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Self-Assembled Monolayer of Wavelength-Scale Core−Shell Particles for Low-Loss Plasmonic and Broadband Light Trapping in Solar Cells Ali Dabirian,*,†,‡ Mahdi Malekshahi Byranvand,§ Ali Naqavi,∥ Ali Nemati Kharat,§ and Nima Taghavinia†,⊥ †

Department of Physics, Sharif University of Technology, Tehran 14588, Iran Institute of MicroEngineering, Ecole Polytechique Fédérale de Lausanne (EPFL), Rue de la Maladière 71, Neuchatel 2002, Switzerland § School of Chemistry, University College of Science, University of Tehran, Tehran 14155-6455, Iran ∥ Optics & Photonics Technology Laboratory, Ecole Polytechique Federale de Lausanne (EPFL), Rue de la Maladiere 71, Neuchatel 2002, Switzerland ⊥ Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588, Iran ‡

S Supporting Information *

ABSTRACT: Scattering particles constitute a key light trapping solution for thin film photovoltaics where either the particles are embedded in the light absorbing layer or a thick layer of them is used as a reflector. Here we introduce a monolayer of wavelength-scale core−shell silica@Ag particles as a novel light trapping strategy for thin film photovoltaics. These particles show hybrid photonic−plasmonic resonance modes that scatter light strongly and with small parasitic absorption losses in Ag (18.2 MΩ), ammonium hydroxide (NH4OH, 25%; Merck), silver nitrate (AgNO3, 99%; Acros), polyvinylpyrrolidone (PVP, MW40000; LOBA Chemie), 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 Silica Spheres. Monodisperse silica spheres were synthesized by a Stöber method.14,25 In a typical synthesis, 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. Subsequently, 4.5 mL of TEOS was rapidly added and then the solution was stirred for 3 h at 30 °C. The white precipitate was centrifuged and washed with absolute ethanol 5 times. The product was dried at 60 °C for 24 h. Preparation of Silica@Ag Spheres. Deposition of Ag shell was carried out on the basis of the process originally proposed by Deng et al.28 A solution of [Ag(NH3)2]+ ions was prepared by addition of NH4OH to AgNO3 drop-by-drop. The solution initially turned dark brown and became transparent with further addition of NH4OH. This transparent solution contains [Ag(NH3)2]+ ions. Subsequently, 20 mL of a freshly prepared 0.1 M [Ag(NH3)2]+ solution was quickly added to a dispersion of 0.001 g of as-prepared silica spheres in ethanol under magnetic stirring at room temperature. After a few minutes, this dispersion was added into 100 mL of ethanol containing 1 wt % PVP, in a three-necked flask and stirred at 80 °C for 7 h. The dark brown product was collected by centrifugation and then redispersed in ethanol for further examination. Deposition of the Silica Shell. A thin layer of silica was coated on the as-prepared silica@Ag particles using a modified sol−gel process.29 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 spheres was added to this solution. After stirring at 500 rpm for 1 h, the PVPmodified spheres 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 particles dispersion, followed by stirring at 500 rpm for 2 h. The particles were collected by centrifugation and then they were redispersed in ethanol. DSC Device Fabrication. FTO substrates were immersed in 40 mM TiCl4 aqueous solution at 70 °C for 30 min and then rinsed with DI water and ethanol. The TiO2 paste was then screen-printed onto the TiCl4-treated FTO substrates in three and five steps to obtain 7 and 11 μm-thick layers, respectively. These layers were then annealed in air in 3 steps: 325 °C for 5 min, 375 °C for 5 min, and finally 450 °C for 45 min. After cooling down, the layers were treated with 40 mM TiCl4 aqueous solution once more, followed by annealing at 500 °C for 30 min. A monolayer of silica-coated silica@Ag (silica@Ag@silica) particles was deposited onto the photoanodes by spin coating. The optimal condition was spin-coating a 5000 ppm ethanolic solution of these particles at 1000 rpm for 30 s. The device was then annealed at 450 °C for 30 min to sinter the silica@Ag@silica particles together. The layers were then immersed in N719 dye solution (0.2 mM) for 24 h followed by thorough rinsing using acetonitrile. Dye-sensitized photoanodes were assembled with a Pt counter electrodes (CE) into a sandwich-type cell using a hot-melt film (Surlyn) as the spacer between the two electrodes at 120 °C. The electrolyte injection was carried out via vacuum backfilling through a hole in CE. The electrolyte solution contains 1.0 M 1-butyl-3methylimidazolium 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. CEs were fabricated by casting a drop of 5 mM

have not shown comparable enhancements in overall device PCE. Despite the large scattering efficiency of high refractive index dielectric particles,22,23 they have the major drawback of their scattering efficiency being strongly influenced by the refractive index of the surrounding medium. In fact, their scattering efficiency fades away as the surrounding medium refractive index approaches the particle refractive index. Here we show that the scattering efficiency of dielectric particles can be largely decoupled from the surrounding medium refractive index if they are coated with a thin semitransparent metallic shell. In fact, these dielectric@metal particles, in which the dielectric core is large enough to support photonic resonance modes, scatter light as a hybrid plasmonic− photonic resonator.24 Full-wave electromagnetic modeling is used to theoretically study the scattering and absorption of light by these particles. Subsequently, we synthesize silica@Ag particles in a Stöber process14,25 followed by Ag coating using a polyol process.26 A thin silica layer is deposited onto the Ag shell to protect it from oxidation and corrosion in the electrolyte. A monolayer of these particles is applied to DSCs via self-assembly and the enhancements in cell performance are evaluated. The optical absorption and scattering of these particles is further evaluated by measuring optical properties of a monolayer and a submonolayer of these particles. These measurements show solid confirmation of strong light scattering and low optical dissipation losses by these particles.

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MATERIALS AND METHODS

Particles and Device Configuration. Figure 1a shows the configuration of the scattering particles proposed in this work. The

Figure 1. Panel a shows the schematic of the cross section of the silica@Ag@silica particles studied in this work. Panel b illustrates the configuration of a dye-sensitized solar cell (DSC) in which a monolayer of these particles is applied as the scattering layer at the interface of the dye-sensitized layer (DSL) and the electrolyte. particle consists of a transparent dielectric (here silica) core that is large enough to support Mie resonance modes. This particle is coated with a thin (∼15 nm) and semitransparent Ag shell, which is coated with a thin (10−20 nm) silica shell to protect Ag from corrosion and oxidation. Such a thin silica shell is not expected to have a significant impact of the optical properties of the particle.26,27 A monolayer of these particles is applied to a dye-sensitized solar cell (DSC) as shown in Figure 1b to improve the optical absorption in the device. The standard DSC comprises a mesoporous TiO2 layer composed of 15− 20 nm basic particles coated on a SnO2:F (FTO)-coated glass.12 We consider a mesoporous layer that is sensitized with N719 dye molecules and it is completely infiltrated with either a liquid or a solid electrolyte. We assume sunlight illumination from the glass side, which is the standard configuration of DSC operation. Materials and Reagents. Tetraethyl orthosilicate (TEOS, 98%; Merck), absolute ethanol (C2H6O, 99%; Merck), deionized water (DI B

DOI: 10.1021/acsami.5b08560 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Panels a and b show the spectra of scattering cross section (Csca) and absorption cross section (Cabs) of a single silica (450 nm)@Ag (15 nm) particle with an extra 15 nm silica shell at three different refractive index values of the surrounding (ns) medium. Both graphs are normalized to the maximum of the scattering cross section. Panel c shows the profiles of electric field amplitude for wavelengths marked on Panels a and b.

dielectric, need to be larger than λ/(2·n) where λ is the wavelength, in the range of interest, and n is the refractive index of the sphere. This condition of dielectric core dimension is set by Mie theory32,33 so that the particle can support photonic resonance modes. Then a semitransparent thin layer of metal, here Ag, is used to decouple significantly its light scattering properties from the refractive index of the surrounding medium (ns) while still allowing large portion of the light to couple into the resonance modes of the dielectric core. We assume an Ag shell of 15 nm thickness coated onto 450 nm silica sphere for our study. The reasons for choosing 15 nm are (i) this thickness can be easily obtained by most Ag solution processing techniques and (ii) the skin depth of Ag at the 300− 800 nm wavelength range is in the 11−13 nm (Figure S1 of the Supporting Information); therefore, a fraction of the incident light passes through the Ag shell and couples to the silica core. Skin depth is defined as the depth in which the amplitude of the electric field of the incident light at wavelength λ decays to the 1/e value of the electric field amplitude at the interface. It is given by the λ/(4πk) where λ is the wavelength and k is the extinction coefficient of Ag.34 In solution processing, Ag is usually deposited by processes on the basis of Tollens’ process in which an activated Ag salt is reduced to Ag by a chemical reducing agent. In Tollens’ process, it is difficult to deposit dense layers much thicker than 15 nm due to the chemistry of the process.35 A number of attempts have been made to deposit thicker Ag layers; however, they had resulted in Ag shells with a granular structure in which the shell is composed of small Ag nanoparticles.36,37 Such Ag layers are not suitable for low-loss light scattering because light couples to localized surface plasmon resonances (LSPR) of these small nanoparticles causing significant optical absorption losses. Optical Modeling of a Single Particle. A fraction of the incident light passes through the 15 nm Ag shell and reaching the photonic modes of the silica core. This light is subject to interaction with the plasmonic resonance modes of the metallic shell and the photonic resonance modes of the dielectric core.

H2PtCl6 ethanolic solution onto a 1 cm2 FTO-glass followed by annealing at 450 °C for 15 min. Characterizations and 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. Extinction spectra of silica@Ag spheres were measured in highly diluted ethanolic suspension of the particles using a PerkinElmer, Lambda 25UV−visible photospectrometer. X-ray diffraction (XRD) was carried out using a Rigaku D/max-rB equipment in Bragg−Brentano configuration using Cu Kα1 radiation (λ = 1.540 56 Å). Diffuse reflection (DRS), transmittance (DTS), and optical absorption 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. Incident photon-tocurrent conversion efficiency (IPCE) was measured using a setup consisting of a Jarrel-Ash monochromator, a 100 W halogen lamp, and a calibrating photodiode (Thorlabs). Dye loading of mesoporous TiO2 layers was measured by desorption of dye through soaking the dyesensitized TiO2 films in a 0.1 M NaOH aqueous solution. Concentration of desorbed dye molecules was estimated from optical absorption of the 0.1 M NaOH aqueous solution containing the desorbed dye molecules. Optical Calculations. We calculated the optical absorption and scattering cross section of these silica@Ag@silica particles by numerically solving the full-wave Maxwell equations over the entire sphere geometry under a plane-wave incidence of E̅ (r)̅ = r̂exp(−jk̅·r)̅ . In the modeling process, the total electromagnetic field, E̅ t = E̅ i + E̅ s, is calculated, which is the summation of the incident and scattered fields (E̅s). The absorption cross section of the particles is defined by Cabs = (1/Ii) ∫ E̅ ·Jd̅ v where Ii is the intensity of the incident light Ii = (1/2) cϵ0ϵr|E̅ i|2, J ̅ is the Ohmic current density induced by electromagnetic fields in the sphere, and the integration is carried out over the entire sphere volume.30,31 Scattering cross section is defined by Csca = ∫ S̅·da,̅ in which S̅ = (1/2)(E̅ × H̅ *) is the Poynting vector describing the outgoing electromagnetic power per unit area. The integration is carried out over a closed surface surrounding the particle.

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RESULTS AND DISCUSSION The basic design rules of the silica@Ag@silica particles shown in Figure 1a are as follows. The core, silica or any transparent C

DOI: 10.1021/acsami.5b08560 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Therefore, it is expected to create hybrid modes24,38,39 and results in more broadband optical spectrum. Figure 2a,b shows the normalized Csca and Cabs spectra of a single silica@Ag particle as the surrounding medium refractive index varies. The spectra are normalized to the maximum of the strongest Csca among the three curves. The spectra with absolute values are shown in Figure S2 of the Supporting Information. The spectra of Csca are composed of several peaks that are associated with the photonic resonance modes of the particle. We observe that if the refractive index of the surrounding medium varies from ns = 1 to 1.9, the Csca amplitude varies within a range of smaller than 10%. We should mention that the Csca of a silica sphere disappear at ns = 1.4 due to its refractive index of 1.44. Another interesting feature is that the Cabs values on average are smaller than 1.5% of the Csca. This highlights the low-loss plasmonic feature of these particles. The profiles of electric fields (|E̅ sca|) upon interaction of a plane wave incident on the particle from the left-side of the page at certain wavelengths marked in Figure 2a,b are shown in Figure 2c. These wavelengths correspond to the peaks in the Csca spectrum and the resonance in Cabs spectra of the particle with ns = 1.9. At these wavelengths, a significant amount of the incident light passes through the Ag shell and we observe the resonance modes arising from mutually interaction of the Ag shell and the silica optical cavity. The mechanism is somewhat similar to other coupled plasmonic-photonic resonance system in which the large fraction of the optical resonance modes are set by the photonic resonance modes. The accurate analysis of the resonance modes of these particles is important for understanding different physical effects that could occur in these particles; however, in this paper, we solely focus on those modes that are related to our structure for the purpose of low parasitic absorption light trapping. The field profile at λ = 565 nm shows that there is a significant coupling into the resonance modes of the silica core where a quadrupole Mie resonance mode is excited. The peak observed at λ = 675 nm corresponds to the dipole resonance mode of the silica core. In the Cabs spectra a Fano-like behavior is observed with a dip at λ = 770 nm and a peak at λ = 780 nm. The Fano-like resonance in spherical resonances is known and has been intensively studied in the past years.40,41 In an actual PV device, an array of silica@Ag@silica particles is used. Previous studies, both theoretical and experimental, have shown that the results of isolated particles cannot be directly translated to large area devices composed of a large number of particles.10,18,30,31 This is due to coupling among adjacent particles and the presence of multiple scattering effects taking place among these particles. Therefore, presence of adjacent particles is expected to influences the optical absorption enhancement induced by these particles in a DSC device. DSC Device Application. We experimentally evaluate the proposed ideas by synthsizing silica@Ag particles in a two-step process. In the first step, monodisperes spherical silica particles are synthesized in a Stöber process (Figure 3a). These particles are then coated with a rather smooth Ag shell (Figure 3b,c) in an optimized polyol process. XRD is carried out on a layer of silica@Ag particles drop-casted onto a fused silica plate confirm formation of Ag (Figure 3d). Optical extinction spectrum of the particles (Figure S3 of the Supporting Information) illustrates the broadband interaction of these particles with incident light. The particles are later coated with a thin silica shell and then a

Figure 3. Structural characterization of the particles and the device. Panels a−c show the SEM images of silica, silica@Ag and TEM image of silica@Ag particles, respectively. Panel d shows the XRD pattern of a layer of silica@Ag particles drop-casted onto a silica plate substrate. Panel e shows the cross section of a mesoporous TiO2 electrode on which the silica@Ag@silica particles are loaded. The inset shows the top-view SEM image of the particles layer. The scale-bar of the inset is 3 μm.

nearly packed monolayer of them is coated onto mesoporous TiO2 scaffold followed by a multistep annealing procedure described in detail in the Materials and Methods section (Figure 3e). Although Ag is known to oxidize at temperatures around 200 °C, it reduces to Ag at temperatures higher than 400 °C.42,43 Therefore, we rule out the oxidation of Ag during the fabrication of the electrodes. Photovoltaic performance of the cells with different photoanode structures is measured on the masked area of 25 mm2 under simulated AM1.5G illumination. Figure 4a shows the photocurrent density versus voltage (J−V) curves of 7 μm-thick standard cell and similar cell with the monolayer of silica@Ag@ silica particles at the surface. PV performance data for these cells and cells with 11 μm-thick layer are summarized in Table 1. We observe that significant improvements take place in the short circuit current density (JSC) of the cells. The enhancement of JSC is 38% and 19% for the 7 and 11 μm-thick DSCs, respectively (Table 1). These enhancements are significant for plasmonic enhancements of optical absorption in thin film solar cells. The enhancement of the 11 μm-thick is relatively weaker because in DSC the light passes through the sensitized layers and then it is scattered by the silica@Ag@silica particles. The enhancement due to scattering layer is weaker because the single-path absorption of 11 μm-thick layer is higher than that of the 7 μm-thick one. Figure 4b shows the spectra of IPCE for 7 and 11 μm-thick DSCs with and without the monolayer of silica@Ag@silica particles on the surface. Presence of the scattering particles improves the IPCE over the entire wavelength range confirming that the effect is a broadband effect. This enhancement is particularly strong at λ > 600 nm because in this wavelength range the optical absorption of the N719sensitized layer is weak. Further confirmation of the enhanceD

DOI: 10.1021/acsami.5b08560 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

broadband light scattering due to their rough surface morphology. Although these particles are proven to be significantly more efficient than nanoparticles, they do not completely inhibit the parasitic light absorption induced by the plasmonic absorption losses in the constituent small nanoparticles. On the other hand, they have the possibility of improving optical absorption due to the near-field enhanced optical absorption. Nevertheless, the essential implication of these research efforts is that using plasmonic particles of wavelength-scale dimension is the key to achieve efficient and low-parasitic absorption plasmonic light trapping. Statistical analysis are conducted to ensure that the improvement in device efficiency after using silica@Ag@silica particles is due to enhanced light trapping rather than statistical variation in device processing.46 For instance, the experimental variations in thickness of the dye-sensitized layer leads to variations in the device JSC and PCE. Its impact can be safely decoupled from the light trapping effect of the monolayer of silica@Ag@silica particles through statistical analysis. Therefore, 10 reference devices and 10 devices with a monolayer of silica@Ag@silica particles are processed. The thickness of the DSL in all these experiments is 7 μm. The PV performance metrics (JSC, VOC, FF, and PCE) of the individual devices are measured and reported in Tables S2 and S3 of the Supporting Information. The histograms of the data of these devices are shown in Figure 5 in which average (μ) and standard deviation (σ) of the measurement data are shown. The JSC of the reference devices varies from 8.9 to 9.6 mA·cm−2 whereas the JSC of the devices with scattering particles varies in the 12.5− 13.4 mA·cm−2 range. This confirms that there is a visible improvement in JSC due to the presence of the scattering layer of silica@Ag@silica particles. This improvement is clearly beyond the statistical variation induced by device processing reproducibility. The VOC and FF are not expected to be affected significantly after application of the scattering particles because the presence of the scattering particles is intended to increase the JSC. The statistical analysis shows that these parameters also vary in closely similar ranges. PCE of the reference devices varies in the 4.3%−4.8% range in contrast to the 6%−6.4% for the devices with monolayer of silica@Ag@silica scattering particles showing more than 1% PCE improvement. Scattering Measurements. The scattering efficiency of a particle is intuitively described as the fraction of the incident light that acquires a different direction than the incident light upon interaction with the particle.47 Practically the measurements of the scattering efficiency are conducted by a photospectrometer equipped with an integrating sphere in which the specular and diffuse components of the transmitted and the reflected light can be decoupled (Figure 6a).48 The extent of light scattering by silica@Ag@silica particles is evaluated by conducting optical measurements on monolayer and submonolayres of these particles. The following two structures are used: (i) a nearly packed monolayer of the

Figure 4. Photovoltaic characterizations. Panels a and b show the current density versus voltage (J−V) and the incident photon-toelectron conversion efficiency (IPCE) of 7 and 11 μm-thick standard cell and similar cell with the monolayer of silica@Ag@silica particles (scattering layer) at the surface.

ment of optical absorption is obtained in the optical absorbance spectra of the dye-sensitized layers shown in Figure S4 of the Supporting Information. The optical transmittance spectra of the dye-sensitized layers with a monolayer of silica@Ag@silica particles show that a fraction of the light transmitted through the layer without being absorbed (Figure S5 of the Supporting Information). In principle, this allows further improvement of device performance using an Ag or a TiO2 scattering particles reflecting layer.14,15 Addition of extra layers of silica@Ag@silica scattering particles deteriorates device performance (Figure S6a of the Supporting Information). This is likely due to the stronger light absorption in multilayers of silica@Ag@silica particles leading to the darker appearance of these multilayers as shown in inset of Figure S6b of the Supporting Information. Recently, large plasmonic particles with rough surface morphology have been used for light trapping in thin film solar cells.44,45 These particles provide the advantage of

Table 1. Photovoltaic Properties of the DSCs Prepared with Different Thicknesses and Either with or without the Monolayer of Silica@Ag@Silica Scattering Particles device

JSC (mA·cm−2)

VOC (V)

FF (%)

PCE (%)

reference cell, 7 μm reference cell, 11 μm cell with silica@Ag@silica monolayer, 7 μm cell with silica@Ag@silica monolayer, 11 μm

9.54 11.81 13.13 14.02

0.744 0.744 0.745 0.739

0.68 0.68 0.65 0.67

4.82 5.97 6.36 6.94

E

DOI: 10.1021/acsami.5b08560 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. Panels a−d and e−h show the histogram of PV performance metrics of the 10 reference DSCs and the 10 DSCs with a monolayer of silica@Ag@silica particles, respectively.

reflectance where diffuse reflectance constitutes more than 80% of the reflected light. The spectra of total reflectance (TR) from both samples are nearly similar and relatively constant over the entire wavelength ranges of study (Figure 6c,e). The fraction of DR of the sample with densely packed monolayer of the silica@Ag@silica particles is larger than the DR of the one with smaller coverage of the particles. This indicates that a significant fraction of the reflectance is caused by the particles and not by Fresnel reflectance from the layers underneath. The substrate optical absorption is decoupled from the absorption in the Ag shell by depositing a submonolayer of silica@Ag@silica particles on a fused silica plate (Figure 6d,e). The optical absorption of the structure is less than 5% at λ > 500 nm. This highlights the negligible optical absorption in these particles as suggested by the calculation results of Figure 1. The haze of this structure with nearly 15% coverage of silica@Ag@silica particles (Figure S8 of the Supporting Information) almost linearly decay from 40% at λ = 300 nm to 20% at λ = 800 nm, further confirming the significant light scattering by these particles. This measurement confirms that the silica@Ag@silica particles strongly scatter light with small optical absorption losses.

silica@Ag@silica particles deposited onto a mesoporous TiO2 layer on FTO glass (Figure 6b and Figure S7 of the Supporting Information) and (ii) a submonolayer of these particles with nearly 15% coverage deposited onto the fused silica plate (Figure 6d). The former structure has the configuration of the DSCs used in this work, however, without dye loading. Therefore its optical measurements give information how the light is scattered into the mesoporous TiO2 layer by silica@ Ag@silica particles. In the latter, the mutual interaction of the particles is not significant due to the low concentration of these particles on the surface. Therefore, the measurements give information about the scattering and absorption efficiency of individual silica@Ag@silica particles. The optical absorption spectrum of the sample schematically shown in Figure 6b is depicted in Figure 6c and it shows that the structure absorbs light strongly at λ < 400 nm and significantly at λ < 500 nm. This wavelength-dependent behavior of the optical absorption indicates that the optical absorption is mainly taking place in TiO2 and FTO. The optical absorption at long wavelengths falls to about 10%, illustrating the small optical absorption (TA) by silica@Ag@silica particles. It is observed from comparing the DT with the TT that the large fraction of the transmitted light is diffused. This can be quantified as the haze of the structure using H = DT/TT.49 The structure maintains more than 40% haze at λ > 700 nm, as shown in Figure S8 of the Supporting Information. This directly confirms that the enhancement in the cell performance is caused by the strong light scattering from silica@Ag@silica particles. The effect of light scattering is more significant in the

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CONCLUSIONS Spherical core−shell of dielectric@metal with thin metallic shell and a transparent dielectric core of wavelength-scale dimensions constitute efficient scattering elements with low parasitic absorption losses. These particles, depending on the F

DOI: 10.1021/acsami.5b08560 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 6. Panel a shows the general case of light scattering from a sample in which diffuse reflectance (DR) and diffuse transmittance (DT) are separated from the specular components of the reflectance and transmittance. Total transmittance (TT) or total reflectance (TR) are the summation of the specular and diffuse components and total optical absorption (TA) is defined by TA = 100 − TT − TR. Panels b−e show the schematic and measured optical parameter of the two structures shown. Inset of panel e shows the SEM image of the actual structure produced.

thickness of the metallic shell, function either as plasmonic particles or as hybrid plasmonic-photonic resonators. Relative to dielectric scattering particles, the scattering cross section of these particles shows small variations with the change in the refractive index of the surrounding medium. Silica@Ag particles of 450 nm core and 15 nm shell synthesized in a two stepprocess, then coated by a protective silica shell, proved to be effective in light trapping in dye-sensitized solar cells with several micrometer-thick light absorbing layers. Strong scattering efficiency and low parasitic absorption losses of the particles makes them interesting as alternative to dielectric scattering particles in light trapping in photovoltaics or light out-coupling in LEDs.

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ethanolic dispersion of silica@Ag particles, optical absorbance spectra, and dye adsorption data of the dye-sensitized layers, J−V curves and optical absorption spectra of DSC devices with multilayers of silica@Ag@ silica scattering particles, haze spectra, SEM images of silica@Ag@silica monolayer on the mesoporous TiO2 layer, and the photovoltaic performance data of devices processed for the statistical evaluation (PDF).

AUTHOR INFORMATION

Corresponding Author

*A.D. E-mail: ali.dabirian@epfl.ch. Notes

ASSOCIATED CONTENT

The authors declare no competing financial interest.

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08560. Skin depth of Ag, Csca and Cabs spectra with absolute values, optical extinction spectra of highly diluted

ACKNOWLEDGMENTS The authors thank Iran Nanotechnology Initiative (Grant number 93981) for financial support of this work. G

DOI: 10.1021/acsami.5b08560 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.5b08560 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX