- Email: [email protected]
Highly catalytic nickel sulfide counter electrode for dye-sensitized solar cells
Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Highly catalytic nickel sulfide counter electrode for dye-sensitized solar cells Dinah Punnoose a , Hee-Je Kim a, * , CH.S.S. Pavan Kumar b , S. Srinivasa Rao a , Chandu V.V.M. Gopi a , Sang-Hwa Chung c a b c
Department of Electrical and Computer Engineering, Pusan National University, Gumjeong-Gu, Jangjeong-Dong, Busan 609-735, South Korea Department of Physics, Pusan National University, Gumjeong-Gu, Jangjeong-Dong, Busan 609-735, South Korea Department of Computer Engineering, Pusan National University, Gumjeong-Gu, Jangjeong-Dong, Busan 609-735, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 November 2014 Received in revised form 17 January 2015 Accepted 18 March 2015 Available online 19 March 2015
Dye-sensitized solar cells (DSSCs) have been the epicenter of attention for past few decades as a potentially cost-effective substitute for silicon-based solar cells. In DSSCs, the counter electrode (CE) plays a pivotal role in amassing electrons from the external circuit and catalyzing the I3 reduction in the electrolyte. In this paper, the development of nickel sulfide (NiS) CEs for DSSCs are studied, emphasizing significant crescendos that assure economical, competent, and stable DSSC systems. Specially, we pinpoint on the design of highly efficient NiS CE, including design ideas, fabrication approaches and characterization techniques that serve as practical replacements to conventional noble metal Pt electrodes. DSSC fabricated with the NiS CE achieved a power conversion efficiency of 5.69% under 1 sunlight illumination (100 mW cm2, AM 1.5 G). ã 2015 Elsevier B.V. All rights reserved.
Keywords: Counter electrode Dye-sensitized solar cells (DSSCs) Nickel sulfide
1. Introduction Dye-sensitized solar cells (DSSCs) have been the focal point of harnessing solar energy from the past few decades owing to their capability in converting solar energy to electrical energy [1–3]. Enhancements in the efficiency and stability of DSSCs are required for widespread hands-on usage. The DSSCs are fabricated primarily with two electrodes and a redox-active electrolyte. In DSSCs, the photo electrodes are made from TiO2 which when exposed to light prevents photo corrosion due to their extraordinary chemical stability, subsequently surging the life time [4,5]. The titanium dioxide (TiO2) centered photo-anodes sensitized with dye (ruthenium based dye), an electrolyte with the I3/I redox couple and Pt (platinum) counter electrode (CE) forms a DSSC. In DSSCs, dye absorbs the molecules for photo generation which plays an integral role in increasing the power conversion efficiency and stability. The redox electrolyte, iodide/triiodide (I/I3) is an essential inorganic constituent in DSSCs that helps in diminishing the power conversion efficiency losses. Prominent optimizations of all cell components are indispensable for promising performance of DSSCs. The CE is a vital component of DSSCs which is composed of Pt materials. However, Pt being expensive and scarce in nature is replaced with an inferior substitute. Thus, there exists a tradeoff between the cost and quality of the solar cells. The prerequisite of
* Corresponding author. Tel.: +82 51 510 2364; fax: +82 51 513 0212. E-mail address: [email protected] (H.-J. Kim). http://dx.doi.org/10.1016/j.jphotochem.2015.03.015 1010-6030/ ã 2015 Elsevier B.V. All rights reserved.
the substitute material is to overcome the challenges associated with Pt-based electrodes and should provide high electrical conductivity and superior catalytic activity simultaneously. The Pt-free materials for DSSCs such as the inorganic materials have fashioned good performances for DSSCs [6]. Among the inorganic materials, nickel sulfide (NiS) has been considered as a favorable alternative to substitute Pt for DSSCs due to its high conductivity, easy fabrication, and excellent catalytic activity. Many other researches have been reported based on NiS CE [7,8]. In this work, we efficaciously synthesized DSSCs assembled with NiS CEs and accomplished comparable photoelectric conversion properties to that based on Pt CE. Chemical bath deposition (CBD) is frequently and economically used methods for the deposition of metal chalcogenide, metal oxide and metal sulfide thin films and is presently drawing and expedient for large area deposition [9]. The low charge transfer resistance (RCT) at the NiS CE/electrolyte interface is detected based from electrochemical impedance spectroscopy, indicating a good electro-catalytic activity of NiS CE ability towards the I/I3 electrolyte. The uniqueness in this present work describes the methodical study accomplished for appreciative and applicable composition of materials, which improves the photovoltaic properties of DSSCs which has not been reported previously. In this work, the CBD method has been used for the deposition of NiS thin films. The morphological, optical, and structural properties of the optimized films are deliberated. This report deals with the challenge in fabricating DSSCs with the optimized CE. We have achieved quite
42
D. Punnoose et al. / Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46
good photovoltaic conversion efficiency of 5.69%. A representational structure of the TiO2/dye/NiS DSSC with nano structured NiS CE is depicted in Fig. 1. The photons are absorbed by the dye and the electrons are driven towards the TiO2 conduction band and hole transfer into the electrolyte upon 1 sun illumination. At the CE, reduction of the oxidized redox system regenerates the reduced redox species. 2. Experimental 2.1. Materials All chemicals used were purchased from Aldrich and employed without any further purification. FTO substrate with a sheet resistance of 10 V/square (Sigma–Aldrich) was used to prepare photo-anodes and CE. 2.2. Preparation of TiO2 electrode films
Fig. 1. Schematic structure of TiO2/dye DSSC based NiS CE.
The substrates were cleaned ultrasonically using acetone, ethanol, and deionized water for 10 min each and dried with nitrogen gas. The cleaned FTO glasses were dipped in 50 mM of TiCl4 solution for 40 min at 60 C to form a compact layer of TiO2 followed by annealing at 450 C for 30 min. The photo electrodes were prepared using a commercially available nano-porous TiO2 paste with a particle size of 20 nm (Ti-Nanoixide HT/SP Solaronix) on electrodes having an active area of 0.25 cm2. Subsequently, the samples were sintered at 450 C for 30 min for solvent evaporation. 2.3. Fabrication of dye sensitized photoanode The TiO2 electrodes were kept in N719 dye for 24 h and then dried with N2 gas and only then used further. 2.4. Fabrication of NiS counter electrode NiS thin films were deposited on FTO substrates using CBD. Erstwhile to deposition, FTO substrates were ultrasonically cleaned with acetone, ethanol, and distilled water each for 10 min. The substrate was immersed horizontally into the solution which contains nickel sulfate hexa- hydrate and thioacetamide (TAA) which acts as a source of Ni2+ and S2 ions urea and/or TEA (triethanolamine) reagents for depositing NiS thin films. Table 1 shows the in-depth preparation of the NiS CEs thin film. The substrate with growth solution was kept in hot air oven at different conditions of temperature of 70 C, 80 C, and 90 C for 2 h and 1 h deposition time. No visible coating was observed at 70 C and 80 C while semi transparent films were observed at 90 C for 2 h deposition time and thus the optimized condition was achieved. Finally, at different conditions the NiS coated FTOs were cleaned with DI water and are denoted as Sample A–D. The Pt CE is denoted as Sample E. To fabricate the Pt electrode, the cleaned FTO glass substrate was coated with a Pt paste (Pt-Catalyst T/SP, Solaronix) in the active area of 0.7 cm2 using the doctor blade method and sintered at 450 C in air for 10 min.
2.5. Cell fabrication The dye sensitized TiO2 electrode and the NiS counter electrode were assembled and sandwiched using a transparent 60 mm thick Surlyn spacer (Dupont). The iodide/triiodide (I/I3) electrolyte was injected through the pin-hole made in the CE. 2.6. Characterization The surface morphology of NiS thin films was characterized using field emission scanning electron microscope (FE-SEM, S-4200, Hitachi) equipped with an on-system energy dispersive X-ray spectroscopy (EDS) analysis with an operating voltage of 15 kV for elemental compositions. The current–voltage characteristics of the DSSCs was performed under 1 sun illumination (AM 1.5 G, 100 mW cm2) using ABET Technologies (USA) solar simulator. Electrochemical impedance spectroscopy (EIS) was conducted on DSSC with NiS and Pt CEs using BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) under 1 sun illumination and the frequency ranged from 100 mHz to 500 kHz. 3. Results and discussion 3.1. Surface morphology Fig. 2 shows the surface morphology of deposited NiS thin films on FTO at different preparation conditions such as concentrations of TAA with and without TEA. The NiS film thickness was found to be 190 nm for Sample B and thickness varied between 150 and 200 nm for other NiS thin films. Adhesion of CE active materials on FTO substrate plays a vital role which determines the power conversion efficiency [10]. If the active material does not adhere properly to the substrate, it may be peeled off from the substrate and released into the electrolyte which could cause the decrease in efficiency of DSSCs. In this study, good adhesion of NiS thin films on
Table 1 NiS thin films deposited on FTO substrate at different preparation conditions. Nickel sulfide CE
Nickel (M)a
Thioacetamide (M)a
Urea (M)a
Triethanolamine (ml)b
Sample Sample Sample Sample
0.1 0.1 0.1 0.1
0.4 0.4 0.8 0.8
0.8 0.8 0.8 0.8
– 2.5 2.5 –
a b
A B C D
M: molarity. ml: milliliter.
D. Punnoose et al. / Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46
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Fig. 2. SEM images of NiS thin films deposited at (c) Sample A, (b) Sample B, (c) Sample C, and (d) Sample D thin films.
FTO substrate is obtained. For Sample A the gaps are more and rough surface structured with accumulated nanoparticles that condemn the sulfides with low catalytic activity. NiS thin films prepared using Sample B consisted of nano wheat like structure (Fig. 2b) with the presence of triethanol amine and thus concentrates a high surface area with uniform distribution. Whereas Samples C and D exhibited nano platelets like morphology (Fig. 2(c) and (d), respectively). However, a complete changeover of morphology from nanoparticles to nano-platelets like structure was observed upon adding more concentration of TEA with urea as shown in Fig. 2(b) and (c). Moreover, TAA from 0.4 to 0.8 M causes a change in surface morphology due to which the electrocatalytic activity changes and there could be an effect on the photovoltaic performance.
peaks in NiS (ICDD file no. 75-0613). However, the XRD pattern of NiS films could not exhibit any characteristic peaks, which might 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 is reported that the wet deposited composite films are amorphous and/or nano crystalline. The atomic percentage of Ni:S in Samples A, B, C, and D is found to be (45:50), (51:46), (46:51), and (50:50), respectively. The EDX result confirms that the formation of Ni is faster than the S source for Sample B.
3.2. Optical properties The optical properties of the counter electrode give an important information to the researchers DSSCs. Fig. 3 shows the UV absorption spectrum of the NiS thin films on an FTO substrate, which exhibits the exponentially declining behavior of absorbance from 300 nm to about 560 nm subsequently improvement in absorbance from about 560 nm onwards. Such absorption performance 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. Although a clear absorption peak in visible light region could not be observed, the presence of broad lump at around 400–500 nm could be realized. In contrast, an exponentially continuous decreasing behavior of absorbance from UV to near infrared region has also been reported for remaining samples. Such contrast absorption behaviors of various reported NiS thin films could be due to different morphology and crystal structures. Fig. 4 shows the X-ray diffraction pattern of NiS deposited films on FTO. The diffraction peaks appeared at 2u = 30.15 , 34.5 , 45.10 , 53.49 , 61.01, 64.74 , and 73.1 matches the observed diffraction
Fig. 3. UV of NiS thin films deposited at (a) Sample A, (b) Sample B, (c) Sample C, and (d) Sample D.
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D. Punnoose et al. / Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46 Table 2 Photovoltaic performance of DSSC fabricated with different CEs and electrochemical parameters from EIS.
Fig. 4. X-ray diffraction pattern of NiS thin films deposited (a) Sample A, (b) Sample B, (c) Sample C, and (d) Sample D.
3.3. Performance of NiS CE in DSSCs Fig. 5 shows the J–V curves of the TiO2/dye/NiS. Table 2 represents the the performance of DSSCs in terms of Voc,Jsc, fill factor (FF, and efficiency (h). In this illustration a similar photo anode (TiO2/dye) was used for NiS as well as for the Pt CE. The outstanding conversion efficiency (h) of 5.69% was obtained for the DSSC using Sample B CE. All compositions of CEs showed almost analogous Voc (open circuit voltage) in the range of 0.6–0.7 V and FF (fill factor) in the range 0.70–0.74 paralleled to Pt DSSCs (Voc = 0.66 V and FF = 0.66) respectively, which is credited to highest electro-catalytic activity towards I3 reduction. The conversion efficiency and FF of the Pt CE was 4.55% and 0.66, which was ascribed to lowest electro-catalytic activity towards I3 reduction. The Jsc (short circuit current) was found to be much higher for NiS CE than Pt CE. The JSC relies on the charge transfer resistance at the CE/electrolyte interface, light harvesting efficiency, electron and hole injection and charge collection efficiency [11,12]. Whereas it showed a lower Voc value of 0.63 than the Pt based cell (0.66), the highest h (5.69%) was observed, which could be due to its highest FF (0.72) than the Pt-based DSSC (0.66). The Sample B CE exhibited high Jsc of 12.68 mA cm2, open circuit voltage (Voc) of 0.63 V, fill factor (FF) of 0.72, leading to the high
Fig. 5. Current density voltage characteristics of DSSCs based on NiS and Pt CE.
Parameters
Sample A
Sample B
Sample C
Sample D
Sample E
Voc (V) Jsc (mA cm2) FF h (%) Rs (V) RCT (V) RCE (V) Cm (mF) Zw (V)
0.70 7.58 0.74 3.97 11.85 3.64 24.71 239 1.7
0.63 12.68 0.72 5.69 7.93 5.22 12.49 118 0.03
0.64 8.11 0.74 3.89 11.79 11.62 24.23 270 2.29
0.63 9.62 0.73 4.42 10.73 7.15 13.82 361 0.60
0.66 10.39 0.66 4.55 10.95 5.46 11.72 273 0.78
conversion efficiency. The pragmatic higher Jsc and FF in NiS CEs are attributed to considerable increase in charge transfer at the CE/electrolyte interface that reduces the internal resistances, concentration inclines in the electrolyte, and improves the electrocatalytic activity [13]. The enhancement of efficiency of Sample B CE may be due to the change of morphological structure and also due to the fact that at 90 C urea and TEA plays a key role to form the metal sulfide complex on FTO substrate. The above results show that NiS CE will assist as an efficient CE in DSSCs using I/I3 electrolyte. In the interim, the DSSC using Samples A, C, and D CEs revealed equitable photovoltaic properties where the conversion efficiencies are 3.97%, 3.89%, and 4.42%, respectively. The decrease in the FF can be endorsed to the higher charge transfer resistance (RCE) and sheet resistance (Rs) than Sample B [14,15]. Therefore, the use of the NiS CE ensures effective charge transfer at the CE/electrolyte interface, and enhances the electrocatalytic activity in the DSSC, which ultimately escalates the conversion efficiency. The EIS of a DSSC using NiS CEs (Samples A–D) are shown in Fig. 6 and the results are compared with Pt CE. The inset in Fig. 6 shows an equivalent circuit model used for the DSSCs. EIS was used to measure the charge transfer resistance (RCE) at the CE/electrolyte interface in the high frequency region, which was correlated with the electro-catalytic activity for I3 reduction, RCT at the interface of the photo-electrode/dye/electrolyte in the intermediate frequency region, and the Warburg diffusion resistance (Zw) of I/I3 in the electrolyte at the low frequency region, Zw is merged with RCT [16]. The RCE, RCT, and Zw were attained straightly from the impedance analysis software called Z-VIEW. Table 2 lists the measured EIS parameters. In relation to the equivalent circuit, the series resistance (RS) denotes the
Fig. 6. Nyquist plot for symmetric cells of NiS and Pt CE; inset shows the magnified plot of Pt at higher impedance range and equivalent circuit.
D. Punnoose et al. / Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46
high-frequency nonzero intercept of the real axis, the left semicircle at high-frequency region which denotes the parallel combination of charge transfer resistance RCT at the CE/electrolyte interface and the constant phase element (CPE) of electrical double layer, the small semicircle (low frequency region) at the end was assigned to the Warburg impedance (Zw) of the redox couple in the electrolyte [17]. The RCT values of Samples A, B, C, D, and E (Pt) CE are found to be 3.64, 5.22, 11.62, 7.15, and 5.46 V, respectively. The power conversion efficiency also relies on the charge transfer resistance at the CE/electrolyte interface. The low charge transfer resistance of 5.22 V was obtained for Sample B CE which is lower than that of the Sample E (Pt) CE (5.46 V). The lowest value of RCT indicates superior electrocatalytic activity of NiS CE compared to Sample E (Pt), suggesting an acceleration of the high electron transfer process at the interface of the CE/electrolyte [18]. The Zw values of Samples A, B, C, D, and E (Pt) CEs were found to be 1.7, 0.03, 2.29, 0.60, and 0.78 V, respectively. The lower value of Zw shows better electrolyte diffusion, facilitates faster mass transport of electron and improves the performance of the DSSC by increasing the fill factors [19]. The CPE value of Samples A, B, C, D, and E (Pt) CE were observed to be 268, 341, 132, 336, and 2.99 F, respectively. The NiS electrodes exhibited larger CPE values than that of the Pt electrode, which is a crucial factor for the high electro-catalytic activities [20]. The higher value for CPE corresponds to higher surface area, leading to better electro-catalytic activity [21]. This shows that NiS CEs have great potential for the fabrication of highly efficient DSSCs. Tafel polarization was performed to investigate the electrocatalytic activity of CEs. Fig. 7 shows the Tafel polarization curves of the symmetric dummy cells measured under dark conditions. The Tafel polarization curves showed a logarithmic current density (J0) as a function of the voltage (V). The electro-catalytic activity of the CE is related directly to the exchange current density (J0). The exchange current density (J0) was obtained from the intercept value by extrapolating the linear region of the curve at zero over potential. Since the slope and the logarithmic current densities on cathodic and anodic branches for the Pt CE are lower than the NiS CEs, it can be suggested that the exchange current density (J0) for the Pt CE is lower than that for the NiS CEs in terms of the Tafel equation [22] J0 ¼
RT nFRCT
(1)
where RCT is the charge transfer resistance at the CE/electrolyte interface obtained from EIS spectra, n is the number of electrons involved in the I32 reduction at the counter electrode, T is
Fig. 7. Tafel curve for symmetric cells based on NiS and Pt CE.
45
temperature, R is the gas constant, and F is the Faraday constant. J0 entirely depends on the RCT value and decrease with the increase in RCT. The larger anodic or cathodic slope suggests a greater exchange current density (J0) on the electrode. The Tafel polarization curve of Sample B exhibited a larger exchange current density (J0) than the Pt electrode, suggesting that Sample B CE has superior electrocatalytic activity for I32 reduction. A steep slope obtained from the electrodes indicates the presence of large exchange current density (J0) on the electrode surface. 4. Conclusion We have exploited nano wheat structured NiS counter electrode in the fabrication of DSSCs. An active NiS CE could be formed by taking advantage of the various conditions. Impressively, the appropriate composition of materials in thin films on the NiS CE had a mammoth impact on the reduction of electrolyte and thereby boosting up the overall efficiency. In comparison with the Pt CE, the NiS CE at Sample B with nano wheat structure yields a low charge transfer resistance towards the I/I3 electrolyte and exhibited an outstanding electro-catalytic activity and power conversion efficiency of 5.69%. The imposing optical and electronic features of NiS counter electrode can inflate the comprehensive performance of DSSCs to the next level. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0014437). References [1] B. O’Regan, M. Gratzel, A low-cost, high efficiency solar cell based on dye sensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cell, Chem. Rev. 110 (2010) 6595–6663. [3] N. Papageorgiou, W.F. Maier, M. Gratzel, An iodine/triiodide reduction electro catalyst for aqueous and organic media, J. Electrochem. Soc. 144 (1997) 876–884. [4] A.L. Viet, R. Jose, M.V. Reddy, B.V.R. Chowdari, S. Ramakrishna, Nb2O5 photoelectrodes for dye-sensitized solar cells: choice of the polymorph, J. Phys. Chem. C 114 (2010) 21795–21800. [5] W. Mingxing, M. Tingli, Platinum-free catalysts as counter electrodes in dye-sensitized solar cells, ChemSusChem 5 (2012) 1343–1357. [6] T. Zhu, Z.Y. Wang, S.J. Ding, J.S. Chen, X.W. Lou, Hierarchical nickel sulfide hollow spheres for high performance supercapacitors, RSC Adv. 1 (2011) 397–400. [7] Z. Li, F. Gong, G. Zhou, Z.S. Wang, NiS2/reduced graphene oxide nanocomposites for efficient dye-sensitized solar cells, J. Phys. Chem. C 117 (2013) 6561–6567. [8] S.M. Pawar, B.S. Pawar, J.H. Kim, O.S. Joo, C.D. Lokhande, Recent status of chemical bath deposited metal chalcogenide and metal oxide thin films, Curr. Appl. Phys. 11 (2011) 117–254. [9] A. Hauch, A. Georg, Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells, Electrochim. Acta 46 (2001) 3457–3473. [10] M. Tachan, Shalom, I. Hod, S. Ruhle, S. Tirosh, A. Zaban, PbS as a highly catalytic counter electrode for polysulfide-based quantum dot solar cells, J. Phys. Chem. C 115 (2011) 6162–6166. [11] A.D. Savariraj, K.K. Viswanathan, K. Prabakar, Influence of Cu vacancy on knit coir mat structured CuS as counter electrode for quantum dot sensitized solar cells, Appl. Mater. Interface 22 (19) (2014) 702–19709. [12] D.B. Mitzi, Templating and structural engineering in organic–inorganic perovskites, J. Chem. Soc. Dalton Trans. (2001) 1–12. [13] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Use of highlyordered TiO2 nanotube arrays in dye-sensitized solar cells, Nano Lett. 6 (2006) 215–218. [14] Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, I/I3 redox reaction behavior on poly(3,4-ethylenedioxythiophene) counter electrode in dye sensitized solar cells, J. Photochem. Photobiol. A 164 (2004) 153–157. [15] F.F. Santiago, J. Bisquert, E. Palomares, L. Otero, D. Kuang, S.M. Zakeeruddin, M. Gratzel, Correlation between photovoltaic performance and impedance
46
[16]
[17]
[18]
[19]
D. Punnoose et al. / Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46 spectroscopy of dye-sensitized solar cells based on ionic liquids, J. Phys. Chem. C 111 (2007) 6550–6560. S.Q. Fan, B. Fang, J.H. Kim, B. Jeong, C. Kim, J.S. Yu, J. Ko, Ordered multimodal porous carbon as highly efficient counter electrodes in dye-sensitized and quantum dot solar cells, Langmuir (2010) 13644–13649 Article 26. E. Ramasamy, W.J. Lee, D.Y. Lee, J.S. Song, Spray coated multi-wall carbon nanotube counter electrode for tri-iodide I3 reduction in dye-sensitized solar cells, Electrochem. Commun. 10 (2008) 1087–1089. M. Wu, Y. Wang, X. Lin, N. Yu, L. Wang, L. Wang, A. Hagfeldt, T. Ma, Economical and effective sulfide catalysts for dye-sensitized solar cells as counter electrodes, Phys. Chem. Chem. Phys. 13 (2011) 19298–19301. M.H. Kumar, B.S. Kumar, M. Rao, R.R. Prabhakar, N. Mathews, Y.M. Lam, S.G. Mhaisalkar, Solution processed transition metal sulfides: application as
counter electrodes in dye sensitized solar cells (DSCs), Phys. Chem. Chem. Phys. 13 (2011) 19307–19309. [20] H. Chen, L. Zhu, H. Liu, W. Li, ITO porous film supported metal sulfide counter electrodes for high-performance quantum dot sensitized solar cells, J. Phys. Chem. C 117 (2013) 3739–3746. [21] I. Mora-Sero, S. Gimenez, F. Fabregat-Santiago, R. Gomez, Q. Shen, T. Toyoda, J. Bisquert, Recombination in quantum dots sensitized solar cells, Acc. Chem. Res. 42 (2009) 1848–1857. [22] T. Shu, P. Xiang, Z.M. Zhou, H. Wang, G.H. Liu, H.W. Han, Y.D. Zhao, Meso-scopic nitrogen-doped TiO2 spheres for quantum dot-sensitized solar cells, Electrochim. Acta 68 (2012) 166–171.
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Highly catalytic nickel sulfide counter electrode for dye-sensitized solar cells Dinah Punnoose a , Hee-Je Kim a, * , CH.S.S. Pavan Kumar b , S. Srinivasa Rao a , Chandu V.V.M. Gopi a , Sang-Hwa Chung c a b c
Department of Electrical and Computer Engineering, Pusan National University, Gumjeong-Gu, Jangjeong-Dong, Busan 609-735, South Korea Department of Physics, Pusan National University, Gumjeong-Gu, Jangjeong-Dong, Busan 609-735, South Korea Department of Computer Engineering, Pusan National University, Gumjeong-Gu, Jangjeong-Dong, Busan 609-735, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 November 2014 Received in revised form 17 January 2015 Accepted 18 March 2015 Available online 19 March 2015
Dye-sensitized solar cells (DSSCs) have been the epicenter of attention for past few decades as a potentially cost-effective substitute for silicon-based solar cells. In DSSCs, the counter electrode (CE) plays a pivotal role in amassing electrons from the external circuit and catalyzing the I3 reduction in the electrolyte. In this paper, the development of nickel sulfide (NiS) CEs for DSSCs are studied, emphasizing significant crescendos that assure economical, competent, and stable DSSC systems. Specially, we pinpoint on the design of highly efficient NiS CE, including design ideas, fabrication approaches and characterization techniques that serve as practical replacements to conventional noble metal Pt electrodes. DSSC fabricated with the NiS CE achieved a power conversion efficiency of 5.69% under 1 sunlight illumination (100 mW cm2, AM 1.5 G). ã 2015 Elsevier B.V. All rights reserved.
Keywords: Counter electrode Dye-sensitized solar cells (DSSCs) Nickel sulfide
1. Introduction Dye-sensitized solar cells (DSSCs) have been the focal point of harnessing solar energy from the past few decades owing to their capability in converting solar energy to electrical energy [1–3]. Enhancements in the efficiency and stability of DSSCs are required for widespread hands-on usage. The DSSCs are fabricated primarily with two electrodes and a redox-active electrolyte. In DSSCs, the photo electrodes are made from TiO2 which when exposed to light prevents photo corrosion due to their extraordinary chemical stability, subsequently surging the life time [4,5]. The titanium dioxide (TiO2) centered photo-anodes sensitized with dye (ruthenium based dye), an electrolyte with the I3/I redox couple and Pt (platinum) counter electrode (CE) forms a DSSC. In DSSCs, dye absorbs the molecules for photo generation which plays an integral role in increasing the power conversion efficiency and stability. The redox electrolyte, iodide/triiodide (I/I3) is an essential inorganic constituent in DSSCs that helps in diminishing the power conversion efficiency losses. Prominent optimizations of all cell components are indispensable for promising performance of DSSCs. The CE is a vital component of DSSCs which is composed of Pt materials. However, Pt being expensive and scarce in nature is replaced with an inferior substitute. Thus, there exists a tradeoff between the cost and quality of the solar cells. The prerequisite of
* Corresponding author. Tel.: +82 51 510 2364; fax: +82 51 513 0212. E-mail address: [email protected] (H.-J. Kim). http://dx.doi.org/10.1016/j.jphotochem.2015.03.015 1010-6030/ ã 2015 Elsevier B.V. All rights reserved.
the substitute material is to overcome the challenges associated with Pt-based electrodes and should provide high electrical conductivity and superior catalytic activity simultaneously. The Pt-free materials for DSSCs such as the inorganic materials have fashioned good performances for DSSCs [6]. Among the inorganic materials, nickel sulfide (NiS) has been considered as a favorable alternative to substitute Pt for DSSCs due to its high conductivity, easy fabrication, and excellent catalytic activity. Many other researches have been reported based on NiS CE [7,8]. In this work, we efficaciously synthesized DSSCs assembled with NiS CEs and accomplished comparable photoelectric conversion properties to that based on Pt CE. Chemical bath deposition (CBD) is frequently and economically used methods for the deposition of metal chalcogenide, metal oxide and metal sulfide thin films and is presently drawing and expedient for large area deposition [9]. The low charge transfer resistance (RCT) at the NiS CE/electrolyte interface is detected based from electrochemical impedance spectroscopy, indicating a good electro-catalytic activity of NiS CE ability towards the I/I3 electrolyte. The uniqueness in this present work describes the methodical study accomplished for appreciative and applicable composition of materials, which improves the photovoltaic properties of DSSCs which has not been reported previously. In this work, the CBD method has been used for the deposition of NiS thin films. The morphological, optical, and structural properties of the optimized films are deliberated. This report deals with the challenge in fabricating DSSCs with the optimized CE. We have achieved quite
42
D. Punnoose et al. / Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46
good photovoltaic conversion efficiency of 5.69%. A representational structure of the TiO2/dye/NiS DSSC with nano structured NiS CE is depicted in Fig. 1. The photons are absorbed by the dye and the electrons are driven towards the TiO2 conduction band and hole transfer into the electrolyte upon 1 sun illumination. At the CE, reduction of the oxidized redox system regenerates the reduced redox species. 2. Experimental 2.1. Materials All chemicals used were purchased from Aldrich and employed without any further purification. FTO substrate with a sheet resistance of 10 V/square (Sigma–Aldrich) was used to prepare photo-anodes and CE. 2.2. Preparation of TiO2 electrode films
Fig. 1. Schematic structure of TiO2/dye DSSC based NiS CE.
The substrates were cleaned ultrasonically using acetone, ethanol, and deionized water for 10 min each and dried with nitrogen gas. The cleaned FTO glasses were dipped in 50 mM of TiCl4 solution for 40 min at 60 C to form a compact layer of TiO2 followed by annealing at 450 C for 30 min. The photo electrodes were prepared using a commercially available nano-porous TiO2 paste with a particle size of 20 nm (Ti-Nanoixide HT/SP Solaronix) on electrodes having an active area of 0.25 cm2. Subsequently, the samples were sintered at 450 C for 30 min for solvent evaporation. 2.3. Fabrication of dye sensitized photoanode The TiO2 electrodes were kept in N719 dye for 24 h and then dried with N2 gas and only then used further. 2.4. Fabrication of NiS counter electrode NiS thin films were deposited on FTO substrates using CBD. Erstwhile to deposition, FTO substrates were ultrasonically cleaned with acetone, ethanol, and distilled water each for 10 min. The substrate was immersed horizontally into the solution which contains nickel sulfate hexa- hydrate and thioacetamide (TAA) which acts as a source of Ni2+ and S2 ions urea and/or TEA (triethanolamine) reagents for depositing NiS thin films. Table 1 shows the in-depth preparation of the NiS CEs thin film. The substrate with growth solution was kept in hot air oven at different conditions of temperature of 70 C, 80 C, and 90 C for 2 h and 1 h deposition time. No visible coating was observed at 70 C and 80 C while semi transparent films were observed at 90 C for 2 h deposition time and thus the optimized condition was achieved. Finally, at different conditions the NiS coated FTOs were cleaned with DI water and are denoted as Sample A–D. The Pt CE is denoted as Sample E. To fabricate the Pt electrode, the cleaned FTO glass substrate was coated with a Pt paste (Pt-Catalyst T/SP, Solaronix) in the active area of 0.7 cm2 using the doctor blade method and sintered at 450 C in air for 10 min.
2.5. Cell fabrication The dye sensitized TiO2 electrode and the NiS counter electrode were assembled and sandwiched using a transparent 60 mm thick Surlyn spacer (Dupont). The iodide/triiodide (I/I3) electrolyte was injected through the pin-hole made in the CE. 2.6. Characterization The surface morphology of NiS thin films was characterized using field emission scanning electron microscope (FE-SEM, S-4200, Hitachi) equipped with an on-system energy dispersive X-ray spectroscopy (EDS) analysis with an operating voltage of 15 kV for elemental compositions. The current–voltage characteristics of the DSSCs was performed under 1 sun illumination (AM 1.5 G, 100 mW cm2) using ABET Technologies (USA) solar simulator. Electrochemical impedance spectroscopy (EIS) was conducted on DSSC with NiS and Pt CEs using BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) under 1 sun illumination and the frequency ranged from 100 mHz to 500 kHz. 3. Results and discussion 3.1. Surface morphology Fig. 2 shows the surface morphology of deposited NiS thin films on FTO at different preparation conditions such as concentrations of TAA with and without TEA. The NiS film thickness was found to be 190 nm for Sample B and thickness varied between 150 and 200 nm for other NiS thin films. Adhesion of CE active materials on FTO substrate plays a vital role which determines the power conversion efficiency [10]. If the active material does not adhere properly to the substrate, it may be peeled off from the substrate and released into the electrolyte which could cause the decrease in efficiency of DSSCs. In this study, good adhesion of NiS thin films on
Table 1 NiS thin films deposited on FTO substrate at different preparation conditions. Nickel sulfide CE
Nickel (M)a
Thioacetamide (M)a
Urea (M)a
Triethanolamine (ml)b
Sample Sample Sample Sample
0.1 0.1 0.1 0.1
0.4 0.4 0.8 0.8
0.8 0.8 0.8 0.8
– 2.5 2.5 –
a b
A B C D
M: molarity. ml: milliliter.
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Fig. 2. SEM images of NiS thin films deposited at (c) Sample A, (b) Sample B, (c) Sample C, and (d) Sample D thin films.
FTO substrate is obtained. For Sample A the gaps are more and rough surface structured with accumulated nanoparticles that condemn the sulfides with low catalytic activity. NiS thin films prepared using Sample B consisted of nano wheat like structure (Fig. 2b) with the presence of triethanol amine and thus concentrates a high surface area with uniform distribution. Whereas Samples C and D exhibited nano platelets like morphology (Fig. 2(c) and (d), respectively). However, a complete changeover of morphology from nanoparticles to nano-platelets like structure was observed upon adding more concentration of TEA with urea as shown in Fig. 2(b) and (c). Moreover, TAA from 0.4 to 0.8 M causes a change in surface morphology due to which the electrocatalytic activity changes and there could be an effect on the photovoltaic performance.
peaks in NiS (ICDD file no. 75-0613). However, the XRD pattern of NiS films could not exhibit any characteristic peaks, which might 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 is reported that the wet deposited composite films are amorphous and/or nano crystalline. The atomic percentage of Ni:S in Samples A, B, C, and D is found to be (45:50), (51:46), (46:51), and (50:50), respectively. The EDX result confirms that the formation of Ni is faster than the S source for Sample B.
3.2. Optical properties The optical properties of the counter electrode give an important information to the researchers DSSCs. Fig. 3 shows the UV absorption spectrum of the NiS thin films on an FTO substrate, which exhibits the exponentially declining behavior of absorbance from 300 nm to about 560 nm subsequently improvement in absorbance from about 560 nm onwards. Such absorption performance 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. Although a clear absorption peak in visible light region could not be observed, the presence of broad lump at around 400–500 nm could be realized. In contrast, an exponentially continuous decreasing behavior of absorbance from UV to near infrared region has also been reported for remaining samples. Such contrast absorption behaviors of various reported NiS thin films could be due to different morphology and crystal structures. Fig. 4 shows the X-ray diffraction pattern of NiS deposited films on FTO. The diffraction peaks appeared at 2u = 30.15 , 34.5 , 45.10 , 53.49 , 61.01, 64.74 , and 73.1 matches the observed diffraction
Fig. 3. UV of NiS thin films deposited at (a) Sample A, (b) Sample B, (c) Sample C, and (d) Sample D.
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D. Punnoose et al. / Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46 Table 2 Photovoltaic performance of DSSC fabricated with different CEs and electrochemical parameters from EIS.
Fig. 4. X-ray diffraction pattern of NiS thin films deposited (a) Sample A, (b) Sample B, (c) Sample C, and (d) Sample D.
3.3. Performance of NiS CE in DSSCs Fig. 5 shows the J–V curves of the TiO2/dye/NiS. Table 2 represents the the performance of DSSCs in terms of Voc,Jsc, fill factor (FF, and efficiency (h). In this illustration a similar photo anode (TiO2/dye) was used for NiS as well as for the Pt CE. The outstanding conversion efficiency (h) of 5.69% was obtained for the DSSC using Sample B CE. All compositions of CEs showed almost analogous Voc (open circuit voltage) in the range of 0.6–0.7 V and FF (fill factor) in the range 0.70–0.74 paralleled to Pt DSSCs (Voc = 0.66 V and FF = 0.66) respectively, which is credited to highest electro-catalytic activity towards I3 reduction. The conversion efficiency and FF of the Pt CE was 4.55% and 0.66, which was ascribed to lowest electro-catalytic activity towards I3 reduction. The Jsc (short circuit current) was found to be much higher for NiS CE than Pt CE. The JSC relies on the charge transfer resistance at the CE/electrolyte interface, light harvesting efficiency, electron and hole injection and charge collection efficiency [11,12]. Whereas it showed a lower Voc value of 0.63 than the Pt based cell (0.66), the highest h (5.69%) was observed, which could be due to its highest FF (0.72) than the Pt-based DSSC (0.66). The Sample B CE exhibited high Jsc of 12.68 mA cm2, open circuit voltage (Voc) of 0.63 V, fill factor (FF) of 0.72, leading to the high
Fig. 5. Current density voltage characteristics of DSSCs based on NiS and Pt CE.
Parameters
Sample A
Sample B
Sample C
Sample D
Sample E
Voc (V) Jsc (mA cm2) FF h (%) Rs (V) RCT (V) RCE (V) Cm (mF) Zw (V)
0.70 7.58 0.74 3.97 11.85 3.64 24.71 239 1.7
0.63 12.68 0.72 5.69 7.93 5.22 12.49 118 0.03
0.64 8.11 0.74 3.89 11.79 11.62 24.23 270 2.29
0.63 9.62 0.73 4.42 10.73 7.15 13.82 361 0.60
0.66 10.39 0.66 4.55 10.95 5.46 11.72 273 0.78
conversion efficiency. The pragmatic higher Jsc and FF in NiS CEs are attributed to considerable increase in charge transfer at the CE/electrolyte interface that reduces the internal resistances, concentration inclines in the electrolyte, and improves the electrocatalytic activity [13]. The enhancement of efficiency of Sample B CE may be due to the change of morphological structure and also due to the fact that at 90 C urea and TEA plays a key role to form the metal sulfide complex on FTO substrate. The above results show that NiS CE will assist as an efficient CE in DSSCs using I/I3 electrolyte. In the interim, the DSSC using Samples A, C, and D CEs revealed equitable photovoltaic properties where the conversion efficiencies are 3.97%, 3.89%, and 4.42%, respectively. The decrease in the FF can be endorsed to the higher charge transfer resistance (RCE) and sheet resistance (Rs) than Sample B [14,15]. Therefore, the use of the NiS CE ensures effective charge transfer at the CE/electrolyte interface, and enhances the electrocatalytic activity in the DSSC, which ultimately escalates the conversion efficiency. The EIS of a DSSC using NiS CEs (Samples A–D) are shown in Fig. 6 and the results are compared with Pt CE. The inset in Fig. 6 shows an equivalent circuit model used for the DSSCs. EIS was used to measure the charge transfer resistance (RCE) at the CE/electrolyte interface in the high frequency region, which was correlated with the electro-catalytic activity for I3 reduction, RCT at the interface of the photo-electrode/dye/electrolyte in the intermediate frequency region, and the Warburg diffusion resistance (Zw) of I/I3 in the electrolyte at the low frequency region, Zw is merged with RCT [16]. The RCE, RCT, and Zw were attained straightly from the impedance analysis software called Z-VIEW. Table 2 lists the measured EIS parameters. In relation to the equivalent circuit, the series resistance (RS) denotes the
Fig. 6. Nyquist plot for symmetric cells of NiS and Pt CE; inset shows the magnified plot of Pt at higher impedance range and equivalent circuit.
D. Punnoose et al. / Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46
high-frequency nonzero intercept of the real axis, the left semicircle at high-frequency region which denotes the parallel combination of charge transfer resistance RCT at the CE/electrolyte interface and the constant phase element (CPE) of electrical double layer, the small semicircle (low frequency region) at the end was assigned to the Warburg impedance (Zw) of the redox couple in the electrolyte [17]. The RCT values of Samples A, B, C, D, and E (Pt) CE are found to be 3.64, 5.22, 11.62, 7.15, and 5.46 V, respectively. The power conversion efficiency also relies on the charge transfer resistance at the CE/electrolyte interface. The low charge transfer resistance of 5.22 V was obtained for Sample B CE which is lower than that of the Sample E (Pt) CE (5.46 V). The lowest value of RCT indicates superior electrocatalytic activity of NiS CE compared to Sample E (Pt), suggesting an acceleration of the high electron transfer process at the interface of the CE/electrolyte [18]. The Zw values of Samples A, B, C, D, and E (Pt) CEs were found to be 1.7, 0.03, 2.29, 0.60, and 0.78 V, respectively. The lower value of Zw shows better electrolyte diffusion, facilitates faster mass transport of electron and improves the performance of the DSSC by increasing the fill factors [19]. The CPE value of Samples A, B, C, D, and E (Pt) CE were observed to be 268, 341, 132, 336, and 2.99 F, respectively. The NiS electrodes exhibited larger CPE values than that of the Pt electrode, which is a crucial factor for the high electro-catalytic activities [20]. The higher value for CPE corresponds to higher surface area, leading to better electro-catalytic activity [21]. This shows that NiS CEs have great potential for the fabrication of highly efficient DSSCs. Tafel polarization was performed to investigate the electrocatalytic activity of CEs. Fig. 7 shows the Tafel polarization curves of the symmetric dummy cells measured under dark conditions. The Tafel polarization curves showed a logarithmic current density (J0) as a function of the voltage (V). The electro-catalytic activity of the CE is related directly to the exchange current density (J0). The exchange current density (J0) was obtained from the intercept value by extrapolating the linear region of the curve at zero over potential. Since the slope and the logarithmic current densities on cathodic and anodic branches for the Pt CE are lower than the NiS CEs, it can be suggested that the exchange current density (J0) for the Pt CE is lower than that for the NiS CEs in terms of the Tafel equation [22] J0 ¼
RT nFRCT
(1)
where RCT is the charge transfer resistance at the CE/electrolyte interface obtained from EIS spectra, n is the number of electrons involved in the I32 reduction at the counter electrode, T is
Fig. 7. Tafel curve for symmetric cells based on NiS and Pt CE.
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temperature, R is the gas constant, and F is the Faraday constant. J0 entirely depends on the RCT value and decrease with the increase in RCT. The larger anodic or cathodic slope suggests a greater exchange current density (J0) on the electrode. The Tafel polarization curve of Sample B exhibited a larger exchange current density (J0) than the Pt electrode, suggesting that Sample B CE has superior electrocatalytic activity for I32 reduction. A steep slope obtained from the electrodes indicates the presence of large exchange current density (J0) on the electrode surface. 4. Conclusion We have exploited nano wheat structured NiS counter electrode in the fabrication of DSSCs. An active NiS CE could be formed by taking advantage of the various conditions. Impressively, the appropriate composition of materials in thin films on the NiS CE had a mammoth impact on the reduction of electrolyte and thereby boosting up the overall efficiency. In comparison with the Pt CE, the NiS CE at Sample B with nano wheat structure yields a low charge transfer resistance towards the I/I3 electrolyte and exhibited an outstanding electro-catalytic activity and power conversion efficiency of 5.69%. The imposing optical and electronic features of NiS counter electrode can inflate the comprehensive performance of DSSCs to the next level. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0014437). References [1] B. O’Regan, M. Gratzel, A low-cost, high efficiency solar cell based on dye sensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cell, Chem. Rev. 110 (2010) 6595–6663. [3] N. Papageorgiou, W.F. Maier, M. Gratzel, An iodine/triiodide reduction electro catalyst for aqueous and organic media, J. Electrochem. Soc. 144 (1997) 876–884. [4] A.L. Viet, R. Jose, M.V. Reddy, B.V.R. Chowdari, S. Ramakrishna, Nb2O5 photoelectrodes for dye-sensitized solar cells: choice of the polymorph, J. Phys. Chem. C 114 (2010) 21795–21800. [5] W. Mingxing, M. Tingli, Platinum-free catalysts as counter electrodes in dye-sensitized solar cells, ChemSusChem 5 (2012) 1343–1357. [6] T. Zhu, Z.Y. Wang, S.J. Ding, J.S. Chen, X.W. Lou, Hierarchical nickel sulfide hollow spheres for high performance supercapacitors, RSC Adv. 1 (2011) 397–400. [7] Z. Li, F. Gong, G. Zhou, Z.S. Wang, NiS2/reduced graphene oxide nanocomposites for efficient dye-sensitized solar cells, J. Phys. Chem. C 117 (2013) 6561–6567. [8] S.M. Pawar, B.S. Pawar, J.H. Kim, O.S. Joo, C.D. Lokhande, Recent status of chemical bath deposited metal chalcogenide and metal oxide thin films, Curr. Appl. Phys. 11 (2011) 117–254. [9] A. Hauch, A. Georg, Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells, Electrochim. Acta 46 (2001) 3457–3473. [10] M. Tachan, Shalom, I. Hod, S. Ruhle, S. Tirosh, A. Zaban, PbS as a highly catalytic counter electrode for polysulfide-based quantum dot solar cells, J. Phys. Chem. C 115 (2011) 6162–6166. [11] A.D. Savariraj, K.K. Viswanathan, K. Prabakar, Influence of Cu vacancy on knit coir mat structured CuS as counter electrode for quantum dot sensitized solar cells, Appl. Mater. Interface 22 (19) (2014) 702–19709. [12] D.B. Mitzi, Templating and structural engineering in organic–inorganic perovskites, J. Chem. Soc. Dalton Trans. (2001) 1–12. [13] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Use of highlyordered TiO2 nanotube arrays in dye-sensitized solar cells, Nano Lett. 6 (2006) 215–218. [14] Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, I/I3 redox reaction behavior on poly(3,4-ethylenedioxythiophene) counter electrode in dye sensitized solar cells, J. Photochem. Photobiol. A 164 (2004) 153–157. [15] F.F. Santiago, J. Bisquert, E. Palomares, L. Otero, D. Kuang, S.M. Zakeeruddin, M. Gratzel, Correlation between photovoltaic performance and impedance
46
[16]
[17]
[18]
[19]
D. Punnoose et al. / Journal of Photochemistry and Photobiology A: Chemistry 306 (2015) 41–46 spectroscopy of dye-sensitized solar cells based on ionic liquids, J. Phys. Chem. C 111 (2007) 6550–6560. S.Q. Fan, B. Fang, J.H. Kim, B. Jeong, C. Kim, J.S. Yu, J. Ko, Ordered multimodal porous carbon as highly efficient counter electrodes in dye-sensitized and quantum dot solar cells, Langmuir (2010) 13644–13649 Article 26. E. Ramasamy, W.J. Lee, D.Y. Lee, J.S. Song, Spray coated multi-wall carbon nanotube counter electrode for tri-iodide I3 reduction in dye-sensitized solar cells, Electrochem. Commun. 10 (2008) 1087–1089. M. Wu, Y. Wang, X. Lin, N. Yu, L. Wang, L. Wang, A. Hagfeldt, T. Ma, Economical and effective sulfide catalysts for dye-sensitized solar cells as counter electrodes, Phys. Chem. Chem. Phys. 13 (2011) 19298–19301. M.H. Kumar, B.S. Kumar, M. Rao, R.R. Prabhakar, N. Mathews, Y.M. Lam, S.G. Mhaisalkar, Solution processed transition metal sulfides: application as
counter electrodes in dye sensitized solar cells (DSCs), Phys. Chem. Chem. Phys. 13 (2011) 19307–19309. [20] H. Chen, L. Zhu, H. Liu, W. Li, ITO porous film supported metal sulfide counter electrodes for high-performance quantum dot sensitized solar cells, J. Phys. Chem. C 117 (2013) 3739–3746. [21] I. Mora-Sero, S. Gimenez, F. Fabregat-Santiago, R. Gomez, Q. Shen, T. Toyoda, J. Bisquert, Recombination in quantum dots sensitized solar cells, Acc. Chem. Res. 42 (2009) 1848–1857. [22] T. Shu, P. Xiang, Z.M. Zhou, H. Wang, G.H. Liu, H.W. Han, Y.D. Zhao, Meso-scopic nitrogen-doped TiO2 spheres for quantum dot-sensitized solar cells, Electrochim. Acta 68 (2012) 166–171.