Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
1999-10-07
2001-07-17
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S255000, C136S261000, C136S258000, C257S432000, C257S437000, C257S461000, C257S464000, C438S057000, C438S069000, C438S072000, C438S083000, C438S089000, C438S098000
Reexamination Certificate
active
06262359
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improved solar cell and a process for fabricating thereof. More particularly, the present invention relates to a solar cell including a p-n junction located near a non-illuminated surface of the solar cell and a process for fabricating thereof.
Solar cells are widely used because they convert easily accessible energy from a light source, such as the sun, to electrical power to operate electrically driven devices, e.g., calculators, computers, and heaters used in homes.
FIG. 1
shows a cross-sectional view of a layered stack that makes up a conventional silicon solar cell
10
. Conventional silicon solar cell
10
typically includes a p-n junction
24
sandwiched between a p-type base layer
18
and an n-type layer
16
, which is located near an illuminated (front) surface
11
. The term “illuminated surface,” as used herein refers to the surface of a conventional solar cell that is exposed to light energy when the solar cell is active or under operation. Thus, the term “non-illuminated surface” refers to a surface that is opposite the illuminated surface. The basic structure of p-n junction
24
includes a heavily-doped (about 10
20
cm
−3
) n-type emitter layer (n
+
)
16
at or near the illuminated surface
11
and disposed above a moderately-doped (about 10
15
cm
−3
) p-type base layer (p)
18
. Commercial embodiments of conventional solar cells typically include an optional antireflective coating
14
and a p
+
layer
20
that is formed between p-type base layer
18
and p-type silicon contact
22
.
A typical depth of p-n junction
24
from the top of the n
+
emitter layer
16
measures about 0.5 &mgr;m. A shallow front p-n junction
24
is desired in order to facilitate the collection of minority carriers that are created on both sides of p-n junction
24
. Each photon of light that penetrates into p-type base layer
18
and is absorbed by base layer
18
surrenders its energy to an electron in a bound state (covalent bond) and thereby frees it. This mobile electron, and the hole in the covalent bond it left behind (which hole is also mobile), comprise a potential element of electric current flowing from the solar cell. In order to contribute to this current, the electron and hole cannot recombine, but rather are separated by the electric field associated with p-n junction
24
. If this happens, the electron will travel to n-type silicon contact
12
and the hole will travel to p-type silicon contact
22
.
In order to contribute to the solar cell current, photogenerated minority carriers (holes in the n
+
emitter layer and electrons in the p-type base layer) should exist for a sufficiently long time so that they are able to travel by diffusion to p-n junction
24
where they are collected. The average distance over which minority carriers can travel without being lost by recombining with a majority carrier is called the minority carrier diffusion length. The minority carrier diffusion length generally depends on such factors as the concentration of defects in the silicon crystal (i.e. recombination centers) and the concentration of dopant atoms in the silicon. As the concentration of either defects or dopant atoms increases, the minority carrier diffusion length decreases. Thus, the diffusion length for holes in the heavily-doped n
+
emitter layer
16
is much less than the diffusion length for electrons in moderately-doped p-type base layer
18
Those skilled in the art will recognize that the n
+
emitter layer
16
is nearly a “dead layer” in that few minority charge carriers created in emitter layer
16
are able to diffuse to p-n junction
24
without being lost by recombination. It is desirable to have n
+
emitter layer
16
that is shallow or as close to surface of emitter layer
16
as possible for various reasons. By way of example, a shallow emitter layer allows relatively few photons to be absorbed in n
+
emitter layer
16
. Furthermore, the resulting photogenerated minority carriers created in n
+
emitter layer
16
find themselves close enough to p-n junction
24
to have a reasonable chance of being collected (diffusion length >junction depth).
Unfortunately, in the conventional solar cell design, the depth of n
+
emitter layer is limited and cannot be as shallow as desired. Metal from emitter contacts
12
, especially those formed by screen-printing and firing, can penetrate into p-n junction
24
and ruin or degrade it. The presence of metal in the p-n junction
24
“shorts” or “shunts” the junction. Therefore, although a shallow and lightly-doped n
+
emitter layer
16
is desired in order to enhance the current produced by the cell, in practice, however, n
+
emitter layer
16
is relatively deeper and more heavily-doped than desired to avoid shunting p-n junction
24
. Consequently, in conventional solar cells, the deep location of n
+
emitter layer
16
compromises the amount of current produced by the cell.
What is needed is a structure and process for fabricating a silicon solar cell, which has a high minority carrier diffusion length, eliminates shunting of the p-n junction and does not compromise the amount of current produced.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides a solar cell. The solar cell includes a base layer having dopant atoms of n-type conductivity and being defined by an illuminated surface and a non-illuminated surface. The illuminated surface has light energy impinging thereon when the solar cell is exposed to the light energy and the non-illuminated surface is opposite the illuminated surface. The solar cell further includes a back surface emitter layer that is made from an aluminum alloy to serve as a layer of p-type conductivity. The solar cell further still includes a p-n junction layer disposed between the non-illuminated surface of the base layer and the back surface layer.
In another aspect, the present invention provides a process for fabricating a solar cell. The process includes: (1) providing a base layer, (2) fabricating an emitter layer of p-type conductivity on a same side as the non-illuminated surface of the base layer to provide a strongly doped p-type emitter layer and a p-n junction between the n-type base layer and the p-type emitter layer. The base layer of the present invention has n-type conductivity and is defined by an illuminated surface and a non-illuminated surface. The illuminated surface has light energy impinging thereon when the solar cell is exposed to the light energy and the non-illuminated surface is opposite the illuminated surface.
These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures.
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patent: 3895975 (1975-07-01), Lindmayer
patent: 4056879 (1977-11-01), Lindmayer
patent: 4106047 (1978-08-01), Lindmayer
patent: 4131486 (1978-12-01), Brandhorst, Jr.
patent: 4226017 (1980-10-01), Lindmayer
patent: 4916503 (1990-04-01), Uematsu et al.
patent: 5538564 (1996-07-01), Kaschmitter
patent: 5641362 (1997-06-01), Meier
patent: 5951742 (1999-10-01), Tange et al.
patent: 5973260 (1999-10-01), Tange et al.
patent: 6071753 (2000-06-01), Arimoto
patent: 0776051 (1997-05-01), None
patent: WO 97/13280 (1997-04-01), None
Hu et al, Solar Cells from Basic to Advanced Systems, McGraw-Hill (1983), pp. 81-83.*
Salami, Jalal, et al.,Elsevier Science, Solar Energy Materials and Solar Cells 48, “Self-aligned locally diffused emitter (SALDE) silicon solar cell,” (1997) pp.:159-165.
Davis Hubert P.
Garcia Ruth A.
Meier Daniel L.
Salami Jalal
Diamond Alan
Ebara Solar, Inc.
Squire Sanders & Dempsey LLP
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