Ethoxylated polyethylenimine as an efficient electron injection layer for conventional and inverted polymer light emitting diodes

Organic Electronics 15 (2014) 2387–2394

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Ethoxylated polyethylenimine as an efficient electron injection layer for conventional and inverted polymer light emitting diodes Xiaohui Yang a,b,⇑, Ruixue Wang a,1, Changjun Fan a,1, Guoqing Li a,1, Zuhong Xiong a,1, Ghassan E. Jabbour b,⇑ a b

School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China Material Science and Engineering, Arizona State University, 7700 S. River Parkway, Tempe, AZ 85284, USA

a r t i c l e

i n f o

Article history: Received 17 May 2014 Received in revised form 6 July 2014 Accepted 6 July 2014 Available online 16 July 2014 Keywords: Conventional polymer light emitting devices Inverted polymer light emitting devices Work function Electron injection layer

a b s t r a c t We report inverted light emitting devices using ethoxylated polyethylenimine (PEIE) as a single electron injection layer for indium tin oxide cathode, which possess comparable efficiency to those using ZnO/PEIE double electron injection layers. Implementation of a PEIE layer between light emitting polymer layer and aluminum has been shown to significantly enhance device efficiency as well. Improvement of device efficiency can be attributed to increased electron injection due to the reduced work function of PEIE modified cathode as well as the hole blocking effect of PEIE layer. Furthermore, PEIE serves as an efficient electron injector for a range of light emitting polymers with wide distribution of energy levels. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Polymer light emitting diodes have been actively studied for applications of flat-panel displays and solid-state lighting due to their unique advantages of flexibility, self-emitting, low-cost and large area processing [1]. The typical configuration of conventional devices is indium tin oxide (ITO)/hole injection layer/light emitting polymer (LEP)/metal, where commonly adopted hole injection layer is poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS). Efficient injection of electrons and holes from the electrodes into LEPs is required to achieve high efficiency. To facilitate electron injection, low work⇑ Corresponding authors. Address: School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China. Tel.: +86 2368254913 (X. Yang). E-mail addresses: [email protected] (X. Yang), [email protected] (G.E. Jabbour). 1 Tel.: +86 2368254913. http://dx.doi.org/10.1016/j.orgel.2014.07.009 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

function (WF) metals such as calcium and barium are generally used [2], which on the other hand cause devices very sensitive to ambient moisture and oxygen. Various materials such as alkaline/alkaline earth metal fluorides [3], poly(ethylene glycol) [4] or polyelectrolytes [5] have been incorporated into devices for efficient electron injection from high WF metals such as aluminum. Among them, poly(ethylene glycol) and polyelectrolytes can be deposited onto LEP layer from alcohol or aqueous solution, representing a facile approach toward fully solutionprocessed light emitting devices. Alternatively, inverted devices with the structure of ITO/metal oxide/LEP/metal oxide/metal, which utilize air-stable low WF metal oxides such as ZnO and TiO2 as electron injection layers and high WF metal oxides such as MoO3 as hole injection layers, exhibit improved environmental stability [6]. In such devices, ITO serves as the cathode as the work function difference is important for the polarity of the devices [7]. The conduction band minimum of TiO2 and ZnO lies at

2388

X. Yang et al. / Organic Electronics 15 (2014) 2387–2394

ca. 4 eV below the vacuum level and thus enables electron injection into a few LEPs with the deep-lying Lowest Unoccupied Molecular Orbital (LUMO) level such as poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), achieving reasonable device efficiency. But there is significant energy barrier for electron injection from such metal oxides into the vast majority of LEPs having LUMO energy in the range of 2–3 eV [8]. To address this problem, manifold electron injection materials including dipolar self-assembled (SAM) materials [9], polyelectrolytes [10] and metal salts such as cesium carbonate [11] and barium hydroxide [12] have been placed between the metal oxide and LEP layer. Highly efficient inverted light emitting devices using a thick F8BT layer in combination with a Cs 2CO 3 or Ba(OH) 2 interlayer were reported by Lu et al. [12]. The superior efficiency of the Ba(OH) 2 device compared to the Cs 2 CO 3 device was attributed to higher luminescence quantum yield of F8BT film, more effective hole-blocking capability and less disturbance of bulk hole-transport in F8BT. ZnO and TiO2 layers are typically prepared by thermal-conversion of metal salt ‘‘sol’’ at ca. 300–450 °C, which is time-consuming, increases the cost and cannot be used for plastic substrates. Further efforts are directed to reduce the annealing temperature and simplify device fabrication process. For example, ZnO [13] and SnO 2 [14] nano-particles are used as electron-injection layers, significantly reducing the heat treatment temperature. Parallel studies indicate the WF of ITO can be adjusted to a large extent via molecular absorption method. For example, chemical absorption of acid or base on ITO surface dramatically affects its WF and the chemical composition of ITO surface prior to the treatment plays an important role in determining the WF shift as well [15]; Expose of tetrakis(dimethylamino)ethylene (TDAE) was reported to significantly diminish the WF of ITO from 4.6 to 3.7 eV, which was attributed to the formation of interfacial dipole due to charge transfer from TDAE to ITO [16]; Incorporation of a water-soluble polyelectrolyte layer reduced the WF of ITO by ca. 0.4 eV as measured by Kelvin probe [17]; Zhong et al. [18] described the synthesis of amino-functionalized polyfluorenes and application of such polymers as ITO surface modifiers, which shifted the WF of ITO by ca. 0.44 eV and enabled efficient electron injection into LEPs. Organic electron injectors are more compatible with the adjoining organic layer than inorganic counterparts, leading to the formation of the integral contact [10]. Zhou et al. [19] reported that aliphatic amine-containing polymers such as ethoxylated polyethylenimine (PEIE) can significantly reduce the WF of manifold substrates such as metals, metal oxides, conducting polymers and graphene, which allowed flexible device structure designs and improved properties of various organic electronic devices. In particular, incorporation of a PEIE layer between 4,7-diphenyl-1,10-phenanthroline electron transport layer and Al led to high-efficiency light emitting devices. Despite the fact that PEIE as a surface modifier possesses many tempting properties such as a wide range of adaptability, durability and robustness, polymer light emitting devices using PEIE electron-injection

layer, in particular inverted devices with PEIE modified ITO as the electron injection contact, remain largely unexplored. In this manuscript, we report incorporation of PEIE layer at either the ITO/LEP or LEP/Al interface can significantly enhance the luminance efficiency of inverted and conventional polymer light emitting devices. Comparison of the current density–voltage characteristics of electron-only devices with or without a PEIE layer indicates that addition of a PEIE layer greatly increases electron injection. We also examine inverted light emitting devices employing various LEPs and find out that PEIE works as an efficient electron injector for a range of LEPs with wide distribution of energy levels. 2. Experimental section Materials: 80% ethoxylated polyethylenimine (PEIE, Mw = 70,000 g mol1), zinc acetate dihydrate (Zn(OAc)22H2O) and PEDOT: PSS (Clevios P VP AI4083) were purchased from Sigma–Aldrich and Heraeus Clevios GmbH, respectively and were used as received. Preparation of ZnO layer: 100 mg Zn(OAc)2.2H2O was dissolved in 1 mL 2-methoxyethanol at 80 °C and 56 lL ethanolamine as stabilizer was added into Zn(OAc)2 solution. The mixture was then heated and stirred at 60 °C for 12 h. The solution was spin-coated onto UV–ozone treated ITO substrates and the resultant film was annealed at 300 °C for 1 h under ambient conditions, giving a 30 nm ZnO layer. Fabrication of inverted light emitting devices: PEIE was diluted using 2-methoxyethanol by a factor of 50 and the solution was then spin-coated onto either bare or ZnO-covered ITO substrates. Part of PEIE layers were spin-rinsed with 2-methoxyethanol for 2–3 times. A 100 nm poly[2-methoxy5-(20 -ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), poly[9,9-dioctylfluorene-co-(bis-thienylene)benzothiadiazole (PF-TBT) or a fluorene–amine copolymer (PF-A) layer was deposited from the respective chlorobenzene solution. The MEH-PPV, PF-TBT and PF-A samples were annealed on a hotplate at 60, 180 and 180 °C for 10 min, respectively. 10 nm MoO3 and 100 nm aluminum were evaporated as the hole-injection layer and anode for bipolar devices. The CsF (1 nm)/Al (100 nm) cathode was deposited on top of a 300 nm MEH-PPV layer for electron-only devices. Fabrication of conventional light emitting devices: A 50 nm PEDOT: PSS layer was prepared by spin-coating its aqueous solution, which was subsequently tempted at 170 °C for 10 min under ambient conditions to remove the moisture. A 100 nm MEH-PPV layer, followed by a PEIE layer, was deposited onto PEDOT: PSS layer. The samples were annealed at 60 °C for 10 min. A CsF electron injection layer with nominal thickness of 1 nm was deposited onto MEH-PPV layer. Conventional devices were completed by thermal-deposition of 100 nm Al. Electron-only devices, where PEDOT: PSS layer in the above bipolar devices was replaced by a 30 nm ZnO layer to block hole injection, were prepared. With the exception of the deposition of PEDOT: PSS layer, all processes were carried out in a dry nitrogen atmosphere. Characterization: The thickness of ZnO, PEDOT: PSS, PEIE and LEP layers was determined by a Dektak 6M stylus

X. Yang et al. / Organic Electronics 15 (2014) 2387–2394

profiler. The voltage–current density–luminance (V–I–L) characteristics of devices were measured by a programmed Keithley 2400 SourceMeter and a Konica-Minolta CS-100A Chroma Meter. Electroluminescence (EL) spectra of devices were recorded using an Ocean Optics USB4000-UV–VIS spectrometer. XPS measurements were performed with a Kratos Axis Ultra DLD spectrometer. The morphology of films was measured with a Hitachi atomic force microscopy (AFM).

3. Results and discussions To investigate the morphology of PEIE deposited onto different underlayers, the AFM images of PEIE layers on top of ITO or MEH-PPV, as well as those of ITO and MEH-PPV, are measured. As presented in Fig. 1, MEH-PPV layer shows a smooth surface with root-mean-square (RMS) roughness value of 1.1 nm, while PEIE appears to form small aggregates on MEH-PPV, leading to slightly enhanced RMS roughness value of 1.3 nm. The AFM image of ITO shows crystalline grains of ITO with the size of a few tens of nanometers and RMS roughness value of ITO is 3.9 nm. PEIE covers ITO inhomogeneously, forming island-like structures with a thick layer interlaced with a thin layer. RMS roughness value of PEIE layer deposited on top of ITO is 3.4 nm, slightly lower than that of the underlying ITO. We start the device characterization with the optimization of PEIE layer thickness and find out that a 10 nm PEIE layer is optimal for the luminance efficiency of inverted devices. Fig. 2a shows the V–I–L characteristics of the inverted devices with or without a PEIE layer, i.e. the devices with the structures of ITO/PEIE(10 nm)/ MEH-PPV(100 nm)/MoO3(10 nm)/Al and ITO/MEH-PPV (100 nm)/MoO3(10 nm)/Al. Addition of a PEIE layer decreases the current density as shown in Fig. 2a. Despite the fact that the device without using a PEIE layer possesses large current density, light emission can barely be observed. Implementation of a PEIE layer initiates light emission and at the same time reduces the current density, implying that PEIE layer works as an electron-injection and hole-blocking layer. Fig. 2b presents the energy level diagram of inverted devices, in which the WF of electrodes and energy levels of LEPs are cited from the literatures [1,19–23]. Barrier-less hole injection from MoO3 layer [24] and the existence of large energy barrier for electron injection due to the mismatch between the WF of ITO (ca. 4.7 eV) [1] and LUMO level of MEH-PPV (ca. 2.7 eV) [20] suggest the current in the ITO/MEH-PPV(100 nm)/ MoO3(10 nm)/Al device is unipolar and predominated by holes. Fig. 2c shows the luminance efficiency–current density properties of the devices. The luminance efficiency of the device with a PEIE layer is ca. 1.25 cd/A at 20 mA/ cm2. We fabricate the electron-only devices with the structures of ITO/PEIE(10 nm)/MEH-PPV(300 nm)/CsF(1 nm)/Al and ITO/MEH-PPV(300 nm)/CsF(1 nm)/Al to investigate the effect of PEIE layer incorporation on electron injection properties. The ITO contact is negatively biased and the CsF/Al contact is employed to inhibit hole-injection due to its low WF (2.6 eV) [25] as shown in the inset of

2389

Fig. 2d. Fig. 2d displays the current density–voltage properties of the electron-only devices. Incorporation of a PEIE layer between ITO and MEH-PPV layer increases the electron current density by a factor of ca. 5, which in turn improves the balance of carrier injection and transport, resulting in enhanced luminance efficiency. We prepare inverted devices with ZnO/PEIE electron injection layers to study the influence of the configuration of electron transport layer on device properties. Fig. 3a shows the V–I–L characteristics of the inverted devices with the structures of ITO/PEIE(10 nm)/MEH-PPV (100 nm)/MoO3(10 nm)/Al and ITO/ZnO(30 nm)/PEIE (10 nm)/MEH-PPV(100 nm)/MoO3(10 nm)/Al. The current density of the device with ZnO/PEIE electron injection layers is lower than that of the device with a single PEIE electron injection layer under the same drive voltage. Both devices show the luminance on-set voltage (ca. 0.1 cd/m2) of ca. 3 V. Fig. 3b presents the luminance efficiency versus current density plots of the devices. Two types of devices show comparable luminance efficiency (1.15 versus 1.2 cd/A at 20 mA/cm2 for the device with ZnO/PEIE electron injection layers and that with a PEIE electron injection layer), indicating that PEIE can serve as a single electron injection layer for ITO cathode without compromising device efficiency. In contrast, the inverted light emitting device with the combination of Cs compound and TiO2 electron injection layer was reported to possess much higher luminance efficiency than the device using Cs compound electron injection layer [26]. The effect of PEIE layer incorporation at the LEP/Al interface on device properties has been studied as well. Fig. 4a presents the V–I–L characteristics of the device with the structure of ITO/PEDOT: PSS(50 nm)/MEH-PPV (100 nm)/PEIE(3 nm)/Al, as well as those of the control devices with the structures of ITO/PEDOT: PSS(50 nm)/ MEH-PPV(100 nm)/Al and ITO/PEDOT: PSS(50 nm)/MEHPPV(100 nm)/CsF(1 nm)/Al. Addition of a PEIE layer between MEH-PPV and Al only slightly affects the current density. Overall weaker impact of PEIE layer incorporation on the current density for the conventional device with respect to the case for the inverted device (Fig. 2a) may be understood by the penetration of Al into the underlying organic layers during thermal-evaporation process. On the other hand, addition of a PEIE layer increases the luminance by a factor of ca. 30. The luminance on-set voltage for the device with a PEIE layer is ca. 2 V, which is lower than that of the device using bare Al cathode (4 V), but close to that of the device using widely-adopted CsF electron injection layer. The V–I–L characteristics of the PEIE device are slightly shifted toward higher drive voltage compared to those of the CsF device. Fig. 4b shows the luminance efficiency versus current density plots of the devices. Incorporation of a PEIE layer between MEH-PPV layer and Al enhances the luminance efficiency by more than one order of magnitude. The luminance efficiency of the PEIE device (1.85 cd/A) is comparable to that of the CsF device (1.6 cd/A) in the present study and reported in the literature [27], indicating that PEIE works equally well as CsF to promote electron injection from Al. Besides, PEIE layer can be readily solution-processed under ambient conditions and is more compatible with the neighboring

2390

X. Yang et al. / Organic Electronics 15 (2014) 2387–2394

Fig. 1. Shows AFM images of MEH-PPV layer (a), PEIE on top of MEH-PPV (b), ITO (c) and PEIE deposited onto ITO (d).

organic layer than metal fluorides. To understand the mechanism underlying device performance improvement, the electron-only devices with the structures of ITO/ ZnO(30 nm)/MEH-PPV(300 nm)/Al and ITO/ZnO(30 nm)/ MEH-PPV(300 nm)/PEIE(3 nm)/Al are prepared, where

ZnO layer is employed as a hole-blocking layer due to its deep-lying valence band maximum located at ca. 6.6 eV blow the vacuum level [12] as shown in the inset of Fig. 4c and the top aluminum electrode is negatively biased to function as electron injection contact. Fig. 4c compares

2391

X. Yang et al. / Organic Electronics 15 (2014) 2387–2394

-1.5 -2.1

250

-2.9 100

200 150

10 100

-3.4

Al -4.2

ITO -4.7

1

50

-5.2

(a) 0

(b)

1000

ITO ITO/PEIE

Luminance (cd/m2)

Current density (mA/cm2)

300

3

-5.4

6

Voltage (V)

-6.7

-7.7

-9.7 PEIE

(c)

0.009

Current density (mA/cm2)

Luminance efficiency (cd/A)

1.5

1.2

0.9

0.6

0.3

0.0

-2.9

ITO/PEIE -3.1

LEP

MoO3

CsF/Al -2.6

(d)

0.006 ITO -4.7 0.003

-5.2 MEH-PPV

0.000 50

100

150

Current density (mA/cm2)

0

3

6

9

Voltage (V)

Fig. 2. Presents the energy level diagram and properties of the inverted devices: the energy level diagram, in which energy levels of LEPs are presented in lines having the same color as their emission i.e. blue for PF-A, orange for MEH-PPV and red for PF-TBT (a); the V–I (solid symbols)-L (open symbols) characteristics (b); the luminance efficiency–current density plots (c); the current density–voltage curves of the electron-only devices (d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the current density–voltage properties of the electron-only devices. The current density of the device with a PEIE layer is ca. 5–7 times higher than that of the device without using a PEIE layer, indicating that incorporation of a PEIE layer at the MEH-PPV/Al interface greatly improves electron injection. The above results illustrate that implementation of a PEIE layer at either the ITO/MEH-PPV interface or the MEH-PPV/Al interface can largely improve electron injection, resulting in significant enhancement of device efficiency. In the initial report of Zhou et al. [19], the reduction of substrate WF upon incorporation of a PEIE layer was mainly attributed to the dipole layer formation and the WF shift was insensitive to PEIE layer thickness. It should be

also noted that non-negligible doping of organic layers by PEIE has also been reported. More recently, the WF shift of ITO upon addition of a PEIE layer has been reported to be 1.62 eV by Son et al. [28]. We carry out XPS measurements of Al and PEIE modified Al samples and the results are shown in Fig. 5. Addition of a PEIE layer decreases the WF of Al by ca. 1.0 eV, which can be attributed to the fact that the dipole moment of PEIE layer and the formation of interfacial dipole elevate the vacuum level. We find that the luminance efficiencies for both the conventional and inverted devices are dependent of PEIE layer thickness and propose that varied hole-blocking property and bulk resistance of different thickness PEIE layers are relevant to device properties. Comparable electron current density

2392

X. Yang et al. / Organic Electronics 15 (2014) 2387–2394

1.5

ZnO/PEIE

1000

rinsed PEIE layer

200

100

150

100

10

50

1

Luminance efficiency (cd/A)

PEIE

Luminance (cd/m2)

Current density (mA/cm2)

250

0

3

6

0.9

0.6

0.3

0.0

0.1

0

1.2

50

9

100

150

200

Current density (mA/cm2)

Voltage (V)

Fig. 3. Compares the V–I (solid symbols)-L (open symbols) (a) and luminance efficiency–current density (b) characteristics of the inverted devices with a single PEIE electron injection layer or with ZnO/PEIE electron injection layers. Part of PEIE layers for the ITO/PEIE/MEH-PPV/MoO3/Al devices have been rinsed with 2-methoxyethanol prior to the deposition of MEH-PPV layer.

PEIE/Al Al CsF/Al

300

1000

250

100

200 10

150

1

100 50

Luminance (cd/m2)

Current density (mA/cm2)

350

0.1

(a) 0 0

3

6

9

Voltage (V) 0.006

1.6

1.2

0.8

0.4

0.0

-2.9

(c)

(b) Current density (mA/cm2)

Luminance efficiency (cd/A)

2.0

PEIE/Al -3.2

-3.6

0.004

Al -4.2 ITO -4.7

-5.2 MEH-PPV

0.002 -6.6 ZnO

PEIE/Al Al

0.000 0

50

100

150

Current density (mA/cm2)

0

3

6

Voltage (V)

Fig. 4. Describes properties of the conventional devices: the V–I (solid symbols)-L (open symbols) characteristics (a); the luminance efficiency–current density plots (b); the current density–voltage curves of the electron-only devices (c).

for the ITO/ZnO(30 nm)/MEH-PPV(300 nm)/PEIE (3 nm)/Al and ITO/PEIE(10 nm)/MEH-PPV(300 nm)/CsF(1 nm)/Al electron-only devices reveals that the PEIE/Al and ITO/PEIE contact possess similar electron injection capability. We find that rinsing PEIE layer with 2-methoxyethanol prior to the deposition of MEH-PPV layer results in significant reduction of the operating voltage. The V–I–L and

luminance efficiency–current density characteristics of the device using a 2-methoxyethanol rinsed PEIE layer are also included in Fig. 3a and b for ease of comparison. While the luminance efficiency of the device with a rinsed PEIE layer is comparable to that of the device with an untreated PEIE layer, the operating voltage of the former device is greatly reduced, e.g. the drive voltage at ca.

2393

X. Yang et al. / Organic Electronics 15 (2014) 2387–2394

6000

4000

2000

0 66

68

70

72

74

76

78

80

Binding energy (eV) Fig. 5. Displays Al 2P core level spectra for Al and PEIE modified Al samples.

20 mA/cm2 for the device with an untreated PEIE layer is 4.6 V, which is reduced to ca. 3.7 V for the device with a rinsed PEIE layer. Increased photoresponse of an organic photodetector using a deionized water washed PEIE layer was reported by Saracco et al. [29], and improvement of the detectivity was attributed to the elimination of non-physisorbed PEIE or contaminants, which work as carrier traps. Since PEIE and MEH-PPV layer are co-annealed in our case, we speculate that the reduced intermixing of

3.0

(a)

(b)

1000 PF-TBT PF-A

150

100

100

10

1 50 0.1 0

Luminance (cd/m2)

Current density (mA/cm2)

200

Luminance efficiency (cd/A)

Intensity (a.u)

PEIE and MEH-PPV as a result of the removal of nonphysisorbed PEIE by rinsing with 2-methoxyethanol and in turn the decreased probability of n-doping MEH-PPV [19], which is expected to disturb hole-transport in MEH-PPV, may be responsible for increased current density. We fabricate inverted devices with LEPs possessing various energy levels and emission wavelengths to examine if PEIE can increase electron injection into such LEPs as well. The LUMO level of MEH-PPV (ca. 2.7 eV below the vacuum level) is located between that of PF-A (ca. 2.1 eV) [21] and PF-TBT (3.4 eV) [22]. Fig. 6a and b shows the V–I–L and luminance efficiency–current density characteristics of the PF-TBT and PF-A devices. The luminance on-set voltage for the PF-TBT or PF-A device is ca. 1.8 or 2.8 V, which is very close to the emitting photon energy of PFTBT and PF-A divided by elementary charge, indicating very small energy barrier for electron injection from PEIE modified ITO cathode. The PF-TBT and PF-A devices show the maximum luminance efficiencies of 0.8 and 2.2 cd/A, which are comparable to those of the analogous devices using low WF metal cathode [30,31]. Fig. 6c shows the EL spectra of the devices. The EL spectra are identical to the photoluminescence (PL) spectra of PF-A and PF-TBT film. Thus, our results indicate PEIE modified ITO works as an efficient electron injection contact for a range of LEPs with wide distribution of LUMO energy. It should also be noted that efficient electron injection into PF-A and PF-TBT is

Al PEIE/Al

8000

2.4

1.8

1.2

0.6

0.0 0

3

6

0

50

100

Current density (mA/cm2)

Voltage (V)

(c)

Intensity (a.u)

0.9

0.6

0.3

0.0 400

500

600

700

800

Wavelength (nm) Fig. 6. Depicts properties of the inverted devices using either PF-TBT or PF-A: the V–I (solid symbols)-L (open symbols) plots (a); the luminance efficiency– current density characteristics (b); the EL spectra (c).

2394

X. Yang et al. / Organic Electronics 15 (2014) 2387–2394

maintained even after thermal treatment of PF-A/PF-TBT and PEIE layer at 180 °C is included during the process of device fabrication, manifesting remarkable thermal stability of electron injection properties of PEIE modified ITO contact. 4. Conclusion Taking into account of all above results, it can be concluded that incorporation of a PEIE layer at either the ITO/MEH-PPV interface or the MEH-PPV/Al interface significantly enhances the luminance efficiencies of the inverted and conventional light emitting devices. The PEIE based light emitting devices show comparable luminance efficiency to the analogous devices using a CsF or n-type metal oxide electron injection layer. Improvement of device efficiency can be attributed to increased electron injection due to the reduced work function of PEIE modified cathode as well as the hole-blocking effect of PEIE layer. PEIE works as an efficient electron injector for a range of light emitting polymers with wide distribution of energy levels. Apart from excellent electron injection capability, PEIE can be readily processed from alcohol or aqueous solution under ambient conditions and is more compatible with the neighboring organic layer than metal fluorides. All tributes represent PEIE a promising electron injector for polymer light emitting devices. Acknowledgements Financial support by the National Natural Science Foundation of China (Grant Nos. 61177030 and 11374242), the Chinese Ministry of Education under the program for New Century Excellent Talents in Universities (Grant No. NCET-11-0705), the start-up Grant (SWU111057) and college students’ innovation fund (1218004) from Southwest University is acknowledged. References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredás, M. Logdlund, W.R. Salaneck, Electroluminescence in conjugated polymers, Nature 397 (1999) 121. [2] D. Braun, A.J. Heeger, Visible-light emission from semiconducting polymer diodes, Appl. Phys. Lett. 58 (1991) 1982. [3] G.E. Jabbour, B. Kippelen, N.R. Armstrong, N. Peyghambarian, Highly efficient and bright organic electroluminescent devices with an aluminum cathode, Appl. Phys. Lett. 73 (1998) 1185. [4] Y.H. Niu, H. Ma, Q.M. Xu, A.K.Y. Jen, High-efficiency light-emitting diodes using neutral surfactants and aluminum cathode, Appl. Phys. Lett. 86 (2005) 083504. [5] H.B. Wu, F. Huang, Y.Q. Mo, W. Yang, D.L. Wang, Q.B. Pei, Y. Cao, Efficient electron injection from a bilayer cathode consisting of aluminum and alcohol-/water-soluble conjugated polymers, Adv. Mater. 16 (2004) 1826. [6] K. Morii, M. Ishida, T. Takashima, T. Shimoda, Q. Wang, M.K. Nazeeruddin, M. Grätzel, Encapsulation-free hybrid organic– inorganic light-emitting diodes, Appl. Phys. Lett. 89 (2006) 183510. [7] M.F. Lo, T.W. Ng, T.Z. Liu, V.A.L. Roy, S.L. Lai, M.K. Fung, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 96 (2010) 113303. [8] M. Sessolo, H. Bolink, Hybrid organic–inorganic light-emitting diodes, Adv. Mater. 23 (2011) 1829. [9] J.S. Park, B.R. Lee, J.M. Lee, J.S. Kim, S.O. Kim, M.H. Song, Efficient hybrid organic–inorganic light emitting diodes with self-assembled dipole molecule deposited metal oxides, Appl. Phys. Lett. 96 (2010) 243306.

[10] H. Choi, J.S. Park, E. Jeong, G.H. Kim, B.R. Lee, S.O. Kim, M.H. Song, H.Y. Woo, J.Y. Kim, Combination of titanium oxide and a conjugated polyelectrolyte for high-performance inverted-type organic optoelectronic devices, Adv. Mater. 23 (2011) 2759. [11] For example H. Bolink, E. Coronado, J. Orozco, M. Sessolo, Efficient polymer light emitting diode using air-stable metal oxides as electrodes, Adv. Mater. 21 (2009) 79. [12] L.P. Lu, D. Kabra, R.H. Friend, Barium hydroxide as an interlayer between zinc oxide and a luminescent conjugated polymer for lightemitting diodes, Adv. Funct. Mater. 22 (2012) 4165. [13] M. Sessolo, H. Bolink, H. Brine, H. Prima-Garica, R. Tena-Zaera, Zinc oxide nanocrystals as electron injecting building blocks for plastic light sources, J. Mater. Chem. 22 (2012) 4916. [14] H. Lee, C.M. Kang, M. Park, J. Kwak, C. Lee, Improved efficiency of inverted organic light-emitting diodes using tin dioxide nanoparticles as an electron injection layer, ACS Appl. Mater. Interfaces 5 (2013) 1977. [15] F. Nuesch, L.J. Rothberg, E.W. Forsythe, Q.T. Le, Y.L. Gao, A photoelectron spectroscopy study on the indium tin oxide treatment by acids and bases, Appl. Phys. Lett. 74 (1999) 880. [16] W. Osikowicz, X. Crispin, C. Tengstedt, L. Lindell, T. Kugler, W.R. Salaneck, Transparent low-work-function indium tin oxide electrode obtained by molecular scale interface engineering, Appl. Phys. Lett. 86 (2004) 1616. [17] S.I. Na, T.S. Kim, S.H. On, J.K. Kim, S.S. Kim, D.Y. Kim, Enhanced performance of inverted polymer solar cells with cathode interfacial tuning via water-soluble polyfluorenes, Appl. Phys. Lett. 97 (2010) 223305. [18] C.M. Zhong, S.J. Liu, F. Huang, H.B. Wu, Y. Cao, Highly efficient electron injection from indium tin oxide/cross-linkable aminofunctionalized polyfluorene interface in inverted organic light emitting devices, Chem. Mater. 23 (2011) 4870. [19] Y.H. Zhou, C. Fuentes-Hernandez, J.W. Shim, J. Meyer, A. Giordano, H. Li, P. Winget, T. Papadopoulos, H.S. Cheun, J.B. Kim, M. Fenoll, A. Dindar, W. Haske, E. Najafabadi, T. Khan, H. Sojoudi, S. Barlow, S. Graham, J.L. Brédas, S. Marder, A. Kahn, B.A. Kippelen, Universal method to produce low-work function electrodes for organic electronics, Science 336 (2012) 327. [20] L. Holt, J.M. Leger, S.A. Carter, Electrochemical and optical characterization of p- and n-doped poly[2-methoxy-5-(2ethylhexyloxy)-1,4-phenylenevinylene], J. Chem. Phys. 123 (2005) 044704. [21] B.M.W. Langeveld-Voss, S.L.M. van Mensfoort, M.M. de KoK, R. Coehoorn, Tuning the voltage dependence of the efficiency of blue organic light-emitting diodes based on fluorene–amine copolymers, Org. Electron. 11 (2010) 755. [22] P. de Bruyn, D.J.D. Moet, P.W.M. Blom, All-solution processed polymer light-emitting diodes with air stable metal–oxide electrodes, Org. Electron. 13 (2012) 1023. [23] J. Meyer, S. Hamwi, S. Kroger, W. Kowalsky, T. Riedl, A. Kahn, Transition metal oxides for organic electronics: energetics, device physics and applications, Adv. Mater. 24 (2012) 5408. [24] M.T. Lu, P. de Bruyn, H. Nicolai, G.H. Wetzelae, P.W.M. Blom, Holeenhanced electron injection from ZnO in inverted polymer lightemitting diodes, Org. Electron. 13 (2012) 1693. [25] T.M. Brown, F.J. Cacialli, Contact optimization in polymer lightemitting diodes, J. Polym. Sci.: Part B 41 (2003) 2649. [26] K. Morii, T. Kawase, I. Satoshi, High efficiency and stability of the encapsulation – free hybrid organic–inorganic light emitting diode, Appl. Phys. Lett. 92 (2008) 213304. [27] X.H. Yang, Y.Q. Mo, W. Yang, G. Yu, Y. Cao, Efficient polymer light emitting diodes with metal fluoride/Al cathodes, Appl. Phys. Lett. 79 (2001) 563. [28] D.I. Son, H.H. Kim, D.K. Hwang, S. Kwon, W.K. Choi, Inverted CdSe– ZnS quantum dots light-emitting diode using low-work function organic material polyethylenimine ethoxylated, J. Mater. Chem. C 2 (2014) 510. [29] E. Saracco, B. Bouthinon, J. Verilhac, C. Celle, N. Chevalier, D. Mariolle, O. Dhez, J. Simonato, Work function tuning for high-performance solution-processed organic photodetectors with inverted structure, Adv. Mater. 25 (2013) 6534. [30] F. Laquai, D. Hertel, Influence of hole transport units on the efficiency of polymer light emitting diodes, Appl. Phys. Lett. 90 (2007) 142109. [31] Q. Hou, Q.M. Zhou, Y. Zhang, R.Q. Yang, Y. Cao, Synthesis and electroluminescent properties of high-efficiency saturated red emitter based on copolymers from fluorene and 4,7-di(4hexylthien-2-yl)-2,1,3-benzothiadiazole, Macromolecules 37 (2004) 6299.