JP2005244071A - Solar cell and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar cell which can further increase a photoelectric conversion efficiency. <P>SOLUTION: The solar cell comprises a photoelectric conversion layer of a layer 7a of a first conduction type, an i type layer 7b, and a layer 7c of a second conduction type, each made of a crystallite silicon and stacked in this order. The crystallite silicon is provided between the i type layer 7b and the second-conduction-type layer 7c, and its crystallinity index is lower than that of the i type layer 7c. Since an interface level between the i type layer 7b and the second-conduction-type layer 7c is decreased, a photoelectric conversion efficiency is increased. Since a buffer layer 13 is made of a crystallite semiconductor, an electric field to be applied to the buffer layer 13 is smaller than that to be applied to a buffer layer 13 made of an amorphous semiconductor. For this reason, a strong internal electric field is applied to the i type layer 7b. As a result, carrier recombination in the i type layer 7b is decreased and the photoelectric conversion efficiency is increased. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a solar cell and a manufacturing method thereof.

  At present, fossil fuels such as oil, which are used as a major energy source, have a problem of carbon dioxide emission, which is concerned about future supply and demand and causes global warming. Thus, solar cells have attracted attention as an energy source that can replace fossil fuels such as petroleum.

  A solar cell uses a pn junction as a semiconductor photoelectric conversion layer that converts light energy into electric power, and silicon is generally used as a semiconductor material constituting the pn junction.

  In this thin film solar cell using silicon, it is preferable to use single crystal silicon in terms of photoelectric conversion efficiency, but in order to realize a larger area and lower cost, an amorphous silicon material is used as a photoelectric conversion layer. Some thin film solar cells have been put into practical use. On the other hand, in order to further stabilize the characteristics and increase the efficiency, a thin film solar cell in which a microcrystalline silicon material is applied as a photoelectric conversion layer has been studied. This microcrystalline silicon, like amorphous silicon, can be formed into a thin film by plasma CVD.

  As a general method for forming thin film silicon by plasma CVD, there is a method of using a source gas obtained by diluting a silane gas 10 times or more with hydrogen. When deposited using this source gas, a microcrystalline silicon film is obtained, and the crystallization rate can be increased by increasing the hydrogen dilution rate. It is known that when the silicon film is increased in volume while keeping the hydrogen dilution rate constant, the crystallization rate generally increases as the film thickness increases.

  Note that the microcrystalline silicon film is a thin film including a mixture of crystalline silicon and amorphous silicon.

  The photoelectric conversion efficiency of the current microcrystalline silicon solar cell is about 10% lower than the photoelectric conversion efficiency of 20% of the single crystal silicon solar cell. In the case of the microcrystalline silicon solar cell of about 1 μm, the amorphous silicon solar cell is It is about 7 to 8%, which is equivalent to the photoelectric conversion efficiency.

 In recent years, it has been strongly desired to improve the photoelectric conversion efficiency of microcrystalline silicon solar cells.

As one of the methods for that purpose, a method is known in which an interface layer made of amorphous silicon is inserted between an i-type microcrystalline silicon layer and a conductive type layer (for example, an n-type microcrystalline silicon layer) (for example, (See Patent Document 1). In this method, due to the presence of the interface layer, impurity diffusion from the conductive layer into the i-type microcrystalline silicon layer is reduced, and the photoelectric conversion efficiency is improved.
JP 2002-343991 A

 However, the photoelectric conversion efficiency cannot be sufficiently improved even by the above method, and further improvement of the photoelectric conversion efficiency is desired.

 This invention is made | formed in view of the situation which concerns, and provides the solar cell which can further improve a photoelectric conversion efficiency.

  The solar cell of the present invention includes a photoelectric conversion layer having a first conductive type layer made of a semiconductor, an i type layer made of a microcrystalline semiconductor, and a second conductive type layer made of a semiconductor stacked in this order, A buffer layer made of a microcrystalline semiconductor and having a crystallization rate lower than that of the i-type layer is provided between the second conductivity type layer.

  The present invention is characterized in that a buffer layer made of a microcrystalline semiconductor and having a crystallization rate lower than that of the i-type layer is provided between the i-type layer and the second conductivity type layer. Conversion efficiency is improved.

  The effect | action which improves a photoelectric conversion efficiency by providing a buffer layer is demonstrated as follows.

  When there is no buffer layer, the i-type layer and the second conductivity type layer are in direct contact. The i-type layer is made of a microcrystalline semiconductor in which a crystalline semiconductor and an amorphous semiconductor are mixed. Of the i-type layer, the crystalline semiconductor portion comes into contact with the second conductivity type layer to generate an interface state. On the other hand, even if the amorphous semiconductor portion of the i-type layer is in contact with the second conductive type layer, the interface does not generate many defects and hardly causes an interface state.

  In the present invention, a buffer layer made of a microcrystalline semiconductor is formed between the i-type layer and the second conductivity type layer. This buffer layer has a crystallization rate smaller than that of the i-type layer. That is, in this buffer layer, the proportion of the crystalline semiconductor portion is smaller than that of the i-type layer. That is, the proportion of the portion that often generates interface states is small. Therefore, according to the present invention, the interface state is reduced. Since the interface state is reduced, the recombination of carriers is reduced and the photoelectric conversion efficiency is improved.

  The “microcrystalline semiconductor” that forms the buffer layer does not include an entire semiconductor layer made of an amorphous semiconductor. When the entire buffer layer is made of an amorphous semiconductor, there may be a problem that the electric field applied to the buffer layer is increased and the internal electric field applied to the i-type layer is decreased. However, in the present invention, such a problem does not occur. . In the present invention, at least a part of the buffer layer is made of a crystalline semiconductor, so that the electric field applied to the buffer layer is reduced and the internal electric field applied to the i-type layer is increased. As a result, carrier recombination in the i-type layer is reduced, and the photoelectric conversion efficiency is improved.

  The above contents will be described with reference to energy band diagrams (FIGS. 4 to 6). 4 to 6 are reference diagrams used only for understanding of the present invention, and do not limit the scope of the present invention. In each figure, (a) is a sectional view and (b) is an energy band diagram.

  FIG. 4 shows a solar cell according to Conventional Example 1 that does not have a buffer layer. This solar cell includes a photoelectric conversion layer configured by laminating a first conductivity type layer 51, an i-type layer 53, and a second conductivity type layer 57 in this order. Interface states 59 and 61 are formed at the interface between the first conductivity type layer 51 and the i-type layer 53 and at the interface between the i-type layer 53 and the second conductivity-type layer 57, respectively. Here, the i-type layer 53 is formed from the left side to the right side. The crystallization rate of the i-type layer is initially small and increases as the film thickness increases. As described above, since the number of defect states increases as the crystallization rate increases, the number of interface states 59 in the left interface is smaller than the number of interface states 61 in the right interface. In the i-type layer 53, electrons 63 and holes 65 serving as carriers are generated and travel toward the first conductivity type layer 51 and the second conductivity type layer 57, respectively.

  FIG. 5 is a solar cell of the present invention. The difference from FIG. 4 is that a buffer layer 55 is formed between the i-type layer 53 and the second conductivity type layer 57. It should be noted that the number of interface states 61 is reduced due to the formation of the buffer layer 55. The reason is as described above.

  FIG. 6 shows a solar cell according to Conventional Example 2 having a buffer layer 67 made of an amorphous semiconductor. Since the buffer layer 67 is made of an amorphous semiconductor, a large electric field is applied to the buffer layer 67. Therefore, as can be seen from FIG. 6B, the internal electric field in the i-type layer 53 is reduced. As a result, the recombination probability of carriers in the i-type layer 53 increases, and the photoelectric conversion efficiency decreases.

 According to the present invention, the interface state between the i-type layer and the second conductivity type layer is reduced. As a result, the photoelectric conversion efficiency is improved.

 In addition, since the buffer layer is made of a microcrystalline semiconductor, the electric field applied to the buffer layer is smaller than when the buffer layer is made of an amorphous semiconductor or the like. Therefore, a strong internal electric field is applied to the i-type layer. As a result, carrier recombination in the i-type layer is reduced, and photoelectric conversion efficiency is improved.

  The solar cell of the present invention includes a photoelectric conversion layer having a first conductive type layer made of a semiconductor, an i type layer made of a microcrystalline semiconductor, and a second conductive type layer made of a semiconductor stacked in this order, A buffer layer made of a microcrystalline semiconductor and having a crystallization rate lower than that of the i-type layer is provided between the second conductivity type layer.

  Specifically, the solar cell of the present invention is implemented, for example, in the following manner.

1. Super Straight Structure The solar cell according to the first embodiment of the present invention includes a transparent conductive layer, the above-described photoelectric conversion layer, and a back electrode stacked in this order on a translucent substrate. You may provide a back surface reflection layer between a photoelectric converting layer and a back surface electrode.

1-1. Translucent substrate The translucent substrate includes a glass plate, a translucent resin (such as polyimide, polyvinyl, PET, PEN, PES, or Teflon (registered trademark)), ceramics, or a laminate of these. Although it is preferably used, it is not particularly limited as long as it has high light transmittance and supports and reinforces the entire solar cell. The translucent substrate preferably has a heat resistance of about 200 ° C. The translucent substrate preferably has a thickness of 0.1 to 10 mm. Moreover, the translucent board | substrate may be equipped with the unevenness | corrugation in the surface. In addition, an insulating film, a conductive film, a buffer layer, or the like may be stacked over the light-transmitting substrate, or a composite layer combining these may be stacked.

1-2. Transparent conductive layer The transparent conductive layer is made of a transparent conductive material. For example, a single layer or a stacked layer of transparent conductive films such as ITO, tin oxide, zinc oxide, and InO ₃ can be used. The transparent conductive layer is preferably formed of zinc oxide in order to improve plasma resistance.

Since the transparent conductive layer plays a role as an electrode, it is preferable that the electrical conductivity is high, and a layer having improved electrical conductivity by adding a small amount of impurities can be used. Examples of the impurities include group III elements such as gallium and aluminum, and the concentration is preferably 5 × 10 ^{20 to} 5 × 10 ²¹ atoms / cm ³ .

  As a manufacturing method of a transparent conductive layer, it can form using a vacuum evaporation method, EB evaporation method, an atmospheric pressure CVD method, a low pressure CVD method, a sol gel method, an electrodeposition method etc. The transparent conductive layer is preferably formed by a sputtering method in order to easily control the transmittance and resistivity.

  Further, it is desirable that an uneven shape is formed on the surface of the transparent conductive layer. The unevenness can scatter and refract incident light incident from the translucent substrate side to extend the optical path length, so that the light confinement effect in the photoelectric conversion layer is enhanced and the short circuit current can be expected to be improved.

1-3. Photoelectric Conversion Layer The photoelectric conversion layer has a first conductive type layer made of a semiconductor, an i-type layer made of a microcrystalline semiconductor, and a second conductive type layer made of a semiconductor in this order. The photoelectric conversion layer includes a buffer layer made of a microcrystalline semiconductor and having a crystallization rate lower than that of the i-type layer between the i-type layer and the second conductivity type layer.
In other words, the photoelectric conversion layer has a first conductive type layer made of a semiconductor, an i-type layer made of a microcrystalline semiconductor, a buffer layer made of a microcrystalline semiconductor, and a second conductive type layer made of a semiconductor in this order. The buffer layer is formed under the condition that the crystallization rate is lower than that of the i-type layer.

1-3-1. First Conductive Type Layer and Second Conductive Type Layer The first conductive type layer and the second conductive type layer are layers made of n-type and p-type semiconductors, or vice versa. Each layer is preferably made of a non-single-crystal (non-single-crystal, for example, polycrystalline, microcrystalline, or amorphous) semiconductor, more preferably a microcrystalline semiconductor, and more preferably microcrystalline silicon. . Each layer preferably has a thickness of about 1 to 100 nm. Each layer may be made of Si _x C _1-x doped with carbon and Si _x Ge _1-x doped with germanium.

Each layer can be formed by a CVD method using a high frequency in a frequency band from RF to VHF, an ECR plasma CVD method, or a CVD method combining these. For example, when the plasma CVD method is used, the conditions are as follows: a frequency of about 10 to 200 MHz, a power density of about 10 mW / cm ^{2 to 1} W / cm ² , a chamber pressure of about 0.1 to 20 Torr, and a substrate temperature of room temperature to 600. It is about ℃.

Examples of the silicon-containing gas used at this time include SiH ₄ , Si ₂ H ₆ , SiF ₄ , SiH ₂ Cl ₂ , and SiCl ₄ , and are used together with H ₂ gas that is a dilution gas. The mixing ratio of the silicon-containing gas and the diluent gas may be constant or may be formed while being changed. For example, the volume ratio is about 1: 1 to 1: 100.

Note that the p-type semiconductor layer is a gas containing a group III element arbitrarily as a doping gas, for example, B ₂ H ₆ or the like is mixed, and the n-type semiconductor layer is a dopant gas, for example, PH ₃ It can form by mixing the gas containing V group elements, such as. The impurity concentration as a dopant is preferably about 10 ¹⁸ to 10 ²⁰ pieces / cm ³ .

 The crystallization rate of each layer depends on film formation parameters such as a mixing ratio of silicon-containing gas and hydrogen gas as a dilution gas, input power, and film formation pressure. For example, when the mixing ratio of hydrogen gas is increased or decreased, the crystallization rate is increased or decreased accordingly.

Here, the crystallization rate Ic / Ia is (1) exposing the layer to be measured, (2) irradiating the surface of the layer with an argon ion laser (514.5 nm) at about 10 mW, and (3) The peak height of the crystalline semiconductor near 520 cm ⁻¹ obtained by measuring the Raman scattering spectrum is Ic, the peak height of the amorphous semiconductor near 480 cm ⁻¹ is Ia, and (4) Ic and Ia It can obtain | require by the method provided with the process of calculating | requiring each peak intensity ratio Ic / Ia. Note that the crystallization rate has a positive correlation with the volume fraction of the crystalline semiconductor portion included in the microcrystalline semiconductor layer.
The layer to be measured can be exposed by removing the upper layer by an oblique polishing method or the like.

1-3-2. i-type layer The i-type layer is made of a substantially intrinsic microcrystalline semiconductor, preferably made of microcrystalline silicon. The thickness of the i-type layer is preferably about 1 to 5 μm. The i-type layer may be made of Si _x C _1-x added with carbon and Si _x Ge _1-x added with germanium.

  The i-type layer can be formed by the same method as in 1-3-1 above, and the crystallization rate can be controlled. The i-type layer preferably has a crystallization rate of 2 to 12, and more preferably 4 to 10. This is because when the crystallization ratio is in the range of 2 to 12, the effect of the buffer layer can be obtained and the number of defect levels existing in the i-type layer can be reduced, so that high photoelectric conversion efficiency can be obtained. The i-type layer crystallization rate means the crystallization rate in the vicinity of the buffer layer in the i-type layer.

1-3-3. Buffer Layer The photoelectric conversion layer includes a buffer layer made of a microcrystalline semiconductor and having a crystallization rate lower than that of the i-type layer between the i-type layer and the second conductivity type layer.

The buffer layer is made of a microcrystalline semiconductor. The “microcrystalline semiconductor” here is preferably not included when the entire surface of the buffer layer is amorphous at the interface between the buffer layer and the second conductivity type layer. This is because the electric field applied to the buffer layer is increased in this case. When a layer in which the entire surface of the buffer layer is amorphous is formed in the vicinity of the interface between the buffer layer and the second conductivity type layer, the thickness of the layer is preferably 0.1 μm or less, and Preferably, it is 0.05 μm or less, and more preferably 0.02 μm or less. The buffer layer is preferably made of microcrystalline silicon. The buffer layer may be made of Si _x C _1-x added with carbon and Si _x Ge _1-x added with germanium. The buffer layer preferably has a thickness of 0.02 to 0.2 μm. This is because if it is smaller than 0.02 μm, it is difficult to control the crystallization rate, and if it is larger than 0.2 μm, the crystallization rate may be too low and the electric field applied to the buffer layer may be too high. It is.

  The buffer layer can be formed by the same method as in 1-3-1 above, and the crystallization rate can be controlled. The buffer layer and the i-type layer may be formed using a silicon-containing gas diluted with hydrogen, and the buffer layer may be formed under a condition in which the silicon-containing gas has a lower hydrogen dilution rate than the i-type layer. preferable. This is because the crystallization rate of the buffer layer can be easily reduced.

1-4. Back surface reflection layer Preferably, a back surface reflection layer is formed on the photoelectric conversion layer. The back reflective layer is formed using a transparent conductive material such as SnO ₂ , InO ₂ , ZnO, ITO, or the like by a magnetron sputtering method or the like. The thickness of the back reflective layer is preferably about 50 nm.

1-5. Back Electrode A back electrode is formed on the photoelectric conversion layer or on the back reflective layer. The back electrode is formed of a single layer of a metal film or a plurality of layers using materials such as Ag, Al, Cu, Au, Ni, Cr, W, Ti, Pt, Fe, and Mo by sputtering or vacuum deposition. A metal film is laminated. The thickness of the back electrode is preferably about 0.1 to 1 μm.

2. Substrate-type structure The solar cell according to the second embodiment of the present invention has the above-described photoelectric conversion layer and transparent conductive layer stacked in this order on a metal substrate or a substrate whose surface is covered with metal. Prepare. A grid electrode may be formed on the transparent conductive layer.

2-1. Substrate As the substrate, a substrate such as a metal such as stainless steel (SUS) or aluminum can be used. Further, as the substrate, glass, a heat-resistant polymer film (such as polyimide, PET, PEN, PES, or Teflon (registered trademark)), ceramics, or the like coated with metal may be used. Moreover, you may use what laminated | stacked these for a board | substrate.

  The substrate preferably has a heat resistance of about 200 ° C. The substrate preferably has a thickness of 0.1 to 10 mm. Further, the substrate may be provided with irregularities on its surface. In addition, an insulating film, a conductive film, a buffer layer, or the like may be stacked on the substrate, or a composite layer combining these may be stacked.

2-2. Photoelectric conversion layer A photoelectric conversion layer is formed on the substrate. The configuration and manufacturing method of the photoelectric conversion layer are the same as those described in 1-3.

2-3. Transparent conductive layer A transparent conductive layer is formed on the photoelectric conversion layer. The configuration and manufacturing method of the transparent conductive layer are the same as those described in 1-2.

2-4. Grid electrode A grid electrode is preferably formed on the transparent conductive layer. Known configurations and manufacturing methods of the grid electrode can be used.

3. Others In this specification, the phrase “on the substrate” includes the case of “on the substrate via a protective film or an insulating film”. The same applies to other phrases such as “on the membrane” and “on the layer”.

1. Structure FIG. 1 is a cross-sectional view showing the structure of a solar cell 1 according to Example 1 of the present invention. A solar cell 1 according to the present example includes a transparent conductive layer 5 made of zinc, a transparent conductive layer 5 made of zinc oxide, a photoelectric conversion layer 7, a back reflective layer 9 made of zinc oxide, and a back electrode made of silver. 11 are provided in this order.

  In addition, the photoelectric conversion layer 7 includes a p-type layer 7a, an i-type layer 7b, and an n-type layer 7c each made of microcrystalline silicon, which are stacked in this order, and between the i-type layer 7b and the n-type layer 7c, A buffer layer 13 made of microcrystalline silicon and having a crystallization rate lower than that of the i-type layer 7b is provided.

2. Manufacturing method The solar cell 1 is manufactured in the following steps.

  First, the transparent conductive layer 5 made of zinc oxide is formed on the translucent substrate 3 made of glass with a thickness of 800 nm. Next, the surface of the transparent conductive layer 5 is etched with an aqueous acetic acid solution to form irregularities.

  Next, on the obtained substrate, a p-type layer 7a having a thickness of 30 nm, an i-type layer 7b having a thickness of 1.9 μm, and a buffer having a thickness of 0.1 μm, each made of microcrystalline silicon, by high-frequency plasma CVD. The layer 13 and the n-type layer 7c having a thickness of 30 nm are stacked in this order to form the photoelectric conversion layer 7.

When the p-type layer 7a is formed, a material gas (hydrogen dilution rate 100 times) obtained by diluting SiH ₄ gas with H ₂ gas 100 times in flow rate ratio is used. Further, 0.1% B ₂ H ₆ gas is added to the source gas.

When the i-type layer 7b is formed, a material gas obtained by diluting SiH ₄ gas with H ₂ gas 70 times in flow rate ratio is used.

When the buffer layer 13 is formed, a material gas obtained by diluting SiH ₄ gas with H ₂ gas 50 times in flow rate ratio is used. That is, the buffer layer 13 is formed under the condition that the hydrogen dilution rate of the source gas is lower than that of the i-type layer 7b.

When forming the n-type layer 7c, a material gas obtained by diluting SiH ₄ gas with H ₂ gas 50 times in flow rate ratio is used. Also, 0.01% PH ₃ gas is added to the source gas.

  Next, the back reflective layer 9 made of zinc oxide is formed with a thickness of 50 nm by magnetron sputtering. Next, the back electrode 11 made of silver with a thickness of 500 nm is formed by electron beam evaporation, and the manufacture of the single-junction solar cell 1 with a super straight type structure is completed.

3. Test result The solar cell which concerns on a present Example was manufactured using the said manufacturing method. About each obtained solar cell, the relationship between the film thickness which match | combined the i-type layer 7b and the buffer layer 13, and its crystallization rate was investigated. The method for measuring the crystallization rate is as described in 1-3-1. The result is shown in FIG.

  As can be seen from FIG. 2, the crystallization rate of the i-type layer 7b increases as the thickness of the i-type layer 7b increases. It can be seen that the buffer layer 13 starts from the dotted line portion in the figure, and in the buffer layer 13, the crystallization rate is rapidly reduced.

  Since the conventional solar cell does not have the buffer layer 13, the crystallization rate does not decrease. Therefore, a lot of interface states are generated between the i-type layer 7b and the n-type layer 7c formed on the i-type layer 7b, causing a decrease in photoelectric conversion efficiency.

  In this embodiment, since the buffer layer 13 is provided, the buffer layer 13 is in contact with the n-type layer 7c with the crystallization rate lowered. Therefore, the interface state is reduced and the photoelectric conversion efficiency is improved.

  Example 2 is similar to Example 1, except that Example 2 has a hydrogen dilution ratio of 35, 40, 45, 50, 55, and 60 times.

  As a comparative example, a hydrogen dilution rate of 70 times, that is, the same hydrogen dilution rate as that when forming the i-type layer 7b was manufactured.

About each obtained solar cell, photoelectric conversion efficiency etc. were measured. The measurement was performed using a solar simulator capable of irradiating light having a spectrum AM of 1.5 and a light intensity of 100 mW / cm ² while maintaining the temperature of the sample at 25 ° C. The current-voltage characteristics of this solar cell under AM1.5 (100 mW / cm ² ) irradiation conditions were measured. The measured solar cell characteristics are shown in Table 1.

  According to Table 1, when the hydrogen dilution rate is 45 to 55 times, it can be seen that the conversion efficiency is improved as compared with the comparative example. When the hydrogen dilution rate was less than 45 times (35 times and 40 times), the conversion efficiency was not substantially improved. The reason is that in this case, the entire surface of the buffer layer 13 becomes amorphous at the interface between the buffer layer 13 and the n-type layer 7c, and as a result, the electric field applied to the buffer layer 13 increases, and the i-type layer This is considered to be because the electric field applied to 7b has become smaller. Note that the crystallization rate reflects not only the crystal state of the surface of the layer to be measured but also the crystal state inside the layer, so that the surface of the buffer layer 13 (the buffer layer 13 and the n-type layer 7c Even if the surface of the buffer layer 13 at the interface between them is completely amorphous, the crystallization rate may not be zero. Therefore, when the hydrogen dilution ratio is 35 times and 40 times, the crystallization ratios of 2.5 and 2.6 respectively indicate that the surface of the buffer layer 13 is not amorphous. It is not a thing. It can also be seen that when the hydrogen dilution rate was 60 times, the crystallization rate did not decrease and the conversion efficiency did not substantially improve.

  Example 3 is similar to Example 1, except that Example 3 is different in that the thickness of the buffer layer 13 is changed between 0.02 and 0.3 μm.

  Using the same manufacturing method as in Example 1, the buffer layer 13 having a thickness of 0.02, 0.05, 0.15, 0.2, and 0.3 was manufactured. About each obtained solar cell, photoelectric conversion efficiency etc. were measured. The measuring method is the same as described above. The results are shown in Table 2.

  According to Table 2, when the film thickness of the buffer layer 13 is 0.02-0.2 micrometer, it turns out that conversion efficiency becomes large especially. When the film thickness of the buffer layer 13 was larger than 0.2 μm (film thickness 0.3 μm), the conversion efficiency was a small value. The reason is that in this case, the entire surface of the buffer layer 13 becomes amorphous at the interface between the buffer layer 13 and the n-type layer 7c, and as a result, the electric field applied to the buffer layer 13 increases, and the i-type layer This is considered to be because the electric field applied to 7b has become smaller.

  Next, the relationship between the film thickness of the buffer layer 13 and the hydrogen dilution rate of the source gas to obtain high photoelectric conversion efficiency was examined. The results are shown in Table 3.

  According to Table 3, when the buffer layer 13 is thin (0.02 μm), a high photoelectric conversion efficiency can be obtained by reducing the hydrogen dilution rate of the source gas and rapidly decreasing the crystallization rate of the buffer layer 13. You can see that Further, when the buffer layer 13 is thick (0.3 μm), a high photoelectric conversion can be achieved by increasing the hydrogen dilution rate of the source gas and suppressing an increase in the proportion of the amorphous portion of the buffer layer 13. It turns out that efficiency is obtained.

  Example 5 is similar to Example 1, except that Example 5 changed the crystallization rate of the i-type layer 7b by changing the hydrogen dilution rate when forming the i-type layer 7b. Is different.

  About each obtained solar cell, photoelectric conversion efficiency etc. were measured. The measuring method is the same as described above. The result is shown in FIG. The code | symbol 21 shows the result about the solar cell which concerns on this invention, and the code | symbol 23 shows the result about the conventional solar cell.

According to FIG. 3, it can be seen that the photoelectric conversion efficiency of the solar cell of the present invention is higher in almost all regions. It can also be seen that particularly high photoelectric conversion efficiency is obtained when the crystallization rate is about 2 to 12, and even higher photoelectric conversion efficiency is obtained when the crystallization rate is about 4 to 10.

It is sectional drawing which shows the structure of the solar cell which concerns on Example 1 of this invention. It is a graph which shows the relationship between the film thickness of i type layer + buffer layer, and the crystallization rate based on Example 1 of this invention. It is a graph which shows the relationship between the crystallization rate of an i-type layer, and photoelectric conversion efficiency based on Example 5 of this invention. About the solar cell which concerns on the prior art example, (a) The sectional drawing, (b) The energy band figure. It is (a) the sectional view and (b) the energy band figure about the solar cell concerning the present invention. About the solar cell which concerns on the prior art example 2, (a) The sectional drawing, (b) The energy band figure.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Solar cell 3 Translucent substrate 5 Transparent conductive layer 7 Photoelectric conversion layer 7a p-type layer 7b i-type layer 7c n-type layer 9 Back surface reflection layer 11 Back surface electrode 13 Buffer layer 21 Result about the solar cell according to the present invention 23 Conventionally Result 51 for Solar Cell First Conductive Layer 53 i-Type Layer 55 Buffer Layer 57 Second Conductive Layer 59, 61 Interface Level 63 Electron 65 Hole 67 Buffer Layer

Claims (5)

  A photoelectric conversion layer having a first conductive type layer made of a semiconductor, an i type layer made of a microcrystalline semiconductor, and a second conductive type layer made of a semiconductor in this order, and comprising an i type layer and a second conductive type layer A solar cell comprising a buffer layer that is made of a microcrystalline semiconductor and has a crystallization rate lower than that of the i-type layer. A first conductive type layer made of a semiconductor, an i-type layer made of a microcrystalline semiconductor, a buffer layer made of a microcrystalline semiconductor, and a photoelectric conversion layer having a second conductive type layer made of a semiconductor stacked in this order. A solar cell formed under the condition that the crystallization rate is lower than that of the i-type layer. The solar cell according to claim 1 or 2, wherein the i-type layer has a crystallization ratio of 4 to 10. The solar cell according to claim 1, wherein the buffer layer has a thickness of 0.02 to 0.2 μm. The buffer layer and the i-type layer are formed using a silicon-containing gas diluted with hydrogen, and the buffer layer is formed under a condition in which the hydrogen dilution rate of the silicon-containing gas is lower than that of the i-type layer. 4. The solar cell according to any one of 4.

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