1. Field of the Invention
The invention relates to heterojunction photovoltaic devices and, more particularly, to the utilization of heterojunction photovoltaic devices for producing electricity from semiconductor-insulator-semiconductor junction photovoltaic cells and for sensing objects by radiometric detection cells.
2. Description of the Prior Art
The solid-state heterojunction photovoltaic cell has recently received considerable attention. Such a cell utilizes at least two layers of materially different solids, one of which is generally a semiconductor to convert energy from light (usually solar energy) into electrical energy. These solid materials, according to the band theory of solids, contain atoms whose discrete electronic energy states have merged into energy bands of allowed energies for electrons. The energy required to excite electrons in such solid materials from a maximum energy in the valence band to a minimum energy in the conduction band represents the band gap energy. At approximately room temperature, valence and conduction energy bands of conductors such as metallic solids are not separated, i.e. they have a band gap of about 0. Semiconductor solid materials, on the other hand, are typically separated by a band gap of above 0 to less than about 4.0 e.V., while higher values are associated with insulator materials.
The most efficient utilization of terrestrial solar energy by semiconductor materials has been observed to occur with the absorption of photons associated with near-infrared light. Light-absorbing semiconductor materials having a band gap of approximately 1.4 e.V. tend to maximize the efficiencies of the conversion from solar to other forms of energy. In photovoltaic cells, electron-hole pairs are generated by the absorption of light in semiconductors. The electron and the hole of electron-hole pairs are separated at a metal-semiconductor (M-S) junction, a metal-insulator-semiconductor (M-I-S) junction, a semiconductor-semiconductor (S-S) junction, or at the junction of two semiconductors having a thin layer of insulator material between them (S-I-S), and are injected at respective sides of the junction to produce electrical energy. Holes and electrons move to the surface or bulk of the semiconductor, depending on their resistivity category, i.e. n-type or p-type.
One of the problems with heterojunction photovoltaic devices containing semiconductor materials is efficiency. A number of approaches have been taken to increase the efficiency of photovoltaic devices. In one approach, a cell with an M-I-S or S-I-S junction includes a coating material, such as a highly conductive metal like platinum, or a semiconductor having a wide band gap (i.e., &gt;3.0 e.V.), such as InO.sub.2 or SnO.sub.2, on the surface of the photovoltaic device. However, the search continues for coating materials that impart higher efficiencies to semiconductors useful in heterojunction photovoltaic cells.
Accordingly, it is an object of the present invention to provide a photovoltaic device that exhibits high efficiency when employed in a photovoltaic cell.
Another object of the invention is to provide a coating material for a semiconductor-containing photovoltaic device exhibiting high efficiency in a photovoltaic cell.
Yet another object still is to provide a method for producing a photovoltaic device exhibiting highly efficient photocurrent generation in a photovoltaic cell.
These and other objects and advantages of the invention will become apparent from the following description.