Orange-Red Thermally Activated Delayed Fluorescence Conjugated Polymers toward Highly Efficient Solution-Processable Red and White OLEDs

Abstract

Due to the difficulty in achieving a suitable match between blue emitters and orange-red thermally activated delayed fluorescence (TADF) polymers, the development of solution-processable white organic light-emitting diodes (OLEDs) based on TADF polymers remains a significant challenge. Herein, a series of orange-red TADF polymers are developed using a strategy involving embedding the acceptor of quinoxaline-6,7-dicarbonitrile (QC) into the backbone and attaching the donor triphenylamine (T) with long alkyl chains as pendants. The synthesized polymers PCzTQCx exhibit orange-red emissions with emission peaks ranging from 571 to 621 nm. Non-doped, solution-processable organic light-emitting diode (OLED) devices based on these polymers exhibit pure white emission with a high color rendering index (CRI) of 80, achieved by using a TADF polymer with a low content of the TQC unit. In addition, warm white emission with a maximum external quantum efficiency (EQE_max) of 14.3%, CRI of 54 and red emission with an EQE_max of 9.3% and an emission peak at 609 nm can also be achieved. Through combination with a blue TADF small-molecule emitter, the single-emissive layer doped devices achieve warm white emission with EQE_max of 15.7%, luminance of 4325 cd m^–2, and CRI of 64.

1. Introduction

After three decades of development, organic light-emitting diodes (OLEDs) have become practically applicable in flat-panel displays for smartphones and televisions. However, commercial OLED devices are generally prepared using vacuum evaporation, a process that requires precise co-evaporation techniques, complex preparation procedures, and high power consumption. In comparison, solution-processed OLEDs offer several advantages such as low cost, ease of large-scale preparation, and flexible displays. , By virtue of their theoretical 100% internal quantum efficiency (IQE), thermally activated delayed fluorescence (TADF) polymers demonstrate great potential for large-scale industrial applications. Up to now, the blue, green, yellow, and red TADF polymers have made steady progress, accompanied by maximum external quantum efficiencies (EQE_maxs) of 23.2%, 32.4%, 24.8%, and 21.2%, respectively. However, the development of white OLEDs based on TADF polymers lags far behind.

Currently, there are two approaches for constructing TADF emitters: (1) multi-resonance (MR) effect-induced TADF emitters − with narrowband emission spectra (Scheme a) and (2) donor–acceptor (D-A) type TADF emitters , with wide emission spectra (Scheme b). Owing to the minimal structure relaxation in the excited state due to their high rigidity, the former emitters exhibit narrowband emission with small full width at half-maximum (FWHM) values (≤50 nm), which is advantageous for achieving high-definition displays. However, for white OLEDs with wide emission spectra, employing D-A type TADF emitters featuring large FWHMs of 70–120 nm is a more suitable choice. Evidently, using a blue D-A type TADF small-molecule emitters in combination with yellow, orange, or red D-A type TADF polymers is advisable to prepare solution-processable white OLEDs (Scheme c). This approach is easier to implement compared to a single white polymer (SWP) for white emission, which requires more complex synthesis routes. On the other hand, Förster resonance energy transfer (FRET) generally occurs from the blue emitter to the red one. Therefore, balancing the complex energy transfer processes between the blue TADF emitter and red TADF polymers while maintaining the emission efficiency of both emissions presents a formidable challenge. To date, several reports have highlighted blue D-A type TADF small-molecule emitters with high EQE_max values approaching 40%. − However, efficient orange and red TADF polymers have been scarcely reported to date. ,, Therefore, developing efficient orange and red TADF polymers with an appropriate emission peak is pivotal for achieving highly efficient white OLEDs.

1. Two Kinds of TADF emitters: (a) MR-TADF Emitters with Narrowband Emission spectra, (b) D-A Type TADF Emitters with Wide Emission Spectra, and (c) This Work for Constructing Solution-Processable White OLEDs by Combining a Blue D-A Type TADF Small Molecule and Orange D-A Type TADF Conjugated Polymers.

Generally, two primary factors need to be satisfied for designing efficient TADF emitters: one is a small energy gap (ΔE _ST) between the lowest singlet (S ₁) and triplet (T ₁), and another is a high photoluminescence quantum yield (PLQY). However, these two essential conditions are often in conflict. A small ΔE _ST can be achieved by minimizing the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), but this inevitably reduces the oscillator strength (f) and deteriorates the PLQY. In addition, according to the energy gap law, the non-radiative rate constant (k _nr) increases exponentially as the emission wavelength increases. Therefore, selecting a red TADF chromophore with high rigidity is vital for the development of efficient red TADF polymers. Quinoxalinedicarbonitrile, an ideal acceptor for red emitters, has been widely used in the design of efficient red emitters, benefiting from its strong electron-withdrawing ability and planar, rigid structure. , For example, Xu et. al reported a deep-red emitter, pCNQ-TPA, which uses quinoxaline-5,8-dicarbonitrile as the acceptor and triphenylamine as the donor. Based on this small-molecule emitter, an EQE_max of up to 30.3% with an emission peak at ≈690 nm was achieved in its device via vacuum evaporation. As reported in the literature, six strategies have been proposed for constructing TADF polymers, considering both the methods of D-A connection and approaches for incorporating TADF units. However, for red TADF polymers, the available strategies are limited. For example, in our previous work, a series of red TADF conjugated polymers, PCzAQCx, were prepared by embedding the acceptor of quinoxaline-6,7-dicarbonitrile into the backbone and attaching donor 9,10-dihydroacridine as pendant. The resulting solution-processed device exhibited a high EQE_max of 12.5% with an emission peak at 620 nm. After optimization of the hole injection layer, a record-high EQE_max of 21.2% was achievable in the device. Therefore, incorporating a TADF unit into the polymeric backbone using an appropriate approach is crucial for designing efficient orange/red TADF polymers.

Herein, a series of orange to red TADF conjugated polymers were synthesized by embedding an acceptor into the polycarbazole backbone and attaching donor as pendant. In this skeleton, the quinoxaline-6,7-dicarbonitrile (QC) was used as the acceptor, and triphenylamine (T) with long alkyl chains was used as the donor. Through adjustment of the molar ratios of the TADF unit TQC from 0.1% to 5%, a series of new polymers, PCzTQCx, were obtained. Generally, the LUMO of the TADF unit is primarily distributed on the acceptor. It is conceived that the motion of the TADF unit in the excited state was restricted when embedding the acceptor into the backbone, leading to a high radiative rate constant (k _r) and photoluminescence quantum yield (PLQY). As a result, PCzTQCx exhibited long-wavelength emission with main emission peaks at 571–621 nm, and a high PLQY of 60%. For the non-doped OLEDs with these polymers as emissive layers (EMLs), impressive EL performance was demonstrated as follows: (1) PCzTQC0.1, with a 0.1% molar content of the TQC unit, achieved perfect white emission with high CRI of up to 80 and Commission Internationate de I’Eclairage coordinates (CIE) of (0.32, 0.31), which was close to standard white emission; (2) PCzTQC0.5 exhibited warm white emission with the EQE_max of 14.3% and CRI of 54; (3) PCzTQC5 demonstrated pure red emission with an EQE_max of 9.3%, emission peak at 609 nm, and CIE coordinates of (0.61, 0.39). Furthermore, in combination with a blue TADF small molecule, 10-(4-((4-(9H-carbazol-9-yl)­phenyl)­sulfonyl)­phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF), which has a broad FWHM of 77 nm, the doped devices demonstrated warm white emission with a EQE_max of 15.7%, CIE coordinates of (0.44, 0.41), and CRI of 64 (Scheme c).

2. Results and Discussion

2.1. Molecular Design and Synthesis

Scheme illustrates the synthesis routes of the polymers PCzTQCx. Compound 1 was synthesized by reacting 1,2-bis­(4-bromophenyl)­ethane-1,2-dione with bis­(4-octylphenyl)­amine, resulting in a yield of 91%. Subsequently, monomer M₁ (2Br-TQC) was obtained through the reaction of compound 1 with 4,5-diamino-3,6-dibromophthalonitrile. The polymers PCzTQCx were synthesized by Suzuki polycondensation of M₁ with two carbazole derivatives (M₂ and M₃), and their feed ratios were 0.001:0.499:0.5 for PCzTQC0.1, 0.005:0.495:0.5 for PCzTQC0.5, 0.01:0.49:0.5 for PCzTQC1, and 0.05:0.45:0.5 for PCzTQC5, respectively. Poly­(2,7-carbazole) (PCz) was selected as the backbone instead of poly­(3,6-carbazole) due to its superior conjugation, which facilitates red-shifting of the photoluminescence (PL) spectra. , To improve the solubility of the monomer 2Br-TQC, the long alkyl chain octyl was introduced, compensating for the inferior solubility of phthalonitrile. The detailed synthetic routes of intermediates, monomer 2Br-TQC, and polymers were provided in the Supporting Information (Scheme S1). The common characterizations including ¹H NMR, ¹³C NMR, and mass spectroscopy were conducted to manifest the chemical structures of these compounds. The polymers were characterized by ¹H NMR.

2. Synthetic Routes of the Polymers .

^a Reagents and conditions: (i) Pd­(OAc)₂, t-Bu₃P·HBF₄, K₂CO₃, toluene, 120 °C. (ii) AcOH, 120 °C. (iii) Pd­[P­(o-tol)₃]₂Cl₂, 2 M K₃PO₄ (aq.), THF, 75 °C. (iv) Phenylboronic acid, 75 °C; bromobenzene, 75 °C.

As expected, all polymers show good solubility in common organic solutions, such as chlorobenzene, tetrahydrofuran, and dichloromethane. According to the measurements of gel permeation chromatography (GPC), the number-average molecular weight (M _n) of these polymers decreased progressively from 69.8 to 14.6 kDa as the molar ratios of the TQC unit increased from 0.1% to 5% (Table S1, Supporting Information), hinting at the negative impact of TQC unit on the M _n of polymer. Correspondingly, their polydispersity indexes (PDIs) ranged from 1.96 to 2.73 (Table S1, Supporting Information). Of note, PCzTQCx exhibited higher M _n values than other TADF polymers , with a poly­(3,6-carbazole) backbone due to the better linear configuration of the poly­(2,7-carbazole) backbone, which is beneficial for achieving high M _n. Additionally, these polymers exhibit good thermal stability, with decomposition temperatures (T _d, with 5% weight loss) ranging from 355 to 452 °C and glass transitions temperatures (T _g) between 75 and 84 °C (Figure S4 and Table S1).

2.2. Theoretical Calculations

To give insight into the geometric structure and frontier molecular orbital (FMO) distribution of these polymers, theoretical calculations based on density functional theory (DFT) were conducted on the model compound Cz-TQC-Cz (Figure ). In the ground state, the torsion angels between triphenylamine and the QC unit are around 33.3°, which are significantly lower than those of previous red TADF polymers, PCzAQCx, with donor acridine groups as pendants. Meanwhile, the dihedral angels between the QC unit and the its adjacent carbazoles are approximately 50°. Similar to typical D-A type TADF polymers, the distribution of FMOs of the model compound demonstrated good separation, featuring the HOMO located on the two triphenylamine groups and the LUMO primarily delocalized on the QC unit. In addition, the LUMO extend slightly to the two phenyl bridges and the adjacent carbazoles. The energy gap (E _g) of the model compound is 2.84 eV, which is suggestive of the yellow/red emissions of these polymers. To further investigate the electronic structures in the excited states, the time-dependent DFT (TD-DFT) calculations were performed on Cz-TQC-Cz. The distributions of the hole and particle states of S ₁ and T ₁ are in good agreement with the HOMO and LUMO distributions of the model compound, respectively. The moderate overlap of the particle and hole in S ₁ provides a high oscillating strength (f) of 0.344. To further dig into electronic structures of the model compound, the spin–orbit coupling (SOC) values between singlet and triplet states were calculated (Figure S2). Impressively, the SOC matrixes ⟨S ₁|Ĥ _SOC|T ₁⟩ of 0.435 cm^–1, ⟨S ₁|Ĥ _SOC|T ₂⟩ of 0.182 cm^–1, and ⟨S ₁|Ĥ _SOC|T ₃⟩ of 0.674 cm^–1 are comparable to those of TADF emitters , with the heavy atom effect, indicating the effective RISC process of the PCzTQCx polymers. In addition, the Huang–Rhys factors (HRFs) at different vibration modes (v) were also stimulated (Figure S3), and the dominated HRFs of below 20 involve low-frequency twisting vibration at carbazoles (v = 4.37 and 6.15 cm^–1) and rotation of the methyl on the triphenylamine (v = 7.13 cm^–1). These results were consistent with the strategy’s effect of reducing non-radiative loss by embedding acceptor into backbone and attaching donor as pendant for orange-red TADF polymers.

1.

Theoretical calculations of model compound Cz-TQC-Cz.

2.3. Photophysical and Electrochemical Properties

To probe the photophysical properties of PCzTQCx, the absorption and PL spectra of polymers in both dilute solution and film states were measured, as shown in Figure , and Table lists the detailed photophysical parameters. In toluene solution, the strong absorption peaks around 387 nm were attributed to the n-π* and π-π* transitions of PCz. When the content of the TQC unit increased to 5%, new peak at 530 nm emerged, corresponding to the charge transfer (CT) transition from the TQC unit. The profiles of the absorption spectra of polymers in film were found to be quite similar to those in solution.

2.

(a) UV/Vis absorption and (b) PL spectra of polymers in toluene solution. (d) UV/Vis absorption spectra and (e) PL spectra of polymers in the neat film. (c) Prompt decay curves and (f) full-range decay curves of the transient decay spectra of the neat films of polymers under deaerated atmosphere at room temperature.

1. Photophysical and Electrochemical Properties of the Polymers.

Polymers	λabs [nm] sol/film	λPL[nm] sol/film	HOMO /LUMO [eV]	E S1/E T1 [eV]	ΔE ST [eV]	PLQY [%]	τ P(ϕPF) [ns (%)]	τ d (ϕDF) [μs (%)]	k r [108 s–1]	k nr [108 s–1]	k RISC [104 s–1]
PCzTQC0.1	387/396	417/427, 452, 571	–5.70/–2.86	2.45/2.34	0.11	38	2.2(25.3)	25.6(12.7)	1.2	1.9	5.9
PCzTQC0.5	387/395	415, 593/426, 450, 587	–5.69/–2.86	2.36/2.16	0.20	55	2.9(35.3)	31.5(19.7)	1.2	1.0	5.0
PCzTQC1	387/395	417, 602/425, 450, 588	–5.69/–2.89	2.33/2.13	0.20	60	2.6(33.2)	31.2(26.8)	1.3	0.9	5.8
PCzTQC5	384, 530/392, 534	415, 602/621	–5.65/–3.32	2.24/2.03	0.21	54	3.5(35.8)	25.8(18.2)	1.0	0.9	5.9

^aRecorded in toluene solution at room temperature.

^bDetermined in neat film at room temperature.

^cMeasured by cyclic voltammetry in neat film.

^dCalculated from the HOMO and the optical energy gap (E _g) values.

^eSinglet (E _S1) and triplet (E _T1) excited energy levels determined from the onset wavelengths of fluorescence spectra at room temperature and phosphorescence spectra at 77 K in the neat film, respectively.

^fΔE _ST is equal to E _S1 – E _T1.

^gAbsolute PL quantum yield evaluated using an integrating sphere under an argon atmosphere in a neat film.

^hThe prompt fluorescence (τ _P) and delayed fluorescence lifetime (τ _D) and the component of the prompt fluorescence (ϕ_PF) and delayed fluorescence (ϕ_DF) to the PLQY for the neat film in an argon atmosphere.

ⁱThe radiative rate constants (k _r) and non-radiative rate constants (k _nr) from S ₁ to S ₀ and the rate constants of reverse intersystem crossing (k _RISC).

In solution, PCzTQC0.1, with a low content of the TQC unit, exhibited blue emission peaking at 417 nm. As the molar ratio of the TQC unit increased from 0.5% to 5%, PCzTQCx showed a progressively intensified emission peak in the range of 593–602 nm, in addition to the blue emission. Evidently, like the similar red TADF polymers PCzAQCx as reported in the literature, the blue emission in the new polymers PCzTQCx was from the emission of PCz, while the emissions at the long wavelength area were attributed to Cz-TQC-Cz. In the film case, as the molar ratio of the TQC unit increased from PCzTQC0.1 to PCzTQC1, these polymers showed dual emissions with gradually red-shifted main peaks from 571 to 588 nm, along with a weak peak at around 427 nm. These phenomena were attributed to incomplete energy transfer from PCz to the Cz-TQC-Cz unit. Meanwhile, as the molar ratio of the TQC unit further increased to 5%, PCzTQC5 showed complete energy transfer, with red emission peaking at 621 nm. The emission at 621 nm of PCzTQC5 was blue-shifted to that (645 nm) of the analogous red TADF polymer PCzAQC5 with donor acridine as the pendant. This shift indicates the weaker electron-donating ability of diphenylamine relative to that of acridine.

From PCzTQC0.1 to PCzTQC5, the S ₁/T ₁ energy levels of the polymers progressively decreased from 2.45/2.34 eV to 2.24/2.03 eV, leading to their small ΔE _STs in the range of 0.11–0.21 eV. These small ΔE _STs indicate their TADF characteristics. To verify this, the transient PL decay spectra of these polymers in neat films were recorded . As shown in Figure c and f and Table , all polymers exhibited evident prompt fluorescence within 2.2–3.5 ns and delayed fluorescence in the range of 25.6–31.5 μs. In addition, all polymers exhibited evident quenching of the delayed fluorescence under an oxygen atmosphere (Figure S6), further demonstrating the TADF characteristics of these polymers. Impressively, all polymers demonstrated high radiative rate constants (k _r) exceeding 10⁸ s^–1, with non-radiative rate constants (k _nr) at comparable levels. The rate constants of reverse intersystem crossing (k _RISC) were in the range of 5.0 × 10⁴ to 5.9 × 10⁴ s^–1. Moreover, these polymers showed moderate PLQYs lying within 38% to 60%.

2.4. Electroluminescence Properties

As promoted by the good PL performance, the EL performance based on these polymers was evaluated. The device structure was as follows: ITO/PEDOT:PSS (40 nm)/EML (40 nm)/POT2T­(20 nm)/ANT-BIZ (30 nm)/Liq­(1 nm)/Al (100 nm). In these devices, the commercial indium tin oxide (ITO) served as the anode, poly­(ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) was employed as hole injection layer through spin-casting, the polymers PCzTQCx or the mixtures of polymer and small-molecule emitter acted as EMLs via solution-processing, (1,3,5-triazine-2,4,6-triyl)­tris­(benzene-3,1-diyl)­tris­(diphenylphosphine oxide) (PO-T2T) served as the hole blocking layer, 1-(4-(10-([1,1′-biphenyl]-4-yl)­anthracen-9-yl)­phenyl)-2-ethyl-1H-benzo­[d]-imidazole (ANT-BIZ) acted as the electron transport layer, 8-hydroxyquinolinato lithium (Liq) was used as the electron injection layer, and Al (aluminum) was used as the cathode. Among these layers, POT2T/ANT-BIZ/Liq/Al were deposited in sequence through vacuum evaporation. The key information on these devices, including the energy level diagram and chemical structures of materials used, is presented in Figure . The EL performance is also illustrated, encompassing EL spectra, plots of current density and voltage versus luminance, curves of current efficiency and luminance versus power efficiency, and the EQE–luminance traces. Table lists the key data for these EL devices.

3.

Performance of the non-doped devices based on polymers: (a) energy level diagram (eV), (b) chemical structures of materials used in EL devices, (c) EL spectra (recorded at 11 V), (d) plots of current density and luminance versus voltage, (e) curves of current efficiency and power efficiency versus luminance, and (f) EQE–luminance traces.

2. Summary of the Key Data for EL Devices.

			Max Performance
EMLs	EL [nm]	V on [V]	L [cd m–2]	CE [cd A–1]	PE [lm W–1]	EQE [%]	CIE (x, y)	FWHM [nm]	CRI​
PCzTQC0.1	428, 454, 581	5.5	1094	10.0	5.7	4.1	(0.32, 0.31)	118	80​
PCzTQC0.5	427, 455, 582	4.5	3217	33.8	21.2	14.3	(0.46, 0.42)	91	54​
PCzTQC1	427, 456, 583	3.0	3749	25.1	26.3	11.9	(0.51, 0.44)	89	47​
PCzTQC5	609	3.5	4562	13.6	12.3	9.3	(0.59, 0.40)	95	0​
CzAcSF:PCzTQC0.5 (20 wt %)	427, 454, 477, 583	4.5	2348	31.7	19.9	14.3	(0.42, 0.40)	101	70​
CzAcSF:PCzTQC0.5 (50 wt %)	427, 456, 479, 585	5.0	4325	35.2	20.1	15.7	(0.44, 0.41)	98	64​

^aRecorded at 11 V.

^bRecorded at 12 V.

Initially, considering that these polymers could exhibit dual emissions, their non-doped devices were evaluated. Remarkably, the device based on PCzTQC0.1 demonstrated nearly standard white emission with CIE coordinates of (0.32, 0.31) and a high color rendering index (CRI) of 80% due to its complementary dual emissions, although an EQE_max of only 4.1% was achieved. As the molar ratio of the TQC unit increased from 0.1% to 5%, the emission at long wavelength in the devices based on these polymers gradually intensified. Ultimately, the blue emission in the device based on PCzTQC5 vanished due to the complete energy transfer, which is consistent with its PL spectra. Among these devices, the one based on PCzTQC0.5 exhibited warm white emission with the highest EQE_max of 14.3%, emission peak at 582 nm, FWHM of 91 nm, and CRI of 54. Of note, the EQE_max of 14.3% was relatively higher than that of the similar polymer PCzAQC0.5 (12.5%) reported in our previous work. In addition, PCzTQC1 achieved a relatively lower EQE_max of 11.9%. Notably, all of these devices, when the molar ratio of the TQC unit in PCzTQCx was below 1%, exhibited color-tunable and intelligent lighting characteristics under voltage-driven operation (Figure S8–S10). Finally, PCzTQC5 demonstrated red emission with an EQE_max of 9.3%, emission peak at 609 nm, and CIE coordinates of (0.59, 0.40), accompanied by relatively stable EL spectra (Figure S11 and Table S4).

To fully utilize the high EL performance of PCzTQC0.5 and further improve the CRI value of the white OLED, the doped devices based on PCzTQC0.5 were also fabricated by combining it with a typical D-A type blue TADF small-molecule emitter CzAcSF. CzAcSF was selected as both the host material and sensitizer due to its excellent sensitizing effect and compatibility with orange-red TADF polymers, enabling white emission. This was achieved thanks to its wide FWHM and suitable emission peak at 477 nm. By adjusting the doping ratio of PCzTQC0.5 from 20 to 50 wt %, two doped devices were evaluated. As shown in Figure , their EL characteristics were presented, with key EL data listed in Table . Excitingly, warm white emission with an EQE_max of 14.3%, CIE coordinates of (0.42, 0.40)), high CRI of 70, and FWHM of 101 nm was achievable by using 20 wt % PCzTQC0.5. Evidently, the FWHM of 101 nm in this doped device was larger than that of its non-doped device (91 nm), indicating the positive effect of CzAcSF in broadening the EL spectra for white emission, although it did not enhance the EQE. Moreover, by further increasing the doped ratio of PCzTQC0.5 to 50 wt %, the device achieved a higher EQE_max of 15.7%, with a luminance of 4325 cd m^–2, CRI of 64, and FWHM of 98 nm. As the voltage increased from 3.5 to 13 V, the blue emission progressively intensified, leading to gradually increased CRI values in these two devices. This phenomenon is similar to that in our previous TADF polymer PCzT1-Q0.1 with green and red TADF units simultaneously incorporated in poly­(2,7-carbazole) backbone. To the best of our knowledge, these results based on PCzTQCx are among the best in terms of the performance of white OLEDs based on TADF polymers (Table S6).

4.

Performance of the doped devices with PCzTQC0.5:CzAcSF as emissive layers: (a) EL spectra (recorded at 12 V), (b) current density–voltage–luminance characteristics, (c) curves of current efficiency and power efficiency as functions of luminance, and (d) traces of EQE–luminance.

3. Conclusion

By using the strategy of embedding acceptor into backbone/attaching donor as pendant, a series of TADF polymers PCzTQCx with long wavelength emissions were developed. The neat films of these polymers demonstrated orange-red emissions with EL peaks from 571 to 621 nm and large FWHMs of over 110 nm. The non-doped device based on PCzTQCx with a low content of the TQC unit achieved perfect white emission with a high CRI of 80 and CIE coordinates of (0.32, 0.31). In addition, the highest EQE_max of 14.3% with emission peak at 582 nm, FWHM of 91 nm, and CRI of 54 could be achieved. Simultaneously, a red emission with EQE_max of 9.3%, emission peak at 609 nm, and CIE coordinates of (0.59, 0.40) was achievable in the EL device based on the polymer with a high molar ratio of the TQC unit (5%). By using a blue TADF small molecule as both the host and sensitizer, the doped devices exhibited warm white emission with the highest EQE_max of 15.7%, high luminance of 4325 cd m^–2, and CRI of 64. These results indicated that simultaneously using a D-A type blue small-molecule TADF emitter and orange-red TADF polymers featuring broad FWHMs as emissive layers is a versatile approach for developing highly efficient solution-processable white OLEDs.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/polymscitech.5c00046.

• Experimental section, synthesis and characterizations, theoretical calculations, thermal properties and molecular weights, photophysical properties, transient PL decay spectra, electrochemical properties, electroluminescent device and measurements, and ¹H NMR and ¹³C NMR spectra (PDF)

The authors declare no competing financial interest.