BaKhabar, January 2011, Page-6

better light a candle than curse the darkness

Power of Sun ... a scientific outlook

... By Shahzada Ahmad*

Currently, renewable sources of energy are mainly considered to be water (hydroelectric, wave, tidal), wind, biomasses and earth (geothermal) and to some extent sun (thermal and photovoltaics). Globally, fossil fuels currently account for more than 80% of energy demand with the remainder met by a mixture of nuclear (6%), hydro (2%) and other renewable energy sources (12%) including biomass combustion which represents two thirds of the latter developing sustainable energy systems. Energy, is much more important today than at any time in human history and directly or indirectly it is playing a role to make middle east politically unstable for one after another reason.

It is projected that the world population will rise to 10 billion by 2050 and to cater the societal need and to build the required infrastructure and industrialization there will a huge energy gap (in Terawatt) which can not be fulfilled by current petroleum resources. The major economies of the world are investing money in the research and development of renewable energy sources and are working to obtain 20% of all energy from renewable sources by the year 2020. This can be realized through a number of ways; by generating energy from solar cells and additionally saving 20% of energy through improvements in energy efficiency lighting (with the use of compact fluorescent lamp, organic light emitting diodes). In the present scenario Sun is the answer of our energy crisis. Sun rays can be utilized to generate electricity without any possible carbon dioxide emission and is an ideal way to utilize the renewable energy. Currently most of the commercial solar cells are made up of Silicon which was invented almost five decades back, is by far the most widely used material and has a strong dominance in the market of solar cells. Despite significant developments over the past decades, from the government in the form of subsidy the high cost of Si based solar cells was major hindrance for the implementation of solar electricity on a large scale. This aroused interest to many researchers worldwide in both academics and industry. The developments of new materials which are more efficient and less expensive for the production of solar cells become a topic of interest to researchers. The perfect example comes from Mother Nature in which plants make their food by photosynthesis for last 2 million years. By using some chemicals this process can be mimicked in the lab to perform artificial photosynthesis and produce energy from light. This was realized almost two decade back by Prof. Micheal Graetzel and his team who works at Ecole Polytechnique Federale de Lausanne, Switzerland where he invented this process.

Prof. M.K. Nazeeruddin showing the solar bag on which solar panels are on the front to charge cell phone/laptop (left), and self charging torch to use in night and inset the back view (right) showing lithium battery which charge itself from sun light using front solar panels.

Dye-sensitized solar cells are now being also called Graetzel cells use organic dyes on titanium dioxide to capture sunlight. The use of nano-materials could potentially lead to the realization of highly efficient and low-cost solar cells. Silicon based solar cells, which hold highest power conversion efficiencies, also represents the highest market share in terms of solar cells which are being produced, are regarded as first generation solar cells, suffer from high pay back time (read high cost) of >20 years, while the new generation solar cells which are either made of dye sensitized solar cells or polymer based bulk heterojunction are being popular due to there flexibility, light weight and low pay back time of less than 1 year ( read low cost). Mohammad Khaja Nazeeruddin (Senior Scientist at Switzerland and World Class University Professor at Korea University), who is working on this project with Graetzel for last 23 years and an expert of this field, says that dye sensitized solar cells will be a cheap and green alternative for energy production in coming years, without costing the environment. Dye sensitized solar cells consists of porous network of nanometer sized semiconductor particles, on which a thin layer of dye, chemically adsorbed that absorb the incident light and generate the excitons. "The basic principle is like photosynthesis", In light, the dye –adsorbed titanium dioxide injects an electron into the conduction band. This electron flows through the external load to the counter electrode. The oxidized dye is regenerated by electron donation from electrolytes, while at the counter electrode, ions are regenerated through the donation of an electron from the external circuit" and thus electrical circuit gets closed.

Titanium dioxide is white particle which we use in our everyday life in toothpaste, paints or in making a piece of paper whiter. But here the same titanium dioxide is being used as semiconductor particles. Titanium dioxide films can be made by different film forming techniques on conducting substrates which can be plastics or glass, and then it is dipped in the dye solution to absorb. Then second piece of conducting substrate (counter electrode) which has a small hole in it to fill electrolytes is put on top of the dye coated titanium dioxide films and clipped together and the device is ready to work.
These solar cells have major potential as a significant contributor to the diverse portfolio of future energy technologies and energy is a vital part of our daily lives. Graetzel cells are likely to have an important role in low-cost, large-scale solutions for renewable energy production to produce electricity-generating windows and low-cost solar panels. Learning from the concepts used by green plants, we have been able to develop a solar cell that shows a light-to-electricity conversion of 11.3% which is still lower than silicon based solar cells (>18%) but has many benefits which can not be dreamt of by using silicon based solar cells. These solar cells have recently been launched in consumer products, including battery-charging backpacks and night lights (See image). These new type of solar cells can work under cloudy or rainy days with the same efficiency and will be a boon for remote villages in India and Africa which are still not connected to the power supply or where there is power crisis. Due to the abundance of sun rays in Africa, European Union is working on mega project to install the solar grids in Africa and transfer the power to Europe by connecting with grids, making Africa next economy. These solar cells will also play an important role in tackling climate change and holds the key to the future development of clean energy conversion and storage devices essential to mitigate CO2 emissions and hence global warming.

*The author is Scientist at Max Planck Institute, Germany and currently works in the area of Materials Science for energy applications. His email ID: shahzada80@gmail.com