Thermally Activated Delayed Fluorescence Pendant Copolymers with

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Thermally Activated Delayed Fluorescence Pendant Copolymers with Electron and Hole-Transporting Spacers Chensen Li, Yukun Wang, Dianming Sun, Huihui Li, Xiaoli Sun, Dongge Ma, Zhongjie Ren, and Shouke Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00136 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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ACS Applied Materials & Interfaces

Thermally

Activated

Delayed

Fluorescence

Pendant

Copolymers with Electron and Hole-Transporting Spacers Chensen Li†, Yukun Wang‡, Dianming Sun†, Huihui Li†, Xiaoli Sun†, Dongge Ma§, Zhongjie Ren,†* Shouke Yan†* †State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China. ‡State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry Chinese Academy of Sciences, Changchun 130022, China. University of Chinese Academy of Sciences, Beijing 10039, China. § Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. E-mail: [email protected], [email protected] Keywords: Thermally Activated Delayed Fluorescence; Copolymers; Hole-Transporting; Electron-Transporting; Organic Light-Emitting Diodes; Photophysical Properties.

Abstract To study the effect of hole- and electron-transporting spacers in copolymers on the TADF properties and device efficiency of copolymers, two series of copolymers PCzPT-x and POPT-x have been designed and synthesized successfully. In these copolymers

2-(10H-phenothiazin-10-yl)dibenzothiophene-S,S-dioxide

units

give

green-yellow TADF, while hole-transporting 9-(4-vinylphenyl)-9H-carbazole units or electron-transporting diphenyl(4-vinylphenyl)phosphine oxide act as spacers or hosts. Their thermal, electrochemical, photophysical, and electroluminescent (EL) properties, and theoretical calculations are systematically investigated to illustrate the relationships between molecular structures and photophysical properties. By optimizing the up-conversion and radiative decay rate and managing the energy transfer, a green-yellow device based on POPT-25 achieves an EQEmax of 5.2%, CE of 16.8 cd/A and PE of 7.8 lm/W with CIE coordinates of (0.36, 0.50). Moreover, EQE 1

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of 3.5% at the practical luminescence of 100 cd/m2 is obtained. Introduction Thermally activated delayed fluorescence (TADF) material has emerged as a promising alternative for highly efficient organic light-emitting diodes (OLEDs), in which TADF emitters exhibit excellent performance by harvesting the triplet for delayed fluorescence through the reverse intersystem crossing (RISC) process.1,2 In terms of designing the TADF small molecules, it is very important to choose suitable donors and acceptors as well as their linkage mode. A spatially separated configuration between donors and acceptors can minimize the energy gap (∆EST) between the lowest singlet state (S1) and triplet state (T1), and thus enable efficient exciton-harvesting via spin up-conversion from nonradiative T1 states to radiative S1 states.3 In addition, a relatively large radiative decay rate constant (kr) of the singlet exciton transition from the S1 to the S0 state is also important to get efficient TADF emission. However, the large kr of S1→S0 and the small ∆EST are usually antithetical. Therefore, fine molecular design is required to realize synergism of them for efficient TADF OLEDs.4 For example, Adachi et al.5 synthesized Spiro-CN with the spirobifluorene moiety as bridge which finally leads to efficient TADF due to the extreme small ∆EST (0.057 eV). However, Spiro-CN also exhibited a low photoluminescence quantum yield (PLQY) of 27% and its OLED device gave a relatively low maximum external quantum efficiency (EQEmax) of 4.4% because of the large kr of T1 excitons. For increasing the kr from S1 to S0 states with small ∆EST, two approaches have been applied. On the one hand, increasing the overlap density 2


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(ρ10) distribution between the electronic wave functions of the ground state, in which S1 state can promote S1-S0 radiative decay to improve the PLQY of the TADF emitter;6 on the other hand, large delocalization of molecular orbitals with suitable overlap of HOMO and LUMO levels can suppress the decrease in radiative decay rate for fluorescence (kf), meanwhile lowering ∆EST.7 However, it is notably difficult to realize these structural requirements for polymers which are favorable for low-cost mass production of electroluminescent (EL) devices. Up to now, there are only several examples of TADF polymers.8-15 Furthermore, EQEs of the most of polymer-based light-emitting diodes are obviously lower than those of their small molecular counterparts. Generally, TADF polymers are prepared by imbedding the TADF units into the polymer backbone8,9 or grafting the TADF emitters to the side chain of polymers.10-15 The common copolymerizing TADF units mainly include styrene4,6,7 or hole-transporting groups, such as carbazole,9,12-14 triphenylamine,13 acridine9 and dibenzothiophene.10,15 However, no TADF polymers based on copolymerizing TADF units with electron-transporting groups has yet been reported. Hole-transporting based TADF polymers usually cause an imbalance of electron and hole injection in OLED devices. Therefore, hole injection predominates and charge recombination occurs near the interface of the emission layer/electron-transporting layers, which results in low electroluminescent efficiency due to quenching of excitons.16 One approach to solve this problem is to use electron-transporting materials as the host to lower the energy barrier of electron

3



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injection. Therefore, designing and synthesizing polymeric TADF materials with electron-transporting hosts or spacers is meaningful and promising. A diphenylphosphine oxide derivative, bis(2-(di(phenyl)phosphino)-phenyl)ether oxide (DPEPO), is one of the most popular high-energy-gap electron-transporting TADF hosts, due to the excellent suppression of intermolecular interaction.3,17-19 However, OLED devices with DPEPO generally suffer from serious efficiency roll-off due to its weak electroactivity. A series of phosphine oxide hosts and electron-transporting materials were modified to support blue TADF devices with high efficiency.20-26 For example, Huang et al.26 reported a universal TADF host, 4’-diphenylphosphinoylspiro[fluorene-9,9’-xanthene] (SFXSPO) constructed with a highly distorted and asymmetric configuration, and disordered molecular packing in its solid state was obtained. SFXSPO successfully endowed its full-color TADF diodes with state-of-the-art EQE of 22.5% and 19.1% for its yellow TADF diodes and single-host full-TADF nearly-white-emitting devices, respectively. Recently, several reports investigated the photophysical properties of an efficient

green

TADF

emitter

2-(10H-phenothiazin-10-yl)-8-vinyldibenzothiophene-S,S-dioxide (PTZ-DBTO2)27-29 which has also been grafted as side chains onto non-conjugated polymers.10,11,15 In this work, to explore the relationship between hole or electron-transporting hosts or spacers and device efficiency of TADF polymers, a hole-transporting unit (9-(4-vinylphenyl)-9H-carbazole

(PhCz))9

and

an

electron-transporting

unit

(diphenyl(4-vinylphenyl)phosphine oxide (Ph3PO))24 are copolymerized as hosts or 4


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spacers

with

TADF

unit

2-(10H-phenothiazin-10-yl)-8-vinyldibenzothiophene-S,S-dioxide (PTZ-DBTO2). A series of copolymers, PCzPT-x and POPT-x, are obtained by tuning the ratio of spacers and TADF units. OLED device data demonstrate that the strongly polar copolymers POPT-x give dramatically better performance than the weakly polar copolymers PCzPT-x, establishing the benefit of efficient up-conversion and radiative decay rate of TADF polymers. A green-yellow device of POPT-25 achieve an EQEmax as high as 5.2%, with EQE 3.5% at 100 cd/m2. 2. Results and discussion 2.1. Synthesis and characterizations The copolymers series with carbazole (PCzPT-x) or triphenylphosphine oxide (POPT-x)

spacers

were

prepared

by

radical

copolymerization

of

9-(4-vinylphenyl)-9H-carbazole or diphenyl(4-vinylphenyl)phosphine oxide with 2-(10H-phenothiazin-10-yl)-8-vinyldibenzothiophene-S,S-dioxide

under

different

molar feed ratios, as shown in Scheme 1, respectively. The polymers PCzPT-10, PCzPT-19

and

PCzPT-43

were

prepared

by

copolymerization

9-(4-vinylphenyl)-9H-carbazole

of and

2(10H-phenothiazin-10-yl)-8-vinyldibenzothiophene-S,S-dioxide with molar feed ratios of 12:1, 6:1 and 3.3:1, in which the actual content of TADF unit, PTZ-DBTO2, is ca. 10%, 19%, and 43% as estimated from elemental analysis data. Similarly, POPT-13, POPT-25 and POPT-41 were obtained with molar feed ratios of 10:1, 5:1 and

3.75:1

for

diphenyl(4-vinylphenyl)phosphine 5


oxide

and


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2(10H-phenothiazin-10-yl)-8-vinyldibenzothiophene-S,S-dioxide, respectively. And the actual content of TADF unit determined by elemental analysis is ca. 13%, 25%, and 41%. The product ratio of PTZ-DBTO2 units are relatively higher than the feed ratio of those, indicating the slightly greater reactivity of PTZ-DBTO2 monomers. These copolymers are readily soluble in chloroform, toluene and tetrahydrofuran. The weight average molecular weight (Mw) was determined to be 11.7, 15.9, and 15.1 kDa with PDI of 1.62, 1.81, and 1.78 for PCzPT-10, PCzPT-19 and PCzPT-43, respectively. The Mw of 11.9, 27.5 and 96.8 kDa with PDI of 1.26, 1.51 and 1.50 are obtained for POPT-13, POPT-25 and POPT-41, respectively (Table 1 and Figure S1, Supporting Information). The structures of the copolymers were confirmed by 1

HNMR spectra (Figure S2, Supporting Information).

Scheme 1. Synthetic routes for PCzPT-x and POPT-x. Table 1. Characterization data for PCzPT-x and POPT-x. Mw/PDIa

Tg b

Tdc

λabsd

λPLe

Eg f

HOMOg

LUMO h

ET i

∆ESTj

ΦPL,tolk

ΦPL,toll

(℃ ℃)

(℃ ℃)

(nm)

(nm)

(eV)

(eV)

(eV)

(eV)

(eV)

(%)

(%)

PCzPT-10

11,700/1.62

199

388

328,341

535

2.67

-5.54

-2.87

2.65

0.07

14

18

PCzPT-19

15,900/1.81

218

389

328,341

538

2.65

-5.55

-2.89

2.61

0.06

16

25

PCzPT-43

15,100/1.78

389

328,341

543

2.64

-5.57

-2.93

2.58

0.05

15

POPT-13

11,900/1.26

176

226

375

328,355

538

2.76

-5.35

-2.59

2.62

0.13

16

31

POPT-25

27,500/1.51

180

378

328,355

541

2.75

-5.36

-2.61

2.58

0.11

16

52

6


22

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POPT-41

96,800/1.50

181

379

328,355

542

2.72

-5.37

-2.65

2.40

0.09

16

39

a

Determined by GPC with polystyrene standards. bTg was determined at a heating rate of 10 oC/min under nitrogen atmosphere. cTemperature at 5% weight loss. dMeasured e in toluene solution at room temperature. Measured in doped f 1,3-di(9H-carbazol-9-yl)benzene (mCP) film at room temperature. Optical energy gap (Eg) deduced from the absorption onset in toluene solution. gCalculated according to EHOMO = − (E(onset, ox vs Fc+/Fc) + 5.1) by CV. hCalculated according to LUMO = HOMO + Eg. iTriplet energy was calculated from the maximum peak wavelength of phosphorescent emission. j∆EST = S1 –T1. kAbsolute PL quantum yield in toluene solution determined by a calibrated integrating sphere in air; error ± 2%. lAbsolute PL quantum yield in toluene solution determined by a calibrated integrating sphere in nitrogen; error ± 5%.

2.2. Thermal analysis The thermal properties of polymers were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). As shown Table 1 and Figure 1b, the decomposition temperatures (Td) with 5% weight loss under nitrogen are in the range of 388-389 ℃ for PCzPT-x and 375-379 ℃ for POPT-x, showing their excellent thermal stability. It is clear that while the POPT-13 shows a 3-4 ℃ decrease in decomposition temperature (Td), the Td of other POPT-x remains almost the same with an increased ratio of PhCz and Ph3PO units. In addition, PhCz is more helpful in enhancing the Td of copolymers than is Ph3PO. Figure 1a and Table 1 show that there is a distinct glass transition temperature (Tg) for each polymer with values ranging from 199 to 226 ℃ for PCzPT-x and from 176 to 181 ℃ for POPT-x. Additionally, there are no exothermic peaks resulting from crystallization within the scan ranges for all the copolymers, indicating that of all the copolymers are amorphous. The high Td and Tg values favour long-term stability.

7



POPT-13 POPT-25 POPT-41 50

100

150

200

PCzPT-10 PCzPT-19 PCzPT-43

50

100

150

200

Weight (%)

(a) Endotherm (up)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

100 80 60 40 20 0

250

250

POPT-13 POPT-25 POPT-41

(b) 100

100 80 60 40 20 0

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200

300

400

500

400

500

600

700

800

600

700

800


100

200

300

O

Temperature ( C)

O

Temperature ( C)

Figure. 1 DSC traces (a) and TGA measurements (b) of PCzPT-x and POPT-x recorded at a heating rate of 10 ℃/min under nitrogen atmosphere. 2.3 Electrochemical analysis and theoretical calculations The electrochemical behaviour of the two series of copolymers was investigated by cyclic voltammetry (CV) in degassed anhydrous acetonitrile solution. The CV curves show quasi-reversible oxidation and reduction processes for all copolymers (Figures 2a&b). PCzPT-x have similar oxidation and reduction potentials with the unsubstituted phenothiazine donor and dibenzothiophene-S,S-dioxide acceptor units, respectively.10 A slight decrease in oxidation potential is observed when PhCz is added, i.e., the PTZ-DBTO2 (HOMO: -5.4 eV, LUMO: -2.95 eV)10 are easier to oxidize than PCzPT-x. Also, the oxidation potential of POPT-x series is approximately that of PTZ-DBTO2 and changes with increasing Ph3PO units, since the contributions of oxidation potential primarily come from the phenothiazine donor. The reduction potential of PCzPT-x shows an increased trend with increasing PhCz units. And for POPT-x, the reduction potential increases with elevated content of Ph3PO units. The cyclic voltammograms and phosphorescent spectra of PhCz and Ph3PO can be found in Figure S3 of Supporting Information. 8


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In order to predict the TADF feature of these copolymers, the distributions of the HOMOs and LUMOs of copolymers were calculated by density functional theory simulation with B3LYP 6-31G(d) level. Specifically, PCzPT-19 and POPT-25 were chosen to study and compare. As shown in Figures 2c&d, the two repeat units of PCzPT-19 and POPT-25 were optimized and calculated. The LUMOs and HOMOs of PCzPT-19

are

mainly

localized

on

the

electron

acceptor

of

dibenzothiophene-S,S-dioxide and the hole-transporting unit of PhCz, respectively. PhCz can stabilize the HOMO due to its a little stronger electron-donating ability compared with phenothiazine. The low overlap density distribution between the electronic wave functions of the ground state decrease the oscillator strength (f = 0.00032) between S1 and S0 (Table S1),6,30 resulting in a small ∆EST (0.06 eV) and low PLQY yield (24.8 %). As a comparison, the LUMOs and HOMOs of POPT-25 (Figure 2d) are mainly localized respectively on the electron acceptor and donor of PTZ-DBTO2 unit, indicating the inherited characteristics of the TADF small molecule. Although the HOMOs and LUMOs of the two polymers are similar, other frontier molecular orbitals show the distinguishing difference. As shown in Figure S4 of supporting information, the HOMO-1 of PCzPT-19 is localized on phenothiazine instead of PhCz because of its lower energy level. The HOMO-1 and HOMO values of PCzPT-19 are almost the same, which indicate that they are degenerate orbitals and the HOMO-1 → LUMO transitions are allowed for S1. In consideration of the very small HOMO – HOMO-1 energy gap and large LUMO – LUMO+1 energy gap of 0.57 eV, the whole PCzPT-19 would be involved in hole injection and transport. In 9



comparison with PCzPT-19, the HOMO-1 and HOMO distributions of the POPT-25 show almost no difference, and the LUMO and LUMO+1 distributions are almost the same as well. The LUMO − LUMO+1 energy gap of 0.51 eV is smaller than the HOMO – HOMO+1 energy gap of 1.05 eV, suggesting the dominating electron-transport property of POPT-25.

Figure 2. Cyclic voltammograms of (a) PCzPT-x and (b) POPT-x in acetonitrile. The HOMO and LUMO distributions obtained by density functional theory simulations for (c) two repeat units of PCzPT-19 and (d) two repeat units of POPT-25. 2.4. Photophysical properties Figures 3a&c show the UV-Vis absorption, fluorescence spectra and phosphorescent spectra of PCzPT-x and POPT-x. The absorption spectra of PCzPT-x in toluene solution show similar electronic transitions of PTZ-DBTO2 from 328 to 10


Page 10 of 28

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341 nm, while the absorption spectra of POPT-x extend to 355 nm owing to their stronger intramolecular charge transfer (ICT). In addition, π-π* transitions of the carbazole moiety at 293 nm can be observed in PCzPT-x. Furthermore, the relatively wide energy gaps (Egs) of 2.78 to 2.64 eV for PCzPT-x were obtained from the onset of absorption, which reduce with the increasing ratio of TADF units. Similarly, the

Egs of POPT-x decrease from 2.76 to 2.72 eV with the elevated content of PTZ-DBTO2. These results indicate that TADF units play an important role in tuning the energy gaps of these copolymers. The fluorescence spectra of PCzPT-x in doped mCP thin films display a similar profile with λmax range from 535 to 543 nm. The corresponding λmax of POPT-x shows relatively narrow range of 538 to 542 nm. Moreover, a small emission band at low wavelength can be seen in PL spectra. This band can be attributed to the emission of hole- or electron-transporting units instead of 1LE state of the TADF unit, because a corresponding band is not observed in PTZ-DBTO2.10 The PL peaks gradually redshift with increasing ratio of PTZ-DBTO2 units, indicating that PhCz and Ph3PO units can effectively disperse the TADF units. To estimate the TADF characteristic of copolymers, the triplet energies (ETs) of two series of polymers were determined according to the phosphorescent spectra as shown in Figures 3b&d. As shown in Table 1, ETs of PCzPT-x range from 2.65 to 2.58 eV, which increase with the increased content of TADF units and a similar trend can be observed in POPT-x ranging from 2.62 to 2.40 eV. In these copolymers, the TADF units mostly behave as isolated chromophores with larger proportion of PhCz or Ph3PT spacers, and thus the phosphorescence is at the higher energy. The 11



phosphorescent spectra are well resolved and show a characteristic vibrational structure, indicating that the T1 states are locally excited (3LE) states. The singlet charge transfer state energies (1CT) of these polymers were calculated by the onset of broad emission band of fluorescence spectra. The ∆EST values are calculated to be 0.07-0.05 eV for PCzPT-x and 0.13-0.09 eV for POPT-x and ∆EST reduces with increasing content of TADF units as well. The RISC process is strongly dominated by spin-orbit coupling (SOC) between 1CT and 3LE, and thus achieving a very small energy gap between 3LE and 1CT is key for efficient thermally activated RISC. Moreover, PCzPT-x have smaller energy gaps than those of POPT-x, which is

1.0

(a)

UV/PCzPT-10 UV/PCzPT-19 UV/PCzPT-43 PL/PCzPT-10 PL/PCzPT-19 PL/PCzPT-43

0.8 0.6

Normalized Intensity (a.u.)


consistent with the results of molecular simulation.

0.4 0.2 0.0 300

400

500

600

1.0

PH/PCzPT-10 PH/PCzPT-19 PH/PCzPT-43

(b)

0.8 0.6 0.4 0.2 0.0 400

450

Wavelength (nm)

1.0

UV/POPT-13 UV/POPT-25 UV/POPT-41 FL/POPT-13 FL/POPT-25 FL/POPT-41

0.8 0.6

(c)

0.4 0.2 0.0 300

400

500

500

550

600

650

Wavelength (nm)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

600

1.0

PH/POPT-13 PH/POPT-25 PH/POPT-41

(d)

0.8 0.6 0.4 0.2 0.0 400

450

500

550

600

650

Wavelength (nm)

Wavelength (nm)

Figure 3. UV-Vis absorption, fluorescence and phosphorescence spectra of PCzPT-x (a, b) and POPT-x (c, d). UV-Vis absorption spectra were measured in toluene solution at room temperature; fluorescence spectra (excited at 340 nm) were measured 12


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in doped mCP thin film with 10 wt % PCzPT-x or POPT-x at 300 K; phosphorescence spectra (excited at 320 nm) were measured in doped mCP thin film with 10 wt % PCzPT-x or POPT-x at 77 K. To

further

confirm

the

TADF

feature

of

these

copolymers,

the

temperature-dependent transient photoluminescence (PL) decays of PCzPT-x and POPT-x from 80 to 300 K were investigated (Figures 4 a&b). The fluorescence of neat films decays in a double exponential model comprised of a nanosecond-scale component and a microsecond-scale one, which are attributed to the prompt fluorescence and delayed fluorescence, respectively. It is very clear that the delayed fluorescence lifetimes decrease with the elevated temperatures, indicating acceleration of RISCs from the triplet to singlet excited states by thermal activation. In addition, as the temperature increases from 80 to 300 K, the delayed fluorescence decay time decreases from 18.2 to 12.6 µs for PCzPT-10, 17.1 to 9.9 µs for PCzPT-19 and 8.1 to 2.9 µs for PCzPT-43. Meanwhile, the delayed time decreases from 8.9 to 3.8 µs for POPT-13, 9.1 to 5.6 µs for POPT-25 and 7.8 to 5.2 µs for POPT-41. With increased ratio of TADF units, the delayed time of two series of polymers exhibits a downward trend due to the increase of their radiative decay rates. Basically, POPT-x show faster decays of delayed fluorescence than do PCzPT-x. This fast decay involves efficiently reduced nonradiative mechanisms such as triplet-triplet annihilation, triplet-polaron annihilation, and so on.31 POPT-x may be the better TADF emitters than PCzPT-x because of their shorter τ of delayed fluorescence. Accordingly, the ratio of delayed component increases with the temperature, which improves from 64.3 to 77.6% for 13



PCzPT-19 and 86.6 to 96.5% for POPT-25 when the temperature is elevated from 80 to 300 K (Figures 4c&d). The results are consistent with the lifetime of delayed fluorescence. The corresponding results of PCzPT-10, PCzPT-43, POPT-13 and POPT-41 are shown in Figures S5&S6 of Supporting Information. The fluorescence spectrum of TADF polymer is sensitive to oxygen. As the atmosphere changes from air to vacuum, the long-lived component increases because oxygen can quench triplet excitons. PCzPT-x and POPT-x also display similar characteristics (Figures 4e&f and Figures S5&S6 of Supporting Information). The calculated emission ratio of DF in the overall emission is 25.3% for PCzPT-10, 16.2% for PCzPT-19 and 7.2% for PCzPT-43, which means the more spacers cause few exciton annihilation stemming from the aggregation of TADF units. The values of 42.6% for POPT-13, 29.7% for POPT-25 and 17.5% for POPT-41 are larger than those of PCzPT-x. Accordingly, PLQYs (ΦPL) of PCzPT-10, PCzPT-19 and PCzPT-43 in toluene solution under air atmosphere respectively are 14%, 16% and 15%, which increases to 18%, 25% and 22% after N2 bubbling, respectively (Table 1). The ΦPL of POPT-13, POPT-25 and POPT-41 are the same value of 16% under air and 31%, 52% and 39% under nitrogen atmosphere, respectively. The rise of ΦPL is consistent with the fluorescence decay and PL under air and nitrogen atmosphere. The contribution from the triplet state in the overall luminescence of POPT-x is stronger than that of PCzPT-x, suggesting that electron-transporting groups could maintain the LUMO distribution in TADF units without spreading to Ph3PO and thus keep the relatively high PLQY, which is consistent with the results of the corresponding 14


Page 14 of 28

Page 15 of 28

quantum chemistry calculation. Time resolved fluorescence spectra and overlay plots of PL, delayed PL (1.5 µs), and EL spectra of these polymers are presented in Figures S7&S8 of Supporting Information to further illustrate their photophysical properties. 104

10

3

10

2

80

80 K 150 K 220 K 300 K

τd (µs) τd:rel (%)

70

64.3

(c)

77.6

66.1

600.0k

60 20

17.1

(e)

Air Vacuum

800.0k

73.6

Counts

Counts

(a)

15.4 12.1

9.9

400.0k 200.0k

10 101

0 0

10

20

30

40

50

0.0

80 K

150 K

220 K

400

300 K

τd (µs)

100

τd:rel (%) 86.6

88.2

96.5

(d) 89.6

80 9.1

101

6.8

100 20

30

600

650

700

40

(f)

5.7

5.6

800.0k 400.0k 0.0

0 10

550

1.2M

2

0

500

Air Vacuum

1.6M

Counts

10

80 K 150 K 220 K 300 K

(b)

103

450

Wavelength (nm)

Time (µs)

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

80K

150K

220K

300K

400

500

600

700

Wavelength (nm)

Time (µs)

Figure 4. Transient photoluminescence decay (excited at 340 nm) curves at 550 nm for (a) PCzPT-19 and (b) POPT-25 from 80-300 K. Excited state lifetimes and ratios for delayed emission of (c) PCzPT-19 and (d) POPT-25 were calculated by transient photoluminescence decay curves. PL spectra of (e) PCzPT-19 and (f) POPT-25 in film state under air atmosphere and vacuum. To further reveal the effect of spacers or hosts on transition rate constants for PCzPT-x and POPT-x, inter-conversion and decay rate constants were calculated according to the integration of PF and DF components and lifetimes of the transient PL decay together with PLQY values. These data of PCzPT-x and POPT-x were measured in doped mCP (10 wt%) films at room temperature under air (Table S1 of supporting information). The prompt fluorescence rate constants (kF) are 2.4, 3.1 and 2.5×106 s−1 for PCzPT-10, PCzPT-19 and PCzPT-43 which is much lower than that of 15



POPT-13, POPT-25 and POPT-41 (3.9, 4.0 and 5.1×106 s−1, respectively). Besides, their nonradiative transition rate constants (kIC) are also calculated as kIC = 11.5, 11.6 and 13.0×106 s−1 for PCzPT-10, PCzPT-19 and PCzPT-43 and kIC = 7.3, 7.2 and 10.9×106 s−1 for POPT-13, POPT-25 and POPT-41, respectively. Obviously, kF and

kIC is at the same order; however, kF is smaller than kIC. Moreover, POPT-x show a lower kIC and a higher kF than those of PCzPT-x. Among the two series of copolymers, POPT-25 exhibits the lowest kIC and relatively higher kF. The Förster energy transfer rate (kFET) is calculated to be 1.0, 3.1 and 5.3×105 s−1 for PCzPT-10, PCzPT-19, PCzPT-43 and 0.9, 2.2 and 4.1×105 s−1 for POPT-13, POPT-25, POPT-41, respectively (Table S1 of Supporting Information). The results demonstrate that the energy transfer rates can be accelerated by increasing the ratio of TADF units, ensuring efficient energy transfer between the host and guest; on the other hand, the nonradiative transition rates would unavoidably increase resulting from concentration quenching or Dexter energy transfer from host to guest and thus the efficiency loss at high ratios of TADF units increases.32 Therefore, the appropriate proportion of TADF units needs to be optimized by balancing the two factors. The reverse intersystem crossing rate (kRISC) from the 3LE to 1CT is also determined from the TADF decay time and the ratio of DF and PF decays. The kRISC is 2.2, 1.9 and 1.3×105 s−1 for PCzPT-10, PCzPT-19 and PCzPT-43, respectively, while the value are 6.8, 8.1 and 3.9×105 s−1 for POPT-13, POPT-25 and POPT-41. POPT-25 shows the highest kRISC among all of them. 2.5 OLED Performances 16


Page 16 of 28

Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


OLEDs of PCzPT-x and POPT-x with the device structures of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene:-poly(styrenesulfonic

acid)

(PEDOT:PSS)

(50 nm)/emitting layer (EML) (45 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) are fabricated, where PEDOT:PSS and LiF serve as the hole- and electron-injection layer, respectively; 1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB) acts as the electron transporting layer. These emitting materials were dissolved in chlorobenzene with the concentration of 10 mg/mL. The OLEDs devices based on the pristine copolymers are presented in Figure S9 and Table S1, Supporting Information. The turn-on voltages of 4.4-5.3 V for PCzPT-x and 7.3-8.7 V for POPT-x can be observed, respectively. The maximum luminance of these devices are below 1100 cd/m2. Low EQEmax of ranging from 0.31 to 0.86%, can be found. The EL peaks are located at ca. 570 nm, which is slightly redshifted compared with PL spectra of copolymers. The higher onset voltages of devices based on POPT-x probably can be attributed to the higher LUMOs located on Ph3PO. The low efficiency of non-doped devices may be caused by the severe annihilation of excitons in TADF copolymers. Therefore, mCP is chosen as a host to reduce the concentration quenching of excitons. EMLs are mCP/copolymers (9:1) for all of devices. In this case, all devices based on PCzPT-x and POPT-x produce green-yellow EL, which is similar to the corresponding PL of their doped films, indicating that EL is generated by the copolymer emitters through the same radiative decay process. The EL characteristics of all devices are summarized in Table 2. The EQEmax of 0.6%, 1.2% and 1.0% is achieved in the device based on PCzPT-10, PCzPT-19 and PCzPT-43, as shown in Figure 5. All 17



devices based on PCzPT-x reach their maximum luminance, ranging from 1962 to 4388 cd/m2, at around 550 nm and exhibit relatively low turn-on voltages of 4.5-5.2 V. Among them, PCzPT-19 displays the best efficiency, which may be caused by the appropriate content of PhCz to disperse the TADF units and high PLQY. Meanwhile, the devices based on POPT-x exhibit higher efficiency and turn-on voltages than that of PCzPT-x. EQEmax of 3.5%, 5.2% and 4.1%, CEmax of 10.6, 16.8 and 13.6 cd/A, PEmax of 4.5, 7.8 and 6.0 lm/W, Von of 6.7, 6.4 and 6.4 V can be obtained for POPT-13, POPT-25, POPT-41, respectively. At practical luminance of 100 cd/m2, the EQE remains as high as 3.1%, 3.5% and 3.3% for POPT-13, POPT-25 and POPT-41, indicating a moderate efficiency roll-off which is presumably caused by the short TADF lifetime for POPT-x. Among POPT-x-based devices, POPT-25 shows the best EL performances, owing to the high PLQY (Table 1), indicating that the ratio of around 20% TADF units in copolymers is the optimal proportion. OLEDs performance is closely related to the charge transporting of copolymers. Therefore, the charge transport properties of PCzPT-x, POPT-x and their doped films were measured by hole-only (ITO/PEDOT:PSS (50 nm)/polymers or doped films (60 nm)/Al) and electron-only (ITO/polymers or doped films (60 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al) devices. As seen in Figure S10 of Supporting Information, neat PCzPT-x show better hole-transporting and worse electron-transporting properties than those of neat POPT-x. In addition, the hole-transporting properties of PCzPT-x increases with elevated ratio of PhCz, and the electron-transporting properties of POPT-x are enhanced with increasing content of Ph3PO. After doping the mCP host, 18


Page 18 of 28

Page 19 of 28

the hole-transporting ability is almost the same as that before doping, and thus the undoped devices based on PCzPT-x demonstrate turn-on voltages are similar to those of the doped ones. In addition, for doped POPT-x with mCP, an increasing hole-transporting ability can be observed. The doped POPT-x shows more balanced bipolar properties. Through all of the above, the device efficiencies of POPT-x are better than those of PCzPT-x due to the higher PLQY and kRISC of POPT-x, as well as more balanced

102 400 101

200

(a)

0 2

4

6

8

100 12

10

102

1

1

10

0.1

100

0.01

10-1

(b)

150 2

10 100

101

50

(d)

0 5

Luminance (cd/m2)

103

100 15

10

400

102

104

101

101

103

100

102

10-1

101

-2

100

10

Current Efficiency (cd/A)

POPT-13 POPT-25 POPT-41

200

0.01

1E-3

Voltage (V) 250

0.1

10-2

1E-3 2

104

1

EQE (%)

600

10

Power Efficiency (lm/W)

103

10

10-3

Voltage (V)

3

10

Luminance (cd/m2)

(e) 1

10

100

1000

500

700

800

(c)

103

Luminance (cd/m2)

100

10-1 400

10-2

10-1

600

Wavelength (nm) 102

EQE (%)


800

Power Efficiency (lm/W)

1000

Luminance (cd/m2) Current Efficiency (cd/A)

Current Density (mA/cm2)

charge transport properties.

Current Density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

600

700

800

Wavelength (nm) 1

Luminance (cd/m2)

10

100

(f)

1000

Luminance (cd/m2)

Figure 5. Current density-voltage-luminance (J-V-L) curves of (a) PCzPT-x and (b) POPT-x; Current and power efficiency versus luminance for (c) PCzPT-x and (d) POPT-x; (c) EQE versus luminance for (e) PCzPT-x and (f) POPT-x; the insets in parts (c) and (f) are electroluminescent spectra at 100 cd/cm2. Table 2. OLED Performance. Devices

V a on (V)

L

b max (cd/m2)

PCzPT-10

4.7

1962

PCzPT-19

5.2

4388

CE

c max (cd/A)

PE

d max (lm/W)

EQE e max (%)

EQE e 100 (%)

f peak (nm)

f 100 (x,y)

2.0

0.9

0.6

0.5

548

0.38,0.53

4.1

1.8

1.2

0.9

552

0.40,0.55

19


λ

CIE


Page 20 of 28

PCzPT-43

4.8

2924

3.2

1.4

1.0

0.8

556

0.42,0.54

POPT-13

6.7

2952

10.6

4.5

3.5

3.1

536

0.36,0.49

POPT-25

6.4

3386

16.8

7.8

5.2

3.5

540

0.36,0.50

POPT-41

6.4

4069

13.6

6.0

4.1

3.3

544

0.37,0.51

a

The voltage at 1 cd/m2. bMaximum luminance. cMaximum current efficiency. Maximum power efficiency. eEQE at maximum and 100 cd/cm2. fEL emission peak and CIE coordinates at 100 cd/cm2. d

Conclusion In summary, we successfully developed a series of TADF copolymers PCzPT-x and POPT-x with hole-transporting and electron-transporting spacers. These copolymers showed relatively small ∆EST values, which enabled an efficient TADF process. Among these copolymers, POPT-25 exhibited the best luminous efficiency because of its optimized TADF units concentration and energy transfer. A yellow device based on POPT-25 achieved an EQEmax of 5.2%, CE of 16.8 cd/A and PE of 7.8 lm/W with the CIE coordinates of (0.36, 0.50). Moreover, EQE of 3.5% at 100 cd/m2 was obtained. In addition, POPT-x have better TADF performance than PCzPT-x due to balanced up-conversion and radiative decay rate. We believe that these results can provide a molecular design strategy for continuing exploration of efficient TADF polymers and the development of solution-processed TADF based OLEDs.

Experimental Section General synthetic procedure for the copolymers: A mixture of AIBN (10 mg, 0.06 mmol),

toluene

(8.0

mL)

/

THF

(20

mL),

and

different

ratios

of

9-(4-vinylphenyl)-9H-carbazole (M1) or diphenyl(4-vinylphenyl)phosphine oxide 20


Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(M2) and 2-(10H-phenothiazin-10-yl)-8-vinyldibenzothiophene-S,S-dioxide (M3) were placed in an ampule, which was cooled, degassed, and sealed in vacuo. After stirring at 60 ℃ for 20 h, the reaction mixture was poured into a large excess of methanol. The yellow polymer was obtained by filtration and then was dried in vacuo. The polymer was fractionated by Soxhlet extraction using hexane. PCzPT-10: M1 (810 mg, 3.0 mmol) and M3 (110 mg, 0.25 mmol) were used in the polymerization (yield: 81%). Elemental analysis. Found: C 83.13; H 5.00, N 4.43, S 1.94%. PCzPT-19: M1 (810 mg, 3.0 mmol) and M3 (220 mg, 0.5 mmol) were used in the polymerization (yield: 87%). Elemental analysis. Found: C 83.14; H 5.17, N 4.25, S 3.61%. PCzPT-43: M1 (672.5 mg, 2.5 mmol) and M3 (330 mg, 0.75 mmol) were used in the polymerization (yield: 83%). Elemental analysis. Found: C 79.53; H 4.56, N 3.99, S 4.96%. POPT-13: M2 (760 mg, 2.5 mmol) and M3 (110 mg, 0.25 mmol) were used in the polymerization (yield: 84%). Elemental analysis. Found: C 75.83; H 5.56, N 0.46, S 2.42%. PoPT-25: M2 (608 mg, 2.0 mmol) and M3 (176 mg, 0.4 mmol) were used in the polymerization (yield: 82%). Elemental analysis. Found: C 76.18; H 5.54, N 0.97, S 3.74%.

21



Page 22 of 28

PoPT-41: M2 (570 mg, 1.88 mmol) and M3 (246.4 mg, 0.56 mmol) were used in the polymerization (yield: 89%). Elemental analysis. Found: C 75.84; H 5.77, N 0.53, S 4.94%. Acknowledgements The authors would to thank Prof. J. Schultz at University of Delaware, USA, for his kind help in English editing. The financial support of the National Natural Science Foundations of China under Grant No. 51521062 is gratefully acknowledged.

Supporting Information Available: The supporting information is available free of charge on the ACS Publications website. General information about the experiments. Precursor and polymers synthesis procedure

and

characterization.

The

phosphorescent

spectra

and

cyclic

voltammograms of PhCz and Ph3PO. The HOMO−1 and LUMO+1 distributions of PCzPT-19 and POPT-25. Transient photoluminescence decay, excited state lifetimes and ratios for delayed emission, and PL spectra under air atmosphere and vacuum of PCzPT-10, PCzPT-43, POPT-13, and POPT-41. Time resolved fluorescence spectra and overlay plots of PL, delayed PL, and EL spectra of polymers. Performances of the device with non-doped polymers as emitting layers as well as of doped and non-doped hole-only and electron-only devices. Physical properties of the polymers: mCP films. References [1] Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient 22


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Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234-238. [2] Sun, J. W.; Lee, J. H.; Moon, C. K.; Kim, K. H.; Shin, H.; Kim, J. J. A Fluorescent Organic Light-Emitting Diode with 30% External Quantum Efficiency. Adv. Mater. 2014, 26, 5684-5688. [3] Zhang, Q.; Li, B.; Huang, S.; Nomura, H.; Tanaka, H.; Adachi, C. Efficient Blue Organic

Light-Emitting

Diodes

Employing

Thermally

Activated

Delayed

Fluorescence. Nat. Photon. 2014, 8, 326-332. [4] Yang, Z.; Mao, Z.; Xie, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Xu, J.; Chi, Z.; Aldred, M. P. Recent Advances in Organic Thermally Activated Delayed Fluorescence Materials. Chem. Soc. Rev. 2017, 46, 915-1016. [5] Nakagawa, T.; Ku, S. Y.; Wong, K. T.; Adachi, C. Electroluminescence Based on Thermally Activated Delayed Fluorescence Generated by a Spirobifluorene Donor– Acceptor Structure. Chem. Commun. 2012, 48, 9580-9582. [6] Shizu, K.; Noda, H.; Tanaka, H.; Taneda, M.; Uejima, M.; Sato, T.; Tanaka, K.; Kaji,

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Delayed-Fluorescence Emitters with Large Overlap Density Between Luminescent and Ground States. J. Phys. Chem. C 2015, 119, 26283-26289. [7] Hirata, S.; Sakai, Y.; Masui, K.; Tanaka, H.; Lee, S. Y.; Nomura, H.; Nakamura, N.; Yasumatsu, M.; Nakanotani, H.; Zhang, Q.; Shizu, K.; Miyazaki, H.; Adachi, C. Highly Efficient Blue Electroluminescence Based on Thermally Activated Delayed Fluorescence. Nat. Mater. 2015, 14, 330-336. 23



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[8] Nikolaenko, A. E.; Cass, M.; Bourcet, F.; Mohamad, D.; Roberts, M. Thermally Activated Delayed Fluorescence in Polymers: A New Route toward Highly Efficient Solution Processable OLEDs. Adv. Mater. 2015, 27, 7236-7240. [9] Lee, S. Y.; Yasuda, T.; Komiyama, H.; Lee, J.; Adachi, C. Thermally Activated Delayed

Fluorescence

Polymers

for

Efficient

Solution-Processed

Organic

Light-Emitting Diodes. Adv. Mater. 2016, 28, 4019-4024. [10] Nobuyasu, R. S.; Ren, Z.; Griffiths, G. C.; Batsanov, A. S.; Data, P.; Yan, S.; Monkman, A. P.; Bryce, M. R.; Dias, F. B. Rational Design of TADF Polymers Using a Donor-Acceptor Monomer with Enhanced TADF Efficiency Induced by the Energy Alignment of Charge Transfer and Local Triplet Excited States. Adv. Opt. Mater. 2016, 4, 597-607. [11] Ren, Z.; Nobuyasu, R. S.; Dias, F. B.; Monkman, A. P.; Yan, S.; Bryce, M. R. Pendant Homopolymer and Copolymers as Solution-Processable Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Macromolecules 2016, 49, 5452-5460. [12] Zhu, Y.; Zhang, Y.; Yao, B.; Wang, Y.; Zhang, Z.; Zhan, H.; Zhang, B.; Xie, Z.; Wang, Y.; Cheng, Y. Synthesis and Electroluminescence of a Conjugated Polymer with Thermally Activated Delayed Fluorescence. Macromolecules 2016, 49, 4373-4377. [13] Luo, J.; Xie, G.; Gong, S.; Chen, T.; Yang, C. Creating a Thermally Activated Delayed Fluorescence Channel in a Single Polymer System to Enhance Exciton Utilization Efficiency for Bluish-Green Electroluminescence. Chem. Commun. 2016, 24


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52, 2292-2295. [14] Xie, G.; Luo, J.; Huang, M.; Chen, T.; Wu, K.; Gong, S.; Yang, C. Inheriting the Characteristics of TADF Small Molecule by Side-Chain Engineering Strategy to Enable Bluish-Green Polymers with High PLQYs up to 74% and External Quantum Efficiency over 16% in Light-Emitting Diodes. Adv. Mater. 2017, 29, 1604223. [15] Li, C.; Nobuyasu, R. S.; Wang, Y.; Dias, F. B.; Ren, Z.; Bryce, M. R.; Yan, S. Solution-Processable Thermally Activated Delayed Fluorescence White OLEDs Based

on

Dual-Emission

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with

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Aggregation-Enhanced Emission Properties. Adv. Opt. Mater. 2017, DOI: 10.1002/adom.201700435. [16] Hughes, G.; Bryce, M. R. Electron-Transporting Materials for Organic Electroluminescent and Electrophosphorescent Devices. J. Mater. Chem. 2005, 15, 94-107. [17] Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. Design of Efficient Thermally Activated Delayed Fluorescence Materials for Pure Blue Organic Light Emitting Diodes. J. Am. Chem. Soc. 2012, 134, 14706-14709. [18] Wu, S.; Aonuma, M.; Zhang, Q.; Huang, S.; Nakagawa, T.; Kuwabara, K.; Adachi, C. High-Efficiency Deep-Blue Organic Light-Emitting Diodes Based on a Thermally Activated Delayed Fluorescence Emitter. J. Mater. Chem. C 2014, 2, 421-424. [19] Lee, D. R.; Kim, M.; Jeon, S. K.; Hwang, S. H.; Lee, C. W.; Lee, J. Y. Design Strategy for 25% External Quantum Efficiency in Green and Blue Thermally 25



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H.;

Huang,

W.

Dibenzothiophene-Based Phosphine Oxide Host and Electron-Transporting Materials for Efficient Blue Thermally Activated Delayed Fluorescence Diodes through Compatibility Optimization. Chem. Mater. 2015, 27, 5131-5140. [22] Li, J.; Ding, D.; Wei, Y.; Zhang, J.; Xu, H. A “Si-Locked” Phosphine Oxide Host with Suppressed Structural Relaxation for Highly Efficient Deep-Blue TADF Diodes.

Adv. Opt. Mater. 2016, 4, 522-528. [23] Zhang, J.; Ding, D.; Wei, Y.; Xu, H. Extremely Condensing Triplet States of DPEPO-Type Hosts Through Constitutional Isomerization for High-Efficiency Deep-Blue Thermally Activated Delayed Fluorescence Diodes. Chem. Sci. 2016, 7, 2870-2882. [24] Zhang, J.; Ding, D.; Wei, Y.; Han, F.; Xu, H.; Huang, W. Multiphosphine-Oxide Hosts for Ultralow-Voltage-Driven True-Blue Thermally Activated Delayed Fluorescence Diodes with External Quantum Efficiency beyond 20%. Adv. Mater. 2016, 28, 479-485. [25] Duan, C.; Fan, C.; Wei, Y.; Han, F.; Huang, W.; Xu, H. Optimizing the Intralayer and Interlayer Compatibility for High-Efficiency Blue Thermally Activated 26


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Table of Contents Graphic Title: Thermally Activated Delayed Fluorescence Pendant Copolymers with Electron and Hole-Transporting Spacers Author: Chensen Li, Yukun Wang, Dianming Sun, Huihui Li, Xiaoli Sun, Dongge Ma, Zhongjie Ren,* Shouke Yan*

Air Vacuum

42.6% DF

450

500

550

600

650

Wavelength (nm)

28


Air Vacuum

25.3% DF

700

450

500

550

600

Wavelength (nm)

650

700

Thermally Activated Delayed Fluorescence Pendant Copolymers with

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