CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201402146

Coupled Near- and Far-Field Scattering in Silver Nanoparticles for High-Efficiency, Stable, and Thin Plasmonic Dye-Sensitized Solar Cells Gede Widia Pratama Adhyaksa,[a] Se-Woong Baek,[a] Ga In Lee,[b] Dong Ki Lee,[b] JungYong Lee,*[a] and Jeung Ku Kang*[a, b] Here, we report plasmonically enhanced thin dye-sensitized solar cells (DSSCs) in an imidazolium-dicyanamide based ionic liquid, in which size-controlled metal (silver) nanoparticles (AgNPs) with passivation layers of a few nanometers are arranged into the electrolyte and photo-electrodes. It was revealed that the AgNPs in the electrolyte and the photo-electrode have distinct effects on device performance via different coupling mechanisms. Strong far-field scattering is critical in the electrolyte while near-field scattering is efficient in the photo-electrode. Indeed, we find that the power conversion efficiency of the DSSC can be substantially improved by a synergistic arrangement of the AgNPs in the electrolyte and the photo-electrode. Furthermore, an imidazolium-dicyanamide based nonvolatile ionic liquid electrolyte for MNPs is demonstrated to provide thin plasmonic DSSCs with good stability.

The most critical challenge in solar cell systems is to harvest photons with energies in the visible region of the spectrum,[1] thereby improving photon collection. Compounds in the class of quantum molecular dyes absorbs in the visible region,[2–4] and these have been used extensively as photon collectors in dye-sensitized solar cells (DSSCs), which represent one of the more promising options for the realization of low-cost solar cells. However, they have yet to reach satisfactory levels of power conversion efficiency (PCE) and stability. Molecular-complex dyes[5] absorb in the visible region and have been extensively used as photon collectors in dye-sensitized solar cells (DSSCs). However, their absorption intensities (e ~ 103 m 1 cm 1)[6] are roughly 100 times lower than those of other inorganic nanostructure sensitizers (e ~ 105 m 1 cm 1).[7] For this reason the intensity has been improved by adding thick light scattering layers. However, this approach makes such solar cells thicker than preferable, thus preventing the realization of [a] G. W. P. Adhyaksa,+ S.-W. Baek,+ Prof. J.-Y. Lee, Prof. J. K. Kang Graduate School of Energy, Environment, Water, and Sustainability (EEWS) Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305-701 (Korea) E-mail: [email protected] [email protected] [b] Dr. G. I. Lee, D. K. Lee, Prof. J. K. Kang Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305-701 (Korea) [+] These authors contributed equally to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402146.

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thin solar cells and leading to high resistance values and expensive devices.[8] Hence, strategies to retain or reduce the thickness of the materials while maintaining efficient photon absorption are necessary.[9–14] For this purpose, the localized surface plasmon resonance (LSPR)[15–19] of metal nanoparticles (MNPs) is of great interest. In principle, to enhance of absorption efficiency, the LSPR in DSSCs can be controlled by adjusting: 1) the physical morphology of MNPs, and 2) the distance between MNPs and absorbing dyes. The electrolyte offers a more open space for the MNPs compared to the photo-electrode, because it occupies at least 60 % of the total volume in a DSSC. In addition, the use of MNPs in the photo-electrode enables the dye to experience the LSPR more intensively owing to the close proximity of the dye and MNPs. However, the number of MNPs placed in the photo-electrode could be limited because MNPs have to compete with the dyes for the limited space on the TiO2 surfaces. In addition, electrolytes containing redox compounds could decompose the MNPs, resulting in stability issues,[20] unless protection layers are applied on the surfaces of the MNPs. Indeed, DSSCs with a volatile electrolyte and an added thick scattering layer have reached respectable efficiencies. However, the use of volatile solvents such as acetonitrile and thick scattering layers in the high-efficiency cells is a big hurdle for the practical application of photovoltaic technologies, because both the cost-expensive sealing of volatile electrolytes and also the added thickness of the scattering layers counteract the merits of DSSCs as a photovoltaic technology with a high performance. Herein, we find that synergistic near- and far-field coupling of MNPs in the photo-electrode and in the non-volatile electrolyte can result in both high efficiency as well as high stability for thin DSSCs without requiring the use of thick scattering layers. For this study, silver nanoparticles (AgNPs) with different sizes (8.6, 29, 48, and 98 nm; see Supporting Information, Figure S1) were selected because their plasmon resonance energies in the range 3.76 ~ 2.63 eV are well matched to the dye, having its first excitonic peaks at 3.81 ~ 2.72 eV. Figure 1 shows three possible domains: the electrolyte, the photo-electrode, and on a combination of the electrolyte and the photo-electrode. The AgNPs were treated with three different passivation agents: poly(vinylpyrrolidone) (PVP), 2-dimethyl amino ethane-thiol (DMAET), and sodium dicyanamide (NaDCA). The passivation layers added a thickness of ca. 2 to 3 nm compared to the neat AgNPs (Supporting Information, Figure S2). The excitonic peaks of the N719 dye were still within the resonance locations of the AgNPs of four different sizes, ChemSusChem 0000, 00, 1 – 8

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the presence of Ag atoms, accounting for approximately 10.41 wt % out of 67.68 wt %. The surface coverage of the well-distributed AgNPs on the TiO2 photo-electrode is estimated at 15 %. Figure 2 demonstrates the performance of the plasmonic DSSCs compared to a control device without AgNPs. First, we investigated the optimum concentration for each AgNP size Figure 1. Three spatial arrangements of AgNPs in DSSCs: (a) in the electrolyte; (b) in the photo-electrode; and (c) in both the electrolyte and the photo-electrode. and the arrangement in the DSSCs (Figure 2 a). It was revealed that different sizes have different optimum concentrations. In general, to achieve an despite a small red-shift of the resonance (Figure S2j). Upon optimum concentration at the same size, a higher number addition of the passivation agents, the homogeneous size disdensity of AgNPs is required in the electrolyte than in the case tribution of the AgNPs and their dispersion in a hydrophilic of a photo-electrode. Moreover, at the same arrangement, the imidazolium-type ionic liquid containing 1-ethyl-3-methylimismaller AgNPs need a higher number density of AgNPs to achdazolium cation (EMIm + ) and dicyanamide anion (DCA ) were ieve higher PCEs. More details are described in the Supporting also well-maintained. The purpose of the passivation agents Information (Figures S3 and S4). For all DSSCs containing was: 1) protection against corrosion by redox compounds; AgNPs in their electrolyte, a broadband external quantum effi2) avoiding agglomeration; and 3) preventing direct contact of ciency (EQE) enhancement in the wavelength range of 350 ~ AgNPs with the dyes. In this study, the AgNPs of different sizes 650 nm could be seen (Figure 2 b). In particular, the DSSC with were loaded into the entire body of the DSSCs. In terms of the 98 nm AgNPs yielded the best enhancement (~ 8.2 %), at loadable amount of AgNPs in the DSSC, the electrolyte has a wavelength of 455 nm. This enhancement was reduced to a great potential. Because approximately 60 % of the device 7.5, 6.4, and 4.1 % for embedded 48, 29, and 8.6 nm AgNPs, revolume is filled by its electrolyte, it can offer a more space spectively (Figure 2 b and Supporting Information, Figure S7a). than the photo-electrode (Supporting Information, Figure S3). Also, the good efficiency of 7.11 % was achieved upon embedIn the case of the photo-electrode, the main restriction in placding 98 nm AgNPs in the electrolyte, as illustrated in Figing the MNPs is that the sites on the TiO2 should be shared ure S7c. The open-circuit voltage (Voc) and the fill factor (FF) with the dyes. A large amount of MNPs on the photo-electrode could also increase the possibility of direct contact between values were not significantly affected, and only the short-circuit MNPs and dyes, which could stimulate an undesirable charge current (Jsc) had an influence on the PCE enhancement due to recombination process, thereby reducing the PCE (Supporting the effect of plasmonic scattering by AgNPs, as summarized in Information, Figure S4). Figure S5 (Supporting Information) Table 1. In contrast, upon embedding the AgNPs in the photoshows the arrangements of the AgNPs in the ionic liquid, electrode, the EQE enhancement generally decreased with inwhich were incorporated into the DSSCs. Figure S5a shows creasing AgNPs size; it reached an optimum for 8.6 nm AgNPs, a photograph of the ionic liquid solution containing AgNPs, as shown in Figure 2 c and Figure S7b. A broadband EQE enwhich was used as the electrolyte of the DSSCs. The AgNPs in hancement at wavelength of 350 ~ 700 nm was also achieved the solution are well-dispersed and their size distribution is ho(Figure S7b). The Jsc enhancement mainly boosted the PCE of the DSSCs, leading to a PCE of 8.05 % when using 8.6 nm mogeneous (Figure S5b–5e). Figure S5f shows two photoAgNPs, as shown in Figure S7d. The Voc and FF values were not graphs of the photo-electrode (PE) of the DSSCs before (top) and after (bottom) deposition of AgNPs. Notably, the PE with significantly affected either, as summarized in Table 1. AgNPs is darker than the reference PE, implying improved abTo clarify this result, experimental absorption spectra of the sorption by the dyes aided by the AgNPs via light trapping. devices for each size arrangement compared to the reference Figure S5 g is a cross-section scanning electron microscopy device are also provided (Supporting Information, Figure S8a– (SEM) image of the PE deposited with AgNPs, showing that b). The EQE enhancement (Figure 2 b–c) follows the same AgNPs are homogeneously dispersed deep into the ca. 10 mm trends as the absorption enhancement (Figure S8c–d) in the thick TiO2 PE, where each elemental mapping scan for Ag, Ti, broadband range 350 ~ 700 nm. Noticeably, the EQE enhancement as well as the absorption enhancement are higher than and O is shown in a white rectangular area. The top surface of for the devices with AgNPs in the electrolyte by a factor of apthe PE also shows that the 48 nm AgNPs contrast well with the proximately 2 to 3, indicating that the near-field effect by the TiO2 nanoparticles of 15 to 25 nm in size (Figure S5 h). The size MNPs improves optical absorption more effectively than their of the AgNPs was unchanged after being deposition onto the far-field absorption with regards to the distance effect from TiO2 photo-electrode (Supporting Information, Figure S6). Analthe dye molecules. The multiple scattering by the MNPs in the ysis by energy dispersive X-ray spectroscopy (EDS) validates  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 2. Photovoltaic characterizations. (a) Optimized concentration: DSSC performance as a function of the volumetric number density of AgNPs in the DSSCs. (blue) 8.6 nm; (red) 29 nm; (olive) 48 nm; (green) 98 nm. Open circles and filled circles represent the electrolyte (EL) and the photo-electrode (PE), respectively. The effect of AgNPs size on the external quantum efficiency (EQE) enhancement of the DSSCs: (b) in the electrolyte; (c) in the photo-electrode. (blue) 8.6 nm; (red) 29 nm; (olive) 48 nm; (green) 98 nm. Effect of the AgNPs spatial arrangement: (d) EQE (open circles) and EQE enhancement (filled circles) of the optimized DSSCs; (black dashed line) a reference device; (green) AgNPs in the electrolyte; (blue) AgNPs in the photo-electrode; (pink) AgNPs in combinations. (e) Current density–voltage (J–V) characteristics of the optimized DSSCs; (black dashed line) a reference device; (green) AgNPs in the electrolyte; (blue) AgNPs in the photo-electrode; (pink) AgNPs in combinations. (f) Contour map of the power conversion efficiency (PCE) along the size of AgNPs in both domains. For Figure 2 b–f, the optimized concentrations in the electrolyte are 7.56  1016, 1.21  1015, 1.84  1014, and 1.08  1014 particles cm 3 for 8.6 nm, 29 nm, 48 nm, and 98 nm, respectively while those in the photo-electrode are 2.68  1016, 4.05  1014, 2.14  1014, and 4.28  1013 particles cm 3 for 8.6 nm, 29 nm, 48 nm, and 98 nm, respectively. All characterizations were conducted under simulated AM 1.5G illumination at 100 mW cm 2.

Table 1. Photovoltaic characteristics of DSSCs containing AgNPs in the electrolyte and the photo-electrode (single arrangement). Data were measured under simulated AM 1.5G illumination at 100 mW cm 2. Arrangement

electrolyte

photo-electrode

AgNPs

Voc [V]

Jsc [mA cm 2]

FF [%]

h [%]

Control device 8.6 nm 29 nm 48 nm 98 nm Control device 8.6 nm 29 nm 48 nm 98 nm

0.72 0.72 0.72 0.72 0.73 0.72 0.72 0.72 0.73 0.73

11.97 12.16 12.52 12.64 12.89 11.97 15.12 14.55 13.86 13.25

0.73 0.73 0.74 0.75 0.76 0.73 0.74 0.75 0.73 0.74

6.34 6.41 6.67 6.85 7.11 6.34 8.05 7.83 7.40 7.16

electrolyte could result in optical loss, because the scattered light can be re-absorbed by other MNPs in the electrolyte before it reaches the dyes of the photo-electrode. In case of absorption enhancement, as shown in Figure S8c and S8d, we found that the absorption enhancement of the device was not clearly matched to the plasmonic resonance peaks of AgNPs at different sizes (Supporting Information, Figure S1). Because, in real device configurations, the various factors should be additionally considered to analyze the absorption enhancement of device such as the absorption spectrum of absorption material (dye)[21] and non-dominant or other scattering effects in different regimes. A detailed explanation is given in the Supporting Information (Figure S8). In addition, an off-resonance enhance 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ment can be used to explain the broad optical enhancement, which is not only specific for the plasmonic range of the AgNPs. This process takes place due to an accumulation of “hot-spot” enhanced electric fields, comprising dye surrounded by multiple AgNPs and the high dielectric constant of the dispersing medium (ionic liquid). Results determined on a simple model to support this contribution are summarized in the Supporting Information (Figure S9). In addition, to investigate the synergistic effect of AgNPs in both the electrolyte and also the photo-electrode, we fabricated devices with a combination of these two. Device performances at the optimum conditions for each arrangement were analyzed based on the spectral response of the EQE and the EQE enhancements (Figure 2 d), and the current density-voltage (J–V) characteristics (Figure 2 e). We examined 16 different size combinations. The contour map of the PCEs of the plasmonic DSSCs is depicted in Figure 2 f, and detailed characteristics are summarized in Table 2. To ensure a reliable analysis, 9 ~ 15 devices were fabricated for each size combination. A PCE of 8.33 % was acquired in the device through a synergistic configuration of 8.6 and 98 nm AgNPs in the photo-electrode and the electrolyte, respectively (see Supporting Information, Table S2 for complete characteristics). In general, smaller AgNPs in the photo-electrode domain result in higher enhancement of the EQE while larger AgNPs in the electrolyte are beneficial for improving the EQE, as demonstrated in Figure 2 b and c. The marked tendencies between the two domains clearly explain that two different mechanisms apply, depending not only on the size of the AgNPs but also ChemSusChem 0000, 00, 1 – 8

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while it was enhanced in two separate regions, 350 to 500 and 550 to 700 nm, by the latter. Size of AgNPs in electrolyte Size of AgNPs in photo-electrode These distinct characteristics also ref. cell 8.6 nm 29 nm 48 nm 98 nm manifest the different effects PCE [%] PCE [%] PCE [%] PCE [%] PCE [%] due to their arrangements. This ref. cell 6.34 8.05 7.83 7.40 7.16 observation suggests that the 8.6 nm 6.41 8.06 7.85 7.46 7.17 AgNPs could be more effectively 29 nm 6.67 8.09 7.90 7.54 7.29 coupled with the dyes, for which 48 nm 6.85 8.19 8.02 7.72 7.32 98 nm 7.11 8.33 8.17 7.80 7.38 the absorption is concentrated at a wavelength of 533 nm, than TiO2 SL. To further exploit the change in interaction between phoon the distance between the dyes and the AgNPs: coherent tons and molecular dyes upon incorporating the AgNPs, we near-field coupling and incoherent far-field coupling.[22, 23] The characterized the DSSCs by using Raman spectroscopy near-field effect concentrates a strong electromagnetic field (Figure 3). There is a Raman peak at 1577 cm 1 representing vinear the MNPs. Field enhancement is well-known to increase brational states from the central ruthenium atom with dicarwith decreasing AgNP size. In addition, the near-field effect boxybipyridine (dcbpy), one of its constituent ligands conrapidly decays as the distance from the MNP surface increastained in the dye molecules[25, 26] as a function of the embedes.[24] Therefore, it can be concluded that shorter distances between the AgNPs and the dye, and smaller AgNPs are advantaded AgNPs size. In addition, it shows that the presence of geous to obtain the greatest near-field enhancements. AgNPs in the electrolyte enhanced the Raman intensities by The AgNPs embedded in the photo-electrode are in very a factor or approximately 2, while there was a 3- to 4-fold enclose proximity to the dyes anchored on the TiO2. This implies hancement in the photo-electrode, and these enhanced signals that the near-field effect plays a crucial role in enhancing the were dependent on the size of the AgNPs. The combination of dye absorption; hence, the higher EQE was attained with both arrangements demonstrated an even higher Raman insmaller sizes as shown in Figure 2 c. The smaller AgNP size is tensity, as observed by combining 98 nm AgNPs in the electroalso favorable with respect to the distance between the AgNPs lyte with different AgNPs sizes in the photo-electrode (Figand the dye, because smaller AgNPs can more easily infiltrate ure 3 a). In the electrolyte, the Raman intensity increased prointo the TiO2 porous layer comprised of NPs of ca. 15 to portionally to the AgNPs size with a maximum intensity for the 98 nm AgNPs, while in contrast the intensity was inversely pro25 nm. The smaller AgNPs can be placed the closer to the dye, portional to the increased size, peaking when 8.6 nm AgNPs causing the maximum EQE to occur for the 8.6 nm AgNPs. The were embedded in the photo-electrode. The Raman intensity slight red-shift of the EQE peaks for the larger AgNPs also illusin the combination arrangement is approximately 4 to 5 times trates the signature of the plasmonic resonance peak, as seen greater than that of the reference cells, and 1 to 2 times greatin Figure 2 c and Supporting Information, Figure S6. In contrast, er than those for the single arrangements, suggesting that the the far-field effect of the AgNPs is dominant when they are arAgNPs in the photo-electrode exert a synergistic effect in comranged in the electrolyte. Because the AgNPs in the electrolyte bination with the AgNPs in the electrolyte. Raman intensity deare far (ca. 20 ~ 60 mm) from the dye sensitizers, the interaction pendency in each domain on the AgNPs size agrees well with occurs through far-field coupling, which strongly scatters the that the device performance. A detailed analysis is described in incident light. The far-field effect increases the optical path the Supporting Information (Figure S11). length of an incident photon via a widely scattering electroThe efficiency of DSSCs cells based on the nonvolatile ionic magnetic radiation. Optical simulations predict that the scatterliquid and the volatile conventional acetonitrile were tested ing power of the AgNPs should increase with increasing AgNP periodically up to 1056 h. The presence of AgNPs in the elecsize, as shown in Supporting Information, Figure S2k. The PCE, trolyte based on acetonitrile, which is a volatile electrolyte, deEQE, and light absorption were enhanced as the AgNPs size ingraded the device efficiency.[27] However, the nonvolatile ionic creased, as summarized in Table 1, suggesting that the far-field scattering effects by the AgNPs enhance the device perliquid-based DSSCs exhibited good stability (Figure 4 a). The formance. Figure 2 d shows that the signature of the plasmonic DSSCs using the ionic liquid-based electrolyte, with and withband for the EQE is less pronounced in the electrolyte than in out AgNPs, retained approximately 86.3 and 91.8 % of their inithe photo-electrode. The EQE reduction with decreasing tial efficiencies after 1056 h, respectively. Meanwhile, the effiAgNPs size is attributed to the pronounced self-absorption of ciencies of the devices using acetonitrile exhibited only 42.9 the AgNPs without a photocurrent contribution. In fact, disand 52.9 % of their initial efficiencies with and without AgNPs, persed AgNPs in an ionic liquid electrolyte play a role similar respectively. Figure 4 b–e shows that the AgNPs without the to that of the conventional TiO2 scattering layer (SL) in increaslayers were corroded after 2 months. In contrast, the dispersion of AgNPs with the layers was maintained without significant ing the optical path length of incident light, but they show difcorrosion after the same period of time. In general, the volatile ferent EQE characteristics, as clearly illustrated in Figure S10 electrolyte containing redox compounds corrodes the neat (Supporting Information). The EQE was enhanced in a broad AgNPs, thus damaging long-term stability of the device. The spectral range of 350 to 650 nm using the former approach, Table 2. PCE of the DSSCs containing AgNPs both in the electrolyte and the photo-electrode. Data were measured under simulated AM 1.5G illumination at 100 mW cm 2.

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Figure 3. Raman characterization. (a) Raman intensity at 1577 cm 1 for different AgNPs sizes and different spatial arrangements. (green) AgNPs in electrolyte; (blue) AgNPs in photo-electrode; (pink) AgNPs in combination. In combination results, the AgNPs sizes indicates the one in the photo-electrode while 98 nm AgNPs are embedded in the electrolyte. (b) Contour map of Raman intensity in the combinational arrangement. Raman responses of the devices were measured at room temperature, where the optimized concentrations in the electrolyte are 7.56  1016, 1.21  1015, 1.84  1014, and 1.08  1014 particles cm 3 for 8.6 nm, 29 nm, 48 nm, and 98 nm, respectively while those in the photo-electrode are 2.68  1016, 4.05  1014, 2.14  1014, and 4.28  1013 particles cm 3 for 8.6 nm, 29 nm, 48 nm, and 98 nm, respectively.

presence of the single passivation agent on the AgNPs was ultimately not effective enough to prevent the corrosion. Hence, we utilized the ionic liquid-based electrolyte alternatively, because it has a large number of excess ions, which help to protect the surface of AgNPs from the access of redox compounds by electrostatic stabilization. The cation (EMIm + ) and anion (DCA ) from the ionic liquid could create a double protection layer on the surface of AgNPs,[28] thus reducing the possibility of corrosion by hindering the AgNPs from directly contacting the redox compounds in the electrolyte, as schematically illustrated in Figure 4 f. In addition, the thermally driven motion of AgNPs could be suppressed due to the relatively higher viscosity of the ionic liquid than that of acetonitrile,[29] thus minimizing the contact of AgNPs with redox compounds. Further details are given in the Supporting Information. In summary, we demonstrate that spatial arrangement of silver nanoparticles (AgNPs) with passivation layers of a few nanometers enables high-performance thin dys-sensitized solar cells (DSSCs). The high efficiencies, achieved without thick added scattering layers, can be attributed to synergy between near- and far-field coupling when the AgNPs reside in both the electrolyte and the photo-electrode. Moreover, thin DSSCs with AgNPs in an ionic liquid (IL)-based electrolyte are demonstrated to have long-term stability.

Experimental Section Nanoparticles Synthesis and Functionalization: We synthesized various sizes of AgNPs by polyol and borohydride reduction methods. The ratio between the Ag precursor and capping agents determined the nanoparticle size. Briefly, for the 8.6 nm AgNPs, 1 mL of 0.01 m silver perchlorate (AgClO4, Aldrich) dissolved in Type III water was mixed with the 1 mm sodium borohydride (NaBH4, Aldrich) and 0.3 mm sodium citrate (Na-cit, Aldrich) in a 99 mL cold ice water bath (4 8C) with vigorous stirring. For the 29 nm AgNPs,  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

1.5 g of PVP (polyvinylpyrrolidone, Mw: 10 000, Aldrich) and 1/ 37.5 wt % of silver nitrate (AgNO3, Aldrich) were dissolved in 75 mL of ethylene glycol (EG, Aldrich) and gradually heated to 120 8C at a rate of 1 8C min 1. The 48 nm AgNPs were obtained with 1/ 25 wt % AgNO3 and the solution was gradually heated to 120 8C at a rate of 5 8C min 1. The reactions were maintained for 1 h and then cooled to room temperature. The 98 nm AgNPs were obtained with mixing ratio of PVP to AgNO3 at 6:1 in 20 mL of EG. The reaction was maintained for 4 h at 140 8C with vigorous stirring. After cooling to room temperature, all solutions were washed with ethanol at least 5 times and stored in a refrigerator under dark condition. In addition, additional passivation agents were introduced onto the surfaces of the as-synthesized AgNPs by mixing them with sodium dicyanamide (Na-DCA, Aldrich), 2-dimethyl amino ethane-thiol (DMAET, Aldrich), and the ionic liquid, 1-ethyl3-methylimidazolium dicyanamide (EMIm-DCA, Aldrich). Prior to the mixing process, the absorption intensities of all as-synthesized AgNPs were measured to determine the number density of AgNPs (# cm 3) by UV/Vis spectrometry. UV/Vis absorption spectra were obtained using a UV/Vis spectrometer (Cary 300 Conc, Varian) with a 150 mm integrating sphere. The DMAET in methanol (0.4 m), and Na-DCA in deionized (DI) water (0.4 m) were added into the AgNPs dispersed in DI water, and then stirred overnight under an argon atmosphere. The suspension was then filtered and washed with DI water twice to remove the unreacted silver nitrite. During the stirring process, EMIm-DCA (80 wt %, 20 mL) was gradually added to the AgNPs suspension while the temperature was simultaneously increased to 50 8C over 5 h at a rate of 5 ~ 7 8C min 1. In addition, the remaining water and methanol were removed from the mixture using rotary pump. Photo-electrode Paste: The paste for reference photo-electrode was produced by mixing TiO2 paste with nitrogen-doped carbon nanotubes (N(sp2)-CNTs, 0.032 wt %).[30] The length and diameter of the N(sp2)-CNTs were 40 mm and 15 nm, respectively. The paste for photo-electrode containing AgNPs was made as follows: Various concentrations of AgNPs were directly mixed with the reference paste. During the mixing process, trace water was gradually evapo-

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Figure 4. Stability of the DSSCs. (a) Normalized efficiency of the DSSCs as a function of time in the ionic liquid- (green) and the acetonitrile based-electrolyte (purple) with AgNPs on combination of 8.6 nm in the photo-electrode to 98 nm in the electrolyte, where the optimized concentrations of 1.08  1014 particles cm 3 for 98 nm in the electrolyte and 2.68  1016 particles cm 3 for 8.6 nm in the photo-electrode have been used. The efficiencies were measured periodically at consecutive hours: 0, 24, 48, 72, 120, 168, 336, 720, 888, 1008, and 1056 h. (b–e) TEM images of AgNPs: (b) without a protection layer, and (c) with a protection layer; and after 2 months in storage: (d) without, and (e) with a layer. The AgNPs without protection layer were corroded after 2 months. (f) Schematic of stabilized AgNPs in the ionic liquid. The ionic liquid helps to protect the surface of AgNPs from the penetration of redox compounds, thus preventing them from corrosion. The dark and light orange color on the surface of AgNPs represents the double layer of cation-anion of ionic liquid.

rated from the mixture by using a vacuum rotary evaporator at 80 8C for 6 h until the paste became a slurry. Device Fabrication: FTO glass (TEC15, 15 W/sq, Pelkington, USA) of 1.5 cm  2 cm in size, was sequentially cleaned with triton-X, acetone, and ethanol for 20 min each in an ultra-sonication bath. The clean FTO was then dried using nitrogen gas flow before treatment with TiCl4. The TiCl4 treatment was conducted using a chemical bath deposition technique by immersing the clean FTOs into

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40 mm TiCl4 solution at 80 8C for 30 min, then rinsing with water and ethanol, and sintering at 450 8C for 30 min. The photo-electrode pastes were then applied to the FTO substrate by using a doctor blade technique, followed by a gradual sintering process at 150 8C for 20 min, at 350 8C for 15 min, and at 500 8C 30 min. The purpose of treating the FTO with TiCl4 is to achieve a good contact between the FTO and the doctor-bladed TiO2. For the photo-electrode containing AgNPs, the additional electrophoretic deposition[31] was performed as follows: FTO deposited with

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CHEMSUSCHEM COMMUNICATIONS photo-electrode paste was connected to the positive terminal, while the bare FTO was connected to the negative terminal, and both were immersed in an aqueous solution containing the desired concentration of AgNPs. The distance between the two terminals was maintained at 3 mm. A DC voltage of 60 V was applied for 15 min to initiate the electro-deposition process, and then, the substrate was rinsed with ethanol. This process was repeated 3 times. As a result, the AgNPs uniformly covered the TiO2 with a total surface coverage of approximately 15 % when viewed from above (see Supporting Information, Figure S5). A surface of the photo-electrode was cleaned by UV-ozone treatment for 30 min before immersing the photo-electrode into a 0.3 mm acetonitrile/ tert-butanol (1:1) solution of N719 dye for 20 h at 2 ~ 8 8C. The counter-electrodes were prepared by dip-coating clean FTO in an ethanol solution of 0.06 mm chloroplatinic acid hexahydrate (H2PtCl6·6 H2O, Aldrich) followed by sintering at 380 8C for 20 min. For device assembly, the photo-electrode and counter-electrode were sandwiched with the desired thickness of Surlyn film used both as a spacer and a sealing agent. The electrolyte, based on EMIm-DCA ionic liquid consisting of 1-ethyl-3-methylimidazolium iodide (EMIm-I, 0.5 m), 4-tert-butylpyridine (tBP, 0.36 m), lithium iodide (LiI, 0.13 m), and iodine (I2, 0.01 m) was injected into the cell through holes drilled at the counter-electrode side. The holes were then sealed with transparent tapes. For the non-ionic liquid-based electrolyte, we used commercially available acetonitrile electrolyte (Iodolyte AN-50, Solaronix), which consists of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIm, 0.6 m), guanidinium thiocyanate (0.1 m), tBP (0.5 m), and I2 (0.03 m). Raman Spectroscopy: High-resolution dispersive Raman microscopy (Horiba Jobin Yvon, France) was utilized to obtain the Raman spectra generated from the bulk devices. A 514.5 nm CW laser (Ar ion, 5 mW) source, closely corresponding to the first excitonic peak of the Ru-complex dye (N719) at 533 nm, was directed toward the photo-electrode side of the devices. Before collecting data, the surfaces of the active area of the photo-electrodes were verified using a confocal microscope (BXFM, Olympus) in order to ensure clean surfaces. The laser beam was focused on the clean and defect-free area. The spectra were scanned from 100 to 3000 cm 1 with a resolution of 2 cm 1, and only spectra in the range 900–1800 cm 1 were analyzed, which is an appropriate range for detecting the vibrational modes of N719 dyes. Photovoltaic Characterization: Current density–voltage (J–V) data were measured under simulated AM 1.5G illumination at 100 mW cm 2 (PEC-L 11, Peccell Technologies) with less than 25 % spectral mismatch and a thermostat controller (set to 23 ~ 26 8C), and these components were calibrated with a reference silicon solar cell. The active area of each cell was carefully determined with a digital microscope (0.16  0.02 cm2). EQE spectra were recorded using a spectrum measurement system (PEC-S2026, Bunkoukeiki Co. Ltd.), composed of a Xenon lamp light source (150 W), a monochromator with a 600-grooves/mm grating, and a 0.1 mm slit that provides a wavelength accuracy of  2 nm or less. Morphological Characterization: Size, shape, and morphological features were acquired by transmission electron microscopy (TEM) operated at 200 kV (ARM200F, JEOL Japan). Samples were prepared by drop casting the solutions onto carbon-coated copper grids. A scanning electron microscopy instrument (SEM, JEOL JSM-7401F and HITACHI) equipped with an EDX instrument was used to observe the attachment of AgNPs on the TiO2. Cross-sectional SEM images were used to determine the average thickness of the photo-electrodes.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org Data Processing: The concentration of AgNPs in Figure 2 b–f, 3 a– b, and 4 a are based on the optimum concentration for each AgNP size in each arrangement (see Figure 2 a), where the optimized concentrations in the electrolyte are 7.56  1016, 1.21  1015, 1.84  1014, and 1.08  1014 particles cm 3 for 8.6 nm, 29 nm, 48 nm, and 98 nm, respectively while those in the photo-electrode are 2.68  1016, 4.05  1014, 2.14  1014, and 4.28  1013 particles cm 3 for 8.6 nm, 29 nm, 48 nm, and 98 nm, respectively. In addition, the supplementary information of Figure S3 shows the thickness effect of the electrolyte and the photo-electrode on the EQE enhancement of AgNPs-embedded DSSCs. The electrolyte thickness was adjusted by the thickness of separator tapes. We examined 5 different electrolyte thicknesses of 20, 30, 40, 50, and 60 mm when the thickness of photo-electrode was maintained at 10.5 mm. As the thickness of electrolyte increases, the EQE is gradually improved regardless of the sizes of AgNPs. For instance, when using 98 nm AgNPs in the electrolyte, the EQE enhancement at the wavelength of 450 nm is progressively increased from 5.9 % to 8.46 % as the electrolyte thickness is increased from 20 mm to 60 mm (Figure S3d). The origin of such a EQE enhancement is attributed to a larger amount of AgNPs capable of being stored in a thicker electrolyte. Therefore, more photons can be optically scattered via far field coupling to yield a higher EQE enhancement. Similarly, to investigate the thickness effect on the photo-electrode, 5 different thicknesses of 4 mm, 7 mm, 8.76 mm, 10.5 mm, and 11.6 mm were examined by using the screen printing machine when the electrolyte thickness was fixed at 60 mm. Interestingly, the EQE enhancement is almost unchanged along the thickness of the photo-electrode as shown in Figure S3e–h. Furthermore, the maximum EQE is shifting from 380 nm to 450 nm as the sizes increased from 8.6 nm to 98 nm (Figure S3e–h). This fingerprint is matched with the LSPR spectra of AgNPs (Figure S2j). As a result, the 60 mm and 10.5 mm were selected as the optimum thicknesses of electrolyte and photo-electrode, respectively. Furthermore, for the data mapping, we plot the x-y axis with the interval of 15 nm for clarity. In addition, the contour maps for 8.6, 29, 48, and 98 nm are shown in Figure 2 f and 3 b.

Acknowledgements We appreciate valuable comments by Dr. Isnaeni and Prof. YongHoon Cho. This research was mainly supported by the National Research Foundation (2011-0028737) and the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning. In addition, J. K. Kang was supported mainly by the National Research Foundation of Korea (2009-0094038, 2010-0029042, 2011-0031407), the Korea Center for Artificial Photosynthesis (2009-0093881) funded by the Ministry of Science, ICT & Future Planning, and the Center for Inorganic Photovoltaic Materials (2012-0001174). Also, J.Y.L. was supported by the New & Renewable Energy Core Technology Program of KETEP (No. 20133030000130) and the international collaborative R&D program of KIAT (No. 1415134409). Keywords: dye-sensitized solar cells · nanoparticles scattering · silver · surface plasmon resonance

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COMMUNICATIONS Near, far, wherever you are: Synergy through coupling of near- and far-field scattering of size-controlled silver nanoparticles (AgNPs) improves the efficiency of thin dye-sensitized solar cell (DSSCs). Far-field scattering is critical in the electrolyte while near-field scattering is efficient in the photo-electrode. The size of the AgNPs affects both the photon-to-current efficiency in the electrolyte and in the photo-electrode. Use of a nonvolatile ionic liquid prevents corrosion of the nanoparticles. In addition, the DSSCs show excellent stability.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

G. W. P. Adhyaksa, S.-W. Baek, G. I. Lee, D. K. Lee, J.-Y. Lee,* J. K. Kang* && – && Coupled Near- and Far-Field Scattering in Silver Nanoparticles for High-Efficiency, Stable, and Thin Plasmonic Dye-Sensitized Solar Cells

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