Polarity- and molecular orbital-engineered host materials for stable and ecient blue thermally activated delayed uorescence

To utilize thermally activated delayed uorescence (TADF) technology for future displays, it is necessary to develop host materials that can harness the full potential of blue TADF emitters. We suggest an elaborative approach for designing host molecules for blue TADF devices with simultaneously improved eciency and stability. We signicantly enhanced the delayed uorescence quantum yield by engineering the molecular geometry, polarity, and excited-state dipole moment of host molecules based on 3′,5-di(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-carbonitrile. The engineerd hosts stabilized the charge-transfer excited states of TADF emitters, suppressed exciton quenching, and improved the charge balance in the emitting layer. Moreover, because the hosts are phosphine-oxide bond-free molecules, they are photochemically and electrochemically stable compared to bis[2-(diphenylphosphino)phenyl] ether oxide, the most popular high-polarity host. The devices employing the hosts exhibited a two-fold increase in external quantum eciency and a 37-fold increase in operation lifetime compared to control devices with the same TADF emitter. operation: i) high polarity stabilizes the CT excited state of the TADF emitter; ii) a relatively low excited-state dipole moment suppresses intermolecular CT-induced quenching of excitons; iii) electron-transporting character allows hole transport/electron transport mixed-host-like behavior in the emitting layer owing to the hole-transporting ability of the highly doped TADF emitter; and iv) P–O bond-free design improves photochemical stability. The novel hosts adopt asymmetric molecular structures and polar groups appropriately to provide a trade-off between high ground-state dipole moments and low excited-state dipole moments, while simultaneously modulating their molecular orbital energy levels. Our strategy for designing hosts with particular characteristics will promote the commercialization and success of TADF devices and technology. 3.5 mW with an excitation wavelength of 325 nm. The lms for the PL stability test were glass-encapsulated in a nitrogen-lled glove box after vacuum deposition. The angle-dependent PL was measured using a continuous wave laser (325 nm, Melles Griot). The incident angle of the excitation source was xed at 45°. The angle dependent PL spectra of P-polarized light were detected using charge-coupled device (MAYA 2000, Ocean Optics).


Introduction
In host-guest organic light-emitting diodes (OLEDs), the photophysical properties of the light-emitting molecules are strongly impacted by the host material, [1][2][3][4] Therefore, the host should be carefully considered to ful ll the intricate requirements of a given emitter. For example, to achieve highperformance OLEDs based on thermally activated delayed uorescent (TADF) emitters, polarity of the host materialis particularly important owing to the strong charge transfer (CT) character of the excited states in TADF emitters. Host materials with high ground-state polarity are capable of stabilizing the CT excited states of TADF emitters due to electrostatic interactions between the excited-state dipole moment of the TADF emitter and the ground-state dipole momentof the host material. [5][6][7] A high-polarity hosts can reduce the lowest excited-state singlet energy (S 1 ) of the TADF emitter; therefore, the energy gap between S 1 and the lowest excited-state triplet energy (T 1 ) can be reduced. The emission peaks of the TADF emitters are thus shifted to longer wavelengths and their reverse intersystem crossing rate (k rISC ) and delayed uorescence quantum yields (Φ DF ) are enhanced, which improves the maximum external quantum e ciency (EQE max ) of the TADF device. [8][9][10][11] Bis [2-(diphenylphosphino)phenyl]ether oxide (DPEPO) exhibits high polarity, making it a popular host for blue TADF emitters with several studies demonstrating the outstanding EQE max of DPEPO-based blue TADF device. [9][10][11] However, the band gap of DPEPO is too wide and its charge transporting ability is poor, which results in a high driving voltage, poor electrochemical stability, and relatively short device operation lifetime. Moreover, DPEPO has photochemically unstable phosphine-oxide (P-O) bonds which induce faster degradation. [12][13][14] Hence, no studies have reported long operation lifetimes of DPEPO-based TADF devices. On the other hand, several highly stable blue TADF devices have been obtained with stable hosts such as 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP) and 3',5-di(9H-carbazol-9-yl)-[1,1'-biphenyl]-3carbonitrile (mCBP-CN), yet at the expense of e ciency. [14][15][16] Herein, we introduce alternative host materials that are capable of stabilizing the CT excited-states of TADF emitters owing to their relatively high ground-state polarity, while simultaneously providing long OLED operation lifetimes owing to their narrower band gaps, better charge transporting ability, and greater material stability compared to P-O bond-based DPEPO. The novel host material-based blue TADF devices exhibit similar performance to those using DPEPO, and are as stable as those using mCBP-CN. The simultaneous achievement of high e ciency and stability of these high-polarity and P-O bond-free compounds makes them suitable alternative host materials for DPEPO-and mCBP-CN.

Results
Molecular asymmetry and polar groups introduce high polarity to P-O bond-free host materials The ability to stabilize the CT excited state of TADF emitters primarily depends on the geometry and electrostatic properties of the host materials. The dipole moment can serves as a convenient polarity scale because of its dominant contribution to the electrostatic interactions between the emitter and host molecules. With the aim of designing a P-O bond-free host with high polarity, we introduced asymmetry and polar groups into the molecular geometry of a leading molecule to control the dipole moment. As a leading molecule, we considered mCBP-CN (Figure 1), an electron-transporting host derivatized from mCBP by attaching a polar cyano (CN) group to one of the two phenyls. The asymmetrically attached CN group increased the ground-state dipole moment, µ GS , to 3.4 D (debye, where 1 D ≈ 3.34 x 10 -30 C·m).
However, its µ GS still much lower than that of DPEPO. By modifying the asymmetry and polar groups of mCBP-CN, we obtained four novel host materials, H1-H4, with improved µ GS values (Figure 2a, rst panel). H1 was obtained by modifying the bond position of one of the carbazole groups of mCBP-CN from meta to ortho to induce asymmetry,t hereby increasing µ GS to 4.18 D. H2 was obtained by attaching two additional CN groups to the second carbazole group of H1, which induced a much higher µ GS of 6.84 D. H3 was achieved by substituting the second carbazole group of H1 with a benzofuran group; the introduction of a heteroatom, oxygen, slightly improved the µ GS value (4.63 D) over that of H1. Finally, an additional CN-functionalization on the dibenzofuran group of H2 resulted in H4, which exhibited a much higher µ GS of 7.64 D. Notably, the µ GS values of H2 and H4 are close to the value of 8.05 D for DPEPO.
To experimentally compare the ground-state polarities of the host materials, we measured their retention times with high-performance liquid chromatography (HPLC). The elution order of solutes in HPLC is governed by polarity. 17 Therefore, HPLC measurements provide details about the polarity of materials. H1-H4 and DPEPO showed shorter HPLC-retention times than mCBP and mCBP-CN (Figure 2a, second panel), indicating that H1-H4 and DPEPO have higher ground-state polarities. DPEPO had the shortest HPLC-retention time, suggesting that it is still the most powerful host for enhancing TADF e ciency. Consequently, the HPLC-retention times can scale the examined host materials as well as the ground-state dipole moments does but they have an inverse tendency. Figure 2b shows the correlation between them. The correlation coe cient, R 2 is −0.75 when considering DPEPO and −0.88 without DPEPO.
To explore their suitability as host materials for TADF emitter molecules, we investigated whether the excited-state energies (S 1 and T 1 ) of H1-H4 would be high enough to con ne excitons within a given blue TADF emitter, 1PCTrz which has a typical structure of twisted intramolecular charge transfer (TICT) molecules ( Figure 2c). 18 Our calculations indicated that H1-H4 had su ciently higher S 1 and T 1 values ( Figure 2a, third panel) than the given TADF emitter. To verify the ground-state polarities of the host materials and investigate their effect on the photophysical properties of the TADF emitter, we compared photoluminescence (PL) spectra of thin lms composed of the given TADF emitter and host material.
According to the work of Lippert et al., 19 polar solvents can redshift the emission of TICT molecules which are popular TADF emitters. In addition, the polarity of the solvents changes the emission state of TICT molecules between a locally excited (LE) state and CT state by changing the twist angle between the electron accepting moiety (A) and electron donating one (D). 20 When fabricated with H1-H4 and DPEPO, 1PCTrz exhibited redshifted and broadened PL emission compared to that with mCBP and mCBP-CN, as shown in Figure 2d. The PL peak wavelengths and full-width at half maximum (FWHM) values for all the tested host:1PCTrz (host:1PCTrz) lms are summarized in Table 1.
High polarity hosts enhance the reverse intersystem crossing rate and delayed uorescence quantum yield The primary purpose of employing high-polarity hosts for TADF devices is to deduce their potential for stabilizing the CT excited states of TADF emitters, in the manner of DPEPO. The improved EQE max of DPEPO-based TADF devices originates from the enhanced reverse intersystem crossing rate, k rISC , and delayed uorescence quantum yield, Φ DF , of the TADF molecules owing to the high-polarity DPEPO matrix. Such enhancements are the origin of the improved EQE max of TADF devices employing DPEPO as a host. Based on the results of PL experiments in Figure 2d, we expected H1-H4 to have the same effect as DPEPO.
The k rISC values were calculated using the equations presented by Zeng et al. 21 Table 1 shows that 1PCTrz exhibited higher Φ DF and k rISC values when hosted in H1-H4 than in mCBP and mCBP-CN. This is ascribed to the high polarity of H1-H4, as indicated by the enhanced µ GS values. However, Φ DF did not monotonically increase with µ GS , even when comparing hosts with the same core structure. H2 and H4, which have µ GS values near 7 D exhibited lower Φ DF values than H1 and H3. It should be noted that the dipole moment of the host has two sides in relation with Φ DF . In the ground state, a high dipole moment enhances delayed uorescence; while in the excited state, it facilitates exciton quenching through intermolecular CT owing to interactions with the emitter dipoles in the CT excited state. 7 The excited-state dipole moments (µ ES ) are given in Figure 2a and Table 1. H2 and H4, which have high µ GS values, exhibit higher µ ES values and relatively low Φ DF values. This implies that their CT-character is strengthened in the excited state, and thereby enhancing exciton quenching. The formation of intermolecular CT complexes depends on the energy-level structures of the host and emitter molecules. Although µ ES value of DPEPO is slightly below that of H2 (11.06 vs. 11.73 D), its Φ DF value is higher (46.5% vs. 37.9%). This is because the extremely wide bandgap of DPEPO suppresses the formation of intermolecular CT complexes.
The effect of these excited-state dipole moments on the performance of TADF devices is discussed later. Considering the higher Φ DF and k rISC values and redshifted and broadened emission of 1PCTrz in the novel high-dipole moment host materials (H1-H4), we expect them to be strong candidates for replacing both high-polarity hosts such as DPEPO and stable hosts such as mCBP-CN.
Ground-state characteristics of the high-dipole moment host materials Although donor-acceptor (D-A) type TADF emitters have bipolar characteristics, they largely show holetransporting behavior when used with electron-transporting host materials such as mCBP-CN. 14 TADF devices typically have high emitter/host compositional ratios compared to other technologies such as a uorescent and phosphorescent OLEDs; therefore, to provide high e ciency and stability, an electron transporting-host such as mCBP-CN is required, which forms a mixed-host-like emitting layers with holetransporting TADF emitters. In this work, the employed TADF emitter, 1PCTrz, exhibits hole-transporting behavior rather than electron-transporting behavior, although it is basically a bipolar D-A molecule as shown in Figure 3. Thus, the host should exhibit electron-transporting properties.
The designed hosts, H1-H4, all exhibit electron-transporting behavior ( Figure 3). According to our calculations, H1-H4 have comparable LUMOs to that of 1PCTrz, yet much lower highest occupied molecular orbitals (HOMOs) than both 1IPCTrz and mCBP, in a similar manner to mCBP-CN. Our previous work 14 revealed that electron-transporting hosts can form mixed-host-like emitting layers with highly doped TADF emitters, which can improve the stability of TADF devices. Note that H1 and H2, which have the same core structure show different HOMO and LUMO energy levels. Speci cally, H2 has lower-lying HOMO and LUMO energy levels than H1, which indicates that H2 has stronger electron-transporting behavior. H4 also has stronger electron transport than than H3. It is interesting to compare H2 with H1 and H4 with H3, particularly from view point of stability, whch we will discuss in the following section.

Device performances
The device characteristics of the tested OLEDs are shown in Figure 4and their performances are summarized in Table 2. The OLEDs were fabricated with the following layer stacks: indium tin oxide The normalized electroluminescence (EL) spectra (Figure 4a) of the TADF devices look similar to the nomalized PL spectra (Figure 2d), with only small differences ascribed to weak micro-cavity effects in multilayered OLEDs. 23 Thus, in the same manner as the PL spectra, the electroluminescence spectra of the TADF devices employing high-polarity hosts (H1-H4, DPEPO) were slightly redshifted and broader than those of the OLEDs based on mCBP or mCBP-CN. This result indicates that host polarity has the same effect in the electrically driven multilayered OLEDs as in the optically pumped single-layer thin lms. Figure 4b shows the current density-voltage characteristics of the OLEDs. The DPEPO-based OLED, which has the widest bandgap (5.43 eV, calculated) showed the highest device resistance; while mCBP-CN has the second narrowest bandgap (3.73 eV, Figure 3) shows the smallest device resistance. Although H1 has the narrowest bandgap (3.71 eV, Figure 3), the device resistance of H1 is higher than that of mCBP-CN, because the difference in device resistance is very small, yet mCBP-CN has a lower LUMO, which may facilitate charge transport within the highly doped hole-transporting TADF emitter. 14 Figure 4c shows the EQE-luminance characteristics of the OLEDs. The devices employing H1-H4 exhibit higher EQE max of 13.6%-18.0% than the mCBP-based (10.0%) and mCBP-CN-based (9.1%) OLEDs. These high EQE max values can be attributed to the increased delayed uorescence of 1PCTrz in H1-H4, as these hosts provide highly a polar environment. The H3:1PCTrz lm exhibits the highest Φ DF among the 1PCTrzdoped lms employing P-O bond-free hosts (H1-H4, mCBP, and mCBP-CN) and consequently the H3:1PCTrz-based OLED exhibits the highest EQE max . Notably, the EQE max of the H3:1PCTrz-based OLED is the closest to that of the DPEPO:1PCTrz-based OLED (18.0% vs. 21.1%). Furthermore, at practical brightnesses (500 or 1,000cd m -2 , for example), the EQE of the H3:1PCTrz-based OLED exceeds that of the DPEPO:1PCTrz-based OLED as e ciency roll-off is suppressed in the former. Despite the short delayed uorescence lifetime (τ DF ) of the DPEPO system, DPEPO:1PCTrz-based OLEDs exhibit signi cant e ciency roll-off owing to the inferior property of DPEPO. 14 Although τ DF for the H1-H4 systems were longer than those for the DPEPO system, the devices based on H1-H4 showed higher EQEs at a high luminance (1,000 cd m -2 ). This is attributed to the improved charge balance in the emitting layer, where a lower LUMO level of H1-H4 leads to increased injection of electrons.
It is noteworthy that the EQE max values for the devices based on H1-H4 varied consistently with Φ DF , indicating that the effects of host polarity observed in the optical experiments are also applicable to the electrical devices. Among H1-H4, H3 exhibits the highest EQE max , despite its relatively low µ GS (4.63 D, Table 1). This can be explained by its low µ ES (5.04 D). We believe that excitons from the 1PCTrz can be signi cantly quenched by hosts (H2 and H4) as their excited-state polarities are much higher than that of H3, inducing easier formation of intermolecular CT complexes. In contrast, in H3, 1PCTrz can avoid serious exciton quenching in H3 owing to the low µ ES of H3 and small µ ES gap (1.84 D) between H3 and 1PCTrz, with affects the in uence of the excited-state dipole eld of the host. 7 Nevertheless, the DPEPObased OLED still has the greatest EQE max (21.1%), despite DPEPO having the highest µ ES (11.06D). This is because the extremely wide band gap of DPEPO makes it di cult for it to reach the excited state or form an intermolecular CT complexes with other molecules. Although the µ ES of H1 is lower than that of H3 (3.8 vs. 5.04 D), its EQE max is only very slightly lower (17.9% vs. 78.0%) owing to its lower µ GS (4.18 vs.

D)
. From the perspective of effciency, H1 and H3 are probably the most promising candidates as alternative hosts for blue TADF devices.
To investigate the in uence of the host materials on the device stability, the operation lifetimes of OLEDs with different host materials were measured at an initial luminance of 500 cd m -2 . The decreases in luminance with increasing OLED operation time was measured using the LT50 parameter, which represents the time at which the luminance decreased to 50% of the initial luminance. Figure 4d shows the normalized luminance of the OLEDs as a function of OLED operation time under a constant current density at the initial luminance of 500 cd m -2 . Consistently with the previous report in which mCBP-CN was suggested as a stable host, 14 the LT50 of the OLED fabricated using mCBP-CN (36.2 h) was much longer than that of the DPEPO-based OLED (0.6 h) and the conventional mCBP-based OLED (3.3 h). Nevertheless, the H2-, H3-and H4-based OLEDs notably exhibit much longer operational lifetimes than not only the DPEPO-and mCBP-based OLEDs, but also the mCBP-CN-based OLED. In particular, the H3based OLED exhibited an operational lifetime comparable to that of the stable mCBP-CN-based OLED, while simultaneously exhibiting remarkably improved e ciency (Figure 4c). Furthermore, the H2-and H4based OLEDs exhibited much longer LT50 (122.4 and 86.9 h, respectively) than even the mCBP-CN-based OLED. While the mCBP-CN-based OLED had the best charge transport characteristics as shown in Figure  4b, and the most suppressed e ciency roll-off, as shown in Figure 4c, indicating the best charge balancing ability, its EQE at a practical brightness and LT50 were inferior to those of the OLEDs based on H2-H4.
As we discussed with Figure 1, the simulataneously high e ciency and OLED operational stability are attributed to the high polarity of the novel hosts, stabilizing CT excited states of 1PCTrz, and the P-O bond-free molecular structure with strong electron-transporting character, which enhances operational stability. However, the LT50 of the H1-based OLED was shorter than that of the mCBP-CN-based OLED (21.7 vs. 36.2 h), while its EQE was superior. This exception can be explained by considering the charge transporting character of H1. Owing to the CN-functionalization, the four novel host materials (H1-H4) all have low-lying HOMO and LUMO levels (Figure 1b), which provides electron-transporting characteristics and high device stability with highly doped (20vol% here) hole-transpoting TADF emitters. 14 With this knowledge, the shorter LT50 of the H1-based OLED can be explained by considering the shallower LUMO energy level of H1, which indicates weaker electron-transporting character. This explanation is valuable when compariing the device operation lifetimes of the OLEDs based on the high-polarity host materials with the same core strucutres (H1 and H2; H3 and H4). The H2-based TADF device exhibits a much longer device operation lifetime than the H1-based device (1224.4 vs. 21.7 h), probably because H2 has a lower LUMO than H1 (-2.04 vs. -1.75 eV). Meanwhile, the H4-based TADF device exhibits a longer device operation lifetime than H3-based device (86.9 vs. 55.1 h) for the same reason; the LUMO energy levels of H4 and H3 are-1.79 eV and-1.64 eV, respectively. Through those comparisons, we can conclude that host materials with lower lying LUMO are more suitable for enhancing device stability of TADF devices that employ highly doped hole-transporting TADF emitters.

Characteristics and stability of hole-only devices (HODs) and electron-only devices (EODs)
To investigate the material stability of the novel hosts and explain the longer operational lifetimes of their OLEDs, we performed charge transport stability test of the host lms in hole-only devices (HODs) and electron-only devices (EODs). Figure 5a and b show the current density-voltage(J-V) characteristics of the HODs and EODs, respectively. The EODs with the high-polarity hosts had much lower turn-on voltages than the HODs, indicating that they are electron-transporting hosts, similarly to mCBP-CN. Electrontransporting hosts are well-suited touse with hole-transporting TADF emitters with a shallow HOMO and high doping concentration, which facilitates the fabrication of stable OLEDs. 14 Figure 5cand 24 For all the novel high-polarity hosts (H1-H4), the HODs and EODs showed much smaller increments in V d during operation than DPEPO-based HOD and EOD. This indicates that the novel high-polarity hosts will degrade much slower than DPEPO under the same electrical stress, in the same manner as mCBP-CN.

Photoluminescence stability
To investigate and compare the excited-state stabilities of the hosts, we performed a PL-stability study by exposing the lms to a UV laser for 3h. Figure 6a and b show normalized PL spectra of 50-nm H2 and mCBP lms, respectively, in the as-deposited and degraded (3-h UV-laser-exposed). Normalization was performed by dividing the spectra by the PL peak intensity of the as-deposited lm. The peak intensity for the H2 lm decreased to 76% of its initial value after 3-h exposure to UV-laser, while that of mCBP decreased to 36%. This signi cant difference in PL stability correlates with the great difference in device operation lifetime between the H2-and mCBP-based OLEDs (LT50 = 122.4 and 3.3h, respectively, Table  2). The other high-polarity hosts also exhibited high PLstability (Supplementary Figure 2), as did mCBP-CN in the previous report. 14 We were unable to perform PL stability studies on DPEPO because it requires a higher energy laser for excitation. However, a few articles have reported low photochemical stability of the P-O bond, 12,13 and we have previously discussed the poor PL stability of DPEPO and its substitutions with two P-O bonds. 14 We performed further PL stability studies with 1PCTrz-doped host lms, which enables the excited-state stability to be investigated without detrimental host interactions such as concentration quenching, triplettriplet annihilation, exciton migration, etc. [25][26][27][28][29] Figure 6c, d and e show normalized PL spectra of 50-nm H2:1PCTrz, mCBP:1PCTrz and DPEPO:1PCTrz lms, respectively. The peak intensity for the H2:1PCTrz lm decreased to 80% of its initial value after 30h UV-laser exposure, while that for the mCBP:1PCTrz and DPEPO:1PCTrz lms decreased to71% and 68%, respectively. This result also explains the longer operational lifetime of the H2:1PCTrz-based OLEDs, although the decrement of the peak intensity for mCBP:1PCTrz and DPEPO:1PCTrz lms was reduced when compared to that for the host-only lms. The improved PL stability after 1PCTrz doping is attributed to the host-to-guest energy transfer for mCBP. However, for DPEPO, the unexpectedly low loss of intensity after 1PCTrz doping is related to the fact that DPEPO was not excited during the DPEPO:1PCTrz lm PL measurement, so it did not suffer the degradation induced by the weak P-O bond. The other emitting layer lms (H1:1PCTrz, H3:1PCTrz, and H4:1PCTrz) also exhibited higher PL stabilities than the mCBP:1PCTrz and DPEPO:1PCTrz lms. Their normalized PL spectra before and after 3-h UV-laser exposure are presented in Supplementary Figure S2.

Orientation of transition dipole moments
To further verify the mechanism of host polarity-induced e ciency enhancement, we performed angledependent PL measurements with various host lms doped with 1PCTrz, which enables the cause of the improved EQE max to be more closely examined. In particular, we can observe if it originates from light outcoupling effects instead of host polarity. Figure 7a shows the angle-dependent PL spectrum of ppolarized light from the 50-nm-thick 1PCTrz-doped H2 host lm, while the angle-dependent PL intensities for H2:1PCTrz are depicted in Figure 7b (magenta squares). A theoretical t of the data reveals a horizontal orientation of the transition dipole moments (h/(h+ν)) of 0.89. This value is quite far from the value for an isotropic orientation (h/(h+ν) = 0.67), which indicates that the orientation effect is signi cant. However, the mCBP-CN:1PCTrz lm also exhibits a comparable h/(h+ν) value of 0.88. This means that the signi cant difference in EQE max between the H2-and mCBP-CN-based OLEDs is not attributed to orientation effects. Moreover, in spite of the small h/(h+ν) of 0.79 of the H3:1PCTrz emitting layer, which indicates low out-coupling e ciency, the OLED with the H3:1PCTrz emitting layer exhibited the highest EQE max among the OLEDs with P-O bond-free hosts. For the tested lms, there were no signi cant differences in the h/(h+ν) values, with the exception of the H3:1PCTrz lm. This indicates that the enhanced EQE max observed with high-polarity host-based TADF OLEDs is not attributed to the orientation effect. Considering the results of the PL spectrospcopic study, including the redshifted and broadened emission peaks and higher delayed uorescence quantum yields with high-polarity host-based lms, we conclude that the enhanced e ciencies of the TADF devices are ascribed to the stabilization of the CT excited states of the TADF emitter when surrounded by high-polarity host molecules. The values of h/(h+ν) for all the tested lms are summarized in Table 3 along with their expected out-coupling e ciencies. The angle-dependent PL intensities for the other hosts and their theoretical t are presented in the Supplementary Figure 3).

Discussion
We designed, synthesized and veri ed four new P-O bond-free host materials, H1-H4, for use with a blue TADF emitter molecule, 1PCTrz. The host materials had molecular asymmetry and large electric dipole moments. The TADF devices based on the novel hosts exhibited high e ciencies approaching that of the DPEPO-based TADF device. At the same time, they achieved much longer operational lifetimes than those based on DPEPO, mCBP and mCBP-CN. This simultaneous improvement in e ciency and stability is attributed to the intricate properties of the host materials that works consonantly toward enhanced device operation: i) high polarity stabilizes the CT excited state of the TADF emitter; ii) a relatively low excitedstate dipole moment suppresses intermolecular CT-induced quenching of excitons; iii) electrontransporting character allows hole transport/electron transport mixed-host-like behavior in the emitting layer owing to the hole-transporting ability of the highly doped TADF emitter; and iv) P-O bond-free design improves photochemical stability. The novel hosts adopt asymmetric molecular structures and polar groups appropriately to provide a trade-off between high ground-state dipole moments and low excited-state dipole moments, while simultaneously modulating their molecular orbital energy levels. Our strategy for designing hosts with particular characteristics will promote the commercialization and success of TADF devices and technology.

Methods
Calculations of dipole moment. The molecular structures of the P-O bond-free hosts considered in this study have rotatable bonds and thus allow various conformations, which might give a wide range of dipole moments. We estimated that the dipole moment may affect the polarity of the host matrix. In order to nd an appropriate set of conformers, we used a conformational search module included in Schrödinger software 30 that collects structures using the mixed torsional/low-mode sampling method starting from a given structure. A set of representative conformers were determined from the local minimization starting from numerous structures. In the minimization, the energies were evaluated in the classical level using the OPLS3e force eld. For all conformers, we performed optimizations through density functional theory (DFT) calculations at the level of B3LYP/6-31G(d) and obtained the values of dipole moments using Jaguar software. 31 We determined the representative value of dipole moment, denoted by µ GS , to be the one calculated from the structure with the minimum energy among the conformers. The minimum and the maximum values of the dipole moments in company with µ GS for all hosts are displayed in Supplementary Fig. 1  Device fabrication and measurement. The organic layers were deposited on pre-cleaned ITO glass substrates using a thermal evaporation system with a vacuum pressure of <     mCBP-CN, those in H1-H4 and DPEPO are redshifted (red dotted arrow) and broadened (black solid arrows). µGS, µES, T1, and S1 are represented as the values corresponding to the minimum energy structure for each host (See also Figure S1 and Table 1.).  Characteristics of hole-only devices (HODs), and electron-only devices (EODs). a. and b. Current densityvoltage curves of HODs and EODs, respectively, for various host lms. c. and d. Changes of the driving voltages of HODs and EODs, respectively, for various host lms as a function of operation time with constant driving current. V0 is the initial voltage.

Supplementary Files
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