Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2000-06-29
2002-04-09
Dye, Rena L. (Department: 1772)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S086000
Reexamination Certificate
active
06368892
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to photovoltaic devices and more particularly to multi-junction solar cells fabricated of amorphous silicon and copper indium diselenide and their alloys.
Over the years numerous solar cells have been developed which have met with varying degrees of success. Single junction solar cells are useful but often cannot achieve the power and conversion efficiency of multi-junction solar cells. Unfortunately, multi-junction solar cells and single junction solar cells have been constructed of various materials which are able to capture and convert only part of the solar spectrum into electricity. Multi-junction solar cells have been produced with amorphous silicon and its alloys, such as hydrogenated amorphous silicon carbon and hydrogenated amorphous silicon germanium, with wide and low bandgap intrinsic i-layers. Amorphous silicon solar cells have a relatively high open circuit voltage and low current but can only respond to capture and convert into electricity wavelengths of sunlight from 400 to 900 nanometers (nm) of the solar spectrum.
Copper indium disclenide (CIS) polycrystalline solar cells have a relatively low bandgab of approximately 1 eV and are able to respond, capture and convert into electricity a great spectrum of sunlight from 400 to 1350 nm. Copper indium diselenide solar cells can generate more current but at lower voltage than amorphous silicon solar cells and their alloys. Copper indium diselenide polycrystalline solar cells, however, generally are more temperature dependent than amorphous silicon solar cells and can lose as much as 60% of their power at higher temperatures in a manner somewhat similar to polycrystalline silicon solar cells.
The segments, layers or cells of multi-junction solar cells are electrically interconnected, such as by laser scribing. High current CIS polycrystalline solar cells generate greater power losses (I
2
R) due to the resistance at the front and rear contacts, e.g. tin oxide contacts, than do amorphous silicon solar cells. Such power losses can be partially overcome by laser scribing more scribe lines and dividing the solar cell into smaller segments, such as 60 segments of 1 cm width. More scribe lines, however, decreases the active area of utilization of the solar cell which is available to capture and convert solar energy into electricity. Furthermore, deviations, voids and imperfections in the composition of polycrystalline can adversely effect the performance of polycrystalline solar cells.
Current output of a photovoltaic device is maximized by increasing the total number of photons of differing energy and wavelength which are absorbed by the semiconductor material. The solar spectrum roughly spans the region of wavelength from about 300 nanometers to about 2200 nanometers, which corresponds to from about 4.2 eV to about 0.59 eV, respectively. The portion of the solar spectrum which is absorbed by the photovoltaic device is determined by the size of the bandgap energy of the semiconductor material. Crystalline silicon (c-Si) has a bandgap energy of about 1.1 eV. Solar radiation (sunlight) having an energy less than the bandgap energy is not absorbed by the semiconductor material and, therefore, does not contribute to the generation of electricity, current, voltage and power, of the photovoltaic device.
Solar cells and other photovoltaic devices convert solar radiation and other light into usable electrical energy. The energy conversion occurs as the result of the photovoltaic effect. Solar radiation (sunlight) impinging on a photovoltaic device and absorbed by an active region of semi-conductor material, e.g. an intrinsic i-layer of amorphous silicon, generates electron-hole pairs in the active region. The electrons and holes are separated by an electric field of a junction in the photovoltaic device. The separation of the electrons and holes by the junction results in the generation of an electric current and voltage. The electrons flow toward the region of the semiconductor material having an n-type conductivity. The holes flow toward the region of the semiconductor material having a p-type conductivity. Current will flow through an external circuit connecting the n-type region to the p-type region as long as light continues to generate electron-hole pairs in the photovoltaic device.
Single-junction devices comprise three layers. These are p- and n-layers which are extrinsic or doped and i-layer which is intrinsic or undoped (at least containing no intentional doping). The i-layer is much thicker than the doped layers. This is because mainly light absorbed in the i-layer is converted to electrical power which can be used in an external circuit. The thickness of the i-layer (sometimes called the absorber layer) determines how much light is absorbed. When a photon of light is absorbed in the i-layer it gives rise to a unit of electrical current (an electron-hole pair). However, this electrical current will go nowhere on its own. Hence, the p- and n-layers. These layers, which contain charged dopant ions, set up a strong electric field across the i-layer. It is this electric field which draws the electric charge out of the i-layer and sends it through an external circuit where it can do work (i.e. power a light bulb).
An amorphous silicon solar cell is comprised of a body of hydrogenated amorphous silicon (a-Si:H) material, which can be formed in a glow discharge of silane. Such cells can be of the type described in U.S. Pat. No. 4,064,521 entitled Semiconductor Device Having A Body Of Amorphous Silicon which, issued to David E. Carlson on Dec. 20, 1977. Within the body of the cell there is an electric field which results from the different conductivity types of the semiconductor regions comprising the body.
Amorphous silicon solar cells are often fabricated by the glow discharge of silane (SiH
4
). The process of glow discharge involves the discharge of energy through a gas at relatively low pressure and high temperature in a partially evacuated chamber. A typical process for fabricating an amorphous silicon solar cell comprises placing a substrate on a heated element within a vacuum chamber. A screen electrode, or grid, is connected to one terminal of a power supply, and a second electrode is connected to the substrate. While silane, at low pressure, is admitted into the vacuum chamber, a glow discharge is established between the two electrodes and an amorphous silicon film deposits upon the substrate.
Amorphous hydrogenated silicon (a Si:H) based solar cell technology is currently the leading candidate for large area, low-cost photovoltaic applications. The basic device structure is a single p-i-n junction or an n-i-p junction in which all layers are traditionally amorphous and are made in a continuous plasma deposition process.
The substrate of the solar cell can be made of glass or a metal, such as aluminum, niobium, titanium, chromium, iron, bismuth, antimony or steel. If a glass substrate is used, a transparent, conductive coating, such as tin oxide (SnO
2
) can be applied to the lass substrate prior to forming the amorphous silicon. A metallic contact can be formed on the back of the substrate.
The doped layers in the device play a key role in building up the strong internal electric field across the i-layer, which is the predominant force in collecting photocarriers generated in the i-layer. In particular, the doped layers in the recombination junction of a multi-junction solar cell have to support large electric fields extending into the intrinsic layers, in addition to the high field in the recombination junction itself. The interface region must promote efficient recombination of electrons, generated in the first i-layer, with holes from the second i-layer. Also, the tunnel junction layers should provide minimal optical absorption. However, the electrical properties of amorphous doped layers are relatively poor as compared to their crystalline counterparts. For instance, the conductivities are typically only
~
1×10
−6
(&OHgr;·cm)
−1
for a-Si:H
BP Corporation North America Inc.
Dye Rena L.
Miggins Michael C.
Nemo Thomas E.
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