Quantum Dots In Solar Cells

High Efficient Solar Cells A. Kathalingam and Mi-Ra Kim Millimeter-wave INnovation Technology Research Center Dongguk University, Seoul, Korea

The world’s energy demand is projected more, expected to double by 2050 and to more than triple by the end of the century. Considering the devastating and environmental polluting nature of conventional fossil fuels, sunlight becomes a “compelling solution” to the need of clean and cheap energy. It poses no threat to our environment through pollution or to our climate through greenhouse gas emissions. Sunlight is plenty on earth, unimaginable amount of energy from sun strikes the Earth. The sun light falls on the earth in one hour is more than the energy we consume in a year through out the globe. The vast majority of solar panels today are made of silicon. These devices are called first generation, and make for highly stable and efficient solar cells, but, because of the material processing necessary, it is expensive to make first generation solar cells and levels of efficiency in electricity production range from around 10 to 20 percent. A more recent alternative involves constructing solar cells using thin films with the potential to produce solar energy at a reduced cost. These thin film cells are called second generation, and are cheaper, but they have more difficulty absorbing radiation and are not very efficient. Scientists have been seeking a third generation - a low cost semiconductor material that would have a tunable bandgap, allowing the manufacturer to control the absorptive properties of the solar cell. Present semiconductor PV devices based on a single-bandgap absorber, which have a theoretical thermodynamic conversion efficiency of 32% in unconcentrated sunlight. However, multiple-bandgap absorbers in a cascaded junction configuration can result in high photoconversion efficiencies, particularly when cells are designed to sustain the operating conditions (e.g., elevated temperatures) associated with highly concentrated sunlight. A major challenge in the conversion of solar energy is tapping the full spectrum of solar radiation. The absorbing materials used in today’s solar cells capture only a fraction of the wavelengths in sunlight. So, it is must to designing composite materials that absorb full spectrum of solar light spectrum and convert efficiently to useful form of fuel and to generate electricity, it would be a crosscutting breakthrough in this area. Advancements in science have made a revolutionary change in the concept of the solar cell materials and design. Increasing the efficiency of solar cells more than 50% is not a science fiction. It is very easy to increase the solar cell efficiency more the 50%, is not a fiction, one day in near future we can use such a solar cell. Today's fast progress on the scientific frontiers of nanotechnology and biotechnology will provide a strong foundation for future breakthroughs in solar energy conversion.

New technologies are coming up and these will materialize in the near future. Thin layers of materials consisting of a network of interpenetrating regions can facilitate effective charge separation, conduction and collection. Quantum dots come into prey and it could find to fill the need. Nanostructured materials such as quantum dots, quantum wires and tubes incorporated nanocomposite materials could improve the performance of the solar cell. Only bridging the technologies available with the viable use of solar energy method is the need of the day. Bridging this gap requires revolutionary breakthroughs that come only from basic research to understand the fundamental principles of solar energy conversion and develop new materials with efficient conversion ability. Array of QD Quantum dots are semiconductor crystals typically between 1 and 10 nanometers in diameter, a nanometer being a billionth of a meter. Each quantum dot contains a tiny droplet of free electrons. Quantum dots offer tunable optical and electronic properties that can work around the natural limits of traditional semiconductors. They could be useful as the basis of new solar electric cells. Quantum dots are especially exciting for their tunable absorption wavelength, their quantum conversion efficiency above 100% through multiple-exciton generation, and their easy fabrication through self-assembly. Quantum dots can be made into flexible sheets, put into liquid form, or made to be transparent, and they cost relatively little compared with bulk silicon semiconductor material and thin films. In addition to electric energy, solar radiation can also be converted to heat energy using a feasible strategy, this can replace much of the heat now supplied by burning fossil fuels such as oil, gas and coal and thereby could possible to reduce green house gases. Material scientists are doing wonders, one day in near they could produce an ultimate material "smart materials" having capacity of self-repairing, storage capacity. One of the key problems in solar energy capturing and conversion is how to separate charge efficiently over macroscopic distances without using expensive, highly pure, semiconductor materials. This effort requires the development of new materials and methods.