Efficiency Enhancement of Organic/GaAs Hybrid Photovoltaic Cells ...

480

IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 6, NO. 2, MARCH 2016

Efficiency Enhancement of Organic/GaAs Hybrid Photovoltaic Cells Using Transparent Graphene as Front Electrode Chi-Hsien Huang, Shu-Chen Yu, Yi-Chun Lai, Gou-Chung Chi, and Peichen Yu

Abstract—Large-area graphene of high quality and uniformity was successfully grown by chemical vapor deposition (CVD) using surface oxidation treatment of copper foil prior to the graphene growth step. The graphene was transferred to the polyethylene terephthalate (PET) substrate (G/PET) to act as a transparent front electrode of hybrid heterojunction photovoltaic (PV) cells; these cells were based on a structural motif of poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as the p-type semiconductor, GaAs (1 0 0) as the inorganic n-type semiconductor, and a silver comb electrode atop of PEDOT:PSS. By using G/PET as a transparent front electrode, the power conversion efficiency (PCE), under simulated AM1.5G illumination conditions, was greatly enhanced by up to 26% (from 6.85% to 8.60%). All PV characteristics, including open-circuit voltage (Vo c ), shortcircuit current (Jsc ), and fill factor (FF), contributed to this PCE enhancement. The reflectance, external quantum efficiency, and dark current were investigated to explain this observed PCE enhancement. Although two layers of graphene can efficiently reduce the sheet resistance, the reduction of transmittance in multilayer cells resulted in lower short-circuit current density, leading to lower PCE, in comparison with those with only one layer of graphene. Index Terms—Graphene, organic/inorganic hybrid, photovoltaic cells, thin film devices, transparent electrode.

I. INTRODUCTION N recent years, the emergence of hybrid organic and inorganic semiconductor photovoltaic (PV) cells has attracted much attention due to the urgent need for a new third generation of solar cells, which would ideally be solution-processable PV cells with high power conversion efficiency (PCE) and low manufacturing costs [1]–[4]. Due to the inexpensive fabrication of thin films using solution processes and the mild temperatures at which organic semiconductors can function, they are very promising materials for low-cost PV cells. Unfortunately, the PCE of this class of solar cells cannot compete with that of inorganic solar cells, owing mainly to the low carrier mobility of the former [5]–[7]. Although the PCEs of perovskite-based

I

Manuscript received September 15, 2015; revised October 26, 2015; accepted November 15, 2015. Date of publication December 1, 2015; date of current version February 18, 2016. This work was supported by the Ministry of Science and Technology in Taiwan under Grant 103-2221-E-131-011. C.-H. Huang is with the Department of Materials Engineering, Ming Chi University of Technology, New Taipei City 243, Taiwan (e-mail: chhuang@ mail.mcut.edu.tw). S.-C. Yu, Y.-C. Lai, G.-C. Chi, and P. Yu are with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu 300, Taiwan (e-mail: [email protected]; welkinshining@ yahoo.com.tw; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2015.2501718

PV cells with planar heterojunctions have been demonstrated to be as high as 15.9% [8], the stability of these systems is still a major challenge [9]. By comparison, inorganic waferbased PV cells with four junctions comprised of high-mobility semiconductor materials have high PECs of up to 44.7% [10], yet their fabrication involves high deposition temperatures and ultrahigh vacuum processes, which makes them unsuitable for the development of a new class of solar cells on a large scale. In addition, the inorganic materials for PV cells have become increasingly scarce due to the demand for high PCE PV cells in several sectors. In order to reduce the material cost of this class of PV cells while allowing for wide-spread applications, different cell layouts (such as thin-wafer cells) have been developed to meet these requirements. Given the issues mentioned above, hybrid organic and inorganic semiconductor PV cells have been proposed as a promising candidate for the thirdgeneration PV cells. This class would combine the manufacturing advantages of the second-generation PV cells with the high PCE found in the first-generation cells. These hybrid PV cells would offer a practical way to reduce the cost of manufacturing PV cells by adopting a low-temperature, scalable, and solution-based process of organic materials to form heterojunctions with a crystalline semiconductor at the interface [11], [12]. By further adapting a thin-film template, the hybrid heterojunction approach can be used to make wafer-based PV cells a very viable option for the third-generation solar cells. Transparent electrodes covering 100% of the semiconductor layer as a front electrode play an important role in hybrid PV devices because they can solve the issue of low carrier collection when traditional comb electrodes are used. Furthermore, these electrodes act as a protective layer to prevent degradation of the semiconductor layer resulting from exposure to ambient conditions. Indium tin oxide (ITO) has been widely used as a transparent electrode material for solar cell applications due to its conductivity and transparency in the visible range. However, there are some significant disadvantages, including the limited supply of available indium, high production costs, low transparency in the near-infrared region, and brittleness, which would limit its application in the third-generation solar cells [13]–[15]. In particular, the mechanical properties of ITO make it unsuitable for use in flexible solar cells. Similar to the third-generation solar cells, graphene has also attracted great attention since 2004 due to its outstanding electrical and optical properties [16]. In addition, this material exhibits excellent flexibility and mechanical strength and has a much more abundant and inexpensive source material (carbon) compared with ITO.

2156-3381 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.

HUANG et al.: EFFICIENCY ENHANCEMENT OF ORGANIC/GAAS HYBRID PV CELLS USING TRANSPARENT GRAPHENE

Fig. 1.

481

(a) Transfer of graphene onto PET. (b) GaAs/PEDOT:PSS hybrid PV cell fabrication. (c) Transfer of G/PET to HYBRID PV cell.

Therefore, it is a highly attractive candidate to replace ITO as the front electrode material in solar cells. Besides micromechanical exfoliation of highly ordered pyrolytic graphite, several alternate methods have been developed to achieve reliable, repeatable, and scalable graphene sheets [17]–[22]. Among these methods, chemical vapor deposition (CVD) has been demonstrated as an applicable method for the production of continuous large-scale graphene sheets [23]. In this study, GaAs was used as an n-type inorganic substrate in the construction of PV cells because of its high carrier mobility and direct bandgap, and poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) was used as a p-type organic emitter to form the heterojunction. Although GaAs is relatively more expensive than Si, its high absorption coefficient enables thinner GaAs layers to be used in PV cells by using the lift-off process to reuse the GaAs wafer [24]. Comb electrodes were fabricated on the PEDOT:PSS layer, and then high-quality graphene sheets grown by CVD were transferred onto the PV cells as transparent front electrodes. Prior to the graphene transfer, the shielding ratio of comb electrodes was optimized. A systematic investigation was also conducted on the effect of the number of graphene layers on the open-circuit voltage (Vo c ), short-circuit current (Jsc ), fill factor (FF), and PCE.

II. EXPERIMENTAL STUDY A. Graphene Growth and Transfer Large-area single-layer graphene films were prepared on copper foil by CVD in a tubular quartz furnace. The Cu foil was placed at the center of the quartz tube and then the system was heated to 1050 °C under a constant flow of H2 (20 sccm) at 2.8 × 10−1 torr for 50 min. Then, oxygen gas (3 sccm) was

introduced into the furnace at 7.4 × 10−2 torr for 3 min to grow a thin copper oxide layer. After that, a gas mixture of CH4 (50 sccm) and H2 (20 sccm) was introduced into the system at 1050 °C and the pressure was kept at 6.5 × 10−1 torr for growth of the monolayer graphene; the graphene and Cu foil were then cooled to 25 °C. As shown in Fig. 1(a), the as-grown graphene was transferred from the Cu foil to a PET substrate using a roller press. The PET substrate contains a thin film of silicone layer acting as an adhesive layer to fix the graphene layer onto it and a buffer layer for making the front electrode that will be described in Section IIB. The graphene and PET combination, (G/PET) was then separated from the Cu foil through chemical etching of the Cu in a Fe(NO3 )3 ·9H2 O solution. Finally, the G/PET samples were intensively rinsed with deionized water and then dried using N2 gas. After graphene transfer, Raman spectra of the G/PET samples were collected in a LabRAM HR, Horiba Raman system (laser wavelength: 488 nm; laser spot size: ca., 1 μm). The Si peak at 520 cm−1 was used as a reference for wavenumber calibration prior to each measurement. The transmittance (T) and reflectance (R) spectra were measured using a UV-Vis-NIR spectrophotometer (Hitachi U-4100), and the sheet resistances were obtained by performing 4-probe measurements. B. Hybrid Photovoltaic Cell Fabrication and Characterization The fabrication of the organic/GaAs hybrid PV cells with graphene front electrodes, as shown in Fig. 1(b), started with an n-type GaAs (1 0 0) substrate purchased from AXT, Inc., with a doping concentration of 1–4 × 1017 cm−3 and a thickness of 350 μm. The GaAs substrates were then cut into 2 × 2 cm chips and then immersed in a solution of a buffered oxide etching agent to remove native oxide. Right after the removal of native

482


Fig. 2. (a) Representative Raman spectrum of CVD-grown graphene. (Inset) Raman mapping. (b) Representative transmittance spectrum of CVD-grown graphene. Inset shows the image of G/PET, and the graphene is labeled with red frame.

oxide, the samples were loaded into a thermal evaporator system in order to apply a 100-nm-thick Al layer as the back electrode. Then, a highly conductive aqueous PEDOT:PSS dispersion (Levios PH 1000) with 5 wt.% dimethyl sulfoxide as a secondary dopant were spin coated onto the GaAs substrates at the spin rate of 8000 r/min. The coated organic layers were heated to 115 °C for 15 min in ambient conditions. The thickness and sheet resistance of coated PEDOT:PSS layer are ∼40 nm and ∼3500 Ω/, respectively. After that a 100-nm-thick Ag comb electrode (as the anode) with a shielding ratio of 14.4% was thermally evaporated onto the substrate through a shadow mask. The shielding ratio was optimized for PCE (see Fig. S1 in the Supporting Information). Finally, the G/PETs were placed onto the samples by roller press, as shown in Fig. 1(c). Due to the existing buffer layer of silicone, the graphene can contact both Ag electrode and PEDOT:PSS conformally. PV characterization of the fabricated hybrid PV cells was conducted under simulated AM1.5G (Air Mass 1.5, Globe) illumination conditions. The measurement system was comprised of a 1000-W Class A solar simulator (Newport 91192A) with a xenon lamp and an AM1.5G filter, a power supply (Newport 69920), a probe stage, and a source-meter (Keithley 2400) using a four-wire connection mode. The spectrum of the solar simulator was calibrated by a PV measurement (PVM-154) monocrystalline Si solar cell (NREL calibrated), and a Si photo diode with KG-5 color filter (Hamamatsu, Inc.) was used to check the irradiation of the exposed area (100 mW/cm2 ). The cells under measurement were performed using a black stainless metal mask with an opening area of 1 cm2 , and the temperature was actively controlled at 25 ± 0.5 °C during the measurements [25]. The system used to measure the external quantum efficiency (EQE) consisted of a 450-W xenon lamp (Oriel Instruments, model 6266) light source, a water-based IR filter (Oriel Instruments, model 6123 NS), and a monochromator (Oriel Instruments, model 74100). The incident light was focused onto a 1 mm × 3 mm spot between the comb electrodes. A calibrated silicon photodetector (Newport 818-UV) was used to calibrate the EQE system before each measurement. A lock-in amplifier (Standard Research System, SR830) and an optical chopper controller (SR540) in the voltage mode were also equipped to

Fig. 3. Current density–voltage characteristics of the hybrid PV cells without and with G/PET under simulated AM1.5G illumination conditions. TABLE I PV CHARACTERISTICS, INCLUDING V o c , Jsc , FF, AND PCE OF HYBRID PV CELLS WITHOUT AND WITH G/PET PV cells Without G/PET With G/PET

V o c (V)

J s c (mA/cm2 )

FF(%)

PCE(%)

0.69 0.71

17.55 18.14

63.06 72.38

6.85 8.60

lock the output signal, and the photocurrent was converted to voltage using a 1-Ω resistor, which was in parallel with the sample. The cell’s temperature was also actively controlled at 25 ± 0.5 °C during the measurements. III. RESULTS AND DISCUSSION Raman spectroscopy is commonly used to characterize the graphene nanostructures. The typical features that are observed are a G band (ca., 1580 cm−1 ) and a 2-D band (ca., 2700 cm−1 ), which are characteristics of the C–C bonds of the sp2 -hybridized C atoms. In addition, a D band (ca., 1345 cm−1 ) represents the bonds involving non-sp2 -hybridized C atoms, derived from atomic-scale defects or lattice disorder. Fig. 2(a) shows a representative Raman spectrum of the CVD-grown graphene on Cu foil fabricated in this study, which features an intensity ratio for


483

Fig. 4. (a) Dark current density–voltage characteristic in a semilogarithmic plot and (b) reflectance (R) and EQE spectra of the fabricated hybrid PV cells without and with G/PET.

the 2-D to G bands [I(2-D)/I(G)] of approximately 2.9 and a narrow full-width at half-maximum of the 2-D band (ca., 28 cm−1 ), consistent with a monolayer of graphene [26]. In addition, the D band was almost invisible in the spectrum, suggesting that the monolayer graphene used in this experiment was of high quality [27]. Raman mapping of graphene was also carried out, as shown in the inset of Fig. 2(a), where it can be seen that an I(2-D)/I(G) ratio of > 2.5 was observed for more than 90% of the measured area (9 × 9 mm2 ), indicating that high-quality graphene is largely enough for transparent electrode application. The high-quality graphene with large area is owing to the surface treatment of oxidation for copper foil during the CVD process [28], [29]. Fig. 1(b) shows the transmittance spectrum of G/PET, and the weightage transmittance (T) ranging from 300 to 1100 nm was 87.0%, calculated using 1100 T =

T (λ)IAM 1.5G (λ)dλ 1100 300 IAM 1.5G (λ)dλ

300

(1)

where T(λ) is the transmittance and IAM 1.5G (λ) is the intensity illuminated under M1.5G at various wavelengths. Furthermore, the sheet resistance of G/PET obtained by four-point measurement was 1325 Ω/sq. After confirming the high quality of CVD-grown graphene, the G/PET was layered onto the PEDOT:PSS/GaAs hybrid PV cells featuring comb electrodes. Fig. 3 shows the representative current density–voltage (J–V) curves of the hybrid PV cells without and with graphene. From these, it is clear that the hybrid PV cells with graphene have better PV performance than the cells without graphene. The PV characteristics, including open-circuit (Vo c ), short-circuit current density (Jsc ), FF, and PCE of the hybrid PV cells without and with G/PET are summarized in Table I. The hybrid PV cells with graphene possess a Vo c of 0.71 V, Jsc of 18.14 mA/cm2 , and FF of 72.38%, yielding a PCE of 8.60%, which improves upon that of the cells without graphene by 26%. The Vo c and FF were increased from 0.69 to 0.71 V and from 63.06% to 72.38%, respectively, after applying G/PET as a transparent front electrode. Both characteristics are highly related to the conductivity of PV cell. We examined the dark J–V characteristics of hybrid PV cells. As shown in Fig. 4(b), it can be seen that the saturation current density of hybrid PV cells with G/PET is lower than

that of the cells without G/PET at reversed bias. We considered that the higher conductivity of graphene layer improves the carrier conduction through the interface between PEDOT:PSS and GaAs leading to the lower carrier recombination. As a result, the lower saturation current was obtained resulting in higher Vo c and FF. It is also worthwhile to note that a significant increase in Jsc also contributes to PCE enhancement. In order to explore possible avenues for improvement, reflectance (R) and EQE measurements were performed for hybrid PV cells with and without G/PET, as shown in Fig. 4(b). The hybrid PV cells with G/PET exhibit a lower surface reflection, which is consistent with recent reports [30], and this lower surface reflection leads to a higher light harvesting capability. On the other hand, the EQE of the hybrid PV cells with G/PET shows higher efficiency across a broad spectral range (425–875 nm), indicating low surface recombination in the cell. Furthermore, the contact area between the electrode and the PEDOT:PSS layer is drastically enlarged by using graphene as a transparent electrode, which greatly improves the efficiency of carrier collection. Accordingly, the hybrid PV cells fabricated with G/PET demonstrated higher Jsc . Multilayer graphene can efficiently reduce the sheet resistance to enhance the carrier collection [31]. In this study, an additional graphene layer was added to G/PET following the same transfer process as mentioned in Section II. After adding one more layer of graphene, the sheet resistance of G/PET decreased to 772 Ω/sq. Fig. 5(a) shows the J–V characteristics of hybrid PV cells with 0, 1, and 2 layers of graphene on PET. The dependence of PV characteristics such as Vo c , Jsc , FF, and PCE on the number of graphene layer was displayed in Fig. 5(b). Unfortunately, the PCE decreased slightly when adding one more layer of graphene to G/PET as a transparent front electrode. One possible explanation is the significant reduction of Jsc resulting from the decrease of transmittance, as shown in Fig. 5(c), while the FF and Vo c did not change obviously. As the transmittance of three layers of graphene further decreased, as shown in Fig. 5(c), in this study, hybrid PV cells with three layers of graphene on PET were not synthesized for the investigation of relevant PV characteristics. Given these results, it is apparent that a single graphene layer on an Ag comb electrode with a shielding ratio of 14.4% had the best PCE for the hybrid PEDOT:PSS/GaAs PV cell.

484


Fig. 5. (a) Current density–voltage characteristics and (b) PV characteristics V o c , Jsc FF, and PCE of the hybrid PV cells with various numbers of graphene layers. (c) Weightage transmittance of various number of graphene layer on PET.

IV. CONCLUSION Graphene was grown via a CVD process, with surface oxidation treatment prior to the graphene growth step. Using Raman spectroscopy, it was observed that the graphene area of I(2-D)/I(G) occupied more than 90% of 1 × 1 cm with sheet resistance of about 1.3 kΩ/sq. Then, the CVD-grown graphene was transferred to PET to act as a transparent front electrode for hybrid heterojunction PV cells based on PEDOT:PSS/GaAs and Ag comb electrodes atop of PEDOT:PSS. By using G/PET as a transparent front electrode, the PCE of this system was greatly enhanced by up to 26% compared with similar systems without G/PET. By using G/PET as a transparent front electrode, the FF was significantly increased due to the high conductivity of graphene. The Vo c was also slightly increased after applying G/PET as a transparent front electrode, which resulted from the lower saturation current in the dark J–V curve. Moreover, the lower surface reflection, higher EQE, and additional contact area between graphene and PEDOT:PSS further improved the Jsc and FF boosting the efficiency up to 8.60%. Although two layers of graphene can efficiently reduce the sheet resistance, the reduction of transmittance resulted in lower short-circuit current density leading to lower PCE of these PV cells than those cells with only one layer of graphene. ACKNOWLEDGMENT The authors would like to thank Prof. H.-F. Meng, Institute of Physics, National Chiao Tung University, for fruitful discussions.

REFERENCES [1] D. Chi, B. Y. Qi, J. Z. Wang, S. C. Qu, and Z. G. Wang, “High-performance hybrid organic-inorganic solar cell based on planar n-type silicon,” Appl. Phys. Lett., vol. 104, no. 19, pp. 193903-1–193903-4, May 2014. [2] S. H. Eom et al., “Roles of interfacial modifiers in hybrid solar cells: Inorganic/polymer bilayer vs inorganic/polymer:fullerene bulk heterojunction,” ACS Appl. Mater. Interfaces, vol. 6, no. 2, pp. 803–810, Jan. 2014. [3] Y. Zhang et al., “Heterojunction with organic thin layers on silicon for record efficiency hybrid solar cells,” Adv. Energy Mater., vol. 4, no. 2, p. 1300923, Sep., pp. 1300923-1–1300923-7, 2014. [4] M. Pietsch, M. Y. Bashouti, and S. Christiansen, “The role of hole transport hybrid inorganic/organic silicon/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) heterojunction solar cells,” J. Phys. Chem. C, vol. 117, no. 18, pp. 9049–9055, Apr. 2013. [5] P. L. Ong and I. A. Levitsky, “Organic/IV, III-V semiconductor hybrid solar cells,” Energies, vol. 3, no. 3, pp. 313–334, Mar. 2010. [6] T. Kirchartz, T. Agostinelli, M. Campoy-Quiles, W. Gong, and J. Nelson, “Understanding the thickness-dependent performance of organic bulk heterojunction solar cells: The influence of mobility, lifetime, and space charge,” J. Phys. Chem. Lett., vol. 3, pp. 3470–3475, Nov. 2012. [7] B. Ray, M. R. Khan, C. Black, and M. A. Alam, “Nanostructured electrodes for organic solar cells: Analysis and design fundamentals,” IEEE J. Photovolt., vol. 3, no. 1, pp. 318–329, Jan. 2013. [8] G. Hode, “Perovskite-based solar cells,” Science, vol. 342, no. 6156, pp. 317–318, Oct. 2013. [9] J. D. Fan, B. H. Jia, and M. Gu, “Perovskite-based low-cost and high-efficiency hybrid halide solar cells,” Photon. Res., vol. 2, no. 5, pp. 111–120, Aug. 2015. [10] F. Dimroth et al., “Wafer bonded four-junction GaInP/GaAs/ GaInAsPGaInAs concentrator solar cells with 44.7% efficiency,” Prog. Photovolt. Res. Appl., vol. 22, no. 3, pp. 277–282, Jan. 2014. [11] J. Ackermann, C. Videlot, A. E. Kassmi, R. Guglielmetti, and F. Fages, “Highly efficient hybrid solar cells based on an octithiophene-GaAs heterojunction,” Adv. Funct. Mater., vol. 15, no. 5, pp. 810–817, May 2005. [12] T. Song, S. T. Lee, and B. Sun, “Prospects and challenges of organic/group IV nanomaterial solar cells,” J. Mater. Chem., vol. 22, no. 10, pp. 4216–4232, Feb. 2012.


[13] A. Kumar and C. Zhou, “The race to replace tin-doped indium oxide: which material will win,” ACS Nano, vol. 4, no. 1, pp. 11–14, Jan. 2010. [14] C. Soldano et al., “ITO-free organic light-emitting transistors with graphene gate electrode,” ACS Photon., vol. 1, no. 10, pp. 1082–1088, Sep. 2014. ¨ [15] Y. Wang, S. W. Tong, X. F. Xu, B. Ozyilmaz, and K. P. Loh, “Interface engineering of layer-by-layer stacked graphene anodes for high performance organic solar cells,” Adv. Mater., vol. 23, no. 13, pp. 1514–1518, Jan. 2011. [16] K. S. Novoselov et al., “Electric field effect in atomically thin carbon film,” Science, vol. 306, no. 5956, pp. 666–669, Oct. 2004. [17] C. Berger et al., “Electronic confinement and coherence in patterned epitaxial graphene,” Science, vol. 312, no. 5777, pp. 1191–1196, Apr. 2006. [18] G. Eda, G. Fanchini, and M. Chhowalla, “Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material,” Nature Nanotechnol., vol. 3, no. 5, pp. 270–274, Apr. 2008. [19] P. W. Sutter, J. I. Flege, and E. A. Sutter, “Epitaxial graphene on ruthenium,” Nature Mater., vol. 7, no. 5, pp. 406–411, Apr. 2008. [20] S. Stankovich et al., “Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide,” Carbon, vol. 45, no. 7, pp. 1558–1565, Jun. 2007. [21] A. N. Obraztsov, E. A. Obraztsova, A. V. Tyurnina, and A. A. Zolotukhin, “Chemical vapor deposition of thin graphite films of nanometer thickness,” Carbon, vol. 45, no. 10, pp. 2017–2021, Sep. 2007. [22] X. S. Li et al., “Large-area synthesis of high-quality and uniform graphene films on copper foils,” Science, vol. 324, no. 5932, pp. 1312–1314, May 2009. [23] S. Bae et al., “Roll-to-roll production of 30-inch graphene films for transparent electrodes,” Nature Nanotechnol., vol. 5, no. 8, pp. 574–578, Jun. 2010.

485

[24] A. van Geelen et al., “Epitaxial lift-off GaAs solar cell from a reusable GaAs substrate,” Mater. Sci. Eng. B, vol. 45, nos. 1–3, pp. 162–171, Mar. 1997. [25] T. G. Chen et al., “Flexible silver nanowire meshes for high-efficiency microtextured organic-silicon hybrid photovoltaics,” ACS Appl. Mater. Interfaces, vol. 4, no. 12, pp. 6857–6864, Nov. 2012. [26] C. H. Huang, C. Y. Su, C. S. Lai, Y. C. Li, and S. Samukawa, “Ultralow-damage radical treatment for the highly controllable oxidation of large-scale graphene sheets,” Carbon, vol. 73, pp. 244–251, Jul. 2014. [27] M. A. Pimenta et al., “Studying disorder in graphite-based system by Raman spectroscopy,” Phys. Chem. Chem. Phys., vol. 9, no. 11, pp. 1276–1291, Jan. 2007. [28] Y. Hao et al., “The role of surface oxygen in the growth of large singlecrystal graphene on copper,” Science, vol. 342, no. 6159, pp. 720–723, Oct. 2013. [29] J. Pan et al., “Oxidation as a means to remove surface contaminants on Cu foil prior to graphene growth by chemical vapor deposition,” J. Phys. Chem. C, vol. 119, no. 23, pp. 13363–13368, Jun. 2015. [30] X. Jiang et al., “Anti-reflection graphene coating on metal surface,” Surf. Coatings Technol., vol. 261, pp. 327–330, Nov. 2015. [31] Y. Wu et al., “Graphene transparent conductive electrodes for highly efficient silicon nanostructures-based hybrid heterojunction solar cells,” J. Phys. Chem. C, vol. 117, no. 23, pp. 11968–11976, May 2013.

Authors’ photographs and biographies not available at the time of publication.