Enhanced electrocatalytic activity of electrodeposited

Journal of Power Sources 316 (2016) 53e59

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Enhanced electrocatalytic activity of electrodeposited F-doped SnO2/ Cu2S electrodes for quantum dot-sensitized solar cells Vu Hong Vinh Quy a, Jae-Hong Kim a, Soon-Hyung Kang b, Cheol-Jong Choi c, ***, 1, John Anthuvan Rajesh a, **, 1, Kwang-Soon Ahn a, *, 1 a b c

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

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Electrodeposited F-doped SnO2 (FTO)/Cu2S films are fabricated. They consist of vertical-standing nanosheets with small nanosheets in between. They exhibit the facilitated ion transport and large surface area. They are used as the counter electrode of the quantum dot-sensitized solar cells. Significantly enhanced cell efficiency of 4.58% was achieved.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 December 2015 Received in revised form 15 March 2016 Accepted 21 March 2016

Copper sulfide (Cu2S) films were deposited on F-doped SnO2 (FTO) substrates via the electrodeposition (ED) of copper (Cu) nanoparticles followed by sulfurization. The Cu nanoparticles were deposited on FTO substrates for various ED times ranging from 10 to 30 min at a constant 0.4 V. The FTO/Cu films consisted of flower-like nanoparticles comprised of randomly-clustering nanoflakes. The Cu nanoparticles electrodeposited for 10 min (FTO/Cu (10 min)) were dispersed sparsely over the FTO substrate, whereas the FTO/Cu (20 and 30 min) provided increased coverage. Unlike FTO/Cu2S (10 min), the FTO/ Cu2S (20 and 30 min) consisted of vertically-standing large Cu2S nanosheets with numerous small nanosheets in between. This was attributed to the sufficient number of Cu seed nanoflakes, which not only facilitate ion transport of the redox couple but also increased the surface area, leading to significantly enhanced electrocatalytic activity. The quantum dot-sensitized solar cell (QD-SSC) with FTO/Cu2S (20 min) exhibited a significantly improved cell efficiency of 4.58%, compared to those with Pt and FTO/ Cu2S (10 min). The QD-SSC with the FTO/Cu2S (30 min) showed similar cell efficiency to that with the FTO/Cu2S (20 min), despite the larger surface area because of its amorphous crystallographic structure offsetting the electrocatalytic activity. © 2016 Published by Elsevier B.V.

Keywords: Copper sulfide Counter electrode Electrodeposition Electrocatalytic activity Quantum dot-sensitized solar cell

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (C.-J. Choi), [email protected] (J.A. Rajesh), [email protected] (K.-S. Ahn). 1 K. eS. Ahn, J. A. Rajesh and C. eJ. Choi contributed equally to this work as the corresponding authors. http://dx.doi.org/10.1016/j.jpowsour.2016.03.075 0378-7753/© 2016 Published by Elsevier B.V.

54

V.H. Vinh Quy et al. / Journal of Power Sources 316 (2016) 53e59

1. Introduction

2. Experimental

In recent years, the demand for renewable energy, such as solar energy, has increased due to the limited fossil fuel reserves and global warming issues caused by CO2 emissions from the use of these fuels [1]. Up to now, a range of solar technologies have been evaluated. Among them, quantum dot-sensitized solar cells (QDSSCs), in which semiconductor quantum dots (QDs), such as CdS [25], CdSe [2,3,6,7] and PbS [5,8], are commonly used as photosensitizers, are attractive candidates for third-generation solar cells because of their favorable characteristics of QDs, such as a tunable band, high absorption coefficient [3,9,10], the possibility of hot electron injection [11], and multiple exciton generation effects [12]. QD-SSCs follow the concept of dye-sensitized solar cells (DSSCs) [13]. QD-SSCs have a sandwich structure in which the redox electrolyte is filled between QD-assembled mesoporous TiO2 on a transparent conducting oxide (TCO) electrode and an electrocatalytic counter electrode (CE). QD-sensitizers can be excited by photo absorption. The electrons are photo-generated in the excited QDs and injected into the conduction band of TiO2. They reach the TCO by diffusing through the mesoporous TiO2 network and moving to the CE via an external circuit. The CE supplies electrons to the electrolyte via an electrocatalytic reaction and the original state of the QDs are then recovered by electrons supplied from the electrolyte [3]. Despite the advantages of QDs, the overall energy conversion efficiency of QD-SSCs with cadmium chalcogenide QDs is low. Considerable efforts to improve the performance of QD-SSCs focused mainly on the light-harvesting ability of the QD photosensitizers, charge injection from the QDs to TiO2 and the charge recombination rate at the TiO2/QD/electrolyte interfaces [9,14-16]. In addition, the design and optimization of the CE, which is responsible for catalyzing the reduction reaction in the electrolyte redox system, also plays an important role in further improving the performance of QD-SSCs. Polysulfide electrolytes stabilize the photoanodes against photocorrosion [17], particularly for CdS and CdSe. On the other hand, the conventional Pt CE, which has superior conductivity and electrocatalytic activity for the iodide/triiodide redox couple in DSSCs [18], is unsuitable for QD-SSCs with a polysulfide redox system (S2/S2 n ) [19,20] because of the poisoning effect caused by S2 chemisorbed onto the Pt surface, which decreases the surface activity of the catalyst. Recently, alternative electrocatalysts to Pt in a polysulfide electrolyte, such as Au [21], PbS [22], copper sulfide (Cu2S) [23,24], CoS [25,26], NiS [26,27], and CoS/NiS [26], have been reported. Among them, Cu2S CE exhibits relatively high electrocatalytic activity toward the reduction of polysulfide species in QD-SSCs. Despite its importance, the morphological effects of Cu2S films on the electrocatalytic activities has attracted little research attention. This paper reports Cu2S CEs deposited on F-doped SnO2 (FTO) substrates via the electrodeposition (ED) of the flower-like Cu nanoparticles followed by sulfurization. By varying the ED times from 10 to 30 min at a constant cathodic potential of 0.4 V, the mean size and coverage of the flower-like Cu particles on the FTO substrate were increased gradually. The flower-like Cu nanoparticles consisted of randomly-clustering Cu nanoflakes, whose coverage strongly affected the petal-shaped Cu2S morphology. The QD-SSC with the FTO/Cu2S (20 min) exhibited significantly enhanced electrocatalytic activity, leading to significantly increased cell efficiency (4.54%), because of its distinct morphology consisting of large vertically-standing nanosheets with numerous small nanosheets in between. These results were studied systematically in terms of nucleation and growth, electrocatalytic activity, chargetransfer resistance, and electrochemical active sites.

2.1. Preparation of FTO/Cu2S and Pt counter electrodes The FTO/Cu2S CE was fabricated according to a published procedure with minor modifications [24]. First, the Cu film was electrodeposited potentiostatically on the FTO substrate with a platinum (Pt) mesh and Ag/AgCl (sat. KCl) used as the counter and reference electrodes, respectively. Prior to electrodeposition, the FTO substrates were cleaned sequentially with acetone, ethanol and deionized (DI) water under sonication for 30 min each. The electrolyte, which was always degassed prior to use, consisted of 5 mmol,L1 CuSO4, 1 mol,L1 Na2SO4 and 0.5 mol L1 H3BO3 in DI water. The potentiostatic ED of Cu was carried out at a constant cathodic potential of 0.4 V, which was determined by linear sweep voltammetry (Fig. 1). The ED duration time for Cu deposition was varied for 10, 20 and 30 min, followed by rinsing with DI water and drying in an oven at 70 C. Subsequently, the FTO/Cu films were immersed in a polysulfide aqueous solution containing 1 mol,L1 S and 1 mol,L1 Na2S followed by rinsing with DI water and drying in an oven at 70 C. The Pt CEs were prepared by doctor-blading a Pt nanoclustercontaining Pt paste (PT-1, Dyesol. Ltd.) onto FTO substrates followed by calcination at 450 C for 30 min. 2.2. Preparation of CdSe QD-assembled TiO2 films Compact TiO2 blocking layers were always deposited on the FTO substrates. These blocking layers were formed by dipping the FTO substrates in a 40 mmol,L1 TiCl4 solution at 70 C for 30 min. Mesoporous TiO2 films were then prepared by doctor-blading a TiO2 paste (Ti-Nanoxide T/SP, Solaronix SA) onto the FTO substrates followed by drying at 70 C for 30 min and sintering at 450 C in air for 30 min. The double-layer TiO2 was fabricated to a total thickness of 10 mm and an active area of 0.16 cm2. The CdSe QDs were assembled onto the TiO2 films using a successive ionic layer adsorption and reaction (SILAR) technique in a glove box, as described elsewhere [7]. The CdSe QDs were synthesized from 30 mmol L1 Cd(NO3)2 and sodium selenide (Na2Se) using ethanol as the solvent. The Na2Se solution was prepared from 30 mmol,L1 SeO2 and 60 mmol,L1 NaBH4 in ethanol, as described in equation (1).

-2

Current density (mAcm )

SeO2 þ 2NaBH4 þ 6C2H5OH / Se2 þ 2Naþ þ 2B(OC2H5)3 þ 5H2 þ 2H2O (1)

0.0 -0.4 -0.8 -1.2 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Voltage (vs. Ag/AgCl) Figure 1. Linear sweep voltammetry (LSV) cathodic curve of the FTO in the electrolyte consisting of 5 mmol,L1 CuSO4, 1 mol,L1 Na2SO4 and 0.5 mol L1 H3BO3 in DI water.


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, 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, the TiO2/CdSe film was coated with a ZnS passivation layer via the SILAR process, where the films were dipped alternately into an ethanol solution of 0.1 mol L1 Zn(NO3)2$6H2O for 1 min, rinsed with ethanol and dried with a drier, followed by dipping into a methanol/DI water (1:1 by volume) solution of 0.1 mol,L1 Na2S$9H2O, rinsing with methanol and drying with a drier. This SILAR procedure was repeated for 2 cycles. 2.3. Cell fabrication To produce the QD-SSCs, the CdSe QD-assembled TiO2 films and CEs (Pt and FTO/Cu2S) were sandwiched using a 60 mm-thick sealing material. The electrolyte was polysulfide, which had been prepared from a DI water solution containing 1 mol,L1 Na2S, 1 mol,L1 S and 0.1 mol,L1 NaOH. 2.4. Characterizations The morphology of the Cu2S CEs was characterized by scanning electron microscopy (SEM, Hitachi FE-SEM S4800). The crystalline characteristics and chemical composition were analyzed by X-ray diffraction (XRD, PANalytical X'Pert) and X-ray photoelectron spectroscopy (XPS, Omicron Nanotechnology ESCA Probe system), respectively. The photovoltaic currentvoltage (JV) characteristics were measured using a solar simulator (PEC-L11 m Peccell Ltd.) under 1 sun illumination (100 mWcm2, AM 1.5G), which was verified using an AIST-calibrated Si-solar cell. Electrochemical impedance spectroscopy (EIS) was performed between 101 and 105 Hz using symmetrical cells (CE/electrolyte/CE) with a 60 mm spacer thickness. 3. Results and discussion The morphology, crystalline structure and composition of the electrodeposited Cu and sulfurized Cu thin films were characterized by SEM, XRD and XPS, respectively. The morphologies of the electrodeposited Cu and sulfurized Cu thin films were examined by SEM (Fig. 2). Fig. 2aec shows SEM images of the electrodeposited Cu thin films for different ED duration times ranging from 10 to 30 min. The Cu films on the FTO substrate were composed of Cu nanoparticles. The inset in Fig. 2a shows flower-like nanoparticles consisting of two-dimensional nanoflakes. The mean size and coverage of the Cu nanoparticles on the FTO surface increased with increasing electrodeposition time from 10 to 30 min. A similar trend was observed for the sulfurized Cu thin films but the morphology of the sulfurized Cu thin films changed to a typical petal-shaped Cu2S morphology consisting of thin nanosheets. Note that the Cu2S nanosheets fabricated from the Cu electrodeposited for 10 min did not fully cover the FTO substrate (Fig. 2d). In contrast, as the ED time was increased further up to 30 min (Fig. 2e and f), the Cu2S nanosheets almost covered the entire area of the FTO substrate by the formation of small nanosheets between the verticallystanding large nanosheets. Enlarged images of Fig. 2gei clearly show these distinct morphological differences. The nanoflakes of the flower-like Cu nanoparticles were seed particles for the nucleation of Cu2S. The nucleation and growth of Cu2S during the sulfurization proceeded in all directions due to the randomlyclustering nanoflakes. The Cu electrodeposited for 10 min (referred to as Cu (10 min))

55

exhibited sparsely-dispersed Cu nanoparticles over the FTO substrate, providing a much smaller number of nucleation sites for Cu2S. On the contrary, the Cu electrodeposited for 20 min exhibited much better coverage, leading to a larger number of the nucleation sites for Cu2S formation. The surrounding numerous nucleation suppresses the lateral growth of Cu2S but facilitated the vertical growth of Cu2S, resulting in large vertically-standing Cu2S nanosheets and small nanosheets between them. Because of no sufficient powders from the thin films, the quantitative surface area cannot be estimated. However, the qualitative estimation can be drawn from those morphological features. The vertically-standing nanosheets make ion transport of the redox couple more efficient and the numerous small Cu2S nanosheets lead to a significantly increased surface area, which may be beneficial for the electrocatalytic activity. The thickness of the different electrodeposited Cu films and the Cu2S films were determined by cross-sectional SEM. Fig. 3aec presents cross-sectional SEM images of the Cu films on the FTO substrate. A film thickness of 250 nm (Fig. 3a), 380 nm (Fig. 3b) and 520 nm (Fig. 3c) was obtained for an electrodeposition time of 10 min, 20 min and 30 min, respectively. This result confirmed that increasing the electrodeposition time from 10 to 30 min increases the film thickness from 250 to 520 nm. In the case of the Cu2S films (Fig. 3def), the measured thicknesses for electrodeposition times of 10, 20 and 30 min were 520 nm, 850 nm and 1.07 mm, respectively. The cross-sectional images (Fig. 3def) showed that the Cu2S nanosheets have close contact with the FTO substrate, which benefits the electrocatalytic activity of the CE. The crystalline phases of the Cu and Cu2S films were characterized by XRD, as shown in Fig. 4. Fig. 4a presents XRD patterns of the FTO/Cu (10 min), FTO/Cu (20 min) and FTO/Cu (30 min). The main XRD peak at 43.29 2q was the assigned to the (111) plane of the face-centered cubic Cu phase (JCPDS No ¼ 004-0836). All the other peaks were assigned to the XRD planes of the FTO substrate. The intensity of Cu main peak increased with increasing electrodeposition time from 10 to 30 min due to the increased Cu film thickness. On the other hand, Fig. 4b shows that there are no diffraction peaks related to Cu2S, indicating that amorphous Cu2S had formed after sulfurization. This result is consistent with previous studies of amorphous CuxS films reported by Fuwei et al. [28] and Ke et al. [29]. XPS was performed to examine the accurate chemical composition of the sulfurized Cu film. Fig. 5a and b shows the highresolution Cu 2p and S 2p spectra, respectively, of the Cu2S film sulfurized from the Cu film electrodeposited for 20 min Fig. 5a presents two peaks at 932.5 and 952.0 eV corresponding to Cu 2p3/2 and Cu 2p1/2, respectively [30,31]. The binding energies of the S 2p3/ 2 and S 2p1/2 peaks were 161.48 and 162.5 eV, respectively (Fig. 5b). These binding energies (BEs) are consistent with those previously reported for Cu and S in Cu2S [32], confirming the chemical composition of Cu2S. The photovoltaic performance of the QD-SSCs employing Pt, FTO/Cu2S (10 min), FTO/Cu2S (20 min), and FTO/Cu2S (30 min) as the CEs was measured under 1 Sun illumination, where the FTO/ Cu2S (x min) refers to the sulfurized Cu (x min) film. Fig. 6 presents the JV curves of QD-SSCs, and the detailed photovoltaic parameters are listed in Table 1. Compared to the QD-SSCs with the Pt CE, all the QD-SSCs with the FTO/Cu2S CEs exhibited a significantly improved overall energy-conversion efficiency (h) in the sequence of FTO/Cu2S (20 min) > FTO/Cu2S (30 min) > FTO/Cu2S (10 min) > Pt. All the photovoltaic parameters (short-circuit current, open-circuit potential, and fill factor) were enhanced significantly in the QD-SSCs with the FTO/Cu2S. In particular, the QD-SSC with the FTO/Cu2S (20 min) CE exhibited the highest cell efficiency, as high as 4.58%, which is 86.9% and 21.8% higher than the QD-SSCs

56


Figure 2. Top-view images of (aec) FTO/Cu (10, 20 and 30 min) films, respectively, and (def) FTO/Cu2S (10, 20 and 30 min) films, respectively. (gei) Enlarged top-view images of the (def), respectively. Inset of (a) shows an enlarged image of the electrodeposited Cu.

Figure 3. Cross-sectional SEM images of (aec) FTO/Cu (10, 20 and 30 min) films, respectively, and (def) FTO/Cu2S (10, 20 and 30 min) films, respectively.

with the Pt and FTO/Cu2S (10 min) CEs, respectively. To examine the electrocatalytic activities of all the CEs used, sandwiched symmetrical cells consisting of CE/electrolyte/CE were constructed and the currentevoltage (IeV) characteristics were measured using the polysulfide electrolyte (1 mol,L1 S, 1 mol,L1

Na2S and 0.1 mol,L1 NaOH). Fig. 7 presents Tafel plots of the FTO/ Cu2S (10 min), FTO/Cu2S (20 min), FTO/Cu2S (30 min) and Pt CEs. The Tafel polarization curve measurement is a simple and powerful tool for evaluating the electrocatalytic activity because the exchange current density (i0), which reflects the electrocatalytic


57

(a)

Intensity (a.u.)

Cu 2p3/2

Cu 2p1/2

965

960

955

950

945

940

935

930

925

Binding energy (eV) (b)

S 2p3/2

Intensity (a.u.)

S 2p1/2

172

170

168

166

164

162

160

158

Binding Energy (eV) Figure 5. XPS spectra of (a) Cu 2p and (b) S 2p of the FTO/Cu2S (20 min).

18

activity, can be estimated easily from the extrapolated intercepts of the anodic and cathodic branches of the Tafel curves. The larger slope of the Tafel curves indicates a higher i0 value, suggesting superior electrocatalytic activity. In addition, the i0 is related to the charge transfer resistance (Rct) according to the following equation:

RT i0 ¼ ; nFRct

(2)

where R is the gas constant, T is the absolute temperature, n is the number of electrons involved in the polysulfide reduction, and F is the Faraday constant. The results show that all the Cu2S CEs exhibited significantly enhanced electrocatalytic activities for the 2e reduction of S2e n to S , compared to the Pt CE. The poor electrocatalytic activity of Pt can be attributed to poisoning by S2e ions chemisorbed on the Pt surface, which hinder the reduction S2e n þ 2e 2ee (CE) / S2e at the CE interface [19,20,33]. The electron1 þ S catalytic activity was enhanced significantly in the FTO/Cu2S (20 and 30 min) CEs compared to the FTO/Cu2S (10 min) CE. This is because the FTO/Cu2S (10 min) obtained from the Cu electrodeposited for 10 min are composed of laterally-lying nanosheets with poor coverage over the FTO substrate. On the other hand, the FTO/Cu2S (20 min) consisted of small Cu2S nanosheets between the

Current density (mAcm-2)

Figure 4. XRD patterns of (a) the FTO/Cu (10, 20 and 30 min) films and (b) the FTO/ Cu2S (10, 20 and 30 min) films.

16 14 12 10 8 6

Pt Cu2S (10 min)

4

Cu2S (20 min)

2

Cu2S (30 min)

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Voltage (V) Figure 6. Photovoltaic JeV performance of the QD-SSCs with the Pt, FTO/Cu2S (10, 20, and 30 min) CEs.

vertically-standing large nanosheets. The vertically-standing nanosheets facilitate the diffusion of the redox couple and the small nanosheets increase the surface area (or electrochemical active site) remarkably, giving rise to significantly enhanced electrocatalytic activity [34-36]. The FTO/Cu2S (30 min) exhibited similar electrocatalytic activity to the FTO/Cu2S (20 min), despite


Samples QD-SSC QD-SSC QD-SSC QD-SSC

with with with with

Pt CE FTO/Cu2S (10 min) CE FTO/Cu2S (20 min) CE FTO/Cu2S (30 min) CE

Jsc/mAcm2

Voc/V

FF/%

h/%

11.56 15.64 16.54 16.48

0.50 0.54 0.54 0.53

42.39 44.50 51.75 52.01

2.45 3.76 4.58 4.54

25 1500

Pt

20

-Z'' (Ohm)

Table 1 Photovoltaic JeV characteristics of the QD-SSCs with the Pt, FTO/Cu2S (10, 20 and 30 min) CEs.

-Z'' (Ohm)

58

15

1000

500

0 0

1000

2000

3000

Z' (Ohm)

Current density (mAcm-2)

10

1

10

Pt Cu2S (10 min)

5

Cu2S (20 min) Cu2S (30 min)

0

10

0 10

20

30

40

50

60

70

Z' (Ohm)

-1

10

Figure 8. Nyquist plots of the symmetrical cells consisting of a CE/electrolyte/CE for the Pt and FTO/Cu2S (10, 20 and 30 min) CEs. The inset shows the Nyquist plot for the Pt CE and the electrical equivalent circuit model used for data fitting.

Pt Cu2S (10 min)

-2

10

Cu2S (20 min) Cu2S (30 min)

-3

10

-1.0

-0.5

0.0

0.5

Table 2 EIS characteristics estimated from the Nyquist plots of the symmetrical cells for the Pt and FTO/Cu2S (10, 20 and 30 min) CEs.

1.0

Overpotential (V) Figure 7. Tafel polarization curves of the sandwiched symmetrical cells consisting of a CE/electrolyte/CE, where the CE is the Pt and the FTO/Cu2S (10, 20 and 30 min).

being thicker with a larger surface area. This is because the Cu2S films exhibited an amorphous crystallographic structure, which has poor carrier mobility, offsetting the electrocatalytic activity. EIS was carried out using symmetrical sandwiched cells (CE/ electrolyte/CE) with two identical CE films to examine the charge transfer kinetics at the interface of the CE/electrolyte [37]. The EIS measurements were conducted at 0 V in the dark with the same electrolyte composition and the effective area was approximately 0.96 cm2 for each cell. The Nyquist plots in Fig. 8 show the impedance characteristics of various CEs. The EIS results were fitted using Z-view software using the inserted equivalent circuit model in Fig. 8, and the EIS parameters, such as the series resistance (Rs) and the charge-transfer resistance (Rct) of the CEs were estimated and listed in Table 2. All the FTO/Cu2S CEs exhibited lower Rs values to the Pt, indicating good electrical contact of the Cu2S films with the FTO substrates. The Rct values for the Cu2S/FTO (10 min), Cu2S/ FTO (20 min), Cu2S/FTO (30 min), and Pt CEs were estimated to be 40.67, 15.44, 16.65, and 3233.8 U, respectively. The very high Rct of the Pt CE confirms its poor electrocatalytic activity, due to the poisoning effect. The lowest Rct of the FTO/Cu2S (20 min) CE indicates significantly enhanced electrocatalytic activity because of its distinct nanostructures consisting of vertically-standing large nanosheets with small nanosheets in between. The FTO/Cu2S (30 min) exhibited a similar Rct to the FTO/Cu2S (20 min), despite being thicker because of the offsetting effect caused by the poor carrier mobility in the amorphous phase. The FTO/Cu2S (10 min) exhibited moderate electrocatalytic activity, giving rise to a moderate cell efficiency of 3.76% when employed in QD-SSCs because of its low surface area and laterally-lying nanosheets. On the other hand, the FTO/Cu2S (20 min) contributed to the best photovoltaic cell efficiency of 4.58% because of its significantly enhanced electrocatalytic activity. The QD-SSC with the FTO/Cu2S (30 min) exhibited similar cell efficiency to that with the FTO/Cu2S (30 min)

Samples

Rs /U

Rct/U

Pt FTO/Cu2S (10 min) FTO/Cu2S (20 min) FTO/Cu2S (30 min)

25.02 19.60 17.95 17.69

3233.8 40.67 15.44 16.65

due to its similar electrocatalytic activity.

4. Conclusions This paper reported Cu2S CEs on FTO substrates via the ED of the flower-like Cu nanoparticles followed by sulfurization. By varying the ED times for 10, 20 and 30 min at a constant cathodic potential of 0.4 V, the mean size and coverage of the flower-like Cu particles on the FTO substrates were increased gradually. The flower-like Cu particles consisted of randomly-clustering Cu nanoflakes. The FTO/ Cu2S films had petal-like morphologies consisting of nanosheets, which were strongly affected by the electrodeposited Cu films. The Cu nanoparticles of the Cu (10 min) were sparsely dispersed over the FTO, whereas the Cu electrodeposited over 20 min exhibited much better coverage, leading to an increase in the number of nucleation sites for Cu2S. The randomly-oriented Cu nanoflakes allowed the nucleation and growth of Cu2S in all directions. The FTO/Cu2S (10 min) exhibited nanosheets mainly lying down on the FTO that did not fully cover the FTO surface due to the sparsely distributed Cu nanoparticles in the Cu (10 min). In contrast, the FTO/Cu (20 and 30 min) films provided a sufficient number of nucleation sites, facilitating the vertical growth of Cu2S nanosheets and suppressing lateral growth. The resulting FTO/Cu2S (20 and 30 min) consisted of vertically-standing big Cu2S nanosheets and numerous small nanosheets in between, which fully cover the FTO surface. The QD-SSCs with the FTO/Cu2S (20 min) exhibited the best overall energy-conversion efficiency of 4.58%, which was 86.9% and 21.8% higher than that of the QD-SSCs with the Pt and FTO/Cu2S (10 min) CEs, respectively. This is because the vertically-standing large nanosheets facilitated efficient ion transport of the redox couple and the numerous small Cu2S nanosheets increased the electrochemical active sites significantly, giving rise to significantly


enhanced electrocatalytic activity. The QD-SSCs with the FTO/Cu2S (30 min) exhibited similar cell efficiency to that of the FTO/Cu2S (20 min), despite its larger surface area, because of the amorphous crystallographic structure of the Cu2S, which offsets the electrocatalytic activity. These results provide insight into alternative electrocatalysts for applications to photoelectrochemical cells, fuel cells, supercapacitors, and batteries. Acknowledgements This study was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF- 2015R1D1A3A01016158). This study was also supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning, granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20154030200760). References [1] P.K. Nayak, G.G. Belmonte, A. Kahn, J. Bisquert, D. Cahen, Photovoltaic efficiency limits and material disorder, Energy Environ. Sci. 5 (2012) 6022e6039. [2] P.V. Kamat, K. Tvrdy, D.R. Baker, J.G. Radich, Beyond photovoltaics: semiconductor nanoarchitectures for liquid-junction solar cells, Chem. Rev. 110 (2010) 6664e6688. [3] P.V. Kamat, Quantum dot solar cells. Semiconductor nanocrystals as light harvesters, J. Phys. Chem. C 112 (2008) 18737e18753. [4] C.H. Chang, Y.L. Lee, Chemical bath deposition of CdS quantum dots onto mesoscopic TiO2 films for application in quantum-dot-sensitized solar cells, Appl. Phys. Lett. 91 (2007) 053503 (1-3). [5] H. Lee, H.C. Leventis, S.J. Moon, P. Chen, S. Ito, S.A. Haque, T. Torres, F. Nuesch, T. Geiger, S.M. Zakeeruddin, M. Gratzel, M.K. Nazeeruddin, PbS and CdS quantum dot-sensitized solid-state solar cells: “Old concepts, new results”, Adv. Funct. Mater 19 (2009) 2735e2742. [6] H. Zhang, K. Cheng, Y.M. Hou, Z. Fang, Z.X. Pan, W.J. Wu, J.L. Hua, X.H. Zhong, Efficient CdSe quantum dot-sensitized solar cells prepared by a postsynthesis assembly approach, Chem. Comm. 48 (2012) 11235e11237. [7] H. Lee, M. Wang, P. Chen, D.R. Gamelin, S.M. Zakeeruddin, M. Gratzel, M.K. Nazeeruddin, Efficient CdSe quantum dot-sensitized solar cells prepared by an improved successive ionic layer adsorption and reaction process, Nano Lett. 9 (2009) 4221e4227. [8] R. Plass, S. Pelet, J. Krueger, M. Gratzel, U. Bach, Quantum dot sensitization of organic-inorganic hybrid solar cells, J. Phys. Chem. B 106 (2002) 7578e7580. [9] G. Hodes, Comparison of dye- and semiconductor-sensitized porous nanocrystalline liquid junction solar cells, J. Phys. Chem. C 112 (2008) 17778e17787. [10] I. Robel, M. Kuno, P.V. Kamat, Size-dependent electron injection from excited CdSe quantum dots into TiO2 nanoparticles, J. Am. Chem. Soc. 129 (2007) 4136e4137. [11] W.A. Tisdale, K.J. Williams, B.A. Timp, D.J. Norris, E.S. Aydil, X.Y. Zhu, Hotelectron transfer from semiconductor nanocrystals, Science 328 (2010) 1543e1547. [12] J.B. Sambur, T. Novet, B.A. Parkinson, Multiple exciton collection in a sensitized photovoltaic system, Science 330 (2010) 63e66. [13] H.K. Jun, M.A. Careem, A.K. Arof, Quantum dot-sensitized solar cellsperspective and recent developments: a review of Cd chalcogenide quantum dots as sensitizers, Renew. Sustain. Energy Rev. 22 (2013) 148e167. [14] M. Shalom, S. Buhbut, S. Tirosh, A. Zaban, Design rules for high-efficiency quantum-dot-sensitized solar cells: a multilayer approach, J. Phys. Chem. Lett. 3 (2012) 2436e2441. [15] S.W. Jung, J.H. Kim, H. Kim, C.J. Choi, K.S. Ahn, CdS quantum dots grown by in situ chemical bath deposition for quantum dot-sensitized solar cells, J. Appl. Phys. 110 (2011) 044313 (1-4). [16] H.J. Yun, T. Paik, M.E. Edley, J.B. Baxter, C.B. Murray, Enhanced charge transfer

[17] [18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33] [34]

[35]

[36]

[37]

59

kinetics of CdSe quantum dot-sensitized solar cell by inorganic ligand exchange treatments, ACS Appl. Mater. Interface 6 (2014) 3721e3728. S. Licht, Electrolyte modified photoelectrochemical solar cells, Sol. Energy Mater. Sol. Cells 38 (1995) 305e319. S. Thomas, T.G. Deepak, G.S. Anjusree, T.A. Arun, S.V. Nair, A.S. Nair, A review on counter electrode materials in dye-sensitized solar cells, J. Mater. Chem. A 2 (2014) 4474e4490. J.G. Radich, R. Dwyer, P.V. Kamat, Cu2S reduced graphene oxide composite for high-efficiency quantum dot solar cells. Overcoming the redox limitations of S-2/S2n at the counter electrode, J. Phys. Chem. Lett. 2 (2011) 2453e2460. M.H. Yeh, C.P. Lee, C.Y. Chou, L.Y. Lin, H.Y. Wei, C.W. Chu, R. Vittal, K.C. Ho, Conducting polymer-based counter electrode for a quantum-dot-sensitized solar cell (QD-SSC) with a polysulfide electrolyte, Electrochim. Acta 57 (2011) 277e284. Y.P. Yoon, J.H. Kim, S.H. Kang, H. Kim, C.J. Choi, K.K. Kim, K.S. Ahn, Enhanced electrocatalytic activity of the Au-electrodeposited Pt nanoparticles-coated conducting oxide for the quantum dot-sensitized solar cells, Appl. Phys. Lett. 105 (2014) 083116 (1-5). Z. Tachan, M. Shalom, I. Hod, S. Ruhle, S. Tirosh, A. Zaban, PbS as a Highly catalytic counter electrode for polysulfide-based quantum dot solar cells, J. Phys. Chem. C 115 (2011) 6162e6166. S.Y. Lee, M.A. Park, J.H. Kim, H. Kim, C.J. Choi, D.K. Lee, K.S. Ahn, Enhanced Electrocatalytic activity of the annealed Cu2-xS counter electrode for quantum dot-sensitized solar cells, J. Electrochem. Soc. 160 (2013) H847eH851. K. Meng, P.K. Surolia, O. Byrne, K.R. Thampi, Efficient CdS quantum dot sensitized solar cells made using novel Cu2S counter electrode, J. Power Sources 248 (2014) 218e223. N. Balis, V. Dracopoulos, K. Bourikas, P. Lianos, Quantum dot sensitized solar cells based on an optimized combination of ZnS, CdS and CdSe with CoS and CuS counter electrodes, Electrochim. Acta 91 (2013) 246e252. H.J. Kim, S.W. Kim, C.V.V.M. Gopi, S.K. Kim, S.S. Rao, M.S. Jeong, Improved performance of quantum dot-sensitized solar cells adopting a highly efficient cobalt sulfide/nickel sulfide composite thin film counter electrode, J. Power Sources 268 (2014) 163e170. H.J. Kim, D.J. Kim, S.S. Rao, A.D. Savariraj, K. Soo-Kyoung, M.K. Son, C.V.V.M. Gopi, K. Prabakar, Highly efficient solution processed nanorice structured NiS counter electrode for quantum dot sensitized solar cells, Electrochim. Acta 127 (2014) 427e432. F. Zhuge, X. Li, X. Gao, X. Gan, F. Zhou, Synthesis of stable amorphous Cu2S thin film by successive ion layer adsorption and reaction method, Mater. Lett. 63 (2009) 652e654. A. Ghahremaninezhad, E. Asselin, D.G. Dixon, Electrodeposition and growth mechanism of copper sulfide nanowires, J. Phys. Chem. C 115 (2011) 9320e9334. C.S. Kim, S.H. Choi, J.H. Bang, New insight into copper sulfide electrocatalysts for quantum dot-sensitized solar cells: composition-dependent electrocatalytic activity and stability, ACS Appl. Mater. Interface 6 (2014) 22078e22087. W. Ke, G. Fang, H. Lei, P. Qin, H. Tao, W. Zeng, J. Wang, X. Zhao, An efficient and transparent copper sulfide nanosheet film counter electrode for bifacial quantum dot-sensitized solar cells, J. Power Sources 248 (2014) 809e815. J. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, p. 220, p. 235, Perkin-Elmer Corp, Eden Prairie, MN, 1992. M.J.H. Gary, C. David, Electrocatalytic electrodes for the polysulfide redox system, J. Electrochem. Soc. 127 (1980) 544e549. €tzel, M.D. McGehee, MeltC.D. Bailie, E.L. Unger, S.M. Zakeeruddin, M. Gra infiltration of spiro-OMeTAD and thermal instability of solid-state dyesensitized solar cells, Phys. Chem. Chem. Phys. 16 (2014) 4864e4870. T. Leijtens, I.K. Ding, T. Giovenzana, J.T. Bloking, M.D. McGehee, A. Sellinger, Hole transport materials with low glass transition temperatures and high solubility for application in solid-state dye-sensitized solar cells, ACS Nano 6 (2012) 1455e1462. W. Yuan, H. Zhao, G.L. Baker, Low glass transition temperature hole transport material in enhanced-performance solid-state dye-sensitized solar cell, Org. Electron 15 (2014) 3362e3369. M. Adachi, M. Sakamoto, J.T. Jiu, Y. Ogata, S. Isoda, Determination of parameters of electron transport in dye-sensitized solar cells using electrochemical impedance spectroscopy, J. Phys. Chem. B 110 (2006) 13872e13880.