Enhancing efficiency of organic light-emitting diodes using a. carbon nanotube-doped hole injection layer

Transcription

1 Enhancing efficiency of organic light-emitting diodes using a carbon nanotube-doped hole injection layer Shui-Hsiang Su*, Wang-Ta Chiang, Chung-Ting Kuo, Yu-Che Liu, Meiso Yokoyama Department of Electronic Engineering, I-Shou University 1, Section 1, Hsueh-Cheng RD, Ta-Hsu Hsiang, Kaohsiung County, Taiwan * shsu@isu.edu.tw Abstract Nanocomposite layers consisting of multi-walled carbon nanotubes (MWCNTs)-doped poly(3,4,-ethylene dioxythiphene):polystyrene sulfonic acid (PEDOT:PSS) were employed as a hole injection layer (HIL) in organic light-emitting diodes (OLEDs). The structure of OLED is glass/indium-tin oxide (ITO)/MWCNTs-doped PEDOT:PSS/PEDOT:PSS/ tris(8-hydroxyquinolinato)aluminium (Alq 3 )/LiF/Al. The luminous efficiency of the OLED is as high as 2.1 cd/a, which is 70% higher than that of a conventional device without a MWCNTs-doped HIL. A device with a hole-only structure, ITO/MWCNTs-doped PEDOT:PSS/Al, was constructed to elucidate the mechanism of carrier injection and transporting. The MWCNTs-doped PEDOT:PSS layer exhibits a low carrier transporting ability. Keywords: Multi-wall carbon nanotubes; PEDOT:PSS; organic light-emitting diodes

2 Introduction Efficient organic light-emitting devices (OLEDs), that based on small molecular organic materials or polymers, have attracted extensive research interest because of their potential application to full-color flat panel displays. Significant progress has recently been made in improving the characteristics of device s structure for emissive display applications [1,2]. Fluorescent emission in OLEDs is associated with the radiative recombination of singlet excitons. One of them is an imbalance of electron and hole mobilities [3,4], which results in a shift of the recombination zone toward an electrode, lowering the device efficiency due to exciton quenching by the metal electrode. Polymer/carbon nanotube (CNT) nanocomposite has been regarded as a solution to low electron mobility [5-8], and the polymer/cnt junctions act as exciton quenching centers, improving charge transport and quenching photoluminescence [9-12]. More balanced charge injection is associated with higher external quantum efficiency of the OLED. A key discovery of recent years is that doping improves emission efficiency and intensity. The predominant electronic interaction in the nanocomposite s is known to be polymer-to-cnt photoinduced charge (electron) transfer [13-15], while photoinduced energy (exciton) transfer has been suggested to occur in the

3 polymer/cnt bilayer [16]. However, while various potential applications of CNTs have been suggested, few practical uses have been realized. The main difficulties concern the poor purity and process ability of MWCNTs that contain powder. In this investigation, the prepared MWCNTs were purified and then doped as a very small fraction of up to 2.5 wt% into extensively used PEDOT:PSS. Double-layer and hole-only devices were fabricated, and the device characteristics were studied to examine the role of MWCNTs in OLEDs. Experimental Details The MWCNTs in this investigation were synthesized using chemical vapor deposition (CVD) and purified following a standard procedure. 100mg of MWCNTs powder was added to 50ml of a mixture of sulfuric acid and nitric acid in a 3:1 ratio, and the mixture was stirred for 15min on a hot plate at 100 C. The suspension was then diluted to 200ml. Finally, the MWCNTs were collected by membrane filtration (0.45 μm pore size), and washed with sufficient deionized water to remove residual acids. The PEDOT:PSS solution was filtered through a 0.45 μm polyvinyl difluoride (PVDF) syringe filter. Purified MWCNTs was doped into PEDOT:PSS solution with concentrations

4 of 0, 0.7, 1.5 and 2.5 wt%. The mingle solution in an ultrasonic bath for approximately 24 hr. OLEDs were fabricated as follows. ITO-coated glass substrates (with a sheet resistance of 7Ω/ ) were cleaned by ultrasonic in a detergent solution and deionized water. A HIL of MWCNTs-doped PEDOT:PSS nanocomposite was spin-coated on the optically transparent ITO glass substrates at 3000 rpm, which were then backed for 1 hr at 120 C in a vacuum oven. The other layers (organic layer and Al cathode) of the devices were fabricated by conventional thermal evaporation in a high vacuum chamber at a base pressure of Torr without breaking the vacuum. The deposition rate of the organic layer was 0.05 nm/s and that of the Al metal cathode was 0.5 nm/s. Figure 1 shows the structure and energy-level diagrams of the OLEDs. The device consists of a 120 nm-thick anode layer of ITO, a 60 nm HIL of MWCNTs-doped PEDOT:PSS, a 60 nm emissive layer and electron-transporting layer (ETL) of Alq 3, a 0.7 nm electron injection layer of lithium fluoride (LiF) and a 150 nm cathode layer of aluminum (Al). The transmission electron micrographs of MWCNTs-doped PEDOT:PSS nanocomposite s film were obtained using an FEI Tecnai transmission electron microscope (TEM) with a point-to-point resolution of 0.14 nm. The surfaces of the films were analyzed using atomic force microscopy (AFM). The

5 current-voltage characteristics were measured using a Keithley 2400 source meter, and the luminance of OLEDs was measured using a PR-650 spectrometer. All the measurements were made in the dark at room temperature.

6 Results and discussion Untreated pristine MWCNTs were doped into the PEDOT:PSS to act as an HIL in an OLED. However, poor optoelectronic characteristics of OLED have been observed- even an absence of light emission at a very high driving current. Several investigations have suggested that acid treatment can cut MWCNTs and shorten them, while reducing the diameter of MWCNTs [17-20]. It can also remove amorphous carbon and contaminating metals. Figure 2 shows TEM images of MWCNTs after acid treatment. As shown in Fig. 2(a), acid treatment removed contaminating metallic catalyst from the top of the graphite tube wall. Figure 2(b) demonstrates that treatment with 1:3 HNO 3 :H 2 SO 4 acidic solution for 48 hr produces a sharp graphite tube wall, indicating that the MWCNTs had been cut off and thereby shortened. Figure 3 plots the J-V characteristics of OLEDs with PEDOT:PSS doped with various concentrations of MWCNTs. As the fraction of MWCNTs increases, the current density declines and the turn-on voltage increases. The purification of the prepared MWCNTs is critical to making the MWCNTs an effective material in the fabrication of OLEDs. Impurities such as amorphous and metallic catalytes in solution remarkably degrade the physical and chromatic characteristics of the device - especially at the interface and inside the bulk.

7 Figure 4 plots the luminance-current density characteristics of the OLEDs. OLEDs with MWCNTs-doped PEDOT:PSS HIL exhibit higher luminance. The OLEDs with 1.5 wt% MWCNTs-doped PEDOT:PSS HIL had a luminance of 1650 cd/m 2 at a current density of 100 ma/cm 2. Figure 5 presents the calculated luminous efficiency. The overall trend in which associated property in the observed luminance of the undoped devices equals that of the doped ones. The luminous efficiency of MWCNTs-doped devices increases with the concentration of MWCNTs. One plausible cause is the retard of hole-transporting and polymer-nanotube interactions, which rigidify the polymer chains. The device with 1.5 wt% doping has higher luminous efficiency. However, the luminous efficiency of the 2.5 wt% MWCNTs-doped device falls sharply as the current density rises above 30 ma/cm 2. Doping PEDOT:PSS with excess MWCNTs may destroy the polymer chain, reducing luminous efficiency at high current density. The variation of the surface roughness caused by MWCNTs-doping in PEDOT:PSS might affect the performance of OLEDs. An OLED employing an MWCNTs-doped PEDOT:PSS HIL covered by PEDOT:PSS had been prepared. Luminous efficiency can be increased to 2.1 cd/a at a current density of 60 ma/m 2. It is 70% higher than that of the conventional device without a MWCNTs-doped

8 PEDOT:PSS HIL. The surface roughness of MWCNTs-doped PEDOT:PSS on ITO was investigated by the tapping mode AFM and shown in Figs. 6 (a) and (b). The root-mean-square (RMS) surface roughness of MWCNTs-doped PEDOT:PSS was approximately nm, whereas bestrew smooth layer the RMS of the PEDOT:PSS was approximately 1.74 nm. Obviously, the smooth layer can provide the better surface contact between HIL and EML interface. It was reported that the HIL with smaller roughness led to the enhanced device performance. It is well known that the driving voltage depends on the carrier injection and transporting, while current efficiency depends not only on carrier injection, but also on the balance of electrons and holes. To verify the hole-transporting ability of the MWCNTs-doped PEDOT:PSS HIL, the devices with hole-only structures were fabricated by sandwiching the MWCNTs-doped PEDOT:PSS or pristine PEDOT:PSS films between two electrodes (ITO and Al) and their electrical properties were examined. The device structure was ITO/MWCNTs-doped PEDOT:PSS/Al. The concentrations of MWCNTs were 0 and 2.5 wt%. Figure 7 plots the current density voltage curves of the hole-only devices. At the driving voltage of 5 V, 2.5 wt% MWCNTs-doped PEDOT:PSS device produced a current density of 318 ma, which was much lower than 645 ma for the

9 undoped device. On the other hand, under the current density of 200 ma/cm 2, the voltage drop of 2.5 wt% MWCNTs-doped PEDOT:PSS device is 3.3 V, which is much larger than 1.7 V for the undoped device. This indicates that the MWCNTs-doped PEDOT:PSS layer exhibits a low carrier transporting ability. Conclusion This study investigated the feasibility of using MWCNTs-doped PEDOT:PSS as an HIL in OLEDs. The structure of OLED was glass/ito/mwcnts-doped PEDOT:PSS/PEDOT:PSS/Alq 3 /LiF/Al. Research into a nanocomposite of PEDOT:PSS and MWCNTs has clearly demonstrated that the MWCNTs can be easily purified, processed and used as an effective material in OLEDs. The luminous efficiency of the OLED is 2.1 cd/a, which is 70% higher than that of a conventional device without a MWCNTs-doped PEDOT:PSS HIL. A smooth layer covering on the MWCNTs-doped PEDOT:PSS improves the surface roughness and further enhances the luminous efficiency. The hole-only device reveals that the MWCNTs-doped PEDOT:PSS layer exhibits a low carrier transporting ability. MWCNTs can be a candidate in such a form in organic solar cells or other organic optoelectronic devices. Acknowledgements

10 The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC E and NSC E The authors would also like to thank the MANALAB at ISU, Taiwan.

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13 Figure captions Fig. 1 (a) Structure and (b) energy-level diagram of OLEDs. Fig. 2 TEM images of MWCNTs treated with acidic H 2 SO 4 /HNO 3 mixture. Fig. 3 J-V characteristics of OLEDs with various weight percentages of MWCNTs doped in PEDOT:PSS films. Fig. 4 Luminance-current density characteristics of OLEDs with various weight percentages of MWCNTs doped in PEDOT:PSS films. Fig. 5 Luminous efficiency-current density characteristics of OLEDs with various weight percentages of MWCNTs doped in PEDOT:PSS films. Fig. 6 AFM images of (a) ITO/MWCNTs-doped PEDOT:PSS and (b) ITO/MWCNTs-doped PEDOT:PSS/PEDOT:PSS. Fig. 7 Current-voltage characteristics of hole-only devices. The inset is structure of the hole-only device.

14 e - LiF/Al Alq 3 MWCNTs-doped PEDOT:PSS ITO Glass ITO 4.7ev PEDOT :PSS MWNT 3.1ev Alq 3 LiF/Al 3.5ev h + 5.2ev 5.7ev Figure 1 Su et. al.

15 (a) (b) Figure 2 Su et. al.

16 500 Current Density (ma/cm 2 ) wt% 0.7wt% 1.5wt% 2.5wt% Voltage (V) Figure 3 Su et. al.

17 Luminance (cd/m 2 ) wt% 0.7 wt% 1.5 wt% 2.5 wt% Current Density (ma/cm 2 ) Figure 4 Su et. al.

18 Luminous Efficiency (cd/a) wt% 0.7wt% 1.5wt% 2.5wt% 1.5wt%+smooth layer Current Density (ma/cm 2 ) 100 Figure 5 Su et. al.

19 (a) (b) Figure 6 Su et. al.

20 1200 Current Density (ma/cm 2 ) wt% 2.5 wt% Voltage (V) 10 Figure 7 Su et. al.