Nanomaterials and their Applications in Solar Cells - Term Paper Example

Nanomaterials And their Applications In Solar Cells Nanomaterials possess advantage in electric and optical properties, and when these carefully designed and fabricated nanostructures serve as a photo electrode film of solar cells, they can effectively improve the energy conversion efficiency by offering a large specific surface area, providing direct pathways for electron transport, or generating light scattering so as to extend the traveling distance of photons within the photo electrode film. Nowadays, the world consumes about 13 TW of continuous worldwide energy. By the year 2050, according to many energy experts we need about 30 TW to accommodate the increasing requirement of energy & economic growth. Moreover, this new energy has to come from a environmental friendly source so that we don’t face a tragedy climate change. Solar energy is the best energy source in terms of its availability, distribution and environmental impact. Nanotechnology, with its unprecedented control over the structure of materials, can manufacture efficient and inexpensive solar cells on large scale. Nanotechnology is producing some significant advantages in solar cells to increase the efficiency/cost ratio by using nanomaterials with suitable bandgaps (Zweibel 2000). Nanomaterials enhance the effective optical path and significantly decrease the probability of charge recombination. Nano-structured CdS as window layer, CdTe as absorber layer and TiO2 electrodes are of interest as in thin film solar cells. Our study focus on the synthesis of nano structured oxides and the application of these nanostructures on the fabrication, working cycle and implementation of dye-sensitized solar cells, their application and also discussion of the materials of the thin film solar cells. 1. Introduction: The quest for alternative fuels has become one of science’s major pre-occupations and finding ways cost effectively produce energy from the sun is a key battlefront. The usage of solar cells results in many useful features like require less maintenance effort, avoid release of toxic gases or other noises and mainly avoid transmission losses and costs. Also the solar cells available today are not considered feasible when compared to the non renewable sources.1 The solar cells manufactured at present are low in efficiency and very expensive if they have to be used to generate electricity on a large scale . We are interested in using the valuable features of nanotechnology so that we can manufacture efficient and cheaper solar cells in large quantities. It is essential to understand the fundamental process that a basic solar cell uses before the introduction of new solar products in which nanotechnology is used. The solar cells which have been in use conventionally are called photovoltaic cells. A semi conducting material, most commonly Silicon, is used to make these kinds of cells. Energy is absorbed by the cells, through the light which falls on them. Electrons are knocked out by the energy which is absorbed which allows them to flow. Various impurities are added to Silicon, for example Boron, to establish an electric field. As the electrons are allowed to flow in only one direction, the electric field establish functions as a diode. As a result of the flow of electrons electricity is generated. The two main disadvantages of solar cells used conventionally are: Their efficiency is very low, around 10% The cost to manufacture them is too high The disadvantage of low efficiency cannot be avoided with solar cells made of silicon, because the approaching light, consisting of photons should have the band gap energy, the correct energy so that an electron can be knocked out. When the energy of a photon is lower than the band gap energy it passes through. On the other hand excess energy is dissipated as heat if the energy is higher than the band gap. Out of the total energy incident on the cell, these two effects described are responsible for losing roughly 70% of it. The efficiency which can be achieved for a solar cell is 25%, when carefully prepared in a laboratory(Kazmerski 1997). The solar cells produced on large scale have lower efficiency than this. This problem can be solved through nanotechnology, which increases the efficiency. Still, the main advantage of using nanotechnology is the lower cost of manufacturing. Nanomaterials materials can prove very beneficial in future. Their advantages are being researched in applications such as electro-optical, sensor and micromechanical devices. When the dye molecules are attached to a nanostructured TiO2 electrode the light absorption occurs forming an electro chemical solar cell known as Dye sensitized nano structured solar cell (DSSC) (Kallioinen et al. 2001). . In field of thin film solar cells, these nanostructured layers have primarily three advantages. As the light undergoes multiple reflections, the optical path for absorption is increased. This optical path is much larger than the actual film thickness. The losses occurring through recombination are reduced considerably. This is because the generated electrons and holes have to cover a much shorter distance and thus the thickness of the absorber layer can be very thin, around 150 nanometer, which is very less compared to several micrometers in conventional cells. The designs are more flexible for the absorber and window layers. This is a result of the fact that the size of nano-particles can be changed which allows the energy gap to be customized according to the required design value. Mass produced large-area solar cells necessitate production of periodic arrays of semiconductor nanostructures which permit: Customization of composition and size of nanostructures Inclusion of the semiconductor nanostructures within a strong host material Freedom in choosing variety of substrate materials Suitable inclusion in standard Silicon fabrication processes The process which fulfils these criteria is self assembly process. This process can be used to produce various nano-structured films, like CdTe and CdS on ITO-glass substrates.Using nanoparticles in the manufacture of solar cells has the following benefits: Reduced manufacturing costs as a result of using a low temperature process similar to printing instead of the high temperature vacuum deposition process typically used to produce conventional cells made with crystalline semiconductor material. Reduced installation costs achieved by producing flexible rolls instead of rigid crystalline panels. Cells made from semiconductor thin films will also have this characteristic. Currently available nanotechnology solar cells are not as efficient as traditional ones; however their lower cost offsets this. In the long term nanotechnology versions should both be lower cost and, using quantum dots, should be able to reach higher. 2. Photovoltaic cells: Photovoltaics is a method of generating electrical power by solar current into direct current electricity using various semiconductors that exhibit the photovoltaic effect. Where as the photovoltaic effect is the process of transferring the generated electrons between different bands within the material resulting in the building of a voltage between two electrodes. The materials used for this process include copper indium selenide, cadmium telluride, polycrystalline silicon, monocrystalline silicon, amorphous silicon. The photovoltaic devices are known as solar cells since the radiation in most of these applications is sunlight. Solar cells produce direct current electricity from sunlight that can be used to recharge battery and other purposes. Solar cells are need to protected from the environment and are usually packaged tightly behind a glass sheet. The solar cells are arranged in an array and form modules when the requirement of energy is high. Nowadays the silicon crystalline modules are replaced by the thin film solar cells such as CdS ,CdTe , CIGS, amorphous Si, microcrystalline Si for improved efficiency (Maycock 2000).. Other improvements such as concentrator modules ,silver cells , continous printing process, casting wafers instead of sawing are used. The photovoltaic cells are being implemented in various commercial applications such as Power stations Standalone devices In transport In buildings Rural electrification In solar roadways In solar power satellites Despite of the greater usage of the photovoltaic cells it also include the practical difficulties which includes huge installation cost , its not productive at nights, and reduced in cloudy conditions therefore storage power system is needed, also solar cells produce DC electricity must be converted to AC which incurs some energy loss.2 A number of solar cells electrically connected to each other and mounted in a frame is called a photovoltaic module as shown in the Figure 1. Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system. The current produced is directly dependent on how much light strikes the module. The more the module the more amount of current is generated. Figure: 1 Multiple modules can be wired together to form an array. (Source: www.blueplanet-energy.com/images/solar/PV-cel.) 3. Dye-Sensitized Nanostructured Solar Cells: Among all the solar cells the dye sensitized nano structured solar cell is a photo electrochemical solar cell which offers high efficiency and long term stability. It uses a liquid electrolyte and other ion conducting phase as a charge transport medium (McEvoy & Grätzel 1994). . As shown in Figure 2 the dye sensitized solar cell consist of a transparent conducting glass electrode coated with porous Nano crystalline TiO2 , molecules are attached to the surface of the nc- TiO2, T an electrolyte containing a reduction-oxidation couple such as I-/I3 and a catalyst coated counter-electrode. At the time of illumination the cell generates voltage over and current through an external load connected to the electrodes. The dye molecules cause the absorption of light in Dye sensitized nano structured solar cell and the charge separation by electron injection from the dye to the TiO2 at the semiconductor electrolyte interface (ORegan & Grätzel 1991). .3 The optical thickness of the layer can be increased by stacking the dye molecules on top of each other to obtain a thick dye layer to increase the absorption of the incoming light. Here the dye molecules are in direct contact to the semiconductor electrode surface can separate charges and contribute to the current generation (Shah et al. 1999).. It can be recovered by the use of porous Nano crystalline TiO2. In order to increase the internal surface area of the electrode to allow large amount of dye to be contacted at the same time by both the electrode and the electrolyte Figure 2. A schematic representation of the structure and components of the dye-sensitized solar cell. (Source: http://photochemistryportal.net/category/dye-sensitized-solar-cells/) 3.1 Light Scattering Effect: In dye-sensitized solar cells there is increase the light gathering efficiency of DSSC. Due to the light scattering generated by oxide accumilates, the traveling distance of photons within the photo electrode film can be increased significantly. This will result in an improvement in the light harvesting efficiency of photo electrode and thus helps to the solar-to-electricity conversion efficiency of the cells. The aggregates ensure the photo electrode film having a large surface area in order to the porous structure of individual aggregate.4 Figure 3: this diagram shows the incident of light and diffuse reflection towards the closed group of the cells. (Source http://depts.washington.edu/solgel/) As shown in the Figure 3 the schematic drawing presented to represents the propagation and multiple scattering of light in highly disordered films consisting of spherical aggregates. Multiple scattering results in the increase of light traveling distance and the formation of optical confinement when the light scattering is trapped in closed loop thus increase in the productivity 3.2 The Working cycle of the Dye-sensitized solar cell: The working cycle of the Dye-sensitized solar cell includes the following process. The dye molecule absorbed on the surface of the Nano crystalline TiO2 particle absorbs the incoming photon and an electron from a molecular ground state S0 is exited to a higher lying excited state S*. the excited electron is injected to the conduction band of TiO2 particle leaving the dye molecule to an oxidised state S+. the injected electron passes through the porous Nano crystalline structure to transparent conducting oxide layer of the glass substrate. To yield iodine the electron is transferred to the tri-iodide in the electrolyte at the counter electrode. By the reduction of the oxidised dye by the iodine in the electrolyte the cycle is closed5. By using the efficient sensitizer dye and a successful control of the morphology and properties of the nanostructured thin film are the most important factors for reaching high efficiencies in Dye Sensitized Solar Cell. 6 Figure 4: Dye-sensitized solar cell tile from the STI production line (Source: http://www.sta.com.au/newimages/hhtp12.jpg). The fundamental operating differences between the traditional pn junction solar cells and the dye-sensitized nanostructured solar cells are, In pn junction solar cells the light absorption and charge transport are occurs in the same material where as in Dye sensitized solar cells photons are absorbed by the dye molecules and the charge transport are carried out by the TiO2 electrode and electrolyte. In the pn junction solar cell the charge separation is induced by the electric field across the junction but in dye sensitized solar cells they occur by the kinetic and energetic reasons at the semiconductor electrolyte interface. The produced opposite charges travel in the same material in the pn junction solar cells, where as in dye sensitized the electrons travel in the TiO2 network and holes in electrolyte.7 3.3 Implementation Of Dye sensitized solar cells In commercial: In figure a demonstration of an architectural use of Dye sensitized solar cell implemented in STI is shown. The DSSC is mostly used when compared to other photovoltaic devices since it contains relatively simple manufacturing process and the inexpensive equipment in DSSC the old fashioned screen printing machines can be used which results in low manufacturing cost. Figure 5: A demonstration of an architectural use of DSSC by STI (Source:http://www.sta.com.au/images/nz1.jpg) 3.4 Outlook on DSSC: In future the DSSC can be enhanced by the development of the new dyes with enhanced absorption properties ,dye cocktails, semiconductor quantum dot dyes, UV and IR absorbing dyes for transport photovoltaic windows and further development in the materials of DSSC will results in the effective and efficient methodology. 3.5 Hybrid Solar Cells: Hybrid solar cells combine advantages of both organic and inorganic semiconductors.These include both the characteristics of organic and inorganic semi conducting substances. The organic materials consist of conjugated variety of quality polymers that absorb light and transport holes, while the inorganic materials in the structure are used as the acceptor and electron transporter. Thus the way of generating electricity is more effiecient. And it can be relatively manufactured in a low manufacturing cost when combined with successful development techniques. The use of nanostructured inorganic materials in hybrid solar cells may significantly raise the interface area of p-n junction and meanwhile provides direct pathway for electron transport so as to reduce the electron trapping that improves the process.8 4. Materials of thin film solar cells: The following are the materials of the thin film solar cells, 4.1 Nano-crystalline CdS /CdTe thin film solar cells: Nano-crystalline Cadmium telluride films are used in a large scale for the thin film devices technology, such as solar cells, IR and γ detectors, whose electrical and optical properties determine their efficiency. Therefore, these properties need to be carefully examined as a highly and stable efficiency of respective devices can be guaranteed by determining the best conditions. The research done on these materials shows that Nano-crystalline thin film CdTe is the most important material for the fabrication of cheaper and dependable photovoltaics.9 As discussed in the introduction part there are two key properties of material that -make this possible (Green 2001).. The ideal band gap for photovoltaic conversion efficiency is (1.45 eV), and it has high optical absorption coefficient. Band gap of CdTe is 1.5 eV, which is very close to the desired value .Thin film CdTe solar cells are usually hetero-junctions with CdS being the n-type partner (or window layer). Efficiencies as high as 16.5% have already been achieved. Still, there is great scope for increasing them further (Kay & Grätzel 1996). . The structure of a CdTe/CdS solar cell is composed of 4 layers as shown in Figure 6. 1. A transparent and conducting oxide (TCO) which acts as a front contact; 2. A CdS film which acts as a window layer; 3. A CdTe film: It is the absorber layer made on top of CdS; 4. The back contact on top of the CdTe layer. Figure 6: Schematics of a typical superstrate CdS/CdTe solar cell structure.(Source: http://www.iopscience.iop.org) Each component has different physical and chemical properties as the materials used for each is different. Thus, the overall performance of the device is affected by each component in some form or the other. Therefore to design a device, a detailed understanding of the properties of these different components is necessary. The various interfaces between the different layers also hold an important role as the interfaces can cause stresses, defect and interface states, surface recombination centres, photon reflection/transmission/scattering, interdiffusion and chemical changes with attendant electro-optical changes because each layer has different crystal structure, microstructure, lattice constant, electron affinity/work function, thermal expansion coefficient, diffusion coefficient, chemical affinity and mobility, mechanical adhesion and mobility, etc Solaronix (2000). .. 4.2 Substrate: Two types of structure: superstrate or substrate structure are used to configure thin-film solar cells devices. In the superstrate configuration, the substrate is transparent on which the contact is made by a conducting oxide coating. In the substrate configuration, the substrate is metal or metallic coating on a glass/polymer substrate, which also acts as the contact. Substrate is a passive component in the device. It should be mechanically stable, with compatible thermal expansion coefficient with deposited layers and inert during the fabrication. These criteria are the parameters on which suitable substrates are selected for various processes (Chmiel et al. 1998), . Flexible substrates, like stainless steel foils/polymer films, are most suitable for roll-to-roll deposition which enables a compact deposition system design, together with flexibility in device handling. The development of front and rear-side conduction cells is enabled by electrically conductive substrate but an insulating substrate enables fabrication of monolithically interconnected cells for modules. Deposition, which involves high-temperature processes, usually desires rugged and expensive substrates like high-temperature glass or ceramics. Processes involving lower temperatures allow usage of cheaper flexible substrate. The substrate could be a major reason for high cost in TFSC technology. At present, both superstrate and substrate device structures are being used for CIGS device fabrication. The film growth and interdiffusion, and thus the device properties are predicted by the device structure. The copper indium gallium selenide (CIGS) solar cells based on superstrate structure is has the disadvantage of the interdiffusion of CdS during high-temperature CIGS film growth as compared to substrate structure. The best device efficiency achieved on superstrate device configuration is 10.2%, which has a ZnO buffer layer.31, whereas, a substrate configuration with CdS buffer layer reported a 19.2% efficiency. To improve the photovoltaic performance of the CIGS absorber materials, a critical role is played by the substrate. When an efficient CIGS device fabrication is desired, the Na in the soda-lime glass substrate is an important prerequisite. This is because the Na diffuses from the substrate into CIGS absorber, which improves the grain growth and cell performance. Sodium is essentially present in soda-lime glass (SLG). While, Na precursors (Na2Se, Na2S, NaF) are intentionally incorporated in the device fabrication in case of Na free substrate. As the CdTe surface is exposed for contacting, CdTe devices are fabricated usually in superstrate configuration. Also, the lattice mismatch between CdTe and CdS is reduced by the benign feature of CdS diffusion during the processing. CdTe solar cells make the use of borosilicate glass for high-temperature deposition, around 600oC, and soda-lime glass for low-temperature deposition, 60-500oC. CdTe has also been deposited on thin metallic foils like stainless steel, Ni and Mo. Out of these, Mo is most compatible for CdTe deposition, because of better thermal matching. At high temperatures, during the process of deposition, the possibility of oxide layer formation on the metallic substrate exists, and this can create a barrier for electrical conduction (Shah et al. 1999). Amorphous and nanocrystalline Si solar cells can also be fabricated on a various metallic and non-metallic substrates, for example, p–i–n cells are generally fabricated with glass substrate, superstrate configuration. On the other hand, n–i–p cells are usually grown on metallic substrates, substrate configuration. Substrate properties are known to determine the structural properties of the microstalline Si cells. In this context, large substrate area deposition techniques have displayed the capabilities of thin-film deposition techniques in this regard. Techniques like roll-to-roll deposition on stainless steel and glass-in-module-out technologies for a-Si solar cells have already been started in production (Goetzberger & Hebling 2000).10 . 4.3 Transparent conducting oxide: Transparent conducting oxides(TCO) usually are n-type degenerate semiconductors with high transparency and good electrical conductivity in the visible spectrum. Therefore, a low-resistance contact to the device and transmission of most of the incident light to the absorber layer is assured. Carrier concentration and mobility determine the conductivity of a TCO. An increase in the carrier concentration can result in enhanced free carrier absorption hence the transparency of the TCO in the higher-wavelength region is reduced. Thus, increasing the mobility by improving crystalline properties is considered to be the best practice for a good TCO. Along with these optoelectronic properties, the mechanical, thermal, chemical, and plasma-exposure stability and passivity of TCOs play an important role (Kay & Grätzel 1996, McEvoy et al. 1998). . Various studies have demonstrated that only ZnO-based TCOs can withstand H-bearing plasma and are also stable up to 800K. Thus, ZnO-based materials are now more commonly used in TFSC technologies. Bilayer makes it possible to make use of differing properties of two TCOs. High-efficiency CIGS and CdTe devices are usually developed with these bilayer structures which consists of a highly conducting layer for low-resistance contact and lateral current collection, and a much thinner high-resistivity layer, called HR layer by CdTe groups and buffer layer by CIGS groups, of a suitable material, to minimize forward current through pinholes in the window layer. with the inclusion of a 50-nm-thick resistive SnO2, In2O3, ZnO, or Zn2SnO4 layer, the CdS layer thickness can be reduced to less than 20 nm, which greatlly improves the blue response of the CdTe devices. Also, to improve the CdS film morphology by providing large grains during chemical bath deposition, the presence of the smoother high-resistive layer is used. Bilayer ZnO is used in CIS solar cells on the front side for substrate configuration (Chmiel et al. 1998) . By controlling the microstructure, textured single- and double-layer TCOs can be deposited and are used in a- Si solar cells to improve the scattering-assisted light absorption. The texture angle plays a major role in light trapping and internal reflections. If the angle is increased, the angle causes better internal trapping in the i-layer, but also higher SnO2/a-Si reflection losses, as well as SnO2 and metal absorption losses. 11 4.4 Window layer: The most important function of a window layer in a heterojunction is to form a junction with the absorber layer while permitting the maximum amount of light to the junction region and absorber layer. No photocurrent generation occurs in the window layer. For high optical throughput, with minimal resistive loss, the bandgap of the window layer should be kept as high as possible. It should also be kept in mind that any potential ‘spike’ in the conduction band at the heterojuction be minimized for optimal minority carrier transport. Lattice mismatch at the junction is critical for epitaxial or highly oriented layers. For microcrystalline layers, mismatch varies spatially. Thus, the complicated effect, if present, averages out (Shah et al. 1999). . The CIGS solar cells generally use a CdS window layer. This layer is deposited by a chemical bath deposition (CBD) technique, which results in a superior device performance as compared with that deposited by a physical vapour deposition (PVD) technique. This is partially because of the improvement in the interface chemistry between CIGS and CdS during the chemical processes. The chemical bath is advantageous as it removes the natural oxides from the CIGS film surface and it also allows Cd to diffuse into the Cu-poor surface layer of the CIGS films. Also, it is possible to use thinner layers of CdS as CBD deposition provides good surface coverage of the rough polycrystalline CIGS surface (even at a film thickness of 10 nm) (Kazmerski 1997). . When compared to near perfect lattice match between CIGS (112) and CdS (001), the lattice mismatch is 9.7% between CdTe (111) and CdS (001). Even after considering the large lattice mismatch, CdS remains the best hetero junction partner for CdTe. This is because high-efficiency devices with reduced lattice mismatch can be developed by forming an interfacial CdS & Te alloy layer, also because the role of mismatch in a sub micrometer-grained polycrystalline films may not be important. The blue response is reduced due to the relatively low bandgap of CdS as a window layer, but the effect is mitigated in both CdTe and CIGS devices by utilizing thinner CdS films. To maximize the blue response in CdTe devices, it is essential to utilize the thinnest possible CdS layer in conjunction with a bilayer TCO, which is done to ensure uniformly low dark current (Goetzberger & Hebling 2000). . In another alternative approach, improved optical transmission is achieved by making use of a wide-bandgap semiconductor alloy of higher resistivity as window layer. x can be changed continuously from CdS to ZnS by changing the chemical precursors used for CBD film deposition (Shah et al. 1999).12 Therefore, the optical transmission, thickness and film resistivity can be optimized to enhance the device output. In devices which are based on I–III–VI films, there are several alternative window layers which currently being studied to replace CdS, as there is concern about the toxicty of Cd. It is also being done to improve the blue response in devices. To integrate the heterojunction formation into in-line fabrication process efforts are being made to replace the CBD process by a PVD process. In case of a-Si solar cells, depending on device configuration, the n- or p-layer is very thin and acts like a window layer that permits all the photons to be transmitted to the i-region. As these films are higly absorbent, a very thin doped layer (10 nm) is required. Alloy films having good photoconductivity and excellent optical transparency have been used as the window layers (Zweibel 2000) . 4.5 Cadmium Telluride: Due to its optoelectronic and chemical properties, Cadmium telluride (CdTe) is a perfect absorber material for high-efficiency, low cost thin film polycrystalline solar cells. It is a direct bandgap material with an energy gap of 1.5 eV. It has an absorption coefficient of 105­­/cm in the visible region, which means that a layer thickness of a few micrometers is sufficient to absorb 90% of the incident phtons. Since most cases are the high temperature deposition the films are deposited with Cd deficiency, this gives rise to p-type conductivity. Owing to the high ionicity (72%) of CdTe, the crystallite formed are well-passivated. Also, the strong chemical bonding (5.75 eV) results in high chemical and thermal stability. CdTe solar cell devices have shown to be exceptionally tolerant to the deposition methods, as devices with efficiency greater than 10% have been fabricated by several deposition techniques. Out of these techniques, close-spaced sublimation (CSS), PVD, electrodeposition and screen-printing have been appropriately scaled to result in large area modules. The solar cells based on CdS/CdTe junction have resulted in an efficiency of 16.5% in small areas, as compared with the theoretical maximum efficiency of 29%. In the last decade, the efficiency has only increased from 15.8% to 16.5% (Kalyanasundaram and Grätzel (1998). The reason behind this slow progress seems to be a lack of R&D efforts on such issues like the requirement of activation treatment that changes the interfacial, bulk and grain boundary properties, and the difficulty of forming an ohmic contact without reducing the quality of device. The issue related to the environmental problem of Cd raised primarily by European nations, despite their abundant use of Cd in batteries, needs to be resolved without further delay, and is addressed in the literature (Green 2001). 5. Recent progress on CdTe/CdS Thin Film Solar Cells: 5.1 The Back Contact: Recently, researchers made a back contact on CdTe with a material containing Cu, like Cu–Au alloy, Cu2Te, ZnTe:Cu and Cu2S). It is known that Cu isessential to make an ohmic contact on CdTe since CdS/CdTe solar cells made with contacts not containing Cu have a higher series resistance. It is probable that the series resistance is not a result of the contact but it comes from CdTe which is more conducting at the interface than in the bulk. The higher conductivity, close to the interface, can be a result of the fact that CdTe mixes with CdS and its gap decreases. Still, in useful cells Cu should not be used since in the long run as it segregates at grain boundaries resulting in degradation of solar cell. It should be noted that the highest efficiency solar cell achieved so far has been done with some copper at the back contact. Copper, by diffusing into CdTe, lowers its resistivity and thus, for a while it gives a higher efficiency.13 5.2 The Cadmium Chloride Treatment: This Cadmium chloride (CdCl2) treatment is essential irrespective of the technique used to deposit CdTe. If this technique is not performed, the short circuit current of the solar cell will be very low and also, the efficiency is very low. This treatment consists of depositing 300–400 nm of CdCl2 on top of CdTe with a subsequent annealing at 400oC for 15–20 min in air or in an inert gas like Argon. During this process the sulphur(S) diffusion on CdTe grains are put in vapour phase which recrystallize resulting in a better-organized CdTe matrix, and following the reaction shown below: CdTe(S) + CdCl2(S) → CdCl2(S) + CdTe(S) can be deposited easily by sputtering, also it can be deposited by vacuum evaporation. It has also been verified that Sb2Te3 makes an ohmic contact with CdTe thin films. Researchers have also found out that Sb2Te3 makes an ohmic contact on a p-type CdTe single crystal whose resistivity is on the order of 107Ωm. With Sb2Te3 as a back contact, CdS/CdTe thin film solar cells were obtained with a maximum efficiency of 15.8%. These cells are heated up to 200oC under 20 suns without degrading, but they generally show a higher efficiency after the 200oC treatment (Kazmerski 1997). . 6. Other Nanostructures with Potential Application in Solar Cells: Nowadays nanostructured materials are used in combination with the solar cells for many domestic and large scale applications. It includes many possible advantages such as independent of any economic crisis regarding the environment in all aspects.14 It also causes no environmental pollution, renewable source of energy and growth in industries and economy since low maintenance cost and labor. The variety of potential applications that the nanostructures are being used are stated below, TiO2 Nanoparticle Aggregates TiO2Nanorods TiO2 Nanotube array ZnO Nanorod array TiO2-B Nanorods ZnO Nanotubes Figure 7: This is a picture of nano tubes contains nano walls. Picture taken from (Source: http://depts.washington.edu/solgel) 6.1 Nano Tubes and Nano walls: As shown in the figure 7 the nanotubes are formed by using various nano structured CdS, CdTe , TiO2 and ZnO. The nano walls as shown in the diagram helps in the reflection of the light and the nano clusters perform the loop result in the efficient production of output. 7. Materials choices to achieve absorption at lower cost: First choice used would be a polymer / nanoparticle/ supramolecular structures as they achieve absorption with less material and provide the same amount of result. Next best are thin films of direct gap materials. Silicon is the most inefficient and should be least to use. Broaden absorption bands of polymer/nanoparticle/supra-molecular structures helps in the improved result Improve durability in oreder to attain less implementation cost Control nanostructure also helps in the reduction of initial cost applied. Other managemental issues such as minimizing the reflection, maximizing internal reflection, use plasmonics to concentrate light at hetero junction interfaces would also attain expecting result. 6. Conclusion: The performance of all thin-film solar cells which are being investigated currently is improving at a steady pace because of the increasingly better understanding of the unique and wide range of chemical, structural and optoelectronic characteristics of thin-film materials. Also, due to the effective exploitation of some properties for such functions as passivation, activation, photon scattering/recycling, generation of surface electric fields, graded bandgaps, etc. In future, more improvement is desired by closing the gap between the achieved efficiencies and the theoretically expected ones, with deeper understanding of the electronic role of interfaces in the layered structure of the devices and with more accurately audited view of photons and excited carriers in the device.in addition to this "multijunction" cells also called "cascade" or "tandem" cells are being developed and in currently being experimenting in many practical applications. Multijunction devices can achieve a higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity. Even after the great effort for developing a rigid, reliable and stable ohmic contact, the results are not satisfactory yet which is one of the major shortcomings for large scale industrialization of CdS/CdTe solar cell modules. A combination of researchers with a background in different areas is evidently needed for a successful basic research with the DSSC.The better usage of other measurement equipment should of course be decided on he basis the research problem and the availability of suitable equipment in the cooperating laboratories. Many of the techniques described above have been developed in order to achieve high efficiency CdS/CdTe solar cells. In this regard, the team lead by Wu in the NREL in USA, has achieved the highest efficiency for these kinds of solar cells. References 1) G.M. 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