Rod-coating all-solution fabrication of double functional graphene oxide films for flexible alternating current (AC)-driven light-emitting diodes

Abstract

Graphene oxide (GO) as dielectric layer and reduced graphene oxide (rGO) as thin-film electrode have been introduced into alternating current light emitting diode devices (AC LEDs) using the large-area Meyer rod-coating technique. Size-dependent effects of GO nanosheets ranging from 10–1200 μm² have been investigated on the conductivity and transparency of the rGO thin-film electrodes on polyethylene terephthalate (PET) substrates. The optimized rGO films show a low sheet resistance of 2.6 kΩ sq⁻¹ at a transmittance of 69% at 550 nm, exhibited higher stability over ITO thin films on PET after repetitive external tensile stress is applied. The prototype flexible AC LEDs have been fabricated on PET with a four-layer configuration of PET/rGO/ZnS:Cu phosphor/GO/rGO or PET/rGO/ZnS:Cu phosphor/BaTiO₃/rGO. As a result, luminance in GO-based devices rises steadily with the increasing frequency of the driving voltage (up to 1500 Hz), while its luminance is surpassed in BaTiO₃-based devices when the frequency reaches 700 Hz. Finally, a rod-coating method allows us to fabricate double functional graphene oxide-based flexible AC LED devices with a size of 14 cm × 18 cm, which opens a promising way for large-area LED displays.

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Introduction

Graphene, a two-dimensional (2D) carbon sheet, has attracted great attention due to its high transparency, excellent electrical properties and chemical and thermal stability. Graphene oxide (GO), a material derived from the exfoliation of oxidized graphite crystals, has also drawn particular interest because of its compatibility with solution processes and tuneable electrical properties from insulating to conductive by controlling the reduction degree.^1,2 Hence, graphene as a conductive material and GO as a dielectric material have been used in many research fields, such as supercapacitors,^3–5 field-effect transistors (FETs),^6–8 touch screens,^9,10 solar cells,¹¹ memory devices,^12–14 light emitting diodes (LED),^15,16 lithium-ion batteries,¹⁷ etc. In the LED devices, graphene and doped graphene thin films were widely used as transparent electrodes,^18–20 and several groups have also integrated the graphene quantum dot hybrid structures as phosphor layers.^21,22 However, to the best of our knowledge, most of them use the energy-consumptive CVD growth or high temperature thermal annealing reduction of GO to prepare highly conductive graphene or rGO films as electrodes. Furthermore, GO have been used as an effective hole injection layer to enhance the luminance intensity of OLEDs.²³ Solution processed rGO and GO that are compatible for the mass-productive roll-to-roll procedures make it possible for low-cost LEDs, however, using rGO as transparent electrode and GO as dielectric layer in one single LED device hasn't been reported yet.

LED lighting sources are becoming the market leader due to its high power efficiency and long lifetime. According to a recent report by Robert Weissbourd of the University of Chicago, LEDs will capture up to “60 percent of the market globally in the next ten years”.²⁴ Compared with the direct current (DC) powered LED, AC LED do not need the AC/DC converter, which can simplify the supporting circuits and save the energy consumed by AC to DC transformation process.²⁵ At the same time, AC LED can also work without strict requirements on the proper metal electrode with their work functions matched for efficient electron/hole injection or the proper p–n heterojunction based active layers with their energy band structures matched for good charge accumulation and then recombination at the interface.

AC LED usually consists of a front electrode and a rear electrode, with a phosphor layer sandwiched between two electrodes to emit light, such as ZnS-based phosphor for blue lighting. To protect AC LED from short circuits and arcing, a dielectric layer is inserted between the rear electrode and the phosphor layer.²⁶ The most used dielectric materials are SiO₂ or BaTiO₃, which are deposited by sputtering.^27–29 However, the sputtering procedure can damage the front electrode and even the substrate. To solve this issue and make it compatible with “roll-to-roll coating” process for large-scale production, a facile method named screen printing, was developed to coat the dielectric layer using commercial SiO₂ or BaTiO₃ pastes (such as Dupont 5540 paste from Dupont Electronics).³⁰ Generally, the thickness of dielectric layer is ∼5–100 μm. It is noteworthy that when the density of the particles is too low (due to no heat-treatment and large size of the particles) there exist some voids between the particles which will result in dielectric loss and low breakdown frequency and/or voltage,³¹ which also weakens the effective electric field on the phosphor layer.³¹ As a result, the performance of AC LEDs with reference to electroluminescence, light stability and power efficiency is not satisfying on their current state.

In this work, we have developed a double functional-graphene oxide based AC LED device by using GO film instead of metal oxide (e.g. SiO₂ or BaTiO₃) pastes as the dielectric layer, and rGO instead of ITO as the front and rear electrodes. Since the major parts of the device are composed of graphene oxide, the device is flexible and has been solely fabricated by Meyer rod-coating method, a lab-scale roll-to-roll coating process. Fig. 1 shows the mayer rod coating procedure we have developed for the preparation of AC LEDs. So far we could fabricate lighting area up to A5 paper size (148 mm × 210 mm). Larger sized AC LEDs are possible by using industrial roll-to-roll coating equipment. From the performance point of view, high-frequency (>700 Hz) AC driven LED device based on GO as dielectric layer, shows more stability and higher luminance in comparison with those using commercial BaTiO₃ paste. The brightness of the GO based device exhibits excellent stability with the frequency increasing, while the brightness of the BaTiO₃ based device starts to drop dramatically from ∼300 Hz due to the large dielectric loss of the BaTiO₃ layer.

Fig. 1 AC LED with rGO as electrode and GO as dielectric layer were fabricated by rod coating.

Results and discussion

Rod coating to prepare flexible GO thin film on PET substrate

The opto-electronic properties of the rGO films are dependent on the GO size effect. In this work, we first investigated the effect of GO size, which was fabricated from natural graphite powder by the modified Hummer's method.³² The as-produced GO was composed of single-layer and few-layer nanosheets with the lateral size of tens of micrometers.³³ Then, different sizes of GO sheets were prepared by utilizing a strong probe sonication for the as-received large GO sheets dispersed in deionized (DI) water. The GO sheets with different sizes we prepared were listed and summarized in Table 1. From the SEM images of column 2, the largest size of the prepared GO sheets is around 1000–1200 μm² without the further strong probe sonication, i.e. probe sonication for 0 min. Longer time probe sonication can break GO sheets into smaller pieces. The GO sheets with the size of ∼100–200 μm² or 10–20 μm² were obtained after the probe sonication for 10 min and 20 min, respectively. Note here, in order to achieve a well dispersed GO solution, all solutions were treated with a normal ultra-sonication for 20 min before the subsequent strong probe sonication. GO thin films on PET substrate were prepared via Meyer rod coating process by utilizing GO sheets with three different sizes as the ink. At first, oxygen plasma treatment was used to wet the PET substrate. Then the GO ink was dropped on the treated PET substrate and Meyer rod was pulled over the solution to form a uniform GO thin film. Finally, the film was dried in oven at 80 °C for 3 min. The thickness of GO film can be well controlled by tuning the concentration of GO ink or the diameter of Meyer rod.

Table 1 The optoelectronic properties of rGO film with different sizes of GO sheets

Time^a (min)	GO area/size^b (μm^2)	rGO film on PET substrate^b	Sheet resistance (kΩ sq^−1)	Transmittance (550 nm)
a Probe sonication time for GO solution.b SEM images.
0			15.6	87.7%
10			18.3	81.4%
20			26.8	78.7%

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To obtain conductive rGO film, the oxygen functional groups in GO film should be removed by reduction, such as high-temperature annealing (at least 200 °C) under protective gas (H₂), hydrazine^11,34 or sodium borohydride (NaBH₄).³⁵ In our case, PET substrate can't bear temperature higher than 120 °C. On the other hand, the electrical conductivity of the rGO films reduced by hydrazine or NaBH₄ is still limited. It should be noted that by using 55 wt% hydroiodic (HI) acid as reductant, GO can be successfully reduced. Moreover, the integrity and flexibility of the reduced GO film can be kept as good as that of original GO film.^36,37 In our experiments, we used the mild HI acid to reduce GO within short treating time (∼30 s) at low temperature (∼100 °C). To study the effect of the removal process of oxygen-containing groups after reduction, the GO films were characterized by X-ray photoelectron spectroscopy (XPS) before and after reduction, as shown in Fig. 2. The typical XPS results from GO films are presented in Fig. 2a. There are three peaks in the XPS spectrum of GO sheet, located at 284.0, 286.2 and 287.7 eV, respectively. These peaks can be assigned to the carbon bonds of C–C, C–O and C=O. In Fig. 2b, the signals of C–O and C=O peaks become very weak for the rGO film. The area radios of several groups are summarized in the Table S1 in ESI.† The area radios of C–O and C=O decrease from 49.96% to 13.97% and 7.31% to 4.48%, respectively. In contrast, the area radio of C–C increases from 42.72% to 81.55% after the reduction. The value of C/O ratio improved from 2.5 to 9.8 after the reduction process. These results indicate GO film has been reduced effectively.

Fig. 2 XPS spectra of (a) GO (inset is the photo of GO thin film on PET) and (b) rGO (inset is the photo of rGO thin film on PET reduced by HI acid).

Optoelectronic and mechanical properties of rGO thin film

Table 1 shows three types of GO sheets with the sizes of 1000–1200 μm², 100–200 μm² and 10–20 μm². The resultant rGO films exhibit significantly different opto-electronic properties, as shown in Table 1, Fig. 3a and S1 (in ESI†). The rGO thin film derived from 1000–1200 μm² GO sheets exhibits the lowest sheet resistance, only 2.6 kΩ sq⁻¹ at the transmittance of 69% (at 550 nm) with the thickness around 15 nm as measured by atomic force microscopy (AFM) section analysis (Fig. S2 in ESI†). The sheet resistance increases with the transmittance enhancing. In addition, the sheet resistance of rGO film increases with the decreasing of (r)GO sheet size at the same transmittance. This is due to more inter-sheet connections between smaller rGO sheets which results in an increase in the whole film resistance. It's worth noting that the opto-electronic properties of our rGO film derived from the largest GO sheets are superior to that of the reported flexible transparent conductive rGO films prepared by other methods. For example, the rGO films obtained by combining hydrazine vapour exposure and proper thermal annealing at 200 °C of vacuum filtrated GO film shows sheet resistance of 43 kΩ sq⁻¹ at transmittance of 73% at 550 nm,³⁴ or by exposure to hydrazine vapour to reduce Langmuir–Blodgett assembled GO film with 1.9 × 10⁷ kΩ sq⁻¹ at transmittance of 95.4% at 650 nm,³⁸ or by combining ammonia and hydrazine to reduce GO in aqueous solution followed by spray coating to form rGO film with 2 × 10⁷ kΩ sq⁻¹ at transmittance of 96% at 600–1000 nm.³⁹ The opto-electronic properties of our rGO thin film are comparable with the value reported by Wang et al.,¹⁰ where they employed similar rod coating method to prepare rGO film, but they had to rely on the complicated palladium assisted reduction with the pressurized hydrogen in autoclave, which will induce high cost and safety hazard during the working process.

Fig. 3 (a) The opto-electronic and mechanical properties of rGO and ITO thin films on PET. Sheet resistance vs. transmittance of rGO films fabricated from GO nanosheets with different sizes obtained by controlling probe sonication time for 0 min (P0), 10 min (P10) and 20 min (P20). (b) Resistance change rate of rGO and ITO thin films vs. the bending times. SEM images of the rGO film (c) before bending and (d) after bending for 500 times. SEM images of ITO film on PET substrate (e) before bending and (f) after bending for 500 times.

To investigate the flexibility of rod-coated rGO films, the resistance change of rGO thin films and commercial ITO films on PET substrate under bending test were characterized after 500 times bending experiments, with a bending time of 500. Fig. 3a shows the resistance changing rate ((R_n − R₀)/R₀) of ITO and rGO thin films after bending for different times, where R₀ and R_n represent the resistance before and after the bending, respectively. The resistance changing rate of ITO film sharply increased from 1.2 to 48.2 when the film was bent from 1 time to 3 times, and the changing rate increased to 607.6 after 500-time bending. The resistance changing rate of rGO films was nearly 0.2, i.e. increasing only by 20%, with the bending times from 0 to 500. The superior flexibility of the rGO films in comparison with ITO films was further proved by the microstructures changes from the two films after the same bending of 500 times. As shown in Fig. 3c and d, many cracks were formed in the ITO film after bending, but there is no visible change in the rGO film. Therefore, the rGO/PET film exhibits much better flexibility than ITO/PET in terms of their resistance and the conductivity retention after the application of external tensile stress, i.e. the bending. This advantages the application in flexible areas for such rod-coated rGO films.

Fabrication and characterization of AC LEDs with double functional GO films

Fabrication of the AC LEDs with GO and rGO films as dielectric and front electrode, respectively was completed with layer-by-layer Meyer rod coating. The GO and rGO films are coated from the large-size GO sheets mentioned above. The thickness of rGO film as front electrode for all devices is ∼15 nm, and that of GO film as dielectric layer is 10–25 cm. There is no thickness requirement for the rear electrode with rGO film as the light emits from the front rGO electrode. Therefore, a thick (>50 μm) rear electrode can be coated to decrease its sheet resistance. For all devices, the ZnS:Cu phosphor layer is controlled with ∼50 μm, and the substrates are PET.

In order to study the device's properties for different dielectric layers, device A and device B have been made with four-layer structure rGO/ZnS:Cu phosphor/GO/rGO and rGO/ZnS:Cu phosphor/BaTiO₃/rGO, respectively. Device C, as a control device, is fabricated to investigate the thickness effect of GO film as dielectric layer on device performance. The GO dielectric layer thickness is ∼10 μm in device C, which is thinner than that of ∼25 μm for device A and the BaTiO₃ thickness of 35 μm for device B. These could be seen from the cross sectional SEM images for device A (Fig. 4a), B (Fig. S3a in ESI†) and C (Fig. S4a in ESI†). Note here, we took cross sectional SEM images before the rear rGO electrode coating as rGO and GO have similar morphology, difficult to distinguish whether they are stacked/coated together. Meanwhile, too thin GO dielectric layer doesn't work effectively as it will not completely cover the rough ZnS:Cu phosphor layer below.

Fig. 4 (a) SEM images of the cross section of devices A. (b) The magnified morphology of GO dielectric layer. (c) Electroluminescence (EL) spectra of devices A, B and C under applied with 80 V AC bias at 1500 Hz. (d) Luminance–voltage (L–V) curves of devices A and B at 50 Hz. (e) Luminance–frequency (L–F) characteristics of devices A, B and C under 80 V AC bias. (f) The lighting large-area AC LEDs with the structure: rGO/ZnS:Cu phosphor/GO/rGO under the bending state. (The structures of device A and C are rGO/ZnS:Cu phosphor/GO(∼25 μm)/rGO and rGO/ZnS:Cu phosphor/GO(∼10 μm)/rGO, respectively. Device B, rGO/ZnS:Cu phosphor/BaTiO₃(∼35 μm)/rGO.

As shown obviously in Fig. 4b and S4b,† two-dimensional wrinkled GO sheets formed. The generation of wrinkles is probably induced by the rough surface of ZnS:Cu phosphor layer and the drying process after GO coating. The SEM image (Fig. S5a†) shows the size of commercial ZnS:Cu phosphor particles is as large as ∼15 μm, resulting in an unsmooth layer surface. In Fig. S6,† the EL spectra of the three devices have been tested and presented systematically by varying the frequency under applying a constant AC voltage, and vice versa. Consistently, the brightness of all the devices increases when the voltage increases under the application of a constant frequency. The AC-EL spectra for devices A, B and C, driven with 80 V AC bias at 1500 Hz, were shown in Fig. 4c. The emission peaks of the three devices are all located at ∼500 nm because their active layers are composed of the same commercial ZnS:Cu phosphor. Fig. 4d shows the luminance intensity of device A and B at 50 Hz. It can be see that with the increase of voltage, the intensity of the two devices increase. Device B exhibits higher luminance than device A under the same voltage. Impressively, as the L–F curves shown in Fig. 4e, device A with 25 μm thick GO dielectric layer shows the highest luminance intensity within the high frequency region (>700 Hz) under the same bias. As the frequency increases, an enhanced luminance is observed in device A, while a decreasing trend in device B over 300 Hz. Especially, at the frequency of 1500 Hz, device A exhibits much higher luminance of 16.4 cd cm⁻², up to twice higher than that of device B with BaTiO₃ layer (7.6 cd cm⁻²) and greatly higher than that of device C with 10 μm thick GO dielectric layer. The high luminance of device A can be well retained even after the bending test (Fig. S7†).

Compared with device B which has large dielectric loss of BaTiO₃ layer, the higher performance of device A at high frequency region can be attributed to the small dielectric loss of GO layers. As shown in Fig. S5b,† the size of BaTiO₃ particles are large, thus the layer continuity will be problematic when it was rod coated onto the unsmooth ZnS:Cu phosphor layer. Note here, the smooth surface (Fig. S3b†) from large BaTiO₃ particles based film rod-coated on PET/rGO/ZnS:Cu phosphor results from the filling of the gaps between particles by the commercial binders. The gaps between BaTiO₃ particles cause dielectric loss, which is further confirmed by the similar luminance drop phenomenon from device C with a thinner GO layer compared with that of device A. The thin GO layer probably leads to its incomplete coverage on ZnS:Cu phosphor layer and results in large dielectric loss at high frequency. Meanwhile, GO sheets have flexible 2D morphology and can cover the ZnS:Cu phosphor layer completely and continuously compared with the large particle of BaTiO₃. Moreover, the GO dielectric layer can be fabricated by solution process at room temperature, with the merit of flexibility, low cost, little toxicity and thin dielectric layer. Therefore, GO dielectric layer can not only enhance the lighting efficiency of LED devices, but also improve the lighting stability at high working frequency.

In order to demonstrate the up-scalability of our device fabrication method, an AC LED device with large-area of 14 cm × 18 cm has been developed, as shown in Fig. 4f. We found that this AC LED device can sustain the bending without visible degradation of the luminance. Such high flexibility arises from the introduction of highly flexible GO and rGO double functional layers, i.e. GO as dielectric layer and rGO as conductive electrodes, for AC LED. Based on the results and discussions above, it is promising that larger LEDs can be fabricated by using the industrial roll-by-roll equipment.

Conclusions

By using Meyer rod coating method, flexible transparent conductive rGO thin films have been prepared on PET substrate using GO sheets with different sizes (10–1200 μm²) followed by the hydroiodic (HI) acid reduction at 100 °C for 30 s. The rGO film derived from 1000–1200 μm² sized GO exhibits a low sheet resistance of 2.6 kΩ sq⁻¹ at the transmittance of 69% (550 nm). Its resistance/conductivity can be well retained even after bending up to 500 times. The double functional GO based AC LEDs, which use GO as dielectric material and rGO as front/rear electrode, have been developed successfully with ZnS:Cu phosphor as the phosphor layer. The luminance of the device, based on 25 μm thick GO as dielectric layer, increases steadily with the frequency rise (up to 1500 Hz) at 80 V AC bias, while the device employing 35 μm thick BaTiO₃ as dielectric layer significantly drops due to large dielectric loss. Since under lower frequency (50 Hz), BaTiO₃ dielectric layer based AC LED exhibits better performance, the luminance of GO-based device is twice higher than that of BaTiO₃ based device under high frequency (1500 Hz). Moreover, the outstanding properties of GO nanosheets, such as flexibility and 2D morphology, offer the flexibility, low cost, little toxicity and thin dielectric layer of the fabricated AC LED devices. Meanwhile, GO exhibits low dielectric loss property, the dielectric constant (∼4.3)⁷ is close to that of SiO₂ (κ = 3.9),⁶ SiO₂ is one of the common dielectric materials. Therefore, GO has potential use as ideal dielectric material in AC LED device or other electroluminescent devices.

Experimental section

Fabrication of rGO transparent conducting thin films on PET

The rGO transparent conducting thin films on PET were made by Meyer rod coating method. Firstly, for the preparation of GO solutions, the dried GO sheets synthesized by a modified Hummer's method (45 mg) and DI-water (15 mL) were mixed and sonicated by the ultra sonication for 20 min, and then divided this solution into three parts. One of them was used as control solution with the largest size GO sheet (1000–1200 μm²), and the others were sonicated by the probe sonicator for 10 min or 20 min, respectively, to get small sized GO sheets. Secondly, the PET substrate was rinsed by ethanol and dried by N₂ blowing or in the oven for several minutes at 80 °C, then treated by oxygen plasma (power 200 W, O₂ 75%, Ar 20%) for 90 s. Thirdly, the GO thin films were fabricated using Meyer rod coater. Then the wet films were dried in the oven for 3 min at 80 °C. At last, the rGO thin films were obtained by the reduction of HI acid at 100 °C for 30 s.^36,37 Then the residual HI acid on the films was rinsed by ethanol.

The fabrication and characterization of AC LEDs

Several steps for making AC LEDs based on PET/rGO thin film via the rod coating technique are shown as follows: (1) the phosphor layer was coated on the rGO film and dried in oven at 120 °C for 30 min. (2) The dielectric layer was coated on the top of phosphor layer, then dried at 120 °C for 30 min in oven. (3) On the top of dielectric layer, rGO suspension was rod-coated with the thickness >50 μm as rear electrode since it doesn't have light transparency requirement. At this step, it is worth notice that the GO must be reduced into rGO by HI acid first, and then the received rGO was dispersed into DI-water assisted with strong probe sonication. This avoids the reduction of GO dielectric layer during the rear electrode fabrication. Then the devices were dried at 120 °C for 30 min. (4) Using the copper tape to contact the front electrode and rear electrode. Finally, the received devices are ready for characterization. The testing on the properties of all devices was finished in the atmospheric environment at room temperature by using the AC power source 105AMX and the photo research spectra scan PR655.

Acknowledgements

This work was supported by the National Research Foundation (NRF) Proof-of-Concept (POC) grant funded by the Singapore government (no. NRF2011NRF-POC001-048) and by the National Basic Research Program of China (973 Program, no. 2009CB930600), National Natural Science Foundation of China (Grants no. 20974046, no. 20774043, no. 51173081, no. 50428303, no. 61136003), The Program for New Century Excellent Talents in University (NCET-11-0992), the Natural Science Foundation of Jiangsu Province (BM2012010, BK2011761, no. 11KJB510017, no. BK2008053, no. BK2009025, no. 10KJB510013 and no. BZ2010043). Funding of Jiangsu Innovation Program for Graduate Education (CXZZ11_0413, CXZZ12_0457). L. H. Xie thanks the Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06147k

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