters

260  Chapter 7

Fig. 7.36 Chemical structures of solution-processable, TADF small molecules.

LEDs Based on Small Molecules  261 mentioned here were fabricated. A high EQE of 18.3% was achieved using t4CzIPN as the dopant, which was even higher than the efficiency of the dry-processed device. However, limitation exists in the strategy because the strong electron-donating character of tert-butyl may cause a red shift of the emission color for blue emitters (Cho et al., 2014a, b). Su et al. developed a novel compound, TBP-DMAc, based on benzene-1,3,5-trityltris (phenylmethanone) (TBP) and 9,9-dimethyl-9,10-dihydroacridine. Due to the flexibility of the TBP moiety, a small ΔEST can be achieved by the twisted DA structure, while the created multiple DA transition channels can maintain a large transition integral for a large radiative rate ksr. Furthermore, with the n-π* transition character of TBP, the valid conjugation length for 3LE could be shortened, assuring an effective RISC from 3CT to 1CT (Fig. 7.37). The authors found that by increasing dopant concentration and using more polarized surroundings, the reverse upconversion and triplet concentration quenching processes can be promoted, while the radiative rate will be reduced. This trade-off can be attributed to a condensed-state solvation effect. OLEDs based on the TADF emitter TBP-DMAc achieved high EQEs of 26% and 22% via vacuum-evaporation and solution-processing methods, respectively. To date, this wet-processed device achieves the highest efficiency and an extremely low efficiency roll-off among its counterparts (Cai et al., 2018). Polymeric materials are considered as ideal emissive materials for solution-processed devices. A simple solution process such as spin-coating or ink-jet printing can be utilized to improve the efficiency of the process and reduce the production cost. Wang et al. developed the first blue TADF nonconjugated polymers based on through-space charge transfer (TSCT), as shown in Fig. 7.38. The authors used polyethylene as the backbone and 9,9-dimethyl-10-phenyl-acridan (Ac) or 9,9-bis(1,3-ditert- butylphenyl)-10-phenylacridan (TBAc) as the pendant electron donor and 2,4,6-triphenyl-1,3,5-triazine (TRZ) as the pendant electron acceptor. Four polymers were synthesized: P-Ac50-TRZ50, P-Ac95-TRZ05,

Fig. 7.37 Schematic diagram of the TADF process and correlated energy-level relationship of TBP-DMAc.

262  Chapter 7

Fig. 7.38 (A) Chemical structures of TSCT-based polymers and (B) the corresponding control polymers.

P-TBAc50-TRZ50, and P-TBAc95-TRZ05. Pendant units are physically separated but spatially proximate to allow TSCT, and tert-butyl groups are introduced to investigate the influence of distance between the D and A units on the CT interactions. A small ΔEST of 0.019 eV and high PLQY of up to 60% in the film state can be obtained, and a device based on the resulting polymer P-Ac95-TRZ05 exhibits EQEmax of 12.1%, with a very small roll-off of 4.9% (11.5% at 1000 cd/m2) (Shao et al., 2017). Dendrimers are perfectly branched polymers with fixed molecular weight, good solubility, and an amorphous nature. They have unique structures with a high steric hindrance that can isolate chromophores at the core to prevent concentration quenching. Ban and Jiang et al. designed and synthesized two encapsulated TADF molecules, Cz-3CzCN and Cz-4CzCN, as a solution-processable host and a blue guest, respectively (Fig. 7.39). The emitter Cz-CzCN was used to fabricate the green-light devices. The device EQEs are among the highest values of reported solution-processed blue (23.5%) and green (23.8%) TADF-OLEDs. Nonencapsulated counterparts [namely, 3CzBN:4CzBN (3CzBN: 2,4,6- tri(9H-carbazol-9-yl) benzonitrile and 4CzBN: 2,3,5,6-tetra(9H-carbazol-9-yl) benzonitrile)] were studied as well,

N

(CH 2) 6

(CH 2)6 O

O N

N (CH 2) 6

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N

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N (CH 2) 6 O

N N

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N (CH 2) 6

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O

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Cz-3CzCN

Fig. 7.39 Chemical structures of Cz-CzCN, Cz-3CzCN, and Cz-4CzCN.

Cz-4CzCN

LEDs Based on Small Molecules  263

Cz-CzCN

(CH 2) 6

264  Chapter 7 showing a much lower performance than the dendrimer systems. The results indicate that the triplet-triplet interactions between adjacent molecules can be reduced for the alkyl-linked carbazole units, and the molecular aggregation-induced energy leakage of the excitons in the host can be restricted. Moreover, according to DFT calculations, the triplet state locations of the encapsulated molecules are independent of the HOMO and LUMO distributions, indicating that the separated energy transfer and CT channels would reduce the triplet-polaron quenching (TPQ) effect during device operation. In conclusion, encapsulated materials are a promising design strategy for solution-processed TADF-OLEDs, and further exploration can be focused on the reduction of efficiency roll-off by efficient molecular modulation (Ban et al., 2017). 7.2.6.2  TTA (also known as P-type delayed) materials Triplet-triplet annihilation (P-type) delayed fluorescence has a mechanism such that one singlet state is generated by quenching two triplets, and then fluorescence is generated by a singlet. This phenomenon is mainly obtained from anthracene derivatives and four benzene derivatives. The advantage of TTA lies in achieving the high efficiency and long lifetime for deep-blue emitting materials, which presents great potential in commercial applications. Monkman et al., from Durham University, designed and synthesized a series of anthracene derivatives and introduced diphenylamine groups as hole transport groups, as shown in Fig. 7.40. These materials possess an intramolecular CT state and high PL efficiency. The efficiency of the optimized device based on DF4 can reach 5.5%, and the power

N

N

DF5

N

N

DF4

DF1

Fig. 7.40 TTA fluorescence.

LEDs Based on Small Molecules  265 efficiency reaches 8 lm/W at 100-cd/m2 brightness. By adding an electronic barrier layer, the EQE of the device can reach 6%, while the power efficiency of the device at 260 cd/m2 is 11.2 lm/W, which is higher than the upper limit of the efficiency calculated according to the fluorescence quantum efficiency of the DF4. This increase in efficiency comes mainly from the contribution of TTA-delayed fluorescence. The abovementioned materials promised 2T1≤Tn, so the efficiency of TTA producing singlet is 50%. The triphenylamine group is one of the reasons for the high efficiency of the materials. It can enhance hole mobility and increase the CT-state properties of materials. This research provides a theoretical basis for designing more efficient fluorescent materials. 7.2.6.3  HLCT state materials The hot exciton fluorescent material, also known as HLCT material, was first proposed by Ma et al., at Jilin University. For the OLED material, the delocalization of the exciton (small binding energy) fosters the improvement of the Xs (the singlet exciton ratio), while the localization of the exciton (large binding energy) is beneficial to the enhancement of the luminescent efficiency, and the two are contradictory. For TADF materials, the increase of Xs is often not conducive to the increase of the S1 state’s exciton radiative transition rate. In order to obtain the new material with moderate exciton binding energy, the researchers have proposed a new theory: to wit, that the CT state and the LE state are combined to make a new and hybrid excited state—that is, the HLCT. This is a new strategy to break the exciton statistical law (Xs=25%). The TADF materials enhance the exciton utilization by the transfer from the T1 excited state to the S1 excited state (RISC), while the HLCT materials apply the RISC process in the high-energy excited state (Tm−Sn, m, n>1) to realize the conversion of the triplet state to the singlet state. In material science, the process of using a low-energy excited state (T1−S1) is called cold exciton, and the process of using a high-energy excited state (Tm−Sn, m, n>1) is called hot exciton, so the HLCT material is called hot exciton material. Because the HLCT material is implemented by an independent hot-CT exciton channel to allow the RISC process to increase the generation ratio of the single-state S1 exciton, the S1 exciton has the high radiation transition efficiency of the LE state. In theory, the exciton binding of this material can be moderate. On the one hand, it can maximize the conversion efficiency of T to S exciton; on the other hand, it is beneficial to the radiation transition of S1 to S0, which resolves the contradiction between the two. The difference in the path between TADF and HLCT leads to the difference in the delocalization state of the generated S1 exciton. Therefore, although the two methods can increase the proportion of the S1 exciton in the radiation, the rates of the radiation transition of the S1 exciton are different, the LE of the S1 exciton obtained by the HLCT path is stronger, and the efficiency of radiation transition is higher. The mechanism is illustrated in Fig. 7.41.

266  Chapter 7 Electron/hole recombination

kRISC S1

DES

1T3

kfl

T3

»0 kIC(T)

kph

DET1T3 >> DES1T3 T1

Ground state (S0)

Fig. 7.41 The mechanism of HLCT.

As shown in Fig. 7.41, the designed molecular needs to be characterized by both LE and CT excited states (i.e., the characteristics of the HLCT state), and the CT energy is higher than the LE excited state as follows: 1. S1 and T1 have significant LE excited state characteristics, and the large electron/hole wavefunction overlaps to ensure the high radiative transition probability of the S1 state. 2. Sn and Tm have significant CT excited-state characteristics, and small electrons/hole wavefunctions overlap to ensure a sufficiently small ΔEST (Sn/Tm), so that RISC (Tm to Sn) can be carried out at high speed. As an important prerequisite for ensuring that the mechanism is effective, a sufficiently large gap between Tm and Tm−1 is necessary to effectively reduce the rate of internal conversion and make the rate of RISC (Tm to Sn) large enough to compete with the IC (Tm to Tm−1) rate of the T exciton, which eventually leads to the change of the relaxation path of the T exciton, which deviates from the T1 state. In 2008, Ma et al. from Jilin University pioneered the research of HLCT materials. First, the mechanism was proposed in the triphenylamine derivative system, showing significant EL efficiency and device stability. The Xs was about 50%. Compared with the pure LE compound, the EQE and lifetime of the devices made by the HLCT material improved. The theoretical calculation showed that the improved efficiency is due to the combination of anthracene LE and triphenylamine-anthracene CT emission characteristics. In 2014, based on a similar system, the triphenylamine-naphthothiadiazole material TPA-NZP was studied. In those findings, the emission wavelength was 632 nm, the EQE was 2.8%, Xs reached 93%, the device showed high stability and a low efficiency roll-off with high current density.

LEDs Based on Small Molecules  267 In 2012, a deep-blue EL device was prepared with triphenylamine-phenanthroimidazole (TPA-PPI) as the emitting layer material. The maximum current efficiency of the device is 5.7 cd/A, the EQE exceeds 5%, and Xs=28%. In 2013, the researchers introduced the CN group to the benzene of TPA-PPI to enhance acceptor strength in order to further improve the components of the CT-state exciton. Based on this material as the emitting layer, they obtained the saturated blue device with the largest current efficiency of 10 cd/A. The EQE of the device is 7.8%, and Xs=98%. The authors consider that the delocalized CT exciton with low binding energy is beneficial to increase the value of Xs. In addition, in 2014, Ma et al. studied the phenothiazine-benzothiadiazole PTZ-BZP, and the emitting wavelength was 700 nm, EQEmax=1.54%, and Xs=48%. HLCT is an ideal excited state. The high-energy CT state is responsible for building an exciton channel to improve the exciton utilization, while the lowest-excitation S1 state is responsible only for the luminescence, so that the high-exciton utilization and highfluorescence efficiency can be carried out with no interference. The large ΔETm−Tm−1 effectively reduces the ISC rate and reduces the triplet aggregation as a result of the reduction of T-T annihilation and roll-off at high current density. The RISC between high energy levels avoids the occurrence of delayed fluorescence, and this molecular device is able to obtain higher efficiency without doping, while it is simple to operate and the cost is greatly reduced. But at present, in the mechanism of HLCT, there is only a theoretical calculation, which we cannot verify by experimentation. In addition, the molecular design is difficult. Although the efficiency of the device based on HLCT breaks through the limit of the traditional fluorescence, it has not realized the expected 100% internal exciton utilization. 7.2.6.4  Materials employing new luminescent mechanisms Due to limitations in spin statistics, the upper limit of the IQE of organic EL devices (i.e., OLEDs) is typically only 25%. Although organic phosphorescent materials can exceed the 25% limit, the IQE of phosphorescent devices can reach 100%, but the phosphorescent materials require the use of heavy metal elements, which are scarce and carry a high price, making OLED products expensive. Therefore, exploring new luminescence mechanisms in inexpensive fluorescent luminescent materials with the goal of breaking the 25% limit has become a hot topic in current OLED research. Li et al. of Jilin University used a kind of open-shell organic molecule TTM-1Cz as a luminescent material to prepare a novel OLED based on dual-state exciton luminescence (Fig. 7.42). TTM-1Cz consists of a trityl radical attached to a carbazole group. Because the highest occupied orbit of TTM-1Cz has only one electron, when the electron is excited to the lowest unoccupied orbit, the highest occupied orbit is empty, which makes the transition of the excited state to the ground state completely spin-allowed. Because an electron has only two spin states, this excited state is called a dual state. This study cleverly bypasses the “triplet exciton utilization” problem in the OLED field, providing a new way to achieve 100% quantum efficiency in OLEDs.

268  Chapter 7

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10–1

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100 0 10–2

(D)

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100 200 300 t (ns) 8

10

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Fig. 7.42 (A) The chemical structure of TTM-1Cz. (B) The steric configuration of TTM-1Cz calculated by DFT. The unpaired electron is surrounded by six chlorine atoms. (C) The J-V-L characteristics of the OLEDs. (D) The EQE of the OLEDs as a function of voltage. The inset shows the lifetime of the excited states of TTM-1Cz in toluene solution.

Li et al. of Jilin University and Shuai Zhigang of Tsinghua University proposed a new mechanism of triplet- to single-state upconversion between molecules induced by triplet-polaron interactions. The new mechanism was demonstrated from both experimental and quantitative calculations. It can break through the 25% limit of IQE, providing a new way to obtain highefficiency and low-cost OLEDs (Fig. 7.43). The design of the synthetic material TPA-TAZ is shown in Fig. 7.43, and the EQE of the deep-blue device based on the material is 6.8%. 7.2.6.5  Materials with record high efficiencies Wu and Wang et al. from National Taiwan University designed and synthesized a series of TADF materials based on triazine/acridine derivatives. The molecules have a high horizontal orientation, which can improve the light extraction efficiency of the device. The EQE reached 37% (Fig. 7.44).

LEDs Based on Small Molecules  269

Fig. 7.43 Schematic diagram for the TPI‐induced conversion from triplet to singlet. (A) The TPI between triplet and positive‐charge polaron, (B) The TPI between triplet and negative‐charge polaron.

Fig. 7.44 Highest-efficiency blue fluorescent device.

270  Chapter 7

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Simulation isotropic (Q = 0.67) Simulation horizontal (Q = 1) 10% CzDBA:CBP (Q = 0.84) 10% tBuCzDBA:CBP (Q = 0.83)

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CzDBA 2.0

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1.0 0.5 0.0 400

500 600 Wavelength (nm)

700

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105

Fig. 7.45 Highest-efficiency green fluorescent device.

A team lead by Chien-Hong Cheng at Tsinghua University designed and synthesized two kinds of diboron-based molecules: CzDBA and tBuCzDBA. This D-A-D rod compound has excellent TADF characteristics, and the PLQY in the film is about 100% and has a horizontal orientation of 84%. The green OLED based on CzDBA achieves a high EQE of 37.8±0.6% (Fig. 7.45). Yang et al. of Wuhan University used rigid acridine groups as electron donors and 1,8-naphthalimide as electron acceptors to design and synthesize two rigid red-red TADF molecules with large torsion angles: NAI-DMAC and NAI-DPAC. The molecule has a high PLQY (ΦPL) and a horizontal orientation. The orange-red OLED based on NAI-DMAC and NAI-DPAC has a microcavity effect, and the EQE is as high as 21%–29.2%, which significantly exceeds the previously reported, TADF-based, orange-red OLED performance (Fig. 7.46).

7.2.7  Phosphorescent Materials Since Ma and Forrest et al. reported on the OLEDs based on triplet exciton emitting in 1998, phosphorescent materials have been rapidly developed. Phosphorescence comes from the triplet excited state of the materials. In the solid state at room temperature, the phosphorescence of general organic compounds is very weak, while heavy metals such as ruthenium (Ru), osmium (Os), iridium (Ir), platinum (Pt), and other heavy-metal complexes have strong phosphorescence emissions, which can be used as phosphorescent materials for OLED.

LEDs Based on Small Molecules  271

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Fig. 7.46 (A) Chemical structures, (B) external quantum efficiencies, power efficiencies, and (C) transient PL decay curves of NAI-DMAC and NAI-DPAC.

Transition metal complexes, such as Ru, Pt, Rh and especially Ir complexes, have been regarded as excellent phosphorescent materials in the field of organic EL. Such complexes can enhance the SOC of the molecules through the ISC after electron-hole recombination, changing the original spin-free resistance to local permission, which promotes the increase of the original triplet state in the system and improves the probability of radiation transition from the triplet excited state to the ground state, realizing the mixed singlet and triplet exciton radiation. Thus, the luminous efficiency is three times higher than that of the fluorescent materials and the theoretical IQE can reach 100%. In recent years, the iridium complexes have received a great deal of attention and have wide application in phosphorescent materials because of its excellent luminescent properties, such as extremely high quantum yield and rich, luminescent color. Neutral iridium complexes are widely used in the preparation of high-performance phosphorescent OLEDs, while ionic iridium complexes are outstanding in light-emitting electrochemical cells (LECs) due to their ionic nature, and they are still immature as new phosphorescent materials. The following discussion mainly covers two aspects: neutral iridium complexes and ionic iridium complexes. 7.2.7.1  Neutral iridium complexes In 2011, Suh et al. designed a new series of red phosphorescent materials with 2-phenylquinoline as the ligand and the PL wavelength in the range of 580–595 nm, as shown

272  Chapter 7

Ir N

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O O

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Ir(mphmq) 2tmd

Fig. 7.47 Neutral iridium complexes with red phosphorescence.

in Fig. 7.47. These dyes have high steric hindrance due to the introduction of methyl or tertbutyl groups in the ligand, which reduces the self-quenching effect of the dye molecules. Therefore, the devices achieved high EQEs, which are 15.7%, 19.8%, 21.9%, 22.2%, 24.6%, respectively. Later, Kim et al. used Ir(phq)3, Ir(mphq)2acac, Ir(MDQ)acac, and Ir(mphmq)2tmd as the dyes, as well as B3PYMPM, a fluorescent material with horizontal orientation characteristics, as the host materials. The EQEs of the devices reached 20.9%, 27.6%, 27.1% and 35.6%, respectively. Compared with the devices prepared with NPB, which horizontal orientation characteristics do not have, the performance of these devices is greatly improved. It is difficult to prepare blue phosphorescent dyes that have good monochromaticity and thermal stability. To tackle this problem, researchers have conducted many studies. Parthasarathy et al. designed a series of iridium complexes with N-heterocyclic carbene (NHC) as the ligand in order to achieve high brightness in deep-blue luminescence. They designed two isomeric complexes, as shown in Fig. 7.48. The device based on fac-Ir(pmp)3 achieved a brightness of over 7800 cd/m2 and CIE coordinates of (0.16, 0.09), while devices based on mer-Ir(pmp)3 achieved a brightness of over 22,000 cd/m2, and the CIE coordinates are (0.16, 0.15). The reason for the sharp increase in brightness is that the Ir-NHC bond in the material is extremely strong. This is of great significance for the design and improvement of blue phosphorescent materials. In 2014, the group also reported a stacked device with dual luminescent layers and gradient doping in the luminescent layer. This new device structure is applied to a blue phosphorescent material to make it luminescent, achieving a long lifetime of T80 = 616±10 h. In 2013, Kim et al. obtained devices with EQE of over 30% by using a green-light dye, Ir(ppy)2acac, with good horizontal orientation, and B3PYMPM and TCTA as cohosts. Again,

LEDs Based on Small Molecules  273 N N N

N

N

N

N

Ir

N N

N

N

N

Ir

N N

N

N N

N

fac-Ir(pmp)3

mer-Ir(pmp)3

Fig. 7.48 Neutral iridium complexes with blue phosphorescence

it was proved that the horizontally oriented luminescent molecules, which can cause an increase in light extraction efficiency, will significantly improve device performance. In 2009, Yu Liu et al. of Jilin University designed a iridium complex with N,Ndiisopropylbenzylhydrazine as the ligand (Fig. 7.49), and later designed a similar ligand. The iridium complexes with these two ligands emit at 500 and 605 nm, respectively. The materials are characterized by high mobility and solid-state luminous efficiency. Red-emitting devices based on these materials have EQEs as high as 26.3%. In 2017, Yun Chi’ s group at Tsinghua University in Hsinchu, Taiwan, reported a deep-blue iridium complex with a dicarbene-structured, tridentate ligand (Fig 7.50). The luminescence peak in solution was 447 nm, and the EQE of the device was up to 20.7% with CIE coordinates of (0.15, 0.17). In 2018, Liu et al. designed and synthesized iridium complexes with a triazolyl-biphenylstructured ligand (Fig. 7.51). The luminescence peaks in the solution were 431 nm and

F F N F

N

N Ir

N

N Ir

N F

(F2ppy) 2Ir(dipcca)

N N

N N

(bzq)2 Ir(dipig)

Fig. 7.49 Neutral iridium complexes with N,N-diisopropylbenzylhydrazine as the ligand.

274  Chapter 7

Fig. 7.50 Neutral iridium complexes with a dicarbene-structured, tridentate ligand.

Fig. 7.51 Neutral iridium complex with a triazolyl-biphenyl-structured ligand.

458 nm, respectively, and the quantum efficiencies were 41.3% and 64.4%, respectively. The PL efficiency of the doped film in the DPEPO body is more than 80%. The EQE of the device reaches 22.5%, and the CIE coordinates are (0.15, 0.11), which is the highest efficiency in the deep-blue range. 7.2.7.2  Ionic iridium complexes As early as 2005, Plummer et al. first reported on phosphorescent OLEDs based on ionic iridium complexes. They successfully prepared yellow-emitting OLEDs by mixing a classical complex, [Ir(ppy)2(bpy)] [PF6], into poly(N-vinylcarbazole) (PVK). The device structure was ITO/PEDOT: PSS (100 nm)/PVK: [Ir(ppy)2(bpy)][PF6] (70 nm)/TPBi (60 nm)/Ba (5 nm)/Al (100 nm). The conductive polymer layer (poly(3,4-ethylenedioxythiophene))—polystyrene sulfonate, PEDOT:PSS and luminescent layer PVK: [Ir(ppy)2(bpy)][PF6]—were prepared by spin-coating; HBL (1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene, TPBi) and barium/aluminum (Ba/Al)

LEDs Based on Small Molecules  275 cathodes were prepared by vacuum evaporation-deposition. The device has an EL wavelength of 560 nm and a maximum current efficiency of 22.5 cd/A. Later, researchers developed efficient OLEDs based on ionic iridium complexes with different emission. In 2010, Park et al. reported a single-layer green OLED based on the ionic iridium complex [Ir(dFppy)2(bpy)] [PF6], with a maximum current efficiency of 12 cd/A and an EL wavelength of 512 nm. Yong Qiu’s research group used a series of ionic iridium complexes containing pyrazole and imidazole auxiliary ligands, [Ir(dFppy)2(pzpy)] [PF6], [Ir(ppy)2(pzpy)] [PF6], [Ir(dFppy)2(pyim)] [PF6], [Ir(ppy)2(pyim)] [PF6], [Ir(dFppy)2(pybi)] [PF6], [Ir(ppy)2(pybi)] [PF6], [Ir(ppy)2(qlbi)] [PF6], and [Ir(ppy)2(bid)] [PF6], as phosphorescent dyes. Also, 1,3-bis[5-(4-tert-butyphenyl)-1,3,4-oxadiazol-2-yl] benzene (OXD-7), an ETM suitable for blue and blue-green dyes, or 2-(4-biphenyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole (PBD), an ETM for green to red dyes, was doped into PVK hosts, preparing OLEDs by solution-processing. The device structure was ITO/PEDOT:PSS (60 nm)/PVK: OXD-7 or PBD: complex (85 nm)/TPBi (30 nm)/Cs2CO3 (2 nm)/Al (100 nm). The EL wavelength of the devices ranged from 458 nm to 620 nm, and color coordinates were between (0.16, 0.22) and (0.61, 0.38). Among them, the blue OLEDs based on [Ir(dFppy)2(pzpy)][PF6] have a maximum current efficiency of 2.5 cd/A, and the EQE of 1.8%; the green OLEDs based on [Ir(ppy)2(pyim)][PF6] have maximum current efficiency as high as 25.3 cd/A, EQE of 8.1%, and maximum luminance of more than 38.5×103 cd/m2; the red OLEDs based on [Ir(ppy)2(qlbi)][PF6] have the maximum current efficiency of 4.2 cd/A and the EQE of 3.2%. In 2012, Tang et al. reported two red-emitting ionic iridium complexes containing 1,10-phenanthroline-based ancillary ligands, which further increased the EQE of red OLEDs to 7.1%. The ionic iridium complexes have the characteristics of simple design and synthesis, easy adjustment of luminescent color, rich photophysical properties, good redox stability, and solubility in polar solvents. However, it is severely limited in applications in highperformance OLEDs due to its ionic nature, which forbade it to be processed by vacuum evaporation deposition. In 2007, Weiyang Huang from Hong Kong Baptist University reported for the first time two sublimable ionic iridium complexes, [Ir(dpfd)2(dMebpy)] [PF6] and [Ir(dpfd)2(dMephen)] [PF6], using 4,4′-dimethyl-2,2′-dipyridine (dMebpy) and 4,7-di Methyl-1,10-phenanthroline (dMephen) as ancillary ligands and (9,9-diethyl-7pyridinylfluoren-2-yl) diphenylamine (dpfd), which contains a dendritic large sterically hindered group, as the cyclometalated ligand (Fig. 7.48). They successfully prepared yellow OLEDs by vacuum evaporation deposition. The EL wavelength is at 565 nm, and the maximum current efficiency is 19.7 cd/A, with EQE at 6.5%. In 2016, Zhang et al. also reported three sublimable ionic iridium complexes containing large sterically hindered ligands. A yellow-green OLED was prepared by vacuum evaporation, with an EL wavelength of 540 nm and a maximum EQE of 6.8%.

276  Chapter 7 In 2016, Yong Qiu’s group proposed a general molecular design strategy for prepare sublimable ionic iridium complexes that are suitable for vacuum evaporation deposition. They introduced large, sterically hindered, charge-distributed, tetraphenyl boron derivatives as counterions, which can significantly weaken the anion-cation interaction in the iridium complexes (Fig. 7.52), which can effectively improve their volatile property, thereby obtaining a kind of evaporable ionic iridium complexes. This method can convert most of the currently reported ionic iridium complexes into sublimable phosphorescent dyes by a simple and easy ion exchange reaction at room temperature, which break through the limitations of applying ionic transition metal complexes in the vapor-deposited OLED and enrich the PV material system.

7.2.8  White OLEDs (WOLEDs) and Optical Out-Coupling 7.2.8.1  All fluorescent WOLEDs As a result of spin statistics, conventional fluorophores suffer from an IQE of no more than 25%. Accordingly, all fluorescent white OLEDs (WOLEDs) based on conventional fluorescent emitters have lagged in recent years owing to relatively low efficiency. Apart from this, the triplet energy levels of commonly used blue fluorophores are even lower than some red phosphorescent materials. The reverse energy transfer from phosphorescent emitters to blue fluorophores with low T1 will lead to severe exciton quenching and efficiency roll-off. Therefore, the use of TADF materials, which can harness both singlet and triplet excitons, has shown tremendous potential for the development of high-efficiency, all fluorescent WOLEDs. Adachi and coworkers developed all fluorescent WOLEDs with EQEs of over 12% based on blue TADF materials (DMAC-DPS) combined with green and red fluorescent emitters, which provides a new way to fabricate high-efficiency, all fluorescent WOLEDs (Adachi et al., 2015). However, there is still a great disparity in the efficiency and color rendering index (CRI) between all fluorescent WOLEDs and all phosphorescent WOLEDs. Additionally, a TADF sensitized fluorescent emitting system with TBPe and TBRb doped in CzAcSF as a TADF host was applied to fabricate all fluorescent WOLEDs, realizing maximum EQEs of up to 15.2% (Lee et al., 2015a, b). Apart from this, an adjustable yellow TADF material, 2-(4-phenoxazinephenyl) thianthrene9,9’,10,10’-tetraoxide (PXZDSO2), combined with traditional fluorescent emitters, were utilized to fabricate WOLEDs (Su et al., 2016). The optimized WOLEDs achieved a high CRI of 95 and EQE of 19.2% (Fig. 7.53). Because of its good charge transporting properties and high exciton utilization ratio, a TADF exciplex consisting of CDBP and PO-T2T was reported as a universal host of TADF emitters, including 2CzPN, 4CzIPN, and AnbCz. All blue, green, and orange devices based on TADF

LEDs Based on Small Molecules  277

Fig. 7.52 Sublimable ionic iridium complexes.

278  Chapter 7

Fig. 7.53 (A) Schematic configuration of monochromatic WOLEDs. (B) Molecular structures of NI-1-PhTPA, PXZDSO2 and DBP.

emitters with an exciplex as host showed low turn-on voltage and high efficiency. WOLEDs with simplified structure achieved maximum EQE and power efficiency of 19% and 63 lm/W, respectively (Zhang et al., 2016c). Precise management of singlet and triplet excitons is crucial to fabricate high-efficiency, all fluorescent WOLEDs. Using a blue TADF host and a conventional fluorescent emitter, the resulting WOLEDs based on DPEPO:DMAC-DPS:TBRb as light-emitting layers showed maximum EQE and power efficiency of 14.6% and 51.6 lm/W, respectively. Moreover, by strategic management of electrically generated excitons within an efficient, sandwich-type emissive zone, high-performance all fluorescent WOLEDs achieved a maximum EQE of 18.4% and a CRI of 82. Considering that blue TADF materials can serve as both hosts and emitters in all fluorescent WOLEDs, edge-spiro effects were adopted to develop host-featured blue TADF dyes to achieve high performance in such devices. The maximum efficiencies are as high as 22.9% for EQE and 52.4 lm/W of all fluorescent WOLEDs based on SFI34pTz:DTPATXO (Wang et al., 2018). Recently, a series of hosts for high-performance all fluorescent WOLEDs were developed (Cheng et al., 2016; Su et al., 2015). A twisted spirocyclic phosphine oxide (SFXSPO) was reported as a universal host for WOLEDs, and the resulting device showed a maximum EQE of 19.7%, (Huang et al., 2016), which indicated that a rational design of hosts and dyes is important to achieving high-efficiency, all fluorescent WOLEDs. 7.2.8.2  All phosphorescent WOLEDs Single-EML WOLEDs have attracted increasing attention due to the advantages of low-cost and solution-processed approaches (Wu et al., 2010; Xue et al., 2008). All phosphorescent

LEDs Based on Small Molecules  279 WOLEDs incorporating blue phosphorescent dyes (FIrpic) and orange phosphorescent dyes (fbi)2Ir(acac) were developed and achieved EQE, current efficiency, and power efficiency of 19.3%, 52.8 cd/A, and 42.5 lm/W (Ma et al., 2009b). However, the CRI and color shift with CIE coordinate variations still needed to be improved. The high-performance, all phosphorescent WOLEDs were achieved by using FIrpic, Ir(ppy)2(acac), and Ir(MDQ)2(acac) as phosphorescent dyes with EQE of 15.7%, CRI > 80, and good color stability (Ma et al., 2012a). By incorporating green-emitting Ir(ppy)3 and red-emitting Ir(phq)3 into blue EMLs, the single-EML WOLEDs showed maximum EQE and power efficiency of 22% and 45 lm/W (Kido et al., 2014). Although the device performances of single-EML WOLEDs have made great progress in recent years, the fabrication procedures and device engineering still face huge challenges in modulating the doping concentration and balance of charge carriers (Ma et al., 2011, 2012b). An effective strategy is to utilize a mixed host composed of an ETM and an HTM. As a result, utilization of a mixed host with bipolar charge transport and a broad recombination zone can greatly improve the efficiency and lifetime of devices. Compared with single-EML devices, multi-EML, all phosphorescent WOLEDs allow the precise regulation of charge and exciton distribution by well-designed EMLs or charge generation layers. With the increase of functional layers in devices, the fabrication procedures are relatively more complicated, but multi-EML WOLEDs have great potential in industrial applications due to their insensitivity to doping concentration. Employing the same hosts in different EMLs is widely used to enhance energy transfer and reduce energy loss. Based on this strategy, dual-emissive-layer WOLEDs based on blue and orange phosphorescent dyes were developed, with maximum EQE and power efficiency as 16.9% and 44.1 lm/W, respectively (Ma et al., 2015a). In order to realize 100% exciton utilization efficiency, employing a single-host system to manipulate charges and excitons in the optimized WOLEDs achieved a maximum EQE of 20.1%, power efficiency of 41.3 lm/W, and a high CRI of 85 (Ma et al., 2009a). Owing to the influence of TTA and TPQ, the development of all phosphorescent WOLEDs suffer from a bottleneck to reduce severe efficiency roll-off. To make full use of excitons and reduce efficiency roll-off, a novel design of device structure was employed to fabricate all phosphorescent WOLEDs with three separated EMLs. The resulting WOLEDs realized an EQE of 22.4%, power efficiency of 46.6 lm/W, and current efficiency of 46.4 cd/A (Ma et al., 2015b). Moreover, the performances of devices maintained 22%, 41.3 lm/W and 46.2 cd/A at a high luminance of 1000 cd/m2 (Fig. 7.54). To satisfy the requirement of white-LED illumination application, there are high demands for high-performance WOLEDs with ideal CRI and reduced efficiency roll-off at high luminance (Sasabe et al., 2010). To develop WOLEDs with high CRI, a rational strategy is to combine more than three emitters. Four-color, all phosphorescent WOLEDs were achieved by codoping of multiple phosphorescent emitters into EML. The resulting WOLEDs show a high EQE of 24.5%

280  Chapter 7

Fig. 7.54 Proposed energy diagram and exciton dynamics for the WOLEDs.

and CRI of 81 at a luminance of 1000 cd/m2. Further, the EQE and CRI remain as high as 20.4% and 85, respectively, at a luminance of 5000 cd/m2, respectively (Fig. 7.55) (Lu et al., 2013). High-efficiency blue OLEDs are dispensable to improve the overall performance of WOLEDs. Therefore, a bipolar host with orthogonal molecular structure was utilized to fabricate sky-blue, phosphorescent OLED with large white light-emitting panels (150×150 mm) with a maximum power efficiency of 75.9 lm/W (Liao et al., 2015). Tandem WOLEDs can be fabricated from vacuum and solution deposition. Solutionprocessed, multilayer WOLEDs suffer from a trade-off between efficiency and solutionprocessed limitations. By introducing PEDOT:PSS/ZnO/PEIE as an interconnecting conductive layer between emissive units, solution-processed, all phosphorescent WOLEDs with a stack structure achieved a high EQE (over 20%) and current efficiency of 69 cd/A at a luminance of 5000 cd/m2, providing a new way to fabricate solution-processed WOLEDs for solid-state lighting (Chiba et al., 2015). To simplify device structures and circumvent adopting doped-EML WOLEDs, ultrathin, nondoped EMLs (about 0.1 nm) were introduced between an HTL and an ETL (Fig. 7.56). Based on this cost-effective method, the maximum EQE reached over 17% for blue, green, orange, and red monochrome OLEDs, respectively. Moreover, RGB WOLEDs and blue/ orange WOLEDs showed maximum EQEs of 18.5% and 16.4%, respectively, with low efficiency roll-off and a luminance of 1000 cd/m2, indicating that ultrathin, nondoped EMLs offer potential applications in high-performance WOLEDs (Ma et al., 2013).

LEDs Based on Small Molecules  281

Fig. 7.55 (A) Device configurations of WOLED W1-W4. The dopants employed are FIrpic for blue (B), Ir(ppy)2(acac) for green (G), Ir(BT)2(acac) for yellow (Y), and Ir(MDQ)2(acac) for red (R). (B) Energy level diagrams of WOLED W1-W4. (C) A photo of a large area (80 mm × 80 mm) WOLED (W3) illuminating with a color rendering index of 85.

7.2.8.3  Hybrid WOLEDs Due to the short lifetimes and high requirements for triplet energy levels of phosphorescence materials, blue phosphorescent materials are prone to cause poor color stability and lifetimes of white light devices. But the strategy of blending blue fluorescent materials into yellow, red, and green phosphorescent materials can avoid TTA and increase the lifetime of white light devices.

282  Chapter 7 ROGB LiF(1 nm)/Al(150 nm) TmPyPB(40 nm)

RGB

Flrpic(0.1 nm)

LiF(1 nm)/Al(150 nm)

TCTA(1 nm)

TmPyPB(40 nm)

BO

(ppy)2Ir(acac)(0.05 nm)

Flrpic(0.1 nm)

LiF(1 nm)/Al(150 nm)

TCTA(1 nm)

TCTA(1 nm)

TmPyPB(40 nm)

(fbi)2Ir(acac)(0.1 nm)

(ppy)2Ir(acac)(0.05 nm)

(fbi)2Ir(acac)(0.05 nm)

TCTA(1 nm)

TCTA(1 nm)

TCTA(1 nm)

(piq)2Ir(acac)(0.1 nm)

(MDQ)2Ir(acac)(0.1 nm)

Flrpic(0.1 nm)

TCTA(12 nm)

TCTA(13 nm)

TCTA(14 nm)

NPB(40 nm)

NPB(40 nm)

NPB(40 nm)

ITO/PEDOT

ITO/PEDOT

ITO/PEDOT

Glass substrate

Glass substrate

Glass substrate

Fig. 7.56 Device structure of WOLEDs with ultrathin nondoped EMLs.

Note that 4P-NDP has a very high luminous efficiency and triplet energy level, making it a very good blue fluorescent material. Ma et al. used 4P-NPD as a blue fluorescent dye to design a luminescent WOLED device (Sun et al., 2014a, b, c). To prevent the energy transfer from the triplet state of Ir(ppy)3 to 4P-NDP, the green and blue light-emitting layers are separated by a red light-emitting layer, Ir(MDQ)2(acac) (Fig. 7.57). The triplet exciton energy of 4P-NDP can be transferred to the red luminescent layer so that an IQE close to 100% can be achieved. The achieved WOLED device has a turn-on voltage of 3.3 V, a maximum EQE of 21.2%, a maximum power efficiency of 40.7 lm/W, and a maximum current efficiency of 49.6 cd/A. Although the intermediate layer plays an important role in hybrid white-light devices, there are still some disadvantages. First, the voltage drop caused by the intermediate layer cannot be ignored because it will lead to a decrease in the power efficiency of the device. Second, the intermediate layer increases the number of interfaces, increases the probability of formation of the exciplex, and affects the efficiency and lifetime of the device. Third, the energy transfer of the triplet excitons through the intermediate layer causes energy loss and reduces device efficiency. Finally, the intermediate layer causes the device preparation process to be complicated. Ma et al. designed and fabricated hybrid white light devices without intermediate layers (Sun et al., 2014a, b, c). The blue light-emitting layer was prepared by using a mixed host with bipolar transmission performance (Fig. 7.58). The device has a turn-on voltage of 3.1 V and maximum EQE, current efficiency, and power efficiency of 19%, 45.2 cd/A, and 41.7 lm/W,

LEDs Based on Small Molecules  283

Fig. 7.57 The hybrid white-light device with 4P-NPD as the blue fluorescent dye, as designed by Ma et al.

respectively. At the brightness of 1000 cd/m2, the EQE, current efficiency, and power efficiency of the device were 17%, 40.5 cd/A, and 34.3 lm/W, respectively. As the brightness changes, the luminescence spectrum of the device remains stable. When the 4P-NDP is replaced by DPAVBi, with a lower triplet level, the maximum power efficiency still reaches 40.3 lm/W. Exciton utilization in hybrid WOLED devices has a huge impact on device efficiency. Especially for hybrid white light devices using low-triplet, blue fluorescent dyes, improving the triplet exciton utilization of blue fluorescent dyes in devices is an urgent problem that needs to be solved (Fig. 7.59). Duan et al. proposed increasing the triplet utilization of lowtriplet, blue fluorescent dyes by TTA luminescence, and using α, β-ADN as a blue fluorescent dye and Ir(MDQ)2acac and Ir(ppy)3 as red and green phosphorescent dyes to construct a hybrid WOLED device with BPBiPA as the ETL. Due to the good electron transport performance and exciton-blocking performance of BPBiPA, the probability of occurrence of TTA in the light-emitting layer is increased and the device efficiency is improved. The device’s current efficiency, EQE, and power efficiency are 65.7 cd/A, 28%, and 57.3 lm/W, respectively, and the lifetime exceeds 7000 h at the brightness of 1000 cd/m2. Blue TADF materials can also be used to construct high-efficiency WOLED devices. Using the blue TADF material as the host, 100% IQE can be achieved due to the RISC of the TADF

284  Chapter 7

Fig. 7.58 (A) Energy-level diagram of the hybrid white-light device without an intermediate layer. (B) Device efficiency-brightness curve and luminescence spectrum.

Exciton energy

3.0 LiF/Al

T1

Förster S1

S1

T1 ISC

Upconversion by TTA T1

Dexter

Fluorescence & delayed fluorescence

Phosphorescence

5.6

5.3 5.6

5.6

6.4

6.4

5.8

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Phosphorescent EML

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W1 with Alq3

5.0k

100

0.0 6 Voltage (V)

8

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100

10 1.0

1

0.8 0.6 0.4

W1 100 cd/m2 W1 1000 cd/m2 W2 100 cd/m2 W2 1000 cd/m2

0.2 0.0

0.1

(D)

W1 W2

400 440 480 520 560 600 640 680 720 760 Wavelength (nm)

0

5000

10000 15000 Brightness (cd/m2)

1

0.1 20000

Fig. 7.59 (A) Hybrid white-light device structure. (B) Exciton energy-level diagram of hybrid white-light device. (C) The I-V and B-V curves of hybrid white-light device. (D) The current and power efficiency of hybrid white light device at different brightnesses.

LEDs Based on Small Molecules  285

10.0k

101

Brightness (cd/m2)

15.0k

4

Fluorescent EML

Power efficiency (Im/W)

W2 with BPBiPA

102

2

Interlayer

(B)

EL intensity (a.u.)

103

10–1

S1

Alq3

2.9

2.7

BPBiPA

2.9

2.5

Current efficiency (cd/A)

5.5

Current density (A/m2)

2.9

2.3

2.3

(A)

(C)

2.3

S0

5.1

ITO

BPAPF: Ir(MDQ)2acac

NPB

2-TNATA:F4TCNQ

2.7

α,β-ADN: DACrs

2.3

BPAPF: SBFK

2.2

BPAPF: SBFK Ir(ppy)3

2.2

286  Chapter 7

Fig. 7.60 (A) Energy-level diagram of hybrid white light device. (B) Energy-transfer diagram of hybrid whitelight device.

material. Duan et al. used a blue-light TADF material, DMAC-DPS, as the host and blue dye, and PO-01 as an orange dye to achieve a single light-emitting-layer hybrid WOLED device (Fig. 7.60) (Zhang et al., 2015a, b, c). The triplet excitons of DMAC-DPS can be transferred to singlet excitons through the RISC, and the energy is transferred to the dye molecules through long-range energy transfer, which reduces the loss of triplet excitons and efficiency roll-off and improves device efficiency. The maximum EQE and power efficiency of the device are 20.8% and 51.2 lm/W, respectively. The CIE coefficients were (0.398, 0.456) at a luminance of 500 cd/m2. TADF materials based on exciplex systems can also be used in hybrid WOLED devices. Zhang et al. achieved a single emitting-layer, hybrid WOLED device by incorporating green dye Ir(ppy)2(acac) and red dye Ir(MDQ)2(acac) into the blue TADF exciplex system CDBP:POT2T. The maximum EQE, current efficiency, and power efficiency of the device are 25.5%, 67 cd/A, and 84.1 lm/W, respectively (Fig. 7.61). The EQE, current efficiency, and power efficiency are 14.8%, 37 cd/A, and 24.2 lm/W, respectively, at the brightness of 1000 cd/m2. Despite high maximum EQE, the efficiency roll-off of the device is severe, and the spectrum changes significantly as the brightness changes. These problems need to be solved through subsequent research (Liu et al., 2015a, b). Duan et al. found that in the single emitting-layer, hybrid white-light device, due to the heavy atom effect of the phosphorescent dye, the ISC rate of the fluorescent dye from singlet to triplet increases, and then the fluorescence quenches and the device efficiency is reduced. However, when the fluorescent dye is replaced by the TADF material, due to the increasing rate of the reversed intersystem of the TADF material caused by the heavy atom effect of the phosphorescent dye, the luminous efficiency increases. Duan et al. achieved

LEDs Based on Small Molecules  287

Fig. 7.61 (A) CDBP and PO-T2T molecular structures. (B) UV-visible absorption and PL spectra of CDBP, PO-T2T, and CDBP: PO-T2T films. (C) Phosphorescence spectra of CDBP and PO-T2T and fluorescence and phosphorescence spectra of CDBP:PO-T2T films.

288  Chapter 7

Fig. 7.62 Schematic diagram of energy transfer of hybrid white-light devices.

a hybrid WOLED device with 2CzPN as the blue TADF material and PO-01 as the yellow phosphorescent dye (Fig. 7.62) (Zhang et al., 2015a, b, c). The maximum EQE was 19.6%, and the power efficiency was 50.2 lm/W. Wang Yue et al. of Jilin University designed and achieved a high-efficiency WOLED device with the TADF material PHCz2BP as both the blue dye and the host for phosphorescent dye. A high CRI, an EQE of 25.1%, a power efficiency of 24.1 lm/W, and a CRI of 87 was achieved (Fig. 7.63) (Liang et al., 2018). Ma et al. of Jilin University uses the HTM NPB and the ETM bis[2-(2-hydroxyphenyl)pyridine] beryllium (Bepp2) between phosphorescent dyes and fluorescent dyes to construct a heterojunction to separate singlet and triplet excitons, thereby avoiding the quenching of triplet excitons by fluorescent dyes and improving the performance of hybrid white-light devices (Fig. 7.64) (Shi et al., 2018). 7.2.8.4  Tandem WOLEDs The tandem white light device has more than one EL unit. They usually consist of a unique charge-generation layer that connects the light-emitting units in series. Compared to traditional OLEDs, stacked devices have many advantages. The brightness of the tandem OLED is proportional to the number of light-emitting units (Fig. 7.65). At the same time, the lifetime of series OLED devices is longer than that of traditional OLED devices at the same brightness (Gather et al., 2011; Jou et al., 2015). Kido et al. designed an OLED device with dual illuminating cells and a single chargegenerating layer by solution-processing. PEIE/ZnO was used as an electron-injecting layer.

LEDs Based on Small Molecules  289

Fig. 7.63 (A) The device structure and the energy levels of the corresponding materials. (B) EL spectrum of the luminescent material at a luminance of 1000 cd/m2. (C) Current density-voltage-luminance curves for red, orange, and green materials in the device. The inner graph is the current densityvoltage-luminance curve for the blue material. (D) Power efficiency-luminance-EQE curves of red, orange, and green materials in the device. The inner graph is the power efficiency-luminance-EQE curve of the blue-light material.

The driving voltage and brightness of the device are the sum of each of the light-emitting units. It is indicated that the charge-generation layer prepared by the solvent method can also generate electrons and holes well. This provides a new method of fabrication of stacked devices (Fig. 7.66) (Chiba et al., 2015). Currently, the main challenge in developing high-performance tandem OLEDs is to find suitable charge-generation-layer materials. The early charge-generation layer is V2O5. Later, MoO3 was able to provide better light transmission with a work function of 5.3 eV. Recently, the use of HAT-CN/HAT-CN:TAPC/TAPC has achieved efficient charge generation, thus achieving high EQE and CE, while PE has also greatly improved. Kwon et al. designed a tandem white-light device with a microcavity effect by using HATCN and TAPC as charge-generation layers with the help of theoretical optical simulation. The red

290  Chapter 7

Fig. 7.64 (A) Structure and energy level in the device. (B) Specific structural parameters of a particular planar heterojunction in a hybrid white-light device.

Fig. 7.65 Solution-processed phosphorescent tandem white-light device.

and green parts were phosphorescent materials Ir(mphmq)2(tmd) and Ir(ppy)2acac, and the blue part was fluorescent material BCzVBi. Finally, a power efficiency of 33.4 lm/W and a CRI value of 93 were achieved (Fig. 7.67) (Park et al., 2017). High stability has always been the advantage of tandem white-light devices. Parthasarathy et al. used a five-layer structure featuring a red-blocking layer, stable charge

LEDs Based on Small Molecules  291

Fig. 7.66 Tandem dual-color white-light device.

Fig. 7.67 Stacked three-color white-light-device structure and device performance.

generation layer, gradient doping, and thermal exciton management to achieve a long-lifetime white light: a full-phosphorescence white-light device with T70 = 80,000 h under the conditions of power efficiency of 50 lm/W and CRI value of 89 (T70 is the time it takes for the brightness to decay to 70% of the initial brightness) (Fig. 7.68) (Coburn et al., 2017).

292  Chapter 7

Fig. 7.68 The high-efficiency and stable tandem white-light device realized by Parthasarathy et al.

7.2.8.5  Progresses in optical out-coupling For OLED devices, due to the refractive indices of air, glass, and organic layers, according to conventional radiation theory, a large part of the light cannot leave the device due to internal reflection. In order to be able to reduce these losses, a number of strategies can be employed, such as flat surface, substrate with a specific shape, microlens arrays, microcavity effect, and nanopatterned and nanoporous film. They can be broadly classified into four categories: internal light extraction techniques, external light extraction techniques, internal and external light extraction techniques, and regulatory molecular orientation (Jou et al., 2015). Compared with the other three methods, the horizontal orientation of regulatory molecules has gradually become a main focus of research. Studies have shown that when the molecular dipole direction is completely perpendicular to the substrate, the light extraction efficiency is about 2%. When the dipole direction of the molecular transition is isotropic, the light extraction efficiency is about 30%. When the dipole direction of the molecular transition is completely horizontal, the light extraction efficiency can reach up to 45%. Therefore, the development of new molecules with horizontal orientation is critical to improving the efficiency of OLED devices. Yokoyama et al. proposed a design strategy for horizontally oriented molecules. For example, if traditional organic fluorescent molecules were “disk-” or “rod”-like, they would tend to

LEDs Based on Small Molecules  293 have a strong horizontal orientation. The host material, evaporation temperature, substrate temperature, and other elements will have a greater impact on the horizontal orientation of the fluorescent molecules (Yokoyama et al., 2011). Recently, some phosphorescent materials have also been found to have a horizontal orientation, and the corresponding phosphorescent devices achieve an EQE of more than 30%. Ir(ppy)3 is a typical isotropic phosphorescent material with a horizontally oriented dipole ratio of 67%. Ir(ppy)2acac has a certain horizontal orientation, with a horizontal orientation dipole ratio of 77%. Due to the presence of the horizontal orientation, the efficiency of the device based on Ir(ppy)2acac is 19% higher than that of Ir(ppy)3 (Liehm et al., 2012). Subsequently, Kim et al. incorporated Ir(ppy)2acac into the host material TCTA/ B3PYMPM of the exciplex. Because of the high-level orientation of the phosphorescent material, the EQE of the device can be as high as 30.2% (Kim et al., 2013a, b; Kim et al., 2014a, b, c, d). The group then further doped a horizontally oriented, high-PL-efficiency, green phosphorescent material Ir(ppy)2tmd in the excimer host, achieving an EQE of up to 32.3% (Lee et al., 2014a, b). In 2015, the Kim group mixed the orange red phosphorescent material Ir(mphmq)2tmd into the NPB/B3PYMPM host to achieve an EQE of 35.6%. Its high efficiency is mainly due to the high horizontal dipole orientation (82%) and high EQE (96%) of Ir(mphmq)2tmd dye (Kim et al., 2014a, b, c, d). As more and more horizontally oriented phosphorescent molecules have been developed, the reasons for affecting the horizontal orientation of phosphorescent molecules have been extensively studied. Kim’s group found that the phosphorescent molecule itself should have a specific orientation relative to the substrate, and the triplet of the phosphorescent molecule should have a horizontal transition dipole moment (Fig. 7.69)

CBP

Ir Horizontal

PMMA 4

UGH-2 4

Ir

Ir

Random

Vertical

Substract

Fig. 7.69 Effect of host material on the orientation of phosphorescent dyes.

294  Chapter 7 (Moon et al., 2015). In addition, the orientation of the host molecule does not affect the emission dipole orientation of the guest molecule. When the host and guest are linearly bonded, the bonding energy is large and parallel to the transition dipole moment, and the guest molecule will have a horizontal emission dipole orientation. When the host and the guest repel each other, a nonlinear structure is formed, resulting in a vertical emission dipole orientation. In addition to phosphorescent molecules, horizontally oriented TADF molecules achieve ultraefficient OLED devices. Since 2012, Adachi et al. have reported on the 4CzIPN molecule, which became a model molecule for green TADF. Subsequent studies have found that the 4CzIPN molecule has a certain horizontal orientation (73%) and an extremely high PL efficiency (97%) (Sun et al., 2014a, b). Device efficiency based on 4CzIPN has exceeded 30%. Subsequently, a series of TADF molecules with high PL quantum efficiency and high horizontal orientation were developed. In 2014, Adachi et al. designed and synthesized the horizontally oriented TADF molecule PXZ-TRZ (Komino et al., 2014). The effect of deposition temperature on device efficiency was also examined. The results show that when the deposition temperature is 200 K, 250 K, and 300 K, the efficiencies are 9.6%, 10.7%, and 11.9%, respectively. This is similar to the results of traditional fluorescent molecules. Recently, Kaji et al. designed blue-light TADF molecules, CCX-I and CCX-II. Its horizontal dipole orientation ratio is 0.75 and 0.83, while its PL quantum efficiency is as high as 97.2% and 104%. Because CCX-II has both high PL quantum efficiency and high horizontal orientation, its EQE is as high as 25.9% (Miwa et al., 2017). In 2016, Wong and Wu et al. reported a series of pyrimidine-based, deep-blue TADF materials (Lin et al., 2016). Compared to other materials, spiro-AC-TRZ has up to 100% PL quantum efficiency and an ultrahigh horizontal orientation (83%). The spiro-AC-TRZ-based, blue OLED device achieves an EQE of up to 36.7%. This is currently the highest value in the TADF field. Tang et al. from Suzhou University used ZnO as an EIL in an inverted OLED device, with light extraction of a corrugated structure device formed by ZnO (Fig. 7.70). Finally, the highest EQE of 42.4% and the power efficiency of 85.4 lm/W were achieved (Zhao et al., 2017). For solution-processed OLED devices, Lemmer et al. in Germany introduced a 10-micron optical enhancement layer of high-reflectivity titanium dioxide nanoparticles between the ITO anode and the glass substrate, which achieved a luminous efficiency of 56%. Because this nanoparticle is realized by screen printing, it is expected to be applied to future largescale OLED panels (Fig. 7.71) (Preinfalk et al., 2017).

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Fig. 7.70 The ZnO optical enhancement layer used by Tang et al.

Fig. 7.71 Screen-printed titanium dioxide optical enhancement layer.

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302  Chapter 7

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LEDs Based on Small Molecules  303 Kim, S.K., et al., 2009. Synthesis and electroluminescent properties of highly efficient anthracene derivatives with bulky side groups. Org. Electron. 10, 822–833. Kim, D.H., et al., 2011. Highly efficient red phosphorescent dopants in organic light-emitting devices. Adv. Mater. 23, 2721–2726. Kuo, H.-H., et al., 2017. Bis-Tridentate Ir(III) metal phosphors for efficient deep-blue organic light-emitting diodes. Adv. Mater. 29, 1702464. Lee, T.C., et al., 2009. Rational design of charge-neutral, near-infrared-emitting osmium(II) complexes and OLED fabrication. Adv. Funct. Mater. 19, 2639–2647. Li, W.J., et al., 2012. A twisting donor-acceptor molecule with an intercrossed excited state for highly efficient, deep-blue electroluminescence. Adv. Funct. Mater. 22, 2797–2803. Li, X., et al., 2018. Deep blue phosphorescent organic light-emitting diodes with CIEy value of 0.11 and external quantum efficiency up to 22.5%. Adv. Mater. 30, 1705005. Lin, C.H., et al., 2012. Phosphorescent OLEDs assembled using Os(II) phosphors and a bipolar host material consisting of both carbazole and dibenzophosphole oxide. J. Mater. Chem. 22, 10684–10694. Ly, K.T., et al., 2017. Near-infrared organic light-emitting diodes with very high external quantum efficiency and radiance. Nat. Photonics 11, 63–69. Ma, B., et al., 2006. Platinum binuclear complexes as phosphorescent dopants for monochromatic and white organic light-emitting diodes. Adv. Funct. Mater. 16, 2438–2446. Ma, D., et al., 2016a. New insights into tunable volatility of ionic materials through counter-ion control. Adv. Funct. Mater. 26, 3438–3445. Ma, D.G., et al., 2016b. Management of singlet and triplet excitons: a universal approach to high-efficiency all fluorescent WOLEDs with reduced efficiency roll-off using a conventional fluorescent emitter. Adv. Opt. Mater. 4, 1067–1074. Ma, D.G., et al., 2016c. Managing excitons and charges for high-performance fluorescent white organic lightemitting diodes. ACS Appl. Mater. Interfaces 8, 28780–28788. Obolda, A., et al., 2016. Triplet–polaron-interaction-induced upconversion from triplet to singlet: a possible way to obtain highly efficient OLEDs. Adv. Mater. 28, 4740–4746. Park, B., et al., 2010. Solution processable single layer organic light-emitting devices with a single small molecular ionic iridium compound. J. Appl. Phys. 108, 094506. Peng, T., et al., 2009. Very high-efficiency red-electroluminescence devices based on an amidinate-ligated phosphorescent iridium complex. J. Mater. Chem. 19, 8072–8074. Peng, Q., M., et al., 2015. Organic light-emitting diodes using a neutral p radical as emitter: the emission from a doublet. Angew. Chem. Int. Ed. 54, 7091–7095. Plummer, E.A., et al., 2005. Electrophosphorescent devices based on cationic complexes: control of switch-on voltage and efficiency through modification of charge injection and charge transport. Adv. Funct. Mater. 15, 281–289. Shahroosvand, H., et al., 2002. Red electroluminescence of ruthenium sensitizer functionalized by sulfonate anchoring groups. Dalton Trans. 43, 9202–9215. Shahroosvand, H., et al., 2013. Going from green to red electroluminescence through ancillary ligand substitution in ruthenium(II) tetrazole benzoic acid emitters. J. Mater. Chem. C 1, 6970–6980. Su, Y.J., et al., 2003. Highly efficient red electrophosphorescent devices based on iridium isoquinoline complexes: remarkable external quantum efficiency over a wide range of current. Adv. Mater. 15, 884–888. Tang, H., et al., 2012. Two novel orange cationic iridium(III) complexes with multifunctional ancillary ligands used for polymer light-emitting diodes. Org. Electron. 13, 3211–3219. Tao, P., et al., 2016. Facile synthesis of highly efficient lepidine-based phosphorescent iridium(III) complexes for yellow and white organic light-emitting diodes. Adv. Funct. Mater. 26, 881–894. Tsuboyama, A., et al., 2003. Homoleptic cyclometalated iridium complexes with highly efficient red phosphorescence and application to organic light-emitting diode. J. Am. Chem. Soc. 125, 12971–12979. Tsuzuki, T., et al., 2003. Color tunable organic light-emitting diodes using pentafluorophenyl-substituted iridium complexes. Adv. Mater. 15, 1455–1458. Tung, Y.L., et al., 2006. Orange and red organic light-emitting devices employing neutral Ru(II) emitters: rational design and prospects for color tuning. Adv. Funct. Mater. 16, 1615–1626.

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