Layer-by-Layer Self-Assembled Graphene Multilayers as Pt-Free

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Layer-by-Layer Self-Assembled Graphene Multilayers as Pt-Free Alternative Counter Electrodes in Dye-Sensitized Solar Cells Adila Rani,† Kyungwha Chung,† Jeong Kwon,‡ Sung June Kim,‡ Yoon Hee Jang,† Yu Jin Jang,† Li Na Quan,† Minji Yoon,† Jong Hyeok Park,§ and Dong Ha Kim*,† †

Department of Chemistry and Nano Science, Division of Molecular and Life Sciences, College of Natural Sciences, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Korea ‡ School of Chemical Engineering and SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 440-746, Republic of Korea § Department of Chemical and Biomolecular Engineering, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea S Supporting Information *

ABSTRACT: Low cost, charged, and large scale graphene multilayers fabricated from nitrogen-doped reduced graphene oxide N-rGO(+), nitrogen and sulfur codoped reduced graphene oxide NS-rGO(+), and undoped reduced graphene oxide rGO(−) were applied as alternative counter electrodes in dye-sensitized solar cells (DSSCs). The neat rGO-based counter electrodes were developed via two types of layer-bylayer (LBL) self-assembly (SA) methods: spin coating and spray coating methods. In the spin coating method, two sets of multilayer films were fabricated on poly(diallyldimethylammonium chloride) (PDDA)-coated fluorine-doped tin oxide (FTO) substrates using GO(−) combined with N-GO(+) followed by annealing and denoted as [rGO(−)/N-rGO(+)]n or with NS-GO(+) and denoted as [rGO(−)/NS-rGO(+)]n for counter electrodes in DSSCs. The DSSCs employing new types of counter electrodes exhibited ∼7.0% and ∼6.2% power conversion efficiency (PCE) based on ten bilayers of [rGO(−)/N-rGO(+)]10 and [rGO(−)/NS-rGO(+)]10, respectively. The DSSCs equipped with a blend of one bilayer of [rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)] on PDDA-coated FTO substrates were prepared from a spray coating and showed ∼6.4% and ∼5.6% PCE, respectively. Thus, it was demonstrated that a combination of undoped, nitrogen-doped, and nitrogen and sulfur codoped reduced graphene oxides can be considered as potentially powerful Pt-free electrocatalysts and alternative electrodes in conventional photovoltaic devices. KEYWORDS: counter electrode, DSSCs, electrocatalyst, layer-by-layer self-assembly, nitrogen-doped reduced graphene oxide, nitrogen and sulfur codoped reduced graphene oxide good electrocatalytic activity in the I−/I3− pair. It serves to inject electrons in the I−/I3− pair in the redox electrolyte.4 Therefore, a counter electrode is an important component of the DSSC device because it catalyzes the reduction of the redox species. Currently, Pt is the most commonly used catalyst for the I−/ − I3 redox pair as the counter electrodes of DSSCs due to its superior electrocatalytic activity, good chemical and corrosion stability, low sheet resistance, and high transparency.5 However, Pt is the most expensive component in DSSCs and can be decomposed in the I−/I3− redox pair in long-term use. Thus, it is highly desirable to explore Pt-free counter electrode with

1. INTRODUCTION Dye-sensitized solar cells (DSSCs), reported by O’Regan and Gratzel in 1991,1 have stood out as a prominent third generation category in photovoltaic devices due to low cost production, high efficiency, ease of fabrication, and short energy payback.2,3 The generic DSSC device consists of a dyeadsorbed mesoporous titanium dioxide (TiO2) nanoparticles photoanode, platinum (Pt)-based photocathode or counter electrode, sensitizer (dye), and a redox mediator such as iodide electrolyte. The operation mechanism of the DSSCs device involves photoinduced oxidation of a dye molecule on a working TiO2 photoanode, the reduction of the oxidized dye molecules by the iodide/triiodide (I−/I3−) pair, and the diffusion of the oxidized species from the surface of the dye to the counter electrode. The regeneration of the sensitizer is possible by the assistance of counter electrode (Pt) due to its © XXXX American Chemical Society

Received: February 11, 2016 Accepted: April 25, 2016

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DOI: 10.1021/acsami.6b01770 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

different charged and doped graphene oxide structures. In spin coating methods, multilayers of [GO(−)/N-GO(+)]n and [GO(−)/NS-GO(+)]n were deposited on PDDA-coated fluorine-doped tin oxide (FTO) substrates followed by annealing in an Ar atmosphere.34 In the spray coating method, a monolayer of [GO(−):N-GO(+)] and [GO(−):NS-GO(+)] blend was deposited on PDDA-coated FTO substrates followed by annealing in an Ar atmosphere. The performance of DSSCs equipped with different types of reduced graphene oxide-based counter electrodes was systematically compared along with the controlled thickness and surface coverage. Electrochemical impedance spectroscopy (EIS) measurement and cyclic voltammetry (CV) supported the observed comparable performance, leading us to conclude that neat, charged, and doped/undoped reduced graphene oxide prepared from LBL SA can be promising candidate materials for the low-cost, easily fabricable, and highly efficient counter electrode of DSSCs.

stable, cheaper, abundant, and eco-friendly materials for practical applications. Many carbon-based hybrid materials have been suggested as counter electrodes.6−15 Among them, solution processed graphene-based counter electrodes have recently shown promise as an alternative counter electrode. Graphene films for counter electrodes have been fabricated by various techniques, including thermal exfoliation of graphite oxide,16 the oxidative exfoliation of graphite followed by hydrazine reduction,17 the chemical reduction of graphene oxide colloids under microwave irradiation,18 electrophoretic deposition followed by an annealing treatment,19 hydrothermal method to get metal-free nitrogen-doped graphene,20 nitrogen-doped graphene foams,21 nitrogen-doped graphene nanoplatelets using the electrospray (e-spray) method,22 and layer-by-layer (LBL) self-assembly (SA) of poly(diallyldimethylammonium chloride) (PDDA) and graphene oxide followed by electrochemical reduction.23 Much work has also been demonstrated using graphene-polymer hybrid materials.24,25 In all the cases, it was found that numerous issues related to functional groups, reduction method, lattice defects, surface adhesion, and thermal annealing of the graphene sheets play a role in the electrocatalytic activities in counter electrodes. Previous works presented that the defects and the amount of oxygen-containing groups are intimately correlated with the catalytic activity.26,27 It has also been reported that incorporation of heteroatoms can improve the electrocatalytic properties of graphene materials. Xu et al. reported that incorporation of nitrogen in the carbon framework of graphene oxide led to enhanced catalytic property23 due to the disparity between the electronegativity of the carbon atoms and nitrogen of ammonium ions. Yen et al. reported metal-free nitrogen-doped reduced graphene prepared by the hydrothermal method and used as counter electrode.20 Further, Xue et al. proposed nitrogen-doped graphene foam as metal-free counter electrodes and argued that doping with nitrogen atoms led to performance improvements resulting from the alteration of electronic structure of graphene.21 Furthermore, Luo et al. demonstrated the synthesis of nitrogen and sulfur dual-doped reduced graphene oxide and used a disulfide/thiolate redox shuttle mediator as counter electrode.28 Most recently, it was reported that nitrogen and sulfur codoped graphene nanosheets fabricated from a hydrothermal method showed synergistically enhanced performance as counter electrodes.29 Therefore, tuning the chemical parameters and synthetic techniques of graphene may lead to promising DSSC characteristics and open wide opportunities to manipulate their electrocatalytic properties. Further, the LBL SA technique provides highly ordered nanoscale-level control of the thickness and composition of different materials. It is achieved through the sequential adsorption of oppositely charged components by electrostatic interactions or other attractive forces while preserving the unique characteristics of their oppositely charged components. Thus, the highly ordered multilayered architectures via LBL SA can provide exquisite design protocol, reproducible fabrication, and synergistic effect from constituents.30 Although the LBL SA-based deposition method of graphene thin films has already been reported,30−33 viable incorporation of charged doped analogue structures of reduced graphene oxide into counter electrodes based on the LBL SA technique has not been systematically investigated. Herein, we introduce for the first time graphene-based counter electrodes in DSSCs using LBL SA in spin- and spray-coating methods using

2. EXPERIMENTAL PROCEDURES 2.1. Preparation of Negatively Charged Graphene Oxide and Positively Charged Graphene Oxide. Negatively charged graphene oxide GO(−) was prepared using the modified Hummer’s method35,36 followed by the ultrasonication. Graphite flakes (2.5 g) were added to a flask containing H2SO4 (57 mL) with vigorous stirring. To the stirred dispersion, NaNO3 (1.5 g) was added, and after 1 h of stirring, the dispersion was cooled to 0 °C in a bath with ice water. Next, KMnO4 (7.5 g) was gradually and slowly added to the flask while maintaining the temperature below 20 °C. Then, the resulting suspension was maintained at 35 °C for 2 h. At the end of 2 h, cold deionized water was added slowly. The diluted suspension was stirred at this temperature for 15 min. Then, the mixture was further diluted by adding 350 mL of warm deionized water, and 50 mL of 30% H2O2 was added to the suspension to reduce the residual permanganate and manganese dioxide to colorless soluble manganese sulfate. Finally, the resulting yellow brown solution was centrifuged at 8000 rpm for 30 min to avoid precipitation of the slightly soluble salt of mellitic acid formed as a side product. The resulting suspension was washed and centrifuged with a mixture of HCl and deionized water to remove the remaining metal ions until no sulfate ions were detected in a BaCl2 solution test. The light brown colored GO solution was then washed repeatedly with deionized water until the pH of the filtrate became neutral. The resulting washed GO slurry was freeze-dried and stored in a desiccator. After that, the required amount of GO in deionized water was ultrasonicated, centrifuged, and used as negatively charged graphene oxide GO(−). Positively charged nitrogen-doped graphene oxide N-GO(+) was synthesized by decorating an amine functional group on the surface of GO(−).37 GO(−) suspension (50 mL) was stirred at 60 °C for 4 h with 1-ethyl-3-[3-(dimethylamino) propyl] carbodiimide hydrochloride EDC (100 mg), ethylamine (2 mL), and triethyl amine (1 mL) and used as positively charged nitrogen-doped graphene oxide. Positively charged nitrogen and sulfur codoped graphene oxide NS-GO(+) was synthesized by decorating amine and thiol functional groups on the surface of GO(−). GO(−) suspension (50 mL) was stirred at 60 °C for 4 h with EDC (100 mg), ethylamine (1 mL), triethyl amine (1 mL), and 4-amino thiophenol in ethanol (2 mL) and used as positively charged nitrogen and sulfur codoped graphene oxide. 2.2. Preparation of Photocathode: PDDA[rGO(−)/N-rGO(+)]n, PDDA[rGO(−)/NS-rGO(+)] n , PDDA[rGO(−):N-rGO(+)], and PDDA[rGO(−):NS-rGO(+)]. Fluorine doped tin oxide (FTO) substrates were cleaned by piranha solution followed by oxygen plasma treatment for 10 min (50 sccm and 100 W) to make the surface hydrophilic. (CAUTION: “Piranha” solution reacts violently with organic materials; it must be handled with extreme care.) After that, the FTO substrates were coated with 2% aqueous solution of PDDA. Then, different numbers of bilayers were spin-coated at 1000 rpm for 1 min driven by electrostatic interaction between GO(−) and N-GO(+) or GO(−) and NS-GO(+) followed by annealing at 400 °C for 2 h in B

DOI: 10.1021/acsami.6b01770 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Synthetic route of N-GO(+) and NS-GO(+) from GO(−), (b) colloidal stability of prepared N-GO(+), NS-GO(+), and GO(−) suspensions in water, (c) schematic illustration of layer-by-layer (LBL) assembly of N-GO(+) or NS-GO(+) with GO(−) on PDDA-coated FTO substrates using spray coating and spin coating methods. After LBL assembly of [GO(−):N-GO(+)] and [GO(−):NS-GO(+)] by spray coating, the films were annealed at 400 °C in an Ar atmosphere in order to reduce [GO(−):N-GO(+)] and [GO(−):NS-GO(+)] to [rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)], respectively, and (d) schematic diagram of a DSSC equipped with reduced graphene oxide-based counter electrode. C

DOI: 10.1021/acsami.6b01770 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) Raman spectra of rGO(−), N-rGO(+), and NS-rGO(+), with inset showing the 2D band, (b) UV-vis absorbance of diluted solutions of GO(−), rGO(−), N-rGO(+), and NS-rGO(+), (c) wide scan XPS spectra of GO(−), N-GO(+), and NS-GO(+), (d) peak deconvolution spectra of C 1s of GO(−), (e) peak deconvolution spectra of C 1s of N-rGO(+), (f) peak deconvolution spectra of C 1s of NS-rGO(+), (g) AFM image of rGO(−), (h) AFM image of N-rGO(+), and (i) AFM image of NS-rGO(+). an Ar environment. After each spin-coating, the substrates were washed once with deionized water for 1 min at 1000 rpm. The fabricated devices using the spin coating method were designated as [rGO(−)/N-rGO(+)]n and [rGO(−)/NS-rGO(+)]n, respectively. In the spray coating method, 2 mL of (GO(−)) solution (0.05%) was first spray-coated using a nitrogen stream on preheated PDDA-coated FTO substrates at 120 °C and maintained for 5 min, and then, 2 mL of N-GO(+) or NS-GO(+) solution was spray-coated on the same substrates. After annealing at 400 °C for 2 h in an Ar environment, the one bilayer devices were designated as [rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)], respectively. The reference or standard photocathode was prepared using ∼2.5 nm thick Pt coating on cleaned FTO substrate. 2.3. Preparation of Photoanode. FTO substrates were sequentially cleaned by ultrasonication in acetone, IPA, and deionized water baths for 15 min each, followed by drying with nitrogen stream. Nanocrystalline TiO2 paste was deposited on the cleaned FTO substrates using a doctor blade and annealed at 550 °C for 90 min. After that, the resulting substrates with an area of 10 mm2 were immersed into a solution of ruthenium dye (cis-diisothiocyanatobis(2,20-bipyridyl-4,40-dicarboxylato) ruthenium(II) bis(tetrabutylammonium), N-719, Solaronix) by soaking in 0.2 mM dye/ethanol solution for 18 h at room temperature followed by washing with ethanol and dried using blowing nitrogen gas. 2.4. DSSC Device Fabrication. Both the photocathode and photanode were combined using spacers (50 μm thick hot-melt sealing foil, SX1170-25, Solaronix), and an ionic liquid electrolyte, 0.60 M

BMIM-I, 0.03 M I2, 0.50 M TBP, and 0.10 M GTC in acetonitrile/ valeronitrile 85/15 (v/v) (no. ES-0004, io.li.tec), was then injected into the small gap between the two electrodes driven by capillary force. The electrochemical properties and energy conversion efficiency of the DSSCs were measured by using a POLARONIX K3000 Solar Cell I-V measurement system under simulated AM 1.5G illumination with an intensity of 100 mW cm−2. 2.5. Characterization. The quality of GO(−), N-GO(+), and NSGO(+) was analyzed on silicon substrates by Raman Spectroscopy using HORIABA Jobin Yvon, at excitation wavelength of 630 nm. The chemical identity of GO(−) and annealed samples at 400 °C (NrGO(+) and NS-rGO(+)) was investigated by X-ray photoelectron spectroscopy (XPS) using an ESCALab spectrometer (Thermo VG, U.K.) with monochromated Al Kα radiation. The deposition behavior of LBL assembly of different bilayers was analyzed by surface plasmon resonance (SPR) spectroscopy (Resonant Technologies GmbH/ RT2005 SPR spectrometer) using p-polarized laser light (He−Ne, 632.8 nm, 10 mW) illuminating the Au-film through the prism. All the samples were deposited on Au-coated glass substrates by spray and spin coating methods. Calculation of GO thickness was carried out using Winspall software (RES-TEC, Germany) with a complex refractive index value of 3 + 1.149106i for GO.38 It was assumed that GO(−), N-GO(+), and NS-GO(+) have the same refractive index values. UV−visible spectra were collected by a UV−vis-NIR spectrometer (Cary 5000, Varian Inc.) on quartz and FTO substrates. The surface morphology was investigated by atomic force microscopy (AFM) using a Dimension 3100 scanning force microscope in tapping D

DOI: 10.1021/acsami.6b01770 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces mode (Digital Instrument) and field emission scanning electron microscopy (FE-SEM, JSM-6700F, JEOL). The photoactive area was tested with a mask after measuring the area using image analysis software, ImageJ. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical analyzer (IVIUMSTAT.XR, IVIUM Technologies). The EIS spectra were fitted to the equivalent circuit by Z-View software.

analyzed by XPS after annealing at 400 °C in an Ar atmosphere (Figure 2c−f). The survey XPS spectrum of GO(−) shows only C 1s and O 1s peaks at 282 and 230 eV, respectively. In the case of N-rGO(+), the XPS spectrum shows an additional N 1s peak at 399 eV. Compared to the GO(−) and N-rGO(+), the XPS spectrum of NS-rGO(+) reveals both N 1s and S 2p peaks. High resolution spectra of N 1s and S 2p of both N-rGO(+) and NS-rGO(+) are shown in Figure S1. The peak at 399 eV corresponds to graphitic nitrogen, and additional S 2p peaks in NS-rGO(+) around 167 and 162 eV are attributed to the C− Sn−C (n = 1, 2), C−SOx−C (x = 2, 3, 4), and SO2−C (Sulphone).28,44−46 Figure S1a shows three different types of nitrogen moieties in the deconvoluted spectra for N-rGO(+), i.e., pyrrollic N (398.8 eV), pyridinic N (400.05 eV), and graphitic N (401.4 eV).44−46 Figure S1b reveals two different sulfur moieties in the deconvoluted spectra for NS-rGO(+), i.e., C−Sn−C (n = 1, 2) and C−SOx−C (x = 2, 3, 4). From the deconvoluted spectra in Figure 2d−f, we found that a proportion of C−O−C groups of GO(−) was converted to C−N and C−S groups in both N-rGO(+) and NS-rGO(+).20 XPS results confirmed that the doping of nitrogen in N-rGO(+) and codoping of nitrogen and sulfur in NS-rGO(+) were successfully performed, and C 1S peaks in the spectra for NrGO(+) and NS-rGO(+) show that the delocalization of πelectrons was increased by the incorporation of doped nitrogen and codoped nitrogen and sulfur after annealing. Figure 2g−i displays the AFM images of rGO(−), N-rGO(+), and NSrGO(+), and Figure S1c−e shows the thickness of GO(−), NGO(+), and NS-GO(+) measured from the height profile images, respectively. The results show that graphene oxide sheets are irregular in shape with a height of ∼1 nm in all the samples. To analyze the deposition behavior and thickness of the multilayers obtained from the suspensions of GO(−), NGO(+), and NS-GO(+), UV−visible spectroscopy and SPR spectroscopy measurements were carried out. The buildup of different bilayers of [GO(−)/N-GO(+)]n and [GO(−)/NSGO(+)]n was monitored using UV−visible spectroscopy, and the growth of the film was assessed by a regular increase in absorbance upon adsorption of each graphene oxide (GO) bilayer (see Figure S2). SPR is a unique optical phenomenon observed at the gold−dielectric interface in nanoscale dimension, and the resonance band position depends on various parameters including optical property of the prism, type of metal (gold), thickness and refractive index of the medium in contact with the metal, wavelength of the light source, etc.47 It provides useful information about the thickness and refractive index of the adsorbate layers on gold film.48−51 All the samples were deposited on Au-coated glass substrates by spray and spin coating methods. Calculation of GO thickness was carried out using Winspall software (RES-TEC, Germany) with complex refractive index value of 3 + 1.149106i for GO.38 It was assumed that GO(−), N-GO(+), and NS-GO(+) have the same refractive index values. Figure 3a,b represents the scan mode SPR angular curves of the multilayers ([GO(−)/N-GO(+)]n and [GO(−)/NS-GO(+)]n) obtained from stepwise spin coating of 0.05% GO solutions. SPR angle shifted progressively to higher incident angle as the number of deposited GO bilayers increased from one bilayer to 15 bilayers in both [GO(−)/N-GO(+)]n and [GO(−)/NS-GO(+)]n. Broadening of curves and increase of reflectivity at SPR angles were observed with an increase in the number of layers due to the increase of surface roughness inducing scattering of incident

3. RESULTS AND DISCUSSION Figure 1a describes the synthetic procedure of nitrogen-doped graphene oxide N-GO(+) and nitrogen and sulfur codoped graphene oxide NS-GO(+) derived from graphene oxide GO(−) in the presence of 1-ethyl-3-[3-(dimethylamino) propyl] carbodiimide hydrochloride (EDC) with ethylamine and triethylamine and/or 4-amino thiophenol in ethanol.37 The resulting graphene oxides in solution (Figure 1b) were used to fabricate photocathode using spin coating and spray coating methods. Different types of LBL SA bilayers consisting of GO(−) and N-GO(+) or GO(−) and NS-GO(+) were deposited on solid substrates followed by annealing at 400 °C in the presence of an Ar stream.34 Figure 1c represents the schematic illustration of LBL SA of different bilayers by spin and spray coating methods. The sample codes prepared by the spin coating method was represented as [rGO(−)/N-rGO(+)]n and [rGO(−)/NS-rGO(+)]n. For the samples prepared by the spray coating method, the counter electrodes were notated by [rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)]. The schematic diagram of the entire cell equipped with the spray-coated counter electrode [rGO(−):N-rGO(+)] was illustrated in Figure 1d. Further, Raman spectroscopy of rGO(−), N-rGO(+), and NS-rGO(+) was investigated and presented in Figure 2a. The peaks of G-band for rGO(−), N-rGO(+), and NS-rGO(+) were observed at 1585, 1595, and 1597 cm−1, respectively. It corresponds to the first order scattering of the optical E2g mode and is attributed to the bond stretching or vibrations of sp2 carbon pairs.39,40 The D-band is a measure of disorder and structural imperfections created by the attachment of hydroxyl, epoxide, and other functional groups on the carbon lattice and appears at 1342, 1332, and 1339 cm−1 in rGO(−), N-rGO(+), and NS-rGO(+), respectively. Next, the overtone 2D band of rGO(−), N-rGO(+), and NS-rGO(+) was observed around ∼2699 cm−1, representing the formation of bilayers of graphene and significant structural changes.40 The Raman spectra of (NrGO(+)) and (NS-rGO(+)) show lower D/G ratio (0.831 and 0.836) than (rGO(−)) (1.20) which might be due to the larger domain size of the sp2 by providing free electrons upon reaction with amines and/or thiol group in graphene oxide to fill the gaps in the electrical structure, resulting in significant improvement of electrocatalytic properties.40−42 Overall, Raman spectra show significant disorder in the carbon lattice of the (rGO(−)) suspension than (N-rGO(+)) and (NSrGO(+)). Figure 2b shows the UV−visible absorption spectra of GO(−), rGO(−), N-rGO(+), and NS-rGO(+) solutions on quartz substrates. The absorption peak of GO(−) appeared at 232 nm. After annealing and doping of nitrogen (N-rGO(+)) and codoping of nitrogen−sulfur (NS-rGO(+)), it shifted to 260 nm. Further, the absorption peak of rGO(−) appeared at 260 nm in the same annealing condition without doping, indicating that the electronic conjugation of GO(−) is restored in rGO(−), N-rGO(+), and NS-rGO(+).43 The doping status of the annealed samples of N-rGO(+) and NS-rGO(+) was E

DOI: 10.1021/acsami.6b01770 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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in intensity after the deposition of each layer (Figure S3c,d). Figure 4 shows SEM images of the different multilayers of [rGO(−)/N-rGO(+)]n and [rGO(−)/NS-rGO(+)]n on FTO substrates. It is observed that the FTO substrates are covered with thin layers of different bilayers by increasing the number of bilayers. Figure 5 shows the histograms of the distribution of device performances of DSSCs fabricated with different multilayers of [rGO(−)/N-rGO(+)]10 and [rGO(−)/NS-rGO(+)]10 prepared by the spin coating method as counter electrodes on FTO substrates. The cell equipped with 10 bilayers of [rGO(−)/NrGO(+)]10 and [rGO(−)/NS-rGO(+)]10 at 0.05% concentration showed higher power conversion efficiency (PCE) by increasing the number of multilayers from 5 to 15 bilayers. In contrast, Pt-based reference DSSCs exhibited PCE of 7.14%. Overall, in both [rGO(−)/N-rGO(+)]n and [rGO(−)/NSrGO(+)]n, the PCE was increased upon increasing the number of bilayers from 5 to 10 and decreased after 15 (Table 1 and Figure S4). We investigated the performance of each type of device using almost 30 different samples. It was observed that the current density was increased from ∼15 to ∼20 mA/cm2 in spin-coated devices ([rGO(−)/N-rGO(+)]10 and [rGO(−)/ NS-rGO(+)]10). The better performance of spin-coated devices might be due to the higher specific surface area and more uniform coating.20 Furthermore, low FF values were observed in all the devices presumably due to thin coating of the bilayers on the rough surface of FTO substrates as analyzed by SEM images. The higher FF value of Pt compared is ascribed to the high electronic conductivity; i.e., Pt metal has lower charge transfer resistance which is favorable for faster reduction of I3− ions. Also, the surface morphology of the thin coating of the bilayers is less dense compared to the compact surface of Pt, which prevents the charge transfer at the composite electrode, resulting in a lower FF value.52,53 However, due to the high Jsc value of the spin-coated device [rGO(−)/N-rGO(+)]10, the overall efficiency close to that of Pt electrode was obtained. To check the effect of concentration of different solutions, devices with different bilayers were fabricated using thicker solutions (0.1%) using the spin coating method and the results were summarized in Figure S5 and Table S1. Next, the spray coating technique was employed to prepare blend films of [rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)] as counter electrodes in DSSCs to improve the FF value. The

Figure 3. Surface plasmon resonance (SPR) curves of spin-coated multilayers of (a) [GO(−)/N-GO(+)]n and (b) [GO(−)/NSGO(+)]n.

light. A regular shift in SPR angle upon each bilayer deposition is observed as shown in Figure 3, indicating that the deposition of GO bilayers was regular with a linear increase in thickness.38 To evaluate the thickness changes during the LBL SA process, a simulation study was conducted using Winspall software and it was observed that an ∼1 nm thick GO layer was loaded on the Au surface by each deposition.47 Thus, it was estimated that the 10 bilayers of [GO(−)/N-GO(+)] and [GO(−)/NS-GO(+)] have ∼20 nm thickness. The SPR angular shift was also checked for the films prepared from higher concentration solutions (0.1%) of GO(−), N-GO(+), and NS-GO(+), and the results are presented in Figure S3a,b. The deposition of multilayers was also analyzed by UV−visible spectroscopy, and a regular deposition is assessed, evidenced by a linear increase

Figure 4. SEM images of different multilayers on FTO prepared by the spin coating method. (a) [rGO(−)/N-rGO(+)]5, (b) [rGO(−)/NrGO(+)]10, (c) [rGO(−)/N-rGO(+)]15, (d) [rGO(−)/NS-rGO(+)]5, (e) [rGO(−)/NS-rGO(+)]10, and (f) [rGO(−)/NS-rGO(+)]15. F

DOI: 10.1021/acsami.6b01770 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Histogram of current−voltage characteristics of the DSSCs devices: (a−d) [rGO(−)/N-rGO(+)]10 and (e−h) [rGO(−)/NS-rGO(+)]10.

Table 1. Photovoltaic Parameters of DSSCs Fabricated with Different Multilayers of [rGO(−)/N-rGO(+)]n and [rGO(−)/NSrGO(+)]na counter electrodes Pt coating [rGO(−)/N-rGO(+)]5 [rGO(−)/N-rGO(+)]10 [rGO(−)/N-rGO(+)]15 [rGO(−)/NS-rGO(+)]5 [rGO(−)/NS-rGO(+)]10 [rGO(−)/NS-rGO(+)]15 a

Voc [V] 0.73 0.68 0.73 0.68 0.63 0.69 0.65

± ± ± ± ± ± ±

JSC [mA cm−2]

0.08 0.02 0.03 0.02 0.03 0.02 0.03

14.95 7.96 20.59 15.01 13.58 20.53 16.75

± ± ± ± ± ± ±

0.91 0.82 0.81 0.82 0.83 0.90 0.81

FF [%] 64.75 45.09 48.91 39.32 43.80 45.23 40.98

± ± ± ± ± ± ±

PCE [%] 2 1 4 4 3 4 4

7.14 2.44 7.03 3.69 3.75 6.22 4.38

± ± ± ± ± ± ±

0.05 0.05 0.09 0.08 0.06 0.89 0.49

The values were estimated from Figure 5.

deposition of the blend films was analyzed by SPR spectroscopy and UV−visible spectroscopy (Figure S6). Figure 6a,b represents the SPR spectroscopy results obtained from both [GO(−):N-GO(+)] and [GO(−):NS-GO(+)], respectively. Due to the thick blend coating, the SPR peak showed a larger shift with broadening of the curve after spray coating of N-rGO(+) and NS-rGO(+) in [GO(−):N-GO(+)] and [GO(−):NS-GO(+)], respectively. The surface morphology and thickness during the spray coating process was also analyzed using SEM images (Figure 7). It was observed that the entire surface was uniformly covered with the multilayer films and the thickness was measured to be in the range of 230−250 nm (Figure S7) in both [rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)] films. The histogram and performance characteristics of the DSSC devices with thick coatings were presented in Figure 8 and Table 2 (also see Figure S8). The [rGO(−):N-rGO(+)] device exhibited higher PCE than the [rGO(−):NS-rGO(+)] device. It is interesting to note that the PCE of the devices [rGO(−)/NrGO(+)]10 and [rGO(−):N-rGO(+)] are higher than that of the [rGO(−)/NS-rGO(+)]10 and [rGO(−):NS-rGO(+)], respectively. This can be ascribed to the formation of aggregated sheets of NS-GO(+) in both spin and spray coating methods. It is easily discerned in AFM spectroscopy (Figure S1c,d) that NS-GO(+) sheets were more aggregated and took an irregular shape compared to N-GO(+). This would reduce the electrochemical catalytic activity toward reduction of tri-iodide ions, resulting in low PCE and electrocatalytic activities as compared to the cell with N-GO(+).

In order to evaluate the catalytic activities and the performance of all the spin- and spray-coated devices, cyclic voltammetry and electrochemical impedance measurements were carried out. Figure 9a shows the cyclic volatammograms of different spin- and spray-coated devices in LiClO4 solutions. It was observed that all the graphene-based electrodes exhibited higher current density due to the high specific surface area and high incidence of defects. Two pairs of peaks were observed for all electrodes, similar to what has been reported for comparative systems in the literature.21,29,54 The results suggested that the devices showed similar electrocatalytic activities in terms of the current density, onset potential, and the value of peak-to-peak separation. Figure 9b shows the Nyquist plots obtained from the cells with different types of rGO-based devices using both spin coating ([rGO(−)/NrGO(+)]10 and [rGO(−)/NS-rGO(+)]10) and spray coating ([rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)]) methods. It was observed that the charge transfer resistance value (Rct) decreased in the following order, Pt > [rGO(−):N-rGO(+)] > [rGO(−):NS-rGO(+)] > [rGO(−)/N-rGO(+)]10 > [rGO(−)/ NS-rGO(+)]10 coating, as summarized in Table 3. The Rct value is closely related with the electrocatalytic performance of DSSCs. It was observed that the spin-coated device [rGO(−):N-rGO(+)] showed lower Rct values and better electrocatalytic activity than other spin- and spray-coated devices. The Rct value [rGO(−):N-rGO(+)] (4.3 Ω cm2) was nearly the same as that of Pt-coated electrode (3.3 Ω cm2), implying that the graphene multilayer thin films can be employed as alternative promising electrodes. G

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Next, the performance was also investigated by the lifetime measurement of photoelectrons (τt) using the Bode plot. In the entire Nyquist plots, two well-defined semicircles were observed in the high frequency (>1 kHz) and medium frequency (1−100 HZ) regions (see Figures S9−S13). The high frequency semicircle represents the redox reaction of I−/ I3− at the counter electrode, and the medium semicircle reflects the electron transfer at the interface of FTO substrate including dye-absorbed TiO2 with electrolytes. The lifetime of photoelectrons in a photoanode can be calculated using the equation: τt = 1/2πft

where f t is the peak angular frequency and oscillation frequency region. From the Bode plot, τt is calculated for all the devices and summarized in Table 3 together with the charge transfer resistance values of all spin- and spray-coated devices. However, direct comparison can not be made for the results utilizing different counter electrodes based on spin- and spray-coated devices because the lifetime of electrons reflects the activity of photoanodes, as the lifetime of electrons is closely related to the condition of TiO2 and depends on how effectively the recombination at the electrolyte/TiO2 interface can be blocked for the generated carriers in the same condition.55 The Nyquist plot revealed that the spray-coated device ([rGO(−):NrGO(+)]) showed almost similar electrocatalytic activity with the Pt-coated device. Figure S14 shows EQE spectra of all the devices prepared by spray coating ([rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)]) and spin coating ([rGO(−)/NrGO(+)]10 and [rGO(−)/NS-rGO(+)]10) methods. The higher values of spin-coated devices ([rGO(−)/N-rGO(+)]10 and [rGO(−)/NS-rGO(+)]10) may be due to the higher current density as compared to spray- and Pt-coated devices. As control experiments, the devices based on a single spray coating layer of rGO(−), N-rGO(+), and NS-rGO(+) were also fabricated and their performance were summarized in Figure S15 and Table S2. It was observed that the PCE value of all devices was in the range of 4%. The effect of the presence of a PDDA layer between the LbL films and substrate was also analyzed, and the PDDA-free cell showed a poorer PCE value than the PDDA-coated ([rGO(−)/N-rGO(+)]10) device (see Figure S16). The above results demonstrated that the catalytic

Figure 6. SPR spectroscopy for the deposition of one bilayer using the spray coating method. Scan mode SPR curves of (a) [GO(−):NGO(+)] and (b) [GO(−):NS-GO(+)].

Figure 7. SEM images of different one bilayers on FTO substrates (a) [rGO(−):N-rGO(+)] and (b) [rGO(−):NS-rGO(+)].

Figure 8. Histogram of current−voltage characteristics of the DSSCs devices: (a−d) [rGO(−):N-rGO(+)] and (e−h) [rGO(−):NS-rGO(+)]. H

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Table 2. Photovoltaic Parameters of DSSCs Fabricated with One Bilayer of [rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)] Using the Spray Coating Methoda

a

counter electrodes (spray coating method)

Voc [V]

JSC [mA cm−2]

FF [%]

PCE [%]

Pt coating [rGO(−):N-rGO(+)] [rGO(−):NS-rGO(+)]

0.731 ± 0.08 0.735 ± 0.07 0.723 ± 0.06

14.95 ± 0.9 15.63 ± 0.9 14.76 ± 0.6

65 ± 0.02 59 ± 0.09 45 ± 0.04

7.14 ± 0.05 6.43 ± 0.09 5.29 ± 0.06

The values were estimated from Figure 8.

counter electrode in DSSCs. The DSSCs employing new types of counter electrodes exhibited reasonably high power conversion efficiency (∼6.0% to 7.0%) comparable with conventional Pt-coated counter electrode-based cells (∼7.14%). One bilayer of spray-coated blend films ([rGO(−):N-rGO(+)] and [rGO(−):NS-rGO(+)]) was also utilized as the counter electrode, and the resulting cell efficiency showed ∼6.43% and ∼5.6%, respectively. It was confirmed that the doped rGOs with lower D/G band intensity ratio in both N-rGO(+) and NS-rGO(+) retained modified electronic structure and led to good performance of DSSCs. Thus, it is proposed that a combination of undoped, nitrogen-doped, and nitrogen sulfur codoped reduced graphene oxides can be employed as a new class of highly active electrocatalysts and Ptfree alternative electrodes in conventional photovoltaic devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01770. High resolution N 1s and S 2p XPS spectra of N-rGO(+) and NS-rGO(+); AFM image and height profile of GO(−), N-rGO(+), and NS-rGO(+); UV−vis absorbance spectra of multilayers prepared by the spin coating method; SPR and UV−vis spectra of [GO(−)/NGO(+)]n and [GO(−)/NS-GO(+)]n using 0.1% solution; current−voltage characteristics of the DSSCs devices; EIS data of different DSSCs devices; EQA spectra; current−voltage characteristics of the DSSCs device without PDDA coating. (PDF)

Figure 9. (a) Cyclic voltammograms for the oxidation and reduction process using the Pt and different working electrodes of ten bilayers obtained by the spin coating method and one bilayer obtained by the spray coating method (scan rate: 10 mV/s). (b) Nyquist plots of the different reduced graphene oxide-based dummy cells measured at 0 V.

Table 3. Summary of the EIS Parameters of DSSCs Devices in Figure 9 counter electrodes

Rs [Ω cm2]

Rct [Ω cm2]

τt [s]

Pt coating [rGO(−):N-rGO(+)] [rGO(−):NS-rGO(+)] [rGO(−)/N-rGO(+)]10 [rGO(−)/NS-rGO(+)]10

4.0 4.3 4.2 4.1 4.2

3.3 4.3 5.3 9.9 12.5

0.021 0.023 0.022 0.022 0.020

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-2-3277-4517. Fax: +82-2-3277-3419. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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activity of different counter electrodes is significantly dependent on different types of doping and thickness of the LBL SA films. N-doping and NS-co-doping can produce local strains in a hexagonal carbon network, leading to structural deformations, and the additional lone pair electrons of nitrogen atoms can bring charges with respect to the delocalized π-system of an sp2 hybridized carbon framework, which can enhance electrontransfer ability and electrocatalytic activities.40−42

ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea Grant funded by the Korean Government (2014R1A2A1A09005656, 2011-0030255).

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REFERENCES

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4. CONCLUSION In summary, we established a viable fabrication protocol of thickness- and composition-controlled multilayer thin films via alternative deposition of negatively charged undoped graphene oxide GO(−) and positively charged doped graphene oxide (NGO(+) and/or NS-GO(+)) and demonstrated their function as I

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K

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