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Autorin: Hyun-Joo Kang

THE HOLY GRAIL FOR RESEARCHERS IN THE FIELD OF SOLAR PHOTOVOLTAIC (SPV) ELECTRICITY IS TO GENERATE POWER AT A LOWER COST THAN THAT OF GRID ELECTRICITY. REACHING THIS GOAL WOULD FREE US FROM OUR DEPENDENCE ON CHEAP, BUT HIGHLY POLLUTING FOSSIL FUELS. Lower electricity costs are, unavoidably coupled with higher efficiency. They say «every cloud has a silver lining». With the hefty rise in electricity and gas prices, that lining is solar power. Nevertheless, tapping into this voluminous energy reservoir proves a colossal challenge, since even though sunlight is a ubiquitous form of energy, it is not as of yet an economic one. Many researchers propose that by understanding nature’s inherent processes and implementing them as a model, we could greatly advance the construction and efficiency of developing technologies.

Robotics researchers have incessantly been seeking for the ideal conditions where robots would act flexibly, smoothly and autonomously. They found their inspiration in nature, in particular, human cognitive mechanisms, learning, recognition of others and social behaviors. Likewise, solar researchers should be marching along with photosynthesis as their lodestar in order to manufacture better solar cells. Energy usage is the transformation of energy from one form to another. The burning of wood, coal, and even oil and gas, nuclear fission, wind drifts, water tides and torrents, chemical decomposition of organic materials, geothermal energy from inner Earth and solar radiation all render energy. No- netheless, it is solar power that is receiving intense inspection as a means of producing electricity, thanks to its many unique properties, such as a renewable, decentralized energy source, relatively independent of location (e.g. off-grid), available in vast quantities and, in addition, exposing no pollution. Every year the earth‘s surface receives about 10 times as much energy from sunlight as is contained in all the known reserves of coal, oil, natural gas and uranium combined. This energy equals 15 000 times the world‘s annual consumption by humans. By harnessing this heat, we would be able to solve all our energy problems. Furthermore, solar energy has been used by nature for centuries to facilitate photosynthesis, which is an essential source of energy for the food we eat. Hydroelectric power depends on the sun to evaporate water that later falls as rain to fill the reservoir. Wind power depends on convection currents produced by the sun heating the atmosphere. Even fossil fuels are remnants of plants that grew by photosynthesis with light from the sun.

In solar cells, sunlight can be converted directly into direct current electricity by photovoltaic conversion, hence the name «photovoltaic» cells. The traditional solar cell, developed in 1954 at the Bell Laboratories is a semiconductor (usually crystalline silicon). When light with photon energies greater than the band gap shines on it, the electrons in the material are able to absorb the photons (Photoelectric Effect). In turn the electrons overcome the band gap, jump from the low-energy valence band to a higher-energy conduction band, and flow freely as a steady current. Commercial silicon cells have efficiencies ranging from 15% to 20%. In the laboratory, the highest efficiency attained until now was 42.8%. The figure for non-traditional cells is far lower. A typical cell based on electrically conductive plastic has an efficiency of a mere 3% to 4%.
The traditional solar cell has several disadvantages. First, there exists an upper limit on the efficiency of silicon solar cells. Second, sunshine is free, but converting it into electricity is not. There have been a number of different approaches to reduce costs over the last decade, notably the thin-film approaches, but to date there has been limited application due to a variety of practical problems. Even if manufacturing costs decline over time, many researchers are convinced that only within three to eight years will the price of solar power be cost-competitive with electricity from the grid.

In recent years, a new class of low-cost solar cells has gradually emerged. Grätzel cell, Dye-Sensitized Solar Cell (DSSC) and Dye Sensitized Nanocrystalline Solar Cell (DYSC) all encompass the term for the invention by Michael Grätzel and Brian O‘Regan at the École Polytechnique Fédérale de Lausanne in 1991. These cells are highly promising because they are made of low-cost materials and do not require elaborate apparatus to manufacture. Although their conversion efficiency is less than the best traditional solar cells, their price/watt ratio should be high enough to allow them to compete with the fossil fuel electrical generation in Europe. These cells use dye-adsorbed highly porous nanocrystalline titanium oxide (nc-TiO 2 ) to produce electricity. The interaction of sunlight with various organic dyes produces solar power in a way that is perhaps more similar to photosynthesis than to the semiconductor processes in normal solar cells. These cells are more environmentally-friendly than silicon-based models, since titanium dioxide is a plentiful, renewable and non-toxic mineral; already used in consumer products such as toothpaste, white paint and cosmetics. In the long run, nanotechnology-based solar cells, using quantum dots, should be able to reach higher efficiency levels than conventional solar cells.
The development of these new types of solar cells is promoted by increasing public awareness that the Earth‘s oil reserves could deplete during this century. Yet, despite the growing infusion of capital at a torrid pace, solar power provides only a tiny fraction of the world‘s electricity needs today and will not be able to supply more than 1% of its needs for at least another decade. Instead of focusing on traditional solar cells, further investments and research should be directed towards emerging solar cell technologies.

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