Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2001-09-20
2004-08-31
Nguyen, Nam (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S251000, C136S259000, C136S261000, C052S173300
Reexamination Certificate
active
06784361
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to photovoltaic devices, particularly, to solar cells comprising amorphous silicon. More particularly, this invention relates to photovoltaic devices comprising amorphous silicon wherein the amorphous silicon in the form of an intrinsic layer or i-layer is thick, and wherein the photovoltaic devices having such thick i-layers perform unexpectedly well at elevated temperatures.
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 photovoltaic devices comprise three layers. These are p- and n-layers which are extrinsic or doped and the i-layer which is intrinsic or undoped (at least containing no intentional doping). The i-layer is 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. As discussed above, 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 provide power for electrical components.
Thin film solar cells, which are one type of solar cells, are typically constructed of amorphous silicon-containing semiconductor films on a substrate. 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. Soda-lime glass is often used as a substrate because it is inexpensive, durable and transparent. If a glass substrate is used, a transparent conductive coating, such as tin oxide (SnO
2
) can be applied to the glass substrate prior to forming the amorphous silicon-containing semiconductor films. A metallic contact can be formed on the back of the solar cell. Solar cells are often placed in metal frames to provide attractive photovoltaic modules.
In an amorphous silicon-containing solar cell, the amorphous silicon component is comprised of a body of hydrogenated amorphous silicon (a-Si:H) material. This can be formed in a glow discharge of silane (SiH
4
). 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.
The process of glow discharge of silane and other suitable materials 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 a good 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 as described above. The collection of layers resulting in a p-i-n or n-i-p component, are referred to herein, at times, as a cell.
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. 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.
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. An important quality for the doped layers used in solar cells, besides good electrical properties, is low optical absorption. In contrast to single crystalline devices where p-n junctions can be used, photons absorbed in amorphous doped layers can be lost because the diffusion length of photo-carriers is extremely short in those layers. This requirement is especially important for the p-layer through which light enters into the device. It is partly for this reason that amorphous silicon carbon (a-SiC:H) p-layers with an optical bandgap of about 2.0 eV have been used instead of amorphous silicon.
On of the most important objectives in designing and manufacturing a photovoltaic device such as a solar cell is to maximize the efficiency of the device in converting light energy into electric energy. It would be desirable in some applications to use the photovoltaic device in a climate, or under other conditions, as on a roof top with poor or no air circulation around the device. Under such conditions, the photovoltaic device may reach elevated temperatures of, for example, more than 50° C., or more than 60° C. It would be advantageous to have a photovoltaic device that operates efficiently at these elevated temperatures and, more particularly, it would be highly advantageous to have an amorphous silicon-type photovoltaic device, which operates more efficiently at these high temperatures compared to prior devices. The present invention provides for such photovoltaic devices and a process for using them to generate electricity.
SUMMARY OF THE INVENTION
This invention is a photovoltaic device comprising an amorphous silicon-containing i-layer that is efficient at elevated operation temperatures. This invention is a photovoltaic device comprising an amorphous, silicon-containing i-layer wherein the i-layer has a thickness of at least about 3000 Å, more preferably at least about 3500 Å. Preferably the i-layer has a thickness of about 3000 to about 5500 Å, more preferably about 3500 to about 5000 Å. It can have a thickness of about 4000 to about 5000 Å, or from greater than 4000 Å and, suitably, up to about 5000 or 6000 Å. The i-layer can be at least about 5000 Å, at least about 5500 Å, and can be at least about 6000 Å. We have determined that photovoltaic devices having such thick i-layers function much more efficiently at elevated operation temperatures, for example, temperatures of at least about 50° C., than prior photovoltaic devices having thinner amorphous i-layers. The photovoltaic devices
Carlson David E.
Ganguly Gautam
Lin Guang H.
BP Corporation North America Inc.
Mutschler Brian L.
Nemo Thomas E.
Nguyen Nam
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