Md. Shamim, Doctoral Student, Department of Chemistry, IIT Delhi

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   Nowadays, there are immense needs of energy in every field of life. Ever-growing energy need is one of the most important challenges of the 21st century. With the growing demand of energy the use of fossil fuels increases day by day. Due to this high consumption rate, they will run out of stock in near future. Moreover, burning of fossil fuels is also a threat for mankind, as it releases large amounts of carbon dioxide and sulfur dioxide in the environment. In a recent report of the environment cell of United Nations, if such pollution is not stopped, life itself will be wiped off the face of the planet by 2150. In this respect, solar energy is inded the best option. 120,000 terawatts of solar power irradiates Earth, out of which humans currentlt consume only 15 terawatt.
   Basically, a solar cell is a device that converts the energy of sunlight directly into electrical energy. The basic mechanism of all solar cells is similar - the first step is light absorption by the cell to generate excitation in the semiconductor material of the panel. The electron-hole pairs generated are inject into an energetically-favorable charge-separating junction, through a diffusion driven process. The photo-generated charge carriers are then transferred into the electrodes, after which the collected electrons dissipate their energy in the external circuit, returning to the solar cell, merging with the holes.
   Three generations of solar cells are mainly considered - a first generation cell mainly consists of crystalline p-n silicon, dominant in the commercial market. Due to the use of pure silicon, the manufacturing cost is high, but it is still the best commercial and stabilized solar cell availiable.
Second generation solar cells are thin-film cells of amorphous silicon, poly-crystalline silicon, cadmium telluride, copper indium gallium selenide etc. These solar cells are significantly less expensive due to reduced materials and processing costs and often have stability problems as well as the low efficiency.
Third generation solar cells are the solution processed solar cells. This technology comprises of dye sensitized solar cells (DSSCs), organic solar cells, solution-processed bulk inorganic photovoltaics, colloidal quantum dot solar cells etc. It is the most promising type of solar cell as it has very lower cost as well as its maximum theoretical efficiency is much higher (68% for multi-junction cell) as compared to the single junction silicone solar cell.

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