Catalytic and photoelectrochemical performances of Cu–Zn–Sn–Se thin films prepared using selenization of electrodeposited Cu–Zn–Sn metal precursors

Journal of Power Sources 286 (2015) 47e57

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Catalytic and photoelectrochemical performances of CueZneSneSe thin films prepared using selenization of electrodeposited CueZneSn metal precursors Pin-Wen Shao a, Chun-Ting Li b, Kuo-Chuan Ho b, Kong-Wei Cheng a, * a b

Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan

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


CZTSe film was prepared via selenization of electrodeposited CueZneSn sample.
The direct energy band gaps of CZTSe samples are in the range of 0.95 e1.02 eV.
CZTSe sample with [Zn]/[Sn] ratio of 0.66 has the maximum PEC performance.
DSSCs using CZTSe as counter electrode had maximum cell efficiency of 7.98%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 February 2015 Received in revised form 12 March 2015 Accepted 16 March 2015 Available online 17 March 2015

In this study, Cu2ZnSnSe4 (CZTSe) films are deposited onto the fluorine-doped-tin-oxide-coated glass substrate via the selenization of electrodeposited CueZneSn metal precursors in an acidic solution with the applied potential of 0.9 V vs. an Ag/AgCl electrode. X-ray diffraction patterns reveal that the samples are the quaternary tetragonal CZTSe phase. The thicknesses and direct band gaps of the samples are in the ranges of 2.3 to 2.7 mm and 0.95 to 1.02 eV, respectively. All samples are p-type semiconductors with carrier density, mobility and flat-band potential in the ranges of 3.88  1017 to 1.37  1018 cm3, 10.31 to 12.6 cm2 V1 s1 and 0.01 V to 0.08 V vs. Ag/AgCl reference electrode, respectively. The sample with [Cu]/[Zn þ Sn] and [Zn]/[Sn] molar ratios of 0.87 and 0.66, respectively, has a maximum photo-enhanced current density of 0.41 mA cm2 at an applied bias of 0.5 V vs. an Ag/AgCl electrode in 0.5 M H2SO4 solution under illumination. The best photo-conversion efficiency of dye-sensitized solar cells using CZTSe with [Cu]/[Zn þ Sn] and [Zn]/[Sn] molar ratios of 0.87 and 0.66, respectively, as the counter electrode was 7.98%. The results show the high quality CZTSe films have potentials in applications of photoelectrochemical water splitting and dye-sensitized solar cells. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

Keywords: Electrodeposition Photoelectrochemical Dye-sensitized solar cell Counter electrode

1. Introduction

* Corresponding author. 259 Wen-Hwa 1st Rd., Kwei-Shan, Taoyuan 333, Taiwan. E-mail address: [email protected] (K.-W. Cheng). http://dx.doi.org/10.1016/j.jpowsour.2015.03.101 0378-7753/Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

There has been increasing interest in the conversion of solar light energy into electrical power or hydrogen energy. One possible

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approach is water splitting using a photoelectrochemical (PEC) cell, in which water is oxidized to oxygen at the surface of n-type semiconductors and reduced into hydrogen at the counter electrode (CE), or a dye-sensitized solar cell (DSSC), which can directly convert solar light into electrical power without a net chemical change [1,2]. The structures of PEC cells and DSSCs have similar components, such as photoelectrodes as photo-absorber layers, electrolytes, and CEs. Semiconductors such as metal oxides [1e3], metal sulfides [4,5], and metal selenides [6,7] can be used as photoelectrodes in PEC cells. TiO2/ruthenium complex dyes are used as photoelectrodes in DSSCs [2]. A Pt electrode and aqueous solutions containing H2SO4, NaOH, Na2SO4, K2SO3, NaS or methanol [3e8] are used as the CE and electrolyte, respectively, in PEC cells, whereas a Pt layer deposited on a transparent conductive oxide (TCO)-coated (e.g., indium-tin-oxide, ITO) glass substrates and I/I 3 redox couples are the CE and electrolyte, respectively, in DSSCs. For PEC water splitting, although metal oxides are stable photocatalysts in electrolytes, their low PEC performances in the visible-light region limit their industrial application. Binary metal sulfides such as CdS are good photocatalysts for hydrogen production under visible-light irradiation because of their suitable band edge position for hydrogen production [9,10]. However, the toxicity and photocorrosion problem of CdS in electrolytes cause environmental problems. Some studies have reported that multi-component metal sulfides with similar photocatalytic activities and higher stability in electrolytes than those of binary metal sulfides [11,12]. PEC water splitting using various multi-component metal sulfide semiconductors, such as AgIn5S8, AgGaS2/AgInS2, and CuInS2/AgInS2/ ZnS, as solid solution photocatalysts has been reported [13e15]. In DSSC studies, a Pt layer grown on TCO-coated glass substrates is generally used as the CE because of its high electrocatalytic ability and chemical stability in the I/I 3 electrolyte. However, the cost of Pt is a obstacle for low-cost DSSC applications. Binary metal sulfides/selenides such as NiSe2 [16], CoSe2 [16], CoS [17], NiS [18], MoS2 [19], and RuSe2 [20] have been reported as possible CEs in DSSCs due to their good stability under light irradiation and excellent electrocatalytic activity for the reduction of I 3 ions in electrolytes. Recently, I-IV-VI/II-VI solid solution semiconductors such as Cu2ZnSnSe4 (CZTSe) have been applied in thin film solar cells [21], as CEs in DSSCs [22] and as photoelectrodes in PEC cells [23]. Septina et al. [23] prepared CZTSe thin films on Mo-coated glass substrates using one-step electrodeposition and annealed the samples under S vapor at a temperature of 500  C to obtain CZTS/Se samples. Their samples showed a maximum PEC response of 0.5 mA cm2 at an external bias of 0.45 V (vs. an Ag/AgCl electrode) in a solution containing Eu3þ ions. The highest external quantum efficiency of their CZTSe samples was around 2% with the external bias kept at 0.4 V vs. an Ag/AgCl reference electrode. Wang et al. [22] prepared CZTSe nanocrystals using the hot injection method. Their DSSCs with CZTSe before and after ligand exchange treatment as CEs showed the photo-conversion efficiencies (PCEs) of 0.67% and 7.06%, respectively. Du et al. [24] synthesized CZTSe nanoparticles with diameters of 200e300 nm using a onestep solvothermal method without surfactants or templates. The maximum PCE of their DSSCs with these CZTSe nanoparticles dropcasted onto fluorine-doped tin oxide (FTO)-coated glass as the CEs was 3.80%. Although some studies have reported the catalytic response of CZTSe CEs in I/I 3 redox couples and the PEC performance of CZTSe samples in Eu3þ solutions, few studies have reported the PEC water splitting of CZTSe in electrolytes. In the present study, CZTSe quaternary samples were prepared via the reactive selenization of electrodeposited CueZneSn metal precursors. The effect of the [Zn]/[Sn] molar ratio in the precursor solution on the PEC and catalytic performances of CZTSe thin films are studied.

2. Experimental details 2.1. Preparation of CZTSe thin films on substrates CueZneSn metal precursors were grown on FTO-coated glass substrates (sheet resistance ¼ 10 U/square, Union Chemical Co., Taiwan) in a conventional three-electrode system using a potentiostat (CHI 600C, CH Instruments). The working, reference, and CE were the FTO-coated glass substrate, an Ag/AgCl (sat. KCl) electrode, and a Pt plate electrode, respectively. The solution baths, which were well stirred, contained 6 mM copper(II) nitrate (Cu(NO3)2$2.5H2O, Merck, 99%), 0.1 M zinc nitrate (Zn(NO3)2$6H2O, Fluka, 99%), 7e10 mM tin (II) chloride (SnCl2, Merck Co, 99.9%), 50 mM potassium sulfate (K2SO4, Merck Co., 99%), and 12 mM ethylenediaminetetraacetic acid disodium salt (Na2-EDTA, Riedel€n, 99%). The pH value of the bath was kept at 1 using deHae concentrated H2SO4 (BASF Co., Taiwan, 96%) in order to avoid the formation of metal complexes such as Zn(OH) or Sn(OH)þ [25]. The process of electrodeposition was similar to that in our previous study [26]. A silver wire was attached to the conducting layer of the FTO-coated glass substrate by silver paste. The contacts and edges of the sample electrodes were sealed with epoxy resin and dried at room temperature. The [Zn]/[Sn] molar ratio in the solution bath was varied for the deposition of samples, which was carried out at 25 ± 1  C using a recirculating water bath (nRC-10L, Cheng Seng Scientific Co., Taiwan). After electrodeposition, the metal precursors were loaded into an evacuated quartz tube under a rapid thermal annealing (RTA) process. The temperature of the RTA system was increased from room temperature to 550  C in 300 s and maintained at 550  C for 10 min in order to obtain the CueZneSn metal precursors. The selenization of CueZneSn metal precursors was carried out in a closed graphite container with a suitable amount of Se powders (around 1 g). The graphite box was loaded into an evacuated quartz tube under an RTA process in order to obtain highly crystalline CZTSe thin films on substrates. The chamber for RTA was first evacuated to 5.0  103 Torr in order to avoid the influence of oxygen gas. The temperature of the RTA system was increased from room temperature to 550  C in 300 s and maintained at 550  C for 10 min in order to obtain the highly crystalline CZTSe thin films on substrates. 2.2. Fabrication of DSSCs Lithium iodide (LiI), iodine (I2), and poly(ethylene glycol) (PEG, MW ¼ 20,000) were obtained from Merck Co. 4-tert-butylpyridine (TBP, 96%) and tert-butyl alcohol (t-BA, 96%) were obtained from Acros Co. and Ti (IV) tetraisopropoxide (TTIP, 98%), acetonitrile (ACN, 99.99%), acetylacetone (AA, >99.5%), ethanol (99.5%), and isopropyl alcohol (IPA, 99.5%) were obtained from Aldrich Co. Dimethyl sulfoxide (DMSO, 99.7%) and 2-methoxyethanol (99.95%) €n, Fluka, J. T. Backer, and Sigmawere obtained from Riedel-deHae eAldrich Co., respectively. 1,2-dimethyl-3-propylimidazolium iodide (DMPII) was obtained from Tokyo Chemical Industry Co. 3methoxypropionitrile (MPN, 99%) was obtained from Alfa Aesar Co. 0.5 M TTIP in 0.1 M nitric acid aqueous solution was stirred at a temperature of 88  C for 8 h. When the mixture cooled down to room temperature, the resultant colloids were transferred to an autoclave (PARR 4540, USA.), and then heated at 240  C for 12 h in order to make the TiO2 powders uniform. The TiO2 paste for the preparation of the transparent layer was a mixture of 8 wt% TiO2 colloids in PEG solution. Another TiO2 paste for the scattering layer was obtained by adding 8 wt% commercial TiO2 particles (ST-41, 200 nm, Ishihara Sangyo, Ltd., Japan) into the transparent layer paste. The conducting layer of the FTO was treated with a solution of TTIP (1 g) in 2-methoxyethanol (99.5%, SigmaeAldrich, 3 g) to

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obtain good mechanical contact between the conducting glass and TiO2 film, as well as to isolate the conducting glass surface from the electrolyte. TiO2 pastes were coated onto the FTO-coated glass substrates using the doctor blade technique. For the coating of each TiO2 layer, the dried TiO2 film was gradually heated to 500  C in an oxygen atmosphere and maintained at 500  C for 30 min. The TiO2 photoanodes in the DSSCs were composed of a 10-mm-thick transparent TiO2 layer and a 5-mm-thick for TiO2 scattering layer. With the TiO2 photoanodes cooled to 80  C, the TiO2 film was immersed in a 5  104 M solution of N719 dye at room temperature for 24 h. N719 (Solaronix S.A., Aubonne, Switzerland) was dissolved in ACN and t-BA (volume ratio of 1:1) as a standard dye solution. The TiO2 photoanode was coupled with the CZTSe film on the FTO-coated glass substrate. The distance between these two electrodes was fixed and sealed by heating 60-mmethick Surlyn® (SX1170-60, Solaronix S.A., Aubonne, Switzerland). The electrolyte, which consisted of 0.6 M DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in MPN/ACN (volume ratio ¼ 1:1), was injected into the gap between these two electrodes via capillarity. The 50-nm thick Pt film was prepared on the FTO-coated glass substrate using DC sputtering as the standard CE in the DSSCs. 2.3. Characterization of CZTSe samples The crystal phases of the samples were characterized using an X-ray diffractometer (XRD D5005, Siemens) with Cu Ka (l ¼ 1.5405 Å) radiation in the 2q range of 20e80 and a Raman spectroscope (Protrustech, UniRaman, 523-nm YAG laser). The surface morphology and composition of the samples were analyzed using a field emission scanning electron microscope (FESEM, JEOL JSM-7500F) equipped with an energy-dispersive analysis of X-ray (EDAX). The accelerating voltage and working distance of SEM were set at 10 kV and 20 mm, respectively. The surface microstructure and roughness of the samples were analyzed using an atomic force microscope (AFM XE70, Park Systems) operated in non-contact mode. The thickness of the samples was estimated using a surface profiler (Surfcorder ET3000, Kosaka Laboratory Co.). The optical transmittance and reflectance spectra of samples were obtained using a UVevisibleenear-infrared (UVeViseNIR) spectrophotometer (V-670, JASCO) with an integrating sphere in a wavelength range of 400e1600 nm at room temperature. The mobility and carrier density of the samples on substrates were measured using room temperature Hall measurements (HMS3000, Ecopia) with a magnetic flux of 0.57 T. A computer-controlled potentiostat (CHI 600C) equipped with a frequency response analyzer was used for the measurements for MotteSchottky plots. The procedures of MotteSchottky measurements were similar to those reported in the literature [6,15]. Aqueous 0.5 M H2SO4 was used as the electrolyte. All experiments were carried out in a nitrogen environment at a temperature of 25  C. The applied potentials were in the range of 0.7 to 0.1 V vs. an Ag/AgCl electrode. The PEC performance of the samples was measured in a quartz electrolytic cell with the CZTSe sample (average area ¼ 1.0 cm2), a Pt plate electrode (average area ¼ 1.0 cm2), and an Ag/AgCl electrode used as the working, counter, and reference electrodes, respectively. The PEC measurement system was similar to those reported in the literature [27]. Aqueous 0.5 M H2SO4 solution was used as the electrolyte. The electrolyte, prepared using double-DI water and degassed by purging with nitrogen gas (99.99% purity), was put into an ultrasonic bath for 30 min before each experiment in order to decrease the influence of gas solutes. All measurements were carried out in a nitrogen environment at a temperature of 25  C. The current densities, as a function of the applied voltage (0.5 to 0.0 V vs. an Ag/AgCl electrode) for the samples, were

49

recorded with a computer-controlled potentiostat (CHI 600C) for all PEC experiments. A 300-W Xe short arc lamp (PE300BF, Perkin Elmer) with a white light intensity of 100 mW cm2 was employed to simulate solar light. The intensity of incident light from the Xe lamp was measured using a photometer (818P-015-19, Newport). The electrocatalytic abilities and electrochemical properties of the CZTSe films on FTO-coated glass substrates in the electrolyte were investigated using cyclic voltammetry (CV) and Tafel polarization curves. The CV curves of CZTSe samples in the electrolyte were recorded using a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, the Netherlands). A three-electrode setup was employed, where the samples with an area of 1 cm2, a Pt foil, and an Ag/Agþ electrode were used as the working, counter, and reference electrodes, respectively. The electrolyte for CV measurements contained 10 mM LiI, 1.0 mM I2, and 0.1 M LiClO4 in ACN. The Tafel polarization curves of a symmetric cell with various CZTSe film electrodes were measured using a potentiostat/galvanostat (PGSTAT 30). The electrolyte for the Tafel polarization curve measurement was composed of 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M tBP in MPN/ACN (volume ratio of 1:1). The cell conversion efficiencies (h) of DSSCs with various [Zn]/[Sn] molar ratios in the CZTSe films on FTO-coated glass substrates as CEs were measured using a potentiostat/galvanostat (PGSTAT 30) under 100 mW cm2 light illumination by a class A solar simulator (XES-301S, AM1.5G, San-Ei Electric Co., Ltd., Osaka, Japan). The incident light intensity was calibrated with a standard Si cell (PECSI01, Peccell Technologies, Inc., Kanagawa, Japan). 3. Results and discussion The quaternary CZTSe samples were deposited onto FTO-coated glass substrates using the selenization of electrodeposited CueZneSn metal precursors. The [Zn]/[Sn] molar ratio in the precursor solution was verified to study its effects on the growth, catalytic and PEC performances of samples. Fig. 1 shows CV curves of Cu2þ, Zn2þ and Sn2þ ions in aqueous solution baths containing EDTA chelating agent. The scan rate was kept at 5 mV s1 for all CV curve measurements. The reduction current at 0.25 V (A0) and 0.52 to 0.75 V (A1) (vs. Ag/AgCl) corresponded to the reduction of Cu2þ to Cuþ ions and Cuþ to Cu0 on the substrate, respectively. In acidic solution, Cu2þ ions are first reduced to Cuþ ions, which are then reduced to Cu0 on the substrate when the

Fig. 1. CV curves of (I) 6 mM Cu(NO3)2 þ 50 mM K2SO4 þ 12 mM Na2-EDTA solution, (II) 0.1 M Zn(NO3)2 þ 50 mM K2SO4 þ 12 mM Na2-EDTA solution and (III) 10 mM SnCl2 þ 50 mM K2SO4 þ 12 mM Na2-EDTA aqueous solution.

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applied potentials is swept to more negative potentials [28,29]. When the applied voltage was more negative than 0.8 V (vs. Ag/ AgCl), the cathodic current of the samples in the electrolyte increased, which corresponds to Hþ ions in the solution bath being reduced to H2. The overpotentials for the reduction of H2/Hþ and Cu/Cu2þ redox couples are due to EDTA chelating agent being added to the solution bath. The concentration of Hþ in the electric double-layer decreases because some EDTA chelating agent adsorbs to the electric double-layer. Because EDTA has a larger molecular volume than that of H2O, the concentration of Hþ ions in the double-layer decreases, which results in the reduction peak of Hþ/ H2 shifting to more negative potentials. The formation constant of the Cu(EDTA) complex is 1019.7 [26], so most Cu2þ ions in the solution bath form Cu(EDTA) complexes, which results in a more negative potential of the reduction of Cu/Cu2þ redox couple in the solution bath containing EDTA chelating agent. For the CV curve of Zn2þ ions in aqueous solution with EDTA chelating agent, the onset potential of the cathodic polarization curve for the Zn2þ/Zn0 redox was 0.8 V vs. Ag/AgCl (B1). With the applied potential swept to a more negative voltage, the current of the cathodic curve increased due to the Zn deposited on the surface of the substrates. For the CV curve of Sn2þ ions in aqueous solution with EDTA chelating agent, the onset potential of the cathodic polarization curve for the Sn2þ/ Sn0 redox couples was 0.6 V vs. Ag/AgCl (C1). The C1 peak is due to the Sn deposited on the surface of the substrates. Fig. 2 shows the CV curve of the electrolyte containing 6 mM Cu(NO3)2 þ 0.1 M Zn(NO3)2 þ 10 mM SnCl2 þ 50 mM K2SO4 þ 12 mM Na2-EDTA. The peaks in the CV curve at around 0.2 to 0.3 V, 0.4 to 0.6 V, 0.6 to 0.7 V, and 0.75 to 1.0 V (vs. Ag/ AgCl) correspond to the reduction reactions of Cu2þ to Cuþ, Sn2þ to Sn0, Cuþ to Cu0, and Zn2þ to Zn0, respectively. The results indicate that the potential of electrodeposited CueZneSn metal precursors has to be set in the range of 0.75 to 1.0 V vs. Ag/AgCl. In order to obtain an optimal applied potential during the electrodeposition of the CueZneSn metal precursors, the CueZneSn metal precursors were prepared in an aqueous solution of 6 mM Cu(NO3)2 þ 0.1 M Zn(NO3)2 þ 10 mM SnCl2 þ 50 mM K2SO4 þ 12 mM Na2-EDTA with various applied potentials (0.7, 0.75, 0.8, 0.9, and 1.0 V vs. Ag/AgCl) and a deposition time of 30 min. The XRD patterns of CueZneSn metal precursors prepared with various applied potentials and annealed using the RTA system show that all CueZneSn metal precursors are a mixture of low-crystallinity hexagonal CuSn (JCPDS no. 65-3434), tetragonal Sn (JCPDS no.

Fig. 2. CV curve of 6 mM Cu(NO3)2 þ 0.1 M Zn(NO3)2 þ 10 mM SnCl2 þ 50 mM K2SO4 þ 12 mM Na2-EDTA aqueous solution.

65-7657), and monoclinic CuZn (JCPDS no. 08-0349) phases. Similar results were reported by Chen et al. [30]. The selenization process of CueZneSn metal precursors is important for obtaining highquality CZTSe thin films. With the selenization process at high temperature, the highly crystalline CZTSe thin films can be obtained. However, tin (IV) selenide may decompose during hightemperature thermal treatment. Han et al. [31] discussed the selenization of Sn and Zn metals on substrates with various selenization temperatures and times. Sn melted at a temperature of 230  C and the orthorhombic SnSe phase appeared at 230e280  C. The SnSe phase transformed into the hexagonal SnSe2 phase under an oversupply of Se vapor. The SnSe2 phase transformed into the SnSe phase at a temperature of around 500  C. The ZnSe phase appeared at a selenization temperature of 330  C. Binary SnSe2, ZnS, and Cu2Se can be obtained at the selenization temperatures of below 500  C. In order to obtain highly crystalline CZTSe thin films on substrates, a selenization temperature of above 500  C is necessary. Han et al. [31] also reported that the intensities of peaks in the XRD pattern of the SnSe2 phase decreased for a selenization time of 14 min and a temperature of 520  C. In order to obtain the highly crystalline CZTSe samples without phase change, the selenization of CueZneSn metal precursor was carried out using the RTA system in the present study. Fig. 3 shows the XRD patterns of CZTSe samples after selenization of electrodeposited CueZneSn metal precursors fabricated in 6 mM Cu(NO3)2 þ 0.1 M Zn(NO3)2 þ 10 mM SnCl2 þ 50 mM K2SO4 þ 12 mM Na2-EDTA aqueous solution at various applied potentials (vs. Ag/AgCl) and the electrodeposition time kept at 30 min. The intensities of peaks for CZTSe thin films on substrates prepared using the selenization of electrodeposited CueZneSn metal precursor increased when the applied potentials was increased from 0.7 V to 0.9 V vs. Ag/AgCl. The intensity of peaks for CZTSe samples prepared using the selenization of electrodeposited CueZneSn metal precursor with the applied potential of 1.0 V vs. Ag/AgCl decreased compared with the CZTSe samples prepared using the selenization of electrodeposited of CueZneSn metal precursors with applied potentials kept 0.9 V vs. Ag/AgCl. In this study, the electrodeposition of CueZneSn metal precursors was conducted with various [Zn]/[Sn] molar ratios in the precursor solution with applied potential kept

Fig. 3. XRD patterns of CZTSe samples after selenization of electrodeposited CueZneSn metal precursor fabricated in 6 mM Cu(NO3)2 þ 0.1 M Zn(NO3)2 þ 10 mM SnCl2 þ 50 mM K2SO4 þ 12 mM Na2-EDTA aqueous solution at various applied potentials (vs. Ag/AgCl).

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Table 1 Physical properties of CueZneSneSe samples. Sample

(A) (B) (C) (D)

Molar ratio of metal precursors (from EDAX analysis)

Molar ratios of sample (from EDAX analysis)

[Cu]/[Zn þ Sn]

[Zn]/[Sn]

[Cu]/[Zn þ Sn]

[Zn]/[Sn]

[Se]/[Cu þ Zn þ Sn]

0.65 0.73 0.85 0.95

0.71 0.72 0.91 1.02

0.67 0.77 0.87 1.06

0.47 0.57 0.66 0.67

0.67 0.76 0.70 0.71

Thickness/nm

Eg/eV

Carrier density/cm3

2709 2565 2450 2361

1.02 1.01 0.99 0.95

3.88 4.12 5.50 1.37

   

1017 1017 1017 1018

Mobility/ cm2 V1 s1

EFB/V vs. Ag/AgCl

12.6 12.51 11.67 10.31

0.01 0.03 0.05 0.08

at 0.9 V vs. Ag/AgCl. Table 1 shows the molar ratios of [Cu]/ [Zn þ Sn] and [Zn]/[Sn] in CueZneSn metal precursors prepared using electrodeposition. The molar ratios of [Cu]/[Zn þ Sn] and [Zn]/[Sn] in metal precursors are in the ranges of 0.65e0.95 and 0.71e1.02, respectively. Fig. 4 shows the XRD patterns of CZTSe samples on FTO-coated glass substrates fabricated via the reactive selenization of electrodeposited CueZneSn metal precursors with various [Zn]/[Sn] molar ratios in the solution bath. For sample (A) ([Zn]/[Sn] molar ratio of 0.71 in the metal precursors), the five major diffraction peaks correspond to the (112), (220/204), (312/ 116), (400/008), and (332/316) planes of the tetragonal CZTSe phase (JCPDS card no. 52-868), respectively. With an increase in the [Zn]/ [Sn] molar ratio in the metal precursors, the intensity of all peaks of the samples decreased. However, the peaks in the XRD patterns for binary metal selenides such as ZnSe and CuxSe are almost at the same positions as those for CZTSe due to their similar crystal structure. Raman spectroscopy was used to confirm the crystalline phases of the samples. The peak at 263 cm1 in the Raman spectra corresponds to the CuxSe phase and those at 173, 196 and 231 cm1 corresponded to the CZTSe phase [23,32e34]. Fig. 5 shows the Raman shift of the CZTSe samples prepared in this study. Samples (A)e(C) are tetragonal CZTSe phase while sample (D) is a CZTSe phase with a minor CuxSe phase. A composition analysis was carried out using EDAX to analyze the molar ratios of Cu, Zn, Sn, and Se in the samples. The molar ratios of [Cu]/[Zn þ Sn], [Zn]/[Sn] and [Se]/[Cu þ Zn þ In] obtained from EDAX for the CZTSe samples prepared in this study are shown in Table 1. When the molar ratio of [Zn]/[Sn] in the metal precursors was increased from 0.71 to 1.02 (samples (A)e(D)), the molar ratios of [Cu]/[Zn þ Sn] and [Zn]/[Sn] in the CZTSe samples increased from 0.67 to 1.06 and 0.47e0.67, respectively. The molar ratios of [Zn]/

[Sn] in the samples are smaller than those in the metal precursors due to some loss of volatile species such as ZnSe. Bhaskar et al. [35] reported some loss of volatile ZnSe at annealing temperatures of above 400  C. The [Se]/[Cu þ Zn þ Sn] molar ratio of the samples was in the range of 0.67e0.76. The EDAX analysis shows that there was an Se deficit in samples (A)e(D) ([Se]/[Cu þ Zn þ Sn] < 1). Sample (D) is a Cu-rich CZTSe sample with some CuxSe impurity because its [Cu]/[Zn þ Sn] molar ratio is greater than 1. The results obtained from EDAX analysis agree well with the Raman results. The thicknesses of the samples obtained from surface profile measurements, shown in Table 1, are in the range of 2.3e2.7 mm. FE-SEM and AFM images were used to observe the microstructures of the sample surfaces. Fig. 6(I), (III), (V), and (VII) shows FESEM images of samples (A), (B), (C), and (D), respectively. The SEM images show that the samples were dense. Flower-like microstructures with small grains and pinholes can be observed for sample (A). With an increase in the [Zn]/[Sn] molar ratio in the metal precursors, flower-like microstructures with relatively large grains and fewer pinholes can be observed on the surfaces of samples (B)e(D). The large grain size results in fewer grain boundaries and lower recombination loss, thus leading to a high PEC response of samples. The microstructures of the CZTSe samples prepared in this study are similar to those reported by Septina et al. [23] and Zhu et al. [36]. Fig. 6(II), (IV), (VI), and (VIII) show AFM images of samples (A)e(D), respectively. The surface roughness values of samples (A)e(D) obtained from AFM are 806, 937, 936, and 913 nm, respectively. The high surface roughness of samples results in a higher active area for the reactions with the electrolyte. The transmittance and reflectance spectra of the CZTSe samples prepared in this study are shown in Fig. 7. Sample (A) exhibits a transmission of 40% in the NIR region. The transmittance of the

Fig. 4. XRD patterns of CZTSe samples prepared via selenization of electrodeposited CueZneSn metal precursors with various [Zn]/[Sn] molar ratios in solution.

Fig. 5. Raman shift of CZTSe samples with various [Zn]/[Sn] molar ratios.

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Fig. 6. SEM and AFM images of CZTSe samples.

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samples decreases with increasing [Zn]/[Sn] molar ratio in the samples (samples (A)e(D)). Sample (D) shows the lowest transmission (15e20%) in the wavelength range of 1400e1600 nm. The reflectance values of the samples were less than 30% in the wavelength range of 400e1600 nm. The energy gap (Eg) of the samples is an important property for PEC applications. The absorption coefficients (a) of samples can be estimated using the Manifacier model [37]:

a ¼ 1=t ln½ð1  RÞ=T

(1)

where t is the thickness of the sample, and R and T are the reflectance and transmittance values, respectively. The relationship between the absorption coefficient (a) in the high-absorption region (a > 104 cm1) and the incident photon energy (hv) can be written as [38]:


n ðahvÞ ¼ A hv  Eg

(2)

Generally, n is 2 for an indirect band gap and 1/2 for a direct band gap. The optical energy band gaps of samples can be obtained by plotting (ahn)1/n with respect to hv and extrapolating the curve

Fig. 7. (I) Transmittance and (II) reflectance spectra of CZTSe samples with various [Zn]/[Sn] molar ratios.

53

to (ahn)1/n ¼ 0. The CZTSe is a direct energy band gap reported in the literature [35,39]. Fig. 8 shows the plot of (ahn)2 vs. hn for the CZTSe samples. The energy band gaps of samples obtained from Fig. 8 are shown in Table 1. The values of energy band gap of the samples are in the range of 1.02e0.95 eV. The energy band gap of CZTSe is around 1.0 eV [35,39]. The optical energy band gap of samples (A)e(D) is almost the same as that of bulk CZTSe. The CZTSe/electrolyte interface is also important in PEC water splitting. One method based on the capacitance (C) vs. applied potential (E) measurement is the MotteSchottky equation [15,26]:

i . h . 1 C 2 ¼ B  2 εε0 eND A2 ½E  EFB  ðkT=eÞ

(3)

where ε is the dielectric constant of the samples, A is the surface area at the interface, ND is the carrier density of the samples, EFB is the flat-band potential of the samples, e is the electronic charge, and ε0 is the permittivity of a vacuum. For n- and p-type semiconductors, B equals 1 and 1, respectively. The flat-band potential of the samples was obtained using the intercept of the C2 versus E plot. The electrolyte for measurements of MotteSchottky plots for samples with various [Zn]/[Sn] molar ratios was aqueous 0.5 M H2SO4 solution. An impendence analysis of the samples in the electrolyte was employed by developing a reasonable equivalent circuit for the sample in the electrolyte, and then fitting the measured impendence data to the equivalent circuit. The frequency of impendence was set to 10 kHz because the equivalent circuit of the samples in the electrolyte can be simplified into a resistancecapacitance (RC) circuit. Fig. 9 shows the MotteSchottky plots for the samples in aqueous 0.5 M H2SO4 solution. Because the B value of MotteSchottky plots is 1 for samples (A)e(D), the samples are p-type semiconductors, which agrees well with the literature [21,35]. Table 1 shows the values of flat-band potentials of the CZTSe samples prepared in this study. The flat-band potentials of these samples are in the range of 0.01 to 0.08 V vs. an Ag/AgCl reference electrode. Negative shifts in the flat-band potential were observed with an increase in the [Zn]/[Sn] molar ratio in the CZTSe samples. Sample (D) had the most negative flat-band potential in aqueous 0.5 M H2SO4 solution. The conduction type, carrier density, and mobility of samples on substrates were determined using room-temperature Hall measurements. All CZTSe samples were found to be p-type semiconductors, which agrees well with the MotteSchottky plots and reports published in the literature [21,35]. The values of carrier

Fig. 8. Plot of (ahn)2 vs. hn for CZTSe thin films on substrates.

54

P.-W. Shao et al. / Journal of Power Sources 286 (2015) 47e57

4H þ þ 4e /2H2

Fig. 9. MotteSchottky plots of CZTSe samples in H2SO4 aqueous solution.

density and mobility of the samples obtained using Hall measurements are shown in Table 1. The carrier densities and mobilities of the CZTSe samples on substrates are in the ranges of 3.88  1017e1.37  1018 cm3 and 10.31e12.6 cm2 V1 s1, respectively. The carrier densities of the CZTSe samples obtained via the selenization of electrodeposited CueZneSn metal precursors are similar with those reported in the literature [21,22]. With an increase in the [Zn]/[Sn] molar ratio in the samples (samples (A)e(D)), the carrier density increased and the mobility decreased. The electrical properties of CZTSe samples can be adjusted with small deviations from stoichiometry in the crystal. In CZTSe samples, the Cu vacancy (VCu) and Cu occupied the Zn site (CuZn) defects in the lattices are the main acceptors while the Zn occupied the Cu site (ZnCu) and Sn occupied the Cu site (SnCu) defects in the lattices are the main donors [40e43]. Sample (A) are the Cu-poor CZTSe ([Cu]/[Zn þ Sn] molar ratio <1), making it p-type semiconductors. However, the excessive Sn4þ ions ([Zn]/[Sn] molar ratio <1) would substitute the Cu sites to form the SnCu defects in CZTSe samples. The donors forming from SnCu defects resulted in the drop of carrier density for sample (A). With an increase in the Zn content in CZTSe samples (samples (A)e(C)), both the concentrations of Cu vacancies and Sn4þ ions occupied the Cu site in samples decreased, rendering a slight increase of the carrier density with an increasing [Zn]/[Sn] molar ratio in samples. Since sample (D) is the Cu-rich CZTSe ([Cu]/[Zn þ Sn] molar ratio >1), the excessive Cuþ ions would substitute Zn sites in the lattice to form CuZn defects, making sample (D) p-type semiconductor with the highest carrier density. It was found in our study that the mobility increased with decreasing carrier density of samples. This is due to the elimination of some defects in the samples. The numbers of defects forming in the samples result in the change of mobility of samples. With an increasing in the defects in samples, the hole density of samples increases while the mobility of samples decreased. Similar results are also reported in literature [40,41,43]. The current density-applied voltage measurements of samples in 0.5 M H2SO4 aqueous solution were carried out using the chopping method to examine the PEC water splitting performance. The PEC responses in 0.5 M H2SO4 aqueous solution under illumination can be expressed as: hv

samples!e ðCBÞ þ hþ ðVBÞ

(4)

2H2 O þ 4hþ /4Hþ þ O2

(5)

(6)

Fig. 10 shows the current density-applied voltage plots of the samples with an applied voltage in the range of 0.5 to 0 V vs. an Ag/AgCl electrode in 0.5 M H2SO4 aqueous solution. All samples showed enhanced cathodic current density under illumination in the solution, which confirms that the samples are p-type semiconductors. These results agree well with those obtained from MotteSchottky and Hall measurements. The onset of current density-applied voltage plots of samples under light irradiation can be treated as the flat-band potential, EFB [44]. The values of flatband potentials were determined from the current densityapplied voltage plots of the samples are in the range of 0.01 to 0.15 V vs. Ag/AgCl. The flat-band potentials obtained from MotteSchottky plots agree well with those obtained from current density-applied voltage plots of samples. The differences in current density measured in the dark and under light irradiation for samples (A)e(D) at an applied voltage of 0.5 V vs. Ag/AgCl are 0.30, 0.31, 0.41 and 0.26 mA cm2, respectively. The PEC response of samples increased when the [Zn]/[Sn] molar ratio in the samples increased from 0.47 to 0.66. When the [Zn]/[Sn] molar ratio in the sample was 0.67 (sample (D)), the PEC response of samples decreased. This indicates that there is an optimum composition that leads to the maximum photoresponse under illumination. Septina et al. [23] reported that several factors such as recombination at the surface of bulk CZTSe and sluggish charge transfer in the electrolyte affect the PEC performance of CZTSe samples. Bhaskar et al. [35] reported that large grain size results in fewer grain boundaries and lower recombination loss, thus leading to high PEC. With an increase in the [Zn]/[Sn] molar ratio in samples, the grains of CZTSe samples increase in size, decreasing on recombination loss. However, impurities such as CuxSe can trap the carriers, increasing the possibility of recombination. Samples (C) with a [Cu]/[Zn þ Sn] ratio of 0.87 and a [Zn]/[Sn] ratio of 0.66 had the maximum PEC performance in 0.5 M H2SO4 aqueous solution. Septina et al. [23] reported a PEC response of 0.5 mA cm2 for CZTSSe samples in aqueous solution containing Eu3þ ions at an applied voltage of 0.45 V vs. Ag/AgCl. The maximum PEC performance values of the CZTSe samples in this study are similar to those reported by Septinal et al. [23] for aqueous solution containing Eu3þ ions. In order to examine the possible applications of CZTSe in DSSCs, the current density (J)evoltage (V) characteristics of the DSSCs using CZTSe with various [Zn]/[Sn] molar ratio as the CEs are shown in Fig. 11. The photovoltaic parameters are listed in Table 2. When the [Zn]/[Sn] molar ratios in the sample was increased (samples (A)e(D)), the open-circuit voltage (Voc) of the DSSCs using the CZTSe samples as CEs increased. The maximum Voc of the DSSCs was 0.8 V, obtained with a [Zn]/[Sn] molar ratio of 0.67 in the CZTSe sample. When the [Zn]/[Sn] molar ratio in the sample was below 0.66 (samples (A)e(C)), both the short-current density (Jsc) and filled factor (FF) of the DSSCs increased. With a [Zn]/[Sn] molar ratio of 0.67 in the sample (sample (D)), both Jsc and FF deceased. A possible reason for the tendency of the Jsc and FF of DSSCs is the same as that discussed regarding the PEC performance of the samples in the electrolyte. The best PCE of DSSCs using CZTSe as CEs was 7.98% with Voc, FF, and Jsc values of 0.79 V, 0.68, and 14.74 mA cm2, respectively. The standard DSSC using Pt as the CE had a cell efficiency of 8.15% under the same measurement conditions. The best PCE of DSSCs using CZTSe (sample (C)) as the CE is slightly lower than that of the DSSC using Pt as the CE. To further analyze the electrocatalytic activities of CZTSe CEs in DSSCs, the CV measurements of CZTSe samples were employed. The CV analysis of CZTSe in electrolyte containing the Ie/Ie 3 couple can obtain the

P.-W. Shao et al. / Journal of Power Sources 286 (2015) 47e57

55

Fig. 10. Current densityeapplied voltage plots in range of 0.5 to 0 V vs. Ag/AgCl electrode for CZTSe samples in H2SO4 aqueous solution.

electrocatalytic activities of CZTSe in electrolyte by using two key factors: (I) the cathodic peak current density (Jpc) and (II) peak separation (DE), which indicate the electrocatalytic ability and kinetic reduction capability for Ie 3 , respectively. A larger Jpc indicates better electrocatalytic ability in the electrolyte containing the Ie/Ie 3 redox couples and a smaller DE indicates the faster kinetic reduction capability of the CEs. The reduction reaction at the CE can be expressed as:

I3  þ 2e /3I 

Fig. 11. JeV curves of DSSCs with various CZTSe CEs under 100 mW cm2 (AM1.5G) light irradiation.

(7)

Fig. 12 shows the CV curves of the CZTSe and Pt CEs in an electrolyte containing the I/I 3 redox couple obtained using a three-electrode electrochemical system. The tendencies of Jpc and DE of the CZTSe samples are (C) > (D) > (B) > (A) and (C) < (D) < (B) < (A), respectively. Sample (C) shows the highest value of Jpc and the lowest values of DE, which indicates that it has superior electrocatalytic activity in the electrolyte containing the I/I 3 redox couple. The Jpc and DE values obtained from Fig. 12 are slightly lower and higher than those of the Pt CE, respectively. The Tafel polarization curve measurement was carried out in this study in order to confirm the electrocatalytic activity of CZTSe CEs in the electrolyte. The interface charge transfer properties of a symmetric cell can be studied using Tafel polarization curve measurement. The Tafel polarization curves of the symmetric cell based on samples (A)-(D) and the Pt CE are shown in Fig. 13. The exchange current

56

P.-W. Shao et al. / Journal of Power Sources 286 (2015) 47e57

Table 2 Photovoltaic parameters and electrochemical properties of Pt and CueZneSneSe samples as CEs in DSSCs. Sample

h(%)

Voc(V)

Jsc(mA cm2)

FF

Jpc(mA cm2)

DE(V)

J0(mA cm2)

Rct(U cm2)

(A) (B) (C) (D) Pt

6.28 7.20 7.98 7.58 8.15

0.74 0.75 0.79 0.80 0.74

13.79 14.32 14.74 14.58 16.53

0.62 0.67 0.68 0.65 0.68

0.66 0.92 1.45 1.19 2.00

0.48 0.46 0.38 0.44 0.31

2.71 3.91 6.43 4.62 7.49

9.47 6.57 3.99 5.56 3.43

density (J0) can be obtained from the intercept of the linear fitting lines of anodic and cathodic data. The charge transfer resistance (Rct) of various CZTSe and Pt CEs can be calculated as:

J0 ¼

RT nFRct

(8)

where T is the absolute temperature, R is the gas constant, F is the Faraday constant, n is the total number of electrons involved in the reaction, and Rct is the charge transfer resistance. Table 2 listed the values of J0 and Rct for CZTSe and Pt CEs obtained from the Equation (8). A larger value of J0 and a smaller value of Rct indicate better electrocatalytic ability. The tendencies of J0 and Rct for samples are (C) > (D) > (B) > (A) and (C) < (D) < (B) < (A), respectively. Sample (C) had the highest value of J0 and the smallest value of Rct. The two factors that can affect the value of Rct are the electrocatalytic activity and the effective surface area of samples, which can be determined by the cathodic peak current density (Jpc) and surface roughness [45]. A lower cathodic peak current density (Jpc) results in a higher charge transfer resistance (Rct). The tendency of surface roughness of CZTSe samples is (B) > (C) > (D) > (A). However, the difference of surface roughness of samples (B), (C) and (D) is very small, which indicates that the tendency of Rct was controlled by the cathodic peak current density (Jpc). The tendency of Rct agrees well with that of cathodic peak current density. The DSSCs with CZTSe with a [Zn]/[Sn] molar ratio of 0.66 in the samples as the CE exhibited best PCE. 4. Conclusion In this study, CueZneSneSe samples were grown on FTO-

Fig. 13. Tafel polarization curves of various CZTSe CEs in electrolyte of 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M tBP in MPN/ACN.

coated glass substrates via the reactive selenization of electrodeposited CueZneSn metal precursors. The effects of the [Zn]/[Sn] molar ratio in the solution bath on the structural, optical, PEC and catalytic properties of the samples were investigated. The CZTSe samples obtained via the selenization of electrodeposited CueZneSn metal precursors with the applied potential kept at 0.9 V vs. Ag/AgCl were highly crystalline. The XRD patterns show that the intensity of peaks for CZTSe samples decreased with increasing [Zn]/[Sn] molar ratio in the solution bath. The [Cu]/ [Zn þ Sn], [Zn]/[Sn] and [Se]/[Cu þ Zn þ In] molar ratios in the samples estimated from EDAX analysis were in the ranges of 0.67e1.06, 0.47e0.67 and 0.67e0.76, respectively. The thicknesses and optical energy band gaps of samples were in the ranges of 2.3 to 2.7 mm and 0.95 to 1.02 eV, respectively. All samples were p-type semiconductors with a carrier density of 3.88  1017 to 1.37  1018 cm3 and a mobility of 10.31 to 12.6 cm2 V1 s1. The flat-band potentials of the samples in 0.5 M H2SO4 aqueous solution were in the range of 0.01 to 0.08 V vs. an Ag/AgCl electrode. With an irradiation of 100 mW cm2 from an Xe lamp, the highest photo-enhanced current density of samples was 0.41 mA cm2 at an external potential of 0.5 V vs. an Ag/AgCl electrode in aqueous 0.5 M H2SO4 solution. The best PCE of DSSCs using CZTSe as the CE was 7.98% with a Voc of 0.79 V, a Jsc of 14.74 mA cm2, and an FF of 0.68. This study shows that CZTSe is a potential material for replacing Pt as the CE of DSSCs and PEC water splitting. Acknowledgments

Fig. 12. CV curves of various CZTSe CEs in electrolyte of 10 mM LiI, 1.0 mM I2, and 0.1 M LiClO4 in ACN.

This study was supported by the Ministry of Science and Technology of Taiwan, R.O.C., under grants MOST 101-2221-E-182-018MY3 and MOST-103-2221-E-182-059-MY3.

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