Highly efficient white-emitting thermally activated delayed fluorescence polymers: Synthesis, non-doped white OLEDs and electroluminescent mechanism

Nano Energy 65 (2019) 104057

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Highly efficient white-emitting thermally activated delayed fluorescence polymers: Synthesis, non-doped white OLEDs and electroluminescent mechanism

T

Chensen Lia, Yuwei Xub, Yuchao Liua, Zhongjie Rena,∗, Yuguang Mab,∗∗, Shouke Yana,c,∗∗∗ a

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, PR China b State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou, 510640, PR China c Key Laboratory of Rubber-Plastics, Ministry of Education, Qingdao University of Science & Technology, Qingdao, 266042, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Single white-emitting polymers Thermally activated delayed fluorescence polymers Non-doped solution processed light-emitting diodes

Single white-emitting polymers are greatly desired to develop solution-processed white organic light-emitting diodes (WOLEDs) with simple fabrication processes and low-energy-consumption. Thermally activated delayed fluorescence (TADF) polymers harvesting both triplet and singlet excitons without heavy metals are promising candidates for solution-processable white-emitting devices. However, all-TADF polymers with efficient white emissions have not been well-developed. Herein, a new strategy is proposed to develop a series of all-TADF single white-emitting polymers for highly efficient non-doped solution-processed WOLEDs. A WOLED based on one of these polymers as the emitting layer achieves an EQEmax of 14.2%, a CEmax of 38.8 cd A−1, and a PEmax of 20.3 lm W−1 with the CIE coordinates of (0.33, 0.42) and (0.31, 0.39) at 100 cd m−2 and 2000 cd m−2, respectively. The double-channel trap-assisted and Langevin recombinations account for broad recombination zones, reduced formation of high-energy excitons on hosts and enhanced efficiency of PDTPT-1, 2&3 based OLEDs. As far as it is known, PDTPT-1 is the highest efficient white-emitting TADF polymer so far.

1. Introduction White organic light-emitting diodes (WOLEDs) have received persistent attention due to their extensive application in solid-state lighting [1–4]. Especially, solution-processed WOLEDs are believed to be a promising field due to its simple fabrication processes and low cost [5–7]. To this end, polymeric emitting materials are ideal choice for large-area solution-processed WOLEDs because of their good solubility, homogeneous film morphology and tuneable emission colours [8]. However, the electroluminescence (EL) performance of solution-processed WOLEDs is still inferior to that of vacuum-deposited WOLEDs, and thus further improvement is urgently required for the practical applications [9,10]. Recently, thermally activated delayed fluorescence (TADF) materials have been developed to produce efficient fluorescence, utilizing both singlet and triplet excitons and thus an inner quantum efficiency (IQE) of 100% can be reached [11–13]. So far,

highly efficient TADF emitters with a wide range of emissive colours have been reported, including red, green, and blue emitters [14–18]. Therefore, TADF polymers incorporating with two or three emissive species were also developed for solution-processed WOLEDs [19,20]. For example, our group developed the first TADF polymers based warm-white OLEDs, showing dual yellow TADF and blue fluorophores emission with maximum external quantum efficiency (EQEmax) of 10.4%, Commission Internationale de l’Eclairage (CIE) coordinates of (0.37, 0.38) and high colour rendering index (CRI) of 77 [18]. Similarly, Cheng et al. [19] reported a series of WOLEDs based on twocomponent TADF/fluorophor polymers achieving an EQEmax of 9.9% with the CIE coordinates of (0.42, 0.45). However, those solution-processed TADF/fluorophores hybrid WOLEDs consisting of multi-component systems (hosts and polymers) intrinsically suffer from phase separation and inadequate excitons utilization efficiency, leading to the deterioration of device efficiency and an unwanted colour shift.

∗

Corresponding author. Corresponding author. ∗∗∗ Corresponding author. Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, PR China. E-mail addresses: [email protected] (Z. Ren), [email protected] (Y. Ma), [email protected] (S. Yan). ∗∗

https://doi.org/10.1016/j.nanoen.2019.104057 Received 17 June 2019; Received in revised form 16 August 2019; Accepted 27 August 2019 Available online 29 August 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.

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2. Experimental section

Therefore, it is necessary to develop all-TADF single white emission polymers (SWPs) to avoid phase separation, achieve adequate excitons utilization efficiency and thus to improve the EL performances [21]. Generally, SWPs are developed by covalently integrating complementary emissive species into one polymer chain, which can produce broad-band white emissions via partial energy transfer from highenergy species to low-energy ones [6]. However, the highly efficient energy transfer would result in a low fluorescence radiative efficiency for high-energy emitters, and thus resulting in an inadequate short wavelength emission. In contrast, the low energy transfer rate might decrease the energy capture in low-energy components, bringing about a decline of long wavelength emission. Therefore, the trade-off between energy transfer and radiative transition is an essential factor for high efficiency white emission of polymers [9]. In addition, in terms of energy transfer, it is anticipated that enhancing long-distance Förster resonance energy transfer (FRET) and declining insufficient short-distance Dexter energy transfer (DET), which can be realized in polymers with very low concentration of emissive species (< 1%) [7]. Meanwhile developing SWPs with low concentration of emitting components also remains a tough task [22]. Design and synthesis of polymeric emitter with high emitting-component ratio with sufficient spacers among emitters provide a promising method for preparing high-efficiency SWPs. Moreover, the rational design of spacers (hosts) among emitters in the SWPs is also favourable for improvement of the emission efficiency. A host with high triplet energy (ET) is required to confine the triplet excitons all on the emitters. However, enhancing ET would inevitably increase the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the host, which leads to a large barrier for charge injection [21]. In addition, lack of an appropriate deep-blue TADF emitters has undoubtedly restricted the development of efficient full-TADF SWPs. So far, few of deep-blue TADF emitters can achieve shallow HOMO level (< 5.6 eV), high EL efficiency, low efficiency roll-off and good solutionprocessability simultaneously [23–28]. Herein, a series of bichromophoric full-TADF SWPs were developed to fabricate highly efficient non-doped solution-processed WOLEDs. Specifically, we designed and synthesized a series of non-conjugated TADF polymers with polyethylene as the backbone, pendant 9,9-dimethyl-10-phenyl-acridan (BDMAc) [29,30] as host units, 2-(9,9-dimethyl-acridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DMATXO2) [31,32] as deep-blue emitters and 2-(10H-phenothiazin-10-yl) dibenzothiophene-S,S-dioxide (PTZ-DBTO2) [32–36] as yellow emitters. The host BDMAc with a high T1 of 3.38 eV and a shallow HOMO level of −5.25 eV, which can prevent triplet energy back-transfer (TEBT) from the blue and yellow emitters to the hosts, and give a very small energy barrier of only 0.05 eV for hole injection from ITO/PEDOT:PSS anode (−5.20 eV) to the polymer. For deep-blue TADF emitter, DMA-TXO2 still remains high PLQY of 90% and triplet energy level (3.08 eV) compared with 2,7-bis(9,9-dimethyl-acridin-10-yl)-9,9-dimethylthioxan-thene-S,S-dioxide (DDMA-TXO2), but the HOMO energy of DMA-TXO2 shifts to a higher energy from −6.10 eV to −5.50 eV, easily matching with the host. For yellow emitters, PTZ-DBTO2 with low T1 of 2.45 eV has moderate HOMO energy of −5.33 eV and low LUMO energy of −2.63 eV, which well matches with LUMO of TmPyPB and consequently improves the injection efficiency of electrons. The resulting polymers with different ratios of three components display obvious TADF features and white emission. Non-doped WOLEDs based on these polymers as the emitting layers achieve an EQEmax of 14.2% with the CIE coordinates of (0.33, 0.42) at 100 cd m−2, which surpasses those of the solution-processed hybrid WOLEDs, and can even be comparable to those of all phosphorescent WOLEDs [19,20].

2.1. General synthetic procedure for the polymers A mixture of AIBN (5 mg, 0.03 mmol), toluene (8.0 mL)/THF (20 mL), and the different ratios of 2-(9,9-dimethyl-acridin-10-yl)-8vinyl dimethylthioxanthene-S,S-dioxide (DMA-TXO2), 9,9-dimethyl-10(3-vinylphenyl)-9,10-dihydroacridine (BDMAc) and 2-(10H-phenothiazin-10-yl)-8-vinyldibenzothiophene-S,S-dioxide (PTZ-DBTO2) were placed in an ampule, which were cooled, degassed, and sealed in vacuum. After stirring at 60 °C for 20 h, the reaction mixture was poured into a large excess of methanol. The white polymers were obtained by filtration and then were dried in vacuum. The polymers were fractionated by Soxhlet extraction using hexane. PDTPT-1: DMA-TXO2 (133 mg, 0.43 mmol), BDMAc (500 mg, 1.6 mmol) and PTZ-DBTO2 (73 mg, 0.17 mmol) were used in the polymerization (yield: 81%). Elemental analysis. Found: C 82.89; H 6.07, N 3.98, S 1.73%. PDTPT-2: DMA-TXO2 (86 mg, 0.13 mmol), BDMAc (525 mg, 1.7 mmol) and PTZ-DBTO2 (40 mg, 0.09 mmol) were used in the polymerization (yield: 87%). Elemental analysis. Found: C 85.68; H 6.64, N 4.19, S 0.98%. PDTPT-3: DMA-TXO2 (45 mg, 0.09 mmol) and BDMAc (600 mg, 1.93 mmol) and PTZ-DBTO2 (23 mg, 0.05 mmol) were used in the polymerization (yield: 83%). Elemental analysis. Found: C 86.13; H 6.53, N 4.32, S 0.61%. 2.2. XRR measurements The films of TADF polymers with a thickness of 30 nm were spincoated on Si(100) substrates. XRR patterns were measured using a SmartLab diffractometer (In-plane) with attachment of Chi phi Z cradle using CuKα radiation (λ = 0.15418 nm) at 40 kV and 200 mA. The film densities were estimated from the critical angles for the total external reflection of the organic layers. Curve fitting and calculations were performed using the GlobalFit program package. 2.3. Devices fabrication and characterization The hole-injection material PEDOT:PSS (Al 4083), electron-transporting and hole-blocking material TmPyPB were obtained from commercial sources. ITO-coated glass with a sheet resistance of 10 Ω per square was used as the substrate. Before device fabrication, the ITOcoated glass substrate was thoroughly cleaned in ultrasonic bath of tetrahydrofuran, isopropyl alcohol, detergent, deionized water, and isopropyl alcohol and treated with O2 plasma for 10 min in sequence. PEDOT:PSS was then spin-coated onto the clean ITO substrate as a holeinjection layer. Next polymer solution in chlorobenzene was spincoated (10 mg mL−1; 2000 rpm) to form a ca. 30 nm thick emissive layer and then annealed at 80 °C for 30 min to remove the residual solvent. Finally, a 40 nm thick electron-transporting layer of TmPyPB was vacuum deposited, and a cathode composed of a 1 nm thick layer of LiF and aluminum (100 nm) was sequentially deposited through shadow masking with an array of 3 mm × 3 mm openings under a pressure of 10−5 Torr. Deposition rates are 1–2 Å. s−1 for organic materials, 0.1 Å. s−1 for LiF, and 6 Å. s−1 for Al, respectively. EL spectra were recorded by an optical analyzer, Photo Research PR745. The current density-voltage-luminance (J-V-L) characteristics of the devices were measured using a Keithley 2400 Source meter and Konica Minolta chromameter CS-200. The EL spectra were recorded using a JYSPEX CCD3000 spectrometer. The EQE values were calculated from the luminance, current density, and electroluminescent spectrum. For measurement of the transient electroluminescence characteristics, shortpulse excitation with a pulse width of 20 μs was generated using a pulse generator (Tektronix AFG3011C). The period was 50 μs, delayed time was 30 μs, and duty cycle was 30%. The decay curves of EL were detected using a transient spectrometer (Princeton Instruments, Acton 2

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Fig. 1. (a) Synthetic route for PDTPT-1, 2&3. Absorbance and PL spectra of PDTPT-1, 2&3 in toluene (b) and in film (c) at 300 K. (d) PL spectra of PDTPT-1 in different solvents and the corresponding Lippert-Mataga plot (inset). (e) Arrhenius curve of reverse intersystem crossing rate plotted versus various temperatures for the evaluation of thermally activated energy of PDTPT-1, 2&3 at 520 nm.

thermal stability (Fig. S3 and Table S1). Fig. 1b&c shows the ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectra of PDTPT-1, 2&3 both in toluene and the film states. All of them exhibit the same π-π* transition peak at ca. 290 nm assigned to the host (Table 1), indicating that blue and yellow pendants have small contribution to the absorption because of the low loading contents. In toluene solution, the PL profiles of PDTPT-1, 2&3 are constituted by the emission of DMA-TXO2 at ca. 448 nm, a slight emission of host at ca. 370 nm and a low shoulder peak of PTZ-DBTO2 at ca. 520 nm. The emission of host can be attributed to the inadequate energy transfer from host to emitters in solution. In addition, the tiny emission of yellow emitter (PTZ-DBTO2) may be assigned to the weak intramolecular energy transfer and the quenching of charge transfer state of PTZ-DBTO2 in the polar solvent. Typically, in addition to the emission of host at ca. 370 nm and DMA-TXO2 at ca. 428 nm for PDTPT-1 in nonpolar hexane, a relatively obvious emission of PTZDBTO2 at ca. 530 nm can also be observed (Fig. 1d). However, PL peak of PTZ-DBTO2 become weaker with increasing solvent polarity. Meanwhile, the obvious redshift of PL peak at ca. 430 nm (DMA-TXO2)

SpectraPro SP-2300). All measurements were performed at room temperature under ambient conditions.

3. Results and discussion As depicted in Fig. 1a, the polymers were prepared by radical copolymerization of monomers DMA-TXO2, BDMAc and PTZ-DBTO2 with different feed molar ratios. Three polymers with DMA-TXO2/BDMAc/ PTZ-DBTO2 feed molar ratios of 13.3/78.6/8.1, 7/89.5/3.5, and 4.4/ 93/2.6 were denoted as PDTPT-1, PDTPT-2 and PDTPT-3, respectively. The actual contents of the blue and yellow units were estimated from elemental analysis data to be ca. 14.1% and 6.2% for PDTPT-1, 7.8% and 4.4% for PDTPT-2, 6.3% and 2.3% for PDTPT-3. Their structures and molecular weights are confirmed by 1H NMR spectra and GPC, respectively (see Fig. S1&S2, Table S1). These polymers can be readily dissolved in common organic solvents, such as chloroform, toluene, and chlorobenzene. Their thermal decomposition temperatures (Td) with 5% weight loss range from 385 to 394 °C by TGA, and glass transition temperatures (Tg) are in the range of 178–186 °C, indicating good 3

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Table 1 The photophysical data of PDTPT-1, 2&3.

PDTPT-1 PDTPT-2 PDTPT-3 a b c d e f g h i j k

λabsa (nm)

λPLb (nm)

HOMOc (eV)

LUMOd (eV)

PLQYe (%)

τd/ratiof [μs]/[%]

τd/ratiog [μs]/[%]

kRISCh (105 s−1)

kRISCi (105 s−1)

ΔESTj (eV)

ΔESTk (eV)

288 290 290

445,521 442,516 436,515

−5.38 −5.28 −5.22

−1.63 −1.54 −1.49

69 57 56

6.9/31 8.3/32 11.1/37

15.5/88 13.5/87 12.9/87

2.10 1.78 1.43

7.26 6.98 6.80

0.03 0.03 0.05

0.02 0.03 0.03

Measured in toluene solution at room temperature. Measured in pristine film at room temperature. Calculated according to EHOMO = −(E(onset,ox vs Fc+/Fc) + 5.1) by CV. Calculated according to LUMO = HOMO + Eg. Absolute photoluminescence quantum yield (PLQY) in film determined in N2 atmosphere error ± 2%. Measured in pristine film at room temperature in vacuum at 440 nm. Measured in pristine film at room temperature in vacuum at 520 nm. The rate constant of reverse intersystem crossing at 440 nm. The rate constant of reverse intersystem crossing at 520 nm. Energy gap between singlet and triplet at 440 nm. Energy gap between singlet and triplet at 520 nm.

to achieve a balanced two-color white emission. The singlet-triplet energy splitting (ΔEST) values for blue emitters in PDTPT-1, 2&3 are estimated experimentally from the onset of the PL and phosphorescence spectra at 77 K (Fig. S8), which are 0.03 eV for PDTPT-1, 0.03 eV for PDTPT-2 and 0.05 eV for PDTPT-3. Furthermore, the ΔEST values of yellow emitters are quantitatively evaluated by fitting Arrhenius plot according to the following equation [43,44]:

with increasing solvent polarity is found, i.e. solvatochromic effect. The Lippert-Mataga plot (νabs -νem against polarity of solvent) exhibits a slope of ~7168 cm−1 for PDTPT-1, indicating that the excited state of blue emitter possesses strong charge transfer character [37]. The PL spectra of PDTPT-2&3 in different solvents can be seen in Fig. S4. In contrast, all polymers show two distinctive emission peaks at about 445 (DMA-TXO2) and 520 nm (PTZ-DBTO2) in the thin films (Fig. 1c and Table 1), which can be attributed to the stronger intermolecular energy transfer (Fig. S6). Besides, the PL peaks of these polymers gradually redshift with the increasing contents of TADF units. The PL spectra in PMMA doped films and toluene solution with the different concentrations are also measured to get insight on the concentration-induced quenching properties (Fig. S7). The partial overlap of absorption spectra of blue emitter (DMATXO2) and yellow emitter (PTZ-DBTO2) with PL spectrum of host (BDMAc) can be found in Fig. S6. Therefore, the efficient energy transfer from high-energy host (S1 = 3.65 eV) to middle-energy DMATXO2 (S1 = 3.23 eV) and low-energy PTZ-DBTO2 (S1 = 2.63 eV) occurs in PDTPT-1, 2&3. It can also be confirmed by PL spectra of PDTPT-1, 2& 3 in films, in which no emission of host is observed. The critical Förster energy transfer radius (R0) is also calculated according to the overlap integral of absorption of the emitters and PL spectrum of the host (equation S(1)). R0s are 11.8 nm from host to DMA-TXO2 emitter and 9.6 nm from host to PTZ-DBTO2 emitter, suggesting efficient FRET from host to the two emitters [9]. In addition, the energy transfer from DMA-TXO2 to PTZ-DBTO2 can also occur as evidenced by the partial overlap of absorption of PTZDBTO2 with PL spectrum of DMA-TXO2. Only small overlap can be observed, indicating their relatively slight energy transfer as shown in Fig. S6. The rate constants of FRET (kFET) from DMA-TXO2 to PTZDBTO2 in PDTPT-1, 2&3 can be obtained from equation (1) [38]. The kFET for PDTPT-1 is 4.41 × 106 s−1, which is faster than those of PDTPT-2&3 (4.23 and 1.89 × 106 s−1 for PDTPT-2 and PDTPT-3, respectively) due to the closer average distance between donor (DMATXO2) and acceptor (PTZ-DBTO2) (R) in PDTPT-1 [39,40]. kFET = 1/τDA ‒ 1/τD = (1/τD) (R0/R)6

(1)

ΦFET = 1 ‒ τDA/τD

(2)

kTADF = 1/3kp exp(−ΔEST/kBT)

(3)

kTADF, kp, kB, and T denote thermally activated delayed fluorescence rate constant, prompt fluorescence decay rate, Boltzmann constant and temperature, respectively. As shown in Fig. 1e, the ΔEST values of yellow emitters in polymers are calculated to be 0.02 eV for PDTPT-1, 0.03 eV for PDTPT-2 and 0.03 eV for PDTPT-3. The small ΔEST ensures the efficient reverse intersystem crossing (RISC) and thus harvesting triplet excitons by up-conversion. The cyclic voltammetry (CV) curves of the polymers are shown in Fig. S9. All the polymers have similar oxidation and reduction potentials with the unsubstituted phenothiazine donor and dibenzothiophene-S,S-dioxide acceptor units of PTZDBTO2, respectively. This is consistent with the distribution of HOMO/ LUMO orbitals of the simplified repeat units of polymers estimated by density functional theory calculations [B3LYP, 6-31G (d)] (Fig. S10). TD-DFT calculations with the B3LYP functional were carried out to find the electronic levels involved in the excitations and to calculate UV–vis spectra of the monomers (Fig. S11) [45]. The theoretical UV–vis spectrum of DMA-TXO2 features two major peaks at 243 and 306 nm. The π-electron excitation could be considered at around 306 nm. The S0→S4 excitation is the dominated electronic transition accounting for 65.5% composition and the proportions of S0→S5 and S0→S3 excitations are 19.9% and 10.1%. The UV–vis spectrum of PTZ-DBTO2 shows a main peak at 272 nm and a shoulder peak at 313 nm. For the first intense absorption peak, the predominant S0→S2 excitation and subordinate S0→S4 excitation make up 72.6% and 7.5% contributions, respectively. Small ΔEST values and the sizable spin-orbit coupling matrix elements should be simultaneously realized to facilitate RISC [46–48]. Considering that three T sub-states (m = 0, ± 1) are degenerate, the spin-orbit coupling matrix elements (SOCME), ⟨S1|ĤSO|T1 or T2⟩, are calculated using the ORCA 4.1.1 package. We speculated RISC via the T2 state for the cases of PTZ-DBTO2 and DMA-TXO2, where the energy difference between the S1 and T2 states is small and the corresponding SOC matrix element play a significant role (Fig. S11). A small ΔEST of 0.01 eV is not adequate to provide an efficient direct T1 →S1 RISC up-conversion due to the predominant CT nature of T1 [28,32] and the small ⟨S1|ĤSO|T1⟩ values (0.028 and 0.117 cm−1). However, the next T2 state is more appropriate for this process. The energy splittings between the T2 and 1CT states for PTZ-DBTO2 and DMATXO2 are 0.25 and 0.26 eV, which is usually sufficient for an efficient

Where τDA and τD are the radiative decay time of the prompt part of donor components in the presence and absence of acceptor components, respectively. Furthermore, FRET efficiencies (ΦFRET) from the blue to yellow emitters in polymers are estimated to be 17.9% for PDTPT-1, 17.2% for PDTPT-2 and 8.5% for PDTPT-3 according to equation (2) [41,42]. The relatively low ΦFET could both obtain sufficient singlet excitons for blue emission and partial energy for radiation transition of yellow emission

4

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Fig. 2. PL decay spectra excited at 300 nm (a) collected at 440 nm for PDTPT-1 at 300 K; (b) collected at 520 nm for PDTPT-1 at 300 K; (c) collected at 440 nm for PDTPT-1 in vacuum with the different temperatures; (d) collected at 520 nm for PDTPT-1 in vacuum with the different temperatures; (e) collected at 440 nm for PDTPT-1, 2&3 at 300 K; (f) collected at 520 nm for PDTPT-1, 2&3 at 300 K.

thermally activated population of the 1CT state by RISC [49]. We propose that the CT proportions decrease whereas the LE proportions increase for the T2 states, which is favourable for enhancing the ⟨S1|ĤSO|T2⟩ to 0.788 and 0.526 cm−1. To determine the TADF feature in SWPs, we recorded the transient PL decay spectra for their thin films in air and in vacuum. At ambient temperature, they all exhibit longer delayed fluorescence (DF) lifetimes and higher DF ratios at the wavelengths of 440 and 520 nm in vacuum than that in air (Fig. 2a&b and Fig. S12). DF lifetimes of PDTPT-1 increase from 5.4 μs in air to 6.9 μs in vacuum at 440 nm and from 7.1 μs in air to 15.5 μs in vacuum at 520 nm. This feature indicates that DF originates from the triplet states of the two TADF pendants. For comparison, we have also measured the transient PL decay spectra of blue monomer (DMA-TXO2) and yellow monomer (PTZ-DBTO2) in vacuum (Fig. S13). DF lifetimes of 13.6 μs for DMA-TXO2 and 11.3 μs for PTZDBTO2 in vacuum can be observed. Comparing to the monomers, the decreased lifetime of DMA-TXO2 and the increased lifetime of PTZDBTO2 in the polymers are obvious, indicating the energy transfer from the DMA-TXO2 to PTZ-DBTO2 emitter in polymer films again. To further figure out the origin of DF, the temperature dependence of their transient PL properties was measured in the film states (80–300 K). All of them obviously display the prompt and delayed components in vacuum at both 440 nm and 520 nm (Table 1). As shown in Fig. 2c&d, with elevated temperature, the DF lifetime of PDTPT-1 increases from 3.6 to 6.9 μs at 440 nm and decreases from 15.5 to 11.5 μs at 520 nm. Meanwhile, the ratio of DF increases up from 20% to 31% at 440 nm and from 82% to 88% at 520 nm, which further confirms that DF component comes from TADF. The similar phenomenon has been observed for PDTPT-2&3 (Fig. S14). In addition, at 300 K, with increasing concentration of the blue and yellow monomers, the DF lifetimes and ratios of the polymers increase from 6.9 μs (31%) for PDTPT-1 to 11.1 μs (37%) for PDTPT-3 at 520 nm, and decrease from 11.5 μs (88%) for PDTPT-1 to 11.0 μs (87%) for PDTPT-3 at 440 nm (Fig. 2e&f). This opposite trend can be considered to support the large extent of FRET efficiency from DMA-TXO2 to PTZ-DBTO2 emitter with increasing content of emitters in polymers. The PL quantum yields

(PLQYs) of PDTPT-1, 2&3 exhibit the same trend as the ratios of TADF units increase, following the order PDTPT-1 (69%) > PDTPT-2 (57%) > PDTPT-3 (56%) (Table 1). The rate constants of reverse intersystem crossing (kRISC) of polymers were also calculated. In the film states, the kRISC of PDTPT-1 at 520 nm is 7.26 × 105 s−1, which is larger than those of PDTPT-2 (6.98 × 105 s−1) and PDTPT-3 (6.80 × 105 s−1). The result is consistent with the smaller ΔEST of PDTPT-1 than those of PDTPT-2&3. Moreover, kRISCs of polymers at 440 nm are slightly lower than those measured at 520 nm, which are 2.10 × 105 s−1 for PDTPT-1, 1.78 × 105 s−1 for PDTPT-2, 1.43 × 105 s−1 for PDTPT-3 (Table 1 & Table S2) [50]. These results indicate that the RISCs of the polymers from T1 to S1 of PTZ-DBTO2 are much faster and more efficient than that of DMA-TXO2. To investigate their EL performance, white polymer light-emitting diodes (PLEDs) based on the three polymers as emitting layer (EML), respectively, were fabricated. As well known, the efficiency of solutionprocessed PLEDs depends remarkably on the film morphology of the EML. The film-forming ability of these polymers was revealed by atomic force microscopy (AFM). Observed from the surface images (Fig. S15), they all display the smooth and homogeneous film morphologies with small values of root-mean-square (RMS) roughness in the range of 0.30–0.35 nm, which indicates that these polymers would be desirable materials for EML of PLEDs. As sketched in Fig. 3a, a double-layer device configuration of indium tin oxide (ITO)/PEDOT:PSS (40 nm)/ EML (30 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) was used, in which, the PEDOT:PSS (poly(3,4-ethylenedioxythiophene:poly(styrene sulfonate)) and the TmPyPB (1,3,5-tris(3-pyridyl-3-phenyl)benzene) serve as the hole-injecting layer and the electron-transporting layer, respectively. Fig. 3b&d shows current density-voltage and luminance-voltage characteristics, as well as luminance dependence of efficiency for the devices. The device performance is summarized in Table 2. PDTPT-1 based device achieves a maximum luminance of 2900 cd m−2, a maximum current efficiency (CE) of 38.8 cd A−1, a maximum power efficiency (PE) of 20.3 lm W−1 with a CIE coordinates of (0.33, 0.42). In addition, a low turn-on voltage of 3.5 V can be obtained due to the good 5

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Fig. 3. (a) Energy-level diagrams and structures of OLED devices. (b) Current density-voltage-luminance (J-V-L) curves of PDTPT-1, 2&3. (c) EL spectra of PDTPT-1 at different voltages. The inset is EL image of PDTPT-1. (d) EQE and power efficiency vs luminance for PDTPT-1, 2&3.

EQE = IQE × ηout = γ × ηST × ηPL × ηout

matching of energy levels. The efficiency and luminance of PDTPT-1 are the best among the prepared three polymers probably due to its low hole-transporting abilities confining the “majority” charge carrier within the EL layer and inhibiting excitons migration [51]. From the JV curves of the hole- and electron-only devices in Fig. S16, the current density of the electron-only device is negligible under below the bias of 8 V (in the region where the OLEDs achieve their maximum EQEs), which means that the polymers are hole-transporting only. Moreover, high EQEmax of 14.2% for PDTPT-1, 10.8% for PDTPT-2 and 9.1% for PDTPT-3 can be observed, making this polymer the most efficient white-emitting TADF polymer ever reported [18,19,52]. At the luminance of 100 cd m−2, the EQE of PDTPT-1 is maintained at 13.4%, corresponding to a roll-off of only 7.5% compared to the EQEmax. As we known, the theoretical EQE for OLEDs is generally expressed as

(4)

where ηout is the light out-coupling efficiency, γ is the charge balance factor (ideally γ = 1), ηST is the fraction of radiative excitons and ηPL is PLQY of the emissive material. The ηST of PDTPT-1 can reach 82.3%, assuming a ηout of 25% [29]. The EL spectra of PDTPT-1 at various applied voltages show almost bias-independent white emission (Fig. 3c). Typically, when the driving voltage of PDTPT-1 is changed from 5 to 9 V, the CIE coordinates of PDTPT-1 slightly shifts from (0.33, 0.42) with CRI of 70 to (0.31, 0.39) with CRI of 73. EL spectra of PDTPT-2&3 can be seen in Fig. S17. The stable spectra herein may be attributed to the stable recombination zones. The excellent colour stability is as crucially important as high efficiency for white-light devices [53–55]. Additionally, in EL spectra, the broad emission from 600 to 700 nm can be observed, which cannot

Table 2 OLED performance. Devices

Vona (V)

Lmaxb (cd m−2)

CEmaxc (cd A−1)

PEmaxd (lm W−1)

EQEmaxe (%)

EQE100f (%)

EQE500g (%)

CIE5Vh (x,y)

CRI5Vi

PDTPT-1 PDTPT-2 PDTPT-3

3.5 4.0 3.9

2900 1448 1808

38.8 31.3 24.5

20.3 17.9 11.8

14.2 10.8 9.1

13.4 10.8 8.9

9.9 5.7 5.8

(0.33,0.42) (0.35,0.46) (0.32,0.40)

70 68 70

a b c d e f g h i

The voltage at 1 cd m−2. Maximum luminance. Maximum current efficiency. Maximum power efficiency. Maximum EQE. EQE at 100 cd cm−2. EQE at 500 cd cm−2. CIE coordinates at 5 V. CRI at 5 V. 6

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Fig. 4. (a) The scheme of Langevin recombination. (b) XRR patterns of spin-coated PDTPT-1, 2&3 film on a silicon substrate. The calculated film density is also shown. (c) The scheme of trap-assisted recombination. (d) The HOMOs and LUMOs of BDMAc, DMA-TXO2, PTZ-DBTO2 and the adjacent PEDOT:PSS, TmPyPB. (e) The EL decay curves of the devices based on PDTPT-1, 2&3.

1.08, 0.81 and 0.76 g cm−3, respectively. With the same film thickness, the higher film density of PDTPT-1 than those of PDTPT-2&3 suggestes much closer packing of molecular chains and thus the smaller average intermolecular distances in the films of PDTPT-1. A close physical contact between intermolecular chains of PDTPT-1 facilitates to enhance FRET and DET from hosts to the emitters. Therefore, highly efficient energy transfer in PDTPT-1 accounts for its higher EL efficiency than PDTPT-2&3. Trap-assisted recombination has recently been studied in TADF based OLEDs, where electrons and holes will directly recombine on emitters and emit light [60]. In this mechanism, it exists a large energy barrier for charge injection into the host from the adjacent layers due to a wide energy gap. Meanwhile, there exists the deeper LUMO and shallower HOMO energy levels of the emitter than those of host (Table S3, Fig. S18). And thus the charges can directly inject and combine into the emitter and then decay to emit [61]. For our case, direct charge injection into the emitters can be anticipated as illustrated in Fig. 4c. The obvious evidences are that HOMO level of BDMAc has no big difference with two kinds of emitters, which is also much close to HOMO of the PEDOT:PSS as shown in Fig. 4d. However, the large LUMO gap between BDMAc (−1.41 eV) and adjacent TmPyPB (−2.54 eV) can be found. In contrast, there is only tiny energy barrier of less than 0.1 eV between TmPyPB and DMA-TXO2 or PTZ-DBTO2. Therefore, direct charge injection into the emitters can be anticipated as illustrated in Fig. 4d. This mechanism could be evidenced by the current densities of the devices based on PDTPT-1, 2&3. As shown in Fig. 3b, with the same bias voltages, current densities of polymers increase with elevated concentration of TADF units. That is, PDTPT-1 with highest content of TADF units displays the highest current density among three polymers. Furthermore, for the electron-only devices as shown in Fig. S16, when the concentration of TADF units is increased, the electron current density obviously enhances, indicating that the TADF units function as the electron traps. Since the electron is mainly transported through the

been found in PL spectra. This may be caused by the different formation mechanism of excitons under electroluminance and photoluminance. The red region emission from 600 to 700 nm may be attributed to the formation of exciplex under electro-excitation. Generally, in OLEDs, Langevin recombination is one of the main recombination pathways, where excitons form on hosts and then transfer their energy to the emitters [56]. For PDTPT-1, 2&3 based OLEDs, the efficient energy transfer from hosts to the emitters have been verified and disscused above. As shown in Fig. 4a, singlet and triplet excitons on hosts transfer to singlet and triplet of the emitters by FRET and DET, respectively. The high RISC rates of the emitters DMATXO2 and PTZ-DBTO2 are helpful to suppress intersystem crossing from singlet to triplet, and thus the singlet excitons up-converted from the triplet can be effectively utilized. Therefore, the reduced concentration of triplet excitons of the emitters can prevent the triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), benefiting the device efficiencies [57]. Moreover, the singlet excitons of DMA-TXO2 can also transfer to PTZ-DBTO2 with high RISC rate by FRET and thus the quenching of relatively high energy excitons is reduced. The FRET from the blue emitter to the yellow one can promote the ratio of the singlet excitons (> 25%) of the DMA-TXO2 while suppressing the triplet ones (< 75%), which suppresses DET from blue emitter to yellow one. Regarding of the energy transfer efficiency from hosts to the emitters in PDTPT-1, 2&3, we have also simply estimated and compared by the average intermolecular distances. X-ray reflectivity (XRR) measurements were performed and the film density of polymers can be derived from the critical angles of the total external reflection by X-ray. As shown in Fig. 4b, the typical Kiessig fringes are observed for them. The thickness of their spin-coated films is estimated by calculating the period of the fringes to be ~30 nm [57–59], which is consistent with the thickness of EML of PLEDs. In addition, the fitting analysis using theoretical curves indicates that the film density of PDTPT-1, 2 &3 are 7

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hopping between the neighbouring TADF units, the increased concentration of TADF units would benefit the electron transport and thus promote electron current density of the devices [21]. However, the hole-only devices based on polymers reveal that the current densities of the devices drop with the raised ratios of TADF units. DMA-TXO2 and PTZ-DBTO2 can act as hole scattering centres in polymers, leading to an increase of transit path for holes and thus a decrease of hole current density. The reduced hole current density and increased electron current density in PDTPT-1 device could achieve the more balanced charge carrier transport, leading to broaden recombination zone and reduced excitons quenching [6]. The EL transient decay curves of the devices were also measured as shown in Fig. 4e. A turning spike is clearly observed for all the devices, indicating the direct charge trapping on the TADF emitters [62]. With the careful observation, comparing to PDTPT2&3, the much obvious spike in EL decay of PDTPT-1 is displayed, suggesting PDTPT-1 is trap-assisted recombination dominant device. This may be caused by the high content of TADF units in PDTPT-1, which is also confirmed by the respective EL decays at 420 nm and 550 nm. As shown in Fig. S19, the much higher spike in EL decay at 550 nm than that at 420 nm can be seen because of the deeper LUMO level and shallower HOMO level of yellow TADF units than blue TADF units, and thus the easier charge trapping on the yellow TADF emitters. Trap-assisted recombination together with Langevin recombination ensure the balanced charge fluxes, broad recombination zones, and reduced formation of high-energy excitons on hosts. Eventually, double-channel recombinations would reduce excitons quenching and enhance the efficiency of PLEDs based on PDTPT-1, 2&3.

[2]

[3] [4] [5]

[6]

[7]

[8]

[9]

[10]

[11] [12]

[13] [14]

4. Conclusion In summary, we developed a series of bichromophoric all-TADF SWPs to fabricate highly efficient non-doped solution-processed white PLEDs. The use of picked host with high HOMO level and high ET can confine the triplet excitons all on the emitters, leading to the occurrence of an efficient cascade energy transfer from hosts to blue and yellow fluorophores and from blue to yellow fluorophore. Meanwhile, appropriate HOMO/LUMO energy levels allow direct charge trapping on the TADF emitters. PLEDs based on PDTPT-1 as the emitting layer achieve an EQEmax of 14.2%, a CEmax of 38.8 cd A−1, and a PEmax of 20.3 lm W−1 with the CIE coordinates of (0.33, 0.42) and (0.31, 0.39) at 100 cd m−2 and 2000 cd m−2, respectively. These devices surpasses those of the solution-processed hybrid WOLEDs, and can even be comparable to those of all phosphorescent WOLEDs. This strategy might be a guide for preparing high-efficiency and low-cost WOLEDs and paves a way toward practical applications of WOLEDs.

[15]

[16]

[17]

[18]

[19]

[20]

Author contributions Conceptualization, C. L., Z. R.; Methodology, C. L., Y. L. and Z. R.; OLEDs Measurement and Analysis, C. L., Y. X., Y. M.; Writing-Original Draft, C. L.; Writing-Review&Editing, C. L., Z. R., S. Y.; Supervision, Z. R., Y. M., S. Y.

[21]

[22] [23]

Acknowledgements The financial support of the National Natural Science Foundation of China (Nos. 51922021 and 51221002) is gratefully acknowledged.

[24]

[25]

Appendix A. Supplementary data [26]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.104057.

[27]

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Chensen Li received a BSc in 2014 from Inner Mongolia Normal University, China. He is currently a doctoral student at State Key Laboratory of Chemical Resource Engineering of Beijing University of Chemical Technology. His research interests include design and synthesis of TADF polymer for organic optoelectronic devices.

Yuwei Xu received his Bachelor degree in 2014 in Light Chemical Engineering from Shaanxi University of Science & Technology. Now, he is a Ph.D. candidate working in State Key Laboratory of Luminescent Materials and Devices (SKLLMD), South China University of Technology under supervision of Prof. Yuguang Ma. He is currently conducting research in the field of organic light-emitting materials and devices focusing on “hot excitons” organic lightemitting diodes (OLEDs).

Yuchao Liu received his B.E. degree from the Department of Materials Science and Engineering, Tianjin University, China, in 2015. He studied for his master degree in Department of Materials Science and Engineering, Beijing University of Chemical Technology (BUCT), China, from 2015 to 2017. Now, he is a PhD candidate in BUCT under the supervision of Prof. Shouke Yan and Dr. Zhongjie Ren. His present interests are in the research of TADF conjugated polymer emitting materials and solution-processed devices.

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Nano Energy 65 (2019) 104057

C. Li, et al. Zhongjie Ren received his PhD from the State Key Laboratory of Polymer Physics and Chemistry at Institute of Chemistry, Chinese Academy of Sciences, in 2009. He then worked for two years as a postdoctoral research fellow at Beijing University of Chemical Technology. From December 2011, he was appointed as an associate professor at Beijing University of Chemical Technology. From 2014 to 2015, he moved to Durham University as a visiting scholar/postdoc. His research interests cover the design, synthesis and application of thermally activated delayed fluorescence polymeric materials, organosilicon-based optoelectronic materials in OLEDs.

Shouke Yan received his PhD at Changchun Institute of Applied Chemistry, The Chinese Academy of Sciences (CIAC-CAS) under the joint guidance of Prof. Decai Yang and Prof. J. Petermann (Dortmund University, Germany) and then worked at Dortmund University associated with Prof. J. Petermann. In 2001, he joined the Institute of Chemistry, the Chinese Academy of Sciences as a full Professor. In 2008, he moved to Beijing University of Chemical Technology. He has obtained the Excellent Hundred Talents Award and an NSFC Outstanding Youth Fund. His current research involves surface-induced polymer crystallization, orientation-induced polymer crystallization, and phase transitions of crystalline polymers.

Yuguang Ma is a Professor of the State Key Laboratory of Luminescent Materials and Devices and School of Materials Science and Engineering, South China University of Technology (SCUT), China. He received his B.S. degree and Ph.D. degree from Jilin University in 1985 and 1991. Then he earned a faculty position in Jilin University until 2012 and was promoted professor in 1998. During this time, he visited University of Hong Kong in 1995 and 1997. He visited the Department of Chemistry at University of Cambridge in Britain in 1999–2000. His research interests lie in organic/polymer optoelectronic materials and devices.

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