Ordered and ultrathin reduced graphene oxide LB films as hole injection layers for organic light-emitting diode
© Yang et al.; licensee Springer. 2014
Received: 16 May 2014
Accepted: 19 September 2014
Published: 1 October 2014
In this paper, we demonstrated the utilization of reduced graphene oxide (RGO) Langmuir-Blodgett (LB) films as high performance hole injection layer in organic light-emitting diode (OLED). By using LB technique, the well-ordered and thickness-controlled RGO sheets are incorporated between the organic active layer and the transparent conducting indium tin oxide (ITO), leading to an increase of recombination between electrons and holes. Due to the dramatic increase of hole carrier injection efficiency in RGO LB layer, the device luminance performance is greatly enhanced comparable to devices fabricated with spin-coating RGO and a commercial conducting polymer PEDOT:PSS as the hole transport layer. Furthermore, our results indicate that RGO LB films could be an excellent alternative to commercial PEDOT:PSS as the effective hole transport and electron blocking layer in light-emitting diode devices.
KeywordsReduced graphene oxide Conducting polymer LB films OLED Hole injection layer
The two-dimensional (2D) single-layer carbon material, graphene, has emerged as a rising star in the field of materials . Owing to its unique electrical, chemical, and mechanical properties , graphene has been developed for various applications in optoelectronics , sensors [4, 5], and electrochemistry [6, 7]. Meanwhile, many studies on graphene-based photovoltaic applications have been carried out, in which graphene was used as active layer materials, electron transport bridges, and transparent electrodes [8–10]. However, the general approaches used to prepare graphene, for instance CVD thermal evaporation, result in high cost fabrication process [11, 12].
As the surging interest in graphene-based materials, graphene oxide (GO) has regained significant attention as a solution-processable precursor for bulk production of graphene used on transparent conductors and supercapacitors . The reduced graphene oxide (RGO) can be obtained by reducing GO through chemical and thermal treatment . It has been also demonstrated that RGO exhibits high mechanical strength, as well as combined with interesting physical properties, including high performance of electrical, thermal conductivity, and electrochemical activity [15–17].
Due to the high transparent and electrical performance, RGO has been utilized as an electrode layer for optoelectronic devices such as organic light-emitting diode (OLED) and organic photovoltaic devices [18, 19]. RGO film deposition methods, such as spin-coating, spray coating etc., have been demonstrated as effective methods to deposit RGO on indium tin oxide (ITO) electrode as a hole injection layer [20, 21]. However, it is still a challenge to address ordered and thickness-controlled arrangement of RGO on ITO as high performance hole injection layers. The arrangement feature of carbon sheets results in the remarkable change of electrical ability that influences the performance of OLED devices dramatically. So, it is worthwhile to obtain well-arranged RGO sheets on ITO as hole injection layer, which provide a superior hole injection performance to prepare OLED devices with high luminescent efficiency.
It is well known that GO can float on a water surface without the need for surfactants or stabilizing agents and some reports about the Langmuir-Blodgett (LB) deposition of GO films have been reported [22, 23], but few works focused on the optoelectronic electrode applications of these GO-based LB films. In our previous work, a RGO/conducting polymer composite was prepared as high performance electrochemical capacitor electrode . In this paper, we demonstrate the preparation of a well-ordered and thickness-controlled RGO layer on ITO surface as hole injection layer by using the LB method. This RGO hole injection layer offers tunable arrangement and loading of RGO on a substrate. Limited work is focused on fabricating RGO LB layers as hole injection layer and the performance of related device. Owing to the flexible nature of RGO, the LB deposition technique can substantially suppress the folding and wrinkling of graphene oxide sheets, and the sheets are able to be transferred onto a substrate with defined structure, which could provide preferred film structure for effective hole carrier injection.
Graphite flakes used for GO preparation were purchased from Sigma-Aldrich (St. Louis, MO, USA). GO was synthesized from natural graphite flakes through Hummer's method . The size of graphite flakes was 380 μm (grade 3061). In order to obtain stable GO dispersion for LB deposition, 20 mg GO was dissolved in 80 ml methanol/deionized (DI) water (volume ratio 4:1) mixture solution, and the solution was subjected to ultrasonication for 30 min followed by centrifugation at 2,500 rpm. N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) and Tris(8-hydroxyquinolinato)aluminium Alq3 were also purchased from Sigma-Aldrich. A commercial conducting polymer PEDOT:PSS (product code as Clevios™ P Jet) was purchased from Bayer company (Leverkusen, Germany). Aluminum as a cathode was purchased from Dongyang Inc. (Shenzhen, China). All solvents used in experiment are high purity level.
Film and device fabrication
The preparation of different-layer GO sheets was carried out in a KSV-5000 LB system (KSV Instruments Ltd., Helsinki, Finland). The self-assembly performance of GO at air-water interface was characterized by a BAM-300 Brewster angle microscopy (KSV Instruments Ltd., Helsinki, Finland). Before the film preparation, the trough was carefully cleaned with chloroform and then filled with DI water. GO solution was dropwise spread onto the water surface by using a glass syringe. Surface pressure was monitored through a tensiometer attached to a Wilhelmy plate. The film was compressed by the barriers at a speed of 1 mm/min. The GO monolayers were transferred to substrates at various points during the compression by vertically dipping the substrate into the trough and slowly pulling it up (1 mm/min). The substrate was first processed with a hydrophilic treatment in order to deposit uniform GO LB layers. By repeating this vertical dipping process, different-layer GO sheets were deposited on substrate uniformly with a layer-by-layer structure. After the deposition of GO LB films, the substrate was treated in a water vapor oven at 200 Co for 6-h GO reduction. Surface morphology of GO and RGO films were investigated by SP 3800 atomic force microscopy (AFM; Seiko Instrument Industry Corporation, Tokyo, Japan) with a tapping mode. The morphological properties of GO and RGO were also investigated with Hitachi S-2400 scanning electron microscopy (SEM; Hitachi, Ltd., Chiyoda, Tokyo, Japan). UV-vis spectrum of the film was recorded on a UV 1700 spectrometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan).
After the preparation of RGO on substrate, the following active layers for OLED were prepared through spin-coating method. The TPD and Alq3 solutions were spin-coated on RGO-covered substrate at 2,000 rpm. The devices used a 50-nm TPD as the hole transport layer and 50-nm Alq3 as the electron transport layer. After the deposition of active layer, an Al cathode electrode was deposited onto the active layer by thermal evaporation in vacuum with a thickness of 60 nm. The OLED with a structure of ITO/RGO LB films/TPD/Alq3/Al-electrode was fabricated, and the device performance was measured at room temperature. For electrical performance testing of GO and RGO films, A Si/SiO2 substrate with prepatterned electrodes was used for current-voltage (I-V) measurement. The interdigitated electrodes had a 15 and 30 μm channel length. The I-V curves were characterized by a Keithley 4200 semiconducting testing system (Cleveland, OH, USA).
Results and discussion
Performance of devices with different films as hole injecting layer
Driving voltage (V) for 100 mA/cm2
Max luminance (cd/m2, 12 V)
Luminance efficiency (cd/A, 12 V)
PEDOT:PSS spin-coating film
GO spin-coating film
RGO spin-coating film
RGO LB film
Influence of heating temperature during thermal reduction on device luminance performance
Heating temperature (°C)
Driving voltage (V) for 100 mA/cm2
Max luminance (cd/m2, 12 V)
Luminance efficiency (cd/A, 12 V)
The LB assembly and a following reduction process produce the high-conductivity and well-ordered-structure RGO LB films. The results of I-V tests indicate that the thermal treatment changes the electrical performance of GO films dramatically. The RGO LB films are successfully incorporated between ITO and active layer as a hole injection layer. The incorporation of well-ordered and thickness-controlled RGO leads to an increase in recombination of electrons and holes as well as the block to the electrons. Our results indicate that RGO LB films are an excellent alternative to commercial PEDOT:PSS as the effective hole transport and electron blocking layer in OLED for improving luminance efficiency.
WY and XY are students of a Master's degree at the School of Optoelectronic Information, University of Electronic Science and Technology of China. YY and SL are associate professors at the School of Optoelectronic Information, University of Electronic Science and Technology of China. JX and YJ are professors at the School of Optoelectronic Information, University of Electronic Science and Technology of China.
The work was supported by the National Science Foundation of China (NSFC) (No. 61101029, 61371046, and 61471085), a plan funding for supporting the New Century Talents (No. NCET-12-0091).
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