JPH02867B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solar cell in which the front and back surfaces of a substrate can be used as electrical energy conversion surfaces.

Solar cells are transducers that have the characteristic of directly converting light into electricity, and since sunlight energy is inexhaustible, power generation using solar cells is an important step as fossil energy sources such as oil are running out. Currently, it has become a promising method of power generation.

However, the density of solar energy falling on the ground is small at about 1 kW/m ² , so a large number of solar cells are required to generate enough power. In addition, there is also the problem that solar cells are expensive and the cost of power generation is high, so it has conventionally been impossible to use solar cells as a general power source due to cost considerations.

Next, conventionally used solar cells will be explained using drawings. FIG. 1 is a cross-sectional perspective view thereof, in which 1 is a p-type substrate, 2 is an n-type diffusion layer, 3 is a light-receiving surface electrode, and 4 is a back surface electrode. This light-receiving surface electrode 3
has a comb-like or mesh-like shape, and is configured to minimize the portion that blocks light and suppress an increase in electrical resistance.

The light-receiving surface of the conventional solar cell shown in Figure 1 is
If light is irradiated from the back side of the n-type diffusion layer 2, electrons generated near the light irradiated surface -
The probability that the hole pair reaches the pn junction without recombining is small. This is because the p-type substrate 1 of a conventional solar cell has a thickness of 250 μm to 500 μm, and the diffusion length of electrons, which are minority carriers, in the p-type substrate is several tens of μm.
This is because the surface recombination rate on the back surface is also high. In fact, it is well known that almost no electromotive force is generated even when the back side of a conventional solar cell is irradiated with light.

Therefore, since the conventional solar cell has only one light-receiving surface, only the light irradiated onto the light-receiving surface formed on one surface of the substrate can be converted into electrical energy.

Furthermore, the silicon used as the substrate for solar cells is expensive and has the same quality as the single crystal silicon used in integrated circuit devices. For this reason, conventional solar cells manufactured using this single crystal silicon are expensive. Therefore, power generation systems using solar cells have had to be expensive.

On the other hand, in order to lower the cost of power generation systems using solar cells, attempts have been made to use lenses etc. to collect light and irradiate it onto the solar cells, but it is difficult to collect light evenly over the entire surface of the solar cells. At the same time, the cost required for condensing the light increases, making it impractical.

From what has been said above, it has two light-receiving surfaces,
And no matter which of the light-receiving surfaces the light is irradiated from,
If a solar cell that can obtain the same electromotive force appears, one light-receiving surface will be directly irradiated with sunlight,
By irradiating the other light-receiving surface with ambient scattered light or light reflected by a mirror, it becomes possible to effectively convert the two light-receiving surfaces into electrical energy, making the power generation system cheaper. be able to.

Furthermore, the thickness of the n-type diffusion layer 2 of the conventional solar cell shown in FIG. . It is practically difficult to increase the thickness of the n-type diffusion layer 2 because the response to short wavelength light that generates electron-hole pairs very close to the surface becomes small. Therefore, in conventional solar cells, care must be taken in forming the light-receiving surface electrode 3, which is one of the reasons for the increase in the price of solar cells. That is, it has been desired to develop a solar cell having a structure in which a light-receiving surface electrode can be easily formed without the risk of p-n junction short circuit.

The purpose of this invention is to have two light-receiving surfaces, front and back,
To provide a solar cell that can effectively convert light irradiated onto any of its light receiving surfaces into electrical energy, and can easily form a light receiving surface electrode without shorting the p-n junction. .

Next, the solar cell according to the present invention will be explained based on Examples. FIG. 2 is a sectional view showing one embodiment of the solar cell according to the present invention, and shows 11
12 is an n type substrate, 12 is an n ⁺ type diffusion layer, 13 is a p type diffusion layer, 14 is a p ⁺ type diffusion layer, 15 is a back electrode, 1
6 is a surface electrode. Such a structure can be easily formed by the following steps. Boron is applied at 150 keV at 2×10 ¹³ /
Inject cm ² . After this, heat treatment is performed to reduce the thickness to 10 μm or less.
A p-type diffusion layer 13 of 50 μm is formed. Next, an oxide film doped with boron is formed on the surface of the p-type diffusion layer 13, and an oxide film doped with phosphorus is formed on the surface of the n-type substrate 11, and heat treatment is performed.
A P ⁺ type diffusion layer 14 and an N ⁺ type diffusion layer 12 having a thickness of about 0.1 μm are formed. Finally, electrodes are formed on both surfaces to complete the process. The p-type diffusion layer 13 may be formed by epitaxial growth or by using a diffusion method other than ^{ion implantation} .
may be formed by ion implantation or by other methods.

To describe the characteristics of the solar cell according to the present invention shown in FIG.
This indicates that the junctions are closer together, and secondly, there are layers of the same conductivity type with different impurity concentrations on the two light-receiving surfaces, which function similar to a pn junction.

The first feature allows electron-hole pairs generated by light irradiated from the back surface to be collected more efficiently than conventional solar cells, and the second feature allows the electron-hole pairs generated by light irradiated from the back surface to be collected more efficiently than in conventional solar cells. A potential difference occurs at the interface between the high impurity concentration region and the low impurity concentration region, which suppresses the diffusion of electron-hole pairs toward surface recombination.

V _hL = KT / qlnN _h /N _L ...(1) where K is Boltzmann's constant, T is absolute temperature,
q is the electron charge, Nh is the impurity concentration in the high impurity concentration region, and N _L is the impurity concentration in the low impurity concentration region. KT/q has a value of about 26 mV at room temperature.

The above relationship will be explained based on the energy band diagram shown in FIG. Reference numbers 11 to 14 in the figure correspond to the respective parts shown in FIG. The hole pair 25, 26 is a potential difference caused by a difference in impurity concentration.

Of the electron-hole pairs 22 that have come to the interface between the p ⁺ type diffusion layer 14 and the p type diffusion layer 13, the electrons are suppressed from moving to the p ⁺ type diffusion layer 14 due to the potential difference 25, and the holes are The migration to the p ⁺ diffusion layer 14 is encouraged.
That is, the interface between the p ⁺ type diffusion layer 14 and the p type diffusion layer 13 functions similar to a pn junction. The interface between the n ⁺ type diffusion layer 12 and the n type substrate 11 also has an electron-hole pair 23 due to the potential difference 26 generated there.
It acts similar to a p-n junction.

As explained above, in the solar cell of the present invention, the interface between the high impurity concentration region and the low impurity concentration region exists near the light-receiving surface, and also exists near the two light-receiving surfaces simultaneously. Electricity can be efficiently generated from light irradiated from either of them. Furthermore, by forming an interface between a high impurity concentration region and a low impurity concentration region near the light receiving surface, the pn junction can be located deep from the surface. As mentioned above, this is because the interface between the high impurity concentration region and the low impurity concentration region is p-
This is because it functions similar to an n-junction, so electron-hole pairs generated by light near the surface can be efficiently collected. If the p-n junction can be formed deep from the surface, there is no risk of short circuiting the p-n junction due to electrode formation, which has the effect of facilitating electrode formation.

Next, the configuration of a solar power generation system using two light-receiving surfaces using the solar cell according to the present invention will be briefly described.

FIG. 4 shows an example of a power generation system using the solar cell according to the present invention, in which 31 is the solar cell according to the present invention, 32 is a reflecting mirror, 33 is incident light,
34 is reflected light.

A portion of the incident light 33 directly enters the solar cell 31 , and the rest is reflected by the reflecting mirror 32 and becomes reflected light 34 and enters the solar cell 31 from the back surface.
In other words, the solar cell 31 according to the present invention can convert light into electricity over twice the area as a conventional solar cell of the same size. The reflecting mirror 32 is
It is less expensive than general solar cells, and since it is a plane mirror, it can evenly irradiate the back surface of the solar cell 31 with light. Therefore, the configuration shown in FIG. 4 can reduce power generation costs compared to the conventional case where two solar cells are used to generate power.

As described above, the present invention has the following remarkable effects.

(b) A high impurity concentration-low impurity concentration junction is provided on the surface to collect electron-hole pairs, and this allows the p-n junction to be formed with a diffusion length of minority carriers or less and with a thickness that is sufficient to absorb light. depth greater than
It can be provided at a thickness of 10 μm to 200 μm, and furthermore, it becomes possible to efficiently collect electron-hole pairs generated by light in any region from the surface to the deep part.

(b) Since the front and back surfaces of the substrate can be used as light-to-electrical energy conversion surfaces without complicating the electrode structure, expensive silicon single crystal substrates can be used effectively, and solar power generation costs can be reduced.

[Brief explanation of drawings]

Figure 1 is a cross-sectional perspective view of a conventional solar cell;
The figure is a cross-sectional view showing one embodiment of a solar cell according to the present invention, and FIG. 3 is an energy level diagram for explaining the potential difference that occurs at the interface between a high impurity concentration region and a low impurity concentration region. , Figure 4 is
1 is a diagram showing an embodiment of a power generation system using a solar cell according to the present invention. 11...N-type impurity concentration substrate, 12...N ⁺ type impurity high concentration diffusion layer, 13...P type impurity concentration low concentration diffusion layer, 14...P ⁺ type impurity high concentration diffusion layer.

Claims (1)

[Claims] 1 A first region with a high impurity concentration having p-type conductivity and a thickness of 0.1 μm to 1 μm, a second region with a low impurity concentration having p-type conductivity and a thickness of 10 μm to 200 μm, and an n-type It consists of a third region having high impurity concentration having conductivity and a thickness of 0.1 μm to 1 μm, and a fourth region having low impurity concentration having n-type conductivity and having a thickness of 10 μm to 200 μm; and the fourth region are adjacent to each other to form a p-n junction, the first region is formed on the surface of the second region, and the third region is formed on the surface of the second region.
A solar cell characterized in that a region is formed on the surface of the fourth region.