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Abstract
To obtain solution-processable graphene in bulk-quantities, the chemical route was widely used to prepare graphene oxide (GO); however, the reduction process was inevitably needed to transform insulating GO into semi-metallic reduced graphene oxide (rGO). Therefore, to obtain stable rGO dispersions, many researchers introduced insulating surfactants that prevent irreversible aggregation. Although insulating surfactants were introduced to produce the stable dispersion of rGO, some properties of rGO, such as its dispersibility, processability, and electricity, are still problematic. In order to solve these problems, we synthesized novel rGO (F-OH-rGO) using the reduction process after blending GO and fullerenol (F-OH). By replacing insulating polymers with semiconducting and highly water-soluble F-OH, F-OH-rGO with not only higher conductivity but also a high dispersion concentration as well as film-forming properties could be achieved. We introduced various GO derivatives as hole-transporting layers (HTLs) to organic solar cells (OSCs) with discrepancies in important properties, so the fabricated solar cells manifested significant differences in their performances as we expected.
© 2017 Optical Society of America
1. Introduction
Among the carbon-based materials such as 0-dimensional (0-D) fullerene, 1-D carbon nanotube, 2-D graphene, and 3-D graphite, graphene has attracted great attention due to its extraordinary properties [1,2]. Geim et al. demonstrated the superior properties of mono-layered graphene in 2004 by the Scotch tape method [1]. Although mono-layered graphene obtained by the method possesses decent properties and quality, the method should be replaced with an alternative to make it suitable for mass production (i.e., commercialization and manufacturing of large area devices). Therefore, numerous researchers have introduced various methods to obtain few-layer graphene sheets and even mono-layered graphene [3–13].
To acquire decent few-layer graphene, two representative ways are commonly used: the top-down (ultrasound [3,4], ball milling [5,6], and oxidation [7–10] and bottom-up (chemical vapor deposition (CVD) [11,12], epitaxial growth [13]) approaches. Among these methods, the CVD method was recognized as the best way to obtain high quality graphene. However, with almost every CVD method, it is hard to avoid commercially unattractive procedures, such as a vacuum condition [14] and a high temperature processing [15]. In the case of the top-down approach by employing exfoliation of graphite, ultrasound methods are confronted by a cooling problem due to the high thermal energies that occur during the exfoliation of graphite. According to a report [16], although the ball milling method has been suggested as one of the efficient ways to obtain high quality graphene sheets and even mono-layered graphene, it has a limitation in terms of the inevitable detaching process. Among these various methods for obtaining decent graphene layers, the exfoliation of graphene by oxidizing graphite was widely implemented due to its relatively facile procedure, monolayer acquirability, introduction of functional groups to graphene sheets (both basal plane and edge plane), and solution dispersibility. During the oxidization step, many sp2 conjugations in graphene were broken by employing oxide functional groups to the edge and basal planes. To recover sp2 π-π bonding between carbon atoms, the reduction process was used with a modified version of Hummers’ method in order to remove oxide functional groups [17].
Reduced graphene oxide (rGO) has better metallic properties than graphene oxide (GO) after the reduction process due to significant recovery of sp2 conjugations as mentioned. However, its dispersibility in aqueous solvents deteriorates after the reduction process because of the occurrence of aggregation phenomena (i.e., Van der Waals interactions between graphene originating from the 2-D sheet structure of graphene [18]). To prevent the aggregation of graphene during the reduction process, surfactants having amphiphilic functionalities were introduced to stabilize the dispersion of rGO in aqueous solvents because they interacted with both the hydrophobic graphene sheet and hydrophilic solvent molecules [17]. However, representative surfactants, such as poly(sodium 4-styrenesulfonate) (PSS), and poly(ethylene oxide) derivatives possess an insulating characteristic that has an adverse effect on the conductivity of graphene. To overcome this problem, a novel surfactant is essential for both stabilizing rGO in aqueous solution even water and mitigating graphene conductivity losses.
In the present paper, we synthesized water-soluble fullerene derivatives with many hydroxyl groups, fullerenol (F-OH) to stabilize rGO during the reduction process. F-OH was synthesized using a modified version of the method presented in previous reports [19], and it was soluble in water up to 30 mgml−1. Consequently, we obtained the novel rGO (F-OH-rGO), stabilized with F-OH. It dispersed in water at a concentration of 2 mgml−1, indicating that it was far superior to PSS based rGO [17], and it had a conductivity value that was almost 100 times higher than that of PSS-based rGO. To verify its potential for use with electronic applications, we fabricated organic solar cells (OSCs) with F-OH-rGO as a hole-transporting layer (HTL), and devices fabricated via novel rGO HTL showed better performances than conventional devices.
2. Experiments
The materials, Sodium hydroxide (reagent grade, 97%, flakes, DC chemical), Fullerene C60 (99%, Nano-C), Polyethylene glycol 400 (PEG 400, 400 g/mol, TCI), Graphite power (< 20 μm, powder, synthetic, Aldrich), Sulfuric acid (ACS reagent, 95-98%, Aldrich), Potassium permanganate (ACS reagent, 99%, Aldrich), Hydrogen peroxide (contains inhibitor, 30 wt %, solution in water, ACS reagent, Aldrich), and all solvents were used as received without further purification.
Synthesis of fullerenol (F-OH): Fig. 1(a) was briefly shown the synthetic scheme of fullerenol (F-OH) and this synthesis procedures are very similar to the previous report [19]. The C60 (50 mg) in benzene solution (70 ml) were vigorous stirred for at least 30 min in a reaction flask. Sodium hydroxide (NaOH, 2 g) and PEG 400 (50 μl), as a surfactant, solution dissolved in DI water (2 ml) mixing via vortexes (220 V, 60 Hz). The reaction was taken place in air at room temperature during 24 h by adding NaOH and PEG 400 solution to C60 solution. After then, the originally deep purple colored solution in flask turned to colorless and precipitated black sludge. After vaporizing of benzene solvent by rotary pump evaporator, DI water (30 ml) was added to the black sludge and stirred for 1 h to dissolve the sludge completely. The water-insoluble product was filtered by glass filter and the filtered solution was concentrated in vacuum. Methanol (MeOH) was added to the concentrated solution and the brown precipitate (F-OH) was produced. The precipitate was washed with MeOH until the pH of rinsed solvent was fixed and then the precipitate step was proceeded twice. However, fullerenol solution was still exhibited pH> 9, indicating incomplete removal of NaOH. Consequently, the further purification step was needed. The diffusion dialysis process was carried out during 3 days for removal of residual NaOH [19]. After the diffusion dialysis process step, the aqueous fullerenol solution with a pH at or below that of the deionized water was used to dissolve the sample. A brown solid product (40 mg/yield: 49%) was obtained by precipitation of fullerenol solution and drying in vacuum oven through overnight. IR (KBr) ν: 3400, 1702, 1605, 1446, 1384, 1080 cm−1; XPS At %: C 60.09, Na 4.06, O 28.28; Anal. Calcd for C60(OH)20.5(ONa)7 wt %: C, 53.69; H, 1.54. Found: C, 53.93; H, 1.55.
Fig. 1 (a) Synthetic scheme of fullerenol (F-OH), and (b) Schematic illustration of F-OH-rGO, F-OH(1/4)-rGO, and rGO preparation.
Synthesis of graphene oxide (GO): The graphite powder (1 g) was added to flask of concentrated sulfuric acid (95%, 50 ml), and then totally 3 g of potassium permanganate was added for three times in order to avoid the rapidly rising of temperature after that the flask heated to 40 °C during 6 h and then, pouring the enough crumbling ice which is containing H2O2 (30 wt %, 10 ml) over the GO slurry. Originally dark green slurry was turned to dark brown slurry through the diffusion dialysis process during one week and the resultant suspension was filtered over a PTFE (polytetrafluoroethylene) filter membrane. The remaining solid was thoroughly washed with 1M HCl followed by acetone and water. The GO powder obtained by freeze-dried at −60 °C for 24 h. Finally, the GO dispersion in deionized water was obtained at the concentration of ~1 mgml−1, and used for device applications [20].
Synthesis of reduced graphene oxide (rGO): Ultrasound was applied to a mixture of PSS (0.105 g) and aqueous GO solution (1 mgml−1, 30 ml) during 15 min, and then hydrazine (35 wt % in water, 110 μl) was added to a mixture at 100 °C under a reflux condenser at N2. The reduction of GO was implemented for 3 h. Finally, a stable black suspension of rGO was obtained at a concentration of ~1 mgml−1, and used for device applications [20].
Synthesis of reduced graphene oxide with fullerenol (F-OH-rGO): For the synthesis of the F-OH-rGO, the 15 min sonication was applied to a mixture of F-OH (30 mg) and aqueous GO solution (1 mgml−1, 30 ml), and then hydrazine (35 wt % in water, 110 μl) was added to a mixture at 100 °C under a water-cooled condenser with N2 purging. In addition, higher concentration of F-OH-rGO (~2 mgml−1) solution was able to prepare by two times concentrated GO solution. Finally, the stable F-OH-rGO solution was obtained at a concentration of ~1 mgml−1, and used for device applications [20].
Synthesis of reduced graphene oxide with fullerenol (F-OH(1/4)-rGO): The synthetic procedures of F-OH(1/4)-rGO were identically equal to the F-OH-rGO, excluding the weight ratio of fullerenol about GO. The mixture is reacted which is sonicated F-OH (7.5 mg) with aqueous GO solution (1 mgml−1, 30 ml). Finally, the stable F-OH(1/4)-rGO solution was obtained at a concentration of ~1 mgml−1, and used for device applications [20].
3. Results and discussion
To prepare the water-dispersible rGO, water soluble fullerene stabilizers were synthesized as shown in Fig. 1(a) [19]. The synthesized fullerene oxide (F-OH) was introduced as a dispersion stabilizer in the reduction step to prevent of the re-aggregation of GO in water, and introduce the hydrogen bonding interaction between GO and F-OH. As shown in Fig. 1(b), two kinds of rGO in water were prepared with different stabilizers (similar to the surfactant) corresponding to the previously reported [17] surfactant PSS and synthesized F-OH. The aqueous GO solution to which PSS was added during the reduction process was named “rGO”, GO with equivalent weight ratio of F-OH was named “F-OH-rGO”, and GO with a quarter weight ratio of F-OH was named “F-OH(1/4)-rGO”. The novel F-OH-rGO had a dispersion concentration in water that was seven times higher than that of conventional rGO, and it obtained more uniform and stable dispersion without aggregation and precipitation over one month.
The film status of GO derivatives was evaluated by atomic force microscopy (AFM). As shown in Fig. 2, the full coverage of the film GO and F-OH-rGO images indicated that the graphene sheets with high dispersion concentration and uniformity could be coated on substrates even in such a high-speed spin-coating condition. The root mean square (RMS) roughness of GO, rGO, F-OH-rGO, and F-OH(1/4)-rGO were 1.09, 1.78, 1.32, and 1.14 nm, respectively. rGO had the highest RMS roughness value due to the PSS polymers attached to the basal plane of the graphene sheet [17]. In the case of GO and rGO with F-OH, the RMS roughness values increased as more F-OH was introduced as a stabilizer. A thicker single sheet than GO (~1 nm) [8] in F-OH(1/4)-rGO (~1.16 nm) was derived from the attachment of the F-OH stabilizer to the basal plane of the graphene sheet. It is highly suspected that the smaller sheet size in rGO than GO originated from the ultrasound step in the synthetic procedure. According to the AFM analysis, a higher concentration of F-OH leads to a more covered film morphology and affects film roughness. When we spin-coated the GO derivatives on the top of Glass/ITO substrates, the thickness of films were GO (~6 nm), rGO (~38 nm), F-OH-rGO (~9 nm), and F-OH(1/4)-rGO (~8 nm), respectively.
To investigate the amount and species of oxygen groups in GO derivatives, an X-ray photoelectron spectroscopy (XPS) analysis was implemented for the four samples. The most variable peak of the C1s spectra in Fig. 3, with a binding energy of ~286.7 eV indicated that the representative oxygen peaks of GO such as the hydroxyl and epoxy groups, were in the basal plane of the graphene sheet [21]. The remarkably reduced oxygen peaks in rGO compared with GO showed that the oxygen groups generated in the oxidization step were almost removed by the modified version of Hummers’ method implemented in our work. The F-OH-rGO series might have contained more oxygen groups than rGO prepared by the commonly used PSS surfactant due to the oxygen groups from the F-OH molecules. According to the amount of F-OH, the fact that F-OH(1/4)-rGO possessed lower oxygen peaks than F-OH-rGO is consistent with this former statement.
Fig. 3 The XPS C 1s spectra of GO derivatives via various reduction process and observed their differences of C-O functionality.
To evaluate the hole-transporting properties of novel GO derivatives, OSCs were fabricated based on the Poly(3-hexylthiophene-2,5-diyl) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PC61BM) active layers which are the most common active materials in OSCs. As shown in Fig. 4(a), GO derivatives were spin-coated on the top of an indium tin oxide (ITO)/Glass substrate as normal structure. According to I-V curves and Table 1 of the fabricated solar cells, solar cell with F-OH(1/4)-rGO showed the best power conversion efficiencies (PCEs) at ~3.15% (1% higher than pristine GO). In the case of open circuit voltage (Voc), pristine GO had a higher Voc value than the others due to the lowest fill factor (FF) value in rGO and the presence of F-OH in F-OH-rGO HTLs. Every device possessed a similar current density except pristine GO, which had an insulating property due to the existence of numerous uncovered broken sp2 orbitals [21]. The FF values followed the tendency of RMS roughness in the AFM images, so F-OH(1/4)-rGO obtained the highest FF value, and GO had a lower FF value than the others due to its insulating nature, which increased the series resistance.
Fig. 4 (a) Schematic device structure of OSCs with interfacial layers by GO derivatives. (b) The current density-voltage (J-V) characteristics of OSCs with various GO derivatives.
Table 1. Photovoltaic parameters for P3HT:PC61BM BHJ solar cells based on different interfacial layers.
To determine the differences in the device parameters by following each HTL, the work function (WF) and conductivity of materials were evaluated by ultraviolet photoelectron spectroscopy (UPS) and 4-point probe, respectively as shown in Fig. 5.
The highest Voc (0.61 V) value in pristine GO was well matched with the WF data (5.0 eV) in Table 2. The lowest WF value of F-OH (4.5 eV) led to a slightly higher WF value in F-OH(1/4)-rGO (5.0 eV), than F-OH-rGO (4.9 eV), which had equal amounts of F-OH and GO. Although the GO and F-OH(1/4)-rGO showed the same WF values, F-OH(1/4)-rGO had a lower Voc in solar cells. This was because of the F-OH acting as a recombination site at the interface due to fullerene’s intrinsic high electron affinity [22] and GO’s hole-transporting ability [23]. The higher photovoltaic performance of F-OH(1/4)-rGO than F-OH-rGO derived from the higher WF value of F-OH(1/4)-rGO that highly related to the Voc of photovoltaic device and also the lower concentration of F-OH mitigated the surface recombination at interfacial layers between HTL and active layer that led to a higher Voc value. To measure the conductivity of GO derivatives, the following equation was used:
where R (Ω) is the sheet resistance, d (m) is the film thickness, and σ (Sm−1) is the conductivity.Table 2. Work function and conductivity values of GO derivatives.
After reduction process, every GO derivative possessed 10 to 100 times higher conductivity than GO. According to the conductivity differences, GO derivatives after the reduction process showed higher current densities than GO in OSCs. Replacing an insulating PSS, which is conventionally used as a surfactant for preventing the aggregation of graphene sheets during reduction, with semiconducting F-OH for higher conductivity was successfully completed to achieve an even higher dispersion concentration in water (2 mgml−1). In doing so, the conductivities of GO with the F-OH stabilizer were obtained 100 times higher than that of pristine GO and 10 times higher than that of conventional r-GO. As shown in the AFM images, attaching of an insulating PSS polymer to the whole graphene basal plane, which was determining the electrical properties of graphene, induced the lowest conductivity value among the rGO films. Therefore, F-OH(1/4)-rGO possessed the highest solar cell performance owing to its higher dispersion concentration in water and higher electric conductivity than pristine GO and conventional rGO.
4. Conclusion
In conclusion, we successfully synthesized rGO in aqueous solution with a novel highly water-soluble F-OH (> 30 mgml−1) stabilizer. A higher dispersion concentration and stability of F-OH-rGO than conventional rGO could be originated from the proper adhesion of the F-OH particles to basal plane of the GO sheets. In addition, the replacement of insulating PSS surfactant with semiconducting F-OH induced higher electrical conductivity, which is important to the resistance of interfacial layers in OSCs. Finally, replacing the hydroxyl groups in F-OH with other functional groups is expected to open new possibilities in terms of increasing their solubility to other solvents and tuning electrical, optical, chemical, and even mechanical properties for various applications.
Funding
National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF-2015R1A2A1A10054466); the Korea Institute of Energy Technology Evaluation and Planning (KETEP); the Ministry of Trade, Industry and Energy (MOTIE) of the South Korea (No. 20163030013900); and the GIST Research Institute (GRI) grant funded by the GIST in 2017.
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