稀有金属(英文版) 2019,38(04),316-320
Novel dye-sensitized solar cell architecture using TiO2-coated Ag nanowires array as photoanode
Ming Wang Sam Zhang Zhao-Fu Du Li-Dong Sun Dong-Liang Zhao
Department of Functional Material Research, Central Iron and Steel Research Institute
School of Mechanical and Aerospace Engineering, Nanyang Technological University
作者简介:*Zhao-Fu Du,e-mail:duzhf@hotmail.com;
收稿日期:17 December 2013
基金:financially supported by the National Natural Science Foundation of China(No.51301041);
Novel dye-sensitized solar cell architecture using TiO2-coated Ag nanowires array as photoanode
Ming Wang Sam Zhang Zhao-Fu Du Li-Dong Sun Dong-Liang Zhao
Department of Functional Material Research, Central Iron and Steel Research Institute
School of Mechanical and Aerospace Engineering, Nanyang Technological University
Abstract:
With the aim of reducing series resistance and increasing dye loading, novel dye-sensitized solar cell architecture was designed with TiO2 nanoparticle-coated Ag nanowires array as the photoanode. Ag nanowire array was prepared by anodic aluminum oxide(AAO) templateassisted electrochemical deposition route. Then, Ag nanowires were coated by TiO2 nanoparticles in hydrothermal process. The structures of the photoanode were characterized by field emission scanning electron microscopy(FESEM). Ag nanowires are covered by a layer of very fine nanoparticles with a diameter of less than 5 nm. X-ray diffraction(XRD) and selected-area electron diffraction(SAED) show that Ag nanowires have a strong preferred orientation in(220) direction and the TiO2 coating layer is a polycrystalline structure. With this photoanode, 3.2%conversion efficiency is achieved for the cell sensitized with N3 dye.
Keyword:
Dye-sensitized solar cell; Nanowires array; Photoanode; Anodic aluminum oxide;
Received: 17 December 2013
1 Introduction
Dye-sensitized solar cell (DSSC) is thought as a potential and hopeful thin film solar cell because of its low cost.Since it was first reported by O'Regan and Gratzel
[
1]
,DSSC has been intensively studied during the past two decades
[
2,
3,
4]
.In general,DSSCs comprised photoanode(the most frequently used material is anatase TiO2) decorated by the monolayer of dye molecules (sensitizers),the platinized counter electrode,and the electrolyte solution with a dissolved iodide ion/triiodide ion redox couple between the electrodes.According to the architectures of the photoanodes,most frequently studied DSSCs can be pided into two types:nanoparticle based (the most efficient is TiO2 nanoparticles)
[
1,
5,
6]
and one-dimensional based (such as TiO2 nanotube and ZnO nanorod)
[
7,
8,
9]
.In TiO2 nanoparticle-based DSSC,trapping and detrapping events of electrons frequently happen at the boundaries of the nanoparticles during the transport process.These events not only slow down the mobility of the electrons but also increase the recombination between electrons and triiodide ions in the electrolyte.As such,further improvement in nanoparticle-based DSSC is difficult.One-dimensional nanostructured photoanodes are designed to overcome those shortcomings.Transporting of electrons through nanowires or nanotubes largely reduces the probability of trapping and detrapping,and hence improves the photoanode conductivity.However,the low surface area of the one-dimensional structure results in low dye loading,which effectively hinders efficiency.Lengthening of the nanotubes or nanowires helps but only to a certain extent,as there is a length limitation for the low-conducting semiconductors.Jennings et al.
[
10]
calculated that the electron diffusion length (depending on the diffusion coefficient and lifetime of the photoinjected electron) in titania nanotube was about 100 (μm.While in experiments,the optimum lengths for TiO2 nanotubes are even low(16-25 (μm)
[
7,
11,
12]
.
This paper introduces a new type of photoanode architecture to increase electrical conductivity and reduce the recombination between electrons and triiodide ions in the electrolyte:Ag nano wire array coated with TiO2 nanoparticles (Fig.1).TiO2 nanoparticles serve to assist in generation of photoelectrons as well as in the conventional photoanode.Ag nano wires act as the“highway”to transport electrons from the conduction band of the TiO2 to the electrode.In conventional nanoparticle-based DS SCs,TiO2nanoparticles are spread on a flat fluorine doped tin oxide(FTO) or TiO2 sheet,which resulted in the overlap of particles and low conductivity.This new architecture can be thought as the extending of the anode in nanosize with the purpose of increasing conductivity.The Ag nano wire array was fabricated by template-assisted electrochemical deposition.TiO2 nanoparticles were coated onto the nanowires by hydrothermal process.The as-prepared photoanode was assembled into a dye-sensitized solar cell for the characterization of conversion efficiency.This initial study demonstrates the effectiveness of the new architecture.By increasing the length of the Ag nanowires in further study,the amount of the dye loading can be increased.
2 Experimental
Commercially available anodic aluminum oxide (AAO)template in this study had a pore diameter of 60 nm,inter space of 20 nm,and a thickness of 60μm.Before electrochemical deposition,a thin layer of Ag was deposited on one side of the template by magnetic sputtering.The AAO template was then set as the cathode and a platinum foil as the anode,leaving a 10-mm-diameter area of the template exposed to the electrolyte.The distance between the anode and the cathode was 10 mm.The electrolyte contained3.4 g silver nitrate (AgNO3),23.5 g trisodium citrate dihydrate (C6H5Na3O7·2H2O),8.5 g potassium sodium tartrate tetrahydrate (C4H4O6KNa·4H2O),12.6 g sodium sulfite (Na2SO3),16.7 g boric acid (H3BO3),and 250 ml deionized water
[
13]
.Deposition was carried out in galvanostatic mode.The current density was set as 2 mA·cm-2.Time duration was set from 2 to 8 h.After the deposition,a copper sheet of 2 mm in thickness was adhered to the Ag-sputtered side of the template with silver colloid.The copper sheet worked as a support layer of the thin film and also the electrode of the cell.Etching of the AAO template was done in agitated NaOH solution (2 mol·L-1) via magnetic stirring for 2-10 min at 40℃,followed by rinsing in 3.7%HCl solution and deionized water.An area of5 mm in diameter was exposed to the etchant.The hydrothermal treatment was frequently described in DSSC research articles to increase the surface area of the photoanode
[
14]
.The etched sample was immersed in 15 ml0.1 mol·L-1 TiCl4 solution in a 20 ml Teflon-lined stainless-steel autoclave for coating TiO2
[
15]
.The autoclave was sealed and heated at 90℃for 40 min.The as-prepared sample was immersed into 2×10-4 mol·L-1 N3-type dye solution for 24 h at room temperature,and then carefully sealed into a back-side-type cell similar to the conventional DSSCs.The electrolyte used was 0.50 mol·L-1 LiI,0.05 mol·L-1 I2,and 0.5 mol·L-1 tert-butyl pyridine in methoxypropionitrile.Effective area of the cell was0.196 cm2 (5 mm in diameter).The whole experimental procedure is briefly shown in Fig.2.
The morphologies of the etched and coated samples were characterized by field emission scanning electron microscopy (FESEM,Hitachi S-4800).The X-ray diffraction (XRD) measurements were performed with a Bruker D8 advance diffractometer with Cu Kαradiation.Transmission electron microscopy (TEM) was used to examine the coating structures (JEOL JEM 21 00F).The crystal structure of Ag nanowire and the TiO2 coating layer were further confirmed by selected-area electron diffraction(SAED).The photocurrent-voltage characteristic of the cell was measured with I-V test model SOLARIV 100under AM 1.5 (100 mW·cm2) irradiating condition.
3 Results and discussion
3.1 Morphologies
Figure 3 a shows free-standing Ag nano wire array prepared by electrochemical deposition combined with etching process.At the deposition current density of 2 mA·cm2,electrochemical treatment for 6 h was enough to nearly fill the pores of the template (60μm in thickness) with Ag deposition.Too long deposition time results in the formation of Ag dots on the template (seen as local metallic luster).Some unbalanced growths of Ag nano wires are from place to place,as seen from Fig.3a.This is likely inherited from the surface unevenness of the AAO template.Etching is very sensitive to the temperature,concentration of the NaOH solution,and even the speed of the magnetic stirring.Etching rate of AAO template is not a constant.After first several minutes,etching accelerates until A AO template is totally dissolved after about 9 min.This might be due to the increase of surface area of etched sample exposed to the etching solution.Magnetic stirring is necessary in the etching process;otherwise,etching is inhomogeneous in the whole sample.Figure 3a shows the partially etched result of etching carried out in 3 mol·L-1 NaOH for 3 min at 40°C(the remaining part can serve as the support for the freestanding Ag nano wires).The stirring speed was 10 r·s-1.The average length of the Ag nanowire out of the AAO template is about 8μm (the bottom inset in Fig.3b).The diameter of Ag nanowires is about 60 nm,the same as the pore diameter of AAO template.Figure 3b shows the top view of TiO2-coated Ag nano wire array.The top inset in Fig.3b shows a magnification of the coating morphology.The coating process makes the nano wires bundle together due to the bonding effect of TiO2 nanoparticles,which might reduce the surface area of the photoanode.From the comparison of Fig.3c and d,obvious difference can be seen in the surface morphologies.After coating,the surface of Ag nano wires is covered by a layer of very fine nanoparticles.Figure 3d shows that the thickness of the coating layer ranges from 5 to 20 nm.The crystal size of TiO2 nanoparticles is less than 5 nm (inset in Fig.3d).
Fig.1 Schematic of dye-sensitized cell with TiO2 nanoparticle-coated Ag nanowire array as photoanode
Fig.2 Schematic of experimental procedure
Fig.3 FESEM images of Ag nanowire array before coating a and c,and after coating b and d.Top and bottom insets in b being magnification and demagnification of Ag nano wire coated with TiO2,respectively.Inset in c being top view of nano wire array.Inset in d being TEM image of a single Ag nano wire coated with TiO2 prepared by ultrasonic dispersion
Fig.4 XRD pattern of Ag nanowire array without coating.Inset being SAED pattern of Ag nano wire
Fig.5 SAED pattern of TiO2 coating layers
Fig.6 I-V curve of cell with as-prepared sample as photoanode with N3 dye as sensitizer
3.2 Structures
Figure 4 shows the XRD pattern of the etched sample without coating.The Ag nano wires display three peaks in(111),(200),and (220) directions and show a strong preferred orientation in (220) direction,which is the same as the results of Sun et al.
[
13]
.The inset of Fig.4 is the SAED pattern of a single Ag nano wire taken from (220)direction.It seems that the area selected in Ag nano wire is a single crystal structure.Besides that,unconspicuous bump between 20°and 40°,which may be introduced by amorphous AAO residual,can be observed.
XRD was also used to measure the coated samples.However,TiO2 does not show any peaks.This may be due to the poor crystallization of TiO2 and the very strong peak of Ag (220) peak,making it difficult to obtain sharp peaks from XRD pattern.SAED was then carried out to test the structure of TiO2.The SAED pattern shows that the TiO2 coating layer is a poly crystalline structure (Fig.5).It is calculated that the phase of Ti02 layer is anatase.
3.3 Efficiency
The as-prepared sample was then sensitized with N3 dye and then carefully sealed into a back-side-type cell.Effective area of the cell is 0.785 cm2 (5 mm in diameter).The photocurrent-voltage characteristic of the cell was measured with an I-V test model irradiating condition.The short-current of the cell is measured as 8.7 mA·cm2.The open-voltage of the cell is measured as 0.65 V.Fill factor of the I-V curve is 0.55 and the efficiency of the cell is calculated as 3.2%(Fig.6).Although the efficiency is still low,the initial study proves that this new architecture is effective.Improvement of the cell sealing and studies on the influence of the nano wire length on the cells,efficiency are now undergoing.
4 Conclusion
Ag nano wire array coated with TiO2 nanoparticles was fabricated as a new architecture of DSSC photoanode.The nanowires prepared by electrochemical deposition show a high orientation in (220) direction.TiO2 particles coated on the Ag nanowires are anatase.The preliminary efficiency of the cell from this architecture is 3.2%.This initial study demonstrates the effectiveness of the new architecture.
Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No.51301041).
参考文献
[1] O'Regan B, Gratzel M. A low-cost high efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature. 1991;353(6346):737.
[2] Gr?tzel M. Photoelectrochemical cells. Nature. 2001;414(6861):338.
[3] Chiba Y, Islam A, Watanabe Y, Komiya R, Koide N, Han L.Dye-sensitized solar cells with conversion efficiency of 11.1%.Jpn J Appl Phys. 2006;45(25):L638.
[4] Hardin BE, Snaith HJ, McGehee MD. The renaissance of dyesensitized solar cells. Nat Photonics. 2012;6(3):162.
[5] Senthil TS, Muthukumarasamy N, Velauthapillai D, Agilan S,Thambidurai M, Balasundaraprabhu R. Natural dye(cyanidin3-O-glucoside)sensitized nanocrystalline TiO2 solar cell fabricated using liquid electrolyte/quasi-solid-state polymer electrolyte. Renewable Energy. 2011;36(9):2484.
[6] Guo GQ. Preparation and characterization of high tap density silver powder for solar cell. Chin J Rare Metals. 2013;37(6):922.
[7] Roy P, Kim D, Lee K, Spiecker E, Schmuki P. TiO_2 nanotubes and their application in dye-sensitized solar cells. Nanoscale.2010;2(1):45.
[8] Hossain MF, Biswas S, Zhang ZH, Takahashi T. Bubble-like CdSe nanoclusters sensitized TiO_2 nanotube arrays for improvement in solar cell. J Photochem Photobiol A. 2011;217(1):68.
[9] Salazar R, Delamoreanu A, Levy-Clement C, Ivanova V. ZnO/CdTe core-shell nanowire arrays for extremely thin absorber solar cells. Energy Procedia. 2011;10(1):122.
[10] Jennings JR, Ghicov A, Peter LM, Schmuki P, Walker AB. Dyesensitized solar cells based on oriented TiO2 nanotube arrays:transport, trapping, and transfer of electrons. J Am Chem Soc.2008;130(40):13364.
[11] Zhu K, Neale NR, Miedaner A, Frank AJ. Enhanced chargecollection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO_2 nanotubes arrays. Nano Lett.2007;7(1):69.
[12] Zhu K, Vinzant TB, Neale NR, Frank AJ. Removing structural disorder from oriented TiO2 nanotube arrays:reducing the dimensionality of transport and recombination in dye-sensitized solar cells. Nano Lett. 2007;7(12):3739.
[13] Sun BL, Jiang XH, Dai SX, Du ZL. Single-crystal silver nanowires:preparation and surface-enhanced Raman scattering(SERS)property. Mater Lett. 2009;63(29):2570.
[14] Charoensirithavorn P, Ogomi Y, Sagawa T, Hayase S, Yoshikawa S. Improvement of dye-sensitized solar cell through TiCl_4-treated TiO2 nanotube arrays. J Electrochem Soc. 2010;157(3):B354.
[15] Yan CH, Xie ZY, Wang ZF, Zhang ZJ, Wu YY, Zhang M.Preparation and ER performance of hard-shell composite TiO_2/PS microspheres. Chin J Rare Met. 2013;37(4):650.