Boosting external quantum efficiency to 38.6% of sky-blue delayed fluorescence molecules by optimizing horizontal dipole orientation.

To achieve high electroluminescence efficiency, great efforts are devoted to enhancing photoluminescence quantum yield (ΦPL) and exciton utilization of luminescent molecule, while another important factor, light out-coupling efficiency (ηout), receives less attention in molecule design. Here, we focus on horizontal dipole orientation engineering of the molecule to increase ηout and external quantum efficiency (ηext). A series of tailor-made luminescent molecules consisting of an electron-accepting carbonyl core plus double electron-donating groups of spiro[acridine-9,9′-fluorene] and carbazole derivatives [e.g., 1,3,5-tri(carbazol-9-yl)benzene] are developed and systematically investigated. These molecules hold distinguished merits of strong sky-blue delayed fluorescence with excellent ΦPL values, large horizontal dipole ratios, and balanced bipolar carrier transport, which furnish record-high ηext values of up to 26.1 and 38.6% in nondoped and doped sky-blue organic light-emitting diodes (OLEDs), respectively. Moreover, the state-of-the-art nondoped hybrid white OLED and all-fluorescence single-emitting layer white OLED are also realized, demonstrating great potentials in OLED industry of these molecules.

INTRODUCTION

As a promising display technique, organic light-emitting diodes (OLEDs) have attracted continuously increasing interests from academia and industry (–). To realize superb performances of OLEDs for commercial application, the developments of organic functional materials and device fabrication technologies are of critical importance. In general, the external quantum efficiency (ηext) of OLED is the product of internal quantum efficiency (ηint) and light out-coupling ratio (ηout). Because ηint is determined by photoluminescence (PL) quantum yield (ΦPL), exciton utilization efficiency (ηr), and carrier balance factor (γ), tremendous efforts have been devoted to exploring luminescent materials with high values in these aspects. Considerable current research interest is focused on purely organic luminescent materials with thermally activated delayed fluorescence (TADF), owing to their excellent theoretical ηr values of 100% by virtue of reverse intersystem crossing (RISC) process (–). The twisted electron donor (D)–acceptor (A) structures of TADF emitters may also bring about bipolar carrier transport ability and thus a high γ value. However, even if the molecule has a high ηint value of nearly 100%, for bottom-emitting OLED, the ηout value is generally in the range of 20 to 30%, corresponding to a top limit of 30% for ηext (, ). From this point of view, there is still much room for further improvement of ηext by enhancing ηout.

In addition to adopting light extraction techniques, such as microlens arrays (, ), scattering layers (, ), and corrugated structures (, ), raising horizontal dipole ratio (Θ//) is a feasible molecular engineering approach to reduce optical loss and enhance ηout. Elongating molecular conjugation along the direction of transition dipole moment (–) and expanding molecular plane where the transition dipole moment lies (, –) are two major molecule design strategies in the literatures, but often at the cost of sacrificing emission color and efficiency of TADF emitters because of varied internal D-A interaction strength or strong intermolecular interactions in the aggregated state. To address this issue, recent studies proposed that the incorporation of spiro-shaped donor to create D-A backbones for TADF emitters could effectively enhance Θ// without causing serious emission quenching (, –), due to high steric hindrance. On the other hand, Byeon et al. () successfully realized nearly 100% horizontal dipole orientation of TADF emitters by increasing donor plane with carbazole units, but the energy level and emission color were greatly dependent on the number of carbazole units. Xiang et al. () succeeded in efficient modulation of horizontal dipole orientation without affecting electronic structures by the expansion of acceptor plane of TADF emitters and obtained highly efficient green OLEDs.

Taking the above into consideration, here, we wish to report the design and synthesis of a series of tailor-made luminogenic molecules with strong delayed fluorescence for highly efficient sky-blue OLEDs (Fig. 1A). In these molecules, carbonyl is selected as the electron-accepting core because carbonyl can undergo n-π* transition, availing the spin-orbit coupling (SOC) and thus promoting RISC process (). Electron-donating spiro[acridine-9,9′-fluorene] (SFAC) is connected to one side of carbonyl core via a phenyl bridge to construct a D-A framework for the generation of delayed fluorescence. In addition, hole-transporting functional groups of 9-phenylcarbazole (CP), 1,3-di(carbazol-9-yl)benzene (mCP), and 1,3,5-tri(carbazol-9-yl)benzene (TCP) are introduced to the other side of carbonyl core to facilitate the balance of carrier injection and transport of the molecules. It is envisioned that, in addition to the spiro group SFAC (), the presence of these carbazole-based functional groups can enlarge molecular plane for transition dipole moment and thus have supplemental effect for optimizing Θ// values. Meanwhile, these rigid and space-demanded functional groups (e.g., SFAC and TCP) can also reduce intramolecular motion and inhibit intermolecular π-π interactions, ensuring efficient emissions in solid films (). As expected, the generated molecules not only exhibit strong sky-blue delayed fluorescence with excellent ΦPL values reaching 99% but also prefer horizontal dipole orientation in film states with large Θ// values of up to 88.0%. On the basis of these new molecules, efficient sky-blue nondoped OLEDs are fabricated, furnishing outstanding ηext values of 26.1% and small efficiency roll-offs. Moreover, superb sky-blue doped OLEDs with even higher ηext values of up to 38.6% are achieved, which could be the most efficient OLED with similar color reported so far. Further, nondoped hybrid white OLED and all-fluorescence single-emitting layer white OLED with the state-of-the-art performances are realized on the basis of the new sky-blue molecules.

Fig. 1.

New molecules studied in this work.

(A) Chemical structures and (B) geometrical structures of the new molecules, optimized by time-dependent density functional theory (DFT). Dihedral angles are indicated in the geometrical structures, and transition dipole moment of S1 state is indicated by red arrow. (C) Surface distance projection maps in the x-y plane of P-BP-SFAC, CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC.

Theoretical calculations of P-BP-SFAC, CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC.

Frontier molecular orbital distributions, spin-density distributions of T1 states (TSDD), and the length, width, and height of the molecules. The optimized geometrical and electronic structures were calculated by PBE0-D3/6-31G (d, p).

RESULTS

Synthesis and characterization

The new luminogenic molecules (CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC) and a control molecule (P-BP-SFAC) were prepared facilely according to the straightforward synthetic routes outlined in fig. S1. The detailed synthetic procedures and characterization data are described in the Supplementary Materials. CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC enjoy brilliant thermal stability (fig. S2), with high decomposition temperatures of 504°, 559°, and 589°C, respectively, which are much higher than that of P-BP-SFAC (371°C), disclosing that the introduction of carbazole units has greatly improved the thermal stability of the molecules. No clear glass-transition temperatures are observed for these molecules, indicative of inconspicuous thermal transitions and thus good morphological stability. Moreover, these molecules have reversible oxidation and reduction processes, as measured by cyclic voltammetry (fig. S3), demonstrating their good electrochemical stability. From the onsets of oxidation and reduction waves, the energy levels of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are obtained. CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC have approximate LUMO energy levels of ca. −2.67 eV, and HOMO ones of ca. −5.36 eV, which are very close to those of P-BP-SFAC (−2.75 and −5.36 eV), implying that LUMO and HOMO energy levels are mainly determined by the D-A framework based on carbonyl and SFAC.

The geometrical and electronic structures of these molecules are studied by theoretical calculation. In optimized molecular structures, the carbonyl and SFAC are connected in a highly twisted conformation via the phenyl bridge, with large torsion angles of ~90° between SFAC and phenyl bridge (Fig. 1B), which is favored for the separation of HOMOs and LUMOs to endow the molecules with small energy splitting between lowest singlet (S1) and triplet (T1) states (ΔEST). On the contrary, the carbazole groups are conjugated better to carbonyl core in CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC with small torsion angles of 26° to 29°. In consequence, the HOMOs are mainly concentrated at the acridine segment, while the LUMOs are distributed on carbonyl core, phenyl bridge, and part of carbazole unit adjacent to carbonyl core. These almost the same transition orbital distributions on the D-A framework based on carbonyl and SFAC moieties as well as very close energy levels of HOMOs and LUMOs imply the similar PL property. In addition, because of the separated distribution of HOMOs and LUMOs, the theoretical ΔEST values are calculated to be 0.106, 0.080, and 0.061 eV for CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC, respectively, which are small enough for the occurrence of RISC process ().

The calculated spin-density distributions of T1 states (TSDD) indicate that the triplet excitons are mainly located on the carbonyl core and adjacent phenyl rings, while barely on the peripheral rigid aromatic groups, probably due to the low-lying energy level of carbonyl core (Fig. 2). As the increase of carbazole units, the entire molecules become more space-demanded, which can protect the excitons on the central carbonyl core from getting close, namely, the peripheral rigid aromatic groups without obvious exciton distributions can act as a defense against exciton annihilation. On the other hand, the transition dipole moments of all the molecules are calculated to be preferentially oriented along the molecular axis (in the x-y plane) of the D-A framework based on carbonyl and SFAC (Fig. 1B). The introduction of carbazole units can expand the effective projected area in the x-y plane (Fig. 1C) for the transition dipole moment to lie in, suggesting a positive effect on horizontal dipole orientation. However, the enhancement of projected areas by mCP and TCP are somewhat limited relative to that of CP, because the additional carbazole units in mCP and TCP are actually highly twisted out of the molecular plane of transition dipole moment, implying similar effects from CP, mCP, and TCP for horizontal dipole orientation.

CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC show similar absorption maxima at 335 to 337 nm in tetrahydrofuran (THF) solutions, assigned to the π-π* transitions of the molecules. The intramolecular charge transfer (ICT) states are reflected as absorption tails extending to 430 nm rather than discernible absorption peaks, indicating that the ICT effect should be weak in these molecules. On the contrary, P-BP-SFAC exhibits an apparent absorption band with a peak at about 377 nm, indicative of a stronger ICT effect. Similar absorption spectral profiles are also observed in nonpolar toluene solutions (fig. S4). These findings disclose that the electron-withdrawing ability of the carbonyl core has been weakened by the direct connection to the 3-position of electron-donating carbazole group, leading to decreased ICT effect between carbonyl and SFAC. In comparison to strong ICT effect, the relatively weak ICT effect is actually more favorable to achieve stronger and bluer PL emissions.

CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC exhibit strong sky-blue delayed fluorescence in neat films, with PL peaks at ~483 nm and delayed fluorescence lifetimes (τdelayed) of 5.4 to 5.7 μs. The ΦPL values are increased from 77% of CP-BP-SFAC to 86% of mCP-BP-SFAC and to 92% of TCP-BP-SFAC. The doped films of these molecules in diphenyl-4-triphenylsilylphenyl-phosphine oxide (DPEPO) host at concentrations of 20 or 25 weight % (wt %) exhibit similar PL peaks at 478 to 482 nm, close τdelayed values of 5.5 to 5.6 μs, and outstanding ΦPL values reaching 99%. These ΦPL and τdelayed values in both neat and doped films are much larger than those in THF or toluene solutions (Table 1). Similar enhancements in emission intensity and lifetime can also be detected in their nanoaggregates formed by adding a large amount of water into THF solutions (Fig. 3C, figs. S5 and S6, and Table 1). The interesting phenomenon of obviously enhanced delayed fluorescence in films or nanoaggregates can be attributed to the restriction of intramolecular motion and molecular structure rigidification by spatial constraint, which blocks nonradiative decay of excited states, and thus facilitates RISC process and radiative decay (–). By increasing temperature from 77 to 300 K, the proportion of delayed components of these molecules in both neat and doped films is enhanced apparently, disclosing that the delayed fluorescence can be promoted by thermal energy, namely, TADF characteristic (Fig. 3D, figs. S7 and S8, table S1).

Table 1.

Photophysical properties of P-BP-SFAC, CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC.

Emitter	THF/toluene*				Neat film/doped film†
	λabs (nm)	λem (nm)	ΦPL‡ (%)	<τ>§ (ns)	λem (nm)	ΦPL‡ (%)	<τ>§ (μs)	τdelayed║ (μs)	Rdelayed¶ (%)	ΔEST# (meV)	Θ//** (%)
P-BP-SFAC	377/372	518/485	24/49	126/327	493/491	88/94	1.7/2.3	4.8/5.2	34.6/43.5	55/50	71.5/72.5
CP-BP-SFAC	335/342	507/475	36/31	499/414	482/481	77/99	1.7/2.4	5.4/5.5	33.0/44.6	86/57	86.5/84.5
mCP-BP-SFAC	337/338	509/475	36/33	262/448	484/478	86/99	1.8/2.4	5.5/5.5	33.0/43.0	95/58	87.5/86.0
TCP-BP-SFAC	336/337	512/476	26/40	143/568	483/482	92/99	1.6/2.5	5.7/5.6	27.2/43.9	94/59	85.5/88.0

*Measured in THF and toluene solutions (10−5 M) bubbled with nitrogen.

†Doped films are 20 wt % P-BP-SFAC:DPEPO, 25 wt % CP-BP-SFAC:DPEPO, 20 wt % mCP-BP-SFAC:DPEPO, and 25 wt % TCP-BP-SFAC:DPEPO.

‡Absolute PL quantum yield determined by a calibrated integrating sphere under nitrogen at room temperature.

§Mean fluorescence lifetime evaluated at 300 K under nitrogen.

║Lifetime of delayed decay component evaluated at 300 K under nitrogen.

¶Proportion of delayed component evaluated at 300 K under nitrogen.

#Estimated from the onsets of fluorescence and phosphorescence spectra at 77 K.

**Horizontal emitting dipole ratio.

Fig. 3.

Photophysical properties of P-BP-SFAC, CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC.

(A) Absorption spectra in THF solutions (10−5 M). (B) PL spectra in neat films (solid line) and doped films (dash line) of P-BP-SFAC, CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC. (C) PL decay curves of TCP-BP-SFAC in THF/water mixtures with different water fractions (fw) (10−5 M) and (D) temperature-dependent PL decay curves of 25 wt % TCP-BP-SFAC:DPEPO–doped film, measured under nitrogen. au, arbitrary units.

CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC have quite close values of PL peak, ΦPL, τdelayed, and other relevant constants in RISC process (table S2) in doped films, and similar phenomenon is also observed for these molecules in solutions. However, the ΦPL values of CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC in neat films are enhanced progressively along with the increase of carbazole unit. These findings further validate that, at the molecularly dispersed states, the photophysical properties of these molecules are mainly determined by the D-A framework based on carbonyl and SFAC, and the increase of carbazole units shows little interference. However, in neat films, the presence of more carbazole units, particularly the star-burst TCP, is conducive to preventing emission quenching and improving emission intensity, rendering a highest ΦPL value of 92% of TCP-BP-SFAC. These experimental results are in good agreement with the theoretical calculations as discussed above.

Host tuning effect on ΔEST

CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC show ΔEST values of 0.086, 0.095, and 0.094 eV in neat films (fig. S9) and 0.057, 0.058, and 0.059 eV in doped films with a DPEPO host (fig. S10), measured from the fluorescence and phosphorescence spectra at 77 K. These small ΔEST values can ensure the occurrence of RISC process and thus delayed fluorescence. However, in comparison with the neat films, the doped films have smaller ΔEST values and higher ratios of delayed components (table S2), disclosing that a DPEPO host benefits the delayed fluorescence of these molecules. To further confirm the ΔEST values, TCP-BP-SFAC is taken as an example to evaluate the ΔEST values of its neat and doped films by fitting an Arrhenius plot according to kRISC ∝ exp.(ΔEST/kBT) (), where kRISC, kB, and T denote the RISC rate constant, Boltzmann constant, and temperature, respectively. The ΔEST values are estimated to be 0.094 and 0.052 eV for neat film and doped film, respectively, which are quite close to the ΔEST values obtained from the fluorescence and phosphorescence spectra (fig. S11).

As shown in table S3, the decrease of ΔEST values in doped films results from the more increments of energy levels of T1 than S1 states, which can be attributed to the different impacts of a DPEPO host on S1 and T1 transitions of these molecules. Such kind of host tuning effect in this system is virtually associated with the different polarities and the valence shell transition types of S1 and T1 states. The theoretical calculation results disclose that the S1 states of these molecules are dominated by π-π* transition, while the T1 states are governed by n-π* transition (Fig. 4) (). The n orbital is sensitive to the changes in matrix polarity because of its high polarity caused by strong electron-withdrawing oxygen atom in C═O double bond, but the π orbital is relatively polarity insensitive because of the evenly distributed electron cloud (). Therefore, the energy levels of T1 states can be changed in a larger degree than those of S1 states when the matrix polarity is altered.

Fig. 4.

Transition types of S1 and T1 states.

Natural transition orbital (NTO) analysis of S1 and T1 states for P-BP-SFAC, CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC. The encircled and enlarged parts indicate the n orbital character.

On the other hand, the calculated molecular polarity index (MPI) values of these molecules in S1 and T1 states are 14.68 to 19.95 and 8.65 to 10.01 kcal mol−1, respectively (table S4). The MPI value of a DPEPO host is 11.20 kcal mol−1, which is smaller than those of S1 states, but larger than those of T1 states. Therefore, when these molecules are in S1 states, the doped films with a DPEPO host have lower polarity than the neat films formed by these molecules. In consequence, the energy levels of S1 states are increased. On the contrary, in T1 states, the doped films have higher polarity than the neat films, resulting in enhanced energy levels as well. However, the increments of T1 energy levels are larger than those of S1 ones, leading to decreased ΔEST values in doped films finally (Fig. 5), which is in excellent agreement with the experimental measurement.

Fig. 5.

The energy levels in neat film and doped film.

The energy level changes of (A) S1 and (B) T1 states and (C) varied ΔEST values from neat films (NF) to doped films (DF) with a DPEPO host. MPINF and MPIDF denote the MPI values in neat film and doped film, respectively. ΔET, DF-NF and ΔES, DF-NF denote the energy level changes of T1 and S1 states from neat film to doped film, respectively. ΔEST, NF and ΔEST, DF denote the ΔEST in neat film and doped film, respectively.

To verify the reliability of the account, a low-polar host TCP, which has a lower polarity (MPI = 8.42 kcal mol−1) than those of DPEPO as well as TCP-BP-SFAC in the T1 state (MPI = 8.72 kcal mol−1), is chosen to fabricate doped film of TCP-BP-SFAC with a concentration of 25 wt %. Compared with the neat film, the doped film of TCP-BP-SFAC exhibits an obviously increased S1 energy level, and the increment is much larger than that in the doped film with a DPEPO host. On the contrary, the T1 energy level of the doped film is only slightly altered. Consequently, the ΔEST value of the doped film is greatly enhanced to 0.146 eV (fig. S10E). These results demonstrate that the host materials have a substantial tuning effect on the ΔEST values of the delayed fluorescence molecules.

Horizontal dipole orientation

The orientation of the transition dipole moments of these molecules in neat and doped films deposited on quartz substrates is investigated by using the angle-dependent p-polarized PL spectra (). TCP-BP-SFAC shows large Θ// values of 85.5% in neat film and 88.0% in doped film, indicative of enhanced ηout values relative to those of common molecules with random dipole orientation (Fig. 6, A and B). The Θ// values of P-BP-SFAC are 71.5% in neat film and 72.5% in doped film (Fig. 6, C and D), which are larger than those of common molecules but still smaller than those of TCP-BP-DMAC (9,9-dimethylacridine). On the other side, the Θ// values of the other two control molecules P-BP-DMAC and TCP-BP-DMAC containing DMAC instead of SFAC are measured as 63.0 and 76.0%, respectively (fig. S12), rendering the same trend as that of P-BP-SFAC and TCP-BP-SFAC. These results demonstrate that the presence of TCP is conducive to horizontal dipole orientation, owing to the expanded molecular plane, where the transition dipole moment lies (, ). Besides, the Θ// values of P-BP-SFAC and TCP-BP-SFAC are also larger than those of P-BP-DMAC and TCP-BP-DMAC, verifying that the spiro group SFAC is beneficial for horizontal dipole orientation, which is consistent with the reported results (). Therefore, it comes to a conclusion that the combined horizontal dipole orientation effects from SFAC and TCP endow a larger Θ// value of TCP-BP-SFAC than those of P-BP-SFAC and TCP-BP-DMAC with individual effect from SFAC and TCP, respectively. CP-BP-SFAC and mCP-BP-SFAC also exhibit a high trend for horizontal dipole orientation with large Θ// values similar to those of TCP-BP-SFAC in neat and doped films (Table 1 and figs. S12 and S13).

Fig. 6.

The characterizations of horizontal dipole orientation by angle-dependent p-polarized PL spectra.

Measured horizontal transition dipole moment ratios of neat and doped films of (A and B) TCP-BP-SFAC and (C and D) P-BP-SFAC.

Bipolar carrier transport

To investigate the impact of carbazole groups on the carrier transport behaviors of these molecules, their hole and electron mobilities are measured by the space-charge limited current (SCLC) method on the hole- and electron-only devices with configurations of indium tin oxide (ITO)/1,10-bis(di-4-tolylaminophenyl)cyclohexane (TAPC; 10 nm)/emitter (80 nm)/TAPC (10 nm)/Al and ITO/1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB; 10 nm)/emitter (80 nm)/TmPyPB (10 nm)/LiF (1 nm)/Al, respectively, in which TAPC and TmPyPB function as buffer layers to shield off electrons and holes, respectively. As shown in Fig. 7, the hole and electron mobilities become close as the increase of carbazole units, and TCP-BP-SFAC shows an impressive balance in carrier mobility in a wide range of electric field from 4 × 104 to 1 × 106 V cm−1. The eminent balanced bipolar carrier mobility should be beneficial to exciton recombination efficiency and exciton recombination zone located in the middle of the emitting layer.

Fig. 7.

The characterizations of bipolar carrier mobility by SCLC method.

Electric field–dependent carrier mobility of (A) P-BP-SFAC, (B) CP-BP-SFAC, (C) mCP-BP-SFAC, and (D) TCP-BP-SFAC neat films in single-carrier devices.

Moreover, close survey on the hole and electron mobilities of these molecules unveils that the electron mobilities are only altered to a very small extent but the hole mobilities exhibit relatively large changes as the introduction of carbazole units. To gain a deep insight into the effects of carbazole units on the carrier mobilities, molecular orbitals and volumes are taken into consideration. As mentioned above, the distributions of the HOMOs and LUMOs of these molecules are almost the same. Hence, the difference in carrier mobility may be caused by other orbitals. In general, the energy gaps between the unoccupied orbitals are wider than those of the occupied orbitals as described by the variational principle, and furthermore, the antibonding nature of the unoccupied orbitals except the LUMOs makes them hardly contribute to electron mobility (–). Therefore, for these molecules, the electron mobilities are dominated by the LUMOs that are mainly located on central carbonyl and nearby fragments. Although the introduction of carbazole units can increase molecular volumes and affect molecular packing, the almost identical LUMO distributions and relatively long distances among electron-transporting carbonyl cores render only slight fluctuations in electron mobilities of these molecules.

As for the hole mobility, the calculation results show that, in addition to the HOMOs, the HOMO-1 orbitals of these molecules are able to contribute to hole mobility because their energy levels are relatively closer to those of HOMOs. The HOMO distributions are almost identical, but the HOMO-1 orbitals are located on different fragments (fig. S14). The HOMO-1 orbitals of P-BP-SFAC and CP-BP-SFAC are centered on fluorene units, while those of mCP-BP-SFAC and TCP-BP-SFAC are located on a peripheral carbazole of mCP and TCP moieties. The molecular volumes and stacking can have large impacts on the distances and interactions among peripheral fluorene and carbazole units than the central carbonyl. Given the same distributions of the HOMO and HOMO-1 orbitals of P-BP-SFAC and CP-BP-SFAC, the lower hole mobility of CP-BP-SFAC than P-BP-SFAC is probably caused by the looser stacking of CP-BP-SFAC due to its more branched conformation, which is unfavorable for hole mobility via a hopping model. This is also true for the slightly lower hole mobility of TCP-BP-SFAC than mCP-BP-SFAC. On the other hand, the lower hole mobility of CP-BP-SFAC than mCP-BP-SFAC is attributed to the weaker hole transport ability of fluorene than carbazole. In short, the introduction of carbazole units mainly changes the distribution of the HOMO-1 orbitals and the molecular packings, both of which can influence the carrier mobilities.

Electroluminescence performances

In view of the strong delayed fluorescence, horizontal dipole orientation, and balanced bipolar carrier mobility of these molecules in neat films, their electroluminescence (EL) performances in nondoped OLEDs are initially evaluated. The device configuration is ITO/hexaazatriphenylenehexacabonitrile (HATCN; 5 nm)/TAPC (50 nm)/tris[4-(carbazol-9-yl)phenyl]amine (TCTA; 5 nm)/mCP (5 nm)/emitter (20 nm)/DPEPO (5 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al, in which the neat films of these molecules are used as emitting layers, and the functional layers of HATCN, TAPC, TCTA, and TmPyPB are adopted for hole injection, hole transport, hole buffer, and electron transport layers, respectively. mCP and DPEPO work as exciton blocking layer to confine the excitons within the emitting layer on account of their high T1 energy levels (2.9 and 3.0 eV) (, ). These nondoped OLEDs can be turned on at low voltages of 2.7 to 2.8 V with high maximum luminance (Lmax) values of 28,590 to 39,460 cd m−2, indicative of facile carrier injection, balanced carrier transport, and efficient exciton recombination in devices (Fig. 8, B and C, and fig. S15A). The device of CP-BP-SFAC shows an EL peak at 490 nm and Commission Internationale de I’Eclairage coordinates (CIE) of (0.19, 0.36). Its maximum ηext (ηext,max), current efficiency (ηC,max), and power efficiency (ηP,max) are 22.5%, 51.2 cd A−1, and 57.4 lm W−1, respectively. The devices of mCP-BP-SFAC and TCP-BP-SFAC radiate strong sky-blue EL emissions with slightly blue-shifted peaks at 488 nm (CIE = 0.17, 0.34) and provide excellent ηext,max, ηC,max, and ηP,max values of 25.0%, 53.8 cd A−1, and 56.0 lm W−1 and 26.1%, 56.4 cd A−1, and 63.3 lm W−1, respectively. The device of TCP-BP-SFAC enjoys the best efficiency stability, whose ηext remains as 23.8% at a high luminance of 1000 cd m−2, equal to a small efficiency roll-off of 9.6%, which is much superior to that of P-BP-SFAC–based device (31.3%). According to the calculated results of TSDD, the space-demanded star-burst TCP can better suppress exciton annihilation via short-distance Dexter energy transfer. The perfectly balanced carrier transport of TCP-BP-SFAC makes exciton recombination occur as much as possible at the middle region of TCP-BP-SFAC layer and thus reduce exciton quenching at the interfaces of active layers. These factors are considered to account for the smallest efficiency roll-off of the nondoped device of TCP-BP-SFAC. Given the highest ηext,max and smallest roll-off, as far as we know, the nondoped device of TCP-BP-SFAC is the best nondoped sky-blue OLED ever reported (table S5) (, , , ).

Fig. 8.

Device performance of sky-blue OLEDs.

(A) Device architecture, energy diagram, and functional layers for the vacuum-deposited OLEDs. Plots of (B and D) luminance–voltage–current density and (C and E) external quantum efficiency–luminance of nondoped and doped OLEDs. Inset in (C) and (E): EL spectra and photos (TCP-BP-SFAC) of the nondoped and doped devices at 5 V.

To further examine their EL performances, doped OLEDs with a configuration of ITO/HATCN (5 nm)/TAPC (50 nm)/TCTA (5 nm)/mCP (5 nm)/x wt % emitter:DPEPO (20 nm)/DPEPO (5 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al are fabricated (Fig. 8A), where DPEPO functions not only as an exciton blocking layer but also as a host material. A series of doped devices with different doping concentrations are fabricated to screen the optimal concentrations, which are set to 20 wt % for P-BP-SFAC and mCP-BP-SFAC and 25 wt % for CP-BP-SFAC and TCP-BP-SFAC finally (table S6). The doped devices of CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC exhibit sky-blue EL emissions with peaks in the range of 478 to 484 nm (CIE = 0.16, 0.28) and remarkable EL efficiencies (Fig. 8, D and E, and fig. S15B). Because of the outstanding ΦPL values and large Θ// values, remarkable EL performance is achieved, with ηext,max, ηC,max, and ηP,max of 36.6%, 73.8 cd A−1, and 72.4 lm W−1 for CP-BP-SFAC; 38.0%, 73.1 cd A−1, and 67.5 lm W−1 for mCP-BP-SFAC; and 38.6%, 74.3 cd A−1, and 68.6 lm W−1 for TCP-BP-SFAC (Table 2). Impressively, to the best of our knowledge, the eximious ηext,max of 38.6% is the highest value for sky-blue OLEDs reported so far (table S7) (, , , –), demonstrating the great success of our molecular design.

Table 2.

EL performances for nondoped and doped OLEDs based on the new molecules.

Emitting layers of doped OLEDs: 20 wt % P-BP-SFAC:DPEPO, 25 wt % CP-BP-SFAC:DPEPO, 20 wt % mCP-BP-SFAC:DPEPO, and 25 wt % TCP-BP-SFAC:DPEPO; Von, turn-on voltage at 1 cd m−2; ηC,max, maximum current efficiency; ηP,max, maximum power efficiency; ηext,max, maximum external quantum efficiency; Lmax, maximum luminance; λEL, EL peak; CIE, Commission Internationale de I’Eclairage coordinates.

Emitter	Von (V)	ηC,max (cd A−1)	ηP,max (lm W−1)	ηext,max (%)	Lmax (cd m−2)	λEL (nm)	CIE (x, y)
Nondoped OLEDs
P-BP-SFAC	2.8	60.8	66.7	24.4	37880	500	(0.19, 0.42)
CP-BP-SFAC	2.7	51.2	57.4	22.5	39460	490	(0.19, 0.36)
mCP-BP-SFAC	2.8	53.8	56.0	25.0	28590	488	(0.17, 0.34)
TCP-BP-SFAC	2.7	56.4	63.3	26.1	33030	488	(0.17, 0.34)
Doped OLEDs
P-BP-SFAC	3.0	66.6	69.7	28.9	27650	492	(0.18, 0.38)
CP-BP-SFAC	3.1	73.8	72.4	36.6	21900	484	(0.17, 0.30)
mCP-BP-SFAC	3.3	73.1	67.5	38.0	15880	478	(0.16, 0.28)
TCP-BP-SFAC	3.3	74.3	68.6	38.6	17130	480	(0.16, 0.28)

Encouraged by the excellent EL performance of TCP-BP-SFAC, its potential application in the fabrication of white OLEDs is further explored (Fig. 9). First, the feasibility of TCP-BP-SFAC on the fully nondoped hybrid two-color white OLED is studied, which is rarely reported to date. The nondoped white OLED W1 with a configuration of ITO/HATCN (5 nm)/TAPC (50 nm)/TCTA (5 nm)/mCP (5 nm)/Ir(tptpy)2acac (0.05 nm)/TCP-BP-SFAC (5 nm)/Ir(tptpy)2acac (0.05 nm)/TCP-BP-SFAC (5 nm)/DPEPO (5 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al is fabricated, in which iridium(III)bis(4-(4-t-butyl-phenyl)thieno[3,2-c]pyridinato-N,C2)acetylacetonate (Ir(tptpy)2acac) is used as the orange emitting layer and TCP-BP-SFAC functions as the blue emitting layer. Device W1 has a low turn-on voltage of 2.8 V and displays warm-white light (CIE = 0.44, 0.52), with an Lmax of 37310 cd A−2. High ηext,max, ηC,max, and ηP,max values of 17.1%, 58.6 cd A−1, and 62.6 lm W−1, respectively, with a small efficiency roll of 4.1% at 1000 cd A−2 are obtained, which are comparable to those efficient nondoped white OLEDs in the literature. In addition, all-fluorescence two-color white OLED W2 with a single-emitting layer is further fabricated with a configuration of ITO/HATCN (5 nm)/TAPC (50 nm)/TCTA (5 nm)/mCP (5 nm)/0.5 wt % 2,3,5,6-tetrakis(3,6-di-(tert-butyl)carbazol-9-yl)-1,4-dicyanobenzene (4CzTPNBu):TCP-BP-SFAC (20 nm)/DPEPO (5 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al, in which TCP-BP-SFAC serves as both blue emitter and host, and a reported TADF molecule, 4CzTPNBu, works as an orange emitter. Device W2 also shows a low turn-on voltage of 2.8 V and radiates intense warm-white light (CIE = 0.37, 0.51) with an Lmax of 29,190 cd m−2. Notably, device W2 achieves outstanding EL efficiencies of ηext,max, ηC,max, and ηP,max values of 26.2%, 89.6 cd A−1, and 100.5 lm W−1, respectively, with a small efficiency roll-off of 12.9% at 1000 cd m−2. To the best of our knowledge, device W2 is the most efficient all-fluorescence single-emitting layer white OLEDs reported so far (table S8) (, –). These results further validate the success of our molecular design of robust and efficient luminescent materials for high-performance OLEDs.

Fig. 9.

Device performance of warm-white OLEDs.

Device architectures and energy diagrams of (A) the nondoped white device and (B) the doped white device. (C) Molecular structures of Ir(tptpy)2acac and 4CzTPNBu. (D) Luminance–voltage–current density curves of devices W1 and W2. External quantum efficiency–luminance–power efficiency curves of devices (E) W1 and (F) W2. Inset in (E) and (F): EL spectra of devices W1 and W2 at 1000 cd m−2.

DISCUSSION

In this work, an effective molecular design strategy for highly efficient luminescent molecules is proposed and a series of robust carbonyl-cored molecules with strong sky-blue delayed fluorescence are developed. In these molecules, carbonyl and SFAC form a twisted D-A framework via a phenyl bridge, which is responsible for the generation of sky-blue delayed fluorescence. The presence of carbonyl core is favored for enhancing SOC and promoting RISC because of n-π* transition, which facilitates the occurrence of delayed fluorescence. Moreover, the n orbital on carbonyl core makes the T1 states of these molecules that are governed by n-π* transition more sensitive to the changes in matrix polarity than the S1 states dominated with π-π* transition. In comparison with neat films, the doped films of these molecules in a DPEPO host have larger increments of T1 energy levels than S1 ones, leading to reduced ΔEST values. Such kind of host tuning effect associated with the different polarities and transition types of S1 and T1 states can be used to effectively modulate ΔEST values and regulate delayed fluorescence property.

The introduction of carbazole units on the other side of carbonyl core barely interferes the electronic structures of HOMOs and LUMOs and thus shows no destructive effect on delayed fluorescence nature, but weakens the ICT effect to some degree, rendering blue-shifted emissions of the molecules. The presence of the carbazole units not only exerts apparent positive impacts on enhancing thermal stability and preventing emission quenching and exciton annihilation but also affects the carrier mobility by regulating the distribution of HOMO-1 orbitals and the stacking of the molecules. As the increase of carbazole units, the electron mobility of the molecules changes slightly, but hole mobility shows relatively larger variations. In TCP-BP-SFAC, the hole mobility and electron mobility achieve a best balance. In addition to the incorporation of SFAC, the expansion of molecular plane containing transition dipole moment by introduction of carbazole units can further promote horizontal dipole orientation of these molecules in film sates.

The synergistic effect of the above factors endows TCP-BP-SFAC with an excellent ΦPL value of 99%, a large Θ// value of 88.0%, and highly balanced hole and electron mobilities. Because of these distinguished merits, outstanding ηext,max values of up to 26.1 and 38.6% are attained in the state-of-the-art sky-blue nondoped (CIE = 0.17, 0.34) and doped (CIE = 0.16, 0.28) OLEDs of TCP-BP-SFAC, respectively. In addition, high-performance nondoped hybrid white OLED with a ηext,max of 17.1% and a ηP,max of 62.6 lm W−1, and all-fluorescence single-emitting layer white OLEDs with a ηext,max of 26.2% and a ηP,max of 100.5 lm W−1 are realized using TCP-BP-SFAC as blue emitter. These results not only eloquently demonstrate the superb competence of these luminescent molecules in OLED industry but also unveil that horizontal dipole orientation engineering of the molecules may bring about further breakthroughs in OLED researches.

MATERIALS AND METHODS

General information

All the chemicals and reagents were purchased from commercial sources and used as received without further purification. The final products were subjected to vacuum sublimation to further improve purity before PL and EL property investigations. 1H and 13C nuclear magnetic resonance (NMR) spectra were measured on a Bruker AV 500 spectrometer or a Bruker AV 400 spectrometer in CDCl3, CD2Cl2, or C2D6SO at room temperature. High-resolution mass spectra were recorded on a GCT premier CAB048 mass spectrometer operating in the matrix-assisted laser desorption/ionization–time-of-flight (MALDI-TOF) mode. Ultraviolet (UV)–visible absorption spectra were measured on a Shimadzu UV-2600 spectrophotometer. PL spectra were recorded on a Horiba Fluoromax-4 spectrofluorometer. PL quantum yields were measured using a Hamamatsu absolute PL quantum yield spectrometer C11347 Quantaurus_QY. Transient PL decay spectra were measured using Quantaurus-Tau fluorescence lifetime measurement system (C11367-03, Hamamatsu Photonics Co., Japan). Cyclic voltammograms were measured in a solution of tetra-n-butylammonium hexafluorophosphate (nBu4NPF6, 0.1 M) in dichloromethane (anodic) and dimethylformamide (cathodic) at a scan rate of 50 mV s−1, using a platinum wire as the auxiliary electrode, a glass carbon disk as the working electrode, and Ag/Ag+ as the reference electrode, standardized for the redox couple ferricenium/ferrocene (Fc/Fc+) (HOMO = −[Eonsetox +4.8] eV, and LUMO = −[Eonsetox +4.8] eV) (). Eonsetox and Eonsetox represent the onset oxidation and reduction potentials relative to Fc/Fc+, respectively. Thermogravimetric analysis (TGA) was carried out on TA TGA Q5000 under dry nitrogen at a heating rate of 20°C min−1. Thermal transition was investigated by TA DSC Q1000 under dry nitrogen at a heating rate of 10°C min−1. The ground-state geometries were optimized using the density functional theory (DFT) method with PBE0-D3 functional at the basis set level of 6-31G (d, p), and the excited-state geometries were optimized using the time-dependent DFT method with PBE0-D3 functional at the basis set level of 6-31G (d, p). The ΔEST values were calculated by the vertical excitation based on S0 geometry. The above calculations were performed using Gaussian16 package. The spin-density distributions of T1 states, the length, width, and height of P-BP-SFAC, CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC, the MPI values, and natural transition orbital analysis were analyzed with Multiwfn ().

P-BP-SFAC, CP-BP-SFAC, mCP-BP-SFAC, and TCP-BP-SFAC were synthesized and characterized according to the methods described in fig. S1. The detailed procedures and characterization data are given in the Supplementary Materials.

Horizontal dipole ratio measurement

For the measurement of orientation of emitting dipoles in molecules, a setup RSQX-01 made by Changchun Ruoshui Technology Development Co. Ltd. was used. The dipole orientation of doped and nondoped films was determined by angle-resolved and polarization-resolved PL on a half quartz cylinder prism. A continuous-wave He:Cd laser (325 nm) with a fixed angle of 45° to the substrate was used as excitation source. p-Polarized emitted light was detected at the respective peak wavelength of the PL spectrum of each film.

Carrier mobility measurement

The SCLC property can be described via the Mott-Gurney equation (Eq. 1), and the carrier mobility (μ) of organic semiconductors can be calculated according to the Poole-Frenkel formula (Eq. 2), where ε0 is the free-space permittivity (8.85 × 10−14 C V−1 cm−1), εr is the relative dielectric constant (assumed to be 3.0 for organic semiconductors), E is the electric field, μ0 is the zero-field mobility, γ is the Poole-Frenkel factor, and L is the thickness of the neat film of each molecule

By fitting the current density–voltage curves in the SCLC region according to Eq. 1 (), the μ0 and γ values are obtained, thus generating the field-dependent carrier mobility by Eq. 2.

OLED fabrication and measurement

The glass substrates precoated with a 90-nm layer of ITO with a sheet resistance of 15 to 20 ohms per square were successively cleaned in ultrasonic bath of acetone, isopropanol, detergent, and deionized water, respectively, taking 10 min for each step. Then, the substrates were completely dried in a 70°C oven. Before the fabrication processes, to improve the hole injection ability of ITO, the substrates were treated by O2 plasma for 10 min. The vacuum-deposited OLEDs were fabricated under a pressure of <5 × 10−4 Pa in a Fangsheng OMV-FS450 vacuum deposition system. Organic materials, LiF, and Al were deposited at rates of 1 to 2 A s−1, 0.1 A s−1, and 5 A s−1, respectively. The effective emitting area of the device was 9 mm2, determined by the overlap between anode and cathode. The luminance–voltage–current density and external quantum efficiency were characterized with a dual-channel Keithley 2614B source meter and a PIN-25D silicon photodiode. The EL spectra were obtained via an Ocean Optics USB 2000+ spectrometer, along with a Keithley 2614B source meter. For white OLEDs, the luminance–voltage–current density characteristics and electroluminescent spectra were obtained using a PhotoResearch PR670 spectroradiometer, with a Keithley 2400 source meter. All the characterizations were conducted at room temperature in ambient conditions without any encapsulation, as soon as the devices were fabricated.