Enhanced electrocatalytic activity and electrochemical

Thin Solid Films 660 (2018) 46–53

Contents lists available at ScienceDirect

Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Enhanced electrocatalytic activity and electrochemical stability of copper(I) sulfide electrode electrodeposited on a Ti interlayer-coated fluorine-doped tin oxide substrate and its application to quantum dot-sensitized solar cells

T

Jiyoung Chaea, Munsik Ohb, Vu Hong Vinh Quya, JongMyeong Kwona, Jae-Hong Kima, ⁎ ⁎ ⁎ Soon-Hyung Kangc, Hyunsoo Kimb, ,1, Elayappan Vijayakumara, ,1, Kwang-Soon Ahna, ,1 a b c

School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea Department of Chemistry Education, Chonnam National University, Gwangju 500-757, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Copper sulphide Titanium interlayer Potentiodynamic electrodeposition Counter electrode Quantum dot-sensitized solar cells

Copper(I) sulfide (Cu2S) films are deposited on 15 nm-thick Ti (Titania) interlayer-coated fluorine-doped tin oxide (FTO) substrates. This is performed using potentiodynamic electrodeposition at selective numbers of cycles (10−20) in the potential range of −0.7 V to −0.2 V (vs. Ag/AgCl), followed by sulfurization. The results of Cu2S films on the FTO/Ti substrates are subsequently employed as counter electrodes for cadmium selenide quantum dot-sensitized solar cells (QDSSC). The Ti interlayer facilitates the Cu (copper) nucleation, during the Cu electrodeposition and leads to side-by-side packing of small Cu2S nanosheets after sulfurization. In contrast, conventional Cu2S grown on the FTO consists of a mixture of large and small Cu2S nanosheets. The distinct nanostructure of the FTO/Ti/Cu2S counter electrodes enhances the electrocatalytic activity and electrochemical stability comparing to those of FTO/Pt (Platinum) and FTO/Cu2S films. This is due to the increased number of electrochemically active sites, fast ion transport in the Cu2S nanosheets perpendicular to the substrate, and good adhesion to the Ti interlayer. The optimized FTO/Ti/Cu2S electrode, deposited in 15 cycles, contributes to significantly increase the cell efficiency (4.11%) of the QDSSC, resulting in 140% and 35.2% performances improvements for the QDSSCs with the Pt (1.71%) and FTO/Cu2S (3.04%) counter electrodes, respectively.

1. Introduction Recently, the paucity of non-renewable energy sources has brought about an increase in the demand for renewable energy sources, such as solar, wind, and thermal energy. Among the various renewable energy sources, sunlight is abundant on the earth surface. Conversion of sunlight into electricity may be achieved using solar cells [1]. Of the various types of solar cells, quantum-dot sensitized solar cells (QDSSCs) have a number of distinct advantages, such as high extinction coefficients, large intrinsic dipole moments, the ability to generate multiple excitons, band-gap controllability by control of quantum dot size, and ease of fabrication [2]. In addition, the theoretically achievable maximum power conversion efficiency (PCE) for the QDSSCs is 44%, which is considerably higher than that of dye-sensitized solar cells (DSSCs, 31%), as calculated by Shockley and Queisser [3,4]. QDSSCs have similar structures and working mechanisms to DSSCs. The distinctive feature to distinguish between these two types of solar cells is the

⁎

1

sensitizer used, with the former being sensitized by quantum dots and the latter, by organic dyes [5]. The power conversion efficiency of QDSSCs primarily depends on the materials used in each component. A typical QDSSC is assembled with the components in the following order: fluorine-doped tin oxide (FTO)/titanium dioxide (TiO2) sensitized with quantum dot/polysulfide electrolyte/counter electrode/FTO [6,7]. In recent years, multiple efforts have been made to increase the overall power conversion efficiency of QDSSCs by modifying the above materials by optimize their chemical and physical properties. There are many factors that significantly contribute to improvements in the PCE of QDSSCs, such as the type of quantum dot used, the nanostructure of the photoanode, the types of electrolyte(s) used (liquid, gel, and solid), and the counter electrode materials [8,9]. Among the components of QDSSCs, the counter electrodes (CEs) play a key role in completing the electrical circuit and in the regeneration of sulfur redox couple through a reduction process [10–12]. In addition, the design and optimization of the

Corresponding authors. E-mail addresses: [email protected] (H. Kim), [email protected] (E. Vijayakumar), [email protected] (K.-S. Ahn). K. S. Ahn, H. S. Kim and E. Vijayakumar contributed equally to this work as the corresponding authors.

https://doi.org/10.1016/j.tsf.2018.05.049 Received 8 February 2018; Received in revised form 26 May 2018; Accepted 26 May 2018

Available online 27 May 2018 0040-6090/ © 2018 Published by Elsevier B.V.


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electrochemically onto Ti-coated FTO substrate by potentiodynamic electrodeposition, using Pt mesh and Ag/AgCl counter and reference electrodes, respectively. The aqueous electrolyte contained 5 mmol·L−1 CuSO4, 0.2 mol·L−1 sorbitol, and 3.0 mol·L−1 NaOH. Potentiodynamic electrodepositions were carried out in a voltage range of −0.7 to −2.0 V (vs. Ag/AgCl) at a scan rate 10 mV/ s for 10, 15, or 20 cycles, followed by rising with DI water before drying in hot air oven at 70 °C. Subsequently, to bring about the formation of the Cu2S films, the deposited Cu films were immersed in a polysulfide aqueous solution containing 1 mol·L−1 S and 1 mol·L−1 Na2S for 6 min, followed by rising with DI water and ethanol prior to drying in hot air oven at 70 °C. For comparison, Cu2S films were also prepared on FTO substrates not coated with Ti, using the above method. The FTO/Ti/Cu2S films prepared at n electrodeposition cycles are denoted as FTO/Ti/Cu2S (n cycles). A Pt CE was also prepared by doctor-blading a Pt nanocluster-containing Pt paste (PT-1, Dyesol. Ltd.) on the FTO surface, followed by sintering at 450 °C for 30 min.

CE can also bring about an overall increase in the PCE of the QDSSCs through effectively catalyzing the reduction reaction in the polysulfide redox system. The widely used conventional Pt (Platinum) CE, with iodide/triiodide electrolyte, used in DSSCs is not an effective catalyst in QDSSCs with a polysulfide redox system (S2−/Sn2−) due to the poisoning effect caused by S2− chemisorbed onto the Pt surface, which decreases the electrocatalytic activity of the CE. To overcome this problem, significant efforts have been devoted to improving the electrocatalytic activity of CEs), in the past few years. Many of these studies focused on the optimization and design of alternative CE materials for QDSSCs. Recently, a large number CE materials, such as noble metals [13,14] (e.g. Au), conducting polymers [3,15] (e.g. polyaniline, polypyrrole, and Poly (3,4-ethylenedioxythiophene), metallic compounds [16,17] (e.g. copper(I) sulfide (Cu2S) and nickel sulfide), carbonaceous materials [18–20] (e.g. carbon nanotube, graphene, mesoporous carbon, activated carbon, carbon black, and carbon foams) and heterogeneous materials have been extensively studied [18–26]. Among these materials, Cu2S is a promising candidate for a CE materials, primarily due to its favorable electrochemical properties, low cost, and high stability compared to other CE materials [27–32]. Therefore, Cu2S CEs have been the focus of intensive research. The method of preparation is a further important consideration in the fabrication of CE films at FTO surfaces. A number of preparation techniques have been considered, such as doctor-blade method, electrospinning, electrodeposition, thermal reduction, and sputtering. Among the above-mentioned methods, electrodeposition has attracted significant attention in recent decades, due to its many advantages, such as controllability of particle size and film thickness, strong adhesion, ease of fabrication, less time-consuming method, reduced internal stress, and uniform film morphology, compared to the other techniques. Despite its importance, the effects of Cu2S films on the electrocatalytic activity have not been widely investigated. In this paper, we report on the preparation of Cu2S CEs deposited on 15 nm-thick titania (Ti) interlayer-coated FTO substrate. The Cu2S is prepared by potentiodynamic electrodeposition of copper (Cu) nanoparticles followed by sulfurization. We investigated the effects of the FTO/Ti/Cu2S films on the electrocatalytic activity and the application to the QDSSCs. We found that the Ti interlayer on the FTO substrate provides numerous nucleation sites, leading to the distinct petal-like, vertically aligned, small Cu2S nanosheets that provide significantly increased electrochemical active sites and facilitated ion transport in and out of the film. The electrocatalytic activity, charge transfer resistance, electrochemical stability, and photovoltaic performance of the resulting materials were systematically investigated.

2.3. Preparation of CdSe (cadmium selenide) QD-assembled TiO2 photoanode The mesoporous TiO2 films were prepared by doctor-blading TiO2 paste (Ti-Nanoxide T/SP, Solaronix SA) onto the FTO glass substrates, followed by drying at 70 °C for 30 min prior to sintering at 450 °C for 30 min. The total thickness of TiO2 film was 10 μm and the active area was 0.16 cm2. The CdSe QDs were assembled onto the mesoporous TiO2 films by a successive ionic layer adsorption and reaction (SILAR) technique in a glove box, as described elsewhere [33]. The CdSe QDs were synthesized from 30 mmol·L−1 Cd(NO3)2 and Na2Se in ethanol. The Na2Se solution was prepared from 30 mmol·L−1 SeO2 and 60 mmol·L−1 NaBH4 in ethanol, as described in Eq. (1).

SeO2 + 2NaBH 4 + 6C2 H5 OH → Se2 − + 2Na+ + 2B(OC2 H5 )3 + 5H2 + 2H2 O

(1)

The whole SILAR process including the reaction (1) was performed inside the glove box with inert Argon gas environment. The SILAR process involved immersing the TiO2 films in a Cd(NO3)2 solution for 1 min, rinsing with ethanol and drying with a drier, followed by further immersion in a Na2Se solution for 1 min, and again rinsing with ethanol and drying. Each two step immersion process constituted a single SILAR cycle. The SILAR process was carried out on all TiO2 samples for 8 cycles. Finally, a ZnS passivation layer was coated over the TiO2/CdSe film via the SILAR process, where the films were dipped alternately into the ethanol solution containing 0.1 mol·L−1 Zn(NO3)2.6H2O for 1 min, rinsed with ethanol and dried with a drier, followed by dipping into the methanol/DI water (1:1 by volume) solution containing 0.1 mol·L−1 Na2S·9H2O, rinsing with methanol and drying with a drier. This SILAR procedure was repeated (i.e. two cycles) [34].

2. Experimental details 2.1. Materials Cadmium nitrate, selenium dioxide, sodium sulfide, sulfur, zinc nitrate, sorbitol, copper sulfate, sodium boro hydrate and sodium hydroxide were obtained from Aldrich. Solvents (methanol, ethanol, and acetone) were obtained from Merck. Standard TiO2 paste (Ti-Nanoxide T/SP, Solaronix SA) and Pt paste (PT-1, Dyesol. Ltd.) were used as received. Deionized (DI) water was obtained from a Millipore Derect-Q3 UV system. FTO glass with sheet resistance approximately ~14 Ω cm−2 was purchased from Asahi Glass (Japan). All aforementioned chemicals were analytical grade and used as received without any further purification.

QDSSCs were fabricated by sandwiching the CdSe QD-sensitized TiO2 films and CEs (Pt, FTO/Ti/Cu2S and FTO/Cu2S) using a 60 μmthick thermoplastic sealing material (Surlyn). The cells were filled with a polysulfide redox electrolyte containing 1 mol·L−1 Na2S, 1 mol·L−1 S, and 0.1 mol·L−1 NaOH in DI water. We fabricated three solar cells for each type. All of three samples exhibited almost the same performance.

2.2. Preparation of CEs

2.5. Characterization techniques

Initially, the FTO glass substrates were cleaned by sequentially sonicating in acetone, ethanol, and deionized water for 30 min each. A thin Ti layer (15 nm thick) was coated onto the cleaned FTO substrates by electron beam evaporation. The Cu thin films were deposited

The surface morphology was examined by field emission scanning electron microscopy (FESEM, Hitachi FE-SEM S4800). The crystalline characteristics were analyzed by X-ray diffraction (XRD, PANalytical X'Pert) techniques using Cu-Kα radiation and scanning from 20 to 70°

2.4. Assembly of QDSSC

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a)

b)

5 μm

5 μm

c)

d)

5 μm

5 μm

Fig. 1. FESEM top view: (a, b) low and high resolution images of FTO/Cu2S (15 cycle). (c, d) Low and high resolution images of FTO/Ti/Cu2S (15 cycle). Insets in (a) and (c) show the surface images of FTO/Cu and FTO/Ti/Cu prior to sulfurization.

electrodeposited FTO/Ti/Cu2S (10, 15 and 20 cycles) thin films and those corresponding thicknesses are tabulated in the Table. 1. The crystalline phases of the FTO/Cu, FTO/Cu2S, FTO/Ti/Cu, and FTO/Ti/Cu2S were characterized by XRD. The resulting diffraction patterns are shown in Fig. 3. The main XRD peak at 2θ = 43.29° in FTO/Cu and FTO/Ti/Cu samples are assigned to the (111) plane of the face-centered cubic Cu phase (JCPDS No = 004–0836) [33]. The FTO/ Ti/Cu sample exhibited wider full width at half maximum at the (111) peak than the FTO/Cu sample. This indicates that the crystallite size is smaller in the FTO/Ti/Cu material than in the FTO/Cu materials, in a good agreement with the SEM images. After the sulfurization of FTO/ Cu and FTO/Ti/Cu, the Cu peaks disappeared completely, indicating that all of the crystalline Cu is converted into an amorphous Cu2S phase upon sulfurization. This result is consistent with previous studies of amorphous CuxS films reported by Fuwei et al. [35]. To confirm the Cu2S phase, XPS (X-ray photoelectron spectroscopy) analysis were performed. Fig. 4 shows the high-resolution Cu 2p and S 2p spectra, respectively, of the electrodeposited Cu2S film. Fig. 4 (a) presents two peaks at 932.5 and 952.0 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. The binding energies of the S 2p3/2 and S 2p1/2 peaks were 161.48 and 163 eV, respectively (Fig. 4(b)). These binding energies) are well consistent with those previously reported for Cu and S of the Cu2S [33]. Fig. 5 shows energy dispersive X-ray (EDAX) analyses of the FTO/Ti, FTO/Ti/Cu2S and FTO/Cu2S materials, respectively. After the sulfurization, the sulfur appeared in the films at atomic percentages approximate to the composition of Cu2S, confirming the Cu phases were transformed completely into the Cu2S by the sulfurization. To investigate the interfacial charge transfer properties and/or electrocatalytic activity of all the CEs materials prepared, sandwiched symmetrical dummy cells, consisting of CE/polysulfide electrolyte (S2−/Sn2−)/CE, were assembled and their current-voltage curves were measured and Tafel polarization curves prepared. In general, the exchange current density (Jo) is calculated by extrapolation of linear segments in the Tafel plots. Fig. 6 shows the Tafel plots of the FTO/Ti/

with a step scanning of 0.05°. The photovoltaic current-voltage (J-V) characteristics were measured using a solar simulator (PEC-L11 m Peccell Ltd.) under 1 sun illumination (100 mW cm−2, AM 1.5G), which was verified using an AIST-calibrated Si-solar cell. Electrochemical impedance spectroscopy (EIS) was performed between 10−1 and 105 Hz for the symmetrical cells (CE/electrolyte/CE) with a 60 μm-thick spacer. Tafel polarization measurements were also carried out for the symmetrical cells to confirm the electrocatalytic activity of the CEs.

3. Results and discussion The surface morphology and crystalline structure of the Cu2S thin films, with and without Ti thin interlayer, was characterized by SEM and XRD analysis. Fig. 1 shows the surface morphologies of the FTO/ Cu2S and FTO/Ti/Cu2S films. Fig. 1 (a,b) indicates that the FTO/Cu2S prepared herein consist of a mixture of small and large Cu2S nanosheets. Fig. 1 (c,d) shows SEM images of the Cu2S film with the Ti interlayer and indicates the formation of uniformly grown, small, petalshaped, Cu2S nanosheets packed side-by-side. Furthermore, these nanosheets are almost perpendicular to the surface of Ti-coated FTO substrate. To investigate the difference in the morphologies between the FTO/Cu2S and the FTO/Ti/Cu2S, the surface morphologies of the electrodeposited Cu films were measured prior to sulfurization, and are shown in the insets of (a) and (c), respectively. The Cu films consist of clusters comprising many small nanoparticles. The cluster sizes of the electrodeposited Cu were estimated to be in the range of 90–140 nm and 10–40 nm for FTO/Cu and the FTO/Ti/Cu, respectively, indicating that the thin Ti layer provides more Cu nucleation sites than the FTO surface. Smaller Cu clusters correspond to smaller Cu2S nanosheets, because the Cu2S is formed and grown by the surface diffusion, rather than bulk diffusion, during the sulfurization reaction. The uniformly synthesized small Cu2S nanosheets, packed side-by-side, of the FTO/Ti/ Cu2S material, provide significantly more electrochemical active sites than the FTO/Cu material and facilitates ion transport through the vertical channels. Fig. 2 show the cross-sectional view of the 48


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(a) 10 cy

(c) 20 cy

(b) 15 cy

1.05 μm

893 nm

744 nm

1 μm

1 μm

1 μm

Fig. 2. Cross-sectional SEM images of the FTO/Ti/Cu2S (10, 15 and 20 cycles) films.

Cu2S (10, 15, and 20 cycles), FTO/Cu2S, and FTO/Pt CEs. The current densities in the Tafel curves are rapidly increased from the 0 V, indicating so fast electron transfer kinetics. That is, the Tafel curves were limited by mass transport, which makes it difficult to define exact linear segments in the Tafel plots. Nevertheless, the Tafel plots are very useful to estimate the electrocatalytic performances, because the larger slope of the Tafel curve indicates a better exchange current density. Furthermore, the Jo is related to the charge transfer resistance (Rct) according to the following Eq. (2) [36]:

Table 1 Thicknesses of FTO/Ti/Cu2S (10, 15 & 20 cycles) films. Counter electrode

Thickness (~nm)

FTO/Ti/Cu2S (10 cycle) FTO/Ti/Cu2S (15 cycle) FTO/Ti/Cu2S (20 cycle)

750 900 1050

Jo =

RT nFR ct

(2)

where R, T, F, and n represent the gas constant, absolute temperature, Faraday's constant, and number of electrons involved in the polysulfide reduction, respectively. Here, the value of n is 2, since 2 electrons are involved in the electrochemical reduction reaction, Sn2− + 2e− (CE) → Sn−12− + S2−, at the CE interface. The larger the slope of the Tafel curve, the higher the exchange current density. The value for Jo follows the order of FTO/Ti/Cu2S (15 cycle) > FTO/Ti/Cu2S(20 cycle) > FTO/Ti/Cu2S (10 cycle) > FTO/Cu2S(15 cycle) > FTO/Pt, indicating that all the FTO/Ti/Cu2S CEs exhibit superior electrocatalytic activity in the S2−/ Sn2− redox couple than the other CEs (FTO/Cu2S and FTO/Pt). The poor electrocatalytic activity of the FTO/Pt can be attributed to poisoning by chemisorption of S2− ions onto the Pt surface, thereby hindering the reduction at the CE/electrolyte interface. The FTO/Ti/Cu2S (10 cycle) material exhibited less electrocatalytic activity than FTO/Ti/ Cu2S (15 cycle), due to the slightly lower coverage of the Cu2S on the substrate. The FTO/Ti/Cu2S (20 cycle) material exhibited reduced electrocatalytic activity compared to the FTO/Ti/Cu2S (15 cycle)

Fig. 3. XRD patterns of FTO/Cu (15 cycle), FTO/Cu2S (15 cycle), FTO/Ti/Cu (15 cycle) and FTO/Ti/Cu2S (15 cycle) films.

(a) Cu 2p

(b) S 2p

Cu 2p3/2

Cu 2p1/2

930

935

940

945

950

955

Intensity (a.u.)

Intensity (a.u.)

S 2p3/2

960

Binding Energy (eV)

S 2p1/2

160

162 Binding Energy (eV)

Fig. 4. Cu 2p and S 2p XPS curves of the FTO/Ti/Cu2S 15-cycle sample. 49

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Element

Wt%

At%

OK

30

75

Ti K

3

2

Sn L

68

23

material. This may be due to slower charge transfer arising from the presence of a thicker film. The Jlim is directly related to the diffusion of the redox couple in the electrolyte and inside the nanostructures. For the relative comparison of the diffusion coefficient, the intersection of the cathodic branch with the y axis is regarded as the limiting diffusion current density (Jlim) of the CEs, as shown on the Fig. 6. The diffusion coefficients (D) can be estimated according to the following Eq. (3) [36]:

C

VQ 1 6K

6K VQ 6K

%W

VQ

Element

Wt%

At%

OK

13

41 13

5 1 6K

SK

8

Ti K

2

2

Cu K

24

20

Sn L

53

23

%W

6K

%W 1

5

D=

Wt%

At%

OK

13

42

SK

8

13

Cu K

27

23

Sn L

52

23

VQ

D

E

%W %W

Fig. 5. EDAX of (a) FTO/Ti, (b) FTO/Ti/Cu2S, and (c) FTO/Cu2S films. The insets present the compositional analysis in mass (wt%) and atomic (at.%) percentages.

Log Current density (mA/cm2)

(3)

where the D is the diffusion coefficient of the electrolyte between the Cu2S sheets, l is the spacer thickness between the electrodes, C is the concentration of the Sn2− ions, F is Faraday's constant, and n is the electron number involved in the poly sulfide redox couple. To calculate the D values, the concentration of the polysulfide ions should be calculated. However, it is too hard to determine its concentration in the polysulfide electrolyte, because the sulfide ions exist in the form of Sn2− ions where the n is in the range of 5–8. The D values can be compared qualitatively, because the D is proportional to the Jlim. All of the FTO/ Ti/Cu2S CEs exhibited higher Jlim values than the FTO/Cu2S CEs, indicating the enhanced diffusion coefficient for the FTO/Ti/Cu2S material. Unlike FTO/Cu2S, the FTO/Ti/Cu2S material consists of the small Cu2S nanosheets, uniformly packed side-by-side perpendicular to the FTO/Ti surface. Therefore, the significantly enhanced Jo and Jlim of the FTO/Ti/Cu2S (15 cycle) CEs can be attributed to the presence of more electrochemical active sites and more efficient ion transport through the open vertical channels between the nanosheets. To investigate the electronic charge transport kinetics of the CEs in more detail, EIS analysis was undertaken using symmetrical sandwiched cells (CE/electrolyte/CE) with effective areas of approximately 0.96 cm2 and incorporating electrolyte with the S2−/Sn2− redox couple. Nyquist plots were acquired in the frequency range of 10−1 to 105 Hz at 0 V, in the absence of light. Fig. 7 shows Nyquist plots of the symmetrical cells with different CEs. The corresponding electrochemical parameters are listed in Table 2. The series resistance (Rs) and the Rct of the CEs were estimated using Z-view software by fitting to the equivalent circuit shown in the inset of Fig. 7. All of the FTO/Ti/Cu2S CEs exhibited lower Rs values than the FTO/ Pt and FTO/Cu2S CEs, indicating that the presence of the thin metallic Ti interlayer improves the electronic conductivity. The Rct values were estimated to be 17.74, 12.45, 13.09, 20.10, and 5188 Ω, for the FTO/ Ti/Cu2S (10, 15, and 20 cycles), FTO/Cu2S, and FTO/Pt CEs,

%W

Element

l Jlim 2nFC

Jlim 10

1 FTO/Ti/Cu2S (10 cycle)

0.1

0.01

FTO/Ti/Cu2S (15 cycle) FTO/Ti/Cu2S (20 cycle) FTO/Cu2S (15 cycle) Pt

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Potential (V) Fig. 6. Tafel polarization curves for the symmetrical cells assembled with Different CEs.

Fig. 7. Nyquist plots of the symmetrical cells consisting of CE/electrolyte/CE for the different CE materials. The insets show the Nyquist plot of the symmetrical cell with the Pt CE and the electrical equivalent circuit model for data fitting. 50


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Table 2 Photovoltaic performances of the QDSSCs and EIS parameters of the symmetrical cells with different CEs. Samples

Jsc (mA cm−2)

Voc (V)

FF (%)

ɳ (%)

Rs (Ohm)

Rct (Ohm)

CPE (F)

FTO/Pt FTO/Ti/Cu2S (10 cycle) FTO/Ti/Cu2S (15 cycle) FTO/Ti/Cu2S (20 cycle) FTO/Cu2S (15 cycle)

13.50 14.87 16.31 15.15 14.74

0.49 0.56 0.57 0.56 0.56

26 39 44 43 37

1.71 3.25 4.11 3.60 3.04

18.29 17.24 17.28 17.92 20.96

5188 17.74 12.45 13.09 20.10

0.00003 0.0142 0.0098 0.0114 0.0058

due to inhibition of the reduction reaction (Sn2− + 2e−(CE) → Sn2− + S2−) thereby slowing the transportation of the Sn2− from the 1 photoanode to the CE. More accumulated Sn2− ions near the photoanode facilitates the recombination by the back-electron transfer reaction (Sn2− + 2e−(TiO2) → Sn-12− + S2−) from the TiO2 to the electrolyte, leading to a decrease in voltage (Voc) and FF [34]. The QDSSCs incorporating FTO/Ti/Cu2S CEs exhibited superior photovoltaic performances compared to those incorporating FTO/Cu2S (15 cycle) CEs. In addition, the QDSSCs incorporating FTO/Ti/Cu2S CEs showed the improved performance in the order of: FTO/Ti/Cu2S (15 cycle) > FTO/Ti/Cu2S(20 cycle) > FTO/Ti/Cu2S (10 cycle). These results are in a good agreement with the Tafel and EIS results. The superior cell performance of the FTO/Ti/Cu2S(15 cycle) CEs can be attributed to the distinct features of the material. Firstly, the Ti metallic interlayer not only improves the electronic conductivity but also facilitates the electron transfer between the Cu2S and FTO, leading to the reduced Rs and Rct values. Secondly, the Ti interlayer increases the Cu nucleation rate during the Cu electrodeposition, leading to the formation of small Cu2S nanosheets packed side-by-side after sulfurization. These Cu2S nanosheets are perpendicular to the FTO/Ti surface, resulting in open vertical channels which facilitate ion transport. This distinct nanostructure of the Cu2S increases the surface area (or electrochemical active sites) of the electrode and enhances ion transport efficiency. The long term compatibility of electrodes with an electrolyte may be assessed by recording of multiple successive cyclic voltammograms (CVs) [38,39]. The long-term electrochemical stability of the CE is very important for their practical application. The CV curves shown in Fig. 9 were measured using three electrode cell where the Cu2S electrode served as working electrode, Pt mesh as the counter electrode, and Ag/ AgCl as the reference electrode. Fig. 9 shows the 50 successive CV curves of the (a) FTO/Ti/Cu2S (15 cycle) and (b) FTO/Cu2S (15 cycle), where the electrolyte was 1 mol·L−1 Na2S, 1 mol·L−1 S, and 0.1 mol·L−1 NaOH in DI water and the scan rate was 10 mV/s. The Cu2S CEs exhibited redox peaks corresponding to the redox reaction of Sn2− + 2e− ⇌ Sn-12− + S2−. The current density of the FTO/Ti/Cu2S was much higher than that of FTO/Cu2S, indicating significantly

Photocurrent density (mA/cm2)

18

14

FTO/Cu2S (15 cycle) FTO/Ti/Cu2S (10 cycle) FTO/Ti/Cu2S (15 cycle)

12

Pt

16

FTO/Ti/Cu2S (20 cycle)

10 8 6 4 2 0 0.0

0.1

0.2 0.3 0.4 Potential (V)

0.5

0.6

Fig. 8. Photocurrent density-voltage (J-V) curves of the QDSSCs with different CEs.

respectively. The very large Rct value of the FTO/Pt CE (5188 Ω) indicates very poor electrocatalytic activity. This is attributed to the strong chemisorption of S2− ions on the Pt surface (the so-called poisoning effect) which inhibits reduction of the redox couple and decreasing the energy-conversion efficiency when incorporated into QDSSCs [25,37]. All of the FTO/Ti/Cu2S CEs exhibited lower Rct than the FTO/Cu2S CEs, indicating that the Ti interlayer facilitated electron transfer from the FTO to the catalytic Cu2S nanosheets. The FTO/Ti/ Cu2S (15 cycle) exhibited the lowest Rct value, confirming its excellent electrocatalytic activity determined from Tafel plots of Fig. 6. Fig. 8 shows the photovoltaic performance of the QDSSCs with different CEs under one sun illumination (A.M 1.5 G, 100 mW/cm2). The resulting photovoltaic parameters are summarized in Table 2. These results were reproducible. The QDSSCs based on the Pt CE exhibited very poor photovoltaic performance due to the aforementioned poisoning effect [2]. Poor electrocatalytic activity of the Pt CE arises

30

30

20

10

b) FTO/Cu2S (15 cycle)

Current (mA)

Current (mA)

a) FTO/Ti/Cu2S (15 cycle) 1st 20

th

30

th

40

th

50

th

1st 20th 30th

10

40th 50th

0

0 -0.8

20

-0.4

0.0

0.4

0.8

Potential (V)

-0.8

-0.4

0.0 0.4 Potential (V)

0.8

Fig. 9. 50 successive CV curves for (a) FTO/Ti/Cu2S (15 cycle) and (b) FTO/Cu2S (15 cycle) CEs in polysulfide electrolyte. 51


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enhanced electrocatalytic activity of the FTO/Ti/Cu2S. The long term stability of the FTO/Ti/Cu2S maintained good mechanical integrity after first 20th cycles. Initial degraded stability of the FTO/Ti/Cu2S may be due to the partial passivation of active electrochemical sites and/or initial irreversible reaction, the detailed study of which is underway. On the contrary, the FTO/Cu2S without Ti interlayer exhibited gradually decreased stability. These results indicate that the Ti interlayer plays a role as an adhesive layer preventing the Cu2S nanosheets from peeling off the substrate and the FTO/Ti/Cu2S has an excellent electrochemical stability in the polysulfide electrolyte.

[8]

[9]

[10]

[11]

4. Conclusion [12]

Herein we have reported the synthesis of Cu2S films on the Ti layercoated FTO substrates using a two-step procedure. The Cu films were first deposited on the FTO/Ti substrates using the potentiodynamic electrodeposition at different number of cycles (10–20 cycles). Subsequently these FTO/Ti/Cu films were immersed into the polysulfide solution to prepare Cu2S films. For comparison, the conventional FTO/Cu2S and FTO/Pt films were also prepared. For the first time, it is demonstrated that the Ti interlayer increased the Cu nucleation rate during the Cu electrodeposition and led to the formation of small Cu2S nanosheets packed side-by-side after the sulfurization. In contrast, the FTO/Cu2S films, prepared without a Ti interlayer are composed of a mixture of large and small Cu2S nanosheets. The distinct nanostructure of the FTO/Ti/Cu2S CEs produced the following in benefits for the resulting QDSSCs. Firstly, small Cu2S nanosheets, packed side-by-side, perpendicular to the substrate, give a significantly increased number of electrochemical active sites and more efficient ion transport. Secondly the Ti interlayer not only increased the electronic conductivity of the CE, but also facilitated the electron transfer between the FTO and the Cu2S. Thirdly, the Ti interlayer acts as the adhesion layer and improves the mechanical integrity of the Cu2S, leading to significantly improved electrochemical stability. As a result, the QDSSC incorporating the FTO/Ti/Cu2S (15 cycle) CEs exhibited significantly increased cell efficiency (4.11%), compared to QDSSCs with the Pt (1.71%), and FTO/ Cu2S (3.04%), CEs. These findings should also provide insights into the electrodeposited nanostructures for the different applications, such as fuel cells, batteries, supercapacitors, and photoelectrochemical cells.

[13]

[14] [15]

[16]

[17] [18]

[19]

[20]

[21]

[22]

[23]

[24]

Acknowledgements [25]

This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (grant number 2018R1D1A3B05042787) This work was also supported by the “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), via a grant from the Ministry of Trade, Industry, & Energy, Republic of Korea (No. 20174030201760).

[26]

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