Electrocatalytic activity of chemically deposited CuxS thin film for counter electrode in quantum dots-sensitized solar cells

Materials Chemistry and Physics xxx (2014) 1e7

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Electrocatalytic activity of chemically deposited CuxS thin film for counter electrode in quantum dots-sensitized solar cells Iseul Lim a, Deok Yoon Lee a, Supriya A. Patil a, Nabeen K. Shrestha a, Soon Hyung Kang b, Yoon-Chae Nah c, Wonjoo Lee d, *, Sung-Hwan Han a, * a

Department of Chemistry, Hanyang University, Seoul 133-791 Republic of Korea Department of Chemistry Education, Chonnam National University, Gwangju 500-757, Republic of Korea School of Energy Materials Chemical Engineering, Korea University of Technology and Education, Chungnam 330-708, Republic of Korea d Department of Defense Ammunitions, Daeduk College, Daejeon 305-715 Republic of Korea b c

h i g h l i g h t s
We investigated the preparation of Cu2S thin film by chemical bath deposition.
The Cu2S films were characterized using electrochemical techniques.
In addition, the Cu2S films were investigated as a counter electrode in QSSCs.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 July 2013 Received in revised form 21 May 2014 Accepted 5 August 2014 Available online xxx

The compact (c-CuxS) and the porous (p-CuxS) with particle decorated films of coppersulfidearesynthesized using a chemical bath deposition technique, and the films are characterized using electrochemical techniques. In addition, the chemically deposited CuxS films are investigated as a counter electrode in quantum dots-sensitized solar cells (QSSCs). The available redox active reaction sites of the p-CuxS film are found to be 57.9% higher than those available in the c-CuxS film. From the electrochemical impedance spectroscopy, the effective diffusion coefficients of the polysulfide electrolyte in the c-CuxS and p-CuxS films are estimated to be 3.67  105 and 6.35  105 cm2 s1, respectively. These results can be ascribed to the improvement in the available redox active reaction sites and the electrocatalytic activity of the CuxS counter electrode. As compared to the c-CuxS film, the p-CuxS film as a counter electrode exhibits an enhanced photovoltaic performance of the QSSCs with the power conversion efficiency of 3.17%, short-circuit current of 11.89 mA cm2, open-circuit voltage of 0.50 V, and fill factor of 53.29. The improved performance of the QSSCs is ascribed to the improvements on the available redox active reaction sites, electrocatalytic activity and the diffusion coefficients, which are directly related to the surface morphology of the sulfide films. © 2014 Elsevier B.V. All rights reserved.

Keywords: Semiconductor Nanostructures Chemical synthesis Electrochemical techniques

1. Introduction Quantum dots-sensitized solar cells (QSSCs) have attracted considerable attention as a next generation photovoltaics due to their potentially high power conversion efficiency [1e4].It has been expected that the generation of multiple excitons by impact ionization in colloidal semiconductor quantum dots (QDs) could drive the thermodynamic photovoltaic conversion efficiency limit up to 44% from the 31% of the Shockley-Queisser limit [5e8]. The

* Corresponding authors. Tel.: þ82 42 866 0222; fax: þ82 42 866 0359. E-mail addresses: [email protected] (W. Lee), [email protected] (S.-H. Han).

configural composition and the working principle of a QSSC are similar to that of a dye-sensitized solar cell. In general, a QSSC consists of a quantum dots-sensitized TiO2photoanode, redox electrolyte containing polysulfide (S2/S2 x ) and a counter electrode with catalysts [1e3]. As an alternative to the ruthenium-complex sensitizers in DSSCs, metal chalcogenide quantum dots such as CdS [9e12], CdSe [13e18], and PbS [5,19,20] have been reported as promising sensitizers, and these QD sensitizers have attractive advantages over ruthenium-complex sensitizers such as tunable band gap, high extinction coefficients and large intrinsic dipole moment [1e3]. Like in DSSCs, a Pt film is commonly used as a counter electrode in QSSCs [9e11]. However, the performance of the Pt film for redox

http://dx.doi.org/10.1016/j.matchemphys.2014.08.008 0254-0584/© 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: I. Lim, et al., Electrocatalytic activity of chemically deposited CuxS thin film for counter electrode in quantum dots-sensitized solar cells, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.08.008

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I. Lim et al. / Materials Chemistry and Physics xxx (2014) 1e7

reaction of polysulfide electrolyte is not satisfactory because the electrocatalytic activity is reduced by a barrier layer formed by the self-assembled monolayers (SAMs) of sulfide ion on the surface of Pt film [21]. Therefore, in order to improve the performance of the QSSCs, great attentions have been paid on the substitution of Pt counter electrodes [21e29]. As an alternative, Au [21,27], CoS [23,28] and CuxS [24e29] films have also been suggested as the potential counter electrodes in polysulfide electrolyte based QSSCs. Among these materials, CuxS exhibited relatively higher electrocatalytic activity toward polysulfide redox system [24e29]. Generally, the in-situ reaction of brass sheet with polysulfide solution [24,27,29] or screen printing of CuxS powders prepared by chemical precipitation [25] is employed to prepare the CuxS counter electrodes for QSSCs. Although numerous other methods are available to prepare CuxS films [24e30], chemical bath deposition technique [30], which is one of the solution phase methods useful for the preparation of compound semiconductors from aqueous solutions, produces good deposits on suitable substrates by controlled precipitation of the compound. As a result, this technique offers many advantages over the other well-known methods of vapor phase synthetic routes. In addition, this technique facilitates the easy control on the growth factors such as film thickness, deposition rate and crystalline quality, which could be controlled by varying the solution pH, temperature and bath concentration [31,32].Nevertheless, to the best of our knowledge, chemically deposited CuxS film from solution phase has not been reported yet to fabricate QSSCs. Moreover, the previously reported CuxS counter electrodes prepared by other techniques has also not been fully characterized especially using electrochemical techniques although the techniques can help to design the efficient optoelectronic devices based on quantum dots. For this reason, in the present work, we investigated the preparation of nanostructured CuxS counter electrode in QSSCs by chemical bath deposition, and characterized its electrocatalytic activity in polysulfide electrolyte using electrochemical techniques.

SILAR process of CdSe was similar to that of CdS. However, a higher reaction temperature (~60  C) was needed for dipping the sample in to Na2SeSO3 solution, which was used as Se precursor for the formation of CdSe QDs. A total of three and four SILAR cycles were performed for the deposition CdS and CdSe QDs, respectively. Finally, as a passivation layer, ZnS was deposited using a one SILAR cycle [19,27], and subsequently the films were sintered at 150  C for 30 min at the rate of 5  C min. 2.3. Chemical bath deposition of CuxS thin film Chemical bath deposition of a thin compact thin film (c-CuxS) was carried out using the previously reported procedure [30]. Briefly, a mixture solution consisting of 5 ml 1 M CuCl2, 8 ml 28% NH4OH, 4 ml triethanolamine, 6 ml 1 M thiourea and 63 ml H2O, which has a final pH of 9.4was used as the precursor solution. After immersing the FTO substrates into the above precursor solution, the temperature of the reaction mixture was raised and kept constant at 40  C for 30 min using a water bath. The thin films thus produced were washed with DI water, dried in air, and finally annealed at 100  C for 30 min. To obtain the porous film (p-CuxS), pH of the precursor solution was adjusted to ~10.2 by adding concentrated solution of NH4OH into the above precursor. 2.4. Cell fabrication The CdS/CdSe QD-sensitized TiO2 electrode and the CuxS counter electrode were assembled and sealed as a sandwich using a transparent 60 mm thick Surlyn spacer (Dupont). The electrolyte composed of 0.5 M Na2S, 0.1 M S and 0.2 M K Clin MeOH/H2O (v/v % ¼ 7/3) was injected through the pin-hole made in the counter electrode. The active area of the QSSCs was about 0.4 cm2, which was measured precisely using an image analyzer equipped with a CCD camera (Moticam 1000). 2.5. Characterization

2. Experimental details 2.1. Materials Chemicals were purchased from Aldrich and were used as received. F-doped SnO2 conducting glass (FTO) electrodes were obtained from Pilkington (TEC8, 8 U/square, glass thickness of 2.3 mm). TiO2 nanoparticle-paste was prepared by the method described elsewhere [33]. 2.2. Preparation of CdS/CdSe-sensitized TiO2 films Before formation of porous TiO2 films on an FTO electrode, a thin TiO2compact layer were deposited as a hole blocking layer using a chemical deposition method [34]. For this, 0.15 M Titanium (IV) bis(ethylacetoacetato) di isopropoxide in 1-butanolprecursor solution was dropped on an FTO electrode, and spanned at the rate of ~2000 rpm for 40 s, followed by sintering at 500  C for 20 min. Porous TiO2 films were deposited on the TiO2 blocking layer-coated FTO electrodes using a doctor blade technique, followed by sintering at 500  C for 30 min at the rate of 5  C min1. The thickness of the TiO2 film thus prepared was approximately 12 mm. The formation of CdS and CdSe QDs on TiO2 film was carried out using a successive ionic layer adsorption and reaction (SILAR) method [9,12,21]. The SILAR process involved the dipping of TiO2 film into a 0.05 M CdCl2-ethanol solution for 5 min, rinsing it with ethanol and again dipping it for another 5 min in 0.05 Na2Smethanolsolutionfollowed by rinsing it with methanol. The twostep dipping procedure is considered here as one SILAR cycle. The

The CuxS thin film was characterized using optical absorption spectrometer (Varian, CARY 5000), X-ray powder diffractometer (XRD, Siemens D-5005 diffractometer), scanning electron microscopy (SEM, Hitachi S-4100), energy dispersive X-ray analyzer (EDAX, EM912) and cyclic voltammetry (CHI 620A Electrochemical Analyzer, CH Instruments, Austin, TX). Photocurrentevoltage measurements were performed using a Keithly-2400 source meter. A class-solar simulator (Newport) equipped with a 150 W Xe lamp was used as a light source, and the light intensity was calibrated and adjusted to 1 sun light using an NREL-calibrated Si solar cell with KG-1 filter. Incident photon-tocurrent conversion efficiency (IPCE) was measured as a function of wavelength from 400 nm to 800 nm using a specially designed IPCE system for the DSSCs (PV measurements, Inc.). For this, a 75-W Xenon lamp was used as a light source for generating monochromatic beam. A calibration was performed using a silicon photodiode (NIST-calibrated photodiode G425 as a standard), and IPCE values were collected under a halogen bias light at a low chopping speed of 4 Hz. Electrochemical measurements were performed using a single compartment glass cell consisting of an Ag/AgCl reference electrode and a Pt plate as a counter electrode. An exposed active area of 0.5 cm2of the CuxS/FTO electrode was used for the electrochemical characterization and the aqueous phase measurements were carried out under nitrogen atmosphere. The specific capacitance and the available redox active reaction sites were measured using cyclic voltammogram in an aqueous solution of 1 M H2SO4 þ 0.1 M CuSO4. A linear sweep voltammetry in a MeOH/H2O (v/v ¼ 7/3)electrolytecontaining0.5 M Na2S, 0.1 M S and 0.2 M KCl

Please cite this article in press as: I. Lim, et al., Electrocatalytic activity of chemically deposited CuxS thin film for counter electrode in quantum dots-sensitized solar cells, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.08.008

I. Lim et al. / Materials Chemistry and Physics xxx (2014) 1e7

was performed to obtain the currentevoltage characteristic curves, which were used to study the electrocatalytic performance of the counter CuxS electrode. The electrochemical impedance spectroscopy(EIS) was obtained using a potentiostat(Solartron 1287) equipped with a frequency response analyzer (Solartron 1260) in the frequency range of102 e106 Hz. The magnitude of the alternative signal was 10 mV and the measurements of EIS were carried out at opencircuit potential under AM 1.5G one sun light illumination. Impedance parameters were determined by fitting the impedance spectrum using the Z-view software. 3. Results and discussion 3.1. Deposition mechanism Chemical bath deposition of a compact CuxS thin film was carried out using the previously reported procedure [30]. Actually, synthesis of CuxS thin films was carried out in a relatively high alkaline medium (pH > 9) on FTO substrates, which were previously washed with acetone, ethanol and deionized water in an ultrasonication bath for 15 min each, and were finally washed with isopropanol. The Cu2þ and S2 ions, from the precursor solution as described in the experimental Section 2.3, react with each other and precipitate as neutral molecules either spontaneously or very slowly in the highly alkaline medium. Fast precipitation implies that no film formation takes place on the substrate. In contrast, if the rate of precipitation is slow and controllable (with the reaction additives like triethanolamine or ammonia solution), then neutral molecules could get enough time to adhere firmly on the substrate, and thereby initiation for the formation of a thin solid deposit on the substrate takes place [32]. In particular, NH3 solution is added to the deposition bath, which acts as a complexing agent for metal ions and controls the precipitation rate by changing ionic activity. Therefore, the amount of reaction additives (i.e. NH3) solution in chemical bath deposition has a strong influence on film growth rate, surface morphology, optical and chemical properties of the CuxS film. The mechanism of chemical bath deposition of CuxS film can be described by the following reactions. CuCl2 þ (NH3)n 4 [Cu(NH3)n]2þ þ 2Cl

(1)

[Cu(NH3)n]2þ 4 Cu2þ þ (NH3)n

(2)

CH3eCSeNH2 þ Hþ þ 2H2O 4 CH3COOH þ H2S þ NHþ 4

(3)

H2S 4 S2 þ 2Hþ

(4)

Cu2þ þ S2 / CuS

(5)

Due to the presence of ammonia in the aqueous medium, the formation of hydrolyzed species in the solution leads to the deposition of spherical grain structures on the substrate. It has been observed that the average growth rate increases as pH of the bath is increased because it avoids the formation of hydrolyzed species, thereby leads to the growth of nano-spheres. Recently, such a change in CuS surface architecture upon increasing the pH of the alkaline bath has been reported [30].

3

of a bare FTO glass substrate and the chemically deposited CuxS thin film form solution phase on an FTO substrate. The surface morphology of the bare FTO electrode has a tetragonal structure, where the thickness of FTO layer on the glass is ~600 nm [Fig. 1(aec)]. In case of pH 9.4, the surface morphology of the CuxS film appears mainly as a compact and smooth [Fig. 1(def)]. This film is named here as c-CuxS film. However, some local region in the film also exhibits sunken morphology, which is consisted of large particles with the size of ~80e100 nm. On the other hand, in case of pH 10.2, the surface morphology of the CuxS film shows the deposition of large particles having the size of ~150e200 nm [Fig. 1(g) and (h)]. Assembling of these particles has created grain boundaries with inter particle space, which has thus resulted the very narrow nanoporous structures of the film [Fig. 1(g) and (h)]. This film is named here as c-CuxS film. Furthermore, these large particles of ~150e200 nm of the p-CuxS film are decorated by the deposits of nanoparticles having the size of ~10e20 nm on their surface [Fig. 1(i)]. In addition to the nanoporous structure, such particle decorations further increase the film surface area. The thickness of the above two CuxS films is about 100e150 nm [Fig. 1(f) and (j)]. The film composition estimated with an EDAX [Fig. 1(k)] shows the atomic composition of 52.4% Cu and 47.6% S, which suggests a composition close to a 1: 1stoichiometry.As shown in Fig. 1(l), the color of the CuxS thin film is dark brown (in web version) while the color of Pt/FTO is black.

3.3. Optical properties It would be appropriate here to discuss briefly about the case of back illumination when the photoanode consists of a metallic substrate, for instance TiO2 nanotube/Ti electrode. In such a case, the incident light is needed to be passed through the counter electrode. In doing so, the actual intensity of the incident light on the surface of the photoanode differs from the intensity of the source due to the partial absorption of the incident light by the counter electrode. Therefore, the photoanode utilizes only the remaining spectrum which is filtered out by the counter electrode. Although the QSSCs in the present study are front illuminated device systems, optical properties of the counter electrode provide an important information to the researchers who need to study the back illumination system in QSSCs. Fig. 2(a) shows the UVeVis. absorption spectrum of the CuxS thin films on an FTO substrate, which exhibits the exponentially decreasing behavior of absorbance from 300 nm to about 560 nm followed by again raise in absorbance from about 560 nm onwards. Such absorption behavior with an increased absorbance near infrared region has been well reported. However, a broad absorption peak at 465 nm has been reported in this case [35]. Although a clear absorption peak in visible light region could not be observed in Fig. 2(a), presence of a broad hump at around 400 nm could be realized. In contrast, an exponentially continuous decreasing behavior of absorbance from UV to near infrared region has also been reported [36]. Such contrast absorption behaviors of various reported CuxS thin films could be due to different morphology and crystal structures of the deposits. Based on the absorption range, clearly the CuxS film is not very attractive candidate for those solar cell devices in which back illumination is needed as the film partially absorbs light to some extent in visible region. The UVeVis. absorption spectrum was further used to estimate the energy band gap of the CuxS thin film using the Eq.(6) [37,38]

3.2. Surface morphology The surface morphology of the CuxS thin film is controlled by adjusting the pH of the deposition baths. Fig. 1 shows SEM images


1=2 ahn ¼ A hnEg

(6)

Please cite this article in press as: I. Lim, et al., Electrocatalytic activity of chemically deposited CuxS thin film for counter electrode in quantum dots-sensitized solar cells, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.08.008

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Fig. 1. SEM images of (aec) bare FTO, (def) c-CuxS/FTO and (gej) p-CuxS/FTO. (k) EDAX spectrum of CuxS/FTO. (l) Photographic images of bare FTO and CuxS/FTO. (c, f, j) Crosssectional SEM views of samples.

where, A is a constant related to the refractive index and the electron/hole masses. An extrapolation of the linear portion a plot of (ahn)2 versus hn (photon energy) on the x-axis [Fig. 2(b)] is used to estimate the band gap of the CuxS thin films. The calculated band gap of the CuxS thin film is thus found to be~ 2.53 eV, and this value is not very far from the previously reported value (~2.48 eV) for CuxS thin film deposited by solution phase [38]. In order to confirm the crystal structure, XRD analysis of the CuxS thin film on a FTO sunstrate was made [Fig. 2(c)]. However, the XRD pattern of the CuxS thin film sample did not exhibit any characteristic peaks, which could be due to very thin thickness of the materials to be

analyzed and/or due to the interference of the XRD signals from the high crystallinity of the FTO substrate. In general, it has been reported that the wet deposited composite films are amorphous and/ or nanocrystalline [32]. 3.4. Measurement of available redox reaction sites The amount of available redox reaction sites in the film has a strong influence on the electrocatalytic activity for redox electrolyte as well as on the efficiency of the devices. Therefore, the specific capacitance and the available redox reaction sites of the above

Fig. 2. (a) UVevis absorption spectrum, (b) (ahv)2 vs hv plot of CuxS thin film on an FTO substrate, and (c) XRD patterns of bare FTO (solid line, :), c-CuxS/FTO (dash line, -) and p-CuxS/FTO (dot line, C).

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mentioned two CuxS films were investigated using cyclic voltammetry. Fig. 3 shows the cyclic voltammogram of the chemically deposited CuxS thin film in an already reported aqueous electrolyte containing 1 MH2SO4and 0.1 M CuSO4 [39].However, the cyclic voltammogram of the both compact and porous CuxS films did not show the distinctive peaks during oxidation and reduction cycles (Fig. 3). Oxidation of the CuxS is not a simple, but is made up of a series of consecutive reactions. Different researchers have argued different series of assumptions about the mechanism of electrochemical oxidation of CuxS [40e45]. However, the most acceptable mechanism presumes that the metal ions from the mineral crystal lattice are transferred into the solution, leaving a surface region with the higher content of sulfur. That sulfur can be treated as adsorbed species giving rise to the pseudo-capacitance exhibited by CuxS. From the voltammograms, a specific capacitance value of each electrode type was calculated using the following relation [43];

C ¼ ðjqa j þ jqc jÞ=ð2  m  DVÞ

(7)

where, C is the specific capacitance, qa and qc are the anodic and the cathodic voltammetric charges on anodic and cathodic scans, respectively, m is the mass of active material and DV is the potential window of cyclic voltammogram. Thus, at the scan rate of 10 mV s1, the specific capacitance values of the compact CuxS and porous CuxS film electrodes were found to be14.1 and 22.9 F g1, respectively (Table 1). The inner and outer charges shown in Table 1 were calculated using qa, qc, scan rate and DV. The specific capacitance of the p-CuxS electrode is thus 62.4% higher than that of the cCuxS. The specific capacitance is easily influenced by the electrical conduction in the system, the ion diffusion into active material and the redox active sites of active material. Based on themorphology of the CuxS films, it is apparent that the porous p-CuxS film provides significant amount of redox active sites to have more redox reactions. To justify this argument, the available redox reaction active sites (Z)were determined from the specific capacitance of the CuxS film according to the following equation [46,47,48];

Z ¼ C  ðDVÞ  M=F

(8)

where, DV is the potential window (here 0.8 V), M is the molecular weight of CuxS (159.15 g mol1), F is the Faraday constant, and C is the specific capacitance (F g1), respectively. For instance, if all the electro active sites are involved in the film, the Z value comes out to be 1. As shown in Table 1, the redox reaction sites of the c-CuxS and the p-CuxS films are 1.9  102 and 3.0  102, respectively. Hence, the higher active sites of the p-CuxS film are attributed to the higher surface area available for the redox reaction due to the porous and

Fig. 3. Cyclic voltammogram of c-CuxS(dash line, -) and p-CuxS (dot line, C) in 1 M H2SO4 þ 0.1 M CuSO4at scan rate of 10 mV s1.

5

Table 1 Parameters obtained from the cyclic voltammogram measurement of c- and p-CuxS film. Samples

Specific capacitance (F g1)

Available redox active reaction sites (Z)

Inner charge (C g1)

Outer charge (C g1)

c-CuxS p-CuxS

14.1 22.9

1.9  102 3.0  102

14.5 22.3

8.01 14.4

particle decorated surface morphology [48]. Therefore, based on the calculated Z values, it is reasonable to assume that the surface area of the p-CuxS counter electrode is relatively higher than that of the c-CuxS film, which is obviously due to the porous structure as well as due to the decoration of small nanoparticles on the surface of the larger particles in the film electrode. 3.5. Measurement of electrocatalytic activity The electrocatalytic activity of the above two CuxS thin films was determined by investigating the currentevoltage characteristics of the films in MeOH/H2O (v/v ¼ 7/3) containing0.5 M Na2S, 0.1 M S and 0.2 M KCl at the scan rate of 50 mV s1 (Fig. 4). In the currentevoltage curves, cathodic currents are due to the reduction 2 reaction of [S2 x ] to [S ] and the anodic currents are due to the oxidation reaction of [S2] to [S2 x ]. Both the films exhibit their electrocatalytic performance as demonstrated in Fig. 4. However, as compared to the c-CuxS film, the catalytic current of the p-CuxS film appears at the relatively anodic direction indicating the higher oxidative catalytic performance. We also measured the redox potentials (Eredox) of the polysulfide electrolytes with the above two CuxS thin films, and the Eredox of the c-CuxS and the p-CuxS films are found to be 0.57 and 0.71 V, respectively. As a result of more negative Eredox, the hole recovery rate from the p-CuxS photoelectrode to the electrolyte increases [23,28], which leads to increase in the values of short-circuit currents in QSSCs. 3.6. Measurement of diffusion coefficients EIS measurements are powerful tool to characterize the performance of each component in QSSCs [3]. In the present study, the EIS measurements were carried out to determine the diffusion coefficient of S2 x ions in QSSCs consisting of the compact and the porous CuxS counter electrodes at open-circuit potential under AM 1.5G one sun light illumination and the as-measured EIS spectrum was fitted with the corresponding equivalent circuits by the Z-View software [34].

Fig. 4. Current vs. voltage curves of c-CuxS/FTO (dash line, -) and p-CuxS/FTO (dot line, C) in presence of polysulfide electrolyte [0.5 M Na2S/0.1 M S/0.2 M KCl in MeOH/ H2O (v/v ¼ 7/3)].

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Fig. 5. (a) Nyquist plot of QSSCs based on c-CuxS/FTO (-) and p-CuxS/FTO (C) counter electrode. (b) Equivalent circuit model.

Fig. 5(a) shows the Nyquist plots of the QSSCs with different CuxS counter electrodes. Fig. 5(b) is the equivalent circuit model to fit the obtained impedance spectra. Here, Rs denotes the resistance of the electrolyte and the FTO substrate. R1 and R2 are the charge transfer resistance across the interfaces of the counter electrode/electrolyte and the porous electrode/electrolyte, respectively. The finite Warburg impedance related elements, which are associated with diffusion processes, are influenced by the nature of the porous electrode/ electrolyte interface and the counter electrode/electrolyte. The symbol, CPE, describes the constant phase element of the capacitance, meaning a non-ideal frequency dependent capacitance due to a non-uniform distribution of current by the material heterogeneity [49]. The elements with subscripts 1 and 2 are related to the capacitance of the interfaces of the electrolyte/CuxS counter electrode and photoelectrode/electrolyte, respectively. The CPE was defined by two values, CPE-T and CPE-P.The finite Warburg impedance (Ws), which depends on the parameters, WseR, WseT and WseP, accounts for the finite-length Warburg diffusion (ZDif) in one dimension. In the diffusion interpretation WseP ¼ l2/D, l is the effective diffusion thickness (approximately 30 mm) and D is the effective diffusion coefficient of the species. Here, ZDif is defined as follows [50];

ZDif ¼

i h RDif  tanhðjTuÞp =ðjTuÞp

(9)

where, R, T, and p were abbreviated from WseR, WseT, and WseT, respectively. The measured EIS spectrum shown in Fig. 5 is similar to the previously reported EIS spectrum, which showed three semicircles in the Nyquist plot. Table 2 summarizes the quantitatively fitted results using the equivalent circuit. Rs and RDif describe the electrolytic resistance and diffusion related resistance of the electrolyte. As shown in Fig. 5 and Table 2, the Rs, R1, R2 and RDif of the pCuxS counter electrode are7.8, 6.4,32.7 and 13.1 U, respectively. Similarly, each corresponding values of the c-CuxS counter electrode are7.8, 16.8,33.2 and 15.9U, respectively. Here, the R1 and RDif of the p-CuxS film are lower than those of the c-CuxS film. On the other hand, Rs and R2 of the p-CuxS film are similar with those of the c-CuxS film. AsR1and RDif are related with the charge transfer resistance of the interfaces of the counter electrode/electrolyte and the diffusion processes, the decreased interfacial resistances is attributed to the improvement of surface area.

Table 2 Parameters obtained from measurement of electrochemical impedance spectra. Samples

Rs (U)

R1 (U)

R2 (U)

Rd (U)

ZDif (cm2 s1)

c-CuxS p-CuxS

7.8 7.8

16.8 6.4

33.2 32.7

15.9 13.1

3.67  105 6.35  105

The diffusion coefficient (D) of S2 x ions in QSSCs was calculated using the equation, WseT ¼ l2/D. The D values of the S2 x ions in the c-CuxS and the p-CuxS films are found to be approximately 3.67  105 and 6.35  105 cm2 s1, respectively. This value is close to the previous reports based on the electrochemical behavior of polysulfides in tetrahydrofuran (D ¼ ~105 cm2 s1) [51]. This finding indicates that the permeation of the polysulfide electrolyte was improved by enhancements of electrocatalytic activity at the counter electrode, which can increase the fill factor in QSSCs. 3.7. Performance of chemically deposited CuxS counter electrode in QSSCs Fig. 6 shows the photocurrentevoltage curves and IPCE of the chemically deposited CuxS counter electrode based QSSCs, and Table 3 summarizes the photovoltaic parameters of the QSSCs with different counter electrodes. It is important to note here that the QSSCs with the p-CuxS counter electrode shows power conversion efficiency (Eff) of 3.17% with short-circuit currents (Jsc) of 11.89 mA cm2, open-circuit voltage (Voc) of 0.50 V, and fill factor (ff) of 53.29. On the other hand, the Eff of the c-CuxS counter electrode based QSSCs is 2.77% with Jsc ¼ 11.30 mA cm2, Voc ¼ 0.48 V, and ff ¼ 51.12. Thus, as compared to the c-CuxS film, the p-CuxS counter electrode has demonstrated an improved photovoltaic parameters of the QSSCs. The improved photovoltaic performance can be directly related to the improvement on the electrocatalytic activity, which actually depends on the surface morphology. For instance, the enhanced Eff is attributed to the improvement of diffusion coefficient of the redox electrolyte caused by the surface morphology shown by the p-CuxS counter electrode. In addition, the enhanced Jsc can be ascribed to the improved rate of electron regeneration in quantum dots by enhancements of the hole recovery rate. On the other hand, the enhanced Voc can be attributed to the change of conduction band edge by improvement of Jsc or reduction of recombination process. Consequently, the Eff of the QSSCs is enhanced by improving the surface morphology of the CuxS counter electrode from compact to the porous together with particle decorated surface architecture. 4. Conclusion In conclusions, we have described a chemical bath deposition method to prepare thin CuxS films with different surface morphology and its electrochemical characterization to measure the electrocatalytic activity for polysulfide electrolyte in QSSCs. The available redox active reaction sites and the electrocatalytic activity of the CuxS thin film were determined from the current-voltage characteristics obtained by measuring the cyclic and linear sweep

Please cite this article in press as: I. Lim, et al., Electrocatalytic activity of chemically deposited CuxS thin film for counter electrode in quantum dots-sensitized solar cells, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.08.008

I. Lim et al. / Materials Chemistry and Physics xxx (2014) 1e7

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Fig. 6. (a) Photocurrent vs. voltage curves and (b) IPCE of QSSCs based on c-CuxS/FTO (-) and p-CuxS/FTO (C) counter electrode.

Table 3 Performances of quantum dots-sensitized solar cell using CuxS films as counter electrode. Samples

Voc (mV)

Jsc (mA cm2)

FF

Eff. (%)

IPCE

c-CuxS p-CuxS

0.48 0.50

11.30 11.89

51.12 53.29

2.77 3.17

41.96 44.06

voltammetry. The available redox active reaction sites, diffusion coefficient and the electrocatalytic activity of the p-CuxS thin film electrode for the polysulfide electrolyte are found to be higher than those of the c-CuxS thin film electrode. Due to the better performance of the p-CuxS electrode, the QSSCs with the p-CuxS counter electrode demonstrated Eff of 3.17% with Jsc ¼ 11.89 mA cm2, Voc ¼ 0.50 V, ff ¼ 53.29, and these values are higher than those demonstrated by the c-CuxS counter electrode. The improved photovoltaic performance is ascribed to the improvement of the surface morphology of the p-CuxS counter electrode. It is our strong belief that the present method of preparation and the characterization of the CuxS counter electrode in QSSCs will help to design and fabricate highly efficient QSSCs in future. Acknowledgments This research was supported by a grant (code #: 2010K000335) from 'Center for Nanostructured Materials Technology' under '21st Century Frontier R&D Programs' of the Ministry of Education, Science and Technology, Korea. This work is also in part supported by KRCF (Korea Research Council of Fundamental Science & Technology) and KIST (Korea Institute of Science & Technology) for 'NAP (National Agenda Project) program. One of the authors (N.K. Shrestha) is supported by The Korean Federation of Science and Technology Societies under Brain Pool program. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2014.08.008. References [1] P.V. Kamat, J. Phys. Chem. C 112 (2008) 18737. [2] S. Rühle, M. Shalom, A. Zaban, ChemPhysChem 11 (2010) 2290. [3] I. Mora-Ser, S. Gimnez, F. Fabregat-Santiago, R. Gmez, Q. Shen, T. Toyoda, J. Bisquert, Acc. Chem. Res. 42 (2009) 1848. [4] J. Tang, E.H. Sargent, Adv. Mater. 23 (2011) 12. [5] J.B. Sambur, T. Novet, B.A. Parkinson, G. Decher, Science 277 (1997) 1232. [6] A.J. Nozik, Phys. E 14 (2002) 115. [7] A.J. Nozik, Chem. Phys. Lett. 457 (2008) 3. [8] A.J. Nozik, Inorg. Chem. 44 (2005) 6893. [9] Y.-L. Lee, C.-H. Chang, J. Power Sources 185 (2008) 584.

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Please cite this article in press as: I. Lim, et al., Electrocatalytic activity of chemically deposited CuxS thin film for counter electrode in quantum dots-sensitized solar cells, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.08.008