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Hole-Transporting Side-Chain Polymer Bearing Thermally Crosslinkable Bicyclo[4.2.0]octa-1,3,5-trien-3-yl Group for HighPerforming Thermally Activated Delayed Fluorescence OLED Cheol Hun Jeong, Mallesham Godumala, Jiwon Yoon, Suna Choi, Yong Woo Kim, Dae Hyuk Choi, Min Ju Cho, and Dong Hoon Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03446 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Materials & Interfaces

Hole-Transporting Side-Chain Polymer Bearing Thermally Crosslinkable Bicyclo[4.2.0]octa-1,3,5-trien-3-yl Group for High-Performing Thermally Activated Delayed Fluorescence OLED Cheol Hun Jeonga, Mallesham Godumalaa, Jiwon Yoona, Suna Choia, Yong Woo Kimb, Dae Hyuk Choib, Min Ju Cho*a and Dong Hoon Choi*a a

Department of Chemistry, Research Institute for Natural Sciences, Korea University, 145 Anam-ro, Sungbuk-gu, Seoul 02841, Korea. b

LT Materials, 113-19, Dangha-Ro, Namsa-Myeon, Cheoin-Gu, Yongin-Si, Gyeonggi-Do 17118, Korea. *Corresponding authors: M. J. Cho (E-mail: [email protected]); D. H. Choi (E-mail: [email protected])

ABSTRACT A new side-chain polymer (X-TPACz) bearing hole-transporting pendant groups accompanying a thermally crosslinkable entity was synthesized using N-([1,1'-biphenyl]-4-yl)N-(4-(9-(4-vinylbenzyl)-9H-carbazol-3-yl)phenyl)bicyclo[4.2.0]octa-1(6),2,4-trien-3-amine (6) via an addition polymerization. The X-TPACz could be spontaneously crosslinked without using any further reagents and showed a good film-forming property upon low-temperature thermal treatment. The thermal curing temperature for X-TPACz film was optimized to be 180 C based on differential scanning calorimetry thermogram. Moreover, the thermal degradation temperature of XTPACz was measured as over 467 C using thermogravimetric analysis demonstrated that it shows an excellent thermal stability. In particular, X-TPACz exhibits HOMO energy level to be -5.26 eV which is beneficial for facile hole injection and transportation. Consequently, the thermally activated delayed fluorescent organic light-emitting diodes fabricated using X1

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TPACz as the hole-transporting material showed state-of-the-art performances with a low turnon voltage (Von) of only 2.7 V and high external quantum efficiency (EQE) of 19.18% with with a high current efficiency (CE) of 66.88 cd/A and a high power efficiency (PE) of 60.03 lm/W, which are highly superior than the familiar poly(9-vinylcarbazole) (PVK)-based devices (Von=3.9 V, EQE of 17.42%, with CE of 58.33 cd/A and PE of 33.32 lm/W). The extremely low turn-on voltage and high EQE were found to be due to the higher-lying highest occupied molecular orbital energy level (EHOMO=-5.23 eV) and better hole-transporting property of XTPACz than PVK.

Keywords: hole transport material; thermal crosslinking; solution process; thermally activated delayed fluorescent; organic light-emitting diode.

INTRODUCTION Recently, solution-processable organic light-emitting diodes (OLEDs) have been highly desired due to their application to low-cost, large-area, and flexible displays.1-7 However, in order to fabricate the above multilayer OLEDs by a solution process, materials having high solubility in an organic solvent are required, and there is another problem to be solved simultaneously. When a multilayer thin film is prepared by a solution process, the lower layer can be dissolved when the upper layer material solution is applied to the already formed lower layer. In order to manufacture an OLED device by a solution process, a hole-injection layer (HIL) or a hole-transport layer (HTL), which is mainly present in the lower layer, should have excellent solvent resistance and excellent compatibility at the interface with other surrounding layer materials. Obviously, if the HIL or HTL material does not satisfy the above two characteristics, the device performance of the multilayer OLED is negatively affected. 2


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Predominantly, the usage of an orthogonal solvent is an effective method to solve the problem of dissolution occurring in the bottom layer.5-13 For instance, water-soluble poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) used as a hole injection material (HIM) can form an insoluble thin film in various organic solvents.3,4,6,8-12 Moreover, poly(9-vinyl carbazole) (PVK) used as a hole-transporting material (HTM) can control the solubility by varying molecular weight (MW).13 High-MW PVK is highly soluble in chlorobenzene but poorly soluble in toluene. It is possible to fabricate an upper emitting layer using toluene solution containing many types of emitting materials including polymers, oligomers, and small molecules. Although indium-tin-oxide (ITO)/PEDOT:PSS/PVK as an anode/HIL/HTL system is one of the most effective device structures, the lower-lying highest occupied molecular orbital (HOMO) level of PVK can occasionally interfere with the smooth hole transport between PEDOT:PSS and the emitting layer. However, the formation of the HTL through crosslinking, which results in an insoluble layer in many common organic solvents, is considered to be the most effective method. Compared to an orthogonal solvent system, a crosslinking system can yield various HTMs such as small molecules, star-shaped macromolecules, and main-/side-chain polymers. To date, many types of crosslinkable HTMs have been proposed.5-7,14-20 Among various reactive moieties, vinyl units were often employed to design thermally crosslinkable small molecules and polymers.21-23 Tseng and his colleagues demonstrated a thermal crosslinkable hole injection small molecule (VB-DATA) based on 4,4',4''-tris[(3-methylphenyl)phenylamino]triphenylamine having three peripheral moieties of styryl (vinylbenzene) moiety. The crosslinked VB-DATA thin films could be prepared by spincasting followed by thermal treatment at 190 C and applied to green emitting OLEDs.21 Very recently,

Cha’s

group

reported

poly(indenofluorene-co-triphenylamine)

a

thermally

copolymer

crosslinkable (X-IFTPA)

3


hole-transporting

containing

a

vinyl-


functionalized triphenylamine moiety. Crosslinking between vinyl units was efficiently induced by thermally annealing the film at 150 C. Green emitting polymer light-emitting diodes bearing crosslinked X-IFTPA film as the HTL exhibited 11.57 cd/A of luminous efficiency.23 Another effective moiety of benzocyclobutene was also incorporated into the HTL polymers for fabricating solution processed OLEDs.5,24,25 A thermally crosslinkable copolymer containing

benzocyclobutene

(BCB)

units

and

N,N’-bis(3-methylphenyl)-N,N’-

diphenylbenzidine) was synthesized and reported. Insoluble polymer films with a smooth surface could be obtained by thermal annealing at 200 °C. Using the crosslinked HTL, solution processed green OLEDs were fabricated and exhibited high performance with 10.4% external quantum efficiency at a brightness of 350 cd/m2.5 In addition, the styrene polymer bearing 3,6bis(carbazol-9-yl)carbazole and BCB was suggested as a good HTM and the film showing high triplet energy was applied to green phosphorescence OLEDs.25 According to the results of previous studies mentioned above, it is considered very easy to introduce the electron donating moiety and the crosslinkable moiety into the side chain when designing the HTM polymer. Polymers bearing hole transporting unit as a side-chain also can exhibit easily tuned energy levels owing to the nonconjugated bridge between the main-chain and HTM entity. Additionally, the purification of side-chain polymers via addition polymerization is easier than that of main-chain conjugated polymers synthesized using a metallic catalyst. Herein, we demonstrate a new side-chain hole-transport polymer (X-TPACz) comprised of a thermally crosslinkable pendant, N-(4-(9H-carbazol-3-yl)phenyl)-N-([1,1'-biphenyl]-4yl)bicyclo[4.2.0]octa-1(6),2,4-trien-3-amine. A familiar bicyclo[4.2.0]octa-1,3,5-trien-3-yl 4


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group is selected as a thermally crosslinkable unit. The resulting X-TPACz can be easily crosslinked by thermal treatment under 200 C. In particular, a thermally cured thin film of XTPACz showed a fine/smooth surface morphology and became insoluble in various common organic solvents such as chloroform, toluene, chlorobenzene etc. The HOMO energy level of X-TPACz was estimated to be −5.26 and −5.23 eV for the pristine and cured film, respectively. To investigate the ability of X-TPACz as a HTM, green thermally activated delayed fluorescent

(TADF)-OLEDs

were

explored.

An

ITO/PEDOT:PSS/X-

TPACz/EML/TPBi/LiF/Al based device displayed a remarkably low turn-on voltage (Von) of only 2.7 V, along with a high external quantum efficiency (EQE) of 19.18%, whereas a wellknown PVK-based device exhibited a relatively higher Von of 3.9 V and lower EQE of only 17.42%.

EXPERIMENTAL SECTION Materials In this study, all chemical materials for synthesizing X-TPACz were purchased from MERCK and Thermo Fisher Scientific, and used without further purification. The azobisisobutyronitrile (AIBN) was recrystallized using MeOH before using for reaction. Nphenyl-[1,1'-biphenyl]-4-amine (1) was prepared following previous reports.26

Synthesis Synthesis of N-([1,1'-biphenyl]-4-yl)-N-phenylbicyclo[4.2.0]octa-1(6),2,4-trien-3-amine. (2) Compound 1 (2.4 g, 9.78 mmol), 3-bromobicyclo[4.2.0]octa-1,3,5-triene (1.8 g, 9.83 mmol), 5



and potassium tert-butoxide (1.1 g, 9.9 mmol) were dissolved in dried toluene (20 mL) under a nitrogen atmosphere. Pd2(dba)3 and P(t-butyl)3 were added to the mother mixture and purged with nitrogen for 10 min. The resulting mixture was kept under stirring at 90 C for 4 h. After the solution was cooled to room temperature (RT), it was filtered. After concentrating the filtered solution, purified by silica-gel column chromatography with n-hexane as the eluent. Compound 2 was obtained as a white powder after recrystallization using MeOH (yield : 3.0 g, 88.3%). 1H NMR (500 MHz, CDCl3): δ (ppm) 7.57 (d, J = 7.3, 2H), 7.47 (d, J = 8.85 Hz, 2H), 7.42 (t, J = 7.48 Hz, 2H), 7.30 (t, J = 6.7 Hz, 1H), 7.26 (t, J = 7.35 Hz, 2H), 7.11 (d, J = 7.95 Hz, 4H), 7.02-6.98 (m, 3H), 6.91 (s, 1H), 3.17 (s, 2H), 3.14 (s, 2H).

Synthesis of N-([1,1'-biphenyl]-4-yl)-N-(4-bromophenyl)bicyclo[4.2.0]octa-1(6),2,4-trien3-amine. (3) Compound 2 (3.0 g, 8.63 mmol) was dissolved in dimethylformamide (DMF) (15 mL). Nbromosuccinimide (NBS) (1.65 g, 9.27 mmol) solution in DMF (5 mL) was added into the mother solution. The mixture was kept under stirring for 1.5 h at RT. The solution was extracted with n-hexane and water, and the organic layer was dried with sodium sulfate. The crude solution was concentrated, then purified by silica-gel column chromatography (DCM:n-hexane = 1:20 v/v) (yield: 2.1 g, 57.1%). 1H NMR (500 MHz, CDCl3): δ (ppm) 7.56 (d, J = 8.25 Hz, 2H), 7.46 (d, J = 8.85 Hz, 2H), 7.42 (t, J = 7.30 Hz, 2H), 7.33-7.30 (m, 3H), 7.09 (d, J = 8.85 Hz, 2H), 6.98 (s, 2H), 6.96 (d, J = 8.85 Hz, 2H), 6.88 (s, 1H), 3.17-3.13 (m, 4H).

Synthesis

of

N-([1,1'-biphenyl]-4-yl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-26


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yl)phenyl)bicyclo[4.2.0]octa-1(6),2,4-trien-3-amine. (4) Compound 3 (2.1 g, 4.93 mmol), bis(pinacolato)diboron (1.88 g, 7.39 mmol), potassium acetate (1.12 g, 11.33 mmol), and Pd(PPh3)4 were dissolved together in DMF and the mixture was stirred overnight at 90 C under a nitrogen atmosphere. Subsequently, water was added to the reaction mixture, followed by filtering. Then, the filtered solid was dissolved in DCM and the solution was dried with sodium sulfate, and then concentrated. The compound was purified by silica-gel column chromatography (DCM:n-hexane = 1:8 v/v) (yield: 1.66 g, 71.19%). 1H NMR (500 MHz, CDCl3): δ (ppm) 7.66 (d, J = 8.55 Hz, 2H), 7.56 (d, J = 8.25 Hz, 2H), 7.46 (d, J = 7.65 Hz, 2H), 7.42 (t, J = 7.65 Hz, 2H), 7.31 (t, J = 7.65 Hz, 1H), 7.14 (d, J = 8.55 Hz, 2H), 7.04 (d, J = 8.55 Hz, 2H), 7.00 (m, 2H), 6.90 (s, 1H), 3.17-3.13 (m, 4H), 1.35-1.33 (s, 12H).

Synthesis of N-(4-(9H-carbazol-3-yl)phenyl)-N-([1,1'-biphenyl]-4-yl)bicyclo[4.2.0]octa1(6),2,4-trien-3-amine. (5) Compound 4 (0.450 g, 0.95 mmol) and 3-bromo-9H-carbazole (0.234 g, 0.95 mmol) were dissolved in dried toluene. A freshly prepared potassium carbonate (0.790 g, 5.72 mmol) solution, aliquat 336 and Pd(PPh3)4 were added to the reaction mixture, which was then purged with nitrogen for 10 min. The mixed solution was kept under stirring overnight at 90 C in a nitrogen atmosphere. The solution was extracted with DCM and water, and the organic layer was dried with sodium sulfate. The crude product was purified by silica-gel column chromatography (DCM:n-hexane = 1:1 v/v) (yield : 0.522 g, 85.3%). 1H NMR (500 MHz, CDCl3): δ (ppm) 8.26 (s, 1H), 8.11 (d, J = 7.35 Hz, 1H), 8.05 (s, 1H), 7.65 (d, J = 8.25 Hz, 7



1H), 7.59-7.57 (m, 4H), 7.48-7.47 (m, 3H), 7.44-7.40 (m, 4H), 7.29 (t, J = 7.30 Hz, 1H), 7.20 (d, J = 7.00 Hz, 2H), 7.16 (d, J = 7.00 Hz, 2H), 7.07 (d, J = 7.35 Hz, 1H), 7.00 (d, J = 7.65 Hz, 1H), 6.96 (s, 1H), 3.17-3.16 (m, 4H).

Synthesis of N-([1,1'-biphenyl]-4-yl)-N-(4-(9-(4-vinylbenzyl)-9H-carbazol-3-yl)phenyl) bicyclo[4.2.0]octa-1(6),2,4-trien-3-amine. (6) NaH (0.0234 g, 0.975 mmol) was added to a stirred solution of compound 5 (0.5 g, 0.975 mmol) dissolved in DMF (20 mL) at 0 C. After stirring for 30 min, 4-vinylbenzyl chloride (0.1637 g, 1.072 mmol) was directly added to the mixture at 0 C, which was then stirred for a further 3 h at RT. The mixture was extracted with EA and water, and the organic layer was dried with sodium sulfate. The compound was purified by silica-gel column chromatography (DCM:n-hexane = 1:1 v/v) and precipitated using MeOH to obtain a white solid (yield: 0.522 g, 85.3 %). 1H NMR (500 MHz, CDCl3): δ (ppm) 8.31 (s, 1H), 8.16 (d, J = 7.60 Hz, 1H), 7.65 (d, J = 8.50 Hz, 1H), 7.60-7.57 (m, 4H), 7.48 (d, J = 8.55 Hz, 2H), 7.45-7.35 (m, 5H), 7.327.29 (m, 3H), 7.20 (d, J = 8.55 Hz, 2H), 7.17 (d, J = 8.55 Hz, 2H), 7.12 (d, J = 8.25 Hz. 2H), 7.07 (d, J = 7.90 Hz, 1H), 7.00 (d, J = 7.95 Hz, 1H), 6.96 (s, 1H), 6.65 (m, 1H), 5.68 (d, J = 17.65 Hz, 1H), 5.53 (s, 2H), 5.21 (d, J = 11.0 Hz, 1H), 3.17-3.16 (m, 4H). MS (MALDI-TOF) m/z [M]+ Calcd. for C47H36N2, 628.2878; found 628.1982.

Synthesis of Crosslinkable Polymer (X-TPACz) Compound 6 (300 mg, 0.477 mmol) and AIBN (1.5 mg, 0.009 mmol) were dissolved in dry toluene (1.5 mL) in a 3.0 mL sealed tube, and purged with nitrogen for 10 min. The mixed 8


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solution was kept under stirring at 80 C for 48 h. After the solution was cooled to room temperature, the crude solution was precipitated in MeOH (100 mL). To remove the unreacted monomers and oligomers, Soxhlet extraction was conducted sequentially with acetone, nhexane, and DCM. The DCM fraction was then precipitated into MeOH (100 mL) and dried under vacuum at 50 C for 24 h. (Mn = 11.69 kDa, PDI = 2.47). Elemental Anal. Calcd. for C47H38N2 : C, 89.49; H, 6.07; N, 4.44. Found: C, 89.14; H, 5.77; N, 4.56.

Instrumental Analysis A Bruker 500 MHz spectrometer was utilized for acquiring 1H nuclear magnetic resonance (NMR) spectra. The number average molecular weight of the newly synthesized X-TPACz was measured using gel-permeation chromatography (polystyrene standard and odichlorobenzene as the eluent, with an Agilent GPC 1200 series instrument). Differential scanning calorimetry (DSC) was employed to investigate the polymer thermal properties under a nitrogen atmosphere on a Mettler DSC 821e (Mettler, Greifensee, Switzerland) instrument. Thermogravimetric analysis (TGA) was carried out using a SCINCO TGA-N 1000 thermal analysis system. Absorption spectroscopy was conducted using a UV–vis absorption spectrometer (Agilent HP 8453, PDA type). In order to prepare the film sample, the X-TPACz was dissolved in chlorobenzene, and the solution was deposited by spin-coating on a cleaned glass. The films were annealed at elevated temperatures of 160, 180, and 200 C for a fixed curing time. Absorption spectroscopy was also used to investigate the solvent resistance after thermal curing. The thermally cured films were rinsed with chloroform and the spectra were acquired sequentially. Cyclic voltammetry (CV) was performed using an eDAQ EA161 potentiostat (scan rate = 50 mV s–1, reference electrode = Ag/AgCl, electrochemical 9



workstation model). The sample was prepared by dissolving the X-TPACz in DCM (1.0 wt.%) and the solution was dropped on a platinum (Pt) plate. CV was performed on the dropped Pt film before and after curing at 180 C. The film surface morphology on glass was observed before and after curing using an atomic force microscope (AFM) (Trapping mode, XE-100, PSIA).

Fabrication of Green TADF-OLED. The TADF-OLEDs were manufactured in a general device configuration of ITO/PEDOT:PSS/HTL/EML/TPBi/LiF/Al. ITO and Al work as the anode and cathode, respectively. PEDOT:PSS and LiF work as hole-injection and electron-injection layers, respectively, and TPBi is an electron-transporting layer. A new host material, 5-(6-(9Hcarbazol-9-yl)pyridin-3-yl)-7,7-dimethyl-13,13-diphenyl-7,13-dihydro-5H-indeno[1,2b]acridine (IAPC) and the known material 2,4,5-tetra(3,6-di-tert-butylcarbazol-9-yl)-1,3dicyanobenzene (t4CzIPN)2 as a green TADF emitter were utilized as a dopant. Further, the newly synthesized X-TPACz was implemented as a HTM and the control PVK-based device were fabricated. The fabrication process was performed by the following procedures. The ITO-coated glass substrate was successively pre-cleaned with deionized water and isopropanol. After drying at 100 C overnight, PEDOT:PSS was deposited as a HIL, followed by annealing at 155 °C for 15 min. The X-TPACz solution in chlorobenzene was deposited by spin-coating and cured at 180 °C for 30 min, and the PVK solution in chlorobenzene was spin-coated and annealed at 130 °C for 20 min. The emissive layer was spin-coated from a toluene solution made with IAPC and t4CzIPN, and then 2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), LiF, and the Al electrode were prepared using the thermal evaporator. 10


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To find the best OLED performance conditions, we fabricated the devices at three different dopant concentrations (i.e., 3, 6, and 9 wt.%). We observed the current density–voltage– luminance (J–V–L) using a Keithley 236 source-measure unit instrument and a SpectraScan colorimeter (PR-655) using the devices fabricated above. All the device performance measurements were carried out under ambient conditions.

RESULT AND DISCUSSION Synthesis and Characterization of X-TPACz A new thermally cross-linkable hole-affine polymer, X-TPACz was designed and synthesized to function as a HTM in solution-processable OLEDs (Scheme 1). Compound 1 was prepared using the previous report.15 Compound 2 was synthesized using compound 1 and commercially available 3-bromobicyclo[4.2.0]octa-1,3,5-triene probing the Buchwald– Hartwig reaction. Compound 3 was ascertained by the bromination of compound 2, following borylation reaction with bis(pinacolato)diboron to obtain compound 4. Further, Suzuki– Miyaura coupling conditions were conducted with compound 4 and 3-bromo-9H-carbazole to produce compound 5. Compound 5 was treated with 4-vinylbenzyl chloride using NaH as a base to yield compound 6.

11



i

H N

ii

N

N

2

1

Page 12 of 27

iii

Br

O B O

N

3

4 iv

n

N

N

vi

N

N

X-TPACz

6

v

N

NH

5

Scheme 1. Synthetic procedure for the crosslinkable hole-transporting polymer, X-TPACz. i) 3-bromobicyclo[4.2.0]octa-1,3,5-triene, Pd2(dba)3, P(t-butyl)3, KOtBu, toluene, 90 C, 4 h; ii) NBS, DMF, room temperature, 1.5 h; iii) bis(pinacolato)diboron, Pd(PPh3)4, KOAc, DMF, 90 C, overnight; iv) 3-bromo-9H-carbazole, Pd(PPh3)4, 6M K2CO3, toluene, aliquat 336, 90 C, overnight; v) 4-vinylbenzyl chloride, NaH, DMF, room temperature, 3 h; vi) AIBN, toluene, 80 C, 48 h.

Finally, the targeted polymer, X-TPACz was synthesized using compound 6 via radical polymerization with AIBN as a radical initiator. X-TPACz is highly soluble in chloroform, chlorobenzene, and o-dichlorobenzene at RT. The corresponding solution showed a good uniform film-forming capability. The number average molecular weight (Mn) and polydispersity index (PDI) of X-TPACz were measured using GPC and calibrated against a polystyrene standard. The Mn and PDI of X-TPACz were found to be 11.69 kg mol−1 and 2.47, respectively.

Thermal Properties of X-TPACz TGA and DSC were employed to study the thermal properties of X-TPACz and the 12


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corresponding plots are depicted in Figures 1a and 1b, respectively. The TGA measurements demonstrate that the polymer had an excellent thermal stability with a thermal degradation temperature (Td) of 467 C (Figure 1a). Moreover, DSC measurements were carried out at a heating (cooling) rate of 10 (−10) C/min under nitrogen. As illustrated by Figure 1b in the first heating cycle, the glass transition temperature (Tg) and crosslinking temperature (Tcrosslinking) of X-TPACz can be observed as 199 and 258 C in nitrogen atmosphere, respectively. Nevertheless, in the second heating cycle, because the X-TPACz polymer was already crosslinked, the previous thermal transition temperatures could not be observed at all. The exothermic transition peak observed at 258 C was considered as a thermal crosslinking temperature owing to the opening of the cyclobutane of a typical bicyclo[4.2.0]octa-1,3,5triene, consistent with the results reported in the literature.27

13



Figure 1. TGA (a) and DSC (b) curves of X-TPACz. Crosslinking mechanism between XTPACz polymer chains (c).

The newly prepared X-TPACz contains a high density of thermally curable side chain moiety, which is hole-affine in nature. This can be expected to spontaneously induce a highly efficient crosslinking process between the polymer chains without adding any crosslinking agent. The crosslinking process of X-TPACz polymer chains by bicyclo[4.2.0]octa-1,3,5triene is depicted in Figure 1c. The bicyclo[4.2.0]octa-1,3,5-triene present in the polymer side chain undergoes a ring opening reaction by heat treatment and forms an octagonal structure by a cycloaddition reaction between the ring-opened bicyclo[4.2.0]octa-1,3,5-triene moieties.5,27

Absorption, Photoluminescence, and Electrochemical properties Absorption (Abs) and photoluminescence (PL) spectra of X-TPACz in the film state are displayed in Figure 2a and CV of the polymer thin films are shown in Figure 2b for electrochemical analysis. X-TPACz film shows a strong absorption band located at 341 nm and featureless emission spectra with the maximum intensity wavelength at 421 nm. The optical energy bandgap (Egopt) was calculated from the onset wavelength of the spectrum. Both the optical energy bandgap (Egopt) of pristine and crosslinked X-TPACz film were calculated as 3.12 eV, which indicates no significant variation in the frontier orbital structure under thermal crosslinking. This is attributed to the fact that the crosslinking reaction only occurred at the terminal end of the side chain, effect of hole transporting moiety in the side chains. The HOMO levels of X-TPACz before and after thermal curing was estimated from the CV 14


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measurements. The HOMO levels of pristine and crosslinked X-TPACz film were estimated to be −5.26 and −5.23 eV from the oxidation-onset potentials of +0.82 and +0.79 V, respectively. The lowest unoccupied molecular orbital (LUMO) levels of pristine and crosslinked X-TPACz film was determined to be −2.14 and −2.11 eV, respectively. They did not show significant variation under crosslinking, which could be beneficial for aligning the energy levels of each layer in practical multilayered OLEDs. The relevant optical and electrochemical data are tabulated in Table 1.

Figure 2. (a) UV–vis absorption and photoluminescence (PL) spectra of X-TPACz in film state. (b) Cyclic voltammograms of X-TPACz films before and after thermal curing. (Curing condition = 180 C, 30 min)

Table 1. Optical and electrochemical properties of X-TPACz before and after thermal curing. Sample

Curing condition

max a (nm)

onset a (nm)

Egopt. b (eV)

Eox c (V)

Energy levels (eV) HOMO d LUMO e

Pristine 341 397 3.12 0.82 −5.26 −2.14 film Cured 180 C, 30 341 397 3.12 0.79 −5.23 −2.11 film min c a Film state. b 1240/ onset. The oxidation potentials were obtained from cyclic voltammograms d in the film state. HOMO = −e(4.44 V +Eox). e LUMO = (HOMO + Egopt). 15



Solubility Test of X-TPACz Film After Thermal Curing We effectively investigated the feasibility of thermally crosslinked X-TPACz films as suitable HTLs for multilayered OLED devices fabricated via solution processes. After deposition of the emitting layer (EML) on the surface of the HTM layer, the HTL should be sustained without swelling and dissolution. In this regard, the solubility of the crosslinked XTPACz film in chloroform was investigated using absorption spectroscopy.

Figure 3. Schematic diagram for preparing cured thin films (a). Absorption spectra of XTPACz film before (black solid line), after thermal curing (red solid line), and after rinsing (blue solid line) in chloroform for thermal curing time; thermal curing time at 180 C: (b) : 10 min, (c) : 20 min, (d) : 30 min.

Figure 3 illustrates the results of the rinsing test in chloroform for investigating the solvent resistance of films prepared using the X-TPACz film crosslinked at a temperature of 180 °C and the resistance was monitored by changing the thermal curing time. The films were cured 16


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for various time intervals: 10, 20, and 30 min. The cured X-TPACz film was rinsed by immersion in chloroform for 1 min. As shown in Figure 3, the films cured for 10 and 20 min, after rinsing in chloroform, showed lower absorbance intensity, which indicated the partial dissolution of X-TPACz in chloroform. However, the film cured for 30 min showed a very small decrement in absorbance. Therefore, the film cured for 30 min at 180 C showed the highest stability and it was demonstrated that thermally crosslinkable X-TPACz is an excellent polymer for solution processing in multilayered OLED fabrication. Therefore, in this study, the curing conditions of X-TPACz films were optimized to 180 C for 30 min.

Surface Morphology of X-TPACz Films Before and After Thermal Curing

17



Figure 4. AFM topography (a and b) and phase (c and d) images (5 μm × 5 μm) of before and after thermally cured X-TPACz film (curing conditions = 180 C, 30 min).

In the case of thermally cured polymers, the film surface often becomes coarse and less homogeneous after thermal treatment.28 This can cause a charge trapping phenomenon at the interface between the HTL and EML in an OLED device consisting of a multistacked structure. AFM was used to measure changes in the surface morphology of X-TPACz before and after curing. (Figure 4) In Figures 4a and 4b, the surface of the X-TPACz film shows a fine morphology with a very small root mean square (RMS) roughness value. Likewise, the surface morphology showed no significant change and small root mean square roughness (Rq) after thermal crosslinking, which demonstrates that the X-TPACz films are suitable for use as HTLs in multilayered OLEDs, without having a detrimental effect on the interface between the HTL and emitting layer.

Hole-Only Device (HOD) of HTM To investigate the electrical properties of the HTM layer, a HOD was fabricated with the following structure: ITO/PEDOT:PSS (40 nm)/pristine or crosslinked X-TPACz or PVK film (100 nm)/MoO3 (10 nm)/Ag (100 nm). In this case, a control device using PVK instead of the newly synthesized X-TPACz was also fabricated as a control device. The structure of the HOD device and a plot of the current density–voltage characteristics of the device are shown in Figure S1a. The hole mobility of 1.32 × 10−5 cm2 V−1s−1 was obtained for the device using PVK as the HTL (Figure S1b), whereas the pristine and crosslinked X-TPACz films showed 18


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enhanced mobilities of 3.52 × 10−4 cm2 V−1s−1 and 2.01 × 10−4 cm2 V−1s−1, respectively. It was also found that the charge transport properties of X-TPACz as the HTL did not exhibit any significant changes after thermal crosslinking, and were better (nearly 15-26 times higher) than that of PVK.

Fabrication and Characterization of Green TADF-OLED In this study, green TADF OLEDs were fabricated to investigate the feasibility of the function of X-TPACz as solution-processed HTM. In this regard, t4CzIPN and a newly synthesized host, IAPC in the form of indenoacridine-pyridine-carbazole, have been utilized as a green TADF emitter and a host, respectively. The synthesis and characterization of IAPC are well-described in the supporting information. (Figure S2–S4, Scheme S1), Optical and electrochemical data of IAPC are summarized in Table S1. The device structure is as follows: ITO (150 nm)/PEDOT:PSS (40 nm)/PVK (Device A, t=20 nm) or X-TPACz (Device B, t=8 nm)/IAPC:t4CzIPN (20 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (100 nm). Figure 5a displays the device structure and the chemical structures of the components used in this study. Figure S5 shows the energy alignment of each layer between two electrodes.

19



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Figure 5. (a) Device configuration and components of green TADF-OLEDs. (b) Current density–voltage (J-V) curve, (c) luminance–voltage (L–V) curve, (d) external quantum efficiency (EQE)–current density curve, (e) electroluminescence (EL) spectra obtained from the optimized solution-processed green TADF OLED devices bearing PVK or crosslinked XTPACz as HTL.

Table 2. Device parameters of green TADF-OLED device (IAPC:t4CzIPN = 94:6 wt.%) with different film thickness of PVK or crosslinked X-TPACz as HTL. HTM PVK

XTPACz

HTL Von a Thickness (V) (nm) 20 3.9

CEmax b (cd/A)

PEmax c (lm/W)

EQEmax d (%)

Lmax e (cd/m2)

CIE f (x,y)

58.33

33.32

17.42

14410

(0.31, 0.59)

35

4.7

53.55

30.59

16.24

10350

(0.31, 0.59)

40

4.9

45.69

23.92

14.10

10280

(0.30, 0.58)

8

2.7

66.88

60.03

19.18

18030

(0.33, 0.60)

20

2.8

63.48

56.98

18.17

16030

(0.33, 0.60)

20


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40 2.9 56.10 46.37 16.21 8558 (0.33, 0.60) a Turn-on voltage at a luminance of 1 cd/m2. b Maximum current efficiency. c Maximum power efficiency. d Maximum external quantum efficiency. e Maximum luminance. f CIE color coordinates at the luminance of 1000 cd/m2.

The doping concentration of the emitter in the EML was examined in the range 3–9 wt.%, and the device performance was also optimized by adjusting the thickness of the HTL (Figure S6–S9, and Table 2 and S2). The devices using PVK as the HTM with the dopant concentrations of 3, 6, and 9 wt.% are denoted as A-1, A-2, and A-3, and the devices using X-TPACz with the dopant concentrations of 3, 6, and 9 wt.% are represented as B-1, B-2, and B-3. (Figure S6, S7, and Table S2). Among these, device A-2 showed the highest EQE of 17.05% with a current efficiency (CE) of 57.82 cd/A, and a power efficiency (PE) of 33.02 lm/W. Similar to device A-2, device B-2 with 6% doping concentration exhibited the highest EQE of 17.77%, with a CE of 60.88 cd/A and PE of 45.82 lm/W. In order to investigate the effect of the HTL thickness in 6% doped TADF-OLEDs, the thickness of PVK and X-TPACz were varied by using different solution concentrations and the corresponding device performance and parameters are displayed in the Figure S8, S9, and Table 2. Among the devices, the device with 8 nm thick X-TPACz as HTL showed the highest EQE of 19.18%, with a high CE of 66.88 cd/A and a high PE of 60.03 lm/W, whereas the device with 20 nm thick PVK HTL showed the highest EQE of 17.42%, with CE of 58.33 cd/A and PE of 33.32 lm/W, as shown in Figure 5 and Table 2. Most importantly, the turn-on voltage (Von) at a luminance of 1 cd/m2 of device B-2 (2.7 V) with 8 nm thick X-TPACz HTL was significantly lower than that of Device A-2 (3.9 V) with 20 nm thick PVK HTL. It is 21



ascribed to the fact that the crosslinked X-TPACz film exhibited higher hole mobility than PVK film and the device B-2 has a very small injection barrier (~0.13 eV) between the PEDOT:PSS and crosslinked X-TPACz layer. (Figures S1 and S5) Therefore, more facile hole transport property could be achieved in the device B-2. As a result, the turn-on voltage in the device with crosslinked HTL was significantly reduced.

CONCLUSION In this study, we introduced a new HTL material that facilitates hole injection/transport properties and induces balanced charge recombination in an emitting layer in a multilayered OLED device fabricated by solution processing. The X-TPACz synthesized in this study can undergo crosslinking between its polymer chains by thermal curing at a moderately low temperature, and it was confirmed that the molecular energy level and charge transporting property are well maintained after the crosslinking reaction. Finally, the efficiency of device with a new HTM is superior to that of other devices with PVK as the HTM. In particular, when using the crosslinked X-TPACz HTL, a much lower turn-on voltage was observed compared with the conventional PVK-based device, owing to a very low charge injection barrier. To our knowledge this is the highest performance and lowest turn-on voltage for the t4CzIPN based device reported till date. Our work unambiguously demonstrates that new HTLs made from thermally cured X-TPACz films exhibit effective hole transport and solvent resistance and show excellent performance when applied to multilayered OLED devices. This material technology can be fully utilized to implement various TADF-OLED devices with various hosts and dopants through solution processing.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acsami.xxxxxxx Synthesis, Details of the general experimental procedures, electrical properties of HODs, optical and photophysical properties, UV-vis absorption spectra, PL spectra, cyclic voltammogram, energy level diagrams, OLED device performances, and comparison of the performance using the t4CzIPN as a dopant.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

ACKNOWLEDGEMENTS This research was supported by LT Materials (2019). It was also supported by the Key Research Institute Program (NRF 2019R1A2C2002647, 20100020209). We also thank Korea Basic Science Institute (Seoul) for allowing to use the MALDI-TOF MS instrument. REFERENCES

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Hole-Transporting Side-Chain Polymer Bearing ... - ACS Publications

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