Recent advances in plasmonic dye-sensitized solar cells

Author’s Accepted Manuscript Recent advances in plasmonic dye-sensitized solar cells Won-Yeop Rho, Da Hyun Song, Hwa-Young Yang, Ho-Sub Kim, Byung Sung Son, Jung Sang Suh, Bong-Hyun Jun www.elsevier.com/locate/yjssc

PII: DOI: Reference:

S0022-4596(17)30426-7 https://doi.org/10.1016/j.jssc.2017.10.018 YJSSC19980

To appear in: Journal of Solid State Chemistry Received date: 8 May 2017 Revised date: 11 October 2017 Accepted date: 13 October 2017 Cite this article as: Won-Yeop Rho, Da Hyun Song, Hwa-Young Yang, Ho-Sub Kim, Byung Sung Son, Jung Sang Suh and Bong-Hyun Jun, Recent advances in plasmonic dye-sensitized solar cells, Journal of Solid State Chemistry, https://doi.org/10.1016/j.jssc.2017.10.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Recent advances in plasmonic dye-sensitized solar cells Won-Yeop Rhoa1, Da Hyun Songb1, Hwa-Young Yangc, Ho-Sub Kimb, Byung Sung Sona, Jung Sang Suhb, Bong-Hyun Juna* a

Department of Bioscience and Biotechnology, Konkuk University, Seoul 143-701, Republic of Korea.

b

c

Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea.

Department of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-

756, ,Republic of Korea. *Correspondence: Bong-Hyun Jun: [email protected]

Abstract Dye-sensitized solar cells (DSSCs) are among the best devices in generating electrons from solar light energy due to their high efficiency, low-cost in processing and transparency in building integrated photovoltaics. There are several ways to improve their energy-conversion efficiency, such as increasing light harvesting and electron transport, of which plasmon and 3-dimensional nanostructures are greatly capable. We review recent advances in plasmonic effects which depend on optimizing sizes, shapes, alloy compositions and integration of metal nanoparticles. Different methods to integrate metal nanoparticles into 3-dimensional nanostructures are also discussed. This review presents a guideline for enhancing the energy-conversion efficiency of DSSCs by utilizing metal nanoparticles that are incorporated into 3-dimensional nanostructures. Graphical abstract Recent advances in plasmonic effects which depend on optimizing sizes, shapes, alloy compositions and integration of metal nanoparticles are discussed. Different methods to integrate metal nanoparticles into 3dimensional nanostructures are also discussed. This review presents a guideline for enhancing the energyconversion efficiency of DSSCs by utilizing metal nanoparticles that are incorporated into 3-dimensional nanostructures.

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W.-Y. Rho and D. H. Song were equal contributors to the work. 1

Keywords: Dye-sensitized solar cells (DSSCs), plasmonic effect, metal nanoparticles, 3-dimensional nanostructures, energy conversion efficiency

1. Introduction Human have used fossil fuel as the main source of energy for almost two centuries, but it is now causing serious problems such as environmental damage and continuous rise in fuel prices [1, 2]. Due to ever increasing energy consumption, there is a great need for novel and inexpensive sources of renewable energy. Research on renewable energy is being actively pursued across the globe. Since sunlight is a plentiful, inexhaustible and ecofriendly source of energy, solar energy is one of the sustainable energy sources that has been aggressively studied at numerous research institutes in various countries [3, 4]. Current research on solar cells has achieved a significant progress in areas such as silicon-based solar cells, thin-film solar cells using III–V group elements, organic photovoltaic cells using organic compounds, quantum dot solar cells and dye-sensitized solar cells (DSSCs) [5-11]. Among these cells, silicon-based solar cells generally provide an energy-conversion efficiency of more than 20% to residential end users [12-15]. However,

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there are some drawbacks related to their complex and difficult manufacturing process as well as high overall cost [16, 17]. DSSCs are predicted to be the next generation of solar cells. The early version of DSSCs, developed by O’Regan and Grӓ tzel, used a ruthenium (Ru)-based dye and a porous, 10-μm-thick, TiO2 nanoparticle film. In these cells, the energy-conversion efficiency was 7%, which was high enough to motivate subsequent research in exploring other cells with similar structures [5, 18-26]. DSSCs have several unique advantages, namely 1) high energy-conversion efficiency in conjunction with relatively low cost; 2) low handling expenses; 3) mechanical durability; and 4) light weight. Moreover, DSSCs can be fabricated in a variety of colors with varying transparency and can be efficiently operated over a wide range of wavelengths. Yet, their main disadvantage is the energy-conversion efficiency which is lower than that of other solar cells [27-30]. Numerous studies on DSSCs are being conducted to find ways to overcome this disadvantage [31-34]. In this review, we discuss the recent advances in plasmonic DSSCs, specifically on metal nanomaterials for plasmonic DSSCs and on the plasmonic effect modified by different methods of integration within the structure of DSSCs.

2. Metal nanoparticles in DSSCs In general, the following components are essential to structure DSSCs: a TiO2 nanoparticle layer coated on a transparent conductive oxide (TCO) glass as photoanode; a dye (mostly Ru-based complexes, such as N3, N719, N749, Z907, black dye, etc.) [35-44] that is adsorbed onto the photoanode; an electrolyte such as I-/I3- redox mediator; and a counter-electrode (platinum deposited on TCO is commonly used). The photosensitized dye molecules absorb incident solar photons and excite electrons. These excited electrons are transferred to a conduction band of a TiO2 nanoparticle layer [45-47]. The oxidized dye molecules are then regenerated by collecting electrons from the electrolytes, and subsequently the electrolytes are reduced by accepting electrons from the counter-electrode as shown in Fig. 1(a) [29, 48-52]. However, in the plasmonic structure, the photosensitized dye molecules can absorb greater incidental solar energy and excite more electrons due to increased light absorption, reflection and scattering by metal nanoparticles as shown in Fig. 1(b). Compared to the normal structure of DSSCs, the plasmonic structure can generate more electrons due to subwavelength antennas, light scattering properties, or plasmonon-polaritons from incident light.

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Figure 1. General concept of (a) normal and (b) plasmonic structure in DSSCs.

In developing highly efficient DSSCs, several approaches can be applied such as 1) utilization of various materials that cover wideband solar spectral absorption by employing strong absorbing dyes [43, 53]; 2) efficient generation of electron-hole pairs [54]; 3) transportation of generated charge carriers; and 4) reduction of the loss of charge carriers at defects or interfaces [55-58]. As strong plasmonic near-fields significantly enlarge the cross-section of photon scattering, and thereby increase the overall dye absorption and the efficiency by many different routes (such as light trapping), applying a plasmon to DSSCs is likely be a very effective method in devising highly efficient DSSCs [59-62]. A plasmon is a type of quasiparticle consisting of free electrons that collectively vibrate within a metal. Especially, plasmons in metal nanoparticles are referred to as “surface plasmons” because they exist only on the surface of the metal nanoparticles. The term, surface plasmon resonance (SPR), refers to a phenomenon that typically occurs along an interface between a metal having a negative dielectric constant and a medium having a positive dielectric constant. Electromagnetic waves in the visible or near-infrared band can be combined with a plasmon, and the combination generates a plasmon-polariton which leads to optical absorption and higher electric field intensity (normally, a plasmon-polariton refers to another quasiparticle generated by the combination of a plasmon with a photon) [63, 64]. A plasmon-polariton generated by this process has stronger energy than incident light. It also has the same properties and forms as an evanescent wave which decays exponentially as it recedes in the vertical direction away from the interface. This feature of SPR indicates that light energy is accumulated on the surface of metal nanoparticles and that optical control is possible in a range narrower than the limit of optical diffraction [65-68].

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This phenomenon of SPR, i.e., having the capability for optical control, can be used to trap light in solar cells which is applicable to three types of methods used in solar cells (see Fig. 2). Firstly, nanoparticles can function as light scattering sites. As metal nanoparticles increase the number of optical pathways and allow more light to stay for longer duration within an element, absorption of more light can be achieved [69-73]. Secondly, nanoparticles can play a role of subwavelength antennas by the localized surface plasmon resonance (LSPR) effect. This effect assists in generation of more electric energy by increasing the range of electric field wavelength [74-77]. Lastly, if a corrugated metallic film is used, light is trapped by excitation states of surface plasmon-polaritons at the interface between a metal and a semiconductor, which also promotes absorbing more light energy [78-81]. In this regard, the SPR effect on light absorption can be a very effective method to overcome the limitation in DSSCs performance which is imposed by the limited (extremely thin) thickness of the photoactive layer [9, 82-84].

Figure 2. Illustration of the principle of SPR phenomenon applied to a solar cell [9]. Copyright 2010 Nature.

2.1. Au and Ag nanoparticles The SPR in noble metal nanoparticles can greatly enhance light absorption of DSSCs by utilization of subwavelength antennas, strong light scattering properties, and/or by generation of plasmon-polaritons from incident light, thereby increasing photocurrent and enhancing the efficiency of the device [85-87]. To increase the efficiency of DSSCs, various applications of noble metal nanoparticles have been attempted [70, 88]. Among them, Au and Ag nanoparticles are the most widely investigated metallic materials that can provide plasmonic enhancement in solar cells with their remarkable SPR optical properties [89, 90]. Au nanoparticles are commonly used because of their chemical stability and lower energy band, compared to those of Ag nanoparticles [68, 82, 91]. Ag nanoparticles, on the other hand, have a high energy band and scattering efficiency. Therefore, to enhance the energy-conversion efficiency of DSSCs by capitalizing the plasmonic effect, both Au and Ag nanoparticles have been utilized in DSSCs. In the following section, we focus on the effects of different structures and shapes (spherical nanoparticles, nanocubes, nanorods, nanoplates, etc.) of Au and Ag nanoparticles in developing plasmonic DSSCs.

2.1.1. Spherical nanoparticles Au and Ag spherical nanoparticles, whose maximum absorption wavelengths are approximately 400 nm and 530 nm, respectively, have been used to improve the optical absorption of dye molecules by the plasmonic effect 5

and thereby enhancing the energy-conversion efficiency of DSSCs. The absorption bands of these nanoparticles are well matched to those of the most commonly used Ru-based dye molecules, such as N719 (393 nm and 533 nm). Park et al. developed DSSCs by incorporating Au nanoparticles of ∼100  nm diameter into TiO2 nanoparticles. At an Au/TiO2 mass ratio of 0.05, the energy-conversion efficiency of the DSSCs improved from 2.7% to 3.3%, corresponding to a 20% enhancement, which is attributable to the plasmonic effect from the Au nanoparticles. The DSSCs based on TiO2 nanoparticles with Au nanoparticles had stronger light absorption of the dye and more prolonged optical paths by light scattering when compared with the DSSCs without Au nanoparticles [92]. However, when the metal nanoparticles are directly exposed to dye molecules or electrolytes, the results can cause several problems, including corrosion of metal nanoparticles, recombination or back reaction of charge carriers leading to a loss of charge carriers and poor charge separation at the electrodes. One of the promising approaches to bypass these issues is to coat the metal nanoparticles with an oxide shell, typically with TiO2 or SiO2. This shell not only preserves the metal nanoparticles from corrosion by I -/I3- electrolytes, but also increases the dye adsorption. Qi et al. fabricated DSSCs with their core-shell Ag@TiO2 nanoparticles. Since the TiO2 coating of Ag nanoparticles can protect the metal core from electrolytes with the Ag nanoparticles having high extinction coefficient, the energy-conversion efficiency of the DSSCs was enhanced from 7.8% to 9.0% [33]. Au nanoparticles coated with silica shells which were utilized in DSSCs were also reported. The DSSCs incorporating Au@SiO2 nanoparticles demonstrated a stable device performance for more than one month, which provides good evidence that the SiO2 shells can prevent the metal core from being damaged by electrolytes and dye molecules [93]. The energy-conversion efficiencies of plasmonic DSSCs with two different types of shell materials (TiO 2 vs. SiO2) were compared. Choi et al. prepared Au@TiO2 and Au@SiO2 nanoparticles to investigate the effect of different shell materials on the energy-conversion efficiency of DSSCs. In the DSSCs with Au@TiO2 nanoparticles, it was found that electrons were transferred to the Au core and charged the metal core, resulting in higher open circuit voltage (Voc) from the charging effect. On the other hand, in the DSSCs with Au@SiO2 nanoparticles, the SiO2 shell prevented electron charging of the metal core, which created higher short-circuit current density (Jsc) resulted from the surface plasmonic effect [94]. The thickness of the metal oxide shell, which is the distance between the dye molecule and the metal nanoparticle, can influence the plasmonic effect in the DSSCs. Liu et al. have studied the effect of shell thickness on the enhancement of plasmonic DSSCs using Au@TiO2 core-shell nanoparticles. When Au nanoparticles with a 5-nm TiO2 shell were incorporated in the DSSCs, the energy-conversion efficiency of the DSSCs improved from 6% to 7.38%, corresponding to an enhancement of 23%. When Au nanoparticles with a TiO2 shells thinner than 5 nm were incorporated into the DSSCs, the photocurrent of the DSSCs increased. When Au nanoparticles with a TiO2 shells thicker than 5 nm were incorporated into the DSSCs, the open-circuit voltage of the DSSCs increased [95]. Standridge et al. also demonstrated shell-thickness dependent plasmonenhancement properties in DSSCs using Ag@TiO2 nanoparticles, wherein DSSCs with a 2-nm TiO2 shell 6

thickness exhibited superior efficiency. As the thickness of the TiO2 shell increased, the efficiency of the DSSCs with Ag@TiO2 nanoparticles decreased due to the reduction in plasmon coupling between the dye molecule and the metal nanoparticle. Since the optimum shell thickness exists for the highest energy-conversion efficiency of DSSCs, it is necessary to adjust the thickness of the oxide shell to improve light harvesting capacity and stability [96]. The energy-conversion efficiency of DSSCs is highly dependent on the concentration of core-shell metal nanoparticles. Hossain et al. prepared a series of DSSCs with various concentrations of Ag@SiO 2 nanoparticles to explore the effect of concentrations of nanoparticles on the energy-conversion efficiency. As the concentration of Ag@SiO2 nanoparticles on a photoanode in DSSCs increased, the intensity of the lightabsorption spectra of the photoanode increased gradually, whereas the amount of dye adsorption decreased. At an optimal Ag@SiO2 nanoparticle concentration, the DSSCs had a 43.25% increase in the energy-conversion efficiency due to increased light absorption of the dye and light coupling caused by the plasmonic effect from Ag@SiO2 nanoparticles. However, an excessive amount of Ag@SiO2 nanoparticles (more than 3 wt%) adversely affected the energy-conversion efficiency because of the reduced effective surface area of the films, decreased amount of dye absorbed and increased charge-carrier recombination [97]. Therefore, it is essential to select the optimal concentration of metal nanoparticles to improve the energy-conversion efficiency of DSSCs. In comparison to using individual metal nanoparticles, the light-harvesting efficiency is enhanced by incorporating two types of metal nanoparticles into DSSCs due to their broader absorption region. Recently, our group reported double-layered DSSCs incorporated with both Ag and Au nanoparticles to maximize the plasmon-enhanced absorption of N719 dye. The energy-conversion efficiency of the DSSCs based on a component film containing both Au and Ag nanoparticles was higher than those with either Au or Ag nanoparticles. As shown in Fig. 3, the energy-conversion efficiency was improved from 8.42% to 10.03% due to the close optical matching between two visible absorption bands of N719 dye at 393 and 533 nm [98].

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Figure 3. (Top) UV-visible absorption spectrum of Ag (orange) and Au nanoparticles (purple) solution and N719 dye (blue) and (Bottom) the transmission electron microscope (TEM) images of Ag (left) and Au (right) nanoparticles [98]. Copyright 2015 Royal Society of Chemistry.

Andrei et al. investigated the plasmonic effect of Au NPs on DSSCs, based on various Ru dyes, as shown in Fig. 4. The levels of increase in efficiency were significantly different across different Ru dyes due to their varying ability to convert photons within the absorption wavelength of Au NPs. As a result, by incorporating Au NPs, the increase in efficiency for N3 dyes was the highest at 25.5 %, while that for N719 dyes was the lowest at 14.6 % when compared to that without Au NPs [99].

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Figure 4. Comparison of the J-V curves of DSSCs with/without Au NPs based on N3, N749, N719 dyes [99]. Copyright 2014 Andrei et al. Public Library of Science

2.1.2. Various shapes of nanoparticles The optical properties of metal nanoparticles inducing the plasmonic effect can also be influenced by their structures and shapes. As shown in Fig. 5, compared to spherical nanoparticles, anisotropic nanoparticles such as nanocubes, nanoplates, nanorods and nanostars have higher absorption efficiency and extinction, better electromagnetic field localization at edges and corners, and larger surface areas [100]. Since the distribution of solar energy is 43–45% in the visible region and 48–52% in the near-infrared (NIR) region [101], it is important to modulate the plasmonic effect in both visible and NIR region. One of the ways to achieve this is controlling the shapes of the nanoparticles. For example, the spherical Au or Ag nanoparticles are commonly restricted to a particular absorption region at approximately 400 nm or 500 nm, whereas the anisotropic nanoparticles exhibit a broader absorption band which includes the NIR region [102].

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Figure 5. Extinction (black), absorption (red) and scattering (blue) spectra calculated for Ag nanoparticles of various shapes: (a) a sphere, (b) a cube, (c) a tetrahedron, (d) an octahedron and (e) a triangular plate. (f) extinction spectra from rectangular bars with 3 kinds of different aspect ratios [102]. Copyright 2006 American Chemical Society.

Zarick et al. incorporated Au@SiO2 nanocubes into the photoanode of DSSCs to improve light harvesting. Finite-difference time-domain (FDTD) simulations showed that the light extinction by Au@SiO 2 nanocubes was very high in the 700–900 nm region due to their strong light scattering and absorption at this range as shown in Fig. 6. This result occurred because the intensity of electromagnetic fields at the corners and edges of the nanocubes was far greater than those of spherical nanoparticles, increasing the plasmonic molecular coupling and amplifying the carrier generation and hence the DSSCs efficiency [103]. Moreover, the photocurrent behavior and incident photon-to-current efficiency (IPCE) spectra showed that the performance of the device 10

was controlled by the concentration of Au@SiO2 nanocubes. For example, Jsc was significantly increased with 1.8 wt% of Au@SiO2 nanocubes, enhancing the energy-conversion efficiency from 5.8% to 7.8%, which corresponds to a 34% improvement in the performance of DSSCs.

Figure 6. (a) Low-magnification TEM images and (inset) high-magnification image of Au@SiO2 nanocubes (b) Schematic illustration of plasmon-enhanced DSSCs with nanocubes embedded within a TiO2 layer, (c) extinction spectra of Au nanocubes and (inset) the electromagnetic intensity profile. (d) The absorption, scattering and extinction spectra of Au nanocubes in a TiO2 layer [103]. Copyright 2014 American Chemical Society.

In addition, Elbohy et al. incorporated Au nanostars, which have broad and strong NIR light absorption at λmax ca 785 nm, into DSSCs based on N719 dye. The energy-conversion efficiency of the DSSCs with Au nanostars increased by 20%, from 7.1% to 8.4%. Meanwhile, with DSSCs based on N749 dye, which has a longer maximum absorption wavelength (at 410 nm and 610 nm) than N719, the energy-conversion efficiency of the DSSCs with Au nanostars was enhanced by 30%, from 3.9% to 5.0%. This improvement of energyconversion efficiency was attributed to the strong and broad plasmonic effect in the NIR region from 500 nm up to 1000 nm [104]. By simply modulating the aspect ratio of the nanorods (NRs), Li et al. fabricated DSSCs with an Au NR@SiO2 composite photoanode. Two spectrally separated plasmonic bands were created by the dependence of optical properties of NRs on their aspect ratio. When Au NR@SiO 2, which has its maximum absorption 11

wavelengths at 514 nm and 656 nm, was incorporated into the DSSCs, the efficiency was increased from 5.86% to 7.21%, showing a 23% enhancement, compared with that of the DSSCs without Au NR@SiO 2. The improvement of the energy-conversion efficiency of the DSSCs were attributed to the enhancement of the spectral response of the dye that resulted from the longitudinal plasmon absorption of the Au NRs [105]. Various concentrations of Ag nanowire (NW)@SiO2 core–shells have also been applied to DSSCs by Guo et al. Their results showed an enhancement of energy-conversion efficiency of DSSCs from 5.45% to 6.26% with the improvement of photocurrent from 9.69 mA/cm2 to 11.83 mA/cm2. These results are due to a significant increase in the ability to absorb light and capture photon of the dye molecules by incorporating the AgNW@SiO2, as the light coupling is increased by the plasmonic effect at 350 nm and 388 nm by the AgNW@SiO2 in the photoanode [106]. As can be seen in Fig.7, the effects of incorporating triangular nanoprisms and nanoplates were explored in DSSCs. In contrast to spherical Ag nanoparticles that have plasmonic absorption at ca 400 nm, red-shifted extinction is exhibited in anisotropic nanoparticles such as triangular silver nanoprisms and nanoplates. By changing their aspect ratio, the position of the plasmonic effect can be tuned from a blue-violet to NIR region. Incorporation of 0.05 wt% triangular Ag@SiO2 nanoprisms to the photoanode of DSSCs significantly improved the energy-conversion efficiency by 32% from their enhanced ability to light-harvest in the red and NIR regions (550–750 nm) [107].

Figure 7. The schematic illustration (left) and the J-V curve (right) of DSSCs with Ag@SiO2 nanoprisms [107]. Copyright 2013 American Chemical Society.

Three kinds of Ag nanoplates, which can operate in the entire visible region, were applied to DSSCs. By employing a rapid thermal process, our group prepared Ag nanoplates whose extinction maximum wavelengths were 470, 540, or 620 nm. As shown in Fig. 8, a panchromatic quasi-monolayer of Ag nanoparticles was fabricated by immobilizing three types of Ag nanoplates on a film coated with poly(4-vinyl pyridine) (P4VP). The plasmonic effect of the panchromatic quasi-monolayer with these Ag nanoplates took place across the entire visible region. The efficiency of the DSSCs incorporating the quasi-monolayer of Ag nanoparticles improved up to 11.4% as light absorption increased [108]. 12

Figure 8. Scanning electron microscope (SEM) images of three kinds of Ag nanoparticles, that have maximum absorption wavelengths at 470 (a), 540 (b) and 620 (c) nm, immobilized on a surface; and (d) a schematic drawing of Ag nanoparticles immobilized on a surface with poly(4-vinyl pyridine) (P4VP). The inset in (b) shows a TEM image of Ag nanoparticles [108]. Copyright 2015 Royal Society of Chemistry.

2.1.3. Alloy nanoparticles Compared with individual nanoparticles, alloy nanoparticles incorporated to DSSCs can be an effective solution to achieve a broad range of plasmonic absorption with high intensity and with the capacity to tune optical properties. To generate better optical properties than those of mono-metal nanoparticles, mostly mixed alloy (Ag-Au alloy) nanoparticles and core-shell bimetal nanoparticles (Au@Ag) were applied to DSSCs. For example, Al-Asawi et al. prepared Ag-Au alloy nanoparticles by a two-step synthesis method, mixing and irradiating the as-prepared Au and Ag colloids. After being irradiated by laser pulses, the mixed Au and Ag suspension exhibited a broad single absorption peak at 501 nm. The DSSCs with Au-Ag alloy nanoparticles had maximum values of photocurrent and energy-conversion efficiency of 11.67 mA/cm2 and 5.81%, respectively. Compared with DSSCs without Au-Ag alloy nanoparticles, the energy-conversion efficiency in the DSSCs with Au-Ag alloy nanoparticles had a 52.1% of improvement. This result is attributable to the extensive optical absorption of dye molecules from the plasmonic effect of Au-Ag alloy nanoparticles as shown in Fig. 9 [109].

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Figure 9. UV-visible absorption spectra of colloidal suspensions of Au, Ag and Au-Ag alloy nanoparticles before (black, red, blue lines) and after (green line) irradiation with 532-nm laser pulses [109]. Copyright 2015 American Chemical Society.

DSSCs with popcorn-shaped Au-Ag alloy nanoparticles were fabricated by Huang et al.. The irregular popcorn-shaped Au-Ag alloy nanoparticles enhanced the absorption band from 350 nm to 800 nm. Their broad light absorption band improved the energy-conversion efficiency of the DSSCs by 16%, from 5.26% to 6.09%, in 2.38 wt% of popcorn-shaped Au-Ag alloy nanoparticles [110]. Alloy nanoparticles with core-shell bimetal nanoparticles have also been applied to DSSCs. Yun et al. synthesized hollow TiO2 nanoparticles (HNPs) decorated with Au@Ag core-shell nanoparticles (Au@Ag/TiO2 HNPs). In the UV-visible spectra, the Au@Ag/TiO2 HNPs exhibited stronger and broader absorption from 350 nm to 800 nm. When 0.2 wt% of Au@Ag core/shell nanoparticles were incorporated into the photoanode, the energy-conversion efficiency was increased from 7.8% to 9.7%, corresponding to a 24% enhancement. This improvement is due to the broad light-absorption band which resulted in enhanced light-harvesting efficiency [111]. Dong et al. introduced Au@Ag NRs to the photoanode to achieve both increasing the adsorption by the dye and broadening the light-absorption band on the photoanode. The Au@Ag NRs had a wider plasmonic region from 350 to 900 nm with stronger peaks than simple Au nanostructures. In DSSCs with 3.68 wt% Au@Ag NRs, the energy-conversion efficiency reached 8.43% with optimized photocurrent of 16.53 mA/cm2, which was almost a 40% increase in the efficiency compared to that of the DSSCs without Au@Ag NRs. As shown in Fig. 10, the improvement is mainly attributed to the strong plasmonic absorption and effective light-harvesting efficiency in the visible and NIR region, caused by the introduction of Au@Ag NRs [112].

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Figure 10. Solar spectrum of reference (AM 1.5) and absorption spectra of N719, Au@Ag NR and Au NR [112]. Copyright 2015 Royal Society of Chemistry.

Au NRs can be encapsulated by an Ag2S layer to prevent recombination. Chang et al. incorporated Au NR@Ag2S into the photoanode of DSSCs. Compared to DSSCs based on photoanodes without Au NR@Ag2S, the DSSCs based on Au NR@Ag2S embedded in the photoanode had an improvement of 37.6% in photocurrent, resulted from the longitudinal plasmon resonance of Au NR@Ag2S in the 600–720 nm region [113].

2.2 Other metal nanoparticles In addition to Ag and Au nanoparticles, other metal nanoparticles such as Al, Cu, Pt, etc., are used in plasmonic DSSCs. Compared with Ag or Au nanoparticles, Al nanoparticles has plasmonic effect and chemical stability to iodide/triiodide electrolytes. Liu et al. investigated DSSCs by incorporating Al nanoparticles into TiO2 nanoparticles. The energy-conversion efficiency of the DSSCs improved from 6.15% to 6.95%, corresponding to about 13% increase due to the reduction of the quenching process and the plasmonic effect from Al nanoparticles [114]. Cu nanoparticles also used in plasmonic DSSCs to increase the energy-conversion efficiency of DSSCs [115-117]. Mahesh et al. achieved an enhancement of 26 % compared to TiO2 DSSCs by incorporating 0.3 mole% Cu nanoparticles into TiO2 nanoparticles and by plasmonic effect from Cu nanoparticles [116]. Zhang et al. synthesized a Cu nanowires@TiO2 core-shell nanostructure (Cu NWs@TiO2) and incorporated it into TiO2 photoanodes of DSSCs. By introducing 3.90 wt% Cu NWs@TiO2 into the TiO2 photoanodes, the energyconversion efficiency of the DSSCs improved from 7.65% to 9.44%, corresponding to a 23 % increase. This improvement is due to the improvement of the light absorption of dye molecules and the separation of carriers from the plasmonic effects of Cu NWs@TiO2 [117].

3. Integration within the structure of DSSCs The integration of metal nanoparticles into the structure of DSSCs is a critical factor for light harvesting. Plasmonic effects have been triggered by different geometries of integration on the active layer of DSSCs such 15

as front-side, rear-side, or active-layer integration. In the front-side integrated structure, metal nanoparticles were modified on a fluorine tin oxide (FTO) glass as a photoanode prior to being coated with metal oxide. In the rear-side integrated structure, metal nanoparticles were modified on a FTO glass as a counter electrode. In this case, the metal nanoparticles were directly modified on the FTO glass, core-shell type, etc. Lastly, in the activelayer integrated structure, metal nanoparticles in a photoanode were incorporated on metal oxide nanoparticles with different configurations - at the top, middle or bottom - and they were combined across all three positions.

3.1. Front-side integration When metal nanoparticles are integrated on the front-side, the energy-conversion efficiency of DSSCs is improved by the plasmon effect. Since the plasmon effect depends on the size of metal nanoparticles, DSSCs with metal nanoparticles of different sizes were explored in front-side integrated DSSCs. Zhang et al. investigated a series of DSSCs with a modified FTO glass using various sizes of immobilized Au nanoparticles (15–80 nm). The DSSCs with FTO containing immobilized 25 nm Au nanoparticles showed the highest efficiency of 6.69% with high values of Jsc (12.84 mA/cm2) and Voc, (0.78 V). The DSSCs with FTO exhibited a 15% enhancement compare to DSSCs with bare FTO (5.84%). A further increase in the size of the Au nanoparticles (greater than 60 nm) caused a decrease in the efficiency of the DSSCs due to an increase in the Rct2 value. This result represents the electron transfer that occurrs at the TiO2/dye/electrolyte interface by aggregation and formation of Au islands [118]. Density distribution of metal nanoparticles was also considered in front-side integrated DSSCs. Choi et al. studied DSSCs that used a modified FTO glass with immobilized Ag or Au nanoparticles. The energyconversion efficiency of the DSSCs with FTO/Ag nanoparticles/TiO2 was 7.49%, which was better than that of the DSSCs with FTO/TiO2 (6.54%). The improvement can be attributed to plasmonic absorption by the Ag nanoparticles. On the other hand, the energy-conversion efficiency of the DSSCs with FTO/Au nanoparticles/TiO2 (6.27%) was lower than that of the DSSCs with FTO/TiO2. The result can be explained by the denser distribution of small sized Au-nanoparticles on FTO which caused blocking of incident light [119]. Recombination is another factor that needs to be considered in the integration of metal nanoparticles in frontside DSSCs. Lin et al. fabricated DSSCs with FTO/Ag nanoparticles/TiO 2 electrode geometry. The photovoltaic properties of the DSSCs with FTO/Ag nanoparticles/TiO2 electrodes were similar to those of the DSSCs without Ag nanoparticles. In the DSSCs, the Ag nanoparticles were not able to trap incident light in the active layer nor improve the ability to collect excited electrons to the FTO/Ag nanoparticles/TiO2 electrode in charge separation and transfer process. The result is due to a smaller Schottky barrier at the interface between the Ag nanoparticles and the TiO2, which influenced efficient light trapping [120]. In sum, when metal nanoparticles are integrated in front-side DSSCs, it is necessary to consider the size, density distribution and work function of the metal nanoparticles in order to enhance the energy-conversion efficiency of the DSSCs by plasmonic effects.

3.2. Rear-side integration To facilitate more light absorption to the photoanode, the light reflection and scattering by a counter 16

electrode is best achieved by introducing metal nanoparticles to induce a plasmon effect. In this case, the metal nanoparticles can provide higher catalytic and better optical properties to DSSCs. Wang et al. fabricated DSSCs whose counter electrode was modified by introducing an Au layer. The DSSCs with the counter electrode modified by the Au layer showed an 18% improvement, from 5.61% to 6.64%. The improvement can be attributed to enhanced light scattering, reflection, light collection and electron-hole pair generation by the Au layer. Consequently, these factors resulted in an increase in the length of optical path, which improved the photocurrent and energy-conversion efficiency of the DSSCs [121]. Wu et al. synthesized DSSCs based on a Pt counter electrode with an assembly of Ag nanoparticles. By integrating the assembly of Ag nanoparticles in the counter electrode, the energy-conversion efficiency of the DSSCs was increased from 6.95% to 7.96%. This improvement corresponds to an enhancement by a factor of 1.145 when compared to the DSSCs without Ag nanoparticles. The result is due to their greater surface area, stronger and diffused reflection of light and the injection of hot electrons caused by the plasmonic effect of the Ag nanoparticles [122]. Wu et al. investigated DSSCs with integrated Au nano-islands (NIs) at a counter electrode. By the integration of Au NIs at the counter electrode, the energy-conversion efficiency of the DSSCs was improved from 5.31% to 6.32%, corresponding to a 19% enhancement compared to that of the DSSCs without Au NIs. The improvement was attributed to the increased photocurrent as well as suppression of R1 at the counter electrode and R2 at the photoanode, caused by the plasmonic effect and the increase in the surface area of the counter electrode from the integration of the Au Nis [122]. Lacroix et al. employed PEDOT (poly[3,4-ethylenedioxythiophene]) films incorporated with Au nanoparticles as counter electrodes in DSSCs. Compared to DSSCs with a plain PEDOT film as a counter electrode, the efficiency of the DSSCs with the Au nanoparticles-PEDOT film improved from 1.4% to 3.2%, which was a 130% increase. The enhancement of the energy-conversion efficiency is mainly due to an increase in the conductivity of the counter electrode. In summary, metal nanoparticles have been integrated into the rear-side of DSSCs to enhance the energyconversion efficiency by means of plasmonic and reflection effects [123].

3.3. Active-layer integration For better energy-conversion efficiency of the DSSCs, metal nanoparticles can be integrated into an active layer to reduce recombination and to enhance the amount of photocurrent. The effect of metal nanoparticles within the active layer has been studied. Jung et al. fabricated DSSCs with Au NIs at different positions (i.e., the top, the middle and the bottom of the active layer) to identify plasmonic effects in the DSSCs. Compared to DSSCs without any integration of metal nanoparticles on the photoanode (5.92%), the energy-conversion efficiencies of the DSSCs with Au NIs, integrated at the top, the middle and the bottom of the active layer were increased to 6.49%, 6.37% and 6.52%, respectively. The different positions of integration did not significantly influence the energy-conversion efficiency of the DSSCs, as their optical properties and photovoltaic performances were similar [124]. Well-aligned TiO2 nanotube arrays in DSSCs have better electron transport, trapping, and transfer to redox 17

electrolyte than simple TiO2 nanoparticles. The volume fraction is larger in channels by about 56%. However, the energy-conversion efficiency of the DSSCs with TiO2 nanotube arrays is still less than those with TiO2 nanoparticles, because the surface area for dye adsorption is much smaller. The plasmonic effect in TiO2 nanotube arrays is one of the best ways to improve the energy-conversion efficiency. Wang et al. fabricated DSSCs with Ag nanoparticles deposited on the TiO2 nanotube array. The energy-conversion efficiency of the DSSCs based on the modified TiO2 nanotube array with Ag nanoparticles increased from 1.20% to 1.68%, corresponding to a 40% enhancement compared to that of the DSSCs based on a bare nanotube array. The enhancement is due to the enhanced light absorption and electron-hole separation resulting from the plasmonic effect [125]. Our group investigated plasmonic DSSCs with TiO2 nanotube arrays. The TiO2 nanotube arrays were dipped in Ag precursor solution and then irradiated with a 254 nm UV lamp. The Ag + can diffuse into the channel of the TiO2 nanotube arrays since it is dissolved in an aqueous solution. When the TiO2 nanotube arrays are exposed to UV irradiation at 254 nm, the valence band of TiO 2 is excited to the conduction band, which can reduce the Ag+ to Ag. DSSCs with and without Ag nanoparticles were fabricated as shown in Fig.11 [126]. For the DSSCs with Ag nanoparticles, the energy-conversion efficiency was increased from 4.64% to 6.14%, equivalent to an improvement of 32% compared to DSSCs without Ag nanoparticles. This result indicates that the total active layer of TiO2 nanotube arrays was affected by the Ag nanoparticles. Moreover, there are three ways to improve the energy conversion efficiency of DSSCs with Ag nanoparticles on TiO 2 nanotube arrays: 1) modify the TiO2 nanotube arrays [127]; 2) reduce the Ag nanoparticles [128]; and 3) blend different types or shape of metal nanoparticles [98, 108, 129]. Firstly, large TiO2 nanoparticles (400 nm) or carbon materials decorated on TiO2 nanotube arrays [127, 129] are one of best methods to enhance the energy conversion efficiency in DSSCs due to the fact that large TiO2 nanoparticles are well-known scattering materials [127] and also that carbon materials are well-recognized electron transport materials from their π-π interaction [130]. The second method is to reduce the core-shell type of Ag@Ag2O nanoparticles that are formed during the sintering step. The oxide layer of Ag nanoparticles is disturbed by the plasmonic effect as the effect is influenced by distance [96], and hence this layer could be removed and applied to DSSCs for better energy conversion efficiency. The last method is to match the absorption band of metal nanoparticles and dyes. Some metal nanoparticles have a narrow absorption band and different absorption wavelength compared to those of the dye. In this case, to provide a better plasmonic effect, metal nanoparticles could be prepared whose absorption is similar to that of dye. Hence, to improve the performance of DSSCs with TiO2 nanotube arrays, different types of metal nanoparticles could be homogeneously integrated into a channel for greater light harvesting and better electron transport.

18

Figure 11. (A) Overall schema of plasmon DSSCs based on TiO2 nanotube arrays, (B) Schematic diagram of assembled cells [126]. Copyright 2014 Elsevier.

Table 1. The parameters of the DSSCs devices including Jsc, Voc, Jsc, ff and power conversion efficiencies (η) with different metallic types or shapes by integrated structures in DSSCs. Integrated structure Activelayer Activelayer Activelayer Activelayer Activelayer Activelayer Activelayer

Metal Type

Shape

Au@SiO2 Nanospheres Au@TiO2 Octahedral Ag@SiO2 Nanospheres Au@ SiO2

Nanocubic

Au

Nanostars

Au@SiO2

Nanorods

Ag@SiO2

Nanowires

Size

Jsc (mA/cm2)

2.14 to 3.37 11.55 to 23 nm 14.73 10.20 to 10 nm 13.85 13.5 to 45 nm 18.3 15.1 to 60–70 nm 17.2 L: 40-45 nm 13.15 to D: 17-20 nm 15.88 9.69 to D: 55 nm 11.83 15 nm

Voc (V)

ff

0.74 to 0.76 0.73 to 0.72 0.63 to 0.67 0.67 to 0.67 0.75 to 0.78 0.69 to 0.73 0.72 to 0.71

0.66 to 0.76 0.71 to 0.70 0.67 to 0.67 0.64 to 0.60 0.62 to 0.62 0.64 to 0.62 0.68 to 0.63

η (%) 1.05 to 1.95 6.00 to 7.38 4.30 to 6.16 5.8 to 7.3 7.00 to 8.45 5.86 to 7.21 5.45 to 6.26

Enhancement ref 85.71%

[93]

23.00%

[95]

43.25%

[97]

25.86%

[103]

20.71%

[104]

23.04%

[105]

14.86%

[106] 19

Activelayer Activelayer Activelayer Activelayer Activelayer

11.2 to 14.4 470 nm, 540 16.23 to Ag Nanospheres nm, 620 nm 19.27 9.08 to Au-Ag Nanospheres 25.12 nm 11.67 Popcorn 13.07 to Au-Ag 200 nm shape 19.51 Au: 15 nm 15.8 to Au@Ag Nanospheres Ag: 25 nm 17.3 Au L: 65 nm ActiveD: 15 nm 11.69 to Au@Ag Nanorods layer Ag 16.53 L: 70 nm D: 28 nm ActiveL: 48 nm 8.79 to Au@Ag2S Nanorods layer D: 21 nm 11.10 11.90 to Front-side Au Nanospheres 25 nm 12.84 12.83 to Front-side Ag Nanospheres 6–7 nm 13.49 Hexagonal 12.6 to Rear-side Au D: 100 nm structure 14.6 14.27 to Rear-side Ag Nanospheres 70 nm 16.50 9.4 to Rear-side Au Nanoislands 9 nm 17.5 Rear-side

Activelayer

Ag@SiO2

Au

Au

ActiveAg layer ActiveAg@TiO2 layer ActiveAg@TiO2 layer ActiveAu and Ag layer

Nanoprism

Nanospheres

70 nm

15 nm

Nanoislands

33 nm

Nanospheres

10–40 nm

Nanospheres

30 nm

Nanospheres

20 nm

Multi-shaped Ag: 29 nm nanospheres Au: 19 nm

3.6 to 7.6 Ref: 14.4 Bottom: 15.5 Middle: 15.3 Top: 15.6 All: 16.8 2.27 to 4.37 8.47 to 11.65 12.46 to 16.46 15.86 to 19.76

0.65 to 0.70 0.80 to 0.79 0.68 to 0.76 0.73 to 0.71 0.74 to 0.75

0.76 to 5.6% to 0.74 7.4% 0.69 to 9.3 to 0.71 11.0 0.62 to 3.82 to 0.65 5.81 0.62 to 5.94 to 0.57 7.85 0.71 to 8.3 to 0.72 9.4

0.71 to 0.73

0.70 to 0.70

5.91 to 8.43

0.73 to 0.79 0.77 to 0.78 0.82 to 0.82 0.69 to 0.69 0.74 to 0.75 0.73 to 0.66 0.70 to 0.71 Ref: 0.67 Bottom: 0.69 Middle: 0.67 Top: 0.68 All: 0.69 0.78 to 0.79 0.81 to 0.78 0.86 to 0.87 0.76 to 0.75

0.67 to 0.64 0.64 to 0.67 0.61 to 0.67 0.65 to 0.64 0.66 to 0.65 0.77 to 0.55 0.56 to 0.60 Ref: 0.61 Bottom: 0.60 Middle: 0.61 Top: 0.61 All: 0.60 0.68 to 0.49 0.67 to 0.68 0.75 to 0.74 0.70 to 0.69

4.3 to 5.6 5.84 to 6.69 6.54 to 7.49 5.6 to 6.6 6.95 to 7.96 5.31 to 6.32 1.4 to 3.2 Ref: 5.92 Bottom: 6.52 Middle: 6.37 Top: 6.49 All: 7.01% 1.20 to 1.68 4.64 to 6.14 8.04 to 10.60 8.44 to 10.22

32.14%

[107]

18.28%

[108]

52.09%

[109]

32.15%

[110]

13.25%

[111]

42.63%

[112]

30.23%

[113]

14.55%

[118]

14.52%

[119]

17.86%

[121]

14.53%

[122]

19.02%

[122]

128.57%

[123]

Bottom: 10.13% Middle: 7.60% Top: 9.63% All: 18.41%

[124]

40%

[125]

32.32%

[126]

31.84%

[129]

21.09%

[129]

4. Conclusion & Perspective One of the great methods to improve energy-conversion efficiency of DSSCs is by utilizing the plasmonic effect with metal nanoparticles for higher light harvesting and more electron generation. Another way is by charge separation with 3-dimensional nanostructures for better electron transport and longer electron lifetime. 20

Different properties of metal nanoparticles such as sizes, shapes, assembled structures, or alloy types have been extensively studied in DSSCs to enhance the energy-conversion efficiency by the plasmonic effect, which are summarized in Table 1. To optimize light harvesting for enhanced energy-conversion efficiency, broad light absorption by mixing different types of nanoparticles is required. In integrated DSSCs structures, homogeneous distribution of nanoparticles in the active layer improves light harvesting. In particular, integrating the metal nanoparticles on 3-dimensional nanostructures is very promising as it improves light harvesting and electron transport. In this regard, integrating different types of metal nanoparticles for broad light absorption in the active layer is probably the best way to enhance the energy-conversion efficiency, the method of which can be applied to other types of photovoltaic devices.

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