Overcoming power efficiency limitation of white fluorescence light‐emitting diodes via multilevel‐hydrogen‐bond matrix

High power efficiency and low efficiency roll‐off at practical luminance are two requirements for new‐generation energy‐saving lighting technologies, which are still bottlenecks of thermally activated delayed fluorescence (TADF) white organic light‐emitting diodes (WOLED), despite the advantages of TADF materials and devices in low cost and high sustainability. Herein, we developed a spiro phosphine oxide host named SSOXSPO, which can form multiple and multidirectional intermolecular hydrogen bonds (IHB). The resulted multilevel IHB network integrates long‐range ordered and short‐range disordered alignments for suppressing triplet‐polaron quenching (TPQ) and triplet‐triplet annihilation (TTA). Electronic characteristics of SSOXSPO matrix are further regulated, leading to the optimal exciton allocation through balancing energy and charge transfer. As consequence, using SSOXSPO as host, the single‐emissive‐layer TADF WOLEDs realized the record performance, including ultralow operation voltage as ∼4.0 V, power efficiency beyond fluorescent tube (70.1 lm W−1) and negligible external quantum efficiency roll‐off (3%) at 1000 nits for indoor lighting. This work demonstrates that multiple interplays supported by host matrixes in TADF WOLEDs can facilitate the synergistic effects of TADF emitters on 100% exciton utilization.

It is noted that TADF molecules are commonly based on donor-acceptor (D-A) structures with small singlet-triplet splitting energy (ΔE ST ), rendering effective reverse intersystem crossing (RISC) and efficient delayed fluorescence (DF). [11]Simultaneously, the ambipolar characteristics of D-A molecules benefit the carrier injection and transportation.Nonetheless, high molecular polarities of TADF emitters render strong short-and long-range intermolecular interactions, resulting in collisional quenching [12] and dipole-dipole quenching [13] .7l,14] Therefore, "ideal" hosts in all-TADF WOLEDs should effectively suppress quenching effects, and simultaneously balance carrier flux and exciton allocation. [15]The previous reports showed that regular intermolecular hydrogen bond (IHB) networks in host matrixes can establish effective carrier transportation channels to balance carrier flux, which facilitates carrier recombination, therefore reduces excessive hole or electron induced triplet-polaron quenching (TPQ) (Figure 1A). [16]7g,17] Obviously, the "ideal" strategy is combining the advantages of these two modes through constructing host matrixes with multilevel IHBs, which can form IHB networks featuring long-range order and short-range disorder for simultaneous TPQ and TTA suppression.
Regarding to this complementary design of IHB networks, host molecules with several different hydrogen bond acceptors are promising, since the differentiation of hydrogen bond intensities, densities and orientations is the basis of forming locally diverse but whole consistent networks.Herein, we demonstrate the effectiveness of this "multilevel hydrogen bond" strategy for developing high-performance hosts in TADF WOLEDs.A spirocyclic phosphine oxide (PO) host SSOXSPO is developed with the structure of ortho-diphenylphosphine oxide (DPPO) substituted spiro[thioxanthene-9,9′-xanthene]-S,Sdioxide (SSOX), whose σ-O, sulfuryl π-O S and phosphoryl π-O P can play the roles of hydrogen-bond acceptors with totally different polarities and orientations (Figure 1B).It is shown that functions of SSOXSPO molecules are markedly dependent on their hydrogen-bond environment.Triplet excited states are localized on single SSOXSPO, which are protected by adjacent three molecules, leading to effective quenching suppression and doping concentration-insensitive DF; meanwhile, the matrix provides two linear and cyclic carrier transportation modes, which harmonize carrier flux balance and recombination.SSOXSPO-optimized energy transfer between sky-blue TADF emitter named bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) and yellow TADF emitter named 2,3,5,6-tetrakis(3,6di-t-Butylcarbazol-9-yl)−1,4-dicyanobenzene (4CzTPNBu), resulting in ∼100% photoluminescence quantum yield (PLQY, ϕ PL ).Based on single-EML, SSOXSPO endowed its TADF WOLEDs with a record-high maximum η PE reaching 113.6 lm W −1 , which is the combined result of the ultralow turn-on voltage of 2.6 V and the state-of-the-art maximum external quantum efficiency (EQE, η EQE ) of 30.3%.Especially, at 1000 cd m −2 , η PE beyond 70 lm W −1 from TADF WOLEDs was demonstrated for the first time, which is equal to that of fluorescent tube.Meanwhile, EQE roll-off was significantly limited to a record-low value of 3%, reflecting the optimal exciton formation, allocation and radiation in EMLs.

Molecular design and structure
To construct multilevel IHB networks, two key structural factors should be considered: (i) Balance between IHB intensities and directions.The different kinds of strong IHBs lead to diverse local sub-structures and thereby short-range disorder; while, the comparable weak IHBs support consistent interactions with long-range order.Various IHB orientations are crucial for establishing relatively uniform and widely distributed network in long range; (ii) balance between steric hindrance and intermolecular interactions.Appropriate steric effect of bulky groups is desired to avert π-π stacking and make IHBs predominant in intermolecular interactions.Thus, we adopt spiro cyclic core, whose steric hindrance is further amplified by ortho-DPPO group.However, single crystal Xray diffraction result shows that the respective and mutual dihedral angles of xanthene and thioxanthene S,S-dioxide rings are 164.1B).Furthermore, markedly inclined thioxanthene leads to its intersection angle to xanthene of only 86.6 • , due to steric effect of DPPO.As consequence, highly twisted configuration of SSOXSPO completely suppresses intermolecular π-π stacking, which is in accord to previous reports of distorted spiro hosts. [18]ifferently, SSOX core provides an σ-O and two sulfuryl π-O S as hydrogen bond acceptors, in addition to π-O P of DPPO group.σ-O at the molecular center forms three strong intramolecular hydrogen bonds with thioxanthene and phenyls of DPPO (bond lengths of only ∼2.7 Å).Moreover, the hydrogen bond length between P=O and ortho-H of xanthene is also as short as ∼2.5 Å.It is convincing that the highly rigid structure of SSOXSPO can effectively restrain ground-state vibration and excited-state relaxation.Single crystal packing diagram further reveals the basic repeat unit as an octamer, which is composed of four dimers (Figures 1C  and S1 and S2).Two strong IHBs with short lengths of ∼2.6 Å between π-O P and thioxanthene link two adjacent SSOXSPO molecules to form the dimer.Sulfuryl π-O S and phosphoryl π-O P further generated weaker but multiple IHBs with bigger lengths of >3 Å at two opposite directions, respectively resulting in two different tetramers.It is clear that the multiple hydrogen bond acceptors indeed support multilevel IHB network in SSOXSPO matrix, in which the F I G U R E 1 Molecular design, structure, and steady-state photophysical properties.(A) Hydrogen-bond networks in host matrixes and the corresponding influences on carrier and exciton processes: (i) ordered hydrogen-bond networks can establish the effective and balanced carrier transporting channels to facilitate carrier combination and therefore alleviate triplet-polaron quenching (TPQ); (ii) disordered hydrogen-bond networks can avert too strong intermolecular interaction induced triplet-triplet annihilation (TTA); (iii) hydrogen-bond networks with long-range order and short-range disorder can integrate carrier flux balance and intermolecular interaction limitation for simultaneous TPQ and TTA suppression; (B) chemical and single crystal structures of SSOXSPO with three different types of hydrogen bonds.Solid and hollow arrows represent intermolecular and intramolecular hydrogen bonds, respectively.Intramolecular hydrogen bonds are highlighted with red dash lines, which are based on oxygen atoms of xanthene and P=O moieties; (C) single-crystal packing diagrams of SSOXSPO dimer and octamer (along a axis), in which red and blue dash lines refer to intermolecular hydrogen bonds (IHB) formed by SSOX cores and P=O groups, respectively, and green dash lines correspond to π-π stacking in dimer; (D) Electronic absorption, fluorescence (FL) and time-resolved phosphorescence (PH) spectra of SSOXSPO in dilute dichloromethane solution, and emission spectra of SSOXSPO, and DMAC-DPS and 4CzTPNBu as blue and yellow TADF emitters in film; (E) photoluminescence (PL) spectra of vacuum-evaporated SSOXSPO:30% DMAC-DPS:y% 4CzTPNBu (y = 0.5, 1.0, 1.5 and 2.0), SSOXSPO:30% DMAC-DPS (blue dash line) and SSOXSPO:5% 4CzTPNBu (yellow dash line) films.Inset indicates the energy transfer process from SSOXSPO to TADF dopants.heterogeneous molecular densities reflect the combination of local disorder and general order, therefore establishing the basis for balancing carrier transportation and quenching suppression in EMLs of devices.
Taking virtue of the different ortho-hydrogen activities for O and S atoms, SSOXSPO can be selectively synthesized from spiro[thioxanthene-9,9′-xanthene] (STX) through a successive three-step reaction of direct lithiation, phosphorization and oxidation with a good total yield of ∼50% (Scheme S1).Its aromatic structure renders the high enough decomposition temperature of 420 • C, making film formation through thermal vacuum evaporation feasible (Figure S3 and Table S1).The strong intramolecular hydrogen bonds render the glass transition temperature as high as 233 • C, which is beneficial for alleviating phonon relaxation induced energy loss and exciton quenching.It is noted that the melting point of SSOXSP only increases by 63 to 296 • C, verifying the comparable intensities of intramolecular and intermolecular hydrogen bonds, and the exclusion of strong π-π interactions in aggregation states.

Photophysical properties
In dilute dichloromethane solution, SSOXSPO shows four electronic absorption bands respectively peaked at 332, 300, 264, and 232 nm, corresponding to n→π* transitions of SSOX and P=O moieties and π→π* transitions of aromatic rings (Figure 1D).In neat film, all the absorption bands and peak wavelengths are basically retained, revealing the limited intermolecular interactions in solid state (Figure S4 and Table S1).Owing to its high structural rigidity, SSOXSPO reveals narrow fluorescent (FL) emissions respectively peaked at 329 and 325 nm in solution and neat film, which are mainly ascribed to SSOX centered radiative transitions (Figure 1D and Table S1).Besides the smaller full width at half maximum (FWHM) of neat film (37 nm) than that in solution (40 nm), FL spectrum of SSOXSPO in dilute solution is markedly broadened in long-wavelength range, due to solvation effect-induced structural relaxation.It demonstrates that IHB network formed in SSOXSPO film effectively restrains phonon vibration and regulates the intermolecular interactions.DMAC-DPS (S 1 /T 1 ≈ 2.8 eV) and 4CzTPNBu (S 1 /T 1 ≈ 2.2 eV) are used as dopants to prepare singly and dually doped films through vacuum evaporation.Estimated with absorption edge, the first singlet energy level (S 1 ) of SSOXSPO is 3.86 eV (Table S1).According to timeresolved phosphorescence (PH) spectrum in solution at 77 K (Figure 1D), its first triplet energy level (T 1 ) is 2.99 eV, which is equal to the value of its neat film (3.00 eV, corresponding to 0-0 transition at 413 nm) (Figure S4).Such high T 1 energy level can support positive energy transfer to the S 1 /T 1 energy levels of DMAC-DPS and 4CzTPNBu (∼2.8 eV and ∼2.2 eV).Therefore, photoluminescence (PL) spectra of SSOXSPO films respectively doped with 30% DMAC-DPS and 5% 4CzTPNBu only consist of dopants originated emissions.PL quantum yield (ϕ PL ) of SSOXSPO:30% DMAC-DPS is as high as 97%, owing to the suitable hostdopant triplet energy gap.In contrast, too big triplet energy gap markedly reduces ϕ PL of SSOXSPO:5% 4CzTPNbu to 63%.The successive host-dopant-dopant energy transfer ren-ders the continuously changed white emissions of dually doped SSOXSPO:30% DMAC-DPS:y% 4CzTPNBu films, whose ratios of blue and yellow components are strongly dependent on y (y = 0.5, 1.0, 1.5 and 2.0).It is shown that two dopants are complementary in exciton utilization at y = 0.5 and 1.0, but become mutually competitive at y = 1.5 and 2.0.As a result, ϕ PL reaches the maximum value of 99% at y = 1.0, but rapidly decreases to 82% at y = 2.0.
Time decay curves of SSOXSPO:x% DMAC-DPS:y% 4CzTPNBu films show that at x = 30, the lifetimes of both blue prompt fluorescence (PF) and delayed fluorescence (DF) are in reverse proportion to y, due to blue-to-yellow energy transfer (Figure 2A).Moreover, the variation of blue PF lifetime is remarkably larger than that of blue DF lifetime, manifesting the predominance of singlet-featured Förster resonance energy transfer (FRET) in dopant-dopant energy transfer.Thus, yellow PF lifetimes of dually doped films are longer than that of yellow-emitting films, and also reversely proportional to y, because of the energy transfer from DMAC-DPS.Yellow DF lifetimes of white-emitting films show the similar situation, however, they are shorter than that of SSOXSPO:5% 4CzTPNBu (Figure 2B).Furthermore, compared to singly doped film, both yellow PF and DF decays of dually doped films reveal the rising ups at the beginning, indicating the involvement of DMAC-DPS in energy transfer.It verifies that DMAC-DPS provides intermediate excited energy levels to accelerate the energy transfer from SSOXSPO to 4CzTPNBu.
Time-resolved emission spectra (TRES) indicate that PF intensities of singly doped blue and yellow-emitting films are markedly lower then those of dually doped white-emitting films; while, DF proportions of the former are higher in turn (Figure 2C).In accord to spectral overlapping result, triplet DXET is predominant in SSOXSPO-to-dopant energy transfer.In contrast, dopant-dopant energy transfer in dually doped films is highly dependent on y: (i) at y = 0.5, yellow PF proportion as the majority also indicates the predominance of singlet FRET; (ii) at y = 1.0, the comparable yellow PF and DF intensities reflect that singlet FRET and triplet DXET are nearly equal; (iii) at y ≥ 1.5, more yellow DF components reflect more predominant triplet DXET.The beginning stage of exciton formation and energy transfer is investigated with sliced TRES in time range of 0-6 μs after excitation (Figure 2D).It shows that owing to higher concentration and smaller triplet energy gap, the exciton formation in SSOXSPO:30% DMAC-DPS is ∼0.3 μs earlier than SSOXSPO:5% 4CzTPNBu, manifesting the faster host-dopant energy transfer in the former.More importantly, for SSOXSPO:30% DMAC-DPS:y% 4CzTPNBu films, the starting times of blue components are unchanged, but yellow components can be observed earlier through increasing y.Consequently, at y = 0.5, yellow emission is mainly based on energy transfer from DMAC-DPS; at y = 1.0, blue and yellow components are observed at nearly the same time, indicating the synergism between two dopants in exciton utilization; at y ≥ 1.5, yellow components can be observed earlier since direct exciton capture by 4CzTPNBu, in turn markedly reducing excitons confined on DMAC-DPS.So, it is rational that RISC by DMAC-DPS and 4CzTPNBu can complement at y = 1.0, therefore improving triplet utilization.
It is noted that in SSOXSPO matrix, the rate constants of prompt (PF, k PF ) and delayed fluorescence (DF, k DF ) of DMAC-DPS and 4CzTPNBu are comparable (Table S2).However, the rate constant of single radiation (k S r ) of DMAC-DPS is twice of that of 4CzTPNBu, and the rate constants of singlet (k S nr ) and triplet nonradiation (k T nr ) of the former are less than a half of those for the latter.Furthermore, DMAC-DPS has markedly higher ratio of RISC efficiency (ϕ RISC ) to intersystem crossing (ISC) efficiency (ϕ ISC ).For SSOXSPO:30% DMAC-DPS:y% 4CzTPNBu films, along with y increasing, k PF and k DF values of DMAC-DPS persistently decerase, but those values of 4CzTPNBu are firstly reduced at y = 0.5, and then recovered.Noteworthily, k PF of 4CzTPNBu is even doubled at y = 2.0.It is rational that the singlet FRET from DMAC-DPS facilitates exciton convergency to the S 1 state of 4CzTPNBu, leading to enhanced yellow PF.For the same reason, k S r values of 4CzTPNBu also markedly increase by ∼4 folds, but its k T nr values only slightly decrease.It indicates that at y ≥ 1.5, 4CzTPNBu already competes with DMAC-DPS in triplet capture, rendering ϕ PL decreases.Nevertheless, it should be noticed that despite much higher total doping concentration of SSOXSPO:30% DMAC-DPS:2% 4CzTPNBu, its ϕ PL is still one third higher than that of SSOXSPO:5% 4CzTPNBu, reflecting the dopant-dopant synergistic effect in exciton utilization amplified by SSOXSPO.

White OLED performance
Cyclic voltammetric analysis shows that the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of SSOXSPO are −6.52 and −2.74 eV (Figure S5a), which are respectively deeper and shallower than those of DMAC-DPS and 4CzTPNBu.So, holes and electrons can be effectively confined on these TADF dopants (Figure 3A).This case will facilitate direct exciton formation on the dopants during EL process, and simultaneously alleviate field-induced exciton decomposition.The intrinsic carrier transporting ability of SSOXSPO was further estimated with volt-ampere characteristics of single-layer single-carrier transporting devices (Figure S5b).It is shown that besides markedly lower threshold voltage, current density (J) of electron-only devices is 1-2 orders of magnitude larger than hole-only analogs.According to space charge limited current (SCLC) model, the electron mobility of SSOXSPO reaches 7.8 × 10 6 cm 2 V −1 s −1 , which is comparable to conventional electron-transporting materials, and two orders of magnitude higher than its hole mobility (Table S1).Obviously, SSOXSPO displays electron-predominant injecting and transporting properties.as electron transporting layer, respectively (Figures 3A  and S6).For comparison, monochromic devices of SSOXSPO:x% DMAC-DPS and SSOXSPO:5% 4CzTPNBu were also fabricated with the same configuration.For sEML white TADF diodes, y was further tuned in the range of 0.5%-2.0%with an interval of 0.5%, to figure out the influence of 4CzTPNBu concentration on exciton allocation and utilization.Furthermore, the control devices with dual EMLs (dEML) of SSOXSPO:30% DMAC-DPS|SSOXSPO:5% 4CzTPNBu, and sEML of SSOXSPO:10% FIrpic:y% 4CzTPNBu (y = 0.1-1.0)were also fabricated.For blue devices, x was tuned in a range of 10%-40% to confirm the highest η EQE of 21.2% achieved at x = 30 (Figurs S7), which was further adopted to fabricate white devices.Nonetheless, it is noted that driving voltages, the maximum brightness and efficiencies of blue devices are actually comparable at different x (Table S3).It means at x ≥ 10, SSOXSPO and DMAC-DPS can effectively mutually complement in carrier injection, transportation and recombination.In contrast, the efficiencies of SSOXSPO:5% 4CzTPNBu-based yellow devices were only about a half of those of blue devices, because of inefficient SSOXSPO-to-4CzTPNBu energy transfer (Figure S8 and Table S4).
In accord to PL spectra of dually doped films, sEML white TADF diodes of SSOXSPO:30% DMAC-DPS:y% 4CzTPNBu revealed the complementary white emissions, whose correlated color temperatures (CCT) gradually decreased from ∼7000 to ∼3700 K in reverse proportion to y, corresponding to cool white and warm white, respectively (Figures 3B and Figure S9a and Table S4).The angular distributions of EL emission for the devices indicated the increased forward ratio, which is roughly consistent with Lambertian emission (Figure S10).The devices showed the stable EL spectra nearly independent on driving voltage, rendering negligible Δx/Δy < 1 and CCT variation within 1000 K.It should be noted that different to gradually changed blue/yellow ratio of PL spectra, increasing y from 0.5 to 1.0 resulted in a sharp decrease of EL blue components (Figure S9b).So, in addition to energy transfer, direct carrier capture by 4CzTPNBu also made significant contributions to yellow emission.However, the further deceases of blue components at y = 1.5 and 2.0 were relatively smaller.In this sense, y = 1.0 was the turning point of exciton allocation.At y < 1.0, the dopantdopant energy and carrier transfer were not sufficient enough; while, at y > 1.0, the competition between the dopants on exciton allocation became dominant.In contrast, dEML TADF devices showed markedly stronger blue components, corresponding to cool-white emission with a high CCT value of ∼8000 K.It indicates spatially restrained dopant-dopant energy transfer and the main recombination zone in the EML of SSOXSPO:30% DMAC-DPS (Figure S11).
Compared to blue analogs, the driving voltages of the sEML TADF WOLDs were reduced by 0.4-0.8V, indicating the synergistic effect of DMAC-DPS and 4CzTPNBu on carrier recombination and exciton utilization (Figure S9c and Table S4).It is noted that at y = 1.0, the driving voltages of 2.6, <3.6, and <4.1 V at 1, 100 and 1000 cd m −2 were realized (Figure 3C), which are record-low values for all-TADF WOLEDs, and make the devices among the best low-voltage-driven WOLEDs.This is the first reported all-TADF WOLED can be directly driven by a commercial single-cell polymer lithium battery with working voltage of 4.2-4.4V for indoor lighting (∼300 lumens).The influence of y on current density (J) was rather limited, indicating the predominance of SSOXSPO and DMAC-DPS in carrier injection and transportation.Furthermore, it is noteworthy that accompanied with one order of magnitude decreased luminance, the driving voltages of dEML devices dramatically increased by 2-10 V, which were nearly equal to those of yellow analogs.Thus, the markedly reduced driving voltages of sEML TADF WOLEDs should be owing to increased luminance by 4CzTPNBu codoping and markedly improved exciton radiation, which is further verified by their doubled maximum luminance in comparison to blue/yellow analogs (Table S4).Obviously, because of its intermediate energy levels of the S 1 and T 1 states and frontier molecular orbitals, DMAC-DPS serves as the "distributor" of exciton and carriers, therefore can cooperate with 4CzTPNBu for all exciton utilization in sEML, rather than mutually compete in dEML.
It is shown that the efficiencies of sEML TADF WOLDs were strongly dependent on y (Figure S9d and Table S4).At y = 0.5, the devices achieved the maximum values of 67.7 cd A −1 for current efficiency (CE, η CE ), 78.7 lm W −1 for η PE and 22.7% for η EQE , which were already among the best results of all-TADF WOLEDs.Then, the highest efficiencies were realized at the turning point of y = 1.0, which were up to 94.1 cd A −1 and 30.3% (Figure 3D).More importantly, the combination of 30.3% as one of the highest η EQE values and the record-low driving voltages gives rise to the recordhigh η PE value of 113.6 lm W −1 for all-TADF WOLEDs.It is interesting that the maximum η EQE at y = 1.0 was almost equal to the sum of those of blue and yellow diodes, reflecting the desired complementarity and cooperation of DMAC-DPS and 4CzTPNBu in exciton utilization.Further increasing 4CzTPNBu concentration induced efficiencies decreased to 67.2 cd A −1 , 81.2 lm W −1 and 20.2%, and 61.1 cd A −1 , 73.8 lm W −1 and 18.1% at y = 1.5 and 2.0, respectively, due to increased ratio of exciton directly captured by relatively low-efficiency 4CzTPNBu.The importance of dopant-dopant synergy to exciton utilization is further verified by dEML TADF WOLEDs as a counter example, whose spatially separated dopants rendered markedly lower maximum η EQE of 12.9% as the mean value of those of blue and yellow diodes.
Inspiringly, sEML TADF WOLDs further displayed the excellent efficiency stability (Figure S9d and Table S4).At y = 1.0, η PE still remained as high as 81.1 and 70.1 lm W −1 at 100 and 1000 cd m −2 , respectively, demonstrating SSOXSPO-hosted devices to be the first practical all-TADF lighting source with the energy-saving performance at fluo-rescent tube level (Figure 3D).Besides low driving voltages, the state-of-the-art efficiencies at the practical brightness is based on extremely low roll-offs.Especially at y = 0.5 and 1.0, η EQE values were 25.2% and 29.3% at 1000 cd m −2 , respectively, corresponding to negligible EQE roll-offs of 1.6% and 3.3%.Notably, roll-offs were directly proportional to y, which is identical to the variation tendency of η EQE .It implies that the triplet exciton confinement on 4CzTPNBu should be the main reason inducing triplet quenching and EQE roll-offs, on account of much more serious roll-offs for yellow diodes (55.3%).In contrast, for blue diodes of SSOXSPO:x% DMAC-DPS, increasing x from 10% to 30% reduced roll-off by 8%.The long-range order feature of SSOXSPO further in turn renders the relative ordered distribution of high-concentration DMAC-DPS.In this case, SSOXSPO and DMAC-DPS are complementary in carrier injection and transportation, facilitating carrier flux balance in EMLs for effective TPQ suppression.Nonetheless, different to dEML TADF diodes with the roll-off (56.6%) equal to those of yellow diodes, the dramatically reduced roll-offs of sEML TADF WOLDs were simultaneously lower than those of blue and yellow diodes.Moreover, the perfomances of sEML TADF WOLEDs were markedly higher than those of PH analogs, due to the stronger carrier capture ability and higher host compatability of the TADF dopants than phosphores (Figure S12 and Table S5).Obviously, the complementarity of DMAC-DPS and 4CzTPNBu in RISC and limited dopant-dopant interaction at y = 1.0 in SSOXSPO matrix led to effective TTA suppression.
As a result, SSOXSPO endowed its sEML diodes with the highest comprehensive performances among reported TADF WOLEDs (Figure S13).The record values of η PE and roll-offs at 1000 nits demonstrate the competence of TADF systems for new-generation daily lighting.

Exciton kinetics
EL time-decay curves of blue and white diodes based on SSOXSPO:30% DMAC-DPS:y% 4CzTPNBu show that EL lifetimes of blue components were basically in reverse proportion to y, but the variation was limited, especially at y ≥ 1.5 (Figure 4A).The combination of small reductions of blue EL lifetimes and markedly increased yellow intensities verifies the effectively suppressed dopant-dopant quenching during exciton allocation processes.In contrast, yellow EL lifetimes of the WOLEDs revealed the opposite tendency, which were proportional to y, but still shorter than that of yellow diodes.Obviously, in the ternary system, DMAC-DPS actually accelerated radiation of 4CzTPNBu.It indicates that the dependences of PL and EL time decays on doping concentrations were nearly identical.Since both energy transfer and carrier capture were involved in EL process, SSOXSPO matrix provided a uniform and highly effective exciton and carrier migration network, making these two exciton allocation channels equally efficient and consistent.In this case, the energy and carrier transfer from DMAC-DPS to 4CzTPNBu were also readily modulated by y variation.Time resolve EL emission spectra (TREES) were measured to figure out the detailed exciton kinetic processes in the devices (Figure 4B).It is shown that in accord to PL result, at y = 0.5, blue EL DF intensity was nearly unchanged, reflecting the predominance of singlet FRET in exciton allocation between dopants.In contrast, when y ≥ 1.0, blue EL DF was gradually but slightly decreased, which verified the triplet confinement on 4CzTPNBu mainly through direct carrier capture rather than Dexter energy transfer.For yellow EL DF, increasing y from 0.5 to 1.0 led to marked acceleration of DF, owing to the enhanced FRET.However, further increasing y to ≥1.5 rendered the opposite tendency, namely yellow EL DF intensity and duration directly proportional to y, since direct triplet utilization by 4CzTPNBu became dominant.
To further confirm the details of carrier recombination and EL PF stages, sliced TREES of the devices are compared in the time ranges of 0-50 and 50-100 μs, respectively (Figure 4C).It is noted that the start time of carrier recombination for yellow diodes was ∼5 μs earlier than that of blue analogs, but their times for high exciton concentrations are nearly identical, which reflects the predominance of exciton formation through carrier capture by the dopants at the biginning and then host-dopant energy transfer.For WOLEDs, the beginning times of exciton formation on 4CzTPNBu were independent on y.However, carrier recombination on DMAC-DPS was markedly postponed when y increasing, reflecting the significance of direct carrier capture by 4CzTPNBu, which is different to PL processes.Nonetheless, it is noteworthy that the times of high exciton concentrations for yellow components reversely became ear-lier, which should be ascribed to improved energy transfer from SSOXSPO matrix to 4CzTPNBu through DMAC-DPS.Furthermore, different to blue EL DF, blue EL PF intensity was linearly decreased, and almost in direct proportion to y, manifesting the effective singlet FRET to 4CzTPNBu.Consistently, yellow EL PF durations of all WOLEDs were longer than that of yellow diodes, and independent on y, owing to high FRET efficiency.
Therefore, it is convincing that at the stage of exciton formation, carrier capture was predominant at the stage of carrier recombination, assistant with energy transfer; while, at the stage of exciton allocation, singlet FRET should be the predominant channel for dopant-dopant exciton allocation, accompanied by carrier transfer.This case facilitated the complementarity of blue and yellow dopants in exciton utilization.But at high y ≥ 1.5, DXET between dopants became inevitable, leading to the undesired competition between DMAC-DPS and 4CzTPNBu with respect to triplet utilization.
Obviously, the exciton allocation processes are mainly controlled by intermolecular interplays in host.To illustrate the crucial influences of SSOXSPO matrix on exciton and carrier migration, the electronic characteristics of the ground (S 0 ), S 1 and T 1 states for monomer and oligomers in single crystal matrix are simulated with density functional theory (DFT) and time-resolved DFT (TDDFT) methods (Figure 5).LUMO mainly localized on one xanthene ring of another molecule, rendering the effective electron confinement.So, SSOXSPO matrix provides two complementary carriermigration modes, namely linear mode for wide-range and balanced carrier transportation and cyclic mode facilitating carrier capture and recombination by local dopants.Furthermore, it is noted that the cyclic-type tetramer has the shallowest HOMO and the deepest LUMO.In this case, after rapid dispersion in whole matrix, carriers would be uniformly localized for energy or charge transfer to the dopants.
Natural transition orbital (NTO) analysis shows that similar to monomer, the S 1 state of dimer is locally excited (LE) state, but uniformly dispersed on two xanthene rings of two SSOXSPO molecules.In contrast, linear-type tetramer reveals LE state centralized on single xanthene ring, but the S 1 state of cyclic-type tetramer features charge transfer (CT) with "hole" and "particle" respecitvely localized on two adjacent xanthene rings.Compared to the other oligomers, cyclic-type tetramer has the lowest S 1 energy level with the gap of ∼0.1 eV.Since the low oscillator strength of singlet CT state, this feature of SSOXSPO can in turn facilitate energy and charge transfer to dopants.Furthermore, the T 1 states of monomer, dimer, and tetramers are identically localized on a single xanthene ring, corresponding to triplet LE states.The centralized S 1 and T 1 states of tetramers are isolated and protected by surrounding molecules, which is beneficial to collisional quenching suppression.The multilevel IHB network in SSOXSPO matrix reveals progressive effects on optoelectronic properties, which can not only control molecular alignment but also effectively regulate electronic characteritics.The balanced carrier diffusion and confinement, CT-featured S 1 and single-molecule localized T 1 states of SSOXSPO matrix promote carrier capture and FRET, and suppress TTA and TPQ during host-dopant and dopant-dopant exciton allocations.

CONCLUSION
A "multilevel hydrogen bond" strategy is demonstrated with a spirocyclic PO host SSOXSPO containing three hydrogen-

F
I G U R E 2 Transient emission properties of SSOXSPO hosted films.(A) Time decay curves of (A) blue and (B) yellow prompt fluorescence (PF, insets) and delayed fluorescence (DF) at 460 and 560 nm, respectively, from SSOXSPO:x% DMAC-DPS:y% 4CzTPNBu films.x = 0 for yellow-emitting film and 30 for blue and white-emitting films; y = 5.0 for yellow-emitting film and 0.5-2.0 for white-emitting films; (C) time-resolved emission spectra (TRES) of SSOXSPO:x% DMAC-DPS:y% 4CzTPNBu films dependent on doping concentrations.Sky-blue and yellow arrows mark the variation tendencies of blue and yellow DF components; (D) Concentration dependence of sliced TRES spectra for SSOXSPO:x% DMAC-DPS:y% 4CzTPNBu films in allocation (2-3 μs) and decay (>3 μs) stages after excitation.

F I G U R E 3
Electroluminescence characteristics of white organic light-emitting diodes (OLEDs).(A) Device structures and energy level diagrams of single emissive-layer (sEML) and double emissive-layer (dEML) thermally activated delayed fluorescence (TADF) and sEML phosphorescence (PH) white organic light-emitting diodes (WOLED); (B) electroluminescent (EL) spectra (left) and corresponding Commission Internationale de IEclairage (CIE) coordinates on chromatic panel for three kinds of WOLEDs; (C) current density (J)-voltage-luminance curves of the devices with the optimized doping concentrations.Arrow indicates the curves related to luminance; (D) efficiencies versus luminance relationships of the WOLEDs.η PE and η EQE values at the maximum, 100 and 1000 cd m −2 of sEML TADF WOLDs were highlighted with arrows; (E) Summary of the reported η PE values at the maximum (symbols) and 1000 cd m −2 (columns) and EQE roll-offs (dash line) for selected all-TADF WOLEDs.Values reported in this work are highlighted with red star symbols.

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Exciton kinetics of organic light-emitting diodes (OLEDs).(A) Electroluminescent (EL) time decays, (B) EL time-resolved emission spectra (TRES) (ELTRES) contours of blue and yellow components and (C) sliced EL contours of WOLEDs based on SSOXSPO:x% DMAC-DPS:y% 4CzTPNBu, in comparison to blue (x = 30, y = 0) and yellow (x = 0, y = 5) devices.Sliced EL contours were recorded in the time ranges of 0-50 μs at recombination stages and 50-100 μs at decay stages, respectively.

F I G U R E 5
Theoretical simulation of SSOXSPO oligomers at the ground (S 0 ), S 1 and T 1 states.The relationships between intermolecular hydrogen bonds (IHB) and electronic characteristics of oligomers under different IHB interactions, including the highest occupied (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of the S 0 state, and "holes" and "particles" of natural transition orbitals (NTO) for the first singlet (S 1 ) and triplet (T 1 ) states.Hydrogen-bond linkage styles are illustrated on the arrows, in which blue and red lines represent P=O⋯H and S=O⋯H, respectively.FMO, 1 CT and 3 LE refer to frontier molecular orbitals, singlet charge transfer and triplet locally excited states.f S and σ are singlet oscillator strength and transition weight.The single molecule indicates the HOMO and the LUMO overlapped on xanthene ring.Its LUMO simutaneously disperses to peripheral thioxanthene S,S-dioxide, revealing the electron-transporting predominance.It is noted that IHBs have significant influences on FMO distributions.Despite the slight LUMO dispersion on thioxanthene and DPPO, IHBs between thioxanthene S,S-dioxide and P=O render the FMOs of dimer are mostly localized and nearly thoroughly overlapped on xanthene rings.Remarkably enhanced bimolecular electron exchange is verified by the continuous and cyclic LUMO distribution on whole dimer.IHB linkage of adjacent dimers gives rise to two kinds of tetramers respectively through up-down S=O⋯H and right-left P=O⋯H, corresponding to linear and cyclic types.Different to monomer and dimer, the HOMOs and LUMOs of tetramers are thoroughly separated, whose HOMOs are localized on a single xanthene ring.Meanwhile, the LUMO of linear-type tetramer is widely and relatively uniformly dispersed on the other three molecules, establishing a continuous electron migration channel.In opposite, cyclic-type tetramer reveals the bond acceptors of σ-O, sulfuryl π-O S and phosphoryl π-O P .The different intensities and orientations of P=O⋯H and S=O⋯H give rise to IHB network in SSOXSPO matrix, simultaneously controlling molecular alignment and electronic characteristics of the S 0 , S 1 , and T 1 states.As consequence, SSOXSPO supports the efficient carrier flux balance and rational exciton allocation in EMLs through regulating host-dopant and dopant-dopant energy and charge transfer processes.As consequence, its sEML TADF WOLEDs realized the record-high maximum η PE beyond 110 lm W −1 .At 1000 nits, the devices revealed the record values of ultralow operation voltage of ∼4.0 V and η PE of 70.1 lm W −1 , which was equal to fluorescent tubes.Such high efficiency stability was further verified by the record-low EQE roll-off of 3%.This work demonstrates the synergistic and complementary effects of multilevel interactions on optoelectronic optimization of multiple-component systems, and simultaneously opens the window for the practical TADF lighting applications.A C K N O W L E D G M E N T SRD and PM contributed equally to this work.This study was supported by National Natural Science Foundation of China (grant numbers: 92061205, 62175060, 51873056, 61905070, and 22005088), the Changjiang Scholar Program of Chinese Ministry of Education (grant number: Q2021256), and National Science Fund for Excellent Young Scholars of Heilongjiang Province (grant numbers: YQ2020B006 and YQ2022B010).C O N F L I C T O F I N T E R E S T S TAT E M E N TThe authors declare no conflict of interest.O R C I DHui Xu https://orcid.org/0000-0002-2687-5388RE F E R E N C E S