Saturday, August 11, 2012
New research out of Berkeley Lab and the University of California suggests that it is possible to make solar cells from any semiconductor, opening the door to solar panels made from cheaper, more abundant materials.
Until now, phosphides and sulfides of metals have been judged ill-suited for solar cells because of the near-impossibility of chemically doping them with the quality of p-n junctions required. The new approach, dubbed "screening-engineered field-effect photovoltaics" (SFPV), sidesteps the problem by inducing p-n junctions in semiconductors by applying an electric field.
The p-n junctions in a solar cell are the borders between regions of semiconductor with a surplus of charge carriers and those with a shortfall. In a regular solar cell, these positive and negative regions are created by doping the base semiconductor, silicon, with phosphorus and boron, the atoms of which have a surplus and deficit of valence electrons in their outer shells compared to silicon. The p-n junction is crucial to the flow of useful electric current when the cell is exposed to light.
The problem is that the doping process necessitates relatively expensive materials. "Solar technologies today face a cost-to-efficiency trade-off that has slowed widespread implementation," said Alex Zettl, whose Zettl Research Group at UC Berkeley and the Lawrence Berkeley National Laboratory produced the paper Screening-engineered Field-effect Solar Cells. Though the crucial dollar-per-watt metric for photovoltaics is increasingly competitive, the authors argue that its "cost-to-efficiency trade-off" is still a barrier to widespread adoption.
Generally, more readily available semiconductors cannot be chemically doped either because the dopant materials diffuse through the material causing p-n junctions to break down, or because the doping process degrades the semiconductor itself, which can unacceptably compromise the solar cell's efficiency.
Instead, SFPV solar cells exploit the electric field effect. Concentrations of charge carriers within the semiconductor are manipulated by exposure to an electric field. Sustaining the field requires minuscule amounts of energy to maintain compared to the energy generated by the cell when exposed to sunlight.
This principle isn't new, but the key breakthrough is the precise geometry of the top electrode, which gets around the problem of screening and allows p-n junctions to form. Ordinarily, charge carriers in the metal contact move to cancel out the electric field in the nearby region of semiconductor, effectively creating a screen preventing the contact from further affecting charge carriers in the semiconductor.
"The trick behind our new approach is to geometrically structure the top contact in a special way such that the applied electric field doesn't get completely screened out by the top contact and can 'dope' the semiconductor below the contact to create a continuous p-n junction over the whole cell area," lead author Will Regan of the Zettl Group at UC Berkeley told Ars. And in fact the researchers have identified two ways of achieving this.
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at 11:48 AM