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The New Independent Home
by Michael Potts
from chapter 3 :
Harvesting Sunshine: Photovoltaic Cells |
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In the 1950s, when the photoelectric effect was explored, it was an expensive laboratory curiosity, yet scientists saw that its promise, a local, lasting, and trouble-free supply of electricity, was very important. The world's finest technology was applied to put this scientific breakthrough to use, with a clear objective: develop a reliable energy source (1) that liberates as many electrons as possible (2) using the cheapest and most abundant materials (3) manufactured and operating in a nonpolluting way. This last consideration was new at the time; humanity was just beginning to admit that its works were having unintended effects. The manufacturing problem for semiconductors, the stuff of photovoltaics, is not trivial because their processing, purification, and assembly uses toxic chemicals and prodigious amounts of energy. Semiconductors produced for electronics are noteworthy principally for their smallness. This parallel development made semiconductor use in photovoltaics affordable, but PVs harvest electricity from sunlight in direct proportion to their size, and so bigger is better. Materials scientists have dramatically decreased the costs and increased efficiency at every step in the production of silicon-based devices, growing purer and larger boules in pressurized ovens, slicing them with ever-thinner diamond saws so less material is lost to the saw kerf, doping them with other materials, and devising robots for assembling them into finished cells. Amorphous and multicrystalline cells are now grown in thin sheets, eliminating the boules and wasteful slicing; this process continues to allow decreases in cost per watt. Other materials besides silicon, and many doping and fabrication strategies, are being evaluated, but there appear to be no profound breakthroughs close at hand. Costs will continue to decrease because of the technological learning curve, as materials and techniques are fine-tuned, and modules are produced in ever-larger volumes. Photovoltaic technology is mature and ready for deployment.
Photons falling on a photovoltaic cell can make electrons flow. In this schematic, the first and third photons strike atoms in the silicon lattice, liberating outer valence-band electrons and corresponding "holes" that begin their random walks down the electrical gradient created by the dopant in the semiconductor. The electrons in this semiconductor migrate toward the collection grid on the cell's surface while the holes move toward the backplane.
When electrons reach the collector, electricity is conducted through a wire to charge the battery. When the electrons get all the way back around to the backplane, they reunite with their long-lost holes, completing their circuit. A majority of incoming photons do not liberate electrons, but dissipate their energy as heat within the lattice or against the backplane. |
sidebar:
The Photovoltaic Effect
By far the most magical energy source uses the photovoltaic effect to turn light directly into electricity.
Billions of inbound photons (light) enter the cell past a conductive collecting grid. Somewhere in the thin cell, each photon gives up its energy. In the diagram, the leftmost photon sails through the whole cell and encounters the opaque backplane, where it gives up its energy as heat. Photons two and four make it part way through, before whacking one of the electrons in the outermost layer (the valence band) of the silicon atoms with sufficient force to liberate an electron-hole pair into the silicon lattice. Photon three hits an impurity or silicon nucleus and also dissipates as heat.
The liberated electrons, encouraged by a charge gradient created by doping the upper part of the silicon lattice with a rare earth such as phosphorus, wander from silicon atom to silicon atom generally "downhill" (for them, due to the charge gradient) away from the positive doping of the p-type silicon, toward the conductor on the cell's face, where they flow along the circuit to the battery where they are stored. The holes (spaces in the valence band "wanting" an electron) do the same random walk in the opposite direction 'downhill' away from the negatively-doped n-type silicon.
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