Cross

ARTICLE pubs.acs.org/cm

Highly Efficient Electron Injection from Indium Tin Oxide/Cross-Linkable Amino-Functionalized Polyfluorene Interface in Inverted Organic Light Emitting Devices Chengmei Zhong,† Shengjian Liu,† Fei Huang,* Hongbin Wu, and Yong Cao Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China ABSTRACT: We show that a series of solution processable and cross-linkable aminofunctionalized polyfluorene materials can effectively enhance the electron injection from the indium tin oxide (ITO) electrodes, and for the first time, highly efficient inverted organic light emitting devices are fabricated using cross-linkable conjugated polymers as a single electron injection layer without the need for n-type metal oxide layers. The mechanism for the electron injection enhancement effect of the ITO/cross-linkable conjugated polymer interface is studied by X-ray photoelectron spectroscopy, Kelvin probe, and photovoltaic measurement. It was found that the amino groups among these polymers can effectively lower the work function of ITO, which will greatly enhance the electron injection in the resulting devices. KEYWORDS: cross-linkable conjugated polymers, inverted structure, organic light-emitting diodes, electron injection layer

1. INTRODUCTION It is widely accepted that the performance of organic electronic devices such as organic light emitting diodes (OLEDs)1,2 and organic solar cells (OSCs)3,4 is critically dependent on the electronic properties of the interfaces between the active layer and the electrodes.5 Thus, the interface engineering of the most widely used transparent electrode indium tin oxide (ITO) is an important research topic in the field of organic electronics.6,7 In principle, ITO is capable of injecting or collecting either electrons or holes because its work function (4.5
4.7 eV) lies between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of most organic semiconducting materials for OLED and OSC applications. Accordingly, the charge selectivity of ITO can be modified to efficiently inject or collect either charge carriers by coating respective functional thin films or self-assembled monolayers on top of it.8
10 However, the electron injecting or collecting ability of ITO is much less studied compared to its hole injecting or collecting ability, because it is much more difficult to realize efficient electron injection or collection from ITO. This problem should be emphasized because the electron injecting or collecting ability of ITO interface is crucial to the development of highly efficient inverted structure organic electronic devices, such as inverted organic light emitting diodes (IOLEDs) and inverted organic solar cells (IOSCs).11,12 Inverted device structure is a developing trend in OLED and OSC research because it greatly improves the device stability and eliminates the need for expensive encapsulation technologies by moving the air stable anode to the top of the device, which is exposed to air, and by using air stable metal oxide materials as electron injection/transporting layers (EILs) instead of low work function metals such as Ba and Ca.13
15 Specifically in the case of r 2011 American Chemical Society

IOLEDs, the design of effective EIL materials is the most important problem for researchers.11,16,17 The most widely used EIL materials in IOLEDs by now are n-type metal oxides (n-MO) such as zinc oxide (ZnO), titanium oxide (TiOx), and zirconium oxide.15,18
20 However, in most cases, the n-MO EIL layer is obtained by the sol
gel process, and high temperature annealing is required to achieve better electron transporting properties, which is not compatible with industry scale solution process techniques.12 On the other hand, a variety of electron-injecting conjugated polymer (EICP) materials, especially water/alcohol soluble polyfluorenes, have been reported to be able to realize efficient electron injection from high work function metals through dipole formation on the metal/EICP interface.21
27 Compared to n-MOs, EICPs have better compatibility with industry scale solution process technologies, and they are superior to n-MOs in terms of versatility because their electronic properties can be easily adjusted by fine-tuning the chemical structures. These attractive properties make EICP materials potential candidates for EILs in IOLEDs and IOSCs. It was reported that some ionic EICP materials can improve the electron injecting properties of TiOx or ZnO layer in IOLEDs.28,29 We have also demonstrated that similar ionic EICP materials can act as independent EIL materials for IOLEDs when semitransparent Au was inserted between a EICP and ITO cathode.30 However, no attempts have been made to apply EICP materials as independent EILs in IOLEDs without n-MO or metal layers on top of ITO, though this would effectively simplify the fabrication of IOLEDs. To realize this, there are two Received: August 28, 2011 Revised: October 7, 2011 Published: October 17, 2011 4870

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Chemistry of Materials Scheme 1. Structure of Various Cross-Linkable Electron Injecting Conjugated Polymers and Water/Alcohol Soluble Conjugate Polymer PF-NR2

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Table 1. HOMO, LUMO, and Band Gap Information for Various EICPs EICP

HOMO (eV)

LUMO (eV)

band gap (eV)

PFN-OX


5.55


2.64

2.91

PF-OX PFN-S


5.56
5.43


2.65
2.52

2.91 2.91

PF-S


5.54


2.62

2.92

PF-NR2


5.61


2.70

2.91

Scheme 2. Synthetic Route of the Copolymer PFN-S and PF-Sa

major challenges that need to be overcome. First, although the electron injection properties of EICP/metal cathode interface have been well studied,21,22 it is not known whether EICPs would facilitate efficient electron injection from ITO electrode. Second, the EIL based on EICPs needs to have good solvent resistance and thus be able to avoid the interface erosion during the subsequent solution processing of emission layer (EML). The second challenge is also the main reason why nonionic EICP materials are not used in current literature as EILs in IOLEDs. Although nonionic EICPs are better than their ionic counterparts in terms of OLED device response time and stability,21,22 they do not possess orthogonal solubility with EML materials like the ionic ones and thus can be greatly affected by solvent wash-off effect. In this article, we show that a series of solution processable and cross-linkable EICP materials can effectively enhance the electron injection from the ITO electrodes, and for the first time in literature, highly efficient IOLEDs were fabricated using crosslinkable conjugated polymers as single layer EIL materials without the need for n-MO layers. For demonstration, the classical high efficiency green emitting polymer poly-[2-(4-(30 ,70 -dimethyloctyloxy)-phenyl)-p-phenylene vinylene] (P-PPV) was used as an EML, and the IOLED device with a cross-linkable EICPs as a single layer EIL can achieve a maximum luminous efficacy (LE) of 14.8 cd A
1, which is comparable to the 14.1 cd A
1 achieved by P-PPV devices using optimized conventional device structure (ITO/PEDOT:PSS/P-PPV/Ba/Al). Our results offer a new approach to fabricate high efficiency inverted organic electronic devices by solution process.

2. MATERIALS AND SYNTHESIS 2.1. Polymer Design and General Properties. The design of cross-linkable amino-functionalized polyfluorene materials is based on the water/alcohol soluble EICP poly[(9,9-bis(30 -(N, N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)] (PF-NR2),27,31 and the chemical structures are shown in Scheme 1. It has been well-known that the amino groups among them can generate interface dipoles and greatly

a

Reagents and conditions: (i) K2CO3, CH3CN, reflux, 5 h; (ii) THF, CH3P+ Ph3Br
, tBuOK, 5h, 0 °C, 2 h, argon; (iii) Pd(PPh3)4, Na2CO3/ H2O, toluene, and THF, 90 °C, 24 h, argon.

enhance the electron injection from high work function metal cathodes.21 Besides the amino groups, PFN-OX and PFN-S contains cross-linkable oxetane groups and styrene groups, respectively, which enable them to be solution processed into uniform thin films and then photo-cross-linked or thermally cross-linked to form EILs with good solvent resistance for IOLEDs.32
34 For comparison, the analogous polymers without electron injection amino groups PF-OX and PF-S were also developed. The HOMO, LUMO, and band gap data of these EICP materials are shown in Table 1. The HOMO was measured by cyclic voltammetry, the band gap was determined by the onset of optical absorption spectrum, and the LUMO value was calculated by adding the band gap value to the HOMO value. It is clear from Table 1 that the five EICP materials in this study all possess similar HOMO and LUMO levels. The detailed synthesis of PFN-OX is described elsewhere,35 and the synthesis of PF-OX has already been reported in the literature.36 The synthesis route for PFN-S and PF-S are shown in Scheme 2. Generally, all reagents, unless otherwise specified, were purchased from commercial sources (Sigma-Aldrich Chemical Co., J&K Chemical Ltd., or Alfa Aesar Chemical Co.), and all reagents were used without further purification. All solvents were dried under standard procedure prior to use. The monomer 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborlan-2-yl)-9,9-bis(60 -(N,Ndiethylamino)hexyl)-fluorene (3) was prepared according to the already published procedures.27 4871

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Chemistry of Materials 2.1.1. 4,40 -(((2,7-Dibromo-9H-fluorene-9,9-diyl)bis(hexane6,1-diyl))bis(oxy))dibenzaldehyde (1). In a two-neck flask, the solution of 2,7-dibromo-9,9-bis(6-bromohexyl)-fluorene (3.71 g, 7 mmol), 4-hydroxybenzaldehyde (3.66 g, 30 mmol), and potassium carbonate (5.4 g, 40 mmol) in CH3CN (100 mL) was heated to reflux under stirring for 5 h. After the reaction system was cooled to room temperature, water (100 mL) was added, and the organic layer was extracted three times with CH2Cl2. After drying with anhydrous magnesium sulfate, the crude product was purified by silica gel column chromatography by using a mixed solvent (petroleum ether/ethyl acetate = 4:1) as the eluent; the target product was obtained as colorless solid (yield: about 80%). 1 H NMR (300 MHz, CDCl3), δ (ppm): 9.86 (s, 2H), 7.82
7.78 (tt, 4H, J = 9.45 Hz), 7.77
7.51 (dd, 2H, J = 7.77 Hz), 7.50
7.44 (t, 4H, J = 4.67 Hz), 6.95
6.90 (dd, 4H, J = 11.28 Hz), 3.94
3.48 (t, 4H, J = 6.47 Hz), 1.97
1.41 (m, 4H, J = 4.14 Hz), 1.67
1.65 (m, 4H, J = 6.90 Hz), 1.29
1.19 (m, 8H, J = 5.55 Hz), 0.66
0.56 (m, 4H, J = 6.20 Hz). 13C NMR (75 MHz, CDCl3), δ (ppm): 190.72, 164.15, 152.23, 139.09, 131.93, 130.32, 129.79, 126.10, 121.55, 121.22, 114.71, 68.20, 55.09, 40.09, 29.48, 28.86, 25.58, 23.57. Anal. calcd for C39H40Br2O2: C, 63.94; H, 5.50; Br, 21.82; O, 8.74. Found: C, 63.85; H, 5.36; Br, 21.75; O, 9.04. 2.1.2. 2,7-Dibromo-9,9-bis(6-(4-vinylphenoxy)hexyl)-fluorene (2). To a solution of methyltriphenylphosphonium bromide (10.33 g, 28.9 mmol) in 30 mL of THF, potassium-t-butoxide (tBuOK) (3.25 g, 28.9 mmol) was added under ice water bath, during which the solution changed to a yellow color. The reaction solution was stirred for another 30 min, and then, monomer 1 (9.5 g, 13.15 mmol) was added. The reaction was stirred for 5 h at room temperature, whereby the color changed to orange. The reaction system was then poured into water (100 mL), and the organic layer was extracted three times with CH2Cl2. After drying with anhydrous magnesium sulfate, monomer 2 was purified by column chromatography with petroleum ether as eluent (yield: 77%). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.53
7.52 (dd, 2H, J = 7.50 Hz), 7.50
7.49 (dd, 4H, J = 7.86 Hz), 7.46
7.29 (tt, 4H, J = 9.54 Hz), 6.81
6.75 (tt, 4H, J = 9.63 Hz), 6.68
6.59 (m, 2H, J = 9.50 Hz), 5.61
5.55 (dd, 2H, J = 17.55 Hz), 5.11
5.08 (dd, 2H, J = 10.86 Hz), 3.85
3.81 (t, 4H, J = 4.34 Hz), 1.95
1.90 (m, 4H, J = 4.14 Hz), 1.61
1.54 (m, 4H, J = 6.89 Hz), 1.25
1.12 (m, 8H, J = 5.93 Hz), 0.63
0.59 (m, 4H, J = 5.17 Hz). 13C NMR (75 MHz, CDCl3), δ (ppm): 158.87, 152.32, 139.08, 136.28, 130.28, 130.24, 127.32, 126.13, 121.54, 121.20, 114.47, 111.38, 67.83, 55.61, 40.10, 29.69, 29.09, 25.66, 23.61. Anal. calcd for C41H44Br2O2: C, 67.59; H, 6.09; Br, 21.93; O, 4.39. Found: C, 67.01; H, 5.68; Br, 21.06; O, 6.25. 2.1.3. Poly[9,9-bis(60 -(N,N-diethylamino)hexyl)-fluorene-alt9,9-bis(60 -(4-vinylphenoxy)hexyl)-fluorene] (PFN-S). 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(60 -(N,Ndiethylamino)hexyl)-fluorene (3) (632 mg, 1.0 mmol), 2,7dibromo-9,9-bis(6-(4-vinylphenoxy)hexyl)-fluorene (2) (728 mg, 1.0 mmol), and tetrakis(triphenylphosphine) palladium [(PPh3)4Pd(0)] (10 mg) were dissolved in a mixture of 10 mL of toluene, 10 mL of THF, and 4 mL of 2 M Na2CO3 aqueous solution in a 50 mL two-necked round-bottomed flask under argon. The mixture was refluxed with vigorous stirring for 24 h under an argon atmosphere. The end groups were capped by refluxing for 2 h with phenylboronic acid and then bromobenzene, respectively. Right after the mixture was cooled to room temperature, 200 mL of methanol was added into the mixture. The precipitated material was recovered by filtration through a funnel. The resulting solid

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material was washed for 24 h using acetone in a Soxhlet apparatus to remove oligomers and catalyst residues, and the resulting material was dissolved in 30 mL of toluene. The solution was filtered with a 0.45 μm PTFE filter, concentrated, and precipitated from methanol to yield PFN-S as a yellow fiber solid (805 mg, 78%). 1 H NMR (300 MHz, CDCl3) δ (ppm) 7.55
7.52 (d, 2H), 7.50
7.49 (d, 4H), 7.48
7.47 (d, 4H), 7.46
7.29 (d, 4H), 7.25
6.77 (d, 4H), 6.68
6.59 (m, 2H), 6.68
6.59 (d, 2H), 5.61
5.55 (d, 2H), 5.11
5.08 (d, 2H), 3.85
3.81 (t, 4H), 2.51
2.44 (q, 8H), 2.30
2.28 (t, 4H), 1.95
1.90 (m, 8H), 1.61
1.54 (m, 4H), 1.30
1.26 (m, 4H), 1.25
1.12 (m, 8H), 1.09
1.08 (m, 8H), 1.07
0.96 (t, 12H), 0.63
0.59 (m, 8H). GPC (THF, polystyrene standard): Mn = 15 000 g mol
1, Mw = 27 000 g mol
1, PDI = 1.8. Anal. calcd for C74H96N2O2: C, 85.01; H, 9.25; N, 2.68; O, 3.06. Found: C, 84.78; H, 8.97; N, 2.75; O, 3.5. 2.1.4. Poly[9,9-dioctyl-fluorene-alt-9,9-bis(60 -(4-vinylphenoxy)hexyl)-fluorene] (PF-S). According to the procedure for PFN-S, 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-fluorene (642 mg, 1.0 mmol), 2,7-dibromo-9,9-bis(6-(4-vinylphenoxy)hexyl)-fluorene (2) (728 mg, 1.0 mmol), and tetrakis(triphenylphosphine) palladium [(PPh3)4Pd(0)] (10 mg) were dissolved in a mixture of 10 mL of toluene, 10 mL of THF, and 4 mL of 2 M Na2CO3 aqueous solution in a 50 mL two-necked round-bottomed flask under argon. The mixture was refluxed with vigorous stirring for 24 h under an argon atmosphere. The end groups were capped by refluxing 2 h with phenylboronic acid and then bromobenzene, respectively. After the mixture was cooled to room temperature, it was poured into 200 mL of methanol. The precipitated material was recovered by filtration through a funnel. The resulting solid material was washed for 24 h using acetone in a Soxhlet apparatus to remove oligomers and catalyst residues, and the resulting material was dissolved in 30 mL of toluene. The solution was filtered with a 0.45 μm PTFE filter, concentrated, and precipitated from methanol to yield PF-S as a yellow fiber solid (670 mg, 70%). 1H NMR (300 MHz, CDCl3), δ (ppm): 9.86 (s, 2H), 7.87
7.78 (m, 4H), 7.77
7.51 (m, 8H), 7.50
7.44 (dd, 4H), 6.95
6.90 (d, 4H), 3.94
3.48 (t, 4H), 2.15 (m, 4H) 1.97
1.41 (m, 4H), 1.67
1.65 (m, 4H), 1.29
1.19 (m, 32H), 0.81
0.56 (m, 10H). GPC (THF, polystyrene standard): Mn = 11 000 g mol
1, Mw = 18 000 g mol
1, PDI = 1.7 .Anal. calcd for C70H86O2: C, 87.63; H, 9.03; O, 3.34. Found: C, 86.25; H, 8.67; O, 5.08. 2. 2. Solvent Resistance of Cross-Linked EICP Thin Films. Solvent resistance is crucial in realizing well-defined multilayer structures in solution processed OLEDs,37
39 and it is one of the key factors in realizing high electroluminescence efficiency in IOLED devices in this work. The solvent resistances of cross-linked EICP thin films are superior to their noncrosslinkable counterparts, thus providing better performance in IOLED devices. To illustrate the solvent resistance of the cross-linked EICP thin films, we wash the cross-linked PFN-OX and PFN-S thin films by spin-casting pristine p-xylene onto the thin film and then observe the thickness variation by optical absorption measurement. It was clear that cross-linking greatly improved the solvent resistance of PFN-OX or PFN-S thin film. As can be seen from Figure 1, 85% of the noncross-linked PF-NR2 thin film was washed away by p-xylene, while the cross-linked PFN-OX or cross-linked PFN-S thin film could retain 99% of its original thickness after washing. 2.3. Device Fabrication and Characterization. Conventional OLED devices were fabricated by published procedures.23 For IOLED devices, ITO substrates were cleaned by standard 4872

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procedure. Then, the cross-linkable EICPs (2 mg mL
1) were spin-cast from methanol, acetic acid =100:1 or p-xylene solution, onto the ITO substrate to form a 20 nm EIL. For the photocrosslinking polymers PFN-OX and PF-OX, protons were provided by acetic acid or photoacid 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (purchased from Sigma-Aldrich) added in the solution, and the cross-linking process was done by heating the half-dried films under 150 °C and fluorescence lamp illumination (no UV lamps required) for 20 min. For the thermally cross-linking polymers PFN-S and PF-S, the cross-linking process

Figure 1. Thickness variation of PF-NR2, cross-linked PFN-OX, and cross-linked PFN-S thin-films before and after p-xylene washing, as measured by UV
vis spectroscopy.

Scheme 3. Schematic Energy Level Diagram of Conventional and Inverted OLED Devices

was done by heating the half dried films under 185 °C for 30 min. Then, a P-PPV (Purchased from Canton OLEDKING Optoelectric Materials Co. Ltd., Guangzhou, China) thin film was spin-cast from p-xylene solution (10 mg mL
1) onto a crosslinked EIL to form a 90 nm EML layer and baked at 90 °C for 20 min. Then, 15 nm of MoO3 and 100 nm of Al are deposited through a shadow mask (defined active area of 0.16 cm2) onto the EML layer by thermoevaporation in a vacuum chamber with base pressure of 3  10
6 mbar. All other device fabrication processes are carried out in a N2-filled glovebox (Braun GmbH). Current density
voltage and luminance
voltage data of the asmade OLED and IOLED devices were collected using a Keithley 236 source meter and a calibrated silicon photodiode in the N2 atmosphere drybox. Devices were then taken out from glovebox, and the external electroluminescence quantum efficiencies (QE) were calibrated by measuring the total light output in all directions in an integrating sphere (IS-080, Labsphere). X-ray photoelectron spectroscopy (XPS) measurement was done on the ESCALAB 250 system (Thermal Scientific), and the error is within 0.1 eV. Photovoltaic built-in potential measurement was carried out under the illumination of a xenon lamp (Thermo Oriel 150 W). The Kelvin probe measurement was carried out on SKP 5050 (KP Technology) in ambient conditions.

3. IOLED DEVICE PERFORMANCE The inverted device structure used in this work was ITO/EIL/ P-PPV/MoO3/Al. The noncross-linkable neutral EICP material PF-NR2 was used as single layer EIL for IOLED devices along with cross-linkable EICP materials PFN-OX, PFN-S, PF-OX, and PF-S to study the effectiveness of cross-linkable EICPs in IOLEDs. Furthermore, OLEDs with the conventional device structure ITO/PEDOT:PSS/P-PPV/EIL/Al were fabricated for comparison. The schematic energy level diagram of conventional and inverted OLED devices is shown in Scheme 3. The summary of OLED and IOLED device performance is listed in Table 2, the current density
voltage (J
V), luminance
voltage (L
V) curves, and current density
luminous efficacy (J
LE) curves are shown in Figure 2. From Table 2, it can be found that, generally, the insertion of an EICP layer between ITO and EML can significantly improve the performance of IOLEDs. The LE increased by at least 2000 times for devices with EILs (PF-OX), compared to the ones without EILs. The turn-on voltage was also significantly reduced,

Table 2. Light Emitting Characteristics of P-PPV OLEDs Using Different Polymer EIL Materials and Device Structures (Device Configuration: ITO/PEDOT:PSS/P-PPV/EIL/Al (OLED); ITO/EIL/P-PPV/MoO3/Al (IOLED)) data at LEmax EIL Ba

device structure OLED

PFN-OX PFN-S

a

V (V)

J (mA cm
2)

QE (%)

Von (V)a

Lmax (cd m
2)

14.1

5.8

31.8

8.0

2.5

23564

13.5

8.2

21.0

7.6

3.7

6973

9.7

9.8

35.5

5.5

3.3

10082

45.8

LE (cd A )

8.9

9.4

PFN-OX

14.8

6.4

PFN-S

11.6

PF-NR2

IOLED


1

PF-OX PF-S

0.20 0.50

No EIL

1.8  10
3

7.8

5.1

3.7

12481

6.21

8.4

3.7

14741

6.05

6.5

3.6

10551

0.11 0.28

8.7 5.9

451 1023

11.0

11

12.4 10.6

219 171

14.7

605

b


2 b

The turn-on voltage at which luminescence reach 0.1 cd m . QE value below detection limit. 4873

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Figure 2. Current density
voltage (J
V) (a), luminance
voltage (L
V) (b) and current density
luminous efficacy (J
LE) characteristics (c) of P-PPV IOLED devices and conventional OLED devices using different EICP materials.

indicating that the electron injection barrier between ITO and EML was greatly reduced by the EICP material. Most importantly, it can be concluded that the choice of pendant group on side chains of EICP materials decisively influenced their electron injection properties. The IOLED devices using EICPs with amino alkyl pendant groups on side chains (PF-NR2, PFN-OX, and PFN-S) as EILs have about an order of magnitude higher LE than those that use EICPs without any polar pendant groups (PF-OX and PF-S). The importance of the amino alkyl group can also be shown from the fact that PF-NR2, which has the same amino alkyl pendant side chains as PFN-OX, can act as an efficient EIL, even though the majority of its film is washed away by solvents during device fabrication. PFN-OX and PFN-S have superior solvent resistance compared to PF-NR2 while inheriting

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the strong electron injection ability from PF-NR2, thus making them better EIL materials for the IOLED device structure, resulting in a 30
60% increase in maximum LE. It was also demonstrated from the data in Table 2 that the electron injection capability of the ITO/EICP interface is at least as good as that of the metal/EICP interface if the detrimental effects of solvent washing are eliminated by cross-linking. The maximum luminance and LE of an ITO/PFN-OX/P-PPV/MoO3/Al device were 14 741 cd m
2 and 14.8 cd A
1, respectively, which were substantially higher than 6973 cd m
2 and 13.5 cd A
1 for the conventional structure counterpart ITO/PEDOT:PSS/P-PPV/ PFN-OX/Al. Similarly, IOLED devices using PFN-S as the EILs were slightly better than conventional OLED devices in terms of maximum LE (11.6 cd A
1 vs 9.7 cd A
1). The other evidence was that the J
LE curves are similar for devices with different structure while using the same EIL material. The maximum LE of the PFN-OX IOLED device is even slightly higher than 14.1 cd A
1 for the best conventional structure device ITO/PEDOT:PSS/PPPV/Ba/Al, and it still has potential for improvement, as the turn-on voltage and maximum luminance is not optimized, as compared to the Ba device. There are also other factors that contribute to the improvement of overall performance of PFN-OX and PFN-S IOLED devices besides the obvious electron injection enhancement effect. As displayed in Figure 2a, the total current densities of the IOLED devices using EICP materials as EILs are smaller than those for the IOLED devices without EILs, the latter of which are clearly hole current dominant devices. This indicates that holeblocking effect is evident in IOLED devices with an EICP layer. In Figure 2a, it is also apparent that the leakage current was suppressed in the low bias region for devices with an EIL, which is beneficial for LE improvement. Therefore, it is the combination of enhanced electron injecting, hole blocking, and leakage current suppressing from the inserted PFN-OX and PFN-S layer on top of ITO that ensured the improved charge balance of the EML layer, thus making the respective IOLED devices most efficient in our study. On the other hand, the carrier balance in the device is not optimized when other EICP materials are used as the EIL in an IOLED. Specifically, the hole blocking ability of PF-NR2 was severely crippled by the solvent washing-off effect, because of the loss of film thickness as depicted in Figure 1. While the crosslinked PF-S and PF-OX were free from such detrimental effects, they still suffered from insufficient electron injection because of the lack of efficient electron injecting groups in their polymer structures. It is apparent from the device data that the performance of IOLEDs with PFN-OX or PFN-S can be further optimized. However, before conducting such optimization, the underlying mechanisms of electron injection enhancement of the ITO/EICP interface must first be elucidated. In the case of metal/EICP interface, it was proved that the electron injection enhancement effect is mainly due to the lowering of the metal work function by the formation of interfacial dipoles21,40 and the amino alkyl group played a key role in the formation of such dipoles. On the other hand, the interface between EICP and ITO or other transitional metal oxides is rarely studied, and it is likely that the electron injection enhancement effect of ITO/EICP interface is governed by similar mechanisms. This speculation was studied and verified by X-ray photoelectron spectroscopy (XPS) work function measurement of ITO/EIL substrates. The XPS secondary electron cutoff of the substrates is shown in Figure 3a. The work functions are determined to be 4.4 eV for a pristine ITO substrate, 4874

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according to Figure 1) can still lower the work function of ITO by 0.4
0.8 eV (the value varies because the solvent washing effect is uncontrollable). This thickness insensitivity of work function change is different from the case of metal/EICP interface.40 Kelvin probe (KP) measurement of ITO substrates also confirmed this finding (see Table 3). The relative insensitivity of ITO work function to EIL thickness indicates that only the amino alkyl groups adjacent to the ITO form interfacial dipoles. It should be noted that the exact value of the work function difference measured by XPS was different from PV or KP measurement results. We attribute this difference to the additional interfacial interaction between the EICP layer and the EML or air (KP measurement was carried out in air), which was present in the PV or KP measurement but absent in the XPS measurement.

Figure 3. (a) Comparison of XPS secondary cutoff of ITO substrates with a PFN-OX layer, with a PF-NR2 layer or without thin film on top. (b) Comparison of the net photocurrent of IOLED (ITO/EIL/P-PPV/ MoO3/Al) devices using different EILs.

Table 3. Kelvin Probe Measurement Results of ITO Substrate with or without an EIL Layer on Top

substrate

ITO

work function (eV) 4.62 a

ITO/

ITO/

ITO/

ITO/

PFN-OX

PFN-OX

PF-NR2

PF-NR2

(20 nm)

(20 nm)a

(20 nm)

(3 nm) a

3.95

3.93

4.06

4.04

EICP layer after p-xylene washing.

4. CONCLUSIONS We have discovered that neutral solution processable crosslinked EICP materials can act as an effective single layer EIL for ITO in IOLEDs without the need for metal oxides and that the EICP/ITO interface can enhance the electron injection properties by lowering its work function through the formation of interfacial dipoles. The IOLED devices with a structure of ITO/ cross-linked EICP/EML/MoO3/Al can achieve high LE and brightness comparable to conventional structure counterparts. The high efficiency is attributed to better charge balance in the EML layer after the introduction of the cross-linked EIL, as a result of the electron injection enhancement brought by amino alkyl pendant group containing side chains and the hole blocking, leakage current suppressing ability brought by the polyfluorene main chain. These findings provide a new way to design high performance solution processed IOLEDs. Compared to metal oxide EIL materials, EICPs are more versatile because the work function and other electronic properties can be easily adjusted by altering their chemical structures. Fine-tuning these parameters by new EICP material designs should be able to further optimize the performance of IOLEDs and finally outperform the conventional structure OLEDs. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions

3.6 eV for 3 nm PF-NR2 covered ITO, 4.3 eV for 5 nm PF-OX covered ITO, and 3.5/3.4 eV for 5 nm/20 nm PFN-OX covered ITO. It can be seen that the work function of ITO can be lowered by ∼1 eV with the introduction of PFN-OX layer, and PF-NR2 layer can also lower the work function of ITO by 0.8 eV, making them suitable for electron injection. Not surprisingly, the EICPs without amino alkyl pendant side chains have much less pronounced work function lowering effect on ITO, which is consistent with their IOLED device performance. The work function difference induced by the PFN-OX or PF-NR2 layer on ITO surface is also confirmed by the photovoltaic built-in voltage (PV) measurement of ITO/EIL/P-PPV/MoO3/Al devices, shown in Figure 3b.23,41 It can be seen that the build-in potential for electron injection in ITO/EICP interface can be lowered by as much as 0.6 eV for devices with PFN-OX EIL. Interestingly, we see from both experiments that the work function difference of ITO is not sensitive to EIL thickness, because the difference between 5 and 20 nm PFN-OX is only about 0.1 eV, and the severely washedoff PF-NR2 layer (the thickness was about 3 nm after washing

†

These authors contributed equally.

’ ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of China (Grant Nos. 50990065, 51010003, 51073058, and 20904011), National Research Project (Grant No. 2009CB623601), National High Technology Research and Development Program 863 (Grant No. 2011AA03A110) from the Ministry of Science and Technology of China (MOST), and the “Fundamental Research Funds for the Central Universities” from South China University of Technology. The authors would like to thank Dr. Fangyan Xie of Sun Yat-San University for the XPS measurements and useful discussions. C.Z. would like to thank Dr. Chunhui Duan for useful discussions. ’ REFERENCES (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. 4875

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