Wide-range work-function tuning of active graphene transparent electrodes via hole doping

Abstract

Graphene is regarded as a potential candidate to replace the transparent conductive (TC) electrodes that are currently used in various optoelectronic applications. However, there is still a lack of methods by which to achieve low sheet resistance (R_s) with stable doping and work functions with a wide range of tunability, which is significant for band alignment at the interface to enhance charge transport and thus to achieve higher device performance. We developed a novel strategy for preparing a TC electrode by doping layer-by-layer (LBL)-stacked graphene with AuCl₃, by which means an excellent TC performance (an R_s of 40 ohm sq⁻¹ at a transmittance (T) of 89.5%) and an extremely wide range of work-function tunability (∼1.5 eV) were successfully achieved. Moreover, a hybrid electrode prepared by transferring doped graphene onto a pre-patterned Cu metal mesh exhibited a low resistance of ∼4.9 ohm sq⁻¹. In addition, we monitored the long-term stability of AuCl₃-doped graphene for 6 months and also constructed a model for accelerated degradation testing. The relevant mechanism of charge transfer between the graphene and the dopants was characterized based on X-ray photoelectron spectroscopy (XPS) spectra to elucidate degradation observed after long-term testing. This work contributes a novel type of “active electrode”; the doped graphene film not only serves as a high-performance TC electrode but also provides a wide range of tunable work functions. The proposed active electrode is prepared using a scalable and facile doping process, which paves the way for its usage in applications such as optoelectronic devices.

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Introduction

Transparent conductive (TC) electrodes are widely used in many applications in current optoelectronic devices, including displays, touch screens, solar cells, organic light-emitting diodes (OLEDs), electrochromics, and photosensors.^1–3 Current TC electrodes are made of semiconductor-based materials, of which the most frequently employed is indium tin oxide (ITO), which is n-doped and has a composition of ∼10% SnO₂ and ∼90% In₂O. Commercial ITO exhibits a sheet resistance (R_s) as low as ∼10 ohm sq⁻¹, with a transparency (T) of ∼80% at 550 nm on glass; this performance is reduced to ∼60–300 ohm sq⁻¹ when the ITO is deposited on flexible polyethylene terephthalate (PET). Although frequently used, ITO suffers from many shortcomings, including the following: (1) degradation of conductivity due to its brittle nature when it is subjected to mechanical bending, which limits its usage in flexible devices; (2) low stability in acidic and basic environments, which leads to short device lifetimes because of the susceptibility of the active materials in the devices to ion diffusion; (3) strong absorption of high-energy light (>4 eV), which hinders the use of ITO in the window layers of many optical devices, such as UV light emitters; and (4) limitations with regard to work-function engineering. Therefore, the discovery of new TC materials will be crucial for addressing these issues. Although several materials, such as metal grids,⁴ metallic nanowires,⁵ and conductive polymers,⁶ have been explored as alternatives, recently reported graphene films offer several advantages over these technologies, such as higher transparency (monolayer graphene absorbs only ∼2.3% of visible light) over a broad range of light wavelengths, higher flexibility and excellent chemical stability. All of these unique properties allow graphene to be considered as a potential candidate to replace the TC materials that are currently in use; in fact, graphene has been widely used in many electronic and optoelectronic applications.¹

A graphene-based TC film can be synthesized using various methods, including methods based on solution-processed reduced graphene oxide (rGO),^7–9 liquid-phase exfoliation¹⁰ and electrochemically exfoliated graphene flakes;^11,12 in this last case, the graphene flakes can be easily dispersed to form a suspension ink, from which a thin film can then be obtained via versatile coating routes, such as Langmuir–Blodgett (LB) assembly,^13,14 spray coating¹⁵ and rod coating.¹⁶ Although these methods have the advantages of low cost and scalable production, the high resistance (approximately a few to a few thousand kohm sq⁻¹) of the films they produce still hinders their usage. Alternatively, in recent efforts, large-area and high-quality graphene films have been successfully achieved via chemical vapor deposition (CVD).^17–21 CVD-grown graphene exhibits scalability and high conductivity as well as controllable transparency; as a result, CVD growth is considered to be a promising route by which to obtain graphene TC electrodes. The typical sheet resistance of intrinsic monolayer graphene ranges from ∼1 kohm sq⁻¹ to 5 kohm sq⁻¹.^20,21 The layer-by-layer (LBL) stacking of up to four layers of graphene film improves this resistance to ∼350 ohm sq⁻¹ while maintaining a transparency of greater than 90%.²² However, this improved sheet resistance is still inferior to the sheet resistance of ITO. To reduce the sheet resistance of CVD-grown graphene, various methods have recently been employed to obtain p-doped graphene with acid, molecular and AuCl₃ precursor,^23–27 in which the transfer of charge (electrons) from the graphene to the dopants significantly increases the hole carrier concentration of the graphene, leading to remarkably lower sheet resistance. For example, four layer (4L)-stacked graphene with nitric acid doping exhibits a low sheet resistance of 30 ohm sq⁻¹ (T ∼ 90%).¹⁸ LBL doping with AuCl₃ also results in high performance, with a sheet resistance of 54 ohm sq⁻¹ (T ∼ 85%).²⁷ Moreover, it has been reported that AuCl₃-doped graphene exhibits higher stability than graphene with other reported dopants because most other dopants readily desorb from the graphene during long-term operation, thereby degrading its stability.

Current electronics and optoelectronics require a so-called “active electrode” with an appropriate work function (WF) to achieve band-structure alignment at the interface, enhancing the efficiency of carrier transport and thus yielding higher device performance. Therefore, the discovery of a new type of electrode with a wide range of work-function tunability would be beneficial for the development of various optoelectronic devices. Typical transparent oxide electrodes, such as ITO, exhibit limited WF tunability (∼0.5 eV) because the Fermi level becomes pinned once the ITO film has been deposited onto a specific substrate. Graphene has been exploited as a novel active electrode in this regard because its high transparency, charge mobility and flexibility make it a promising candidate for application in many bendable optoelectronics, such as organic photovoltaics (OPVs),^26,28,29 organic electronics,^30,31 and OLEDs.³² Various approaches have been reported for altering the WF of graphene, including molecular and chemical functionalization,^33–35 metal contact doping,^36,37 and the intercalation of lithium, fluorine, or iron chloride into few-layer graphene.^38–40 However, the surface functionalization of graphene often destroys the intrinsic graphene lattice, thereby severely degrading its conductivity and charge mobility.⁴¹ Recently, doping with Au precursors (AuCl₃, HAuCl₄, Au(OH)₃, etc.) has been demonstrated to enable the stable modulation of the graphene WF because the as-formed Au nanoparticles (Au-NPs) decorated on the graphene allow charge-transfer doping with long-term stability.^{23,24,27,42–45} A configuration with LBL Au precursor doping on stacked graphene films has been reported to exhibit higher performance in terms of both electrical and optical properties.^27,45 Moreover, this doping technique allows graphene to maintain its high conductivity and transparency during long-term operation. However, the modulated WFs that can be achieved using most reported methods based on the doping of graphene span only a limited range of approximately 0.5–1.0 eV, and precise control of the graphene WF has yet to be demonstrated.^34,46

In this study, we propose a novel approach for preparing graphene-based TC films with a remarkably wide range of work-function tunability (up to ∼1.5 eV). To realize this novel type of film, we perform the LBL stacking of high-quality CVD-grown graphene to form a transparent electrode, followed by a doping process. Several dopants for graphene films, such as TFSA (tetrafluorosuccinic acid, C₄H₂F₄O₄), PBASE (1-pyrenebutyric acid N-hydroxysuccinimide ester, C₂₄H₁₉NO₄), HI (hydroiodic acid) and AuCl₃ precursor, were comprehensively studied and compared. The AuCl₃ doping was optimized under various conditions, resulting in excellent optical and electrical performance (R_s ∼ 40 at T > 89%). The morphology of the Au-decorated graphene was characterized using an electron microscope, revealing the various shapes of the nanostructures. The WFs of the doped graphene electrodes were estimated using ultraviolet photoelectron spectroscopy (UPS) and found to span a wide range, from 4.26 to 5.72 eV. Long-term stability testing for up to 6 months was performed to evaluate the doping stability. Most importantly, the AuCl₃ doping mechanism was investigated in detail via the X-ray photoelectron spectroscopy (XPS) characterization of the chemical bonding status. Moreover, a novel hybrid electrode consisting of a pre-patterned Cu micro-mesh and a doped LBL graphene film was fabricated to improve the electrical conductivity of this novel transparent electrode.

Results and discussion

Fig. S1† schematically illustrates the procedure used to fabricate single-layer or multilayer graphene films (see the Experimental section for details). In brief, CVD-grown graphene films on copper substrates were transferred in an LBL fashion onto the target substrate using the PMMA-transfer method. In this study, several doping precursors were used, including TSFA, PBASE, and AuCl₃, to study and compare the optical and electrical properties of the resulting graphene films. Moreover, the previously un-reported route of doping with HI was also studied here. The doping processes with TSFA, PBASE and AuCl₃ were performed via spin casting, whereas the HI doping was performed by soaking the graphene film in HI vapor; all of these doping processes involve a facile and simple approach. Atomic force microscopy (AFM) was used to characterize the as-prepared graphene film. It should be noted that the AFM measurements were performed under ambient conditions, meaning that layers of water inevitably became trapped at the interface between the graphene and the substrate, resulting in increased step heights. The reported values measured via AFM for single-layer graphene vary between 0.8 and 1.0 nm.^47,48 Fig. 1a shows a typical AFM image recorded in this study and its height profile, which indicates a thickness of 0.78 nm for the as-grown graphene and demonstrates the uniformity of the single-layer graphene film. Raman spectroscopy is often used to characterize the crystallinity of graphene. The G peak (at ∼1580 cm⁻¹) and 2D peak (at ∼2700 cm⁻¹) in the Raman spectrum are characteristic of the sp²-hybridized C–C bonds in graphene,^49,50 whereas the D peak (at ∼1350 cm⁻¹), which is related to non-sp² bonding, is associated with lattice disorder or atomic defects.⁵¹ Fig. 1b shows a typical Raman spectrum for an as-prepared graphene sample, in which the low D-peak intensity indicates a low defect density in the graphene film. In addition, the intensity ratio of the 2D and G peaks (I(2D)/I(G)) is 2.5, and the narrow full width at half maximum (FWHM) of the 2D peak (∼27.8 cm⁻¹) indicates the presence of monolayer graphene.⁵² The Raman spectra for the 1L–4L graphene films are shown in Fig. S2;† the negligible D peaks for the graphene samples with different numbers of layers indicate the reliability of the low rate of defect formation during transfer and LBL stacking. Moreover, a blueshift of the G peak was observed with an increasing number of stacking layers, indicating less p-type doping of the graphene, which can likely be attributed to the weakening of the interaction with the underlying substrate. The optical transmittance spectra of the pristine graphene samples are shown in Fig. 1c, where the transmission levels of the 1L to 4L graphene films are seen to gradually decrease from 97.2% to 90.5%. The corresponding sheet resistance characteristics are shown in Fig. 1d, which shows that the 1L graphene had a sheet resistance of ∼1.3 kohm sq⁻¹; this value decreased as an increasing number of layers were applied via LBL stacking and reached the following values: 865.9 ohm sq⁻¹ (2L), 692.8 ohm sq⁻¹ (3L), and 497.6 ohm sq⁻¹ (4L).

Fig. 1 (a) A typical AFM image of a CVD-grown monolayer graphene film transferred onto a glass substrate. (b) The corresponding Raman spectrum acquired for the film shown in (a). The inset is an optical image of a graphene film with dimensions of 1.0 cm × 2.0 cm on a substrate. (c) The transparencies of different numbers of layers of graphene films on glass substrates as a function of the optical wavelength. The inset shows a plot of the transparency (T%) with respect to the different numbers of layers of graphene film at a wavelength of 550 nm. (d) A statistical analysis of the sheet resistances of samples with different numbers of graphene layers.

To study the doping effect in monolayer graphene, AuCl₃ doping at various concentrations (10, 20, and 30 mM) was performed. Fig. 2a shows the UV-vis absorption spectra for samples at various concentrations. The optical transparency at 550 nm was greater than 95% for all samples. In addition, as shown in Fig. 2b, the sheet resistances of otherwise identical graphene samples exhibited remarkable decreases of up to 90.6% with AuCl₃ doping, from 1.3 kohm sq⁻¹ for the pristine graphene to ∼121.9 ohm sq⁻¹ for the 30 mM AuCl₃-doped graphene. The optical transparencies and sheet resistances of 1L graphene with the other doping agents, TFSA, PBASE and HI, are shown in Fig. S3.† The optimized conditions for each case are summarized in Fig. 2c for comparison; this figure indicates that the doping of the graphene with 20 mM AuCl₃ resulted in the highest performance of the resulting transparent electrode compared with the other doping methods. Although HI-treated samples were expected to lower the sheet resistance due to strong electron affinity of iodine atom on graphene, leading to hole-doping. It is noteworthy that the HI-treated samples did not exhibit obvious sheet resistance reduction (Fig. 2c and S3†). This could be attributed to low adsorption stability of iodine on graphene as been studied in previous report,⁵³ indicating extremely low bonding energy of C–I. As such, the dopants induced p-type doping on graphene was limited.

Fig. 2 (a) The transparencies of pristine and AuCl₃-doped monolayer graphene with different doping concentrations of 10, 20, and 30 mM. (b) A statistical analysis of the sheet resistances of graphene samples with different doping concentrations. (c) Summary of the sheet resistances and transparencies achieved using various dopants, including TFSA, PBASE, HI and AuCl₃ precursor, as a function of the number of graphene layers. (d) The XPS Au 4f core peaks of graphene samples with different AuCl₃ concentrations.

To understand the Au ion doping mechanism underlying this phenomenon, XPS measurements were performed. Fig. 2d shows the Au 4f core-level spectra for 1L graphene films doped with various concentrations of AuCl₃, in which the peaks near 90.3 and 86.6 eV are assigned to the Au ion (Au³⁺) and the peak near 84.1 eV is assigned to neutral Au (Au⁰).⁵⁴ The Au⁰ intensity, as indicated in Fig. 2d, increased along with the doping concentration (i.e., in the sequence 10 mM < 20 mM < 30 mM); this result is ascribed to the reduction of the Au ion to natural Au as a result of its acceptance of a large number of electrons from the graphene. In addition to the electron transfer, the AuCl₃ could also be decomposed and reduced to Au⁰ when it is exposed to light or under heat. It has been reported that AuCl₃, on carbon nanotube, starting transfer to Au⁰ at about 200 °C due to the evaporation of Cl₂.⁵⁵ As such, the thermal treatment (∼120 °C) carried out in this study is considered as another reduction factor. Fig. 3 shows the surface morphologies as observed via SEM for AuCl₃-doped graphene samples with various AuCl₃ precursor concentrations. The Au nanostructure was found to be uniformly distributed on the graphene surface at 10 mM and 20 mM (see (a) and (b), respectively); however, segregation occurred at a concentration of 30 mM (as shown in (c)), indicating that the doping concentration should be optimized to prevent severe metal particle aggregation. Note that different concentrations led to different shapes of the Au nanostructures; for example, Au-NPs with an average diameter of 23 nm were observed with 10 mM doping, as shown in (g). At 20 mM, in addition to Au-NPs, a large number of triangle-shaped Au nanoplates (size: ∼50 μm) began to form, as shown in (e) and (h). When the precursor concentration was increased to 30 mM, the size of the Au plates increased to greater than 150 μm; moreover, Au nanorods began to form. The formation of isotropic Au nanostructures, such as that of Au-NPs, occurs through thermodynamic means (i.e., homogeneous nucleation and subsequent growth of the nuclei), whereas for anisotropic Au nanostructures, such as nanorods and nanoplates, the formation mechanism is a seed-mediated growth process.⁵⁶ Alternatively, a recent report indicates that the graphene itself can induce the transformation of the material structure; for example, the transformation of fcc-Au to hcp-Au was observed when Au ions were precipitated onto a graphene surface.⁵⁷ In addition, Au-NPs were found to preferentially segregate to and decorate the cracked and wrinkled regions of the graphene film (see Fig. 3d); these Au-NPs could spontaneously fill in the cracked regions, which is believed to have reduced the degradation of the electrical conductivity during the transfer process.

Fig. 3 SEM images of AuCl₃-doped monolayer graphene films with different doping concentrations. (a), (d), and (g) show low- and high-magnification images of films with 10 mM AuCl₃ doping, revealing a Au-NP morphology. (b), (e), and (h) show images of films with 20 mM doping, in which triangular Au nanoplates can be observed in addition to Au-NPs. (c), (f), and (i) show images of films with 30 mM doping, which indicate that the graphene surface was decorated with a mixture of Au-NPs, triangular and hexagonal Au plates, and Au nanorods.

Because doping with a high concentration (i.e., greater than 30 mM) of AuCl₃ resulted in severe aggregation, in the subsequent experiment, we selected a concentration of 20 mM for the doping of the graphene film. To investigate the effect of doping on LBL-stacked graphene films, films with different numbers of graphene layers were characterized in terms of their optical transmittance and sheet resistance (Fig. 4a and b). The compiled data are listed in Table 1; the sheet resistance gradually decreased to 40.1 ohm sq⁻¹ with the addition of further graphene layers, and the transmittance remained at >89.5%, indicating that the decoration of graphene with Au-NPs facilitates high electrical conductivity and ultra-low light absorption. These results are superior to those for pristine graphene and comparable to those for currently available ITO electrodes.

Fig. 4 (a) The transparencies of pristine and AuCl₃-doped (at 20 mM) LBL-stacked graphene films. The labels PG and DG indicate “pristine graphene” and “doped graphene”, respectively. (b) A statistical analysis of the sheet resistances of pristine and doped graphene samples with different numbers of graphene layers. (c) Raman spectra of doped graphene samples with different numbers of graphene layers. (d) A statistical analysis of the G-peak positions, as shown in (c), for un-doped and doped graphene samples with different numbers of graphene layers.

Table 1 Characteristics of TC electrode performance for pristine and AuCl₃-doped graphene films

No. of layers	Pristine graphene			LBL graphene doped with AuCl3 (20 mM)
	T (%)	Rs (ohm sq^−1)	Raman G peak position (cm^−1)	T (%)	Rs (ohm sq^−1)	Raman G peak position (cm^−1)	Carrier concentration (cm^−2)
1	97.38	1302.5	1582.2	95.77	127.59	1596.6	4.17 × 10^13
2	95.75	865.89	1574.8	93.91	76.56	1592.5	6.14 × 10^13
3	93.51	692.78	1572.3	92.10	70.56	1590.5	7.98 × 10^13
4	91.04	497.55	1571.9	89.51	40.11	1588	1.24 × 10^14

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To further study the effect of doping, Raman spectroscopy was performed on doped graphene films. It has been reported that the position shift of the G peak is sensitive to doping because of the phonon-stiffening phenomenon that occurs via charge transfer.⁵⁴ The spectroscopic features of the G peak are associated with the degree of p-type doping of graphene; a more prominent upward shift of the G peak is observed with increasing p-type doping. Therefore, the doping degree is more strongly correlated with the G-peak shift (with respect to pristine graphene) than with the G-peak position.⁵³ In this study, the Raman spectra revealed a slight downward shift of the G peak with an increasing number of graphene layers at the same doping concentration (Fig. 4c). By contrast, the doped graphene exhibited a significant upward shift compared with the pristine graphene with the same number of layers (Fig. 4d); the G peak was shifted upward by 16.6 cm⁻¹ from 1571.8 (pristine graphene) to 1588.4 cm⁻¹ (doped graphene) in the case of 4L graphene, which was more prominent than the 14.4 cm⁻¹ shift observed for 1L graphene. As mentioned previously, the Au ions were spontaneously reduced to neutral Au particles through the acceptance of electrons from the graphene, leading to heavy p-type doping because this charge-transfer process withdraws electrons from the graphene, thus leaving behind a large number of hole carriers. The same tendency was observed in 3L graphene samples with different precursor concentrations (Fig. S4†), for which the 2D peak showed a slight shift, whereas the G peak was shifted by ∼30 cm⁻¹ with the increase in the doping concentration from 10 to 30 mM.

The mechanism of doping-induced charge transfer was investigated by measuring the carrier concentrations of the doped graphene. Fig. 6a shows the evolution of the carrier concentration from pristine graphene to doped and stacked graphene, where the hole carrier concentration exhibits an increase of approximately two orders of magnitude, from 1.82 × 10¹² cm⁻² (1L pristine graphene) to 1.24 × 10¹⁴ cm⁻² (4L doped graphene). Note that heavy p-doping of graphene can alter the electronic structure of the graphene by shifting the Fermi level closer to the valence band, resulting in a change in the WF. Fig. 5b and c show the UPS spectra near the secondary electron threshold for 1L (Fig. 5b) and 3L (Fig. 5c) graphene at various concentrations of AuCl₃ doping. The WF ϕ can be extracted from the following equation: ϕ = hν − (E_f − E_cut-off), where hν and E_f are the excitation photon energy (21.2 eV) and the Fermi level edge (0 eV, as shown in the full spectra presented in Fig. S5†), respectively. Here, the value of E_cutoff is determined based on the linear extrapolation and x intercept of the binding energy obtained from the UPS spectra. The extracted WFs are indicated in Fig. 5b and c, showing that the value could be gradually altered over an ultra-wide range of 1.46 eV, from 4.26 eV (pristine 1L graphene) to 5.72 eV (doped 3L graphene). To the best of our knowledge,^23,34,54 this represents the widest reported tuning range for a transparent graphene electrode, which allows this novel method to be used to realize the aforementioned active electrodes required for versatile applications in optical electronics.

Fig. 5 (a) The evolution of the hole carrier concentration, assessed using Hall effect measurements, for pristine and doped LBL-stacked graphene films. (b and c) The UPS spectra in the secondary-electron threshold region for (b) monolayer and (c) 3-layer stacked graphene with different AuCl₃ doping concentrations.

For certain specific applications, transparent electrodes are required to be durable and suitable for long-term operation. Most reported doping technologies, including doping with Au ions (AuCl₃ or HAuCl₂), suffer from degradation and a lack of stability testing. Fig. 6 presents the results of monitoring the long-term stability (in an air atmosphere at room temperature) of a doped 3L graphene sample, the sheet resistance of which instantly dropped to 76.2 ohm sq⁻¹ upon doping and then gradually increased to 316.8 ohm sq⁻¹ after three months (see Fig. 6a). Accelerated degradation tests were also performed by heating the sample to 80 °C and 140 °C; in these tests, the sheet resistance rapidly increased to 255.9 (80 °C) and 315.2 ohm sq⁻¹ (140 °C) after 180 min (see Fig. 6b). The results indicate that 140 °C heating can accelerate degradation analogous to that observed over 3 months of room-temperature operation. Therefore, these conditions were adopted as a baseline for the accelerated degradation testing of Au-ion-doped graphene.

Fig. 6 (a) A long-term stability test of a doped (20 mM AuCl₃) 3L stacked graphene sample over 60 days. (b) The sheet resistances of doped graphene observed following thermal treatment at 80 and 140 °C for accelerated degradation testing, suggesting that the degradation after 180 min at 140 °C is close to that observed after 60 days of room-temperature operation.

Moreover, to detail the mechanism of this degradation, the XPS spectra of carbon 1s (C 1s) and chloride 2p (Cl 2p) were measured to understand the chemical bonding status before and after heat treatment. Fig. 7a shows the XPS C 1s spectra of the doped graphene before and after thermal treatment at 140 °C for 180 min, in which the peaks at ∼284.5 eV and 287 eV correspond to the C=C (sp²) and C–Cl bonds, respectively (refer to Fig. S7† for the XPS survey spectrum). Clearly, the ratio of the integrated area of the C–Cl peak to that of the C=C peak decreased from 0.79 for the initial doped graphene sample to 0.49 for the sample subjected to thermal treatment, indicating that the thermal treatment induced the decomposition of the C–Cl bond. Another key observation is the significant reduction in the intensities of the peaks associated with Cl-2p_2/3 (at ∼198.7 eV) and Cl-2p_1/2 (at ∼199.8 eV) in the Cl(2p) spectra (Fig. 7b), implying the loss of Cl from the doped graphene. The XPS results suggest that AuCl₃ doping can immediately lead to significant p-type doping due to the withdrawal of electrons from the graphene through Cl ion adsorption and C–Cl bonding, in addition to the aforementioned Au ion reduction. The subsequent long-term exposure or thermal treatment of the doped graphene results in Cl ion desorption and the decomposition of the C–Cl bond, leading to lower p-type doping of the graphene and thus gradually increasing the sheet resistance.

Fig. 7 (a) The XPS C 1s binding energy spectra of AuCl₃-doped graphene before and after thermal treatment at 140 °C for 180 min. The de-convoluted chemical bonding signals of C=C, C–O, C–Cl, and C–C(O) are indicated by the fitted peaks. (b) The XPS Cl 2p spectra indicating the bonding status of the AuCl₃-doped graphene before and after thermal treatment.

Recently, the use of TC electrodes based on metal meshes, such as those made of copper and silver, has become a mainstream strategy in the industry in addition to ITO. Recent works have proposed hybrid electrodes fabricated by combining graphene with nanowires or printable metal meshes, which results in a lower sheet resistance because the fine metal wire provides efficient charge carrier transport while the graphene fully covers the interspaces of the mesh and enhances carrier collection without degrading its transparency.⁵⁸ Such a hybrid electrode structure compensates for the high sheet resistance of pristine graphene, allowing the sheet resistance to be improved by more than 3 orders of magnitude with little degradation in transmission. In addition, because of the excellent water/gas anti-permeability properties of graphene, the graphene film covering the metal mesh can significantly suppress its oxidation. To obtain a high-performance transparent electrode, this novel strategy of combining a Cu metal mesh with a doped monolayer graphene was implemented by transferring a doped 1L graphene film onto the top of a pre-patterned Cu mesh (with a grid width of 10, 30, or 50 μm) on a glass substrate, as shown in Fig. 8a–c. Fig. S6† shows that the transmittance was decreased by ∼2% when the 1L graphene fully covered the Cu mesh. The sheet resistance was found to be further improved to approximately 37.5, 15.0 and 4.9 ohm sq⁻¹ at transparency levels of 82.1, 72.7, and 62.7%, respectively (Fig. 8d). The correlations between the sheet resistance and the transmittance achieved using the various doping technologies considered in this report are summarized in Fig. 8e, and the results suggest that AuCl₃-doped 3L graphene or a hybrid electrode prepared by combining such a film with a metal mesh can yield the highest performance among all of the investigated graphene-based TC electrodes.

Fig. 8 Optical micrographs of hybrid electrodes composed of doped monolayer graphene on pre-patterned Cu metal meshes with different grid widths of (a) 10 μm, (b) 30 μm, and (c) 50 μm. (d) The sheet resistances and transparencies of these samples under various conditions. (e) Summary of the correlations between sheet resistance and transparency for all TC electrodes investigated in this work.

Conclusions

In conclusion, we developed a novel strategy for preparing active TC electrodes by doping LBL-stacked graphene with AuCl₃, successfully achieving an extremely wide range of WF tunability of up to ∼1.5 eV. Enhanced hole-carrier doping by two orders of magnitude (1.24 × 10¹⁴ cm⁻²) compared with pristine graphene was obtained. The relevant mechanism of the charge transfer between the graphene and the Au ions was characterized based on XPS spectra to elucidate the degradation observed over long-term operation. The excellent performance of the graphene-based TC films was optimized (40 ohm sq⁻¹ at 89.5% transparency) by altering the number of graphene layers and the doping conditions, and it was found that the optimized graphene-based TC film is capable of replacing ITO. Moreover, the rational design of a hybrid electrode with a Cu metal mesh enabled the achievement of a low resistance of ∼4.9 ohm sq⁻¹. In addition, we monitored the long-term stability of AuCl₃-doped graphene for up to 6 months and also constructed a model for accelerated degradation testing. This work introduces a novel type of “active electrode” that not only is a high-performance TC electrode but also exhibits a broadly tunable work function, which can address the needs of a variety of optoelectronic devices with regard to the critical requirement of band-structure alignment at the interface. This active electrode is prepared using a facile and stable doping process, which paves the way for its use in optoelectronic device applications.

Experimental section

Synthesis of CVD-grown graphene film

The graphene was grown in a 1′′ tubular furnace. Prior to the growth process, the Cu foil on which it was grown (Alfa Aesar 99.8%; 25 μm) was polished to flatten the surface via an electrochemical process for 9 min at a constant voltage of 2.5 V, for which an electrolyte of 70 wt% H₃PO₄ (Nihon Shiyaku 85%) with an additive of polyethylene glycol (Alfa Aesar, polyethylene glycol 6000, PEG6000) was used. For the growth process, the substrate was first loaded into a tubular furnace that was evacuated to 5 × 10⁻³ Torr. The temperature was then ramped to 1060 °C under a mixed gas of H₂/Ar (20/80 sccm), with the pressure maintained at 540 mTorr. Surface annealing was performed under a mixed gas of H₂/Ar (30/1000 sccm) at 760 Torr for 30 min to reduce and flatten the Cu surface. During the graphene growth process, the gas atmosphere was switched to CH₄/H₂/Ar (0.5/30/1000 sccm) at 760 Torr for 5 min. Finally, the sample was cooled to room temperature under H₂/Ar (30/1000 sccm) to complete the process.

The transfer and doping of the graphene film

The transfer process was performed by spin coating a supporting layer of poly(methyl methacrylate) (PMMA-A4, MicroCHEM) onto the as-grown graphene/Cu substrate, followed by baking at 80 °C for 5 min. The Cu substrate was then etched away using 2 wt% ammonium persulfate (J.T. Baker), followed by the application of a large amount of DI water to dilute the residue. The graphene/PMMA film was transferred onto the target substrate (i.e., a cleaned glass substrate in this work), and it was then dried by baking at 80 °C for 10 min. The sample was immersed in warm acetone for 10 min to remove the PMMA, followed by rinsing with IPA and DI water. Few-layered (2L–4L) graphene films were prepared by repeating the LBL stacking approach following the transfer procedures described above.

We used four different dopants in this study: (1) TFSA dissolved in CH₃NO₂, (2) PBASE dissolved in DMF, (3) AuCl₃ in CH₃NO₂, and (4) HI vapor. The concentrations of TFSA in CH₃NO₂ were 1, 10, 20, 50, and 100 mM; the concentrations of PBASE in DMF were 1, 5, and 10 mM; and the concentrations of AuCl₃ in CH₃NO₂ were 10, 20, and 30 mM. For HI vapor doping, the graphene samples were treated with HI (57%) vapor at 120 °C for 1 h.

To fabricate the hybrid transparent electrodes consisting of Cu metal mesh and doped graphene film, metal meshes with different grid line widths of 10, 30, and 50 μm were produced via photolithography. The detailed procedure included the following steps: (1) spin coating the positive photoresist (SPR-220, Dow) in two steps, at 1000 rpm for 10 s and then at 3000 rpm for 30 s; (2) baking at 120 °C for 90 s; (3) exposure to UV light for 45 s; (4) developing the pattern with a developer (MF-319, Dow) and then cleaning the residue with DI water, followed by drying with nitrogen gas; (5) depositing Cu (50 nm)/Ti (50 nm) via electron beam evaporation; and (6) removing the unwanted pattern via a lift-off process using acetone and IPA.

Material characterizations

Raman scattering spectral analyses were conducted using a Horiba HR 550 confocal Raman microscope system (laser excitation wavelength = 532 nm; laser spot size ∼0.5 μm). The Raman scattering peak of Si at 520 cm⁻¹ was used as a reference for wavenumber calibration. The chemical configurations were determined using an X-ray photoelectron spectrometer (Phi V6000) with a Mg Kα X-ray source for sample excitation. UPS analyses were performed in a multi-chamber XPS system using a discharge lamp with He–I radiation for excitation. The ionization energy was estimated from the difference in the UPS spectrum between the onset of photoemission and the valence band edge of the graphene film. Using the known position of the Fermi energy, the WF was then derived from the onset of photoemission. SEM analyses were performed using a JEOL-6330F instrument. Measurements of electrical conductivity as well as carrier concentration and mobility were performed using a Hall measurement system (Swin HALL8800). UV-vis-NIR transmittance spectra were obtained using a spectrophotometer (Jasco V670).

Acknowledgements

This research was supported by the Ministry of Science and Technology, Taiwan (102-2221-E-008-113-MY3) (104-3113-E-008-004).

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Footnote

† Electronic supplementary information (ESI) available: The detailed procedures for the CVD growth of graphene and the transfer process; XPS, Raman and UPS spectra for doped graphene are available. See DOI: 10.1039/c6ra04449b

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