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Bifacial dye-sensitized solar cells with transparent cobalt selenide alloy counter electrodes
Journal of Power Sources 284 (2015) 349e354
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Bifacial dye-sensitized solar cells with transparent cobalt selenide alloy counter electrodes Yanyan Duan a, b, Qunwei Tang a, b, *, Benlin He b, Zhiyuan Zhao b, Ling Zhu b, Liangmin Yu a, c, * a b c
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, PR China Qingdao Collaborative Innovation Center of Marine Science and Technology, Ocean University of China, Qingdao 266100, PR China
h i g h l i g h t s
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Binary Co0.85Se alloy CE is synthesized by a mild solution method. The resultant Co0.85Se alloy CE shows high optical transparency. The DSSC with Co0.85Se CE can generate electricity from either side. Maximum front and rear efficiencies of 8.30% and 4.63% are measured in the DSSC, respectively.
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Article history: Received 3 November 2014 Received in revised form 5 March 2015 Accepted 7 March 2015 Available online 9 March 2015
High power conversion efficiency and cost-effectiveness are two persistent objectives for dye-sensitized solar cell (DSSC). Electricity generation from either front or rear side of a bifacial DSSC has been considered as a facile avenue of bringing down the cost of solar-to-electric conversion. Therefore, the fabrication of a transparent counter electrode (CE) with a high electrocatalytic activity is a prerequisite to realize this goal. We present here the feasibility of utilizing transparent cobalt selenide (CoeSe) binary alloy counter electrode for bifacial DSSC application, in which binary CoeSe alloy electrode is synthesized by a mild solution strategy and the cell device is irradiated by either front or rear side. Due to the high optical transparency, charge-transfer ability, and electrocatalytic activity, maximum front and rear efficiencies of 8.30% and 4.63% are recorded under simulated air mass 1.5 (AM1.5) irradiation, respectively. The impressive efficiency along with fast start-up, multiple start capability, and simple preparation highlights the potential application of cost-effective and transparent CoeSe alloy CE in robust bifacial DSSCs. © 2015 Elsevier B.V. All rights reserved.
Keywords: Bifacial dye-sensitized solar cell Counter electrode Transparent binary alloy Cobalt selenide
1. Introduction
* Corresponding authors. Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China. E-mail addresses: [email protected] (Q. Tang), [email protected] (L. Yu). http://dx.doi.org/10.1016/j.jpowsour.2015.03.045 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Photovoltaic conversion has been a promising electricity generation technique in nowaday's low-carbon economy society [1,2]. Among diversified solar cells, dye-sensitized solar cell (DSSC) has attracted growing interest due to its merits on easy fabrication process, rising space in efficiency, and scalable materials [3e6].
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Since the first prototype reported in 1991 [7], DSSCs have experienced a development period of double decenniums, and power conversion efficiency has also been elevated from 7.1 to 13% [8]. However, the high expense of Pt counter electrode (CE) and limited dye excitation are two of the restrictions for further efficiency enhancement and therefore commercialization. One solution to this impasse is to develop cost-effective CE candidates and to utilize bifacial technique [9e13]. The commonly used conductive polymers [14,15], carbonaceous materials [16] as well as their integrations [17e19] have been proposed respected CE materials to replace Pt, but their stabilities and catalytic activities are still modest in developing robust DSSCs [20]. The bifacial DSSC created €tzel display superiority of generating electricity on either by Gra side and help bring down the cost of solar-to-electric conversion [21,22]. However, the commonly employed Pt electrode in a bifacial DSSC has metallic luster and can reflect incident light on its surface, leading to a low rear efficiency under rear irradiation. Therefore, the exploration of transparent CEs with high chargetransfer ability and catalytic activity toward triiodide reduction reaction has been a prerequisite in designing robust bifacial DSSCs [23,24]. Previous works in elevating electricity generation mainly focus on increasing light harvesting, such as setting blocking layer [25], scattering layer [26], or adding light harvester [27], resulting in tedious procedures without reducing expenses. In our recent work [28], we report the simultaneously irradiation of bifacial DSSC from TiO2 anode (100 mW cm2) and transparent polyaniline CE (68 mW cm2), extracting front, rear, and bifacial efficiencies of 6.70%, 4.15%, and 8.35%, respectively. The incident light from transparent polyaniline electrode has a compensation effect to the light from TiO2 anode, yielding a markedly enhanced dye excitation and electricity generation. Alloy materials have established themselves as robust electrocatalysts for energy conversion [29,30]. With an aim of enhancing light penetration from rear side, enhancing stability and decreasing expense of a CE, in the current work, we pioneerly report the synthesis of binary cobalt selenide (CoeSe) alloy CEs by a mild solution strategy. The resultant CoeSe alloy CE displays high optical transparency, charge-transfer ability, and electrocatalytic activity. The optimal efficiencies of 8.30% and 4.63% are measured for front and rear irradiations, respectively.
2.2. Assembly of DSSCs A layer of TiO2 colloid synthesized by a sol-hydrothermal method with a thickness of ~10 mm was coated by a doctor-blade technique and subsequently calcined at 450 C for 30 min [31,32]. Resultant anodes were further sensitized by immersing into a 0.50 mM ethanol solution of N719 dye. The DSSC was fabricated by sandwiching redox electrolyte between a dye-sensitized TiO2 anode and a CE. A redox electrolyte consisted of 100 mM of tetraethylammonium iodide, 100 mM of tetramethylammonium iodide, 100 mM of tetrabutylammonium iodide, 100 mM of NaI, 100 mM of KI, 100 mM of LiI, 50 mM of I2, and 500 mM of 4-tertbutyl-pyridine in 50 mL acetonitrile. Surlyn film (30 mm) was utilized to seal the device through hot-pressing. 2.3. Electrochemical characterizations The electrochemical performances were recorded on a conventional CHI660E setup comprising an Ag/AgCl reference electrode, a CE of Pt sheet, and a working electrode of FTO glass supported CoeSe alloy. The cyclic voltammetry (CV) curves were recorded in a supporting electrolyte consisting of 50 mM M LiI, 10 mM I2, and 500 mM LiClO4 in acetonitrile. EIS measurements were also carried out in a frequency range of 0.1 Hz ~ 105 kHz and an ac amplitude of 10 mV at room temperature. Tafel polarization curves were recorded by assembling symmetric dummy cell consisting of CEjredox electrolytejCE. 2.4. Photovoltaic measurements The photovoltaic test of the DSSC with an active area of 0.25 cm2 was carried out by measuring the photocurrentevoltage (JeV) characteristic curves using a CHI660E Electrochemical Workstation under irradiation of a simulated solar light from a 100 W Xenon arc lamp (XQ-500 W) in ambient atmosphere. The incident light intensity was controlled at 100 mW cm2 (calibrated by a standard silicon solar cell). A black mask with an aperture area of around 0.25 cm2 was applied on the surface of DSSCs to avoid stray light. Each JeV curve was repeatedly measured at least five times to control the experimental error within ±5%. 2.5. Other characterizations
2. Experimental 2.1. Synthesis of binary CoeSe alloy CEs The feasibility of synthesizing CoeSe alloy CEs was confirmed by the experimental procedures: A mixing aqueous solution consisting of Se powders and CoCl2 at stoichiometric ratios was made by agitating 0.12 g of Se ultrafine powers and CoCl2 (0.059 mmol for Co0.5Se, 0.082 mmol for Co0.7Se, 0.10 mmol for Co0.85Se, 0.118 mmol for CoSe, 0.141 mmol for Co1.2Se) in 10 mL of deionized water. The total volume of reagent solution was adjusted at 35 mL by adding deionized water. 7.5 mL of hydrazine hydrate (85 wt%) was dropped into the above solution, after vigorous agitating for 10 min, the reactant was transferred into a 50 mL of Teflon-lined autoclave and cleaned FTO glass substrate (sheet resistance 12 U square1, purchased from Hartford Glass Co., USA) with FTO layer downward was immersed in. After reaction at 120 C for 12 h, the FTO substrate was rinsed by deion ized water and vacuum dried at 50 C. As references, the pure Se and Co CEs were also prepared according to the above approach at a Se dosage of 0.12 g or CoCl2 dosage of 0.118 mmol. The pristine Pt CE was purchased from Dalian HepatChroma SolarTech Co., Ltd and used as a standard.
The compositions of the alloy CEs were detected by inductively coupled plasma-atomic emission spectra (ICP-AES). Prior to ICP measurements, the alloy CEs were immersed in concentration nitric acid to with agitation for 12 h to dissolve the compounds from FTO glass substrate. The optical transmission spectra of resultant CEs were recorded on a UVevis spectrophotometer at room temperature. X-ray diffraction (XRD) profiles of the resultant alloys were recorded on an X-ray powder diffractometer (X'pert MPD Pro, Philips, Netherlands) with Cu Ka radiation (l ¼ 1.5418 Å) in the 2q range from 10 to 90 operating at 40 kV accelerating voltage and 40 mA current). The morphologies of the resultant Co0.85Se alloy were observed with a transmission electron microscopy (TEM, JEM2010, JEOL). 3. Results and discussion The compositions of resultant CoeSe alloys are determined by ICP-AES measurement, showing atomic ratios of Co0.5Se, Co0.7Se, Co0.85Se, CoSe, and Co1.2Se of 0.482:1.000, 0.713:1.000, 0.858:1.000, 0.986:1.000, and 1.204:1.000, respectively. The measured atomic ratios are close to their stoichiometries, therefore the chemical formulas of the resultant alloy CEs can be represented by
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stoichiometric ratios. The Co0.85Se alloy is subjected to XRD measurement for identifying crystal phase (Fig. 1). At the first glance, all the diffraction peaks corresponding to CoeSe alloy [Pa-3(205), PDF# 88-1712] can be detected in XRD pattern, indicating that the mild solution method is feasible in alloying Co and Se species. Fig. 2 shows the transmission electron microscopy (TEM) photographs of Co0.85Se alloy. Due to the loose structure in Fig. 2a, the alloy CE provides large alloy/electrolyte interface and channels for ion diffusion and more active sites for I 3 reduction. The detection of lattice fringes in Fig. 2b indicates that the Co0.85Se alloy has good crystallinity. The interplanar spacing of d101 derived from the lattice fringes is around 3.13 Å. Moreover, there are many lattice distortions and plenty of defects observed in their alloying region. This result demonstrates the alloying of Co and Se can generate many defects, which provides placeholders for I 3 adsorption and reduction. Optical transparency of a binary alloy CE is one of the crucial characteristics in designing bifacial DSSCs. Fig. 3a compares the optical transparency with a wavelength ranging from 400 to 1000 nm for diversified CEs. All the CoeSe alloy CEs have high optical transparency (>60%) in visible light region in comparison with standard Pt electrode [28]. The transparent CoeSe alloy electrode having high optical transparency is expected to penetrate more light from rear side for dye excitation and electron generation. Fig. 3b displays characteristic JeV curves of DSSCs under front irradiation and the detailed photovoltaic parameters are summarized in Table 1. The cell device with Co0.85Se alloy CE yields a maximum front h of 8.30% (Jsc ¼ 16.80 mA cm2, Voc of 0.742 V, and FF of 0.67) under simulated air mass 1.5 (AM1.5) global sunlight, which is comparable to the cells from Pt and Pt-free CEs [33]. As references, pure Se and pure Co are also deposited on FTO glass substrates for DSSC fabrication, giving front h of 4.86% and 3.61%, respectively. Co is a typical transition metal having catalytic activity toward small molecular compounds, the alloying of Co and Se can favor the electronic perturbation and therefore accelerate the electrocatalytic kinetics of the resultant alloy. On behalf the high optical transmission of CoeSe alloy CEs, the cell devices are also irradiated from rear side to evaluate their electricity-generation capability from rear side. As shown in Fig. 3b, the DSSC with Co0.85Se alloy CE generate an optimal rear h of 4.63%, Jsc ¼ 9.90 mA cm2, Voc of 0.721 V, and FF of 0.65. In comparison with rear h of 2.73% from pristine Pt-based cell [28], the rear electricity-generation capability has been enhanced due to high
Fig. 1. XRD pattern of binary Co0.85Se alloy.
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optical transmission of Co0.85Se alloy electrode. It is noteworthy to mention that Jsc and Voc extracted from rear irradiation are lower than that for front irradiation. Jsc is of highly dependent on accumulative electron density on CB of TiO2 injected from excited dyes. As shown in Fig. 4, the incident light intensity for dye excitation is stepwisely decreased (viewed from anode to CE) under front irradiation, leading to a gradient reduction in electron distribution. In the process of transportation through TiO2 nanocrystallite network, the electrons will suffer shorter lengths under rear irradiation in comparison with front mode. The average electron may experience a million trapping events before percolating to the collecting FTO electrode [34]. Therefore, the electron density on CB of TiO2 and therefore Jsc under front irradiation is higher than that at rear irradiation. Moreover, the maximum Voc is determined by the difference between quasi Fermi energy of electrons in TiO2 and redox potential energy of electrolyte [35], whereas the real Voc of DSSC device is generally smaller than this theoretical limit, and a main reason is a backward reaction between dye molecules and electrolyte. The recombination of electrons by front-irradiation with oxidizing species, predominantly I 3 in the electrolyte can be effectively restricted, giving an elevated Voc. A golden rule in designing an efficient CE is to elevate its electrocatalytic activity toward electrolyte containing redox couples. Fig. 5a shows the CV curves of various CEs for I/I 3 redox couples recorded at a scan rate of 50 mV s1. Two pairs of redox peaks corresponding to I 4 I 3 interconversion [Red1 (I3 þ 2e ¼ 3I )/Ox1 (3I 2e ¼ I3 ); Red2 (3I2 þ 2e ¼ 2I3 )/Ox2 (2I3 2e ¼ 3I2)] are detected in each CV curve. The shapes and peak positions are similar to that from pristine Pt CE, indicating that the alloy electrodes have electrocatalytic activities for liquid electrolyte. The task of a CE is to collect electrons from external circle and to reduce I 3 into I, therefore, the peak current density of Red1 and peak-topeak separation (Epp) between Red1 and Ox1 can be employed to evaluate its catalytic activity. Most of the alloy CEs have higher peak current density for Red1 reaction in comparison with pristine Se and Co electrodes. Among five alloy CEs, Co0.85Se displays the highest current density and smallest Epp (ca. 0.42 V), suggesting a less overpotential loss for Co0.85Se alloy CEs. Moreover, we compare the Bode electrochemical impedance spectra (EIS) of symmetric dummy cells from various CEs, the lifetimes of electrons on CEs are calculated. Once the electrons migrate from external circuit to a CE layer, they participate in the reduction of I 3 species, therefore, the average lifetime of electrons can be employed to compare the reaction kinetics of Red1. From Fig. 5b and Table 2, one can find that Co0.85Se alloy exhibits a lifetime of 8.6 ms, which is shorter than that for other alloys, Se, and Co electrodes. Higher peak current density, lower Epp, and shorter electron lifetime demonstrate that Co0.85Se alloy electrode presents superior catalytic activity in reducing I 3, which is a prerequisite for a robust CE in efficient DSSCs. From the stacking CV curves of Co0.85Se alloy electrode, as shown in Fig. 6a, we can make a conclusion that the cathodic and anodic reactions are diffusion limited for I 3 transportation due to the a linear relationship between square root of scanning rate and peak current density (Fig. 6b). The results account for a fact that the diffusion layer becomes thinner and the electrochemical polarization degree is larger at an elevated scan rate, therefore leading to increasing overpotential and poor reversibility [36]. Except for high power conversion efficiency and costeffectiveness, a robust solar cell for engine or vehicle application should also exhibit fast start-up, multiple start capability, and operational stability. The core component to start the DSSC is the CE, therefore the superiorities of a robust CE include high catalytic activity and stability. Fig. 7a shows the startestop switches of the solar cell with Co0.85Se alloy electrode by alternatively irradiating (100 mW cm2) and darkening (0 mW cm2) the solar devices at an
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Fig. 2. (a) Low-resolution and (b) high-resolution TEM images of Co0.85Se alloy CE.
Fig. 3. (a) Optical transparency of various CEs and characteristic JeV curves of DSSCs for (b) front and (c) rear irradiations.
interval of 25 s and 0 V. No matter irradiation from front or rear side, an abrupt increase in photocurrent density under irradiation means the DSSC has a fast start-up performance, whereas no delay in starting the cell suggests a high catalytic activity for Co0.85Se CE. Table 1 Output photovoltaic characteristics of the DSSCs employing pristine Se, Co, and CoeSe alloy CEs for front or rear irradiation. h: power conversion efficiency; Jsc: short-circuit current density; Voc: open-voltage; FF: fill factor. CEs
Irradiation
h (%)
Voc (V)
Jsc (mA cm2)
FF
Se
Front Rear Front Rear Front Rear Front Rear Front Rear Front Rear Front Rear
4.86 3.22 6.47 4.33 7.46 4.74 8.30 4.63 7.75 4.39 5.37 2.32 3.61 2.90
0.690 0.652 0.721 0.690 0.739 0.724 0.742 0.721 0.743 0.712 0.723 0.682 0.680 0.666
12.21 9.56 13.27 9.63 15.36 9.95 16.80 9.90 15.47 8.97 11.45 5.02 8.29 6.51
0.58 0.52 0.67 0.65 0.66 0.66 0.67 0.65 0.67 0.69 0.65 0.68 0.64 0.67
Co0.5Se Co0.7Se Co0.85Se CoSe Co1.2Se Co
The unchanged in photocurrent density after five startestop cycles is a solid support for multiple start capability. In each “on” stage, no decrease in photocurrent density refers to a low recombination effect between photogenerated electrons on conduction band of TiO2 nanocrystalline and I 3 species in liquid electrolyte. As a reference, the startestop switches for Pt-based DSSC are also measured at the same conditions. As shown in Fig. 7c, the time delay in reaching the maximum photocurrent density under irradiation and reduction in photocurrent density mean a modest
Fig. 4. Schematic diagram for bifacial DSSC with Co0.85Se alloy electrode.
Y. Duan et al. / Journal of Power Sources 284 (2015) 349e354
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1 Fig. 5. (a) CV curves of various CEs for I/I 3 redox couples recorded at a scan rate of 50 mV s . (b) Bode and (c) Nyquist EIS plots and (d) Tafel polarization curves of the symmetric dummy cells from two identical CEs. The inset gives an equivalent circuit.
Table 2 Output parameters extracted from CV and EIS characterizations for the symmetric dummy cells. CEs
Red1 (mA cm2)
Epp (V)
t (ms)
Rct (U cm2)
W (U cm2)
Se Co0.5Se Co0.7Se Co0.85Se CoSe Co1.2Se Co
3.31 3.36 3.56 5.77 4.84 2.43 1.05
1.07 0.62 0.58 0.42 0.55 0.63 0.83
101.4 46.3 42.1 8.6 31.4 97.3 267.3
20.2 12.2 8.2 3.3 5.4 13.4 88.5
114.2 32.9 23.5 3.3 6.5 34.3 78.2
catalytic activity toward I 3 reduction and backward reaction of electrons with I 3 , respectively. Additionally, the photocurrent density stabilities of the solar cells with Co0.85Se and Pt CEs over 10 h are shown in Fig. 7b and d, remaining 91.2% and 75.5% of initial photocurrent density for Co0.85Se-based DSSC, whereas they are
69.1% and 90.8% for Pt-based solar cell under front and rear irradiations, respectively. The results demonstrate that the cell with Co0.85Se alloy electrode has satisfactory stability under durative irradiation.
4. Conclusions In summary, transparent and cost-effective CoeSe binary alloys have been fabricated by a mild solution strategy free of any surfactant or template and employed as CE materials in DSSCs. Co0.85Se alloy CE displays lower Epp and Rct, and higher peak current density than other electrodes. Notably, the DSSC employing Co0.85Se alloy CE displays front and rear efficiencies of 8.30% and 4.63%, respectively, superior to the cell performances from pristine Pt electrode. Additionally, the merits on quick start-up, low electron recombination, high multipleestart capability, and high photocurrent stability motivate the potential applications of such bifacial solar cell
1 Fig. 6. (a) CV curves of Co0.85Se CE for I/I 3 redox species at varied scan rates (from inner to outer: 50, 75, 100, 125, 150, 175, and 200 mV s ), and (b) relationship between peak current density and square root of scan rates.
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Fig. 7. The startestop switches with irradiation from front and rear were achieved by alternatively irradiating (100 mW cm2) and darkening (0 mW cm2) the solar devices from (a) Co0.85Se and (c) Pt electrodes at an interval of 25 s and 0 V. Photocurrent stabilities of the DSSCs with (b) Co0.85Se and (d) Pt electrodes under durable irradiations at 100 mW cm2.
systems in engines, vehicles, and power sources. These promising results along with bifacial irradiation technique may pave the way for study on cost-effective, efficient, and practical DSSCs. Acknowledgment The authors would like to acknowledge financial supports from Fundamental Research Funds for the Central Universities (201313001, 201312005), Shandong Province Outstanding Youth Scientist Foundation Plan (BS2013CL015), Shandong Provincial Natural Science Foundation (ZR2011BQ017), Research Project for the Application Foundation in Qingdao (13-4-198-jch), National Key Technology Support Program (2012BAB15B02), and National High-Tech Research and Development Programme of China (2010AA09Z203, 2010AA065104). References [1] S. Chu, A. Majumdar, Nature 488 (2012) 294e303. [2] S. Thomas, T.G. Deepak, G.S. Anjusree, T.A. Arun, S.V. Nair, A.S. Nair, J. Mater. Chem. A 2 (2014) 4474e4490. [3] A. Hagfeldt, G. Boschloo, L.C. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010) 6595e6663. [4] U. Bach, T. Daeneke, Angew. Chem. Int. Ed. 51 (2012) 10451e10452. [5] J.H. Wu, Y.M. Xiao, Q.W. Tang, G.T. Yue, J.M. Lin, M.L. Huang, Y.F. Huang, L.Q. Fan, Z. Lan, S. Yin, T. Sato, Adv. Mater. 24 (2012) 1884e1888. [6] Y.M. Xiao, G.Y. Han, Y.P. Li, M.Y. Li, Y.Z. Chang, J. Mater. Chem. A 2 (2014) 3452e3460. €tzel, Nature 353 (1991) 737e740. [7] B. O'Regan, M. Gra [8] S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B.F.E. Curchod, N. AshariAstani, I. Tavernelli, U. Rothlisberger, M.K. Nazzeruddin, M. Gr€ atzel, Nat. Chem. 6 (2014) 242e247. [9] Q.D. Tai, X.Z. Zhao, J. Mater. Chem. A 2 (2012) 13207e13218. [10] Y.Y. Duan, Q.W. Tang, B.L. He, R. Li, L.M. Yu, Nanoscale 6 (2014) 12601e12608. [11] Q.D. Tai, B.L. Chen, F. Guo, S. Xu, H. Hu, B. Sebo, X.Z. Zhao, ACS Nano 5 (2011) 3795e3799. [12] D. Song, M. Li, Y. Li, X. Zhao, B. Jiang, Y. Jiang, ACS Appl. Mater. Interfaces 6
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Bifacial dye-sensitized solar cells with transparent cobalt selenide alloy counter electrodes Yanyan Duan a, b, Qunwei Tang a, b, *, Benlin He b, Zhiyuan Zhao b, Ling Zhu b, Liangmin Yu a, c, * a b c
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, PR China Qingdao Collaborative Innovation Center of Marine Science and Technology, Ocean University of China, Qingdao 266100, PR China
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
Binary Co0.85Se alloy CE is synthesized by a mild solution method. The resultant Co0.85Se alloy CE shows high optical transparency. The DSSC with Co0.85Se CE can generate electricity from either side. Maximum front and rear efficiencies of 8.30% and 4.63% are measured in the DSSC, respectively.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2014 Received in revised form 5 March 2015 Accepted 7 March 2015 Available online 9 March 2015
High power conversion efficiency and cost-effectiveness are two persistent objectives for dye-sensitized solar cell (DSSC). Electricity generation from either front or rear side of a bifacial DSSC has been considered as a facile avenue of bringing down the cost of solar-to-electric conversion. Therefore, the fabrication of a transparent counter electrode (CE) with a high electrocatalytic activity is a prerequisite to realize this goal. We present here the feasibility of utilizing transparent cobalt selenide (CoeSe) binary alloy counter electrode for bifacial DSSC application, in which binary CoeSe alloy electrode is synthesized by a mild solution strategy and the cell device is irradiated by either front or rear side. Due to the high optical transparency, charge-transfer ability, and electrocatalytic activity, maximum front and rear efficiencies of 8.30% and 4.63% are recorded under simulated air mass 1.5 (AM1.5) irradiation, respectively. The impressive efficiency along with fast start-up, multiple start capability, and simple preparation highlights the potential application of cost-effective and transparent CoeSe alloy CE in robust bifacial DSSCs. © 2015 Elsevier B.V. All rights reserved.
Keywords: Bifacial dye-sensitized solar cell Counter electrode Transparent binary alloy Cobalt selenide
1. Introduction
* Corresponding authors. Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China. E-mail addresses: [email protected] (Q. Tang), [email protected] (L. Yu). http://dx.doi.org/10.1016/j.jpowsour.2015.03.045 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Photovoltaic conversion has been a promising electricity generation technique in nowaday's low-carbon economy society [1,2]. Among diversified solar cells, dye-sensitized solar cell (DSSC) has attracted growing interest due to its merits on easy fabrication process, rising space in efficiency, and scalable materials [3e6].
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Since the first prototype reported in 1991 [7], DSSCs have experienced a development period of double decenniums, and power conversion efficiency has also been elevated from 7.1 to 13% [8]. However, the high expense of Pt counter electrode (CE) and limited dye excitation are two of the restrictions for further efficiency enhancement and therefore commercialization. One solution to this impasse is to develop cost-effective CE candidates and to utilize bifacial technique [9e13]. The commonly used conductive polymers [14,15], carbonaceous materials [16] as well as their integrations [17e19] have been proposed respected CE materials to replace Pt, but their stabilities and catalytic activities are still modest in developing robust DSSCs [20]. The bifacial DSSC created €tzel display superiority of generating electricity on either by Gra side and help bring down the cost of solar-to-electric conversion [21,22]. However, the commonly employed Pt electrode in a bifacial DSSC has metallic luster and can reflect incident light on its surface, leading to a low rear efficiency under rear irradiation. Therefore, the exploration of transparent CEs with high chargetransfer ability and catalytic activity toward triiodide reduction reaction has been a prerequisite in designing robust bifacial DSSCs [23,24]. Previous works in elevating electricity generation mainly focus on increasing light harvesting, such as setting blocking layer [25], scattering layer [26], or adding light harvester [27], resulting in tedious procedures without reducing expenses. In our recent work [28], we report the simultaneously irradiation of bifacial DSSC from TiO2 anode (100 mW cm2) and transparent polyaniline CE (68 mW cm2), extracting front, rear, and bifacial efficiencies of 6.70%, 4.15%, and 8.35%, respectively. The incident light from transparent polyaniline electrode has a compensation effect to the light from TiO2 anode, yielding a markedly enhanced dye excitation and electricity generation. Alloy materials have established themselves as robust electrocatalysts for energy conversion [29,30]. With an aim of enhancing light penetration from rear side, enhancing stability and decreasing expense of a CE, in the current work, we pioneerly report the synthesis of binary cobalt selenide (CoeSe) alloy CEs by a mild solution strategy. The resultant CoeSe alloy CE displays high optical transparency, charge-transfer ability, and electrocatalytic activity. The optimal efficiencies of 8.30% and 4.63% are measured for front and rear irradiations, respectively.
2.2. Assembly of DSSCs A layer of TiO2 colloid synthesized by a sol-hydrothermal method with a thickness of ~10 mm was coated by a doctor-blade technique and subsequently calcined at 450 C for 30 min [31,32]. Resultant anodes were further sensitized by immersing into a 0.50 mM ethanol solution of N719 dye. The DSSC was fabricated by sandwiching redox electrolyte between a dye-sensitized TiO2 anode and a CE. A redox electrolyte consisted of 100 mM of tetraethylammonium iodide, 100 mM of tetramethylammonium iodide, 100 mM of tetrabutylammonium iodide, 100 mM of NaI, 100 mM of KI, 100 mM of LiI, 50 mM of I2, and 500 mM of 4-tertbutyl-pyridine in 50 mL acetonitrile. Surlyn film (30 mm) was utilized to seal the device through hot-pressing. 2.3. Electrochemical characterizations The electrochemical performances were recorded on a conventional CHI660E setup comprising an Ag/AgCl reference electrode, a CE of Pt sheet, and a working electrode of FTO glass supported CoeSe alloy. The cyclic voltammetry (CV) curves were recorded in a supporting electrolyte consisting of 50 mM M LiI, 10 mM I2, and 500 mM LiClO4 in acetonitrile. EIS measurements were also carried out in a frequency range of 0.1 Hz ~ 105 kHz and an ac amplitude of 10 mV at room temperature. Tafel polarization curves were recorded by assembling symmetric dummy cell consisting of CEjredox electrolytejCE. 2.4. Photovoltaic measurements The photovoltaic test of the DSSC with an active area of 0.25 cm2 was carried out by measuring the photocurrentevoltage (JeV) characteristic curves using a CHI660E Electrochemical Workstation under irradiation of a simulated solar light from a 100 W Xenon arc lamp (XQ-500 W) in ambient atmosphere. The incident light intensity was controlled at 100 mW cm2 (calibrated by a standard silicon solar cell). A black mask with an aperture area of around 0.25 cm2 was applied on the surface of DSSCs to avoid stray light. Each JeV curve was repeatedly measured at least five times to control the experimental error within ±5%. 2.5. Other characterizations
2. Experimental 2.1. Synthesis of binary CoeSe alloy CEs The feasibility of synthesizing CoeSe alloy CEs was confirmed by the experimental procedures: A mixing aqueous solution consisting of Se powders and CoCl2 at stoichiometric ratios was made by agitating 0.12 g of Se ultrafine powers and CoCl2 (0.059 mmol for Co0.5Se, 0.082 mmol for Co0.7Se, 0.10 mmol for Co0.85Se, 0.118 mmol for CoSe, 0.141 mmol for Co1.2Se) in 10 mL of deionized water. The total volume of reagent solution was adjusted at 35 mL by adding deionized water. 7.5 mL of hydrazine hydrate (85 wt%) was dropped into the above solution, after vigorous agitating for 10 min, the reactant was transferred into a 50 mL of Teflon-lined autoclave and cleaned FTO glass substrate (sheet resistance 12 U square1, purchased from Hartford Glass Co., USA) with FTO layer downward was immersed in. After reaction at 120 C for 12 h, the FTO substrate was rinsed by deion ized water and vacuum dried at 50 C. As references, the pure Se and Co CEs were also prepared according to the above approach at a Se dosage of 0.12 g or CoCl2 dosage of 0.118 mmol. The pristine Pt CE was purchased from Dalian HepatChroma SolarTech Co., Ltd and used as a standard.
The compositions of the alloy CEs were detected by inductively coupled plasma-atomic emission spectra (ICP-AES). Prior to ICP measurements, the alloy CEs were immersed in concentration nitric acid to with agitation for 12 h to dissolve the compounds from FTO glass substrate. The optical transmission spectra of resultant CEs were recorded on a UVevis spectrophotometer at room temperature. X-ray diffraction (XRD) profiles of the resultant alloys were recorded on an X-ray powder diffractometer (X'pert MPD Pro, Philips, Netherlands) with Cu Ka radiation (l ¼ 1.5418 Å) in the 2q range from 10 to 90 operating at 40 kV accelerating voltage and 40 mA current). The morphologies of the resultant Co0.85Se alloy were observed with a transmission electron microscopy (TEM, JEM2010, JEOL). 3. Results and discussion The compositions of resultant CoeSe alloys are determined by ICP-AES measurement, showing atomic ratios of Co0.5Se, Co0.7Se, Co0.85Se, CoSe, and Co1.2Se of 0.482:1.000, 0.713:1.000, 0.858:1.000, 0.986:1.000, and 1.204:1.000, respectively. The measured atomic ratios are close to their stoichiometries, therefore the chemical formulas of the resultant alloy CEs can be represented by
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stoichiometric ratios. The Co0.85Se alloy is subjected to XRD measurement for identifying crystal phase (Fig. 1). At the first glance, all the diffraction peaks corresponding to CoeSe alloy [Pa-3(205), PDF# 88-1712] can be detected in XRD pattern, indicating that the mild solution method is feasible in alloying Co and Se species. Fig. 2 shows the transmission electron microscopy (TEM) photographs of Co0.85Se alloy. Due to the loose structure in Fig. 2a, the alloy CE provides large alloy/electrolyte interface and channels for ion diffusion and more active sites for I 3 reduction. The detection of lattice fringes in Fig. 2b indicates that the Co0.85Se alloy has good crystallinity. The interplanar spacing of d101 derived from the lattice fringes is around 3.13 Å. Moreover, there are many lattice distortions and plenty of defects observed in their alloying region. This result demonstrates the alloying of Co and Se can generate many defects, which provides placeholders for I 3 adsorption and reduction. Optical transparency of a binary alloy CE is one of the crucial characteristics in designing bifacial DSSCs. Fig. 3a compares the optical transparency with a wavelength ranging from 400 to 1000 nm for diversified CEs. All the CoeSe alloy CEs have high optical transparency (>60%) in visible light region in comparison with standard Pt electrode [28]. The transparent CoeSe alloy electrode having high optical transparency is expected to penetrate more light from rear side for dye excitation and electron generation. Fig. 3b displays characteristic JeV curves of DSSCs under front irradiation and the detailed photovoltaic parameters are summarized in Table 1. The cell device with Co0.85Se alloy CE yields a maximum front h of 8.30% (Jsc ¼ 16.80 mA cm2, Voc of 0.742 V, and FF of 0.67) under simulated air mass 1.5 (AM1.5) global sunlight, which is comparable to the cells from Pt and Pt-free CEs [33]. As references, pure Se and pure Co are also deposited on FTO glass substrates for DSSC fabrication, giving front h of 4.86% and 3.61%, respectively. Co is a typical transition metal having catalytic activity toward small molecular compounds, the alloying of Co and Se can favor the electronic perturbation and therefore accelerate the electrocatalytic kinetics of the resultant alloy. On behalf the high optical transmission of CoeSe alloy CEs, the cell devices are also irradiated from rear side to evaluate their electricity-generation capability from rear side. As shown in Fig. 3b, the DSSC with Co0.85Se alloy CE generate an optimal rear h of 4.63%, Jsc ¼ 9.90 mA cm2, Voc of 0.721 V, and FF of 0.65. In comparison with rear h of 2.73% from pristine Pt-based cell [28], the rear electricity-generation capability has been enhanced due to high
Fig. 1. XRD pattern of binary Co0.85Se alloy.
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optical transmission of Co0.85Se alloy electrode. It is noteworthy to mention that Jsc and Voc extracted from rear irradiation are lower than that for front irradiation. Jsc is of highly dependent on accumulative electron density on CB of TiO2 injected from excited dyes. As shown in Fig. 4, the incident light intensity for dye excitation is stepwisely decreased (viewed from anode to CE) under front irradiation, leading to a gradient reduction in electron distribution. In the process of transportation through TiO2 nanocrystallite network, the electrons will suffer shorter lengths under rear irradiation in comparison with front mode. The average electron may experience a million trapping events before percolating to the collecting FTO electrode [34]. Therefore, the electron density on CB of TiO2 and therefore Jsc under front irradiation is higher than that at rear irradiation. Moreover, the maximum Voc is determined by the difference between quasi Fermi energy of electrons in TiO2 and redox potential energy of electrolyte [35], whereas the real Voc of DSSC device is generally smaller than this theoretical limit, and a main reason is a backward reaction between dye molecules and electrolyte. The recombination of electrons by front-irradiation with oxidizing species, predominantly I 3 in the electrolyte can be effectively restricted, giving an elevated Voc. A golden rule in designing an efficient CE is to elevate its electrocatalytic activity toward electrolyte containing redox couples. Fig. 5a shows the CV curves of various CEs for I/I 3 redox couples recorded at a scan rate of 50 mV s1. Two pairs of redox peaks corresponding to I 4 I 3 interconversion [Red1 (I3 þ 2e ¼ 3I )/Ox1 (3I 2e ¼ I3 ); Red2 (3I2 þ 2e ¼ 2I3 )/Ox2 (2I3 2e ¼ 3I2)] are detected in each CV curve. The shapes and peak positions are similar to that from pristine Pt CE, indicating that the alloy electrodes have electrocatalytic activities for liquid electrolyte. The task of a CE is to collect electrons from external circle and to reduce I 3 into I, therefore, the peak current density of Red1 and peak-topeak separation (Epp) between Red1 and Ox1 can be employed to evaluate its catalytic activity. Most of the alloy CEs have higher peak current density for Red1 reaction in comparison with pristine Se and Co electrodes. Among five alloy CEs, Co0.85Se displays the highest current density and smallest Epp (ca. 0.42 V), suggesting a less overpotential loss for Co0.85Se alloy CEs. Moreover, we compare the Bode electrochemical impedance spectra (EIS) of symmetric dummy cells from various CEs, the lifetimes of electrons on CEs are calculated. Once the electrons migrate from external circuit to a CE layer, they participate in the reduction of I 3 species, therefore, the average lifetime of electrons can be employed to compare the reaction kinetics of Red1. From Fig. 5b and Table 2, one can find that Co0.85Se alloy exhibits a lifetime of 8.6 ms, which is shorter than that for other alloys, Se, and Co electrodes. Higher peak current density, lower Epp, and shorter electron lifetime demonstrate that Co0.85Se alloy electrode presents superior catalytic activity in reducing I 3, which is a prerequisite for a robust CE in efficient DSSCs. From the stacking CV curves of Co0.85Se alloy electrode, as shown in Fig. 6a, we can make a conclusion that the cathodic and anodic reactions are diffusion limited for I 3 transportation due to the a linear relationship between square root of scanning rate and peak current density (Fig. 6b). The results account for a fact that the diffusion layer becomes thinner and the electrochemical polarization degree is larger at an elevated scan rate, therefore leading to increasing overpotential and poor reversibility [36]. Except for high power conversion efficiency and costeffectiveness, a robust solar cell for engine or vehicle application should also exhibit fast start-up, multiple start capability, and operational stability. The core component to start the DSSC is the CE, therefore the superiorities of a robust CE include high catalytic activity and stability. Fig. 7a shows the startestop switches of the solar cell with Co0.85Se alloy electrode by alternatively irradiating (100 mW cm2) and darkening (0 mW cm2) the solar devices at an
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Fig. 2. (a) Low-resolution and (b) high-resolution TEM images of Co0.85Se alloy CE.
Fig. 3. (a) Optical transparency of various CEs and characteristic JeV curves of DSSCs for (b) front and (c) rear irradiations.
interval of 25 s and 0 V. No matter irradiation from front or rear side, an abrupt increase in photocurrent density under irradiation means the DSSC has a fast start-up performance, whereas no delay in starting the cell suggests a high catalytic activity for Co0.85Se CE. Table 1 Output photovoltaic characteristics of the DSSCs employing pristine Se, Co, and CoeSe alloy CEs for front or rear irradiation. h: power conversion efficiency; Jsc: short-circuit current density; Voc: open-voltage; FF: fill factor. CEs
Irradiation
h (%)
Voc (V)
Jsc (mA cm2)
FF
Se
Front Rear Front Rear Front Rear Front Rear Front Rear Front Rear Front Rear
4.86 3.22 6.47 4.33 7.46 4.74 8.30 4.63 7.75 4.39 5.37 2.32 3.61 2.90
0.690 0.652 0.721 0.690 0.739 0.724 0.742 0.721 0.743 0.712 0.723 0.682 0.680 0.666
12.21 9.56 13.27 9.63 15.36 9.95 16.80 9.90 15.47 8.97 11.45 5.02 8.29 6.51
0.58 0.52 0.67 0.65 0.66 0.66 0.67 0.65 0.67 0.69 0.65 0.68 0.64 0.67
Co0.5Se Co0.7Se Co0.85Se CoSe Co1.2Se Co
The unchanged in photocurrent density after five startestop cycles is a solid support for multiple start capability. In each “on” stage, no decrease in photocurrent density refers to a low recombination effect between photogenerated electrons on conduction band of TiO2 nanocrystalline and I 3 species in liquid electrolyte. As a reference, the startestop switches for Pt-based DSSC are also measured at the same conditions. As shown in Fig. 7c, the time delay in reaching the maximum photocurrent density under irradiation and reduction in photocurrent density mean a modest
Fig. 4. Schematic diagram for bifacial DSSC with Co0.85Se alloy electrode.
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1 Fig. 5. (a) CV curves of various CEs for I/I 3 redox couples recorded at a scan rate of 50 mV s . (b) Bode and (c) Nyquist EIS plots and (d) Tafel polarization curves of the symmetric dummy cells from two identical CEs. The inset gives an equivalent circuit.
Table 2 Output parameters extracted from CV and EIS characterizations for the symmetric dummy cells. CEs
Red1 (mA cm2)
Epp (V)
t (ms)
Rct (U cm2)
W (U cm2)
Se Co0.5Se Co0.7Se Co0.85Se CoSe Co1.2Se Co
3.31 3.36 3.56 5.77 4.84 2.43 1.05
1.07 0.62 0.58 0.42 0.55 0.63 0.83
101.4 46.3 42.1 8.6 31.4 97.3 267.3
20.2 12.2 8.2 3.3 5.4 13.4 88.5
114.2 32.9 23.5 3.3 6.5 34.3 78.2
catalytic activity toward I 3 reduction and backward reaction of electrons with I 3 , respectively. Additionally, the photocurrent density stabilities of the solar cells with Co0.85Se and Pt CEs over 10 h are shown in Fig. 7b and d, remaining 91.2% and 75.5% of initial photocurrent density for Co0.85Se-based DSSC, whereas they are
69.1% and 90.8% for Pt-based solar cell under front and rear irradiations, respectively. The results demonstrate that the cell with Co0.85Se alloy electrode has satisfactory stability under durative irradiation.
4. Conclusions In summary, transparent and cost-effective CoeSe binary alloys have been fabricated by a mild solution strategy free of any surfactant or template and employed as CE materials in DSSCs. Co0.85Se alloy CE displays lower Epp and Rct, and higher peak current density than other electrodes. Notably, the DSSC employing Co0.85Se alloy CE displays front and rear efficiencies of 8.30% and 4.63%, respectively, superior to the cell performances from pristine Pt electrode. Additionally, the merits on quick start-up, low electron recombination, high multipleestart capability, and high photocurrent stability motivate the potential applications of such bifacial solar cell
1 Fig. 6. (a) CV curves of Co0.85Se CE for I/I 3 redox species at varied scan rates (from inner to outer: 50, 75, 100, 125, 150, 175, and 200 mV s ), and (b) relationship between peak current density and square root of scan rates.
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Fig. 7. The startestop switches with irradiation from front and rear were achieved by alternatively irradiating (100 mW cm2) and darkening (0 mW cm2) the solar devices from (a) Co0.85Se and (c) Pt electrodes at an interval of 25 s and 0 V. Photocurrent stabilities of the DSSCs with (b) Co0.85Se and (d) Pt electrodes under durable irradiations at 100 mW cm2.
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