Dye-Sensitized Solar Cells

Carbon Nanotubes for Dye-Sensitized Solar Cells Munkhbayar Batmunkh, Mark J. Biggs, and Joseph G. Shapter*

From the Contents 1. Introduction ...............................................2

As one type of emerging photovoltaic cell, dye-sensitized

2. Overview of Carbon Nanotubes (CNTs) ........3

solar cells (DSSCs) are an attractive potential source of renewable energy due to their eco–friendliness, ease of fabrication, and cost effectiveness. However, in DSSCs, the rarity and high cost of some electrode materials (transparent conducting oxide and platinum) and the inefficient performance caused by slow electron transport, poor light-harvesting efficiency, and significant charge recombination are critical issues. Recent research has shown that carbon nanotubes (CNTs) are promising candidates to overcome these issues due to their unique electrical, optical, chemical, physical, as well as catalytic properties. This article provides a comprehensive review of the research that has focused on the application of CNTs and their hybrids in transparent conducting electrodes (TCEs), in semiconducting layers, and in counter electrodes of DSSCs. At the end of this review, some important research directions for the future use of CNTs in DSSCs are also provided.

3. Transparent Conductive Electrodes (TCEs) ........................................5 4. Semiconducting Layers .............................11 5. Catalyst Layers..........................................16 6. Conclusion and Outlook............................22

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Figure 1. Number of annual publications and patents searched with keywords of “carbon nanotubes” and “dye-sensitized solar cells” since the discovery of CNTs in 1991. Inset shows the number of yearly publications on “carbon nanotubes for dye-sensitized solar cells” since the first use of CNTs in DSSCs, in 2003. The publication and patent numbers were analyzed using Scopus and Espacenet on 27 June, 2014.

1. Introduction Over the years, the world’s energy consumption has dramatically increased because of the rapidly growing global population and the development of modern technologies. The US Department of Energy predicts that the world’s energy demands will double by 2050 and triple by 2100.[1] Although today’s energy requirements are principally met by burning fossil fuels, continued increases in the fuel price must be taken into account. More importantly, potential damage to the environment, caused by the fuel-burning process, has become a serious problem. All these issues have the potential for disastrous consequences and solutions should be pursued with a great sense of urgency. To date, considerable developments have been made in renewable energy technologies including wind power,[2] biofuels,[3] solar cells,[4] and fuel cells.[5] Amongst these renewable energy technologies, solar cells, which convert sunlight directly into electricity, are considered a viable alternative to traditional fossil fuel energy due to their potential to produce power at reasonable cost. Photovoltaic (PV) cells are generally classified into main three generations. In brief: 1) First-generation solar cells based on crystalline (poly + single) silicon, which make up ≈90% of commercial production at present, are estimated to deliver power with approximately 20% efficiency, but they suffer from high manufacturing, installation, and material costs. 2) Second-generation cells (referred as ’thin film tech’) are more cost-effective than traditional PV cells, but their lower performance is the main concern. 3) Third-generation PV cells, which involve organic solar cells and dye-sensitized solar cells (DSSCs), are designed to further lower the costs of

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the second-generation cells (USD $0.8–1.0 W−1 at present) to at least $0.5 W−1, potentially to $0.2 W−1, by maintaining the economical and environmental aspects while increasing the performance. In particular, DSSCs have gained much attention because they are simple to fabricate, have low manufacturing costs, and are eco-friendly, and reasonably high efficiencies (13%[6] for DSSCs) compared to OPVs (10.7%[7] for organic PV (OPV) cells) have been demonstrated. Since the breakthrough work on DSSCs published in the early 1990s,[8] the number of both publications and patents on DSSCs has continuously increased over the years (see Figure 1). Because of the rapidly growing interest in this cutting-edge technology, several companies, (namely Solaronix from Switzerland, Dyesol from Australia, and Dyenamo from Sweden) have been established to contribute to DSSC commercialization. Moreover, in 2009, G24-Power introduced the

M. Batmunkh, Prof. M. J. Biggs School of Chemical Engineering The University of Adelaide Adelaide, South Australia 5005, Australia M. Batmunkh, Prof. J. G. Shapter School of Chemical and Physical Sciences Flinders University Bedford Park Adelaide, South Australia 5042, Australia E-mail: joe.shapter@flinders.edu.au Prof. M. J. Biggs School of Science Loughborough University Loughborough, Leicestershire LE11 3TU, UK DOI: 10.1002/smll.201403155

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world’s first commercial production of DSSCs using a roll-toroll manufacturing process.[9] Typically, a DSSC consists of a transparent conductive oxide (TCO) substrate, dye molecules (Ru-based organic dye) adsorbed onto a nanocrystalline semiconducting oxide film, a platinum (Pt) counter electrode, and an electrolyte containing the iodide/tri-iodide (I−/I3−) redox couple between the two electrodes (see Figure 2). The working principle of a DSSC can be found elsewhere[10–12] but is briefly described in the following. The incident photon, which enters the cell through a titania (TiO2)-coated transparent electrode, is absorbed by photosensitizers (dye) adsorbed onto the TiO2 surface. Electrons in the photosensitizers are excited from the highest occupied molecular orbital (HOMO) level to the lowest unoccupied molecular orbital (LUMO) level (1), as shown in Figure 2. Then the electrons are injected into the conduction band of the semiconducting electrode (2). The injected electrons in the conduction band of TiO2 are transported toward the TCO electrode (3) and travel through the external circuit (4). These electrons then pass through the TCO back electrode (5) and reach the Pt counter electrode (6). The oxidized photosensitizers are finally regenerated to the neutral state by I− in the electrolyte, and I− is reproduced by the reduction of I3− at the Pt counter electrode (7). However, this conventionally structured DSSC suffers from the following significant issues: i) high conversion efficiencies are difficult to obtain due to charge recombination (dashes in Figure 2) between the injected electrons and either the oxidized dye molecules or electron-accepting species in the electrolyte; ii) the high cost and scarcity of materials such as indium-doped tin oxide (ITO) and fluorine-doped tin oxide (FTO) for the TCO electrodes, and; iii) the expense and rarity of Pt. In order to improve the performance and lower the production cost of DSSCs, it is believed that nanostructured materials will play an important role. In particular, CNTs have attracted great attention for use in various applications because of their remarkable properties. The number of publications (more than 10 000 in 2013) and patents (around 400 in 2013) about CNTs, as plotted in Figure 1, clearly indicates the importance of this material. Furthermore, CNTs have attracted much attention in the development of DSSCs due to their unique structure, excellent conductivity, good transparency, high catalytic activity, low cost, and abundance. As shown in the inset of Figure 1, the number of publications on “CNTs for DSSCs” has rapidly increased since the first use of this material in DSSCs in 2003. However, no increase in the number of publications is observed in 2013 as compared to that in 2012, as shown in the inset of Figure 1. This may be due to the fact that other materials, especially “graphene”, has attracted significant attention from the PV research community.[13–16] Some excellent reviews of graphene in DSSCs have been very recently published.[17,18] Indeed, the best efficiency to date (>10%) of carbon-based DSSCs (among pure materials or nonhybrids) was obtained by a vertically aligned CNTs (VACNTs) counter electrode-based cell.[19] On the other hand, among carbonaceous materials incorporated into TiO2 photoelectrode based cells, DSSCs fabricated with a CNT/ TiO2 electrode is the champion cell. All these interesting features have motivated us to review the use of CNTs in DSSCs. small 2015, DOI: 10.1002/smll.201403155

Munkhbayar Batmunkh is currently a PhD candidate at the School of Chemical Engineering at The University of Adelaide, Australia. He is also a visiting researcher at the School of Chemical and Physical Sciences at Flinders University in South Australia. He obtained his BSc in Chemistry from the National University of Mongolia, in 2010, and completed his Masters of Engineering at Gyeongsang National University, South Korea, in 2012. His research involves the modification and characterization of carbon nanotubes and graphene structures for use in dye-sensitized and perovskite solar cells. Mark Biggs, who received his PhD in 1996 from The University of Adelaide (Australia) in Chemical Engineering, is the Professor of Interfacial Science and Engineering and Dean of Science at Loughborough University, UK. He is a visiting professor at The University of Adelaide, and a Fellow of the Institute of Engineers Australia, a Fellow of the Institute of Chemical Engineers (UK), and a Member of the Royal Australian Chemical Institute. Professor Biggs’ research interests focus on understanding and exploiting interfacial phenomena, with particular interest in carbon-based materials. Joe Shapter obtained his PhD from the University of Toronto in 1990 on the detection of small molecules and the determination of their energies. From 1990 to 1996, he worked at the University of Western Ontario. He is now Professor of Nanotechnology and Dean of the School of Chemical and Physical Sciences. He was the founding Director of the Centre of Expertise in Energetic Materials and is currently the Director of the South Australian node of the Australian Microscopy and Microanalysis Facility. His research interests lie in the use of carbon nanotubes for various applications including the production of novel photovoltaic systems.

In this article, we review the major progress of the use of CNTs and their hybrids in DSSCs. The present review will not cover the application of CNT materials as the lightabsorbing material and electrolyte, as there are very limited studies available. We begin by reporting a brief overview of the synthesis and properties of CNTs. Then, preparation methods for CNT-based materials and their use in the TCE, semiconducting layer, and counter electrode of DSSCs are outlined. Finally, we provide important suggestions for the future development of these materials for DSSCs.

2. Overview of Carbon Nanotubes (CNTs) Since their first discovery by Iijima in 1991, CNTs have attracted considerable attention from the scientific as well

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Figure 2. The structure and operational mechanism of a typical DSSC. Thick block arrows on the left represent the sunlight.

as industrial communities owing to their remarkable properties.[20,21] The properties of CNTs, summarized in Table 1, depend on many factors such as the number of walls, type of defects, diameter, length, synthesis method, and concentration. Because of these fascinating properties, CNTs have been used in many applications such as high-strength composites, supercapacitors, batteries, solar cells, various electronic devices, sensors and actuators, drug-delivery systems, transparent conducting films (TCFs), and catalysts.[22–24] CNTs are allotropes of carbon, having a cylindrical structure (1D) composed of one or more layers of graphene with open or closed ends. Depending on the number of walls, they can be classified as single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), or multiwalled carbon nanotubes (MWCNTs). The most widely studied forms are SWCNTs and MWCNTs. SWCNTs consist Table 1. Properties of carbon nanotubes. Property



Density [g cm−3]

1.3 – 1.4[26]



Very high

Elastic modulus [TPa]


7-fold higher than steel

Tensile strength [GPa]


100 times stronger than steel

Thermal stability

2800 °C in vacuum, 750 °C in air[30]

More stable than metal wires in microchips


Twice higher than diamond

Electron mobility, [cm2 V−1 s−1]

1 × 105[31]

70-fold higher than silicon

Electrical conductivity, [S cm−1]

4 × 105[31]

4 times higher than iron at room temperature

1–10 × 109[32]

More than 1000 times greater than copper

Surface area [m2 g−1]

Thermal conductivity, [W mK−1]

Maximum current density, [A cm−2]

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of a single graphene sheet, whereas MWCNTs are made of multiple graphene sheets with an interlayer distance of 0.34 nm, which is close to that of graphene layers in graphite. Typically, the diameters of SWCNTs and MWCNTs are 0.8 to 2 nm and 5 to 20 nm, respectively.[25] Nanotubes have been constructed with a very high aspect ratio (length-todiameter ratio) because the length range of CNTs can be adjusted from the nanometre scale to several centimetres. In order to satisfy the criteria of materials for emerging applications, producing high-quality CNTs with a low defect density and a uniform structure using a scalable, high-yield process is of great interest and is still under investigation. In the past two decades, three main methods including arc discharge, laser ablation, and chemical vapour deposition (CVD) have been developed to synthesize CNTs. The arc discharge and laser ablation techniques have been shown to be unsuitable methods for mass production. In contrast, CVD is could potentially produce large quantities of CNTs and mainly uses fluidized bed reactors, which enable uniform gas diffusion and heat transfer to metal catalyst particles.[33] Compared to the other two techniques, CVD possesses many advantages including easily obtainable pressures and temperatures, easy operation, cost-effectiveness, and high yield. It is well known that the catalyst plays a key role in the synthesis of CNTs. To date, various catalysts such as metallic (Ag,[34] Cr, Sn, Mg, Al,[35] Fe, Ni, Co, Au, Pt, Pd,[34,35] Cu,[34–36] Mn,[35,37] Mo,[35,38] PbS,[39] Re,[40] and metal-free (SiO2,[41–43] TiO2,[43] Si,[43,44] Ge, SiC,[44] ZnO,[45] Al2O3,[43,46] ZrO2,[47] diamond,[48,49] fullerene,[49,50] and even CNTs themselves[51] materials have been developed and used to produce high-quality CNTs. Briefly, in the CVD process, a carbon feedstock is converted with a catalyst support into CNTs. A comprehensive description of CNT synthesis processes is not provided in this review, as this can be found in a recent book edited by Suzuki[52] and in several reviews.[23,24,53] It should be noted that Raymor (Nanotubes for Electronics) and the National Research Council (NRC) in Canada are developing a radio-frequency (RF) induction thermal plasma method (non-CVD method) to scale up nanotube production.

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3. Transparent Conductive Electrodes (TCEs) 3.1. ITO and FTO

Figure 3. The construction of zig-zag, armchair and chiral nanotubes by rolling-up single graphene sheets.[54] Reproduced with permission.[54] Copyright 2009, Royal Society of Chemistry.

Three different types of CNTs (zig-zag, armchair and chiral) can be obtained depending on the orientation of the graphene sheet as it is rolled up (see Figure 3). These tube chiralities can be defined by a chiral vector, Ch = na1 + ma2, where the integers (n, m) are the number of steps along the unit vectors (a1, a2) of the graphene lattice.[54] As shown in Figure 3, the chiral angle, θ, is the angle between the chiral vector and the nearest zig-zag of C–C bonds. This chiral angle determines the amount of “twist” in the nanotube and ranges from 0° to 30°.[55] The rolled sheet with a chircal angle θ = 0° is known as a zig-zag nanotube, whereas the θ = 30° angle is referred as an armchair CNT. All other tube formations in which the chiral angle lies between 0° and 30° are known as chiral.[56,57] Furthermore, the electronic properties (i.e, semiconducting or metallic behaviour) of SWCNTs strongly depend upon the chiral angle, which also depends on the indices (n, m). Therefore, the bandgap of CNTs can be tuned by slightly changing the chiral angle.[55–58] SWCNTs in particular are metallic (m-SWCNTs) if n – m = 3j (with j = 0); but if n – m = 3j (with j ≠ 0), they are small-gap semiconductors (semi-metallic); whereas CNTs can be large-gap semiconductors (s-SWCNTs) when n – m = 3j ± 1.[59] Notably, the bandgap energy of SWCNTs is very dependent on the diameter. Indeed, armchair nanotubes and approximately 33% of all zig-zag CNTs are metallic (no bandgap) at ambient temperatures, whereas the remaining zig-zag nanotubes and all chiral tubes are considered semiconducting.[54,55] Importantly, one of the most critical properties of materials for achieving high-performance PV devices is a suitable semiconductor bandgap. It is worth noting that the bandgap of SWCNTs can vary from zero to 2.0 eV depending on their structure. Thus, one can obtain the desired bandgap energy of nanotubes to optimize the efficiency of PV cells, including DSSCs. small 2015, DOI: 10.1002/smll.201403155

TCEs are a necessary component in a wide range of modern optoelectronic devices such as organic light–emitting diodes (OLEDs), e-paper, touch screens, liquid-crystal displays (LCDs), and thin-film PV cells, all of which are rapidly growing in popularity.[61–63] TCEs are used in both the photoelectrode and counter electrode of DSSCs. An ideal TCE should possess a very low sheet resistance (Rs) at high transmittance and be composed of cheap and abundant materials. To date, ITO and FTO are the most widely used TCEs due to their high conductivity and optical transparency. The Rs of typical TCEs (ITO and FTO) vary from 5 to 100 Ω sq−1 at about 80–97% transparency (often reported at 550 nm wavelength) depending on the film preparation conditions.[64a] These films can be prepared using various techniques such as electron-beam evaporation, radio-frequency sputtering, spray pyrolysis, chemical vapor deposition, magnetron sputtering, molecular beam epitaxy, or pulsed laser deposition.[64b] The use of ITO and FTO, however, has several obstacles: i) ever-increasing material costs due to a shortage of raw product (indium, product of zinc mining,[64c] ii) a significant lifetime issue caused by poor stability at high temperatures and sensitivity to salts and acids, iii) a lack of flexibility due to a brittle nature, iv) a low transparency in the NIR region, and v) leakage of FTO-based devices caused by FTO structural defects. These shortcomings of the current TCEs have inspired researchers to seek other electrode materials that could exhibit comparable sheet resistance at high transmittance while still being readily available, cheap, highly flexible, and lightweight. In the past few years, a number of alternative materials, including conductive polymers,[65] metal grids,[66] metallic nanowires,[67] graphene,[68] and CNTs[69] have been developed to replace ITO and FTO. For a broad perspective of the field, there are several comprehensive reviews of TCEs available.[70–78] Among the aforementioned alternative materials, much attention has been devoted to CNTs as they have been considered the most promising candidate for TCEs because of their unique structure and remarkable properties. CNT films with thicknesses of a few nanometers exhibit excellent electrical conductivity at high optical transmittance due to an inherent network architecture arising from the 1D nature with a high aspect ratio. In the following section, we provide a brief review of the major fabrication techniques of CNTs films and their performance. Based on the advantages and weaknesses of these film preparation methods, we highlight the important factors that have significant influence on the TCE’s performance. We also provide a short discussion of CNT-based hybrid materials for TCEs and their application as CNT–TCEs in DSSCs.

3.2. CNTs and their Hybrids for TCFs Typically, the preparation methods for transparent conductinhg CNT films can be separated into dry processes and wet processes depending on the sample conditions. The list of these methods and their advantages and disadvantages are summarized in Table 2. Schematic figures and scanning

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reviews www.MaterialsViews.com Table 2. Fabrication methods for CNT films. SEM image of CNTs produced by an aerosol method is reproduced with permission.[82] Copyright 2011, American Chemical Society. Schematic illustration and SEM image of array-drawn CNTs are reproduced with permission.[85] Copyright 2010, Wiley-VCH. AFM image of transfer-printed CNT film is reproduced with permission.[89] Copyright 2006, AIP Publishing. SEM image of CNT film prepared by dip-coating is reproduced with permission.[91] Copyright 2012, American Chemical Society. AFM images of spin-coated and spraycoated CNT films are reproduced with permission.[93] Copyright 2010, Wiley-VCH. SEM image of CNT film produced by EPD is reproduced with permission.[100] Copyright 2008, Royal Society of Chemistry. SEM image of printed CNT film is reproduced with permission.[102] Copyright 2009, American Chemical Society. SEM image of brush-painted CNT film is reproduced with permission.[106] Copyright 2014, Elsevier. Production method Dry route

Wet route

Schematic illustration




Rs@T [Ω sq−1 @%]



Production of high performance film

Complicated process, high temperature and pressure and high cost




Very high efficiency

Difficult control and production of small amount of material.

[email protected] (after laser trimming & deposition of Au and Ni metals


Transfer printing

High performance, simple and low cost

Hard to prepare thick CNTs film, limitation of film size and slow filtration process




Very simple, and cheap

Inconsistent thickness, formation of large CNTs aggregate




Cost-effective, simple, Formation of nonsuitable for mass uniform film, loss of production, quick CNT product during process operation

≈128@90 (CNT was dispersed in dichloroethane)



Suitable for mass Formation of rough production, cheap, surface with a partial very simple and quick aggregation of CNTs process

≈57@65 (CNTs were dispersed in H2O:SDS)



Very suitable for large Use of limited subarea film, fast deposi- strates, no guarantee tion, inexpensive of a uniform film



300@90 (sulfuric acid treatment was applied)



Printable on various substrates, very simple and fast deposition, cost-effective

No guarantee of a uniform film


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www.MaterialsViews.com Table 2. Continued Production method

Schematic illustration



Very cheap, suitable Control of thickness is for mass production, not very accurate, no simple and fast guarantee of a uniform deposition film


electron microscopy (SEM) and/or atomic force microscopy (AFM) images of each method are also illustrated with references in Table 2. In the dry processing methods, CNT films can be prepared by the following two routes: i) depositing a CNT aerosol, which is collected in a CVD reactor, on a thin film and transferring it onto the substrate using a dry-transfer technique,[79–82] and ii) drawing from a superaligned CNT forest.[83–85] In 1998, the first route was introduced by Cheng et al.,[79] who deposited CNTs onto the wall of the CVD reactor by floating catalyst particles and then transferred the CNTs onto a transparent substrate. By using the same method, a transparent conductive SWCNT film with a thickness of 100 nm was prepared with an Rs of ≈50 Ω sq−1 at 70% transparency.[80] Furthermore, the performance of the SWCNT film was improved (Rs = 110 Ω sq−1 at high T = 90%) using a post-deposition treatment with nitric acid (HNO3).[81] The effect of HNO3 on the CNT-based TCFs will be discussed later in this review. Jiang et al.[83] were first to report the CNT array-drawing method (the second route named in the previous paragraph) to prepare CNT films. Recently, a CNT-based film was drawn from super-aligned CNT arrays using a straightforward rollto-roll process.[85] Although the as-drawn CNT film showed a high Rs (1 kΩ sq−1) @ 80% transparency, the performance of the transparent conductive CNT electrode was improved remarkably (208 Ω sq−1 @ 90% and 24 Ω sq−1 @ 83.4%) after suitable laser trimming and deposition of Au and Ni metals. Due to the excellent performance of these TCFs, transparent conducting CNT electrodes produced via dry processing methods have been well commercialized. Though the dryprocessed CNT films possess good conductivity, they have some disadvantages such as complicated experimental control, the requirement of high temperatures, limited substrates, and a high cost. Preparing transparent conducting CNT films using wet processing methods is of great interest because solution-processable techniques are simple, compatible with low-temperature processes, applicable for various substrates, as well as cost effective. Since the preparation method for CNT-based TCFs using filtration-transfer and dip-coating techniques was first reported by Wu et al.[69] and Saran et al.,[86] respectively, in 2004, several leading research groups and companies have put considerable effort into this area. Until now, many solution-processing methods such as vacuum filtration followed by transfer printing,[69,87–89] dip coating,[86,90,91] spin coating,[92,93] spray coating,[87,93–98] electrophoretic deposition (EPD),[99–101] printing (rod coating, inkjet printing, and slot small 2015, DOI: 10.1002/smll.201403155


Rs@T [Ω sq−1 @%]


[email protected]


coating)[102–105] and, recently, brush painting[106] have been developed to fabricate CNT-based TCFs. Of course all of these methods have advantages and disadvantages. We provide a detailed description with graphical representations of each technique in Table 2. Comprehensive reviews of solution-processing techniques for the fabrication of CNT-TCFs can be found elsewhere.[72,73,78,107,108] The quality of the CNT-TCFs fabricated by solution-processing methods strongly depends on the dispersion of CNTs in a base fluid. It is well known that CNTs are difficult to disperse in conventional liquids due to the formation of CNT aggregates (bundles), caused by a strong Van der Waals force and their long, winding shape. Though a great deal of work has been done[109–112] and several detailed reviews are available on this topic,[113–115] the search for an effective dispersion strategy is still on. Dispersion of CNTs in a liquid can be prepared by the following three main routes: i) covalently functionalizing CNTs and dispersing them in the solvents, ii) applying noncovalent treatments with some dispersing agents like surfactants and additives and mixing with the solvents, and iii) directly solubilizing pristine CNTs in neat solvents. Covalent functionalization is the most commonly used approach to prepare CNT dispersions,[22,114] but there are a very limited number of studies available in the use of functionalized CNTs for TCFs. We found only two papers about covalently functionalized CNT-TCFs, and both are reported by the same authors. In their first work published in 2006,[116] SWCNTs were first treated with sulfuric acid (H2SO4) and HNO3 and dispersed in an aqueous solution. The TCFs prepared with this CNT structure exhibited an Rs of 6 kΩ sq−1 at 88% transparency. These authors further improved the performance of their CNT-TCFs to an Rs of ≈2.5 kΩ sq−1 at a transparency of 86.5%.[117] This improved performance of the TCF was still far from the industrial requirement for thin films, so it seems that high-performance CNT-TCFs cannot be fabricated using this method (covalent functionalization) because the functional groups disrupt the conjugated sp2 structure of CNTs and reduce the conductivity sharply. Solubilizing CNTs in an aqueous solution with the use of surfactants or additives is one of the most promising approaches for the preparation of CNT-based TCFs. The role of surfactants in the CNT dispersion has been well covered in a previously published review.[113] Dispersion of CNTs in liquid media has been achieved with a variety of surfactants, including sodium cholate (SC), sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), and Triton X-100, all of which can significantly lower the surface

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tension (interfacial tension) between the CNTs and the liquids.[118–120] Surfactants are mainly organic compounds that are amphiphilic, and so they contain both hydrophilic groups (their heads) and hydrophobic groups (their tails). Therefore, surfactants contain both a water-insoluble component and a water-soluble component that makes them perfect for dispersion. In CNT dispersions, the surfactant molecules can be quickly adsorbed onto the surface of the CNTs and thereby completely disperse them in a liquid. The presence of a large amount of surfactant in the CNT-TCFs can significantly decrease the conductivity of the film, but they do not actually alter the natural conductivity of the CNTs. Geng et al.[96] prepared transparent conductive CNT films with a support of SDS surfactant using a spray-coating technique. Their as-prepared film with a thickness of ≈50 nm showed an Rs ≈ 200 Ω sq−1 at a transparency of ≈83%. Interestingly, this Rs was further reduced by 2-fold after washing with HNO3, despite the transparency remaining almost unchanged. This improvement in the film conductivity was due to the removal of residual SDS by strong acid, which simultaneously densified the SWCNT network and enhanced the metallicity of the SWCNTs. Therefore, the SWCNTs still showed high conductivity after the removal of SDS. To our knowledge, the best CNT-TCF (among solution-processed films) to date was fabricated by Wu et al.,[69] who prepared a SWCNT film with the assistance of Triton X-100 using a transfer-printing method, and it achieved an Rs of 30 Ω sq−1 at ≈83% transparency. In the absence of any additive, directly dispersing pristine CNTs in neat solvents with low surface tension is the most facile and ideal strategy to produce TCFs. A comprehensive review on this topic has been published by Coleman.[121] Different types of solvents such as N, N-dimethylformamide (DMF),[122] and dichloroethane (DCE)[123] have been used to fabricate CNT-TCFs. In order to fabricate TCFs, a CNTDCE solution was deposited on a polyethylene terephthalate (PET) substrate using a combination of dip coating and spray coating. The resulting film exhibited an Rs = 340 Ω sq−1 at T = 80%, indicating that this strategy (direct dispersion of CNTs in solvents) is better than films prepared from functionalized CNTs in solution. However, this strategy presents issues because the maximum producible concentration of CNTs in these solvents is lower than 0.1 mg mL−1, while it is generally accepted that industrial application requires >1 mg mL−1 to produce good films at the rates required for viable prodcution.[72,107] The amount of CNTs that can be effectively dispersed in solvents is related to the Hildenbrand solubility parameter.[73] For thin-film fabrication, the rheological properties of a sample are of great importance. Importantly, the requirement for solution viscosity is different depending on the coating method. A 1 mg mL−1 concentration would provide solutions that can be used industrially to give high-quality films. Ideally, it would be of great benefit to find the right solvents with a low boiling point (

Carbon Nanotubes for Dye-Sensitized Solar Cells.

As one type of emerging photovoltaic cell, dye-sensitized solar cells (DSSCs) are an attractive potential source of renewable energy due to their eco-...
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