![]() In addition to using cheaper materials, the scheme separates the absorption spectrum of the cell from the bandgap of the semiconductor, so the cell sensitivity is more easily tuned to match the solar spectrum. The hole left in the dye molecule recombines with an electron carried to it through an electrolyte from the counter electrode by an anion such as I −. The electron is then quickly transferred to the conduction band of a neighboring TiO 2 nanoparticle, and it drifts through an array of similar nanoparticles to the external electrode. The dye, analogous to the light-harvesting chlorophyll in green plants, captures a photon, which elevates one of its electrons to an excited state. 6 Expensive Si, which does both of those jobs in conventional cells, is replaced by a hybrid of chemical dye and the inexpensive wide-bandgap semiconductor titanium dioxide. Achieving high efficiency from inexpensive materials with so-called third-generation cells, indicated in figure 3, is the grand research challenge for making solar electricity dramatically more affordable.ĭye-sensitized solar cells, introduced by Michael Gätzel and coworkers in 1991, create a new paradigm for photon capture and charge transport in solar conversion. Thin-film cells offer advantages beyond cost, including pliability, as in figure 2, and potential integration with preexisting buildings and infrastructure. Cheaper solar cells can be made from other materials, 3 but they operate at significantly lower efficiency, as shown in the table above. The best single-crystal Si cells have achieved 25% efficiency in the laboratory and about 18% in commercial practice. Achieving the efficiency limit of 31% that they established for those conditions remains a research goal. 2 The analysis is based on four assumptions: a single p–n junction, one electron–hole pair excited per incoming photon, thermal relaxation of the electron–hole pair energy in excess of the bandgap, and illumination with unconcentrated sunlight. With their 1961 analysis of thermodynamic efficiency, William Shockley and Hans Queisser established a milestone reference point for the performance of solar cells. ![]() The utilization gap between solar energy's potential and our use of it can be overcome by raising the efficiency of the conversion processes, which are all well below their theoretical limits. It is anticipated that by the year 2030 the world demand for electricity will double and the demands for fuel and heat will increase by 60%. Solar heat provides 0.3% of the energy used for heating space and water. Solar fuel, in the form of biomass, accounts for approximately 11% of world fuel use, but the majority of that is harvested unsustainably. Solar electricity, at between 5 and 10 times the cost of electricity from fossil fuels, supplies just 0.015% of the world's electricity demand. Despite the enormous energy flux supplied by the Sun, the three conversion routes supply only a tiny fraction of our current and future energy needs. Solar photons convert naturally into three forms of energy-electricity, chemical fuel, and heat-that link seamlessly with existing energy chains. It dramatically exceeds the rate at which human civilization produces and uses energy, currently about 13 TW.įigure 1. The enormous power that the Sun continuously delivers to Earth, 1.2 × 10 5 terawatts, dwarfs every other energy source, renewable or nonrenewable. The amount of energy humans use annually, about 4.6 × 10 20 joules, is delivered to Earth by the Sun in one hour. Earth's ultimate recoverable resource of oil, estimated at 3 trillion barrels, contains 1.7 × 10 22 joules of energy, which the Sun supplies to Earth in 1.5 days. The San Francisco earthquake of 1906, with magnitude 7.8, released an estimated 10 17 joules of energy, the amount the Sun delivers to Earth in one second. The Sun provides Earth with a staggering amount of energy-enough to power the great oceanic and atmospheric currents, the cycle of evaporation and condensation that brings fresh water inland and drives river flow, and the typhoons, hurricanes, and tornadoes that so easily destroy the natural and built landscape.
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