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Hybrid film of single-layer graphene and carbon nanotube as transparent conductive electrode for organic light emitting diode
Synthetic Metals 257 (2019) 116186
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Hybrid film of single-layer graphene and carbon nanotube as transparent conductive electrode for organic light emitting diode
T
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Pradeep Kumara,c, Kai Lin Woond, , Wah Seng Wongd, Mohamed Shuaib Mohamed Saheedb,c, ⁎⁎ Zainal Arif Burhanudina,c, a
Department of Electrical and Electronic Engineering, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia c Center of Innovative Nanostructures and Nanodevices (COINN), Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia d Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, Kuala Lumpur, 50603, Malaysia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Carbon nanotube OLED Transparent conductive electrode
Single layer graphene as a transparent conductive electrode (TCE) suffers from high sheet resistance, low work function, and hydrophobicity. In this work, a hybrid TCE of single-layer graphene, carbon nanotube (CNT) and gold nanoparticles (Au NP) decoration is proposed. The hybridization of graphene with CNT enhances the electrical conductivity while maintaining its high transmittance. The Au NP decorated TCE shows a sheet resistance of ∼100 Ω/sq and transmittance of 96%. The potential of TCE is demonstrated in a solution-processed yellow organic light emitting diode (OLED). By introducing a thin layer of Al-doped zinc oxide between the Au NPs and graphene-carbon nanotube networks which increases the hydrophilicity of the surface, OLED with a current efficiency of ∼2.1 cd/A and a turn-on voltage of ∼5 V is obtained suggesting that this TCE can be a viable option for OLED-anode.
1. Introduction Graphene is a two-dimensional crystal of carbon with potential applications in flexible electronics. The use of graphene as a transparent conductive electrode (TCE) is attractive in various optoelectronic applications where flexibility is required. In organic electronic devices such as bottom-emission organic light emitting diodes (OLEDs), indium tin oxide as a TCE is often the first layer where the whole device is built upon. Despite the brittleness of ITO and rareness of indium on earth, it is often used in various device structures such as solar cell [1], super capacitor [2], touch screen [3], LED [4], and OLED [5]. Replacement of ITO with other alternative TCE such as graphene can be a challenge. Firstly, single layer graphene (SLG), despite its high transparency (∼98%) and high intrinsic carrier mobility (∼2 × 105 cm2/Vs) [6], exhibits a high sheet resistance from 600 Ω/sq [7] to 6,000 Ω/sq [6] limiting its application as a TCE. Several alternatives such as carbon nanotubes (CNTs) [8], conducting polymers [9], metallic nanowires (NWs) [10] are also widely investigated for TCE applications with each has its own drawbacks. For CNT networks, high junction resistance between CNTs [11,12] is found to be limiting its
conductivity. Although doping poly (3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS) with silver NWs can provide the high surface conductivity [13], the high surface roughness of silver NWs can affect the reliability of the device. The doping of monolayer graphene or multilayer graphene using chemical (HNO3 and AuCl3) or highly p-type dopants like boron [14–16] is an effective approach to dramatically improve the luminance and current efficiency of OLEDs. However, due to the hydrophobicity of the graphene surface, the conformal coating cannot be achieved. In this work, we investigated the plausibility of hydrophilic composite TCE by combining SLG, CNT, Al-doped zinc oxide (AZO) and gold (Au) nanoparticle (NP) in order to achieve the desired properties. The presence of CNT – networks increase the electrical conduction of graphene while sustaining its high optical transmission. For further improving the sheet resistance of hybrid TCE, the partial decoration of TCE with Au NPs was adopted. The sheet resistance of ∼100 Ω/sq and transmittance of ∼96% are obtained by decorating the Graphene-CNT (GCNT) hybrid TCEs with Au NP. The wettability of the thin film is enhanced by placing a hydrophilic AZO layer between hybrid TCE and Au NPs. Using this electrode, a solution processable OLED exhibiting a
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Corresponding author. Corresponding author at: Center of Innovative Nanostructures and Nanodevices (COINN), Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak, Malaysia. E-mail addresses: [email protected] (K.L. Woon), [email protected] (Z.A. Burhanudin). ⁎⁎
https://doi.org/10.1016/j.synthmet.2019.116186 Received 22 July 2019; Received in revised form 28 August 2019; Accepted 23 September 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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turn voltage of ∼5 V and efficiency as high as ∼2.1 cd/A is demonstrated.
Surface morphology of GCNT hybrid structure was analyzed by Field emission scanning electron microscopy (FESEM, Zeiss Supra55 VP). The quality of the hybrid film was examined by Raman spectroscopy (Horiba Jobin Yvon HR800, laser source length of 514 nm). After analyzing the hybrid nanostructure, the optical and electrical properties of GCNT hybrid film were investigated by measuring the transmittance (T), and sheet resistance (Rs), using Agilent Cary 100 UV-VIS spectroscopy and four-point probe measurement system (Lucas Lab 302 and Keithley 2400 DC source meter), respectively. For transmittance measurement, GCNT hybrid film of size (2.5 cm × 2 cm) was transferred to a microscopic glass slide. For sheet resistance measurements, FESEM and Raman analysis, (1 cm × 1 cm) GCNT hybrid film was transferred on SiO2/Si substrate. HRTEM samples were prepared by transferring the as-grown GCNT film on the standard copper grid. The roughness of films was measured by atomic force microscopy (AFM) analysis. The performance of any TCE is examined on the basis of standard parameters which are T and Rs. But both parameters cannot be well analyzed without specifying any link between them. This link is known as a figure of merit (FoM) [19]. FoM is considered as a trustworthy method to differentiate the graphitic films and transparent conducting films [6,20]. Theoretically, FoM is defined as the ratio of DC conductivity (σDC ) to optical conductivity (σop ) as [19,20],
2. Experimental 2.1. Materials Highly purified (98%) CNT powder was purchased from US Research Nanomaterials Incorporation, USA. 35 μm thick copper foils were provided by Graphene Platform. N-Methyl-2-pyrrolidone (NMP) 99% and Auric Chloride (AuCl3) 99.999% were bought from Sigma Aldrich. In addition, Iron Chloride (FeCl3), acetone and 2-Propanol (IPA) (98%) were procured from Merck KGaA. 2.2. Development and characterizations of TCEs The GCNT hybrid TCE was prepared by fusing the graphene with CNT network. Initially, highly purified CNT powder was dispersed in NMethyl-2-pyrrolidone (NMP) solvent using ultrasonication. The CNT suspension in NMP was kept for 24 h in an ambient environment so that the heavy particles would settle down. Few drops of (0.2 mg/ml) CNT/ NMP suspension were then dropped on Cu foil. The Cu foil, as well as drops of CNT/NMP suspension, was then spin-casted at a rotation speed of 1000 rpm to form a network of CNTs on Cu foil. The metallic surface of Cu acts as a catalytic medium to grow the graphene. The CNT network on Cu foil was put on the hot plate at 60℃ and the temperature was gradually increased up to 130℃. The sample was heated at 130 ℃ for 30 min to evaporate NMP residues and minimizing the possible agglomeration of CNTs. The dispersed CNT network on Cu foil was then placed in the furnace of thermal-chemical vapor deposition (T-CVD) system for graphene synthesis. The pressure in the furnace was lowered to several millitorrs at the beginning of the graphene synthesis process. At 1000 ℃, methane (CH4) gas which acts as a source of carbon was allowed to flow into the T-CVD furnace. At this high temperature, CH4 gas is decomposed into carbon ions which later form a two-dimensional (2D) surface across the dispersed CNT network on Cu foil. The carbon structure which is called graphene on Cu foil was then rapidly cooled to 200 ℃. At room temperature, the sample was taken out from the furnace. For transferring GCNT hybrid film on the target substrates such as SiO2/Si and microscopic glass slides, the hybrid film was spin-coated with poly(methyl methacrylate) (PMMA) followed by baking on a hot plate at 80 ℃ for proper adhesion of PMMA with GCNT hybrid film. The structure of PMMA/GCNT/Cu was then put in the FeCl3 solution to etch Cu foil. The residues of Cu were removed by using 2% diluted hydrochloric acid (HCl). Afterward, PMMA/GCNT structure was cleaned with deionized water (DI water) before scooping out with a target substrate. PMMA was then removed by immersing the PMMA/GCNT/substrate into acetone followed by cleaning with 2-isopropanol (IPA) and DI water. GCNT hybrid film on the substrate was baked at 80 ℃ and 130 ℃ for 5 min and 20 min, respectively for better contact adhesion. The synthesis and transfer processes of GCNT hybrid film are schematically illustrated in Fig. 1. In this work, the pristine graphene was synthesized and transferred to target substrates by following the same method as for GCNT hybrid films. Gold nanoparticle decoration of GCNT hybrid film was accomplished by spin casting the 10 mM aqueous solution of AuCl3 on the GCNT hybrid TCE at 3000 rpm for 60 s. The modified GCNT TCEs were then patterned using CO2 laser ablation at 13% power level of maximum power (30 W). 0.1 M AZO solution was synthesized using the solgel method as reported in the references [17,18]. The transparent thin interlayer of AZO (40 nm) was prepared by spin casting on GCNT hybrid TCE at 3500 rpm for 60 s followed by annealing at 500 ℃ for 1 h in open air conditions. The structure of the GCNT hybrid film was investigated by highresolution transmission electron microscopy (HRTEM, Zeiss Libra 200).
FoM =
σDC Z 1 = 0 σop 2RS (T −0.5 − 1)
(1)
where Z0 is the impedance of free space. The high value of FoM is an indication of high-quality TCE. 2.3. Device fabrication and characterizations OLED device structure is being considered in this work, consists of GCNT hybrid TCE as anode/poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS, 80 nm) as hole injection layer (HIL)/ super yellow (SY-PPV) (80 nm) as emissive layer (EML)/ lithium fluoride (LiF, 0.8 nm) as electron injection layer (EIL)/ aluminum (Al, 100 nm) as cathode. For device fabrication, the patterned TCE was initially cleaned with acetone and IPA sequentially. PEDOT: PSS was then spin-casted on top of anode layer of modified GCNT hybrid TCE at ambient atmosphere. Then, the thin film was immediately baked at 150 °C for 10 min in N2 environment. The SY-PPV dissolved in toluene was spin-casted on top of PEDOT: PSS layer to form an 80 nm EML. Then the structure was baked at 120 °C for 10 min. Finally, the EIL and the cathode of LiF (0.8 nm) and Al (100 nm), respectively were vacuum deposited at a base pressure of 2.5 × 10−4 mbar without breaking the vacuum. All the devices were encapsulated using UV curable epoxy and glass lid in order to avoid the oxygen and moisture from the environment. Performances of the OLED devices were evaluated through the current (I) - voltage (V) – brightness or luminance (L) characteristics, turn-on voltage (VON) and current efficiency (CE). I-V-L characteristics were measured using Konica Minolta CS-200 integrated with Keithley 2612b source-meter. The film thickness was measured using P-6 profilometer (KLA-Tencor). 3. Results and discussion 3.1. Morphological and structural characterizations of GCNT hybrid TCE The quality of thin film is important for low sheet resistance. FESEM and HRTEM are employed to characterize the morphology and the structure of the GCNT hybrid film as illustrated in Fig. 2. The pristine graphene was found to be uniform and continuous free from any void, crack, ripple, and wrinkle as shown in FESEM image in Fig. 2(a), indicating high-quality graphene has been synthesized by CVD method. As seen in Fig. 2(b), the GCNT hybrid nanostructure, the graphene, and CNTs were fused with each other (shown clearly in the inset at a higher 2
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Fig. 1. Synthesis and transfer of GCNT hybrid film on the target substrate.
(FWHM) ∼34.8 of 2D-peak for GCNT hybrid film in comparison to the pristine graphene (∼38) and CNT (∼82) indicates a better crystallinity of hybrid TCE [28]. The effect of Au NP-decoration on GCNT hybrid film was also interpreted by the respective Raman spectrum. The I2D/IG ≈ 2.33 and ID/ IG ≈ 0.44 values indicate that high crystalline graphene is still maintained after Au NP-decoration. The significant blue and redshifts in G, D and 2D peaks of Au NP-decorated GCNT hybrid film is the result of the increased density of charge carriers [29]. A shift in its G-peak ascertains the evidence of carrier transfer on GCNT hybrid film after decoration with gold NPs. The G-peak was found to be blue shifted by ∼8 cm−1 after p-type doping using AuCl3 aqueous solution. The blueshift in Gpeak of Au-decorated GCNT hybrid film indicates the transfer of electrons to gold particles from GCNT hybrid film. As a result, the hole concentrations on GCNT hybrid film increases driven by the potential difference between ionic AuCl−4 and graphene [7,21]. Furthermore, the lowest FWHM value of Au NP-decorated GCNT hybrid film indicates the crystallinity is preserved even after Au NP-decoration. The quality of the TCEs was further verified by analyzing the distance between LD2 = neighboring defects (LD) using equation 4 − 9 [(1.8 ± 0.5) × 10 × λD × (IG / ID ) [22], where, λD is the laser excitation wavelength. The calculated values of LD were found to be 46 ± 6 nm, 27 ± 4 nm, and 17 ± 2 for pristine graphene and GCNT hybrid and Au NP-decorated GCNT hybrid TCEs, respectively as shown in Fig. 4(b), indicating the existence of minimum defects in decorated hybrid TCE.
magnification). Most of the empty space among the randomly scattered CNTs (diameters ∼ 8–26 nm, density ∼146/ μm2) were seen to be bridged by as-grown graphene surface. Thus, it is expected that in GCNT hybrid structure, the presence of interlinked CNT network strengthens the conductivity of the overall thin film. Fig. 2(c) and (d) depict the HRTEM images of the graphene-CNT hybrid structure, where, the CNT is surrounded by graphene surface. This fact is further confirmed by selected area electron diffraction (SAED) pattern of GCNT hybrid film as shown in the inset of Fig. 2(d). The existence of CNT and graphene are confirmed by the presence of ring pattern of (002) graphitic lattice and six-fold symmetric hexagonal pattern, respectively suggesting that high crystallinities of graphene and CNT are maintained after hybridization. High magnification view of the GCNT interface indicates that the CNT structure has ∼13 walls with an outer diameter of ∼10 nm. The GCNT hybrid film surface partially decorated with gold nanoparticle (density ∼1.3 × 106/cm2, diameter 20 nm to 37 nm) is shown in Fig. 3(a) and (b) with a surface coverage of ∼2%. Decoration of gold nanoparticles on the GCNT results in electron transfer from GCNT to the gold nanoparticle [21,22] resulting in free hole carrier in GCNT hybrid film and hence, decrease in sheet resistance of the hybrid film [23]. In order to confirm the quality of the thin film, we carried out Raman spectroscopy of the thin films as illustrated in Fig. 4 with corresponding Raman features tabulated in Table S1. Raman spectrum of GCNT hybrid film exhibited a graphitic G and second-order resonance double resonance 2D peaks at 1586 cm−1 and 2693 cm−1, respectively as depicted in Fig. 4(a). The presence of CNTs in GCNT hybrid structure is indicated by the appearance of D-peak at ∼1346 cm−1 and broadening of 2D-band [24]. The corresponding ID/IG intensity ratio of 0.17 reflects the presence of high-quality graphene with minimal defects [25] in comparison to 0.5 of CNT indicating the presence of some defects. In other words, a highly crystalline graphene layer is still maintained even after hybridization with CNTs. The I2D/IG intensity ratio of 2.2 indicates the presence of monolayer graphene [24] which is ascertained by equation ωG = 1581.6 + [11/(1 + n1.6)] [26,27] that gives number of graphene layer n ≈1. The lower full-width half maximum
3.2. Optoelectrical characterizations of GCNT hybrid TCE The optical transmittance of GCNT hybrid film is presented and compared with transmittances of pristine graphene, CNT, commercially available ITO films as shown in Fig. 5(a). It was observed that GCNT hybrid film showed a high transmittance (> 95%) from the near-ultraviolet (NUV) to near-infrared (NIR) wavelengths. To evaluate the transmittance and sheet resistance of TCEs, 550 nm reference wavelength is often used [13,14,30,31] in the literature as the spectral response of the human eye is the highest [32,33]. At 550 nm, the 3
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Fig. 2. FESEM images of (a) pristine graphene, and (b) as-prepared GCNT hybrid surfaces. The insets show high magnification views of surfaces. The randomly dispersed CNTs were found to be interlinked with graphene (as the background of the image). HRTEM images of (c) GCNT hybrid film on Cu grid. (d) The high magnification image of the selected area marked by dotted lines. CNT was found to be surrounded by graphene surface. The presence of ring patterns of graphitic lattice along with hexagonal patterns as shown in SAED pattern (inset) indicates the presence of CNT and graphene structure on the surface of hybrid GCNT film.
transmittances of pristine graphene, CNT, GCNT hybrid, and ITO films were measured to be 97.1%, 96.4%, 96.6%, and 90.4%, respectively. It is quite clear that ITO film has significantly reduced transparency at near ultraviolet region. The transmittance of pristine graphene was found to be very close to the theoretical limit of monolayer graphene that is 97.7%, indicating high-quality monolayer graphene in our sample [34–36]. The transmittance of GCNT film reduced slightly (∼0.5%) compared to pristine graphene as the reduction is most likely attributed by CNT. After gold nanoparticle decoration, the transmittance of the hybrid film was decreased by 0.5% at 550 nm. The loss in the transparency of hybrid film after decoration is attributed to Au NPs which act as scattering centers for the light [37,38]. The transmittance of GCNT film with AZO interlayer and gold nanoparticle was found to be ∼94% at 550 nm. The measured average RS of TCEs is represented in Fig. 5(b). After hybridization, a lower average RS of ∼272 Ω/sq was measured from GCNT hybrid film in comparison to pristine graphene (RS ∼460 Ω/sq). ∼41% reduction in RS was observed after hybridizing graphene with
CNT networks. This improvement in RS of the hybrid film is attributed to the drop in the contact resistance between graphene and CNT caused by CVD synthesis of graphene across CNTs [39,40] indicating the presence of well-connected CNT networks [25]. The RS of GCNT hybrid film was further improved using gold nanoparticle decoration. After Audecoration, average RS of GCNT hybrid film was measured to be ∼115 Ω/sq, showing a 57% improvement in RS of GCNT. The improvement in the conductivity (reduction in sheet resistance) of hybrid TCE is attributed to the charge carrier transfer from gold nanoparticle to GCNT hybrid TCE [14,21,38]. The charge carrier transfer is mainly governed by work function difference between gold particles (∼5.1–5.54 eV) and graphene based TCE (∼4.5 eV). The electron transfers from GCNT results in the excess holes at GCNT - Au NP interface, leading to p-type behavior of GCNT hybrid TCE. Therefore, GCNT hybrid film became p-doped with AuCl3 during the reduction of Au atoms, leading to the enhancement of electrical conductivity (reduction of sheet resistance) (Fig. S1 and equations S1-S3). After adding the interlayer of AZO layer on top of GCNT hybrid film followed by gold
4
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Fig. 3. (a) The gold nanoparticle decorated GCNT hybrid film. The background is GCNT hybrid film which is partially decorated by Au nanoparticles (white dots).b) higher magnification view of the gold decorated surface of GCNT hybrid film.
the FoM of the as-prepared GCNT hybrid film was compared with reported alternative TCEs of ITO as mentioned in Table S2 [14,19,41–44]. It was found that Au NP-decorated GCNT hybrid film displayed a higher FoM.
nanoparticle decoration, the average RS of modified GCNT hybrid film was maintained at ∼130 Ω/sq. In order to address the fact that TCEs should be useful for a broad wavelength specially for red, green and blue OLED devices, FoM of TCEs was analyzed using Eq. (1) at different wavelength as shown in Fig. 6. At 550 nm, FoM for GCNT hybrid was found to be ∼40 as shown in Fig. 6. Around 2-fold improvement in FoM (∼73) of GCNT hybrid film was observed after Au NP-decoration. It was also noted that Au NPdecorated GCNT hybrid film had around three times higher FoM than as-grown pristine graphene (∼28) film, far above the minimum industry standard for TCE (FoM ∼35) [20]. In addition, at 470 nm, 530 nm, 580 nm, and 700 nm, FoM of Au NP-decorated GCNT TCE is more than 2-fold and 3-fold higher than that of pristine graphene and GCNT TCE, respectively, indicating its higher suitability for blue, green, yellow and red light emission OLEDs. At 400 nm, FoM of GCNT hybrid thin film scores better than commercial ITO. Even adding the AZO interlayer, a high FoM of ∼60, ∼59, and ∼65 was found at 550 nm, 400 nm, and 800 nm wavelength regions respectively. At NIR and NUV wavelengths, the GCNT hybrid film with AZO interlayer and gold nanoparticles shows a higher FoM than gold nanoparticle decorated GCNT hybrid film. To validate the optoelectrical properties of developed hybrid TCEs,
3.3. Performance evaluations of OLEDs Solution processable bottom emission OLEDs were fabricated with GCNT based hybrid TCEs as anodes. For comparison, the OLEDs based on commercial ITO and as-prepared pristine graphene were also fabricated. Fig. 7(a) illustrates the luminance and current density characteristics of fabricated OLEDs as a function of bias voltage. The maximum brightness (Lmax) of OLED on Au-decorated GCNT anode (GCNT/Au NP) anode TCE was found to be ∼100 cd/m2. It was observed to be 5-fold and a 10-fold higher Lmax than to GCNT and SLG anodes based OLEDs, respectively as presented in Table S3. This improvement in the Lmax of Au NP GCNT anode based OLED is attributed by the increased current efficiency with GCNT/AZO/Au NP achieving current efficiency as high as ∼2.03 cd/A. The brightness was further increased up to 650 cd/m2 by introducing interlayer of AZO between the hole injection layer and GCNT hybrid TCE followed by gold nanoparticle decoration. Although Lmax of OLED with GCNT/AZO/Au NP
Fig. 4. (a) Raman spectra and (b) average inter-defect distance (LD) of TCEs. The lower value of LD indicates minimal defects in Au NP-decorated GCNT and asprepared GCNT hybrid TCEs in comparison to pristine graphene TCE, possibly caused by CNTs and gold nanoparticles. 5
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Fig. 5. (a) Transmittance of TCEs for the wavelength range from NUV to NIR. The inset shows the enlarged view of transmittance characteristics pristine graphene, CNT, GCNT, and GCNT with gold nanoparticle decoration. (b) Transmittance (T) versus average sheet resistance (RS) of TCEs.
turn-on voltage (Von) of GCNT/AZO/Au NP OLED was 5.2 V which is lower than that of GCNT/Au NP (6.2 V), GCNT (6.7 V), and pristine graphene (∼6.8 V) anodes. This is mainly due to a lower sheet resistance of TCEs. The maximum current efficiencies of OLEDs with GCNT/AZO/Au NP, GCNT/Au NP, GCNT, and SLG anode TCEs was found to be 2.03 cd/A 0.81 cd/A, 0.53 cd/A and 0.25 cd/A respectively as seen in Fig. 7(b). These efficiencies were noticed to be lower than that of commercial ITO based OLED probably resulted from the combination of energetic barrier between PEDOT: PSS and hybrid GCNT structure, interfacial behavior (such as interfacial adhesion energy) a sheet conductivity of the TCEs. The water contact angle of GCNT /Au NP is reduced from 87° to 19° upon adding AZO between GCNT and Au NP whole adhesion energy increased from 0.076 J/m2 to 0.14 J/m2 (Fig. S2). These results show the superiority of GCNT hybrid TCE over reported modified graphene-based alternatives [13,41,42,45] of ITO as summarized in Table S4, demonstrating the potential of hybrid TCE as anodes for applications in organic optoelectronics. In addition, the role of AZO layer is significant for overcoming the hydrophobicity of graphene-based TCEs.
Fig. 6. Comparison of FoM of TCEs for function of wavelength range (400–800 nm).
decorated anode was found to be lower than that of commercial ITO based OLED, it was found to be around 7, 14, and 64 times higher than that of OLEDs with GCNT/Au NP, GCNT, and SLG anodes, respectively. The I-V-L characteristics for OLED with GCNT/AZO/Au NP anode are shown in Fig. 7(a) along with inset showing the device structure. The
4. Conclusion We have developed hydrophilic GCNT/ Au nanoparticles TCE with high optical transparency exhibiting sheet resistance as low as
Fig. 7. Performance of OLEDs on modified GCNT hybrid TCE anode. (a) Luminance and current density characteristics of fabricated OLEDs as a function of bias voltages. The inset shows the structure of OLED. (b) The current efficiency of OLEDs as a function of current density. 6
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∼100 Ω/sq. Adding AZO between GCNT and Au nanoparticle results in a hydrophilic surface. The potential of this TCEs is demonstrated in OLED devices exhibiting a current efficiency as high as 2.1 cd/A indicating that this TCE can serve as an alternative option of ITO for OLED-anode.
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Acknowledgments The authors thank Kang Chun Hong and Mohammad Umair Shahid for their technical support. The authors also acknowledged the financial support under Graduate Assistantship scheme from Universiti Teknologi PETRONAS, Malaysia. This work was partially funded by the Fundamental Research Grant Scheme from Ministry of Higher Education of Malaysia (Grant Number: FRGS/1/2018/TK04/UTP/02/ 4) and University Malaya UMRG Programme RP038A-17AFR, Malaysia. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116186. References [1] T. Mahmoudi, Y. Wang, Y. Hahn, Nano Energy Graphene and its derivatives for solar cells application, Nano Energy 47 (2018) 51–65, https://doi.org/10.1016/j. nanoen.2018.02.047. [2] J. Ge, G. Cheng, L. Chen, Transparent and flexible electrodes and supercapacitors using polyaniline/single-walled carbon nanotube composite thin films, Nanoscale 3 (2011) 3084–3088, https://doi.org/10.1039/c1nr10424a. [3] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. Ri Kim, Y. Il Song, Y.J. Kim, K.S. Kim, B. Özyilmaz, J.H. Ahn, B.H. Hong, S. Iijima, Roll-toroll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578, https://doi.org/10.1038/nnano.2010.132. [4] M. Oh, W.Y. Jin, H. Jun Jeong, M.S. Jeong, J.W. Kang, H. Kim, Silver nanowire transparent conductive electrodes for high-efficiency III-Nitride light-emitting diodes, Sci. Rep. 5 (2015) 1–11, https://doi.org/10.1038/srep13483. [5] N. Li, S. Oida, G.S. Tulevski, S.-J. Han, J.B. Hannon, D.K. Sadana, T.-C. Chen, Efficient and bright organic light-emitting diodes on single-layer graphene electrodes, Nat. Commun. 4 (2013), https://doi.org/10.1038/ncomms3294. [6] F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Graphene photonics and optoelectronics, Nat. Photonics 4 (2010) 611–622, https://doi.org/10.1038/nphoton.2010. 186. [7] B.-J. Kim, G. Yang, H.-Y. Kim, K.H. Baik, M. a Mastro, J.K. Hite, C.R. Eddy, F. Ren, S.J. Pearton, J. Kim, GaN-based ultraviolet light-emitting diodes with AuCl₃-doped graphene electrodes, Opt. Express 21 (2013) 29025–29030, https://doi.org/10. 1364/OE.21.029025. [8] M. Bansal, R. Srivastava, C. Lal, M.N. Kamalasanan, L.S. Tanwar, Carbon nanotubebased organic light emitting diodes, Nanoscale 1 (2009) 317–330, https://doi.org/ 10.1039/b9nr00179d. [9] T.-B. Song, N. Li, Emerging transparent conducting electrodes for organic light emitting diodes, Electronics 3 (2014) 190–204, https://doi.org/10.3390/ electronics3010190. [10] B. Lim, H. Oh, G.-H. Lim, H. Sim, C. Kim, M.K. Kim, S. Bok, S.M. Cho, Five-minute synthesis of silver nanowires and their roll-to-roll processing for large-area organic light emitting diodes, Nanoscale 10 (2018) 12087–12092, https://doi.org/10. 1039/c8nr02242a. [11] A.L. Gorkina, A.P. Tsapenko, E.P. Gilshteyn, T.S. Koltsova, T.V. Larionova, A. Talyzin, A.S. Anisimov, I.V. Anoshkin, E.I. Kauppinen, O.V. Tolochko, A.G. Nasibulin, Transparent and conductive hybrid graphene/carbon nanotube films, Carbon 100 (2016) 501–507, https://doi.org/10.1016/j.carbon.2016.01.035. [12] A.R. Rathmell, B.J. Wiley, The synthesis and coating of long, thin copper nanowires to make flexible, transparent conducting films on plastic substrates, Adv. Mater. 23 (2011) 4798–4803, https://doi.org/10.1002/adma.201102284. [13] Y. Xu, X. Wei, C. Wang, J. Cao, Y. Chen, Z. Ma, Y. You, J. Wan, X. Fang, X. Chen, Silver nanowires modified with PEDOT: PSS and graphene for organic light-emitting diodes anode, Sci. Rep. 7 (2017) 1–7, https://doi.org/10.1038/srep45392. [14] M. Wei, H. Wang, J. Wang, P. Chen, W. Zhao, X. Chen, J. Guo, B. Kang, Y. Duan, Flexible transparent electrodes for organic light-emitting diodes simply fabricated with AuCl3-modied graphene, Org. Electron. 63 (2018) 71–77, https://doi.org/10. 1016/j.orgel.2018.08.050. [15] T.H. Han, Y. Lee, M.R. Choi, S.H. Woo, S.H. Bae, B.H. Hong, J.H. Ahn, T.W. Lee, Extremely efficient flexible organic light-emitting diodes with modified graphene anode, Nat. Photonics 6 (2012) 105–110, https://doi.org/10.1038/nphoton.2011. 318. [16] T.L. Wu, C.H. Yeh, W.T. Hsiao, P.Y. Huang, M.J. Huang, Y.H. Chiang, C.H. Cheng, R.S. Liu, P.W. Chiu, High-performance organic light-emitting diode with substitutionally boron-doped graphene anode, ACS Appl. Mater. Interfaces 9 (2017)
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Hybrid film of single-layer graphene and carbon nanotube as transparent conductive electrode for organic light emitting diode
T
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Pradeep Kumara,c, Kai Lin Woond, , Wah Seng Wongd, Mohamed Shuaib Mohamed Saheedb,c, ⁎⁎ Zainal Arif Burhanudina,c, a
Department of Electrical and Electronic Engineering, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia c Center of Innovative Nanostructures and Nanodevices (COINN), Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia d Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, Kuala Lumpur, 50603, Malaysia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Carbon nanotube OLED Transparent conductive electrode
Single layer graphene as a transparent conductive electrode (TCE) suffers from high sheet resistance, low work function, and hydrophobicity. In this work, a hybrid TCE of single-layer graphene, carbon nanotube (CNT) and gold nanoparticles (Au NP) decoration is proposed. The hybridization of graphene with CNT enhances the electrical conductivity while maintaining its high transmittance. The Au NP decorated TCE shows a sheet resistance of ∼100 Ω/sq and transmittance of 96%. The potential of TCE is demonstrated in a solution-processed yellow organic light emitting diode (OLED). By introducing a thin layer of Al-doped zinc oxide between the Au NPs and graphene-carbon nanotube networks which increases the hydrophilicity of the surface, OLED with a current efficiency of ∼2.1 cd/A and a turn-on voltage of ∼5 V is obtained suggesting that this TCE can be a viable option for OLED-anode.
1. Introduction Graphene is a two-dimensional crystal of carbon with potential applications in flexible electronics. The use of graphene as a transparent conductive electrode (TCE) is attractive in various optoelectronic applications where flexibility is required. In organic electronic devices such as bottom-emission organic light emitting diodes (OLEDs), indium tin oxide as a TCE is often the first layer where the whole device is built upon. Despite the brittleness of ITO and rareness of indium on earth, it is often used in various device structures such as solar cell [1], super capacitor [2], touch screen [3], LED [4], and OLED [5]. Replacement of ITO with other alternative TCE such as graphene can be a challenge. Firstly, single layer graphene (SLG), despite its high transparency (∼98%) and high intrinsic carrier mobility (∼2 × 105 cm2/Vs) [6], exhibits a high sheet resistance from 600 Ω/sq [7] to 6,000 Ω/sq [6] limiting its application as a TCE. Several alternatives such as carbon nanotubes (CNTs) [8], conducting polymers [9], metallic nanowires (NWs) [10] are also widely investigated for TCE applications with each has its own drawbacks. For CNT networks, high junction resistance between CNTs [11,12] is found to be limiting its
conductivity. Although doping poly (3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS) with silver NWs can provide the high surface conductivity [13], the high surface roughness of silver NWs can affect the reliability of the device. The doping of monolayer graphene or multilayer graphene using chemical (HNO3 and AuCl3) or highly p-type dopants like boron [14–16] is an effective approach to dramatically improve the luminance and current efficiency of OLEDs. However, due to the hydrophobicity of the graphene surface, the conformal coating cannot be achieved. In this work, we investigated the plausibility of hydrophilic composite TCE by combining SLG, CNT, Al-doped zinc oxide (AZO) and gold (Au) nanoparticle (NP) in order to achieve the desired properties. The presence of CNT – networks increase the electrical conduction of graphene while sustaining its high optical transmission. For further improving the sheet resistance of hybrid TCE, the partial decoration of TCE with Au NPs was adopted. The sheet resistance of ∼100 Ω/sq and transmittance of ∼96% are obtained by decorating the Graphene-CNT (GCNT) hybrid TCEs with Au NP. The wettability of the thin film is enhanced by placing a hydrophilic AZO layer between hybrid TCE and Au NPs. Using this electrode, a solution processable OLED exhibiting a
⁎
Corresponding author. Corresponding author at: Center of Innovative Nanostructures and Nanodevices (COINN), Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak, Malaysia. E-mail addresses: [email protected] (K.L. Woon), [email protected] (Z.A. Burhanudin). ⁎⁎
https://doi.org/10.1016/j.synthmet.2019.116186 Received 22 July 2019; Received in revised form 28 August 2019; Accepted 23 September 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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turn voltage of ∼5 V and efficiency as high as ∼2.1 cd/A is demonstrated.
Surface morphology of GCNT hybrid structure was analyzed by Field emission scanning electron microscopy (FESEM, Zeiss Supra55 VP). The quality of the hybrid film was examined by Raman spectroscopy (Horiba Jobin Yvon HR800, laser source length of 514 nm). After analyzing the hybrid nanostructure, the optical and electrical properties of GCNT hybrid film were investigated by measuring the transmittance (T), and sheet resistance (Rs), using Agilent Cary 100 UV-VIS spectroscopy and four-point probe measurement system (Lucas Lab 302 and Keithley 2400 DC source meter), respectively. For transmittance measurement, GCNT hybrid film of size (2.5 cm × 2 cm) was transferred to a microscopic glass slide. For sheet resistance measurements, FESEM and Raman analysis, (1 cm × 1 cm) GCNT hybrid film was transferred on SiO2/Si substrate. HRTEM samples were prepared by transferring the as-grown GCNT film on the standard copper grid. The roughness of films was measured by atomic force microscopy (AFM) analysis. The performance of any TCE is examined on the basis of standard parameters which are T and Rs. But both parameters cannot be well analyzed without specifying any link between them. This link is known as a figure of merit (FoM) [19]. FoM is considered as a trustworthy method to differentiate the graphitic films and transparent conducting films [6,20]. Theoretically, FoM is defined as the ratio of DC conductivity (σDC ) to optical conductivity (σop ) as [19,20],
2. Experimental 2.1. Materials Highly purified (98%) CNT powder was purchased from US Research Nanomaterials Incorporation, USA. 35 μm thick copper foils were provided by Graphene Platform. N-Methyl-2-pyrrolidone (NMP) 99% and Auric Chloride (AuCl3) 99.999% were bought from Sigma Aldrich. In addition, Iron Chloride (FeCl3), acetone and 2-Propanol (IPA) (98%) were procured from Merck KGaA. 2.2. Development and characterizations of TCEs The GCNT hybrid TCE was prepared by fusing the graphene with CNT network. Initially, highly purified CNT powder was dispersed in NMethyl-2-pyrrolidone (NMP) solvent using ultrasonication. The CNT suspension in NMP was kept for 24 h in an ambient environment so that the heavy particles would settle down. Few drops of (0.2 mg/ml) CNT/ NMP suspension were then dropped on Cu foil. The Cu foil, as well as drops of CNT/NMP suspension, was then spin-casted at a rotation speed of 1000 rpm to form a network of CNTs on Cu foil. The metallic surface of Cu acts as a catalytic medium to grow the graphene. The CNT network on Cu foil was put on the hot plate at 60℃ and the temperature was gradually increased up to 130℃. The sample was heated at 130 ℃ for 30 min to evaporate NMP residues and minimizing the possible agglomeration of CNTs. The dispersed CNT network on Cu foil was then placed in the furnace of thermal-chemical vapor deposition (T-CVD) system for graphene synthesis. The pressure in the furnace was lowered to several millitorrs at the beginning of the graphene synthesis process. At 1000 ℃, methane (CH4) gas which acts as a source of carbon was allowed to flow into the T-CVD furnace. At this high temperature, CH4 gas is decomposed into carbon ions which later form a two-dimensional (2D) surface across the dispersed CNT network on Cu foil. The carbon structure which is called graphene on Cu foil was then rapidly cooled to 200 ℃. At room temperature, the sample was taken out from the furnace. For transferring GCNT hybrid film on the target substrates such as SiO2/Si and microscopic glass slides, the hybrid film was spin-coated with poly(methyl methacrylate) (PMMA) followed by baking on a hot plate at 80 ℃ for proper adhesion of PMMA with GCNT hybrid film. The structure of PMMA/GCNT/Cu was then put in the FeCl3 solution to etch Cu foil. The residues of Cu were removed by using 2% diluted hydrochloric acid (HCl). Afterward, PMMA/GCNT structure was cleaned with deionized water (DI water) before scooping out with a target substrate. PMMA was then removed by immersing the PMMA/GCNT/substrate into acetone followed by cleaning with 2-isopropanol (IPA) and DI water. GCNT hybrid film on the substrate was baked at 80 ℃ and 130 ℃ for 5 min and 20 min, respectively for better contact adhesion. The synthesis and transfer processes of GCNT hybrid film are schematically illustrated in Fig. 1. In this work, the pristine graphene was synthesized and transferred to target substrates by following the same method as for GCNT hybrid films. Gold nanoparticle decoration of GCNT hybrid film was accomplished by spin casting the 10 mM aqueous solution of AuCl3 on the GCNT hybrid TCE at 3000 rpm for 60 s. The modified GCNT TCEs were then patterned using CO2 laser ablation at 13% power level of maximum power (30 W). 0.1 M AZO solution was synthesized using the solgel method as reported in the references [17,18]. The transparent thin interlayer of AZO (40 nm) was prepared by spin casting on GCNT hybrid TCE at 3500 rpm for 60 s followed by annealing at 500 ℃ for 1 h in open air conditions. The structure of the GCNT hybrid film was investigated by highresolution transmission electron microscopy (HRTEM, Zeiss Libra 200).
FoM =
σDC Z 1 = 0 σop 2RS (T −0.5 − 1)
(1)
where Z0 is the impedance of free space. The high value of FoM is an indication of high-quality TCE. 2.3. Device fabrication and characterizations OLED device structure is being considered in this work, consists of GCNT hybrid TCE as anode/poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS, 80 nm) as hole injection layer (HIL)/ super yellow (SY-PPV) (80 nm) as emissive layer (EML)/ lithium fluoride (LiF, 0.8 nm) as electron injection layer (EIL)/ aluminum (Al, 100 nm) as cathode. For device fabrication, the patterned TCE was initially cleaned with acetone and IPA sequentially. PEDOT: PSS was then spin-casted on top of anode layer of modified GCNT hybrid TCE at ambient atmosphere. Then, the thin film was immediately baked at 150 °C for 10 min in N2 environment. The SY-PPV dissolved in toluene was spin-casted on top of PEDOT: PSS layer to form an 80 nm EML. Then the structure was baked at 120 °C for 10 min. Finally, the EIL and the cathode of LiF (0.8 nm) and Al (100 nm), respectively were vacuum deposited at a base pressure of 2.5 × 10−4 mbar without breaking the vacuum. All the devices were encapsulated using UV curable epoxy and glass lid in order to avoid the oxygen and moisture from the environment. Performances of the OLED devices were evaluated through the current (I) - voltage (V) – brightness or luminance (L) characteristics, turn-on voltage (VON) and current efficiency (CE). I-V-L characteristics were measured using Konica Minolta CS-200 integrated with Keithley 2612b source-meter. The film thickness was measured using P-6 profilometer (KLA-Tencor). 3. Results and discussion 3.1. Morphological and structural characterizations of GCNT hybrid TCE The quality of thin film is important for low sheet resistance. FESEM and HRTEM are employed to characterize the morphology and the structure of the GCNT hybrid film as illustrated in Fig. 2. The pristine graphene was found to be uniform and continuous free from any void, crack, ripple, and wrinkle as shown in FESEM image in Fig. 2(a), indicating high-quality graphene has been synthesized by CVD method. As seen in Fig. 2(b), the GCNT hybrid nanostructure, the graphene, and CNTs were fused with each other (shown clearly in the inset at a higher 2
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Fig. 1. Synthesis and transfer of GCNT hybrid film on the target substrate.
(FWHM) ∼34.8 of 2D-peak for GCNT hybrid film in comparison to the pristine graphene (∼38) and CNT (∼82) indicates a better crystallinity of hybrid TCE [28]. The effect of Au NP-decoration on GCNT hybrid film was also interpreted by the respective Raman spectrum. The I2D/IG ≈ 2.33 and ID/ IG ≈ 0.44 values indicate that high crystalline graphene is still maintained after Au NP-decoration. The significant blue and redshifts in G, D and 2D peaks of Au NP-decorated GCNT hybrid film is the result of the increased density of charge carriers [29]. A shift in its G-peak ascertains the evidence of carrier transfer on GCNT hybrid film after decoration with gold NPs. The G-peak was found to be blue shifted by ∼8 cm−1 after p-type doping using AuCl3 aqueous solution. The blueshift in Gpeak of Au-decorated GCNT hybrid film indicates the transfer of electrons to gold particles from GCNT hybrid film. As a result, the hole concentrations on GCNT hybrid film increases driven by the potential difference between ionic AuCl−4 and graphene [7,21]. Furthermore, the lowest FWHM value of Au NP-decorated GCNT hybrid film indicates the crystallinity is preserved even after Au NP-decoration. The quality of the TCEs was further verified by analyzing the distance between LD2 = neighboring defects (LD) using equation 4 − 9 [(1.8 ± 0.5) × 10 × λD × (IG / ID ) [22], where, λD is the laser excitation wavelength. The calculated values of LD were found to be 46 ± 6 nm, 27 ± 4 nm, and 17 ± 2 for pristine graphene and GCNT hybrid and Au NP-decorated GCNT hybrid TCEs, respectively as shown in Fig. 4(b), indicating the existence of minimum defects in decorated hybrid TCE.
magnification). Most of the empty space among the randomly scattered CNTs (diameters ∼ 8–26 nm, density ∼146/ μm2) were seen to be bridged by as-grown graphene surface. Thus, it is expected that in GCNT hybrid structure, the presence of interlinked CNT network strengthens the conductivity of the overall thin film. Fig. 2(c) and (d) depict the HRTEM images of the graphene-CNT hybrid structure, where, the CNT is surrounded by graphene surface. This fact is further confirmed by selected area electron diffraction (SAED) pattern of GCNT hybrid film as shown in the inset of Fig. 2(d). The existence of CNT and graphene are confirmed by the presence of ring pattern of (002) graphitic lattice and six-fold symmetric hexagonal pattern, respectively suggesting that high crystallinities of graphene and CNT are maintained after hybridization. High magnification view of the GCNT interface indicates that the CNT structure has ∼13 walls with an outer diameter of ∼10 nm. The GCNT hybrid film surface partially decorated with gold nanoparticle (density ∼1.3 × 106/cm2, diameter 20 nm to 37 nm) is shown in Fig. 3(a) and (b) with a surface coverage of ∼2%. Decoration of gold nanoparticles on the GCNT results in electron transfer from GCNT to the gold nanoparticle [21,22] resulting in free hole carrier in GCNT hybrid film and hence, decrease in sheet resistance of the hybrid film [23]. In order to confirm the quality of the thin film, we carried out Raman spectroscopy of the thin films as illustrated in Fig. 4 with corresponding Raman features tabulated in Table S1. Raman spectrum of GCNT hybrid film exhibited a graphitic G and second-order resonance double resonance 2D peaks at 1586 cm−1 and 2693 cm−1, respectively as depicted in Fig. 4(a). The presence of CNTs in GCNT hybrid structure is indicated by the appearance of D-peak at ∼1346 cm−1 and broadening of 2D-band [24]. The corresponding ID/IG intensity ratio of 0.17 reflects the presence of high-quality graphene with minimal defects [25] in comparison to 0.5 of CNT indicating the presence of some defects. In other words, a highly crystalline graphene layer is still maintained even after hybridization with CNTs. The I2D/IG intensity ratio of 2.2 indicates the presence of monolayer graphene [24] which is ascertained by equation ωG = 1581.6 + [11/(1 + n1.6)] [26,27] that gives number of graphene layer n ≈1. The lower full-width half maximum
3.2. Optoelectrical characterizations of GCNT hybrid TCE The optical transmittance of GCNT hybrid film is presented and compared with transmittances of pristine graphene, CNT, commercially available ITO films as shown in Fig. 5(a). It was observed that GCNT hybrid film showed a high transmittance (> 95%) from the near-ultraviolet (NUV) to near-infrared (NIR) wavelengths. To evaluate the transmittance and sheet resistance of TCEs, 550 nm reference wavelength is often used [13,14,30,31] in the literature as the spectral response of the human eye is the highest [32,33]. At 550 nm, the 3
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Fig. 2. FESEM images of (a) pristine graphene, and (b) as-prepared GCNT hybrid surfaces. The insets show high magnification views of surfaces. The randomly dispersed CNTs were found to be interlinked with graphene (as the background of the image). HRTEM images of (c) GCNT hybrid film on Cu grid. (d) The high magnification image of the selected area marked by dotted lines. CNT was found to be surrounded by graphene surface. The presence of ring patterns of graphitic lattice along with hexagonal patterns as shown in SAED pattern (inset) indicates the presence of CNT and graphene structure on the surface of hybrid GCNT film.
transmittances of pristine graphene, CNT, GCNT hybrid, and ITO films were measured to be 97.1%, 96.4%, 96.6%, and 90.4%, respectively. It is quite clear that ITO film has significantly reduced transparency at near ultraviolet region. The transmittance of pristine graphene was found to be very close to the theoretical limit of monolayer graphene that is 97.7%, indicating high-quality monolayer graphene in our sample [34–36]. The transmittance of GCNT film reduced slightly (∼0.5%) compared to pristine graphene as the reduction is most likely attributed by CNT. After gold nanoparticle decoration, the transmittance of the hybrid film was decreased by 0.5% at 550 nm. The loss in the transparency of hybrid film after decoration is attributed to Au NPs which act as scattering centers for the light [37,38]. The transmittance of GCNT film with AZO interlayer and gold nanoparticle was found to be ∼94% at 550 nm. The measured average RS of TCEs is represented in Fig. 5(b). After hybridization, a lower average RS of ∼272 Ω/sq was measured from GCNT hybrid film in comparison to pristine graphene (RS ∼460 Ω/sq). ∼41% reduction in RS was observed after hybridizing graphene with
CNT networks. This improvement in RS of the hybrid film is attributed to the drop in the contact resistance between graphene and CNT caused by CVD synthesis of graphene across CNTs [39,40] indicating the presence of well-connected CNT networks [25]. The RS of GCNT hybrid film was further improved using gold nanoparticle decoration. After Audecoration, average RS of GCNT hybrid film was measured to be ∼115 Ω/sq, showing a 57% improvement in RS of GCNT. The improvement in the conductivity (reduction in sheet resistance) of hybrid TCE is attributed to the charge carrier transfer from gold nanoparticle to GCNT hybrid TCE [14,21,38]. The charge carrier transfer is mainly governed by work function difference between gold particles (∼5.1–5.54 eV) and graphene based TCE (∼4.5 eV). The electron transfers from GCNT results in the excess holes at GCNT - Au NP interface, leading to p-type behavior of GCNT hybrid TCE. Therefore, GCNT hybrid film became p-doped with AuCl3 during the reduction of Au atoms, leading to the enhancement of electrical conductivity (reduction of sheet resistance) (Fig. S1 and equations S1-S3). After adding the interlayer of AZO layer on top of GCNT hybrid film followed by gold
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Fig. 3. (a) The gold nanoparticle decorated GCNT hybrid film. The background is GCNT hybrid film which is partially decorated by Au nanoparticles (white dots).b) higher magnification view of the gold decorated surface of GCNT hybrid film.
the FoM of the as-prepared GCNT hybrid film was compared with reported alternative TCEs of ITO as mentioned in Table S2 [14,19,41–44]. It was found that Au NP-decorated GCNT hybrid film displayed a higher FoM.
nanoparticle decoration, the average RS of modified GCNT hybrid film was maintained at ∼130 Ω/sq. In order to address the fact that TCEs should be useful for a broad wavelength specially for red, green and blue OLED devices, FoM of TCEs was analyzed using Eq. (1) at different wavelength as shown in Fig. 6. At 550 nm, FoM for GCNT hybrid was found to be ∼40 as shown in Fig. 6. Around 2-fold improvement in FoM (∼73) of GCNT hybrid film was observed after Au NP-decoration. It was also noted that Au NPdecorated GCNT hybrid film had around three times higher FoM than as-grown pristine graphene (∼28) film, far above the minimum industry standard for TCE (FoM ∼35) [20]. In addition, at 470 nm, 530 nm, 580 nm, and 700 nm, FoM of Au NP-decorated GCNT TCE is more than 2-fold and 3-fold higher than that of pristine graphene and GCNT TCE, respectively, indicating its higher suitability for blue, green, yellow and red light emission OLEDs. At 400 nm, FoM of GCNT hybrid thin film scores better than commercial ITO. Even adding the AZO interlayer, a high FoM of ∼60, ∼59, and ∼65 was found at 550 nm, 400 nm, and 800 nm wavelength regions respectively. At NIR and NUV wavelengths, the GCNT hybrid film with AZO interlayer and gold nanoparticles shows a higher FoM than gold nanoparticle decorated GCNT hybrid film. To validate the optoelectrical properties of developed hybrid TCEs,
3.3. Performance evaluations of OLEDs Solution processable bottom emission OLEDs were fabricated with GCNT based hybrid TCEs as anodes. For comparison, the OLEDs based on commercial ITO and as-prepared pristine graphene were also fabricated. Fig. 7(a) illustrates the luminance and current density characteristics of fabricated OLEDs as a function of bias voltage. The maximum brightness (Lmax) of OLED on Au-decorated GCNT anode (GCNT/Au NP) anode TCE was found to be ∼100 cd/m2. It was observed to be 5-fold and a 10-fold higher Lmax than to GCNT and SLG anodes based OLEDs, respectively as presented in Table S3. This improvement in the Lmax of Au NP GCNT anode based OLED is attributed by the increased current efficiency with GCNT/AZO/Au NP achieving current efficiency as high as ∼2.03 cd/A. The brightness was further increased up to 650 cd/m2 by introducing interlayer of AZO between the hole injection layer and GCNT hybrid TCE followed by gold nanoparticle decoration. Although Lmax of OLED with GCNT/AZO/Au NP
Fig. 4. (a) Raman spectra and (b) average inter-defect distance (LD) of TCEs. The lower value of LD indicates minimal defects in Au NP-decorated GCNT and asprepared GCNT hybrid TCEs in comparison to pristine graphene TCE, possibly caused by CNTs and gold nanoparticles. 5
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Fig. 5. (a) Transmittance of TCEs for the wavelength range from NUV to NIR. The inset shows the enlarged view of transmittance characteristics pristine graphene, CNT, GCNT, and GCNT with gold nanoparticle decoration. (b) Transmittance (T) versus average sheet resistance (RS) of TCEs.
turn-on voltage (Von) of GCNT/AZO/Au NP OLED was 5.2 V which is lower than that of GCNT/Au NP (6.2 V), GCNT (6.7 V), and pristine graphene (∼6.8 V) anodes. This is mainly due to a lower sheet resistance of TCEs. The maximum current efficiencies of OLEDs with GCNT/AZO/Au NP, GCNT/Au NP, GCNT, and SLG anode TCEs was found to be 2.03 cd/A 0.81 cd/A, 0.53 cd/A and 0.25 cd/A respectively as seen in Fig. 7(b). These efficiencies were noticed to be lower than that of commercial ITO based OLED probably resulted from the combination of energetic barrier between PEDOT: PSS and hybrid GCNT structure, interfacial behavior (such as interfacial adhesion energy) a sheet conductivity of the TCEs. The water contact angle of GCNT /Au NP is reduced from 87° to 19° upon adding AZO between GCNT and Au NP whole adhesion energy increased from 0.076 J/m2 to 0.14 J/m2 (Fig. S2). These results show the superiority of GCNT hybrid TCE over reported modified graphene-based alternatives [13,41,42,45] of ITO as summarized in Table S4, demonstrating the potential of hybrid TCE as anodes for applications in organic optoelectronics. In addition, the role of AZO layer is significant for overcoming the hydrophobicity of graphene-based TCEs.
Fig. 6. Comparison of FoM of TCEs for function of wavelength range (400–800 nm).
decorated anode was found to be lower than that of commercial ITO based OLED, it was found to be around 7, 14, and 64 times higher than that of OLEDs with GCNT/Au NP, GCNT, and SLG anodes, respectively. The I-V-L characteristics for OLED with GCNT/AZO/Au NP anode are shown in Fig. 7(a) along with inset showing the device structure. The
4. Conclusion We have developed hydrophilic GCNT/ Au nanoparticles TCE with high optical transparency exhibiting sheet resistance as low as
Fig. 7. Performance of OLEDs on modified GCNT hybrid TCE anode. (a) Luminance and current density characteristics of fabricated OLEDs as a function of bias voltages. The inset shows the structure of OLED. (b) The current efficiency of OLEDs as a function of current density. 6
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∼100 Ω/sq. Adding AZO between GCNT and Au nanoparticle results in a hydrophilic surface. The potential of this TCEs is demonstrated in OLED devices exhibiting a current efficiency as high as 2.1 cd/A indicating that this TCE can serve as an alternative option of ITO for OLED-anode.
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Acknowledgments The authors thank Kang Chun Hong and Mohammad Umair Shahid for their technical support. The authors also acknowledged the financial support under Graduate Assistantship scheme from Universiti Teknologi PETRONAS, Malaysia. This work was partially funded by the Fundamental Research Grant Scheme from Ministry of Higher Education of Malaysia (Grant Number: FRGS/1/2018/TK04/UTP/02/ 4) and University Malaya UMRG Programme RP038A-17AFR, Malaysia. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116186. References [1] T. Mahmoudi, Y. Wang, Y. Hahn, Nano Energy Graphene and its derivatives for solar cells application, Nano Energy 47 (2018) 51–65, https://doi.org/10.1016/j. nanoen.2018.02.047. [2] J. Ge, G. Cheng, L. Chen, Transparent and flexible electrodes and supercapacitors using polyaniline/single-walled carbon nanotube composite thin films, Nanoscale 3 (2011) 3084–3088, https://doi.org/10.1039/c1nr10424a. [3] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. Ri Kim, Y. Il Song, Y.J. Kim, K.S. Kim, B. Özyilmaz, J.H. Ahn, B.H. Hong, S. Iijima, Roll-toroll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578, https://doi.org/10.1038/nnano.2010.132. [4] M. Oh, W.Y. Jin, H. Jun Jeong, M.S. Jeong, J.W. Kang, H. Kim, Silver nanowire transparent conductive electrodes for high-efficiency III-Nitride light-emitting diodes, Sci. Rep. 5 (2015) 1–11, https://doi.org/10.1038/srep13483. [5] N. Li, S. Oida, G.S. Tulevski, S.-J. Han, J.B. Hannon, D.K. Sadana, T.-C. Chen, Efficient and bright organic light-emitting diodes on single-layer graphene electrodes, Nat. Commun. 4 (2013), https://doi.org/10.1038/ncomms3294. [6] F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Graphene photonics and optoelectronics, Nat. Photonics 4 (2010) 611–622, https://doi.org/10.1038/nphoton.2010. 186. [7] B.-J. Kim, G. Yang, H.-Y. Kim, K.H. Baik, M. a Mastro, J.K. Hite, C.R. Eddy, F. Ren, S.J. Pearton, J. Kim, GaN-based ultraviolet light-emitting diodes with AuCl₃-doped graphene electrodes, Opt. Express 21 (2013) 29025–29030, https://doi.org/10. 1364/OE.21.029025. [8] M. Bansal, R. Srivastava, C. Lal, M.N. Kamalasanan, L.S. Tanwar, Carbon nanotubebased organic light emitting diodes, Nanoscale 1 (2009) 317–330, https://doi.org/ 10.1039/b9nr00179d. [9] T.-B. Song, N. Li, Emerging transparent conducting electrodes for organic light emitting diodes, Electronics 3 (2014) 190–204, https://doi.org/10.3390/ electronics3010190. [10] B. Lim, H. Oh, G.-H. Lim, H. Sim, C. Kim, M.K. Kim, S. Bok, S.M. Cho, Five-minute synthesis of silver nanowires and their roll-to-roll processing for large-area organic light emitting diodes, Nanoscale 10 (2018) 12087–12092, https://doi.org/10. 1039/c8nr02242a. [11] A.L. Gorkina, A.P. Tsapenko, E.P. Gilshteyn, T.S. Koltsova, T.V. Larionova, A. Talyzin, A.S. Anisimov, I.V. Anoshkin, E.I. Kauppinen, O.V. Tolochko, A.G. Nasibulin, Transparent and conductive hybrid graphene/carbon nanotube films, Carbon 100 (2016) 501–507, https://doi.org/10.1016/j.carbon.2016.01.035. [12] A.R. Rathmell, B.J. Wiley, The synthesis and coating of long, thin copper nanowires to make flexible, transparent conducting films on plastic substrates, Adv. Mater. 23 (2011) 4798–4803, https://doi.org/10.1002/adma.201102284. [13] Y. Xu, X. Wei, C. Wang, J. Cao, Y. Chen, Z. Ma, Y. You, J. Wan, X. Fang, X. Chen, Silver nanowires modified with PEDOT: PSS and graphene for organic light-emitting diodes anode, Sci. Rep. 7 (2017) 1–7, https://doi.org/10.1038/srep45392. [14] M. Wei, H. Wang, J. Wang, P. Chen, W. Zhao, X. Chen, J. Guo, B. Kang, Y. Duan, Flexible transparent electrodes for organic light-emitting diodes simply fabricated with AuCl3-modied graphene, Org. Electron. 63 (2018) 71–77, https://doi.org/10. 1016/j.orgel.2018.08.050. [15] T.H. Han, Y. Lee, M.R. Choi, S.H. Woo, S.H. Bae, B.H. Hong, J.H. Ahn, T.W. Lee, Extremely efficient flexible organic light-emitting diodes with modified graphene anode, Nat. Photonics 6 (2012) 105–110, https://doi.org/10.1038/nphoton.2011. 318. [16] T.L. Wu, C.H. Yeh, W.T. Hsiao, P.Y. Huang, M.J. Huang, Y.H. Chiang, C.H. Cheng, R.S. Liu, P.W. Chiu, High-performance organic light-emitting diode with substitutionally boron-doped graphene anode, ACS Appl. Mater. Interfaces 9 (2017)
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