High Brightness Fluorescent White Polymer Light ... - ACS Publications

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

High Brightness Fluorescent White Polymer Light-Emitting Diodes by Promoted Hole Injection via Reduced Barrier by Interfacial Dipole Imparted from Chlorinated Indium Tin Oxide to the Hole Injection Layer PEDOT:PSS Hong-Ren Syue, Miao-Ken Hung, Yao-Tang Chang, Ge-Wei Lin, Yu-Hsuan Lee, and Show-An Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09515 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

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

ACS Applied Materials & Interfaces

High Brightness Fluorescent White Polymer Light-Emitting Diodes by Promoted Hole Injection via Reduced Barrier by Interfacial Dipole Imparted from Chlorinated Indium Tin Oxide to the Hole Injection Layer PEDOT:PSS Hong-Ren Syue‡, Miao-Ken Hung‡, Yao-Tang Chang, Ge-Wei Lin, Yu-Hsuan Lee and Show-An Chen* Chemical Engineering Department and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing-Hua University Hsinchu 30013, Taiwan, ROC E-mail: [email protected]

Keywords: polymer light-emitting diodes (PLED), fluorescence, chlorinated-ITO (Cl-ITO), polyspirobifluorene (PSBF), interfacial dipole, hole transport layer, PEDOT:PSS.

ABSTRACT

We demonstrated that introducing PEDOT:PSS as a hole transport layer (HTL) on top of chlorinated-ITO (Cl-ITO) anode can lead to a deeper HOMO level of the HTL (promoting from 5.22 eV to 5.42 eV) due to the interfacial dipole imparted by the Cl-ITO, which allows barrier free hole injection to the emitting layer (EL) with polyspirobifluorene (PSBF) doped with the yellow emitter rubrene and significantly prevents excitons quenching by residual chlorine radicals

on

the

surface

of

Cl-ITO.

By

use

of

poly[9,9-bis(6'-(18-crown-

6)methoxy)hexyl)fluorene] chelating to potassium ion (PFCn6: K+) as electron injection (EI) layer and air stable high work function (EФ) metal, aluminum, as the cathode, the performance of fluorescent white polymer light emitting diode (WPLED) achieves the high maximum brightness (Bmax) 61523 cd/m2 and maximum luminance efficiency (ηL,

max)

10.3 cd/A. Replacing PFCn6:

ACS Paragon Plus Environment

1


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 2 of 26

K+/Al cathode by CsF/Al, the Bmax and ηL, max are promoted to 87615 cd/m2 (the record value in WPLED) and 11.1 cd/A, respectively.

1. INTRODUCTION In the past two decades, polymer light emitting diodes (PLEDs) have been extensively studied due to the promising applications in full color flat-panel displays and solid-state lighting.1-3 It can also be fabricated on flexible substrates by solution process to give low-cost and large-area flexible device. For achieving highly efficient WPLEDs, several methods to give white light emission have been reported, including polymer/polymer blends,4-6 polymer/chromophore blends,7-8 single polymer grafted with chromophores

9-11

and multi-emitting layer device

structures.12-14 To get white light emission, the blue light emission is essential, which often comes from polyfluorenes (PFs) as hosts due to their high photoluminescence quantum yields (PLQEs) (>50%),15 thermal stability and good film quality.[16] However, its green excimer induced by electric fields causing color instability cannot be eliminated.17 Thus, instead of using highly coplanar backbone PFs, the spiro-structure of polyspirobifluorenes (PSBFs) having much higher PLQE up to 82% is used.18-19 Unlike that hole mobility (µh) is three orders of magnitude higher than electron mobility (µe) in PFs, the PSBFs in fact possess much more balanced electron and hole mobilities.20 Accordingly, introducing hole transport moiety (triphenyl amine, TPA) into PSBF by copolymerization has been proposed for improving device performance.21-22 Other proposed method was to introduce the hole transport moieties (TPA and Cz) by grafting onto side chains, for promoting hole injection current and therefore the device performance.20 Further hole injection was proposed by the same group by grafting sequentially on same side chains to


2

Page 3 of 26

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


create gradient HOMO energy levels and lead to the promotion of a factor 104 relative to the unmodified sPF. 23 On the other hands, more attentions are given to the stability of device for practical use.24-25 The main problem for the stability is the use of low E metal as the cathode such as Li and Cs, leading to the devices air-sensitive and requiring extra cost in encapsulations. Although in some cases, LiF and CsF capped with Al are used, these metal fluorides are actually reduced to metallic state by the aluminum and the same moisture sensitive problem still persists. To solve this problem, introducing water- or alcohol-soluble EILs based on a conjugated polymer grafted with amino, ammonium salt, and diethanolamino groups such as poly[(9,9-bis(3’-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) has been widely investigated.26-29 Recently, We have demonstrated that an alcohol-soluble EIL based on PF grafted with crown-ether chelated with potassium ions allows the use of high EФ aluminum as the cathode with high electron injection current because the formation of pseudometallic state of the potassium can reduce the EI barrier.30 As to promoting hole current, Cl-ITO was found able to dramatically elevate the EФ up to 6.1 eV, and allows elimination of PEDOT:PSS layer usually used as HTL in PLED.31 This high anode can overcome the hole injection barrier for blue host polymer with deep HOMO level, and could lead to high current density for bright illumination. However, exciton quenching at the interface between electrodes and emitting layer (EML) is still a significant factor to be solved,32 especially for the Cl-ITO anode, which might have residual chloro-carbon fragments on the surface

33-34

able to act as exciton quencher to lower the device

efficiency. Therefore, in the OLED structure with Cl-ITO, an extra electron blocking CBP layer was introduced to avoid this exciton loss; but actually, to find an electron blocking layer without


3


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 4 of 26

introducing hole injection barrier yet with sufficient solvent resistance to prevent from dissolution during applying a coating of next layer is very difficult for solution process. In this work, we introduce PEDOT: PSS as a HTL and buffer layer to prevent Cl-ITO from contact with EML and in additions the latter imparts an interfacial dipole to the former such that a barrier free hole injection is fulfilled. By introducing poly[9,9-bis(6'-(18-crown6)methoxy)hexyl)fluorene] chelating to K+ (PFCn6: K+) as EIL, we demonstrate that the device with PSBF doped with yellow emitter rubrene as the EML can achieve high fluorescent white light emission with Bmax 61523 cd/m2 and ηL, by CsF/Al, the Bmax and ηL,

max

max

10.3 cd/A. Replacing PFCn6: K+/Al cathode

are promoted to 87615 cd/m2 (the record value in WPLED) and

11.1 cd/A, respectively. 2. RESULTS AND DISCUSSION The experimental details are given in the Supporting Information (SI). Scheme 1 shows chemical structures of the materials investigated in this work and their energy level diagram. The synthesis of PFCn6 was carried out according to our previous report.30 PSBF and Rubrene were purchased from Merck KGaA and Luminescence Technology (Taiwan), respectively, and used without further purification. In the device fabrication, PSBF was adopted as the blue emitting polymer and as host for the yellow emitting dopant rubrene which has a HOMO level at 5.3 eV close to the of PEDOT:PSS at 5.22 eV such that the barrier for hole injection is small. The airstable material PFCn6: K+ is introduced as an electron transport layer, which can also well block the hole carriers by its HOMO level with 0.5 eV deeper than that of PSBF. Moreover, the chelated potassium ion by 18-crown-6 can form a so called stable pseudometallic state and can reduce the EI barrier through the formation of interfacial dipole that leads to a reduction of the


4

Page 5 of 26

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


aluminum EФ from 4.28 eV up to 3.16 eV 30; and the interchelate K+ provides an extra route for electron injection beyond the original LUMO level of PF backbone at 2.9 eV.

Scheme 1. The chemical structures of materials investigated in this work and their energy level diagram, where the values of Cl-ITO and PEDOT:PSS are determined by UPS measurement, and those of PSBF, Rubrene and PFCn6: K+ are taken from the literatures.21,30,35 (△ △ denotes the shift of the vacuum level.)

2.1 Electroluminescence and Charge Injection Characteristics of the Blue PLED with Different Anodes. Figure 1 shows the profiles of current density (J) - voltage (V) - B and ηL- B of the blue emitting devices with the structure: anode/PSBF (100 nm)/PFCn6: K+ (1: 3) (20 nm)/Al, where the anodes are ITO, Cl-ITO, ITO/PEDOT:PSS (P-ITO) and Cl-ITO/PEDOT:PSS (P-Cl-ITO), and their characteristic values are listed in Table 1. For the device with Cl-ITO, it


5


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

gives very low Bmax only 337 cd m-2 and ηL,

max

Page 6 of 26

0.4 cd/A; the Bmax is quite low as compared to

that of the P-ITO device 24414 cd/m2. For the ITO device, the barrier (0.3 eV) from ITO to EML is high that leads to the poor hole injection, but the electron injection is much easier with increasing electric field (Figure 2). That is why the efficiency of ITO device drops rapidly after 40 cd/m2 owing to the unbalanced carrier density. For both P-Cl-ITO and P-ITO devices, their efficiencies increase with brightness before 2000 cd/m2, which are resulted from more balanced charge injections as hole current densities are increased. The rapid efficiency growth after 2000 cd/m2 can be attributed to the growth of excimer green emission with applied voltage starting at about 6.5 V (Figure 1a). In addition, for the P-ITO and P-Cl-ITO based blue PLEDs, there is almost no efficiency roll-off in the range of 100 cd/m2 to 2,000 cd/m2; and above 2,000 cd/m2, the efficiency increases with brightness up to 31,616 cd/m2. The inset in Figure 1b shows the normalized EL spectra of the device: P-Cl-ITO/PSBF (100 nm)/ PFCn6:K+(1:3) (20nm)/Al, operated in the voltage range 5~8V. We find that the green peak (~500 nm) grows with voltage from 5V to 8V, and that the origin of green emission is from the generated excimer (peaked at about 500nm) in PSBF under electrical field,17 rather than from the ETL, PFCn6:K+ (1:3), as it emits featureless blue light peaked at 450 nm.30 However such green emission component does not appear in the white light device as to be revealed in more detail later.


6

Page 7 of 26

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


Figure 1. Characteristic blue PLED performance curves of (a) J-B-V and (b) ηL versus B of the devices: anodes/PSBF (100 nm)/PFCn6: K+ (1: 3) (20 nm)/Al. (The inset in Figure 1b shows the normalized EL spectra of device: P-Cl-ITO / PSBF (100 nm)/PFCn6: K+ (1: 3) (20 nm)/Al with various voltages in the range 5-8 V.)


7


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 8 of 26

Table 1. Characteristic parameters of blue emitting PLED with the device structures: Anode/ PSBF (100 nm)/PFCn6: K+ (1: 3) (20 nm)/Al, where the anodes are Cl-ITO, P-ITO or P-Cl-ITO. a)

a)

anode

Von [V]

Bmax2 [cd/m ]

LEmax [cd/A]

Max. EQE (%)

ITO

3.6 V

213

2.2

1.04

Cl-ITO

4.2 V

337

0.4

0.19

P-ITO

4.0 V

24414

2.2

1.05

P-Cl-ITO

3.8 V

31616

2.8

1.33

Von is defined as the voltage at which a luminance of 1 cd/m2 is reached.

By hole carrier only measurements with the device structure anode/PSBF/MoO3/Al (Figure 2), the Jhole profile of Cl-ITO as the anode is 2 to 3 order magnitudes higher than that of P-ITO. Therefore, the very low luminance efficiency 0.4 cd/A can be ascribed to highly imbalanced charge current density; Jhole is much higher than Jelectron from the electron-only device, Al/Ca/PSBF/PFCn : K+ (1:3). For further understanding the characteristic, we prepared the device with P-Cl-ITO as the anode, and found that the Jhole profile is about 1.5 order of magnitudes lower than that with Cl-ITO only as the anode but is still one order higher than that with P-ITO. This indicates that PEDOT:PSS layer can partially reduce the hole injection from Cl-ITO, leading to a more balanced change densities such that the Bmax of the device with P-ClITO is elevated dramatically from 337 cd/m2 to 31616 cd/m2, and the ηL,

max

also increases

significantly from 0.4 to 2.8 cd/A.


8

Page 9 of 26

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


Figure 2. Characteristics curves of single carrier devices of PSBF. The hole-only device structures are anode/PSBF/MoO3/Al and the electron-only device structure is Al/Ca/PSBF/PFCn6: K+ (1: 3)/Al.

In our previous work 33, we have found that exciton residual Cl radicals on Cl-ITO are able to act as quencher. Here, the surface quenching effects giving by impurities on the various anode surface are investigated by comparison of the photoluminance (PL) intensities of PSBF films with same thickness on the various anode surfaces, ITO, Cl-ITO, P-ITO and P-Cl-ITO as shown in Figure 3. The PSBF emission contains two characteristic peaks at 430 nm (0-0 band), 455 nm (0-1 band) and one shoulder peak at 485 nm.18-19 With the same excitation light at 380 nm, the PL emission intensities of the samples with Cl-ITO, P-ITO and P-Cl-ITO relative to that with bare-ITO as the anode are 80%, 92% and 93%, respectively. These results indicate that there are some exciton quenchers on the anode surfaces, and Cl-ITO can quench much more excitons probably due to some chloro-carbon fragments from o-dichlorobenzene (ODCB) during the UV irradiation in the Cl-ITO preparation process.33-34, 36 For PEDOT:PSS coated ITO and Cl-ITO,


9


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 10 of 26

the emission intensities are about the same. Evidently, the exciton quenching effect by Cl-ITO is reduced by inserting PEDOT:PSS as a buffer layer on top of it.

Figure 3. The photoluminescence spectra without normalization of pristine PSBF film with same thickness on substrates of ITO, Cl-ITO, P-ITO and P-Cl-ITO.

2.2 Interfacial Dipole Imparted from Cl-ITO to PEDOT:PSS Measured by Photovoltaic Open Circuit Voltage and Ultraviolet Photoelectron Spectroscopy (UPS). Since the surface layer of Cl-ITO is rich in highly polar Cl-groups, which could impart an interfacial dipole to its top-coated PEDOT:PSS and therefore affect the of PEDOT:PSS, we firstly investigate the open circuit voltage (Voc) of the PLED devices with various anodes under light irradiation from an AM 1.5G solar simulator (Oriel 94021A, 150W from NewPort) and the results are shown in Figure 4a. Such measurement method was proposed by us to detect interfacial dipole imparted by the electron transport moiety oxatriazole grafted onto poly(phenylene vinylene) to the aluminum cathode37. This method was then applied to the electron transport conjugating polymer of the blue emitting PLED with aluminum the cathode 30.


10

Page 11 of 26

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


The Voc values determined as the transition points of photocurrent density-voltage plots 26,38 are 0.03 V, 0.35 V, 1.66 V and 1.90 V for ITO, Cl-ITO, P-ITO and P-Cl-ITO as the anode, respectively. Since the Voc value reflects the built-in potential across the device anode/EML/PFCn6: K+ (1: 3)/Al, the increase of Voc value indicates a decrease in hole-injection barrier.39-40 It means that an interfacial dipole forms at P-Cl-ITO interface which elevates the EФ of PEDOT:PSS by 0.24 eV (1.90-1.66 V). Furthermore, as Cl-ITO is taken as the electrode, the open-circuit voltage is higher than that of bare-ITO by 0.32 eV, which can be attributed to the interfacial dipole induced by indium chloride on the surface of Cl-ITO. Therefore, the decreased hole-injection barriers by similar values with 0.08 eV difference are from the surface bonded chlorine atoms. For further support of the presence of dipole effects, we use UPS to study surface electronic structure on the different anodes as shown in Figure 4b. The EФ was determined from the results of UPS along with the equation, EФ = hν - (Ecutoff - Eonset), where hν is incident photon energy (21.2 eV) of He I, Ecutoff is the high binding energy cutoff and Eonset is the binding energy in HOMO region taking at the turning points. The EФ values of Cl-ITO, PEDOT:PSS atop ITO and PEDOT:PSS atop Cl-ITO are 5.55 eV, 5.22 eV and 5.42 eV, respectively (Table 2). These results indicate that the chlorination of ITO surface can promote the EФ of PEDOT:PSS from 5.22 eV up to 5.42 eV leading to a reduced injection barrier of +0.08 eV (from 5.22 to 5.3 eV) to -0.12 eV (from 5.42 to 5.3 eV). Although the thickness of PEDOT:PSS is about 35 nm (which is significant higher than its thickness atop ITO for EФ measurement about 5 nm) and the PEDOT:PSS in contact with the EL is in the bulk state, the amount of injected holes across the interface and then transporting toward EL is still higher than that from P-ITO. Such EФ promotion of PEDOT:PSS is proposed for the first time.


11


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 26

The introduce of PEDOT:PSS on the Cl-ITO anode not only cause a promoted EФ to reduce the hole injection barrier, but also prevent from direct contact with Cl-ITO where chlorocarbon fragments can quench excitons in the emitting layer. This P-Cl-ITO anode characteristic has not been reported before. By combining with PFCn6:K+ (1:3) as the ETL, high EФ aluminum can be used as the cathode. As depicted in Scheme 1, the dipole effect provided by Cl-ITO leads to the EФ of PEDOT:PSS increasing from 5.22 eV to 5.42 eV and that provided by PFCn6:K+ (1:3) leads to the of aluminum reduces from 4.28 eV to 3.16 eV (taken from our previous work30) . Consequently, both hole and electron currents are dramatically elevated resulting in vary high brightness.


12

Page 13 of 26

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


Figure 4. (a) The current density versus open-circuit voltage of PSBF-based devices with ITO, P-ITO and P-Cl-ITO as anodes, where the device structures are the same as those in the J-V-B measurements in Figure 1; (b) the UPS spectra of Cl-ITO, P-ITO and P-Cl-ITO.

Table 2. Work functions and Voc for the different anodes.

Anodes

EФ

Voc [V]

ITO

5.00 eV a

0.03

Cl-ITO

5.55 eV

0.35

P-ITO

5.22 eV

1.66

P-Cl-ITO

5.42 eV

1.90

a

The value is from Ref. 33.

2.3 White Polymer Light Emitting Diode and its Characteristics. For demonstrating the applicability of the blue PLED to WPLEDs, we introduce the yellow emitting small molecular


13


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 14 of 26

material, Rubrene (which has nearly 100% PLQE 35) into the blue light-emitting polymer PSBF. The J-V-B profiles of the devices: ITO or P-Cl-ITO/PSBF: 0.2 wt% Rubrene (100 nm)/PFCn6: K+ (1: 3) (20 nm)/Al are shown in Figure 5 and their characteristic parameters are listed in Table 3. The current density profile of the latter is significantly higher than that of the former, while the brightness (B) and luminous efficiency (LE) profiles of both devices have intersection points at about 6.7 V and low brightness at 14 cd/m2, above which both B and LE of the latter are significantly better than the former. In the cases as the ETL/cathode is replaced by CsF/Al, the current density profile and the brightness profile of the device with P-ITO are lower than those with P-Cl-ITO, while the LE profiles of both devices intersect at B=15 cd/m2 above which the latter is significantly better than the former. From the performance results of these two sets of devices, as well as the results of the single carrier device measurements, it is obvious that the hole injection dominates at lower applied voltage for all the devices with Cl-ITO and with P-ClITO relative to those devices with P-ITO. The P-Cl-ITO device exhibits Bmax 61523 cd/m2 and LEmax 10.3 cd/A, which are higher than those of P-ITO device 49301 cd/m2 and 8.6 cd/A. Furthermore, Figure 5b shows that all the white PLEDs have mild efficiency roll-off above 2,000 cd/m2. Among the devices, the combination of P-Cl-ITO and CsF/Al shows the best device performance, and its efficiency still maintains at 75% at the maximum brightness (87,615 cd/m2). Furthermore, Figure 5b also shows that both the P-Cl-ITO devices obviously have higher efficiencies than the other two P-ITO devices, which suggests that the extent of charge balance is the dominant factor to affect the device efficiency. From the profile of current versus electrical field from the electron-only device (Figure 2), the electron current density increases monotonically with electrical field, indicating that more electrons under high electrical field needs more holes to reach charge balance. Consequently, the Cl-ITO/PEDOT:PSS anode is the


14

Page 15 of 26

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


better choice for CsF/Al than the ITO/PEDOT:PSS anode owing to its better hole injection ability, since more injected holes to the EML makes more charge balance. Figure 6 exhibits the electroluminescence spectra of the four WPLEDs and the Commission Internationale de L’Eclairage (CIE) coordinates of the emissions from the device with P-Cl-ITO (0.32, 0.35) and P-ITO (0.32, 0.37) devices at the luminance 3000 cd/m2, and both are close to the standard white light (0.33, 0.33), which have considerable potential for light emitting application. For the devices with the cathode CsF/Al without PFCn6:K+(1:3), the maximum brightness and luminance efficiency are 54000 cd/m2 and 8.8 cd/A and 87615 cd/m2 and 11.1 cd/A of P-ITO and P-Cl-ITO as the anodes, respectively. This ultra-high brightness of fluorescent white emission is the highest record in PLED (the one next to it is ~56000 cd/m2 reported by us previously

41

), and slightly higher than that of phosphorescence OLED devices

(~83000 cd/m2) 42. Note also that the green emission centered at 500 nm in the blue device (the inset in Figure 1) does not appear in the white emitting device, which is resulted from energy transfer of the green emission excimer of PSBF to the yellow emission dopant rubrene.


15


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 16 of 26

Figure 5. Characteristic WPLED curves of (a) current density and brightness versus voltage and (b) luminance efficiency versus brightness of the devices: P-ITO/PSBF: Rubrene (0.2 wt%) /PFCn6: K+ (1: 3) /Al, P-Cl-ITO/PSBF: Rubrene (0.2 wt%) /PFCn6: K+ (1: 3) /Al, P-ITO/PSBF: Rubrene (0.2 wt%) /CsF/Al and, P-Cl-ITO/ PSBF: Rubrene (0.2 wt%) /CsF/Al.


16

Page 17 of 26

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


Figure 6. Normalized EL spectra of WPLEDs at a luminance of 3000 cd/m2 for the devices: PITO/PSBF: Rubrene (0.2 wt%)/PFCn6: K+ (1:3)/Al, P-Cl-ITO/PSBF: Rubrene (0.2 wt%)/PFCn6: K+ (1:3)/Al, P-ITO /PSBF: Rubrene (0.2 wt%)/CsF/Al and P-Cl-ITO/ PSBF: Rubrene (0.2 wt%)/CsF/Al.

Table 3. Characteristic parameters of white light emitting PLED with the device structures: Anode/PSBF: Rubrene (0.2 wt%) (100 nm)/cathode, where anodes are P-ITO or P-Cl-ITO, and the cathodes are PFCn6: K+ (1: 3) (20 nm)/Al or CsF (1.5 nm)/Al. anode P-ITO

a

cathode +

PFCn6: K / Al

Bmax

PEmax

Max. EQE

[cd/m ]

[cd/A]

[lm/W]

[%]

[x, y]

2

CIE

a

LEmax

49301

8.6

4.2

3.34

(0.32, 0.35)

P-Cl-ITO

+

PFCn6: K /Al

61523

10.3

4.7

3.74

(0.32, 0.37)

P-ITO

CsF / Al

54000

8.8

4.0

3.39

(0.32, 0.35)

P-Cl-ITO

CsF / Al

87615

11.1

4.7

3.79

(0.35, 0.40)

CIE coordinates are measured at the luminance 3000 cd/m2.


17


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 26

3. CONCLUSIONS We have demonstrated that inserting PEDOT:PSS as a buffer layer between Cl-ITO and EML can significantly reduce the exciton quenching effect caused by the residual radicals on ClITO surface. By open-circuit voltage and UPS measurements, the dipole effect of Cl-ITO provides a promotion of PEDOT:PSS EФ from 5.22 eV to 5.42 eV making the hole injection barrier free. Since P-Cl-ITO as the anode can enhance hole current density by two orders of magnitude as compared to P-ITO for balancing the bipolar carriers, the WPLED of rubrene doped PSBF device along with PFCn6: K+/Al as the cathode can achieve very high Bmax 61523 cd/m2 and ηL, max 10.3 cd/A. Such air-stable blue and white light emitting devices deserve further investigation for pratical application. Replacing PFCn6: K+/Al cathode by CsF/Al, the Bmax and ηL, max are promoted to 87615 cd/m2 (the record value in WPLED) and 11.1 cd/A, respectively.

ASSOCIATED CONTENT Supporting Information Experimental section and origin of the green emission in PSBF This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]


18

Page 19 of 26

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


Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Ministry of Education and the Ministry of Science and Technology for the financial support through project NSC-101-2120-M-007-004, NSC-102-2633-M-007-002, NSC 102-2221E-007-131, MOST 104-2633-M-007-001, MOST 104-2119-M-007-023, and MOST 104-2221E-007-001.

REFERENCES (1) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Santos, D. A. D.; das, J. L. B.; gdlund, M. L.; Salaneck, W. R. Electroluminescence in Conjugated Polymers. NATURE 1999, 397, 121-128. (2) Akcelrud, L. Electroluminescent Polymers. Prog. Polym. Sci. 2003, 28, 875-962. (3) Tang, C.; Liu, X.-D.; Liu, F.; Wang, X.-L.; Xu, H.; Huang, W. Recent Progress in Polymer White Light-Emitting Materials and Devices. Macromol. Chem. Phys. 2013, 214, 314-342. (4) Shih, P. I.; Tseng, Y. H.; Wu, F. I.; Dixit, A. K.; Shu, C. F. Stable and Efficient White Electroluminescent Devices Based on a Single Emitting Layer of Polymer Blends. Adv. Funct. Mater. 2006, 16, 1582-1589.


19


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 20 of 26

(5) Zou, J.; Liu, J.; Wu, H.; Yang, W.; Peng, J.; Cao, Y. High-Efficiency and Good Color Quality White Light-Emitting Devices Based on Polymer Blend. Org. Electron. 2009, 10, 843848. (6) Yu, L.; Liu, J.; Hu, S.; He, R.; Yang, W.; Wu, H.; Peng, J.; Xia, R.; Bradley, D. D. C. Red, Green, and Blue Light-Emitting Polyfluorenes Containing a Dibenzothiophene-S,S-Dioxide Unit and Efficient High-Color-Rendering-Index White-Light-Emitting Diodes Made Therefrom. Adv. Funct. Mater. 2013, 23, 4366-4376. (7) Huang, J.; Hou, W.-J.; Li, J.-H.; Li, G.; Yang, Y. Improving the Power Efficiency of White Light-Emitting Diode by Doping Electron Transport Material. Appl. Phys. Lett. 2006, 89, 133509. (8) Fan, S.; Sun, M.; Wang, J.; Yang, W.; Cao, Y. Efficient White-Light-Emitting Diodes Based on Polyfluorene Doped with Fluorescent Chromophores. Appl. Phys. Lett. 2007, 91, 213502. (9) Zhang, B.; Qin, C.; Ding, J.; Chen, L.; Xie, Z.; Cheng, Y.; Wang, L. High-Performance AllPolymer White-Light-Emitting Diodes Using Polyfluorene Containing Phosphonate Groups as an Efficient Electron-Injection Layer. Adv. Funct. Mater. 2010, 20, 2951-2957. (10) Zhang, L.; Hu, S.; Chen, J.; Chen, Z.; Wu, H.; Peng, J.; Cao, Y. A Series of EnergyTransfer Copolymers Derived from Fluorene and 4,7-Dithienylbenzotriazole for High Efficiency Yellow, Orange, and White Light-Emitting Diodes. Adv. Funct. Mater. 2011, 21, 3760-3769. (11)

Chen, L.; Li, P.; Cheng, Y.; Xie, Z.; Wang, L.; Jing, X.; Wang, F. White

Electroluminescence

from

Star-Like

Single

Polymer

Systems:

2,1,3-Benzothiadiazole

Derivatives Dopant as Orange Cores and Polyfluorene Host as Six Blue Arms. Adv. Mater. 2011, 23, 2986-2990.


20

Page 21 of 26

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


(12) Joo, C. W.; Jeon, S. O.; Yook, K. S.; Lee, J. Y. Multilayer Stacked White Polymer LightEmitting Diodes. J. Phys. D: Appl. Phys. 2009, 42, 105115. (13) Köhnen, A.; Riegel, N.; Kremer, J. H. W. M.; Lademann, H.; Müller, D. C.; Meerholz, K. The Simple Way to Solution-Processed Multilayer OLEDs - Layered Block-Copolymer Networks by Living Cationic Polymerization. Adv. Mater. 2009, 21, 879-884. (14) Chang, Y.-T.; Jen, T.-H.; Chen, S.-A. Voltage Independent White Emission from All Solution Processed Polymer Light-Emitting Diode with Dual Emitting Layers Spaced by an Alcohol Soluble Conjugated Polymer as Interlayer. Org. Electron. 2013, 14, 2948-2952. (15) Ariua, M.; Lidzeya, D. G.; Lavrentieva, M.; Bradleya, D. D. C.; Jandkeb, M.; Strohrieglb, P. A Study of the Different Structural Phases of the Polymer Poly(9,9'-dioctyl fluorene) Using Raman Spectroscopy. Synth. Met. 2001, 116, 217-221. (16) Jen, T.-H.; Wang, K.-K.; Chen, S.-A. Effect of Thermal Stability on Performance of βPhase Poly(9,9-di-n-octylfluorene) in Deep Blue Electroluminescence. Polymer 2012, 53, 58505855. (17) Lu, H.-H.; Liu, C.-Y.; Jen, T.-H.; Liao, J.-L.; Tseng, H.-E.; Huang, C.-W.; Hung, M.-C.; Chen, S.-A. Excimer Formation by Electric Field Induction and Side Chain Motion Assistance in Polyfluorenes. Macromolecules 2005, 38, 10829-10835. (18) Yu, W.-L.; Pei, J.; Huang, W.; Heeger, A. J. Spiro-Functionalized Polyfluorene Derivatives as Blue Light-Emitting Materials. Adv. Mater. 2000, 12, 828-831. (19) Monkman, A. P.; Rothe, C.; King, S. M. Singlet Generation Yields in Organic LightEmitting Diodes. Proc. IEEE 2009, 97, 1597-1605.


21


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 22 of 26

(20) Huang, C.-W.; Peng, K.-Y.; Liu, C.-Y.; Jen, T.-H.; Yang, N.-J.; Chen, S.-A. Creating a Molecular-scale Graded Electronic Profile in a Single Polymer to Facilitate Hole Injection for Efficient Blue Electroluminescence. Adv. Mater. 2008, 20, 3709-3716. (21) Laquai, F. d. r.; Wegner, G.; Im, C.; Bässler, H.; Heun, S. Comparative Study of Hole Transport in Polyspirobifluorene Polymers Measured by the Charge-Generation Layer Time-ofFlight Technique. J. Appl. Phys. 2006, 99, 023712. (22) Nicolai, H. T.; Hof, A.; Oosthoek, J. L. M.; Blom, P. W. M. Charge Transport and Recombination in Polyspirobifluorene Blue Light-Emitting Diodes. Adv. Funct. Mater. 2011, 21, 1505-1510. (23) Huang, C.-W.; Tsai, C.-L.; Liu, C.-Y.; Jen, T.-H.; Yang, N.-J.; Chen, S.-A. Design of Deep Blue Electroluminescent Spiro-Polyfluorenes with High Efficiency by Facilitating the Injection of Charge Carriers through Incorporation of Multiple Charge Transport Moieties. Macromolecules 2012, 45, 1281-1287. (24) Lee, T.-W.; Kim, M.-G.; Park, S. H.; Kim, S. Y.; Kwon, O.; Noh, T.; Park, J.-J.; Choi, T.L.; Park, J. H.; Chin, B. D. Designing a Stable Cathode with Multiple Layers to Improve the Operational Lifetime of Polymer Light-Emitting Diodes. Adv. Funct. Mater. 2009, 19, 18631868. (25) Wang, Z. B.; Helander, M. G.; Qiu, J.; Liu, Z. W.; Greiner, M. T.; Lu, Z. H. Direct Hole Injection in to 4,4-N,N-dicarbazole-biphenyl: A Simple Pathway to Achieve Efficient Organic Light Emitting Diodes. J. Appl. Phys. 2010, 108, 024510. (26) Wu, H.; Huang, F.; Mo, Y.; Yang, W.; Wang, D.; Peng, J.; Cao, Y. Efficient Electron Injection from a Bilayer Cathode Consisting of Aluminum and Alcohol-/Water-Soluble Conjugated Polymers. Adv. Mater. 2004, 16, 1826-1830.


22

Page 23 of 26

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


(27) Oh, S.-H.; Vak, D.; Na, S.-I.; Lee, T.-W.; Kim, D.-Y. Water-Soluble Polyfluorenes as an Electron Injecting Layer in PLEDs for Extremely High Quantum Efficiency. Adv. Mater. 2008, 20, 1624-1629. (28) Hoven, C. V.; Garcia, A.; Bazan, G. C.; Nguyen, T.-Q. Recent Applications of Conjugated Polyelectrolytes in Optoelectronic Devices. Adv. Mater. 2008, 20, 3793-3810. (29) Huang, F.; Zhang, Y.; Liu, M. S.; Jen, A. K. Y. Electron-Rich Alcohol-Soluble Neutral Conjugated Polymers as Highly Efficient Electron-Injecting Materials for Polymer LightEmitting Diodes. Adv. Funct. Mater. 2009, 19, 2457-2466. (30) Lu, H. H.; Ma, Y. S.; Yang, N. J.; Lin, G. H.; Wu, Y. C.; Chen, S. A. Creating a Pseudometallic State of K+ by Intercalation into 18-crown-6 grafted on Polyfluorene as Electron Injection Llayer for High Performance PLEDs with Oxygen- and Moisture-Stable Al Cathode. J. Am. Chem. Soc. 2011, 133, 9634-9637. (31) Helander, M. G.; Wang, Z. B.; Qiu, J.; Greiner, M. T.; Puzzo, D. P.; Liu, Z. W.; Lu, Z. H. Chlorinated Indium Tin Oxide Electrodes with High Work Function for Organic Device Compatibility. Science 2011, 332, 944-947. (32) Mei, J.; Bradley, M. S.; Bulović, V. Photoluminescence Quenching of Tris-(8hydroxyquinoline) Aluminum Thin Films at Interfaces with Metal Oxide Films of Different Conductivities. Phys. Rev. B 2009, 79, 235205. (33) Lee, Y.-H.; Chang, Y.-T.; Syue, H.-R.; Chen, S.-A. Single Layer Deep Blue Polymer Light Emitting Diodes with Chlorinated Indium Tin Oxide after Surface Modification for High Performance. Org. Electron. 2015, 20, 158-163.


23


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 24 of 26

(34) Whitcher, T. J.; Yeoh, K. H.; Chua, C. L.; Woon, K. L.; Chanlek, N.; Nakajima, H.; Saisopa, T.; Songsiriritthigul, P. The Effect of Carbon Contamination and Argon Ion Sputtering on the Work Function of Chlorinated Indium Tin Oxide. Curr. Appl. Phys. 2014, 14, 472-475. (35)

Mattoussi, H.; Murata, H.; Merritt, C. D.; Iizumi, Y.; Kido, J.; Kafafi, Z. H.

Photoluminescence Quantum Yield of Pure and Molecularly Doped Organic Solid Films. J. Appl. Phys. 1999, 86, 2642. (36)

Zhao, Y.; Duan, L.; Zhang, D.; Dong, G.; Qiao, J.; Wang, L.; Qiu, Y. Systematic

Investigation of Surface Modification by Organosiloxane Self-Assembled on Indium-Tin Oxide for Improved Hole Injection in Organic Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2014, 6, 4570-4577. (37) Yu, L.-S.; Chen, S.-A. Full-Range Tunability of Electron and Hole Carrier Mobilities and Density Ratios via Incorporation of Highly Electron-Deficient Moieties in Poly(phenylene vinylene) Side Chains. Adv. Mater. 2004, 16, 744-748. (38) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 1999, 11, 605-625. (39) Crispin, X.; Geskin, V.; Crispin, A.; Cornil, J. r. m.; Lazzaroni, R.; Salaneck, W. R.; Bre´das, J.-L. Characterization of the Interface Dipole at Organic/Metal Interfaces. J. Am. Chem. Soc. 2002, 124, 8131-8141. (40) Huang, F.; Niu, Y. H.; Zhang, Y.; Ka, J. W.; Liu, M. S.; Jen, A. K. Y. A Conjugated, Neutral Surfactant as Electron-Injection Material for High-Efficiency Polymer Light-Emitting Diodes. Adv. Mater. 2007, 19, 2010-2014.


24

Page 25 of 26

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


(41) Peng, K.-Y.; Huang, C.-W.; Liu, C.-Y.; Chen, S.-A. High brightness stable white and yellow light-emitting diodes from ambipolar polyspirofluorenes with high charge carrier mobility. Appl. Phys. Lett. 2007, 91, 093502. (42) Li, N.; Oida, S.; Tulevski, G. S.; Han, S. J.; Hannon, J. B.; Sadana, D. K.; Chen, T. C. Efficient and Bright Organic Light-Emitting Diodes on Single-Layer Graphene Electrodes. Nat. commun. 2013, 4, 2294.


25


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 26 of 26

Table of Contents


26

High Brightness Fluorescent White Polymer Light ... - ACS Publications

Recommend Documents