JP2002222973A - Photoelectric converter and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a bonding layer or a BSF layer in simple steps by forming the boding layer by a thermal diffusion, then switching to an oxygen atmosphere in a furnace, and annealing to form a protective film on a surface of a substrate. SOLUTION: A method for manufacturing a photoelectric converter comprises the steps of forming the bonding layer by the thermal diffusion in an atmosphere containing an N-type or P-type dopant on a P-type or an N-type silicon substrate in a diffusion furnace, then switching the atmosphere in the furnace to the oxygen atmosphere, and starting annealing to form the protective film on the surface of the silicon substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion element and a method for manufacturing the same, and more particularly to a photoelectric conversion element for a lower cell of a tandem solar cell capable of improving the photoelectric conversion efficiency of a practical silicon solar cell and increasing the efficiency. Is applicable as
Furthermore, the present invention relates to a photoelectric conversion element applicable to a thin crystalline silicon solar cell, a photosensor, and the like, and a method for manufacturing the same.

[0002]

2. Description of the Related Art A photoelectric conversion device that directly converts sunlight into electricity using a semiconductor is called a solar cell. At present, amorphous solar cells and the like are partially used according to various uses on the ground, but single-crystal or polycrystalline silicon solar cells having a single junction are mainly used in practical solar cells. .

[0003] This solar cell uses a P-type silicon substrate. As a basic process, phosphorus is thermally diffused on the light receiving surface side, Al paste is sintered on the back surface, and an electric field is formed inside the cell. This electric field is generally generated by BSF (Back Surface Fie
ld: back surface electric field). The cell structure is N
⁺ / P / P ⁺ . In this solar cell, it varies with the substrate materials or the like, the back surface of the P ⁺ structure, the conversion efficiency 16
A good value of 24% is obtained.

As a new attempt, the University of Neuchatel has announced a tandem type solar cell having two junctions shown in a PIN / PN structure (2nd World Conference on Photovoltaics 99). June, Vienna). This is a stacked solar cell in which a PIN upper amorphous silicon photoelectric conversion part and a PN lower microcrystalline silicon photoelectric conversion part are directly connected. In this solar cell, the photoelectric conversion efficiency is 10.7.
%.

[0005]

The BSF solar cell mainly composed of aluminum has a large difference between the coefficient of thermal expansion of the Al-Si layer and the remaining Al layer on the back surface and the coefficient of thermal expansion of silicon. Cells tend to be bent.
In particular, when the thickness of the substrate is 250 μm or less, a large warpage (3.5 mm or more) occurs, so that it is difficult to manufacture a large-area solar cell, and it is difficult to reduce the cost by increasing the area.

Although it is considered that the tandem type solar cell can achieve high efficiency by wavelength division, when the thin film silicon cell is used as the upper cell, it is caused by the P-I- N, and the bottom cell is N
It is necessary to form a P layer on the incident side of the mold substrate. The backside BSF layer requires N ⁺ , and the practical process is limited to phosphorus diffusion in consideration of the process cost.
However, there is a problem that the number of steps increases if thermal diffusion is performed twice on the same substrate.

The present invention has been made in view of such circumstances. After forming a bonding layer by thermal diffusion, the inside of the furnace is switched to an oxygen atmosphere and slowly cooled to perform a protective film on the substrate surface. To form a bonding layer and a BSF in a simple process.
The present invention also provides a photoelectric conversion element which forms a layer and is capable of realizing a solar cell having the above two structures, that is, a large-area solar cell or a tandem-type solar cell at a low cost, and a method for manufacturing the same. Things.

[0008]

According to the present invention, a bonding layer is formed on a silicon substrate of a first conductivity type by thermal diffusion in an atmosphere containing a dopant of a second conductivity type in a diffusion furnace. A method for manufacturing a photoelectric conversion element, comprising a step of switching to an atmosphere and starting slow cooling to form a protective film on a surface of a first conductivity type silicon substrate.

In the present invention, the first conductivity type and the second conductivity type are N-type if the first conductivity type is P-type and second conductivity type if the first conductivity type is N-type. Means that the type is P-type.

According to the present invention, after the formation of the bonding layer by thermal diffusion, a protective film in a subsequent thermal diffusion step of forming a BSF layer, for example, is simultaneously formed in the same furnace. This method can reduce the number of oxidation furnaces and the number of steps, and at the same time, make the substrate diffusion length, which is a semiconductor characteristic that directly affects the characteristics of the photoelectric conversion element, the same value as before diffusion. In addition, a bonding layer for a photoelectric conversion element and a BSF layer can be obtained in a favorable state in terms of both cost and characteristics.

In particular, it is possible to form a protective film in the next diffusion step while obtaining a high oxidation rate by slow cooling by switching to an oxygen atmosphere, and to minimize a decrease in the impurity concentration Cs on the diffusion surface under the oxide film. A photoelectric conversion element with high conversion efficiency can be obtained at low cost.
In other words, since the steep impurity concentration gradient on the cell surface has the effect of increasing the collection efficiency of the generated carriers inside the cell due to the internal electric field, it is necessary to prevent Cs from decreasing so much on the surface.

Slow cooling conditions, ie, oxygen flow rate, slow cooling rate,
According to experiments, the temperature at the time of taking out from the furnace is, for example, by open-tube diffusion, when a cylindrical open tube having an inner diameter of 14 cm (a cross-sectional area of about 150 cm ² ) and a length of 120 cm is used, oxygen during slow cooling is used. The flow rate is desirably 1 to 5 liters / minute.
For example, when boron diffusion is performed before slow cooling, if the flow rate of the boron carrier gas is set to 0.05 to 0.45 liter / min, the density of the boron carrier gas becomes Since it is in the range of 0.25 to 2.5%, it is desirable to apply an oxygen flow rate such that the oxygen density is in the range of 5 to 30%, based on this condition, as the atmosphere condition during slow cooling. . The slow cooling rate is 1 to 10
In the range of ° C./min, the temperature at which the substrate is drawn out of the furnace after slow cooling is desirably in the range of 600 to 750 ° C. In this range, the diffusion length characteristics of the substrate are lower than those without slow cooling. The improvement effect was shown.

According to this method, first, boron is diffused in a nitrogen atmosphere. Next, while performing slow cooling, the atmosphere is switched to an oxygen atmosphere so that oxidation proceeds quickly. Thereby, the necessary thickness of the protective film can be almost secured. In the latter half of the slow cooling, an oxide film having a sufficient thickness can be formed while ensuring the quality more than the thickness. In other words, this simple process of slow cooling alleviates the residual stress in the polycrystalline silicon substrate and suppresses the occurrence of defects inside the crystal due to high temperatures.
After the boron diffusion, the minority carrier diffusion length of the polycrystalline silicon substrate is not deteriorated (see FIG. 4), and the minority carrier diffusion length almost equal to that before the boron diffusion can be maintained, and an oxide film having a sufficient thickness is obtained. Can be.

As the first conductivity type silicon substrate, it is desirable to use a single crystal or polycrystalline silicon substrate having a resistivity in the range of 0.1 to 10 Ω-cm in terms of conversion efficiency (see FIG. 5).

Further, as the first conductivity type silicon substrate,
It is desirable to use a P-type silicon substrate having a thickness of 250 μm or less, and when such a substrate is used, a BSF layer is formed on the back surface of the P-type silicon substrate by the same boron diffusion of the first conductivity type. The electrode baking temperature on the back surface is desirably equal to or lower than the eutectic temperature of the electrode material and silicon. This condition gives a range in which good solar cell characteristics can be obtained in relation to the substrate type, substrate thickness, and electrode forming temperature.

Using a N-type silicon substrate as the first conductivity type silicon substrate and boron as the second conductivity type dopant, a crystalline silicon lower photoelectric conversion element is manufactured. A tandem photoelectric conversion element may be formed by stacking photoelectric conversion elements. When the configuration of the substrate conductivity type and the type of the dopant are specified as described above, when the photoelectric conversion element of the present invention is applied to a solar cell, good characteristics as a tandem solar cell can be obtained.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited by this. In the following embodiments, examples in which the photoelectric conversion element of the present invention is applied to a solar cell using a polycrystalline silicon substrate will be described. First, a method for manufacturing a solar cell according to the present invention will be described. For comparison of manufacturing steps, a conventional manufacturing process will be described as a comparative example.

In the conventional manufacturing process, as shown in FIG. 1, first, the N-type polycrystalline silicon substrate 21 is cleaned (S
TEP1). Next, boron diffusion is performed on the N-type silicon substrate 21 in a diffusion furnace to form a boron diffusion layer 22 (STEP 2).

Next, a protective film 23 is formed on the boron diffusion layer 22 on the front surface (light receiving surface) side in the oxidation furnace (STEP).
3). Next, unnecessary boron diffusion layers 22 on the back surface are removed using a resin mask and chemical wet etching (ST).
EP4). Instead of the chemical wet etching, it is also possible to use dry etching by sputtering.

Next, phosphorus diffusion is performed on the etching-removed surface on the back surface to form a phosphorus diffusion layer 24 (STEP 5). At this time, the protective film 23 prevents the boron diffusion surface on the front surface from invading the phosphorus impurity. Next, a back electrode 25 is formed,
The protection film 23 is removed (STEP 6).

The above is the conventional manufacturing process. next,
The manufacturing process of the solar cell of the present invention will be described. The manufacturing process of the present invention is characterized in that a protective film at the time of the next thermal diffusion, for example, when a BSF layer is formed, is formed simultaneously with the bonding layer.

In the manufacturing process of the solar cell according to the present invention, as shown in FIG. 2, first, the N-type polycrystalline silicon substrate 21 is cleaned (STEP 1). This step corresponds to ST in FIG.
Same as EP1.

Next, boron diffusion is performed on the N-type silicon substrate 21 in a diffusion furnace to form a boron diffusion layer 22. Immediately after the formation of the boron diffusion layer 22, in the same diffusion furnace, the atmosphere in the diffusion furnace is switched to an oxygen atmosphere, and while being cooled slowly, an oxide film (BSG: Boron) is used as the protective film 23 for the next thermal diffusion step. Silicon Glass) is simultaneously formed (STEP 2).

Next, unnecessary boron diffusion layer 22 and protective film 23 on the back surface are removed by using a resin mask and chemical wet etching (STEP 3). This step corresponds to the STE of FIG.
Same as P4.

Next, phosphorus diffusion is performed on the etching-removed surface on the back surface to form a phosphorus diffusion layer 24 (STEP 4). This step is the same as STEP 5 in FIG. Next, a back electrode 25 is formed, and the protective film 23 is removed (STEP 11).
5). This step is the same as STEP 6 in FIG.

In this manufacturing method, as shown in STEP 2 of FIG. 2, the protective film 23 is formed simultaneously with the diffusion of boron. S
The steps after STEP 3 are the same as those after STEP 4 in FIG.

In the method for manufacturing a solar cell according to the present invention, the use of an expensive oxidation furnace can be eliminated. Furthermore, by slow cooling in an oxygen atmosphere after boron diffusion, the minority carrier diffusion length of the substrate, which is a semiconductor characteristic that directly affects the solar cell characteristics, can be maintained at the same quality as before diffusion. Thus, a diffusion layer and a back surface field (BSF) layer for a solar cell can be obtained in a favorable state in terms of cost and characteristics.

In this manufacturing method, after boron diffusion, the atmosphere in the furnace is switched to an oxygen atmosphere to perform slow cooling. The slow cooling effect will be described. FIG. 3 is a graph showing a series of sequences from boron diffusion to slow cooling and removal.

As shown in this graph, in the present manufacturing method, the following process is executed. In the manufacturing process of the polycrystalline silicon solar cell, for example, an inner diameter of 14 cm
An open pipe diffusion is performed using a cylindrical open pipe having a cross section of about 150 cm ² and a length of 120 cm, and a bonding layer is formed on a sample (silicon substrate) by thermal diffusion of boron at a high temperature of about 950 ° C.

At this time, the diffusion atmosphere is set such that the flow rate of the boron carrier gas is 0.05 l / min, the flow rate of N _{2 is} 5 l / min, and the flow rate of O _{2 is} 0.1 l / min. In this case, the volume of the diffusion furnace is about 180
Since it is 00 cm ³ , the density of the boron carrier gas is about 0.28% and the oxygen density is about 0.56%. In this state, the sample is held in a furnace for about 15 minutes to form a bonding layer.

After the formation of the bonding layer, an oxide film (BSG) serving as a protective film is formed on the diffusion layer in the same diffusion furnace.
Is switched to an oxygen atmosphere to form Annealing conditions at this time, the boron carrier gas flow rate gradually cooled Atmosphere 0.05 l / min N ₂ flow rate: None O ₂ flow rate: set to 5.0 liters / min. In this case, considering the volume of the diffusion furnace, the density of the boron carrier gas is about 0.28%, which is the same as before the slow cooling, but the oxygen density changes greatly to about 28%. Then, the furnace temperature is lowered at a cooling rate of 10 ° C./min or less, for example, 5 ° C./min, and when the furnace temperature becomes 750 ° C. or less, a sample is withdrawn.

In the conventional method, there is a problem that the minority carrier diffusion length of the polycrystalline silicon substrate is reduced after boron diffusion. However, in this manufacturing method, boron diffusion is performed in a nitrogen atmosphere, and then, Since the oxidization proceeds quickly by switching to the oxygen atmosphere, the thickness required for the protective film can be substantially secured. This oxidation is effective in ensuring substrate quality in the latter half of slow cooling.

FIG. 4 is a graph showing a comparison between the diffusion length before diffusion and the diffusion length after diffusion without annealing. As shown in this graph, an example of the slow cooling effect by the N-type cast substrate is that, as shown in this graph, the slow cooling process reduces the residual stress in the polycrystalline silicon substrate, suppresses the generation of defects inside the crystal due to high temperature, and increases the boron diffusion. The minority carrier diffusion length of the subsequent polycrystalline silicon substrate did not decrease, and the minority carrier diffusion length almost equal to that before boron diffusion could be maintained. At the same time, an oxide film having a sufficient thickness to serve as a protective film for diffusion is obtained under the oxidizing conditions during the slow cooling.

FIG. 5 is a graph showing the relationship between the resistivity of the silicon substrate and the conversion efficiency of the solar cell. The silicon substrate shows almost the same value for both single crystal and polycrystalline silicon substrates. In the figure, black circles indicate a P-type substrate, and white squares indicate an N-type substrate.

As shown in this graph, the conversion efficiency of the solar cell depends on the resistivity of the silicon substrate. Therefore, the resistivity of the silicon substrate is 0.1 to 10 Ω-c.
It is desirable to use a single crystal or polycrystalline silicon substrate in the range of m.

Embodiment 1 A manufacturing method of the present invention is applied to a large-area 150 mm
An example in which the present invention is applied to the manufacture of a corner polycrystalline silicon solar cell will be described.

FIG. 7 shows a large surface manufactured by the manufacturing method of the present invention.
Explanatory diagram showing the cross-sectional state of a product, a thin single-junction solar cell
It is. In the figure, reference numeral 1 denotes an aluminum back surface electrode.
Pole, 2 is P⁺Layer (P-type silicon layer: BSF layer), 3 is P
Type polycrystalline silicon substrate, 4 is N ⁺Layer (N-type silicon
Layer), 5 is an antireflection film made of SiNx, 6 is a surface electrode
It is.

The P-type polycrystalline silicon substrate 3 is a 150-mm square, 150-μm thick P-type cast silicon polycrystalline substrate obtained by slicing from a cast ingot, and has a substrate resistivity of 0.5 Ω-cm. .

The manufacturing process of the present solar cell is slightly different from the process shown in FIG. 2 because the silicon substrate is a P-type, but after the bonding layer is formed, the atmosphere in the furnace is switched to an oxygen atmosphere, and the furnace is gradually cooled. While forming a protective film in the next thermal diffusion step.

First, after cleaning the polycrystalline silicon substrate 3, the inner diameter is 20 cm (cross-sectional area is about 300 cm ² ) and the length is 20 cm.
Boron was diffused using a 0 cm cylindrical high-temperature boron diffusion furnace (up to 950 ° C.), followed by slow cooling in an oxygen atmosphere to form a P ⁺ layer 2 on the front and back surfaces of the cell. The boron diffusion sequence was performed according to the sequence shown in FIG.

Specifically, the sample was slowly put into a high-temperature boron diffusion furnace, and after the temperature was stabilized, the diffusion atmosphere (BBr ₃ carrier gas flow rate CN ₂ : 0.17 liter / min, nitrogen flow rate N ₂ : 17.0 liter) / Min, oxygen flow O ₂ :
At a rate of 0.34 l / min.), And immediately thereafter, the diffusion atmosphere was changed to an oxygen atmosphere (BBr ₃ carrier gas flow rate CN ₂ : 0.17) in the same diffusion furnace.
Liter / min, nitrogen flow rate N ₂ : 0 liter / mi
n, oxygen flow rate O ₂ : 17 l / min), and the furnace temperature was gradually lowered at a cooling rate of 5 ° C./min. After the furnace temperature was lowered to 750 ° C. or lower, a sample was drawn.

Next, the P ⁺ layer 2 on the surface side is removed with a mixed acid,
While exposing the silicon substrate 3 and leaving the protective film (BSG) grown at the time of boron diffusion on the P ⁺ layer 2 on the back surface, a PO temperature at a furnace temperature of 850 ° C.
Phosphorus diffusion by Cl ₃ source, thickness about 0.32μ
An N ⁺ layer 4 having a thickness of m and an impurity concentration of about 10 ²⁰ cm ⁻³ was formed (PN junction).

FIG. 6 is a graph showing the relationship between the depth of the N ⁺ layer 4 and the concentration. For the N-type diffusion layer 4, the SR (Spread
As a result of measuring the profile of the phosphorus impurity dopant by the ing resistance method, as shown in this graph, a diffusion layer profile 20 capable of performing favorable photoelectric conversion was obtained.

The profile 19 is shown for comparison.
This profile 19 was obtained by a conventional oxidation method using water vapor, and this oxide film was effective as a protective film, but became a profile in which a significant decrease in the surface impurity concentration Cs of the diffusion layer was caused. Was.

Thereafter, plasma C is applied to the surface of the N ⁺ layer 4.
A silicon nitride film was formed as an antireflection film 5 by VD. The surface electrode 6 was formed by applying a paste material made of Ag for an electrode by a screen printing method and firing it.

The back electrode 1 was formed by coating the entire surface with an Al paste and firing at a temperature of about 400.degree. The lead wire was connected to each electrode, and the solar cell of the present invention was completed.

The BSF layer 2 formed by boron diffusion is
The impurity concentration was almost the same as that of the Al paste method, and good cell characteristics were obtained. (Short circuit current Jsc = 36 mA / cm
² , open circuit voltage Voc = 0.625 V, fill factor FF = 0.76, conversion efficiency Eff = 17.
1%). Further, the warpage of the cell can be reduced to 1.5 mm or less, and the problem of large warpage of the cell by the conventional method (3.
5 mm or more).

Although a cast substrate is used in this embodiment, a similar effect can be obtained by using a ribbon substrate or the like.

Embodiment 2 Next, an example in which the manufacturing method of the present invention is applied to a tandem solar cell will be described. FIG. 8 is an explanatory diagram showing an example of a stacked silicon photoelectric conversion device manufactured by the manufacturing method of the present invention. Here, a tandem solar cell having a lower cell manufactured by the manufacturing method of the present invention is taken as an example, and a cross-sectional state thereof is shown.

In the figure, 7 is a three-layer metal electrode made of Ti / Pd / Ag, 8 is an N ⁺ layer (N-type silicon layer), 9
Is an N-type polycrystalline silicon substrate, 10 is a P ⁺ layer (P-type silicon layer), 11 is an intermediate layer, 12 is an N-type amorphous silicon semiconductor, 13 is an I-type amorphous silicon semiconductor, 1
4 is a P-type amorphous silicon semiconductor, 15 is a transparent electrode formed of a transparent conductive film made of ITO, and 16 is A
g is a surface collecting electrode, 17 is an upper photoelectric conversion element part, and 18 is a lower photoelectric conversion element part.

In this tandem solar cell, the upper photoelectric conversion element portion 17 has a PIN structure made of amorphous silicon material having a high energy band gap. The intermediate layer 11 is formed of a zinc oxide film. The lower photoelectric conversion element portion 18 is a polycrystalline silicon solar cell manufactured by the manufacturing method of the present invention. From these three parts, a two-terminal photoelectric conversion device of a two-layer tandem series connection was formed (hereinafter referred to as a tandem solar cell).

In the manufacturing process of the lower photoelectric conversion element portion 18, the boron diffusion sequence was performed according to the sequence shown in FIG. Since the slow cooling process was performed in an oxygen atmosphere, a BSG film of 200 nm or more was simultaneously formed on the surface where boron was diffused to the temperature at which the sample was drawn. The BSG film is used as a protective film for the surface PN junction in the phosphorous diffusion process for forming the N ⁺ layer (BSF layer) 8 on the back surface. With this method, the conventional step of forming a protective film could be omitted.

In the manufacture of the lower photoelectric conversion element portion 18,
Is the crystal plane (100), and the resistivity is 0.1 to 5.0 Ω-cm.
N-type polycrystalline silicon substrate 9 was used. This silicon base
The plate 9 is made alkaline (NH_FourOH: H_TwoO_Two: H_TwoO = 1:
1: 5) and acidic (HCl: H _TwoO_Two: H_TwoO = 1:
1: 5) After cleaning with a chemical solution for 30 minutes each,
High temperature boron diffusion furnace (up to 950 ℃)
Insert slowly and allow the temperature to stabilize before the diffusion atmosphere (BB
r_ThreeCarrier gas flow rate CN_Two: 0.05 liter / mi
n, nitrogen flow rate N_Two: 5.0 liter / min, oxygen flow rate
O_Two: 0.1 liter / min), boron for 15 minutes
Immediately after performing thermal diffusion, in the same diffusion furnace, in a diffusion atmosphere
In an oxygen atmosphere (BBr_ThreeCarrier gas flow rate CN_Two: 0.
05 liter / min, nitrogen flow rate N_Two: 0 liter / m
in, oxygen flow rate O_Two: 5 liter / min)
The furnace temperature was gradually lowered at a cooling rate of 5 ° C./min. Furnace temperature
Pull out the sample after the temperature drops below 750 ° C
Was. Approximately 0.45μm thick by thermal diffusion method, impurity concentration
About 10²⁰cm^-3Was formed.

The backside P-type silicon layer is removed with a mixed acid, and phosphorus is diffused by a POCl ₃ source at a furnace temperature of 850 ° C. while the front surface BSG film remains on the P-type silicon layer 10 to a thickness of about 0.32 μm. Then, an N-type silicon layer 8 having an impurity concentration of about 10 ²⁰ cm ⁻³ was formed (BSF).

After removing the silicon oxide on the front and back surfaces,
A three-layer metal electrode 7 of Ti / Pd / Ag was deposited and annealed in a nitrogen atmosphere. Thus, the lower photoelectric conversion element portion 18 formed by the slow cooling process was obtained.

Next, on the lower photoelectric conversion element portion 18, the zinc oxide intermediate layer 11 having the above-described textured surface is formed.
Was formed by a sputtering method (DC). The atmosphere of the sputter deposition apparatus is a mixed gas of argon (Ar) and oxygen (O ₂ ), and the zinc oxide intermediate layer 11 having a thickness of 30 nm is formed while controlling the degree of vacuum to about 6 × 10 ⁻³ Torr. did. Annealing was performed at 200 ° C. in a nitrogen atmosphere.

Further, an upper photoelectric conversion element portion 17 was formed on the intermediate layer 11 by a plasma CDV method.
The substrate temperature is about 150 to 180 ° C, and the pressure in the film forming chamber is
0.3 Torr, the energy supplied to the plasma is ~
30 mW / cm ² . Source gases include SiH ₄ (silane), H ₂ (high-purity hydrogen), B ₂ H ₆ (diborane), P
H ₃ (phosphine) was used.

Then, an N-type amorphous silicon semiconductor 12, an I-type amorphous silicon semiconductor 13 (thickness 300 nm) to which hydrogen was added, and a P-type amorphous silicon semiconductor 14 were formed. Further, a transparent electrode 15 having a thickness of 107 nm was deposited by a sputtering method. Further, the surface collecting electrode 16 was formed by electron beam evaporation to complete a tandem solar cell.

In this embodiment, the N-type polycrystalline silicon substrate is used, but the same effect can be obtained even when a ribbon substrate is used. Similarly, a P-type polycrystalline silicon substrate may be used.

In the production of the N-type polycrystalline silicon lower photoelectric conversion element in the amorphous cell (PIN) / polycrystalline silicon laminated solar cell (PN), the above-described production method was used. The process of forming the battery cells was simplified, and the characteristics of the lower photoelectric conversion element were significantly improved.

In this stacked solar cell, since the short-circuit current is affected by the upper amorphous silicon current, the important parameters that the lower cell gives to the stacked solar cell characteristics are the open voltage Voc and the fill factor FF. It is. According to the manufacturing method of the present invention, since the deterioration of the minority carrier diffusion length of the substrate due to high-temperature diffusion can be prevented,
The open circuit voltage Voc was improved from 0.57 V to 0.59 V, and the fill factor FF was also improved from 0.74 to 0.79. As a result, the conversion efficiency of the stacked solar cell has been greatly improved from the conventional 12% to 16%. In addition, since the BSG film is used as a protective film, it can be applied to a substrate having an uneven surface.
The manufacturing process of the lower photoelectric conversion element can be simplified.

Further, in the application to a large-area, thin cast solar cell, the impurity concentration due to boron diffusion is higher than that of the Al paste by one digit or more, and the electric field strength of the BSF is increased. can do.

Further, according to the manufacturing method of the present invention, it is possible to lower the firing temperature of the back electrode. This lowering of the temperature can reduce the warpage of the solar cell to 1.5 mm or less, so that the problem of large warpage (3.5 mm above) of the conventional cell could be solved.

That is, according to the present invention, the following effects can be obtained. Since the protective film in the diffusion step of forming the back surface electric field layer is formed at the same time, the oxidation step in the oxidation furnace can be reduced. Since the substrate extension length, which is a semiconductor characteristic that directly affects the solar cell characteristics, can be set to the same value as before diffusion, the quality can be maintained. By minimizing a decrease in the impurity concentration Cs on the diffusion surface below the oxide film, a photoelectric conversion element with high conversion efficiency can be obtained at low cost.

[0065]

According to the present invention, a protective film is simultaneously formed in the next thermal diffusion step of forming a BSF layer, for example, immediately after the formation of the bonding layer by thermal diffusion, so that the oxidation furnace and the number of steps can be reduced. . Also, by forming this protective film,
Since the diffusion length of the semiconductor substrate, which directly affects the characteristics of the photoelectric conversion element, can be set to the same value as before diffusion, the manufacturing process can be simplified while maintaining high quality.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a manufacturing process of a conventional solar cell.

FIG. 2 is an explanatory diagram showing a manufacturing process of the solar battery cell of the present invention.

FIG. 3 shows a gradual cooling from boron diffusion in the production process of the present invention;
It is a graph which shows a series of sequences until taking out.

FIG. 4 is a graph showing a comparison of the diffusion length before and after diffusion with and without slow cooling.

FIG. 5 is a graph showing the relationship between the resistivity of a silicon substrate and the conversion efficiency of a solar cell.

FIG. 6 is a graph showing the relationship between the depth and the concentration of an N-type diffusion layer.

FIG. 7 is an explanatory diagram showing a cross-sectional state of a large-area, thin single-junction solar cell manufactured by the manufacturing method of the present invention.

FIG. 8 is an explanatory view showing an example of a stacked silicon photoelectric conversion device manufactured by the manufacturing method of the present invention.

[Explanation of symbols]

 1, 25 Back electrode 2 P-type silicon layer 3 P-type polycrystalline silicon substrate 4 N-type silicon layer 5 Antireflection film 6 Surface electrode 7 Three-layer metal electrode 8 N-type silicon layer 9, 21 N-type polycrystalline silicon substrate 10 P Type silicon layer 11 Intermediate layer 12 N-type amorphous silicon semiconductor 13 I-type amorphous silicon semiconductor 14 P-type amorphous silicon semiconductor 15 Transparent electrode 16 Surface collecting electrode 17 Upper photoelectric conversion element part 18 Lower photoelectric conversion element part 22 Boron diffusion layer 23 Protective film 24 Phosphorus diffusion layer

Continued on the front page (72) Inventor Toru Fukui 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka F-term (reference) 5F051 AA02 AA03 AA05 AA16 CB14 CB20 CB21 CB24 CB29 DA04 DA15 DA20 FA06 GA04 HA20

Claims (5)

[Claims] After a bonding layer is formed on a silicon substrate of a first conductivity type in an atmosphere containing a dopant of a second conductivity type by thermal diffusion in a diffusion furnace, the atmosphere in the furnace is switched to an oxygen atmosphere, and slow cooling is performed. A method for manufacturing a photoelectric conversion element, comprising a step of starting and forming a protective film on a surface of a first conductivity type silicon substrate. 2. An atmosphere condition during slow cooling is an oxygen density of 5 to 30.
% And the slow cooling rate is in the range of 1 to 10 ° C./min, and the temperature at which the substrate is drawn out of the furnace after slow cooling is 600 to 7
The method according to claim 1, wherein the temperature is in a range of 50C. 3. The method according to claim 1, wherein a single-crystal or polycrystalline silicon substrate having a resistivity in the range of 0.1 to 10 Ω-cm is used as the first conductivity type silicon substrate. Photoelectric conversion element. 4. A silicon substrate having a first conductivity type of 250 μm
The photoelectric conversion element according to claim 1 or 2, wherein a P-type silicon substrate having a thickness of not more than m is used, and an electrode sintering temperature on a back surface of the P-type silicon substrate is set to be equal to or lower than a eutectic temperature of the electrode material and silicon. The photoelectric conversion element manufactured by the manufacturing method of 1. 5. A photoelectric conversion device manufactured by the method for manufacturing a photoelectric conversion device according to claim 1, wherein an N-type silicon substrate is used as the first conductivity type silicon substrate, and boron is used as the second conductivity type dopant. A tandem-type photoelectric conversion element obtained by stacking one or more photoelectric conversion elements on a substrate.