Highly efficient red phosphorescent organic light-emitting diodes based on solution processed emissive layer

Journal of Luminescence 142 (2013) 35–39

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Highly efficient red phosphorescent organic light-emitting diodes based on solution processed emissive layer Baiquan Liu a,b, Miao Xu a,b, Hong Tao a,b, Lei Ying a,b, Jianhua Zou a,b,n, Hongbin Wu a,b, Junbiao Peng a,b a b

Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, PR China State Key Laboratory of Luminescent Materials and Devices, Guangzhou 510640, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 17 October 2012 Received in revised form 1 March 2013 Accepted 25 March 2013 Available online 2 April 2013

Highly efficient red phosphorescent organic polymer light-emitting diodes (PhOLEDs) were fabricated based on a solution-processed small-molecule host 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) by doping an iridium complex, tris(1-(2,6-dimethylphenoxy)-4-(4-chlorophenyl)phthalazine)iridium (III) (Ir(MPCPPZ)3). A hole blocking layer 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBI) with a function of electron transport was thermally deposited onto the top of CBP layer. The diode with the structure of ITO/ PEDOT:PSS (50 nm)/CBP:Ir(MPCPPZ)3 (55 nm)/TPBI (30 nm)/Ba (4 nm)/Al (120 nm) showed an external quantum efficiency (QEext) of 19.3% and luminous efficiency (LE) of 18.3 cd/A at a current density of 0.16 mA/cm2, and Commission International de I'Eclairage (CIE) coordinates of (0.607, 0.375). It was suggested that the diodes using TPBI layer exhibited nearly 100% internal quantum efficiency and one order magnitude enhanced LE or QEext efficiencies. & 2013 Elsevier B.V. All rights reserved.

Keywords: High film quality High EL efficiency Solution-processed small-molecule

1. Introduction Organic light-emitting diodes (OLEDs) have attracted much attention due to their potential applications as one of the important candidates for the next generation of flat-panel displays. Recently, phosphorescent OLEDs (PhOLEDs) generated intense research interests because they can harvest both singlet and triplet excitons so that theoretically 100% internal quantum efficiency can be achieved [1]. Generally, most of the reported PhOLEDs are fabricated based on thermal deposition in vacuum with multilayer device structures, which benefits the fine tuning of electrons and holes' balance in the emissive layer so that highly efficient devices could be achieved [2–5]. The alternative way leading to efficient PhOLEDs with low cost and large area is based on the solution processing technique, which typically employs large band gap conjugated polymers or small molecules as the host and phosphorescent dye dopants as the guest. Even though the devices based on conjugated polymer host such as polyfluorenes or poly (N-vinylcarbazole)s can attain decent device performances [6–12], the batch to batch differences of such polymer hosts could not be avoided, which may generate problems for large scale processing. In comparison with such polymer hosts, solution processed small molecules host materials are of particular interest because of

n Correspondence to: South China University of Technology, Institute of Polymer Optoelectronic Materials and Devices, 381# Wushan Road, Tianhe District, Guangzhou, Guangdong 510640, China. Tel.: +86 020 32203886. E-mail address: [email protected] (J. Zou).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.03.032

their analogous conductivities, facile synthesis and decoration of molecular structures, as well as straightforward purification procedures that can get rid of batch to batch differences. A variety of efficient PhOLEDs based on solution processed small molecules as hosts and phosphorescent dyes as emitters were performed [13–17]. However, the device performances were still lower than that attained by thermal vacuum deposited procedures, which might be due to the poor miscibility of phosphorescent dyes and host materials leading to poor film morphology, or the inappropriately matched triplet energy levels that may cause imbalanced charge carrier in the emissive layer. The red-emitting iridium complex of Ir(MPCPPZ)3, fac-tris (1-(2,6-dimethylphenoxy)-4-(4-chlorophenyl)phthalazine)iridium (III), has been used as an effective emitter to achieve phosphorescent polymer light emitting diodes based on high molecular weight PVK blend with CBP as host materials [18]. It was realized that CBP itself could also be employed for fabrication of solutionprocessed devices because its excellent miscibility with guest phosphorescent dyes, and high quality films could be afforded by spin-coating procedures. In fact, several reports have been focused on the utilization of solution processed CBP as host materials to fabricate efficient PhOLEDs [19–21]. For example, Samuel et al. [20] reported solution processed PhOLEDs by blending green and red emitters with CBP host, and high efficiency devices with tunable color were achieved by adjusting the relative amount of green and red dendrimers in the CBP. In this paper, we reported a highly efficient PhOLED based on solution processed emissive layer, which was composed with a

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B. Liu et al. / Journal of Luminescence 142 (2013) 35–39

commercially available 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) as host, and a red-emitting iridium complex, Ir(MPCPPZ)3 as phosphorescent guest. It was found that CBP with Ir(MPCPPZ)3 exhibited excellent compatibility in halogenated solvents such as chloroform and chlorobenzene, and a very smooth film could be formed by spin casting. In addition, after introducing an additional 2,2′,2″-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole] (TPBI) layer between the emissive layer and cathode, the external quantum efficiency (EQE) and luminous efficiency (LE) could be significantly improved and attained as high as 19.3% and 18.3 cd/A, respectively with CIE coordinate of (0.607, 0.375), indicating that nearly 100% internal quantum yield was achieved.

2. Experimental The chemical structures of CBP, TPBI and Ir (MPCPPZ)3 are showed in Fig. 1. CBP was purchased from American Dye Sources Inc., TPBI was purchased from Aldrich, Ir (MPCPPZ) was homemade [18]. Even though both CBP and Ir(MPCPPZ)3 have good solubility in chloroform and an almost homogenous solution could be formed, the low boiling point of chloroform that caused quick evaporation lead to poor film quality. On the other hand, CBP have poor solubility in high boiling point chlorobenzene. Thus, in order to solve this problem, high boiling point chlorobenzene was mixed into chloroform. We obtained the best film quality afforded based on the ratio of chloroform/chlorobenzene of 3/2 in v/v. The fabrication process of the diodes followed a standard procedure. PEDOT:PSS (Baytron P4083, Bayer AG) was spin coated on a precleaned ITO substrate and dried by baking in vacuum at 80 1C for 12 h. The 55 nm CBP doped with Ir (MPCPPZ)3 was spin coated from a mixed solution of chloroform and chlorobenzene with a volume ratio of 3:2. Profilometry (Tencor Alfa-Step 500) was used to determine the thickness of the films. The TPBI was deposited onto the emitting material layer to confine excitons within the emissive zone. And then, a 4 nm thick layer of Ba and a 120 nm thick capping layer of Al were deposited through a shadow mask defined active area of 0.17 cm2 in a vacuum chamber with a base pressure of 3  10−4 Pa. The layer thickness was monitored upon deposition by using a crystal thickness monitor (Sycon). Device fabrication was carried out in a nitrogen atmosphere dry box (Vacuum Atmosphere Co.). Current density (J),voltage (V), and luminance (L) data were collected using a Keithley 236 source meter and a calibrated by silicon photodiode. External quantum efficiencies (QEext) and photoluminescence quantum efficiency (ηPL) were obtained by measuring the total light output in all

3. Results and discussion Fig. 2 shows the representative atomic force microscopy (AFM) image of the film spin casted from CBP:Ir(MPCPPZ)3 blend on the top of ITO/PEDOT:PSS layer, which has Ir(MPCPPZ)3 content as high as 8 wt% in chloroform/chlorobenzene of 3/2 in v/v. A root mean square (RMS) value of  0.2 nm indicated rather smooth and homogenous film morphology. Good smoothness of the interface is the premise of high performance of devices, thereby good performances of the devices can be expected [13,16,20]. Fig. 3 shows photoluminescence (PL) spectra of Ir(MPCPPZ)3: CBP blend films with various concentrations of 0.5%, 1%, 2%, 4% and 8% (wt/v), and the PL spectrum of pure CBP film is also included for the comparison. The two components of the PL spectra that peaked at  406 nm and  593 nm are corresponding to the emission of the individual CBP and Ir(MPCPPZ)3, respectively. It was noted that the emission from CBP decreased gradually with the increasing the ratio of Ir(MPCPPZ)3. Effective Föster energy transfer from the CBP host to the Ir(MPCPPZ)3 complex could be

Fig. 2. AFM image (2 μm  2 μm) of Ir(MPCPPZ)3:CBP (8 wt/v%) film prepared by spin-casting from chlorobenzene/chloroform (2/3 in v/v).

Cl

1.2

N Ir

CBP

N N O N

N

N

N

Ir(MPCPPZ)3 N

N

3

0.5%

Normalzied PL Intersity

N

directions in an integrating sphere (IS-080, Labsphere). The CIE coordination was recorded using a PR-705 Spectra Scan spectrophotometer (Photo Research).

1

1% 2%

0.8

4% 8%

0.6

CBP

0.4 0.2 0 400

TPBI Fig. 1. Chemical structures of Ir(MPCPPZ)3, CBP and TPBI.

500

600

700

800

Wavelength (nm) Fig. 3. PL spectra of Ir(MPCPPZ)3:CBP films with various blend ratios.

B. Liu et al. / Journal of Luminescence 142 (2013) 35–39

expected since there is an overlap of absorption of guest Ir (MPCPPZ)3 and emission of host CBP (see from Fig. 4). Nonetheless, the emission of CBP at  400 nm was still not completely quenched even at the highest doping content of 8 wt%. The absolute photoluminescence quantum efficiencies (ηPL) of the dopant films increased from 75% to a maximum of 96% with Ir (MPCPPZ)3 content increasing from low to intermediate (0.5–4 wt %), followed by slightly decreasing to 92% at the highest doping content of 8 wt%. The observed 4% optimized doping concentration can be ascribed to the compromise between energy transfer between the host to the guest molecules and the concentration quenching by the guest molecules [22]. Fig. 5 shows the electroluminescence (EL) spectra of the device with architecture of ITO/PEDOT:PSS (50 nm)/CBP: Ir(MPCPPZ)3 (55 nm)/Ba (4 nm)/Al (120 nm). Here the emissive layer was fabricated following the same conditions as that of PL measurement. It was recognized that the emission at  400 nm corresponding to CBP host quenched completely at the lowest Ir (MPCPPZ)3 content of 0.5 wt%. The different PL spectra (Fig. 3) from the emitting layers and the EL spectra of the devices reveals that the EL spectra have a much higher contribution from the narrowband gap Ir(MPCPPZ)3 emitters. As pointed out by many authors [10,22–24], significant deviation of EL spectra with PL emissions of devices from blend systems can be assigned to the dominance of the trapping mechanism in the EL process. This is

CBP PL Ir Abs

Normalized PL intersity

Normalized UV intersity

1

0.8

0.6

0.4

0.2

0

300

400

500

600

700

Wavelength(nm)

37

energetically favored and can be inferred from the energy level chart (Fig. 6). In Fig. 6, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of Ir(MPCPPZ)3 estimated by cyclic voltammetry (CV) were −5.5 eV and −3.2 eV, respectively [18], which lied in between the HOMO and LUMO energy level of CBP of −5.9 and −2.3 eV, respectively [19] Therefore, the injected holes from the anode PEDOT:PSS and electrons from the cathode Ba/Al could be trapped directly by the Ir(MPCPPZ)3 center, demonstrating that the charge trapping was the dominant mechanism in the EL process. The performances of the devices with structure of ITO/PEDOT:PSS (50 nm)/CBP:Ir(MPCPPZ)3 (55 nm)/Ba(4 nm)/Al(120 nm) were summarized in Table 1. It was found that the best device performances were obtained based on the Ir(MPCPPZ)3 content of 4 wt%, which exhibited maximum external quantum efficiency (QEmax) of 1.4% ph/ el and luminance efficiency (LEmax) of 1.2 cd/A at a current density of 6.2 mA/cm2, and a maximum luminance (Lmax) of 1136 cd/m2 was observed. However, such device performances were much lower than expected since the high ηPL of CBP:Ir(MPCPPZ)3 films were exhibited. Therefore, the relatively lower device efficiency might be attributed to the imbalanced carrier in the emissive layer. In order to achieve highly efficient devices, well-balanced charge carriers in the emissive layer are of particular importance. Addressed to this point, a variety of methods have been employed, such as doping additional materials into the host to compensate the minority carrier in the emissive layer, chemical decoration of host materials to attain suitable energy levels, as well as inserting an interfacial layer between a metal cathode and polymer is commonly used to enhance electron injection, and so forth [25,26]. Since CBP is a typical p-type material [19,27], it is reasonable to assume that holes would be majority carriers in the emissive layer. To avoid quenching of the majority hole carriers in the nearby cathode, we herein introduced an additional hole blocking 1,3,5-tris (N-phenylbenzimidizol-2-yl)benzene (TPBI) layer(HBL) between emissive layer and cathode. As can be seen from the energy level diagram of Fig. 6, TPBI has rather deep HOMO energy level of −6.2 eV [28], which is0.3 eV lower than that of CBP, thereby it could effectively reduce the possibility of possible hole/electron recombination at cathode. Moreover, TPBI has LUMO energy level of −2.7 eV, which is identical to the work function of cathode Ba hence the electron injection would not be influenced. Furthermore, the incorporation of an extra TPBI layer can also keep the hole/electron recombination area away from the cathode, which could further limit excitons within the emissive zone while reduce the probability of excitons quenching at the cathode.

Fig. 4. UV absorption spectra of Ir(MPCPPZ)3 film and Photoluminescence (PL) spectrum of a CBP (excited by 380 nm pump light source).

Normalzied EL intersity

1

0.5% 1%

0.8

2% 4%

0.6

8%

0.4

0.2

0 400

500

600

700

800

Wavlength(nm) Fig. 5. EL spectra of devices with structure of ITO/PEDOT (50 nm)/CBP:Ir(MPCPPZ)3 (55 nm)/Ba(4 nm)/Al(120 nm).

Fig. 6. Energy level diagram of the device with structure of ITO/PEDOT/CBP:Ir (MPPZ)3/TPBI/Ba/Al.

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B. Liu et al. / Journal of Luminescence 142 (2013) 35–39

Table 1 Device of performances.a Ir(MPCPPZ)3b (wt%)

HBL

– – – – – TPBI TPBI TPBI

0.5 1 2 4 8 2 4 8 a b

Vth (V)

at maximal quantum efficiency

5.6 5.6 5.6 5.6 6 7.0 7.0 8.0

J (mA/cm2)

L (cd/m2)

LEmax (cd/A)

QEmax (%)

10.1 7.9 12 6.2 9.1 0.1 0.1 0.2

99 86 168 77 103 12 30 34

1 1.1 1.1 1.2 1.1 12.2 18.3 16.3

1 1.2 1.3 1.4 1.3 12.8 19.3 17.2

Lmax (cd/m2)

CIE (x, y) (at 2 mA)

1369 1062 1189 1136 708 2033 4443 5575

(0.618,0.365) (0.617,0.366) (0.617,0.360) (0.618,0.362) (0.618,0.365) (0.603, 0.367) (0.607, 0.375) (0.601, 0.370)

Device structure: ITO/PEDOT:PSS (50 nm)/CBP:Ir(MPCPPZ)3 (50 nm)/HBL (30 nm)/Ba (4 nm)/Al (120 nm). Ir(MPCPPZ)3 content in CBP (wt%). Vth: the turn-on voltage.

20

8x103

6x103

8%

5x10

3

4x103 10-2

3x103 2x103

10-4

1x103 10-6 0

5

10

15

20

Luminous Efficiency (cd/A)

4% 100

2%

7x103

2%

Luminance(cd/m2)

Current Density(mA/cm2)

102

4% 15

8%

10

5

0

0.1

Voltage(V) Fig. 7. I–V–L curves of the devices with structure: ITO/PEDOT/CBP:Ir(MPCPPZ)3/ TPBI/Ba/Al.

In considering that devices based on higher Ir(MPPZ)3 content obtained higher efficiency, and also a much higher PL quantum efficiency was realized in CBP:Ir(MPPZ)3 blend films with Ir (MPPZ)3 content higher than 2%, therefore, the further optimization by insertion of TPBI is mainly focused on blend ratio of 2%, 4% and 8%. The I–V–L curves of these devices shown in Fig. 7, as expected, the device performances remarkably improved after the insertion of TPBI layer, as summarized in Table 1. The best device performances were achieved based on Ir(MPCPPZ)3 content of 4 wt%, which exhibited QEext of 19.3% and LEmax of 18.3 cd/A, and a maximal luminance (Lmax) of 4440 cd/m2 with CIE coordinates of (0.61, 0.38) was observed at a current density of 110 mA/cm2. Considering the efficiency loss due to light out-coupling from the substrate, the internal quantum efficiency of this red phosphorescent device approaches 100%, which also indicated balanced electron and hole recombination, and complete charge trapping for the phosphorescent dye upon electrical excitation. Fig. 8 shows luminous efficiencies as a function of current density of devices based on various blend ratios. The efficiency roll-off at high current density was also realized in this system, which might be attributed to the combination of triplet–triplet annihilation and field-induced quenching effects. Nevertheless, the device based on Ir(MPCPPZ)3 content of 4 wt% still exhibited decent performance with QEext and LE of 8.9% and 8.4 cd/A at a current density of 30 mA/cm2, indicating the promising application as an efficient solution processing emissive layer for PhOLEDs.

1

10

Current Density (mA/cm2) Fig. 8. J–LE curves of the devices with different blend radios in the structure of ITO/ PEDOT/CBP:Ir(MPCPPZ)3/TPBI/Ba/Al.

4. Conclusion We introduced highly efficient red emission PhOLEDs based on the solution processed emissive layer. High quality blend films from CBP host and Ir(MPCPPZ)3 guest could be obtained in chloroform/ chlorobenzene mixing solvent. By optimizing the concentrations of Ir(MPCPPZ)3, it was demonstrated that when the concentration of Ir(MPCPPZ)3 was fixed at 4%, the films showed pretty high PL quantum efficiency of 96%. The quantum efficiency of the device with the structure of ITO/PEDOT:PSS (50 nm)/CBP:Ir(MPCPPZ)3 (55 nm)/Ba(4 nm)/Al(120 nm) is 1.4%, and the quantum efficiency can be significantly increased with one order magnitude higher after the incorporation of an additional TPBI layer as hole blocking layer, up to 19.3%. The result indicated that the obtained PhOLEDs based on the solution processed emissive could be promising candidates for red light emitting diodes.

Acknowledgment The authors acknowledge the financial support by the NSFC (Nos. 60937001 and 61036007), 973 project (2009CB623600) and the China Postdoctoral Science Foundation (2011M500131)

B. Liu et al. / Journal of Luminescence 142 (2013) 35–39

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