Method of making a solar cell having improved anti-reflection passivation layer

A silicon solar cell has increased efficiency by providing an anti-reflection and passivation layer comprising a layer of silicon dioxide thermally grown on a surface of a silicon body and a layer of titanium dioxide deposited on the layer of silicon oxide. In fabricating the composite anti-reflection passivation layer, a layer of aluminum is first deposited on a surface of the thermally grown silicon oxide. After annealing the aluminum layer, the aluminum is removed from the silicon dioxide layer, and the layer of titanium dioxide is then deposited on the surface of the silicon dioxide from which the aluminum was removed. A layer of magnesium fluoride can be deposited on the surface of the titanium dioxide.

BACKGROUND OF THE INVENTION 
This invention relates generally to silicon solar cells, and more 
particularly the invention relates to a solar cell having improved 
anti-reflective surface passivation. 
The silicon solar cell generates electrical charge when exposed to solar 
radiation. The radiation interacts with atoms of the silicon and forms 
electrons and holes which migrate to p- and n-doped regions in the silicon 
body and create voltage differentials between doped regions. 
Typically, the silicon body is coated with a silicon dioxide passivation 
layer. This layer may also serve as an anti-reflection layer to impinging 
radiation, or, alternatively, an additional anti-reflection coating can be 
applied over the passivation layer. Heretofore, a problem with oxide 
passivation layers has been a degradation of the surface recombination 
velocity at the silicon-silicon dioxide interface when exposed to 
concentrated sunlight. This effect is primarily due to hot electrons being 
created by the ultraviolet end of the solar spectrum and being injected 
from the silicon layer into the silicon dioxide layer. 
U.S. Pat. No. 5,030,295 to Swanson, et al. discloses a stable passivation 
for silicon solar cells in which a thin silicon dioxide layer is capped by 
a thin polysilicon layer. Without the presence of the polysilicon, the 
surface is damaged by the ultraviolet portion of the solar spectrum, 
resulting in degradation of cell performance with time. The polysilicon is 
hypothesized to prevent such damage by virtue of either its absorption of 
the ultraviolet light or its prevention of water vapor diffusing through 
the thin silicon dioxide layer. Water is known to reduce the radiation 
hardness of silicon dioxide passivation. 
The present invention is directed to a similar silicon solar cell which has 
an improved anti-reflection passivation layer and to a method of 
fabricating the passivation layer. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a titanium dioxide layer is 
provided over a surface of a silicon dioxide layer formed on a surface of 
a silicon solar cell and forms an improved anti-reflection passivation 
layer for the silicon solar cell. The titanium dioxide is more transparent 
to the desired portion of the solar spectrum while blocking the undesired 
portion including ultraviolet light. 
In accordance with the invention, in fabricating the passivation layer, an 
aluminum film is first deposited on a surface of the silicon dioxide layer 
and then annealed. After annealing, the aluminum is removed from the 
silicon dioxide layer and the titanium dioxide is deposited on the 
surface. 
The invention and object and features thereof will be more readily apparent 
from the following detailed description and appended claims when taken 
with the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, FIG. 1 is a section view of a conventional 
back contact silicon solar cell as disclosed in U.S. Pat. No. 5,030,295. 
The cell comprises a lightly doped or intrinsic silicon body 20 in which a 
plurality of p-doped regions 22 and n-doped regions 24 are all formed in a 
major surface thereof. The surface is normally provided with a passivating 
and reflecting layer to prevent radiation from passing through the silicon 
body and the escape of photons. The opposite surface of the silicon body 
20 has a thin layer of silicon dioxide 26 grown thereon as a passivating 
layer. The silicon dioxide layer has a thickness on the order of 50 .ANG. 
for anti-reflection purposes. The passivation layer further includes a 
phosphorus-doped polysilicon layer 28 and a thicker vapor deposited 
silicon dioxide layer 30. The thin silicon dioxide layer 26 improves the 
problem of water-related traps and produces less stress than thick silicon 
dioxide layers. The phosphorus-doped polycrystalline silicon layer 28 has 
a thickness on the order of 300 .ANG. and provides for improving the 
stability of the passivation layer. It is believed that the function of 
the doped polysilicon layer is to prevent electrons from being injected 
into the silicon dioxide layer from the silicon substrate. Alternatively, 
interface states may be discouraged from being created even if electronic 
injection is occurring. While polycrystalline silicon is more absorptive 
of incoming light than is silicon dioxide, a polysilicon crystalline layer 
of 300 .ANG. will absorb only about 3% of impinging light, which is a fair 
trade off for the increased stability offered thereby. 
FIG. 2 is a section view of a silicon solar cell as in FIG. 1, but with an 
improved passivating layer in accordance with the present invention. In 
this embodiment, the phosphorus-doped polycrystalline layer 28 and the 
thicker vapor deposited silicon dioxide layer 30 are replaced by a single 
layer 32 of titanium dioxide which is more transparent to the desired 
portion of the solar spectrum and also acts as an anti-reflection layer. 
Energy loss in the anti-reflection passivation layer can be reduced from 9% 
for prior art layers to 2% for the titanium dioxide/silicon dioxide 
anti-reflection passivation layer in accordance with the invention. This 
greatly facilitates reaching the theoretical goal of 27.5% efficiency of 
the silicon solar cell in converting sunlight into electricity. 
FIG. 3 is a process flow chart illustrating the steps in fabricating the 
silicon solar cell of FIG. 2. Initially, the silicon dioxide layer 26 is 
thermally grown on a surface of the silicon semiconductor body 20 to a 
thickness of 50 .ANG.-300 .ANG. in dry oxygen (O.sub.2) at a temperature 
of 800.degree. C. The silicon dioxide layer can be undoped or phosphorous 
doped as taught in U.S. Pat. No. 5,030,295. Thereafter, a layer of 
aluminum having a thickness of approximately 0.05 .mu.m is deposited on a 
surface of the silicon dioxide layer 26. The structure is then annealed at 
400.degree. C. in nitrogen (N.sub.2) for 40 minutes followed by annealing 
in a forming gas (20% hydrogen, 80% nitrogen) for 10 minutes. The 
structure is cooled down in the forming gas for 40 minutes and then the 
aluminum is removed by etching in a mixture of nitric and hydrofluoric 
acid (HNO.sub.3 :HF 240:1). The annealing of the aluminum hydrogenates the 
surface and reduces surface defects which enhances cell efficiency. 
Thereafter, a layer 32 of titanium dioxide (TiO.sub.2) having a thickness 
on the order of 500 .ANG. is deposited on the textured surface of the 
silicon dioxide layer 26 from which the aluminum was removed. A layer of 
magnesium fluoride (MgF.sub.2) having a thickness on the order of 1360 
.ANG. can be deposited on the surface of the titanium dioxide to form a 
second anti-reflection layer further reducing reflected light. 
Use of the titanium dioxide and silicon dioxide layers as a composite 
anti-reflection and passivation layer maintains the reflection of 
impinging ultraviolet light while increasing the efficiency in passing the 
desired portion of the solar light spectrum. While the invention has been 
described with reference to a specific embodiment, the description is 
illustrative of the invention and is not to be construed as limiting the 
invention. Various modifications and applications may occur to those 
skilled in the art without departing from the true spirit and scope of the 
invention as defined by the appended claims.