JP6188921B2 - Solar cell and method for manufacturing solar cell - Google Patents

Description

  The present invention relates to solar cell technology, and more particularly to a silicon solar cell having a higher photoelectric conversion efficiency than a conventional solar cell and a method for manufacturing the same.

  In the field of solar cells, tandem (multi-junction) solar cells in which a plurality of photoelectric conversion units are stacked are known in order to improve photoelectric conversion efficiency by photoelectrically converting sunlight in a wide wavelength range. (For example, refer to Patent Documents 1 to 3). In order to further improve the photoelectric conversion efficiency of such a tandem solar cell, it is necessary to further increase the utilization efficiency of light and improve the output current.

  As a solar cell using a single crystal Si wafer, currently high photoelectric conversion efficiency is achieved by a heterojunction solar cell in which amorphous Si is deposited on both sides of the single crystal Si wafer, and on the side opposite to the incident light side. This is a back contact solar cell in which an emitter and a back surface electric field region (BSF region) are formed on the surface.

  In order to further improve the photoelectric conversion efficiency of the tandem Si solar cell, suppression of Auger recombination in the crystalline Si layer is an important issue. One method for solving this problem is to reduce the thickness of the crystalline Si layer. At the research level, a solar cell having a relatively high photoelectric conversion efficiency is obtained by reducing the crystalline Si layer to about 100 μm. .

Japanese Patent Laid-Open No. 10-335683 JP 2001-267598 A JP 2009-260310 A

  As described above, in order to further improve the photoelectric conversion efficiency of the tandem Si solar cell, it is effective to reduce the thickness of the crystalline Si layer and suppress Auger recombination in the crystalline Si layer. . However, on the other hand, when the thickness of the crystalline Si layer is reduced, the light absorption length is shortened, so that the short-circuit current density is reduced. When the entire solar cell is evaluated, there is a problem that the desired photoelectric conversion efficiency is not achieved. Furthermore, it is also required to develop a technique for preventing the thin crystalline Si layer from being damaged in the manufacturing process of the solar cell.

  The present invention has been made in view of such problems, and its object is to reduce the Auger recombination in the crystalline Si layer by thinning the crystalline Si layer of the top cell, and An object of the present invention is to provide a tandem Si solar cell with high photoelectric conversion efficiency and a manufacturing method thereof, in which a thin crystalline Si layer is not damaged even in a manufacturing process.

  In order to solve the above-described problems, a solar cell according to the present invention has a top cell provided on a main surface of a base, and the top cell is sequentially formed from a light incident side, a transparent conductive layer and a first conductive layer. Type amorphous Si material layer, a second conductive type crystalline Si layer opposite to the first conductive type, and a laminated structure having a second conductive type amorphous Si layer, a light receiving surface electrode on the surface of the top cell, The substrate is provided with a back electrode, and the thickness of the second conductivity type crystalline Si layer of the top cell is 30 μm or less.

  Preferably, the thickness of the second conductivity type crystalline Si layer of the top cell is 3 μm to 30 μm.

  Preferably, the thickness of the second conductivity type crystal Si layer of the top cell is 4 μm to 20 μm.

  Further preferably, the thickness of the second conductivity type crystal Si layer of the top cell is 5 μm to 10 μm.

  In one embodiment, the top cell includes an i-type amorphous Si material layer between the first conductivity type amorphous Si material layer and the second conductivity type crystalline Si layer.

  In one embodiment, the top cell includes an i-type amorphous Si layer between the second conductive crystal Si layer and the second conductive amorphous Si layer.

  Furthermore, in an aspect, an insulating transparent passivation layer is provided between the top cell and the substrate.

  Preferably, the insulating transparent passivation layer is a layer made of silicon oxide or aluminum oxide.

  For example, the substrate is made of single crystal Si.

  In one embodiment, the base is made of single crystal Si, and a layer made of indium tin oxide (ITO) is provided between the top cell and the base.

  In one embodiment, the base is a bottom cell made of single-crystal Si, the top cell side is a second conductivity type region, a first conductivity type region is formed below the top cell side, and the bottom cell has the back surface A back electrode is provided and tandemized.

  Further, in one aspect, the bottom cell includes a second conductivity type layer having a donor concentration higher than that of the second conductivity type region on the top cell side of the second conductivity type region.

  In one embodiment, the top cell includes a second transparent conductive layer provided below the second conductivity type amorphous Si layer.

  Furthermore, in one aspect, when the top cell is viewed from above, the surface of the second transparent conductive layer is exposed in a comb shape having a bus bar portion and a plurality of finger portions extending from the bus bar portion. Has been.

  In one embodiment, a first comb-shaped light receiving surface electrode electrically connected to the transparent conductive layer and a surface of the top cell are electrically connected to the second transparent conductive layer. A second comb-shaped light-receiving surface electrode is provided.

  Furthermore, in a certain aspect, the 1st conductivity type area | region formed in the comb-tooth shape which has a bus-bar part and the several finger part extended from this bus-bar part on the back surface side of the said bottom cell, a bus-bar part, and this bus-bar A second second conductivity type region having a donor concentration higher than that of the second conductivity type region is formed, the first conductivity type region having a plurality of finger portions extending from the first portion; The finger portions and the finger portions of the second second conductivity type region are alternately located at a predetermined interval.

  In a preferred embodiment, the back surface of the bottom cell is electrically connected to the first comb-shaped back electrode electrically connected to the first conductivity type region, and to the second second conductivity type region. A second comb-like back electrode is provided.

  In a preferred embodiment, when the solar cell is viewed from above, the bus bar portion of the first comb-shaped light receiving surface electrode and the bus bar portion of the second comb-shaped back surface electrode are on one end side. The bus bar portion of the second comb-shaped light-receiving surface electrode and the bus bar portion of the first comb-shaped back electrode are positioned in parallel on the other end side.

  For example, the transparent conductive layer provided in the top cell is indium tin oxide (ITO).

  Preferably, the transparent conductive layer provided on the light incident side of the top cell also serves as an antireflection layer.

  Preferably, the second conductivity type crystalline Si layer of the top cell is designed to have a thickness that allows the generated currents of the top cell and the bottom cell to be the same.

  Furthermore, preferably, the stacked structure including the transparent conductive layer, the first conductive type amorphous Si material layer, the second conductive type crystalline Si layer, and the second conductive type amorphous Si layer included in the top cell has the solar cell from above. When viewed, it has an array structure in which a plurality of nanowires or walls arranged two-dimensionally at a predetermined interval are aligned in a predetermined direction and partitioned into a plurality of wall-shaped nanowalls arranged two-dimensionally at a predetermined interval, The diameter of the nanowire or the thickness of the nanowall is 10 nm or less at the site of the second conductivity type crystalline Si layer.

  Preferably, the nanowires or nanowalls adjacent to each other are separated by an insulating material.

  A method for manufacturing a solar cell according to the present invention is a method for manufacturing a solar cell having a top cell on a substrate, wherein a second conductivity type amorphous Si layer is formed on a surface region, and the second conductivity type amorphous Si layer is formed. The first second-conductivity-type Si crystal substrate on which a transparent conductive layer is provided and the surface of the substrate on which the transparent conductive layer or the insulating transparent passivation layer is formed are bonded at a temperature of 400 ° C. or lower. And a second step of thinning the first second-conductivity-type Si crystal substrate from the back surface to a thickness of 30 μm or less to form a second-conductivity-type crystal Si layer of the top cell. Yes.

  Preferably, the substrate has a second second conductive layer in which a second conductive type layer having a donor concentration higher than the bulk is formed in a surface region, and an insulating transparent passivation layer is provided on the second conductive type layer. Type Si crystal substrate.

  Preferably, the first step includes a sub-step of performing a surface activation process on at least one of the surface of the first second conductivity type Si crystal substrate and the surface of the base.

  For example, the surface activation treatment is performed by at least one of plasma treatment and ozone treatment.

  For example, the transparent conductive layer is indium tin oxide (ITO), and the insulating transparent passivation layer is a layer made of silicon oxide or aluminum oxide.

  In one aspect, prior to the first step, a step of implanting a predetermined dose of hydrogen into a surface region of the first second conductivity type Si crystal substrate to form a hydrogen ion implanted layer, 2, the second conductivity type crystalline Si layer is peeled off from the first second conductivity type Si crystal substrate by applying a mechanical or thermal shock to the hydrogen ion implantation layer, and the second cell of the top cell. A conductive crystal Si layer is used.

  In one embodiment, after the second step, a third step of forming a first conductivity type amorphous Si material layer opposite to the second conductivity type above the second conductivity type crystalline Si layer is provided. I have.

  Further, in one aspect, the third step includes a plurality of nanowires that two-dimensionally arrange the second conductive crystalline Si layers at predetermined intervals prior to the formation of the first conductive amorphous Si material layer. The second conductivity type crystalline Si layer is a nanowire having a diameter of 10 nm or less, or a plurality of wall-shaped nanowalls whose wall surfaces are aligned in a predetermined direction and are two-dimensionally arranged at predetermined intervals. A sub-step is provided for partitioning into nanowalls having a thickness of 10 nm or less at the portion of the conductive crystal Si layer.

  The solar cell according to the present invention employs a structure in which the second conductivity type crystalline Si layer of the top cell is significantly thinner than the conventional one. As a result, the open-circuit voltage of the top cell is 0.1 V or more higher than that in which the second conductivity type crystalline Si layer is 100 μm, and the current can be taken out at a high voltage, so that the photoelectric conversion efficiency is improved. .

  Also, when the present invention is a tandem solar cell, the output can be taken out from the top cell and the bottom cell independently, so it is necessary to match the generated current required for the series-connected tandem cell. Absent.

  Furthermore, the solar cell manufacturing method according to the present invention applies so-called “bonding” technology, and performs “bonding” between the top cell and the base or the bottom cell at 400 ° C. or lower. There is no separation and no deterioration in film quality, and no new defects are generated in the crystalline Si layer. For this reason, when this invention is used as a tandem solar cell, the deterioration of the heterojunction cell accompanying tandemization does not occur.

It is sectional drawing for demonstrating the outline of the basic structure of the tandem-type silicon solar cell which concerns on this invention. It is a figure which shows the thickness dependence of the top cell n-type crystalline Si layer of photoelectric conversion efficiency. It is a figure which shows the simulation result of the thickness dependence of the n-type crystalline Si layer of the open circuit voltage of a top cell. It is a figure for demonstrating the aspect of the light-receiving surface electrode with which the solar cell concerning this invention is provided. The figure (FIG. 5 (A)) explaining the aspect of the light-receiving surface electrode when it sees from the light-incidence side (upper) of the solar cell which concerns on this invention, and the aspect of the back electrode when it sees from the back side (lower) FIG. 6 is an explanatory diagram (FIG. 5B). It is a figure which shows the outline of the cross-section of the part shown with the broken line in the figure of FIG. 5 (A) and FIG. 5 (B) in the case of setting it as a 2 terminal tandem cell structure. It is a flowchart of the example of a process which manufactures the solar cell concerning this invention. It is a perspective view for conceptually explaining the configuration of a tandem solar cell in the case of an array structure in which a top cell is partitioned into a plurality of wall-like nanowalls that are two-dimensionally arranged at a predetermined interval. It is the transmission electron microscope image which observed a part of top cell which has the array structure divided into the several wall-like Si nanowall arranged two-dimensionally with the predetermined space | interval. It is a diagram showing the wavelength dependence of the reflectance of the reflectance and nanowalls mutual insulation material a is embedded SiO ₂ array structure (B) of the nanowalls compartmented array structure (A).

  Below, with reference to drawings, the solar cell which concerns on this invention, and its manufacturing method are demonstrated. In the following description, the first conductivity type is assumed to be p-type, and the second conductivity type is assumed to be n-type. However, the opposite relationship, that is, the first conductivity type is assumed to be n-type and the second conductivity type is assumed to be second-type. May be p-type. In the following description, the “amorphous Si material layer” is described as an amorphous Si layer, but in addition to this, an amorphous SiO layer, an amorphous SiN layer, or the like may be used.

  Hereinafter, although the case where the solar cell according to the present invention is a tandem solar cell will be described, it is not necessarily required to be a tandem solar cell. The solar cell according to the present invention may be a solar cell in which a top cell is provided on a substrate without including a bottom cell. That is, the “bottom cell” does not necessarily function as a solar battery.

[Outline of basic structure of tandem solar cell according to the present invention]
FIG. 1 is a cross-sectional view for explaining the outline of the basic structure of a tandem silicon solar cell according to the present invention. This solar cell 300 is a tandem solar cell in which a top cell 100 provided on the light incident side (upper side in the drawing) and a bottom cell 200 provided below the top cell 100 are stacked.

  The top cell 100 is produced by a process example described later using the first n-type Si crystal substrate 10. The top cell 100 includes a transparent conductive layer 110, a p-type amorphous Si layer 120 as a p-type amorphous material layer, an n-type crystalline Si layer 130, and an n-type amorphous Si layer 140 sequentially from the light incident side. In the example shown in FIG. 1, a second transparent conductive layer 150 is provided below the n-type amorphous Si layer 140, and the transparent conductive layer 110 and the second transparent conductive layer 150 are connected to two electrode layers. By doing so, the output of the top cell 100 can be taken out independently of the bottom cell 200. The transparent conductive layer 110 is made of indium tin oxide (ITO), for example, and can also serve as an antireflection layer. A light receiving surface electrode (not shown) provided on the surface of the top cell 100 will be described later.

  The p-type amorphous material layer may be a p-type amorphous SiO layer or a p-type amorphous SiN layer instead of the p-type amorphous Si layer 120 described above. Further, an i-type amorphous Si layer as an i-type amorphous material layer may be provided between the p-type amorphous Si layer 120 and the n-type crystalline Si layer 130. Instead of the i-type amorphous Si layer, an i-type amorphous Si layer may be provided. An amorphous SiO layer or i-type amorphous SiN layer may be provided.

  The p-type amorphous Si layer 120, the i-type amorphous Si layer, and the n-type amorphous Si layer 140 described above are almost all hydrogenated amorphous layers. This is the same even when the p-type amorphous Si layer 120 and the i-type amorphous Si layer are the other amorphous Si material layers described above.

  The bottom cell 200 is manufactured by a process example described later using the second n-type Si crystal substrate 20. The bottom cell 200 is made of single crystal Si, the top cell side is an n-type region 210, and a p-type region 220 as an emitter layer is provided below (that is, the back side of the solar cell).

  In the example shown in this figure, an n-type layer 230 having a higher donor concentration than the n-type region as a bulk is provided as a surface electric field layer (FSF) on the top cell side of the n-type region 210 of the bottom cell 200. ing. Further, a second n-type layer 240 having a donor concentration higher than that of the n-type region as a bulk is formed as a back surface field layer (BSF) adjacent to the p-type region 220 as the emitter layer.

  A first back electrode 260 and a second back electrode 270 are electrically connected to the p-type region 220 and the second n-type layer 240, which are emitter layers, via an insulating film 250, respectively. By using the p-type region 220 and the second n-type layer 240 which are layers as two electrode layers, the output of the bottom cell 200 can be taken out independently of the top cell 100.

  In the embodiment described later, the p-type region 220 is formed in a comb-like shape having a bus bar portion and a plurality of finger portions extending from the bus bar portion, and the second n-type region 240 is similarly formed in the bus bar portion. And a plurality of finger portions extending from the bus bar portion, and the finger portions of the p-type region 220 and the finger portions of the second n-type region 240 are alternately positioned at a predetermined interval.

  Here, in the p-type region 220 described above, the bus bar portion does not necessarily have the p-type conductivity type, but for convenience, the bus bar portion is also referred to as a “p-type region”. Similarly, in the second n-type region 240 described above, the bus bar portion does not necessarily have an n-type conductivity type, but for convenience, the bus bar portion is also referred to as an “n-type region”. In other words, the finger portions of the p-type region 220 and the finger portions of the second n-type region 240 are “p-type” and “n-type”, respectively, and these finger portions are alternately arranged in stripes at predetermined intervals. You may make it locate in this.

  A layer denoted by reference numeral 160 provided between the top cell 100 and the bottom cell 200 is an insulating transparent passivation layer, and is a layer used for bonding in a manufacturing process described later. The insulating transparent passivation layer 160 is a layer made of, for example, silicon oxide or aluminum oxide.

  The composition and thickness of each layer of the top cell 100 can be designed as follows, for example. The transparent conductive layer 110 is ITO of about 0.1 μm, the total thickness of the p-type amorphous Si layer 120 and the i-type amorphous Si layer is about 0.01 μm, the thickness of the n-type crystalline Si layer 130 is 30 μm or less, and the i-type amorphous Si layer The total thickness of the n-type amorphous Si layer 140 is about 0.01 μm, and the second transparent conductive layer 150 is about 0.1 μm. In addition, the thickness of the n-type crystalline Si layer of the top cell is preferably 3 μm to 30 μm, more preferably 4 μm to 20 μm, and still more preferably 5 μm to 10 μm. The reason will be described later.

The thickness of each layer of the bottom cell 200 made of single-crystal Si can be designed as follows, for example. The n-type region 210 as a bulk has a thickness of about 200 to 500 μm and a specific resistance of about 1 Ωcm, and the p-type region 220 as an emitter layer has an acceptor concentration of about 5 × 10 ¹⁹ cm ⁻³ and a thickness of about 2 to 3 μm. The n-type layer 230 as the front surface field layer (FSF) has a donor concentration of about 1 × 10 ¹⁹ cm ⁻³ and a thickness of about 0.1 to 1 μm. The second n-type layer as the back surface field layer (BSF) 240 has a donor concentration of about 5 × 10 ¹⁹ cm ⁻³ to 1 × 10 ²⁰ cm ⁻³ and a thickness of about 1 to 2 μm.

Note that SiO ₂ is preferably used as the insulating film 250. Also, SiO ₂ is suitably used for the insulating transparent passivation layer 160 provided between the top cell 100 and the bottom cell 200, and the thickness thereof is, for example, about 0.1 μm.

  In addition, the light receiving surface electrode described later and the above-described back surface electrodes 260 and 270 are formed by patterning a metal (for example, Al or Ag) formed on the entire surface by sputtering or vapor deposition, or a paste such as Al or Ag. It can be formed by baking after screen printing.

[Thickness of n-type crystalline Si layer of top cell]
FIG. 2 is a diagram showing a simulation result of the dependence of photoelectric conversion efficiency on the thickness of the top cell n-type crystalline Si layer. In this simulation, the photoelectric conversion efficiency is obtained by setting the thickness of the bottom cell to 300 μm and using the thickness of the n-type crystalline Si layer of the top cell as a parameter. According to this result, the photoelectric conversion efficiency when the thickness of the n-type crystalline Si layer of the top cell is 100 μm is almost equal to that when the thickness of the n-type crystalline Si layer is 1 μm (23.5%), and the thickness is 100 μm. If it exceeds, it is below this value.

  When the thickness of the n-type crystal Si layer is 30 μm or less, a photoelectric conversion efficiency of 24% or more is obtained, and when the thickness of the n-type crystal Si layer is in the range of 3 to 30 μm, a photoelectric conversion efficiency of 24% or more is obtained. . Further, a photoelectric conversion efficiency exceeding 24.1% is obtained when the thickness of the n-type crystalline Si layer is in the range of 4 μm to 20 μm, and a photoelectric conversion efficiency exceeding 24.2% is obtained in the range of 5 μm to 10 μm.

FIG. 3 is a diagram showing a simulation result of the thickness dependence of the n-type crystalline Si layer of the open voltage V _OC of the top cell. The value of the open voltage V _OC obtained assuming bulk silicon is shown in FIG. It is indicated by a circle. FIG. 3A shows an open circuit voltage when the thickness of the n-type crystalline Si layer is in the range of 1 to 100 μm, and FIG. 3B shows the thickness of the n-type crystalline Si layer in the range of 1 to 10 μm. The open circuit voltage at is shown.

  According to the results shown in these figures, an improvement in the open-circuit voltage is observed when the thickness of the n-type crystal Si layer is around 10 to 20 μm. In particular, when the thickness of the n-type crystal Si layer is 10 μm or less, the open-circuit voltage The improvement is remarkable and a value of 0.8 V or more is obtained. This indicates that the Auger recombination of carriers in the crystalline Si layer is significantly suppressed by reducing the thickness of the n-type crystalline Si layer.

  According to the results shown in FIG. 3, while the n-type crystalline Si layer has a thickness of 10 μm or less, a significant improvement in the open-circuit voltage is recognized. From the results shown in FIG. Considering the fact that the photoelectric conversion efficiency gradually decreases when the thickness is less than 5 μm, it is considered that the most preferable thickness range of the n-type crystalline Si layer is 5 μm to 10 μm.

[Output output from top cell and bottom cell]
FIG. 4 is a diagram for explaining an aspect of the light-receiving surface electrode provided in the solar cell according to the present invention. In this aspect, on the surface of the top cell 100, the first light receiving surface electrode 170 electrically connected to the above-described transparent conductive layer 110 and the second electrically connected to the second transparent conductive layer 150. The light receiving surface electrode 180 is provided and the transparent conductive layer 110 and the second transparent conductive layer 150 are two electrode layers, whereby the output of the top cell 100 can be taken out independently of the bottom cell 200.

  As already described, the output of the bottom cell 200 can be obtained by changing the output of the bottom cell 200 to that of the top cell 100 by using the p-type region 220 as the emitter layer and the second n-type layer 240 as two electrode layers. Can be taken out independently.

  FIG. 5A and FIG. 5B are diagrams for explaining the mode of the light-receiving surface electrode when viewed from the light incident side (upper side) of the solar cell 300 (FIG. 5A) and It is a figure (FIG.5 (B)) explaining the aspect of a back surface electrode when it sees from a back surface side (downward).

  As shown in FIG. 5A, the top cell 100 is provided with a first comb-shaped light-receiving surface electrode 170 and a second comb-shaped light-receiving surface electrode 180 when viewed from above. ing. The surface of the second transparent conductive layer 150 of the top cell 100 is exposed in a comb-like shape having a bus bar portion and a plurality of finger portions extending from the bus bar portion. The surface electrode 170 is electrically connected to the transparent conductive layer 110, and the second comb-shaped light receiving surface electrode 180 is electrically connected to the second transparent conductive layer 150.

  That is, when the top cell 100 is viewed from above, the surface of the second transparent conductive layer 150 is exposed in a comb shape having a bus bar portion and a plurality of finger portions extending from the bus bar portion, A first comb-shaped light-receiving surface electrode 170 electrically connected to the transparent conductive layer 110 and a second comb-shaped light-receiving surface electrode 180 electrically connected to the second transparent conductive layer 150 are provided. Thus, the output of the top cell 100 can be taken out independently of the bottom cell 200.

  Further, in this aspect, on the back surface side of the bottom cell 200, the p-type region 220 formed in a comb-teeth shape having a bus bar portion and a plurality of finger portions extending from the bus bar portion, the bus bar portion and the bus bar portion A second n-type region 240 having a donor concentration higher than that of the n-type region 210 as a bulk is formed in a comb-like shape having a plurality of extending finger portions. The finger portions of the second n-type region 240 are alternately positioned at a predetermined interval.

  A first comb-like back electrode 260 electrically connected to the p-type region 220 and a second comb tooth electrically connected to the second n-type region 240 are formed on the back surface of the bottom cell 200. A back electrode 270 is provided so that the output of the bottom cell 200 can be taken out independently of the top cell 100.

  The structure of the light receiving surface electrode in this manner is particularly effective when the loss due to the high sheet resistance of the transparent conductive layers (110, 150) of the top cell 100 cannot be ignored. Since the output of the top cell 100 and the output of the bottom cell 200 can be taken out independently, it is not necessary to match the generated currents of the two like a two-terminal tandem cell electrically connected in series. There are few restrictions.

  Moreover, in the above-mentioned aspect, since the output terminal of the top cell 100 is on the incident light surface side and the output terminal of the bottom cell 200 is on the back surface side, a module is manufactured by incorporating several cells. In some cases, the top cells or the bottom cells are connected in series, and the terminal box of the module has four terminals.

  It is possible to make the generated currents of the top cell 100 and the bottom cell 200 the same by adjusting the thickness of the crystalline Si layer 130 of the top cell 100. In this case, two terminals can be realized. For example, the output of the top cell and the bottom cell may be connected in series and incorporated in the module, or the output from the top cell and the output from the bottom cell may be connected in series in the terminal box of the module to form two terminals.

  In order to obtain a two-terminal tandem cell structure, for example, the following interelectrode connection relationship can be employed.

  FIG. 6 is a diagram showing an outline of a cross-sectional structure of a portion indicated by a broken line in FIGS. 5 (A) and 5 (B). First, the thickness of the crystalline Si 130 layer of the top cell 100 is adjusted so that the optimum operating currents of the top cell 100 and the bottom cell 200 are substantially equal. Thereafter, the comb-teeth electrode electrically connected to the bus bar of the comb-teeth electrode 260 for taking out the output from the emitter of the bottom cell 200 and the second transparent conductive layer 150 provided on the n-type amorphous Si layer side of the top cell 100 The 180 bus bars are connected by a conductive material 280 so as to cross the end of the bottom cell 200 in the cell thickness direction.

  In this case, the comb electrodes of the comb electrodes to be connected are arranged so as to be the same end edge of the cell. That is, when the solar cell 100 is viewed from above, the bus bar portion of the second transparent conductive layer 150 that is the second comb-shaped light-receiving surface electrode and the first comb-shaped back surface electrode are the first. The bus bar portion of the back electrode 260 is positioned in parallel on the other end side. On the other hand, the bus bar portion of the first transparent conductive layer 110 that is the first comb-shaped light-receiving surface electrode and the bus bar portion of the second back electrode 270 that is the second comb-shaped back surface electrode are on the other end side. It will be located in parallel.

  The connection between the electrodes is performed so that the voltage drop due to the generated current is sufficiently small. When the insulating film 250 on the back surface of the cell is an oxide film formed by thermal oxidation or plasma CVD, the end face of the cell is also covered with such an insulating film 250, and the thickness of the cell In view of the fact that the thickness is several hundred μm, it is preferable to use a conductive paste such as an Ag paste having a thickness of several μm or more. If such a conductive paste is used, the voltage drop at the connection location due to the generated current is compared to the generated voltage just by applying and baking the end of the cell so as to be in contact with the two buses described above. It can be made sufficiently small.

  Below, the outline | summary of the manufacturing method of the solar cell which concerns on this invention of the structure mentioned above is demonstrated by an illustration. In the following embodiments, the first conductivity type is p-type and the second conductivity type is n-type. However, the opposite relationship, that is, the first conductivity type is n-type and the second conductivity type is n-type. The type may be p-type as described above. In addition, as described above, the “amorphous Si material layer” can be an amorphous SiO layer or an amorphous SiN layer instead of an amorphous Si layer.

  FIG. 7 is a flowchart of an example process for manufacturing a solar cell according to the present invention. First, the two n-type single crystal Si substrates (10, 20) described above are prepared. Although there is no special restriction | limiting in the thickness of Si substrate, Generally, it is 200-500 micrometers. The first n-type Si crystal substrate 10 is for making a top cell, and the second n-type Si crystal substrate 20 is for making a bottom cell. In the following embodiment, the first n-type Si crystal substrate 10 may be a single-side polished finish, but the second n-type Si crystal substrate 20 is a double-side polished finish. Further, the specific resistance value of the Si substrate is a design item of the solar cell, but here, a specific resistance value of about 1 Ωcm is used.

  An n-type hydrogenated amorphous Si layer 140 is formed on the surface of the first n-type Si crystal substrate 10 (S101), and a transparent conductive layer 150 made of ITO is formed on the n-type hydrogenated amorphous Si layer 140. Form (S102).

On the other hand, on the rear surface of the second n-type Si crystal substrate 20, n ⁺ region (phosphorus concentration 10 ¹⁹ ~10 ²⁰ cm ^-3 or so) and the p ⁺ region (boron concentration 10 ¹⁹ ~10 ²⁰ cm ^-3 of about p ^{The +} layer is formed in a stripe shape (or comb shape), and these are used as the second n-type layer 240 as the back surface field layer (BSF) and the p-type region 220 as the emitter layer (S201). These n ⁺ regions and p ⁺ regions are formed by ion implantation, thermal diffusion, laser doping, or the like.

Next, an n-type layer 230 (n ⁺ layer having a phosphorous concentration of about 10 ¹⁹ cm ⁻³ ) is formed on the surface of the second n-type Si crystal substrate 20 as a surface electric field layer (FSF) (S202). This FSF layer is formed by a thermal diffusion method or an ion implantation method.

  Subsequently, an insulating transparent passivation layer 160 is formed on the n-type layer 230 as the surface electric field layer (FSF) (S203). The passivation film is a thermal oxide film, a deposited oxide film by CVD, a hydrogenated amorphous SiO film by plasma CVD or hot wire CVD, or the like.

  Next, at least one of the surface of the first n-type Si crystal substrate 10 (that is, the transparent conductive layer 150 made of ITO) and the surface of the second n-type Si crystal substrate 20 (that is, the insulating transparent passivation layer 160). A surface activation treatment is performed (S301). This surface activation treatment is, for example, plasma treatment or ozone treatment.

  Subsequent to this surface activation treatment, the surface of the first n-type Si crystal substrate (that is, the transparent conductive layer 150 made of ITO) and the second n-type are applied by applying a known bonding technique between semiconductor substrates. The surfaces of the Si crystal substrates (that is, the insulating transparent passivation layer 160 made of silicon oxide or aluminum oxide) are bonded together (S302). This bonding is performed at a temperature of 400 ° C. or lower. This is because the deterioration of the film quality due to the separation of hydrogen from the hydrogenated amorphous Si layer of the top cell and the introduction of defects into the crystalline Si layer of the top cell are suppressed, and the deterioration of the solar cell characteristics during tandemization is not caused. is there.

  For the purpose of increasing the bonding strength, a hydrogenated amorphous SiO film may be further deposited on the transparent conductive layer 150 made of ITO on the top cell side.

  After this bonding, the crystal part on the back surface side of the first n-type Si crystal substrate is removed, and the thickness is reduced to 30 μm or less (generally 10 μm or less) to form the n-type crystal Si layer 130 of the top cell. (S303).

  This thinning step may be performed by a method such as a so-called “smart cut method” in addition to mechanically polishing the back side of the first n-type Si crystal substrate.

  In the case of the smart cut method, prior to step S101, a predetermined dose amount of hydrogen is implanted into the surface region of the first n-type Si crystal substrate to form a hydrogen ion implanted layer. By applying mechanical or thermal shock to the hydrogen ion implantation layer, the n-type crystal Si layer is peeled from the first n-type Si crystal substrate to form an n-type crystal Si layer of the top cell.

  After such a thinning step, the polishing damage layer is removed by etching as necessary, and the thickness of the crystalline Si layer is adjusted to a desired value.

  Thereafter, a hydrogenated i-type amorphous Si layer, a hydrogenated p-type amorphous Si layer 120, and a transparent conductive layer 110 made of ITO are sequentially laminated on the light incident surface side (S304 to S306).

  Next, as shown in FIG. 4, a part of the second transparent conductive layer 150 is exposed by photolithography technology (S307), and the light receiving surface electrodes 170 and 180 are formed (S308). Thus, the first light receiving surface electrode 170 is electrically connected to the transparent conductive layer 110, and the second light receiving surface electrode 180 is electrically connected to the second transparent conductive layer 150, thereby completing the top cell.

  Finally, contact holes are formed on the back side of the second n-type Si crystal substrate to form back electrodes 260 and 270 as shown in FIG. 4 (S309), and the top cell is completed and a solar cell is formed. Is also completed.

  As already described, the open circuit voltage can be increased by reducing the thickness of the crystalline Si layer of the top cell as in the solar cell according to the present invention. On the other hand, when the thickness of the crystalline Si layer of the top cell is reduced, the light absorption length (optical path length) of the top cell is shortened. Will drop. However, since the solar cell of the present invention has a tandem structure, light that could not be absorbed by the top cell is absorbed by the bottom cell and can be used for power generation. As a result, the conversion efficiency is improved as compared with the case where power is generated by the bottom cell alone by the amount by which the power generation current of the top cell can be extracted at a high voltage.

  Thus, the solar cell according to the present invention employs a structure in which the second conductivity type crystalline Si layer of the top cell is significantly thinner than the conventional one. As a result, the open-circuit voltage of the top cell is 0.1 V or more higher than that in which the second conductivity type crystalline Si layer is 100 μm, and the current can be taken out at a high voltage, so that the photoelectric conversion efficiency is improved. .

  Further, since the output can be taken out independently from the top cell and the bottom cell, it is not necessary to match the generated current required for the series-connected tandem cell.

  Furthermore, the solar cell manufacturing method according to the present invention applies so-called “bonding” technology, and performs “bonding” between the top cell and the bottom cell at 400 ° C. or lower, so that hydrogen is released from the hydrogenated amorphous Si layer. The film quality is not deteriorated and no new defect is generated in the crystalline Si layer, so that the deterioration of the heterojunction cell due to the tandemization does not occur.

  So far, the case where the solar cell according to the present invention is a tandem solar cell has been described. However, the solar cell according to the present invention does not necessarily have to be a tandem type, and may be a solar cell in which the above-described top cell is provided on a base without including a bottom cell. That is, the “bottom cell” described above does not necessarily function as a solar battery.

  When the above-described “bottom cell” is not a solar cell but a so-called substrate, the solar cell according to the present invention has a top cell provided on the main surface of the substrate, and the top cell is from the light incident side. And a laminated structure having a transparent conductive layer, a first conductive type amorphous Si material layer, a second conductive type crystalline Si layer opposite to the first conductive type, and a second conductive type amorphous Si layer, A solar cell characterized in that a light receiving surface electrode is provided on the surface of the cell, and a back electrode is provided on the substrate, and the thickness of the second conductivity type crystal Si layer of the top cell is 30 μm or less. become.

  As the substrate in this case, for example, single crystal Si may be used. Further, when the base is made of single crystal Si, a mode in which a layer made of indium tin oxide (ITO) is provided between the top cell and the base may be adopted.

  Further, the substrate has a second conductivity type layer in which a second conductivity type layer having a donor concentration higher than that of the bulk is formed in the surface region, and an insulating transparent passivation layer is provided on the second conductivity type layer. It is good also as an aspect which is a Si crystal substrate.

  In this case, when the solar cell is manufactured, the above-described step S303 (after bonding, the crystal part on the back surface side of the first n-type Si crystal substrate is removed, and the thickness is reduced to 30 μm or less to form the n-type crystal of the top cell. After the step of forming the Si layer 130, a step of forming a first conductivity type amorphous Si material layer opposite to the second conductivity type above the second conductivity type crystalline Si layer may be provided. Good.

  Further, the top cell structure may have a form in which nanowires and nanowalls are arrayed. By setting it as such a nanostructure, a quantum effect can be improved and the photoelectric conversion efficiency of a solar cell can be improved.

  A solar cell including a top cell having such a structure includes a transparent conductive layer, a first conductive type amorphous Si material layer, a second conductive type crystalline Si layer, and a second conductive type amorphous Si layer provided in the top cell. The structure is such that when the solar cell is viewed from above, a plurality of nanowires or wall surfaces arranged two-dimensionally at a predetermined interval are aligned in a predetermined direction and a plurality of wall-shaped nanowalls arranged two-dimensionally at a predetermined interval The solar cell has a partitioned array structure, and the diameter of the nanowire or the thickness of the nanowall is 10 nm or less at the second conductive crystal Si layer.

  When such an aspect is adopted, it is preferable that the nanowires or nanowalls adjacent to each other are separated by an insulating material.

  In addition, when forming a nanostructure top cell, in the step of forming a first conductivity type amorphous Si material layer opposite to the second conductivity type above the second conductivity type crystalline Si layer, the first conductivity type amorphous Prior to the formation of the Si material layer, a plurality of nanowires in which the second conductivity type crystalline Si layer is two-dimensionally arranged at a predetermined interval and having a diameter of 10 nm or less at a site of the second conductivity type crystal Si layer, or A sub-step of partitioning into a plurality of wall-shaped nanowalls whose wall surfaces are aligned in a predetermined direction and are two-dimensionally arranged at a predetermined interval and having a thickness of 10 nm or less at a portion of the second conductivity type crystalline Si layer; It is preferable to provide.

  FIG. 8 is a perspective view for conceptually explaining the configuration of a tandem solar cell in the case of an array structure in which a top cell is partitioned into a plurality of wall-shaped nanowalls arranged two-dimensionally at a predetermined interval. is there.

  The band gap of Si is about 1.1 eV in bulk, but when nano-order walls and wires are used, it is known that the size becomes larger when the size is smaller than about 10 nm. Then, by changing the size (width) of the nanowall or nanowire, the band gap can be controlled by the quantum confinement effect. In other words, by making the top cell have a structure in which nanowalls and nanowires of such a size are two-dimensionally arranged, the performance as a solar cell can be enhanced by actively utilizing the quantum confinement effect. .

  Assuming nanowalls, theoretically, when the wall thickness is reduced to 2 nm, the effective band gap is about 1.6 eV, which is about 45% of the band gap of bulk silicon (about 1.1 eV). The gap is widened and high efficiency can be expected.

FIG. 9 is a transmission electron microscope image obtained by observing a part of the top cell having an array structure partitioned into a plurality of wall-like Si nanowalls arranged two-dimensionally at a predetermined interval. The size of the nanowall is 10 nm or less, and is about 2 nm in the example shown in this figure. In addition, an insulating material (SiO ₂ or Al ₂ O ₃ ) is embedded between the nanowalls.

FIG. 10 is a diagram showing the wavelength dependence of the reflectance of the array structure (A) partitioned into nanowalls and the reflectance of the array structure (B) in which SiO ₂ that is an insulating material is embedded between the nanowalls. is there.

  As is clear from this figure, by embedding an insulating material between the nanowalls, the reflectance is kept low, and the utilization efficiency of sunlight is increased.

  Such a nanowall can be produced, for example, by the following process. First, a wall having a width of several tens of nm is formed by patterning using immersion lithography. Thereby, for example, a wall having a width of about 75 nm and a height of about 1 μm can be formed. In addition, if the main surface of Si used as a base material is a (1, 1, 0) plane, for example, since the (1, -1, 1) plane is orthogonal to the (1, 1, 0) plane, the wall surface is The (1, -1, 1) plane wall can be formed perpendicular to the main surface. Subsequently, an oxidation process and an etching process are repeated to form nanowalls having a width of several nm.

  It should be noted that, instead of the above-mentioned nanowall, the same quantum effect can be obtained even if the nanowire has an array structure partitioned into a plurality of nanowires arranged two-dimensionally at a predetermined interval, and the nanowire has a diameter of 10 nm or less. It goes without saying that you can get it.

  According to the present invention, Si having a high photoelectric conversion efficiency is obtained by thinning the crystalline Si layer of the top cell to suppress Auger recombination in the crystalline Si layer, and without damaging the thin crystalline Si layer in the manufacturing process. A solar cell and a method for manufacturing the same are provided.

10 first n-type Si crystal substrate 20 second n-type Si crystal substrate 100 top cell 110 transparent conductive layer 120 p-type amorphous Si layer 130 n-type crystal Si layer 140 n-type amorphous Si layer 150 second transparent conductive layer 160 Insulating transparent passivation layer 200 Bottom cell 210 n-type region 220 p-type region as an emitter layer 230 n-type layer having a high donor concentration 240 second n-type layer having a high donor concentration 250 insulating film 260 first back electrode 270 Second back electrode 280 Conductive material 300 Solar cell

Claims (13)

The top cell is provided on the main surface of the single crystal Si substrate,
The top cell has a thickness of 3 μm to 30 μm having a first transparent conductive layer, an amorphous Si material layer having a first conductivity type, and a second conductivity type opposite to the first conductivity type in order from the light incident side. A laminated structure having a single crystal Si layer, an amorphous Si layer having a second conductivity type, and a second transparent conductive layer;
The single crystal Si substrate is a bottom cell of a back contact type crystal Si cell structure having an insulating transparent passivation layer on the top cell side,
The second transparent conductive layer of the top cell and the insulating transparent passivation layer of the bottom cell are joined and tandemized,
The top cell includes an i-type amorphous Si material layer between the amorphous Si material layer having the first conductivity type and the single crystal Si layer, and
An i-type amorphous Si layer is provided between the single-crystal Si layer having the second conductivity type and the amorphous Si layer having the second conductivity type.
A solar cell characterized by that.   The solar cell according to claim 1, wherein the insulating transparent passivation layer is a layer made of silicon oxide or aluminum oxide.   The solar cell according to claim 1, wherein the second transparent conductive layer included in the top cell is made of indium tin oxide (ITO).   The top cell is exposed in a comb-like shape having a surface of the second transparent conductive layer having a bus bar portion and a plurality of finger portions extending from the bus bar portion when viewed from above. The solar cell as described in.   A first comb-shaped light receiving surface electrode electrically connected to the first transparent conductive layer and a second comb tooth electrically connected to the second transparent conductive layer on the top cell surface The solar cell according to claim 1, wherein a light receiving surface electrode is provided. On the back surface side of the bottom cell, a first conductivity type region formed in a comb shape having a bus bar portion and a plurality of finger portions extending from the bus bar portion, and a plurality of bus bar portions and a plurality of bus bar portions extending from the bus bar portion. A second conductivity type region having a finger concentration and a donor concentration higher than that of the bulk portion of the single crystal Si substrate is formed, and the finger portion of the first conductivity type region and the second conductivity type are formed. The finger portions of the mold region are alternately located at predetermined intervals, and
A first comb-shaped back electrode electrically connected to the first conductivity type region and a second comb-shaped electrode electrically connected to the second conductivity type region on the back surface of the bottom cell. A back electrode is provided,
The solar cell of any one of Claims 1-5.   When the solar cell is viewed from above, the bus bar portion of the first comb-shaped light receiving surface electrode and the bus bar portion of the second comb-shaped back surface electrode are positioned in parallel on one end side. The solar cell according to claim 6, wherein the bus bar portion of the second comb-shaped light receiving surface electrode and the bus bar portion of the first comb-shaped back surface electrode are positioned in parallel on the other end side.   The solar cell according to any one of claims 1 to 7, wherein the single-crystal Si layer of the top cell is designed to have a thickness such that the generated currents of the top cell and the bottom cell are the same. The laminated structure including the first transparent conductive layer provided in the top cell, the amorphous Si material layer having the first conductivity type, the single crystal Si layer having the second conductivity type, and the amorphous Si layer having the second conductivity type, When a solar cell is viewed from above, a plurality of nanowires or wall surfaces arranged two-dimensionally at a predetermined interval are aligned in a predetermined direction and are arrayed into a plurality of wall-like nanowalls arranged two-dimensionally at a predetermined interval Having a structure, the diameter of the nanowire or the thickness of the nanowall is 10 nm or less at the site of the single crystal Si layer,
The nanowires or nanowalls adjacent to each other are separated by an insulating material,
The solar cell of any one of Claims 1-8. A method for producing a solar cell having a top cell on a single crystal Si substrate,
A first Si single crystal substrate in which an amorphous Si layer is formed in a surface region, and a transparent conductive layer is provided on the amorphous Si layer, and a second Si single crystal serving as a bottom cell of a back contact type crystal Si cell structure A surface activation treatment is performed on at least one of a plasma treatment and an ozone treatment on the surfaces of the second Si single crystal substrates, which are substrates, each having a transparent conductive layer or an insulating transparent passivation layer formed on the surface region, and 400 A first step of bonding at a temperature of ℃ or less;
A second step of thinning the first Si crystal substrate from the back surface to a thickness of 30 μm or less to form a single crystal Si layer of the top cell;
A method for producing a solar cell comprising:   The method for manufacturing a solar cell according to claim 10, wherein the transparent conductive layer is indium tin oxide (ITO), and the insulating transparent passivation layer is a layer made of silicon oxide or aluminum oxide.   After the second step, a third step is provided in which an amorphous Si material layer having a conductivity type opposite to that of the first Si single crystal substrate is formed above the single crystal Si layer of the top cell. The method for manufacturing a solar cell according to claim 10.   Prior to the formation of the amorphous Si material layer, the third step is a plurality of nanowires in which the single crystal Si layer is two-dimensionally arranged at a predetermined interval, and the diameter of the single crystal Si layer is 10 nm or less. Or a plurality of wall-shaped nanowalls whose wall surfaces are aligned in a predetermined direction and arranged two-dimensionally at a predetermined interval, and are partitioned into nanowalls having a thickness of 10 nm or less at the portion of the single crystal Si layer The method for manufacturing a solar cell according to claim 10, comprising substeps.

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