JP2006229133A - Solar battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar battery which has high conversion efficiency even if the front surface of the rear surface of the solar battery has no structure for generating scattering light, and to provide a method for manufacturing it. <P>SOLUTION: A first conductive semiconductor layer 1 and a second conductive semiconductor layer 2 are laminated, and at least one of the first conductive semiconductor layer 1, and the second conductive semiconductor layer 2 has a refractive index distribution structure. Incident light is scattered at the refractive index distribution structure to increase a light path length. An effective absorption coefficient is increased to improve the conversion efficiency. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

A solar cell is an element that generates electricity using photoexcited carriers generated by the absorption of sunlight, but it improves the effective use and conversion efficiency of sunlight by providing the element with a structure for effective use of light. Has been tried. For example, light confinement by creating a geometric texture structure on the surface of the solar cell, reflection of the light that reaches the back surface by forming a reflective film on the back surface, and the combination of both Etc. have been done.
Such a technique is disclosed in Non-Patent Document 1 and Non-Patent Document 2.

  In Non-Patent Document 1, a periodic mask is formed on the surface of a silicon bulk crystal substrate using photolithography, and a uniform inverted pyramid structure is formed by solution processing, which is used for optical confinement. However, this method requires an advanced lithography technique and is not suitable as a solar cell production technique that requires low-cost production.

  In Non-Patent Document 2, in a thin-film silicon solar cell, light absorption can be increased by fusing light confinement by a geometric texture structure on the front surface and a metal film having high reflectance on the back surface. Has been tried.

Both methods have a problem that the production process becomes complicated in order to produce a homogeneous texture structure. Further, since the light absorption layer is a single substance, there is no refractive index distribution, and light travels straight through the light absorption layer. Therefore, there has been a problem that an optical confinement structure has to be made by the geometric texture structure.
Zhao et al, Progress in Photovoltaics 7, 471 (1999). Yamamoto et al., Appl. Phys. A 69, 179 (1999).

  As described above, the conventional technology requires a complicated structure for generating scattered light on the front or back surface of the solar cell in order to increase the amount of light absorption, and the production process is complicated and expensive. It was. The present invention provides a solar cell having a high conversion efficiency even without a structure for generating scattered light on the front surface or the back surface of the solar cell, and a method for manufacturing a solar cell capable of realizing a light confinement structure by a simple technique. It is an object.

  According to the present invention, the first conductive type semiconductor layer and the second conductive type semiconductor layer are stacked, and at least one of the first conductive type semiconductor layer and the second conductive type semiconductor layer is provided as a semiconductor layer. A solar cell having a refractive index distribution structure is obtained.

  The present invention also provides a solar cell, wherein the refractive index distribution structure is composed of a plurality of layers or a plurality of phases having non-flat boundaries.

  Furthermore, the solar cell is characterized in that at least one of the plurality of layers or the plurality of phases is an island-like crystal formed on a flat surface.

Further, the present invention provides a solar cell, wherein the plurality of layers or the plurality of phases have a mottled pattern.

  Further, according to the present invention, the first conductive type semiconductor layer and the second conductive type semiconductor layer are stacked, and the bonding surface of the first conductive type semiconductor layer and the second conductive type semiconductor layer is non-flat. A solar cell having a structure is obtained.

  Further, according to the present invention, at least two molecular beam sources including components that generate different semiconductors are prepared, and each molecular beam is supplied to the crystal growth substrate at different times, so that semiconductors having different lattice constants can be obtained. A method for manufacturing a solar cell is provided, which includes the step 1 of forming the layers.

  Furthermore, according to the present invention, at least two thin films of different materials are laminated on the substrate, and the substrate obtained in the step 2 is rapidly superheated to a temperature intermediate between the melting points of the different materials. A step 3 of bringing the solid and liquid into coexistence by crystallization into different substances into an equilibrium state, and a step 4 of reducing the temperature of the substrate in the equilibrium state of the step 3 to solidify the whole to generate a semiconductor layer; A method for producing a solar cell characterized by having

  According to the invention, the step 5 of corroding the surface of the first conductivity type semiconductor to form irregularities, and the step 6 of forming the second conductivity type semiconductor layer on the semiconductor surface obtained in the step 5 are formed. A method for producing a solar cell characterized by having

In the present invention, since at least one of the laminated first conductivity type semiconductor layer and second conductivity type semiconductor layer has a refractive index distribution structure, the incident light is a gradient portion of the refractive index in the semiconductor layer. Alternatively, the light is scattered by being refracted or reflected at the boundary. As a result, in the semiconductor layer, the optical path length increases and the amount of light absorption increases.
Therefore, even if there is no complicated structure for generating scattered light on the front surface or the back surface of the solar cell, the effective absorption coefficient increases and the conversion efficiency of the solar cell increases.

  Further, when there are a plurality of layers or a plurality of phases having a non-flat boundary, scattering is effectively performed by the non-flat boundary surface, and high conversion efficiency is obtained. A plurality of layers or a plurality of phases can be formed by a combination of different materials, or in the case of the same material, a combination of crystal and non-crystal, thereby realizing various refractive index distribution structures.

  Furthermore, if there are island crystals formed on a flat surface, scattering is effectively performed by the boundary surfaces of the island crystals. Island crystals have a very stable structure and can be multilayered. Therefore, a solar cell with high conversion efficiency and stable quality can be obtained.

  Further, if the plurality of layers or the plurality of phases are mottled, scattering is effectively performed by the complicated boundary surface of the mottled pattern, and the conversion efficiency is increased.

  Furthermore, in the present invention, since the junction surface between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer has a non-flat structure, incident light is scattered in the vicinity of the junction surface. As a result, the amount of light absorption increases in the semiconductor layer. Therefore, even if there is no complicated structure for generating scattered light on the front or back surface of the solar cell, the effective absorption coefficient increases and the conversion efficiency of the solar cell increases.

  In the present invention, semiconductors having different lattice constants are stacked by supplying different molecular beams to the crystal growth substrate at different times, so that the strain caused by the difference in lattice constants between different semiconductors is alleviated. As a result, an island-shaped semiconductor can be obtained.

  Therefore, a solar cell having a refractive index distribution structure can be realized by forming a plurality of layers or phases having a non-flat boundary with the surface of the island-shaped crystal as a boundary in the semiconductor layer.

  Furthermore, in the present invention, thin films of different materials are used, and the entire solid is formed by reducing the temperature from the equilibrium state where the solid and liquid coexist due to the mixed crystallization, so the mixed crystallization is reflected in the semiconductor layer. Thus, a solar cell having a refractive index distribution structure can be realized by forming a plurality of layers or phases having a complicated mottled pattern boundary.

  Further, in the present invention, the first conductive type semiconductor surface is corroded to form irregularities, and the second conductive type semiconductor layer is laminated on the semiconductor surface. realizable.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional view of an essential part for explaining the basic structure of a solar cell according to the present invention. In FIG. 1, 1 is an n-type semiconductor layer, 2 is a p-type semiconductor layer, 3 is an antireflection film, and the basic structure has a pn junction layer composed of an n-type semiconductor layer 1 and a p-type semiconductor layer 2.

  The n-type semiconductor layer 1 and the p-type semiconductor layer 2 are not homogeneous and each have a refractive index distribution structure. Therefore, the sunlight incident from the direction of arrow A is scattered by the refractive index distribution of the n-type semiconductor layer 1 and the p-type semiconductor layer 2. About the sunlight which reached the p-type semiconductor layer 2, the scattered light which goes to arrow B, arrow c, arrow d, arrow d, arrow f, and arrow f toward many directions arises. As a result, the optical path length increases, and the amount of light absorption in the vicinity of the pn junction layers of the n-type semiconductor layer 1 and the p-type semiconductor layer 2 increases.

  Therefore, an effective absorption coefficient increases in the semiconductor layer itself, and high conversion efficiency can be obtained without providing a complicated structure for generating scattered light on the front surface or the back surface.

(First embodiment)
FIG. 2 is a cross-sectional view of a main part showing a schematic configuration of the solar cell according to the first embodiment of the present invention, showing a configuration in which the refractive index distribution in the semiconductor layer is formed by stacking Ge island crystals and Si thin film crystals. ing. In FIG. 2, 11 is an n-type semiconductor layer, 12 is a p-type Si semiconductor single crystal substrate that becomes a p-type semiconductor layer, 13 is an antireflection film, 14 is a front electrode, 15 is a back electrode, and the n-type semiconductor layer 11 Is a thin film layer N1, a thin film layer N2, or a thin film layer Nm made of Ge island-like crystal 16 and Si thin film crystal 17 stacked in m layers.

  The thin film layer N1 is composed of a number of cross-sectionally chevron-shaped Ge island crystals 16 regularly arranged on the same plane, and a Si thin film crystal 17 formed thereon so as to be flat. The thin film layer N2 to the thin film layer Nm have the same configuration as the thin film layer N1.

  In the n-type semiconductor layer 11 of the solar cell having such a configuration, since the refractive index of Ge is larger than the refractive index of Si, the light scattering effect is produced at the interface between the Ge island crystal 16 and the Si thin film crystal 17. Remarkably expressed.

  Further, since the Ge island-like crystal 16 and the Si thin film crystal 17 have a very stable structure and are laminated, a solar cell having stable quality and high conversion efficiency is obtained.

  Furthermore, since the n-type semiconductor layer 11 composed of the Ge island-like crystal 16 and the Si thin film crystal 17 has a thin film structure, the amount of material consumption is small, and the environmental load is small compared to those using a bulk semiconductor.

  The solar cell according to the second embodiment of the present invention shown in FIG. 2 can be manufactured by the following method, for example.

  First, a p-type Si semiconductor single crystal substrate to be the p-type semiconductor layer 12 is degreased by organic cleaning, and an oxide film is formed on the surface with a sulfuric acid / hydrogen peroxide solution. After that, the surface oxide film is removed by dilute hydrofluoric acid treatment to form a surface terminated with hydrogen. Subsequently, a substrate whose surface is terminated with hydrogen is introduced into the ultrahigh vacuum apparatus and heated at a high temperature to obtain a clean surface. The temperature and time at this time are desirably about 20 minutes at 800 ° C., for example.

  Next, after the substrate temperature is lowered to, for example, 700 ° C., two kinds of molecular beams, that is, a disilane molecular beam and a germane molecular beam are alternately supplied, whereby a Ge island crystal 16 and a Si thin film crystal 17 are formed. A thin film layer N1, a thin film layer N2, or a thin film layer Nm are sequentially formed. The Ge island-like crystal 16 is self-formed by adjusting the supply amount in order to alleviate the strain caused by the difference in lattice constant between Ge and Si.

  Subsequently, for example, phosphorous glass is applied to the surface of the obtained crystal, heat treatment is performed, and the thin film layer N1 or the entire thin film layer Nm, which is a laminated structure of the grown Ge island crystal 16 and Si thin film crystal 17, is n-type. Inverted. The heat treatment condition is, for example, 800 ° C. for 30 minutes.

  Next, phosphorus glass remaining on the surface was removed with a hydrofluoric acid solution, and then an ITO thin film was deposited as an antireflection film 13 by a sputtering method. Further, a comb-shaped silver paste was applied to the front surface, and an aluminum paste was applied to the back surface, followed by firing to form a front electrode 14 and a back electrode 15 to fabricate a solar cell.

  In the above-described embodiment, the p-type Si semiconductor single crystal substrate is used for the p-type semiconductor layer 12, but the p-type semiconductor layer 12 is not limited to the p-type Si semiconductor single crystal substrate. Similar to the n-type semiconductor layer 11, a p-type thin film structure made of Ge island crystals and Si thin film crystals can be used, or a polycrystalline p-type Si semiconductor substrate such as metaradical silicon containing a large amount of impurities. Can also be used. In that case, the effect of light scattering appears even at the non-flat crystal boundary surface of the p-type semiconductor layer 12.

  The formation of island-like crystals is not limited to the combination of Ge and Si, and the combination of InAs and GaAs or the combination of InSb and GaSb can also be formed by their lattice constant difference. These island crystals have a very stable structure and can be multilayered.

(Second Embodiment)
FIG. 3 is a cross-sectional view of a main part showing a schematic configuration of a solar cell according to the second embodiment of the present invention, showing a configuration in which a refractive index distribution in a semiconductor layer is formed by a mottled pattern distribution of SiGe having different compositions. .

  In FIG. 3, 21 is an n-type semiconductor layer, 22 is a p-type semiconductor layer, the n-type semiconductor layer 21 is made of SiGe, and a thin film in which regions A and B having different compositions are distributed in a mottled pattern, a p-type semiconductor The layer 22 is a thin film of uniform composition made of Si.

  In the n-type semiconductor layer 21 of the solar cell having such a configuration, since the refractive index is different between the region A and the region B, light incident from the vertical direction is scattered at the boundary surface between the region A and the region B. As a result, a solar cell with high conversion efficiency is obtained.

Further, since the n-type semiconductor layer 21 and the p-type semiconductor layer 22 have a thin film structure, material consumption is small,
Environmental impact is small compared to bulk solar cells.

  The solar cell according to the second embodiment of the present invention shown in FIG. 3 can be manufactured, for example, by the following method.

  First, a stacked structure of a Si thin film, a Ge thin film, and an oxide film is formed on the p-type semiconductor layer 22 by sputtering or the like.

  Next, the temperature of the sample is rapidly superheated to an intermediate temperature between 938 ° C. which is the melting point of Ge and 1414 ° C. which is the melting point of Si. At this time, only Ge melts first. Furthermore, when Si melts in the Ge melt, mixed crystallization into SiGe occurs, and an equilibrium state in which the solid and the liquid coexist is formed.

  Next, the temperature is lowered. The whole crystallizes into a solid due to a decrease in temperature. At this time, a mottled pattern made of SiGe having a different composition reflecting the coexistence state of the solid and the liquid at a high temperature is formed.

  Thereafter, the entire thin film is made to be n-type to form an n-type semiconductor layer 21 to have a configuration as shown in FIG. The description of the formation of the antireflection film and the electrode is omitted. The surface oxide film acts as a protective film for maintaining the film shape when the surface Ge is melted, and is removed as necessary after the whole crystallizes and becomes solid due to a decrease in temperature.

  In the solar cell of the above-described embodiment, the thin film forming the p-type semiconductor layer 22 may be a thin film having uniform composition of silicon, polycrystalline silicon such as metaradical silicon, or Ge island crystals. There is no particular limitation.

  Moreover, what forms a mottled pattern having a different composition by mixed crystallization is not limited to SiGe but may be InGaAs or the like.

(Third embodiment)
FIG. 4 is a cross-sectional view of a main part showing a schematic configuration of the solar cell according to the third embodiment of the present invention. In FIG. 4, 31 is an n-type semiconductor layer made of SiGe, 32 is a p-type semiconductor layer, and the junction surface between the n-type semiconductor layer 31 and the p-type semiconductor layer 32 has an uneven shape in cross section.

  In the solar cell having such a configuration, light incident from the vertical direction is scattered in the vicinity of the junction surface between the n-type semiconductor layer 31 and the p-type semiconductor layer 32. As a result, a solar cell with high conversion efficiency is obtained.

  The solar cell according to the third embodiment of the present invention shown in FIG. 4 can be manufactured, for example, by the following method.

  First, after degreasing the p-type Si semiconductor single crystal substrate to be the p-type semiconductor layer 32 by organic cleaning, the Si surface is corroded to make the surface uneven. Corrosion of the Si surface can be performed, for example, by immersing in a hydrofluoric acid: nitric acid (1: 6) mixed solution for 1 minute or by immersing in a potassium hydroxide aqueous solution heated to 80 ° C. for 10 minutes.

  Next, the substrate is introduced into the ultrahigh vacuum apparatus and heated at, for example, 800 ° C. for 20 minutes to obtain a clean surface.

  Next, after the substrate temperature is lowered to the growth temperature (500-700 ° C.), a disilane molecular beam and a germane molecular beam are simultaneously supplied to form a SiGe film.

  Subsequently, the SiGe film is n-typed to form an n-type semiconductor layer 31. Note that description of the formation of the antireflection film and the electrode is omitted.

  In the solar cell of the above-described embodiment, the unevenness of the joint surface does not need to be uniform and may be anything as long as it is non-flat.

  The p-type semiconductor layer 32 is not limited to a p-type Si semiconductor single crystal substrate, and the n-type semiconductor layer 31 is not limited to a SiGe thin film.

  As described above, the present invention has been described in detail based on the embodiments of the present invention with specific examples. However, the present invention is not limited to the above contents, and all modifications are possible without departing from the scope of the present invention. And changes are possible.

Example 1
FIG. 5 is an explanatory diagram for explaining an example of the solar cell according to the first embodiment of the present invention shown in FIG.

  In this example, the island height of the Ge island-like crystal 16 was about 10 nm, the diameter of the island was about 100 nm, and the thickness of the Si thin film crystal 17 was about 39 nm.

  In FIG. 5, the horizontal axis indicates the wavelength (unit: nm), the vertical axis indicates the external quantum efficiency, the curve A indicates the characteristics of the solar cell according to the embodiment of the present invention, and the curve B indicates the characteristics of the conventional solar cell. Show. Comparing curve A and curve B, the external quantum efficiency of the solar cell according to the present invention exceeds the external quantum efficiency of the conventional solar cell in a wide wavelength range, and a high photocurrent is obtained. Clearly understood.

(Example 2)
FIG. 6 is an explanatory diagram for explaining an example of the solar cell according to the second embodiment of the present invention shown in FIG. 3.

  In this example, a sample in which a Si thin film having a thickness of 1 micron and a Ge thin film having a thickness of 1 micron were laminated was heated at 1200 ° C. for 1 minute to obtain an equilibrium state in which a solid and a liquid coexisted. Then, it cooled and solidified.

  FIG. 6 shows the results of evaluating the two-dimensional composition distribution of the sample thus prepared by Raman spectroscopy. The range of this scan is 4 microns x 4 microns, and the difference in shading corresponds to the composition. A mottled pattern with a size of about 1 micron is formed, indicating that a refractive index distribution has been introduced.

  According to the present invention, the conversion efficiency of a solar cell can be increased to the same level as that of a bulk crystal using a thin film silicon-based semiconductor. Therefore, it is possible to increase the share of thin-film silicon solar cells that are not widely used due to low conversion efficiency.

  Silicon consumption can be reduced by using thin-film silicon-based semiconductors, solving the problem of securing future raw materials, which is currently a problem in the solar cell industry, and expanding the solar cell industry and solving energy problems Connected.

It is principal part sectional drawing for demonstrating the basic structure of the solar cell by this invention. It is principal part sectional drawing which shows schematic structure of the solar cell by the 1st Embodiment of this invention. It is principal part sectional drawing which shows schematic structure of the solar cell by the 2nd Embodiment of this invention. It is principal part sectional drawing which shows schematic structure of the solar cell by the 3rd Embodiment of this invention. It is explanatory drawing for demonstrating the Example of the solar cell by the 1st Embodiment of this invention. It is explanatory drawing for demonstrating the Example of the solar cell by the 2nd Embodiment of this invention.

Explanation of symbols

1, 11, 21, 31 n-type semiconductor layer 2, 12, 22, 32 p-type semiconductor layer 3, 13 Antireflection film 14 Surface electrode
15 Back electrode 16 Ge island crystal 17 Si thin film crystal

Claims (8)

A first conductivity type semiconductor layer and a second conductivity type semiconductor layer are stacked, and at least one of the first conductivity type semiconductor layer and the second conductivity type semiconductor layer has a refractive index distribution structure. A solar cell characterized by that. The solar cell according to claim 1, wherein the refractive index distribution structure includes a plurality of layers or a plurality of phases having non-flat boundaries. 3. The solar cell according to claim 2, wherein at least one of the plurality of layers or the plurality of phases is an island-like crystal formed on a flat surface. The solar cell according to claim 2, wherein the plurality of layers or the plurality of phases have a mottled pattern. A first conductivity type semiconductor layer and a second conductivity type semiconductor layer are stacked, and a bonding surface between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer has a non-flat structure. Solar cell. Preparing at least two molecular beam sources including components generating semiconductors different from each other, supplying each molecular beam to a crystal growth substrate at different times, and stacking semiconductors having different lattice constants; A method for manufacturing a solar cell. Step 2 of laminating at least two thin films of different materials on a substrate, and rapid superheating of the substrate obtained in Step 2 to a temperature intermediate between the melting points of the different materials to form mixed crystals of different materials And a step 3 for producing a semiconductor layer by lowering the temperature of the substrate in the equilibrium state in the step 3 to solidify the whole and forming a semiconductor layer. A method for manufacturing a solar cell. A step 5 of corroding the first conductivity type semiconductor surface to form irregularities; and a step 6 of forming a second conductivity type semiconductor layer on the semiconductor surface obtained in the step 5. A method for manufacturing a solar cell.