NANO TECHNOLOGY IN POLYMER SOLAR CELLS Mayur Padharia, Hardik Panchal, Keval Shah, *Neha Patni, Shibu.G.Pillai Department of Chemical Engineering, Institute of Technology, Nirma University, S. G. Highway, Ahmedabad-382481, Gujarat, INDIA neha.patni@nirmauni.ac.in ABSTRACT Solar cells, which harvest energy directly from sunlight, may satisfy future energy requirements, but photovoltaic devices are currently too expensive. Polymer solar cell (PSC), on the other hand, are cheaper to produce than conventional inorganic solar cells. Indium tin oxide (ITO) is the material-of-choice for transparent conductors in any optoelectronic application. However, scarce resources of indium and high market demand of ITO have created large price fluctuations and future supply concerns. In this regard, replacing ITO has the potential to dramatically reduce material and processing cost and the energy payback time of PSCs. Finding an alternative for ITO is crucial for the successful commercialization of PSCs. These alternatives belong to four material groups: polymers; metal and polymer composites; metal nanowires and ultra-thin metal films; and carbon nanotubes and graphene. This paper discusses the rapid progress in processing of CNT, graphene, and nanowires and it is not too long before they are commercially utilized as transparent conductors for all optoelectronics in general and organic solar cells in particular. INTRODUCTION Ice sheet disintegration, sea level rise, and other adverse consequences of global warming, have recently emphasized our need for developing inexpensive alternative energy sources. The world's energy demand is expected to double within 40 years as the global population grows. More energy is incident upon the Earth in one hour than is consumed by the world s population in an entire year. Harvesting just a small fraction of this solar energy could provide the solution to our long term energy needs. Solar cells are a promising method of both capturing this energy, and converting it directly into electrical energy. [1] Solar cells based on silicon are currently the dominant photovoltaic technology as they are reliable and highly efficient at converting solar energy into electrical energy. However, the high cost of silicon, especially crystalline silicon which is the most effective material, has
limited the societal impact of solar cell technology and led to an interest in alternative materials.[1] Polymer Solar cells (PSC) have garnered increasing research interest over the last decade. Organic photovoltaic cells are typically produced from either small molecular- weight films or polymer films. They are built from thin films (typically 100 nm) of organic semiconductors including polymers, for example polyphenylene vinylene and small-molecule compounds like copper phthalocyanine, carbon fullerenes and derivatives of fullerene such as PCBM. Even though energy conversion efficiencies are low compared to inorganic materials, we can improve the performance by Nanostructures interfaces, sometimes in the form of bulk heterojunctions. [2] Indium tin oxide (ITO) The transparent conductor industry is dominated by a single material indium tin oxide (ITO). Indium tin oxide (ITO, or tin-doped indium oxide) is a solid solution of indium(iii) oxide (In2O3) and tin(iv) oxide (SnO2), typically 90% In2O3, 10% SnO2 by weight. In the infrared region of the spectrum it acts as a metal-like mirror. It is one of the most widely used transparent conducting oxides because of its two chief properties, its electrical conductivity and optical transparency, as well as the ease with which it can be deposited as a thin film. When used as a conductor, ITO is not very conductive, and as a transparent layer, it is not very transparent and ITO is generally difficult and expensive to apply as a thin film of sufficient quality. Apart from the volatility of indium prices, its incorporation in the processing of ITO requires high preparation temperatures and vacuum-based highly energy intensive deposition techniques such as sputtering, thus further increasing the cost of ITO. [3]
Figure1: Alternatives to ITO Its replacement with cheaper alternative would significantly reduce the cost per watt and energy payback time. The alternatives of ITO could be categorized into four broad material groups: (1) polymer; (2). metal; (3). a combination of polymer and metal; and (4) carbon nanotubes and graphene as shown in figure 1. [3] USE OF NANOTECHNOLOGY Nanotechnology ( nano ) incorporation into the films shows special promise to both enhance efficiency of solar energy conservation & reduce the manufacturing cost. Its efficiency would help preserve the environment, decrease soldiers carrying loads, provide electricity for rural areas, and have a wide array of commercial applications due to its wireless capabilities. In conventional solar cells, ultraviolet light is either filtered out or absorbed by the silicon and converted into potentially damaging heat, not electricity. Ultraviolet light could efficiently couple to correctly sized nanoparticles and produce electricity. To make the improved solar cells, the researchers began by first converting
bulk silicon into discrete, nano-sized particles. Depending on their size, the nanoparticles will fluoresce in distinct colors. [4] Another potential feature of these solar cells is that the nanorods could be tuned to absorb various wavelengths of light. This could significantly increase the efficiency of the solar cell because more of the incident light could be utilized. Nano-structured layers in thin film solar cells offer three important advantages. First, due to multiple reflections, the effective optical path for absorption is much larger than the actual film thickness. Second, light generated electrons and holes need to travel over a much shorter path and thus recombination losses are greatly reduced. As a result, the absorber layer thickness in nano-structured solar cells can be as thin as 150 nm instead of several micrometers in the traditional thin film solar cells. Third, the energy band gap of various layers can be tailored to the desired design value by varying the size of nanoparticles. This allows for more design flexibility in the absorber and window layers in the solar cells. [5] Dye-Sensitized Solar Cells (DSSC) A DSSC comprises a nanocrystalline TiO2 modified with a dye fabricated on transparent conducting oxide, a platinum counter electrode, and an electrolyte solution with a dissolved iodide ion/triiodide ion redox couple between the electrodes. TiO2 nanocrystalline electrode coated with CaCO3 increased both open circuit photovoltage and short-circuit photocurrent and produced cell efficiency of 10.2 %. In order to improve the conversion efficiency, the major research in third-generation PV cells is directed towards absorbing more sunlight using nanotechnology, for example nanotubes, quantum dots (QDs), and hot carrier solar cells. However, implementation of these nanotechnologies in the cell design is a real challenge.
Nanotubes and wires Nanotubes can be either metallic or semiconducting depending on their structures. There are two types of nanotubes, the single walled carbon nanotubes (SWNTs) and the multiwalled carbon nanotubes (MWNTs). SWNTs consist of a single graphite sheet wrapped into a cylindrical tube and MWNTs comprise an array of concentric cylinders. Singlewalled carbon nanotubes to a film made of titanium-dioxide nanoparticles, doubling the efficiency of converting ultraviolet light into electrons when compared with the performance of the nanoparticles alone. Without the carbon nanotubes, electrons generated when light is absorbed by titanium-oxide particles have to jump from particle to particle to reach an electrode. The carbon nanotubes "collect" the electrons and provide a more direct route to the electrode, improving the efficiency of the solar cells. The alignment of the CNT with the polymer composites substrate give very high efficiency in photovoltaic conversion. The nanometer-scale tubes, coated by the special p-type and n- type semiconductor (p/n) junction materials can be used to generate electrical current, which would increase the surface area available to produce electricity.with the increase in nanotube concentration from 0 to 20 wt %, the conductivity of the resulting film increases by six orders of magnitude due to the introduction of conducting paths to the polymer. To improve further, researchers at Indian Association for the Cultivation of Science have introduced functionalized multi-walled carbon nanotubes (MWNTs) in poly (3- hexylthiophene) (i.e.,p3ht) / buckminister fullerence (C60), donor/ acceptor-type photo voltaic devices. The devices were fabricated on indium tin oxide (ITO) coated glass substrate and on top of C60 layer, aluminum (Al) was vacuum evaporated. Dispersed random networks of metal nanowires (NW) can exhibit transmission and conductivity
even superior to ITO. This was first reported by Lee et al. who pioneered the work on solution processing of metal nanowires for application in organic solar cells.[6][7][8] Graphene With less than 0.1% reflectance and 2.3% absorbance for every single graphene sheet, the theoretical transmission limit of a single layer graphene sheet is 97.7%.Sheet resistance of graphene decreases rapidly with increasing stacking of graphene sheets and with doping, however, at the expense of optical transmission. Apart from sheet resistance and transmission, several other properties such as the high chemical and thermal stability, high charge carrier mobility, high current carrying capacity high stretch ability, and low contact resistance with organic materials renders graphene a very favorable alternative to ITO. CONCLUSION Use of nanotechnology into the films shows special promise to both enhance efficiency of solar energy conservation & reduce the manufacturing cost. Although the nanotechnology is only capable of supplying low power devices with sufficient energy, its implications on society would still be tremendous. CNTs, graphene, and metal nanowires have shown remarkable properties that even surpass the benchmark that ITO has set. REFERENCES 1. Buxton G., Robert Morris University, USA, Chapter 9, Nanotechnology and Polymer Solar Cells, 2012. 2. Sethi V.K., Pandey M., Shukla P., International Journal of Chemical Engineering and Applications, 2011. 3. Angmo D.,Krebs F. C., Flexible ITO-Free Polymer Solar Cells, December 2012. 4. Escolano, C., Pérez, J., Bax, L.. Roadmap Report on Thin films & coatings. Nanoroadmap (NRM) Project Working Paper, 2005. 5. Singha R., Rangarib V., Sanagapallia S., Jayaramana V., Mahendraa S., Singha V., Solar Energy Materials & Solar Cells, 82, 315-330. 2004. 6. Hecht D. S.; Hu L.; Irvin G., Advanced Materials, 2011. 7. Hatton R. A.; Miller A. J.; Silva S. R. P., Journal of Materials Chemistry, 2008. 8. Gruner G., Journal of Materials Chemistry, 2006.