Exciplex: An Intermolecular Charge-Transfer Approach for TADF

Spotlight on Applications Cite This: ACS Appl. Mater. Interfaces 2018, 10, 19279−19304

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Exciplex: An Intermolecular Charge-Transfer Approach for TADF Monima Sarma† and Ken-Tsung Wong*,†,‡ †

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan

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‡

ABSTRACT: Organic materials that display thermally activated delayed fluorescence (TADF) are a striking class of functional materials that have witnessed a booming progress in recent years. In addition to pure TADF emitters achieved by the subtle manipulations of intramolecular charge transfer processes with sophisticated molecular structures, a new class of efficient TADF-based OLEDs with emitting layer formed by blending electron donor and acceptor molecules that involve intermolecular charge transfer have also been fabricated. In contrast to pure TADF materials, the exciplex-based systems can realize small ΔEST (0−0.05 eV) much more easily since the electron and hole are positioned on two different molecules, thereby giving small exchange energy. Consequently, exciplex-based OLEDs have the prospective to maximize the TADF contribution and achieve theoretical 100% internal quantum efficiency. Therefore, the challenging issue of achieving small ΔEST in organic systems could be solved. In this article, we summarize and discuss the latest and most significant developments regarding these rapidly evolving functional materials, wherein the majority of the reported exciplex forming systems are categorized into two subgroups, viz. (a) exciplex as TADF emitters and (b) those as hosts for fluorescent, phosphorescent and TADF dopants according to their structural features and applications. The working mechanisms of the direct electroluminescence from the donor/acceptor interface and the exciplex-forming systems as cohost for the realization of high efficiency OLEDs are reviewed and discussed. This article delivers a summary of the current progresses and achievements of exciplex-based researches and points out the future challenges to trigger more research endeavors to this growing field. KEYWORDS: thermally activated delayed fluorescence, reverse intersystem crossing, intermolecular charge transfer, donor−acceptor interface, exciplex, host material

1. INTRODUCTION In the last three decades, organic light-emitting devices (OLEDs) have attracted enormous attention in lighting applications and for usage in flat-panel displays. In OLEDs, recombining the injected carriers, i.e., electrons and holes in the emitting layer (EML) forms bound states of lower energy than that of the unbound particles, which are known as excitons. Radiative decay of the excitons is the main mechanism of light emission from the device. As the holes and electrons are Fermions (i.e., spin-half particles which obey the Fermi−Dirac statistics), their recombination is governed by the spin statistics leading to the generation of singlet and triplet excited states in 1:3 ratio.1 The device external quantum efficiency (EQE, ηext) is given by the product of internal quantum efficiency (IQE, ηint) and light out-coupling efficiency (ηout). The ηint depends on the photoluminescence quantum yield (ΦPL) of the EML, a higher value of which positively increases EQE of the device. In a traditional fluorescence-based OLED, after charge recombination, light emission is primarily due to the radiative decay of singlet excitons leaving the triplet excitons wasted. The ηint of the fluorescence-based OLEDs is thus limited to only 25%, flagging the necessity of recovering triplet excitons for improving the overall emission efficiency. An immediate consequence of this demanding is the breakthrough invention © 2018 American Chemical Society

of phosphorescence-based OLEDs (PhOLEDs), where strong spin−orbit coupling (SOC) in the heavy metal embedded complexes (phosphors) converts the singlet excitons into triplet excitons via efficient intersystem crossing (ISC) followed by phosphorescence emission.2 As harvesting both singlet and triplet excitons is feasible in such systems, the maximum ηint as high as 100% can be realized.2 Though, the majority of commercially available OLED displays at present involve PhOLEDs, however, they also raise some serious concerns such as, higher device fabrication cost due to the rarity of essential heavy metals like Ir and Pt, low operation stability of the blue emitting devices, etc.3 Another photophysical process that has been examined for harnessing triplet excitons is the triplet−triplet annihilation (TTA, also known as P-type delayed fluorescence), which is a bimolecular process. Interestingly, TTA does not necessitate transition metal complexes and can be successfully achieved using organic fluorescent materials with a high singlet−triplet gap (hereafter ΔEST). Though there are some reports using TTA mechanism with pure organic chromophores that demonstrate enhanced OLED efficiency Received: December 2, 2017 Accepted: April 3, 2018 Published: April 3, 2018 19279

DOI: 10.1021/acsami.7b18318 ACS Appl. Mater. Interfaces 2018, 10, 19279−19304

Spotlight on Applications

ACS Applied Materials & Interfaces

Figure 1. (left) Simplified energy level diagram (without zero-point energy consideration) showing various processes during exciplex formation in solution under photoexcitation where the rate constants, k1 = exciplex formation, k−1 = exciplex dissociation, kex = exciplex fluorescence, knr = nonradiative decay, kISC = intersystem crossing, kRISC = reverse intersystem crossing, kf(D/A) = fluorescence of D/A; ΔGex is the Gibbs energy for exciplex formation. (right) Schematic representation of excitons in conventional and exciplex OLEDs.

than those of conventional fluorescent devices. However, the ηint of TTA is still limited to 25% + (0.5 × 75)% = 62.5%, far less than that of PhOLEDs.4 If the singlet−triplet energy separation of a luminophore is in the range of kBT, where kB is Boltzmann constant and T is Kelvin temperature (for example, kBT = 25.6 meV at 298 K), then the triplet excitons have a chance to up-convert into the corresponding singlet through reverse intersystem crossing (RISC) utilizing the thermal energy from the surroundings followed by delayed fluorescence emission and is called thermally activated delayed fluorescence (TADF, also called E-type delayed fluorescence). The rate constant of TADF is temperature dependent. Like phosphors, the TADF emitters have the feasibility of harvesting all the excitons formed under the field bias, thereby leading to 100% IQE from pure organic emitters.5 It has been established by quantum chemical calculations, that ΔEST, which is the most important parameter for TADF materials, is about twice the electron exchange energy (J) expressed by the equation:6 J=

∬ (ϕ)1(ϕ*)1 re

2

bimolecular process where an electronically excited species undergoes complex formation with another ground state molecule through Columbic attraction.7 During this process, a partial charge transfer occurs from a donor-based orbital (D*/ D) to an acceptor-based orbital (A/A*). As the complex formation occurs only in the excited state, the interaction between the donor and acceptor species in the ground state is repulsive in nature and attractive in the excited state. Therefore, the electronically excited state has a minimum in its potential energy surface which corresponds to the exciplex. The exciplex then could either emit radiatively at lower energy than D/A or dissociate into its constituents (D* + A/D + A*), if the possibility of a complete electron transfer is neglected (Figure 1). Unlike in solution where the molecules diffuse together to form the exciplex by photoexcitation, in the OLEDs the donor and acceptor molecules are locked in their positions where the exciplex is formed at the interface under electrical excitation. In the exciplex OLEDs, the carrier injection at the opposite electrodes ultimately leads to injection of an electron in the conduction band of the acceptor and a hole in the valence band of the donor. Therefore, the corresponding electron−hole bound state (exciton) is heteromolecular in nature and this scenario is approximated to an exciplex in solution. This contrasts the conventional OLEDs where the exciton is solely based on the emitter molecules (homomolecular) (Figure 1). As the contributing orbitals in exciplex are purely based on donor and acceptor molecules, they are considered to be similar to the spatially separated FMOs in the TADF molecules. Therefore, analogous to pure TADF emitters, the exciplex emitters display an innately small ΔEST value (∼0−50 meV) and can feasibly undergo TADF emission, indicating a feasible way toward theoretical 100% exciton harvesting ability and rendering them promising candidates for practical applications in OLED-based displays and lighting, together with phosphorescent and pure TADF emitters.8 The Gibbs energy for exciplex formation (ΔGex) is important in the sense that its estimation helps in the selection of appropriate donor and acceptor cunterparts.9 The ΔGex for the formation of exciplex in the solid state could be estimated from a modified Rehm−Weller equation: − ΔGex = [E*exciton (EA* or

(ϕ*)1(ϕ)1dτ1dτ2

12

=

∬

[(ϕ)1(ϕ*)1e][e(ϕ*)1(ϕ)1] dτ1dτ2 r12

Here, ϕ and ϕ* represent the ground and excited electronic states, respectively. Therefore, J can be thought of as an equivalent to Coulombic repulsive interaction between two equal charge densities (ϕϕ*e), wherein a small value of the overlap integral ϕ|ϕ* reduces J and ΔEST. A good TADF material thus should possess a small J value, which is achievable through localized frontier molecular orbitals (FMOs). Therefore, TADF can practically be achieved by tailor-made molecules with weakly coupled electron donor and acceptor components because this molecular configuration leads to spatially separated FMOs, which induces a subtle intramolecular charge transfer (ICT) and leads to a low electron exchange energy, eventually yielding a small ΔEST value. Spatially separated FMOs can also be obtained with intermolecular excited state of donor−acceptor complexes, shortly known as exciplexes. The exciplex formation is a 19280




Figure 2. Device performance features comprising m-MTDATA:3TPYMB emitting layer. (a) EQE versus current density for the device with structure: ITO/m-MTDATA (20 nm)/50 mol % m-MTDATA:3TPYMB (60 nm)/3TPYMB (20 nm)/LiF (0.8 nm)/Al. (b) Emission (PL) spectra of 3TPYMB (blue), m-MTDATA (green) and 50 mol % m-MTDATA:3TPYMB (red) films at room temperature (300 K), and the EL spectrum of the device configured as ITO/m-MTDATA/50 mol % m-MTDATA:3TPYMB/3TPYMB/LiF/Al at a J value of 10 mA cm−2 (black). Reproduced with permission from ref 8. Copyright 2012 Nature Publishing Group.

ED*) − Eexciplex], where E*exciton is the exciton energy of the constituting molecules and Eexciplex is the exciplex energy, which can be determined from fluorescence of the D:A blend.10 The Eexciton could be defined as the difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor and could be obtained from electrochemical data. It could thus be stated that ΔGex has a relationship with the redox potentials of the constituting molecules in solution.10 The optimal condition for efficient exciplex formation is found to be −ΔGex > 0.57 eV while partial exciplex formation takes place when −ΔGex lies between 0.28 and 0.57 eV,9 though a −ΔGex value higher than 0.45 eV could probably act as a reference for the necessary Gibbs energy requirement for exciplex emission.10 The observation of exciplex formation in OLED was reported in earlier times, though their appearance was often regarded as a major concern for reduced device performance.11,12 However, it was also thought that exciplexes could deliver simple possibilities to model a wide variety of emitters as the emission wavelength in such systems is independent of the optical energy gap of solo compounds but dependent on the HOMO−LUMO offset between donor and acceptor molecules. In 2012, Adachi et al. demonstrated an innovative approach to realize radiative-exciton harvesting by virtue of efficient RISC of the exciplex state to overcome the fluorescent EQE limit of 25%. Initially, this group explored a 1:1 blend film of 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, i.e. m-MTDATA: t-Bu-PBD, but its PLQY was measured to be only 20%, leading the corresponding device with an ηext of only 2%. This result implied that PLQY of the blended film is a vital factor in realizing high ηext. Subsequently, to demonstrate superior device performance by the exciplex, a new acceptor, tris-[3-(3-pyridyl)mesityl]borane (3TPYMB) was used instead. The PL efficiency of this blend, m-MTDATA:3TPYMB (1:1) was still found to be as low as 26%. However, the delayed component of the pertinent film in the total photoluminescence efficiency was found to be remarkably higher than that of the m-MTDATA:t-Bu-PBD film (54.2% versus 36.7% of total PL efficiency). The exciplex emission of the mMTDATA:3TPYMB film was observed at 540 nm, and the device was fabricated with a structure: ITO (110 nm)/ mMTDATA (20 nm)/50 mol % m-MTDATA:3TPYMB (60 nm)/3TPYMB (20 nm)/LiF (0.8 nm)/Al (50 nm). The ηext of

this device was found to be 5.4%, a value significantly higher than that of traditional fluorescence-based OLEDs without any TADF features. The performance characteristics of this mMTDATA:3TPYMB exciplex device is shown in Figure 2. This result indeed paved a new pathway to enhance electroluminescence efficiency under electrical excitation, particularly the efficient harvest of all radiative excitons by utilizing the RISC process. In addition to the TADF character of exciplex, it was also evident that the donor HOMO level should be compatible to the acceptor LUMO level and this is another key factor responsible for the enhancement of OLED device performance.8 The remarkable breakthrough achieved by Adachi et al. opened up a new era in this research field and motivated other research groups to delve into the detailed investigation of exciplex-based systems. Since the last five years, there have been substantial reports on usage of exciplexes for modification of emission colors, white emission OLEDs, as well as electrophosphorescent devices. Researchers around the world have also been able to circumvent the problem of device performance in OLEDs using exciplexes.13−18 In fact, substantial researches of exciplexes are still being conducted so as to achieve simultaneously the four main goals of OLEDs: (1) low operation voltage, (2) high IQE, (3) low efficiency roll-off, and (4) high light-out coupling efficiency. Generally, there are two different approaches for exciplex formation in OLEDs: (a) mixing hole-transport (HT) type donor with electron-transport (ET) type acceptor and (b) blending of either D- or A-type material with a D-A-type bipolar material to form an exciplex via an intermolecular CT excited state. Similarly, the electroluminescence performance of an exciplex-based OLED could not only be boosted by the high ΦPL of the exciplex10 but also by the astute choice of donor and acceptor materials based on the following criteria: (a) high hole and electron mobility of the donor and acceptor, respectively, (b) appreciable difference between HOMO of the donor molecule and LUMO of the acceptor molecule, (c) accumulation of a sufficient amount of charge carriers at the D:A interface, (d) higher triplet energy of the individual counterparts than the exciplex for efficient exciton confinement.19 In this Spotlight on Application, we review the most recent developments of exciplex-based OLEDs focusing on donor− acceptor selection, photophysical and other physicochemical properties of the relevant exciplex systems, optimized device structure to obtain high EQE and any other conditions that 19281




Figure 3. Structures of selected acceptor molecules used in exciplex emitters and/or host/cohost systems.

yield highly efficient OLEDs based on exciplexes. In order to make the article more comprehensive, we have divided the content into two subgroups viz. (a) exciplexes as TADF emitters and (b) exciplexes as hosts for fluorescent, phosphorescent and TADF dopants. As this is a spotlight article and there are already some excellent reviews on TADF systems,5 we have concentrated only on those exciplex-based reports which achieved high device performance. Subsequently, we attempt to summarize and analyze the various factors that operate behind the superior device outputs in exciplex-based

OLEDs and provide our prospective points of this emerging research field.

2. EXCIPLEXES AS EMITTERS IN OLEDS 2.1. Exciplex-Based OLEDs by Mixing HT Type Donor Materials and ET Type Acceptor Materials. The design strategy for an exciplex system involves the choice of suitable donor and acceptor molecules without the need of tiresome synthesis for complicated structures. In general, the exciplexbased OLEDs as emitters are mostly composed of a D:A blend 19282




Figure 4. Electroluminescence (EL) features of an ExOLED comprising 8 wt % HAP-3MF:mCP blend film as EML. (a) Device architecture with energy levels (values are in eV). (b) Electroluminescence (EL) spectra at different current densities. (c) The current density−voltage−luminance (J−V−L) profile. (d) External quantum efficiency (EQE) versus current density (J) plot. Reproduced with permission from ref 29. Copyright 2014 Royal Society of Chemistry.

Figure 5. Structures of selected donor molecules (containing carbazole) used in exciplex emitters and host/cohost systems.

significant contribution regarding the elucidation of the underlying mechanism of exciplex-based TADF systems. This discovery was not only about increasing quantum efficiency but also about the advancement toward the reduction in time and cost expended in the development of new emitting materials for efficient OLEDs. They studied three blends made from the familiar electron donor and acceptor pairs, viz. (m-MTDATA:TPBi, TPD:TPBi, and TPD:OXD-7) with the help of timedependent spectroscopy. By their crucial investigation, it was found that TADF mechanism dominates for those systems with an energetically close-lying 1CT and 3LE states to minimize the thermal activation barrier, whereas TTA dominates for system with larger ΔEST. However, mixed TTA and TADF can be

as an EML which is inserted between the HT material and the ET material. Some of the significant acceptor molecules for exciplex formation include oxadiazole, triarylboron, triazine, Nheterocycles, and diphenylphosphine oxides, etc. We shall discuss some notable innovations with these electron-deficient acceptor molecules, which have yielded the best results until date after the revolutionary exciplex approach employing oxadiazole-based acceptor and m-MTDATA donor initiated by Adachi and co-workers.8 The structures of acceptor cores are shown in Figure 3 while structures of donor moieties are presented in Figures 5 and 6. In 2016, Monkman et al. suggested an effective way for developing new emitting materials.20 They presented a 19283




Figure 6. Structures of selected donor molecules (noncarbazole derivatives) used in exciplex emitters and host/cohost systems.

observed with one close-lying and one lower-energy LE state.20 Soon after, Hu et al. reported the first incorporation of exciplex TADF emitters in heterostructured organic light-emitting field effect transistors (OLEFETs) so as to harvest triplet excitons. At the outset, the feasibility of harvesting the triplet excitons was explored using the TCTA:B3PYMPM exciplex system, which exhibits an exciton utilization efficiency of 60.3% (ηext,max: 0.93%). Subsequently, a new kind of exciplex-based TADF emitter (m-MTDATA:OXD-7) was employed, the PLQY of which was found to be ca. 28%. This system was found to have a higher delayed fluorescence to prompt fluorescence (ΦDF/ ΦPF) ratio (∼2.4 times) compared to that of the TCTA:B3PYMPM film, and thus indicated a more efficient TADF process. Though the molar ratios between m-MTDATA and OXD-7 made little influence on the PLQYs of the films, but altering the molar ratios of the same affected the device EQEs. From these results, it is evident that optimizing the molar ratio of the donor and acceptor counterparts for HOLEFETs with exciplex emission is also very important to augment the efficiency of devices. Furthermore, the pertinent exciplex system (m-MTDATA:OXD-7) was also found to decrease the hole injection barrier and thus, enhance the exciton utilization efficiency to about 74.3%. Therefore, it is quite evident that the ratio of donor and acceptor moieties and the injection barrier between HTL and EML play a crucial role to affect the device EQE considerably. Consequently, the device fabricated with exciplex of m-MTDATA:OXD-7 as TADF emitter exhibited much enhanced efficiencies (ηext, max) of 3.76% and a maximum brightness of 1890 cd m−2. However, even though the high EQE for the HOLEFETs based on this exciplex system profits from the efficient utilization of triplet excitons, but it still suffers from a low PLQY. Thus, even though the introduction of oxadiazole group is known to

enhance thermal stability in exciplex forming materials, their PLQYs must be sufficiently increased.21 Triarylboron compounds are another set of electron deficient compounds, which are effective electron acceptors in various photonic and organic optoelectronic materials. In D−A molecular system, a triarylboron acceptor conjugated with an electron donor can lead to significant charge delocalization and exhibit strong ICT properties, which strongly influence their photophysical properties. In particular, tris-[3-(3-pyridyl)mesityl]borane (3TPYMB) is well studied and known as an ETL for OLEDs. Devices employing 3TPYMB as ETL were found to exhibit larger current density and higher luminance compared to those with a typical ET material such as tris(8quinolinolato)aluminum(Alq3) due to the 10 times higher electron mobility of 3TPYMB in comparison to Alq3.22 After the pioneering work by Adachi and co-workers, Monkman et al. examined the influence of electric field on exciplex states using the same exciplex system (m-MTDATA:3TPYMB). They used a simple but novel methodology to investigate the development of the relaxed exciplex, whereby an external electric field diminishes the separation of electron−hole leading to an enhanced emission at the abrupt donor:acceptor interface. The pertinent OLED devices constituted a donor:acceptor interface with a relatively high HOMO−LUMO energy offset. Because of this high interface potential barrier, the injected holes and electrons were unable to prevail over this gap and consequently led to the significant accumulation of exciplex state density pinned through the interface. Thus, as the separation between electron−hole decreases with the increase of applied field, the exciplexes also acquire local excited state nature, which upsurges the radiative decay rate, thereby increasing emission efficiency. Herein, the effect of electric field on the exciplex states was studied using single and double layer OLED devices. The single layer (SL) device structure was ITO/PEDOT: PSS 19284




positive magneto-electroluminescence and magneto-conductivity upon placing the exciplex devices under a 100 mT magnetic field. The magnetic properties obtained were used to confirm that efficient exciplex electroluminescence (EL) originates only from delayed fluorescence via RISC processes and the TTA process is a trivial decay channel of triplet exciplex states. The studies portrayed in this report unveil a new way for designing new efficient exciplex OLEDs.25 An interesting strategy to achieve deep blue OLEDs was also put forward by Jankus et al. using the exciplex formed by NPB and TPBi. The device was configured as ITO/NPB (30 nm)/ NPB:TPBi (35 nm)/TPBi (35 nm)/LiF(1 nm)/Al(100 nm) gave ηext of 2.7% with (CIE) coordinates (0.15, 0.13) and brightness 600 cd m−2. Majority of deep blue EL was harnessed from triplet excitons through triplet fusion (TF) process instead of thermally assisted delayed fluorescence (E-DF). The significance of triplet fusion in fabricating effective deep blue fluorescent OLEDs is therefore noteworthy and consequently the ηext could increase to 12% with enhanced PLQY of the exciplex state.26 Another efficient exciplex system containing TPBi and a boryl compound (TPAPB) containing dimesitylboryl group was reported by Lee et al. The blue exciplex emission for a TPAPB:TPBi (1:1) blended film was found to be peaked at 471 nm having a high PLQY of 44.1%, which was perhaps the best PLQY for blue exciplex reported during that time. Subsequently, a blue-emitting OLED with high efficiency was fabricated having structure ITO/TPAPB (30 nm)/ TPAPB:TPBi (1:1) (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al to give a maximum ηext of 7.0 ± 0.4% with a maximum PE and CE of 7.2 ± 0.5 lm W−1 and 9.1 ± 0.7 cd A−1, respectively. Lee et al. claimed these high device efficiencies were among the finest outcomes for blue fluorescent OLEDs until then. Furthermore, the blended film of TPAPB:TPBi exhibited good bipolar transporting properties due to the effective holeand electron-transport properties of TPAPB and TPBi, respectively, thereby making it a suitable candidate for exciplex host.27 After the breakthrough result,8 the endeavors of Adachi et al. toward achieving better performance of exciplex-based OLEDs were continuously going on, and they explored some phosphine and azine (heterocyclic) based acceptors as well. For instance, in 2012, they reported an exciplex system formed by blending m-MTDATA and diphenylphosphine oxide-based molecule PPT as donor and acceptor, respectively. The emission band of the m-MTDATA:PPT exciplex was found at ∼510 nm, considerably bathochromic shifted compared to the emission observed from its individual counterparts. The obtained results pointed out that efficiency of delayed fluorescence is enhanced if the acceptor has high triplet energy. However, in this study, the confinement of triplet exciplex state by the donor was not sufficient owing to mutual triplet energy transfer between mMTDATA and the exciplex. The device structure was configured as ITO/m-MTDATA (35 nm)/X mol % mMTDATA:PPT (30 nm)/PPT (35 nm)/LiF (0.8 nm)/Al, where X: 30 m, 50 and 70 mol %. Thus, efficient delayed fluorescence of the 50 mol % m-MTDATA: PPT exciplex led to maximum EQE and PE of 10.0% and 47.0 lm W−1, respectively. Furthermore, it was observed that a high concentration (70 mol %) of m-MTDATA in the EML, yielded a slight decrease in PLQY as well as EL efficiency due to triplet energy transfer from the exciplex to m-MTDATA. On the other hand, when there was a low concentration of m-MTDATA in the EML, a decrease in EL efficiency was observed despite the increase in

(40 nm)/MTDATA: 3TPYMB 1:1 (coevaporated 30 nm)/ 3TPYMB (30 nm)/LiF/Al, wheres the double layer, “abrupt interface” double layer (DL) device structure was ITO/ PEDOT:PSS (40 nm)/m-MTDATA (30 nm)/3TPYMB (30 nm)/LiF (0.8 nm)/Al (100 nm). The emission for the blend film was peaked at 540 nm, which was due to exciplex formation. The brightness and device efficiency of the SL device was found to be considerably higher compared to that for the DL device due to enhanced interface area in the mixed system.23 The N-embedding heteroarenes, also known for their electron-deficient nature, are also used as acceptor cores in D−A type materials. In addition to the excellent electrontransport properties, molecules containing these heterocycles deliver good electron injection and hole-blocking abilities. More importantly, they have the added advantage to attain low lying LUMO as well as HOMO energy levels by incorporating additional nitrogen atoms into the aromatic ring. Therefore, the number of N atoms introduced can be adopted to modulate the HOMO and LUMO energy levels. Consequently, such Nheterocyclic compounds have substantial prospective to be used for high-performance OLEDs. Among them, some well-known examples are 4,7-diphenyl-1,10-phenanthroline (BPhen), 2,9dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), benzoimidazole derivative (TPBi), etc. Recently, Grazulevicius and coworkers reported an ambipolar material DPNC as donor for exciplex formation with which a single-layer OLED with device structure ITO/CuI/DPNC/Ca:Al as well as bilayer OLEDs with structure: ITO/CuI/DPNC/Bphen/Ca:Al (Bilayer I) and ITO/CuI/DPNC/TPBi/Ca/Al (Bilayer II) were fabricated. The bilayer OLED with DPNC/Bphen as exciplex emitter exhibited ηext of 3.3% with a maximum brightness of 27 000 cd/ m2, whereas the bilayer device employing DPNC/TPBi as exciplex emitter exhibited ηext of 1.2% with a maximum brightness of 983 cd m−2 at 15 V. The inferior maximum ηext of DPNC/TPBi-based bilayer device was possibly ascribed to shorter lifetime of the DPNC:TPBi exciplex in comparison to that of DPNC:Bphen exciplex, thereby, leading to a diminished reverse intersystem crossing (RISC) efficiency. Furthermore, the fabricated OLEDs were found to exhibit voltage-dependent electroluminescence in a short-range of applied bias which indicated that device emission could be fine-tuned by the adjustment of applied voltage.24 Furthermore, Su et al. reported the exciplex-based OLED based on the blend of m-MTDATA: 70 mol % Bphen, giving the maximum EQE of the optimized device to be 7.79% at 10 mA cm−2 and PE 12.97 lm W−1. Herein, two series of exciplex-based devices comprising mMTDATA:Bphen and m-MTDATA:TPBi mixed films were scrutinized, of which the former gave better result. The device structure of the m-MTDATA:Bphen blend was configured as ITO/m-MTDATA (27 nm)/m-MTDATA:70 mol % Bphen (23 nm)/Bphen (20 nm)/Alq3 (13 nm)/LiF (1 nm)/Al (80 nm). The realization of high efficiency in this m-MTDATA:Bphen exciplex device is mainly ascribed to around zero ΔEST. In an attempt to scrutinize the limiting factors for the device ηext and efficiency roll-off, study on the transient EL process was conducted. The results indicated that existence of unbalanced charge in the excited exciplex EL devices suffer greatly from quenching of singlet exciplex and thus, low EQE and serious efficiency roll-off. The charge imbalance in excited exciplex devices usually arise due to the differences in electron and hole carrier mobility, layer thickness, D:A molar ratio in the blend and other operational settings. This study also observed 19285




molecules, respectively, balanced charge transport, in addition to the remarkably large offsets in energy levels at the donor:acceptor interface. Furthermore, the high ET values of the donor and acceptor moieties facilitated the electrogenerated exciplex to be restricted within the interfacial area, thereby leading to the effective harnessing of exciplex triplet excitons through RISC process and increasing the lightemitting efficiency. The observed electroluminescence spectrum of this device with emission wavelength centered at 544 nm exhibited a CIE chromaticity (0.40, 0.55) which was found to be similar to the photoluminescence spectrum.31 The device characteristics such as brightness and current density can be improved with the blend of TCTA:3P-T2T (1:1) as EML, indicating the beneficial effect of effective D:A contacts. In the very next year, the same group also reported a new triazinebased acceptor PO-T2T, which exhibits a higher LUMO as compared to that of 3P-T2T. PO-T2T was set to combine with mCP exhibiting a lower HOMO compared to TCTA. The larger HOMO (D)-LUMO (A) difference renders the mCP:PO-T2T blend a sky blue (472 nm) exciplex.32 The ηext, CE, and PE for the device with mCP:PO-T2T (1:1) as EML were 8.0%, 15.5 cd A−1, and 18.4 lm W−1. By blending PO-T2T with different donors, exciplex-based OLEDs ranging from blue to red could be successfully achieved. Remarkably, a tandem structure for white OLED (WOLED) constituted of the two parallel blend layers of blue exciplex mCP:PO-T2T and yellow (572 nm) DTAF:PO-T2T exciplex was fabricated to have emission wavelength covering 430 to 730 nm. This tandem architecture with individual emissive units was found to have good color stability and a record high ηext of 11.6% using pure organic materials as emitters.32 Inspired by the superior results obtained with the acceptor PO-T2T, Zhang et al. reported an efficient blue exciplex system, CDBP:PO-T2T (476 nm). This CDBP:PO-T2T blue exciplex was found to exhibit efficient TADF emission and high T1, rendering itself an outstanding prospect for the blue emitter. The device structure for the blue exciplex emitter consisted of ITO/TAPC (30 nm)/ CDBP (10 nm)/ CDBP:50 wt % PO-T2T (30 nm)/PO-T2T (40 nm)/LiF (1 nm)/Al (100 nm). A record-high EQE of 13.0% was obtained for the CDBP:PO-T2T-based blue device.33 Furthermore, in order to restrict the triplet excitons on the exciplex, the donor and acceptor molecules must have sufficiently high triplet energies. As such, Cheah et al. (in 2015) developed a green OLED device by blending the electron donor, TCTA (ET ≈ 2.75 eV) and electron acceptor, Tm3PyBPZ (ET ≈ 2.72 eV) as the emissive layer, the acceptor (Tm3PyBPZ) having a stretched conjugation length than that of PO-T2T, therefore, TCTA:Tm3PyBPZ blend exhibits an emission peak centered at 514 nm. The time-resolved photoluminescence data revealed that the exciplex system was found to display both prompt fluorescence (nanosecond region) and delayed fluorescence (microsecond region). The device consisted of ITO/Hat(CN)6 (5 nm)/TAPC (55 nm)/ TCTA:Tm3PyBPZ (1:1) (30 nm)/Tm3PyBPZ (40 nm)/Liq (2 nm) /Al (100 nm). The pertinent OLED device employing TCTA:Tm3PyBPZ (1:1) as EML was thus found to exhibit high efficiency with maximum ηext of 13.1%, a maximum CE of 44.2 cd A−1, maximum PE of 54.5 lm W−1 along with a low turn-on voltage of 2.4 V. In this study, it was believed that the TADF process is responsible for the excellent device performance of the green OLED.34 In 2016, Hung et al. reported another exciplex system by blending a HT material, Tris-PCz,

the PLQY, owing to nonradiative decay of the triplet excitons in the m-MTDATA layer. It should be important to note that this report suggested that donor molecule with high triplet energy and shallow HOMO level should be designed and synthesized so as to further augment the efficiency of exciplexbased OLEDs.28 In a bid to further improve the device efficiency, Adachi et al. developed a highly efficient exciplex system comprising a heptazine derivative (HAP-3MF) as electron acceptor and mCP as electron donor.29 Although heptazine derivatives are known to be good candidates for OLEDs, however, no exciplex systems based on this derivative was reported until 2014. 2-Fluorotoluene groups were incorporated into the heptazine core so as to enhance solubility of the compound and also to preserve the electron withdrawing ability. The PLQY of this exciplex system (HAP-3MF:mCP) was found to be as high as 55.7%, which is much higher than many exciplex systems reported ever. The device was fabricated as ITO/ α-NPD (30 nm)/TCTA (10 nm)/HAP-3MF:mCP (20 nm)/DPEPO (10 nm)/TPBI (40 nm)/LiF (0.8 nm)/Al (100 nm). It was found that the EQE of HAP-3MF:mCP-based device decreased with the increased weight ratio of HAP-3MF probably due to the concentration quenching effects ascribed to the comparatively planar molecular geometry of the molecule (HAP-3MF). The device containing 8 wt % HAP-3MF in mCP as an EML was found to exhibit a maximum EQE of 11.3% (Figure 4). It was thus quite evident that exciplex system with an innately small ΔEST that yields efficient exciton upconversion and a rather high photoluminescence quantum yield (PLQY) can be achieved.29 For other exciplex systems employing nitrogen-containing heteroarenes as acceptors, Kim et al. reported the utilization of a pyrimidine/pyridine-cored acceptor B3PYMPM to achieve high efficiency exciplex-based OLED in 2013. The codeposited film of TCTA:B3PYMPM exhibited efficient delayed fluorescence and the PL efficiency was found to increase from 36% at room temperature to ∼100% at 35 K. Therefore, the ηext of the devices with the structure configured as ITO/TAPC (30 nm)/TCTA(10 nm)/(1:1)TCTA:B3PYMPM (30 nm)/ B3PYMPM (20−40 nm)/LiF (1 nm)/Al (100 nm) increased from 3.1% at room temperature to 10% at 195 K. The ηext of 10% is considered one of the highest values obtained using exciplex and reveals that a major percentage of the triplet excitons are harnessed via RISC process.30 Furthermore, the highly electron-deficient triazine core has been successfully utilized to develop promising ETLs for OLEDs and is the most heavily used acceptor core. The C3symmetry structure having three potential sites for further modifications signifies that the electron affinity of this core can be easily modified by introducing different electron-donating substituents as its periphery. As such, they can be used as bipolar hosts, D−A emitters or D:A exciplexes. The structural characteristics of triazine open up a good opportunity for designing promising compounds to be used in exciplex formation. In 2013, Hung et al. developed a bilayer-type exciplex OLED by blending the C3-symmetric HT material, TCTA (donor) and ET material 3P-T2T (acceptor) with pyrazolyl groups incorporated into the 2,4,6-triphenyl-1,3,5triazine core. The device was configured as ITO/PEDOT:PSS (30 nm)/NPB (20 nm)/TCTA (5 nm)/ TCTA: 3P-T2T 50 mol % (X nm)/ 3P-T2T (75-X nm)/Liq/Al, where X = 0 and 25. The exciplex device exhibited an EQE as high as 7.8%, CE of 23.6 cd A−1, and PE of 26.0 lm W−1, which is attributed to the high hole and electron mobilities of the donor and acceptor 19286




exciplex OLED exhibiting a (CE)max of 3.6 cd A−1 and ηext of 2.65% was utilized to engage the orange exciplex to fabricate the tandem full exciplex WOLED with a specially designed charge generation layer. The full exciplex-based WOLED was found to provide a (CE)max of 25.4 cd A−1, a peak EQE of 9.17% and (CIE) coordinates of (0.41, 0.44).39 In short, triazine-based acceptors top the list until date with external quantum efficiencies (EQEs) reaching up to 14.4% in devices with the exciplex as emitter systems. A summary of selected high efficiency TADF OLEDs based on exciplex emitters is given in Table 1.

with a newly developed star-shaped triazine-centered molecule CN-T2T as acceptor. CN-T2T was designed by incorporating a benzonitrile onto the meta-position of peripheral phenyl group of 2,4,6-triphenyl-1,3,5-triazine. Both the donor and acceptor components possess high ET values and their balanced hole and electron mobility ensured that sufficient carrier density could be accumulated in the D:A interface for efficient generation of exciplex excitons. The device structure was configured as ITO/ 4% ReO3:Tris-PCz (60 nm)/Tris-PCz (15 nm)/Tris-PCz:CNT2T (1:1) (25 nm)/CN-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm). This green (530 nm) exciplex OLED exhibited an ηext of 11.9% (37 cd A−1, and 46.5 lm W−1) with an extremely low driving voltage of 2.6 V at 100 cd m−2. In a bid to examine the influence of carrier mobility balance on exciplex formation, Tris-PCz was replaced by TCTA (with similar energy levels but higher hole mobility). However, the ηext of the CN-T2T:TrisPCz-based exciplex device was found to be ∼18% higher than that of CN-T2T:TCTA-based exciplex OLED. This result pointed out the importance of charge balances in exciplex formation.35 Additionally, a very recent report by Hung and co-workers also described a unique approach of encompassing remote steric effect onto the donor molecules so as to get an improved performance of the exciplex-based OLED.36 Herein, DTAF and DSDTAF used as donors were blended (1:1) respectively with the acceptor 3N-T2T to form the exciplex systems. To increase the effective contact between the donor and acceptor chromophores, two bulky SiPh3 groups were introduced onto the fluorene bridge of DTAF to create steric hindrance thereby rendering the resulting exciplex to have a higher PLQY. Thus, the PLQY of DSDTAF:3N-T2T film was found to be 59.0%, higher than that of the DTAF:3N-T2T blend exhibiting a PLQY of 51.0%. Devices with DSDTAF:3N-T2T and DTAF:3N-T2T exciplexes as the EML were fabricated having structure: ITO/4% ReO3: DTAF (60 nm)/DTAF (15 nm)/ EML (25 nm)/CN-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm). The green device based on DSDTAF:3N-T2T exciplex (535 nm) not only exhibited a low turn-on voltage (2.0 V) but also higher efficiencies (13.2%, 42.9 cd A−1, 45.5 lm W−1) as compared to the device based on DTAF:3N-T2T exciplex (535 nm) (11.6%, 35.3 cd A−1, 41.3 lm W−1) under the same device structure. Furthermore, the EQE was enhanced from 9.5% to 12.5% when this strategy was further examined for blue (500 nm) exciplex based OLEDs, where the acceptor PO-T2T was blended with CPF and its tert-butyl substituted counterpart CPTBF. As the triazine based exciplexes seemed to augment the OLED efficiencies, more researchers were drawn into using these systems as acceptors in the exciplex blends. For example, Su et al. reported the fabrication of warm WOLEDs based on blue and orange exciplexes. This work was done by the careful choice of donor and acceptor moieties in accordance to their carrier mobility as well as the energy level offset between HOMO (D) and LUMO (A). The TCTA:Bphen blend previously reported by Duan et.al37,38 was selected for the blue exciplex (459 nm), while TAPC:3P-T2T blend was chosen for the orange exciplex (554 nm). The TAPC was chosen as the donor for orange exciplex as it has a comparatively shallow HOMO energy level of 5.5 eV compared to TCTA (5.8 eV). The single device based on TAPC:3P-T2T orange exciplex as emitter offered a current efficiency and EQE of 12.6 cd A−1 and 4.78%, respectively, at current density of 20 mA cm−2, while that of the tandem orange exciplex OLED exhibited a maximum CE of 38.1 cd A−1 and ηext of 14.4%. The blue

Table 1. Summary of Selected High Efficiency TADF OLEDs Based on Exciplex Emitters emitters m-MTDATA: PBD m-MTDATA: 3TPYMB TCTA:B3PYMPM m-MTDATA:OXD-7 DPNC: BPhen DPNC: TPBi m-MTDATA: Bphen m-MTDATA: TPBi NPB: TPBi m-MTDATA:PPT mCP: HAP-3MF TCTA:B3PYMPM TCTA/3P-T2T mCP:PO-T2T DTAF: PO-T2T CDBP:POT2T TCTA: Tm3PyBPZ Tris-PCz:CN-T2T CPF: PO-T2T CPTBF: PO-T2T DTAF: CN-T2T DSDTAF:CN-T2T TCTA:Bphen TAPC:3P-T2T

λPL,exciplex (nm)

ϕfilm (%)

540 540

20 26

490 545 580 540 530

36 28

450 510 550 495 554 472 572 476 514 530

535 535 464 544

Von (V)

2 5

2.3 4.0 2.3

15 17 2.5 28.5 66.1 36 100

51 53 41 44 51 59

EQEmax (%)

4.0 2.8 4.2 2.0 2.0 2.5 2.4 2.6 2.2 2.2 2.0 2.0 2.6 4.1

0.93 3.76 3.3 1.2 7.79 6.85 2.7 10 11.3 3.19(RT) 10.0(LT) 7.8 8 5.7 13.0 13.1 11.9 9.5 12.5 11.6 13.2 2.61 14.4

ref 8

21 24 25 26 28 29 30 31 32 33 34 35 36

39

2.2. Exciplex-Based OLEDs by Blending with Bipolar D−A Materials. The molecules with bipolar feature are typically designed for emission from the intramolecular charge transfer (ICT) excited state and/or balanced charge transport. This decade has witnessed a useful usage of bipolar materials for exciplex-based OLEDs. For example, Jankus et al. demonstrated that a D−A−D type molecule, t-Cbz-SO (2d), where D is carbazole and A is dibenzothiophene S,S-dioxide, is not only capable of emitting via an ICT excited state but also form the exciplex states. The OLEDs configured as the following structure: ITO/NPB (60 nm)/x% t-Cbz-SO:TAPC (30 nm)/TPBi (10 nm)/BCP (20 nm)/LiF (1 nm)/Al, where x = 5, 30, 38, 40, and 55% were fabricated. The photophysical studies revealed that the emissive species is governed by the D:A ratios. It was observed that 30% t-Cbz-SO:TAPC gives better device EQE (14.03%), because of improved injection and balanced charge transport. The exciplex emission of 30% tCbz-SO:TAPC was observed at 540 nm with a PLQY up to 53 ± 4%. At 100 cd m−2, the 30% t-Cbz-SO:TAPC device gave the 19287




Figure 7. Structures of selected D−A bipolar molecules used in exciplex emitters and host/cohost systems.

Figure 8. Diagrams displaying energy transfer in exciplex emitters using (a) two traditional fluorescent materials and (b) a single-molecule TADF emitting material and a traditional fluorescent emitting material. S0, S1, T1, and represents ground state, singlet excited state, triplet excited state, respectively (S1E and T1E are for exciplex). RISC corresponds to reverse intersystem crossing from radiationless T1 to radiative S1. The dashed line denotes intermolecular Dexter energy transfer, whereas FP+D represents prompt and delayed fluorescence. Adapted from ref 43.

maximum PE of 26.74 lm W−1 and CE of 32.25 cd A−1, whereas at high luminance (1000 cd m−2), the maximum PE of 14.82 lm W−1 and CE of 22.47 cd A−1 was obtained.40 This result also highlighted the importance of judicious design of the TADF EML that is required to reach high device efficiency since there might be a competition between exciplex formation and ICT process. The structures of D−A bipolar molecules are shown in Figure 7. Similarly, in the subsequent year, Li et al. demonstrated a series of exciplexes formed between a dipolar material DMAC-DPS and various electron donor and acceptors. The D−A−D−type molecule (DMAC−DPS) consists of a diphenylsulfone (DPS) acceptor and 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor. DMAC−DPS was examined as donor to couple with acceptors such as T2T, B4PyMPm, and PO-T2T. In addition, TPD was also introduced as donor to probe the possibility of forming exciplex with DMAC−DPS. The exciplex emissions for the device with the EML employing DMAC-DPS:T2T, DMAC-DPS:B4PyMPm, DMAC-DPS:POT2T and TPD:DMAC-DPS blends were observed at 480, 493, 535, and 550 nm, respectively. The device architectures

fabricated with these four exciplex systems were: ITO/MoO3 (3 nm)/mCBP (20 nm)/DMAC-DPS:acceptor (25 nm)/ acceptor (40 nm)/LiF (0.8 nm)/Al with acceptor as T2T, B4PyMPm and PO-T2T; and ITO/MoO3 (3 nm)/TPD (20 nm)/TPD:DMAC-DPS (25 nm)/PO-T2T (45 nm)/LiF (0.8 nm)/Al. Among them, the green exciplex DMAC-DPS:POT2T led to the best EQE of 9.08% with maximum luminance above 35 000 cd m−2 and a small efficiency roll-off, whereas the exciplex from TPD:DMAC-DPS only gave an EQE of 1.63%. The results imply that DMAC-DPS is a better candidate to serve as donor for exciplex formation.41 Taking a step forward, Lee et al. also reported a brilliant strategy to develop highly efficient exciplexes by merging two intrinsic type (monomolecular) TADF molecules as donor and acceptor materials. In this report, DMAC-DPS served as a donor molecule due to the stronger donor capability of acridine compared to the carbazole moiety, while DDCzTrz and TCzTrz were used as acceptors because the diphenyltriazine unit holds stronger acceptor capability than the diphenylsulfone unit. Exciplex formation for the films of DMAC-DPS:DDCzTrz 19288




diphenylphosphine oxide-4-(triphenylsilyl)phenyl (5 nm)/ TPBI (30 nm)/ LiF (1 nm)/Al (200 nm). Among these new exciplexes, the green device employing TCTA:CzTrz exciplex gave a highest EQE of 12.6%. The higher PLQY (55%) of the TCTA:CzTrz exciplex was responsible for the increased EQE as compared to TCTA:DTrz exciplex-based system which only exhibited a PLQY of 37%. Though the same trend was observed in the TAPC-based exciplex device, however, due to low PLQY, the EQE in TAPC-based exciplex was relatively low.44 Around the same year (2015), Zhang et al. also reported the utilization of a bipolar compound (DPTPCz) as acceptor having a high triplet energy to design three exciplexes, viz TAPC:DPTPCz, TCTA:DPTPCz, and NPB:DPTPCz. This bipolar compound (DPTPCz) consisting of an electron acceptor 1,3,5-triazine core and an electron donor carbazole group was reported to exhibit excellent device performances.45 The blends, TAPC:DPTPCz, TCTA:DPTPCz, and NPB:DPTPCz were found to exhibit exciplex emissions at 503, 502, and 491 nm with PLQYs of 0.68, 0.55, and 0.15, respectively. Devices were fabricated with the exciplex systems as the EML and consisted of ITO/TAPC (40 nm)/ EML (50 wt %, 30 nm)/ TmPyPB (40 nm)/ LiF (1 nm)/ Al (100 nm). The high PLQY and low ΔEST (47 meV) for high RISC efficiency rendered the TAPC:DPTPCz blend-based device to give a maximum ηext of 15.4% and high PE of 47.9 lm W −1 as well as a low turn-on voltage of 2.7 V. The TCTA:DPTPCzbased device gave ηext of 11.9% and PE of 35.8 lm W−1 with a comparable turn-on voltage of 2.8 V. In sharp contrast, a low ηext of only 0.6% was achieved for the NPB:DPTPCz-based device owing to low PLQY which can be possibly attributed to the lack of RISC due to the low-lying T1 of NPB. Thus, of all the exciplex systems, the TAPC:DPTPCz-based device was found to be among the best performing exciplex OLEDs. These results demonstrated that molecules possessing high triplet energies (ET) were able to prevent energy leakage and facilitate efficient RISC and thus are highly desirable for the design of exciplex emitters with high PLQY.46 Thus, it is clear that bipolar materials are also excellent candidates for the development of high performance exciplex-based OLEDs. The reported results also indicated that the triazine-containing compounds gave the best results until date (up to 17.8%). Another D−A−D molecule t-Cbz-SO (2d) also showed very good efficiency (15%). A summary of selected exciplex-based OLEDs with bipolar D−A materials is given in Table 2. In short summary, exciplex-based emitters are emerging as another generation of electroluminescence emitters with promising features. Under electrical excitation, exciplex formation occurs through the oppositely charged carriers at the frontier molecular orbitals of two different molecules and this has a connotation with the photoinduced electron transfer instead. Nevertheless, exciplex-based OLEDs appear promising on several grounds such as (a) could be obtained by simple mixing of suitable commercially available donor and acceptor materials without the need of complicated synthesis, (b) bipolar nature of the emission layer increases electrical conductivity, (c) simpler device structure. Though the overall device performance of the exciplex-based OLEDs are far behind than that of the conventional TADF emitter based devices which have already achieved EQEs higher than 35%, research efforts to develop the new strategy of exciplex-based TADF OLEDs is praiseworthy. Though the pioneering paper in this area demonstrated just crossing of the 5% EQE limit of fluorescence OLEDs, considerable efforts have made them to

and DMAC-DPS:TCzTrz were observed at 521 and 500 nm, respectively, which were significantly red-shifted from their individual donor and acceptor counterparts. The exciplex emitter derived from DMAC-DPS:DDCzTrz blend showed a PLQY of 0.49 and high EQE of 13.4%, PE = 35.8 lm W−1, CE = 40.4 cd A−1 and CIE coordinate (0.31, 0.53), whereas the exciplex emitter derived from DMAC-DPS:TCzTrz blend displayed a high PLQY of 0.54 and high EQE of 15.3%, PE = 31.4 lm W−1, CE = 41.2 cd A−1m and CIE coordinate (0.25, 0.46). In comparison, Lee et al. also investigated the device performances using different donors, such as, TAPC and TCTA, with the acceptors employed in this study. It was found that the exciplex based on donor material DMAC-DPS exhibited much better device performances than their TAPC or TCTA counterparts.42 In addition to Lee’s work,42 Zheng et al. further proposed an approach for a novel exciplex system, wherein a single-molecule TADF emitter MAC was introduced as donor to combine the PO-T2T acceptor. The proposed strategy was to use a single-molecule TADF emitter as one of the components in the D:A pair, whereby the exciplex TADF emitter can follow two RISC routes on both the singlemolecule TADF emitter and the exciplex emitter (as shown in Figure 8). They thus anticipated that the total RISC efficiency of the exciplex system (MAC:PO-T2T) could be higher than those in conventional exciplex TADF emitters because such a type of emitter could exploit more triplet excitons and thus display higher device efficiencies. Moreover, it was evident from this report that if the S1 and T1 energy levels of the non-TADF material were raised compared to the energy levels of the single-molecule TADF emitter, then the additional RISC process on the latter could effectively harness triplet excitons on the traditional fluorescent emitter at high excitation density, thereby reducing efficiency roll-off. The device was fabricated as ITO/TAPC (40 nm)/mCP (10 nm)/EML (20 nm)/ PO-T2T (45 nm)/LiF (1 nm)/Al (150 nm). By tuning the weight ratios of the donor and acceptor molecules, the device based on MAC:PO-T2T (7:3) was found to exhibit a low Von of 2.4 V, high maximum CE of 52.1 cd A−1, PE of 45.5 lm W−1, and ηext of 17.8%. In comparison to the mCP:PO-T2T device (ηext: 3.2%), the device based on MAC:PO-T2T exciplex displayed a high EQE of 12.3% at a luminance of 1000 cd m−2, which was attributed to the surplus RISC on the MAC molecules. This approach, is still considered to be the best performing among the reported OLEDs concerning exciplex emitters.43 In addition to this, Lee and co-workers had also reported a new bipolar molecule CzTrz consisting of triazine as acceptor and carbazole as donor joined by a biphenyl bridge with meta− meta linkage. For probing the effect of electron-accepting ability of acceptor for the exciplex formation, the carbazole of CzTrz was replaced with a triazine moiety to give a molecule DTrz. Exciplex formation was examined by selecting TAPC and TCTA as the donor materials. The emission of exciplex based on acceptor DTrz was observed to be red-shifted as compared to those of CzTrz-based exciplex irrespective of the donor materials used. It can be reasonably understood that DTrz should exhibit stronger electron-accepting ability than CzTrz, leading to a lower energy gap between acceptor LUMO and donor HOMO. In addition, TCTA-based exciplex system exhibited a hypsochromic PL emission peak as compared to those of TAPC-based exciplex system owing to the relatively weak electron-donating character of TCTA in comparison to TAPC. The device structure was configured as ITO (120 nm)/ PEDOT:PSS (60 nm)/ TAPC (30 nm)/exciplex (25 nm)/ 19289




light on the substantial difference in the OLED efficiency encompassing exciplex-forming cohosts used with fluorescent versus phosphorescent emitters. The working principle of exciplex-hosted phosphorescent OLEDs can be clearly explained as shown in Figure 9. The exciplex excitons can be

Table 2. Summary of Selected Exciplex-Based OLEDs with Bipolar D−A Materials emitters

λPL,exciplex (nm)

ϕfilm (%)

EQEmax (%)

ref

t-Cbz-SO (2d):TAPC DMAC-DPS:T2T DMACDPS:B4PyMPm DMAC-DPS:PO-T2T TPD:DMAC-DPS DMAC-DPS:TCzTrz DMAC-DPS:DDCzTrz MAC:PO-T2T TCTA:CzTrz TCTA:DTrz TAPC:CzTrz TAPC:DTrz TAPC:DPTPCz TCTA:DPTPCz NPB:DPTPCz

540 480 493 535 550 500 521 514 ∼510 ∼515 ∼515 ∼530 503 502 491

53

14.0 4.44 4.40 9.08 1.63 15.3 13.4 17.8 12.62 5.52 8.88 3.27 15.4 11.9 0.6

40 41

54 49 8 55 37 36 16 68 55 15

42 43 44

46

Figure 9. Working principle of exciplex-hosted phosphorescent OLEDs.

over 17% until date and further improvement of efficiency is anticipated. Out of the various efficient exciplex systems, it appears that the triazine acceptor based systems are particularly attractive to realize high device efficiency. Triazine based materials have shown excellent thermal and morphological stability with remarkable optical and electrical properties. Moreover, tailor-made triazine-based derivatives are also reported to possess very low ΔEST values for realizing very efficient TADF and such properties make these materials highly promising for applications in exciplex-based OLED systems.

transferred to phosphorescent dopant from the singlet (Förster resonance energy transfer, FRET) and triplet (Dexter energy transfer) states because the triplet state of phosphorescent dopant is emissive. In this case, the spectral overlap of exciplex’s emission and phosphor’s absorption is less restricted, rendering a wider selection of suitable dopant with lower triplet state as compared to that of a particular exciplex system. The structures of the phosphorescent dopants discussed here are shown in Figure 10. The exciplex hosts/cohost systems based on HT materials, such as, TCTA, NPB, m-MTDATA, mCP, CBP, TCTA, TAPC, mCBP, etc. have delivered high efficiency OLEDs in recent years. For example, in 2013, Kim et al. doped red Ir(mphq)2(acac) and green Ir(ppy)2(acac) phosphors in an exciplex-forming cohost TCTA:B3PYMPM to successfully develop an orange OLED consisting of ITO (150 nm)/ TAPC (20 nm)/TCTA (10 nm)/TCTA:B3PYMPM:Ir(mphq) 2 (acac)(x nm, 3 wt %)/TCTA:B3PYMPM:Ir(ppy)2(acac) (y nm, 8 wt %)/B3PYMPM (45 nm)/LiF (0.7 nm)/Al (100 nm), wherein the thickness of the red EML doped with Ir(mphq)2(acac) and that of the green EML doped with Ir(ppy)2(acac) were varied from 5/25 nm to 10/20 and 15/15 nm. The orange device having red/green thicknesses of 5/25 nm exhibited a CIE value of (0.442, 0.529) and high maximum ηext of 22.8% while that having thicknesses of 10/20 nm exhibited maximum EQE of 23.3%. The OLEDs with red/ green thicknesses of 5/25 nm and 10/20 nm, were found to exhibit EQEs over 19.6 and 18.6%, respectively, at 10 000 cd m−2 since the cohost system dispenses the recombination zone all over the EML and thus diminishes the triplet exciton quenching. Furthermore, it was anticipated that the pertinent orange OLED structure could be blended with a blue fluorescent OLED to develop hybrid tandem WOLEDs exhibiting high efficiencies, high CRI, and low efficiency rolloffs.47 In the same year (2013), by doping green Ir(ppy)2(acac) and red Ir(mphq)2(acac) phosphors into the cohost matrix of TCTA:B3PYMPM exciplex, Kim et al., developed highly efficient orange OLEDs with low driving voltage, a high EQE with a low efficiency roll-off. The molar ratio of TCTA and B3PYMPM used in the EML was kept at 1:1 while the doping concentration of the red dopant was changed from 0.3 to 2 wt % and the green dopant was maintained at 8 wt % to achieve a perceived orange emission. The device was fabricated with the

3. EXCIPLEX FORMING COHOSTS In general, the emission from exciplex performs strong intermolecular charge-transfer character. Therefore, a featureless and wide emission spectrum seems to be inevitable, rendering the usage of exciplex as EML for OLED display technology challenging in terms of color purity. However, the high efficiency of exciplex-based devices indicate that the EML exhibits efficient charge injection and balanced charge transportation, which are highly beneficial for low driving voltages and low efficiency roll-offs. In addition, the high feasibility of triplet harvesting due to the nearly zero exchange energy (leading to low ΔEST), renders exciplex-forming systems to be excellent hosts for highly effective phosphorescent and/or fluorescent OLEDs. For serving as host, the exciplex system must satisfy certain conditions such as (i) the triplet (T1) of the exciplex has to be at a lower level compared to the individual counterparts for the confinement of the excitation energy within the exciplex state, and (ii) the triplet (T1) of the exciplex has to be at a higher level than that of the dopant for confining the emission solely on the emissive dopants, (iii) the emission of exciplex has to possess suitable overlapping with the absorption of dopant for the energy transfer to be effective. Herein, we have collected and discussed recent works regarding the utilization of exciplex as hosts/cohost systems for phosphorescent, TADF and fluorescent emitters to realize high efficiency OLEDs. 3.1. With Phosphorescent Dopants. In an exciplexforming cohost:dopant system, excitons are primarily formed on the exciplex host (donor:acceptor) and not on the dopant. The efficient energy transfer from the exciplex to the dopant and lower triplet-polaron quenching effects are accountable for the efficiency augmentation in PhOLEDs. The results can shed 19290




Figure 10. Structures of selected phosphorescent dopants used in exciplex host/cohost systems.

these exciplex-based OLEDs suffer from short operational lifetimes. Thus, Kim et al. very recently, demonstrated a highly efficient red PhOLED with a long operational lifetime. The red PhOLED was developed using an exciplex-forming cohost (NPB:PO-T2T) doped with two red phosphorescent dyes: [Ir(mphmq)2(tmd)] and (Ir(MDQ)2(acac)). The optimized device structure of the red PhOLEDs Device 1 or Device 2 consisted of: [ITO (100 nm)/(TAPC) (75 nm)/NPB (10 nm)/ NPB:PO-T2T:5 wt % Ir(mphmq) 2 (tmd) or Ir(MDQ)2(acac) (30 nm)/PO-T2T(10 nm)/0.5% Rb2CO3:PO-T2T (40 nm)/Al (100 nm)]. Both the devices exhibited low turn-on voltage with the CIE coordinates being (0.648, 0.352) at 1000 cd m−2 and (0.646, 0.353) at 10 000 cd m−2 for Ir(mphmq)2(tmd) based device, and (0.621, 0.378) at 1000 cd/m2 and (0.618, 0.380) at 10 000 cd/m2 for (Ir(MDQ)2(acac) based device. For a ready comparison, some previously reported red PhOLEDs containing different exciplex-forming cohost as well as unipolar hosts were also fabricated [Device 3: NPB:B3PYMPM; Device 4: single host system (Bebq 2 ) and Device 5: CBP doped with Ir(mphmq)2(tmd) or Ir(MDQ)2(acac)]. Low operating voltages were observed for the exciplex-hosted OLEDs (Devices 1−3), in contrast to the single host OLEDs that exhibited relatively higher Von. The Ir(mphmq)2(tmd) based PhOLED with the NPB:PO-T2T exciplex host (Device 1) was found to exhibit a high maximum ηext of 34.1% with a power efficiency (PE) of 62.2 lm W−1. When PO-T2T was replaced with B3PYMPM (Device 3), the device also yielded remarkable efficiency (maximum ηext of 35.6% with a PE of 66.2 lm W−1) and low efficiency roll-off. The operational lifetime (LT90) for Ir(mphmq)2(tmd) based device (Device 1) was found to be 2243 h and for the (Ir(MDQ)2(acac) based device (Device 2), it was found to be 1102 h. Thus, the efficiencies of the pertinent

structure of ITO (150 nm)/ TAPC (20 nm)/ TCTA (10 nm)/ TCTA:B3PYMPM: Ir(ppy)2(acac):Ir(mphq)2(acac) (30 nm, 8 wt, % x wt % B3PYMPM (45 nm)/ LiF (0.7 nm)/ Al (100 nm). The red device with the doping concentration of 0.3 wt % was found to exhibit well balanced orange emissions with turnon voltage as low as 2.4 V, a very high maximum ηext of 25.0% with a low efficiency roll-off and exhibiting an EQE of 21.2% at 10000 cd m−2. The CIE coordinates of the OLED were also very consistent with only slight variation from (0.501, 0.488) at 1000 cd m−2 to (0.486, 0.491) at 10 000 cd m−2, which was considered one of the best results obtained during that time.48 Thereafter, Zhang and co-workers developed a high powerefficiency solution-processed red PhOLED with a novel heteroleptic iridium complex, [Ir(DPA-Flpy-CF3)2acac]. This red phosphor was found to exhibit efficient photoluminescence (PL) with an emission band positioned at 602 nm, high absorption coefficient at 400−600 nm range and a measured PLQY of 42% in degassed 2-MeTHF. Accordingly, optimized red s-PhOLEDs were designed using the interfacial mMTDATA/TmPyPB exciplex as host with a low doping concentration (x = 1−3 wt %) of [Ir(DPA-Flpy-CF3)2acac]. The device was fabricated with a structure as ITO/ PEDOT:PSS (40 nm)/m-MTDATA: dopant (x wt%) (40 nm)/ TmPyPB (50 nm)/LiF (1 nm)/Al(100 nm). The corresponding s-PhOLED was found to display a maximum EQE as high as 19.3% and a PE of 44.5 lm W− 1 having CIE coordinates of (0.64, 0.36). This performance was one of the highest values reported for red solution-processed PhOLEDs and even similar to the state-of-the-art red thermally evaporated PhOLED (e-PhOLED) counterparts.49 In general, efficiency of PhOLEDs can be enhanced by the use of exciplex-forming cohosts through a barrier-free charge injection into the EML with a broad recombination zone. Nevertheless, a majority of 19291




(CBP:TPBI). Furthermore, the light-emission process of the exciplex TCTA:TPBi hosted device changed as the composition of the mixed film was varied. The OLED device consisted of ITO (120 nm)/ TAPC (75 nm)/ TCTA (10 nm)/EML/ BmPyPb/LiF/Al, where EML:TCTA:TPBi (1:1): Ir(ppy)3 (8 wt %) or CBP:TPBi (1:1):Ir(ppy)3 (8 wt %). It was observed that exciplex formation was favored when the relative ratio of TCTA and TPBi was 50:50. However, an excess amount of TCTA or TPBi quenched exciplex formation induced by charge trapping phenomenon. The strong electron-donating TCTA was responsible for the exciplex formation in the TCTA:TPBi mixed host while no exciplex formation was observed in the CBP:TPBi blend as CBP contains a moderate electrondonating carbazole unit. Thus, exciplex-hosted green PhOLEDs were constructed by employing Ir(ppy)3 as a green triplet dopant. Owing to improved hole injection as well as transport of TCTA as compared to that of CBP, the current density as well as luminance of the green PhOLEDs were found to be higher in the TCTA:TPBI (50:50) hosted device with a driving voltage of 3.74 V at 1000 cd m−2 in comparison to 3.95 V of CBP:TPBi (50:50)-based device. The EQEs of these two devices were rather analogous, 20.5% for TCTA:TPBi and 21.2% for CBP:TPBi hosted devices, where the TCTA:TPBi (50:50) device exhibited a slightly high power efficacy attributed to its low driving voltage.52 Though fabricated OLEDs containing CBP moiety is known to give very good efficiencies, however, there are instances where other systems also outperform the CBP-based devices. In 2014, Zhou et al. used exciplex-forming cohosts consisting of m-MTDATA and TPBi in tandem OLEDs. The exciplex of m-MTDATA:TPBi (1:1) was found to display yellow-green emission with emission at 550 nm. In a bid to investigate the EL properties in two investigations, i.e. utilizing exciplex-forming cohosts versus common hosts, two sets of devices were fabricated. The result suggested that the charge carrier mobility in the Ir(ppy)3-doped m-MTDATA:TPBi EML was much higher compared to that in the Ir(ppy)3-doped CBP EML and thus, the exciplex hosting device delivered a higher current and power efficiency compared to the single host system. Moreover, the turn-on voltages of the devices were found to be mainly governed by the band gap of the host materials, being 3.3 eV for the CBP molecule and 2.2 eV for the exciplex-forming cohost, respectively. Then, tandem OLED was fabricated having the exciplex-forming cohost configured as ITO/MoO3(5 nm)/mMTDATA (35 nm)/ m-MTDATA:TPBi:Ir(ppy)3 (15 nm, 6% by mole)/TPBi (30 nm)/ BPhen: LiNH2 (10 nm, 1:1 by mole)/MoO3 (10 nm)/m-MTDATA(30 nm)/mMTDATA:TPBi:Ir(ppy)3 (15 nm, 6% by mole)/TPBi(40 nm)/LiF(1 nm)/Al(60 nm). However, though current efficiencies of the tandem devices were found to be better than the single devices, the power efficiencies were reduced because stacking two EL cells into tandem devices typically amplified the driving voltage. The enhanced driving voltage in these tandem devices was indicative of the low conductivity of the MoO3 layer in the interfacial layer, thus, a decrease of PE. To upgrade the power efficacy of the tandem OLEDs, MoO3 was replaced with HAT-CN, which consequently was found to exhibit a much lower driving voltage and enhanced electroluminescent (EL) efficiencies. Thus, the authors clearly demonstrated the advantages of using exciplex-forming cohost and HAT-CN as interfacial layer for making tandem OLEDs with a low driving voltage as well as high PE.53 Kim et al. also reported highly efficient PhOLEDs with emitter Ir(ppy)2(acac)

devices are the highest for red PhOLEDs having an LT90 > 1000 h (Figure 11). The obtained results thus signify that high

Figure 11. Normalized electroluminescence (EL) decay curves of the phosphorescent OLEDs as a function of operational time at a preliminary luminance (L0) of 1000 cd m−2. For comparison, the EL decay curves of reported highly efficient red PhOLEDs are shown. (Inset: magnified time region from 0−80 h). Reproduced with permission from ref 50. Copyright 2017 American Chemical Society.

efficiency OLEDs with long operational lifetimes can be obtained via the judicious selection of exciplex cohost comprising very stable charge transporting materials.50 Besides, long operational device lifetimes can also be obtained if there is a large HOMO and LUMO level offset between the HT and ET materials used in the exciplex host.14 With the galore of exciplex-based red or orange PhOLEDs, the recent development by Kim et al. with NPB:PO-T2T exciplex as cohost and Ir(mphmq)2(tmd) as red dopant heads the list with an ηext of 34.1% pertaining to such systems. In the early years of exciplex advancement, initial attempts with CBP as one of the components were made. In 2011, Park et al. probed the energy transfer process taking place from the exciplex to the dopants and fabricated high-efficiency OLEDs. The exciplex was formed at the interface between CBP and B3PYMPM molecules, which was employed as EML host and electron-transporting layer respectively for highly efficient green PhOLEDs. The molar ratio of the codeposited film (CBP:B3PYMPM) is 1:1 with a PL emission peak centered at 425 nm, which was significantly red-shifted from their individual counterparts. The exciplex emission was found to decrease with increase in concentration of the dopant Ir(ppy)3. Thus, an energy transfer mechanism from the exciplex to the dopant could be clearly comprehended. The intensity of the exciplex emission was found to be nearly proportional to the inverse square of concentration of dopant molecule in the EML. Besides, the efficiency of the OLEDs was found to increase with increase in concentration of the emitter molecule, attributed to the higher rate of energy transfer from exciplex system to dopant molecule. The ηext of the OLED was found to be 20.1% when the concentration of the dopant was 6 mol %. Furthermore, it was found that exciplex emission ratio of the OLEDs increases with increase in current density, thereby resulting in additional efficiency roll-off in the OLEDs at high current.51 Soon after, in 2015, Song et al. described the mechanism of light emission by monitoring the current density and luminance of OLEDs with exciplex and exciplex free mixed cohosts. This study revealed that the emission of OLED with exciplex-based mixed cohost (TCTA:TPBi), is dominated by the energy transfer process, while charge-trapping governs the device light emission route with the exciplex free mixed host 19292




Figure 12. (a) Experimental external quantum efficiencies, EQEs (circles) are matched with the EQEs obtained via simulation, using different dipole orientation factors (Θ); (dashed line represents Θ = 1 (fully horizontal), dashed-dotted line represents Θ = 0.79, thick solid line represents Θ = 0.77, short dotted line represents Θ = 0.75, and dotted line represents Θ = 0.67 (isotropic). (b) Contour plot of the simulated outcomes of (EQE) as a function of photoluminescence quantum yield (qPL) and orientation factor of the dipoles (Θ). The two dashed lines point toward the locus of the ηEQE for Θ = 0.77 and qPL = 0.94, respectively while, the dotted lines denotes the ηEQE for Θ = 0.67 (isotropic), 0.75, and 0.79, respectively. Reproduced with permission from ref 54. Copyright 2013 John Wiley & Sons.

29.1%, with a PE of 124.0 lm W−1, with the EQE remaining ∼27.8% up to 10 000 cd m−2. Furthermore, the orientation of the emitter molecule was evaluated to check whether the high EQE corresponded to nearly 100% IQE as horizontally oriented dipoles are known to display higher out-coupling efficiency compared to vertically oriented ones. The emitter was found to have a preferred horizontal emission dipole orientation ratio (77%). Theoretically, it was predicted that with such preferred horizontal orientation and PLQY of the pertinent emitter, the EQE of PhOLED should be about 30%, which matched closely to the experimental data, thereby signifying negligible electrical loss in the PhOLEDs. Moreover, as trap-assisted recombination within the EML is known to reduce the efficiency of OLEDs, therefore, energy-transferdominated light emission using an exciplex-forming cohost is an effective way to eliminate trap-induced efficiency reduction in such devices and thus upsurge efficiency.55 In the subsequent year, Kim and co-workers studied the charge recombination mechanisms in two phosphorescent devices, one comprising the exciplex forming cohost (TCTA:B3PYMPM) and the other with a single host CBP. The exciplex-hosted device consisted of ITO (70 nm)/ TAPC (75 nm)/ (TCTA) (10 nm) /TCTA: (B3PYMPM): [Ir(ppy)2(acac)] (1:1 molar ratio and 8 wt %, 30 nm)/ B3PYMPM (40 nm)/LiF (0.7 nm)/Al (100 nm), while the single host device is composed: ITO (70 nm)/ (MoO3) (1 nm)/CBP (90 nm)/CBP:Ir(ppy)2(acac) (8 wt %, 15 nm)/ TPBi (65 nm)/LiF (0.7 nm)/Al (100 nm). The maximum EQEs of the TCTA: B3PYMPM based PhOLED was found to be 30% with an efficiency roll-off to 27% at 10 000 cd m−2, whereas the CBP-based PhOLED exhibited an EQE of 25% with an efficiency roll-off to 21% at 10 000 cd m−2. The higher device efficiency in the former device compared to the latter case was attributed to better charge balance in the exciplexhosted PhOLED compared to that of the single host PhOLED. In addition, the turn-on voltage for the exciplex-based device was found to be 2.3 V which was about 0.5 V lower compared to the CBP-based PhOLED, thereby signifying efficient charge injection from the electrodes and transportation to the emissive layer under a low external bias in TCTA: B3PYMPM based device.56 This work indicated that excitons in the exciplexforming cohost were largely produced via Langevin recombi-

in an exciplex-forming cohost comprising TCTA and B3PYMPM. The device under study comprised a simple structure consisting of ITO (70 nm)/TAPC (x nm)/TCTA (10 nm)/ TCTA:B3PYMPM:Ir(ppy)2(acac) (1:1 molar ratio 8 wt %) (30 nm)/B3PYMPM (40 nm)/Al (100 nm). The maximum current and power efficacies were found to be 106 cd A−1 and 127.3 lm W−1, respectively, with maximum ηext reaching 30.2%. They also performed an optical simulation experiment based on the PLQY and orientation factor of emitter to predict the maximum EQE that can be achieved for a particular OLED. The experimental results were then compared to the simulated results (shown in Figure 12a) by varying the orientation factors of the dipoles. The close analogy between theoretical and experimental data clearly indicated negligible electrical loss. As the experimental results were found to be suitably described by the simulations, the maximum EQE viable for a definite emitting dye in a host could be easily predicted by merely measuring the PLQY and horizontal dipole ratio (Θ) without the need to fabricate devices. Thus, from the simulated result, the maximum obtainable EQEs as a function of qPL and Θ were evaluated and the contour plot is shown in Figure 12b.54 In addition to these, in the same year (2013), using an Ir(ppy)2(acac) doped in an exciplex-forming cohost system consisting of TCTA and B3PYMPM, Kim et al. developed a PhOLED exhibiting high efficiency with a small driving voltage and an exceptionally reduced efficiency roll-off. The exciplex emission was observed only at low Ir(ppy)2(acac) doping concentrations and disappeared when the concentration of the dopant was over 4 wt %, thereby signifying complete energy transfer from the exciplex to the dopant, Ir(ppy)2(acac) under photoexcitation. In addition, nearly identical S1 and T1 energy levels of the exciplex system led to effective energy transfer from the exciplex to the dopant. A series of devices were explored with different ITO thicknesses (70, 100, and 150 nm) and varying TAPC thickness (60−20 nm) to maximize the outcoupling efficiency. The device architecture was configured as ITO (x nm)/ TAPC (y nm)/TCTA (10 nm)/TCTA:B3PYMPM: Ir(ppy)2(acac) (30 nm)/B3PYMPM (40 nm)/LiF (0.7 nm)/Al (100 nm). The OLED with ITO thickness of 70 nm achieved a low turn-on voltage of 2.4 V, a very high EQE of 19293



ACS Applied Materials & Interfaces nation, while in the CBP-based device, trap-assisted recombination on emitter molecule was more favorable. The reduced charge density in the exciplex-based PhOLEDs was indeed linked to its low efficiency roll-off. Again, in a separate report, considering another phosphor, the same group (Kim et al. in 2014) developed a highly efficient inverted top-emitting OLED using an exciplex-forming cohost system consisting of TCTA and B3PYMPM molecules and which was doped with a phosphorescent green emitter, Ir(ppy)2tmd with a horizontal orientation. The phosphorescent emitter was found to have a horizontal: vertical transition dipole ratio of 0.78:0.22 with a PLQY of 96%. Optically simulated EQE of the device was performed which anticipated an EQE of 31.9% under negligible electrical loss. The optimized device was found to exhibit ηext of 27.6% at 1000 cd m−2 and CE of 120.7 cd A−1, with reduced efficiency roll-off with EQE of 24.5% and luminous efficiency of 107.6 cd A−1 at 20,000 cd m−2. Also, as predicted by simulation results, the device with 30 nm thick electron injection layer (EIL) was found to exhibit the highest luminance.57 Furthermore, in an endeavor to further simplify the devices based on the exciplex host systems, an electron-donor, TPAF with suitable HOMO was blended with the known electronacceptor B3PYMPM and this was reported by Tao et al. in 2017. This exciplex system was found to exhibit bluish-green exciplex emission peaked at 496 nm and ambipolar transporting property was observed when the weight-mixing ratio was 3:7. The phosphorescent devices were constructed by doping the exciplex host (TPAF:B3PYMPM) with the known phosphors Ir(ppy)2acac and Ir(MDQ)2acac and consisted of the structure ITO/TPAF (15 nm)/TPAF:B3PYMPM:10 wt % Ir(ppy)2acac (30 nm)/B3PYMPM (45 nm)/LiF (0.8 nm)/Al (80 nm) and ITO/TPAF (40 nm)/ TPAF:B3PYMPM:1.5 wt % Ir(MDQ)2acac (30 nm)/B3PYMPM (45 nm)/LiF (0.8 nm)/Al (80 nm). Owing to the extremely simple barrier-free structures and the ambipolar transporting property of TPAF:B3PYMPM, both the two devices displayed green and red emission from Ir(ppy)2acac and Ir(MDQ)2acac respectively, indicating full energy transfer from the TPAF:B3PYMPM host to the dopant. The Ir(ppy)2acac- and Ir(MDQ)2acac-doped exciplex-hosted devices exhibited high EQEs of 20.1% and 19.2%, and ultralow turn-on voltages of 2.15 and 2.35 V, respectively. Thus, TPAF:B3PYMPM exciplex was found to be an effective host system in OLEDs, which exhibits high performance with a simplified device structure.58 And, very recently (2017), Meng et al., demonstrated a simplified design whereby a novel tandem structure incorporating both ultrathin emitters and interface exciplex was fabricated. The tandem OLEDs encompass a host-free green phosphorescent thin layer of Ir(ppy)2(acac) as an interface in exciplexforming blend of TAPC and TmPyPB, interconnected by Bphen:LiNH2. The exciplex emission of the blend (TAPC:TmPyPB) was observed at 424 nm. The PL spectrum of the blend was found to overlap well with the absorption spectrum of Ir(ppy)2acac, which warranted efficient energy transfer from the TAPC/TmPyPB exciplex to the Ir(ppy)2acac film.59 Herein, the authors have demonstrated the energy transfer pathway in interface exciplex OLEDs with ultrathin EMLs versus that in conventional host−guest systems. Thus, most carriers in interface exciplex OLED encounter at the interface of the donor (TAPC) and acceptor (TmPyPB) to form excitons and the entire energy transfer process of the pertinent system as well as the conventional host−guest system is depicted in Figure 13.

Figure 13. Triplet energy level Tl of TAPC, TmPyPB, CBP, and Ir(ppy)2(acac). Energy transfer or loss routes of (a) host−guest luminescence system and (b) exciplex:luminescence system. Adapted with permission from ref 59. Copyright 2017 American Chemical Society.

Finally, a device was designed with structure ITO/HAT-CN (10 nm)/TAPC (55 nm)/Ir(ppy)2(acac) (0.8 nm)/ TmPyPB (40 nm)/BPhen:LiNH2 (10 nm, 50% by mole)/HAT-CN(10 nm)/TAPC (55 nm)/Ir(ppy)2(acac)(0.8 nm)/TmPyPB (40 nm)/Liq (2 nm)/Al(120 nm). The designed tandem OLEDs was found to exhibit about twice the luminance and driving voltage compared to that of single-unit OLEDs at the current densities examined. Above all, outstanding efficacy together with comparatively low working voltage was obtained for the designed tandem OLEDs. These OLEDs were found to display a peak CE of 135.74 cd A−1 (EQE = 36.85%) with the exciplexforming cohost, which was almost twice than that of a single emitter device with CE of 66.2 cd A−1 (EQE = 17.97%). Meanwhile, the authors have attributed this enhancement of CE and PE to three factors: (a) plasmon quenching effect; (b) high-quality charge generation unit (CGU) and (c) electricfield-induced quenching effect. Thus, in this report, the authors made an endeavor to shed light on the future progress of OLEDs with high performance, simplified fabrication, low cost and power consumption simultaneously.59 Though a plentiful of exciplex-based green to yellow PhOLEDs have recently been developed, the report by Meng et al. with TAPC:TmPyPB exciplex system and Ir(ppy)2(acac) as green dopant performed the best efficiency with an ηext of 36.8%. In addition, the works by Kim and co-workers consisting of TCTA:B3PYMPM exciplex host and the green dopant Ir(ppy)2(acac) with ηext ≥ 30.0% are also worth mentioning. The procurement of blue PhOLEDs is one of the most challenging tasks for material chemists and there are still limited reports of efficient exciplex-hosted blue PhOLEDs. Among the various Ir-based blue phosphorescent emitters, FIrpic still serves as the most popular example which received extensive research attention as well as serve the benchmark for examining the efficacy of host materials and device configuration designs for delivering high efficiency blue PhOLEDs. In 2014, Sasabe and Kido et al. reported high performance blue PhOLEDs via energy transfer from exciplex to dopant. The exciplex was formed at the interface by blending TAPC as donor and a large optical energy gap host material BTPS as acceptor. The PLQY of the 11 wt % FIrpic-doped exciplex (TAPC/BTPS = 1:1) film was found to be 60 ± 1%. A device was fabricated with the structure of ITO (130 nm)/ TAPC (30−x nm)/TCTA (x nm) with x = 0, 5 nm/11 wt % FIrpic doped BTPS (10 nm)/ B3PyPB (50 nm)/Liq (1 nm)/ Al (100 nm). The optimized 19294




nm)/EML (20 nm)/ TmPyPB (50 nm)/Liq (1 nm)/Al. Deep blue and pure blue exciplex-based OLEDs were realized with EML composed of host:TmPyPB (99:1 wt %), where the host is BTCz-PCz, BTDCb-PCz, and DCb-PCz, respectively. Though the BTCz-PCz- and DCb-PCz-based devices were found to show lower efficiencies, only the BTDCb-PCz-based exciplex OLED displayed maximum EQE of 2.36%, CE of 4.64 cd A−1 and PE of 2.91 lm W−1. Then, by using bilayer neat host/TmPyPB as EML for exciplex OLEDs, it was observed that in comparison to the D:A-blended (doped with 1% TmPyPB) exciplex OLEDs, the BTCz-PCz-and DCb-PCzbased bilayer-type OLEDs exhibited better performance, whereas the BTDCb-PCz-based device showed lower efficiency. Thereafter, these materials were applied as hosts for blue PhOLEDs by using FIrpic as the phosphorescent dopant. Multilayer OLEDs with host: FIrpic (8 wt %) as EML were fabricated and the pertinent blue PhOLEDs exhibited a maximum EQE of 24.6%, CE of 55.6 cd A−1, and PE of 52.9 lm W−1 with low turn-on voltage (Von) at 2.7 V and a very low efficiency roll-off. Thus, these results validate the significance of utilizing exciplex energy in the fabricating efficient blue OLEDs.63 Also, Kim et al. in a very new development (2017) reported an exciplex as cohost composed of mCP as donor and BM-A10 as acceptor for giving deep blue OLEDs. Both the molecules possess high triplet energies greater than 2.8 eV and the resulting exciplex exhibits a very small singlet−triplet energy gap (ΔEST). The emission energy of the mCP:BM-A10 exciplex system was found to be 3.0 eV corresponding to 413 nm, which is sufficiently large enough to be used as host for the deep-blue dopant, FCNIr. The PLQY of the exciplex emission was found to be 0.17, while the FCNIr-doped exciplex was found to be 0.82. The fabricated exciplex-hosted OLED device with structure ITO (70 nm)/6 wt % ReO3-doped mCP (30 nm)/mCP (20 nm)/ EML (30 nm)/BM-A10 (20 nm)/12 wt % Rb2CO3 doped BM-A10 (20 nm)/Al (100 nm) exhibited a maximum EQE of 24% with CIE coordinates of (0.15, 0.21), turn-on voltage of 2.9 V and long operational lifetimes.64 Thus, of the handful of exciplex-based blue PhOLEDs, the mCP:POT2T exciplex host system doped with the blue emitter, FIrpic, dominates in such systems with an ηext of 30.3%. White organic light emitting diodes (WOLEDs) are generally regarded as one of the energy-saving light sources due to their wide-view-angle, high brightness and low driving voltage, and thus researchers are in constant search of WOLEDs with high efficiency and device stability. In 2015, Zhang et al. developed a new mechanism to realize exciplex-fluorescence as well as exciplex-phosphorescence hybrid white OLEDs having simple device structure. This approach circumvents the low RISC efficacy of blue exciplex and complex device structure in conventional WOLEDs, thereby facilitating to achieve efficient hybrid WOLEDs. At the outset, a blue exciplex with PLQY of 34 ± 4% at room temperature was formed between mCBP and PO-T2T (1:1), giving the corresponding exciplex-based OLED a high EQE of 7.66% and a modest brightness of 5016 cd m−2. Thereafter, an excellent warm WOLED was constructed by doping Ir(bt)2(acac) into this blue exciplex (mCBP:PO-T2T) as EML. The best WOLED displayed a very high EQE of 22.21% with CIE coordinates of (0.418, 0.433). This device maintained an EQE higher than 20% even at a high brightness of 1253 cd m−2. In addition to the high triplet energy of the donor and acceptor components as compared to that of exciplex triplet, there was also a large overlap between the absorption spectra of orange dopants Ir(bt)2(acac), Rubrene

PhOLED based on this exciplex host system without TCTA was found to exhibit high power efficiency (PE) of 50.1 lm W−1 and a high EQE of 21.7% at 100 cd m−2, whereas that with TCTA exhibited a power efficiency (PE) of 45.5 lm W −1 with ηext of 22.1% at 100 cd m−2. In addition, the intensity of relative exciplex emission was augmented when the FIrpic concentration was reduced, signifying that energy transfer from the exciplex (TAPC/BTPS) to the emitter FIrpic occurred only in greater doping concentration devices. Furthermore, it was observed that with the increase in the distance between the TAPC/BTPS interface and FIrpic-doped layer, the efficacy of energy transfer was decreased.60 Kim et al. also demonstrated a highly efficient blue PhOLED approaching the theoretical limit using the exciplex-forming cohost (mCP:B3PYMPM) doped with FIrpic. The film of the mCP:B3PYMPM (1:1) blend exhibited red-shifted emission at 415 nm compared to those of mCP and B3PYMPM. Also, the transient PL decay was composed of a long delayed component 0.5 μs indicating an efficient RISC in this system in addition to the prompt decay with the lifetime of 25 ns. Furthermore, the investigation of the angle-dependent photoluminescence spectrum of the FIrpic doped exciplex cohost film indicated that the transition dipole moments of FIrpic in the host matrix was in a preferred horizontal orientation in the EML with a horizontal/vertical dipole ratio of 0.76:0.24. An OLED configured as ITO (70 nm)/mCP:ReO3 (45 nm, 4 wt %)/ mCP (15 nm)/mCP: B3PYMPM (1:1): FIrpic (15 nm, 10 wt %)/ B3PYMPM (20 nm)/B3PYMPM: Rb2CO3 (x nm, 4 wt %)/LiF (0.7 nm)/Al (100 nm) was fabricated, where thickness of the n-doped ETL was varied from (25−55) nm. The OLED exhibited a maximum EQE of 29.5%, CE of 62.2 cd A−1 and a low driving voltage (3.0 V) and reduced efficiency roll-off, which agrees quite well with the theoretical anticipation based on PLQY measurement and the transition dipole moment orientation of FIrpic under the hypothesis of trivial electrical loss. The authors claimed that such device efficiency was the highest for blue PhOLEDs during that time.61 In another report, the same group together with Wong et al. reported the utilization of skyblue exciplex mCP:PO-T2T as host for FIrpic. The triplet level of the exciplex host system was found to be lower than those of the constituent molecules, mCP and PO-T2T, but slightly higher than FIrpic, and thus the excitation energy was constrained in the exciplex state, followed by energy transfer to the phosphor. The mCP:PO-T2T blend displayed a featureless photoluminescence spectrum with the exciplex emission positioned at 470 nm, bathochromically shifted from those of mCP and PO-T2T. Consequently, a highly efficient blue PhOLED was developed with structure of ITO (70 nm)/ 6% ReO3 in mCP (45 nm)/mCP (15 nm)/mCP:PO-T2T:10% FIrpic (30 nm)/PO-T2T (20 nm)/4% Rb2CO3:PO-T2T (25 nm)/Al (100 nm), which exhibited a ηmax,ext of 30.3% and a PE of 66 lm W−1, in addition to low driving voltage of 2.75 V at 100 cd m−2, 3.29 V at 1000 cd m−2, and 4.65 V at 10 000 cd m−2, respectively. The remarkable outcome was credited to the novel FIrpic doped exciplex forming system in addition to the low lying LUMO level of PO-T2T which was responsible for augmenting the efficient electron injection and transport under low external voltage bias.62 In a further study, Wang et al. recently (2017) developed new δ-carboline derivatives having high triplet energies and glass transition temperatures as the donor component for exciplex formation. The exciplex-based OLEDs were examined with device structure ITO/HAT-CN (10 nm)/NPB (10 nm)/ TCTA (5 nm)/TCTA:host (1:1) (5 19295




Table 3. Summary of Selected Phosphorescent OLEDs with Exciplex Forming Systems As Host/Cohost exciplex host/co-host

phosphorescent dopant

λPL/EL (nm)

ϕfilm (%)

Von (V)

EQEmax (%)

TCTA: B3PYMPM:

47 Ir(mphq)2(acac) Ir(ppy)2(acac)

mMTDATA/TmPyPB

2.4

22.8

490

49

Ir(DPA-Flpy CF3)2acac

85.3

2.1

19.3

Ir(mphmq)2(tmd) (Ir(MDQ)2(acac)

91 78 11.0

1.90 1.90

34.1 26.8

NPB:PO-T2T

50

CBP:B3PYMPM

425

51

Ir(ppy)3 TCTA:TPBI

20.1 438

52

Ir(ppy)3 TCTA:B3PYMPM

495

TCTA:B3PYMPM

496 495 [Ir(ppy)2(acac)] [Ir(ppy)2tmd]

2.4

30.2

2.4

29.1

2.3 2.8

30.0 27.6

2.15 2.35 2.43

20.1 19.2 5.4

5.76

36.9

2.5

21.7

3.0

29.5

2.75 2.7 2.9 2.8

30.3 23.6 23.0 24.6 7.66 22.2 6.09 6.16 5.75 4.79

54 55

36 96

56

496 Ir(ppy)2acac Ir(MDQ)2acac DCJTB

TAPC:TmPyPB

424

TAPC/BTPS

474 FIrpic

mCP:B3PYMPM

60 60

415

61

FIrpic mCP: PO-T2T

470

62

FIrpic FIrpic FIrpic FIrpic 473 Ir(bt)2(acac) Rubrene DCJTB-warm DCJTB-cold 4CzTPN-Ph

57 58

59

Ir(ppy)2(acac)

BTDCb-PCz BTCz-PCz DCb-PCz mCBP: POT2T

20.5

36

Ir(ppy)2(acac) TCTA: B3PYMPM

2.5 36

Ir(ppy)2(acac)

TCTA: B3PYMPM TPAF:B3PYMPM

ref

34 2.5

Exciplex: An Intermolecular Charge-Transfer Approach for TADF

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