Electron-Rich Alcohol-Soluble Neutral Conjugated Polymers as Highly Efficient Electron-Injecting Materials for Polymer Light-Emitting Diodes

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Electron-Rich Alcohol-Soluble Neutral ConjugatedPolymers as Highly Efficient Electron-InjectingMaterials for Polymer Light-Emitting Diodes

By Fei Huang, Yong Zhang, Michelle S. Liu, and Alex K.-Y. Jen*

[*] Prof. A. K.-Y. Jen, Dr. F. Huang, Y. Zhang,[+] M. S. LiuDepartment of Materials Science and EngineeringInstitute of Advanced Materials and TechnologyUniversity of Washington Seattle, WA 98195-2120 (USA)E-mail: [email protected]

[+] Current address: Institute of Optoelectronic Materials and Technol-ogy, South China Normal University, Guangzhou, 510631, P. R. China

DOI: 10.1002/adfm.200801898

We report the design and synthesis of three alcohol-soluble neutral

conjugated polymers, poly[9,9-bis(2-(2-(2-diethanolaminoethoxy)

ethoxy)ethyl)fluorene] (PF-OH), poly[9,9-bis(2-(2-(2-diethanol-

aminoethoxy)ethoxy)ethyl)fluorene-alt-4,4(-phenylether] (PFPE-OH) and

poly[9,9-bis(2-(2-(2-diethanolaminoethoxy) ethoxy)ethyl)fluorene-alt-

benzothiadizole] (PFBT-OH) with different conjugation length and electron

affinity as highly efficient electron injecting and transporting materials for

polymer light-emitting diodes (PLEDs). The unique solubility of these

polymers in polar solvents renders them as good candidates for multilayer

solution processed PLEDs. Both the fluorescent and phosphorescent PLEDs

based on these polymers as electron injecting/transporting layer (ETL) were

fabricated. It is interesting to find that electron-deficient polymer (PFBT-OH)

shows very poor electron-injecting ability compared to polymers with

electron-rich main chain (PF-OH and PFPE-OH). This phenomenon is quite

different from that obtained from conventional electron-injecting materials.

Moreover, when these polymers were used in the phosphorescent PLEDs, the

performance of the devices is highly dependent on the processing conditions

of these polymers. The devices with ETL processed from water/methanol

mixed solvent showed much better device performance than the devices

processed with methanol as solvent. It was found that the erosion of the

phosphorescent emission layer could be greatly suppressed by using water/

methanol mixed solvent for processing the polymer ETL. The electronic

properties of the ETL could also be influenced by the processing conditions.

This offers a new avenue to improve the performance of phosphorescent

PLEDs through manipulating the processing conditions of these conjugated

polymer ETLs.

Adv. Funct. Mater. 2009, 19, 2457–2466 � 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Wein

1. Introduction

Polymer light-emitting diodes (PLEDs) isone of the most investigated areas amongorganic electronics, where significantimprovements in efficiency, brightness,and drive voltage have led to the realizationof high-efficiency full-color and white lightPLEDs.[1] It is well known that the efficientinjection and transport of electrons andholes from both electrodes is essential forachieving high efficiency PLED devices.[2]

As a consequence, most of the advancedPLEDs adopt the multilayer device struc-ture with one or multi-layer of holetransport/injection layer (HTL) or electrontransport/injection layer (ETL) integratedin the devices.[3] Thereby, in addition to theneed for excellent electroluminescent (EL)materials, it is also critical to have suitablematerials for HTL/ETL to achieve high-performancePLEDs. Ingeneral,most of theHTLs are based on the electron-rich amine-containing materials, which are easy tooxidize for hole injection,[3d,4] whilemost ofthe ETL are based on electron-deficientaromatic nitrogen-containing heterocycles,such as derivatives of 1,3,4-oxadiazole,pyridine, benzothiadizole, etc.[3f,g]

Recently, the polyfluorene-based poly-electrolytes and their precursors are foundto be goodETLmaterials for PLEDs.[5] Theirgood solubility in polar solvents andexcellent electron injecting ability render

them as ideal materials for ETL in solution processed, multilayerPLEDs. This allows scientists to fabricate PLEDs using the well-controlled multilayer device structures that can only be accom-plished in small molecule-based OLEDs. Different from thetraditional ETL materials, this kind of materials can effectivelyenhance electron injection from high work-function metals (suchasAl,Ag,Au).As a result, high-efficiencyprintablePLEDswithAg-paste as cathode has been realized.[6]

It has beenproposed that thisunique electron injectionpropertyorigins from the charged or polar groups on the side chains, whichcan generate positive interfacial dipole between cathode and theorganic layer. This results in reduced injection barrier height at theinterface.[7] However, the influence of the polymermain chains on

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Scheme 1. Synthetic route for monomer.

Scheme 2. Synthetic route for polymers.

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their electron injection ability remains poorly understood.According to the literature, polyfluorenes represent a class ofelectron-rich, p-type semiconductors which is not ideal forETL.[3f,g] The hole-blocking capacity of polyfluorene ETL is alsoquestionable when they are used in blue-emitting devices, wherethe emitting complex or the host material usually have higherenergy in its highest occupied molecular orbital (HOMO) energylevel.[8]

The alcohol-soluble neutral conjugated polymers combine theadvantages of conjugated polyelectrolytes[5] and the traditionalneutral surfactants,[9] which can be processed from environ-mental-friendly alcohol solutions and the polar groups on theirside chains can also facilitate electron injection from high work-function metal cathodes.[10] Meanwhile, the neutral character ofthese polymers eliminates the concern ofmobile ions in the deviceduringoperation. Therefore, theymay enhance lifetimeof devices.In this paper, three neutral conjugated polymers, poly[9,9-bis(2-(2-(2-diethanolaminoethoxy)ethoxy)ethyl) fluorene] (PF-OH),poly[9,9-bis(2-(2-(2-diethanolaminoethoxy)ethoxy) ethyl)fluorene-alt-4,40-phenylether] (PFPE-OH) and poly[9,9-bis(2-(2-(2-dietha-nolamino-ethoxy) ethoxy)ethyl)fluorene-alt-benzothiadizole](PFBT-OH) were developed for use as electron injecting layer inPLEDs to probe the effect of main chain structure on electroninjecting ability. These polymers possess the same surfactant-likeside chain but different polymer backbones. PFBT-OH wasdesigned to have the same electron-deficient main chain aspoly(9,90-di-n-octylfluorene-alt-benzothiadizole) (PFBT)which is atypical electron transporting material with good electron mobil-ity.[11] Whereas PFPE-OH and PF-OH were designed to have aelectron-rich backbone with different conjugation lengths(Scheme 2). Surprisingly, PFBT-OH showed the poorest electroninjecting ability compared to the electron-rich PFPE-OH and PF-OH, despite of its electron-deficient main chain. This is contra-dictory to the design guideline often followed in the literature forefficient electron injecting/transporting materials.[3f,g] Theseresults suggest that polymer’s main chain may also play a criticalrole in forming dipole at the interface between the ETL andcathode, which dictates the efficiency of electron injection fromcathode. Moreover, when these polymers were used as ETL inphosphorescent PLEDswith poly(N-vinylcarbazole) (PVK) as host,it was discovered that the highly doped phosphorescent emissionlayer could be significantly influenced by the processing conditionof the subsequent ETL polymers. By changing the processingcondition of these polymers, the interface between these layerscould be easily controlled and result in greatly improved deviceperformance.

� 2009 WILEY-VCH Verlag GmbH &

2. Results and Discussion

2.1. Design, Synthesis and Characterization

Schemes 1 and 2 show the synthesis and structures of themonomers and polymers used in these studies. Monomer 2,7-dibromo-9,9-bis(2-(2-(2-chloroethoxy)ethoxy)ethyl)fluorene 1 wassynthesized by reacting 2,7-dibromofluorene with 1,2-bis(2-chloroethoxy)ethane in the presence of excess NaOH. The keyintermediate 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(2-(2-(2-chloro-ethoxy)ethoxy)ethyl)fluorene 2 was synthe-sized in 63% yield by heating a mixture of monomer 1 andbis(pinacolato)diborane in the presence of KOAc and dioxane at80 8C for 12 h. The copolymerization of 2 with either 1, 4,40-dibromophenylether, or 4,7-dibromo-2,1,3-benzothiazole usingthe Suzuki cross-coupling conditions with Pd(PPh3)4 and K2CO3

as catalyst affords the precursor polymers PF-Cl, PFPE-Cl, andPFBT-Cl in 65–76% yields.

The precursor polymers are soluble in common organicsolvents such as toluene, tetrahydrofuran (THF), and chloroformand can be purified through repetitive precipitation from their

Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19, 2457–2466

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Figure 1. UV-vis absorption and PL emission spectra of polymers in

methanol, water solution and in solid-state films. a) PF-OH, b) PFPE-

THF solutions into methanol. Characterization by NMR spectros-copy confirms their corresponding chemical structures. Thenumber average molecular weight (Mn) estimated by gelpermeation chromatography (GPC) using polystyrene as thestandard and THF as the eluent is 36 900 (with a polydispersity of3.9) for PF-Cl, 26 800 (with apolydispersity of 3.7) for PFPE-Cl, and19200 (with a polydispersity of 3.1) for PFBT-Cl, respectively. Thefinal conjugated polymers PF-OH, PFPE-OH and PFBT-OH wereobtained by treating the corresponding precursor polymers withexcess diethanolamine in a mixed solvent of THF and N,N-dimethylformamide (DMF) at 60 8C for 5 days.

After the post polymerization treatment, all the resultingpolymers comprised conjugated mains chain and surfactant-likeside chains. The conjugated main chains endow them goodconductivity, while the surfactant-like side chains enhance theirsolubility in polar solvents (such as alcohol and DMF), which isorthogonal to most of the solvents utilized to dissolve ELconjugated polymers. This feature prevents interfacial mixingbetweenEL layer and adjacent electron injection layer,which is oneof themost challenging problems in fabricatingmultilayer PLEDsby solution process. In addition, the ethoxy-, hydroxyl-, and amino-groupson theside chains also render themgoodelectron- injectingability from high work-function metals due to the interaction ofthese polar groups withmetals.[7,9] By changing the co-monomersin thepolymerization, all thesepolymershavedifferentmainchainstructures (Scheme 2). PF-OHhas the same fully conjugatedmainchain as the typical electron-rich polyfluorene homopolymer.[12]

PFPE-OH also has a electron-rich polyfluorene-like main chain,however, its conjugation length is effectively confined by theoxygen atoms on the main chain leading to a larger band gap thanthe polyfluorene homopolymer.[13] Different from PF-OH andPFPE-OH, PFBT-OH has a fully conjugated but electron-deficientmain chain as PFBT,[11] which fulfills the common requirementsfor a good electron injecting material.[3f,g]

OH, c) PFBT-OH.

2.2. Optical and Electrochemical Properties

Figure 1 shows the UV-vis absorption and PL spectra of thepolymers in methanol, deionized water, and in the solid-state.These neutral conjugated polymers show similar spectralchange as that of charged conjugated polymers which red-shiftwhen solvent polaritywas increased.[14] As summarized inTable 1,the absorption peaks of PF-OH in methanol and water wereobserved at 392 nm and 394 nm, respectively. The correspondingPL spectrum peaked at 415 nm and 418 nm with their vibronicshoulder around439 nmand442 nm, respectively.Similar spectral

Table 1. UV-vis absorption, photoluminescence and electrochemical propert

Polymer Methanol H2O

labs (nm) lPL (nm) labs (nm) lPL (nm) labs (nm) lPL

PF-OH 392 415 394 418 404 4

PFPE-OH 338 391 339 396 348 4

PFBT-OH 442 557 447 587 468 5

[a] Highest-occupied-molecular-orbital (HOMO) energy values were deduced

energy values were estimated from the p–p� gap, together with HOMO valu

Adv. Funct. Mater. 2009, 19, 2457–2466 � 2009 WILEY-VCH Verl

shift of theabsorbanceandPLspectrawere alsoobserved forPFPE-OH and PFBT-OH. Due to the effective confinement ofconjugation length, PFPE-OH exhibited blue-shifted absorptionpeaks at 338 nm in methanol and 339 nm in water, withcorresponding PL spectrum peaked at 391 nm and 396 nm,respectively. PFBT-OH exhibited an absorption peak at 442 nm inmethanol and its corresponding PL peak at 557 nm. In water, theabsorption peak of PFBT-OH was red-shifted to 447 nm, whereasits PL spectrum peaked at 587 nm. These results indicate that inpolar solvents, these conjugated polymers tend to form aggregate

ies of the polymers.

Film

(nm) Optical band Gap (eV) [a] Eox (V) HOMO (eV) LUMO (eV) [b]

28 2.9 1.01 �5.7 �2.8

01 3.3 1.20 �5.9 �2.6

70 2.4 1.19 �5.9 �3.5

from cyclic voltammetry. [b] Lowest-unoccupied-molecular-orbital (LUMO)

es.

ag GmbH & Co. KGaA, Weinheim 2459

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to minimize the exposure of their hydrophobic main chains.[14] Itshouldbenoted that PFBT-OHshoweda significantly red-shift andmuch broader PL emission compared to PF-OH and PFPE-OH.The similar phenomenawas also observed among the PFBT-basedconjugatedpolyelectrolytes,which couldbe attributed to itsuniqueelectronic structure.[15] Compared to PF-OH and PFPE-OH, thebenzothiadizole units on PFBT-OH main chain act as goodaccepter which renders PFBT-OH with a strong intramolecularcharge transfer character. Therefore, the excited state of this PFBT-OH is more sensitive to the polarity of its environment.[16]

In films, all the polymers show more pronounced changes inoptical properties (Fig. 1 and Table 1). Transparent and uniformfilms of the polymers could be prepared on quartz substrate byspin-casting from their methanol solutions at room temperature.The main absorption peaks of these polymers were at 404 nm,348 nm, and 468 nm for PF-OH, PFPE-OH, and PFBT-OH, whiletheir PL spectra peaked at 428 nm, 401 nm, and 570 nm,respectively. PF-OH exhibited a very broad PL spectrum withobvious excimer emission at around 485 nm, indicating the strongaggregate formation in the polymer films.[17] The optical band gapof the polymers were estimated from the absorption onset of thefilms and listed in Table 1. Among the three polymers, PFPE-OHhad the largest band gap of 3.3 eV due to its shorter conjugationlength, while the green-emitting PFBT-OH exhibited the smallestband gap of 2.4 eV. All the polymers showed a similar optical bandgap to those analogous polymers with the same main chains,indicating that the surfactant-like side chains do not really influenttheir optical properties.[11–13]

The energy levels of the polymers were investigated by cyclicvoltammetry (CV) which were carried out in 0.1 M of tetrabutyl-ammonium hexafluorophosphate (Bu4NPF6) in acetonitrile at ascan rate of 100mVs�1 at room temperature under argon. An ITOglass was coated with polymer thin film and was used as theworking electrode.APtwirewas used as the counter electrode, andAg/Agþ was used as the reference electrode. The onset of theoxidation potentials for PF-OH, PFPE-OH, and PFBT-OH were1.01V, 1.20V, and 1.19V, respectively (Table 1). The HOMOenergy level of PF-OH,PFPE-OH, andPFBT-OHis calculated to be�5.7 eV, �5.9 eV, and �5.9 e V, respectively using ferrocence(FOC) value of �4.8 eV as the internal standard.[18] By decreasingthe conjugation length or introducing the electron withdraw unitson the polyfluorenes main chain, the HOMO levels of PFPE-OHandPFBT-OHdecreasedby0.2 eVcompared to that of PF-OH.Thelowest unoccupied molecular orbital (LUMO) energy levels of thepolymers were estimated from its optical band gap and HOMOenergy level. The LUMO levels of PF-OH, PFPE-OH, and PFBT-OHwere�2.8 eV,�2.6 eV, and�3.5 eV, respectively. According tothe energy levels of these three polymers, PFBT-OH should have

Table 2. Device performance of PLEDs using different cathode in device con

Cathode lmax [nm] Von[a] Voltage [b] Luminance [cd/m

Al 620 8.0 11.4 14

Ba/Al 620 6.4 9.0 317

PF-OH/Al 628 6.2 9.6 973

PFPE-OH/Al 624 6.6 9.4 1040

PFBT-OH/Al 624 11.8 12.8 21

[a] The driving voltage at a brightness of 1 cd/m2. [b] Device performance at


the best electron-injecting ability compared to PF-OH and PFPE-OH. Moreover, as material for ETL, the hole-blocking ability ofPFBT-OHand PFPE-OH should also be better than PF-OH, due totheir relatively high HOMO level.

2.3. As ETL in Fluorescent PLEDs

The initial investigation of the electron-injecting properties ofthese polymers were conducted by fabricating multilayer deviceswith the configuration of ITO/PEDOT:PSS/PVK/PFDBT02/ETL/Al, where poly(3,4-ethylenedioxy thiophene)-poly(styrenesulfonicacid) (PEDOT:PSS) and PVK were used as the hole-injecting and-transporting layer, respectively, and a red-emitting polymerPFDBT02,[19] was used as the emissive layer. The ETL polymerswere spin-coated from their methanol solutions on top of theemissive layer as the electron-injecting layer followed byevaporating a 120 nm thick of aluminum (Al) as cathode. Forcomparison, the devices based on using Al or Ba/Al as cathodewere also fabricated.

Table 2 lists the performances of all the devices at the samecurrent density of 35mA cm�2. Due to poor electron injectionfrom high work-function Al, the device based on Al cathodeshowed a very poor device performance with a luminescenceefficiency (LE) of 0.04 cd A�1 and a luminance of 14 cd m�2. Byusing the low work-function metal Ba as cathode, the deviceshowed an almost 20 times increased LE of 0.79 cd A�1 andbrightness of 317 cd m�2, due to greatly enhanced electroninjection from cathode. However, when PF-OH was used as theETL, the device with PF-OH/Al cathode showed an even higherperformance than using Ba as cathode, which exhibited a LE of2.80 cdA�1 and a luminanceof 973 cdm�2 at the current density of35mA cm�2. The greatly enhanced device performance indicatesthat electron injection from Al cathode through PF-OH issignificantly enhanced because all the devices have the sameanode. It is interesting to find that the polymer with shortconjugation length,PFPE-OHalsoworkswell as electron-injectingmaterial. The device with PFPE-OH/Al cathode showed a LE of2.89 cd A�1 and a luminance of 1040 cdm�2 at the similar currentdensity, which is comparable to the device using PF-OH/Alcathode. Moreover, the use of PFPE-OH does not cause anysignificant increase in the voltage for device operation (Table 2).However, when the electron-deficient polymer, PFBT-OH wasused as the electron-injecting material, the device only exhibited avery poor device performance (LE of 0.06 cd A�1 and a luminanceof 21 cd m�2). Figure 2 shows the luminous efficiency (LE) versuscurrent density characteristics of the devices with differentcathodes. Clearly, the devices with PF-OH/Al and PFPE-OH/Al

figuration ITO/PEDOT:PSS/PVK/PFDBT02/cathode.

2] [b] QE [%] [b] LE [cd/A] [b] Maximum LE [cd/A] Voc

0.04 0.04 0.08 1.1

0.80 0.79 0.91 1.8

2.84 2.80 3.49 2.1

2.94 2.89 3.48 2.2

0.06 0.06 0.09 1.3

current density of 35mA/cm2.


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Figure 2. Luminous efficiency versus current density characteristics of the

devices with different cathodes. (Device configuration: ITO/PEDOT:PSS/

PVK/PFDBT02/cathode.)

Figure 3. Photovoltaic characteristics of the devices with configuration

ITO/PEDOT:PSS/PVK/PFDBT02/cathode.

cathode show much better device performance than the others,while the LE of the devices with Al and PFBT-OH/Al cathode aresimilar to each other and are the lowest among all the devices.

Since all threepolymershave similar surfactant-like side chains,the reason for different device performance should be due to theirmain chain structures. Comparing the efficiency of devices madefromPF-OH/Al,PFPE-OH/Al, andPFBT-OH/Al cathode, theoneswith electron-rich PF-OHandPFPE-OHare�40 times better thanthat with electron-deficient PFBT-OH as the electron-injectingmaterial. This does not follow the trend of conventional designrule, where the electron-deficient molecule is considered moresuitable for electron injection, instead of the electron-richones.[3f,g] Considering the LUMO level of PFBT-OH (�3.5 eV),the electron injection barrier fromAl (work-function 4.3 eV) to theemissive layer (LUMO�2.8 eV)[20] should be effectively decreasedleading to better device performance. However, the device withPFBT-OH/Al as cathode only shows similar device performance asthe devicewithAl cathode. Its turn-on voltage (11.8 V) is alsomuchhigher than the device made from Al cathode (8.0 V) (Fig. 2 andTable 2), which is contrary to the anticipated results according tothe energy level of ETL, EML, and thework-function of Al. In orderto get more insight of the origin of this unexpected electron-injecting property, photovoltaicmeasurements were carried out todetermine the device’s built-in potential across the junction. It wasreported earlier that the polar groups on the polymer (such asamino, hydroxyl groups et al.) could interact with the high work-function metals and form positive interfacial dipole between thecathode and theETL.This results in a decreased injectionbarrier atthe interface.[7] This change can be reflected by the open-circuitvoltagemeasuredwhich is thebuilt-inpotential across the junctionand the anode in devices.[21] Figure 3 shows the photovoltaiccharacteristics of the devices with different cathodes. The devicewith Al cathode showed an open-circuit voltage (Voc) of 1.1 Vand itincreased to 2.1 V for PF-OH/Al device and 2.2 V for PFPE-OH/Aldevice, respectively. It should be noted that the Voc for PF-OH/Aldevice and PFPE-OH/Al are even higher than that of Ba/Al device(�1.8 V). All these results indicate that the effective barrier height


for electron injection is substantially loweredby inserting aPF-OHor PFPE-OH layer between Al and EML. This led to a morebalanced injection of electrons and holes. As a result, the deviceswith PF-OH/Al and PFPE-OH/Al as cathode exhibited a greatlyimproved device performance. In the case of PFBT-OH/Al device,the device’s open-circuit voltage (1.3 V) was much smaller thanthose of PF-OH/Al and PFPE-OH/Al devices, indicating no effectof decreasing the electron injection barrier from Al cathode. Onepossible reason is that PFBT-OH’s main chains play a critical rolein the dipole formation at the cathode interface, which directlydetermines the electron injection efficiency.

In the literature, there are many reports discussing the use oforganic self-assembled monolayer (SAM) to change the dipolemoment at the SAM/metal interface to tune the metal workfunction.[22] It was proved that both the head- and the tail-groups ofthe SAM molecules can influence the dipole interactions at theSAM/metal surface. For example, alkyl thiolate-based SAMs withdifferent tail-groups exhibit totally different interface dipole on themetal surface, leading to either an increase or decrease of themetalwork function.[22] Both the experimental results and the theoreticalcalculations indicate that the alkyl thiolate with electron-rich tail-groups (such as�NH2) will decreasemetal’s work-function, whilethe ones with electron-withdrawing tail-groups (such as fluorin-ated alkyl group) will increase metal’s work-function.[22] Fromthese examples, it is possible that the electron-deficientmain chainof PFBT-OH also act similarly as electron-withdrawing tail groupsto disrupt the dipole formation between the surfactant side chainsand Al cathode, therebymade the electron injectionmore difficultfromAl cathode comparing to the electron-rich PF-OHand PFPE-OH polymers.

2.4. As ETL in Phosphorescence PLEDs

Compared to fluorescent emitters, the phosphorescent tripletemitters have the advantage of achieving very high efficiency(�100% internal quantum efficiency) in their electroluminescentdevices.[23] To examine the general applicability of theseconjugated polymers for electron injection, all these materialswere investigated as ETLs in a series of LEDs with the well


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devices with ETL processing from different solvents. a) PF-OH, b) PFPE-

OH device configuration: ITO/PEDOT:PSS/PVK:OXD-7(30wt %):FIr6

(5wt %)/Cathode.

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established phosphorescent blue emitter, bis(40,60-difluorophe-nylpyridinato)-tetrakis(1-pyrazolyl)borate (FIr6)[24] and greenemitter, tris(2-phenylpyridine)iridium (Ir(ppy)3)

[25] as dopants.The device configurations used were ITO/PEDOT:PSS/PVK:PBD(30wt%):Ir(ppy)3(1 wt%)/ETL/Al and ITO/PED-OT:PSS/PVK:OXD-7(30wt %):FIr6(5wt %)/ETL/Al for green-and blue-emitting phosphorescent devices, respectively. 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxidiazolyl] phenylene (OXD-7) wereincorporated to improve electron transport of the emissive layer.For comparison, the reference devices with plain Al cathode werealso made.

These polymers also work well for the phosphorescent PLEDs.However, the performance of these phosphorescent devices ishighly dependent on the processing condition of these polymers.As shown in Figure 4 and Table 3, when only Al was used ascathode, the blue-emitting phosphorescent device showed verypoor device performance due to large electron injection barrierbetweenAl and the EL layer. The device only exhibited amaximumLEof0.041 cdA�1witha luminanceof4.6 cdm�2.Witha thin layerof PF-OH processed from methanol solution as ETL, themaximum LE of the device was significantly improved to 2.4 cdA�1 with a luminance of 18.5 cd m�2. This indicates that PF-OHinserted between EML and Al can effectively improve electroninjection. The device performance can be enhanced significantlyby modifying the processing condition of the ETL. When PF-OHlayer was processed from a mixed solvent of water and methanol(W/M, 1/4, v/v), the maximum LE reached 14.1 cd A�1 with aluminance of 259 cd m�2. Similar trends were also found indevices with PFPE-OH as the ETL (Fig. 4b and Table 3). The blue-emitting phosphorescent device with PFPE (W/M)/Al as cathodeexhibited a maximum LE of 10.6 cd A�1 with a luminance of274 cd m�2, which is much better than that of the PFPE (M)/Alcathodedevicewhichhas amaximumLEof0.96 cdA�1. In the caseof PFBT-OH device, all the devices showed very poor performanceregardless of what kind of solvents the ETL was processed from(Table 3). Figure 5 showed the photovoltaic characteristics of theblue-emitting phosphorescent devices with different cathodes,where the ETL was processed from a mixed solvent water/methanol (1/4 v/v) (Fig. 5a) or onlymethanol (Fig. 5b). ComparingFigure 5a and b, it revealed that two types of devices have verysimilar open-circuit voltages, indicating the electron-injectingability of the ETL does not change much although the processing

Table 3. Device performance of PLEDs at maximum luminescence efficiency u7(30wt%):FIr6(5wt%)/Cathode.

Cathode Von[a] Voltage J [mA/cm2] Lu

Al 8.4 9.3 11.1

PF-OH (M)b/Al 4.6 5.0 0.77

PF-OH(W/M)c/Al 4.1 4.8 1.84

PFPE-OH (M)/Al 5.1 5.8 2.21

PFPE-OH (W/M)/Al 4.6 6.0 2.58

PFBT-OH (M) /Al 7.3 7.6 1.47

PFBT-OH (W/M) /Al 6.4 6.7 3.12

[a] The driving voltage at a brightness of 1 cd/m2. [b] ETL(M) film spin-coated fo

(1/4 v/v) solution.


conditionof theETLpolymerswas changed. It shouldbenoted thatthe devices with PFBT-OHas the ETL showedmuch smaller open-circuit voltages than those devices with PF-OH and PFPE-OH asthe ETL. This might be the reason for the poor performanceobserved for the PFBT-OH-based blue-emitting phosphorescentdevices.

sing different cathode in device configuration ITO/PEDOT:PSS/PVK:OXD-

minance [cd/m2] QE [%] h [lm/W] Maximum LE [cd/A]

4.6 0.022 0.016 0.041

18.5 1.31 1.68 2.40

259 6.17 10.2 14.1

21.2 0.509 0.578 0.96

274 4.88 6.18 10.6

2.28 0.060 0.072 0.156

2.11 0.027 0.035 0.068

rmmethanol solution. [c] ETL(W/M) film spin-coated formwater/methanol


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ITO/PEDOT:PSS/PVK:OXD-7(30wt%):FIr6(5wt%)/Cathode. a) ETL pro-

cessed from water: methanol (1:4 v/v) solution. b) ETL processed from

methanol solution.


devices with ETL processing from different solvents. a) PF-OH, b) PFPE-

OH Device configuration: ITO/PEDOT:PSS/PVK:PBD(30wt%):Ir

(ppy)3(1wt%)/Cathode.

Similar solvent effect on the performance of devices could alsobe found in the green-emitting phosphorescent devices. As shownin Figure 6 and Table 4, the Ir(ppy)3-based green-emittingphosphorescent deviceswith PF-OHor PFPE-OHETLs processedfrom water/methanol (1/4 v/v) solution showed much betterperformance than the devices with ETL processed frommethanolsolution. The device with PF-OH (W/M)/Al cathode exhibited amaximum LE of 40.2 cd A�1 with a luminance of 243 cd m�2,which is comparable to those obtained from the CsF/Al device.[26]

Interestingly, the devicewith PFPE-OH (W/M)/Al cathode showedan even higher maximum LE of 43.5 cd A�1 with a luminance of436 cd m�2. It should be noted that the device with PFPE-OH

Table 4. Device performance of PLEDs at maximum luminescence efficiePVK:PBD(30wt%):Ir(ppy)3(1wt%)/Cathode.

Cathode Von[a] Voltage J [mA/cm2] Lu

Al 8.8 11.5 6.3

PF-OH (M)[b]/Al 6.3 8.6 2.04

PF-OH(W/M)[c]/Al 5.2 6.3 0.61

PFPE-OH (M)/Al 6.3 7.9 0.633

PFPE-OH (W/M)/Al 6.1 8.2 1.00

PFBT-OH (M) /Al 9.0 14.6 128

PFBT-OH (W/M) /Al 8.6 10.0 0.282

[a] The driving voltage at a brightness of 1 cd/m2. [b] ETL(M) film spin-coated fo

(1/4 v/v) solution.


(W/M)/Al cathode showed a lower power efficiency (18.5 lmW�1)than that (22.3 lm W�1) of PF-OH (W/M)/Al cathode device,despite of its higher LE. This is due to its non-conjugated mainchain, which decreases its conductivity relative to the fullyconjugated PF-OH polymer.

Figure 7 showed the photovoltaic characteristics of the green-emitting phosphorescent devices with different cathodes. Similar

ncy using different cathode in device configuration ITO/PEDOT:PSS/

minance [cd/m2] QE [%] h [lm/W] Maximum LE [cd/A]

107 0.481 0.517 1.70

148 2.08 2.95 7.26

243 11.3 22.3 40.2

83.7 3.73 5.85 13.2

436 12.2 18.5 43.5

714 0.157 0.134 0.56

8.83 0.87 1.09 3.13

rmmethanol solution. [c] ETL(W/M) film spin-coated formwater/methanol


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ITO/PEDOT:PSS/PVK:PBD(30wt%):Ir(ppy)3(1 wt%)/ Cathode. a) ETL

processed from water: methanol (1:4 v/v) solution. b) ETL processed from

methanol solution.

2464

to those observed for blue-emitting phosphorescent devices, thedeviceswithETLprocessed fromdifferent solvents showed similartrend in their open-circuit voltages.And thedeviceswithPFBT-OHasETL showed the lowest open-circuit voltage compared to deviceswith PF-OH or PFPE-OH as ETL. As a result, the devices withPFBT-OH/Al cathode exhibited much poorer device performancethan the devices with PF-OH or PFPE-OH as ETL.

2.5. Possible Reasons for the Solvent Effect on the

Performance of the Phosphorescence Devices

It is clear that the interface erosion by the solvent of upper layerwillsignificantly influence the device performance. One of theconcerns is that high content (30wt%) of OXD-7 small moleculardopants in the EML will be washed out during the processing ofETL polymers,[27] although the high molecular weight PVK hostwill not be influenced by the highly polar solvents (methanol).Indeed, the absorption peak at around 297 nm of PVK: (30wt%)OXD-7 film, which is corresponding to the absorption of OXD-7,was greatly decreased after treated with methanol, showing thatalmost 50% OXD-7 were washed out by methanol solvent.However, in the case of film washed by methanol/water mixedsolvent, only around 20%OXD-7were erased from the film,which


can be estimated from the intensity change of the absorptionspectra (Fig. S1, Supporting Information). These results indicatethat the erosion of OXD-7 by methanol can be greatlysuppressed by adding small amount of water into the methanol.As a result, the phosphorescent devices with ETLs processed fromwater/methanol mixed solvent exhibited a much better perfor-mance than those of devices with ETLs processed methanolsolution.

Another possible reason is that the ETLs processed fromdifferent solvents possess different electronic properties, leadingto different injection and transport properties of ETLs. Previousstudies showed that electronic properties of conjugated polymersare strongly dependent on the morphology of their thinfilms which can be manipulated by varying the processingparameters, such as solvent polarity, solution concentrations andtemperatures, spin-coating procedures, and annealing tempera-tures.[28] Moreover, morphological control of polymer films hasbeen demonstrated to improve the performance of the PLEDseffectively and it has been shown that optical and electronicproperties of conjugated polymers could be affected by controllingchain conformation and degree of aggregation.[29–31]

Since all these polymers are only slightly soluble in water, thesepolymers will form more aggregates by adding water to theirmethanol solution, leading to rougher surface of the prepared ETLfilms.[31] The results from atomic forcemicroscopy (AFM) showedthat the surface ofPF-OHfilmcast frommethanol solution is quitesmooth anduniform,whereas the surface of PF-OHfilmcast fromwater/methanol solution shows very rough and aggregatedfeatures (Fig. S2, Supporting Information). The root mean square(rms) surface roughness of PFN-OH film increases from 0.44 nmto 0.73 nm by changing solvent from ethanol to water/ethanol.Similarly, in thecase ofPFPE-OHandPFBT-OH, thefilmcast frommethanol solution exhibits a relatively smooth surface with a rmsof 0.71 nm (PFPE-OH) and 0.73 nm (PFBT-OH) compared to thefilm processed from water/methanol mixture solution (rms0.93 nm and 0.81 nm for PFPE-OH and PFBT-OH, respectively).It isproposed that this kindof aggregatedmorphology inETLcouldpossibly increase its electron-injecting ability due to enhancedcontact with metal cathode[5f ] or enhanced local electric fieldintensity.[9b] Moreover, due to the poor solubility in water, thepolymer films cast fromwater/methanol solvent are comprised ofmany loosely agglomerated domains and all these domainsmay generate very poor electrical connectivity with the emissivelayer,[31a] which leads to poorer hole injection from the emissivelayer. Therefore, holes could be confined more effectivelywithin the emissive layer, resulting in enhanced device perfor-mance.

3. Conclusions

In summary, three kinds of alcohol-soluble neutral conjugatedpolymers with different conjugation length and electron affinitywere designed and synthesized as electron-injecting materials forPLEDs. All these materials can be processed from polar solvents(such as methanol), which is suitable for multilayer integration ofsolution processed PLEDs. Both fluorescent and phosphorescentPLEDsbased on these conjugatedpolymerETLswere fabricated. It


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is interesting to find that electron-deficient polymer (PFBT-OH)shows very poor electron-injecting ability compared to polymerswith electron-rich main chain (PF-OH and PFPE-OH). Thisphenomenon is quite different from that obtained fromconventional electron-injecting materials. The PFPE-OH whichhas confined conjugation length, exhibits similar electron-injecting ability as that of fully conjugated PF-OH.

The performance of the phosphorescence PLEDs devices ishighly dependent on the processing conditions of the polymers. Itwas found that the interface erosion of OXD-7 by the methanolcould be greatly suppressed by adding small amount of water intothemethanol.Moreover, theETLsprocessed fromwater/methanolmixed solvents possessmore aggregatedfilmmorphology than theETLs processed form methanol solution, resulting in a differentETL/EML interfacial property and ETL’s electronic properties. As aresult, the phosphorescence devices with ETLs processed fromwater/methanol mixed solvent exhibited a much better perfor-mance than those of devices with ETLs processed methanolsolution. This provides a new direction for further optimization ofconjugated polymer-based ETL and offers a new route to improvethe performance of phosphorescent PLEDs.

Acknowledgements

Financial support from the National Science Foundation (NSF-STCProgram under Agreement Number DMR-0120967), Boeing Task Number2008-015-5 on Polymer Light-Emitting Diodes and the Boeing-JohnsonFoundation are acknowledged. Supporting Information is available onlinefrom Wiley InterScience or from the author.

Received: December 23, 2008

Revised: February 19, 2009

Published online: June 10, 2009

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Documents

Electron-Rich Alcohol-Soluble Neutral Conjugated Polymers as Highly Efficient Electron-Injecting Materials for Polymer Light-Emitting Diodes