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Upconversion-Enhanced Dye-Sensitized Solar Cells
CHAPTER 9
Upconversion-Enhanced DyeSensitized Solar Cells Deyang Li1,2 and Guanying Chen1,2 1
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, P.R. China 2 Key Laboratory of Micro-systems and Micro-structures, Ministry of Education, Harbin Institute of Technology, Harbin, P.R. China
9.1 INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted extensive research interests in the photovoltaic field because of their advantages of lower product cost, simple fabrication, and being environmentally friendly. Since the first fabrication of DSSCs (efficiency, 7.12%) by Michael Grätzel and his coworkers in 1991 [1], the efficiency has now reached 14.5% [2], comparable to that of the commercial Si-based solar cell. However, the inability to use light in the infrared (IR) spectral range, which accounts for almost half the energy of solar irradiation, poses a stringent limit for further increasing the conversion efficiency of DSSCs. The typically used dyes in DSSCs, for example, ruthenium-based dyes, are only able to absorb light in the region of 300 800 nm [3], while the absorption range of crystalline silicon (c-Si) solar cell is 300 1150 nm [4]. This indicates that DSSCs have a narrower solar spectrum response region than c-Si solar cells, and thus a higher transmission loss. The transmission loss arises from the fact that solar cells can only absorb photons with higher energy than the bandgap, while the photons with energy lower than the bandgap are transparent for solar cells. As a result, the absorption range of dyes determines the overall efficiency of DSSCs [5]. Finding ways to expand DSSCs’ spectral response to the IR region (typically $ 700 nm) are under intensive investigations. The near-infrared (NIR)-responsive photosensitizers are explored for obtaining panchromatic DSSCs [6 11]. Unfortunately, this type of panchromatic DSSC was found to have a problem of poor electron injection efficiency and serious charge recombination [12]. Upconversion (UC) materials, with the ability to convert NIR photons to visible light, have been considered as a promising approach for Dye Sensitized Solar Cells DOI: https://doi.org/10.1016/B978-0-12-814541-8.00009-4
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enhancing light harvesting in the IR region for solar cell devices [13 15]. By employing doped rare-earth ions with real ladder-like energy-level structures, visible or ultraviolet photons can be generated, whereby two or more photons in the IR range are sequentially absorbed via real intermediate long-lived energy levels. Generally, the upconverting light produced from rare-earth ions is sharp because this emission arises from 4f to4f orbital electronic transitions and the 4f electrons are shielded by the outer complete 5s and 5p shells [16 17]. As a consequence, the upconverting light produced by rare-earth ions can exhibit high resistance to photobleaching and photochemical degradation. In addition, the required power density to excite rare-earth ions for light UC is as low as B0.1 W/cm2 [18], close to the level of solar irradiation on earth. Indeed, it was reported that the application of UC materials to a silicon solar cell can impressively improve its overall performance due to the enhanced NIR light harvesting [19]. Moreover, it has been predicted that, with the application of upconverters possessing a narrow absorption bandwidth of B0.5 eV, the Shockley Queisser efficiency limit of the solar cell with a bandgap of 1.7 eV (close to that of the dye N719 typically used in DSSCs), can be boosted from 28.2% to as high as 43.6% [20]. It should be noted that the optimization of UC materials and solar cell structure can be done separately; therefore, the UC technology can be applied to all the existing types of solar cells without altering their structures. Alongside the role of UC, UC materials can also provide light scattering for the light-harvesting capability by increasing the optical trapping effects of a DSSC device [21]. The combination of UC and scattering effect from UC materials entails a useful strategy for the improved performance of DSSCs. In this chapter, we discuss the operational principle of UC for UC material-incorporated DSSCs, the related three typical solar cell structures, the strategies to enhance UC materials for light absorption and scattering, and the perspectives for future directions.
9.2 THE OPERATIONAL PRINCIPLE OF UPCONVERSION IN DYE-SENSITIZED SOLAR CELLS For a conventional DSSC without UC (e.g., employing N719 dye), the bandgap (B1.8 eV) determines that the device is responsive to light with wavelength shorter than 689 nm, while being unresponsive to longer wavelength light. Namely, the visible light can excite the N719 dye to produce photo-induced electrons, which are then injected into the
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conduction band of the TiO2 film (the photoanode), and finally flowed out of indium tin oxide glass to the external circuit to form a photocurrent. At this point, the amount of photo-induced electrons largely determines the efficiency of DSSCs. The upconverting effect of UC materials can increase the yield of photo-induced electrons by harvesting NIR light, which is unresponsive for conventional DSSC solar cells. UC materials are typically composed of two elements: (1) host materials and (2) lanthanide ions. Generally, host materials are dielectric materials with good chemical stability and low phonon energy, for example, NaYF4 (phonon energy of B350 cm21) that incorporate trivalent lanthanide ions. Low phonon energy allows to minimize nonradiative losses at the intermediate states, thus leading to UC emission at higher efficiency. The Er31, Ho31, Tm31, and Pr31 are commonly used as emitter ions in UC materials, while Yb31 ions are often codoped as the sensitizer, which have B10 times higher absorption cross-sections than the emitter ions. The basic UC mechanisms are comprised of excited-state absorption, energy transfer upconversion (ETU), cooperative sensitization UC, and photon avalanche. The ETU process is the most used mechanism for lanthanide ions, for example, Er31/Yb31, toward the application of UC materials to increase the efficiency of DSSCs. Taking the Er31/Yb31 codoped UC materials with an ETU mechanism, for example, when being incorporated in DSSC cells, the operational principle of UC effect is schematically shown in Fig. 9.1. First, NIR light (B980 nm) is absorbed by the Yb31 ion (the sensitizer) that has two energy states of 2F2/7 and 2F5/2, resulting in a Yb31 ion being excited to 2F5/2 from 2F2/7 energy state. Second, the absorbed energy by the sensitizer is then transferred to an Er31 ion, an activator, exciting it to the energy state 4I11/2. Then, the Er31 ion at this state is further excited to the energy state 4F2/7 by absorbing more energy from a neighboring excited Yb31 ion. Subsequently, the Er31 ion relaxes to the ground state by emitting visible upconverted light at 525/545 nm. Since the light-absorbing dye (e.g., N719) has a strong absorption in the visible range, the emitted visible upconverted light can excite the dye to generate extra photo-induced electrons, thus enhancing the photocurrent of DSSCs. Note that when the dye is placed in close contact with the UC materials (within a few nanometers), the Er31 ion at the 4F2/7 state can nonradiatively transfer its contained energy to the dye (e.g., N719 dye), in which the energy transfer efficiency is defined by the distance between them, and the spectral overlap between the UC emission and the
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Figure 9.1 The operational principle for the utilization of upconversion (UC) materials to enhance the performance of dye-sensitized solar cells. The ultraviolet and visible sunlight can be efficiently harvested by the dye and TiO2 layer, generating separated charge carriers. Meanwhile UC materials are able to harvest the unutilized near-infrared light, and then convert it into high-energy visible light utilizing the real energy levels of rare-earth ions. The converted visible luminescence can then be captured by the dyes to produce charge carriers for the photocurrent.
absorption of the dye. The described operation principle of UC here can be applied to all other types of rare-earth UC for use in DSSCs. Alongside UC, the scattering effect can also enhance the performance of DSSCs, which refers to the optical path of incident light in the lightabsorbing layer which can be increased due to the size-induced scattering effect of UC materials. Since scattering is a universal phenomena which applies to light of all wavelengths, the scattering effect enhances the spectral response of a DSSC over all wavelength ranges. This is in marked contrast to the UC effect, which contributes to the NIR wavelength range by producing light harvesting. The different types of host lattice for UC materials significantly influence the UC contribution to the performance enhancement of DSSCs, as they impact the UC efficiency greatly. The major standards for choosing excellent host lattice are aspects of low lattice phonon energy and high chemical stability. As a result, materials of fluoride (e.g., NaYF4 and LaF3), oxide (e.g., TiO2, Y2O3, Gd2O3, and GeO2), and oxyfluoride (e.g., YOF) have often been used as the matrix of UC materials.
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Particularly, titanium oxide (TiO2) can also be utilized as the UC host material, and has the advantage of having compatibility with the mesoporous TiO2 film of a DSSC, entailing a strong interaction with the dye. It is known that the open-circuit photovoltage (Voc) is mainly determined by the difference between the electron quasi-Fermi level of the TiO2 film and of the redox mediator. The doping of rare-earth ions can elevate the Femi level of TiO2 [22], due to the replacement of Ti41 with Er31 or Yb31, thus increasing the Voc of the DSSCs. This favors the overall performance of DSSCs.
9.3 THREE TYPES OF UPCONVERSION-ENHANCED DYESENSITIZED SOLAR CELL STRUCTURES A typical DSSC is composed of FTO glass, mesoporous TiO2 film coated with dye, redox electrolyte, and FTO glass coated with Pt positioning UC materials into different parts of DSSCs will produce different types of UC-enhanced structures. When placing UC materials in the internal of the cell, there are three integration modes (Fig. 9.2). (1) One way is to mix UC materials and TiO2 nanoparticles to form a hybrid TiO2 film (structure A). (2) The other way is to replace the noble conterelectrode metal Pt with UC materials that possess the catalytic activity for the iodine-based redox couple (structure B). (3) The third type is to place UC materials outside the cell. Usually, UC nanoparticles are coated at the rear side of FTO glass which is coated with Pt (structure C).
Figure 9.2 The three typical structures of upconversion-enhanced dye-sensitized solar cells. (A) Placing upconversion (UC) nanoparticles into the light-harvesting layer of a mesoporous TiO2 film (inside the cell); (B) employing an upconverting counterelectrode (inside the cell); (C) placing UC materials in the device back as a lightconverting layer (outside the cell).
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The fundamental difference between these three structures lies in the distance between the upconverting materials and the light-harvesting dyes. Structure A can make full advantage of the produced upconverting processes due to the shortest distance (within the nanometer scale) between the UC materials and the dye. While, for structures B and C, before reaching the dye, the upconverting light has to pass through an electrolyte and counterelectrode, thus weakening the intensity of the UC light. At the same time, the macroscopic distance between UC materials and light-harvesting dyes also prevents the Förster resonance energy transfer (FRET) process from occurring, which allows the upconverted energy to be efficiently accepted by light-harvesting dyes. However, the main advantage of structure B lies in its ability to lower the material cost by replacing the noble metal, Pb, that plays an important role in catalyzing 2 I2 circulation, while the main advantage for structure C is that the 3 =I UC materials are placed outside the solar cell device, leaving the device intact.
9.4 STRATEGIES FOR THE IMPROVEMENT OF UPCONVERSION-ENHANCED DYE-SENSITIZED SOLAR CELLS The performance of the aforementioned three structures hinges on the surface condition and optical profiles of UC materials, including the charge recombination on the surface, the upcoversion efficiency, the light-scattering ability, as well as the spectral absorption range of light. Here, we describe four strategies that can be utilized for the improvement of UC-enhanced DSSCs: (1) surface treatment of UC materials for favorable charge transportation; (2) enhancing the UC efficiency; (3) enhancing the light-scattering effect; and (4) broadening the absorption range of UC materials.
9.4.1 Surface Treatment of Upconversion Materials for Favorable Charge Transportation The surface of UC nanomaterials is important to the performance of DSSCs, as there are plenty of long-chain organic ligands and defects on the surface of UC materials, which can significantly affect the charge transportation properties when placed in the internal cell using structure A. Though the UC processes are favorable for NIR light harvesting, the positioning of nonconducting UC nanomaterials inside the mesoporous TiO2 film can impede the electron transfer among semiconductor TiO2
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nanoparticles, and lower the loading amount of dyes into the mesoporous pores by reduction of the internal surface area of pores. Furthermore, the surface defects of UC nanoparticles can also act as charge recombination centers by capturing photon-induced electrons. The insulating properties of the surface and the existence of defects on the surface, thus substantially impair the performance of UC materials on DSSCs. There are two approaches for optimizing the surface of UC materials. One approach is to solve the problem of UC materials acting as charge recombination centers. The strategy is to grow an insulating shell, such as an SiO2 shell, onto the surface of UC materials. For example, a core shell structured β-NaYF4:Er31/Yb31@SiO2 nanoparticle was used in DSSCs using structure A, which can efficiently eliminate the problem of charge recombination by avoiding the direct contact between the UC materials with the TiO2 photoanode. Indeed, a recent experimental result shows that an absolute increase of 0.38% in efficiency of the DSSC was obtained when compared to the one using β-NaYF4:Er31/Yb31 UC nanoparticles without the SiO2 shell [23]. The second is to address the problem of UC materials which impede the electron transfer and the loading amount of dyes. The solution is to coat a TiO2 shell onto the silica-coated UC nanoparticles. As discussed, the medium SiO2 shell layer could protect the photon-induced electrons from being trapped by surface recombination centers, while the outermost TiO2 shell could guarantee the photoanode with UC materials to absorb dyes as much as pure TiO2 photoanodes, and perform efficient transportation of conducting electrons. Indeed, it was reported that despite the β-NaYF4:Er31/ Yb31@SiO2@TiO2 material possessing poorer luminescent properties than the β-NaYF4:Er31/Yb31 core, the core double shell structure could generate a relative efficiency enhancement as high as 10.9% for the DSSC [24].
9.4.2 Enhancing the Upconversion Efficiency The efficiency of UC materials significantly affects the generated amount of photo-induced electrons after light harvesting in the NIR range. It refers to the ratio between the emitted UC light power and the power of the absorbed light by UC materials. In fact, the parameter of UC efficiency defines the UC materials’ ability to convert the IR light to visible luminescence, that is, the energy relay role of UC materials. The higher the UC efficiency, the better the UC-induced efficiency enhancement of DSSCs.
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For a given UC nanosized material, there are two prominent strategies to significantly enhance its UC efficiency: (1) suppression of surfacerelated deactivations; and (2) localized surface plasmon resonance (LSPR) enhancement. In fact, the surface characteristics of UC nanoparticles determine their luminescent efficiency to a great extent because UC nanoparticles possess a high surface-to-volume ratio at the nanometer dimension, resulting in a number of doped lanthanide ions exposed to surface deactivations (surface defects, ligands, and solvents, owning high phonon energy), which can capture photoelectrons. These are so-called surface-related deactivation processes. To deal with this problem, it is feasible to fabricate an epitaxial core shell structure by applying an inert shell layer with appropriate thickness onto the surface of UC core nanoparticles, isolating the core nanoparticles from surrounding quenching sites, thus producing more efficient UC [21]. LSPR occurs when the incident light photons resonate with the surface charge carriers of metallic nanomaterials. As for Au and Ag metallic nanomaterials, the plasmon resonant frequency for plasmonic nanostructures usually achieves its maximum number in visible and close NIR ranges [25 26]. The LSPR can improve the UC efficiency by enhancing the electromagnetic field near the surface of the plasmonic nanostructure that greatly increases the luminescence intensity of the emitter located in that area [27]. Among the reported works, the most appropriate nanostructure for efficiently generating LSPR is the core shell nanostructure of metallic@silica@UC materials or UC materials@silica@metallic. On the one hand, this structure provides a single platform to couple LSPR and UC luminescence at the nanoscale. On the other hand, the distance between UC materials and metallic nanoparticles can be precisely adjusted by varying the silica shell thickness to reach maximized UC luminescence. It has been shown in many reports that using the silica as a spacer to control the distance can result in UC luminescence enhancement by orders of magnitude [28 34]. LSPR-enhanced UC materials may have an important role in DSSCs, as not only the significantly enhanced UC luminescence can be employed, but also the plasmon effect can also be directly used for solar cell improvement.
9.4.3 Enhancing the Scattering Effect of Upconversion Materials In addition to the effect of the UC process on the improvement of solar cell efficiency, the scattering effect provided by UC materials is also
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beneficial for the performance of DSSCs, and sometimes it dominates the contribution of UC materials with respect to solar cells’ efficiency enhancement, especially when the UC process is inefficient. Since the scattering direction of incident light is random, multiple times scattering of the incident photon can take place before the escape of the lightabsorbing layer. As a result, the light traveling path within the lightabsorbing layer is multiplied, thus escalating its absorbing odds by dyes before exiting the light-harvesting layer. Because light scattering is a fundamental physical phenomenon which applies to photons of all wavelengths, the incident photon-to-current efficiency of DSSCs over the entire spectral range can be elevated. The size of scatterers matters for the process of scattering. The larger the size, the higher the scattering capability. Generally, UC nanoparticles are of larger size than TiO2 nanoparticles in the photoanode film, and thus they have better light-scattering ability. For a given UC material, the strategy to increase its scattering ability is to produce a core shell structure, which increases the size while reserving the upconverting capability of the core. The other approach is to couple the UC materials with other scattering materials to form a composite with a bigger size and preferably benefiting the upconverting process. As such, the aforementioned coating layer of SiO2 and TiO2 shell on UC materials (see Section 9.4.1) can increase the UC materials’ scattering effect to an extent. However, a larger nanoparticle means a smaller surface-to-volume ratio, which can lower the TiO2 photoanode’s dye absorption amount, thus hampering the efficiency increase of DSSCs. That is to say, a proper size of UC nanoparticles is needed for the use of UC materials to DSSCs. The use of metallic structures, that is, coating/coupling Au or Ag shells or nanoparticles to the surface of UC materials (Section 9.4.2) can also lead to an enhanced scattering effect, while exploiting the LSPR effect to enhance UC. Moreover, nanomaterials with hollow structure can facilitate the stronger scattering effect by substantially increasing the light travel path [35 37]. For example, as shown in Fig. 9.3, the material of rare-earth ion-doped TiO2 hollow shell was used as a scattering layer. And when compared with pristine TiO2 photoanode, the total relative efficiency enhancement reaches B32.7%, with the scattering-induced enhancement by B29.5% and UC-induced enhancement by merely B3.2%. Therefore, the observed efficiency enhancement effect benefits greatly from the efficient light-scattering process. However, owing to the structure of hollow shell, more rare-earth ions are placed on the surface, which aggravates UC
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Figure 9.3 The structure of a upconversion TiO2 photoanode consisting of rareearth ion-doped shell in shell TiO2 hollow materials. Modified from X. Wu, G.Q. Max Lu, L. Wang, Dual-functional upconverter-doped TiO2 hollow shells for light scattering and near-infrared sunlight harvesting in dye-sensitized solar cells. Adv. Energy Mater. 3 (6) (2013) 706.
luminescence quenching. Therefore, it is encouraging to optimize the scattering effect of UC materials for higher efficiencies of DSSCs.
9.4.4 Broadening the Absorption Band of Upconversion Materials The efficiency enhancement effect of a solar cell induced by UC materials strongly depends on the absorption bandwidth of the employed upconverters, which defines the amount of harvested IR light energy. In fact, the nature of 4f 4f electronic transitions of doped rare-earth ions leads to a weak and narrow-band NIR absorption of upconversion nanoparticles (UCNPs), which caps their photon-harvesting capability. Typically, UC materials employ doped Yb31 ions (sensitizers) to absorb IR radiation and nonradiatively transfer the excitation to the doped lanthanide activators X31 (X 5 Er, Ho, Tm) which can subsequently emit visible or ultraviolet upconversion luminescence (UCL) through radiation transition. However, the absorption range and absorption cross-section of the sensitizer ion Yb31 is just B10,260 10,660 cm21 (B10 times narrower than that of an organic dye) and B10220 cm2 (B1000 10,000 times lower
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than that of a dye molecule, B10217 10216 cm2). Therefore, it is nontrivial to broaden the NIR light harvesting [38]. To date, there are two reported approaches for broadening the absorption band of UC materials. One is to introduce a range of distinct types of rare-earth ions into a hierarchical nanostructure, but without introducing deleterious cross relaxations between different types of rare-earth ions. For example, NaYbF4:Er31@NaYF4:Nd31 core shell UC nanocrystals have been prepared for this purpose because, except for the NIR absorption provided by Yb31 ions, Nd31 ions in the shell possessing the multiple absorption bands enable the material to harvest broader NIR light. In addition, this structure not only exploits the strong absorbing abilities of Nd31 ions, but also has the advantage of avoiding deleterious crossrelated quenching effects through spatial isolation of the Nd31 ions from the Er31/Yb31 ions. Moreover, this UC material provides a new efficient upconverting path of Nd31 - Yb31 - Er31 to convert NIR light to visible light. Furthermore, a 15.6% relative efficiency enhancement of the DSSC with UC material owning IR dual absorption band (785 and 980 nm), was achieved with respect to the DSSC with pure TiO2 photoanode [39]. The working principle of this dual-band UC for use in DSSCs is schematically shown in Fig. 9.4. The second approach to broaden the absorption band is to utilize organic dyes to sensitize the rare earth-doped UC nanoparticles. The organic IR dyes feature strong and broadband NIR absorption bands over more than 100 nanometers, which act as antennas on the nanoparticle surface to efficiently harvest light, and then nonradiatively transfer the absorbed energy to surface rare-earth ions, such as Yb31 and Nd31 ions, to produce efficient UC with upconverting quantum yield as high as B10% [40 43]. As such, dye-sensitized rare-earth UC materials are promising for UC-enhanced DSSCs. Indeed, the organic dye (IR-783)-sensitized UC NaYF4:Yb31, Er31@NaYF4:Nd31 nanoparticles have been utilized for DSSCs (Fig. 9.5), which has an absorption band over 190 nm, about B10 times broader than that of rare-earth ions. Application of this type of UC material into DSSCs, employing structure A in Fig. 9.2, has resulted in a 7.1% relative efficiency increase of DSSCs caused by UC function, while among the previous reports the highest value is just 2.1% [44]. This result provides a paradigm on utilizing dye-sensitized rare-earth UC toward more efficient DSSCs.
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Figure 9.4 A schematic illustration of NaYbF4:Er31@ NaYF4:Nd31 material working in dye-sensitized solar cell. HOMO, highest occupied molecular orbital; LOMO, lowest unoccupied molecular orbital. Reproduced with permission from C. Yuan, et al., Simultaneous multiple wavelength upconversion in a core-shell nanoparticle for enhanced near infrared light harvesting in a dye-sensitized solar cell. ACS Appl. Mater. Interfaces 6 (20) (2014) 18081.
Figure 9.5 Broadband near-infrared sunlight harvesting and then spectral conversion into the visible range to activate N719 dye for the improvement of a dye-sensitized solar cell device. Reproduced with permission from S. Hao, et al., Enhancing dyesensitized solar cell efficiency through broadband near-infrared upconverting nanoparticles. Nanoscale 9 (20) (2017) 6712.
9.5 CONCLUSION UC provides a promising approach for DSSCs to minimize the energy loss caused by spectrum mismatch between solar cells and the incident solar spectrum due to its ability of converting subbandgap sunlight to photons in the wavelength, where solar cells have a strong response. In
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this chapter, the working principle of UC material-induced DSSCs is presented, as well as the strategies for enhancing the performance of DSSCs with the use of UC materials. The basic difference between three typical structures of UC material-induced DSSCs is the distance between the UC materials and the dyes absorbed on the TiO2 surface. The improvement of UC-induced performance in DSSCs can be enhanced by optimizing UC materials in four aspects. (1) Surface treatment of UC materials for favorable charge transportation; coating UC materials with one or a double shell, such as a TiO2 and SiO2 shell, can effectively alleviate the problem of surface charge recombination. (2) Enhancing the UC efficiency; plasmon-enhanced UC and suppression of surface-related luminescence quenching using an epitaxial core shell structure will play important roles toward this. (3) Enhancing the light-scattering effect; the scattering effect can be intensified by increasing the size of nanoparticles and various core shell structures, in particular, the growth of a hollow shell structure. (4) Broadening the absorption range of UC materials. The absorption band range can be extended by employing a set of different types of rareearth ions inside a hierarchical nanostructure to collectively absorb a broad range of IR spectra, or by utilizing IR organic dyes with strong and broad absorption bands as antennas on the surface of upconverting nanoparticles. The objective of optimizing these four aspects is to address two main limiting aspects of UC materials: (1) the low UC efficiency; and (2) the low, narrow-band absorption of rare-earth ions due to the nature of f f transitions. In particular, IR-783 dye-sensitized UC materials have recently shown a significant improvement in the overall efficiency of DSSCs by 13.1%, with a contribution of the UC process by 7.1% [44]. Despite this significant improvement, the utilized dye-sensitized UC system is unoptimized and simply based on just one type of IR-783 dye, further optimization of this system could result in even more promising results, for example, by employing a range of organic dyes to cover a broader IR range. In addition, the optimizations of the four aspects are generally independent of each other. The combination of optimization strategies is also inviting and promising for UC-enhanced DSSCs.
REFERENCES [1] B. O'regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737 740. [2] K. Kakiage, Y. Aoyama, T. Yano, K. Oya, J. Fujisawa, M. Hanaya, Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxyanchor dyes, Chem. Commun. 51 (88) (2015) 15894 15897.
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[3] H. Tian, X. Yang, J. Cong, R. Chen, J. Liu, Y. Hao, et al., Tuning of phenoxazine chromophores for efficient organic dye-sensitized solar cells, Chem. Commun. (41) (2009) 6288 6290. [4] B.S. Richards, Enhancing the performance of silicon solar cells via the application of passive luminescence conversion layers, Sol. Energy Mater. Sol. C 90 (15) (2006) 2329 2337. [5] B.E. Hardin, E.T. Hoke, P.B. Armstrong, J.-H. Yum, P. Comte, T. Torres, et al., Increased light harvesting in dye-sensitized solar cells with energy relay dyes, Nat. Photonics 3 (7) (2009) 406 411. [6] M.K. Nazeeruddin, P. Pechy, T. Renouard, S.M. Zakeeruddin, R. HumphryBaker, P. Comte, et al., Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells, J. Am. Chem. Soc. 123 (8) (2001) 1613 1624. [7] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L.Y. Han, Dye-sensitized solar cells with conversion efficiency of 11.1%, Jpn. J. Appl. Phys. 2 45 (24 28) (2006) L638 L640. [8] P.Y. Reddy, L. Giribabu, C. Lyness, H.J. Snaith, C. Vijaykumar, M. Chandrasekharam, et al., Efficient sensitization of nanocrystalline TiO2 films by a near-IR-absorbing unsymmetrical zinc phthalocyanine, Angew. Chem. 119 (3) (2007) 377 380. [9] Y. Hao, X.C. Yang, J.Y. Cong, H.N. Tian, A. Hagfeldt, L.C. Sun, Efficient near infrared D-Π-A sensitizers with lateral anchoring group for dye-sensitized solar cells, Chem. Commun. (2009) 4031 4033. [10] H. Tian, X. Yang, R. Chen, A. Hagfeldt, L. Sun, A metal-free “black dye” for panchromatic dye-sensitized solar cells, Energ. Environ. Sci. 2 (6) (2009) 674 677. [11] T. Kinoshita, J.T. Dy, S. Uchida, T. Kubo, H. Segawa, Wideband dye-sensitized solar cells employing a phosphine-coordinated ruthenium sensitizer, Nat. Photonics 7 (7) (2013) 535 539. [12] Z.J. Ning, Y. Fu, H. Tian, Improvement of dye-sensitized solar cells: what we know and what we need to know, Energ. Environ. Sci. 3 (9) (2010) 1170 1181. [13] X. Huang, S. Han, W. Huang, X. Liu, Enhancing solar cell efficiency: the search for luminescent materials as spectral converters, Chem. Soc. Rev. 42 (1) (2013) 173 201. [14] J.W. Gong, K. Sumathy, Q.Q. Qiao, Z.P. Zhou, Review on dye-sensitized solar cells (DSSCs): advanced techniques and research trends, Renew. Sust. Energ. Rev. 68 (2017) 234 246. [15] T.F. Schulze, T.W. Schmidt, Photochemical upconversion: present status and prospects for its application to solar energy conversion, Energ. Environ. Sci. 8 (1) (2015) 103 125. [16] G. Chen, H. Qiu, P.N. Prasad, X. Chen, Upconversion nanoparticles: design, nanochemistry, and applications in theranostics, Chem. Rev. 114 (10) (2014) 5161 5214. [17] P.S. Peijzel, A. Meijerink, R.T. Wegh, M.F. Reid, G.W. Burdick, A complete energy level diagram for all trivalent lanthanide ions, J.Solid State Chem. 178 (2) (2005) 448 453. [18] Y. Shang, S. Hao, C. Yang, G. Chen, Enhancing solar cell efficiency using photon upconversion materials, Nanomaterials (Basel) 5 (4) (2015) 1782 1809. [19] A. Shalav, B.S. Richards, T. Trupke, K.W. Krämer, H.U. Güdel, Application of NaYF4:Er31 up-converting phosphors for enhanced near-infrared silicon solar cell response, Appl. Phys. Lett. 86 (1) (2005) 013505. [20] J.A. Briggs, A.C. Atre, J.A. Dionne, Narrow-bandwidth solar upconversion: Case studies of existing systems and generalized fundamental limits, J. Appl. Phys. 113 (12) (2013) 124509 124514.
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[21] N.N. Yao, J.Z. Huang, K. Fu, X.L. Deng, M. Ding, X.J. Xu, Rare earth ion doped phosphors for dye-sensitized solar cells applications, RCS Adv. 6 (21) (2016) 17546 17559. [22] J. Yu, Y. Yang, R. Fan, D. Liu, L. Wei, S. Chen, et al., Enhanced near-infrared to visible upconversion nanoparticles of Ho31-Yb31-F2 tri-doped TiO2 and its application in dye-sensitized solar cells with 37% improvement in power conversion efficiency, Inorg. Chem. 53 (15) (2014) 8045 8053. [23] Z. Zhou, J. Wang, F. Nan, C. Bu, Z. Yu, W. Liu, et al., Upconversion induced enhancement of dye sensitized solar cells based on core-shell structured beta-NaYF4: Er31, Yb31@SiO2 nanoparticles, Nanoscale 6 (4) (2014) 2052 2055. [24] L. Liang, Y. Liu, C. Bu, K. Guo, W. Sun, N. Huang, et al., Highly uniform, bifunctional core/double-shell-structured beta-NaYF4:Er31, Yb31@SiO2@TiO2 hexagonal sub-microprisms for high-performance dye sensitized solar cells, Adv. Mater. 25 (15) (2013) 2174 2180. [25] P.N. Prasad, Nanophotonics, Wiley-Interscience, Hoboken, NJ, 2004. [26] G.Y. Chen, J. Seo, C.H. Yang, P.N. Prasad, Nanochemistry and nanomaterials for photovoltaics, Chem. Soc. Rev. 42 (21) (2013) 8304 8338. [27] J.R. Lakowicz, Radiative decay engineering: biophysical and biomedical applications, Anal. Biochem. 298 (1) (2001) 1 24. [28] Y. Ding, X. Zhang, H. Gao, S. Xu, C. Wei, Y. Zhao, Plasmonic enhanced upconversion luminescence of β-NaYF4:Yb31/Er31 with Ag@SiO2 core shell nanoparticles, J. Lumin. 147 (2014) 72 76. [29] F. Zhang, G.B. Braun, Y. Shi, Y. Zhang, X. Sun, N.O. Reich, et al., Fabrication of Ag@SiO2@Y2O3:Er nanostructures for bioimaging: tuning of the upconversion fluorescence with silver nanoparticles, J. Am. Chem. Soc. 132 (9) (2010) 2850 2851. [30] L. Sudheendra, V. Ortalan, S. Dey, N.D. Browning, I.M. Kennedy, Plasmonic enhanced emissions from cubic NaYF4:Yb:Er/Tm nanophosphors, Chem. Mater. 23 (11) (2011) 2987 2993. [31] W. Xu, B. Chen, W. Yu, Y. Zhu, T. Liu, S. Xu, et al., The up-conversion luminescent properties and silver-modified luminescent enhancement of YVO4:Yb31, Er31 NPs, Dalton Trans. 41 (43) (2012) 13525 13532. [32] W. Ge, X.R. Zhang, M. Liu, Z.W. Lei, R.J. Knize, Y. Lu, Distance dependence of gold-enhanced upconversion luminescence in Au/SiO2/Y2O3:Yb31, Er31 nanoparticles, Theranostics 3 (4) (2013) 282 288. [33] A. Priyam, N.M. Idris, Y. Zhang, Gold nanoshell coated NaYF4 nanoparticles for simultaneously enhanced upconversion fluorescence and darkfield imaging, J. Mater. Chem. 22 (3) (2012) 960 965. [34] W. Deng, L. Sudheendra, J. Zhao, J. Fu, D. Jin, I.M. Kennedy, et al., Upconversion in NaYF4:Yb, Er nanoparticles amplified by metal nanostructures, Nanotechnology 22 (32) (2011) 325604 325612. [35] G. Han, M. Wang, D. Li, J. Bai, G. Diao, Novel upconversion Er, Yb-CeO2 hollow spheres as scattering layer materials for efficient dye-sensitized solar cells, Sol. Energ. Mater. Sol. Cells 160 (2017) 54 59. [36] W. Liao, D. Zheng, J. Tian, Z. Lin, Dual-functional semiconductor-decorated upconversion hollow spheres for high efficiency dye-sensitized solar cells, J. Mater. Chem. A 3 (46) (2015) 23360 23367. [37] X. Wu, G.Q. Max Lu, L. Wang, Dual-functional upconverter-doped TiO2 hollow shells for light scattering and near-infrared sunlight harvesting in dye-sensitized solar cells, Adv. Energy Mater. 3 (6) (2013) 704 707. [38] X.D. Wang, R.R. Valiev, T.Y. Ohulchanskyy, H. Agren, C.H. Yang, G.Y. Chen, Dye-sensitized lanthanide-doped upconversion nanoparticles, Chem. Soc. Rev. 46 (14) (2017) 4150 4167.
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Dye Sensitized Solar Cells
[39] C. Yuan, G. Chen, L. Li, J.A. Damasco, Z. Ning, H. Xing, et al., Simultaneous multiple wavelength upconversion in a core-shell nanoparticle for enhanced near infrared light harvesting in a dye-sensitized solar cell, ACS Appl. Mater. Interfaces 6 (20) (2014) 18018 18025. [40] G.Y. Chen, J. Damasco, H.L. Qiu, W. Shao, T.Y. Ohulchanskyy, R.R. Valiev, et al., Energy-cascaded upconversion in an organic dye-sensitized core/shell fluoride nanocrystal, Nano. Lett. 15 (11) (2015) 7400 7407. [41] G. Chen, W. Shao, R.R. Valiev, T.Y. Ohulchanskyy, G.S. He, H. Ågren, et al., Efficient broadband upconversion of near-infrared light in dye-sensitized core/shell nanocrystals, Adv. Opt. Mater. 4 (11) (2016) 1760 1766. [42] W. Shao, G. Chen, A. Kuzmin, H.L. Kutscher, A. Pliss, T.Y. Ohulchanskyy, et al., Tunable narrow band emissions from dye-sensitized core/shell/shell nanocrystals in the second near-infrared biological window, J. Am. Chem. Soc. 138 (50) (2016) 16192 16195. [43] X. Huang, Broadband dye-sensitized upconversion: a promising new platform for future solar upconverter design, J. Alloys Compd. 690 (2017) 356 359. [44] S. Hao, Y. Shang, D. Li, H. Agren, C. Yang, G. Chen, Enhancing dye-sensitized solar cell efficiency through broadband near-infrared upconverting nanoparticles, Nanoscale 9 (20) (2017) 6711 6715.
Upconversion-Enhanced DyeSensitized Solar Cells Deyang Li1,2 and Guanying Chen1,2 1
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, P.R. China 2 Key Laboratory of Micro-systems and Micro-structures, Ministry of Education, Harbin Institute of Technology, Harbin, P.R. China
9.1 INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted extensive research interests in the photovoltaic field because of their advantages of lower product cost, simple fabrication, and being environmentally friendly. Since the first fabrication of DSSCs (efficiency, 7.12%) by Michael Grätzel and his coworkers in 1991 [1], the efficiency has now reached 14.5% [2], comparable to that of the commercial Si-based solar cell. However, the inability to use light in the infrared (IR) spectral range, which accounts for almost half the energy of solar irradiation, poses a stringent limit for further increasing the conversion efficiency of DSSCs. The typically used dyes in DSSCs, for example, ruthenium-based dyes, are only able to absorb light in the region of 300 800 nm [3], while the absorption range of crystalline silicon (c-Si) solar cell is 300 1150 nm [4]. This indicates that DSSCs have a narrower solar spectrum response region than c-Si solar cells, and thus a higher transmission loss. The transmission loss arises from the fact that solar cells can only absorb photons with higher energy than the bandgap, while the photons with energy lower than the bandgap are transparent for solar cells. As a result, the absorption range of dyes determines the overall efficiency of DSSCs [5]. Finding ways to expand DSSCs’ spectral response to the IR region (typically $ 700 nm) are under intensive investigations. The near-infrared (NIR)-responsive photosensitizers are explored for obtaining panchromatic DSSCs [6 11]. Unfortunately, this type of panchromatic DSSC was found to have a problem of poor electron injection efficiency and serious charge recombination [12]. Upconversion (UC) materials, with the ability to convert NIR photons to visible light, have been considered as a promising approach for Dye Sensitized Solar Cells DOI: https://doi.org/10.1016/B978-0-12-814541-8.00009-4
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enhancing light harvesting in the IR region for solar cell devices [13 15]. By employing doped rare-earth ions with real ladder-like energy-level structures, visible or ultraviolet photons can be generated, whereby two or more photons in the IR range are sequentially absorbed via real intermediate long-lived energy levels. Generally, the upconverting light produced from rare-earth ions is sharp because this emission arises from 4f to4f orbital electronic transitions and the 4f electrons are shielded by the outer complete 5s and 5p shells [16 17]. As a consequence, the upconverting light produced by rare-earth ions can exhibit high resistance to photobleaching and photochemical degradation. In addition, the required power density to excite rare-earth ions for light UC is as low as B0.1 W/cm2 [18], close to the level of solar irradiation on earth. Indeed, it was reported that the application of UC materials to a silicon solar cell can impressively improve its overall performance due to the enhanced NIR light harvesting [19]. Moreover, it has been predicted that, with the application of upconverters possessing a narrow absorption bandwidth of B0.5 eV, the Shockley Queisser efficiency limit of the solar cell with a bandgap of 1.7 eV (close to that of the dye N719 typically used in DSSCs), can be boosted from 28.2% to as high as 43.6% [20]. It should be noted that the optimization of UC materials and solar cell structure can be done separately; therefore, the UC technology can be applied to all the existing types of solar cells without altering their structures. Alongside the role of UC, UC materials can also provide light scattering for the light-harvesting capability by increasing the optical trapping effects of a DSSC device [21]. The combination of UC and scattering effect from UC materials entails a useful strategy for the improved performance of DSSCs. In this chapter, we discuss the operational principle of UC for UC material-incorporated DSSCs, the related three typical solar cell structures, the strategies to enhance UC materials for light absorption and scattering, and the perspectives for future directions.
9.2 THE OPERATIONAL PRINCIPLE OF UPCONVERSION IN DYE-SENSITIZED SOLAR CELLS For a conventional DSSC without UC (e.g., employing N719 dye), the bandgap (B1.8 eV) determines that the device is responsive to light with wavelength shorter than 689 nm, while being unresponsive to longer wavelength light. Namely, the visible light can excite the N719 dye to produce photo-induced electrons, which are then injected into the
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conduction band of the TiO2 film (the photoanode), and finally flowed out of indium tin oxide glass to the external circuit to form a photocurrent. At this point, the amount of photo-induced electrons largely determines the efficiency of DSSCs. The upconverting effect of UC materials can increase the yield of photo-induced electrons by harvesting NIR light, which is unresponsive for conventional DSSC solar cells. UC materials are typically composed of two elements: (1) host materials and (2) lanthanide ions. Generally, host materials are dielectric materials with good chemical stability and low phonon energy, for example, NaYF4 (phonon energy of B350 cm21) that incorporate trivalent lanthanide ions. Low phonon energy allows to minimize nonradiative losses at the intermediate states, thus leading to UC emission at higher efficiency. The Er31, Ho31, Tm31, and Pr31 are commonly used as emitter ions in UC materials, while Yb31 ions are often codoped as the sensitizer, which have B10 times higher absorption cross-sections than the emitter ions. The basic UC mechanisms are comprised of excited-state absorption, energy transfer upconversion (ETU), cooperative sensitization UC, and photon avalanche. The ETU process is the most used mechanism for lanthanide ions, for example, Er31/Yb31, toward the application of UC materials to increase the efficiency of DSSCs. Taking the Er31/Yb31 codoped UC materials with an ETU mechanism, for example, when being incorporated in DSSC cells, the operational principle of UC effect is schematically shown in Fig. 9.1. First, NIR light (B980 nm) is absorbed by the Yb31 ion (the sensitizer) that has two energy states of 2F2/7 and 2F5/2, resulting in a Yb31 ion being excited to 2F5/2 from 2F2/7 energy state. Second, the absorbed energy by the sensitizer is then transferred to an Er31 ion, an activator, exciting it to the energy state 4I11/2. Then, the Er31 ion at this state is further excited to the energy state 4F2/7 by absorbing more energy from a neighboring excited Yb31 ion. Subsequently, the Er31 ion relaxes to the ground state by emitting visible upconverted light at 525/545 nm. Since the light-absorbing dye (e.g., N719) has a strong absorption in the visible range, the emitted visible upconverted light can excite the dye to generate extra photo-induced electrons, thus enhancing the photocurrent of DSSCs. Note that when the dye is placed in close contact with the UC materials (within a few nanometers), the Er31 ion at the 4F2/7 state can nonradiatively transfer its contained energy to the dye (e.g., N719 dye), in which the energy transfer efficiency is defined by the distance between them, and the spectral overlap between the UC emission and the
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Figure 9.1 The operational principle for the utilization of upconversion (UC) materials to enhance the performance of dye-sensitized solar cells. The ultraviolet and visible sunlight can be efficiently harvested by the dye and TiO2 layer, generating separated charge carriers. Meanwhile UC materials are able to harvest the unutilized near-infrared light, and then convert it into high-energy visible light utilizing the real energy levels of rare-earth ions. The converted visible luminescence can then be captured by the dyes to produce charge carriers for the photocurrent.
absorption of the dye. The described operation principle of UC here can be applied to all other types of rare-earth UC for use in DSSCs. Alongside UC, the scattering effect can also enhance the performance of DSSCs, which refers to the optical path of incident light in the lightabsorbing layer which can be increased due to the size-induced scattering effect of UC materials. Since scattering is a universal phenomena which applies to light of all wavelengths, the scattering effect enhances the spectral response of a DSSC over all wavelength ranges. This is in marked contrast to the UC effect, which contributes to the NIR wavelength range by producing light harvesting. The different types of host lattice for UC materials significantly influence the UC contribution to the performance enhancement of DSSCs, as they impact the UC efficiency greatly. The major standards for choosing excellent host lattice are aspects of low lattice phonon energy and high chemical stability. As a result, materials of fluoride (e.g., NaYF4 and LaF3), oxide (e.g., TiO2, Y2O3, Gd2O3, and GeO2), and oxyfluoride (e.g., YOF) have often been used as the matrix of UC materials.
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Particularly, titanium oxide (TiO2) can also be utilized as the UC host material, and has the advantage of having compatibility with the mesoporous TiO2 film of a DSSC, entailing a strong interaction with the dye. It is known that the open-circuit photovoltage (Voc) is mainly determined by the difference between the electron quasi-Fermi level of the TiO2 film and of the redox mediator. The doping of rare-earth ions can elevate the Femi level of TiO2 [22], due to the replacement of Ti41 with Er31 or Yb31, thus increasing the Voc of the DSSCs. This favors the overall performance of DSSCs.
9.3 THREE TYPES OF UPCONVERSION-ENHANCED DYESENSITIZED SOLAR CELL STRUCTURES A typical DSSC is composed of FTO glass, mesoporous TiO2 film coated with dye, redox electrolyte, and FTO glass coated with Pt positioning UC materials into different parts of DSSCs will produce different types of UC-enhanced structures. When placing UC materials in the internal of the cell, there are three integration modes (Fig. 9.2). (1) One way is to mix UC materials and TiO2 nanoparticles to form a hybrid TiO2 film (structure A). (2) The other way is to replace the noble conterelectrode metal Pt with UC materials that possess the catalytic activity for the iodine-based redox couple (structure B). (3) The third type is to place UC materials outside the cell. Usually, UC nanoparticles are coated at the rear side of FTO glass which is coated with Pt (structure C).
Figure 9.2 The three typical structures of upconversion-enhanced dye-sensitized solar cells. (A) Placing upconversion (UC) nanoparticles into the light-harvesting layer of a mesoporous TiO2 film (inside the cell); (B) employing an upconverting counterelectrode (inside the cell); (C) placing UC materials in the device back as a lightconverting layer (outside the cell).
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The fundamental difference between these three structures lies in the distance between the upconverting materials and the light-harvesting dyes. Structure A can make full advantage of the produced upconverting processes due to the shortest distance (within the nanometer scale) between the UC materials and the dye. While, for structures B and C, before reaching the dye, the upconverting light has to pass through an electrolyte and counterelectrode, thus weakening the intensity of the UC light. At the same time, the macroscopic distance between UC materials and light-harvesting dyes also prevents the Förster resonance energy transfer (FRET) process from occurring, which allows the upconverted energy to be efficiently accepted by light-harvesting dyes. However, the main advantage of structure B lies in its ability to lower the material cost by replacing the noble metal, Pb, that plays an important role in catalyzing 2 I2 circulation, while the main advantage for structure C is that the 3 =I UC materials are placed outside the solar cell device, leaving the device intact.
9.4 STRATEGIES FOR THE IMPROVEMENT OF UPCONVERSION-ENHANCED DYE-SENSITIZED SOLAR CELLS The performance of the aforementioned three structures hinges on the surface condition and optical profiles of UC materials, including the charge recombination on the surface, the upcoversion efficiency, the light-scattering ability, as well as the spectral absorption range of light. Here, we describe four strategies that can be utilized for the improvement of UC-enhanced DSSCs: (1) surface treatment of UC materials for favorable charge transportation; (2) enhancing the UC efficiency; (3) enhancing the light-scattering effect; and (4) broadening the absorption range of UC materials.
9.4.1 Surface Treatment of Upconversion Materials for Favorable Charge Transportation The surface of UC nanomaterials is important to the performance of DSSCs, as there are plenty of long-chain organic ligands and defects on the surface of UC materials, which can significantly affect the charge transportation properties when placed in the internal cell using structure A. Though the UC processes are favorable for NIR light harvesting, the positioning of nonconducting UC nanomaterials inside the mesoporous TiO2 film can impede the electron transfer among semiconductor TiO2
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nanoparticles, and lower the loading amount of dyes into the mesoporous pores by reduction of the internal surface area of pores. Furthermore, the surface defects of UC nanoparticles can also act as charge recombination centers by capturing photon-induced electrons. The insulating properties of the surface and the existence of defects on the surface, thus substantially impair the performance of UC materials on DSSCs. There are two approaches for optimizing the surface of UC materials. One approach is to solve the problem of UC materials acting as charge recombination centers. The strategy is to grow an insulating shell, such as an SiO2 shell, onto the surface of UC materials. For example, a core shell structured β-NaYF4:Er31/Yb31@SiO2 nanoparticle was used in DSSCs using structure A, which can efficiently eliminate the problem of charge recombination by avoiding the direct contact between the UC materials with the TiO2 photoanode. Indeed, a recent experimental result shows that an absolute increase of 0.38% in efficiency of the DSSC was obtained when compared to the one using β-NaYF4:Er31/Yb31 UC nanoparticles without the SiO2 shell [23]. The second is to address the problem of UC materials which impede the electron transfer and the loading amount of dyes. The solution is to coat a TiO2 shell onto the silica-coated UC nanoparticles. As discussed, the medium SiO2 shell layer could protect the photon-induced electrons from being trapped by surface recombination centers, while the outermost TiO2 shell could guarantee the photoanode with UC materials to absorb dyes as much as pure TiO2 photoanodes, and perform efficient transportation of conducting electrons. Indeed, it was reported that despite the β-NaYF4:Er31/ Yb31@SiO2@TiO2 material possessing poorer luminescent properties than the β-NaYF4:Er31/Yb31 core, the core double shell structure could generate a relative efficiency enhancement as high as 10.9% for the DSSC [24].
9.4.2 Enhancing the Upconversion Efficiency The efficiency of UC materials significantly affects the generated amount of photo-induced electrons after light harvesting in the NIR range. It refers to the ratio between the emitted UC light power and the power of the absorbed light by UC materials. In fact, the parameter of UC efficiency defines the UC materials’ ability to convert the IR light to visible luminescence, that is, the energy relay role of UC materials. The higher the UC efficiency, the better the UC-induced efficiency enhancement of DSSCs.
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For a given UC nanosized material, there are two prominent strategies to significantly enhance its UC efficiency: (1) suppression of surfacerelated deactivations; and (2) localized surface plasmon resonance (LSPR) enhancement. In fact, the surface characteristics of UC nanoparticles determine their luminescent efficiency to a great extent because UC nanoparticles possess a high surface-to-volume ratio at the nanometer dimension, resulting in a number of doped lanthanide ions exposed to surface deactivations (surface defects, ligands, and solvents, owning high phonon energy), which can capture photoelectrons. These are so-called surface-related deactivation processes. To deal with this problem, it is feasible to fabricate an epitaxial core shell structure by applying an inert shell layer with appropriate thickness onto the surface of UC core nanoparticles, isolating the core nanoparticles from surrounding quenching sites, thus producing more efficient UC [21]. LSPR occurs when the incident light photons resonate with the surface charge carriers of metallic nanomaterials. As for Au and Ag metallic nanomaterials, the plasmon resonant frequency for plasmonic nanostructures usually achieves its maximum number in visible and close NIR ranges [25 26]. The LSPR can improve the UC efficiency by enhancing the electromagnetic field near the surface of the plasmonic nanostructure that greatly increases the luminescence intensity of the emitter located in that area [27]. Among the reported works, the most appropriate nanostructure for efficiently generating LSPR is the core shell nanostructure of metallic@silica@UC materials or UC materials@silica@metallic. On the one hand, this structure provides a single platform to couple LSPR and UC luminescence at the nanoscale. On the other hand, the distance between UC materials and metallic nanoparticles can be precisely adjusted by varying the silica shell thickness to reach maximized UC luminescence. It has been shown in many reports that using the silica as a spacer to control the distance can result in UC luminescence enhancement by orders of magnitude [28 34]. LSPR-enhanced UC materials may have an important role in DSSCs, as not only the significantly enhanced UC luminescence can be employed, but also the plasmon effect can also be directly used for solar cell improvement.
9.4.3 Enhancing the Scattering Effect of Upconversion Materials In addition to the effect of the UC process on the improvement of solar cell efficiency, the scattering effect provided by UC materials is also
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beneficial for the performance of DSSCs, and sometimes it dominates the contribution of UC materials with respect to solar cells’ efficiency enhancement, especially when the UC process is inefficient. Since the scattering direction of incident light is random, multiple times scattering of the incident photon can take place before the escape of the lightabsorbing layer. As a result, the light traveling path within the lightabsorbing layer is multiplied, thus escalating its absorbing odds by dyes before exiting the light-harvesting layer. Because light scattering is a fundamental physical phenomenon which applies to photons of all wavelengths, the incident photon-to-current efficiency of DSSCs over the entire spectral range can be elevated. The size of scatterers matters for the process of scattering. The larger the size, the higher the scattering capability. Generally, UC nanoparticles are of larger size than TiO2 nanoparticles in the photoanode film, and thus they have better light-scattering ability. For a given UC material, the strategy to increase its scattering ability is to produce a core shell structure, which increases the size while reserving the upconverting capability of the core. The other approach is to couple the UC materials with other scattering materials to form a composite with a bigger size and preferably benefiting the upconverting process. As such, the aforementioned coating layer of SiO2 and TiO2 shell on UC materials (see Section 9.4.1) can increase the UC materials’ scattering effect to an extent. However, a larger nanoparticle means a smaller surface-to-volume ratio, which can lower the TiO2 photoanode’s dye absorption amount, thus hampering the efficiency increase of DSSCs. That is to say, a proper size of UC nanoparticles is needed for the use of UC materials to DSSCs. The use of metallic structures, that is, coating/coupling Au or Ag shells or nanoparticles to the surface of UC materials (Section 9.4.2) can also lead to an enhanced scattering effect, while exploiting the LSPR effect to enhance UC. Moreover, nanomaterials with hollow structure can facilitate the stronger scattering effect by substantially increasing the light travel path [35 37]. For example, as shown in Fig. 9.3, the material of rare-earth ion-doped TiO2 hollow shell was used as a scattering layer. And when compared with pristine TiO2 photoanode, the total relative efficiency enhancement reaches B32.7%, with the scattering-induced enhancement by B29.5% and UC-induced enhancement by merely B3.2%. Therefore, the observed efficiency enhancement effect benefits greatly from the efficient light-scattering process. However, owing to the structure of hollow shell, more rare-earth ions are placed on the surface, which aggravates UC
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Figure 9.3 The structure of a upconversion TiO2 photoanode consisting of rareearth ion-doped shell in shell TiO2 hollow materials. Modified from X. Wu, G.Q. Max Lu, L. Wang, Dual-functional upconverter-doped TiO2 hollow shells for light scattering and near-infrared sunlight harvesting in dye-sensitized solar cells. Adv. Energy Mater. 3 (6) (2013) 706.
luminescence quenching. Therefore, it is encouraging to optimize the scattering effect of UC materials for higher efficiencies of DSSCs.
9.4.4 Broadening the Absorption Band of Upconversion Materials The efficiency enhancement effect of a solar cell induced by UC materials strongly depends on the absorption bandwidth of the employed upconverters, which defines the amount of harvested IR light energy. In fact, the nature of 4f 4f electronic transitions of doped rare-earth ions leads to a weak and narrow-band NIR absorption of upconversion nanoparticles (UCNPs), which caps their photon-harvesting capability. Typically, UC materials employ doped Yb31 ions (sensitizers) to absorb IR radiation and nonradiatively transfer the excitation to the doped lanthanide activators X31 (X 5 Er, Ho, Tm) which can subsequently emit visible or ultraviolet upconversion luminescence (UCL) through radiation transition. However, the absorption range and absorption cross-section of the sensitizer ion Yb31 is just B10,260 10,660 cm21 (B10 times narrower than that of an organic dye) and B10220 cm2 (B1000 10,000 times lower
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than that of a dye molecule, B10217 10216 cm2). Therefore, it is nontrivial to broaden the NIR light harvesting [38]. To date, there are two reported approaches for broadening the absorption band of UC materials. One is to introduce a range of distinct types of rare-earth ions into a hierarchical nanostructure, but without introducing deleterious cross relaxations between different types of rare-earth ions. For example, NaYbF4:Er31@NaYF4:Nd31 core shell UC nanocrystals have been prepared for this purpose because, except for the NIR absorption provided by Yb31 ions, Nd31 ions in the shell possessing the multiple absorption bands enable the material to harvest broader NIR light. In addition, this structure not only exploits the strong absorbing abilities of Nd31 ions, but also has the advantage of avoiding deleterious crossrelated quenching effects through spatial isolation of the Nd31 ions from the Er31/Yb31 ions. Moreover, this UC material provides a new efficient upconverting path of Nd31 - Yb31 - Er31 to convert NIR light to visible light. Furthermore, a 15.6% relative efficiency enhancement of the DSSC with UC material owning IR dual absorption band (785 and 980 nm), was achieved with respect to the DSSC with pure TiO2 photoanode [39]. The working principle of this dual-band UC for use in DSSCs is schematically shown in Fig. 9.4. The second approach to broaden the absorption band is to utilize organic dyes to sensitize the rare earth-doped UC nanoparticles. The organic IR dyes feature strong and broadband NIR absorption bands over more than 100 nanometers, which act as antennas on the nanoparticle surface to efficiently harvest light, and then nonradiatively transfer the absorbed energy to surface rare-earth ions, such as Yb31 and Nd31 ions, to produce efficient UC with upconverting quantum yield as high as B10% [40 43]. As such, dye-sensitized rare-earth UC materials are promising for UC-enhanced DSSCs. Indeed, the organic dye (IR-783)-sensitized UC NaYF4:Yb31, Er31@NaYF4:Nd31 nanoparticles have been utilized for DSSCs (Fig. 9.5), which has an absorption band over 190 nm, about B10 times broader than that of rare-earth ions. Application of this type of UC material into DSSCs, employing structure A in Fig. 9.2, has resulted in a 7.1% relative efficiency increase of DSSCs caused by UC function, while among the previous reports the highest value is just 2.1% [44]. This result provides a paradigm on utilizing dye-sensitized rare-earth UC toward more efficient DSSCs.
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Figure 9.4 A schematic illustration of NaYbF4:Er31@ NaYF4:Nd31 material working in dye-sensitized solar cell. HOMO, highest occupied molecular orbital; LOMO, lowest unoccupied molecular orbital. Reproduced with permission from C. Yuan, et al., Simultaneous multiple wavelength upconversion in a core-shell nanoparticle for enhanced near infrared light harvesting in a dye-sensitized solar cell. ACS Appl. Mater. Interfaces 6 (20) (2014) 18081.
Figure 9.5 Broadband near-infrared sunlight harvesting and then spectral conversion into the visible range to activate N719 dye for the improvement of a dye-sensitized solar cell device. Reproduced with permission from S. Hao, et al., Enhancing dyesensitized solar cell efficiency through broadband near-infrared upconverting nanoparticles. Nanoscale 9 (20) (2017) 6712.
9.5 CONCLUSION UC provides a promising approach for DSSCs to minimize the energy loss caused by spectrum mismatch between solar cells and the incident solar spectrum due to its ability of converting subbandgap sunlight to photons in the wavelength, where solar cells have a strong response. In
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this chapter, the working principle of UC material-induced DSSCs is presented, as well as the strategies for enhancing the performance of DSSCs with the use of UC materials. The basic difference between three typical structures of UC material-induced DSSCs is the distance between the UC materials and the dyes absorbed on the TiO2 surface. The improvement of UC-induced performance in DSSCs can be enhanced by optimizing UC materials in four aspects. (1) Surface treatment of UC materials for favorable charge transportation; coating UC materials with one or a double shell, such as a TiO2 and SiO2 shell, can effectively alleviate the problem of surface charge recombination. (2) Enhancing the UC efficiency; plasmon-enhanced UC and suppression of surface-related luminescence quenching using an epitaxial core shell structure will play important roles toward this. (3) Enhancing the light-scattering effect; the scattering effect can be intensified by increasing the size of nanoparticles and various core shell structures, in particular, the growth of a hollow shell structure. (4) Broadening the absorption range of UC materials. The absorption band range can be extended by employing a set of different types of rareearth ions inside a hierarchical nanostructure to collectively absorb a broad range of IR spectra, or by utilizing IR organic dyes with strong and broad absorption bands as antennas on the surface of upconverting nanoparticles. The objective of optimizing these four aspects is to address two main limiting aspects of UC materials: (1) the low UC efficiency; and (2) the low, narrow-band absorption of rare-earth ions due to the nature of f f transitions. In particular, IR-783 dye-sensitized UC materials have recently shown a significant improvement in the overall efficiency of DSSCs by 13.1%, with a contribution of the UC process by 7.1% [44]. Despite this significant improvement, the utilized dye-sensitized UC system is unoptimized and simply based on just one type of IR-783 dye, further optimization of this system could result in even more promising results, for example, by employing a range of organic dyes to cover a broader IR range. In addition, the optimizations of the four aspects are generally independent of each other. The combination of optimization strategies is also inviting and promising for UC-enhanced DSSCs.
REFERENCES [1] B. O'regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737 740. [2] K. Kakiage, Y. Aoyama, T. Yano, K. Oya, J. Fujisawa, M. Hanaya, Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxyanchor dyes, Chem. Commun. 51 (88) (2015) 15894 15897.
338
Dye Sensitized Solar Cells
[3] H. Tian, X. Yang, J. Cong, R. Chen, J. Liu, Y. Hao, et al., Tuning of phenoxazine chromophores for efficient organic dye-sensitized solar cells, Chem. Commun. (41) (2009) 6288 6290. [4] B.S. Richards, Enhancing the performance of silicon solar cells via the application of passive luminescence conversion layers, Sol. Energy Mater. Sol. C 90 (15) (2006) 2329 2337. [5] B.E. Hardin, E.T. Hoke, P.B. Armstrong, J.-H. Yum, P. Comte, T. Torres, et al., Increased light harvesting in dye-sensitized solar cells with energy relay dyes, Nat. Photonics 3 (7) (2009) 406 411. [6] M.K. Nazeeruddin, P. Pechy, T. Renouard, S.M. Zakeeruddin, R. HumphryBaker, P. Comte, et al., Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells, J. Am. Chem. Soc. 123 (8) (2001) 1613 1624. [7] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L.Y. Han, Dye-sensitized solar cells with conversion efficiency of 11.1%, Jpn. J. Appl. Phys. 2 45 (24 28) (2006) L638 L640. [8] P.Y. Reddy, L. Giribabu, C. Lyness, H.J. Snaith, C. Vijaykumar, M. Chandrasekharam, et al., Efficient sensitization of nanocrystalline TiO2 films by a near-IR-absorbing unsymmetrical zinc phthalocyanine, Angew. Chem. 119 (3) (2007) 377 380. [9] Y. Hao, X.C. Yang, J.Y. Cong, H.N. Tian, A. Hagfeldt, L.C. Sun, Efficient near infrared D-Π-A sensitizers with lateral anchoring group for dye-sensitized solar cells, Chem. Commun. (2009) 4031 4033. [10] H. Tian, X. Yang, R. Chen, A. Hagfeldt, L. Sun, A metal-free “black dye” for panchromatic dye-sensitized solar cells, Energ. Environ. Sci. 2 (6) (2009) 674 677. [11] T. Kinoshita, J.T. Dy, S. Uchida, T. Kubo, H. Segawa, Wideband dye-sensitized solar cells employing a phosphine-coordinated ruthenium sensitizer, Nat. Photonics 7 (7) (2013) 535 539. [12] Z.J. Ning, Y. Fu, H. Tian, Improvement of dye-sensitized solar cells: what we know and what we need to know, Energ. Environ. Sci. 3 (9) (2010) 1170 1181. [13] X. Huang, S. Han, W. Huang, X. Liu, Enhancing solar cell efficiency: the search for luminescent materials as spectral converters, Chem. Soc. Rev. 42 (1) (2013) 173 201. [14] J.W. Gong, K. Sumathy, Q.Q. Qiao, Z.P. Zhou, Review on dye-sensitized solar cells (DSSCs): advanced techniques and research trends, Renew. Sust. Energ. Rev. 68 (2017) 234 246. [15] T.F. Schulze, T.W. Schmidt, Photochemical upconversion: present status and prospects for its application to solar energy conversion, Energ. Environ. Sci. 8 (1) (2015) 103 125. [16] G. Chen, H. Qiu, P.N. Prasad, X. Chen, Upconversion nanoparticles: design, nanochemistry, and applications in theranostics, Chem. Rev. 114 (10) (2014) 5161 5214. [17] P.S. Peijzel, A. Meijerink, R.T. Wegh, M.F. Reid, G.W. Burdick, A complete energy level diagram for all trivalent lanthanide ions, J.Solid State Chem. 178 (2) (2005) 448 453. [18] Y. Shang, S. Hao, C. Yang, G. Chen, Enhancing solar cell efficiency using photon upconversion materials, Nanomaterials (Basel) 5 (4) (2015) 1782 1809. [19] A. Shalav, B.S. Richards, T. Trupke, K.W. Krämer, H.U. Güdel, Application of NaYF4:Er31 up-converting phosphors for enhanced near-infrared silicon solar cell response, Appl. Phys. Lett. 86 (1) (2005) 013505. [20] J.A. Briggs, A.C. Atre, J.A. Dionne, Narrow-bandwidth solar upconversion: Case studies of existing systems and generalized fundamental limits, J. Appl. Phys. 113 (12) (2013) 124509 124514.
Upconversion-Enhanced Dye-Sensitized Solar Cells
339
[21] N.N. Yao, J.Z. Huang, K. Fu, X.L. Deng, M. Ding, X.J. Xu, Rare earth ion doped phosphors for dye-sensitized solar cells applications, RCS Adv. 6 (21) (2016) 17546 17559. [22] J. Yu, Y. Yang, R. Fan, D. Liu, L. Wei, S. Chen, et al., Enhanced near-infrared to visible upconversion nanoparticles of Ho31-Yb31-F2 tri-doped TiO2 and its application in dye-sensitized solar cells with 37% improvement in power conversion efficiency, Inorg. Chem. 53 (15) (2014) 8045 8053. [23] Z. Zhou, J. Wang, F. Nan, C. Bu, Z. Yu, W. Liu, et al., Upconversion induced enhancement of dye sensitized solar cells based on core-shell structured beta-NaYF4: Er31, Yb31@SiO2 nanoparticles, Nanoscale 6 (4) (2014) 2052 2055. [24] L. Liang, Y. Liu, C. Bu, K. Guo, W. Sun, N. Huang, et al., Highly uniform, bifunctional core/double-shell-structured beta-NaYF4:Er31, Yb31@SiO2@TiO2 hexagonal sub-microprisms for high-performance dye sensitized solar cells, Adv. Mater. 25 (15) (2013) 2174 2180. [25] P.N. Prasad, Nanophotonics, Wiley-Interscience, Hoboken, NJ, 2004. [26] G.Y. Chen, J. Seo, C.H. Yang, P.N. Prasad, Nanochemistry and nanomaterials for photovoltaics, Chem. Soc. Rev. 42 (21) (2013) 8304 8338. [27] J.R. Lakowicz, Radiative decay engineering: biophysical and biomedical applications, Anal. Biochem. 298 (1) (2001) 1 24. [28] Y. Ding, X. Zhang, H. Gao, S. Xu, C. Wei, Y. Zhao, Plasmonic enhanced upconversion luminescence of β-NaYF4:Yb31/Er31 with Ag@SiO2 core shell nanoparticles, J. Lumin. 147 (2014) 72 76. [29] F. Zhang, G.B. Braun, Y. Shi, Y. Zhang, X. Sun, N.O. Reich, et al., Fabrication of Ag@SiO2@Y2O3:Er nanostructures for bioimaging: tuning of the upconversion fluorescence with silver nanoparticles, J. Am. Chem. Soc. 132 (9) (2010) 2850 2851. [30] L. Sudheendra, V. Ortalan, S. Dey, N.D. Browning, I.M. Kennedy, Plasmonic enhanced emissions from cubic NaYF4:Yb:Er/Tm nanophosphors, Chem. Mater. 23 (11) (2011) 2987 2993. [31] W. Xu, B. Chen, W. Yu, Y. Zhu, T. Liu, S. Xu, et al., The up-conversion luminescent properties and silver-modified luminescent enhancement of YVO4:Yb31, Er31 NPs, Dalton Trans. 41 (43) (2012) 13525 13532. [32] W. Ge, X.R. Zhang, M. Liu, Z.W. Lei, R.J. Knize, Y. Lu, Distance dependence of gold-enhanced upconversion luminescence in Au/SiO2/Y2O3:Yb31, Er31 nanoparticles, Theranostics 3 (4) (2013) 282 288. [33] A. Priyam, N.M. Idris, Y. Zhang, Gold nanoshell coated NaYF4 nanoparticles for simultaneously enhanced upconversion fluorescence and darkfield imaging, J. Mater. Chem. 22 (3) (2012) 960 965. [34] W. Deng, L. Sudheendra, J. Zhao, J. Fu, D. Jin, I.M. Kennedy, et al., Upconversion in NaYF4:Yb, Er nanoparticles amplified by metal nanostructures, Nanotechnology 22 (32) (2011) 325604 325612. [35] G. Han, M. Wang, D. Li, J. Bai, G. Diao, Novel upconversion Er, Yb-CeO2 hollow spheres as scattering layer materials for efficient dye-sensitized solar cells, Sol. Energ. Mater. Sol. Cells 160 (2017) 54 59. [36] W. Liao, D. Zheng, J. Tian, Z. Lin, Dual-functional semiconductor-decorated upconversion hollow spheres for high efficiency dye-sensitized solar cells, J. Mater. Chem. A 3 (46) (2015) 23360 23367. [37] X. Wu, G.Q. Max Lu, L. Wang, Dual-functional upconverter-doped TiO2 hollow shells for light scattering and near-infrared sunlight harvesting in dye-sensitized solar cells, Adv. Energy Mater. 3 (6) (2013) 704 707. [38] X.D. Wang, R.R. Valiev, T.Y. Ohulchanskyy, H. Agren, C.H. Yang, G.Y. Chen, Dye-sensitized lanthanide-doped upconversion nanoparticles, Chem. Soc. Rev. 46 (14) (2017) 4150 4167.
340
Dye Sensitized Solar Cells
[39] C. Yuan, G. Chen, L. Li, J.A. Damasco, Z. Ning, H. Xing, et al., Simultaneous multiple wavelength upconversion in a core-shell nanoparticle for enhanced near infrared light harvesting in a dye-sensitized solar cell, ACS Appl. Mater. Interfaces 6 (20) (2014) 18018 18025. [40] G.Y. Chen, J. Damasco, H.L. Qiu, W. Shao, T.Y. Ohulchanskyy, R.R. Valiev, et al., Energy-cascaded upconversion in an organic dye-sensitized core/shell fluoride nanocrystal, Nano. Lett. 15 (11) (2015) 7400 7407. [41] G. Chen, W. Shao, R.R. Valiev, T.Y. Ohulchanskyy, G.S. He, H. Ågren, et al., Efficient broadband upconversion of near-infrared light in dye-sensitized core/shell nanocrystals, Adv. Opt. Mater. 4 (11) (2016) 1760 1766. [42] W. Shao, G. Chen, A. Kuzmin, H.L. Kutscher, A. Pliss, T.Y. Ohulchanskyy, et al., Tunable narrow band emissions from dye-sensitized core/shell/shell nanocrystals in the second near-infrared biological window, J. Am. Chem. Soc. 138 (50) (2016) 16192 16195. [43] X. Huang, Broadband dye-sensitized upconversion: a promising new platform for future solar upconverter design, J. Alloys Compd. 690 (2017) 356 359. [44] S. Hao, Y. Shang, D. Li, H. Agren, C. Yang, G. Chen, Enhancing dye-sensitized solar cell efficiency through broadband near-infrared upconverting nanoparticles, Nanoscale 9 (20) (2017) 6711 6715.