Cosensitization in Dye-Sensitized Solar Cells - Chemical Reviews

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Cosensitization in Dye-Sensitized Solar Cells Jacqueline M. Cole,*,†,‡,§,∥ Giulio Pepe,† Othman K. Al Bahri,† and Christopher B. Cooper†,§

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Cavendish Laboratory, Department of Physics, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom ‡ ISIS Neutron and Muon Facility, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, United Kingdom § Department of Chemical Engineering and Biotechnology, University of Cambridge, West Cambridge Site, Philippa Fawcett Drive, Cambridge CB3 0AS, United Kingdom ∥ Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ABSTRACT: Dye-sensitized solar cells (DSCs) are a next-generation photovoltaic technology, whose natural transparency and good photovoltaic output under ambient light conditions afford them niche applications in solar-powered windows and interior design for energy-sustainable buildings. Their ability to be fabricated on flexible substrates, or as fibers, also makes them attractive as passive energy harvesters in wearable devices and textiles. Cosensitization has emerged as a method that affords efficiency gains in DSCs, being most celebrated via its role in nudging power conversion efficiencies of DSCs to reach world-record values; yet, cosensitization has a much wider potential for applications, as this review will show. Cosensitization is a chemical fabrication method that produces DSC working electrodes that contain two or more different dyes with complementary optical absorption characteristics. Dye combinations that collectively afford a panchromatic absorption spectrum emulating that of the solar emission spectrum are ideal, given that such combinations use all available sunlight. This review classifies existing cosensitization efforts into seven distinct ways that dyes have been combined in order to generate panchromatic DSCs. Seven cognate molecular-engineering strategies for cosensitization are thereby developed, which tailor optical absorption toward optimal DSC-device function.

CONTENTS 1. Introduction 2. Operational Characteristics of DSCs 2.1. Quantum-Level Energy Characteristics of DSC Circuitry 2.2. DSC-Device-Performance Parameters 2.3. Structural Characteristics of Dyes in DSC Operation 3. Dye Cosensitization Fabrication Techniques 3.1. Cocktail Approach 3.2. Sequential Approach 3.3. Ultrafast Cosensitization 3.4. Fabrication-Control Parameters 4. Analytical Chemistry and Materials Characterization Methods for Cosensitization 4.1. Experiments on Dyes in Solution 4.2. Experiments on TiO2-Adsorbed Dyes 4.3. Experiments on DSC Devices 5. Optical and Chemical Classification of Cosensitizers 5.1. Cosensitizers That Fill the Optical Absorption Gaps of Near-Panchromatic Dyes 5.1.1. Complementing Near-Panchromatic Ruthenium-Based Dyes

© XXXX American Chemical Society

5.1.2. Complementing Near-Panchromatic Porphyrin-Based Dyes 5.2. Cosensitizers That Extend Optical Absorption into the NIR Region 5.3. Cosensitizers with Tunable Optical Absorption Bands 5.4. Cosensitizers Engineered by Combining Chemical Fragments in a “Molecular Lego” Fashion 5.5. Cosensitizers from Pigments in Extracts of Natural Resources 5.6. Cosensitizers That Contribute Proportionally To Achieve Panchromatic Absorption 5.7. Manifold Cosensitizers with Narrow-Band Optical Absorption Peaks 6. Future Outlook and Concluding Remarks Author Information Corresponding Author ORCID Notes Biographies Acknowledgments

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Received: October 19, 2018

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DOI: 10.1021/acs.chemrev.8b00632 Chem. Rev. XXXX, XXX, XXX−XXX

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electron is transported to the counter electrode, i.e., initiating the electrical current in the solar cell. An electrolyte is contained between the electrodes and acts as a redox couple, accommodating the electron from the counter electrode and passing it back to the dye to regenerate its ground state, thus completing the electrical circuit. The redox process is catalyzed by platinum, which is deposited as a thin film onto the counter electrode. Current DSC research is predominantly focused on the improvement of the photovoltaic-device efficiency while keeping DSC production costs low. Since the advent of DSCs, most studies have involved ruthenium-based dyes. This focus is partly driven by inertia, as these dyes were the very first DSC dyes to be used by O’Regan and Grätzel.1 They also offer key attributes such as a broad absorption spectrum and multiple anchoring sites, functional groups of a dye that can adsorb onto a TiO2 surface through a fabrication process called “sensitization”: the soaking of a TiO2 substrate (usually on ITO glass) in a solution of the dye. This sensitization process creates the dye···TiO2 interface that comprises the DSC working electrode. However, the presence of a transition metal in ruthenium-based dyes is also associated with certain problems: transition-metal-based dyes are often toxic, and they are more expensive than metal-free organic dyes owing to their limited availability. The use of organic dyes in DSC devices has gained increasing popularity on account of their low cost and ease of production.10−27 Their high molar extinction coefficients are another attractive property, which permits the fabrication of DSCs with thinner TiO2 films with minimized charge-transport losses.28 This is an important consideration for “smart window” applications, which may sometimes require colorless, rather than transparent but tinted, materials; the TiO2 layer needs to be sufficiently thin in order to not be seen. Moreover, owing to their sensitivity to different solvents, the optical absorption band of many organic dyes can be tuned via solvatochromism, i.e., the spectral shift of absorption bands upon using different solvents.29−33 Despite having potentially good prospects for the replacement of the established ruthenium-based dyes in DSCs,34 organic dyes are limited by their characteristically sharp and narrow absorption bands, which drastically limits the incident-photon-to-electron conversion efficiency (IPCE) of DSC devices that embed such dyes.12 Cosensitization of the semiconducting electrode in a DSC with two or more dyes (Figure 2) is a highly active area of DSC research that is currently under considerable development, as demonstrated in this review. Most cosensitization studies reported thus far have involved a combination of organic and organometallic dyes. In the context of developing DSCs for smart window technology, this is significant, because it has the potential to exploit the advantages of organic material properties for DSC applications while mitigating their disadvantages by including organometallic complexes. Evidently, an entirely organic mix of dyes would represent the best-case scenario from a purely environmentally sustainable perspective. However, as this review will show, the organometallic/organic “hybrid” formulation at least stands to dilute the quantities of metal-containing dyes required for DSC devices while improving their overall performance efficiency, which already represents a “win−win” situation. One of the central problems that DSCs are currently still facing is their merely partial absorption coverage of the solar spectrum. When the solar emission spectrum cannot be fully

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1. INTRODUCTION Solar-powered windows, which generate electricity from sunlight, represent an emerging environmental technology that may be able to satisfy the entire energy demand of buildings in future cities. One of the strongest contenders for such a “smart window” technology is the dye-sensitized solar cell (DSC).1 DSCs represent the only truly transparent type of building-integrated photovoltaic (BIPV) devices.2,3 Moreover, unlike other solar-cell technologies, DSCs excel in poor or ambient sunlight conditions. Accordingly, they are ideally suited to operate in the typical light and temperature conditions of modern city dwellings.2−6 DSCs are also very cost-effective to the extent that their price-to-performance ratio, which governs the economics of solar cells, exceeds “gridparity” status, i.e., their price-to-performance ratio exceeds that of fossil fuels, which renders them competitive with traditional energy technologies. There is also evidence that the performance-efficiency gap between DSCs and other competing BIPV technologies is smaller than anticipated, provided that the real producible energy in a generic BIPV platform is considered.7 DSCs also have several practical advantages: their modules can be mass-manufactured using factory-based “roll-to-roll” processing methods, which would reduce costs once device production is ready for scale-up. Moreover, modules can be made flexible and produced in a range of colors, which is attractive from a commercial perspective when the end consumer is considered, who will integrate these models in highly personalized living environments. DSCs can also be fabricated as fibers, which has provided prospects for their further use as passive energy harvesters in textiles for home interiors or even as wearable devices.8,9 DSCs comprise four key device components: a chemical dye, a working electrode, a counter electrode, and an electrolyte. Figure 1 illustrates the overarching function of a DSC device at

Figure 1. Operational mechanism of a DSC.

the molecular level. First, the chemical dye absorbs light from the sun. Then, in its photoexcited state, the dye molecule injects an electron into the semiconductor electrode (usually TiO2 nanoparticles) onto which it is adsorbed. This generates a potential difference with respect to this working electrode and a counter electrode (a transparent conducting oxide, usually fluorine-doped tin oxide, FTO); as such, this injected B

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Figure 2. Schematic illustration of a DSC cosensitized with two dyes (not to scale). Most cells contain two TiO2 layers, i.e., the scattering layer and the active layer. The purpose of the latter (thickness: 7−20 μm; diameter of the TiO2 nanopores: 20 nm) is to absorb light, while that of the former (thickness: 4 μm; diameter of the TiO2 nanopores: 400 nm) enables the back reflection of incident light that has not been absorbed during the initial passage through the active layer.

may have beneficial or deleterious effects on the function or performance of the DSC. The field of cosensitization is still far from mature, and several limitations that arise from the use of multiple dyes on the same semiconductor layer remain to be addressed. Arguably, unfavorable dye···dye interactions and electron quenching represent the most important issues, as these significantly suppress the power conversion efficiency (PCE).37−41 Furthermore, it has been speculated that the sensitization capacity of TiO2 is already close to its limit, leaving few “free” anchoring sites for additional sensitization, thus increasing competition between the sensitization of multiple dyes.15,41−43 This review thus offers a comprehensive treatise on cosensitized DSCs (co-DSCs). The operational characteristics of a DSC are first introduced, by breaking down the role of each device component into photo- and electrochemical equations and describing their interplay via quantum-energy levels. The various photovoltaic parameters that characterize DSC-device performance are then defined. The molecular characteristics of dye···TiO2 interfacial structures, which comprise DSC working electrodes, are then detailed with particular consideration being given to the structural parameters that influence device function. The specific need for cosensitization prevails through this discussion, before the review describes the fabrication methods for cosensitization and explains how each method affords distinct DSC functionality. Subsequently, studies that have employed various analytical chemistry and materials characterization tools to understand the physical and materials chemistry associated with sensitization are then surveyed. Then, a full literature review of the progress in cosensitization is given, wherein we classify individual cosensitization studies according to the distinct ways that they lead toward panchromatic absorption. This demarcates the various molecular-engineering strategies that are available for tailoring optical absorption in DSCs through cosensitization. The review concludes with a future outlook showing that the need for cosensitization could not be more timely or relevant if we are to succeed in realizing energysustainable buildings for future cities. We set some practical limits to the scope of this review since different cosensitization approaches have been developed over

covered by the optical absorption that stems from the sensitization of a semiconducting surface with a single dye, one viable solution is cosensitization using more than one dye with different absorption spectral onsets to achieve a panchromatic response (Figure 3).35 Organic dyes are

Figure 3. Illustration of how cosensitization can afford panchromatic absorption. Combining dyes 1, 2, and 3, which have different optical absorption peaks, into a single device produces a DSC with a broader absorption spectrum. Adapted with permission from ref 36. Copyright 2016 Royal Society of Chemistry.

generally better suited to this scenario than metal-based complexes, due to their higher molar extinction coefficients which require lower dye concentrations adsorbed onto the TiO2 surface. Considering the physical juxtaposition of organic and organometallic complexes on a TiO2 surface presents further advantages for organic dyes. For example, small organic dyes may fit into the vacant interstices between larger organometallic complexes and adsorb onto the TiO2 surface therein, providing higher overall dye coverage of the nanoparticle surface area. Furthermore, the spatial separation of organic molecules from each other by cosensitizing them with organometallic complexes could potentially circumvent the aggregation of the organic dye molecules, which usually diminishes DSC efficiency. Coumarin dyes represent a classic example of aggregating dyes, as their arene rings are sufficiently exposed to allow intermolecular π···π ring-stacking interactions.30,33 Furthermore, if two distinct chemical dyes are spatially close and their optical absorption and emission spectra overlap sufficiently, Forster resonance energy transfer (FRET) could occur, albeit that the precise nature of FRET C

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Figure 4. Energy-level diagram showing the characteristic relations between the four key device components of a co-DSC (dyes D1 and D2, working electrode, counter electrode, and electrolyte). All processes with arrows between energy states indicate the direction of the electron transfer. Electron recombination processes are shown in gray.

the past decades, rendering a definition of what precisely constitutes a cosensitized DSC ambiguous. For the purposes of this review, DSCs are considered that contain a single film of semiconducting nanopores, sensitized with at least two different dyes that actively participate in the light-harvesting process. Unless otherwise specified, the term “semiconducting nanoporous films” (or derivatives thereof) will refer to pure titanium dioxide (TiO2), considering that it is the most commonly used material for such films. In order to focus the area of interest, novel techniques such as the use of multiple dyes on multiple layers of semiconducting surfaces,44 the selective adsorption of multiple dyes on different areas of the semiconducting surface,45 the use of multiple-energy-relay (FRET) dyes,46 or the use of cosensitized CdSe quantum-sized particles for quantum-dot-sensitized solar cells (QDSCs)47 will not be discussed in this review.

In order to achieve photoexcitation, the energy gap between the ground and the excited states of each dye molecule must be smaller than that of the incident-photon energy. This energy gap in a dye can be approximated as the difference between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO), E0−0, i.e., hν ≥ E0−0. Each photogenerated electron is subsequently injected from the LUMO of the dye molecule into the conduction band (CB) of the nanocrystalline TiO2 substrate: D* → D+ + e−(TiO2 )

This results in an oxidized dye, D+,48 and the electroninjection process typically takes place over a time scale shorter than 25 fs;49,50 the associated kinetic driving force is sufficient to prevent the dye from thermal relaxation prior to completion of the injection process. Nonetheless, electron injection can only occur if the energy of the LUMO for each dye is higher than that of the CB of the semiconductor (cf., Figure 4). The overpotential required to stimulate the electron-injection process is ∼0.2−0.3 eV.51 For nanocrystalline TiO2 grains that are 10−50 nm in size, it is generally assumed that a space-charge layer is not present; i.e., the device lacks a built-in field. Hence, injected electrons are transported through the TiO2 film by diffusion, which is kinetically opposed by electron recombination with the dye or the electrolyte.52 These processes account for the main loss mechanisms in DSCs.53 The dye regeneration requires the HOMO energy of each dye to be lower than the redox potential energy of the electrolyte. Traditionally, the I−/I−3 redox couple has been used most frequently, and this electrolyte is thus used here for the purposes of illustration. The photogenerated hole in the HOMO level of a dye is then captured by the electrolyte, stimulating the electrochemical reaction:54

2. OPERATIONAL CHARACTERISTICS OF DSCS Co-DSCs are essentially a more complex variation of conventional singly sensitized DSCs. Therefore, we initially introduce fundamental DSC concepts. Given that the operational principles of DSCs are governed at the molecular level, we start by considering the quantum-level energy characteristics of a DSC. Subsequently, device-performance parameters that experimentally validate DSC capabilities are considered, followed by a summary of the structural characteristics of the dye···TiO2 interface that affect the function of a DSC working electrode. 2.1. Quantum-Level Energy Characteristics of DSC Circuitry

Figure 4 shows an energy-level diagram that depicts the process of converting light into an electric current within a coDSC. The absorption of a photon, which contains an energy hν, elevates each dye molecule from their ground state (D1 and D2) to their photoexcited state (D1* and D2*) (Figure 4, red and dark blue arrows): D1 + hv → D1*

(1a)

D2 + hv → D2*

(1b)

(2)

D

D+ + e− → D

(3)

3I− → I3− + 2e−

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Figure 5. Characteristics of the dye···TiO2 interfacial structure that influence the function of a DSC working electrode, which lies immediately adjacent to a medium of electrolyte (EL) ions within the device environment.

Upon oxidation, the I− ions form I3− ions, while the released electrons neutralize the holes.54 Then, the I3− ions diffuse to the counter electrode, where they are reduced to I− ions, aided by the platinum catalyst that is deposited on the electrode. I3− + 2e− → 3I−

which is usually plotted as a function of the optical wavelength.52 A convenient way of calculating JSC is to integrate the incident-spectral-photon-flux density, bs(E), multiplied by the IPCE over the entire energy range:56

(5)

∫

JSC = e bs(E)IPCE(E) dE

2.2. DSC-Device-Performance Parameters

As all types of sensitized DSC devices are characterized using standard equipment for solar-cell testing (for further details, see section 4.3), it is pertinent to introduce the key defining parameters that are used to experimentally validate DSC devices. An important quantity representing the overall performance of a solar cell is the PCE (η): η=

This allows a theoretical prediction of the short-circuit current density for a given IPCE and incident-photon flux. Device efficiency losses can then be assessed by comparing this quantity with the measured short-circuit current density. 2.3. Structural Characteristics of Dyes in DSC Operation

The adsorption process that creates a DSC working electrode relies on the self-assembly of monolayers (SAMs) of dye molecules onto a semiconducting (TiO2) substrate. The dye··· TiO2 interfacial structure so afforded can present in a variety of forms. As such, a range of structural parameters has been developed to distinguish them. The adsorption process itself is a primary consideration, and the terms dye anchoring and dye binding are used synonymously to represent the adsorption of dye molecules onto TiO2 surfaces. It is generally considered that the carboxylate group is one of the, if not the, strongest types of anchor; accordingly, dyes normally carry a carboxylic acid as a functional group. A carboxylate ion can bind onto TiO2 in various molecular configurations, although its bidentatebridging mode has been deemed to be the energetically most stable.52,57 Figure 5a depicts this anchoring process which, in turn, depends upon intramolecular charge transfer in the dye delivering electronic charge to its anchoring group, whereupon its contact with the TiO2 surface enables electrons to be injected from the dye into the CB of TiO2. Electron injection can therefore only occur if the dye makes physical contact with the TiO2 surface. Depending on how the dye molecules are oriented once anchored onto the TiO2 surface, various parts of the dye could touch the TiO2 surface; however, it is generally

JSC VOCFF PS

(6)

JSC is the short-circuit current density. VOC is the open-circuit voltage, which is the difference between the Fermi level, EF, of TiO2 and the redox potential of the electrolyte,55 and PS is the incident-light power density. FF is the fill factor, which indicates the intrinsic quality of the fabricated cell FF =

Jmax Vmax JSC VOC

(7)

wherein Jmax and Vmax refer to the values of current density (J) and voltage (V) that maximize their product. Usually, DSCs are assessed under standard test conditions (STCs): AM 1.5 spectrum, T = 25 °C, and PS = 100 mW cm−2.56 Another important measure of solar-cell performance is the incident-photon-to-electron conversion efficiency (IPCE), which is defined as the ratio between the incident photons and the generated charge carriers IPCE = 1240

JSC (λ)[mA cm−2] λ[nm]PS[mW cm−2]

(9)

(8) E

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immediately above the dye···TiO2 working electrode within the DSC device. Such attack is undesirable since it creates an electrical short-circuit on account of the electrolyte ion gaining an electron from the TiO2 surface, effectively removing an electron that has been injected into the TiO2 conduction band from the dye; this process is illustrated in Figure 5f. Such electron recombination is undesirable since it depletes the otherwise available electrical current in a DSC. Such gaps between dyes on TiO2 surfaces could be filled in a number of ways to protect the dye···TiO2 working electrode from electrolyte attack. For example, there are antiaggregation agents, most classically chenodeoxycholic acid (CDCA) or deoxycholic acid (DCA), that can keep dyes apart by inserting themselves in between the dyes on TiO2 surfaces. Since these agents do not have any optical absorption, they are colorless and so their function is purely static. Alternatively, gaps on TiO2 surfaces can be blocked by coating the semiconductor with an atomically thin layer of passive material that has a large bandgap, such as Al2O359 or Ga2O360 deployed by atomic layer deposition, provided that the coating forms exclusively in the interstices and does not cover the dyes so that the electrolyte cannot regenerate them in due course (Figure 5g). These approaches to filling in these gaps are known as surface passivation, as they are additions to the TiO2 surface that do not themselves contribute to the optical or electronic function of the DSC. A nonpassive way that beneficially contributes to DSC function would surely be better; such a method is to fill these gaps in the TiO2 surface with other types of dye molecules, i.e., to create co-DSCs. As shown in this review, the population of TiO2 surfaces with cosensitizers not only overcomes the steric issue of filling in its gaps but also offers a DSC the option to absorb more light; as such, the PCE in the resulting co-DSC is enhanced relative to that of the singly sensitized DSC bearing the original dye. Furthermore, the waveband of light absorbed by the cosensitizer could be tailored in a fashion that is complementary to the optical absorption of the original sensitizer. Therefore, cosensitization not only is useful for mitigating steric issues of TiO2 surfaces but also offers a range of optical tuning options. In particular, cosensitization can be used to design DSCs that exhibit panchromatic absorption, which stand to yield the highest photovoltaic-device efficiencies, given that the solar emission spectrum is most intense in its coverage of the visible part of the electromagnetic spectrum. As we shall see in this review, a number of molecular-engineering strategies have emerged that employ cosensitization to realize panchromatic co-DSCs. The success of these strategies is nonetheless predicated on an ability to control the nature by which SAMs form dye···TiO2 interfacial structures; in turn, this is naturally dependent on the method of sensitization. Accordingly, the fabrication methods of cosensitization are now reviewed.

preferred that such contact is made via the anchoring group since this often doubles up as the electron acceptor of the dye molecule, and so, it leads ICT down the strongest electronic pathway for optimal electron injection. The preferred molecular orientation of dyes relative to the TiO2 surface, Figure 5b, is defined by an angle, θ, that subtends the semiconductor surface and the axis within the dye molecule that maximally extends from the anchoring point to its opposing side in the chromophore. Accordingly, θ helps quantify the extent to which the dye makes contact with the TiO2 surface. For example, dye molecules that stand bolt upright on TiO2 surfaces (θ = 90°) will likely touch the TiO2 surface primarily or exclusively via their anchoring group. At the other extreme, dye molecules that lie flat on the TiO2 surface (θ = 0°) will presumably interact with the TiO2 surface substantially. In practice, this tilt angle, θ, lies in between these two extreme cases for most dyes and is constrained by the geometric nature of the dye-anchoring configuration. Thus, θ provides insights into the likely anchoring configuration that a given dye adopts on TiO2 and affords an indication of its binding strength. Furthermore, θ will indicate the extent by which a dye molecule will impart a physical shadow on TiO2 as it tilts over its surface. This is parametrized by an effective areaper-molecule (APM), which helps assess the extent of dye coverage (Figure 5c) on a DSC working electrode. Dye aggregation can prevent a dye from anchoring onto the TiO 2 surface. 58 Longitudinal or lateral forms of dye aggregation may occur, whereby dye molecules cluster along the direction parallel or perpendicular to the TiO2 surface, respectively. Lateral dye aggregation places dye molecules on top of each other in some fashion such as that illustrated in Figure 5d. Since DSC dyes usually contain carboxylic acid groups that have a high propensity to dimerize, a classic case of lateral dye aggregation would be the manifestation of this phenomenon. However, there are many other forms of steric and electronic effects that cause the stacking of dye molecules on TiO2 substrates. So, while one somewhat presumes that perfect SAMs are created upon sensitization, this is frequently not the case. For every dye molecule that does not reach the surface, one electron is lost in the injection process; so, lateral dye aggregation naturally limits the maximum electrical current, and, thus, the PCE that is achievable in a DSC. Accordingly, a variety of chemical fabrication parameters (such as dye-solution concentration, solvent type, and sensitization time) need to be carefully controlled during the dyesensitization process in order to mitigate lateral dye aggregation. Longitudinal dye aggregation (Figure 5e) also affects DSC operation, albeit in a different way from its lateral counterpart. Dye molecules can agglomerate on a surface, most commonly as a consequence of intermolecular forces such as that seen in the π···π stacking of π-conjugated organic molecules. This presents an uneven interdye separation on the TiO2 surface, which leads to inefficiency in the surface packing of an SAM. The maximum-possible dye coverage could be compromised as a result of this packing inefficiency, which is problematic given that the number of electrons that can be injected into the CB of TiO2 increases with the number of dye molecules that cover the surface of the semiconductor; the extent of electron injection, in turn, governs the electrical current of the device. Uneven interdye separations will also create gaps in the TiO2 surface; if these gaps are sufficiently large, this will expose the semiconductor surface to attack from electrolyte ions that lie

3. DYE COSENSITIZATION FABRICATION TECHNIQUES For the cosensitization of TiO2 with dyes, two principal avenues are usually employed: the cocktail and the sequential approach (Figure 6). 3.1. Cocktail Approach

The cocktail approach involves mixing multiple dyes in a common solvent. The cosensitization happens in one step, by immersing the TiO2 substrate in this cocktail solution. This F

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extent of dye loading onto the TiO2 substrate may vary greatly using the cocktail approach.11,12,14,15,17,21,23,27,29,39 Developing strategies to minimize unwanted competition is a challenging task given the complex and dynamic interplay between the various parameters that influence coadsorption. The most successful approach, so far, has been the use of cosensitizers that exhibit disparate binding preferences on TiO2.26,61 Investigating the influence of parameters such as pH,62 pKa, and binding energy63 may help address this problem. Meanwhile, stabilizing parameters that vary during the dying process such as pH and dye concentrations using acid−base buffers and circulating fresh dying solution,64 respectively, may simplify such investigations. Dyes with high adsorption rates, e.g., due to the presence of numerous dye-anchoring groups, tend to aggregate faster; hence, it is important to synergistically tune the molar ratio of the cosensitizers in the cocktail solution and the immersion time of the substrate to minimize dye aggregation.65 Considering their chemical compatibility, cosensitizing dyes must not react with each other and also need to be soluble in a common solvent to employ the cocktail approach. Even though two miscible solvents may be used under special circumstances, cosensitization via the cocktail approach is usually limited to dyes that share a common solvent. Despite these drawbacks, the cocktail approach is the simplest fabrication technique to test new combinations of dyes together, aided by the fact that dye molecules will selfassemble in the most natural adsorption configuration on TiO2. The limited control afforded by this method may prevent dye molecules from adsorbing onto TiO2 in the most effective dye-loading configuration for optimal DSC lightharvesting properties. Greater control can be achieved through the sequential approach. 3.2. Sequential Approach

In the sequential approach, the TiO2 surface is sensitized using one dye at a time.21 This affords a much higher level of control over dye loading for each dye relative to the cocktail approach; examples of control parameters are the order of sensitization18,19,43 or the sensitization time.13,38,43 Thus, dye competition issues may be minimized, as dominant dyes can be used in later fabrication steps that follow the sensitization of less dominant dyes. When the dye···TiO2 binding energies of the cosensitizers are significantly different, the sequence of sensitization becomes more critical owing to possible desorption effects. For example, when a dye with higher binding energy is used in a later stage of the sensitization sequence, it partly desorbs and replaces previously used dyes that bind more weakly, especially their aggregates; the resulting dye···TiO2 interface tends to feature a greater propensity for dye monolayers.63 The sequential approach is especially useful for maximizing the cosensitization efficiency of dyes that have disparate molecular sizes. For example, large dye molecules can be used in initial sensitization steps, saturating the TiO2 surface, while smaller molecules may be used in a subsequent step to fill residual surface gaps, thus efficiently creating a heterogeneous layer of dye molecules.42 When the molecular size is not substantially different, the shape and geometry of the dyes play a more important role.63 Moreover, the binding energy of each dye on TiO2 influences the relative concentration of the cosensitizers. In both cases, the order of sensitization dictates the organization of the dyes on the

Figure 6. Sequential (top) and cocktail (bottom) cosensitization of TiO2 substrates. Adapted with permission from ref 36. Copyright 2016 Royal Society of Chemistry.

approach allows the dye molecules to find a relatively unperturbed arrangement on the TiO2 surface, in accordance with their thermodynamic and kinetic preferences. Although this approach usually affords less control and combinations of chemically incompatible dyes may cause problems, such as unwanted dye competition, unfavorable dye cosensitization ratios, and dye dissolution, it offers certain advantages such as ease of device fabrication. Unwanted dye competition may occur, when one dye adsorbs more favorably onto the TiO2 surface than the other, thus occupying most of the available anchoring sites. This is one of the main problems that limit the potential of cosensitization. Even if the concentration of the naturally favored dye is reduced relative to that of the other dye, the G

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where D is the diffusion coefficient and τ the diffusion time. For films exceeding L, a large fraction of charge carriers will recombine within the film.79 However, thicker layers will offer an increased TiO2 surface area that can host more dye molecules, thus improving the light-harvesting efficiency (LHE). Additionally, insufficiently thick TiO2 layers may limit the light-harvesting ability of a cell. The extent of this balance between electron recombination and LHE depends, in turn, on the values of the molar extinction coefficients, ϵ, of the dyes and their dye···TiO2 adsorption abilities. The optimal TiO2 film thickness for the fabrication of co-DSCs is therefore a dye-specific parameter. Levels of dye···TiO2 adsorption can also be enhanced by due consideration of the fact that TiO2 offers two types of anchoring sites, i.e., Lewis- and Brønsted-acidic sites. Carefully choosing dye cosensitizers that have different anchoring moieties to each other, such that each preferentially attaches to TiO2 on distinct anchoring sites, has proven to be a viable strategy to minimize competitive adsorption.80 The binding energy of a dye to TiO2 can also be employed as a fabricationcontrol parameter that exploits the chemical complementarity of cosensitizers. This is because the ratio of the binding energies between dye cosensitizers influences the individual dye loading and adsorption rate.63 A vast difference in binding energies may prevent the dye with lower binding energy from anchoring from a cocktail solution. In the sequential method, when the dye with lower binding energy is used first, the second dye will desorb some of the first dye and occupy these sites.63 The difference in binding energies may be a useful parameter to control when combining dyes with different molar extinction coefficients and absorption ranges, whereby a lower loading of the dye with higher molar extinction coefficient and narrow absorption may be preferable. The addition of passive coadsorbers, such as DCA or CDCA (cf. section 2.3), often improves co-DSC performance.10,22,26,28,43,81,82 They act as physical spacers between the functional dye molecules, thus lowering the propensity of dye aggregation,40 which is important as dye aggregation may induce quenching of the photoexcited state of the dye.83 Additionally, coadsorbers occupy adsorption sites on the TiO2 surface, which could potentially be occupied with an active dye molecule. The LHE may thus be limited, even though spatial separation of the dyes usually offers a net benefit in device performance. It is important to point out that, in co-DSCs where the combination of dyes is ideal, each dye will function naturally as a physical spacer for the other. If this is the case, the use of passive coadsorbers such as cholic acids should not be necessary.72,84 The dynamics of dye adsorption onto TiO2, and thus the dye-loading properties, may be affected by the solvent used for dye sensitization.10 Given that each dye dissolves, according to its chemical composition, best in a specific solvent, this effect is dye-specific. However, practical choices tend to gravitate toward solvents such as methanol, ethanol, acetone, or a 1:1 mixture of acetonitrile/t-butanol owing to an initial common usage in DSCs without adverse effects, and their ease of availability. In general, solvents should be kept dry, since any contamination with water will have substantially deleterious (desorption) effects during the sensitization process.82 The addition of tetra-butylammonium perchlorate (TBP) to the electrolyte can improve DSC performance for certain dyes.12 In 1993, Grätzel et al. reported for the first time beneficial effects of this additive,85 which were mostly

surface of the substrate, which is of the utmost importance for device performance. In the sequential approach, sensitization time is an adjustable parameter that may be used to control dye loading. However, surface saturation times on TiO2 are dye-specific, and therefore sensitization time requires experimental finetuning. A prerequisite to such fine-tuning is a detailed understanding of the nature of the dye···dye and dye···TiO2 interactions. For example, ligand-substitution reactions with the anchoring group between different dyes may limit the immersion time.66 Even though the adsorption rate has been linked to the number and nature of anchoring groups on the dye molecules, few details have been discovered in this area so far. Accordingly, the cocktail approach is generally best-suited for initial tests of new combinations of dyes in cosensitization. This explains, at least in part, why the sequential approach has not completely outstripped the cocktail approach, despite its higher control over sensitization. 3.3. Ultrafast Cosensitization

Holliman and co-workers43,67 have shown that ultrafast cosensitization (on the order of minutes) may outperform conventional (on the order of hours) cosensitization since it results in improved DSC efficiency for singly and cosensitized samples. In contrast to the soaking of TiO2-covered FTO glass substrates in dye solutions, ultrafast cosensitization is carried out by pumping dye solutions through holes in fabricated DSC devices (Figure 7).

Figure 7. Solar-cell design for ultrafast dye sensitization. Adapted with permission from ref 43. Copyright 2010 Royal Society of Chemistry.

3.4. Fabrication-Control Parameters

A range of co-DSC fabrication parameters can be fine-tuned in order to optimize device efficiency. For example, in case of the cocktail approach, dye loading may be controlled by adjusting the respective dye concentrations in solution.11,12,14,15,17,21,23,27,29,39,68−73 In the case of the sequential approach, dye loading may be controlled via the order of dye exposure to TiO 2 18,19,43,67,74 or by the dye-exposure time.13,38,43,67,74,75 Recombination of charge carriers has been reported to be affected by the thickness of the films deposited on TiO2,14,21,24,76−78 which is due to the fact that the diffusion length of an electron in the film, L, limits its functional thickness according to L = (Dτ )1/2

(10) H

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spectroscopy that measures changes in the absorbance/ transmittance at a particular wavelength of a sample as a function of time after excitation with light. TAS may be used to investigate mechanistic and kinetic details of very fast chemical processes (>10−15 s) such as nonradiative relaxation of higher electronic states, vibrational relaxations, and radiative relaxation of excited singlet states. TAS can also be used to investigate photochemical reactions, transfer processes (energy, charges, or electrons), conformational changes, thermal relaxation, fluorescence, and phosphorescence. Both the optical stimulation of chemical events (“excitation” or “pump”) and the measurement of transient absorbance (“probe”) are usually achieved by a pulsed laser. In the context of DSCs, TAS is important for determining the rate of electron injection from photoexcited dyes into nanowire and nanoparticle films.87 UV−vis emission and excitation spectroscopy techniques such as photoluminescence excitation spectroscopy88 may be used to predict the external quantum efficiency (EQE) of solar cells.89 Fluorescence spectra of cosensitizers can also prove valuable since the data can be used to approximate the zero− zero excitation energy, E0−0, which is the intercept between the normalized absorption and fluorescence spectra of a dye. For dyes that absorb in the UV or blue regions of the solar spectrum, their fluorescence typically occurs at higher wavelengths in the visible and near-IR regions. The emitted photons can in principle be absorbed by the other cosensitizer if it absorbs in these regions, i.e., if the absorption and fluorescence spectra of the cosensitizers overlap. Unlike FRET, this form of energy transfer is a radiative process that is not the result of dipole−dipole coupling. Another commonly used solution-based analytical chemistry technique for understanding sensitization is cyclic voltammetry (CV), which examines the redox properties of dyes. It provides an experimental means to determine the energy level of the HOMO of a dye, which is a central selection criterion for the suitability of a DSC dye. Such determinations are often complemented by quantum-chemical calculations of the HOMOs and LUMOs using density functional theory (DFT), whereby calculated HOMO values can be normalized to experiment.90 HOMOs and LUMOs of cosensitizers should be carefully considered due to their influence on VOC. Introducing a second dye with lower LUMO than the primary dye lowers the Fermi level of TiO2 relative to the I−/I3− redox potential, which results in a lower VOC.67 On the basis of the available evidence, cosensitizers with similar LUMOs tend to outperform those with drastically different LUMOs, whereby introducing dyes with a lower LUMO tends to be detrimental. Incompatible electronic structures may result in unconstructive charge transfer between dyes or in a modulation of the charge recombination. It is not yet clear how cosensitization alters the electronic structure of the individual dye molecules; while measuring the energies of the charge-separation states for the individual dyes separately is straightforward, such information is difficult to obtain from intimate dye mixtures in co-DSCs. Such measurements, once possible, may help elucidate the effect of cosensitization on the electronic structure of the cosensitizers, which in turn would help with designing more electronically compatible cosensitizers. One possible route is using picosecond photoacoustic calorimetry (PAC).67

attributed to shifts of the TiO2 energy levels that resulted in improvements of VOC.52 Reports that provide a comprehensive understanding of this performance boost are currently still lacking, but it is nevertheless feasible to assume that the specifics of the molecular orbital energy levels of the dyes, relative to this purported change in TiO2 energy levels, will determine the level by which such additives can influence the efficiency of an associated cosensitized DSC device. Treatment of the TiO2 substrate with TiCl476,86 prior to dye sensitization may increase the TiO2 surface roughness, which affords a higher mesoporous surface area and more surface terraces. This increase improves dye adsorption52 and thus the DSC performance.

4. ANALYTICAL CHEMISTRY AND MATERIALS CHARACTERIZATION METHODS FOR COSENSITIZATION The energetic and structural needs of sensitization have been detailed in sections 2.1 and 2.3, while the fabrication needs specific to cosensitization have been explained in section 3. Various analytical chemistry and materials characterization methods are employed to understand these three needs of cosensitization. The respective roles of such methods are hereby described, making specific reference to the current state-of-the-art in methods being used to probe cosensitization. We highlight cases where these methods have been used to study the complementary nature of dye cosensitizers with regard to optical absorption, since this is a key parameter for achieving panchromatic co-DSCs. Dyes need to be characterized at different parts of the DSC fabrication process in order to obtain poignant information about DSC fabrication, energetics, or structure and how this affects DSC-device performance. Experiments can be divided into three categories: (i) experiments that quantify the chemical and optical properties of the dye (in solution); (ii) experiments that quantify the interactions between the dye molecules and the TiO2 surface (on the cosensitized TiO2 substrate); (iii) DSC-performance tests (on the fabricated cell). Experiments in all three categories are crucial to determine the origins of the observed device-performance improvement or deterioration in a co-DSC. 4.1. Experiments on Dyes in Solution

Ultraviolet−visible (UV−vis) absorption spectroscopy is one of the most common and simple methods to analyze the optical properties of dye molecules in solution. The effects of cosensitization, and thus potential effects of dye···dye interactions (at least in solution), can be qualitatively assessed. If present, it is moreover possible to examine the solvatochromism of these dyes, which may offer insights into the type of dye aggregation (J or H).58 The relative quantities of each cosensitized dye can be estimated from the molar extinction coefficient of the combined spectrum with reference to those of the individual dyes. One can also use such spectra of cosensitized dyes to judge the level by which a given dye combination is complementary, e.g., the case where one dye absorbs in a certain UV−vis waveband with good intensity and the other dye absorbs well in a different UV−vis spectral range; as such, their combined absorption properties yield an optical absorption spectrum that emulates that of the solar emission spectrum within its UV−vis region. Transient absorption spectroscopy (TAS) or time-resolved absorption spectroscopy is an extension of absorption

4.2. Experiments on TiO2-Adsorbed Dyes

In order to understand the effect of sensitization, the dye··· TiO2 interfacial structure of DSC working electrodes needs to I

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of ruthenium-based dyes builds up during the early stages of dye···TiO2 adsorption.103 Moreover, a recent study on cosensitized dyes has demonstrated how AFM can be used to calculate average metrics for dye aggregation.104 AFM has also shown that such dyes can anchor onto TiO2 surfaces in multiple types of molecular conformations.105 A concerted scanning tunneling microscopy and scanning tunneling spectroscopy (STM/STS) study revealed that such dyes can coexist on TiO2 in multiple conformations, seven of which were energy-resolved.106 Single-crystal X-ray diffraction studies have shown that ruthenium-based dyes can exhibit structural polymorphism;107 since polymorphism occurs in crystal structures when multiple low-energy conformations manifest in a solution-based crystallization process, this corroborates the notion that multiple molecular conformations of a dye can form during sensitization onto a TiO2 surface. Large variation in the molecular conformations of a series of ruthenium-based DSC dyes was also observed in a survey of their crystal structures.108 X-ray reflectometry (XRR) has complemented single-crystal X-ray diffraction studies by showing that packing effects in crystal structures can translate well to the structure of a dye monolayer on TiO2.109 Furthermore, XRR has demonstrated its use in realizing the preferred molecular orientation of a dye relative to the TiO2 surface, from which the type of dye anchoring and area-per-molecule used by dyes on a TiO2, and overall dye surface coverage have been calculated.109−111 XRR has also determined structural modulations in dye monolayers owing to the pH value of sensitizing solution (affecting dyedeprotonation levels), the type of solvent used, and the presence of dye counterions.111 The preferred dye orientation and surface coverage attributes of dye···TiO2 interfaces have also been determined in the first report of an XRR study on cosensitized dyes, which focuses on assessing dye-aggregation effects.104 Neutron reflectometry (NR) has also proven useful in capturing the dye···TiO2 interfacial structure of DSC working electrodes while the structure is held in its device environment.110 The in situ nature of this study was vital for demonstrating that the preferred orientation of a dye relative to the TiO2 surface can change once the electrolyte precursor has been added to the DSC assembly, owing to complexation of the precursor with the dye.

be examined. To this end, a range of spectroscopy, imaging, calorimetry, and reflectometry tools are being employed. One of the most straightforward experiments concerns the acquisition of UV−vis absorption spectra of dye-sensitized TiO2 substrates. Structural effects such as dye aggregation in dye···TiO2 interfaces can be inferred, via their manifestations as solvatochromic shifts or the emergence of peak shoulders in absorption bands, with reference to their solution-based UV− vis absorption spectra.91 The extent to which dyes adsorb onto TiO2 surfaces can also be probed via UV−vis absorption spectroscopy. Thereupon, dye-loading levels are studied via an analysis of its desorption properties.92,93 Usually sodium hydroxide solution is used to desorb the sensitized dye from the TiO2 surface.94−96 A UV−vis absorption spectrum of the resulting dye solution allows a retrospective quantification of the dye-loading level, by comparing its spectral intensity to that of individual dye solutions of a known concentration. A visible-light imaging method demonstrated by Watson et al.97 can also quantify dye-loading levels, but in a nondestructive fashion since it has no need for desorption; this method additionally offers a real-time analysis of progressive changes in dye surface coverage as a function of sensitization time. The technique achieves this by quantifying a captured time-sequence of digital images of the DSC working electrode across the sensitization period; subsequent image analysis allows an extraction of the red−green−blue (RGB) color information, which permits a diagnosis on the dye-loading levels. Aside from optical methods, a range of other materials characterization techniques has been employed to characterize sensitized TiO2 surfaces, not necessarily for cosensitized DSCs, but this information is nevertheless largely translatable to coDSCs. Spectroscopic techniques include Fourier-transform infrared spectroscopy (FTIR) to analyze the number of active dye-anchoring groups involved in dye···TiO2 adsorption;61 attenuated total reflectance infrared spectroscopy (ATR-IR) to test site-selective adsorption of cosensitizers on TiO2 Lewisand Brønsted-acidic sites;66 vibrational sum-frequency generation (SFG) spectroscopy to realize the preferred molecular orientation of a dye relative to the TiO2 surface98 and associated dye-aggregation effects;99 Auger electron spectroscopy (AES) to probe dye distribution over the TiO2 surface and to rationalize aspects of dye aggregation and loading;17 PAC to measure the energy of charge-separation states formed by dyes;67 X-ray photoelectron spectroscopy (XPS) to examine electronic binding energies associated with dye adsorption;75 and a combination of near-edge X-ray absorption spectroscopy (XANES) with XPS to evaluate the preferred molecular orientation of dyes on TiO2 surfaces.100 Various imaging techniques with nanometer resolution complement these spectroscopy methods by enabling the analysis of dye···TiO2 surface structure at the molecular scale. For example, scanning electron microscopy (SEM), together with energy-dispersive X-ray analysis (EDX), has been used to confirm dye adsorption onto TiO2, while high-resolution transmission electron microscopy (TEM) has been employed to locate the dyes on the TiO2 surface.101 Electron microscopy is often complemented by elemental analysis using an electronprobe microanalyzer (EPMA); to this end, the microscopic distribution of two cosensitizer dyes within a cross-section TiO 2 profile of a DSC working electrode has been determined.102 Meanwhile, atomic force microscopy (AFM) has been used to image the mechanism by which aggregation

4.3. Experiments on DSC Devices

The most common diagnostic tests on DSC devices include spectral measurements of their IPCE in order to quantify the performance of each dye at a specific wavelength, as well as photovoltaic experiments that determine IV curves, leading to the determination of JSC, VOC, FF, and thus η quantifiers. Experiments that analyze the charge injection are also often included in the testing of complete cells. Electrochemical impedance spectroscopy (EIS), for example, allows an evaluation of electron lifetimes (τ) and provides insight into electron transport and charge losses. Increases in VOC in a coDSC can be correlated to an increase in electron lifetime for singly sensitized cells, thus confirming that such increases in a cell are real and occur as a result of reduced electron recombination. Other experiments, such as intensity-modulated photovoltage spectroscopy (IMVS) or the chargeextraction (CE) method, are used for similar purposes. A recent review presents an extensive account of materials characterization of DSC devices, to which the reader is referred for more detailed information.112 J

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In the context of cosensitization, these tests may offer a better understanding of the effects of coadsorbers on the TiO2 surface. However, it is worth highlighting that IPCE tests are especially valuable for cosensitization studies. This is because there is usually a variation in the optical behavior of dyes in solution, once adsorbed onto TiO2, and once embedded into a DSC device. IPCE tests allow one to assess to what extent the innate optical properties of dye combinations remain complementary once dyes have been embedded into a coDSC device. Ideally, the wavelength range of UV−vis absorption should extend as far as possible for a panchromatic co-DSC, while the overall molar extinction coefficient of each dye across this range should be maximized. It is also worth noting that while cosensitized electrodes tend to harvest more photons than their singly sensitized counterparts, the effect is not entirely summative due to overlap in IPCE profiles of the singly sensitized DSCs as well as competitive adsorption that can occur in the cosensitization process.67 The IPCE of co-DSCs at a given wavelength typically ranges somewhere between the IPCEs of the individual cosensitizers on account of the lower concentration of each dye in the co-DSC. In some cases, an auxiliary dye with a narrow absorption band induces a change in the IPCE over the entire solar spectrum. This effect is due to a combination of the change in the aggregation behavior of the primary dye, the added photosensitivity of the auxiliary dye, and the absorption−fluorescence overlap effect at higher wavelengths. This effect can sometimes be used as an indicator of how well each dye suppresses the aggregation of the other dye, although comparing IPCE in the presence and absence of DCA serves as more direct evidence of such changes due to dye aggregation.81 IPCE tests also permit an evaluation of dye-loading levels, an important tuning parameter to optimize spectral complementarity, as the number of available anchoring sites on TiO2 is limited.68 Moreover, studying this parameter offers insights into how cosensitizers with high molar extinction coefficients affect the IPCE; such dyes tend to improve the IPCE significantly already at low dye-loading levels and thus leave sufficient space for the adsorption of the other cosensitizer.113 The optimum relative dye-loading level for the highest optical coverage should result in the maximum JSC value since JSC is approximately a function of the area under the IPCE curve. However, this parameter cannot be treated in isolation since the highest η depends on optimizing JSC, VOC, and FF simultaneously.

cosensitization strategies that tailor optical absorption to achieve panchromatic DSCs. 5.1. Cosensitizers That Fill the Optical Absorption Gaps of Near-Panchromatic Dyes

5.1.1. Complementing Near-Panchromatic Ruthenium-Based Dyes. The success of the ruthenium-based dyes, N719 (1) and N3 (2), in the early demonstrations of DSCs in their modern form1 led to the development of a multitude of derivatives such as N749 (3; also known as Black Dye) which exhibits a far wider spectral response than 1, especially in the longer-wavelength region of the solar spectrum.114 Nonetheless, it is 1 that has become the industrial benchmark for DSC dyes. Many of the Ru-based derivatives developed in this family of dyes have been furnished with bulky substituents since these enable more control of their solubility, minimize aggregation, and maximize the extinction coefficient; DSCs containing some of these dyes have demonstrated photovoltaic performance comparable to those containing the first wave of Ru-based dyes that were developed.115,116

Figure 8. Schematic illustration of how the absorption spectrum of an auxiliary dye (blue dotted line) may complement the nearpanchromatic absorption of a primary dye (black solid line).

Nonetheless, a review of Ru-based dyes shows that they have several significant drawbacks. For example, they tend to exhibit low molar extinction coefficients over their near-panchromatic range, which limits their LHE. Their near-panchromatic response also compromises the photoelectron-injection efficiency into the CB of TiO2 owing to the low LUMO energy of the dye that is required for a panchromatic response, which thermodynamically hinders electron injection.117 Moreover, the near-panchromatic absorption of Ru-based dyes includes regions of poor photoelectron generation and injection, which manifest as dips in their IPCE spectra (Table 1), and their steric bulk tends to result in wide gaps between the anchored molecules on TiO2 surfaces, allowing the electrolyte to penetrate the dye layer and afford charge recombination upon contact with the TiO2 surface. This lowers JSC and VOC, respectively, and consequently the PCE.118 Finally, the commercial up-scaling of Ru-DSC production to meet energy demands remains challenging given the scarcity of natural Ru sources and their commensurately high cost. The combination of favorable properties of Ru-based dyes and the aforementioned drawbacks has made them the most investigated class of dyes in co-DSCs. In the context of compensating for wavelength-dependent dips in the IPCE, the Ru-based dyes 1−12 in Table 1 (and Figure 9) have been combined with 52 non-Ru-based dyes in co-DSCs, 41 and 12 of which are organic and metal-based dyes, respectively. The popularity of organic cosensitizers is mainly due to their narrow and intense absorption bands, which render them

5. OPTICAL AND CHEMICAL CLASSIFICATION OF COSENSITIZERS The overarching goal of cosensitization can essentially be considered as the realization of DSCs that exhibit a wider waveband of optical absorption, and this usually carries the express intent of achieving panchromatic light absorption. Individual studies that employ cosensitization nonetheless manifest with different molecular-engineering strategies which subdivide this overall categorization. Such strategies can be classified according to the manner in which dyes are judiciously combined to afford complementary optical absorption. A full literature review on the progress of cosensitization is now given, using this classification as its overarching framework. That way, progress in cosensitization within each domain of molecular engineering can be collated, which can then be used to further the development of K

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Cheng et al. have demonstrated the use of two panchromatic cosensitizers with complementary absorption by combining 1 with the Zn-porphyrin dye 14.129 The latter compensated for the poor IPCE of the former in the 375−475 nm region, while 1 improved the IPCE at ∼500 nm where 14 absorbs poorly. As a result, the PCE improved by 17% compared to sensitizing a TiO2 substrate with 14 alone. Hu et al. used a similar strategy by combining 1 with 15 or 16. Despite the superior optical properties of 15 relative to 16, cosensitization of 1 with 15 reduced the PCE relative to that of 1, while cosensitization with 16 increased it.130 This is most likely due to the better structural complementarity of 1 and 16, resulting in a compact dye monolayer that mitigates charge recombination with the electrolyte. Out of the 62 entries in Table 2, 31 used the cocktail method, while the sequential and the ultrafast methods were used 27 and 4 times, respectively. For several organic cosensitizers that were adsorbed on TiO2 using the cocktail method, the optimum PCE was obtained with low concentrations. For example, the optimum ratio for 1:17 is 1:0.25 (v/v), as higher concentrations of 17 caused detrimental dye aggregation.72,73 Considering the high molar extinction coefficient of 17, the use of low dye concentration is advantageous since a small amount of 17 on the TiO2 surface leaves enough anchoring sites for 1. Sensitization time is another parameter that can be tuned to optimize the PCE. Sensitizing TiO2 for short periods usually results in low dyeloading levels, while prolonged periods typically result in aggregation. An example of a such an optimization experiment for Ru-sensitized co-DSCs is presented in ref 63. In other instances, the sequential method proved superior to the cocktail method, e.g., when combining 1 and 18, which was attributed to the inability of 18 to compete with 1 for adsorption, given its weak affinity to TiO2.131 While the sequence of sensitization influences the outcome of the cosensitization, the nature of this influence is case-specific. Feng et al. have reported the cosensitization of 1 with the Zn-

Table 1. IPCE Properties of the Ru-Based Dyes 1−12 and Their Wavelength-Band Deficits (Dips) That Are Targeted in the Design of Co-DSCs Ru dye

alternative name

IPCE [nm]

IPCE dips [nm] 375−500 375−500 350−425 375−500 350−450 350−410 350−450 375−425 425−525 400−475, 575−640 350−500 400−525

1 2 3 4 5 6 7 8 9 10

N719 N3 Black Dye (N749) SPS-01 SPS-G3 NCSU-10 HD-2 HD-14 K-60 C106

350−760 325−790 300−860 300−810 300−690 300−750 300−710 310−760 300−780 400−760

11 12

[Ru(H2dcbpy)(dmbpy)(NCS)2] [Ru(dcbppy)(4,4′-bis[di(4-toly) amino]-2,2′-bipyridine) (NCS)2]

300−775 350−700

ref 119 85 40 42 120 121 122 123 124 125 126 127

suitable to compensate for the narrow wavebands of low IPCE in Ru-based dyes. Compensating for the competitive light absorption with the I−/I3− electrolyte in the 350−400 nm region, which appears as a dip in the IPCE of the DSC,128 is one of the most investigated areas of co-DSCs. In this waveband, many nearpanchromatic Ru-based dyes, including 3, intrinsically absorb poorly, evident from their UV−vis absorption spectra in solution and on TiO2. Notable examples of an effective approach to replenish this dip include using a combination of 1 and 13 (strong absorption at ∼410 nm, Figure 10), which enhanced the PCE of 1 by 25% upon cosensitization. This dye combination successfully recovered the IPCE dip in the blue region due to the competitive absorption with the electrolyte and resulted in a plateau-like IPCE over almost the entire visible spectrum.67

Figure 9. Chemical structures of the Ru-based dyes 1−12 discussed in section 5.1.1. L

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Figure 10. continued

M

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Figure 10. continued

N

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Figure 10. Chemical structures of dyes 13−63 used for the cosensitization with 1−12.

formation of J-type aggregates or, alternatively, a slight change in the orientation of the existing H-type aggregates.63 A similar effect was also observed by Cao et al. and Yang et al. The extent of antiaggregation effects due to cosensitization is combination-specific; for example, 20 is better than 25 at suppressing the aggregation of 3, even though 20 is the larger dye.81 These results challenge the notion that smaller molecules are more effective antiaggregation agents, and similar observations have been made by Han et al. with 3 and 26.132 In both cases, the aggregation of 3 was further reduced by adding the archetypal antiaggregation agent CDCA, which indicates that cosensitization may not always eliminate aggregation. However, in other cases, such as 1 and 27, the addition of CDCA did not improve the PCE, which is most likely due to the good structural complementarity of 1 and 27.133 Cosensitization often reduces the amount of adsorbed individual dyes due to competitive adsorption; however, one may overcome this by combining dyes with anchoring groups that differ in preference for Lewis- or Brønsted-acidic anchoring sites on TiO2, such as pyridyl (Lewis) and carboxyl (Brønsted) groups. Notably, Arakawa et al. have taken advantage of the optical complementarity and the different TiO2-adsorption-site selectivity of 3 and 28, leading to an improvement of the PCE by 5% without decreasing the amount of anchored 3, while increasing the overall dye-loading level by 76%.66 Combining 1 and 27, which contains a pyridyl anchor, corroborated this effect.133 Moreover, TiO2-adsorption-site selectivity was achieved using 4 and 29, both of which contain carboxyl anchoring groups, while 29 also contains a neighboring cyano moiety that allows anchoring via its nitrogen atom.42 Performance degradation in DSCs has been attributed to several factors such as the evaporation of volatile electrolyte components37 and the desorption of anchored dye molecules, which decreases JSC and VOC due to the consequent lower LHE and charge recombination with the electrolyte. Chen et al. have demonstrated that cosensitization of 1 and 24 significantly improves device stability, mainly due to an almost constant JSC, over 500 h of light harvesting.128 A similar result was reported by Hu et al. for the cosensitization of 1 and 16 over 1000 h of light harvesting, where VOC remained nearly constant.130 In both cases, the improved stability was attributed to the more compact dye monolayer that limits dye desorption during the device operation.

porphyrin dye 19, where 1 was used after 19 in the sensitization sequence.63 Interestingly, 1 desorbed and replaced up to 55% of the anchored 19 due to its substantially higher binding energy to TiO2 and formed a compact dye monolayer as aggregates of 19 were dissolved by 1. The use of bulky Ru-based dyes in the first sensitization step also proved successful when the auxiliary dye is relatively small, as, for example, in the case of combining 10 with the indoline dye 20, where 20 was used in the second sensitization step to occupy the gaps between the bulky molecules of 10.126 The order of sensitization can also have a marked effect on the optical properties of the cosensitized photoelectrode. For example, when combining 1 and 21, starting the sequence with 1 produces high IPCE in the range 350−750 nm, while the reverse order narrows the range to 350−650 nm.67 This is likely due to the inability of 1 to adsorb after molecules of 21 occupy most anchoring sites. For some dye combinations, only the sequential method is suitable. For example, Arakawa et al. have shown that the pyridyl anchoring group of 22 undergoes a substitution reaction with the NCS ligands of 3 when the dyes are dissolved in one solution, which renders the cocktail method unsuitable.66 The 27 entries in Table 2, which used the sequential method involving two dyes, are split more or less equally in terms of starting with the bulky Ru dye, and regardless of the sequence, the PCE is improved by roughly the same amount. Holliman et al. have reported the first successful ultrafast cosensitization using three dyes by combining 1, 13, and 21.67 While this attempt improved the overall PCE, it was outperformed by combining only 1 and 21. The group of Holliman also reported the ultrafast cosensitization of 1 and 23, which improved the IPCE in the 400−550 nm region with intermittent desorption steps.64 Initially, the ultrafast cocktail method was used to sensitize TiO2. Subsequently, 1 was selectively desorbed using LiOH(aq) and readsorbed via ultrafast sensitization, which resulted in a remarkable 31% and 12.5% improvement in PCE relative to the 1-DSC and the ultrafast cocktail co-DSC, respectively. Cosensitization is a mutually beneficial strategy, and it can be tailored such that the organic dye improves the LHE, while the presence of another chromophore reduces aggregation.72 This is particularly true when small dyes that show a high propensity for aggregation are combined with bulky dyes that are less susceptible to aggregation.84 An example of this strategy is the cosensitization of 1 and 19 that produces a slight bathochromic shift of the IPCE spectra, which indicates the O

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Table 2. Dye Combinations Involving Ru Dyes 1−12 in Co-DSCs and Their Cognate Fabrication (Fab.) Methods and Photovoltaic-Device-Performance Characteristics dyes

electrolyte

(13) + (1)a,b (13)+ (21) + (1)a,b (21) + (1)a,b (1) + (14)a (1)a + (15) (1)a + (16) (1)a + (17) (1)a + (21) (1)a + (23)b (1)a + (24) (1)a + (30) (1)a + (39) (1)a + (42) (1)a + (43) (1)a + (44) (1)a + (45) (18) + (1)a (18) + (1)a (19) + (1)a (19) + (1)a (27) + (1)a (31) + (1)a (32) + (1)a (33) + (1)a (34) + (1)a (35) + (1)a (36) + (1)a (37) + (1)a (38) + (1)a (40) + (1)a,b (41) + (1)a,b

I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

(3)a (3)a (3)a (3)a (3)a (3)a (3)a (3)a (3)a (3)a (3)a (3)a (3)a

(20) (22)b (25) (28)b (46)b (26)b (47)b (48)b (49) (50) (51) (49)b (52)

I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

(2)a + (20) (2)a + (23) (2)a + (20) + (23) (2)a + (37) (4)a + (29) (53) + (5)a (6)a + (54)b (6)a + (55)b (6)a + (56)b (7)a + (57)b (7)a + (58)b (8)a + (59)b (8)a + (60)b

I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

+ + + + + + + + + + + + +

fab.

IPCE [nm]

JSC [mA cm−2] VOC [mV] FF [%]

Cosensitization with N719 (1) UFSc 325−700 15.45 UFCd 13.04 UFCd 14.92 cocktail 350−680 8.57 cocktail 325−710 17.07 cocktail 325−710 18.18 cocktail 350−700 16.39 sequential 350−740 16.22 UFCd and UFSc 17.81 cocktail 310−700 17.90 cocktail 325−710 5.09 sequential 350−750 14.63 sequential 310−700 17.67 sequential 310−700 17.63 sequential 300−680 10.40 sequential 300−705 12.00 sequential 18.69 cocktail 300−710 17.36 sequential 325−730 14.30 cocktail 13.71 sequential 315−715 12.98 sequential 310−380 14.46 sequential 310−660 14.35 sequential 325−370 13.92 sequential 300−395 18.13 sequential 300−700 18.40 sequential 300−690 18.90 sequential 320−650 10.54 sequential 710−730 18.51 sequential 310−700 16.37 sequential 310−700 15.92 Cosensitization with Black Dye (3) cocktail 300−890 22.59 sequential 300−870 20.10 cocktail 375−860 22.11 sequential 300−880 20.70 sequential 300−365 18.30 cocktail 300−860 22.08 cocktail 300−860 21.40 cocktail 300−860 19.53 cocktail 300−860 19.54 cocktail 300−860 20.98 cocktail 300−860 20.44 cocktail 300−860 19.54 cocktail 300−860 20.88 Cosensitization with Other Ru Dyes (2, 4−12) cocktail 14.65 cocktail 14.45 cocktail 15.02 sequential 350−700 11.00 sequential 300−810 15.38 sequential 300−700 16.18 cocktail 300−750 18.47 cocktail 300−755 22.69 cocktail 300−760 20.49 cocktail 300−700 16.12 cocktail 300−720 16.54 cocktail 310−720 16.67 cocktail 310−700 10.09 P

η [%]

Δη [%]

ref

740 730 770 703 709 750 770 680 710 698 561 660 736 758 716 768 730 710 765 735 730 740 730 700 740 750 610 735 711 670 680

65 68 63 67 62 71 70 75 64 63 41 75 66 67 68 66 56 58 70 68 66 62 60 61 58 61 55 69 68 74 74

7.5 6.5 7.2 4.0 7.5 9.7 8.8 8.3 8.1 7.9 1.2 7.2 8.6 9.0 5.1 6.1 7.6 7.1 7.7 6.9 6.2 6.7 6.3 6.0 7.8 8.4 6.4 5.3 8.9 8.1 8.0

25 8 20 18 −10 16 6 43 31 9 −86 25 9 12 45 26 40 31 18 6 18 27 21 14 20 29 34 16 11 17 13

67 67 67 129 130 130 72, 73 134 64 84 130 135 118 118 136 136 131 131 63 63 133 137 137 137 138 138 75 139 140 141 141

696 670 657 680 670 705 698 654 703 685 722 703 743

70 72 71 71 72 72 71 70 71 71 72 71 73

11.0 9.8 10.3 10.0 8.8 11.2 10.7 8.9 9.8 10.2 10.6 9.8 11.3

10 3 3 5 −7 9 4 −14 7 11 15 7 5

142, 81 66, 143 81 66, 143 66 132 132 132 128 128 128 144 40

788 857 767 695 720 640 693 715 687 729 732 650 570

74 76 81 62 75 71 67 62 65 66 66 68 71

8.6 9.5 9.4 4.7 8.3 7.4 8.6 10.1 9.2 7.8 8.1 7.4 4.1

−3 9 7 100 39 30 −3 15 4 3 6 −20 −55

70 70 70 61 42 120 121 121 121 122 122 123 123

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Table 2. continued dyes (8)a + (61)b (9)a + (52) (10)a + (20) (11)a + (62)b (12)a + (63)

electrolyte I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

fab.

IPCE [nm]

JSC [mA cm−2] VOC [mV] FF [%]

Cosensitization with Other Ru Dyes (2, 4−12) cocktail 310−740 21.29 cocktail 300−765 18.26 sequential 380−750 20.60 cocktail 300−775 14.32 (liquid and quasisolid) sequential 350−725 12.99

670 650 766 602 685

65 69 70 73 67

η [%] 9.3 8.2 11.1 6.3 6.0

Δη [%] 1 31 17 21 5

ref 123 124 125 126 127

a

Primary dye: the dye with the highest singly sensitized PCE. bA passive (optically not absorbing) antiaggregation agent such as CDCA was used. UFS: ultrafast sequential. dUFC: ultrafast cocktail.

c

5.1.2. Complementing Near-Panchromatic Porphyrin-Based Dyes. Typical porphyrin-based dyes exhibit an intense blue (Soret) band at ∼450 nm and a relatively weak band at ∼600 nm, which may vary by ∼100 nm. Adsorption on TiO2 typically results in bathochromic shifts and a broadening of the absorption bands; their IPCE spectra follow a similar profile, i.e., the IPCE reaches up to 90% in the blue and red regions, and down to 20% in the green region.78 Several approaches have been used to improve the optical properties and aggregation behavior of individual porphyrin-based dyes, including the use of different cationic centers,145 extending their π-conjugation in a D−π−A fashion where the porphyrinbased chromophore acts as a π-bridge,78 and joining two porphyrin moieties via an ethynyl linker.68 However, three regions of low IPCE remain that can be improved by cosensitization: in the blue region at wavelengths below the Soret band (∼350−410 nm), in the green region (∼500−650 nm), and in the NIR region (>700 nm). These features have made porphyrin dyes the second most commonly used primary cosensitizers after Ru-based dyes, which is reflected in the prevalence of such dyes in cosensitization studies with a total of 44 unique porphyrin-based complexes (Figure 4), 37 of which contain Zn ions. The most common auxiliary cosensitizers in this category are organic dyes, where 24 unique organic dyes constitute 41 out of the 54 combinations in Table 3. The interest in porphyrins as cosensitizers has gained significant momentum since Yella et al. reported the development of the record-breaking dye 64 (Figure 11), the performance of which (the IPCE in the 480−630 nm range) was enhanced via cosensitization with organic dye 65.37 Several other efficient porphyrin-based dyes, such as 66 and 67, were significantly improved in a similar manner. Cosensitization also substantially improved the performance of inefficient porphyrin-based dyes, doubling or tripling the PCE for certain combinations. However, there are also cases where cosensitization depletes the photovoltaic performance of porphyrin-based dyes, which has been attributed predominantly to the electronic incompatibility of the cosensitizers,146 downward shifts of the CB of TiO2, and increased charge recombination rates,145 as well as competitive adsorption that results in lower dye-loading levels, which in turn lowers the IPCE.147 The most common cosensitization strategy for porphyrinbased dyes targets the enhancement of IPCE in the blue and green regions simultaneously using one auxiliary dye. One of the earliest reports on this strategy combined 68 and the organic dye 69, which resulted in a flat IPCE of >80% in the green region, albeit the improvement in the blue region was only marginal. Notably, an IPCE of >80% at 350−700 nm was successfully achieved by Sun et al. using a series of purposely

designed D−π−A organic dyes. In 2015, this strategy was successfully applied to achieve the then highest PCE for Rufree DSCs that use the I−/I3− redox couple. The simultaneous enhancement of the IPCE in the blue, green, and NIR regions has been achieved by combining high-efficiency porphyrinbased dyes with their dimeric analogues; 75 and 64 as well as 76 and 77. Other reports focus on improving the IPCE exclusively in the green region, whereby notable examples include combining 70 and 71 by Bessho et al.78 and combining 72 with 73 by Sharma et al.148 Given the relatively high molecular size of porphyrin-based dyes, they are commonly combined with smaller organic dyes to increase the packing density of the dye monolayers on TiO2 surfaces by using them to fill the interstitial gaps between the anchored porphyrin-based molecules. This is typically achieved using the sequential cosensitization method, where the smaller dye is used second in the sequence. Chang et al.74 have demonstrated that this approach can increase the PCE by 23% relative to the singly sensitized porphyrin-based DSC, which was attributed mainly to a diminution of charge recombination owing to the presence of the smaller dye preventing the electrolyte from penetrating these gaps to reach the TiO2 surface. Li et al. have applied this approach using 74, a small dye with maximum optical absorption at 560 nm, to increase the packing density and enhance the IPCE in the green region simultaneously. However, the overall PCE decreased as a result of competitive adsorption, which resulted in suboptimal dye loading. Furthermore, the ease with which the structure of porphyrins can be modified allows for cosensitization using porphyrins of significantly different molecular sizes. Wu et al. have synthesized 75 by joining two molecules of 64 via an ethynyl linker, and cosensitized TiO2 with 75 and 64.68 A remarkable optical complementarity was observed, as the absorption bands of the dimeric porphyrin are split and shifted relative to that of its singular porphyrin counterpart. This strategy has been successfully applied by Shiu et al., who used 76 in combination with the dimer 77, which improved the PCE by 13%.149 One area that is less explored, in comparison, is cosensitization using more than two dyes. Wang et al. have shown that combining 76 and 78 reduces the PCE relative to that of a DSC based on 76, while adding the organic dye 79 to the combination of 76 and 78 enhances the PCE by 5% relative to that of a DSC based on 76.146 A similar improvement was reported by Wu et al. through a systematic variation of the relative dye concentrations of 64, 75, and 80 in a cocktail solution.68 In both cases, the enhanced photovoltaicdevice performance was attributed to the contribution of the third dye to an upward shift of the CB of TiO2, longer electron lifetimes, and improved IPCEs. Q

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Table 3. Dye Combinations Involving Porphyrin-Based Dyes in Co-DSCs dyes a

(64) + (65) (75) + (64)a (64)a + (75) + (80) (64)a + (80) (66)a + (100)b (66)1 + (101)b (67) + (101)a (104) + (67)a (105) + (67)a (106)a + (67) (68)a + (69)b (70) + (71)a,b (72)a + (73) (113)a + (74) (114)a + (74) (115)a + (74) (75) + (80)a (77) + (76)a,b (76)a + (78) (76)a + (78) + (79) (81)a + (107) (81)a + (108) (81)a + (109) (81)a + (110) (81)a + (111) (81)a + (112) (83) + (82)a (84) + (82)a (85)a + (86) (85)a + (87) (85)a + (88) (96) + (86)a (89)a + (90) (91) + (93)a (92) + (93)a (92) + (93)a (95)a + (94) (120)a + (94) (96)a + (97) (98)a + (100)b (98)a + (101)b (99)a + (100)b (99)a + (101)b (102)a + (103) (104) + (131)a (105) + (131)a (106)a + (131) (116) + (117)a (118)a + (119)b (121)a + (122)b (123) + (124)a (125) + (126)a (127) + (128)a (129)a + (130)

electrolyte II

III

Co /Co I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− (liquid I−/I3− (liquid I−/I3− (liquid I−/I3− (liquid I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− unknown unknown unknown unknown unknown Unknown I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− (liquid I−/I3− (liquid I−/I3− (liquid I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− (liquid I−/I3− (liquid I−/I3− (liquid I−/I3− I−/I3− I−/I3− I−/I3− (liquid I−/I3− I−/I3− I−/I3−

and and and and

quasisolid) quasisolid) quasisolid) quasisolid)

and quasisolid) and quasisolid) and quasisolid)

and quasisolid) and quasisolid) and quasisolid)

fab.

IPCE [nm]

JSC [mA cm−2]

VOC [mV]

FF [%]

η [%]

Δη [%]

ref

cocktail cocktail cocktail cocktail sequential sequential sequential sequential sequential sequential sequential sequential sequential cocktail cocktail cocktail cocktail cocktail cocktail cocktail sequential sequential sequential sequential sequential sequential cocktail cocktail

380−720 350−820 350−820 350−720 320−790 320−780 325−720 350−730 350−730 350−730 350−675 375−710 380−675 300−715 300−710 300−710 350−850 300−860 350−760 350−760 380−630 375−645 375−635 375−630 375−625 375−640 400−660 400−680

cocktail cocktail sequential sequential cocktail sequential sequential cocktail sequential sequential sequential sequential sequential sequential sequential sequential sequential cocktail cocktail

310−700 320−625 310−375 310−380 310−380 400−680 400−660 310−700 320−690 320−690 320−750 320−725 405−670 350−705 350−720 350−720 310−725

17.66 18.38 19.28 16.91 19.52 20.33 18.79 19.62 19.85 19.96 16.74 12.60 11.72 7.73 8.88 6.32 15.81 21.30 16.71 18.76 4.56 6.34 5.60 5.33 4.65 5.96 8.22 9.96 15.60 15.00 11.30 14.40 3.75 16.59 17.03 16.32 17.75 13.42 11.10 17.01 17.70 18.24 19.01 12.40 19.26 19.10 19.89 19.36 16.42 20.27 10.59 7.38 10.40 14.78

935 743 753 760 746 760 774 700 696 728 736 742 720 632 643 653 708 705 698 716 530 500 490 460 430 520 610 640 703 692 621 623 685 676 683 671 755 732 582 764 770 753 765 720 679 688 720 735 730 704 810 610 560 710

74 70 72 72 74 74 72 70 68 74 73 73 73 66 66 67 73 69 71 72 76 73 71 70 65 74 70 67 75 73 72 75 76 70 70 69 67 69 74 72 74 74 76 70 70 69 73 71 71 72 64 73 72 71

12.3 9.6 10.4 9.2 10.6 11.5 10.4 9.6 9.4 10.8 9.0 6.9 6.2 3.2 3.8 2.8 8.2 10.4 8.3 9.7 1.8 2.3 2.0 1.7 1.3 2.3 3.5 4.3 8.2 7.7 5.1 6.7 2.1 7.9 8.1 7.6 9.0 6.8 4.8 9.3 10.1 10.1 11.0 6.3 9.2 9.0 10.5 10.1 8.6 10.3 5.5 3.3 4.2 7.3

3 9 18 5 36 47 20 21 19 34 20 21 35 −15 −11 −25 41 13 −10 5 31 65 39 23 −8 64 −26 −10 67 57 4 139 200 16 20 11 56 17 100 13 23 17 28 29 26 23 30 23 23 3 4 6 19 56

37 68 68 68 157 157 158 159 159 159 153 78 148 147 147 147 68 149 146 146 145 145 145 145 145 145 77 77 150 150 150 156 151 152 152 152 154 154 156 157 157 157 156 82 159 159 159 74 160 161 162 163 163 164

and quasisolid) cocktail cocktail sequential

350−790 350−710 350−690 350−700 375−660

a

Primary dye: the dye with the highest singly sensitized PCE. bA passive (optically not absorbing) antiaggregation agent such as CDCA was used.

The outcome of the cosensitization also depends on the fabrication process of choice. For porphyrin-based cosensitization studies, the cocktail and sequential cosensitization methods were used in equal frequency, but while the sequential

method results, on average, in higher PCEs, the cocktail method increases the PCE in some cases by more than 100%. Most cocktail cosensitization studies use dye molar ratios close to 1:1, but in some cases optimum ratios of 20:1, 10:1, and 7:1 R

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Figure 11. continued

S

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Figure 11. continued

T

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Figure 11. continued

U

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Figure 11. continued

V

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Figure 11. Chemical structures of porphyrin-based dyes and their cosensitization partners (64−131) discussed in section 5.1.2.

investigated for porphyrin-based co-DSCs. Through a careful analysis of the absorption and emission spectra of 85, 86, 87, and 88 on TiO2 and ZrO2 and in the presence or absence of I−/I3−, Choi et al. concluded that little or no energy transfer occurs between the cosensitizers.150 This is because the molar extinction coefficient of 85 is low in the region where the emission and absorption spectra overlap, despite the observed decrease in emission intensity upon cosensitization. Consequently, direct electron injection from each dye into the CB of TiO2 was proposed as the dominant pathway. Zhang et al. have observed a similar trend accompanied by a hypsochromic shift of the emission spectra after cosensitization, which was attributed to the formation of J-type aggregates of the porphyrin-based dyes.145 They proposed that the decrease in emission intensity may be due to a combination of charge transfer between the cosensitizers and more nonradiative

are required, which is probably due to the disparate molecular size of the cosensitizers and the high molar extinction coefficients of the auxiliary dyes. For the sequential method, TiO2 is typically sensitized first with a porphyrin-based dye (immersion for up to 18 h), followed by the organic auxiliary dye (immersion for up to 3 h). Zhang et al. have used the sequential method to form a bilayer of porphyrin-based coordination polymers via axial coordination through the pyridyl moiety on 81.145 In this context, it is worth noting that the optimal thickness of the TiO2 layer itself has rarely been scrutinized. Jeong et al. have shown that the optimal TiO2 layer thickness is less than the effective electron-collection length (Leff) of either cosensitizer when combining 82 with 83 or 84.77 Electron-injection pathways and the possibility of energy or charge transfer between cosensitizers have also been W

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achieved by combining 89 and 90, which are structurally identical but differ in their cationic center, while the addition of CDCA did not show evidence of lower dye aggregation.151 However, Sharma et al. have demonstrated that the addition of CDCA to the cosensitization process of 102 and 103 suppresses dye aggregation further and improves the PCE.82 Finally, the stability of porphyrin-based DSCs over extended periods of light harvesting can be improved via cosensitization. Lan et al. have shown that combining 68 and 69 not only improves the PCE but also significantly stabilizes the photovoltaic parameters over 1000 h of light harvesting.153 A remarkable photostability of highly efficient co-DSCs (PCE > 10%) was also achieved by Xie et al.157 and Liu et al.158

transitions within aggregates. Wang et al. have proposed that energy transfer may be possible between 79, which exhibits fluorescence up to 800 nm, and the porphyrin-based dyes 76 and 78.146 Griffith et al. have argued that energy or charge transfer from 89 to 90 would provide a possible explanation for the significant asymmetry in the photoelectron-injection contribution of the cosensitizers; they hence suggested that the local electric field, which may develop in the dye monolayer due to cosensitization, could ameliorate this process.151 Fan et al. have observed that cosensitization of 91 and 92 with 93 enhanced the luminescence quenching, which may indicate more efficient photoelectron injection into TiO2, to which the impressive increase in IPCE over the 500− 700 nm range was attributed.152 The majority of porphyrin-containing co-DSCs exhibit VOC values that are intermediate relative to those of the individual dyes. This is typically due to a slightly reduced potential edge of the CB of TiO2 (ECB) and an intermediate DSC electron lifetime (τ). However, some porphyrin-based co-DSCs exhibit VOC values that are significantly higher than those of the individual dyes. A common strategy to obtain such high VOC values involves the incorporation of CDCA into the cosensitization process. Lan et al. have shown that using CDCA with 68 causes an upward shift in the ECB without affecting the DSC electron lifetime, while cosensitization of 68 with 69 lowered the ECB and increased the DSC electron lifetime.153 Combining the two approaches, cosensitization in the presence of CDCA, caused an upward shift of ECB and an elongated DSC electron lifetime, which resulted in a VOC value that is higher than those of the individual dyes. Another strategy has been reported by Sharma et al., who induced such a shift of ECB via a pretreatment of the TiO2 substrate with formic acid.148 Competitive adsorption can be an issue for co-DSCs as it may result in suboptimal dye loading, which affords unimproved or diminished IPCE, even after optimizing the photoelectrode fabrication process. One viable strategy to overcome this obstacle involves combining dyes that adsorb selectively on different sites on TiO2. Jia et al. have used picolinic acid anchoring groups in organic dye 94, which showed substantially lower adsorption competition with the carboxyl-anchored porphyrin-based dye 95 while improving the IPCE in the green region.154 The coplanar structure of porphyrins renders them highly susceptible to π···π stacking, which can lead to detrimental aggregation.155 Lan et al. have observed that cosensitization restored the absorption maximum of the Soret band of 68 when combined with 69.153 This indicates that cosensitization realigns 68 on the surface of TiO2 to afford a more stacked configuration (H-type), which provides space for the adsorption of 69. This structural modification of the interface was proposed to lower VOC as the electrolyte additives reach a new equilibrium state, but the compactness of the new interfacial structure retards charge recombination, which in turn increases VOC; in this case, cosensitization resulted in a net increase in VOC. A similar observation has been reported by Kang et al.151 in the cosensitization of 96, 97, and 86.156 By comparing coadsorption of the porphyrin-based dyes 66, 98, and 99 with CDCA and cosensitization with the organic dyes 100 and 101, Xie et al. have shown that while both approaches suppress dye aggregation and enhance VOC, a synergistic increase of both VOC and JSC was achieved only through cosensitization.157 Optimal prevention of dye aggregation was

5.2. Cosensitizers That Extend Optical Absorption into the NIR Region

There is significant interest in developing dyes that can absorb in the NIR region of the solar spectrum, as these wavelengths are often unexploited in current DSCs. Since further extension of the absorption spectral range of near-panchromatic single dyes typically induces adverse effects, cosensitization of a suitable NIR-absorbing dye with a dye that absorbs well in the vis/IR range offers a promising solution, where the properties of each dye can be specifically tailored to enhance absorption in their respective region of the solar spectrum.

Figure 12. Schematic illustration of how the absorption spectrum of an auxiliary dye (blue dotted line) may extend the absorption of a primary dye (black solid line) bathochromically.

The most common NIR dyes are squarine dyes, which possess strong electron-accepting, four-membered rings derived from squaric acid. SQ1 (132, Figure 13) is a representative example for this class of dyes, and originally achieved 6.41% efficiency when cosensitized with 82.13 Since then, a multitude of squarine-based NIR dyes has been developed and tested, including 133 with 134, 135 with 136 and 137, 138 and 139 with 2, 140 with 141−145, and 146 with 147.27,29,65,165−168 However, phthalocyanines have also been used to extend the optical absorption in the NIR region, for example by using 148 to extend the absorption of 149.169 Other examples of NIR-absorbing dyes have been designed by using repeating cyanine units as in 150 with 151;15 or polythiophene units as in 152 with 4.28 The tunable properties of these dyes will be discussed in more detail in section 5.3. Cosensitization with NIR dyes primarily increases the photovoltaic-device efficiency by extending the IPCE range in which a DSC absorbs light from the solar spectrum, thus increasing JSC. For example, Islam et al. have cosensitized 153 with 154 and 155 to extend the IPCE range from 675 to 800 nm,170 while Zhang et al. have shown that 156 cosensitized with 157 extends the absorption from 600 to 725 nm, thus increasing the efficiency by 43%.171 These cosensitizations increase JSC from 13.41 to 18.09 mA/cm2 and from 13.79 to 17.73 mA/cm2, respectively. Cosensitization of 2 with 158 X

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Figure 13. continued

Y

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Figure 13. continued

Z

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Figure 13. Chemical structures of cosensitizers that extend the optical absorption into the NIR region (132−167) discussed in section 5.2.

Table 4. Dye Combinations To Extend the DSC Absorption into the NIR Region dyes a

(1) + (132) (132) + (1)a (132) + (1)a (6) + (1)a + (132)b (2)a + (132) (132) + (82)a (133) + (134)a (135) + (136)a (135) + (137)a (138) + (2)a (139) + (2)a (29)a + (140) (140) + (141)a (142)a + (140) (143)a + (140) (145)a + (140) (145)a + (140) (146) + (147)a (148) + (149)a,b (161)a + (162) (162) + (82)a (163) + (164) (163) + (20) (20) + (165)a (166) + (167)a (150) + (151)a (4)a + (152) (153)a + (154) (153)a + (155) (156)a + (155) (156)a + (157) (2)a + (158) (159)a + (160)

electrolyte −

−

I /I3 I−/I3− I−/I3− I−/I3− I−/I3− binary ionic liquid I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

fab. cocktail UFS UFC UFC cocktail sequential sequential cocktail cocktail cocktail cocktail cocktail cocktail cocktail cocktail cocktail cocktail cocktail cocktail sequential sequential sequential sequential sequential sequential cocktail sequential cocktail cocktail sequential sequential cocktail cocktail

IPCE [nm]

400−750 400−725 325−725

300−750 300−750 400−725 400−740 350−720 350−720 350−720 350−720 400−760 400−725 400−735 375−750 400−750 400−750 400−750 325−750 350−780 300−790 300−800 315−775 315−725 400−775 375−725

JSC [mA cm−2]

VOC [mV]

FF [%]

η [%]

Δη [%]

ref

12.10 16.10 15.00 16.05 14.64 13.62 10.00 7.69 9.74 15.60 17.10 15.13 17.86 16.47 16.62 15.98 12.68 15.20 0.77 8.60 16.20 13.9 15.3 16.0 17.94 15.18 13.70 17.21 18.09 18.71 17.73 15.76 19.18

690 780 820 730 560 665 614 620 670 635 656 640 610 685 714 718 620 600 430 643 666 625 630 650 560 444 700 611 588 657 701 680 721

59 60 63 55 72 71 65 66 65 73 73 66 57 63 63 63 71 45 45 72 72 64 64 71 66 45 72 67 73 71 72 76 71

5.2 7.5 7.9 6.5 5.9 6.4 4 3.2 4.2 7.2 8.2 6.4 6.2 7.1 7.4 7.2 5.6 6.6 0.4 4.1 7.7 5.6 6.2 7.4 6.6 3.0 6.9 7.1 7.8 8.7 9.0 8.1 9.8

−19 25 32 8 −10 6 19 11 32 1 15 17 17 27 24 25 4 13 105 19 9 6 17 37 28 24 26 11 22 38 43 25 23

43 43 43 67 172 13 166 65 65 167 167 23 27 168 168 168 168 29 169 174 175 176 176 177 178 15 28 170 170 171 171 172 173

a

Primary dye: the dye with the highest singly sensitized PCE. bA passive (optically not absorbing) antiaggregation agent such as CDCA was used.

resulted in a co-DSC that absorbs between 400 and 775 nm

Several other factors affect the performance of NIRabsorbing dyes in DSCs. Like other cosensitizers, NIR dyes can suffer from dye aggregation on the TiO2 surface,

with a PCE of 8.0%.172 AA

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an increasing number of thiophene units from 417 (n = 0; 170) to 458 (n = 1; 171) and 465 nm (n = 2; 172).180 More generally, tuning of optical absorption by tailoring the π-conjugation length of a dye can also be accomplished via the addition of a variety of other moieties to cyanine and polythiophene dyes. For example, the replacement of a single thiophene unit with cyclopentadithiophene, as in 173, results in a bathochromic shift of the optical absorption by ∼20 nm as well as in a 170% increase in the molar extinction coefficient.181 Zhu et al. have reported a bathochromic shift of the absorption spectra of 174 and 175 upon addition of a benzoxadiazole and a thiophene unit.182 Meanwhile, Wei et al. have reported a bathochromic shift of the absorption spectrum of 24 upon the replacement of two linked phenyl moieties with linked thiophene units to create 176.183 Yet another example is the organic dye Dyenamo blue (177), which is obtained from extending the π-conjugation of 178 to afford a bathochromic shift of its optical absorption peak from ∼470 to ∼570 nm.184 Using cyanine dyes, Pepe et. al have shown that such tuning processes can also be investigated by computationally analyzing numerous derivatives of parent dyes in order to determine potentially complementary dyes for cosensitization.90 In addition to their tunable absorption properties, cyanine and polythiophene dyes exhibit even more advantageous properties. Most notably, cyanine and polythiophene dyes such as 179−181 exhibit high molar extinction coefficients compared to those of the near-panchromatic dyes discussed in section 5.1.17,185 Cosensitization with auxiliary dyes that exhibit high molar extinction coefficients can significantly improve the overall absorption, provided that the auxiliary dyes absorb wavelengths in a fashion that is complementary to that of the primary dye. Adjusting the molecular structure of the auxiliary dye to increase the number of dye molecules that adsorb onto TiO2 can also be beneficial. For example, Zhu et al. have reported the cosensitization of cyanine dye 182 with the primary dyes 183 or 184.186 Dye 182 has a simple molecular structure, and so, it can adsorb onto TiO2 in between the larger dyes 183 or 184. This work also demonstrates the potential of cyanine and polythiophene dyes to be tuned in order to increase the extent of dye adsorption onto TiO2. Although cyanine and polythiophene dyes generally absorb across a narrow wavelength band, which stands in stark contrast to near-panchromatic dyes, the combination of their absorption tunability and the other previously discussed properties renders these dyes ideal auxiliary dyes in cosensitization. Table 5 shows the cosensitization results of the optically tunable dyes discussed above. Overall, PCEs up to 12.89% have been achieved,185 and cosensitization with tuned dyes leads to increases in PCE of up to 47.73% compared to singly sensitized cells.180 These increases in photovoltaicdevice efficiency are almost exclusively due to enhancements in JSC that arise from the effect of tuning the optical absorption wavelength range and high molar extinction coefficients of the auxiliary dyes. Li et al. have clearly demonstrated these advantages by employing tunable polythiophene dyes 168 and 169 in the cosensitization with polythiophene dyes 170−172 that contain a varying number of thiophene units (170, n = 0; 171, n = 1; 172, n = 2), which resulted in a noticeable improvement in the overall photovoltaic efficiency (Table 5).180 In addition, cosensitization of 169 with 172 affords an overall improvement

potentially leaving areas of the TiO2 substrate exposed to electrolyte attack, which promotes electron recombination.170 A judicious selection of the cosensitizer, however, can lead to significantly reduced aggregation, as reported by Qin et. al with the cosensitization of 2 with 132 and 156.172 Additionally, NIR dyes with high molar extinction coefficients achieve absorption saturation at lower concentrations, and can thus improve photovoltaic-device efficiencies even at low sensitization ratios, evident from the cosensitization of 29 and 140.23 NIR dyes can also be cosensitized via ultrafast fabrication methods (600% has been reported upon installing separate acceptor and anchor (Anc) functional groups. The resulting D−π− A−π−phenyl−Anc structure in that case study yielded reduced electron back transfer to the dye through a diode-like effect.191 This ability to decouple the acceptor and anchor has exciting implications for the systematic molecular design of DSC dyes and could lead to an entirely new range of DSC dyes with a D−π−A−π−Anc architecture.192

Dyes that contain electron-donor and -acceptor groups linked by a π-conjugated bridge (spacer) are referred to as donor−π− acceptor (D−π−A) dyes (Figure 16). The properties of these dyes can be readily tuned by modifying the donor, acceptor, or spacer entities, which renders these dyes particularly attractive for use in cosensitization, where easy modification of optical and chemical dye characteristics is highly advantageous.

5.5. Cosensitizers from Pigments in Extracts of Natural Resources

Many DSCs containing natural dyes that attempt to mimic natural photosynthetic systems have been developed; however, none thus far have exceeded PCEs of 5%. Furthermore, extracts from natural resources invariably contain a mixture of chemicals: many colorless chemicals as well as one or many

Figure 16. Schematic illustration of the “molecular Lego” approach.

Qin et al. have demonstrated the flexibility of the D−π−A molecular architecture by creating two dyes (186 and 187, Figure 17) with a similar structure but complementary absorption spectra for cosensitization.188 Peddapuram et al. have synthesized a dye (188) with two donors and two anchoracceptors that, when 188 was cosensitized with 178, exhibited an IPCE up to 800 nm with a PCE of 7.5% (full sun) and 10.4% (reduced sun).189 Through rational design of the anchor group, Ooyama et al. were able to demonstrate that it is possible to reduce the competitive dye adsorption onto TiO2 by using both Brønsted- and Lewis-acidic sites on the TiO2 surface; however, the co-DSCs developed with this strategy exhibited a reduction in PCE due to significant overlap in the absorption spectra of 189, 190, and 22.190 These examples

Figure 18. Conceptual illustration of plant-based dyes that can be used in co-DSCs.

Figure 17. Chemical structures of the cosensitizers engineered by combining chemical fragments in a “molecular Lego” fashion (186−190) that are discussed in section 5.4. AD

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Table 5. Dye Combinations That Involve “Tunable” Optical Absorption Bands dyes

electrolyte

a

−

(168) + (170) (168)a + (171) (168)a + (172) (169)a + (170) (169)a + (171) (169)a + (172) (173)a + (172) (174)a + (171) (175)a + (171) (24) + (176) (177)a + (178) (179)a + (180) (181)a + (140) (182) + (183)a (182) + (184)a (179) + (185)b

−

I /I3 I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− CoII/CoIII CoII/CoIII I−/I3− I−/I3− I−/I3− I−/I3−

fab. sequential sequential sequential sequential sequential sequential sequential sequential sequential cocktail cocktail sequential cocktail cocktail cocktail sequential

IPCE [nm]

JSC [mA cm−2]

VOC [mV]

FF [%]

η [%]

Δη [%]

ref

9.00 10.90 14.50 9.80 12.80 14.90 18.72 13.25 19.84 16.25 15.60 15.99 21.33 15.16 16.24 19.11

623 621 641 672 674 738 715 662 694 719 797 1034 660 690 720 783

73 74 70 73 72 72 69 67 71 67 70 77 58 67 65 75

4.1 5.0 6.5 4.8 6.2 7.9 9.2 5.9 9.7 7.8 8.70 12.8 8.1 7.0 7.7 11.2

−7 14 48 −14 11 41 19 −5 13 7 19 3 37 7 9 29

180 180 180 180 180 180 181 182 182 183 184 185 17 186 186 187

350−635

350−650 325−760 325−750 325−750 410−570 350−740 400−760 400−525 300−650 300−675 380−730

a

Primary dye: the dye with the highest singly sensitized PCE. bA passive (optically not absorbing) antiaggregation agent such as CDCA was used.

Table 6. Dye Combinations Involving the “Molecular Lego” Approach dyes (186) (188) (189) (189)

a

+ (187) + (178)a + (190)a + (22)a

electrolyte −

−

I /I3 I−/I3− I−/I3− I−/I3−

fab.

IPCE [nm]

JSC [mA cm−2]

VOC [mV]

FF [%]

η [%]

Δη [%]

ref

cocktail cocktail cocktail cocktail

400−760 400−800 380−560 380−560

16.23 17.6 4.52 5.56

571 612 505 561

69 67 61 64

6.43 7.5 1.39 1.99

12 14 −5 −1

188 189 190 190

a

Primary dye: the dye with the highest singly sensitized PCE.

Figure 19. Generic chemical structures of common classes of pigments: porphyrins, chlorophylls, flavonoids (compounds containing the flavonoid base structure shown), carotenoids (featuring combinations of the isoprene base chemical unit, with subclasses carotenes and xanthophylls), betalains (whose subclasses are betacyanins and betaxanthins), anthocyanins (which are known as anthocyanidins when R4 contains a sugar substituent), and anthraquinones.

types of pigments, making extract purification laborious. Consequently, many reports of natural dyes being embedded in DSC devices are, in effect, co-DSCs where pigments are present in multiplicate or DSCs that also contain colorless chemicals that passivate the TiO2 surface or simply present a steric hindrance to the pigments by limiting their access to TiO2 adsorption sites. With this in mind, this section focuses on identifying the photoactive dyes in the naturally extracted pigment where this is possible; certain cases are noted where the chemical identity or purity of the pigment is in question.

These are important cautions to consider when interpreting the associated co-DSC results. Notwithstanding these cautions, it is worth noting that a key advantage of using naturally extracted dyes for DSC applications lies in their origination from natural resources; i.e., these dyes are usually cheap and environmentally sustainable. Cosensitization presents a logical option to augment the photovoltaic-device performance of DSCs containing dyes from natural extracts in order to harness their potential. To this end, light-harvesting antennae in AE

DOI: 10.1021/acs.chemrev.8b00632 Chem. Rev. XXXX, XXX, XXX−XXX

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Table 7. Dye Combinations Involving Plant Pigments That Have Been Extracted from Their Natural Dyes dyes

pigment classes

(191)a + (192) (191)a + (193) (191)a + (194) (191) + (195)a (191) + (196)a (197 + 198) + (199)a (197 + 198) + (199)a pH = 3 (197 + 198) + (199)a pH = 5.4 (197 + 198) + (199)a pH = 8 wormwood chlorophyll + (199)a (197)a +(200−204) chlorophyll + (205 + 206)a (197) + (207 + 208)a chlorophyll beetroota (212) + (208, 209, 213−221)a (212) + (208, 209, 213−221)a Trant + IXa Trant + IXa a

Trant + IX

Trant + IXa Trant + IXa Eugenia jambolanaa + (222) (224)a + (225) (224)a + (225) (224)a + (225) a

(224) +(224−228)

(224) +(226−230)a (224) +(226−230)a

chlorophyll + chlorophyll chlorophyll + chlorophyll chlorophyll + chlorophyll chlorophyll + chlorophyll chlorophyll + chlorophyll chlorophylls + anthocyanins chlorophylls + anthocyanins chlorophylls + anthocyanins chlorophylls + anthocyanins chlorophyll + anthocyanins chlorophyll + anthocyanins chlorophyll + anthocyanins chlorophyll + betalains chlorophyll + betalains anthocyanin + betalains anthocyanin + betalains anthocyanin + anthocyanin anthocyanin + anthocyanin anthocyanin + anthocyanin anthocyanin + anthocyanin anthocyanin + anthocyanin anthocyanin + nonnatural dye carotenoids + anthraquinone carotenoids + anthraquinone carotenoids + anthraquinone carotenoids + monascus dyes carotenoids + monascus dyes carotenoids + monascus dyes

electrolyte

fab.

IPCE [nm]

JSC [mA cm−2]

VOC [mV]

FF [%]

η [%]

Δη [%]

ref

I−/I3−

cocktail

380−720

5.6

500

69

1.9

−44

193

I−/I3−

cocktail

380−750

6.8

500

68

2.3

−32

193

I−/I3−

cocktail

380−750

10.9

570

68

4.3

26

193

I−/I3−

cocktail

380−750

11.9

600

69

5.0

25

193

I−/I3−

cocktail

380−750

14.0

600

64

5.4

42

193

I−/I3−

sequential

0.481

379

54

0.10

−60

197

I−/I3−

cocktail

0.513

436

70

0.16

−36

197

I−/I3−

cocktail

0.915

471

46

0.20

−18

197

I−/I3−

cocktail

0.682

583

80

0.32

29

197

I−/I3−

cocktail

400−600

3.16

660

62

1.29

72

21

I−/I3−

cocktail

400−700

2.8

530

49

0.722

21

198

p-CuI hole collector I−/I3−

cocktail

400−720

4.80

534

51

1.31

30

199

cocktail

4.97

495

46

1.139

69

200

I−/I3−

cocktail

1.244

412

58

0.294

49

201

I−/I3−

cocktail

400−570

0.51

529

64

0.20

−57

202

I−/I3−

sequential

400−570

0.24

507

71

0.09

−81

202

I−/I3−

cocktail (1:4 mix)

4.185

346

50

0.80

43

203

I−/I3−

2:3

3.780

343

49

0.71

27

203

I /I3

1:1

3.150

331

50

0.59

5

203

I−/I3−

3:2

2.914

359

49

0.61

9

203

I−/I3−

4:1

2.074

344

52

0.41

−27

203

I−/I3−

sequential

0.1

215

26

0.14

720

204

I−/I3−

cocktail

1.43

510

43

0.31

−11

205

I−/I3−

sequential (224 first) sequential (225 first) cocktail

1.44

520

50

0.37

6

205

1.55

540

57

0.48

37

205

1.64

600

62

0.61

−8

206

sequential (226− 230 first) sequential (224 first)

2.04

630

64

0.82

24

206

1.79

610

66

0.71

8

206

−

−

I−/I3− I−/I3− I−/I3− I−/I3−

a

Primary dye: the dye whose singly sensitized DSC has the highest PCE value.

following chemical classes (and subclasses) of plant pigments: porphyrins, chlorophylls, flavonoids (anthocyanins, flavones), carotenoids (carotenes, xanthophylls), betalains (betacyanins, betaxanthins), and anthraquinones. The generic chemical schematics of these dyes are given in Figure 19. We now review co-DSCs constructed using pigments from natural extracts. Table 7 presents the co-DSC performance of plant pigments that have been extracted from their natural dyes.

natural photosynthetic systems include the simultaneous use of multiple dyes, which makes cosensitization an approach that mimics nature.193 While there are no reviews that focus on natural dyes being embedded into co-DSCs, there are several reviews that concern the application of natural pigments to singly sensitized DSCs.194−196 These reviews at least set the scene for coDSCs that incorporate natural extracts as they catalogue their various functional light-harvesting constituents into the AF

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Figure 20. continued

AG

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Figure 20. Porphyrins, chlorophylls, flavonoids, carotenoids, betalains, and anthraquinones (191−230) discussed in section 5.5.

Chlorophyll is the quintessential dye in plant science. To this end, Wang et al. explored co-DSCs that employ six porphyrin-based chlorophyll dyes (191−196, Figure 20), the precursors of which were extracted from the seaweed Undaria pinnatifda.193 Dye 191 (Phe a) was selected as a common cosensitizer, since its absorption waveband reaches longer wavelengths and it exhibits high dye···TiO2 electron-injection efficiency. Nonetheless, the changes in PCE due to cosensitization were compared against the primary dye which was not always 191, since 195 (Zn-Phe c) and 196 (Chl c2) issue higher PCE values than 191 in singly sensitized solar cells. Enhanced PCEs were attributed to efficient energy

transfer between the cosensitizers toward the lowest excited singlet states. Cosensitizing the characteristic green color of chlorophyll pigments with the red, purple, or blue dye colors of anthocyanin pigments stands to afford co-DSCs with a wider waveband of optical absorption. Accordingly, Dumbrava et al. have combined chlorophyll dyes from the green alga Enteromorpha intestinalis with pH-tunable anthocyanin dyes extracted from red cabbage, whereby the PCE improved with increasing pH value of the cocktail solution.197 Chromatography has not been reported, and the results discuss a lack of purity in both pigments. Thereby, carotenoids are stated as possible contaminants to the chlorophyll extract which is AH

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example, they will be chemically compatible with each other and with the various nondye constituents of this natural resource, which are typically antioxidants and preservatives that protect these pigments by affording them attributes such as long-term stability under repeated cycles of photochemical stimulus. The solid-state nature of the co-DSCs in this study will at least be likely to attest to improved device stability, on the grounds that these natural pigments are not exposed to the oxidative aspects of the conventional I−/I3− redox couple used as the electrolyte in a liquid-cell DSC. The above descriptions of cosensitizing chlorophyll and anthocyanin dyes essentially represent attempts to achieve coDSCs with panchromatic optical absorption by combining green (from chlorophyll) with the red, purple, or blue dye color from an anthocyanin pigment that is determined by its specific chemical constituents and environment (e.g., pH value). Other studies have targeted panchromatic optical absorption by combining the green pigment from chlorophyll with natural extracts that contain betalain pigments. Figure 19 shows that there are two subclasses of betalains, betacyanins and betaxanthins, whose colors are red-to-violet or yellow-toorange, respectively; indeed, the latter derives from the Greek word, xanthos, for yellow. Kumar et al. produced co-DSCs from natural extracts of Bemuda grass, Cynodon dactylon, and the purple variety of cactus, Opuntia f icus Indica, which contain chlorophyll and betacyanin pigments, respectively.200 Kumar et al. used 197 as the green dye, while they state only the generic structure of betacyanin;200 another study reveals its specific identity to be a mixture of primarily benanidin (207) and betanin (208) with a minor amount of indicaxanthin (209).211 The co-DSCs yielded a 69% PCE improvement relative to the bestperforming singly sensitized DSC that features the cactus pigment (PCE = 0.674%). Explanatory factors for this PCE improvement are (i) the wider optical absorption waveband and higher overall extinction coefficient of the cosensitization mixture; (ii) the ancillary role of the cactus pigment in enhancing the charge-transfer ability of 197, which is otherwise hindered by its lack of a carboxyl group to afford dye···TiO2 adsorption; and (iii) the lower levels of electron recombination in the co-DSC, owing to many trapped sites of TiO2 formed as a consequence of this cosensitization, which induces substantial electron relaxation, as evidenced by the much longer electron-recombination lifetime for the co-DSC (τe = 10 ms; cf., DSCcactus, τe = 0.51 ms; DSCBemuda, τe = 0.03 ms). Sengupta et al. also cosensitized chlorophyll and betalain dyes, from the natural extracts of spinach leaves and beetroot, respectively. A spectroscopic characterization showed that the chlorophyll dye comprised 197 and chlorophyll b (Chl b; 210), while 208 and betaxanthin dyes, mostly likely indicaxanthin (209)201 and vulgaxanthin (211),212 constituted the primary ingredients of the betalain pigment. The green pigment from chlorophyll complemented well the red-purple and yellow components of the betanin and betaxanthin dyes, respectively, to the extent that their mixture produced optical absorption that covered almost the entire UV−vis region of the solar spectrum. Despite the realization of near-panchromatic absorption from cosensitization, the photovoltaic-device attributes of the corresponding co-DSC displayed modest values (Table 7). Nonetheless, the cosensitization produced a 49% improvement in PCE relative to the best-performing singly sensitized DSC that involved the beetroot pigment

expected to consist primarily of Chl-a (197); yet, a minor component of Chl-c (198) was suspected. The specific anthocyanin dye was not identified, although it is likely to be a mixture of c.20 anthocyanins, with cyanidin-3-diglucoside-5glucoside (199) dominating the pigment content, judging from purification studies of red cabbage extracts.207 Evidence of other contaminants in the anthocyanin extract were also noted, having the form of colorless compounds; it was unclear if their presence is detrimental, for example by sterically hindering pigment molecules to adsorb onto TiO2, or beneficial, by acting as antioxidants to preserve the anthocyanins, or even acting as some form of surface passivator. The chlorophyll/ anthocyanin ratio of TiO2 adsorption was found to depend upon the pH value of the cosensitizing solution, as is common in DSC studies that employ vegetal pigments. In this study, higher acidity favored the adsorption of anthocyanins, although the molar ratio of the dyes in the cocktail solution was kept constant. Only basic cosensitizer solution conditions (pH = 8) afforded a co-DSC with a PCE that is better than that of a singly sensitized DSC containing the pigment extracted from red cabbage (Table 7). Chang et al. have reported the cosensitization of chlorophylls from wormwood with anthocyanin dyes from purple cabbage. These anthocyanins are deemed to comprise a similar mixture of pigments to red cabbage,207 while the specific chemical identity of the chlorophyll is not available. Nonetheless, their cosensitization in a 1:1 molar ratio using the cocktail method of dye···TiO2 fabrication resulted in a remarkable increase in PCE by 72% relative to a DSC sensitized only with the purple cabbage pigment. The PCE could be further enhanced to 1.95% by increasing the film thickness of TiO2 in the co-DSC to 24 μm. Chang et al. also fabricated co-DSCs that contain a 1:1 mixture of chlorophyll and anthocyanin pigments from pomegranate leaves and mulberry fruit.198 The specific chemical identity of these two pigments was not given in this study, although the λmax at 665 nm of the UV−vis absorption spectrum of chlorophyll presented therein is characteristic of 197;208 this chemical assignment is corroborated by other work on pomegranate leaves.209 Meanwhile, a chromatography study on the mulberry fruit210 has revealed that the pigment comprises a mixture of five anthocyanins: cyanidin 3-O-(6″-O-α-rhamnopyranosyl-β-D-glucopyranoside) (C3RG; 200), cyanidin 3-O-(6″-O-α-rhamnopyranosyl-β-Dgalactopyranoside) (C3RGa; 201), cyanidin 3-O-β-D-glucopyranoside (C3G; 202), cyanidin 3-O-β-D-galactopyranoside (C3Ga; 203), and cyanidin 7-O-β-D-glucopyranoside (C7G; 204). The resulting co-DSC affords a PCE that is 21% greater than that of the corresponding chlorophyll-based singly sensitized DSC (PCE: 0.597%), which in itself is comparable to its anthocyanin-based counterpart (PCE: 0.548%). Kumara et al. incorporated chlorophyll and anthocyanin cosensitizers within a solid-state co-DSC.199 Thereby, the solid-state electrolyte, CuI, was deposited onto the pigmentcoated surface of the TiO2 semiconductor to act as the electron−hole collector. The precise chemical nature of the chlorophyll was not identified in the study, while the anthocyanin pigment was specified as a mixture of shisonin (205) and malonylshisonin (206), which are close chemical relatives. It is noteworthy that this combination of pigments arose from the same natural source, i.e., shiso leaves. This may provide a benefit since the pigments will have a naturally synergistic operation owing to their singular origin. For AI

DOI: 10.1021/acs.chemrev.8b00632 Chem. Rev. XXXX, XXX, XXX−XXX

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compared to its mix with a different type of pigment. It remains unknown as to how these individual betalain chromophores interact with others while adsorbing onto the TiO2 surface or indeed once adsorbed. Two types of anthocyanins from different natural extracts can also be usefully cosensitized with the goal of panchromatic optical absorption, given the wide range in which this family of chemicals can absorb light. Zolkepli et al. cosensitized two different anthocyanin pigments from extracts of the red petals of Ixora coccinea (IX) and the leaves of Tradescantia spathacea (Trant).203 While the anthocyanin chemical specification is not given for either dye, the authors presented UV−vis absorption spectra that are consistent with the characteristic colors (red and green) of the respective parts of these plants. Their cosensitization employed the cocktail approach, exploring five different ratios of dye concentrations (Table 7). All of these, except for the 4:1 mix of Trant:IX, afforded a PCE improvement relative to the PCE of a DSC with the bestperforming individual dye, IX (PCE: 0.56%). A Trant:IX ratio of 1:4 provided the best results with a 41% improvement in PCE. Particle-size measurements showed that both dyes were prone to aggregation, especially Trant, whose average particle size was 12.4 times that of IX. Mixing Trant and IX via the cocktail cosensitization method reduced dye agglomeration to levels of individual IX. Moreover, there is a clear trend in better PCE values as a function of increasing IX proportions in the Trant:IX mixing ratio. The ability of cosensitization to diminish dye aggregation is thus presumably the cause of the superior PCE. The best-performing 1:4 cocktail mix was shown to be most stable when stored at −20 °C, whereupon the anthocyanins demonstrated a half-life of 1727 days. An anthocyanin from a natural extract, the Eugenia Jambolana fruit, has also been combined with the synthetic organic dye eosin (222).204 The molecular structure of the anthocyanin (223) conforms to the generic structure shown in Figure 19 with R1 = R2 = R5 = R7 = OH, R4 = glucose, R6 = H, while R3 is not stated.201 Sequential cosensitization was employed, dipping the TiO2 substrate in a solution of Eugenia jambalaya first, and then in a solution of 222. While cosensitization afforded a 720% PCE improvement relative to the DSC sensitized solely with the Eugenia jambolana pigment, marked dye aggregation and electron recombination resulted, rendering very low absolute PCE values; the VOC and FF values of both singly sensitized DSCs and co-DSCs were also noticeably modest (see Table 7 for co-DSC values; singly sensitized DSC, VOC = 33 mV, FF = 24%). The yellow-to-red coloring of carotenoids has also been exploited in co-DSCs, by mixing them with naturally extracted pigments from several other families of chemicals. For example, the four crocin-based carotenoids from gardenia yellow, Gardenia jasminoides (224), have been mixed with an anthraquinone-based red pigment, carminic acid, that was extracted from cochineal, Dactylopius coccus, (225).205 The extracted dyes were not purified and therefore contained nonphotoactive compounds as well as four different carotenoid dyes. Nonetheless, these two types of pigments were cosensitized using the cocktail and sequential methods, and both orders of sensitization were explored for the latter method. Table 7 shows that cocktail-based cosensitization resulted in poorer photovoltaic-device performance, while both sequential-based co-DSCs afforded improved PCE relative to the best singly sensitized DSC, which was obtained from the carotenoid-based pigment (PCE: 0.35%). The sequential

(PCE = 0.917); this difference is primarily due to the 66% increase in JSC upon mixing that arises from the broader optical absorption waveband (JSC (co-DSC) = 1.244; JSC (beetroot): 0.75). Unlike other co-DSC reports in this review, this study employed ZnO as the semiconducting surface for dye anchoring, rather than the more common TiO2. However, the dye-adsorption characteristics reported by Sengupta et al.201 appear to show results similar to that which would be expected for TiO2; i.e., chlorophyll dyes struggle to anchor onto the ZnO surface owing to their lack of a carboxylic acid, while the betalains adsorb well onto the ZnO surface since they possess several carboxylic acid groups. However, Sengupta et al. showed that the stability of the beetroot extracts presents an issue if the pigments are elevated above room temperature or if the pH values of the dye solutions extend beyond the range pH = 6−9.201 Betalains have also been cosensitized with anthocyanin pigments from natural extracts. Ramamoorthy et al. constructed co-DSCs from natural pigments of red tamarind, Tamarindus indica, and the common pear, Opuntia dillenii, respectively.202 They proposed the anthocyanin structure as 212 and characterized the optical absorption range of its extract as 400−600 nm; they show that the betalain pigment optically absorbs in a wavelength range similar to that of 212, as well as produces a small extra peak centered at λ = 675 nm, although they did not identify its dye constituents. Fortunately, Betancourt et al. have shown that the betalain pigment of Opuntia dillenii contains nine types of betacyanins and three varieties of betaxanthins.213 Two of the betacyanins are wellknown (betanin, 208; isobetanin, 213) as is one of the betaxanthins (indicaxanthin, 209); yet, six of these betacyanins, i.e., 17-decarboxybetanin (214), 17-decarboxyisobetanin (215), 6′-O-sinapoyl-O-gomphrenin (216), 6′-O-sinapoyl-Oisogomphrenin (217), 2′-O-apiosyl-4-O-phyllocactin (218), and 5″-O-E-sinapoyl-2′-apiosyl-phyllocactin (219), and two of the betaxanthins, i.e., tryptophan-betaxanthin (220) and portulacaxanthin II (221), were identified for the first time, while the identification of one of the betacyanins in Opuntia dillenii remains unknown. Co-DSCs were fabricated by both cocktail and sequential cosensitization methods, and both approaches led to poorer photovoltaic-device attributes compared to DSCs sensitized only with betalain pigments. This is not surprising considering that all betalain dyes contain multiple carboxylic acid groups, which are ideal for dye···TiO2 anchoring, while the anthocyanin does not possess any carboxylic acid substituents; Ramamoorthy et al. have argued that 212 anchors by chelating to its keto and neighboring hydroxy group once in its deprotonated state.202 Dye 212 will thus not bind to TiO2 anywhere near as well as a betalain dye; this inferior anchoring ability of 212 impacts the cosensitization results since the presence of 212 at the TiO2 surface will preclude the betalain dyes from accessing many TiO2 sites that would otherwise be available for anchoring. The sequential cosensitization approach produced worse results than the cocktail approach since the TiO2 substrate was soaked in a solution of 212 before being dipped into a solution of the betalain dyes, which could only access the remaining TiO2 anchoring sites that 212 had not occluded. Either way, cosensitization led to poorer PCE compared to DSCs that were sensitized only with betalain. It is worth remembering that 12 different chemicals make up this betalain pigment, and so, it is hardly a singly sensitized DSC, although it is treated as such for the purposes of comparison in this review since it is AJ

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trying to design the “perfect” dye and allows researchers to use molecular engineering to optimize each dye individually.

cosensitization that first dipped the TiO2 surface into a solution of carminic acid produced slightly better photovoltaicdevice attributes than the alternative order of sensitization. This stands to reason given that molecules of carminic acid contain only one carboxylic acid group and are much smaller than those of the carotenoid-based pigments, wherein crocin-3 and crocin-4 bear two carboxylic acid groups to affect more options for adsorption onto TiO2 surfaces. These differences are manifested by kinetic measurements, whereby Park et al. found that the smaller dye 225 could adsorb onto TiO2 faster, albeit with weaker binding than 224; it could not compete with the steric dominance of 224 when the carotenoid pigments acted as the first sensitizer.205 These carotenoids from gardenia yellow have also been mixed with monascus pigment extracted from fungi.206 The monascus pigment features five chemically related dyes: xanthomonascin A1, xanthomonascin A2, monascin, rubropunctatin, and rubropunctamine, as illustrated by 226−230, respectively.214 Dyes were mixed using both cocktail and sequential cosensitization approaches. The cocktail approach led to co-DSCs with poorer photovoltaic performance than the best-performing singly sensitized DSCs, which employed the monascus pigment (PCE: 0.61%). In contrast, sequential cosensitization improved the photovoltaic performance relative to this singly sensitized DSC, with sensitization of the monascus pigment first yielding slightly better results than the alternative order of sensitization. On the face of it, this result is curious since none of the monascus dyes possess a carboxylic acid, which stands in stark contrast to crocin-3 and crocin-4 of the carotenoid pigment. However, kinetic measurements showed the important detail that the carotenoid dyes could gradually replace monascus dyes owing to their large size, which endows them with slower diffusion as well as stronger intrinsic dye···TiO2 binding capabilities. The cocktail approach does not benefit from this gradual desorption process; rather, Kwon et al. concluded that dye···dye interactions are an issue when employing the cocktail method.206 This underlines the benefit of greater control in cosensitization using the sequential method. In this section, we have surveyed the application of natural extracts to co-DSCs. From these findings, it is helpful to summarize how certain chemical attributes of natural extracts affect the photovoltaic performance of co-DSCs: the extraction temperature, pH value of the sensitizing solution, pH dipping times, and pigment stability in DSCs especially once natural preservatives such as ascorbic acid or their sugar groups have been separated from the plant pigments. The last point is perhaps especially pertinent since the applications of natural extracts to DSCs are reputed for their poor PCE values; yet, there is scope for substantial PCE improvements if the components of the natural extracts, which preserve their lightharvesting pigments, can be harnessed in a way that mimics nature.

Figure 21. Schematic illustration of how the comparable absorption of two dyes may complement each other to achieve nearpanchromatic absorption.

A few examples of this strategy have been reported, and their results are summarized in Table 8. Sharma et al. have cosensitized 21 and 39, which exhibit IPCE ranges of 350− 550 and 500−700 nm, respectively, to generate co-DSCs with panchromatic absorption from 350 to 700 nm and a 26% improvement in PCE.22 Similarly, Guo et al. have reported the cosensitization of organic dyes 231 (absorption between 550 and 725 nm) and 232 (absorption between 400 and 625 nm), which increased the PCE by 17% both through a broader absorption range (400−725 nm) and suppressed aggregation of the individual dyes.11 Aiying et al. have also reported an improvement in PCE by 50% and 33%, upon cosensitizing 164 and 20 as well as 23 and 20, respectively, which resulted in increased light harvesting and inhibited interfacial charge recombination.20 Working with proportionally contributing dyes also allows researchers to experiment with a broader range of molar ratios to find those that best absorb in the solar spectral range and suppress dye aggregation or electron recombination, as exemplified by the cosensitization case study of 233 and 234.215 In addition, some researchers have predicted optimal cosensitization strategies for two proportionally contributing dyes. For example, Bayliss et al. have demonstrated that systematically varying the chemical constituents of the same chemical family of organic dyes may provide a strategy in which complementary optical absorption characteristics can be achieved for a pair of chemically compatible dyes.216 Similarly, Schröder et al. have studied structure−property relationships to predict optimal structures for refunctionalized industrial dyes to be used in the complementary cosensitization of DSCs.217 More recently, Cooper et al. developed a data-mining workflow to identify optimal combinations of organic dyes with complementary optical absorption spectra for cosensitization; the associated predictions of well-matched dye pairings were experimentally validated, resulting in the data-driven materials discovery of dye cosensitizers, 235−239 (Figure 22), the best of which, 235 and 236, exhibit panchromatic optical absorption with a PCE that is comparable to that of N719 (1).104 These results clearly highlight the viability of cosensitizing approaches based on two proportionally contributing dyes as a promising method that improves overall photovoltaic-device efficiencies. As such, this molecularengineering approach warrants further in-depth investigations.

5.6. Cosensitizers That Contribute Proportionally To Achieve Panchromatic Absorption

A less prominent, but potentially promising, cosensitization strategy is to use two dyes that contribute proportionally to achieving panchromatic absorption. For example, combining a dye that absorbs light at 300−500 nm with a second dye that absorbs at 500−700 nm could result in combined absorption from 300 to 700 nm. Such a strategy avoids the pitfalls of AK

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Table 8. Dye Combination Involving Equal Optical Contributions from Two Dyes dyes

electrolyte a

(21) + (39) (231) + (232)a (164)a + (20) (23)a + (20) (233) + (234)a (235) + (236)a (235) + (236)a (235)a + (237) (235)a + (237) (235)a + (239) (235)a + (239) (236)a + (238) (236)a + (238) (237)a + (238) (237)a + (238) (238) + (239)a (238) + (239)a

−

−

I /I3 I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

fab. cocktail cocktail cocktail cocktail cocktail sequential cocktail sequential cocktail sequential cocktail sequential cocktail sequential cocktail sequential cocktail

IPCE [nm]

JSC [mA cm−2]

VOC [mV]

FF [%]

η [%]

Δη [%]

ref

350−700 400−725 325−650 325−700 400−760

12.4 5.8 13.09 13.42 16.85 5.5 6.5 3.9 4.0 2.7 3.6 1.0 1.0 0.6 0.5 0.2 0.3

660 520 767 782 610 700 685 630 640 480 530 570 550 530 500 270 240

76 47 68 70 60 52 50 53 52 43 42 58 54 55 51 46 45

6.26 3.4 6.95 7.30 6.17 2.0 2.2 1.3 1.3 0.6 0.8 0.3 0.3 0.1 0.1 0.03 0.03

26 17 50 33 6 23 38 6 7 −54 −34 −79 −82 −78 −83 −55 −49

22 11 20 20 215 104 104 104 104 104 104 104 104 104 104 104 104

(236 first) (237 first) (239 first) (236 first) (238 first) (238 first)

a

Primary dye: the dye with the highest singly sensitized PCE.

Figure 22. Structures of dyes 235−239 discussed in section 5.6.

5.7. Manifold Cosensitizers with Narrow-Band Optical Absorption Peaks

coefficients and narrow absorption bands, are promising in such scenarios, as each dye can be specially tailored to absorb light near a specific wavelength, while the overall co-DSC, composed of a number of such dyes, exhibits high, broad absorption across the solar spectrum. Nevertheless, the challenges of ensuring chemical compatibility between a set of dyes become increasingly difficult with a larger number of

While cosensitization is mainly focused on the selection of pairs of dyes, cosensitization of more than two dyes with complementary absorption spectra should also be considered. Organic dyes, which typically possess high molar extinction AL

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To this end, we have classified dye combinations on the basis of optical absorption in order to distinguish useful strategies that achieve panchromatic co-DSCs and to facilitate a better understanding of how rational molecular-design concepts can be applied to cosensitization. Indeed, the typecasting of optical absorption spectra as the primary basis of classification is perfect for designing panchromatic co-DSCs since it captures all known relevant dyes in a way that reveals function. This unique and systematic viewpoint allows readers to identify suitable combinations of dyes and improve the light-harvesting efficiency (LHE) and photovoltaic properties of DSCs, especially their incident-photon-to-electron conversion efficiency (IPCE) and power conversion efficiency (PCE) characteristics. To allow for an effective comparison between cosensitization studies, our review has presented the PCEs of all co-DSCs reported using a relative scale, i.e., the changes in η between singly sensitized DSCs and co-DSCs, as well as the absolute scale. The compilation of a large set of relative PCE values, as collated in tables of photovoltaic properties in our review, is important, considering that relative PCE values are reproducible, while absolute values have presented wide discrepancies in the literature to an extent that the field has been compromised. This review therefore allows readers to make a proper comparison of a large set of chemical dyes in terms of their photovoltaic function and use this as a basis to develop new structure−function relationships for co-DSC applications, especially those that are geared to achieving panchromatic absorption. We hope that, by providing both a functional classification of co-DSCs and a comprehensive relative scale with which to compare them, we will encourage and guide future co-DSC research in appropriate and more promising areas. For example, the functional classifications demonstrated in sections 5.6 and 5.7 of this review are notably underexplored and, given the encouraging results reported thus far, deserve more attention. Meanwhile, section 5.2 reveals that success in creating panchromatic co-DSCs by incorporating the near-IR (NIR) waveband is restricted by the limited set of chemical classes currently used for NIR dyes. Investigating a more diverse range of NIR dye cosensitizers would maximize options for chemical compatibility of different combinations of cosensitizers and help ensure good device stability in associated co-DSCs. Section 5.5 surveyed the application of natural extracts to co-DSCs, concluding that there is scope for substantial PCE improvements if the contents of natural extracts, which preserve their light-harvesting pigments, can be harnessed in a way that mimics nature. The paucity of results reported in section 5.4 highlights that the rational design of dye cosensitizers is also underrepresented in the literature. Yet, this “molecular Lego” type of approach has enormous potential since it can be used to build up a dye from its constituent molecular fragments, whose individual functionalities are maximized by combining them in a judicious fashion. Indeed,

dyes; thus, few examples of this cosensitization strategy have been reported so far.

Figure 23. Schematic illustration of how the comparable absorption of more than two dyes with narrow bands of optical absorption may complement each other to achieve near-panchromatic absorption.

Table 9 displays the results of several approaches that align well with this strategy. For example, Cheng et. al have sequentially cosensitized dyes 240−242 (Figure 24) with single absorption peaks, which increased the PCE of a co-DSC by 64% compared to that of singly sensitized DSCs.18 In the other two examples shown in Table 9, at least one of the dyes used possessed multiple absorption peaks; i.e., these examples exhibit a more complex version of the ideal scenario described above, in which each dye absorbs only in a single, narrow region. Wu et al. have cosensitized the metal-based and metalfree dyes 80, 243, and 75, which exhibit absorption peaks at 375, 450, and 500 nm, respectively. The cosensitization afforded a co-DSC with an absorption > 70% (400−700 nm) and an IPCE > 40% (700−800 nm) in combination with an overall PCE of 10.4%.68 Islam et al. have reported the cosensitization of 52, 187, and 155 to yield a panchromatic response with an IPCE > 70% at 400−700 nm and the highest reported PCE for a DSC cosensitized with three organic dyes (7.48%).218 Suitable dye combinations of three or more dyes have been proposed by Pepe et al., which consist of a set of three tailored fluorescein dyes that resulted in a broad optical absorption at 200−650 nm.219 Even though there are only a few reports for this approach, this cosensitization strategy offers a potential mechanism to broaden the absorption spectra of a co-DSC while maintaining high IPCE values by using complementary dyes with high molar absorption coefficients and narrow absorption bands.

6. FUTURE OUTLOOK AND CONCLUDING REMARKS While the literature shows that hundreds of different cosensitizers have been explored as light-harvesting chromophores for DSCs, this review provides the first comprehensive appraisal of all reported forms of cosensitization in DSCs. Cosensitization has become a widely accepted and promising method to tailor the function and performance of DSCs. This review is thus critical to guide future progress in cosensitization by compiling the salient findings from individual cosensitization studies reported in the literature into a holistic framework.

Table 9. Dye Combinations Involving Combinations of Narrow Absorption Bands from More Than Two Dyes dyes

electrolyte a

(240) + (241) + (242) (80) + (243)a + (75) (52) + (187) + (155)a

−

−

I /I3 I−/I3− I−/I3−

fab.

IPCE [nm]

JSC [mA cm−2]

VOC [mV]

FF [%]

η [%]

Δη [%]

ref

sequential cocktail cocktail

300−750 350−820 300−800

20.1 19.28 17.57

597 753 605

68 72 70

8.2 10.4 7.48

64 18 29

18 68 218

a

Primary dye: the dye with the highest singly sensitized PCE. AM

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Figure 24. Structures of dyes 240−243 discussed in section 5.7.

such “molecular Lego” concepts have already proved highly applicable to “design-to-device” strategies in data science, where large-scale data-mining efforts have yielded the materials discovery of new dyes for singly sensitized DSCs.192 Datadriven molecular engineering of DSC cosensitizers presents a natural extension of such strategies. This review has naturally focused on the dye aspects of coDSCs, as this is representative of the literature. However, looking ahead, a more detailed understanding of cosensitization requires the molecular characterization of the entire device architecture of a co-DSC, preferably while it is in operation. This is because the co-DSC function depends on how the dyes interact, not only with each other, but also with the electrolyte and the electrode surfaces. To this end, more detailed analytical chemistry and advanced materials characterization methods are needed. Ideally, one would determine the molecular structure of material components of a co-DSC as probed within their fully assembled device environment (in situ materials characterization methods), and further still, perform this characterization while the co-DSC is in its operational state (in operando materials characterization methods). The latter has yet to be realized, but while the former has not been demonstrated in a co-DSC, it has been realized in a singly sensitized DSC. Thereby, an in situ neutron reflectometry (NR) study has recently enabled the determination of an interfacial structure of a dye···TiO2 working electrode buried within its DSC-device environment.110 This is the first in situ structural study on a DSC within its fully assembled device, made possible by the two key properties of neutrons: their high sample-penetration power together with their contrastmatching capabilities, which enable scattering to be distinguished from the interfacial layer in the device that is of interest. In the meantime, our review has shown how analytical chemistry and materials characterization methods have been applied to study the molecular aspects of the dye···TiO2 interface, which comprises the co-DSC working electrode outside the DSC-device environment (ex situ methods). Such studies have offered new insights into structure−function relationships that govern the intricate workings of co-DSC operation. For example, we have discussed how steric and electronic effects between dye cosensitizers can produce phenomena such as dye aggregation, and electron recombination once adsorbed onto TiO2 surfaces, that will, in turn, influence co-DSC-device performance. We have reviewed the nature by which dye cosensitizers progressively adsorb onto or desorb from TiO2 surfaces to create co-DSC working electrodes and discerned how differences in this process can modulate the photovoltaic properties of the resulting solar cell. The impact that electrolyte attack has on the resulting dye··· TiO2 interface has also been discussed at the molecular level. We have noted how a tightly packed self-assembled monolayer

(SAM) will protect the TiO2 surface from such attack, as well as preclude other detrimental issues, thereby improving the long-term stability of DSC devices. Meanwhile, the cocktail and sequential methods of cosensitization in DSC fabrication have been compared, together with their associated ultrafast methods that are designed to improve control over the cosensitization process. Co-DSCs fabricated using cosensitizers that are chemically compatible, whereby they do not react with each other or compete for TiO2 sites in the adsorption process, will be less susceptible to various degradation pathways; they will thus be more likely to produce photovoltaic devices with increased long-term stability. Current abilities to probe dye···electrolyte interactions are much more restricted than studies on the dye···TiO2 interface. This is partly due to a very limited portfolio of electrolytes that has been showcased for DSC applications, and accordingly, there is a dearth of comparative information. This is curious because it is an accepted wisdom that the choice of electrolyte significantly affects dye function in a DSC, and thus, the overall photovoltaic-device performance. Moreover, the few occasions when multiple electrolyte options are explored within singular studies occur when researchers are attempting to push highperformance DSC dyes to reach PCE world records. In such studies, cosensitization is nearly always employed in order to augment the PCE to reach these world-leading targets; the primary strategy behind cosensitization being exploited there is the ability to fill in a gap in optical absorption, as per the classification of dyes in section 5.1 of this review. A celebrated example of such work is that of Yella et al.37 The optical tuning of absorption bands for cosensitization (section 5.3) has also proved to be a successful strategy in reaching world-leading PCE values; a case in point is that of Kakiage et al.185 It is significant that the work of Yella et al.37 and Kakiage et al.185 both employed a Co2+/Co3+ redox couple as the electrolyte instead of the I−/I3− redox couple. The I−/I3− redox couple is the accepted benchmark for DSC electrolytes as it is by far the most commonly employed DSC electrolyte, as corroborated by the tabulations in this review. In both studies, the change of electrolyte from the I−/I3− to Co2+/Co3+ redox couple augmented the PCE to afford a world-record ranking. Nonetheless, these reports also note the PCE of the analogous co-DSCs that contain the I−/I3− redox couple, and thus allow a framing of the necessary benchmark comparison and distinguish between PCE improvements that arise from the use of a cosensitizer and the change in electrolyte. This distinction is important in drawing out the exclusive impact of cosensitization. Thereby, considering these studies explicitly within an I−/I3− electrolyte restricted scope, Kakiage et al. demonstrate a PCE of 11.2%, which this review suggests is the PCE world record among all DSCs featuring I−/I3− electrolytes. The importance of discovering new DSC electrolytes has become even more evident given the recent leap in DSC AN

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performance achieved last year by Freitag et al.220 via the use of a Cu2+/Cu+-based electrolyte together with a cosensitization approach. That said, this report does not appear to contain comparative data on these co-DSCs using a I−/I3− redox mediator, thus precluding the option herein to quantify the coDSC contribution; accordingly, the study is not listed in an earlier section of this review, although it undoubtedly represents an impressive achievement. The concerted exploration of electrolyte and cosensitization combinations, beyond these studies that celebrate world-record PCE goals, would provide new insights into dye···electrolyte interactions and thus help toward achieving a rational means for their synergistic molecular design. Such design strategies could also prove hugely powerful in optimizing PCE values, pending incorporation of a systematic benchmarking to comparative data that employs an I−/I3− electrolyte. Beyond the use of liquid electrolytes, it would seem prudent to explore holetransporting materials as a feasible solid-state-electrolyte alternative to DSCs, and thus co-DSCs, when looking ahead. While solid-state DSCs are renowned for low PCEs owing to crystallization issues, restricted nanopore filling, and low conductivity properties, a solid-state Cu2+/Cu+-based electrolyte has recently demonstrated a substantial improvement in PCE to 11%.221 This heralds interesting prospects for analogous co-DSC applications. Computational calculations will also hopefully assist more in the future design of cosensitizers. Despite their prominence in complementing experimental studies on singly sensitized DSCs to help support and interpret research findings, such computational calculations have featured little in co-DSC studies. This is perhaps due to the difficulties associated with modeling two disparate dyes on a TiO2 surface in a sufficiently accurate fashion, and this would be a prerequisite for many computational calculations. The few examples of co-DSC dye predictions reported in this review90,216,217,219 present electronic-structure calculations that treat each cosensitizer separately until the final stage of prediction. They nonetheless capitalize on the enormous asset that computational calculations have in providing a measure of energy for a DSC model. Looking ahead, electronic-structure calculations are also becoming increasingly reliable and powerful, given the massive rise in high-performance computing capabilities. Their provision of quantum-energy-level information for co-DSC dye research is impressive since the molecular engineering of cosensitizers requires a good understanding of the interplay between HOMO and LUMO energy levels of each dye, as well as with the electrolyte-redox-energy levels and the conduction band (CB) edge energy of the TiO2 electrode material. Future molecular-design studies of co-DSCs might thus be developed from computational calculations that explicitly probe this interplay of quantum-energy levels, much in the way that calculations have been applied to singly sensitized DSCs.222 For example, the VOC of co-DSCs could be maximized if the choice of cosensitizers is confined to dye matches, whose LUMO energy levels are similar, while their HOMO energy levels should be used for any energy-tuning requirements.67 Computational calculations that predict the photovoltaicdevice characteristics, VOC, JSC, PCE, and IPCE, of a co-DSC would also be extremely helpful, especially given the distinct reliability issues associated with absolute experimental measures of DSC-device parameters.223−226 While this has not yet been achieved for a co-DSC, a computational method that predicts the IPCE, VOC, JSC, and, hence, the PCE for a

given FF of a DSC that is singly sensitized with the C281 dye was recently reported.227 The application of computational calculations and materials discovery strategies to co-DSCs offers exciting prospects for future research and development in cosensitization. As more experimental and computational findings are reported on the various parts of the molecular architecture of a co-DSC device, further developments in the molecular and interface engineering of co-DSCs stand to elucidate better strategies that lead to more rational molecular design of coDSCs. In order to realize commercially viable industrial (niche) applications for co-DSCs, device stability is nonetheless an important potentially limiting factor. The performance degradation of a co-DSC may be due to a multitude of external factors, e.g., thermal and light-soaking stresses, as well as internal factors such as leakage, decomposition or degradation of the electrolyte, thermal stability of the dyes, and mechanical stability of the electrodes. Unfortunately, only a few co-DSC reports contain information on long-term device stability, and even fewer reports address how to improve the device longevity. Nonetheless, these few reports cast a favorable light on the cosensitization approach. In section 5.1, we presented two such examples: (i) Liu et al. reported that DSCs cosensitized with a porphyrin dye (67) and an organic D−A−π−A-type benzotriazole (101) afforded a PCE that retained 98% of its initial value after 1000 h of aging, suggesting desirably high stability for the co-DSC device;158 (ii) Fan et al. reported that DSCs cosensitized with a porphyrin dye (91 or 92) and an organic Y-shaped triphenylamine-based dye (93) exhibited an attractive longterm stability after continuous light soaking for 1000 h.152 While these examples show appreciable long-term stability of specific combinations of dyes, the longevity of co-DSCs may be extended by a molecular-engineering approach, wherein the long-term stability is treated as another performance-design parameter. To this end, some lessons may be learned from singly sensitized DSC approaches that explore different anchoring groups228 or judicious choices of π-spacers in D−π−D−A-type dyes and the use of quasi-solid-state electrolytes.229 Alternatively, the addition of a new DSCdevice component may be considered to afford its long-term stability, for example, the introduction of a multifunctional DSC-device coating such as the photocurable fluorinated polymer demonstrated by Griffini et al. for a singly sensitized DSC, which contains a luminescent europium complex that acts as luminescent down-shifting (LDS) material. This complex converts UV photons into visible light for photovoltaic cells, which improves light-management and UVprotection, and facilitates cleaning. Griffini et al. showed that coating a singly sensitized DSC with this polymer not only resulted in a 70% increase in PCE relative to an uncoated device but also revealed a full preservation of the DSC performance after >2000 h under real outdoor conditions.230 Even though these approaches are promising, they simultaneously highlight the need for more fundamental research in order to establish reliable guidelines on how to compose effective and efficient DSCs. Overall, we have shown that cosensitization has emerged as a technique that can further enhance the growth of DSCs as a next-generation-solar-cell technology. DSCs are well-suited for a diverse set of applications, including solar-powered windows that function particularly well in the low-light conditions AO

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typical of cities, “green” office or textile fabrics that absorb artificial light for energy-sustainable buildings, and passiveenergy harvesters that power wearable electronics. Cosensitization provides a promising pathway to specifically tailor the function and performance of DSCs to meet the diverse operational requirements of these different applications. In this review, we have (i) identified promising cosensitization strategies for further research based on a unique optical absorption classification scheme; (ii) provided both relative and absolute values for each reported cosensitization to allow better comparisons between studies, and (iii) discussed how experimental and computational techniques can be used to predict, analyze, and improve co-DSCs. Providing this comprehensive resource for the solar-cell community will help shape the direction of future cosensitization research in a way that allows DSC technology to become more effective and adaptive to societal needs.

studentship between the Institut Laue Langevin, Grenoble, France, and Durham University. Her university studies began at Durham University where she graduated with first class honors in chemistry in 1994. In her spare time, she has also obtained a B.Sc. Hons. degree in mathematics (2000−4), a diploma in statistics (2004−5), a certificate in astronomy and planetary science (2006−7), a diploma in physics (2007−8), and a B.Eng. Hons. degree in engineering (2010−14) all through the Open University. Giulio Pepe was born in Manduria, Italy, in 1989. He obtained his M.Sci. degree from University College London in 2012 and his Ph.D. from University of Cambridge in 2017, both in physics. During his graduate years, he worked under the supervision of Jacqueline Cole in the Molecular Engineering group. He focused his studies on rationalizing the suitability of various dye molecules as chromophores in dye-sensitized solar cells (DSCs). His research interests span the characterization of structural, optical, and electronic properties of dye molecules via experiments and computation with the aim of enhancing the panchromatic response in cosensitized DSCs.

AUTHOR INFORMATION

Othman K. Al Bahri obtained his B.Sc. in nanoscience from the University of New South Wales, Australia. Currently, he is studying for a Ph.D. in physics at the Cavendish Laboratory under the supervision of Jacqueline Cole. He is a 2017 ministry of Higher Education (Oman) scholar. His current research activities focus on solving the interfacial structures of adsorbed dyes in cosensitized DSCs.

Corresponding Author

*E-mail: [email protected]. ORCID

Jacqueline M. Cole: 0000-0002-1552-8743 Notes

The authors declare no competing financial interest.

Christopher B. Cooper obtained his B.S. in chemical engineering from North Carolina State University in 2017 as a Park Scholar, completing his honor’s thesis under the supervision of Michael D. Dickey. He then completed an M.Phil. in chemical engineering at the University of Cambridge as a Churchill Scholar working with Jacqueline M. Cole. There, his work focused on developing and validating a design-todevice approach for materials discovery of new cosensitized DSCs. Currently, he is pursuing his Ph.D. in chemical engineering at Stanford University working on the design of stretchable and selfhealable electronic devices.

Biographies Jacqueline M. Cole is Head of Molecular Engineering at the University of Cambridge. She concurrently holds the BASF/Royal Academy of Engineering Senior Research Fellowship in Data-Driven Molecular Engineering of Functional Materials, where she is engaged in data science and computational methods that predict and thence experimentally validate materials for photovoltaic, magnetic, and catalytic applications. She holds a joint appointment between the Physics Department (Cavendish Laboratory) and the Department of Chemical Engineering and Biotechnology at Cambridge. She is 50% seconded by the ISIS neutron and muon facility, STFC Rutherford Appleton Laboratory, UK. Her research is highly interdisciplinary. Accordingly, she holds two Ph.D. degrees: one in physics from the University of Cambridge and one in chemistry from the University of Durham. She has received a number of awards and honors including the following: the 1851 Royal Commission 2014 Fellowship in Design (2015−18), a Fulbright Award (all disciplines Scholar, 2013−14), and an ICAM Senior Scientist Fellowship (2013−14) for the smart material design of dye-sensitized solar cells; The Vice-Chancellor’s Research Chair, University of New Brunswick, Canada (2008−2013); a Royal Society University Research Fellowship (2001−11); a Senior Research Fellowship (2002−2009) and Junior Research Fellowship (1999−2002) from St Catharine’s College, Cambridge, UK, for the development and application of in situ light-induced single-crystal Xray diffraction; the Royal Society of Chemistry SAC Silver Medal and Lecture (2009) for her contributions to the development of photocrystallography and advanced methods in neutron diffraction; the Brian Mercer Feasibility Award (2007) for innovation in nanotechnology; the 18th Franco-British Science prize (2006) for collaborative research and cooperation between France and Britain; and the first British Crystallographic Association Chemical Crystallography Prize (2000) for her research on nonlinear optical materials. Before moving to Cambridge, she held a postdoctoral position in physics at the University of Kent at Canterbury, UK. Prior to this, she undertook a Ph.D. in chemistry through an international

ACKNOWLEDGMENTS J.M.C. would like to thank the 1851 Royal Commission for the 2014 Design Fellowship hosted by Argonne National Laboratory, where work done was supported by the DOE Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. J.M.C. is also grateful for the BASF/Royal Academy of Engineering Senior Research Fellowship in Data-Driven Molecular Engineering of Functional Materials. G.P. is indebted to the EPSRC, UK, for a DTA Ph.D. Scholarship (reference: EP/K503009/1). O.K.A. thanks the Ministry of Higher Education (Oman) for sponsoring his Ph.D. research. C.B.C. acknowledges support from the Winston Churchill Foundation of the United States for funding provided via a Churchill Scholarship. REFERENCES (1) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye Colloidal TiO2 Films. Nature 1991, 353, 737−740. (2) Pagliaro, M.; Ciriminna, R.; Palmisano, G. BIPV: Merging the Photovoltaic with the Construction Industry. Prog. Photovoltaics 2010, 18, 61−72. (3) Zhang, K.; Qin, C.; Yang, X.; Islam, A.; Zhang, S.; Chen, H.; Han, L. High-Performance, Transparent, Dye-Sensitized Solar Cells for See-Through Photovoltaic Windows. Adv. Energy Mater. 2014, 4, 1301966. AP

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DOI: 10.1021/acs.chemrev.8b00632 Chem. Rev. XXXX, XXX, XXX−XXX