Single-Crystal Silicon: Photovoltaic Applications
- PDF / 843,755 Bytes
- 3 Pages / 576 x 777.6 pts Page_size
- 15 Downloads / 231 Views
26
become the standard for the fabrication of commercial silicon cells, producing cells of energy conversion efficiencies up to 1415%. Since the early 1980s, there has been a sustained period of silicon cell development, with laboratory cell efficiencies now reaching above 23%.3 In recent years, some of this new technology has started entering commercial production, with pilot production of cells in the 17-18% range reported.4
Properties Relevant to Solar Use Silicon is a natural choice for photovoltaics. It is the second most abundant element in the earth's crust. Silicon's energy bandgap is nearly ideal for photovoltaic use. Moreover, silicon is relatively inert and biologically benign. The most general analysis shows that there are two optimum energy bandgaps for photovoltaic conversion.5 One lies at an energy of 1.15 electron volts (eV), which allows use of all sunlight above the energy of a strong atmospheric absorption band centered at about 1.1 eV The second optimum lies at 1.36 eV. Silicon's bandgap of 1.12 eV lies just below the first optimum. A disadvantage for photovoltaic use is that the bandgap in silicon is "indirect." To excite an electron from the valence band of silicon to the conduction band, not only must the energy of the electron be changed, but also its momentum. Since sunlight can supply the required energy but not the momentum, the momentum change must come from another source, namely lattice vibrations (phonons). Due to the need for both a photon and a phonon to be involved, the absorption of light in silicon is much weaker than in "direct" bandgap semiconductor material such as gallium arsenide. This disadvantage is offset to a certain
extent by the fact that the indirect bandgap also makes it much more difficult for the photo-excited carriers to relax back to their initial state. Recent trends in cell design, particularly those involving the increase of the effective thickness of the silicon by the trapping of light within the cell, offset this advantage to a large extent. However, material quality requirements in relation to carrier lifetimes and crystallographic quality are more severe than in other semiconductor materials. Fortunately, since silicon is an elemental semiconductor, it is easier to achieve such high quality with silicon than with compound materials, where issues such as stoichiometry arise. Silicon also has the advantage of being readily oxidized, with the resulting interface between silicon and its oxide having excellent electronic properties. The quality of this silicon/silicon dioxide interface is one of the main reasons why silicon dominates microelectronics.
Energy Conversion Efficiencies By balancing optical absorption processes in semiconductors with the inverse process of light emission during recombination of excited carriers, it is possible to obtain very general efficiency limits to photovoltaic conversion. For a single material cell, the terrestrial energy conversion efficiency limit is 33% for cells of bandgaps of 1.15 eV and 1.36 eV5 Silicon lies close t
Data Loading...
