**3. Photovoltaics**

different types were made (Fig. 6.). The Type I sensor was assembled simply by depositing SWCNTs onto the glass substrate with Pt electrodes placed by sputter deposition. The Type II sensor was assembled by casting the glass slide with a film of CHIT before being placed into an arc-discharge chamber to deposit SWCNTs. The Pt electrodes were added in a simi‐ lar method. The Type III sensor was assembled using the initial preparation for a Type I sen‐ sor followed by CHIT film coating and Pt electrode deposition. There were slight differences in the interaction of the CHIT film with the SWCNTs. In the Type II sensor, there was some mixing of the CNTs with CHIT but only at the interface. With the Type III sensor, the CNTs

Resistance measurements of the films were made between the electrodes, and the values were around 100 Ω for Type I and II films and around 106 Ω for the Type III film. The high resistance could be accounted for by the contact of the electrode with chitosan, although it

The response of the sensors was measured at room temperature and the results showed 15, 33, and 520% for Type I, Type II, and Type III sensors, respectively. One interesting point made by the authors was that although the Pd decoration of SWCNTs is typically used to enhance hydrogen sensing, the response can be less than the effect of chitosan at 4% H2 gas. This research provided an important step towards the use of CNTs in sensors without the

In summary, the use of CNTs in the hydrogen economy has highlighted some interesting points. Is the race to develop more efficient hydrogen powered devices really producing a sustainable economy? And has the focus on reducing the utility of some of the rare raw ma‐

were immersed in the CHIT matrix.

was noted by the authors that ohmic contacts were present.

422 Syntheses and Applications of Carbon Nanotubes and Their Composites

**Figure 6.** Diagram of the 3 types of sensors. Image adapted from Li *et al.* (2010).

requirement of Pd.

The research field of photovoltaics has certainly become a hot topic over the last few years with a lot of attention based on increasing the efficiency of dye sensitized solar cells (DSSCs) in the hope that they will one day be as prevalent as the silicon based alternative. CNTs are an important addition to the field of photovoltaics with the focus on the nanotubes acting as p-type materials or enhancing/replacing the counter electrodes.

#### **3.1. Dye Sensitized Solar Cells**

If there were an enclave for truly beautiful chemistry, then the research behind dye sensi‐ tized solar cells (DSSCs) would clearly be the centerpiece. The chemistry behind the opera‐ tion of these devices is inspiring a generation of researchers to address the concerns of renewable energy with a different approach to the well established silicon based solar cells. Generally, the DSSCs are comprised of an anode, electrolyte and cathode. The anode is usu‐ ally assembled from nano-crystalline titania particles (TiO2) and a dye attached to the parti‐ cles. The cathode, also known as the counter electrode (CE), is where the catalysis must occur and typically contains platinum. The iodide electrolyte facilitates the iodide/triiodide redox couple where after the excitation of the dye and loss of an electron, it regains one from iodide, oxidizing it to triiodide. The best reported efficiency for DSSCs is 11.4% as docu‐ mented by the National Institute for Material Science (NIMS).

CNTs have been used as a potential replacement for the platinum based CE. In a study by Jo et al. (2012), interconnected ordered mesoporous carbon–carbon nanotube nanocomposites were used to demonstrate Pt-like CE behavior in a dye-sensitized solar cell [22]. CNT fibers have been used as a conductive material to support the dye-impregnated TiO2 particles. The CNTs were first spun from an array synthesized by chemical vapor deposition and resulted in highly aligned macroscopic fibers [23]. The research was novel in the application of these fibers as both the working electrode and the counter electrode.

The CNT/TiO2 composite fiber was produced by submersing the pure CNT fiber in a TiO2 colloid solution which was followed by sintering at 500 °C for 60 min. The thickness of TiO2 layer was determined to be between 4 and 30 µm, depending on the submersion time. The dye used for the cell was cis-diisothiocyanato-bis (2,2′-bipyridyl-4,4′-dicarboxylato) rutheni‐ um(II) bis (tetrabutylammonium) which is better known as N719. For DSSCs with a metal CE the I<sup>−</sup> /I3 <sup>−</sup> couple does eventually cause corrosion, but the CNT fibers exhibit a high stabil‐ ity and are relatively cheap. Fig. 7. shows the schematic of the working device with the two fibers in an electrolytic solution.

work, SWCNTs coated with a positively charged polyelectrolyte showed typical transitions and emission attributes in the visible and near-infrared spectrum. The application of steady state absorption spectra was useful in outlining the superimposition of QD and SWCNT characteristics. The results of the study also confirmed charge transfer between SWCNTs and QDs, underlined by femtosecond transient absorption spectroscopy. Microscopic stud‐ ies suggested that statically formed SWCNT/polyelectrolyte/QD nanohybrids with individu‐ ally immobilized QDs were generated. It is clear that this study focuses on the importance of the interactions between the components of the nanohybrids and creates a pathway for look‐ ing at the development of the layer-by-layer coating of SWNTs and recruitment of photoac‐

With the exception of multi-junction cells and gallium arsenide (GaAs) based devices, crys‐ talline silicon based cells are still the best choice with efficiencies at 20.4% for multicrystal‐ line structures to 27.6% for single crystal based cells. However, there is clearly room for improvement as the increase in efficiency has generally reached a plateau over the last few years. What may be required is a different approach to the design and chemistry of these photovoltaic devices. CNTs have again been applied on the strength of their p-type conduc‐ tion. In one recent example, polyaniline (PANI) and CNTs were used to construct hetero‐ junction diode devices on n-Type silicon [26]. If was found that both PANI and SWCNTs could act as photovoltaic materials in a bilayer configuration with n-type Silicon: n-Si/PANI and n-Si/SWCNT. Four devices were tested (Fig. 8.) and it was determined that the short cir‐

Si/PANI/SWCNT (Fig. 8c). The n-Si/SWCNT/PANI device (Fig. 8d) and its control n-Si/

PANI was synthesized using the MacDiarmid method [27] before being spin-coated at 600 rpm to form a film. The SWCNTs were dispersed in DMF by sonication over a period of 12 h in 3 hour intervals, with the any solids removed by centrifugation. The supernatant was then removed and sonicated for an additional 6 hours before being used to make the devi‐ ces. The devices were assembled by spraying SWCNTs using an airbrush deposition techni‐ que at 150 °C. It was found that the characteristics of the devices were affected by their design structure with better hole transport from PANI to SWCNTs and less efficient trans‐

Other examples of CNT-Silicon hybrid photovoltaic devices include the investigation of the optimal thickness of SWCNT films on n-type silicon in order to maximize photovoltaic con‐ version [28] giving percentage efficiencies between 0.4 and 2.4%, and the effect of the num‐

SWCNT (Fig. 8b) exhibited a decrease in the short-circuit current density.

port of holes from PANI to SWCNTs in the multilayer devices.

ber of walls of MWCNTs on the photon to electron conversion [29].

for n-Si/PANI (Fig. 8a) to 12.41 mA/cm2 n-

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tive particles for photovoltaic applications.

cuit current density increased from 4.91 mA/cm2

**3.3. Silicon Based Solar Cells**

**Figure 7.** Schematic illustration of a wire-shaped DSSC made from two CNT fibers. Figure adapted from Chen *et al.* (2012).

The mechanical properties of the fiber are quite good with tensile strength measure‐ ments that exceed 700 MPa. The enhanced electrical conductivity also ranges from 100 to 1000 S/cm. The fiber-shaped DSSC demonstrated an efficiency of 2.94% which was a sig‐ nificant accomplishment. The fibrous nature of the material would make large-scale com‐ posites easy to fabricate. One of the more exciting applications is that of woven fabrics that may be used for the development of smart textiles for consumers, or extended use for space based electronics.

#### **3.2. Quantum Dot Solar Cells**

Cadmium telluride (CdTe) has been shown to be a promising low-cost component photovol‐ taic material, however the incorporation of quantum dot (QD) based technologies will likely raise fears about the toxicity of cadmium and cadmium based compounds. Significant prog‐ ress has been made during the past several years with the highest efficiency reported for CdTe based photovoltaic devices at 17.3% produced by the company First Solar.

Although research is shifting towards CdTe/graphene composites [24], there is still room for CNT based devices. SWCNT/polyelectrolyte/QD nanohybrids have been produced that take advantage of the negatively charged thioglycolic acid capped CdTe QDs and SWCNTs coat‐ ed with a positively charged polyelectrolyte facilitating electrostatic interactions [25]. In this work, SWCNTs coated with a positively charged polyelectrolyte showed typical transitions and emission attributes in the visible and near-infrared spectrum. The application of steady state absorption spectra was useful in outlining the superimposition of QD and SWCNT characteristics. The results of the study also confirmed charge transfer between SWCNTs and QDs, underlined by femtosecond transient absorption spectroscopy. Microscopic stud‐ ies suggested that statically formed SWCNT/polyelectrolyte/QD nanohybrids with individu‐ ally immobilized QDs were generated. It is clear that this study focuses on the importance of the interactions between the components of the nanohybrids and creates a pathway for look‐ ing at the development of the layer-by-layer coating of SWNTs and recruitment of photoac‐ tive particles for photovoltaic applications.

#### **3.3. Silicon Based Solar Cells**

ity and are relatively cheap. Fig. 7. shows the schematic of the working device with the two

**Figure 7.** Schematic illustration of a wire-shaped DSSC made from two CNT fibers. Figure adapted from Chen *et al.* (2012).

The mechanical properties of the fiber are quite good with tensile strength measure‐ ments that exceed 700 MPa. The enhanced electrical conductivity also ranges from 100 to 1000 S/cm. The fiber-shaped DSSC demonstrated an efficiency of 2.94% which was a sig‐ nificant accomplishment. The fibrous nature of the material would make large-scale com‐ posites easy to fabricate. One of the more exciting applications is that of woven fabrics that may be used for the development of smart textiles for consumers, or extended use for

Cadmium telluride (CdTe) has been shown to be a promising low-cost component photovol‐ taic material, however the incorporation of quantum dot (QD) based technologies will likely raise fears about the toxicity of cadmium and cadmium based compounds. Significant prog‐ ress has been made during the past several years with the highest efficiency reported for

Although research is shifting towards CdTe/graphene composites [24], there is still room for CNT based devices. SWCNT/polyelectrolyte/QD nanohybrids have been produced that take advantage of the negatively charged thioglycolic acid capped CdTe QDs and SWCNTs coat‐ ed with a positively charged polyelectrolyte facilitating electrostatic interactions [25]. In this

CdTe based photovoltaic devices at 17.3% produced by the company First Solar.

fibers in an electrolytic solution.

424 Syntheses and Applications of Carbon Nanotubes and Their Composites

space based electronics.

**3.2. Quantum Dot Solar Cells**

With the exception of multi-junction cells and gallium arsenide (GaAs) based devices, crys‐ talline silicon based cells are still the best choice with efficiencies at 20.4% for multicrystal‐ line structures to 27.6% for single crystal based cells. However, there is clearly room for improvement as the increase in efficiency has generally reached a plateau over the last few years. What may be required is a different approach to the design and chemistry of these photovoltaic devices. CNTs have again been applied on the strength of their p-type conduc‐ tion. In one recent example, polyaniline (PANI) and CNTs were used to construct hetero‐ junction diode devices on n-Type silicon [26]. If was found that both PANI and SWCNTs could act as photovoltaic materials in a bilayer configuration with n-type Silicon: n-Si/PANI and n-Si/SWCNT. Four devices were tested (Fig. 8.) and it was determined that the short cir‐ cuit current density increased from 4.91 mA/cm2 for n-Si/PANI (Fig. 8a) to 12.41 mA/cm2 n-Si/PANI/SWCNT (Fig. 8c). The n-Si/SWCNT/PANI device (Fig. 8d) and its control n-Si/ SWCNT (Fig. 8b) exhibited a decrease in the short-circuit current density.

PANI was synthesized using the MacDiarmid method [27] before being spin-coated at 600 rpm to form a film. The SWCNTs were dispersed in DMF by sonication over a period of 12 h in 3 hour intervals, with the any solids removed by centrifugation. The supernatant was then removed and sonicated for an additional 6 hours before being used to make the devi‐ ces. The devices were assembled by spraying SWCNTs using an airbrush deposition techni‐ que at 150 °C. It was found that the characteristics of the devices were affected by their design structure with better hole transport from PANI to SWCNTs and less efficient trans‐ port of holes from PANI to SWCNTs in the multilayer devices.

Other examples of CNT-Silicon hybrid photovoltaic devices include the investigation of the optimal thickness of SWCNT films on n-type silicon in order to maximize photovoltaic con‐ version [28] giving percentage efficiencies between 0.4 and 2.4%, and the effect of the num‐ ber of walls of MWCNTs on the photon to electron conversion [29].

**4.1. Thermoelectric Fabrics**

generating a thermoelectric current *I*.

One of the more futuristic ideas is that of wearable electronics, and this has been envis‐ aged for many in the field of photovoltaics, but an Interesting alternative can be found in the field of thermoelectrics. Recent advancements in research have shown that composite films of MWCNT and polyvinylidene fluoride (PVDF) assembled in a layered structure can be designed to have the effect of felt-like fabric.[30] A thermoelectric voltage can be gener‐ ated by these fabrics as a result of the individual layers increasing the amount of power produced. More importantly, these fabrics would be more economical to produce clear‐ ing the way for a new generation of energy harvesting devices that could power porta‐ ble electronics. Fig. 9. shows a schematic of a fabric with every alternate conduction layer made with p-type CNTs (B) followed by n-type CNTs (D). The insulating layers allow for alternating p/n junctions when all the layers are stacked, pressed and heated to melt the polymer. It was noted that layers A−D could be repeated to reach a desired number of conduction layers *N*, and when the film is exposed to a change in temperature (ΔT = *T <sup>h</sup>* - *T <sup>c</sup>* ), the charge carriers which can be holes (h) or electrons (e) migrate from *T <sup>h</sup>* to *T <sup>c</sup>*

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**Figure 9.** A Layered arrangement for the multilayered fabric. The CNT/PVDF conduction layers (B,D) are alternated

When more power is required, ΔT would have to be increased. Subsequently, if the heat source were sufficiently large enough, the number of conduction layers could be increased. This would be a huge benefit for manufacturing industries that use high temperature equip‐ ment. In terms of energy output, a fabric composed of 300 layers with a ΔT = 100 K, may

between the PVDF insulation layers (A,C,E). Figure adapted from Hewitt *et al.* (2012).

**Figure 8.** Schematics for (a) n-Si/PANI, (b) n-Si/SWCNTs, (c) n-Si/PANI/SWCNTs, and (d) n-Si/SWCNT/PANI devices. Im‐ age adapted from Bourdo *et al.* (2012).

In summary, photovoltaics have been shown to be very popular within the scientific field and the commercial market. Consumer electronics have been marketed with solar power chargers as a way to promote sustainability and environmental responsibility. The research into ruthenium based DSSCs is very popular but again there are concerns about the use of ruthenium for a sustainable economy. Fortunately, there are many photosensitive dyes that don't contain ruthenium which are currently being explored, but it is clear that the integra‐ tion of interconnected CNTs can play an important role in the development of novel photo‐ voltaic devices.

#### **4. Thermoelectrics**

In 1821 Thomas Johann Seebeck made the first discovery in the series of thermoelectric effects. The Seebeck effect described the electromotive force (emf) produced by heating the junction between two different metals. In essence, the kinetic energy of the electrons in the warmer part of a metal would facilitate the transfer of the electrons to the cooler metal faster than electron transfer from the cooler to the warmer metal, essentially creating an electronic potential where the cooler metal obtains a net negative charge. Harnessing the heat lost from a system and converting it to electricity will help to reduce the strain on electricity providers, but the difficulties surrounding the efficiency of the conversion proc‐ ess need to be addressed.

#### **4.1. Thermoelectric Fabrics**

**Figure 8.** Schematics for (a) n-Si/PANI, (b) n-Si/SWCNTs, (c) n-Si/PANI/SWCNTs, and (d) n-Si/SWCNT/PANI devices. Im‐

In summary, photovoltaics have been shown to be very popular within the scientific field and the commercial market. Consumer electronics have been marketed with solar power chargers as a way to promote sustainability and environmental responsibility. The research into ruthenium based DSSCs is very popular but again there are concerns about the use of ruthenium for a sustainable economy. Fortunately, there are many photosensitive dyes that don't contain ruthenium which are currently being explored, but it is clear that the integra‐ tion of interconnected CNTs can play an important role in the development of novel photo‐

In 1821 Thomas Johann Seebeck made the first discovery in the series of thermoelectric effects. The Seebeck effect described the electromotive force (emf) produced by heating the junction between two different metals. In essence, the kinetic energy of the electrons in the warmer part of a metal would facilitate the transfer of the electrons to the cooler metal faster than electron transfer from the cooler to the warmer metal, essentially creating an electronic potential where the cooler metal obtains a net negative charge. Harnessing the heat lost from a system and converting it to electricity will help to reduce the strain on electricity providers, but the difficulties surrounding the efficiency of the conversion proc‐

age adapted from Bourdo *et al.* (2012).

426 Syntheses and Applications of Carbon Nanotubes and Their Composites

voltaic devices.

**4. Thermoelectrics**

ess need to be addressed.

One of the more futuristic ideas is that of wearable electronics, and this has been envis‐ aged for many in the field of photovoltaics, but an Interesting alternative can be found in the field of thermoelectrics. Recent advancements in research have shown that composite films of MWCNT and polyvinylidene fluoride (PVDF) assembled in a layered structure can be designed to have the effect of felt-like fabric.[30] A thermoelectric voltage can be gener‐ ated by these fabrics as a result of the individual layers increasing the amount of power produced. More importantly, these fabrics would be more economical to produce clear‐ ing the way for a new generation of energy harvesting devices that could power porta‐ ble electronics. Fig. 9. shows a schematic of a fabric with every alternate conduction layer made with p-type CNTs (B) followed by n-type CNTs (D). The insulating layers allow for alternating p/n junctions when all the layers are stacked, pressed and heated to melt the polymer. It was noted that layers A−D could be repeated to reach a desired number of conduction layers *N*, and when the film is exposed to a change in temperature (ΔT = *T <sup>h</sup>* - *T <sup>c</sup>* ), the charge carriers which can be holes (h) or electrons (e) migrate from *T <sup>h</sup>* to *T <sup>c</sup>* generating a thermoelectric current *I*.

**Figure 9.** A Layered arrangement for the multilayered fabric. The CNT/PVDF conduction layers (B,D) are alternated between the PVDF insulation layers (A,C,E). Figure adapted from Hewitt *et al.* (2012).

When more power is required, ΔT would have to be increased. Subsequently, if the heat source were sufficiently large enough, the number of conduction layers could be increased. This would be a huge benefit for manufacturing industries that use high temperature equip‐ ment. In terms of energy output, a fabric composed of 300 layers with a ΔT = 100 K, may produce up to 5 µW. This is certainly a promising material that could potentially be inte‐ grated into many thermal systems and help with waste heat recovery.

mained constant. Aligned CNT bundles may have smaller Seebeck coefficients (thermoelec‐ tric sensitivity) than randomly oriented CNTs. The authors suggested that the difference may be a result of the contribution of inter-tube barriers, relative to Δ*T*, although more work is required to fully understand the effect of CNT films for the integration of them into ther‐

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CNTs have seldom been just another material for novel composites, but their true potential has yet to be transferred from the nano- to macro-scale. More than a two decades after their discovery, their influence has reached almost every aspect of scientific research from engi‐ neering to medicine. Faced with concerns about sustainably and climate change, the use of CNTs have helped to transform our approach to renewable energy. Advances in hydrogen fuel cells with CNT composite electrodes or membranes are helping to reduce and eliminate the need for rare and expensive catalysts. Safety is also another issue for the hydrogen based economy. Many different types of sensors will be required to promote a safe operational en‐ vironment especially when the ignition concentration of hydrogen can be as low as 4%. The same technology that is used in the catalysis process in hydrogen fuel cells can be used to create hydrogen sensors, and work with interconnected CNTs has provided sensitivity val‐

The role of interconnected CNTs in the photovoltaic research field is popular because of the potential to make novel hybrid solar cells, whilst increasing the overall efficiency of the de‐ vice. While the early results look promising, there are still some difficult questions to ad‐ dress, like how does the presence of defects on the CNT surface affect the chemistry and

The integration of CNTs into thermoelectric devices currently does not have the same level of development as the other alternative energy resources, possibly because the field is more geared towards cost saving on an industrial scale and the development of component sys‐ tems for vehicles rather than consumer gadgets or devices, but research into waste heat re‐ covery is substantial. It is likely that thermoelectric devices will conform more to a silent revolution with an uptake in industries that work with high temperature equipment looking at converting some of the heat produced back to electricity. However, the research into ther‐ moelectric fabrics has shown the potential for consumer products that may find a market in

In summary, we are beginning to see a shift towards alternative fuel sources, with a focus on hybrid technologies like those found in the automotive industries, but we need to address

the impact of our current economy as we transition to a more sustainable one.

moelectric devices.

**5. Conclusion**

ues that contend with conventional sensors.

ultimately the efficiency of a DSSC?

the future.

#### **4.2. Micro-Thermal Electrics**

The addition of CNTs to microelectrical mechanical systems (MEMS) typically proceeds by either a bottom-up approach which focuses on the deposition of catalytic nanoparticles to control the location of CNT growth or a top-down which concerns the manipulation of the CNTs to the correct position. A top-down method was use to make a CNT thin film on a microelectrical mechanical system which was then characterized in terms of the thermoelec‐ tric coefficients of the aligned SWCNTs [8]. Using the process of 'super-growth' which incor‐ porates water-assisted chemical vapor deposition, a CNT film was made and patterned by electron beam lithography into the required dimensions. By patterning a formed array of gold–SWCNT thermocouples it was found that under standard room temperature the See‐ beck coefficient of the aligned SWCNT film was between 18 and 20 *μ*V C−1. The Seebeck ef‐ fect of the SWCNT film was documented using thermocouples made of gold–SWCNT (Fig. 10.). Electrodes, a hot end and cold end temperature sensor, and a heater were produced by photolithography, and with a gold lift-off process on top of a silicon substrate that was cov‐ ered by an insulating layer of Si3N4. The SWCNT film was then constructed on the gold sur‐ face using the process of top-down assembly.

**Figure 10.** Schematic of a device for measuring the Seebeck effect in a CNT film. Figure Adapted from Dau *et al.* (2010).

When the device was used, an output voltage of 54 µV was recorded with a temperature difference of 3.07 ◦C. This gave a Seebeck voltage of 19.38 µV K−1 which on average re‐ mained constant. Aligned CNT bundles may have smaller Seebeck coefficients (thermoelec‐ tric sensitivity) than randomly oriented CNTs. The authors suggested that the difference may be a result of the contribution of inter-tube barriers, relative to Δ*T*, although more work is required to fully understand the effect of CNT films for the integration of them into ther‐ moelectric devices.
