Highly efficient, solution-processed organic light emitting diodes

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Highly efficient, solution-processed organic light emitting diodes based on thermally activated delayedfluorescence emitter with a mixed polymer interlayer Yanwei Liu, Xiaofang Wei, Zhiyi Li, Jianjun Liu, Ruifang Wang, Xiaoxiao Hu, Pengfei Wang, Yukiko Yamada-Takamura, Ting Qi, and Ying Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00131 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Applied Energy Materials

Highly Efficient, Solution-Processed Organic Light Emitting Diodes Based on Thermally Activated Delayed-Fluorescence

Emitter

with

a

Mixed

Polymer Interlayer Yanwei Liu,†,‡ Xiaofang Wei,‡,§ Zhiyi Li,‡,§ Jianjun Liu,‡,§ Ruifang Wang,‡,§ Xiaoxiao Hu,‡,§ Pengfei Wang,‡,§ Yukiko Yamada-Takamura,& Ting Qi, †,* and Ying Wang‡,§,* †

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,

Beijing, 100049, China ‡

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute

of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China School of Future Technology, University of Chinese Academy of Sciences, Beijing, 100049,

§

China School of Materials Science, Japan Advanced Institute of Science and Technolgoy, Ishikawa,

&

923-1292, Japan

ACS Paragon Plus Environment

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ABSTRACT: The efficiency of solution-processed organic light emitting diodes based on pure organic thermally activated delayed fluorescence (TADF) emitters are limited by the hole injection/severe

exciton

quenching

of

poly(styreneslphonate)-doped

poly(3,4-

ethylenedioxythiophene) (PEDOT:PSS) and the exction quenching of TADF emitters due to the self-aggregation. In order to effectively enhance the hole injection from PEDOT:PSS and to alleviate the exciton quenching of the PEDOT:PSS, the mixed interlayer of Poly (9vinylcarbazole) (PVK) and Poly [(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4′-(N-(4-secbutylphenyl) diphenylamine)) (TFB) were inserted between emitting layer (EML) and the PEDOT:PSS layer. Additionally, the cohost of 3′,5′-di(carbazol-9-yl)-[1,1′-biphenyl]-3,5dicarbonitrile (DCzDCN) and bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2) were used to suppress the self-aggregation of the TADF emitter of 2-(9-phenyl-9H-carbazol-3-yl)10,10-dioxide-9 thioxanthone (TXO-PhCz) and tune the charge carrier transport. By optimizing the mix ratio, highly efficient, solution-processed green-emitting TADF-OLEDs with the hole transport layer (HTL) of TFB: PVK (1:1) and the EML host of DCzDCN: SimCP2 (1:2) can be turned on at 4 V and show a maximum current efficiency (CE) of 55.6 cd/A, a maximum power efficiency (PE) of 47.2 lm/W, and an external quantum efficiency (EQE) of 18.86%. The devices are among the most efficient OLEDs fabricated by the solution-processed methods. These results indicate that the employment of mixed HTL and EML cohost is a promising way towards the high efficiency solution-processed OLEDs.

Keywords: solution-processed organic light-emitting devices (OLED), thermally activated delayed fluorescence (TADF), mixed polymer interlayer, bipolar cohost


2

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1. INTRODUCTION Organic light emitting diodes (OLEDs) are currently attracting large concern due to their application for high resolution flat panel displays and solid state lighting sources. Extensive research efforts have been focused on the improvement of the efficiency and stability of OLEDs, facilitating their real applications in displays and lightings.1-5 OLEDs with 100% internal quantum efficiency and excellent stability have been achieved, in which multilayer structures formed by thermal evaporation deposition enables the efficient charge carrier injection, transport, and confinement. To make these devices, thermal evaporation method requires high vacuum and excellent stability of all the materials, which results in high cost.6,7 For the high-performance OLEDs, phosphorescent materials are always used as the emitters and dispersed into the host materials with precise control of the doping concentration to avoid the concentration quench.8,9 All these will make the fabrication process very complex and high cost. Hence, solution processed methods, such as spin-coating, inkjet printing, and screen printing, are more desirable for the low cost and large area manufacturability.9,10 In 2011, Shinar’s group demonstrated high efficiency spin-coated electrophosphorescent small molecular OLED based on green emitting tris[2-(p-tolyl)pyridine] iridium (III) (Ir(mppy)3) with a PE of 60 lm/W, a CE of 69 cd/A, and an EQE of 22%.11 Kido et al. reported solution processed blue phosphorescent OLED based on three-coordinated tris(2-(4,6-difluorophenyl)pyridine)iridium(III) with a PE of 35 lm/W, a CE of 43 cd/A, and an EQE of 20%.12 Wang’s group reported high efficiency solution-processed allphosphor-doped white OLEDs based on a dendritic host H2 and an efficient emitter of 5trifluoromethyl-2-(9,9-diethylfluoren-2-yl)pyridine ligand with a PE of 47.6 lm/W, a peak CE of 70.6 cd/A, and an EQE of 26.0%.13 All these exciting results are obtained based on the phosphorescent emitters. However, these emitters always incorporate noble metals and since the


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reserves of the noble metals are limited, they lead to the high cost. Furthermore, serious efficiency roll-off of OLEDs based on these emitters at high luminance and the reliability of the blue phosphorescent emitters obstacles their application in OLEDs.14 Thermally activated delayed fluorescence (TADF) emitters based on pure organic aromatic systems exhibit a sufficiently diminutive energy gap (EST) between singlet and triplet energy levels, enabling the up-conversion of the triplet exciton to singlet exciton to emit stable fluorescence with very high singlet yields.15-17 Thus, OLEDs based on pure aromatic emitters can harvest both singlet and triplet excitons under electric excitation for singlet emission, realizing 100% internal quantum efficiency which are comparable with those of high efficiency phosphorescent OLEDs. TADF emitters stem from pure aromatic systems have been considered as the third generation of OLEDs after the fluorescent and phosphorescent emitters. 14 Enormous efforts have been endeavored to enhance the performance of OLEDs based on pure organic TADF emitters via molecular structure design and device structure construction.18,19 High EQE over 30% for OLEDs originated from pure organic TADF emitters have been achieved, which can be comparable with those of phosphorescent OLEDs.20 Lee et al. reported the pure organic TADF emitter of 2,4,5-tetra(3,6-di-tert-butylcarbazol-9-yl)-1,3-dicyanobenzene (t4CzIPN) with excellent solubility in toluene and demonstrated solution-processed green TADF-OLEDs with an EQE of 18.3% and a PE of 42.7 lm/W.21 Solution-processed blue TADF-OLEDs with an EQE of 20% and a CE of 36.1 cd/A were demonstrated with 2,3,4,6-tetra(9H-carbazol-9-yl)-5fluorobenzeonitrile (4CzFCN).22 Lee et al. constructed solution-processed green TADF-OLEDs with

a

hole

injection

layer

consisting

of

poly(styreneslphonate)-doped

poly(3,4-

ethylenedioxythiophene) (PEDOT:PSS) and tetrafluoroethylene-perluoro-3,6-dioxa-4-methyl-7octene-sulfonic acid copolymer, achieving a CE of 73 cd/A, a PE of 58 lm/W, and an EQE of


4

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24%.23 Nevertheless, reports on the solution-processed OLEDs originated from pure organic TADF emitters are still limited and the reported efficiencies of the devices are still lower than those of the counterparts based on phosphorescent emitters. All these will impede them from practical application in large area displays and lightings. Thus, the construction of solutionprocessed OLEDs based on pure organic TADF emitters is still a challenge and there are many open questions to be answered. Two main reasons can be responsible for the inferior efficiency of solution-processed TADF-OLEDs. First, PEDOT:PSS has been widely used as a holeinjection/transporting layer in solution processed OLEDs due to its high conductivity (1‒10 S/cm) and good stability. Severe exciton quenching will occur at the interface between the PEDOT:PSS layer and the EML, giving rise to the remarkable decreased performance of the devices.24 Also, the hosts of TADF emitters generally have very low HOMO energy level (5.6 eV), affording very high hole injection barrier from PEDOT:PSS. Secondly, TADF emitters with long lifetime of triplet decay are easy to aggregate in the EML, resulting in the serious triplettriplet annihilation and thus low device efficiency.21,22 In this work, for the pursuit of high efficiency of solution-processed TADF devices in simple steps, we first tried to use the mixture of PVK and TFB as the functionalized hole transporting layer (HTL) inserted between the PEDOT:PSS layer and the EML, generating interpenetrating polymer networks to maximize advantages of both, and then, adjusting the hole injection from PEDOT:PSS to the EML. The relationship of the interlayer composition, the interface properties of the polymer interlayer and corresponding performance of organic light emitting diodes was investigated in detail. Bipolar mixed host materials of DCzDCN and SimCP2 (Molecular structure as shown in Figure S1) blended at circumspect ratio were used to prevent the self-aggregation of the emitters and boost an efficient energy transfer (proved by the high PLQY) to TXO-PhCz (Molecular structure as


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shown in Figure S1).25 The devices incorporating them afford a maximum CE of 55.6 cd/A, a maximum PE of 47.2 lm/W, and an EQE of 18.86%. The performance of these devices is comparable to the vacuum evaporated counterparts and among the best of solution-processed OLEDs. 2. RESULTS AND DISCUSSION The interfacial properties between multilayers have large impacts on the characteristics of devices, especially for solution-processed OLEDs. Atomic force microscopy (AFM) observation was performed to elucidate the influences of morphology and interface properties of different polymer interlayers on charge transportation and excition quenching. Figure 1 shows the tapping mode AFM images of the films of PEDOT:PSS, PEDOT:PSS /TFB, and PEDOT:PSS/PVK. All these films are uniform. The root-mean-square (RMS) roughness of the PEDOT:PSS is determined to be 1.26 nm. The RMS roughness of the PEDOT:PSS /TFB and PEDOT:PSS/PVK films are 1.84 and 1.88 nm, respectively. After spin-coating toluene (the solvent for the deposition of EML) onto TFB film, the intensities of the optical absorption and photoluminescence (PL) of TFB are suppressed remarkably, and almost no absorbance or PL peaks can be observed (as shown in Figure 2). These indicates TFB will be readily dissolved by toluene spin-coating during the EML deposition.26 meanwhile, only weak reduction of the counterparts of PVK can be observed, coinciding with the insolubility of the PVK film in toluene. These can be attributed to the elimination of PVK with small molecular weight on the surface of the film. The results above can also be demonstrated by the results of the devices with these HTLs (Figure S2). The devices with both rinsed and unrinsed PVK layers showed similar I-V curve and turn-on voltage. However, the device with the rinsed TFB film exhibits lower turn-on voltage and larger current density than those with unrinsed TFB film due to the thinner TFB film.


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(a)

n

N

N C8H17

C8H17

n TFB

PVK

(c)

(b)

RMS=1.26 nm

(d)

RMS=1.84 nm

RMS=1.88 nm

Figure 1. (a) Molecular structures of TFB and PVK; AFM images of the films: (b) PEDOT:PSS; (c) PEDOT:PSS/TFB; and (d) PEDOT:PSS/PVK.

(a)

PVK PVK-rinsed TFB TFB-rinsed

0.2

0.1

0.0 200

(b) 600 400 200 0

300 400 Wavelength (nm)

PVK PVK-rinsed TFB TFB-rinsed

800

Intensity (a.u.)

0.3

Absorption (a.u.)

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

350

400

450

500

550

Wavelength (nm)

Figure 2. (a) Absorption spectra of PVK and TFB films; (b) PL spectra of PVK and TFB films. Figure 3 shows the tapping mode AFM images of the spin-cast TFB:PVK blend films with different ratio of PVK and TFB (1:2, 1:1, and 2:1). Two main phases existing in two different heights, namely “higher” phase and “lower” phase, can be observed. The distribution of phases in AFM images can be examined by the bearing function, and the depth distribution of the AFM scans is bimodal, demonstrating the presence of the two main phases.27,28 In the mixed polymer


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composite, the two polymers will form interpenetrating network. These domains with micron in size observed in the AFM images are not pure within each domain.27 The surface morphology would be altered if the blend ratio increased. When the blend ratio of PVK and TFB is 1:2, there is a lower matrix phase with a higher enclosed phase of columnar appearance with the lateral size of 0.1-0.8 m. When the PVK proportion in the blend film increased, the higher phase become much larger and the columns merge together with interconnected “ridge-line” appearance. For the 2:1 blend, the enclosed phase become lower phase with the griddle appearance, confirming that a phase inversion occurs between 1:1 and 2:1 composition. Thus, we can conclude that the higher phase is the PVK-rich phase and the lower phase is the TFB-rich phase. The RMS roughness of the blend films is about 9.81 nm. The higher the composition of PVK, the lower the RMS roughness of the blend films is. A similar solubility test is also conducted to the TFBPVK blend. The RMS roughness of all the blend films becomes smaller, and all the height of the “higher” phase become squat. There are some decorations on the higher phase, which can be attributed to the removal of the part of the films with small molecular weight. The optical absorption spectra were also performed before and after rinsing with toluene (as shown in Figure 4). All the films show strong characteristic absorption peaks of both TFB and PVK before rinsing. After toluene rinsing, characteristic absorption peaks of TFB in these films remain still strong, stoutly distinct from that of pure TFB film in which characteristic absorption peaks of TFB is almost undetectable after toluene rinsing. These results demonstrate that the incorporation of PVK enhances the resistance of TFB to the corrosion under the toluene spincoating procedure and the blended HTL can be used for the deposition of EML by solution methods.


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(b)

(a)

RMS= 3.56 nm

RMS= 9.81 nm (d)

(c)

RMS= 9.42 nm

RMS= 3.45 nm (f)

(e)

RMS= 2.84 nm

RMS= 6.51 nm

2.0

Depth Distribution

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


(g) 1.5

PVK:TFB (2:1) PVK:TFB (1:1) PVK:TFB (1:2)

1.0

31.04% 13.55%

0.5

17.45%

0.0

0

20 40 60 Relative Depth (nm)

80


9


Figure 3. The tapping mode AFM images of the spin-cast TFB:PVK blend films with different ratios of PVK and TFB: 1:2 for (a) and (b); 1:1 for (c) and (d); and 2:1 for (e) and (f). The images of (a), (c) and (e) are without rinsing, and the images of (b), (d) and (f) are with rinsing. (g) Analysis of height distribution of phases from AFM data for 2:1 (rectangular), 1:1 (triangle), and 1:2(circle) of PVK: TFB. By using Gaussian fitting, the geometric phase areas of the phases were calculated.

0.4

Absorption (a.u.)

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

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0.3

PVK PVK:TFB (2:1) PVK:TFB (1:1) PVK:TFB (1:2)

TFB PVK:TFB (2:1)-rinsed PVK:TFB (1:1)-rinsed PVK:TFB (1:2)-rinsed

0.2 0.1 0.0 200

300 400 Wavelength (nm)

500

Figure 4. Absorption spectra of the PVK and TFB blend films with different ratios. To elucidate the effect of the film composition on their electronic structure, ultraviolet photoelectron spectroscopy (UPS) was conducted using a He-I source with excitation energy of 21.22 eV in order to assess the frontier energy level offsets at the interface of the polymer interlayers within described multilayer stacks of devices. Valence region and secondary electron cut-off (SECO) region spectra to measure work functions (WFs, ϕ) and interface dipoles are depicted in Figure 5. PEDOT:PSS exhibits a work function of 5.11 eV and the injection barrier for holes to the HOMO of the 4,4'-Bis(9H-carbazol-9-yl)biphenyl (CBP) host (5.9 eV) for devices is 0.8 eV, if we assume no interfacial dipole formed between PEDOT:PSS and CBP. By


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applying the buffer layers, the photoemission onset shifts towards higher binding energy, corresponding to the lower ϕ and thus indicating the formation of an interfacial dipole. The formation of the interfacial dipole will shift down the HOMO band of the HTL, leading to the lower hole injection barrier from HTL to CBP. The TFB component in the HTL is proportional to the interfacial dipole, resulting in the corresponding hole injection barrier from HTL to EML. At the valence band region, the hole injection barriers from PEDOT:PSS are 0.38 eV for TFB and 0.72 eV for PVK, respectively. The hole injection barriers for the blended films are all nested between 0.38 and 0.72 eV, in the increasing sequence of PVK:TFB (1:2) with 0.42 eV, PVK:TFB (1:1) with 0.48 eV, and PVK:TFB (2:1) with 0.56 eV. The cascade energy alignment of PEDOT:PSS, the PVK-TFB blend films, and EML will facilitate the hole injection to the HOMO of CBP and improve the charge carrier balance in EML. Thus, the TFB-PVK blend films are promising as an HTL with low hole injection barrier.

2.0

(a)

(b) 0.38 eV

1.6

PEDOT/TFB

4.95 eV

Intensity (a.u.)

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


0.42 eV PEDOT/PVK:TFB(1:2)

5.02 eV

1.2

0.48 eV PEDOT/PVK:TFB(1:1)

5.04 eV 0.56 eV

0.8

5.05 eV

PEDOT/PVK:TFB(2:1) 0.72 eV PEDOT/PVK

5.09 eV

0.4

PEDOT

5.11eV

0.0 20

18 16 2 1 Bind Energy (eV)

0

Figure 5. UPS spectra of the PVK and TFB blend films with different ratios: (a) Secondary cutoff region and (b) Valence band region.


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The interlayers can also impact the film morphology and the photophysical properties of EML, thus affecting the performance of the device.23,29,30 Hence, the transient PL decay curves of the CBP:TXO-PhCz film (40 nm) on the PVK-TFB blend films with different ratios were further investigated and the results are depicted in Figure 6. All the decay curves exhibit multiexponential decay and the lifetimes can be obtained by the fitting with a bi-exponential function. These indicate two PL decay pathways, the prompt decay from singlet state and the delayed decay via the cycling between singlet and triplet, which coincide with the results reported previously.25,31 Both the prompt and delayed lifetimes for the CBP:TXO-PhCz film (40 nm) on PEDOT:PSS are 72 ns and 56 s, respectively. The delayed lifetime is much shorter than that of TXO-PhCz:mCP (1,3-Bis(carbazol-9-yl)benzene) film via vacuum deposition.25 Assuming the negligible effect of difference in host materials, mCP and CBP, due to their similar polarity, similar delayed lifetimes should be obtained. Thus, the shorter lifetime of TXO-PhCz in CBP can be attributed to the exciton annihilation due to the aggregation of TXO-PhCz. Interestingly, all the CBP:TXO-PhCz films on different TFB-PVK blend films exhibited similar lifetimes with those on PEDOT:PSS. These results demonstrate the serious exciton annihilation of TXO-PhCz due to the aggregation via solution method. The surface morphologies of the EML on different HTL were investigated by atomic force microscopy (as shown in Figure 7). All the images exhibit smooth and uniform surfaces with the RMS roughness of 1.11.3 nm, which is comparable to those of the solution-processed small-molecule films.27,28 These results reveal that the blend ratio of HTLs does not remarkably impact the RMS of the EMLs. These can be ascribed to the effective of the concaves on the surface of the interlayer and form uniform, amorphorous EML. However, some pin-holes can be observed on the surfaces of the EMLs on different HTLs. The large RMS roughness and pinholes of the EML can be ascribed to the self-


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aggregation of the TXO-PhCz and CBP during the thermal annealing, which coincide with the results from the transient PL decay. Thus, it can be concluded that the HTL facilitate the formation of excellent EML morphology for high performance OLEDs.

Intensity (a.u.)

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


PEDOT PVK:TFB (0:1) rinsed PVK:TFB (1:2) rinsed PVK:TFB (1:1) rinsed PVK:TFB (2:1) rinsed PVK:TFB (1:0) rinsed IRF

1000

100

10 0

100

200 Time (s)

300

400

Figure 6. The transient photoemission decay of the spin-cast CBP: 5%TXO-PhCz films on TFB:PVK blend films with different ratios. The IRF stands for reference lifetime from the microsecond pulse flash lamp. (a)

(c)

(b)

RMS=1.10 nm

RMS=1.26 nm

(d)

(e)

RMS=1.18 nm

RMS=1.22 nm

RMS=1.30 nm


13


Figure 7. The tapping mode AFM images of the spin-cast CBP: 5%TXO-PhCz films on TFB:PVK blend films with different ratios: (a) 0:1;(b) 1:2; (c) 1:1; (d) 2:1; and (e) 1:0.

(b)

1000

200 100

150 100 50 0

0

2

4

6 8 10 Voltage (V)

12

14

1

(d)

100

10 N/A PVK:TFB (0:1) PVK:TFB (1:2) PVK:TFB (1:1) PVK:TFB (2:1) PVK:TFB (1:0)

1

0.1

1

10

1000

100

10

10

N/A PVK:TFB (0:1) PVK:TFB (1:2) PVK:TFB (1:1) PVK:TFB (2:1) PVK:TFB (1:0)

1

1

Intensity (a.u.)

(c)

10

100

1 1000

10 100 2 Luminance(cd/m )

1.0

Power Efficiency (lm/W)

250

2

300

Luminance (cd/m )

10000


2

Current Density (mA/cm )

350

Current Efficiency (cd/A)

(a)

EQE (%)

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

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0.8 0.6 0.4 0.2

100

1000

0.0

400

2

500

600

700

Wavelength (nm)

Luminance(cd/m )

Figure 8. Electroluminance characteristics of the TADF-OLEDs based on the TXO-PhCz-CBP EML and mixed polymer interlayer at different mass ratios: (a) Current density-VoltageLuminance curves, (b) Current efficiency-Power efficiency-Lumincance curves, (c) EQELuminance curves, and (d) EL spectra. To verify the effect of TFB-PVK HTL on the performance of the device, multilayers OLEDs were fabricated by capping an electron-transporting/hole-blocking layer, 4,6-Bis(3,5di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PYMPM), on which LiF and Al were deposited. B3PYMPM is chosen as an electron transport and hole blocking material to avoid the charge leakage resulting from the energy shift in the solution-processed EML along with a band tail


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broadening.32 All these devices emit yellowish-green light with the peak of the electroluminescence (EL) spectra at 550 nm (as shown in Figure 8). The introduction of the interlayer increases the thickness of the active layer of the OLEDs and thus suppress the current densities of the devices. The current densities of the devices are proportional to the PVK composition due to its low carrier mobility.33 The TADF-OLEDs with PVK HTL can be turned on at 4.5 V with a CE of 32.94 cd/A, a PE of 18.77 lm/W, and an EQE of 11.06%. The turn-on voltage and the device performance are similar to those of the devices without HTLs, which can be ascribed to the high injection barrier for the hole injection. The use of TFB as HTL can remarkably reduce the turn-on voltage of the devices down to 3.6 V due to its moderate HOMO energy. However, the performance of the device is not satisfying, with a CE of 33.2 cd/A, a PE of 26.1 lm/W and an EQE of 10.4%. These results can be attributed to the low triplet energy level of TFB, resulting in the leakage and waste of the triplet excitons in EML. The charge injection/transport and exciton leakage can be tuned by the combination of PVK and TFB as a mixed HTL due to their synergistic effect. The optimized ratio of PVK and TFB is 1:1, and the device based on the mixed HTL shows the highest performance with a turn-on voltage of 3.7 V, a CE of 45.3 cd/A, a PE of 40.7 lm/W, and an EQE of 14.6%. To verify the effectiveness of the PVK-TFB interlayer, we introduce the interlayer to the orange-red TADF-OLEDs and blue phosphorescent OLEDs (Figure S4-6). Remarkable enhancement of the device performance can also be observed for these devices with the PVK-TFB interlayer (Table S3). Noticeably, the turn-on voltage of the blue phosphorescent OLEDs was reduced down to 3.8 V, and the device affords a maximum CE of 31.9 cd/A, a maximum PE of 25.06 lm/W, and a maximum EQE of 14.3%, which are among the best of the solution-processed phosphorescent OLEDs based on


15


Firpic (Table S4). Thus, it can be concluded that the PVK-TFB interlayer is advantageous to construct the high-performance OLEDs by solution processed method. 1

Intensity (a.u.)

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

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DCZDCN:SIMCP2 (0:1) DCZDCN:SIMCP2 (1:2) DCZDCN:SIMCP2 (1:1) DCZDCN:SIMCP2 (2:1) DCZDCN:SIMCP2 (1:0)

0.1

0.01

1E-3

0

100 Time (s)

200

Figure 9. The transient photoemission decay of the spin-cast DCzDCN:SimCP2: 5%TXO-PhCz films on TFB:PVK (1:1) blend film with different ratios of DCzDCN:SimCP2. As the aggregation exciton quenching of TXO-PhCz is serious due to the unsatisfying miscibility between TXO-PhCz and CBP, bipolar host materials of DCzDCN34, SimCP235 and their blends were used as the host of TXO-PhCz to replace the CBP host. Figure S7 shows the absorption

spectrum

of

TXO-PhCz

and

PL spectra

of

DCzDCN,

SimCP2,

and

SimCP2:DCzDCN:5% TXO-PhCz film. A large overlap between the absorption spectrum of TXO-PhCz film and the PL spectrum of DCzDCN or SimCP2 film can be observed. The PL spectra of SimCP2:DCzDCN:5% TXO-PhCz film exhibit only one broad peak at 532 nm, corresponding to the emission of TXO-PhCz. All these results demonstrate the efficient energy transfer from all these hosts to TXO-PhCz. The transient PL decay curves of the hosts:TXOPhCz film (40 nm) on the PVK-TFB (1:1) blend film also show multi-exponential decay (as shown in Figure 9) and the lifetimes can be obtained by the fitting with a bi-exponential function. The lifetimes of these delayed component of the films are all similar, with a lifetime of 8890 s.


16

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All these lifetimes are much larger than those of the films based on CBP and similar to those of the TXO-PhCz doped-TPBI film (87 s) via vacuum deposition,31 indicating the suppression of the self-aggregation of TXO-PhCz. The suppression of the aggregation exciton quenching of TXO-PhCz can be also testified by the much higher quantum yields (65%) of these EMLs than that (40%) of the EML based on CBP with the same doping concentration. The suppression of the self-aggregation can further be demonstrated by the AFM images of these films (as shown in Figure 10). All the images exhibit smooth and uniform surface with the RMS roughness of 0.560.69 nm, almost half of the film with CBP as the host. No pin holes can be recognized on the surface of the films. These results demonstrate the excellent miscibility of the hosts and the guest. All these RMS roughnesses are comparable to those of the other reported EMLs of highefficiency solution-processed TADF-OLEDs,11,21-23 indicating the applicability of these EMLs for high-efficiency OLEDs.

Figure 10. The tapping mode AFM images of the spin-cast DCzDCN:SimCP2: 5%TXO-PhCz films on the TFB:PVK (1:1) blend film with the different ratio of DCzDCN:SimCP2: (a) 0:1;(b) 1:2; (c) 1:1; (d) 2:1; and (e) 1:0.


17


(b)

150

1000

100

100

50 10 0

2

4

6

8

10

12


100 DCzDCN:SimCP2 (0:1) DCzDCN:SimCP2 (1:2) DCzDCN:SimCP2 (1:1) DCzDCN:SimCP2 (2:1) DCzDCN:SimCP2 (1:0)



200

14

1000 100

10 10 1

0.1

DCzDCN:SimCP2 (0:1) DCzDCN:SimCP2 (1:2) DCzDCN:SimCP2 (1:1) DCzDCN:SimCP2 (2:1) DCzDCN:SimCP2 (1:0)

1

10

2

Intensity (a.u.)

10

0.1

Luminance(cd/m )

(d)

1.0

1


1

0.1 1000

100 2

Luminance(cd/m )

(c) 100

1


(a)

EQE (%)

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 18 of 31


0.8 0.6 0.4 0.2

10 100 2 Luminance (cd/m )

1000

0.0

400

500

600

700

Wavelength (nm)

Figure 11. Electroluminance characteristics of the TADF-OLEDs based on the different DCzDCN:SimCP2 host and the PVK:TFB (1:1) interlayer: (a) Current density-VoltageLuminance curves, (b) Current efficiency-Power efficiency-Lumincance curves, (c) EQELuminance curves, and (d) EL spectra.

Table 1. Summary of the Electroluminescence characteristics of the representative TADFOLEDs by solution processed method. emitter

Host

CIE (x,y)

Von (V)

CE (cd·A-1)

PE (lm·W-1)

EQE (%)

Ref.

4CzIPN

CBP

–

3.8

73

58

24.0

23

t4CzIPN ACRDSO2

CzSi CBP

(0.31,0.59) (0.32,0.58)

– 3.7

– 53.3

42.7 –

18.3 17.5

22 36

PVK

tBuG2TAZ

–

–

4.4

20.9

10.6

8.1

37

– – PEDOT: PSS-

TB-3PXZ CDE1

CzSi –

(0.23,0.54) (0.38,0.56)

– 4.5

41.5 38.9

32.6 17.3

13.9 12.0

38 39

4CzIPN

mCP

4.5

68.5

–

21

40

HTL PEDOT: PSS-PFI PVK –


18

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InCl3 PVKTFB

TXO-PhCz

DCzDCNSimCP2

(0.35,0.56)

4.0

55.6

47.2

18.9

This work

To verify the effect of the ratio of SimCP2:DCzDCN in EMLs, multilayer OLEDs were fabricated with the following device structure: ITO (150 nm)/PEDOT:PSS (20 nm)/PVK-TFB (1:1) (20 nm)/EMLs (40 nm, DCzDCN-SimCP2= 0:1, 1:2, 1:1, 2:1, and 1:0)/ B3PYMPM (50 nm)/LiF (0.9 nm)/Al (100 nm). The electroluminescence characteristics are shown in Figure 11 and summarized in Table S5. The device based on the DCzDCN host can be turned on at 3.8 V with a CE of 47.44 cd/A, a PE of 40.28 lm/W, and an EQE of 14.12%. The device based on the SimCP2 host exhibits a lower performance than that based on DCzDCN, with a turn-on voltage of 4.5 V, a CE of 31.87 cd/A, a PE of 26.25 lm/W, and an EQE of 12.33%. The high turn-on voltage and the inferior performance of the devices are attributed to the large injection barrier from B3PYMPM. To further improve the performance of the device, the mixed hosts of DCzDCN and SimCP2 were used to manipulate the charge transport properties of the EML. The device with the DCzDCN-SimCP2 ratio of 1:2 shows the best performance, affording a turn-on voltage of 4.0 V with a maximum CE of 55.6 cd/A, a maximum PE of 47.2 lm/W, and an EQE of 18.86%. The high efficiency of the device is likely due to the improved balance of the charge injection and transport. This performance is among the best of the solution-processed OLEDs based on TADF emitters, and comparable to those of the spin-coated electro-phosphorescent OLEDs with green emission (Table 1). Noting that the relative long lifetime of TXO-PhCz, pronounced efficiency roll-off at high current density in our devices can be observed due to the severe singlet-triplet annihilation (STA) and triplet-triplet annihilation (TTA), which can be observed ubiquitously for TADF-OLEDs.41-43 These can be suppressed by the design of new TADF emitter with short PL decay time and the new design of the device structure.44-46


19


Page 20 of 31

3. CONCLUSION In conclusion, the mixed HTL of PVK and TFB was introduced to strengthen the holeinjection and alleviate the exciton quenching at the PEDOT:PSS/EML interface by the synergistic effect of high triplet energy level of PVK and moderate LUMO energy level of TFB. The mixed films of TFB and PVK present two main phases, the PVK-rich phase and the TFBrich phase with different heights, leading to the higher RMS roughness of 10 nm. In the mixed films, the incorporation of TFB can effectively enhance the hole injection, and PVK can enhance the resistance of TFB to the corrosion under the EML spin-coating procedure. The 5wt% TXOPhCz:CBP film can form uniform film with a RMS roughness of 1.22 nm and a delayed PL decay lifetime of 56 s on the TFB-PVK mixed film with the ratio of 1:1. The high RMS roughness and short PL decay lifetime of the EML can be attributed to the self-aggregation of TXO-PhCz, which leads to the serious exciton quenching. These films afford the moderate performance of OLEDs with a turn-on voltage of 3.7 V, a CE of 45.3 cd/A, a PE of 40.7 lm/W, and an EQE of 14.6%. The use of the mixed host of DCzDCN-SimCP2 can remarkably suppress the self-aggregation of TXO-PhCz in the EML and tune the charge transport properties of the EML. Highly efficient, solution-processed green-emitting TADF-OLEDs with a turn-on voltage of 4.0 V, a maximum CE of 55.6 cd/A, a maximum PE of 47.2 lm/W, and an EQE of 18.86% were achieved for the devices with the DCzDCN-SimCP2 ratio of 1:2, which is comparable to the best of green-emitting phosphorescent OLEDs by solution method. To the best of our knowledge, these values are among the highest efficiencies in solution-processed OLEDs based on pure organic TADF emitters. Our results provide new insights into the device design of solution-processed TADF-OLEDs to realize large-area displays and solid-state lightings. 4. EXPERIMENTAL SECTION


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4.1 General Methods. All the reagents and solvents were commercially available and used without any further purification. UV-vis spectra and fluorescence spectra of solid films spincoated on quartz substrates were obtained with Hitachi U-3900 and F-4600 spectrophotometers, respectively. The transient photoluminance decay characteristics and temperature dependence experiments were measured using an Edinburgh Instruments FLS920 spectrometer. The temperature dependence experiment is conducted in low temperature refrigeration system from Advanced Research Systems Company. Absolute PL quantum yields of the solid films were measured by Hamamatsu Absolute PL Quantum Yield Spectrometer C11347 under ambient condition without deoxygenation. Different multilayers films were spin-casted onto ITO/glass substrates and annealed in the same way as described for device fabrication. AFM images were obtained with Bruker scanning probe microscope at trapped mode. For UPS studies, polymers were deposited and annealed the same way as described for AFM-sample preparation. The characterization was performed in a Kratos Axis Ultra Dld system using He-I gas discharge lamp source with excitation energy of 21.22 eV and the energy resolution of 150 meV. 4.2 OLED Fabrication and Measurement. Indium tin oxide (ITO; WF ∼4.8 eV) on the glass substrate (3 × 3 mm active area) was used and precleaned carefully before device fabrication. The dried ITO/glass substrate was treated with ultraviolet ozone for 10 min before spin-coating the hole injection layer (HIL) of PEDOT: PSS (Clevios CH4083, LUMTECH Taiwan). The HIL coated substrate was then transferred to the inert condition to conduct the following solution processes. HTL equipped with PVK (ca. 106 g mol1, Xi’an PLT Corp), TFB (ca. 105 g mol-1, Xi’an PLT Corp) and the mixture of them at different mass ratio were spin-casted onto the HIL at the concentration of 5 mg/ml in chlorobenzene followed with an annealing process of 30 mins at 130 oC. The annealed films were spin-rinsed with toluene to remove any soluble residual and


21


Page 22 of 31

baked at the same temperature for 10 mins to remove the solvent. The solutions of TXO-PhCz in toluene dispersed in different hosts (DCzDCN, SimCP2 and CBP, LUMTECH Taiwan) were spin-coated respectively onto the polymer interlayers or PEDOT: PSS as same as above steps and annealed at 90 oC for 15 mins. Then the sample was transferred to the deposition system. The ETL and cathode were deposited by thermal evaporation under high vacuum (∼3×10−5 Pa). The forward-viewing electrical characteristics of these devices were measured with a Keithley 2400 source meter. The electroluminescence spectra and luminance of these devices were obtained on a PR655 spectrometer. All the device fabrications and characterizations were carried out at room temperature under ambient laboratory conditions. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website. Molecular structures, the absorption spectrum of TXO-PhCz, the PL spectra of CBP, DCzDCN, SimCP2 and DCzDCN:SimCP2:5 wt% TXO-PhCz films, energy levels and device structures of OLEDs, and electroluminescence characteristics of OLEDs for comparison (PDF). AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] ORCID Ying Wang: 0000-0001-6638-8026


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Ting Qing: 0000-0003-1347-2608 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Key Research and Development Project (No. 2016YFB0401004), the National Science Foundation of China (Grant No. 61420106002, 21402193 and No.51373189), and the National Program for Support of Top-notch Young Professionals. REFERENCES (1) Yook, K.-S.; Lee, J.-Y. Organic Materials For Deep Blue Phosphorescent Organic LightEmitting Diodes. Adv. Mater. 2012, 24, 3169-3190. (2) Choy, W.-C.; Chan, W.-K.; Yuan, Y. Recent Advances In Transition Metal Complexes And Light-Management Engineering In Organic Optoelectronic Devices. Advanced Materials, 2014, 26,5368-5398.. (3) Kataishi, R.; Lkeda, T.; Sasaki, T.; Toyotaka, K.; Nakamura, D.; Miyake, H.; Lwaki, Y.; Watanabe, K.; Yanagisawa, Y.; Lkeda, H.; Nakashima, H.; Ohsawa, N.; Eguchi, S.; Seo, S.; Hirakata, Y.; Yamazaki, S.; Kurosaki, D.; Ohno, M.; Bower, C.; Cotton, D.; Matthews, A.; Andrew, P. Development Of A High-Resolution RGBW Flexible Display Using A White Organic Light-Emitting Diode With Microcavity Structure And That Of A Side-Roll Touch Panel J. Soc. Inf. Disp. 2015, 8, 381-392.


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Highly efficient, solution-processed organic light emitting diodes

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