Recent Advances of Interface Exciplex in Organic Light-Emitting Diodes

The interface exciplex system is a promising technology for reaching organic light-emitting diodes (OLEDs) with low turn-on voltages, high efficiencies and long lifetimes due to its unique virtue of barrier-free charge transport, well-confined recombination region, and thermally activated delayed fluorescence characteristics. In this review, we firstly illustrate the mechanism frameworks and superiorities of the interface exciplex system. We then summarize the primary applications of interface exciplex systems fabricated by doping and doping-free technologies. The operation mechanisms of these OLEDs are emphasized briefly. In addition, various novel strategies for further improving the performances of interface exciplex-based devices are demonstrated. We believe this review will give a promising perspective and attract researchers to further develop this technology in the future.


Introduction
As global climate warming and energy shortages become aggravated, developing lighting technology with energy conservation and environmental protection becomes a significant world issue [1]. Organic light-emitting diodes (OLEDs) have caught the attention of academia and industry due to their high performance, low power, and flexibility, which exhibit great potential for display and lighting applications [2][3][4][5][6]. In order to overcome the intrinsic low internal quantum efficiency (IQE) of fluorophores-based OLEDs, phosphorescent OLEDs are proposed as a promising technology due to their singlet and triplet harvesting properties [7]. These devices generate phosphorescence emission via the radiative transition process from T 1 excited state to ground state S 0 , which can be enhanced by the intersystem crossing (ISC) process from S 1 to T 1 [8][9][10]. However, phosphorescent dyes are mostly organometallic complexes that contain heavy metal elements of platinum and iridium, increasing the manufacturing costs of commercialization [11].
In 2012, thermally activated delayed fluorescence (TADF) materials were first reported by the Adachi group, and are regarded as new generation emitters by academia and industry [12,13]. Such materials have a small singlet-triplet energy gap (∆E ST ), so that triplet excitons can be upconverted into singlet excitons via reverse intersystem crossing (RISC) at room temperature, thereby generating delayed fluorescence [14][15][16]. Unlike TADF material based on intramolecular charge transfer, the TADF process in exciplex should result from intermolecular charge transfer, which can noticeably improve the utilization of triplet excitons and enhance device efficiency [17,18]. Therefore, exciplexes can be employed as efficient TADF emitters or TADF hosts for dopants with satisfied charge transport properties. In general, exciplex architecture can be constructed by two strategies: The emission peak of exciplex always exhibits a red shift compared with those of pure donor and acceptor [27][28][29][30][31][32][33][34][35][36][37], which can be expressed as follows [38]: where I D is the ionization potential of the donor, A A is the electron affinity of the acceptor, and E C is the coulombic attraction energy. Figure 1a shows the interface exciplex emission process in the EL process. Spin statistics demonstrate that the proportion of 1:3 of singlet excitons to triplet excitons can be generated by electron-hole recombination [39]. In general, a significant energy-level bias between the HOMO of the donor and the LUMO of the acceptor is more likely to provide a small ∆E ST [40]. As for exciplex, the HOMO and LUMO are separated between donor and acceptor molecules, resulting in a small energy gap between singlet level and triplet level [40,41]. A small ∆E ST is essential for exciplex to obtain TADF properties, because minor ∆E ST allows triplet excitons to be upconverted into singlet excitons under thermal excitation via RISC, leading to the efficient radiative transition of singlet excitons [42]. The RISC rate (k RISC ) in the interface exciplex system can be expressed by the Arrhenius equation [43]: where A is frequency factor, k is the boltzmann constant, and T is the temperature. It should be noted that higher k RISC can be achieved when ∆E ST is smaller than kT, which is approximately 26 meV at 300 K [43]. One must note that, owing to large energy level bias between HOMO of the donor and LUMO of the acceptor, the main recombination zone of charge carriers can be confined at the heterojunction [44]. Moreover, for achieving an efficient interface exciplex emitter, the triplet energy level (T 1 ) of exciplex should be lower than either the donor or the acceptor, so that the reverse energy transfer from exciplex to constituent molecular can be prevented [45][46][47][48].

Doping-Free OLEDs Based on Interface Exciplex
As for reaching high-performance OLEDs based on exciplex, effective exciplex systems are essential [58][59][60]. Meanwhile, bulk exciplex has been attested to be a useful emitter and host for dyes that relies on an efficient and balanced charge transportation that can be achieved by employing donor and acceptor materials with high charge mobilities [61][62][63][64][65][66], whereas multi-doping technology and precise control of doping concentration aggravate manufacturing complexity [67][68][69][70][71]. Compared with bulk exciplex-based devices, interface exciplex-based devices can be easily achieved using doping-free

Interface Exciplex as Host
It should be briefly pointed out that the energy-loss pathways in the interface exciplex system can be attributed to the following: (i) owing to the narrow exciton distribution area of the interface exciplex, the concentration quenching effect of triplet excitons is inevitable, especially at high current density [49]; (ii) the RISC rate is always lower than the ISC rate in the interface exciplex system, leading to the non-radiative transition processes of triplet excitons [50][51][52]. It is generally believed that using interface exciplex as an energy transfer host for dye is an effective way to improve the utilization of exciplex triplet excitons. Because the energy transfer process from exciplex to dopant is competitive with the energyloss pathways [51], an efficient energy transfer process can effectively guard against the waste of interface exciplex triplet excitons. For serving as a host, the T 1 of the exciplex should be higher than that of the dopants to prevent reverse energy transfer. Furthermore, the emission spectrum of exciplex should overlap well with the absorption spectrum of the emitter to achieve efficient energy transfer. The expression of energy transfer efficiency (Φ ET ) is [52]: where k ex-g is the energy transfer rate from the host of exciplex to the emitter, k r is the radiative decay rate of exciplex, k nr is the non-radiative decay rate of exciplex, and τ is the delay time of exciplex. As can been seen, higher Φ ET can be reached with improved τ. The energy transfer process in OLEDs based on interface exciplex is shown in Figure 1b. For doping-free OLEDs, the emission layer (EML) and the interface of exciplex are separated by the interlayer so that the short-range Dexter excitation transfer (DET) from exciplex to EML will be hindered. The unutilized triplet excitons can be upconverted into singlet excitons via an effective RISC process, and then the long-range Förster resonance energy transfer (FRET) can be enhanced, thereby improving the energy transfer to the EML [53,54]. As for doped OLEDs, the energy transfer from interface exciplex to dopant is greatly affected by the doping concentration [46,55]. Increasing doping concentration can improve the device efficiency in a reasonable concentration range [56]. However, with doping concentration increasing, the distance between interface exciplex and dopant reduces so that more triplet excitons can transfer energy to the dopant via DET, resulting in the energy loss of triplet excitons [57]. Consequently, an accurate control doping level in the preparation process is essential for efficient and stable energy transfer.

Doping-Free OLEDs Based on Interface Exciplex
As for reaching high-performance OLEDs based on exciplex, effective exciplex systems are essential [58][59][60]. Meanwhile, bulk exciplex has been attested to be a useful emitter and host for dyes that relies on an efficient and balanced charge transportation that can be achieved by employing donor and acceptor materials with high charge mobilities [61][62][63][64][65][66], whereas multi-doping technology and precise control of doping concentration aggravate manufacturing complexity [67][68][69][70][71]. Compared with bulk exciplex-based devices, interface exciplex-based devices can be easily achieved using doping-free technology. In this case, the ultrathin phosphorescent layer sandwiched between the donor and acceptor layers has been widely used to further simplify the fabrication process of OLEDs based on interface exciplex.

Doping-Free Monochrome OLEDs
Monochromatic OLEDs can be achieved by individual interface exciplex or an additional emitter. Although it was mentioned in earlier reports that exciplex formations are unfavorable for the efficiency of devices [72], the EQEs of recently reported OLEDs based on exciplex emission exceeded 5% due to the excellent TADF property of exciplexes [73][74][75]. Hung et al. first reported a simplified yellow exciplex OLED in 2013. As shown in Figure 2a, by employing 4,4 ,4 -tri(N-carbazolyl)triphenylamine (TCTA) and 2,4,6-tris(3-(1H-pyrazol-1-yl)phenyl)-1,3,5-triazine (3P-T2T) as donor and acceptor, respectively, a bilayer structure EML is established, as shown in Figure 2b. Meanwhile, TCTA and 3P-T2T act as a hole transport layer (HTL) and electron transport layer (ETL), respectively [76]. Owing to the excellent charge transport mobilities of the two materials, charge carriers of holes and electrons can reach the donor/acceptor interface without limitation, leading to efficient exciton recombination [77]. The resulting device achieves a maximum CE, PE, and EQE of 22.5 cd/A, 23.6 lm/W, and 7.7%, respectively, along with a low turn-on voltage of 2 V. Because exciplex excitons are confined around the interface of the bilayer because of the relative higher T 1 s of the donor and acceptor, efficient triplet exciton harvest can occur via the RICS process, which is the principal cause of the high performance of the device. This work provides a simplified model of OLEDs based on interface exciplex, which suggests that the interface exciplex could be formed by the befitting combination of donor-acceptor pairs. technology. In this case, the ultrathin phosphorescent layer sandwiched between the donor and acceptor layers has been widely used to further simplify the fabrication process of OLEDs based on interface exciplex.

Doping-Free Monochrome OLEDs
Monochromatic OLEDs can be achieved by individual interface exciplex or an additional emitter. Although it was mentioned in earlier reports that exciplex formations are unfavorable for the efficiency of devices [72], the EQEs of recently reported OLEDs based on exciplex emission exceeded 5% due to the excellent TADF property of exciplexes [73][74][75]. Hung et al. first reported a simplified yellow exciplex OLED in 2013. As shown in Figure 2a, by employing 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) and 2,4,6-tris(3-(1H-pyrazol-1-yl)phenyl)-1,3,5-triazine (3P-T2T) as donor and acceptor, respectively, a bilayer structure EML is established, as shown in Figure 2b. Meanwhile, TCTA and 3P-T2T act as a hole transport layer (HTL) and electron transport layer (ETL), respectively [76]. Owing to the excellent charge transport mobilities of the two materials, charge carriers of holes and electrons can reach the donor/acceptor interface without limitation, leading to efficient exciton recombination [77]. The resulting device achieves a maximum CE, PE, and EQE of 22.5 cd/A, 23.6 lm/W, and 7.7%, respectively, along with a low turn-on voltage of 2 V. Because exciplex excitons are confined around the interface of the bilayer because of the relative higher T1s of the donor and acceptor, efficient triplet exciton harvest can occur via the RICS process, which is the principal cause of the high performance of the device. This work provides a simplified model of OLEDs based on interface exciplex, which suggests that the interface exciplex could be formed by the befitting combination of donor-acceptor pairs. Furthermore, combining interface exciplex with ultrathin layer technology is another promising approach to achieving doping-free OLED [78]. By using complete energy transfer processes, which depend on the high overlapping between the absorption spectra of dyes and the exciplexes emission spectra, efficient utilization of excitons can be anticipated in ultrathin-layer-based devices [79,80].
In 2017, Xu et al. developed a green-emitting phosphorescent OLED based on an ultrathin emission layer (UEML) of 0.8 nm thick Bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) (Ir(ppy)2acac). By inserting the UMEL into the interface of 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-ylphenyl)benzene (TmPyPB), a maximum EQE of 17.97% and CE of 66.2 cd/A were shown in the device. The PL spectra of constituent molecules and exciplex are shown in Figure 3. By further introducing a charge-generating unit (CGU) of Bathophenanthroline (Bphen): LiNH2/HAT-CN, a highly efficient green OLED based on the tandem structure can be constructed, achieving an increased CE of 135.74 cd/A and EQE of 36.85%. The superior performance can be attributed to the following: (i) exciplex formed at the TAPC/TmPyPB Furthermore, combining interface exciplex with ultrathin layer technology is another promising approach to achieving doping-free OLED [78]. By using complete energy transfer processes, which depend on the high overlapping between the absorption spectra of dyes and the exciplexes emission spectra, efficient utilization of excitons can be anticipated in ultrathin-layer-based devices [79,80].
In 2017, Xu et al. developed a green-emitting phosphorescent OLED based on an ultrathin emission layer (UEML) of 0.8 nm thick Bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato) iridium(III) (Ir(ppy) 2 acac). By inserting the UMEL into the interface of 1,1-Bis[(di-4tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-ylphenyl)benzene (TmPyPB), a maximum EQE of 17.97% and CE of 66.2 cd/A were shown in the device. The PL spectra of constituent molecules and exciplex are shown in Figure 3. By further introducing a charge-generating unit (CGU) of Bathophenanthroline (Bphen): LiNH 2 /HAT-CN, a highly efficient green OLED based on the tandem structure can be constructed, achieving an increased CE of 135.74 cd/A and EQE of 36.85%. The superior performance can be attributed to the following: (i) exciplex formed at the TAPC/TmPyPB interface can transfer energy to the ultrathin layer efficiently because the PL spectrum of exciplex overlaps well with the absorption spectrum of the ultrathin layer; (ii) thanks to the higher T 1 s of TAPC and TmPyPB compared to exciplex, the reverse energy transfer from exciplex to the constructed molecules is well blocked, resulting in a sparkling green emission [81]. This OLED achieves efficient exciton harvesting by employing an interface exciplex and a tandem structure, which offers a basis for realizing efficient and simple OLEDs.
Micromachines 2022, 13, x FOR PEER REVIEW interface can transfer energy to the ultrathin layer efficiently because the PL spectr exciplex overlaps well with the absorption spectrum of the ultrathin layer; (ii) tha the higher T1s of TAPC and TmPyPB compared to exciplex, the reverse energy tr from exciplex to the constructed molecules is well blocked, resulting in a sparkling emission [81]. This OLED achieves efficient exciton harvesting by employing an in exciplex and a tandem structure, which offers a basis for realizing efficient and s OLEDs.

Doping-Free WOLEDs
Generally, interface-exciplex WOLEDs with a doping-free structure can be ach by employing complementary emissions (blue and orange) or trichromatic emi (blue, green, red) [82,83]. It is well-known that the health lighting source should high-color rendering index (CRI), which is easily improved by introducing an in exciplex emission due to its broad spectrum composition [84,85].
Sych et al. fabricated an A/D-A/D type WOLEDs by using doping-free techn with sandwich structures of m-MTDATA/pCzPPQ or mCzPPQ/PO-T2T, whe MTDATA is 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine, pCzPPQ i (4-Phenylquinolin-2-yl)phenyl)-9H-carbazole, mCzPPQ is 9-(3-(4-Phenylquin yl)phenyl)-9H-carbazole, and PO-T2T is 2,4,6-tris[3-(diphenylphosphinyl)phenyl] triazine [86]. Meanwhile, D-A-type materials of pCzPPQ and mCzPPQ, which are t mers of 9-phenylcarbazole and quinolone, are synthesized by Friedlander conden and then the Ullmann cross-coupling reaction, as shown in Figure 4a. Therefore, blu orange emissions can be formed at the dual interfaces of m-MTDATA/pCzPPQ (mC and pCzPPQ (mCzPPQ)/PO-T2T without extra donor layers, as shown in Figure 4 thermore, color temperature can be easily recorded by modifying the thickness of A functional system. The optimized all-exciplex-based device achieves a maximum of 3.15% and a high CRI of 76. Recently, Wei et al. adopted a novel approach to con an all-exciplex-based WOLEDs where white emission were formed from the same interface by parallelly depositing acceptor materials of TPBi and PO-T2T on the sam nor layer of TAPC in a vertical direction. The device contains a unique interface ex architecture of TAPC (40 nm)/ TBPi (10 nm) PO-T2T (10 nm) , achieving a maximum CE of 3.17 cd/A with CRI of 71. By using this technology, simplified WOLEDs can be constructed by ex emission without elaborate molecule design engineering, leading to a reduced pr tion complexity of WOLEDs [87].

Doping-Free WOLEDs
Generally, interface-exciplex WOLEDs with a doping-free structure can be achieved by employing complementary emissions (blue and orange) or trichromatic emissions (blue, green, red) [82,83]. It is well-known that the health lighting source should have a highcolor rendering index (CRI), which is easily improved by introducing an interface exciplex emission due to its broad spectrum composition [84,85].
Furthermore, the same group proposed another strategy to fabricate WOLEDs constructed by an exciplex/electroplex system and ultrathin emission [92]. The blue emission is generated by the combination of exciplex and electrople ing to broad EL spectra. The TAPC/TmPyPB interface exciplex and electrople peaks of 425 nm and 468 nm simultaneously under electric excitation. Therefore, a b peak width at half maximum of the blue emissions can be furnished. As shown in The further improvements of efficiency of doping-free WOLEDs can be reached by introducing energy transfer processes from exciplexes to additional emission layers [88]. By employing high energy-state interface exciplex of 26DCzPPy/PO-T2T, Ying et al. realized high-efficiency doping-free WOLEDs using UEML. The main architectures of those WOLEDs are Ir(tptpy) 2 acac (0.1 nm)/26DczPPy (x nm)/FIrPic (0.3 nm)/PO-T2T, where Ir(tptpy) 2 acac is Iridium(III) bis(4-(4-t-butyl-phenyl)thieno [3,2-c]pyridinato-N,C2)acetylace tonate, bipolar 26DCzPPy is 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine, and FIrPic is Iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2 ]picolinate [89]. Under the scenario without the 26DczPPy layer, a weak blue emission and an intense orange emission will be generated from the FIrPic and Ir(tptpy) 2 acac, respectively, because the Ir(tptpy) 2 acac harvests most of the excitons of the low energy-state TCTA/PO-T2T interface exciplex. As the thickness of 26DCzPPy increases to 2 nm, the TCTA/PO-T2T exciplex formation is inhibited by the high energy-state 26DCzPPy/PO-T2T interface exciplex. The excitons energy of exciplex can be effectively transferred to Ir(tptpy) 2 acac and FIrPic, resulting in an EQE of 19.7%. Furthermore, devices achieve the correlated color temperature (CCT) with an adjustable range of 2878~9895 K by changing the thickness of the 26DCzPPy layer, which suggests that the energy transfer from interface exciplex to EMLs can be easily controlled.
However, full phosphorescent emitters always suffer from serious efficiency roll-off, although high efficiencies can be realized [90]. Fluorescence/phosphorescence hybrid WOLEDs have been proven to be a qualified approach to solve the problem. By introducing blue interface exciplex emission, Liu's group realized doping-free WOLED without using additional blue emitters [91]. The m-MTDATA/3-(4-Biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ) exciplex generates blue emissions and can excites orange phosphor of bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl-1Hbenzoimidazole-N,C3)iridium (acetylacetonate) [(fbi) 2 Ir(acac)] due to the higher T 1 . The energy transfer process from exciplex to the (fbi) 2 Ir(acac) can be adjusted by controlling the thickness of m-MTDATA in the m-MTDATA/TAZ interface exciplex forming system, and the balance between orange and blue emissions is achieved with 6 nm thick m-MTDATA. The resulting device achieves CRI of 81, which is one of the highest CRIs of the WOLEDs based on a single-emitter structure. By replacing (fbi) 2 Ir(acac) with the yellow phosphor of Ir(dmppy) 2 (dpp), the device achieves an EQE of 9.9%. Moreover, three-color WOLED is constructed by further employing additional red phosphor, achieving CRI and CCT of 85 and 2376 K, respectively. Furthermore, the same group proposed another strategy to fabricate hybrid WOLEDs constructed by an exciplex/electroplex system and ultrathin emission layers [92]. The blue emission is generated by the combination of exciplex and electroplex, leading to broad EL spectra. The TAPC/TmPyPB interface exciplex and electroplex show peaks of 425 nm and 468 nm simultaneously under electric excitation. Therefore, a broader peak width at half maximum of the blue emissions can be furnished. As shown in Figure 5a, blue emissions come from singlets of the exciplex/electroplex system due to the short diffusion ratio of singlets. Meanwhile, red and yellow emissions can be attributed to multiple factors: (i) the emission of the exciplex/electroplex system itself; (ii) the diffusion processes of triplet excitons from the exciplex/electroplex system to EMLs; (iii) direct charge recombination due to the charge tunneling effect. Consequently, the hybrid WOLED obtains high efficiency by manipulating the exciplex/electroplex system, achieving EQE and CRI of 15.1% and 92.1, respectively (shown in Figure 5b).
Micromachines 2022, 13, x FOR PEER REVIEW 5a, blue emissions come from singlets of the exciplex/electroplex system due to the diffusion ratio of singlets. Meanwhile, red and yellow emissions can be attributed to tiple factors: (i) the emission of the exciplex/electroplex system itself; (ii) the diffusio cesses of triplet excitons from the exciplex/electroplex system to EMLs; (iii) direct c recombination due to the charge tunneling effect. Consequently, the hybrid WOLE tains high efficiency by manipulating the exciplex/electroplex system, achieving EQ CRI of 15.1% and 92.1, respectively (shown in Figure 5b).   By combining ultrathin emission layers with interface exciplex, Xu et al. developed tandem WOLEDs, as shown in Figure 6 [93]. The yellow emission is generated by employing the phosphor of (acetylacetonato)bis[2-(thieno[3,2-c]pyridin-4-yl) phenyl]iridium(III) (PO-01) as an ultrathin emission layer to harvest the singlet and triplet excitons formed by TAPC/TmPyPB exciplex, and the reverse energy transfer from PO-01 to exciplex will be blocked because the T 1 of PO-01 is lower than that of exciplex. Similarly, this mechanism is also applied to the blue emission unit, as shown in Figure 7. By connecting two emission units with the CGU of Bphen: LiNH 2 /HAT-CN, enhanced injections of electrons and holes into the emission units are achieved, with an EQE and CE of 18.59% and 41.5 cd/A, respectively, along with the turn-on voltage of 6.89 V.
Micromachines 2022, 13, x FOR PEER REVIEW 7 5a, blue emissions come from singlets of the exciplex/electroplex system due to the s diffusion ratio of singlets. Meanwhile, red and yellow emissions can be attributed to tiple factors: (i) the emission of the exciplex/electroplex system itself; (ii) the diffusion cesses of triplet excitons from the exciplex/electroplex system to EMLs; (iii) direct ch recombination due to the charge tunneling effect. Consequently, the hybrid WOLED tains high efficiency by manipulating the exciplex/electroplex system, achieving EQE CRI of 15.1% and 92.1, respectively (shown in Figure 5b).

Doped OLEDs Based on Interface Exciplex
The doping concentration is a significant factor determining doped device efficiency [94,95]. Within a reasonable concentration range, the device efficiency increases with the increase of the doping concentration [57]. However, it should be noted that a severe efficiency decline occurs under high doping concentrations owing to the increased possibility of direct electron-hole recombination on the dopant, which can be well inhibited by introducing an interface exciplex system. Thanks to the TADF properties of exciplex, the DET from interface exciplex to dopant will be inhibited, leading to weakening triplet exciton loss in the host-guest system.

Doped Monochrome OLEDs
Understanding exciton behavior is beneficial for manipulating excitons due to the different energy transfer paths between singlet and triplet excitons. For instance, the FRET from S1 of exciplex to dopant can be enhanced by the RISC process, weakening the DET between triplet excitons by augmenting the energy transfer distance; this means excitons become easy to manipulate by using interface exciplex. In the second half of this section, we will introduce several novel methods, including ternary exciplexes and applications of phosphor/TADF sensitization effects in interface exciplex devices Kim et al. fabricated green-emitting phosphorescent OLEDs by doping fac-tris(2-phenyl-pyridine)iridium [Ir(ppy)3] into the donor layer of an N,N′-dicarbazolyl-4-4′-biphenyl (CBP)/ bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM) interface exciplex system [56]. The device efficiency increased with the doping concentration of Ir(ppy)3, which can be ascribed to the enhanced energy transfer efficiency from CBP/B3PYMPM exciplex to the Ir(ppy)3. The maximum EQE of the device was 20.1% with an Ir(ppy)3 doping concentration of 6%. By inserting an undoped CBP layer between the emission layer and the acceptor layer, the efficiency reduces as the thickness of the pure CBP layer rises, suggesting that the exciplex triplet excitons transfer is hindered, which can be explained by the fact that triplet exciton transferring is a short-range process.
By comparing the performance between an interface exciplex device and a bulk-exciplex device, Wang et al. drew the following conclusions: (i) the interface exciplex-based device shows a turn-on voltage of 2.36 V, which is lower than that of the bulk exciplexbased device because of the barrier-free electron-hole recombination; (ii) the interface exciplex-based device reaches a high PE of 97.2 lm/W, which is almost three times than that of the bulk exciplex-based device due to the elimination of charge traps of dye [96]. By employing a homemade material of 9-(4′-(4,6-diphenyl1,3,5-triazin-2-yl)-[1,1′-biphenyl]-

Doped OLEDs Based on Interface Exciplex
The doping concentration is a significant factor determining doped device efficiency [94,95]. Within a reasonable concentration range, the device efficiency increases with the increase of the doping concentration [57]. However, it should be noted that a severe efficiency decline occurs under high doping concentrations owing to the increased possibility of direct electron-hole recombination on the dopant, which can be well inhibited by introducing an interface exciplex system. Thanks to the TADF properties of exciplex, the DET from interface exciplex to dopant will be inhibited, leading to weakening triplet exciton loss in the host-guest system.

Doped Monochrome OLEDs
Understanding exciton behavior is beneficial for manipulating excitons due to the different energy transfer paths between singlet and triplet excitons. For instance, the FRET from S 1 of exciplex to dopant can be enhanced by the RISC process, weakening the DET between triplet excitons by augmenting the energy transfer distance; this means excitons become easy to manipulate by using interface exciplex. In the second half of this section, we will introduce several novel methods, including ternary exciplexes and applications of phosphor/TADF sensitization effects in interface exciplex devices Kim et al. fabricated green-emitting phosphorescent OLEDs by doping fac-tris(2phenyl-pyridine)iridium [Ir(ppy) 3 ] into the donor layer of an N,N -dicarbazolyl-4-4biphenyl (CBP)/bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM) interface exciplex system [56]. The device efficiency increased with the doping concentration of Ir(ppy) 3 , which can be ascribed to the enhanced energy transfer efficiency from CBP/B3PYMPM exciplex to the Ir(ppy) 3 . The maximum EQE of the device was 20.1% with an Ir(ppy) 3 doping concentration of 6%. By inserting an undoped CBP layer between the emission layer and the acceptor layer, the efficiency reduces as the thickness of the pure CBP layer rises, suggesting that the exciplex triplet excitons transfer is hindered, which can be explained by the fact that triplet exciton transferring is a short-range process.
Generally, the unrecombined holes accumulated at the donor/acceptor interface could be a source of the degradation of the device [97]. By using a 5 nm thick emission layer, Colella et al. fabricated a red OLED by interface exciplex architecture of 26DCzPPy: 4% Ir(dmpq) 2 acac (5 nm)/PO-T2T, where Ir(dmpq) 2 acac is bis(2-(3,5-dimethylphenyl)quinoline-C2,N )(acetylacetonato)-iridium(III), achieving a maximum EQE of 28.6%, along with increased lifetime [98]. The key reason for the superior performance is that the carrier recombination region is restrained to the 5 nm thick EML due to efficient Förster and Dexter energy transfer, resulting in a reduced concentration of excitons piled up at the heterojunction. By comparison, the devices without TADF systems exhibit declined performances and additional emissions from host materials at high voltage, which can be attributed to the broader exciton recombination regions in those devices.
In the course of application studies of OLEDs, reaching high-efficiency blue phosphorescent OLEDs with low driving voltages is still a challenge because it is hard to develop host materials with low singlet energy and high triplet energy [99][100][101][102][103]. Kido et al. demonstrated high performance blue-emitting phosphorescent OLEDs by doping FIrPic into the acceptor of 5 ,5 -sulfonyl-di-1,1 :3 ,1 -terphenyl (BTPS), giving a PE of 50.1 lm/W and turn-on voltage of 2.5 V [77]. The excitons behavior is also studied by inserting an undoped BTPS layer (0~10 nm) into the TAPC/BTPS interface. The singlet excitons of TAPC/BTPS exciplex can be harvested by FIrPic via FRET when the thickness of the undoped BTPS is less than 5 nm, which is the main reason for high efficiency. In contrast, as the thickness of undoped BTPS increases to 10 nm, the device shows an efficiency decline, owing to the decrease of FRET efficiency.
Full fluorescent OLEDs are reported for pursuing low-cost and high-stability OLEDs [40,104]. However, fluorescent OLEDs based on conventional hosts always suffer from low EQEs owing to efficient DET from the T 1 of hosts to the T 1 of fluorophores [105,106]. The employment of interface exciplex is proposed to break this bottleneck because the DET from exciplex to fluorescent dopant is expected to be restricted due to the TADF properties of the interface exciplex. Moreover, direct exciton recombination on fluorescent dopants could be suppressed in interface exciplex systems, leading to decreased non-radiative transitions of triplet excitons.
Zhao et al. designed a fluorescent OLED by doping (5,6,11,12)-tetraphenyl-naphthacene (rubrene) into the interface exciplex system [55]. The absorption spectrum of rubrene overlaps well with the emission spectrum of the TCTA/3P-T2T exciplex, leading to an enhanced FRET process from exciplex to rubrene. Thanks to the TADF properties of the TCTA/3P-T2T interface exciplex, an efficient fluorescent OLED is realized, achieving an EQE of 8.1%. It is noteworthy that the doping concentration of rubrene in the device is as low as 1.5%. Thus, the DET process from triplet excitons of exciplex to triplet excitons of rubrene is inhibited, significantly reducing the waste of triplet excitons. By doping TADF polymer emitter PAPTC into the donor layer, Lin et al. developed a green-emitting OLED with the interface exciplex of TAPC/TmPyPB, obtaining a higher PL quantum yield than pure PAPTC, which can be attributed to the restrained aggregation effects of PAPTC polymer chains because the doping into TAPC provides a broad distribution area for PAPTC. Furthermore, thanks to the high k RISC of 1.48 × 10 7 s −1 of the TAPC: PAPTC/TmPyPB exciplex system, a high-efficiency polymer electroluminescence OLED is constructed, achieving an EQE of 14.7% [107].
that of the TPA-3/PO-T2T exciplex. This energy transfer will be more effective because the solution process method induces stronger intermolecular interactions. As a result, an efficient exciplexes-based OLED is developed, achieving an EQE of 24%, which is one of the best values among exciplex-emission devices. Furthermore, the ternary exciplexes device shows a lower efficiency roll-off than the binary TPA-3/PO-T2T exciplex-based device because of the additional RISC path. Various OLEDs based on interface exciplex have been introduced in the above paragraphs. It is worth noting that the donor and acceptor molecules that form the interface exciplex are aggregated in a narrow interface area in the D/A structure, resulting in acute intermolecular interactions and a concentration-quenching effect of triplet excitons [118,119]. Recently, a novel strategy that was constructed by employing spatially separated electron-hole pairs has attracted the attention of researchers, and distributes excitons in a broad region to reduce exciton concentration. Adachi et al. first proposed the donorspacer-acceptor (D-S-A) structure with a spatially separated donor and acceptor [120]. In a device with a substructure of m-MTDATA/3,3-di(9H-carbazol-9-yl)biphenyl(mCBP)/2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), they designed a bipolar material of mCBP that acts as the electron acceptor and electron donor simultaneously to form exciplexes with m-MTDATA and T2T, respectively. Because the molecules of donor and acceptor are directly excited, a charge transfer between D δ+ /S δ− /A and D/S δ+ /A δ− occurs to form exciplexes of D δ+ /S/A δ− by cascade electron transfer between the three molecules. Here, the up-conversion of 3 (D δ+ A δ− )* into 1 (D δ+ A δ− )* occurs via RISC due to the TADF Figure 11. Diagram of energy transfer process of ternary exciplexes.
Various OLEDs based on interface exciplex have been introduced in the above paragraphs. It is worth noting that the donor and acceptor molecules that form the interface exciplex are aggregated in a narrow interface area in the D/A structure, resulting in acute intermolecular interactions and a concentration-quenching effect of triplet excitons [118,119]. Recently, a novel strategy that was constructed by employing spatially separated electronhole pairs has attracted the attention of researchers, and distributes excitons in a broad region to reduce exciton concentration. Adachi et al. first proposed the donor-spacer-acceptor (D-S-A) structure with a spatially separated donor and acceptor [120]. In a device with a substructure of m-MTDATA/3,3-di(9H-carbazol-9-yl)biphenyl(mCBP)/2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), they designed a bipolar material of mCBP that acts as the electron acceptor and electron donor simultaneously to form exciplexes with m-MTDATA and T2T, respectively. Because the molecules of donor and acceptor are directly excited, a charge transfer between D δ+ /S δ− /A and D/S δ+ /A δ− occurs to form exciplexes of D δ+ /S/A δ− by cascade electron transfer between the three molecules. Here, the up-conversion of 3 (D δ+ A δ− )* into 1 (D δ+ A δ− )* occurs via RISC due to the TADF properties of exciplex. It is noteworthy that both the T 1 of m-MTDATA and T2T is lower than that of mCBP, which prevents the energy transfer process between exciplex and mCBP. By changing the thickness of mCBP, the exciton energy and radiative decay lifetime of exciplex can be fine-tuned. Furthermore, long-range coupling CT excited states under electrical excitation are examined, and can be explained by the following expression: D + + A − → 1 (D δ+ A δ− )* → hν ex + D + A. Finally, the feasibility of energy transfer from exciplex to fluorescent dopant is confirmed without direct charge injection into the dopant.
In 2018, Su et al. reported efficient fluorescent OLEDs with high efficiency by adopting a spatially separated exciplex system [121]. Devices with 1-4 nm-thick spacers of mCP showed a higher brightness, increased efficiencies, and restrained efficiency roll-offs than the device without spacer layers. By further doping 1% DBP into TAPC, the device with 3 nm-thick mCP reached the highest EQE of 14.8%, which can be ascribed to the following: (i) the spacer layer provides a broad exciton distribution region that inhibits exciton quenching; (ii) the direct charge trapping is suppressed due to separated hole-electron pairs; (iii) the FRET efficiency from exciplex to DBP is enhanced by using a separated exciplex system. Pu et al. further studied the mechanism of long-range coupling electron-hole pairs. What is impressive is that, although the thickness of the spacer layer of (9,10-bis(3,5dimethoxyphenyl)anthracene) (DMA) increases to 70 nm in the device they studied, the weak long-range coupling CT excited states can still exist, which is beyond expectations and worth further investigate [122]. The EL performances of monochrome OLEDs based on interface exciplex are summarized in Table 1.

Doped WOLEDs
Combining interface exciplex with doping technology, doped WOLEDs can simultaneously reach high efficiencies and low driving voltages while maintaining simple device structures. The EL performances of WOLEDs based on interface exciplex are summarized in Table 2.
Recently, our group developed highly efficient phosphorescence WOLEDs by codoping a complementary emitter into an acceptor of 4,6-bis(3,5-di(pyridin-4-yl)phenyl)-2-phenylpyrimidine (B4PyPPM) (shown in Figure 12a) [123]. A novel interfacial exciplex system 26DCzPPy/B4PyPPM was used to build white OLEDs based on the single-emissionlayer structure, achieving efficiencies of 101.9 lm/W and 81.1 cd/A, along with a low turn-on voltage of 2.4 V, which is one of the best values in WOLEDs based on simplified architecture, as shown in Figure 12b. The enhanced performance could be attributed to the reduced energy loss of triplet excitons in the host through efficient FRET between the exciplex and the dopants, and the DET energy transfer process between the blue and orange dopants is suppressed due to the low doping concentration of orange dye. Additionally, a single-emission-layer white OLED based on an interface exciplex system only requires the doping of three materials simultaneously, with no need for high-performance host materials, providing a promising method to design simplified white OLEDs with high performance for commercial solid-state lighting and display.
Gao et al. reported a high-efficiency and color-stable hybrid WOLED, as shown in Figure 13 [124]. By investigating PL and charge transport properties, the author demonstrates direct electron-hole recombination on a blue TADF fluorophore of 4,5-bis(carbazol-9yl)-1,2-dicyanobenzene (2CzPN), which was well restrained, so that the blue emission from 2CzPN was mainly generated from the energy transfer from the mCP/B3PYMPM interface exciplex. On the other hand, the yellow phosphor Iridium(III) bis(4-(4-tert-butylphenyl) thieno [3,2-c]pyridinato-N,C2 ) acetylacetonate (PO-01-TB) inserted into mCP as the complementary ultrathin emission layer was excited by the following two main paths: (i) the HOMO level of PO-01-TB is shallower than that of TCTA, which means that PO-01-TB is a trapping site for holes, causing direct electron-hole recombination on PO-01-TB; (ii) the T 1 of 2CzPN is higher than that of PO-01-TB, leading to the DET from 2CzPN to PO-01-TB.
Because the exciplex excitons can be efficiently utilized, a warm white device is realized with an EQE of 22.3%. sion-layer structure, achieving efficiencies of 101.9 lm/W and 81.1 cd/A, along with a low turn-on voltage of 2.4 V, which is one of the best values in WOLEDs based on simplified architecture, as shown in Figure 12b. The enhanced performance could be attributed to the reduced energy loss of triplet excitons in the host through efficient FRET between the exciplex and the dopants, and the DET energy transfer process between the blue and orange dopants is suppressed due to the low doping concentration of orange dye. Additionally, a single-emission-layer white OLED based on an interface exciplex system only requires the doping of three materials simultaneously, with no need for high-performance host materials, providing a promising method to design simplified white OLEDs with high performance for commercial solid-state lighting and display. Gao et al. reported a high-efficiency and color-stable hybrid WOLED, as shown in Figure 13 [124]. By investigating PL and charge transport properties, the author demonstrates direct electron-hole recombination on a blue TADF fluorophore of 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN), which was well restrained, so that the blue emission from 2CzPN was mainly generated from the energy transfer from the mCP/B3PYMPM interface exciplex. On the other hand, the yellow phosphor Iridium(III) bis(4-(4-tert-butylphenyl) thieno [3,2-c]pyridinato-N,C2′) acetylacetonate (PO-01-TB) inserted into mCP as the complementary ultrathin emission layer was excited by the following two main paths: (i) the HOMO level of PO-01-TB is shallower than that of TCTA, which means that PO-01-TB is a trapping site for holes, causing direct electron-hole recombination on PO-01-TB; (ii) the T1 of 2CzPN is higher than that of PO-01-TB, leading to the DET from 2CzPN to PO-01-TB. Because the exciplex excitons can be efficiently utilized, a warm white device is realized with an EQE of 22.3%.
By manipulating excitons with CDBP/B4PyPPM interface exciplex, Wang et al. constructed color-stable solution-processed WOLED [125]. The triplet excitons can be well confined to the exciplex because of the relatively low T1 of 4,4′-bis(9-carbazolyl)-2,2′-dimethylbip-henyl (CDBP)/B4PyPPM exciplex, leading to efficient up-conversion from triplet excitons to singlet excitons via RISC to enhance blue emission. The orange-red emissions are generated from the TADF emitter of 2-[4-(diphenylamino) phenyl]-10,10-dioxide-9H-thioxanthen-9-one (TXO-TPA). The energy transfer process from exciplex to TXO-TPA is shown in Figure 14. Note that the CDBP/B4PyPPM exciplex and TXO-TAP have no direct contact because of the 3.5 nm-thick CDBP, resulting in the DET process from blue exciplex to TXO-TPA being well suppressed. Therefore, orange-red emission is enhanced by transferring energy from the blue TADF exciplex to TXO-TPA via FRET, which can be fine-tuned by changing the thickness of the CDBP. By contrast, bulk exciplex of the CDBP: B4PyPP-based device suffers from the shift of the exciton recombination region By manipulating excitons with CDBP/B4PyPPM interface exciplex, Wang et al. constructed color-stable solution-processed WOLED [125]. The triplet excitons can be well confined to the exciplex because of the relatively low T 1 of 4,4 -bis(9-carbazolyl)-2,2dimethylbip-henyl (CDBP)/B4PyPPM exciplex, leading to efficient up-conversion from triplet excitons to singlet excitons via RISC to enhance blue emission. The orange-red emissions are generated from the TADF emitter of 2-[4-(diphenylamino) phenyl]-10,10dioxide-9H-thioxanthen-9-one (TXO-TPA). The energy transfer process from exciplex to TXO-TPA is shown in Figure 14. Note that the CDBP/B4PyPPM exciplex and TXO-TAP have no direct contact because of the 3.5 nm-thick CDBP, resulting in the DET process from blue exciplex to TXO-TPA being well suppressed. Therefore, orange-red emission is enhanced by transferring energy from the blue TADF exciplex to TXO-TPA via FRET, which can be fine-tuned by changing the thickness of the CDBP. By contrast, bulk exciplex of the CDBP: B4PyPP-based device suffers from the shift of the exciton recombination region with increasing voltage or current density due to unfavorable exciton management, leading to unstable spectra. The continuous spectra superimposed by blue emission and orange-red emission spectrum covers the entire visible spectral region, achieving a CRI of 85 in the resulting device, as shown in Figure 15. with increasing voltage or current density due to unfavorable exciton management, leading to unstable spectra. The continuous spectra superimposed by blue emission and orange-red emission spectrum covers the entire visible spectral region, achieving a CRI of 85 in the resulting device, as shown in Figure 15. Figure 13. Schematic diagram of the energy level of hybrid WOLED [124]. Figure 13. Schematic diagram of the energy level of hybrid WOLED [124].     Figure 15.

Conclusions and Outlook
In this review paper, the basic concepts and applications of interface exciplex are demonstrated. Interface exciplex is the charge transfer state formed by donor and acceptor molecules at the donor/acceptor interface, which can be used as an emitter or energy transfer host. The success of interface exciplex as an emitter relies on an efficient RISC process, which improves the fluorescence quantum yield. Therefore, the EQE of OLED based on exciplex emission can exceed 5%. Moreover, introducing the energy transfer process between exciplex and dopant is an effective approach to further improve the efficiency of interface exciplex-based devices, achieving low turn-on voltage and inhibiting direct electron-hole recombination on the dopant. However, the interface exciplex system is still faced with problems such as serious triplet exciton quenching owing to a narrow distribution area and unfavorable fluorescence quantum efficiency because of low RISC rate. Despite providing a broad exciton recombination zone by adopting the long-range coupling CT excited states strategy, the EQE of a device with a D/S/A structure is far below a device with D/A interface exciplex. For now, improving the efficiency of the RISC is the most feasible and practical solution to those problems. Therefore, further studies are necessary to investigate the radiative transition process in interface exciplex, and novel interface exciplex systems with a high-fluorescence quantum yield still need to be developed to accelerate the commercialization process of interface exciplex-based devices.
Author Contributions: Writing, J.S., R.S. and P.C.; review and editing, C.C., W.Z. and E.Z.; validation, W.M. and Y.S. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.