JPH04177880A - Solar cell and its manufacture - Google Patents

Abstract

PURPOSE:To enable energy efficiency to be enhanced inexpensively by depositing a film on a surface of a silicon film and at the same time forming recessed and projecting parts and then enabling a polycrystalline grain diameter to be fully large. CONSTITUTION:A substrate 301 is prepared, a nucleus forming density of a surface of the substrate is controlled by controlling type, flow, deposition temperature, and pressure of a deposition gas, a polycrystalline silicon film 303 whose average grain diameter exceeds 1mum is deposited to a film thickness with an average grain diameter or more from gaseous phase on the substrate 301, thus forming a silicon film with recessed and projecting parts on the surface. Then, by annealing a silicon film at a temperature below a melting point of 1000 deg.C or higher, the crystal grain diameter can be increased to a deposition film thickness or larger and then a solar cell is formed on an obtained polycrystalline silicon film 306 with a large grain diameter, thus obtaining a solar cell with an energy conversion efficiency which is equivalent to the solar cell using a single crystal silicon inexpensively.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a solar cell and a method for manufacturing the solar cell;
BACKGROUND OF THE INVENTION [Related Art] Particularly Related to Solar Cells with High Energy Conversion Efficiency and Methods for Manufacturing the Solar Cells Solar cells are used as drive energy sources in various types of equipment.

A solar cell uses a pn junction as a functional part, and silicon is used as a semiconductor forming the pn junction. From the standpoint of efficiency in converting light energy into electromotive force, it is preferable to use single crystal silicon, but amorphous silicon is considered advantageous from the standpoint of increasing the area and reducing cost.

In recent years, the use of polycrystalline silicon has been considered for the purpose of achieving low cost comparable to amorphous silicon and high energy conversion efficiency comparable to single crystal silicon. However, in the conventionally proposed method, it is difficult to reduce the thickness to 0.3 mm or less because a block of polycrystals (grain size from 100 μm to several mm) is sliced into plate-like bodies. Therefore, the thickness is greater than necessary to absorb a sufficient amount of light, and in this respect, the effective use of the material has not been sufficient.

Therefore, attempts have been made to form thin films of polycrystalline silicon using thin film formation techniques such as chemical vapor deposition (CVD), but with this method, the crystal grain size of the formed film is only a few hundredths of a percent. The energy conversion efficiency is only about microns, and the energy conversion efficiency is lower than that of the bulk polycrystalline silicon slicing method.

In addition, attempts have been made to increase the crystal grain size by irradiating the polycrystalline silicon thin film formed by the above CVD method with laser light and melting and recrystallizing it, but it has not been possible to reduce costs sufficiently, and stable production has not been achieved. It is also difficult.

As mentioned above, polycrystalline silicon solar cells have been researched with the aim of simultaneously obtaining both low cost and high energy conversion efficiency. A solar cell with regular inverted pyramid-shaped irregularities on the cell surface to improve the capture of incident light has been reported by the University of South Wales in Australia (Bakers
,A.W., ,Zhao.

J., ,Wang, A., ,Milne, A.M., ,Dai.
, X, and Green, M.A.

+A 23'1g Efficiency 5i
licon 5olar Ce1l, -Proce
edings of 9th E, C, Photo
ovoltaic Solar Energy Con
ference, pp, 328-329.5ept, 1
989. ).

Such solar cells have achieved the highest level of energy conversion efficiency among solar cells using silicon, but they involve a complicated process of anisotropically etching a silicon X crystal substrate according to a certain pattern. It completely ignores cost reduction.

[Problem to be solved by the invention] In solar cells using polycrystalline silicon films, there are many contradictory factors between low cost and high energy conversion efficiency, and in the end, the power generation cost (1 watt There was no way to outperform amorphous silicon or single-crystal silicon solar cells in terms of the cost of generating electricity. One of the biggest reasons for this is that in order for a polycrystalline silicon solar cell to achieve conversion efficiency comparable to that of a single crystal, the crystal grain size must be on the order of several tens to 100 μm. Another major problem was that when special processes such as processing the cell surface were introduced to improve conversion efficiency, manufacturing costs quickly increased.

An object of the present invention is to solve the above problems and provide a solar cell that is inexpensive and has high energy efficiency, and a method for manufacturing the solar cell.

[Means for Solving the Problems] A solar cell and a method for manufacturing the solar cell according to the present invention include:
By controlling the nucleation density on the surface of the substrate, a polycrystalline silicon film with an average grain size of 1 μm or more is deposited from the vapor phase on the substrate to a thickness greater than the average grain size, and the surface is uneven. a step of forming a silicon film; a step of annealing the silicon film at a temperature of 1000°C or higher and lower than its melting point to increase the crystal grain size to a size equal to or larger than the deposited film thickness; The method is characterized by comprising at least a step of forming a solar cell in a large-grain polycrystalline silicon film.

[Function] The solar cell of the present invention has two major features. That is, 1) without performing any treatment such as etching after film deposition, that is, at the time the film is deposited, irregularities are formed on the film surface that promote the capture of incident light, and 2) annealing is performed after film deposition. Accordingly, the polycrystalline grain size has grown to about the same level as the film thickness or more, and the score is 02.

Next, detailed techniques or phenomena related to these characteristics will be explained in detail.

In general, polycrystalline silicon films deposited from the vapor phase (CVD) always have unevenness on the surface of the film, but the size of the unevenness depends on the average grain size of the film. There's a lot to do. Polycrystalline silicon films deposited by LPGVD typically have a grain size of 0.03 to 0.
．． 1 μm, and its unevenness is about 0.01 to 0.03 μm. Obtained by selective deposition method etc., resulting in 1μ
The unevenness of a polycrystalline silicon film with a crystal grain size of about m is
The average size is about 0.5 μm.

FIG. 1 is a graph showing the results of forming polycrystalline films having various crystal grain sizes by the CVD method and measuring the surface irregularities of each film. The surface unevenness was measured using an alpha step step meter, and the average value of the step difference was plotted.

The average grain size of the crystal is determined by the nucleation density (N, D,) during film deposition.
can be obtained by controlling the The nucleation density is determined by the flow rate of the deposition gas, the flow rate of the etching gas, the deposition temperature, the deposition pressure, the material of the deposition surface, etc. FIG. 2 shows the change in nucleation density when the flow rate of the etching gas (HCII) is changed. That is, the deposition surface material is SiO2 or 5isN4, and the type and flow rate of the deposition gas are 5iH2C1z/Hz = Q, 53/100 (j!/m
in), the deposition temperature, pressure, and time were 950°C and 15°C.
The temperature was fixed at 0 Torr for 20 minutes, and the nucleation density was controlled by changing only the flow rate of the HCf etching gas.

The nucleation density (N, D,) and the average grain size of the deposited film (G, S,
), the following relationship holds true: G, S, = (N, D,) - "", so the average grain size of the deposited film can be controlled by controlling the nucleation density.

Therefore, the surface unevenness of the deposited film can also be controlled.

■ Grain size Grain malleability in crystalline film As mentioned above, when a polycrystalline silicon film is deposited by, for example, the LPCVD method, the average grain size of the film is 0.03 to 0.
．． It will be about 1 μm. This particle size is mainly 600-650℃
This largely depends on the deposition temperature. As the deposition temperature increases, the average grain size of the film also increases, but unless operations such as introducing an etching gas to control the nucleation density as mentioned above are carried out, the average grain size will increase at most. 0.
It becomes only about 8 to 1.0 μm.

However, there is a method of growing small-grain polycrystals by heat treatment to grow the crystal grain size to about the thickness of the crystal film ((:, Daey
Owens and H. Heijligers;
Appl, Phys.

Lett, Vol, 2B, No, 10. pp.5
69, 1975) and the secondary grain growth method (Seco
ndary grain growth) has also been reported (C, V, Thompson, J, Appl, P
hys.

Vol, 58. pp. 763.1985), using the phenomenon reported in these papers, it is possible to form a polycrystalline film with a thickness of 100 μm by simply heat-treating Hayabusa at a temperature of about 1000°C or higher, or about 1200-1300°C. A crystal grain size of 100 μm or more can be easily obtained from a silicon film. Moreover, during this grain growth, only the grain boundaries move, and the surface shape does not change.

Next, an embodiment focusing on the process up to grain growth will be described mainly using FIG. 3.

[Embodiment example] (A) A base 301 is prepared. Basically, any material can be used as the substrate 301 as long as it can withstand the temperature of the heat treatment to be performed later. Examples include a transparent substrate such as a molten metal substrate, a high melting point metal substrate, a ceramic substrate, and even an electrode. When creating a solar cell of the type in which the lower electrode film 302 is formed on both sides, a lower electrode film 302 is formed on the surface of, for example, ceramic or the like. In this case, the lower electrode 302 also needs to be made of a high melting point material that can withstand heat treatment.

(B) A polycrystalline silicon film 303 having an average grain size of 1 μm or more is deposited from the gas phase. An average particle size of 1 μm or more means that
Since the nucleation density (N, D,) is less than 106 (/am2), for example, if the deposition surface material is 5i02, the type and flow rate of the deposition gas can be determined from the graph in Figure 2 by S i H2CJ:12/H2=0.53/10
0 (A/min, ), deposition temperature, and pressure, respectively.
Under conditions of 0° C. and 150 Torr, it is sufficient to flow HCj2 gas at a rate of 0.9 A/min or more.

Other examples of the deposition gas include, for example, S I H4
% S 12 Silane type such as H8, S i CJ24.
S i Chlorosilane type such as HCA3, 5fF4, Si
Examples include fluorosilane-based materials such as H2F2.5iHFs.

The deposition temperature is f! of the gas used. It varies greatly depending on the order, but in order to obtain a polycrystalline film with a grain size of 1 μm or more, the temperature is approximately 600°C.
A temperature higher than that is required. Further, since the deposition temperature greatly affects the film formation rate, it is preferably 900°C or more and 1200°C or less, and optimally 950°C or more and 1100°C or less.

The deposition pressure is several Torr-T; fEE (760 Torr
). However, since it is easier to control the nucleation density at a constant pressure, it is preferably 10 to 250 Torr.
, more preferably from 50 to 50, taking into consideration the deposition rate, etc.
It is 150 Torr.

By changing these conditions, the relationship between the nucleation density and the HC1 flow rate shown in FIG. 2 changes, but it is sufficient that a polycrystalline film with a grain size of 1 μm or more can be formed under each condition.

In addition, the surface unevenness of a film with a crystal grain size of 1 μm or more is 341
From the figure, it can be seen that the unevenness on the surface of the crystal is approximately 0.5 μm or more. This is to increase the efficiency of capturing incident light. It is meant to guide you and use it. Therefore, originally, both the level difference and the shape of the unevenness would be a problem, but in the present invention, the shape is not particularly controlled, and only the level difference is the problem. Regarding this level difference, experimental results show that when it becomes 0.5 μm or less, there is no significant effect. Further, since the increase in the capture efficiency of incident light becomes gradual when the step height is 2 μm or more, it is sufficient that the surface unevenness is 0.5 μm or more and 2 μm or less.

In other words, this means a crystal grain size of 1 μm or more and 7 μm or less. However, the minimum value specified is 1μ.
Considering only m, no upper limit is specified. As the crystal grain size increases, the deposition rate decreases, so when considering the optimum value, the crystal grain size should be 3 μm or more and 6 μm or less.

In order for the solar cell to be created to have properties comparable to those on a single-crystal substrate, it is preferable that the crystal grain size is approximately 100 μm or more, and therefore the deposited film thickness is also preferably approximately 100 μm or more. Taking into consideration the effective use of , the thickness is more preferably 100 μm or more and 200 μm or less, and optimally 120 μm or more and 150 μm or less. However, even if the film thickness is 100 μm or less, it is possible to increase the grain size to 100 μm or more by performing secondary grain growth as described below.

(C) 7: Grain growth is performed using -ru. The atmosphere for annealing is not particularly limited. 9 - For boat fishing, annealing is often carried out in an inert nitrogen atmosphere. The temperature is 1000°C or higher and below the melting point (1420°C). This is because below 1000°C it takes an extremely long time for grain growth, and above the melting point the film begins to melt, causing damage to the surface. This is because the unevenness disappears. In addition, in the case of silicon, when annealed at a temperature of about 1300°C or higher, the crystal grain size becomes larger than the film thickness due to secondary grain growth. Although a larger grain size is advantageous for solar cells, It is necessary to use the primary grain growth region (1000 to 1300° C.) and the secondary grain growth region (1300 to 1400° C.) in consideration of the heat resistance of the substrate, the stress of the film during high-temperature annealing, and the like.

Annealing time depends on film thickness and temperature, but approximately several minutes to 4
It takes about 0 minutes.

[Example] Next, the present invention will be explained in detail.

(Example-1) Example 1 will be explained using FIG. 4.

(A) A transparent fused quartz substrate 401 having a rectangular shape of 10 cm x 15 cm was used as a substrate.

(B) Polycrystalline silicon@4o3 was deposited by CVD method. The deposition conditions at this time are as follows.

Source gas/Yutching gas/Carrier gas: 5i
H2C12/HCI/H2 gas flow rate: 0.5310.
80/100 (It/win,) Deposition temperature=90
0°C Deposition pressure: 150 Torr Deposition time: 80 m1n.

The polycrystalline silicon film 403 thus obtained has a thickness of 1
00μm, average grain size 3μm, average height difference in surface unevenness about 1.
It was 2 μm.

(C) This polycrystalline film was annealed for 40 minutes at 1150't: in a nitrogen atmosphere. This results in an average particle size of approximately 100μ.
A large grain size polycrystalline film 406 of m was obtained.

(D) p′ region 407 and n′ to provide electrodes on the film surface
"regions 408 were formed. These regions were line-shaped and formed by ion implantation so that P+ and n9 were alternated at intervals of 50 μm. Boron and phosphorus ions were used for the ion implantation, respectively. there was.

(E) Aj2 electrode 409 was formed by mask evaporation. Figure 4 CF) shows a plan view of the Den8i409. A1
The thickness is 1 μm, and it is buttered into a comb shape.
P-type and n-type can be extracted from opposite directions. Note that after patterning the electrode, heat treatment was performed at 400° C. for 30 minutes in a hydrogen atmosphere to obtain ohmic properties between An and silicon. Finally, epoxy resin was applied as a passivation film 410.

A pin type solar cell was formed through the above process. This solar cell is of a type in which light enters from the transparent substrate side, but when the light that is perpendicularly incident from the substrate side tries to pass through the silicon, which is the active layer, t
The use efficiency of incident light can be increased by being reflected into the active layer by the unevenness on the 8ii side or by the wired electrodes. The average energy conversion efficiency of this solar cell was 11.8%.

(Example 2) Using the same substrate as in Example 1, a polycrystalline silicon film was deposited by the same method. However, the deposited film thickness was set to 40 μm. That is, the silicon used in Example-1,
Regarding the film deposition conditions, only the deposition time was set to 32 minutes.

Next, this silicon film was heated at 1350°C for 20 minutes in a nitrogen atmosphere.
Heat treatment was performed for 1 minute. The polycrystalline silicon film having a small grain size (3 μm) was heat-treated in the temperature range for secondary grain growth, so that the average grain size became about 80 μm.

After that, a pin type solar cell was formed in the same manner as in Example-1. The average energy conversion efficiency of this solar cell is 12
．． It was 0%.

(Example-3) Example 3 will be explained using FIG. 5.

(A) A 10cm x 15cm alumina ceramic substrate (sintered body) 5o1 is prepared as a substrate, and chromium [502] is vacuum-deposited onto this substrate to a thickness of 1 μm for the lower t8i.
Deposited.

(B) Polycrystalline silicon gso3 was deposited by CVD method. The deposition conditions at this time are as follows.

Source gas/etching gas/carrier gas: 5i
H2Ch/HC: 1/) I2 gas flow rate: 0.53/1
．． 10/100 (j2/min,) Doping gas/flow rate: 50ppm-PHs (H2 dilution)/
72 (sccm) Deposition temperature: 950°C Deposition pressure: 150 Torr Deposition time +90 m1n.

As a result, an n-type child-crystalline silicon film 503 with a film thickness of 100 μm and an average grain size of 4 μm2 and an average height difference of about 1.4 μm between surface irregularities.
was gotten.

(C) This polycrystalline film was heated to 4°C at 1150°C in a nitrogen atmosphere.
Annealed for minutes. As a result, a large-grain polycrystalline film 506 with an average grain size of about 100 μm was obtained. Note that silicide 507 was formed at the interface between the silicon film and the lower electrode, making it possible to establish ohmic contact with the electrode.

(D)! Top on the surface! To obtain the poles, 20 layers 508 were formed. This pI layer was formed by ion implantation.

Boron ions were used for ion implantation.

(E) ITOIIt509, which is a transparent conductive film, was
It was formed to a thickness of 0.0 mm by electron beam evaporation, and this was used as the upper electrode. Finally, epoxy resin was applied as a passivation film 510.

A pn type solar cell was formed through the process described above. The average energy conversion efficiency of this solar cell is 12
．． It was 3%.

(Example-4) Example 4 will be explained using FIG. 6.

(A) A 10 cm x 15 cm tungsten substrate 601 was prepared as a substrate.

(B) A polycrystalline silicon film 603 was deposited by CVD. The deposition conditions at this time are as follows.

Source gas/etching gas/carrier gas: 5i
)12ch/HCI/)12 Gas flow rate: 0.53/1
．． 10/loo (1/win,) Dovinku Gas/
Flow rate: 50ppc-PH1 (H2 dilution)/72
(sccm) Deposition temperature = 950°C Deposition pressure 150 Torr Deposition time: 28 ml n.

As a result, an n-type polycrystalline silicon film 60 with a film thickness of 40 μm, an average grain size of 3.5 μm, and an average step height of 1.3 μm on the surface unevenness.
3 was obtained.

(C) This polycrystalline film was heated at 1350°C for 20 minutes in a nitrogen atmosphere.
Annealed for minutes. As a result, a large-grain polycrystalline film 606 with an average grain size of about 100 μm was obtained. Note that silicide 607 is formed at the interface between the silicon film and the substrate, and ohmic contact with the substrate (lower ti8i>) can be established.

After that, a solar cell was formed using the same steps as in Example-3.

A pn type solar cell was formed through the process described above. The average energy conversion efficiency of this solar cell is 12
．． 2%).

[Effects of the invention] As explained above, in a polycrystalline silicon thin film solar cell, it is possible to create irregularities on the surface of the silicon film (at the same time as the @ deposition) and to make the polycrystalline grain size sufficiently large (several tens to hundreds of μ
m) Grain growth has made it possible to provide inexpensive solar cells with energy conversion efficiency comparable to solar cells using single-crystal silicon.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between grain size and surface unevenness when a polycrystalline silicon film is deposited. FIG. 2 is a graph showing an example of controlling the nucleation density after depositing a polycrystalline silicon film using HCJl etching gas. FIG. 3 is a process diagram illustrating the process of forming a polycrystalline silicon film among the typical solar cell forming processes of the present invention. FIGS. 4 to 6 are diagrams showing examples of the solar cell forming process of the present invention. (Explanation of symbols) 301.401, 501, 601...Substrate, 302.
502...lower electrode film, 303,403°503.6
03... Polycrystalline silicon film, 304... Film surface unevenness, 305... Grain boundary, 308,406°506.606
...Large grain size polycrystalline silicon film, 407.408,50
7,508,607...Ohmic contact layer (n
*, p* or silicide layer), 409,509...
Electrode, 410°510... Passive Basin 5-inch membrane. Figure 1 Particle size (μm) Figure 2 HCj2j flow rate (ff/min,) Figure 6

Claims (1)

[Claims] By controlling the nucleation density on the surface of the substrate, a polycrystalline silicon film having an average grain size of 1 μm or more is deposited from the vapor phase on the substrate to a thickness equal to or larger than the average grain size. , a step of forming a silicon film with an uneven surface, and a step of annealing the silicon film at a temperature of 1000°C or higher and lower than the melting point to increase the crystal grain size to a size equal to or larger than the deposited film thickness. and a step of forming a solar cell on the obtained large-grain polycrystalline silicon film, and a method for manufacturing the solar cell.