JP3203076B2 - Silicon solar cells for space - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon solar cell, and more particularly to an improvement in the output voltage of a silicon solar cell used for space.

[0002]

2. Description of the Related Art Silicon solar cells are widely used as solar cells for converting light energy into electric energy.

FIG. 2 is a sectional view showing an example of a conventional silicon solar cell known in the prior art. In this solar cell, an N ⁺ layer 2 is formed on the front surface of a P-type silicon substrate 1 by thermally diffusing an N-type impurity. On the N ⁺ layer 2, a comb-shaped front electrode 3 is formed. Then, the N ⁺ layer 2 and the front electrode 3 are covered with an antireflection film 4 for reducing surface reflection of incident light.

On the other hand, on the back surface of the P-type silicon substrate 1, BSF (Ba
P ⁺ acting as ck Surface Field) layer
Layer 5 is formed. The BSF layer 5 acts to increase the long wavelength sensitivity of the silicon solar cell. Then, a back electrode 7 is formed on the BSF layer 5.

[0005] However, as shown in FIG.
Open-circuit voltage V _OCIs about 605mV
Is low, and the high efficiency of silicon solar cells
Open circuit voltage V_OCImprovement is essential
Was.

In a report on improving the efficiency of a silicon solar cell, a technology including a plurality of locally formed BSF regions is disclosed in, for example, Conference Record, 21t.
hIEEE, Photovoltaic Specia
list Conference, Florida,
May 1990 pp. 233-238 and pp.
333-335.

[0007] FIG. 333-33
5 shows a cross-sectional view of the silicon solar cell shown in FIG.
In the solar cell of FIG. 3, the front surface of the P-type silicon substrate 1 has a non-reflective shape 2a, and the N ⁺ layer 2 is formed on the front surface. On N ⁺ layer 2, comb-shaped front electrode 3 is formed, and, the N ⁺ layer 2 and the front electrode 3 is covered with the antireflection film 4 for reducing the surface reflection of incident light I have. A silicon oxide film 6 is formed on the back surface of the P-type silicon substrate 1. A plurality of LBSF (Local BSF) regions 5b are formed on the back surface of the silicon substrate 1 through a plurality of openings locally opened in the oxide film 6. Then, a back electrode 7 is formed on the silicon oxide film 6, and the back electrode 7 is connected to a plurality of LBSF regions 5b through a plurality of openings in the oxide film 6.

[0008]

However, the silicon solar cell shown in FIG. 3 is a technology relating to a silicon solar cell for terrestrial use. The front surface of a silicon substrate 1 has a non-reflective shape 2a and is used for space. In this case, there is a problem that the solar absorptivity becomes high. In addition, the resistivity of the base silicon wafer is 0.5 Ωcm and the cell thickness is 280 μm, and there is a problem that radiation resistance is poor for use in space. Therefore, the conditions disclosed in the above article cannot be applied to silicon solar cells for space use, and the conditions for forming the LBSF region 5b suitable for silicon solar cells for space use were unknown.

Accordingly, the present invention provides a space silicon solar cell in which the open circuit voltage V _OC is significantly improved by including a plurality of LBSF regions formed at a preferable ratio with respect to the back surface of the silicon substrate. The purpose is.

[0010]

Silicon solar cell according to the present invention SUMMARY OF THE INVENTION comprises a P-type silicon substrate which surface has a rear flat front, and the N ⁺ layer formed on the front surface of the substrate, N ⁺
A front electrode formed on the layer, a plurality of P ⁺ regions dispersedly formed on the back surface of the substrate to serve as LBSF regions, an insulating layer formed on the back surface of the substrate, and an insulating layer formed on the insulating layer And a back electrode connected to a plurality of P ⁺ regions through a plurality of openings formed in the insulating layer, wherein the area occupied by the plurality of P ⁺ regions in the back surface of the substrate is 1% or more. der less than 9% is, the resistivity of the substrate is substantially 10Ωcm
It is characterized in der Rukoto.

[0011]

In the silicon solar cell according to the present invention, L
Since the area occupied by the P ⁺ regions acting as the BSF regions on the back surface of the silicon substrate is set to 1% or more and less than 9%, the silicon solar cell has a high open-circuit voltage V _OC.
And the efficiency of the solar cell is improved.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a silicon solar cell according to one embodiment of the present invention is schematically shown in a sectional view. FIG.
In the silicon solar cell described above, the thickness of the silicon substrate 1 is, for example, 50 μm to 200 μm and 10 Ωcm.
May be used.
1. The structure of the front surface of the solar cell of FIG. 1 is similar to that of FIG. 2, and corresponding portions are denoted by the same reference numerals.

However, in the solar cell of FIG. 1, a silicon oxide film 6 serving as a passivation film is formed on the back surface of silicon substrate 1 by, for example, thermal oxidation. A plurality of windows 6a are formed in the oxide film 6 by, for example, a photolithography method. These windows 6a have an area ratio occupying the rear surface of the silicon substrate 1 (hereinafter referred to as "LBSF area ratio") of 1% or more and 9%.
It is set to be less than. For example, these windows 6a have a square shape with one side of 20 μm and have a shape of 100 μm.
It can be formed so as to be distributed at a pitch of μm, and the LBSF area ratio at that time is 4%.

A plurality of LBSF regions 5b are formed by diffusing boron using boron tribromide, which is a P-type impurity, or implanting boron ions through the plurality of windows 6a on the back surface of the silicon substrate 1. Have been. Note that, before forming the silicon oxide film 6, the plurality of LBSF regions 5b are formed using paste containing a P-type impurity such as aluminum in a plurality of local regions on the back surface of the silicon substrate 1 using a screen printing technique. , And baking it to form an alloy with the silicon substrate 1. In addition, a P-type impurity material such as aluminum is vacuum-deposited on the back surface of the silicon substrate 1, patterned into a local region by using a known photolithography technique, and baked to be alloyed with the silicon substrate 1. Can also be formed.

The plurality of windows 6a opened in the oxide film 6 are formed by a back electrode 7 formed on the oxide
It is also used as a contact hole for obtaining electrical connection to the back of the device. At this time, the back electrode 7 has a plurality of LBs
Connected to the back surface of the silicon substrate 1 via the SF region 5b,
There is no direct connection to the back surface of the P-type silicon substrate 1 without passing through the LBSF region 5b. Such a back electrode 7 can be formed of a metal layer made of, for example, Al-Ti-Pd-Ag.

FIG. 4 shows the LBS of the silicon solar cell of FIG.
The change of the open circuit voltage V _OC when the F area ratio is changed is shown. In this graph, the horizontal axis represents the LBSF area ratio (%), and the vertical axis represents the open circuit voltage V _OC .

FIG. 5 shows the change of the short-circuit current I _SC when the LBSF area ratio of the silicon solar cell of FIG. 1 is changed. In this graph, the horizontal axis is LBSF
It represents the area ratio (%), and the vertical axis represents the short-circuit current I _SC.

FIG. 6 shows a change of the fill factor FF when the LBSF area ratio of the silicon solar cell of FIG. 1 is changed. In this graph, the horizontal axis is LBSF
The area ratio (%) is represented, and the vertical axis represents the fill factor FF.

FIG. 7 shows the change of the maximum output _Pmax when the LBSF area ratio of the silicon solar cell of FIG. 1 is changed. In this graph, the horizontal axis is LBS
F represents the area ratio (%), and the vertical axis represents the maximum output _Pmax . 4 to 7 show the measured values of AM0 and 28 ° C. of the solar cells each having an area of 2 cm × 2 cm.

As is apparent from the graphs of FIGS.
It can be seen that when the BSF area ratio is 1% or more and less than 9%, V _OC and P _max are remarkably improved.

The reason why the maximum output P _{max of the} solar cell increases with the increase of V _OC is that P _max is expressed by P _max (W) = V _OC (V) × I _SC (A) × FF. , The change of I _SC (FIG. 5) and the change of FF (FIG. 6) are relatively small when the LBSF area ratio is 1% or more and less than 9%, so that P _max changes with V _OC. Because he did. As shown in FIG. 2, the reason that V _OC is improved when the LBSF area ratio is 1% or more and less than 9% is that a plurality of LBs
It is considered that by setting the SF region 5b to 1% or more and less than 9%, the lifetime of minority carriers in the vicinity of the back surface of the silicon substrate 1 can be increased, and the saturation current density decreased. When the LBSF area ratio is 9% or more, the recombination of minority carriers on the back surface of the silicon substrate 1 increases, the lifetime decreases, the saturation current density increases, and as a result, V _OC decreases, and P _It is thought that _max tends to decrease. Also, when the LBSF area ratio is less than 1%, the series connection in the silicon solar cell is increased, and the FF is slightly reduced. Therefore, it is considered that _Pmax tends to decrease.

The present invention is not limited to the embodiment shown in FIG. 1. For example, a P ⁺ diffusion layer is formed on the front surface by using an N-type silicon crystal plate as the silicon substrate 1, and
On the back surface, phosphorus oxychloride can be used as an N-type diffusion impurity . Et al is, P is doped into LBSF region
Aluminum can be used as the type impurity, and the oxide film 6 can also be formed by a CVD method. Although an oxide film has been described as an example of the insulating film here, not only an oxide film but also a nitride film, for example, may be used. Furthermore, as the silicon substrate 1, not only a silicon single crystal plate but also a silicon polycrystal plate can be used. Further, a passivation film may or may not be formed on the surface (front surface) of the silicon substrate.

[0023]

As described above, according to the present invention, it is possible to provide a silicon solar cell having an improved open-circuit voltage V _OC by having an LBSF area ratio of 1% or more and less than 9%.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically illustrating a silicon solar cell according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing an example of a conventional silicon solar cell according to the prior art.

FIG. 3 is a sectional view showing another example of a silicon solar cell according to the prior art.

FIG. 4 is a graph showing a relationship between an LBSF area ratio and an open circuit voltage in the silicon solar cell of FIG.

FIG. 5 is a graph showing a relationship between an LBSF area ratio and a short-circuit current in the silicon solar cell of FIG.

FIG. 6 is a graph showing the relationship between the LBSF area ratio and the fill factor in the silicon solar cell of FIG.

FIG. 7 is a graph showing the relationship between the LBSF area ratio and the maximum output in the silicon solar cell of FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 P-type silicon substrate 2 N ⁺ type layer 2a Front anti-reflection shape 3 Comb-shaped front electrode 4 Antireflection film 5b LBSF area 6 Insulating film 6a Opening formed in insulating film 6 7 Back electrode 8 Front passivation film

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-230776 (JP, A) Conference Record of the Twenty-First First IEEE Photovoltaic Specialties Conferences-1990, p. 333-335, "24% Efficient PERL Structure silicone cell cells", J. Am. Zhao et al. (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 31/04-31/078

Claims (1)

(57) [Claims] 1. A P-type silicon substrate having a front surface and a back surface having a flat surface, an N ⁺ layer formed on the front surface of the substrate, a front electrode formed on the N ⁺ layer, and a back surface of the substrate A plurality of P ⁺ regions which are dispersedly formed and act as LBSF regions; an insulating layer formed on the back surface of the substrate; and a plurality of P ⁺ regions formed on the insulating layer and opened in the insulating layer. through the opening and a back electrode connected to said plurality of P ⁺ regions, the area occupied in the back of said plurality of P ⁺ regions the substrate Ri der less than 9% 1% or more, the Substrate resistivity is substantial
Silicon solar cells for space use that to characterized the 10Ωcm der Rukoto.