Plasmon-enhanced dye-sensitized solar cells using SiO₂ spheres decorated with tightly assembled silver nanoparticles

Abstract

SiO₂ spheres decorated with tightly assembled silver nanoparticles were incorporated into the photoanode of a dye-sensitized solar cell. Localized surface plasmon resonance from the assembled Ag nanoparticles increased the light absorption throughout the wide visible light range. This plasmon-enhanced light absorption resulted in a significant improvement in the device performance.

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Dye-sensitized solar cells (DSSCs) have attracted substantial attention as a renewable energy conversion device because of their low production cost, easy fabrication process, and aesthetically appealing design.^1,2 However, the power conversion efficiency of DSSCs needs to be improved further in order for DSSCs to become an economically feasible alternative to conventional photovoltaic devices. For improving the efficiency of DSSCs, the light absorption performance of photoanode—a key component of DSSCs—should be enhanced as much as possible to increase the production of photocurrent. Thus far, various attempts have been made to enhance the light harvesting efficiency of photoanodes. These efforts include synthesizing new sensitizers with a broader range of absorption wavelength and a higher extinction coefficient,^3–6 designing new semiconducting electrode structures with light scattering properties,^7–11 mixing different dyes for co-sensitization,^12,13 and introducing non-radiative resonance energy transfer concepts.^14–16 Nevertheless, it is still difficult to attain the desired level of light efficiency that includes improved efficiency throughout the whole visible light region.

When free electrons in metal nanoparticles (NPs) are excited by incident light, collective electron charge oscillation called localized surface plasmon resonance (LSPR) occurs. This phenomenon enhances the near-field amplitude and generates the plasmonic scattering at the resonance wavelength, which has been applied on surface plasmon resonance biosensors,¹⁷ surface-enhanced Raman scattering spectroscopy (SERS) based analysis,^18,19 nanoscale optical devices,²⁰ and photocatalysts.²¹ Many researchers have attempted to utilize LSPR for various types of photovoltaic devices including DSSCs in order to improve light absorption by these devices.²²

In previous studies on utilizing LSPR for DSSCs, individual TiO₂ (or SiO₂)-coated noble metal NPs were incorporated into TiO₂ photoanodes, and the cell performance was enhanced via plasmon-enhanced absorption by sensitizers.^23–27 However, performance enhancement due to individual gold or silver NPs is commonly restricted to a particular wavelength region. Generally, the plasmonic absorption characteristics of metal NPs can be easily modulated by controlling their size and shape.²⁸ In addition, the use of an assembly of metal NPs gives rise to plasmon coupling, which contributes to stronger and broader plasmonic absorption.^29,30 Recently, Sheehan et al. reported a coupled plasmonic system for DSSCs that can lead to broadband plasmonic enhancement when used with aggregates composed of core–shell–shell Au@SiO₂@TiO₂ nanostructures.³¹ Therefore, strategies for incorporating plasmonic nanostructures, which are capable of maximizing the enhancement in broad light absorption, are crucial for practical photovoltaic devices.

Herein, we demonstrate a new approach for obtaining plasmon-enhanced DSSCs by utilizing SiO₂ spheres decorated with tightly assembled silver NPs (SiO₂-t-Ag@SiO₂). A schematic illustration of the photoanode employed in this study is shown in Fig. 1. Local electric field (E-field) enhancement and plasmonic scattering from tightly assembled silver NPs improved the cell performance by increasing the light utilization throughout the wide visible light range. Consequently, the overall power conversion efficiency of the cell increased upon the addition of SiO₂-t-Ag@SiO₂, and this was mainly due to the improvement in the photocurrent density owing to plasmonic effects of tightly assembled silver NPs.

Fig. 1 Schematic illustration of a photoanode with SiO₂ spheres decorated with tightly assembled silver nanoparticles (SiO₂-t-Ag@SiO₂).

Silica nanoparticles (SiO₂ NPs) were synthesized by the Stöber method,³² and then thiol groups were introduced on the surface of SiO₂ NPs; the experimental details of this process are described in ESI.† The SiO₂ NPs had diameters of approximately 150–200 nm; the sizes of the SiO₂ NPs were large enough for sustaining a large number of silver nanoparticles (Ag NPs) at a time. Ag NPs with diameters of 20–30 nm were densely grown on the thiol-functionalized SiO₂ surfaces (SiO₂-t-Ag). Then they were coated with a SiO₂ shell (SiO₂-t-Ag@SiO₂) by a similar procedure for electronically and chemically insulating them. Fig. 2(a) and (b) show transmission electron microscopy (TEM) images of SiO₂ spheres decorated with tightly assembled silver NPs before and after covering with a thin SiO₂ shell, respectively. Ag NPs on the SiO₂ core were covered with a ∼5 nm thick amorphous SiO₂ shell.

Fig. 2 TEM images of SiO₂ spheres decorated with tightly assembled silver nanoparticles (a) before and (b) after SiO₂ shell coating. (c) UV-vis absorption spectra of SiO₂-t-Ag (inset: UV-vis absorption spectra of individual Ag NPs) (d) calculated absorption and scattering efficiencies for SiO₂-t-Ag.

Fig. 2(c) shows UV-vis absorbance of the prepared NPs before SiO₂ coating. Across the whole visible light region, SiO₂ spheres decorated with tightly assembled Ag NPs (SiO₂-t-Ag) exhibited plasmonic absorption that was different from that exhibited by individual Ag NPs that generally show a sharp peak at around 400 nm (inset of Fig. 2(c)). This specific broadband absorption of SiO₂-t-Ag was due to coupled plasmon modes that originated from closely located Ag NPs.^33,34 In addition, on the basis of the absorption spectra of the NPs, it is expected that the LSPR of SiO₂-t-Ag can occur even in the near-infrared region. Fig. 2(d) shows calculated absorption and scattering efficiencies of SiO₂-t-Ag. At resonance wavelength, the plasmonic SiO₂-t-Ag particles not only absorb but also scatter light efficiently. The localized surface plasmon of SiO₂-t-Ag is expected to offer increased dye absorption via the interaction of dye molecule dipole and the light trapping by plasmonic scattering. It can be seen the power of plasmonic light scattering decreased more rapidly than the power of absorption with increase of wavelength, and the ratio of integrated area between absorption and scattering is 6.3 : 3.7.

For the introduction of SiO₂-t-Ag@SiO₂ into the photoanode of DSSC, we mixed it with a commercial TiO₂ paste (Dyesol, DSL 18NR-T). To obtain an optimized performance, pastes containing different concentrations of SiO₂-t-Ag@SiO₂ (1 and 3 wt%) were prepared. Further, a TiO₂ paste containing only a SiO₂ core (3 wt% SiO₂) was prepared to confirm plasmon-enhanced absorption by Ag NPs. In order to fabricate photoanode films for UV-vis spectroscopy, the pastes were doctor-bladed onto fluorine-doped tin oxide (FTO) glass plates and were then thermal-annealed at 500 °C for 15 min. The optical properties of the photoanodes fabricated using the prepared pastes are shown in Fig. 3. Generally, photoanode films before and after dye (N719) adsorption showed a similar tendency (Fig. S3†). The UV-vis absorbance of the fabricated photoanodes increased with increasing concentration of SiO₂-t-Ag@SiO₂ because of the strong plasmon absorption of Ag NPs. To investigate the effects of LSPR from Ag NPs on the absorption of N719 dye, the absorbance of pure N719 dye (Abs_dye) was calculated by subtracting Abs_film from Abs_film+dye. Fig. 3(b) shows that light absorption by pure dye molecules increased in the presence of SiO₂-t-Ag@SiO₂. This observation suggested that the increase in dye absorption is mainly due to the LSPR of Ag NPs. Diffused reflectance spectra of the photoanodes (Fig. 3(c)) were not significantly different; however, a slight increase was observed when SiO₂-t-Ag@SiO₂ or a SiO₂ core was incorporated. The increased diffused reflectance of 3 wt% SiO₂ indicated that the relatively large size and different refractive index of the SiO₂ core could cause a small amount of light scattering.

Fig. 3 UV-vis absorption spectra of (a) dye-adsorbed photoanodes, (b) pure N719 dyes of photoanodes (Abs_film+dye − Abs_film), (c) diffused reflectance spectra of dye-adsorbed photoanodes.

The photoelectrochemical characteristics of DSSCs with modified photoanodes were examined to confirm the contribution of LSPR to the cell performance. Fig. 4 shows the current density–voltage (J–V) characteristics of DSSCs with four types of N719-sensitized photoanodes (TiO₂, 1 wt% SiO₂-t-Ag@SiO₂, 3 wt% SiO₂-t-Ag@SiO₂, and 3 wt% SiO₂), which were measured under simulated sunlight conditions (an intensity of 100 mW cm⁻² at AM 1.5G). The main photovoltaic parameters are summarized in Table 1. The short-circuit current density (J_SC), open-circuit voltage (V_OC), fill factor (ff), and power conversion efficiency (η) of a conventional cell (TiO₂ photoanode cell) were 10.38 mA cm⁻², 0.756 V, 0.73, and 5.73%, respectively. On the other hand, the same parameters for a 3 wt% SiO₂-t-Ag@SiO₂ photoanode cell were 12.12 mA cm⁻², 0.784 V, 0.72, and 6.88%, respectively. By using SiO₂-t-Ag@SiO₂, significant improvement (17%) in J_SC was observed, and this led to a 20% increase in the power conversion efficiency of the cell as a consequence. The trend of increase in J_SC with the concentration of SiO₂-t-Ag@SiO₂ is clearly shown in Fig. 4. This increase was mainly due to improved light utilization by LSPR, in agreement with the UV-vis absorption spectroscopy results shown in Fig. 3(a) and (b). In addition, a slightly higher J_SC of 3 wt% SiO₂ cell than that of the TiO₂ cell showed the existence of additional light scattering by the SiO₂ core. These results were well supported by incident photon-to-current efficiency (IPCE) data (Fig. 5(a)). The IPCEs of the cells coincided with the absorbance spectra of photoanode films across the wavelength region in which the light absorption of N719 was dominant, and this was due to the plasmon-enhanced absorption of N719. For further analysis, we calculated IPCE enhancement factors (Fig. 5(b)). A steep increase in the long wavelength region (∼600 nm) was observed for 3 wt% samples, while there was no significant change for the 1 wt% sample in this range. This can be explained by the influence of light scattering, which is an important factor above 600 nm, as can be seen from diffused reflectance spectra shown in Fig. 3(c). It appears that the light scattering ability increased upon the addition of more SiO₂-t-Ag@SiO₂ by 2 wt%. The change in V_OC was another interesting result, as it increased with the concentration of SiO₂. For verification, we also measured J–V curves under dark conditions to obtain information about relative recombination rates. As the SiO₂ concentration increased, the onset of dark current experienced a shift toward a higher voltage range (Fig. 4). A large dark current density indicates that the cell had a high recombination rate, and this caused a decrease in V_OC by increased back reaction. In the SiO₂-blended DSSCs, SiO₂ spheres in TiO₂ nanostructures could passivate the recombination centers on adjacent TiO₂ surfaces, and then suppress electron back-transfer to redox couples in the electrolyte as a consequence. Considering that SiO₂ spheres have relatively large size and insulating property, further analysis is required to fully understand the effect of SiO₂ spheres on the overall cell system.

Fig. 4 J–V curves of DSSCs under illumination and in dark.

Table 1 Photovoltaic parameters of dye-sensitized solar cells

Photoanode	JSC (mA cm^−2)	VOC (V)	ff	η (%)
TiO2	10.38	0.756	0.73	5.73
1 wt% SiO2-t-Ag@SiO2	11.06	0.769	0.75	6.39
3 wt% SiO2-t-Ag@SiO2	12.12	0.784	0.72	6.88
3 wt% SiO2	10.56	0.793	0.74	6.16

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Fig. 5 (a) Incident photon-to-current efficiency (IPCE) spectra of DSSCs. (b) IPCE enhancement factor (IPCE_sample/IPCE_TiO2).

In summary, the plasmonic enhancement effects of a photoanode in DSSCs were investigated by using SiO₂ spheres decorated with tightly assembled silver NPs (SiO₂-t-Ag@SiO₂). Tightly assembled Ag NPs on a SiO₂ core showed broadband plasmonic absorption developed by coupled plasmon modes, which was not limited to a specific wavelength. By incorporating SiO₂-t-Ag@SiO₂ into the photoanodes of DSSCs, light absorption by the photoanode thin films definitely increased. DSSCs with a 3 wt% SiO₂-t-Ag@SiO₂ photoanode exhibited 20% better power conversion efficiency than a conventional cell, which was mainly due to improved J_SC that originated from the plasmon-enhanced light absorption of the photoanode. We expect that the utilization of this broadband plasmonic behavior produced by tightly assembled silver NPs may bring advances to other fields such as different types of solar cells, Raman spectroscopy, and bioimaging techniques.

Acknowledgements

This work was supported by the Institute for Basic Science (IBS) in Korea.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra00596a

‡ Current address: School of Electrical Engineering and Computer Science, Seoul National University, Seoul 151-742, Republic of Korea.

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