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Abstract
Optical properties of benzimidazole (BI)-doped layer-by-layer graphene differ significantly from those of intrinsic graphene. Our study based on transmission electron microscopy and X-ray photoelectron spectroscopy depth profiling reveals that such a difference stems from its peculiar stratified geometry formed in situ during the doping process. This work presents an effective thickness and optical constants that can treat these multi-stacked BI-doped graphene electrodes as a single equivalent medium. For verification, the efficiency and angular emission spectra of organic light-emitting diodes with the BI-doped graphene electrode are modeled with the proposed method, and we demonstrate that the calculation matches experimental results in a much narrower margin than that based on the optical properties of undoped graphene.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, graphene has risen to prominence owing to its superior material properties. The graphene layer is produced by ordering the carbon atoms on the catalytic surfaces of Ni or Cu. The former yields a multilayered graphene film, while the latter yields a monolayer. Thus, chemical vapor deposition (CVD) on Cu foil is advantageous as it can produce a single uniform layer in large area [1,2]. Theoretically, graphene promises to show high electrical sheet conductance and high optical transmittance. However, the actual conductance of graphene is much lower than that shown in theoretical predictions, due to the presence of defects formed during preparation. Two processing limitations degrade the ideal properties of graphene. The first originates from the synthesis because the Cu foil surface is not an atomically smooth single domain. Thus, grain boundaries and pinholes occur during synthesis. The second limitation derives from the transfer process, which can cause local cracking and wrinkles on graphene. These two limitations increase the sheet resistance to an unacceptable level (approximately a few thousand Ω/sq.). To overcome these disadvantages, various doping techniques have been introduced to enhance its conductivity [3–6]. By immersing the graphene layer in a chemical solution such as AuCl3, HNO3, chemically doped graphene with reduced sheet resistance can be obtained [3,4]. Solution-processed chemical doping of graphene using trifluoromethanesulfonic acid also lowers the sheet resistance and increases the work function simultaneously [5]. The surface charge transfers at the interface of graphene with the thermally evaporated metal oxide layer manifests as strong and stable p-type doping of the graphene layers [6]. Defect healing using selective electrochemical deposition on graphene defects was also introduced to improve the electrical conductivity by reducing contact resistance and crack density [7,8].
Various optoelectronic devices using graphene with improved sheet resistance as a transparent electrode have been reported. As an alternative transparent, electrode graphene has been probed in organic light-emitting diodes (OLEDs) and light-emitting diodes (LEDs) with quantum dots (QDs) or perovskite emitters [9–16]. Graphene-based LEDs exhibited optically similar or better efficiency than indium tin oxide (ITO)-based LEDs and were advantageous in their applications in flexible devices. In addition, photovoltaic devices using organic, perovskite, and QD emitters have used graphene in the same manner as proposed for LEDs [17–21]. Our research group has also reported graphene transparent electrode-based OLED [22–27]. Specifically, a 370 × 470 mm2 sized OLED panel and an active-matrix OLED panel using graphene as a transparent electrode was presented [25,26].
The OLED panels demonstrated by our group applied 4-layered graphene doped layer-by-layer with benzimidazole (BI). Graphene electrodes doped with BI exhibited good transmittance-conductance balance with excellent scalability as a transparent electrode [28,29]. The sheet resistance of single-layered graphene is approximately a few hundred Ω/sq. even when a doping technique is applied. Therefore, 2 or more stacked graphene layers are adopted to achieve a sheet resistance lower than 50 Ω/sq. of which value is comparable to that of ITO. BI doping is also advantageous when forming multi-layered graphene. Graphene applied with multi-stacked structure and doping techniques is fabricated in two ways. In the first case, doping is applied on the surface after stacking the whole graphene. In the second case, doping is applied sequentially to individual graphene layers until the desired stack is achieved. The former method is easier to implement, but the latter method is more effective in lowering the sheet resistance. Using BI-doped graphene has both advantages. Since BI doping and Cu etching are performed simultaneously, layer-by-layer doping effect is obtained by simply stacking the BI-doped graphene.
Layer-by-layer doping is excellent electrically, but optical understanding is insufficient. Layer-by-layer doping effectively increases the physical thickness of the graphene-based transparent electrode and changes its optical dispersion. Layer-by-layer molecular doping of graphene embedding tetracyanoquinodimethane (TCNQ) between two graphene layers was reported as an electrode in polymer solar cells [30]. The transmittance of the graphene/TCNQ electrodes decreased and the transmittance difference due to TCNQ doping was noticeable as the number of graphene or deposited TCNQ layers increased. However, analyzing the optical property change by layer-by-layer doping and its relation to the performance of electroluminescent (EL) devices have not been explored in detail. It has been challenging to model optical properties of layer-by-layer doped graphene precisely, making it difficult to design optoelectronic devices made thereof. In this study, the effective optical constants and thickness of layer-by-layer doped graphene are measured. To correlate the number of layer-by-layer doped graphene with the performance of EL devices, we fabricated phosphorescent OLEDs and performed simulations. Our approach can serve as a practical guideline in which hetero-type layered films are used as transparent electrodes.
2. Experimental
2.1 Graphene synthesis, doping and characterization
Graphene was synthesized using hydrogen-free rapid thermal CVD on Cu foil with CH4 a carbon source. Doping of the graphene and etching of the Cu foil was performed by dipping the graphene-grown Cu foil into a liquid solution containing BI [28,29]. The BI dopant layer was covered by a single graphene layer after simultaneous doping and etching. Next, this doped graphene was sequentially transferred to form layer-by-layer doped multilayer graphene electrodes on a target substrate. The repetition of a cycle consisting of transfer of a single-layered graphene and BI-doping then results in a multi-stack structure in which doping-process-induced additional layers are present between the graphene layers. To obtain cross-sectional transmission electron microscopy (TEM) images, dual beam focused ion beam (FIB) system (Helios NanoLab, FEI) was used to prepare samples. Images were obtained using a Cs-Corrected Scanning TEM (JEM-ARM200F, JEOL) facility. Depth profiling of doped graphene films was performed using X-ray photoelectron spectroscopy (XPS) system (K-Alpha+, Thermo Fisher Scientific). Spectroscopy ellipsometry measurements were performed to obtain the complex refractive indices of layer-by-layer doped graphene (M-2000, J.A. Woollam). The transmittance of doped graphene layers was measured using a UV-Vis-NIR spectrophotometer (LAMBDA 750, PerkinElmer).
2.2 OLED fabrication and measurement
To verify the layer-by-layer optical effects on the device level, we fabricated bottom-emission-type phosphorescent green OLEDs having doped graphene as their transparent electrode. A poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, CLEVIOS P VP AI 4083, Heraeus) was spin-coated at 3,000 rpm for 30 s on the UVO-treated graphene-transferred substrate, followed by thermal annealing on a 120 °C hot plate. Organic materials and metal electrodes were deposited using thermal evaporation. The OLED device consisted of a hole transporting layer (HTL), an emission layer (EML), an electron transporting layer (ETL), and a LiF/Al cathode. The HTL layer consists of five pairs of Dipyrazino[2,3-f:2′,3′-h]quinoxaline 2,3,6,7,10,11-hexacarbonitrile (HAT-CN, 10 nm)/ 4,4′-Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC, 40 nm). 2,6-Bis[3-(9H-carbazol-9-yl)phenyl]pyridine (26DCzPPy) doped with Tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3) (20 nm) and 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPhB, 60 nm) was used for the EML and ETL, respectively. A source meter (Keithley 238), a goniometer-equipped spectroradiometer (Minolta CS-2000), and an integrating sphere (HM-series, Otsuka Electronics Korea Co.) was used to measure the electrical and optical properties of the OLEDs. The efficiency of the substrate mode was measured by attaching a half ball lens (10-mm diameter, Edmund optics) to the substrate using index-matching oil.
2.3 Optical simulation
Coherently thin graphene films are deposited on incoherently thick glass substrates. Thus, the transmittances of the graphene samples are predicted using a custom-made calculation tool that is based on the generalized matrix method for mixed coherent and incoherent multilayers [31]. The optical simulation for OLED devices was performed using SETFOS (Fluxim AG), a commercial software. The optical constants of organic materials were measured by spectroscopy ellipsometry and that of Al was obtained from the software. (See Supplement 1, Fig. S1) The emission zone was assumed to be at the EML center. Due to the high electron mobility of 26DCzPPy, electrons can transport more deeply into the EML than the EML/ETL interface [32,33]. In addition, the internal quantum efficiency was adjusted to 98%, so that EQE was 20% in OLEDs with a monolayer graphene electrode.
3. Results and discussion
A previous study has confirmed that the doping materials, BI molecules, are well distributed under the graphene layer using energy dispersive X-ray spectroscopy analysis and sheet resistance mapping of a large-area graphene film [28]. In this study, cross-sectional TEM images confirm the evidence of a uniformly doped graphene film. Because it is difficult to identify doping materials in single-layer graphene, multilayer graphene films were analyzed. Figure 1(a) shows the TEM image of trilayer graphene. Since the doping layer is formed under graphene, an organic layer (HAT-CN, 10 nm) was deposited to distinguish the top graphene layer.
Fig. 1. (a) TEM image and (b) XPS depth profile of trilayer graphene. Inset in (a): molecular structure of BI. Inset in (b): the relative ratio of C 1s and N 1s.
The doping materials are sandwiched between the graphene layers and their physical thickness is about 5 to 10 nm. Although doping materials are not formed by deposition or coating, it is noticeable that the doping layer is formed uniformly with non-negligible physical thickness between the graphene layers. The TEM images of 4-layered and 8-layered graphene spin-coated PEDOT:PSS before fabricating the OLED devices is also shown in Supplement 1, Fig. S2. The physical thickness of the doping layer between graphene layers is similar, except that PEDOT:PSS is 35 nm on the top graphene layer instead of HAT-CN. Even when graphene was transferred several times, the existing BI layers retain their uniformity over graphene surface. Figure 1(b) shows the depth profile of XPS of trilayer graphene on a SiO2/Si substrate. The trilayer graphene contains nitrogen as well as carbon. In particular, nitrogen XPS signal originates from the doping material as shown in the molecular structure of the BI of the inset in Fig. 1(a). In a previous study, a strong N 1s peak was observed in the XPS spectrum of single-layer graphene, which was interpreted as the presence of doping material sandwiched between the overlaying graphene layer and underneath the substrates [28]. As shown in Supplement 1, Fig. S3(a) and (b), the N 1s peak was also observed during a longer etching time in 4-layered and 8-layered doped graphene. The ratio of C 1s and N 1s is maintained before the etch time when the Si 2p or O 1s peak becomes prominent (See the inset of Fig. 1(b)). This result also confirms that the doping materials do not exist only in specific areas, such as the surface of graphene, but exists between graphene layers.
Due to the fine thickness of optoelectronic devices based on organic, QDs, or perovskite materials, the optical resonance effect is significantly important. Considering the dissimilar properties of the doping material and graphene, the actual optical constants of layer-by-layer doped films, which have been overlooked so far in EL devices with graphene films, must be considered. However, it is difficult to obtain the refractive index of the doping material itself because the graphene is doped during the etching process. Therefore, the refractive index of the doped graphene layer is obtained by considering the graphene and doping material as one composite layer. As shown in Fig. 2(a), the doping layer and the graphene layer are effectively regarded as a single medium. In this study, the spectroscopic ellipsometry of the doped graphene monolayer, bilayer, and trilayer is measured. The ellipsometric parameters are fitted assuming that the optical constant of the doped graphene monolayer is the same as in bilayer and trilayer, and the thickness only differs according to the number of doped graphene layers (Supplement 1, Fig. S4). Figure 2(b) shows the measured optical constants of the doped graphene monolayer as well as undoped graphene monolayer reported in previous studies [34–37]. Organic materials such as BI used as doping material have a refractive index (n) and extinction coefficient (k) in the visible wavelength region are significantly low compared with the undoped graphene monolayer. The n of organic materials is approximately 1.5∼2.0 and the k is 0. Therefore, it is reasonable that because graphene and organic doping are modeled as a single layer, both n and k of doped graphene are smaller than the corresponding values of its undoped counterpart. Nevertheless, the difference in the k between the doped graphene and the undoped graphene is much greater than that in the n.
Fig. 2. (a) Single medium model of doped graphene instead of doping layer/graphene composite layer. (b) Effective optical constant of doped graphene compared with the previously reported one.
The thickness determined by the spectroscopy ellipsometry (ellipsometric thickness) of the doped graphene monolayer, bilayer, and triple layer is fitted as 0.875, 1.711, and 2.533 nm, respectively. As the number of layers of doped graphene increases, the fitted ellipsometric thickness increases proportionally. Specifically, the ellipsometric thickness of the doped graphene is measured as 0.875 nm, but that of the undoped monolayer graphene is known as 0.335 nm [34–37]. In addition, the ellipsometric thickness is considerably small compared with the physical thickness of the doping layer shown in Fig. 1(a). To confirm the measured optical constant and ellipsometric thickness, the calculated transmittance was compared with the measurement. Figure 3 shows the transmittance of monolayer, bilayer, and trilayer graphene on glass substrates. The calculated transmittance of the doped graphene layers using effective optical constants and ellipsometric thickness obtained from this study agrees well with the measured transmittance for different number of doped graphene layers. The calculated transmittance using optical constants and ellipsometric thickness obtained from the reference also decreases as the number of graphene layers increases [34]. However, there is a difference in the transmittance measured in the same number of graphene layers, and the difference becomes larger as the number of graphene layers increases. Considering that the transmittance of doped graphene layer is well matched to the calculation with the optical constant and ellipsometric thickness obtained in this study, the optical effect on optoelectronic devices using doped graphene layers must be different from that of undoped graphene layers.
Fig. 3. Measured transmittance (line) and calculated transmittance with optical constants and ellipsometric thickness obtained from this study (open dot) and Ref. [34] (closed dot) of monolayer, bilayer, and trilayer graphene.
The optical effect of doped graphene was confirmed using an OLED device which is one of the representative examples of optoelectronic devices. Figure 4 shows the current density (J), voltage (V), and luminance (L) curves of the OLEDs using various numbers of doped graphene layers as transparent electrodes. As the number of graphene layers increases, the sheet resistance decreases. When a doped graphene layer is stacked up to 1, 2, 4, and 8, the sheet resistance values decrease as 353.5, 153.4, 80.8, and 40.7 Ω/sq., respectively. Although the sheet resistance of the graphene electrode is slightly higher than that of ITO, it is sufficient for use as a pixel electrode with the help of the auxiliary electrode [25,26]. In this study, the emission area of the test device is 2 × 2 mm2, which is small enough that the difference in the sheet resistance affects the conductivity. Thus, the J–V curve is similar for different numbers of graphene layers and the turn-on voltage, where L is 1 cd/m2, is similar. Compared with ITO-based control device, where HAT-CN layer is directly deposited on 70-nm thick ITO without PEDOT:PSS layer, the current density of graphene-based OLEDs is smaller than that of ITO-based OLEDs depending on the operating voltage. However, graphene-based OLEDs have similar turn-on voltage, and achieve about 10,000 cd/m2 at 8 V.
Fig. 4. Current density (J) – voltage (V) – luminance (L) curve of OLEDs with 1-, 2-, 4-, and 8-layered graphene as transparent electrodes.
Figure 5(a) and 5(b) show the external quantum efficiency (EQE) (or out-coupled mode) and substrate mode of OLEDs using 1-, 2-, 4-, and 8-layered graphene as transparent electrodes, respectively. As the number of graphene layers increases, both EQE and substrate mode decrease because the absorption of the graphene layer increases. As shown in Fig. 3, the transmittance of the graphene layer decreases as the number of graphene layers increases. If the decrease in the transmittance of the electrode originates from an increase in the reflectance of the electrode while maintaining low absorption, the efficiency can be increased using the cavity effect. However, the decrease in the transmittance of the graphene layer originates from the absorption. Because the k of graphene is large and its thickness is not negligible as the graphene layers are stacked, the absorption of multilayered graphene is considerable. Therefore, both the EQE and substrate mode linearly decrease as the number of graphene layers increases. Doped monolayer graphene has a smaller k value than that of undoped graphene, but is more than twice as thick as undoped graphene, so it is not beneficial for absorption. OLED devices using doped graphene show slightly lower efficiency when its thickness gets thicker.
Fig. 5. Measured (boxes) and calculated (lines) efficiency of OLEDs with different numbers of graphene layers for (a) air mode and (b) substrate mode using optical constant and ellipsometric thickness obtained from this study for doped graphene and Ref. [34] for undoped graphene.
The increase in the graphene thickness is accompanied by a decrease in efficiency. To confirm this feature, we performed optical simulations. Two optical constants and ellipsometric thicknesses of graphene are compared: one is obtained in this study for the doped graphene and the other is obtained from the reference for the undoped graphene [34]. Both simulation results show that both out-coupled and substrate mode decreases as the number of graphene layers increases as shown in Figs. 5(a) and 5(b). However, the efficiency difference between simulation results in each mode increases as the number of graphene layers increases. The calculated efficiency using the optical constant and the ellipsometric thickness for doped graphene rapidly decreases compared with that for undoped graphene, and is better matched with the measured efficiency in both modes. As a result, when simulating optical properties in an OLED device using doped graphene, it is important to apply measured actual optical constants and the ellipsometric thickness of doped graphene rather than those of the undoped graphene. From the perspective of designing optically optimized OLEDs, attention should be paid to this point.
The validity of the optical constant and ellipsometric thickness of the doped graphene is also confirmed using the angular emission spectra of OLED devices. Figure 6(a) shows the angular emission spectra of the OLED with the monolayer graphene electrode. The emission spectra are maintained as the emission angle increases. When the optical simulations were performed using the optical constants and ellipsometric thicknesses of doped and undoped graphene, respectively, the calculated emission spectra were also similar for different emission angles, as shown in Figs. 6(b) and 6(c). For monolayer graphene, the optical constants of doped and undoped graphene are different, in particular, k is greatly different, but the ellipsometric thickness of both is still less than 1 nm. Therefore, the optical effect on OLED devices is insignificant. The simulation results using different optical constants are similar and agree well with the experimental results. However, as the number of graphene layers increases, the ellipsometric thickness increases, and which result that optical effects gradually appear. Figure 6(d) shows the angular emission spectra of an OLED with 8-layered graphene electrodes. Using different optical constants and ellipsometric thicknesses of doped and undoped graphene, the angular emission spectra are calculated, as shown in Figs. 6(e) and 6(f). Unlike the angular emission spectra for OLED device with monolayer graphene, OLED device with 8-layered graphene shows different angular emission spectra depending on the optical constant and ellipsometric thickness. In the simulation applying the optical constant and ellipsometric thickness of undoped graphene the emission spectrum still does not change significantly as the emission angle increases. However, in the simulation applying the optical constant and ellipsometric thickness of doped graphene, the angular emission spectrum changes according to the emission angle similar to the measurement result. In Supplement 1, Figs. S5(a) – S5(e) show the measured and calculated angular emission spectra of OLED device with 2-layered and 4-layered graphene. The change in the angular emission spectra of them is larger than that with monolayer graphene and smaller than that with 8-layered graphene. As a result, the change in angular emission spectra gradually increases as the number of graphene layers increases. In the optical simulation applying the optical constant and ellipsometric thickness of doped graphene, these changes can be well matched. However, when the optical constant and ellipsometric thickness of undoped graphene is applied, the change in angular emission spectra is insignificant, unlike the experiment.
Fig. 6. Angular emission spectra of OLEDs with monolayer graphene (upper row, (a) – (c)) and 8 layered graphene (lower row, (d) – (f)). The first column shows the measured data, and the second and third columns are calculated using the optical constant and the ellipsometric thickness measured in this study for doped graphene and obtained from Ref. [34] for undoped graphene, respectively.
In the OLED device with 8-layered graphene, the emission spectra of become narrow as the emission angle increases. In particular, as the emission in a long wavelength region decreases, a bandwidth of the emission spectrum decreases. This phenomenon is common in OLEDs with a microcavity effect, in which the resonance wavelength shifts to a shorter wavelength in the emission spectrum [38–40]. A microcavity effect in OLED devices is strongly related to the optical properties of the electrode. When calculating the internal reflectance of 8-layer graphene in the OLED device structure, both doped graphene and undoped graphene have a reflectance of 4% or less, which is low compared to that of a metal thin film widely used as a semt-transparent electrode in cavity OLEDs. (See Supplement 1, Fig. S6(a)) Nevertheless, since doped graphene has higher reflectivity than undoped graphene, the cavity enhancement factor of the doped graphene has a narrower bandwidth than that of undoped graphene as shown in Supplement 1, Fig. S6(b). As a result, the emission spectra in the OLED device with doped graphene are changed, but those with undoped graphene are maintained for different emission angles. Both have similar refractive index, but the thickness of doped graphene is thicker than that of undoped graphene, so the optical path length of doped graphene is larger than that of undoped graphene. Therefore, doped graphene induces a weak resonance effect and there is room for further optical applications such as multilayer electrode architecture in Ref. [10].
4. Conclusions
Owing to the graphene CVD growth processes and the method for transferring graphene to the substrate of interest, the electrical properties of graphene are unsatisfactory. To improve the conductivity of graphene, various doping techniques has been examined. Multi-stack structure of graphene electrodes doped layer-by-layer with BI is implemented easily and effectively enhances conductivity. In this study, we explored the optical effect of these layer-by-layer doped graphene. Cross-sectional TEM images and XPS depth profiles confirmed that the layer-by-layer doped graphene electrode contained a doping layer with a non-negligible thickness. Next, we used OLED devices as a test platform to experimentally and theoretically investigate the change in device performance owing to the modification of the optical properties of doped graphene. By using measured actual optical constant and ellipsometric thickness of doped graphene, it was possible to accurately reproduce both the efficiency and angular emission spectra in simulations. Our results highlight the importance of considering the doping-induced changes in the optical constant and ellipsometric thickness. The optical constant and ellipsometric thickness obtained in this study are limited to BI-doped graphene and cannot be universally applied. However, it should be considered that the optical properties of doped graphene can be significantly affected by the doping material or method. In addition, an optical constant and ellipsometric thickness of doped graphene must be newly obtained as in this study and applied to modeling characteristics of optoelectronic devices.
Funding
Korea Evaluation Institute of Industrial Technology (10044412); Electronics and Telecommunications Research Institute (21ZB1200).
Acknowledgments
The authors thank Hanwha Aerospace for providing graphene layers.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
Supplemental document
See Supplement 1 for supporting content.
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