CNT@rGO@MoCuSe Composite as an Efficient Counter Electrode for

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

CNT@rGO@MoCuSe Composite as an Efficient Counter Electrode for Quantum Dot-Sensitized Solar Cells Chandu V. V. Muralee Gopi,† Saurabh Singh,‡ Araveeti Eswar Reddy,† and Hee-Je Kim*,† †

School of Electrical and Computer Engineering, Pusan National University, Busandaehak-ro 63 beon-gil, Geumjeong-gu, Busan 46241, South Korea ‡ School of Materials Science & Engineering, Pusan National University, Busan 46241, South Korea S Supporting Information *

ABSTRACT: This paper reports an efficient and simple strategy for the synthesis of molybdenum copper selenide (MoCuSe) nanoparticles decorated with a combination of a carbon nanotube (CNT) network and reduced graphene oxide (rGO) nanosheets to form an integrated hybrid architecture (CNT@rGO@MoCuSe) using a two-step hydrothermal approach. The synthesized hybrid CNT@rGO@MoCuSe material onto the Ni foam substrate is applied successfully as an effective counter electrode (CE) in quantum dot-sensitized solar cells (QDSSCs). A highly conductive CNT@rGO network grown on electrochemically active MoCuSe particles provides a large surface area and exhibits a rapid electron transport rate at the interface of CE/electrolyte. As a result, the QDSSC with the designed CNT@rGO@MoCuSe CE shows a higher power conversion efficiency of 8.28% under 1 sun (100 mW cm−2) irradiation, which is almost double the efficiency of 4.04% for the QDSSC with the MoCuSe CE. Furthermore, the QDSSC based on the CNT@rGO@MoCuSe CE delivers superior stability at a working state for over 100 h. Therefore, CNT@rGO@MoCuSe is very promising as a stable and efficient CE for QDSSCs and offers new opportunities for the development of hybrid, effective, and robust materials for energy-related fields. KEYWORDS: CNT@rGO@MoCuSe, counter electrode, quantum dot-sensitized solar cells, electrocatalytic activity, stability photovoltaic performance of the QDDSCs.7 As a result, various non-platinum CEs have been investigated toward polysulfide electrolyte, such as metal chalcogenides,8 conducting polymers,9 and carbon based materials.10 Composite CEs, such as metal chalcogenides combined with carbonaceous materials, have also emerged as potential CEs for QDSSCs because of their large electrical conductivity and excellent catalytic activity with an effective interface area between the CE and redox electrolyte.11−13 Radich et al. prepared a highly efficient reduced graphene oxide (rGO)−Cu2S composite CE with CdS/CdSe QDs and reported a power conversion efficiency (PCE) of 4.4%;14 however, the graphene/PbS composite as CEs delivered a PCE of 2.63% with a similar photoanode.11 Zhang et al. used the CuInS2/carbon composite CE for polysulfide-based CdS/CdSe QDSSCs and delivered a 4.32% PCE and good stability.15 Recently, the CdS/CdSe QDSSCs assembled with the MoS2/carbon nanotube (CNT)−rGO CE displayed a PCE of 3.44%, which was higher than that of the bare CNT−rGO CE because MoS2 with the CNT−rGO support improved the electrocatalytic activity and conductivity of the CE.16 Furthermore, the lower photovoltaic performance of these fabricated composites is still unsatisfactory and their

1. INTRODUCTION High-performance electrochemical energy conversion and energy storage fields, such as solar cells, supercapacitors, Liion batteries, and fuel cells, have attracted considerable attention because of the increasing energy demands and global warming issues.1 As third-generation solar cells, quantum dotsensitized solar cells (QDSSCs) have gained considerable attention in both academic and industrial fields because quantum dots (QDs) have unique properties, such as tunable band gap, multiple exciton generation, hot electron injection, high absorption coefficient, low cost, and easy fabrication.2−4 The structure and working principle of QDSSCs is similar to the concept of dye-sensitized solar cells (DSSCs) with the replacement of molecular dye by semiconductor QDs.3,5 In recent years, enormous efforts have been devoted to improve the performance of QDSSCs, which were mainly focused on the modification and improvement of photoanodes and design of new QD sensitizers and new suitable electrolytes.6 Besides photoanodes, the design and development of new counter electrodes (CEs) with high electrocatalytic activity, good conductivity, and stability are crucial for excellent QDSSC performance. Therefore, much attention has been focused on developing various CEs for QDSSCs. The widely used platinum (Pt) CE in DSSCs is unsuitable for QDSSCs because its surface activity and conductivity are suppressed in a polysulfide redox couple, resulting poor © XXXX American Chemical Society

Received: December 5, 2017 Accepted: March 2, 2018

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DOI: 10.1021/acsami.7b18526 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the synthesis process of the CNT@rGO@MoCuSe composite. The material was then deposited onto the surface of flexible Ni foam for a high-performance QDSSC electrode. ratio was dissolved ultrasonically in ethanol (10 mL). Subsequently, the CNT@rGO solution and 0.1 g of MoCuSe were dissolved in 70 mL DI water, stirred magnetically for 30 min, and treated ultrasonically for 1 h. The mixture was then transferred to a 100 mL autoclave, sealed, heated to 180 °C for 10 h, and cooled to room temperature. The product was filtered, rinsed with ethanol and DI water, and dried overnight at 60 °C for further characterization. The final product was called CNT@rGO@MoCuSe. To fabricate the working electrode, 80% active materials, 10% carbon black, and 10% polyvinylidene fluoride were mixed in Nmethyl-2-pyrrolidone (NMP) to form a homogeneous slurry, which was coated on nickel foam (1.3 × 1.6 cm2). The Ni foam current collector was dried at 120 °C for 12 h. The mass loading of the active material on the Ni foam was controlled at approximately 4 mg/cm2. For comparison, commercially available Pt paste (Pt-catalyst T/SP, Solaronix) was coated on the Ni foam substrate by the doctor-blade method and sintered at 200 °C for 10 min air. 2.3. Fabrication of CdSeTe Sensitized QDSSCs. The synthesis of the CdSeTe sensitizer, deposition of QDs on the TiO2 film, and deposition of ZnS and SiO2 layers were carried out using the literature methods;18,19 the active area of the TiO2 on fluorine-doped tin oxide (FTO) substrate was 0.25 cm2. The QDSSC devices were constructed by assembling the CNT@rGO@MoCuSe CE and sensitized TiO2 photoanode with a sealant (SX 1170-60, Solaronix) at 100 °C; the devices were then filled with a polysulfide electrolyte, which consisted of 1 M Na2S, 2 M S, and 0.1 M KCl in methanol/water (7:3). In the QDSSCs, the back of the porous Ni foam-based CNT@rGO@ MoCuSe was covered with a piece of glass sheet to prevent electrolyte leakage. 2.4. Assembly of Symmetrical Cells for Tafel Polarization and Electrochemical Impedance Spectroscopy (EIS) Analysis. The symmetric cells were sandwiched by two identical CEs, and the space between the electrodes was filled with the polysulfide electrolyte. The active area of the dummy cells was defined as 0.7 cm2. Tafel polarization (scan rate of 10 mV s−1) and EIS analysis (frequency range of 0.1 Hz to 500 kHz) were carried out under dark conditions using an SP-150 biological workstation. 2.5. Characterization. The surface morphology of the samples was investigated by high-resolution scanning electron microscopy (HR-SEM; S-2400, Hitachi). Elemental mapping, selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HR-TEM (CJ111) were performed at an acceleration voltage of 200 kV. The powder X-ray diffraction (PXRD) patterns were recorded with D8 ADVANCE with a DAVINCI diffractometer (Bruker AXS).

studies are limited. Therefore, it is essential to develop highly effective electrocatalysts of CEs with excellent electrocatalytic activity and high electrical conductivity and further improve the QDSSC performance. Therefore, at first, an effective MoCuSe catalyst was fabricated using a facile hydrothermal approach. To date, there are no reports of MoCuSe CE materials for QDSSC applications. Therefore, in this study, multicomponent nanostructure electrode materials were designed by combining the MoCuSe with CNT−rGO acts as an efficient catalyst in QDSSCs. The QDSSC device with the CNT@rGO@MoCuSe CE with delivered an encouraging efficiency of 8.28%, which is far higher than the MoCuSe CE (4.04%). The extraordinary solar performance is contributed by the high electrocatalytic activity of MoCuSe particles and combination of CNT−rGO provides a high surface area and offers shorter diffusion pathways because of the high electrical conductivity of the CNTs and rGO. Owing to the superior catalytic performance, the QDSSC with the CNT@rGO@MoCuSe CE delivered superior electrocatalytic activity with high stability.

2. EXPERIMENTAL SECTION 2.1. Synthesis of rGO. GO was prepared from graphite flakes using the modified Hummer’s method.17 The required amount of GO was dissolved in water and sonicated for 15 min. Subsequently, hydrazine hydrate (N2H4·H2O) was added slowly to the GO solution with constant stirring. The solution was transferred to an 80 mL autoclave and held at 120 °C for 10 h. After cooling, the precipitate was washed sequentially with distilled (DI) water and ethanol and dried in an oven at 60 °C overnight. 2.2. Synthesis and Fabrication of MoCuSe and CNT@rGO@ MoCuSe Composites. The CNT@rGO@MoCuSe hybrid catalyst was prepared using a facile two-step hydrothermal approach. In a typical procedure, 0.1 M sodium molybdate (Na2MoO4·2H2O), 0.1 M copper nitrate hexahydrate (Cu(NO3)2·6H2O), 0.3 M selenium powder (Se), and 0.1 M sodium borohydride (NaBH4) were dissolved in 70 mL of DI water and stirred vigorously for 30 min. The mixture was transferred to a 100 mL autoclave and heated to 180 °C for 10 h, followed by cooling naturally to room temperature. Finally, the product was collected by filtration, washed several times with DI water and ethanol, and dried at 60 °C overnight. The product was called MoCuSe. In the second step, a mixture of rGO and CNTs (purchased from Carbon Nano-Material Technology Co., Ltd.) with a 1:1 weight B


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ACS Applied Materials & Interfaces The J−V curves of the QDSSCs were recorded by an ABET Technologies (USA) solar simulator. The incident photon-to-current conversion efficiency (IPCE) spectra of the QDSSCs were obtained using an Oriel IQE-200. The EIS measurements were conducted using an SP-150 biological workstation.

presents a HR-SEM image of the MoCuSe product, which showed that the product has a three-dimensional (3-D) network structure consisting of a large quantity of agglomerated MoCuSe microparticles. Figure S1 shows a SEM image of the rGO nanosheets. Figure 2c,d presents the morphology of the CNT@rGO@MoCuSe hybrid material, showing that the CNT and rGO network are distributed homogenously over the 3D microparticles. As shown in Figure 2d, the MoCuSe particles are connected and covered with the CNT network and rGO nanosheets. The CNT and rGO deposited on the surface of the MoCuSe particles can enhance the electrocatalytic activity, provide a large active surface, and improve the conductivity, leading to an improvement in the electron transfer capability relative to the single MoCuSe material. The morphology and structure of the CNT@rGO@MoCuSe material were examined by HR-TEM and scanning TEM (STEM). Figure 3a shows a TEM image of the 3D CNT@ rGO@MoCuSe composite; the porous structure of the films can also be observed. The high-resolution TEM image (Figure 3b) clearly showed that the MoCuSe particle was closely surrounded by rGO nanosheets and the CNT network. Figure 3c presents a HR-TEM image of CNT@rGO@MoCuSe, which shows three sets of lattice fringes of ∼0.336, ∼0.28, and ∼0.196 nm, relating to the (002) plane of CNT, the (100) plane of MoSe2, and the (110) plane of CuSe, respectively.20−22 Further, the SAED pattern in Figure 3d showed diffraction rings of the prepared CNT@rGO@MoCuSe material. The SAED pattern mainly showed the (002), (100), and (110) crystal planes of rGO, MoSe2, and CuSe, respectively. Furthermore, the formation of MoCuSe in the presence of CNT and rGO was confirmed by STEM and elemental mapping (Figure 3e). XRD was used to analyze the crystallinity of both MoCuSe and CNT@rGO@MoCuSe. As shown in Figure 4, the asprepared samples exhibited XRD peaks for both the MoSe2 and CuSe. The peaks at 31.4°, 37.8°, 47.5°, and 55.9° 2θ were assigned to the (100), (103), (105), and (110) planes of MoSe2 (JCPDS card no. 29-0914), respectively, and CuSe showed peaks at 26.6°, 28.2°, 30.8°, and 45.9° 2θ, corresponding to the (101), (102), (006), and (110) planes (JCPDS card no. 060427), respectively. The CNT@rGO@MoCuSe material

3. RESULTS AND DISCUSSION The CNT@rGO@MoCuSe catalyst was synthesized using a two-step hydrothermal method. Figure 1 presents a schematic of the preparation process for CNT@rGO@MoCuSe. In the first step, MoCuSe microparticles were prepared using a facile hydrothermal technique. The MoCuSe material was then incorporated into the mixture of CNT and rGO; the resulting mixture was dispersed in ethanol and DI water, followed by a hydrothermal reaction to form the CNT@rGO@MoCuSe composite material. After these processes, the CNT@rGO@ MoCuSe material was compatible with the slurry preparation process for electrode preparation using a Ni foam substrate. The resulting material was applied as an effective CE in QDSSCs and delivered superior electrochemical properties. The morphology of the as-synthesized MoCuSe and CNT@ rGO@MoCuSe materials was verified by HR-SEM. Figure 2a,b

Figure 2. Top-view low- and high-magnification SEM images of the (a,b) MoCuSe and (c,d) CNT@rGO@MoCuSe materials.

Figure 3. (a−c) Low- and high-resolution TEM images and (d) SAED pattern of the CNT@rGO@MoCuSe material. (e) STEM image and relative elemental mapping of C, Mo, Cu, and Se. C


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polysulfide redox couple (Sn2−/S2−).23 As shown in Figure 5a, CNT@rGO@MoCuSe delivered a lower RS (8.36 Ω cm2) than the MoCuSe (11.78 Ω cm2) and Pt (23.30 Ω cm2) CEs. This may be due to its unique structure, which can provide more sites to connect with the substrate. As a result, the reduced RS could lead to a larger FF in QDSSCs.24 In general, the catalytic activity of the electrodes is reflected mainly from the Rct, which was 121.58, 16.64, and 5.40 Ω cm2 for the Pt, MoCuSe, and CNT@rGO@MoCuSe CEs, respectively. The lower Rct value of the CNT@rGO@MoCuSe CE was attributed to the incorporation of the shorter electron paths formed by the CNT and rGO networks onto the highly catalytic activity of the MoCuSe particles. As a result, the CNT@rGO@MoCuSe CE exhibited superior electrocatalytic activity toward Sn2−/S2− reduction. The catalytic activity of the CNT@rGO@MoCuSe, MoCuSe, and Pt CEs was investigated further by the Tafel polarization measurements (Figure 5c). The anodic and cathodic branches of the CNT@rGO@MoCuSe delivered a larger slope than the MoCuSe and Pt CEs, which showed a larger exchange current density (Jo) for the electrode surface. Therefore, the larger Jo value of the CNT@rGO@MoCuSe (4.97 mA cm−2) CE could lead to a higher electrocatalytic activity than the MoCuSe (3.16 mA cm−2) and Pt CEs (0.22 mA cm−2). Moreover, the Jo value of the CE can be used to demonstrate the charge transfer resistance (Rct‑Tafel) at the CE/ electrolyte interface, according to eq 1

Figure 4. XRD spectra of the MoCuSe and CNT@rGO@MoCuSe composite materials.

showed typical (002) XRD peaks at 24° 2θ, showing that the sample has a graphitic structure. To interpret the charge transfer mechanism and electrocatalytic activity of the CEs with photovoltaic performances, EIS analysis was conducted on a symmetrical dummy cell. Figure 5a depicts the EIS curves of the MoCuSe, CNT@rGO@

Jo =

RT nFR ct‐Tafel

(1)

where R is the gas constant, n is the number of electrons contributing to charge transfer at the interface, T is the temperature, and F is Faraday’s constant. According to eq 1, the Rct‑Tafel values of CNT@rGO@MoCuSe, MoCuSe, and Pt CEs are obtained to be 2.60, 4.09, and 58.80 Ω cm2, respectively. The trend in the Rct‑Tafel values from the Tafel measurements was consistent with the variation trend of Rct in EIS. Therefore, the lower Rct and Rct‑Tafel values reveal the large number of electrons transferred through the interface of CE/electrolyte because of the fast electron transfer capability of an electrode. In addition, the limiting current density (Jlim), obtained by the diffusion of ionic carriers at the interface of CE/electrolyte, is related directly to the diffusion coefficient (D) of the Sn2−/S2− redox couple (eq 2): D=

Jlim 2nFC

(2)

where D is the diffusion coefficient of the polysulfide and the other symbols have their usual meaning. As shown in Figure 5c, the CNT@rGO@MoCuSe CE delivers larger Jlim (1.63 mA cm−2) than that of MoCuSe (1.31 mA cm−2) and Pt (0.75 mA cm−2) CEs, revealing the higher diffusion velocity for the CNT@rGO@MoCuSe CE in the polysulfide electrolyte, according to eq 2.25 Figure 6a presents the photocurrent density−voltage (J−V) curves of the QDSSCs based on the CNT@rGO@MoCuSe and MoCuSe CEs under irradiation of 100 mW cm−2, and Table 1 lists the obtained parameters. The QDSSC with the MoCuSe CE delivered a current density (JSC), open-circuit voltage (VOC), and fill factor (FF) of 12.11 mA cm−2, 0.609 V, and 0.548, respectively, resulting in a lower PCE of (η) 4.04% because of the lack of electrocatalytic activity toward polysulfide

Figure 5. Electrochemical properties of the investigated CEs with the polysulfide electrolyte: (a) Nyquist plots of symmetrical cells based on MoCuSe, CNT@rGO@MoCuSe, and Pt CEs. (b) High-impedance range of the Pt CE and equivalent circuit for fitting EIS plots and (c) Tafel polarization curves of various electrocatalytic films.

MoCuSe, and Pt CEs. The high-impedance range of the Pt CE is shown in Figure 5b. The Nyquist plots were fitted with an equivalent circuit (inset of Figure 5b) using Z-View software, where RS is the series resistance, Rct is the charge transfer resistance, Cμ is the chemical capacitance at the CE/electrolyte interface, and Zw is the Warburg diffusion impedance of the D


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line model (Figure 6d) using ZView software proposed by Wang et al.26 ⎡ R wR k ⎤1/2 Z(ω) = ⎢ ⎥ ⎣ 1 + iω/ω k ⎦ coth[(R w /R k )1/2 (1 + iω/ω k )1/2 ]

(3)

where Rw (=rwL) is the electron transport resistance in TiO2, Rk (=rk/L) is the electron recombination resistance at the TiO2/ electrolyte interface, L is the TiO2 film thickness, and ωk is the radian frequency of the electron transfer process. In the equivalent circuit model, RS is the series resistance that stands for the sheet resistance of the FTO; RFTO and CFTO are the resistance and capacitance of the FTO; RCE and CCE represent the charge transfer resistance and capacitance at the CE/ electrolyte interface; and Cμ (=rk/L) correspond to the chemical capacitance, respectively. The resultant fitting parameters are presented in Table 1. In addition, electron life time (τn), diffusion length of electrons (Ln), and diffusion coefficient of electrons (Dn) could be estimated from the following equations27

Figure 6. Photovoltaic properties of CdSeTe QDSSCs based on different CEs. (a) J−V curves and (b) IPCE spectra of QDSSCs assembled with different CEs under 1 sun illumination. (c) Nyquist impedance plots of QDSSCs based on various CEs and (d) equivalent circuit model used to fit EIS plots.

τn = R k × Cμ

reduction. Anchoring the CNT network and rGO nanosheets to the MoCuSe particles supports improved the catalytic activity of the CE and produces enhanced photovoltaic parameters. As a result, the QDSSCs with the CNT@rGO@ MoCuSe CE delivered the highest η of 8.28%, as well as VOC, JSC, and FF values of 0.633 V, 20.54 mA cm−2, and 0.636, respectively. Therefore, the photovoltaic results show that CNT@rGO@MoCuSe can be a promising CE in QDSSC applications. To the best of the authors’ knowledge, the QDSSCs based on the CNT@rGO@MoCuSe CE provide excellent photovoltaic performance compared to the results in several recent reports on metal sulfide/carbonaceous composite CE materials (see Table S1, Supporting Information). On the other hand, the QDSSC with a Pt CE exhibits very poor photovoltaic performance (FF of 0.316 and η of 1.96%) because of low electrocatalytic activity of the Pt catalyst in the presence of polysulfide electrolyte resulting from the strong adsorption of S2− on the surface. The best performance for the CNT@rGO@MoCuSe CE was also analyzed from the IPCE curve (Figure 6b), in which the CNT@rGO@MoCuSe-based QDSSC displays a higher IPCE value than that of the device with MoCuSe and Pt CEs, which is in good accordance with the observed JSC values as listed in Table 1. Furthermore, the electron transport behaviors of the QDSSCs with various CEs were investigated by EIS analysis carried out under open-circuit conditions and 1 sun illumination (100 mW cm−2) by applying a 10 mV ac signal in the frequency range of 0.1 to 500 kHz. The resultant Nyquist plots are shown in Figure 6c and were fitted using transmission

Ln = L

Dn =

(4)

Rk Rw

(5)

Ln 2 τn

(6)

As shown in Table 1, the CNT@rGO@MoCuSe CE exhibits lower RS and RCE values (7.19 and 2.41 Ω cm2) than that of the MoCuSe (11.11 and 4.89 Ω cm2) and Pt (18.21 and 7.70 Ω cm2) CEs, suggesting the higher electrocatalytic activity of the CNT@rGO@MoCuSe CE and implies higher FF and JSC values in the QDSSCs. Because of the enhanced electron transfer rate with reduced recombination at the interface of TiO2/QDs/electrolyte, the CNT@rGO@MoCuSe CE (4.51 Ω cm2) delivers lower Rk value than the MoCuSe (11.93 Ω cm2) and Pt (24.84 Ω cm2) CEs. Moreover, the Rk/Rw values of QDSSCs with CNT@rGO@MoCuSe, MoCuSe, and Pt CEs were 14.09, 8.46, and 6.75, respectively. A higher Rk/Rw value is favorable for the lower charge recombination, a high charge collection efficiency, and a high photovoltaic performance for a solar cell. The QDSSC with the CNT@rGO@MoCuSe CE delivers larger Dn and longer τn and Ln values than the QDSSC based on MoCuSe and Pt CEs, revealing that electron transfer charge transfer in the CNT@rGO@MoCuSe-based QDSSC was unlocked for a longer distance than in the MoCuSe- and Pt-based QDSSCs. The larger values of Dn, τn, and Ln values were very well-matched with the higher FF and JSC values of the CNT@rGO@MoCuSe-based QDSSC.

Table 1. Photovoltaic Parameters and EIS Results of QDSSCs Based on MoCuSe, CNT@rGO@MoCuSe, and Pt CEs in the Presence of the Polysulfide Electrolyte CE MoCuSe CNT@rGO@ MoCuSe Pt

VOC (V)

JSC (mA cm−2)

FF

η%

RS (Ω cm2)

RCE (Ω cm2)

Rw (Ω cm2)

Rk (Ω cm2)

Rk/Rw

Ln (μm)

τn (ms)

Dn (cm2 s−1)

0.609 0.633

12.11 20.54

0.548 0.636

4.04 8.28

11.11 7.19

4.89 2.41

1.41 0.32

11.93 4.51

8.46 14.09

21.81 28.15

6.16 7.19

7.72 × 10−4 11.02 × 10−4

0.595

10.43

0.316

1.96

18.21

7.70

3.68

24.84

6.75

19.48

5.36

7.07 × 10−4

E


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Figure 7. High stability of CdSeTe QDSSCs with the CNT@rGO@MoCuSe CE characterized by (a) VOC, (b) JSC, (c) FF, and (d) η. (e) Statistical results of η for CdSeTe QDSSCs assembled with different CEs.

electrocatalytic activity of the MoCuSe particles, the high surface area, and the superior electrical conductivity of the CNT@rGO networks. The EIS and Tafel polarization measurements revealed the superior electrocatalytic activity of the CNT@rGO@MoCuSe CE. These results show that CNT@rGO@MoCuSe is a promising candidate material for high-performance QDSSC, and constructing similar hybrid composite materials may open the door for advanced electrochemical energy-related applications.

The stability of a photovoltaic device is crucial to practical applications, which restricts the application of QDSSCs. Figure 7a−d shows the temporal evolution of photovoltaic parameters (VOC, JSC, FF, and η) extracted from the working stability tests of QDSSCs for more than 100 h based on the MoCuSe and CNT@rGO@MoCuSe CEs. The stability test shows that photovoltaic parameters increase steadily in the initial stage because of slow electrolyte permeation into the TiO2 pores and the enhanced charge transport due to heating of the electrolyte.28 During the whole measurements, the QDSSC with the CNT@rGO@MoCuSe CE exhibited a slight change in the VOC, JSC, and FF values, whereas the QDSSC with the MoCuSe CE delivered continuous decline of the photovoltaic parameters (Figure 7a−c). As a result, the η of the MoCuSebased QDSSCs changed from an initial 4.02% to a final 3.39% with a negligible drop of ∼18.5%, whereas the η of the CNT@ rGO@MoCuSe-based device remained above ∼98.3% of the initial value even after 100 h light soaking, as shown in Figure 7d. The excellent photovoltaic performance of the QDSSC is mainly due to the efficient electrocatalytic activity of the CNT@rGO@MoCuSe CE. Therefore, the stability test confirmed that the CNT@rGO@MoCuSe can serve as an effective CE in QDSSCs. In addition, the reproducibility for the QDSSCs was examined by testing 40 cells containing MoCuSe and CNT@rGO@MoCuSe as CEs. Figure 7e presents the statistical results of the device performance. The best CNT@ rGO@MoCuSe device efficiency was in the range from 7.35 to 8.74% with a mean efficiency of approximately 8.24%, revealing a reproducible photovoltaic performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18526. SEM image of the rGO and comparison of the performance of the QDSSC based on the CNT@ rGO@MoCuSe CE with that of other composite CEs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82 51 510 2364. Fax: +82 51 513 0212. ORCID

Hee-Je Kim: 0000-0002-3620-0739 Notes

The authors declare no competing financial interest.

4. CONCLUSIONS A CNT@rGO@MoCuSe hybrid composite was synthesized for use as a CE in QDSSC applications. The CNT network and rGO nanosheets were well-anchored to the MoCuSe microparticles. The QDSSC assembled with the CNT@rGO@ MoCuSe CE exhibited an impressive η of 8.28% and good stability at a working state for over 100 h owing to the high

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ACKNOWLEDGMENTS This research was supported by Basic Research Laboratory through the National Research Foundations of Korea funded by the Ministry of Science, ICT and Future Planning (NRF2015R1A4A1041584). F


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