Method for fabricating solar cells

Details Method for fabricating solar cells Method for fabricating solar cells

1999-09-07

2001-08-21

Smith, Matthew (Department: 2825)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S549000, C438S069000, C438S072000

Reexamination Certificate

active

06277667

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method for fabricating solar cells, and in particular to a method for fabricating novel solar cells with low series resistance and enhanced photocurrent.
BACKGROUND OF THE INVENTION
Semiconductor solar cells can transduce light energy into electric energy. Generally speaking, the silicon solar cell consists of a P-N junction. When the photons in sunlight strike the surface of the semiconductor, electron-hole pairs will be induced by high-energy photons. The electron-hole pairs will move to the junction of the semiconductor and produce photocurrent within the loaded semiconductor.
Conventionally, solar cells are fabricated by doping the P-type silicon wafer with concentrated phosphorous to form an N
+
diffusion region in the surface of the P-type silicon wafer. Then, electrodes are formed by means of screen-printing. The conventional method for fabricating solar cells will be illustrated in FIGS.
1
A~
1
D.
CONVENTIONAL EXAMPLE
Referring to
FIG. 1A
, a P-type silicon wafer
10
with a front-side
10
A and a backside
10
B was provided. Then, the P-type silicon wafer
10
was washed or etched by using acidic or basic solvent (e.g. HF or KOH) to form a rough surface as shown in
FIG. 1A-1
. The rough front-side
10
A can reduce the reflection of incident sunlight. Then, an N-type diffusion region was formed by diffusing N-type impurities (e.g. phosphorous or arsenic) into the front-side
10
A of the p-type silicon wafer. The diffusion can be performed by means of furnace diffusion, screen-printing, spin-on or spray.
Referring to
FIG. 1B
, an anti-reflection coating layer (ARC layer)
14
was formed on the front-side
10
A of the P-type silicon wafer by evaporating or vapor deposition. The ARC layer consisted of titanium oxide, tantalum oxide, titanium nitride and so on.
Referring to
FIG. 1C
, a conductive paste (e.g. silver paste) was printed onto the front-side
10
A and backside
10
B of the P-type silicon wafer
10
. Then, electrodes
16
A and
16
B were respectively formed on the front-side
10
A and backside
10
B of the P-type silicon wafer
10
.
Referring to
FIG. 1D
, the product as shown in
FIG. 1C
as transferred to a furnace (e.g. IR-furnace) with a temperature ranging from 500° C. to 1200° C. to sinter the electrodes
16
A and
16
B. The electrode
16
A overlying the ARC layer
14
passed-through the ARC layer
14
after sintering, and contacted to the N-type diffusion region
12
. Thereafter, a solar cell
100
was obtained.
However, the N-type diffusion region
12
is a single-depth junction, thus the conversion efficiency of the solar cell from sunlight to photocurrent is not ideal, and the series resistance of electrodes is high. Consequently, it is hard to enhance the conversion efficiency of the solar cell to 15.5% according to this conventional process.
SUMMARY OF THE INVENTION
This invention discloses a novel method for fabricating solar cells. Using the existing screen-printing, masking or photolithography techniques, a P-type or N-type diffusion source is coated on the sites of an N-type or P-type silicon wafer desired for forming electrodes. Then, a low dose P-type or N-type diffusion source is in situ diffused into the N-type or P-type silicon wafer together with the P-type or N-type diffusion source coated on the N-type or P-type silicon wafer in the furnace. Thereafter, a P
−
/P
+
or N
−
/N
+
diffusion region is formed within the N-type or P-type silicon wafer. Finally, electrodes aligned to the P
+
or N
+
diffusion region are formed by means of screen-printing. Then, a solar cell with high photocurrent and low series resistance can be obtained.


REFERENCES:
patent: 4131488 (1978-12-01), Lesk et al.
patent: 4758525 (1988-07-01), Kida et al.
patent: 5928438 (1999-07-01), Salami et al.

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