Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2002-01-25
2003-12-30
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S256000, C136S261000, C257S053000, C257S052000, C257S065000, C257S066000, C257S070000, C257S431000, C257S464000, C438S089000, C438S166000, C438S097000, C438S486000
Reexamination Certificate
active
06670543
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to thin-film solar cells and method of making.
2. Background Information
Photovoltaic (PV) cells are made of materials referred to as semiconductors, such as, silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy impacts the electrons, allowing them to flow freely. PV cells also all have one or more electric fields which act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, one can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power that the solar cell can produce.
A display screen made with TFT (thin-film transistor) technology is a liquid crystal display (LCD), common in notebook and laptop computers, that has a transistor for each pixel (that is, for each of the tiny elements that control the illumination of your display). Having a transistor at each pixel means that the current that triggers pixel illumination can be smaller and therefore can be switched on and off more quickly. TFT technology is also known as active matrix display technology (and contrasts with “passive matrix” which does not have a transistor at each pixel). A TFT or active matrix display is more responsive to change. For example, when you move your mouse across the screen, a TFT display is fast enough to reflect the movement of the mouse cursor. (With a passive matrix display, the cursor temporarily disappears until the display can “catch up.”) Active matrix (also known as Thin Film Transistor or thin film transistor) is a technology used in the flat panel liquid crystal displays of notebook and laptop computers. Active matrix displays provide a more responsive image at a wider range of viewing angle than dual scan (passive matrix) displays.
In this context, an Si:H film is a film of silicon in which hydrogen is incorporated. The hydrogen content is approximately 3 to 20%.
Solar cells based on the semiconductor material silicon have been known for many years. These solar cells are usually produced from solid monocrystalline or polycrystalline silicon, typical thicknesses of a solar cell of this type being approximately 300 to 500 &mgr;m. These thicknesses are required firstly in order to ensure sufficient mechanical stability and secondly to achieve absorption of the incident sunlight which is as complete as possible. On account of the relatively large film thicknesses and the associated high consumption of material, and on account of the unavoidable need for a high-temperature step for doping of the silicon wafers (T≧1000° C.), solar cells of this type entail expensive manufacture.
As an alternative to these relatively thick silicon solar cells described above, in addition to the thin film solar cells based on amorphous Si:H (referred to below as a-Si:H), which have already been the subject of research for some 20 years, thin-film solar cells made from microcrystalline Si:H (referred to below as &mgr;c-Si:H) have in recent years become an established subject for investigation. This cell material is expected to have a similarly high efficiency to that of monocrystalline silicon, but to involve less expensive production processes, as are also known for a-Si:H. At any rate, the use of &mgr;c-Si:H is supposed to suppress the degradation in the efficiency under intensive illumination, which is inevitable when using a-Si:H. However, a number of significant points still currently stand in the way of commercial utilization of &mgr;c-Si:H as the functional layer in a thin-film solar cell. Unlike the solar cell using a-Si:H, which has a thickness of the photovoltaically active film of approximately 300 nm, the solar cell made from &mgr;c-Si:H, to achieve a similarly good utilization of the incident light, must be approximately 3000 nm thick, i.e. has to be thicker by a factor of 10. Therefore, an economic process must also allow the deposition rate of the microcrystalline material to be higher by this factor than that achieved for a-Si:H. An inexpensive substrate, preferably window glass or even standard plastics, appears to be indispensable as a further necessary feature for commercial utilization of the &mgr;c-Si:H. For this purpose, it is necessary to have available deposition methods which are compatible with the substrates, i.e. low-temperature processes (T<100° C. for plastic or T≦200 to 300° C. for glass which is provided with a transparent conductive film), and these processes must moreover still achieve high film-generation rates.
According to the prior art, microcrystalline silicon (&mgr;c-Si:H) can be applied in thin films to a support material at temperatures of greater than approximately 200° C. using various processes. For example, it can be deposited directly from the gas phase. By way of example, the following deposition methods are known: high-frequency glow discharge deposition (HF-PECVD), electron cyclotron resonance (ECR) process, electron cyclotron wave resonance (ECWR) process, sputter deposition, hot-wire (HW) technique, microwave CVD.
Furthermore, processes are also known in which &mgr;c-Si:H is produced by initially depositing a-Si:H from the gas phase, which is then transformed into &mgr;c-Si:H. The transformation of a-Si:H to &mgr;c-Si:H is known, for example, from the following documents.
For example, U.S. Pat. No. 5,470,619 describes the transformation of a-Si:H into &mgr;c-Si:H by means of heat treatment at a temperature of 450° C. to 600° C.
U.S. Pat. No. 5,486,237 describes a temperature-induced transformation of a-Si:H films into &mgr;c-Si:H films at 550° C. to 650° C. over a period of 3 to 20 hours.
U.S. Pat. No. 5,344,796 describes a process for producing a thin &mgr;c-Si:H film on a glass substrate. In this process, first of all a &mgr;c-Si:H film is generated on the substrate and serves as a seed layer, then a-Si:H is deposited on this seed layer by means of a CVD process. The a-Si:H is transformed into &mgr;c-Si:H by means of a heat treatment, preferably at between 580° C. and 600° C. for a period of from 20 to 50 hours.
U.S. Pat. No. 5,693,957 likewise describes the thermal transformation of a-Si:H films into &mgr;c-Si:H films at 600° C., the transformation of certain a-Si:H films into &mgr;c-Si:H being deliberately prevented by impurities formed by these a-Si:H films.
A microwave plasma CVD process for the production of a-Si:H and &mgr;c-Si:H films is described in U.S. Pat. No. 5,334,423, in which, in saturation mode, 100% of the microwave power is introduced.
Published International Application No. 93/13553 (corresponding to U.S. Pat. No. 5,231,048) describes a microwave CVD process for producing thin semiconductor films, the process pressure lying below the Paschen minimum. A microwave CVD process with controllable bias potential for the production of thin semiconductor films is described in document U.S. Pat. No. 5,204,272.
The production of &mgr;c-Si:H films by means of a microwave CVD process is described in U.S. Pat. No. 4,891,330, in which preferably at least 67% of hydrogen is added to the process or precursor gas in order to assist the formation of the &mgr;c-Si:H phase.
A plasma process for the production of a &mgr;c-Si:H layer is described in document Published International Application No. 97/24769 (corresponding to U.S. Pat. No. 6,309,906), the precursor gas being diluted with hydrogen and/or argon.
Furthermore, a plasma treatment of an a-Si:H film by means of an argon plasma is described in U.S. Pat. No. 4,762,803, and by means of a hydrogen plasma in Published International Application No. 93/10555 (corresponding to U.S. Pat. No. 5,387,542), in order to obtain a &mgr;c-Si:H f
Bauer Stefan
Danielzik Burkhard
Freitag Nina
Lohmeyer Manfred
Möhl Wolfgang
Diamond Alan
Nils H. Ljungman & Associates
Schott Glas
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