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allotropes, why not use carbon in solar cells? This is how carbon nanotubes (a special form of carbon) came to be considered for tapping solar energy. Before studying about carbon nanotubes, we need to know what n a n o t u b e s a r e . Nanotubes are particles that are on the nanoscale, which is are about 1 to 100 nanometres (nm) in size. For comparison, the thickness of a sheet of paper is about 100,000 nm and the width of a hair, about 40,000 – 80,000 nm. Materials that exhibit a dimension below 100 nm have very different and interesting properties than the bulk material. As an example, gold is a very inert metal, but below 100 nm, nanoparticles of gold possess properties that m a k e t h e m g o o d catalysts and sensors. With this in mind, let us s e e w h a t c a r b o n nanotubes are. Carbon nanotubes (CNTs) are allotropes of carbon with a c y l i n d r i c a l nanostructure. These c y l i n d r i c a l c a r b o n molecules have unusual properties, which are v a l u a b l e f o r n a n o t e c h n o l o g y , electronics, optics and other fields of materials science and technology. So, Why use CNTs in Solar Cells? Any solar cell should have good photovoltaic properties to provide maximum number of electron-hole pairs for a g i v e n i n t e n s i t y o f incident light. The cell should be as strong as possible to improve its life and should work at maximum possible efficiency. Above all, it should be cheap. Well, CNTs have many such desirable properties that make it a potential photovoltaic material in the coming years. When light falls on a solar cell, electrons are generated and transferred to the external circuit via metallic conductors and connectors. If the photoreception part of the cell contains CNTs, a lot of positive differences can be observed. This is due t o t h e e l e c t r i c a l properties of CNTs. They are hollow cylinders and hence electrons can only flow over its surface c o m p a r e d t o t h e conventional wires. Due to this reason, CNTs offers lesser resistance to the flow of current and can help reduce losses. As for light harvesting p r o p e r t i e s , c a r b o n nanotubes possess a wide range of direct bandgaps m a t c h i n g t h e s o l a r spectrum and its strong photoabsorption from infrared to ultraviolet, allows it to absorb light belonging to different f r e q u e n c i e s a n d wavelength. It has high carrier mobility and reduced carrier transport scattering which allows C N T s t o t r a n s f e r maximum current to the external load as much as possible with minimum loss of electrons within the bulk of the cell. What good can a solar cell do if it can’t protect itself from mechanical s t r e s s e s ? C a r b o n nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. Recently, SWNTs were directly configured as e n e r g y c o n v e r s i o n materials to fabricate thin-film solar cells, with nanotubes serving as both photogeneration sites and a charge carriers collecting/transport layer. The solar cells consist of a semitransparent thin f i l m o f n a n o t u b e s conformally coated on an n-type crystalline silicon substrate to create highd e n s i t y p - n heterojunctions between nanotubes and n-Si to favor charge separation and extract electrons (through n-Si) and holes (through nanotubes). Initial tests have shown a p o w e r c o n v e r s i o n efficiency of >1\%, proving that CNTs-on-Si is a potentially suitable configuration for making solar cells. For the first t i m e , Z h o n g r u i L i demonstrated that SOCl2 treatment of SWNT b o o s t s t h e p o w e r conversion efficiency of S W N T / n - S i heterojunction solar cells by more than 60%. Later on the acid doping a p p r o a c h i s w i d e l y adopted in the later published CNT/Si works. Even higher efficiency can be achieved if acid liquid is kept inside the void space of nanotube network. Acid infiltration of nanotube networks significantly boosts the cell efficiency to 13.8%, as reported by Yi Jia, by reducing the internal resistance that improves f i l l f a c t o r, a n d b y f o r m i n g photoelectrochemical units that enhance charge separation and transport. The wet acid induced problems can be avoided by using aligned CNT
film. In aligned CNT f i l m , t h e t r a n s p o r t distance is shortened, and the exciton quenching rate is also reduced. Additionally aligned nanotube film has much smaller void space, and b e t t e r c o n t a c t w i t h substrate. So, plus strong a c i d d o p i n g , u s i n g aligned single wall carbon nanotube film can further improve power conversion efficiency (a r e c o r d - h i g h p o w e r- conversion-efficiency of >11\% was achieved by Yeonwoong Jung). *** ARTIFICIAL LEAF: SOLAR CELL THAT PRODUCES HYDROCARBON M r. H . R AV I S A N KA R MENON S8 EEE Researchers have developed a new type of solar cell that is capable of transforming carbon dioxide into usable hydrocarbon fuel using only sunlight as energy. C o m p a r e d w i t h conventional solar cells that convert sunlight into electricity to be stored in batteries, the new solar c e l l s c h e a p l y a n d efficiently convert carbon d i o x i d e i n t h e atmosphere directly into usable fuel. The new device works much like trees and plants that capture and convert carbon dioxide into sugars to store them into energy. Unlike plants that use catalysts to p r o d u c e s u g a r, t h e r e s e a r c h e r s u s e d n a n o f l a k e t u n g s t e n diselenide catalyst to convert carbon dioxide to carbon monoxide. o f o t h e r k i n d s o f hydrocarbons such as oil, gasoline and coal. A solar farm of these soc a l l e d a r t i f i c i a l leaves can also remove significant amounts of the greenhouse gas carbon dioxide in the atmosphere known to significantly drive global warming. In plants, the process of c o n v e r t i n g c a r b o n d i o x i d e i n t o s u g a r involves the organic catalyst enzyme. Carbon monoxide is also a p l a n e t - w a r m i n g greenhouse gas but scientists have already found a way to convert it into usable fuel such as methanol. Converting carbon dioxide into something usable, on the other hand, is more challenging because of it b e i n g r e l a t i v e l y chemically unreactive. packets of light, into p a i r s o f n e g a t i v e l y charged electrons and positively charged holes that separate from each other. When the holes r e a c t w i t h w a t e r molecules, protons and oxygen molecules are created. Along with carbon dioxide, the protons and electrons then react together to p r o d u c e c a r b o n monoxide and water. *** The new solar cells produce usable energydense fuel, which could help address challenges linked with the burning The system involves a reaction very much similar to that found in nature. The artificial leaf converts photons, or
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