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Carbon nanostructures are currently being integrated into photovoltaic cells because they have many valuable properties. Physical benefits of carbon nanostructures include their high surface area and that they are stable across fluctuations in environment, temperature and mechanical stress. Their photochemical benefits range from the good charge transport abilities to conductivity comparable to copper. Carbon based materials are also a viable option because they are much more abundant and less expensive than noble metals such as gold and platinum and the commonly used but very costly ultra-pure silicon.

Carbon Nanotubes
Single wall carbon nanotubes (SWCNT) have been included in dye-sensitized, organic, and polymer-based solar cells for several purposes. They offer the benefit of high surface areas in combination with very narrow widths to create high aspect ratios. Their thermal and mechanical stabilities are promising for long term endurance of the cells. When doped with poly(3-octylthiophene), SWCNT have been shown to have increased charge transfer abilities. In addition to charge transfer roles, nanotubes have been employed as architectural components in photovoltaic devices. Some researchers have taken advantage of their one-dimensional structures to anchor photoactive materials and provide an electron transport path to increase device efficiency. Future applications of SWCNT may include the attachment of quantum dots to the nanostructures to further manipulate their semiconductive behavior.

Stacked Cup Carbon Nanotubes (SCCNT)
Carbon nanocups can be pictured as carbon nanotubes shaped like cones, but with the point truncated. Compared to nanotubes, the cups have a greater amount of exposed inner surfaces and edges for modification sites. The cups have been shown to stack together to form nanobarrels, that can be manipulated as structural components similar to nanotubes. SCCNT have the same chemical and photochemical benefits as carbon nanotubes. Researchers have been able to show that inclusion of SCCNT in solar devices increases the observed photovoltage, and even that display photocurrents greater than observed in similar devices containing SWCNT. It has also been shown that electrons and holes can remain separated for extended periods of time in these structures. Recently, SCCNT have been functionalized with quantum dots to increase their electron/hole separating abilities in photovoltaic devices.

Buckminsterfullerene
Also known as buckyballs, spherical all-carbon C60 has received much attention since its discovery in 1985. C60 and its derivatives have been included in experimental photovoltaic devices for many years. The high electron affinity and ionization energy in this structure give it an energetically lower Lowest Unoccupied Molecular Orbital (LUMO), which can then act as an electron capturing feature in polymer blend type semiconductors. Buckyballs have not received as much attention in the solar cell field as nanotubes, but they have been shown to contribute to faster charge separations.

Carbon Nanohorns (CNH)
Nanohorns are similar in structure to nanocups, without the truncated end. This uniquely combines the structure and functionality of nanotubes and cups at one end of the nanohorn with properties more similar to buckminsterfullerene at the other. This gives CNH the high electron capturing ability along with inherent directionality. Research has shown that CNH are available at high purity and can self-assemble into star-like aggregates without external catalysis. These properties have prompted researchers to explore functionalization of nanohorns to create photovoltaic devices of high efficiency.

Inorganic Solar Cells and Carbon Nanostructures in Inorganic Solar Cells
It has been 170 years since A.E. Becquerel first discovered the photovoltaic effect by shining light onto an AgCl electrode in an electrolyte solution. The prototype of practical inorganic photovoltaic solar cell (IPSC) was developed by Bell Labs which performs efficiency as ~5%. In this IPSC, the semiconductor (single-crystal Silicon for the Bell cell) plays as the light absorber to hold solar energy and then generate electron-hole pairs, and the internal electric field in the Schottky junction separates the photo-excited charges automatically. The two fundamental processes, light absorption and charge separation, are still the basis of all inorganic solar cells today.

Until the most recent data analysis, the photoinduced power conversion efficiency is held at 40.8% by a so-called tandem cell which contains three p-n junctions: GaxIn1-xAs, GayIn1-yP with various compositions. The much better performance lies from the advantage of covering larger scale of the solar spectrum by adopting different band gap distance p-n junctions at one time.

Researchers are devoting efforts to developing new semiconductor materials since the cost and efficiency issues. CNTs possess unique electrical and optical properties that make them an ideal candidate for various components in modern electrics. Photovoltaic effect can be achieved in ideal carbon nanotube diodes. For example, Individual singlewall carbon nanotubes (SWCNT) can form an ideal p-n junction diode. It is mentioned above that semiconductors’ heterostructures have been used in solar cells. In particular, CNTs and silicon can form heterojunctions, and solar cells based on such CNT/silicon heterojunctions have produced a high efficiency of 7%. In this background, CNTs serves as both photogeneration sites and a charge carriers collection/transport layer. The advantages of using CNTs in IPSC are obvious: such as a wide range of direct bandgaps matching the solar spectrum, strong photoabsorption from infrared to ultraviolet, high carrier mobility, and reduced carrier transport scattering. Several groups are focusing on this special topic as heterojunction solar cells associated by p-type CNTs and n-type silicon.

Carbon Nanostructures in Polymer Solar Cells
The high cost and the complex production process of inorganic solar cells call for alternative photovoltaic technologies. Polymer solar cells can be one of them. These cells are constructed from cheap organic materials, employ easy preparation and deposition methods, and can be fabricated on a large scale using flexible substrates. However, there are some key disadvantages associated with these polymer-based solar cells such as low conversion efficiency, limited exciton (bound electron–hole pair) diffusion length, poor hole carriers transport and short lifetime compared to inorganic counterparts.

Recently, efficiency growing has been demonstrated in multilayer structures involving a donor/acceptor bulk heterojunction. Alternatively, a nanomaterial has been added to the polymer active layer to facilitate exciton dissociation and carrier transport through the polymer matrix. The most widely investigated nanomaterials are semiconducting nanocrystals, fullerenes, and carbon nanotubes. Carbon nanotubes, in particular, have extremely high surface area which offers an ideal architecture for exciton dissociation and have high aspect ratio which allows a low percolation threshold, providing the means for high carrier mobility and efficient charge transfer to the appropriate electrodes.

There are mainly two types of carbon nanotubes that are used to incorporate with polymer solar cells. They are single-wall carbon nanotubes (SWCNT) and multiwall carbon nanotubes (MWCNT).

MWCNT was first introduced to polymer solar cells by Ago and co-workers in 2000. They found that a strong interaction in the excited-state occurs in the composite of MWCNT and a conducting polymer. Based on this report, we know that there is energy transfer between the conducting polymer and the MWCNT. This investigation has drawn a lot of attention on exploiting SWCNT to improve carrier generation and collection efficiency in polymer solar cells.

The first composite of SWCNT as acceptor material and a polymer solar cell was reported by Kymakis and co-workers in 2002. The performance of this device increases both open-circuit voltage and short circuit current which can be ascribed to the introduction of internal polymer/nanotube junctions within the polymer matrix. These junctions allow excitons to be dissociated and also create a continuous path for electron transport to the charge-collecting electrode.

In conclusion, CNT-based are incorporated on to polymer solar cells in order to enhance their low efficiency based on their potential of maximizing exciton dissociation, offering higher carrier mobility and efficient charge transfer. However, a lot more research still needs to be done on demonstration novel methods to take better advantage of the beneficial properties of carbon nanotubes.

Applications of Carbon Nanotubes
Carbon nanotubes have a variety of applications due to their incredible strength and versatility. These two properties provide for incorporation into a multitude of products such as clothes, tennis rackets, bicycles, golf balls, and bridges. The carbon nanotubes are also being proposed as a cheaper alternate to the transparent, conductive glass coating indium tin oxide. While such ideas are feasible, another major use for carbon nanotubes is in the area of alternative energy. The carbon nanotubes are proposed for electron transport in solar cells.

Due to the current energy crisis by the year 2050, 10 TW of carbon neutral power needs to be produced, almost equivalent to the power provided by all of today’s energy sources combined. In order to meet this demand, alternative renewable energies need to be improved. There are a variety of sources possible to replace fossil fuels. Such resources include hydrothermal, solar, wind, etc. Solar power provides the best option because it is readily accessible, provides no pollution, and is renewable. The primary goal in solar power research is to capture, convert, and store solar energy. The process is being improved in its efficiency and cost.

The basics of a solar cell are similar to that of any electrochemical cell. There is a cathode and anode, material or solution separating the two, and a shield around the whole system. A photoelectrochemical cell (PEC) is similar to an electrochemical cell except that the the material in the PEC, usually the anode, demonstrates photocurrent. The anode absorbs photons with enough energy to make electron-hole pairs. Upon the separation of the electrons and holes a current can flow. The carbon nanotubes can act as the photoactive source creating and transporting electron-hole pairs. Alternately, the carbon nanotubes can transport the electron from the photoactive material to the back contact for transport to the cathode.

Challenges in the Field
One of the common methods of incorporating CNT into a photovoltaic cell is through electrochemical deposition. However, this method results in a random dispersion of CNT on the surface as opposed to an ordered arrangement. The random dispersion creates the presence of charge recombination pathways which decrease the efficiency of the cell, as well as decreasing the level of control that can be had over the fabrication of the photovoltaic cells. Another factor which decreases efficiency is incomplete excitation dissociation which is particularly a problem at low CNT concentrations. However, at higher concentrations the efficiency of the cell also decreases due to increased chance for short circuit formation. CNT consist of a mixture which posses’ semiconducting and metallic characteristics which cause them to form bundles. The bundles decrease the electron/hole selectivity of the device and favor the recombination of the two carrier species as well as decreasing the surface area which can interact with the matrix. Another problem with the production of CNT is that impurities readily coat the CNT surface affecting the nanotube dispersibility, binding in composites as well as the electrical and mechanical properties of the CNT junctions. Improvements to the efficiency of CNT/polymer based solar cells could be achieved through the development of better processing procedures which would address the fine debundling of CNT in solvents. Also important is the development of synthesis, purification and separation. Current synthetic methods for SWNCT result in major concentrations of impurities. These impurities are typically removed by acid treatment which introduces other impurities, can degrade nanotube length while increasing production cost. These synthetic routes also lead to the formation of a mixture of various semiconducting and metallic nanotubes. Purification steps would allow the deposition of CNT that provide the best frontier orbital energy offset with the polymer. A method for the deposition of aligned CNT arrays would also improve as they outperform the arrays of randomly dispersed CNT.

Stone Wales (SW) Defects
A Stone Wales defect is the rearrangement of the six membered rings of graphene into pentagons and heptagons. This rearrangement is a result of π/2 (90o) rotation of a C-C bond. The density of SW defects is usually small due to the high activation barrier of several electrons volts for the bond rotation however the defects has been imaged using STM as well as resonant-vibrational spectroscopy techniques. A number of theoretical studies have shown the absorption energies and charge transfer energies are larger for single-walled CNT are larger than the corresponding values on pristine CNT. The diminished resonance and higher strain energy of this defect increased the probability of nucleophilic attack which provides a possible explanation for the enhanced reactivity. Research has also shown the incorporation of defects along a nanotube network can program a nanotube circuit to enhance the conductance along a specific path. These defects lead to a charge delocalization which redirects and incoming electron down a given trajectory.