Recent Applications of Interfacial Exciplex as Ideal Host of Power-Efficient OLEDs

Currently, exploring the applications of intermolecular donor-acceptor exciplex couple as host of OLEDs with phosphorescence, thermally activated delayed fluorescence (TADF) or fluorescence emitter as dopant is a hot topic. Compared to other host strategies, interfacial exciplex has the advantage in various aspects, such as barrier-free charge injection, unimpeded charge transport, and the energy-saving direct exciton formation process at the “Well”-like heterojunction interface region. Most importantly, due to a very fast and efficient reverse intersystem-crossing (RISC) process, such a host is capable of regulating singlet/triplet exciton populations in itself as well as in the dopant emitters both under photoluminescent (PL) and electroluminescent (EL) driving conditions. In this mini-review, we briefly summarize and comment on recent applications of this ideal host in OLEDs (including both thermal-evaporation OLEDs and solution-processed OLEDs) with diverse emitters, e.g., fluorescence, phosphorescence, delayed fluorescence, or others. Special attention is given to illustrate the peculiar achievement of high overall EL performance with superiorities of low driving voltages, slow roll-off rate, high power efficiencies and satisfied device lifetime using this host strategy, which is then concluded by personal perspectives on the relevant next-step in this field.

Since their invention in 1987 (Tang and Vanslyke, 1987), OLEDs have received persistent attention considering their great advantage in modern displays and lighting applications (Burroughes et al., 1990;Kido et al., 1995). With the development of phosphorescent emitters (Baldo et al., 1998), efficiencies of OLEDs have been significantly improved . Endo et al. (2011) launched new generation OLEDs by inventing efficient thermal-activated delayed fluorescence (TADF) emitters purely from aromatic carbon materials. On the basis of a high RISC rate and a high radiative decay rate (S 1 → S 0 ) in TADF emitters (Uoyama et al., 2012;Higuchi et al., 2015;Noda et al., 2018), or exciplex couples Hung et al., 2013Hung et al., , 2014Liu et al., 2015a,c), highly efficient monochromatic and white TADF OLEDs have been achieved. Other candidates such as hybridized local and charge-transfer (HLCT) excited state molecules  and radical-based double emission molecules (Ai et al., 2018) have also been reported and well-documented, showing the analogous cost and performance merits.
However, irrespective of emitter categories, e.g., phosphorescent, TADF or fluorescent emitters, it is highly pursued ideal hosts that maximize their EL performance since critical parameters of OLEDs e.g., external quantum efficiency (EQE), power efficiency (PE), roll-off rate and device lifetime, are highly determined by host choices. Among those host strategies (Tao et al., 2011;Yook and Lee, 2014;Wang et al., 2019), interfacial exciplex seems an ideal choice since all these expected characteristics are simultaneously satisfied. Based on relevant publications and our understanding, this mini-review presents a short introduction on its application status and remarks on the future research direction.

INTERFACIAL EXCIPLEX AS HOST IN THERMAL-EVAPORATED OLEDS
At a type II P/N organic/organic (O/O) heterojunction interface between an electron-donating molecule and an electronaccepting molecule, there is a high tendency to form a chargetransfer excited-state complex, also known as an exciplex (Jenekhe and Osaheni, 1994;Itano et al., 1998;Giro et al., 2000;Morteani et al., 2003). Simultaneously, HOMO and LUMO levels of hole transporting material (HTM) and electron transporting material (ETM) display a distinct gap at the heterojunction interface ( Figure 1A). It is barely possible to generate exciton on either constituting molecule. By contrast, exciplex formation is energetically allowed, in which one of them locates in the excited state while another one is in the ground state being coupled. There is an experimental guideline on exciplex formation (Matsumoto et al., 2008), i.e., coexistence of huge gap, e.g., larger than 0.3 eV, for both HOMO and LUMO levels. However, it is not necessarily the case. A certain constituting material couple could be switched to exciplex or not simply by altering the substrate (Ng et al., 2014). From the electronic viewpoint, Ng et al. (2014) illustrated how the local molecular interactions and interfacial energetics at PN heterojunction play a role in exciplex formation ( Figure 1B). It corresponds to P δ− -N δ+ contact at the PN heterojunction, in which the N-type material donates electrons to the LUMO level of the P-type material at the interface. Bounded immobile charges (CTC) formed thus guarantee the exciplex formation. From the classic viewpoint of semiconductor physics, exciplex is a universal concept, i.e., including organic exciplex but not limited to it, such as hybrid exciplex in a lead halide perovskite (MAPbI 3−x Cl x )/quantum dot (core/shell PbS/CdS) heterojunction (Sanchez et al., 2016).
Previously, exciplex was frequently observed during fabrication of thermal-evaporated OLEDs (e-OLEDs), but unwelcomed due to its low PL quantum efficiency (PLQE) as an emitter and quenching effect in host-guest-doping devices. For instance, interfacial exciplex was discovered (or generated but not noticed by researchers) at the interface between the emissive layer (EML) and hole transporting layer (HTL), or between the EML and electron transporting layer (ETL). As the singlet and triplet levels (S 1 /T 1 ) of the exciplex were less than those of emissive constituents of the EML, EL spectra and device efficiency were deteriorated via a back-energy transfer from them to the exciplex (Jenekhe and Osaheni, 1994;Gebler et al., 1997Gebler et al., , 1998Itano et al., 1998;Giro et al., 2000;Matsumoto et al., 2008). One useful exception is white OLEDs, since exciplex featured in color-adjustable and very broad PL/EL spectra (Chao and Chen, 1998). However, restricted by its low PLQE, the corresponding device performance was very limited.
The key results of reported OLEDs using interfacial exciplex host are summarized in the Supplementary Materials.
Duan's group provided an illustration on applications of the exciplex host , and provided direct comparisons on the interfacial exciplex vs. the bulk exciplex host, from the viewpoint of device efficiencies, roll-off performance and device lifetime. The constituting materials were one bipolar host (3 ′ -(4,6-diphenyl-1,3,5-triazin-2-yl)-(1,1 ′ -biphenyl)-3-yl)-9-carbazole (CzTrz), and a donor molecule tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA) to form the CzTrz:TCTA bulk exciplex or CzTrz/TCTA interfacial exciplex host, respectively, where orange phosphor was (acetylacetonato)bis [2-(thieno[3,2-c]pyridin-4-yl)phenyl]iridium(III) (PO-01). Very surprisingly, the interfacial exciplex host based PhOLED comprehensively outperformed the bulk excipelx host based PhOLED. EQE max. /EQE 5000 /EQE 10000 and PE max. /PE 5000 /PE 10000 of the former device reached to 27.0/25.6/24.0% and 73.1/52.1/44.6 lm W −1 , as compared to 23.5/21.5/19.5% and 58.5/41.1/33.2 lm W −1 achieved for the latter. Obviously, the interfacial exciplex host rendered the PhOLED much higher EQEs/PEs along with the alleviated roll-off rate. As disclosed, it was due to the enhanced Förster ET from the CzTrz/TCTA to the dopant in the EML [CzTrz:PO-01(1-3 wt.%)]. Despite relatively high local exciton density at the exciplex interface, fast and efficient long-range Förster ET spread these excitons throughout the EML, thereby overcoming TTA, TPA quenching limitations. Besides, with the CzTrz/TCTA interfacial exciplex host, the device lifetime of PhOLED was enhanced by almost two orders of a magnitude compared to the device counterpart with a bulk exciplex host (L 0 :1,000 cd m −2 ), since this structure avoided the formation of easily dissociated high-energy aromatic amines, TCTA in this case, donor excited states . The formation possibility of unstable high-energy TCTA excitons was largely lowered in the interfacial exciplex host device structure, which was indicated by a condition experiment, i.e., a much longer device lifetime using a lower content of the TCTA constitute. It was not mentioned why the bulk exciplex host based PhOLEDs exhibited much lower efficiencies (as well as quicker roll-off rates), compared to the interfacial exciplex host PhOLEDs. Probably, the TCTA-excitonic involved a degradation process. As indicated, accelerated TPA and/or TTA quenching also distinctly deteriorated device efficiencies and roll-off behaviors. In short, this report might have referential significance in solving efficiency and lifetime issues of other OLEDs with host-guest structures that were not mentioned. The same group further used such geometry in red PhOLEDs, in which the exploration of constituting materials to form suitable interfacial exciplex was proven to be important . Xu et al. (2017) further reported a new emitting sub-unit design in tandem OLEDs, using an ultra-thin emissive layer (UEML), i.e., green-color phosphor bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)-iridium(III) [Ir(ppy) 2 (acac)], sandwiched between a layer of 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and a layer of 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TmPyPB), in which TAPC/TmPyPB could form an interfacial exciplex. It displayed a peak LE/EQE of 135.74 cd A −1 /36.85%, which is among the best efficiencies of OLEDs using non-doped EML without using an out-coupling method. Despite indirect contact, exciplex excited states were generated efficiently via long-range coupling under device operation (Al Attar and Monkman, 2016;Nakanotani et al., 2016), followed by sufficient Förster/Dexter ET to the UEML. As the complexed co-evaporation operation is avoided, such architecture is significant in simplifying the manufacturing process of OLEDs and enhancing yield and repeatability of OLED products.
Su's group conduced systematic works on interfacial exciplex host application in FOLEDs (Li et al., 2018). FOLEDs with different EML structures were fabricated, i.e., TAPC:1% DBP/TmPyTz (device 1), TAPC:1% DBP/TAPC(3 nm)/TmPyTz (device 2), TAPC:1% DBP/mCP(3 nm)/TmPyTz (device 3), in which TAPC/TmPyTz formed exciplex directly at the interface (device 1 and 2) or even long-range distance (device 3, with mCP spacer) (see the Supplementary Materials for a detailed performance) and DBP was a common fluorescent emitter. Among all of them, device 3 with a spatially separated exciplex couple host was the best, i.e., simultaneously achieving a low driving voltage, a high luminance and efficiency, e.g., V on /L max. /EQE max. /PE max of 2.18 V, 2956.8 cd m −2 , 14.8% and 38.8 lm W −1 . As illustrated, the merits of such FOLEDs using a spatially separated exciplex host lie in separating exciton generation and energy transferring areas, and also restraining the charge trapping effect of the dopant emitter, which is basically different from common FOLEDs.
Moreover, Lin et al. (2017) successfully constructed prototypical up-conversion OLEDs on the basis of a so-called exciplex-sensitized TTA (ESTTA) mechanism, featuring ultralow sub-bandgap EL driving voltages, e.g., light turn-on merely at 2.2 V for high energy blue emission (2.9 eV for its bandgap). As verified, low-energy exciplex triplets formed at the interface of 4, 4 ′ , 4 ′′ -tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA)/9,10-bis(2 ′ -naphthyl) anthracene (ADN) are harvested by AND themselves and then trigger their TTA processes to realize high-energy blue emission (S 1 → S 0 ). At first glance, such ESTTA-OLED featured in low-voltage driving but suffered from low EQE performance (0.1%) due to back energy transfer quenching from S 1 of ADN to S 1 of the exciplex. After incorporating the "triplet diffusion and singlet blocking (TDSB)" layer and/or a more efficient fluorescent dopant, e.g., DPAVBi, the corresponding EQE performance was sharply enhanced to 3.8%. Impressively, for TTA emissive material, this configuration theoretically requires only one-half of the driving voltage equal to its singlet photonics energy. This work provides a novel clue toward developing ultralow driving-voltage and power-efficient OLEDs (especially for the blue one).

INTERFACIAL EXCIPLEX AS HOST IN SOLUTION-PROCESSED OLEDS
Solution-processed OLEDs (s-OLEDs) are appealing since the adopted wet-process approach is cost-effective, and more suitable for future flexible, stretchable and large-size displaying and lighting applications via high-speed printing, "roll-to-roll" coating industrial techniques. However, one of the unsolved challenges lies in how to realize sufficient high EL performance, especially low power consumption. To this goal, among various strategies, interfacial exciplex was found to be an ideal host in various types of s-OLEDs, i.e., acquiring sufficient low driving condition, high PE while using a very simplified device structure.
Zhang and Wang et al. first reported power-efficient phosphorescent s-OLEDs (s-PhOLEDs) using an interfacial exciplex host, i.e., m-MTDATA/TmPyPB (Wang et al., 2015). The achieved PE max. of orange s-PhOLED reached a record value of 97.2 lm W −1 , along with ultra-low V on of 2.36 V, and extremely low driving voltages of 2.60/3.03 V at the luminance of 100/1,000 cd m −2 . Such PE is the best among all-reported s-OLEDs and even superior to thermal-evaporated PhOLEDs with the same color. Figures 2A,B depicts the device structure and the proposed EL driving mechanisms, in which both holes and electrons were barrier-freely injected, transported and then combined at the m-MTDATA/TmPyPB heterojunction to form exciplex excitons, and then transferred to dopants of the EML via Förster/Dexter ET. A negligible influence on current density-voltage (J-V) characteristics was observed by doping the guests, indicating that notorious charge trapping/scattering effects of dopants  were absent in such an architecture. By contrast, s-PhOLEDs using bulk exciplex of m-MTDATA:TmPyPB (1:1 w/w) was unsatisfied, corresponding to a low PE max. of 35.2 lm W −1 mainly due to sharply increased driving voltages, e.g., V on /V 100 /V 1000 : 4.15/5.18/5.80 V. Two reasons were involved; i) low charge transporting capability of the bulk exciplex couple due to their intrinsic incompatibility ; ii) the serious charge trapping tendency of dopant in the EML structure of m-MTDATA:TmPyPB:dopant (Wang et al., 2015). Accordingly, interfacial exciplex rather than bulk exciplex shown here is more suitable for s-PhOLEDs as a host.
By replacing the orange phosphor to other ones (Figures 2C-F), power-efficient s-PhOLEDs with similar EL driving features were realized, e.g., 81.1 lm W −1 for green (Wang et al., 2015) and 44.5 lm W −1 for red (Liu et al., , 2018a, respectively, confirming its universal application potential. Komatsu et al. (2015) fabricated efficient TADF s-OLEDs using 4CzIPN as the dopant dispersed within the CBP matrix, and an interfacial exciplex couple of CBP (35 nm)/bis-4,6-(3,5-di-4-pyridylphenyl)-2-methylpyrimidine (B4PyMPM) as the host. With the analogous ET mechanisms shown in Figure 1E and EL driving process shown in Figure 2B, the device was also satisfied in performance, achieving a very low V on of 2.5 V and high PE of 55 lm W −1 . It was one of the best ever developed TADF s-OLEDs with a TADF small molecular emitter (Kim et al., 2016;Liu et al., 2018b). On the topic of TADF polymer s-OLEDs (Nikolaenko et al., 2015;Nobuyasu et al., 2016;Zhang and Cheng, 2019), the interfacial exciplex host strategy was also confirmed as a wise choice. For instance, the application of interfacial exciplex TAPC/TmPyPB as the host of polymer PAPTC endowed the device with a very satisfied overall EL performance, with extremely low voltages of 2.50/2.91/3.51 V at a luminance of 1/100/1,000 cd m −2 , high peak EQE/PE of 14.9%/50.1 lm W −1 , and a wonderful slow roll-off rate, of J 50 of 63.16 cm −2 and L 50 of approx. fifteen thousand cd m −2 . It is distinctly superior to that of a control device using pure PAPTC EML. Especially, efficiency roll-off was enhanced nearly 3-fold. Further studied disclosed that, with respect to the pure PAPTC layer, the optimized structure of TAPC:PAPTC (20 wt.%)/TmPyPB not only gained a much higher PLQE (79.5 vs. 36.3%) by largely restraining aggregation-induced Dexter triplet-quenching , but also sharply reduced triplet population on PAPTC itself by 4-fold enhancement of its k RSIC to as high as 1.48 ×10 7 s −1 (Moon et al., 2017). These two aspects were combined to explain the promotions of the EL performance presented.

CONCLUDING REMARKS
Over the past several years, successful applications of an interfacial exciplex host in OLEDs were presented. Due to its merits in barrier-free exciton generation, "Well"-like exciton confinement and high ET efficiencies, simultaneous low voltage, high EQE/PE, low roll-off rate and even much higher device stability were representatively achieved, irrespective of dopant types. It should be strengthened that, compared to a traditional host, RISC up-conversion of the exciplex host renders the interfacial exciplex with an enhanced Förster ET process, guaranteeing low exciton density on emitters, thereby removing the risk of accelerating exciton-aggregation quenching. Moreover, local high charge/exciton density at the interfacial exciplex region is not a drawback but a special advantage that is not accessible in the bulk exciplex counterpart. For instance, a simple manipulation of the category of the major carrier (hole-rich or electron-rich or balanced) at the donor/accepter heterojunction, was found to induce exciplex recombination or Auger recombination or both (He et al., 2016). After doping with an appropriate guest, the novel EL driving mechanism is anticipated and may be significant. Unfortunately, the corresponding phenomena and mechanisms were not found in bulk exciplex counterparts. In addition, an interesting long-ranging coupling property of exciplex has been discovered recently but has still not been widely used in host applications (Al Attar and Monkman, 2016;Nakanotani et al., 2016). In this respect, interfacial exciplex architecture is also an ideal choice.
It must be noted that compared to bulk exciplex, relevant interfacial exciplex host applications in OLEDs are still limited and required. It is believed that if the structural advantages of interfacial exciplex are further utilized, there are more opportunities to construct OLEDs with a much higher performance and can also enrich our understanding of the related physical processes. Both of them are crucial from an OLED science and technology point of view.