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Broadband and Low-Loss Plasmonic Light Trapping in DyeSensitized Solar Cells Using Micrometer-Scale Rodlike and Spherical Core−Shell Plasmonic Particles Mahdi Malekshahi Byranvand,*,† Ali Nemati Kharat,† Nima Taghavinia,‡,§ and Ali Dabirian*,‡,∥ †
School of Chemistry, University College of Science, University of Tehran, Tehran 1417466191, Iran Department of Physics, Sharif University of Technology, Tehran 14588, Iran § Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588, Iran ∥ Institute of MicroEngineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Rue de la Manadière 71, Neuchâtel 2002, Switzerland ‡
S Supporting Information *
ABSTRACT: Dielectric scattering particles have widely been used as embedded scattering elements in dye-sensitized solar cells (DSCs) to improve the optical absorption of the device. Here we systematically study rodlike and spherical core− shell silica@Ag particles as more effective alternatives to the dielectric scattering particles. The wavelength-scale silica@Ag particles with sufficiently thin Ag shell support hybrid plasmonic−photonic resonance modes that have low parasitic absorption losses and a broadband optical response. Both of these features lead to their successful deployment in light trapping in high-efficiency DSCs. Optimized rodlike silica@Ag@silica particles improve the power conversion efficiency of a DSC from 6.33 to 8.91%. The dimension, surface morphology, and concentration of these particles are optimized to achieve maximal efficiency enhancement. The rodlike silica particles are prepared in a simple one-pot synthesis process and then are coated with Ag in a liquid-phase deposition process by reducing an Ag salt. The aspect ratio of silica rods is tuned by adjusting the temperature and duration of the growth process, whereas the morphology of Ag shell is tailored by controlling the reduction rate of Ag salt, where slower reduction in a polyol process gives a smoother Ag shell. Using optical calculations, the superior performance of the plasmonic core−shell particles is related to the large number of hybrid photonic−plasmonic resonance modes that they support. KEYWORDS: thin-film photovoltaics, dye-sensitized solar cells, plasmonic, polyol synthesis, light trapping
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INTRODUCTION
Dielectric particles have a major drawback of their scattering efficiency being strongly influenced by the surrounding refractive index.20 In fact, the scattering efficiency of these particles decreases as the refractive index contrast of the particle and the surrounding becomes smaller. Plasmonic particles do not have this issue due to the negative permittivity of the metal; therefore, their scattering efficiencies are known to be larger and less sensitive to the surrounding refractive index.21,22 However, optical dissipation in the metal has so far hindered their successful application for light trapping in PV. It has been recently proposed that wavelength-scale plasmonic particles with large scattering to absorption efficiency ratios can address this issue.23,24 In this contribution, we present micrometer-scale rodlike and spherical silica@Ag particles as alternatives to the dielectric particles for application as embedded scattering elements in
Solution-processed thin film photovoltaics (PV) technologies such as dye-sensitized,1,2 quantum dot,3,4 and perovskite solar cells5,6 provide a low-cost alternative to crystalline silicon solar cells and have potential applications in building integrated PV (BIPV) and consumer electronic products.7 In thin film PV, improving the optical absorption is a necessity; therefore, lighttrapping solutions to improve optical absorption in PV devices is strongly pursued.8−12 In dye-sensitized solar cells (DSCs), scattering particles embedded in the sensitized layer are pursued as an easily applicable approach providing significant improvement in optical absorption in the cell.13,14 Compared to other light-trapping strategies, the embedded scattering particles approach has three advantages: (i) the device remains transparent, (ii) the fabrication process is simple because their addition to the paste does not require an extra step in cell fabrication process, and (iii) they do not lead to an increase in cell thickness, which is useful for mechanically flexible devices. Traditionally, dielectric scattering particles such as TiO2,15,16 silica,17,18 and voids19 have been used for this purpose. © XXXX American Chemical Society
Received: January 11, 2016 Accepted: June 2, 2016
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DOI: 10.1021/acsami.6b00348 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
for 7 h. The dark brown product was collected by centrifugation and then redispersed in ethanol and stored at 4 °C for further examination. For synthesis of silica@Ag composites using NaBH4 as reducing agent,28 0.001 g of as-prepared silica particles was dispersed in 80 mL of ethanol containing PVP (1%); then, 20 mL of freshly prepared [Ag(NH3)2]+ (0.1 M) solution was added quickly under magnetic stirring at 30 °C for 30 min. Afterward, colloids were washed with ethanol by centrifugation and ultrasonic dispersion to remove excess [Ag(NH3)2]+ ions, and subsequently they were dispersed in 80 mL of ethanol. Subsequently, 10 mL of NaBH4 solution (0.5%) was added quickly to chemically reduce the [Ag(NH3)2]+ ions adsorbed on silica particles surface; the white color of the solution turned to dark brown at once. After 30 min, the colloids were washed with distilled water to remove the excess NaBH4, dispersed in ethanol, and stored at 4 °C. Coating of Silica@Ag Particles with Silica. A thin silica shell was deposited onto the silica@Ag particles to protect them from corrosion and oxidation in the electrolyte18,29 using a modified sol−gel process.30 In a typical synthesis, 5 g of PVP was dissolved in 50 mL of water by sonication. Then, 0.001 g of the as-prepared silica@Ag particles was added to this solution. After stirring at 500 rpm for 1 h, the PVP-modified particles were collected by centrifugation and then redispersed into 50 mL of ammonia-ethanol solution (5 vol % NH4OH). In the next step, 2 mL of TEOS−ethanol solution (20 vol % TEOS) was added to the particle dispersion, followed by stirring at 500 rpm for 2 h. Finally, silica@Ag@silica (SAS) particles were collected by centrifugation and then redispersed in ethanol. Figure 1a,c shows the schematic of spherical and rodlike SAS particles, respectively.
solar cells. The optical properties of these particles are thoroughly optimized by adjusting the synthesis parameters of the rodlike silica particles. In addition, the surface morphology of the Ag shell is tuned by controlling the reduction rate of AgNO3 precursor used for Ag deposition. The particles are coated with a thin protective silica shell and are then mixed with a standard TiO2 paste with different concentrations to process DSCs. Subsequently, the concentration that delivers the largest power conversion efficiency is identified. The results are also compared to optimized cells with spherical and rodlike silica particles as scattering particles. These plasmonic particles have better performance relative to dielectric ones and are less sensitive to the surrounding refractive index. Finally, the mechanism of light interaction with rodlike silica@Ag particles is studied by modeling light interaction with these particles.
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MATERIALS AND METHODS
Materials. Tetraethyl orthosilicate (TEOS, 98%; Merck), absolute ethanol (C2H6O, 99%; Merck), deionized water (DI water, >18.2 MΩ), ammonium hydroxide (NH4OH, 25%; Merck), 1-pentanol (C 5 H12 O, ≥ 99%; Sigma-Aldrich), sodium citrate dihydrate (C6H9Na3O9, 99%; Aldrich), silver nitrate (AgNO3, 99%; Acros), polyvinylpyrrolidone (PVP, MW-40000; LOBA Chemie), sodium borohydride (NaBH4, 95%; Merck), titanium(IV) chloride (TiCl4, > 99%; Merck), standard transparent TiO2 paste (PST-20T, composed of 20 nm TiO2 nanoparticles; Sharif Solar), fluorine-doped tin oxide substrates (FTO; Dyesol), cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′dicarboxylato) ruthenium(II) bis(tetrabutylammonium) dye (N719; Dyesol), chloroplatinic acid (H2PtCl6, 99.95%; Merck), syrlyn sheets (60 μm; Dyesol), 1-butyl-3-methylimidazolium iodide (Aldrich), guanidinium thiocyanate (GSCN; Merck), iodine (I2; Merck), lithium iodide (LiI; Merck), 4-tertbutylpyridine (TBP; Merck), acetonitrile (Merck), and valeronitrile (Merck) were used without further purification. Synthesis of Spherical Silica Particles. Monodisperse spherical silica particles were synthesized using the standard Stöber method25 where 25 mL of DI water, 62 mL of ethanol, and 9 mL of NH4OH were mixed in a flask and stirred for 30 min until a homogeneous solution was formed. Then, 4.5 mL of TEOS was rapidly added, and the solution was stirred for 3 h at 20 °C. The resulting white colloidal suspension was filtered, washed with DI water and absolute ethanol 5 times, and then dried at 60 °C for 24 h. Synthesis of Rodlike Silica. Rodlike silica particles were synthesized using an one-pot method.26 In a closed 25 mL bottle, 1 g of PVP was dissolved in 10 mL of 1-pentanol by sonication for 2 h. When all PVP was dissolved, an aqueous solution of 1 mL of absolute ethanol, 0.28 mL of DI water, and 0.07 mL of sodium citrate dihydrate (0.18 M) was added. The flask was shaken by hand to mix the contents. Then, 0.23 mL of NH4OH was added, the flask was shaken again, and subsequently 0.1 mL of TEOS was added to the solution. After shaking again, the bottle was left to rest at a certain temperature (Tgrowth), and the reaction was allowed to proceed for a defined duration (tgrowth). We conducted experiments for tgrowth = 1.3, 2.4, and 5 h and for Tgrowth= 0, 20, and 50 °C. The resulting white colloidal solution was filtered, washed with DI water and absolute ethanol 5 times, and then dried at 60 °C for 24 h. Preparation of Silica@Ag Particles. Ag shell was deposited onto the silica particles by reducing [Ag(NH3)2]+ using either PVP at 70 °C or NaBH4 at 30 °C. [Ag(NH3)2]+ was prepared by dropwise addition of NH4OH to AgNO3. First, the solution turns dark brown, and we continued adding NH4OH until the solution became colorless. To synthesize silica@Ag composites using PVP as reducing agent (the polyol process),27 0.001 g of as-prepared silica particles was added to 20 mL of freshly prepared [Ag(NH3)2]+ solution (0.1 M) under magnetic stirring at 30 °C. After a few minutes, the dispersion was added into 100 mL of ethanol containing PVP (1%), in a 250 mL three-necked flask equipped with a reflux system and stirred at 70 °C
Figure 1. (a and c) Schematic of the cross section of spherical and rodlike SAS particles, respectively. (b and d) Configuration of a DSC with embedded spherical and rodlike SAS particles, respectively. DSC Device Processing. DSCs with embedded SAS particles (schematically shown in Figure 1b,d), either rodlike or spherical, were fabricated using the following steps. Different volumes percentages of colloidal solution of SAS particles in ethanol were mixed with standard TiO2 paste (PST-20T), dispersed by ultrasonic bath, and the entire solution was stirred for 3 h. Then, ethanol was removed by rotary evaporator to obtain a viscous paste. FTO substrates were immersed in 40 mM TiCl4 aqueous solution at 70 °C for 30 min and rinsed with DI water and ethanol. Photoanode films were prepared by screen-printing of the paste on TiCl4-treated FTO substrates. Then, these films were annealed in air in 3 steps: 325 B
DOI: 10.1021/acsami.6b00348 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces °C for 5 min, 375 °C for 5 min, and finally 450 °C for 45 min. Photoanode films were treated with 40 mM TiCl4 aqueous solution once more and then annealed at 500 °C for 30 min. After cooling down, the layers were immersed in N719 dye solution (0.2 mM) overnight and then rinsed using an acetonitrile solution. Counter electrodes (CEs) of 1 cm2 surface area were fabricated by coating FTO substrates with one drop of 5 mM H2PtCl6 ethanolic solution followed by annealing at 450 °C for 15 min. Dye-sensitized photoanodes were assembled with Pt CE (with a drilled hole) into sandwich-type cell by heating at 120 °C using a hotmelt film (Surlyn) as the spacer between the electrodes. A drop of electrolyte solution was placed on the hole in CE, and it was driven into the cell via vacuum backfilling. The electrolyte solution contains 1.0 M 1-butyl-3-methylimidazolium iodide, 0.03 M I2, 0.05 M LiI, 0.1 M GSCN, and 0.5 M TBP in acetonitrile and valeronitrile solvent mixture (85:15 volumetric ratio). Finally, the hole was sealed using additional Surlyn and a cover glass. Measurements. Scanning electron microscopy (SEM) was carried out using a Tescan (Czech Republic) Vega II XMU microscope. Transmission electron microscopy (TEM) observation was conducted on a Zeiss-EM10C electron microscope working at 80 kV. X-ray diffraction (XRD) was carried out using a Rigaku D/max-rB equipment in Bragg−Brentano configuration using Cu Kα1 radiation (λ = 1.54056 Å). Optical absorption spectra of silica@Ag colloidal solutions were measured using a highly diluted ethanolic suspension of the particles. A 50 μL aliquot of silica@Ag colloidal solution (5000 ppm) was diluted in 4 mL of absolute ethanol. The measurements were carried out using a Hitachi (Japan) U300 UV−visible photospectrometer. Diffuse reflection, transmittance, and absorptance measurements were carried out using an Avantes photospectrometer (Avaspec-2048TEC) equipped with an integrating sphere. Current− voltage plots were recorded using a Palmsens potentiostat under simulated AM1.5G light (Sharif Solar). Incident photon to current conversion efficiency (IPCE), measured using a setup consisting of a Jarrel-Ash monochromator, a 100 W halogen lamp and a calibrated photodiode (Thorlabs).
confirmed by cryogenic TEM measurements that at this stage an emulsion of water particles in 1-pentanol is formed, which is stabilized by PVP and sodium citrate.26 In step two, a solution of NH4OH and TEOS is added where at first TEOS hydrolyses into silica by reacting with water droplets. Since the hydrolyzed TEOS is hydrophilic, the growing silica nucleus is positioned at the surface of the emulsion droplet. Therefore, the growth is similar to vapor−liquid−solid (VLS) growth where in gasphase nanowires are grown from a metal droplet.34 Growth proceeds from one end of the emulsion; therefore, the emulsion droplet is attached to one end of the rod. When the growth process is terminated, this droplet is washed away, and the resulting rod is flat at one end. Therefore, the rods have a bullet shape, i.e., flat at one end and round at the other end. Basically, the prolongation of the rod takes place at the flat end in a quasi-VLS process.35,36 We set out our study by adjusting the growth time and temperature. In each experiment, we used a similar amount of materials as the original recipe26 in which the amount of the TEOS is expected to be entirely consumed in 17 h if the growth takes place at 20 °C. As a first step of our experimental study, we carried out three experiments at 20 °C with 1.3, 2.4, and 5 h growth times, i.e., the duration after addition of TEOS to the emulsion. Figure 2a shows SEM images obtained when the growth process is quenched after 1.3 h. We observed small bulletshaped particles along with large entities that appear to be agglomerates of the bullet-shaped particles. This shows that to obtain a dispersion of the rodlike particles a minimum growth time is required.37 After 2.4 h, bullet-shaped particles of nearly 300 nm diameter and 500 nm length are formed. As the growth proceeds, diameter does not vary; however, the rods’ length becomes 1700 nm. The next series of experiments were carried out for 5 h but at three different temperatures of 0, 20, and 50 °C (Figure 2b). Spherical particles of 300 nm are formed at 0 °C after 5 h growth, whereas rodlike particles formed at 20 and 50 °C. The rods’ diameter does not vary significantly with growth temperature; however, the rods’ length increases with synthesis temperature. This is due to the faster TEOS hydrolysis reaction at higher temperature following Arrhenius law. Therefore, synthesis temperature does not influence the diameter of the particles. Silica@Ag Particles. The rodlike and spherical silica particles are used as the template for Ag nanoshell coating. Spherical silica particles are synthesized in a standard Stöber process and SEM images of the particles are shown in Figure S1. Ag coating was carried out by reduction of [Ag(NH3)2]+ using either PVP at 70 °C or NaBH4 at 30 °C. PVP, a polyol, is a reducing agent at 70 °C; however, it reduces [Ag(NH3)2]+ to Ag at a significantly slower rate relative to that of NaBH4.27,38 SEM images of silica@Ag particles obtained by reduction of [Ag(NH3)2]+ ions using PVP (Figure 3a,c) show that these particles have a smoother surface relative to those obtained using NaBH4 as the reducing agent (Figure 3b,d). Ag shell obtained using NaBH4 comprises a large number of small and pointy grains, whereas in the polyol process using PVP, an Ag shells with rather large grains is formed. The formation of Ag shells as well as the influence of reducing agent on grain size is studied by TEM and XRD, shown in Figures S2 and S3. The Ag shell thickness is estimated to be 15 nm from TEM and SEM observations.
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RESULTS AND DISCUSSION Rodlike Silica Particles. Figure 2 shows SEM images of rodlike silica particles synthesized by adjusting synthesis parameters of the process originally introduced by Kuijk et al.26,31−33 This synthesis involves the following two steps. In step one, a solution containing water, 1-pentanol, PVP, sodium citrate dehydrate, and ethanol is prepared. Kuijk et al. have
Figure 2. SEM images of rodlike silica particles synthesized (a) at 20 °C for 1.3, 2.4, and 5 h and (b) at 0, 20, and 50 °C for 5 h. C
DOI: 10.1021/acsami.6b00348 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. SEM images of rodlike and spherical silica@Ag particles obtained by reducing [Ag(NH3)2]+ ions to Ag using (a and c) PVP (at 70 °C) and (b and d) NaBH4 (at 30 °C) as reducing agents. Normalized optical absorption spectra of rodlike silica particle (1.5−1.7 μm) and 470 nm silica spheres coated with nearly 15 nm Ag shell by either (e and g) PVP at 70 °C or (f and h) NaBH4 as reducing agent of [Ag(NH3)2]+ to Ag reaction compared against the normalized absorption spectra of 8 nm Ag nanoparticles. The colored labels on the SEM images correspond to the color codes on the optical absorption spectra.
Figure 4. SEM images of silica@Ag particles obtained by reducing [Ag(NH3)2]+ ions to Ag in a polyol (PVP at 70 °C) using silica particles formed at (a) 0 °C/5 h, (b) 50 °C/5 h, (c) 20 °C/1.3 h, and (d) 20 °C/2.4 h. Optical absorption spectra of these particles compared against that of 1.7 μm long rodlike silica@Ag particles of 330 nm diameter (e−h). The colored labels on the SEM images correspond to the color codes on the optical absorption spectra.
film formation on the surface of silica particles leading to a dense and smooth Ag surface. Figure 3e−h shows the optical absorption spectra of silica@ Ag particles synthesized using different reducing agents compared to the absorption spectrum of 8−10 nm Ag nanoparticles (the spectrum designated in gray). The 8−10 nm nanoparticles are synthesized following the standard recipe39 (Figure S4). The optical absorption spectrum of the 8−10 nm Ag nanoparticles covers the 350−450 nm wavelength range, which is a typical feature of Ag nanoparticles (
