JP2013219118A - Solar battery and manufacturing method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a solar battery in which a lead-free solder is formed as a connection material on a surface of a solar battery cell using brittle materials such as silicon, and which is unlikely to cause breaking defects due to a thermal stress and has a semiconductor characteristic which is hard to deteriorate.SOLUTION: In a solar battery manufacturing method, by bonding a surface of a solar battery cell 1 and conductive members 3a, 3b which are opposite materials by using a Sn-Bi-Ag-based lead-free solder, a thermal stress generated when a temperature of the lead-free solder 2 changes from a melting point to an atmospheric temperature is relaxed thereby to inhibit cracks on the surface of the solar battery cell. The solar battery comprises, for example, the thin film solar battery cell 1 using a semiconductor thin film and conductive members 3a, 3b including Cu or a Cu alloy bonded to the surface of the solar battery cell 1 by using the lead-free solder 2 including SnBiAg.

Description

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

  In a general electronic circuit, it is necessary to use a lead-free solder that is easy to form in a ball shape and has good meltability and wettability. For this reason, the mass percent concentration of the alloy which comprises is adjusted, the lead-free solder adjusted in this way is used for a connection material, and the base material and the other party material are connected (for example, patent document 1). .

JP-A-11-347784

`` Materials '', Vol.51, No.4, pp.445-450, Apr.2002.

  The conventional lead-free solder is environmentally friendly because it is lead-free. However, when a solar cell using a brittle material such as silicon is used as the base material, the melting point is higher than that of the conventional lead solder. There was a problem that the thermal stress generated due to the difference in linear expansion between the material and the other material such as the current collecting electrode was large, and the base material was easily cracked.

Further, for example, a heterostructure solar cell in which an amorphous silicon layer or the like is formed on a silicon substrate by CVD, for example, is known to have high power generation efficiency, but the semiconductor characteristics deteriorate at a high temperature of 200 ° C. or higher. Therefore, conventional lead-free solder typified by Sn ₃ Ag _0.5 Cu cannot be used.

  The present invention has been made in view of the above, and is a solar battery in which lead-free solder is formed as a conductive material such as a wiring material on the surface of a solar battery cell using a brittle material such as silicon. It is an object of the present invention to obtain a solar cell in which breakdown failure due to is difficult to occur and the semiconductor characteristics are hardly deteriorated.

  In order to solve the above-described problems and achieve the object, a solar cell according to the present invention includes a thin-film solar cell using a semiconductor thin film, and a Sn-Bi-Ag lead on the surface of the solar cell. And a conductive member joined using free solder.

  According to the present invention, by using Sn-Bi-Ag-based lead-free solder, the temperature of the lead-free solder changes from the melting point to room temperature by joining the surface of the solar battery cell and the conductive member as the counterpart material. The effect of relieving the thermal stress generated during the process and suppressing cracks on the surface of the solar battery cell is exhibited.

FIG. 1 is a perspective view showing the structure of a solar cell according to Embodiment 1 of the present invention. FIG. 2 is a schematic cross-sectional view of the solar battery cell used in the first embodiment. 3A to 3C are process explanatory views showing a method for manufacturing the solar cell. FIG. 4 is a graph illustrating data measured by evaluating the creep characteristics of the lead-free solder according to Embodiment 1 of the present invention by the indentation method. FIG. 5 is a graph showing the relationship between the creep rate and stress at 25 ° C. of the lead-free solder according to Embodiment 1 of the present invention. FIG. 6 is a graph showing the relationship between the creep rate and stress at 100 ° C. of the lead-free solder according to Embodiment 1 of the present invention. FIG. 7 is a perspective view of a test sample and a measuring machine for examining the influence of thermal stress relaxation due to creep deformation of lead-free solder according to Embodiment 1 of the present invention. FIG. 8 is a schematic cross-sectional view of a solar battery cell used in the second embodiment.

  Embodiments of a solar cell and a method for manufacturing the solar cell according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited to the following description, In the range which does not deviate from the summary of this invention, it can change suitably. In the drawings shown below, the scale of each member may be different from the actual scale for easy understanding. The same applies between the drawings.

Embodiment 1 FIG.
FIG. 1 is a perspective view showing a configuration of a solar cell according to Embodiment 1 of the present invention. FIG. 2 is a schematic cross-sectional view of the solar battery cell used in the first embodiment. 3A to 3C are process explanatory views showing a method for manufacturing the solar cell. In the solar cell of the first embodiment, a Sn—Bi—Ag-based material that is a connection material is formed on the surface of a solar cell 1 that is formed by forming a semiconductor thin film on a single crystal silicon substrate that is a brittle material. Via lead-free solder 2, conductive members 3a and 3b for voltage extraction made of Cu or Cu alloy are connected.

As this Sn-Bi-Ag-based lead-free solder 2, Sn ₅₇ Bi ₁ Ag, which is a low melting point solder having a melting point of 146 ° C. and a creep property superior to that of lead solder, is used. Creep is a phenomenon in which strain increases with the passage of time when a continuous stress acts on an object. That is, it can be considered that the higher the creep property, the easier the displacement with respect to the stress and the stress absorption. Therefore, in the present embodiment, with respect to the solder used for connection, not only the melting point but also the creep property contributes to the stress relaxation of the solar battery cell, and the solder material is selected. What is effective in the bonding of the present embodiment is a conventional solder containing Sn as a main component, with Bi and Ag added. By adding Bi, the melting point is lowered, and Ag is added. The creep property is improved. The addition amount of Bi is a range in which the melting point can be lowered to a desired value, and the addition amount of Ag is a range of values in which the creep property can be sufficiently increased and is within a range satisfying both. It is desirable to use -Bi-Ag.

This Sn ₅₇ Bi ₁ Ag not only has a low melting point, but also has a high creep property and a small change in the creep index even when the temperature rises in the solder joint process, and maintains a high creep property. Even when the temperature rises and falls, the buffering property is high, and stress is not applied to the solar battery cell to cause destruction. Therefore, the thermal stress generated when the temperature of the lead-free solder changes from the melting point to room temperature is greatly relieved and cracking of the solar battery cell is suppressed. In other words, since it has a low melting point, it reduces the temperature change that solar cells receive in the bonding process, and has a stable and high creep property without change in all temperature regions. The effect of relaxing thermal stress can be obtained.

  The solar battery cell 1 used in Embodiment 1 is as shown in a schematic cross-sectional view in FIG. 2, and uses an n-type single crystal silicon substrate 101 which is a crystalline semiconductor substrate as a substrate. An intrinsic (i-type) amorphous silicon layer 102, a p-type amorphous silicon layer 103, and a p-side electrode 104 are provided on the light-receiving surface side 101 a of the n-type single crystal silicon substrate 101. An intrinsic (i-type) amorphous silicon layer 105, an n-type amorphous silicon layer 106, and an n-side electrode 107 are provided on the surface (back surface) 101b opposite to the light-receiving surface of the n-type single crystal silicon substrate 101. Yes. The p-side electrode 104 is composed of a translucent electrode such as indium tin oxide (ITO) formed on the entire light-receiving surface side 101 a of the solar battery cell 1. And the n side electrode 107 is comprised by the metal electrode formed in the back surface side 101b of the photovoltaic cell 1. FIG.

  Next, a process of forming a solar battery by connecting the solar battery cell 1 to a conductive member for external connection will be described.

  First, as shown in FIG. 3A, a semiconductor thin film such as an i-type amorphous silicon layer, a p-type amorphous silicon layer, or an n-type amorphous silicon layer is formed on an n-type single crystal silicon substrate 101, and the light-receiving surface side ( Solar cell 1 having electrodes (not shown) is formed on front side 1a and back side 1b (this step will be described later).

Next, as shown in FIG. 3B, lead-free solder 2 made of Sn ₅₇ Bi ₁ Ag is formed on the solar battery cell 1 on which the semiconductor thin film is formed by screen printing.

  Then, as shown in FIG. 3C, conductive members 3a and 3b made of a Cu alloy cross-sectional strip material are arranged on the lead-free solder 2 applied to the front surface 1a and the back surface 1b of the solar battery cell 1. And heated to 150 ° C. The conductive members 3a and 3b are wirings for external extraction, and are connected in series with the adjacent solar cells 1 and are connected to wirings for extracting power from the solar panel at the first stage and the final stage, and processing such as a power conditioner. Connected to the circuit. A plurality of solar battery panels are often connected in series before being connected to the power conditioner.

  As described above, according to the present embodiment, solder bonding is performed at low temperature on the solar battery cell 1 using the lead-free solder 2 having a melting point lower than that of lead solder and having a creep constant equal to or higher than that of lead solder. Therefore, the stress is absorbed by this lead solder, and there is no fear of the solar cell 1 being destroyed, and a highly reliable solder joint is realized. In this embodiment, the lead-free solder 2 is formed in advance on the cell by screen printing. However, it can be similarly manufactured by plating the lead-free solder on the conductive members 3a and 3b made of Cu alloy. It is.

  The p-side electrode 104 and the n-side electrode 107 are connected to the conductive member 3 for voltage extraction, and external extraction is performed. The p-side electrode 104 and the n-side electrode 107 and the conductive member 3 for voltage extraction are connected to each other. A current collecting electrode such as a current collecting grid electrode or bus electrode is formed in the connection portion, but it is not shown here. The grid electrode or the bus electrode for current collection is formed by printing and drying an electrode material paste by a screen printing method, and then baking at a low temperature of 200 ° C. or lower.

  In this solar cell 1, the upper surface side in FIG. 2 is the light receiving surface side 1a (101a), and sunlight is incident. The solar cell 1 is a heterostructure solar cell in which a p-type electrode 104 and an n-type electrode 107 are arranged on the front surface (light receiving surface) side 101a and the back surface side 101b of an n-type single crystal silicon substrate 101, respectively. Thus, in the solar cell 1, the p-type electrode 104 and the n-type electrode 107 are connected to the strips as the conductive members 3a and 3b made of Cu alloy, respectively, and take out the photoelectric conversion voltage.

  Here, it is desirable that the conductive member 3 has a narrow width in order to secure a light receiving area on the light receiving surface side 101 a of the solar battery cell 1. Therefore, it is desirable to increase the thickness in order to reduce the extraction resistance. On the other hand, since it is not necessary to secure the light receiving area on the back surface side 1b (101b) of the solar battery cell 1, the width of the conductive members 3a and 3b can be increased. For this reason, the thickness can be reduced.

  In the case of the solar cell 1 of the present embodiment, the thickness of the conductive members 3a and 3b is 200 to 400 μm on the light receiving surface side 101a, and the thickness of the conductive members 3a and 3b is 20 to 50 μm on the back surface side 101b. The conductive members 3a and 3b are preferably configured to have equal cross-sectional areas on both sides in order to have specific resistance. For example, when the thickness of the conductive member 3a is 200 μm on the light receiving surface side 101a and the thickness of the conductive member 3b is 20 μm on the back surface side 101b, the width of the conductive members 3a and 3b is 20 μm on the light receiving surface side 101a, and the back surface side 101b is The width of the conductive member 3b is set to 200 μm so that the cross-sectional areas are equal.

According to the present embodiment, even when the thickness of the solar battery cell 1 is as thin as about 100 μm, and the thickness and the bonding area of the mating member, that is, the conductive members 3a and 3b connected on both sides are greatly different, the creep property By using the lead-free solder 2 made of Sn ₅₇ Bi ₁ Ag according to the present embodiment which is excellent in heat resistance, the thermal stress generated at the connection portion between the solar cell 1 and the conductive members 3a and 3b even at the reflow temperature in the joining process Can be relaxed by creep deformation, and a highly reliable solder joint is realized.

  Further, the thickness of the solar battery cell 1 is not only 100 μm, but also when it is in the range of 100 μm to 300 μm, it is particularly easy to crack because it is thin, but according to the method of the present embodiment, it is reliable and reliable. Highly bonding is realized. Therefore, in the solar battery of the present embodiment, the particularly effective thickness of the solar battery cell 1 is about 100 μm to 300 μm, and the effect of the present embodiment is great in the case of the thin solar battery cell 1.

  In addition, the thickness of the conductive members a and 3b for the wiring connected to the solar battery cell 1 is 20 μm to 400 μm over a wide range, and according to the method of the present embodiment, reliable and highly reliable bonding is realized. Is done.

  Moreover, since the thickness of the solar battery cell 1 is as thin as about 100 μm to 300 μm and has high brittleness, it is easy to break, and the thickness of the conductive member 3 a for wiring connected to the solar battery cell 1 is 200 μm to When the thickness is 400 μm, cracks are likely to occur in normal bonding, but in the case of the present embodiment, thermal stress generated in the connection portion between the solar cell 1 and the conductive member 3 can be relaxed by creep deformation, and reliability is improved. High solder joint is realized.

  Furthermore, since the thickness of the solar battery cell 1 is as thin as about 100 μm to 300 μm and is highly brittle, it is easy to break, and the thickness of the conductive member 3b for wiring connected to the solar battery cell 1 is 20 μm. In the case of this embodiment, the thermal stress generated in the connection portion between the solar battery cell 1 and the conductive member 3 can be relaxed by creep deformation even in the case of bonding to a thinner and larger area of ˜50 μm. High solder joint is realized.

  Below, the structure of the photovoltaic cell 1 is explained in full detail. The n-type single crystal silicon substrate 101 is a crystalline semiconductor substrate exhibiting an n-type conductivity type by being doped with an n-type dopant (for example, P (phosphorus)).

  The i-type amorphous silicon layers 102 and 105 are laminated so as to cover the light-receiving surface side 101a and the back surface side 101b of the n-type single crystal silicon substrate 101, respectively, and the light-receiving surface side 101a and the back surface side of the n-type silicon substrate 101 are formed. It serves as a surface passivation layer that suppresses carrier recombination on the substrate surface of 101b. By forming such i-type amorphous silicon layers 102 and 105, a passivation effect on the n-type single crystal silicon substrate 101 can be obtained, and an effect of improving the open-circuit voltage and the short-circuit current density can be obtained.

  The p-type amorphous silicon layer 103 is formed on the i-type amorphous silicon layer 102 and forms a pn junction with the n-type single crystal silicon substrate 101 via the i-type amorphous silicon layer 102.

  The n-type amorphous silicon layer 106 is formed on the i-type amorphous silicon layer 105 and contains an n-type dopant (for example, P) at a higher concentration than the n-type single crystal silicon substrate 101.

  The p-side electrode 104 is a take-out electrode for taking out the generated electric power to the outside, and is formed on the p-type amorphous silicon layer 103. Since the p-side electrode 104 is disposed on the light-receiving surface side 101a, it is a translucent electrode. The n-side electrode 107 is a take-out electrode for taking out the generated electric power to the outside, and is formed on the n-type amorphous silicon layer 106.

  Below, an example of the manufacturing method of such a photovoltaic cell 1 is demonstrated easily.

  First, an n-type single crystal silicon substrate 101 containing P as a n-type dopant atom at a predetermined concentration is prepared as a semiconductor substrate. Since the n-type single crystal silicon substrate 101 is manufactured by slicing an ingot formed by pulling up molten silicon and solidifying by cooling with a wire saw, damage on the surface remains on the surface. Therefore, the n-type single crystal silicon substrate 101 is first etched by dipping the n-type single crystal silicon substrate 101 in an acid or heated alkaline solution, for example, an aqueous sodium hydroxide solution, to remove the damaged layer. A damage region that occurs when the substrate 101 is cut out and exists near the surface of the n-type single crystal silicon substrate 101 is removed. Thereafter, in order to further reduce the light reflectivity, a texture etching step for producing a concavo-convex shape on the surface may be performed, and in that case, a solar cell with higher performance can be obtained.

  Next, an i-type amorphous silicon layer 102 and an i-type amorphous silicon layer 105 are formed on both surfaces of the n-type single crystal silicon substrate 101 by plasma CVD. Next, a p-type amorphous silicon layer 103 is deposited on the light-receiving surface side 101a of the n-type single crystal silicon substrate 101 by plasma CVD. Next, an n-type amorphous silicon layer 106 is deposited on the back surface 101b of the n-type single crystal silicon substrate 101 by plasma CVD.

  Next, the p-side electrode 104 is formed on the p-type amorphous silicon layer 103 on the light-receiving surface side 101a of the n-type single crystal silicon substrate 101 by sputtering or the like.

  Next, an n-side electrode 107 is formed on the n-type amorphous silicon layer 106 on the back surface side 101b of the n-type silicon substrate 101 by sputtering or the like.

  Thereafter, a grid electrode and a bus electrode for current collection are formed on the p-side electrode 104 and the n-side electrode 107. The grid electrode and the bus electrode are formed by, for example, printing and drying an electrode material paste by a screen printing method, and then baking it.

  By performing the steps as described above, the solar battery cell 1 according to the present embodiment shown in FIG. 2 can be manufactured. Note that the p-type and the n-type may be interchanged in each of the above parts.

  Here, the solar cell 1 can use not only a single crystal silicon substrate but also a desired crystal substrate such as single crystal germanium or polycrystalline silicon. Moreover, as a structure of the photovoltaic cell 1, another layer of amorphous silicon or the like is formed on a silicon-based substrate such as single crystal silicon or polycrystalline silicon by a CVD method or the like, and a collector electrode is formed thereon, Alternatively, an n-type region or a p-type region is formed by introducing impurities into a silicon substrate, and a current collecting electrode is formed thereon.

Next, the lead-free solder made of Sn ₅₇ Bi ₁ Ag used in the present embodiment will be described. As described above, in this embodiment, in order to prevent cracking of the solar cell, creep characteristics were evaluated based on the discovery that it is necessary to consider not only the melting point but also the creep characteristics. The creep test results comparing the creep characteristics of the lead-free solder containing Sn ₅₇ Bi ₁ Ag with those of other lead-free solders are shown below. The lead-free solder subjected to the creep test here is Sn ₅₇ Bi ₁ Ag, Sn ₃ Ag _0.5 Cu, Sn _2.5 Ag _0.4 Cu ₂₀ In, Sn _2.5 Ag _0.4 Cu ₁₅ In. For reference, Sn ₃₇ Pb containing lead was also subjected to the same test and the results were shown.

The creep test was performed using an indentation method in which an indenter is pressed into an evaluation target (Non-Patent Document 1). In this test, a Vickers indenter was pressed into a test piece of lead-free solder 2 having a thickness of 5 mm at a test temperature of 25 ° C. and 100 ° C. with a maximum indentation load of 300 mN to 1000 mN and a maximum indentation load holding time of 1200 s. Continuous data of indentation load F and indentation amount h were measured. The indentation load F is indicated by a curve a and the indentation amount h is indicated by a curve b. FIG. 4 is an example of continuous data measured when creep characteristics are evaluated by the indentation method. The creep speed ε = dh / dt and the stress σ = F / (3 × 26.3h ² (t)) are calculated using the data of the holding time of the maximum indentation load 700 s to 1200 s, and the steady creep law ε = Cσ ^m Further, the creep constant C and the creep index m were evaluated.

5 and 6 show the relationship between the creep rate and the stress of each lead-free solder at 25 ° C. and 100 ° C., respectively. Table 1 shows the creep characteristic values of these lead-free solders. The creep constant logC and creep index m shown in Table 1 are dimensionless due to the creep characteristics at 25 ° C. of commonly used lead-free solder Sn ₃ Ag _0.5 Cu.

As can be seen from FIGS. 5 and 6, Sn ₅₇ Bi ₁ Ag is a lead-free solder (Sn ₃ Ag _0.5 Cu, Sn _2.5 Ag _0.4 Cu ₂₀ In, Sn _2.5 Ag _0.4 Cu ₁₅ In) at a high temperature (100 ° C.). ), The creep rate (easiness of creep deformation) with respect to the same load stress is particularly large, and creep deformation is likely to occur even when compared with Sn ₃₇ Pb which is lead solder. Even at low temperatures, Sn ₅₇ Bi ₁ Ag is more likely to undergo creep deformation than other lead-free solders. Further, Sn ₅₇ Bi ₁ Ag has a small change in creep rate with respect to a temperature change and is almost constant.

As described in Table 1, the smaller the creep constant logC, the larger the creep rate at the same load stress, so that the creep deformation tends to occur, and the smaller the creep index m, the less likely the creep rate to decrease when the load stress decreases. Therefore, creep deformation is easy. As a result, Sn ₅₇ Bi ₁ Ag is more susceptible to creep deformation than other lead-free solders, and creep deformation is caused by the thermal stress generated at the joint between the base material and the counterpart material in the process of decreasing from the melting point to room temperature. It can be seen that the effect of relaxation is great.

The bottom row of Table 1 shows the product of the creep constant and the creep index m. The smaller the creep constant and the creep index, the easier the creep deformation. Therefore, the smaller the product of the logarithm of the creep constant and the creep index, the easier the creep deformation. Sn ₃ Ag _0.5 Cu of 25 ° C. creep characteristics by dimensionless 25 ° C. and the creep constant at 100 ° C. logarithm log C _{25 ° C.} and _{log C 100 ° C.} and creep index _{m 25 ° C.} and _{m 100 ° C.} of the product (log C _{25 ° C.} × m _{25 ° C.} ) × (log C _{100 ° C.} × m _{100 ° C.} ), Sn ₅₇ Bi ₁ Ag is 0.004, which is particularly small, and it is understood that creep deformation is likely to occur. The solder of the present invention is not limited to Sn ₅₇ Bi ₁ Ag as long as it is lead-free, but the product of the creep index m _{25 ° C.} and m _{100 ° C.} of lead solder Sn ₃₇ Pb (log C _{25 ° C.} × m _{25 ° C.} ) Since × (log C _{100 ° C.} × m _{100 ° C.} ) is 0.006, the solder used in the present invention is the product of the creep index m _{25 ° C.} and m _{100 ° C.} (log C _{25 ° C.} × m _{25 ° C.} ) × (log C _{100 (° C.} × m _{100 ° C.} ) is preferably 0.006 or less.

In the following, in order to investigate the relaxation effect of thermal stress due to creep deformation of Sn ₅₇ Bi ₁ Ag, lead-free solder 2 as a connecting material is formed on one surface of the solar cell 1 as a base material. The calculation result of the curvature which arises by the curvature deformation of the electrically-conductive member 3 at the time of carrying out the soldering joining of the electrically-conductive member 3 which is an other party material is shown.

FIG. 7 is a perspective view showing a test sample and a measuring machine for examining the influence of thermal stress relaxation due to creep deformation of lead-free solder. A silicon wafer is used as the solar cell 1 as a base material, and an element region and an electrode are omitted. Lead-free solder 2 _{Sn 3 Ag 0.5 Cu, Sn 57} Bi 1 Ag, Sn 2.5 Ag 0.4 Cu 20 In, Sn 2.5 Ag 0.4 Cu 15 In, the mating material is a conductive material 3 made of Cu alloy. The silicon wafer constituting the solar cell 1 is 160 μm thick, 10 mm wide and 45 mm long, the lead-free solder is 20 μm thick, 200 μm wide and 45 mm long, and the electrode is 20 μm thick, 200 μm wide and 45 mm long. It was. In addition, the width | variety of the Cu alloy which comprises the photovoltaic cell 1, the lead-free solder 2, and the electrically-conductive material 3 is an example, and is good also as another arbitrary width | variety. Here, the length of the conductive member when the lead-free solder 2 as the connection material is formed on one surface of the solar cell 1 as the base material and the solar cell 1 and the conductive member 3 as the mating member are soldered and joined together. The amount of warpage deformation in the vertical direction is measured by the three-dimensional laser displacement meter 4 to calculate the curvature.

Table 2 shows the effect of thermal stress relaxation by creep deformation of the lead-free solder 2. The magnitude of the curvature indicates a thermal stress relaxation effect, and a larger curvature indicates a smaller thermal stress relaxation effect. When non-dimensionalized by the curvature of commonly used Sn ₃ Ag _0.5 Cu, each curvature is 1 for Sn ₃ Ag _0.5 Cu, 0.45 for Sn ₅₇ Bi ₁ Ag, 0.62 for Sn _2.5 Ag _0.4 Cu ₂₀ In, Sn _2.5 Ag _0.4 Cu ₁₅ In is 0.66, and it can be seen that Sn ₅₇ Bi ₁ Ag has a particularly great stress relaxation effect.

As described above, Sn ₅₇ Bi ₁ Ag is particularly susceptible to creep deformation, and is particularly suitable for alleviating the thermal stress caused by the difference in linear expansion between the base material and the counterpart material at the connection portion of the base material and the counterpart material. I understand that there is.

By the way, as a solar cell, a heterostructure thin film type semiconductor in which an amorphous silicon layer is formed by a CVD method is known to have high power generation efficiency. However, the crystallinity of the film changes at a high temperature of 200 ° C. or higher, and the semiconductor characteristics are improved. Since deterioration occurs, conventional lead-free solder typified by Sn ₃ Ag _0.5 Cu cannot be used. However, the melting point of Sn ₅₇ Bi ₁ Ag is 146 ° C., which is much lower than 200 ° C. Therefore, in such a hetero-structure thin film type semiconductor, if this Sn ₅₇ Bi ₁ Ag is used to join the semiconductor cell and the counterpart material, such as Cu or Cu alloy, by soldering, the melting point in soldering is changed to room temperature. It is possible to obtain a solar cell that has a large stress relaxation effect due to the temperature change, is less prone to breakage due to thermal stress, and has little characteristic deterioration of the heterostructure thin film semiconductor.

Note that the thickness of the solar cell 1 such as silicon of the semiconductor cell as a base material has been gradually reduced in recent years, and as a result, the stress generated between the solar cell 1 and the conductive member 3 which is the counterpart material is also increased. ing. Therefore, Sn ₅₇ Bi ₁ Ag, such as Sn ₃₇ Pb, which is more susceptible to creep deformation than conventional lead solder and has a large stress relaxation effect, is particularly effective for stress relaxation in further thinning of semiconductor cells in the future. .

  In the above experimental example, the thickness of the solar cell 1 as a base material is 160 μm, the thickness of the lead-free solder 2 is 20 μm, and the thickness of the conductive member 3 as a counterpart material (Cu alloy) is 20 μm. For example, when the thickness of the solar cell 1 is selected from the range of 100 μm to 300 μm, the thickness of the lead-free solder 2 is selected from the range of 10 μm to 40 μm, and the thickness of the conductive member 3 is selected from the range of 20 μm to 400 μm. Great effect.

Embodiment 2. FIG.
FIG. 8 is a schematic cross-sectional view of solar battery cell 1 used in Embodiment 2 of the present invention. The present embodiment is different only in that solar cell 1 in which a pn junction is formed by impurity diffusion on the surface of a single crystal silicon substrate is used. That is, as in the solar cell of the first embodiment, the voltage extracting conductive member 3 made of Cu or Cu alloy is connected via the Sn—Bi—Ag-based lead-free solder 2 that is a connecting material. It is.

As this Sn—Bi—Ag-based lead-free solder 2, Sn ₅₇ Bi, which is a low melting point solder having a melting point of 146 ° C. and a creep property superior to that of lead solder, as in the solar cell of the first embodiment. ₁ Ag is used. This Sn ₅₇ Bi ₁ Ag not only has a low melting point, but also has a high creep property, and even when the temperature rises in the solder bonding process, the creep index does not change and maintains a high creep property. Even when the temperature rises and falls, the buffering property is high, and the solar battery cell 1 is stressed and does not break. Therefore, the thermal stress generated when the temperature of the lead-free solder changes from the melting point to room temperature is greatly relaxed, and cracking of the solar battery cell 1 is suppressed. In other words, since it has a low melting point, the temperature change that the solar battery cell 1 receives in the joining process is reduced, and since it has a stable and high creep property in all temperature ranges, it has a high creep property throughout the entire process. Thus, a thermal stress relaxation effect can be obtained.

  The solar cell 1 used in Embodiment 2 is as shown in a schematic cross-sectional view in FIG. 8, and as the silicon substrate 201, for example, a p-type single crystal silicon substrate can be used. For example, phosphorus is diffused into the surface layer of the silicon substrate 201 to form a second conductivity type layer 202 which is an impurity diffusion layer (n-type impurity diffusion layer) to form a pn junction. Further, on the back side of the silicon substrate 201, a first conductivity type layer 203 which is a diffusion layer in which an electrode material of the back side electrode 205, for example, aluminum, is diffused is formed on the surface layer on the back side of the silicon substrate 201. Note that the silicon substrate 201 is not limited to this, and a p-type polycrystalline silicon substrate, an n-type single crystal silicon substrate, an n-type polycrystalline silicon substrate, or the like may be used. The insulating layer 204 is provided as an antireflection film, and is formed of, for example, a silicon nitride film or a silicon oxide film. Reference numeral 206 denotes a surface side electrode. Note that a transparent conductive film may be formed over the entire surface of the second conductivity type layer 202 between the surface side 206 and the second conductivity type layer 202 in order to improve current collection.

The conductive member 3 is joined to the front surface side electrode and the back surface side electrode by lead-free solder made of Sn ₅₇ Bi ₁ Ag as in the first embodiment. The joining method is the same as the process described in the first embodiment.

  On the light-receiving surface side of the silicon substrate 201 of the solar cell 1 according to the second embodiment configured as described above, the height and shape are uniform and fine as in the solar cell 1 of the first embodiment. Pyramidal texture T is uniformly formed on the entire surface. The texture T having a uniform height and shape increases the probability that the light reflected by the texture T re-enters the adjacent texture T, and increases the light use efficiency of the solar battery cell 1. The increase in the light use efficiency can increase the number of carriers generated in the solar battery cell 1 and realize an improvement in photoelectric conversion efficiency.

  Therefore, according to the photovoltaic cell 1 concerning Embodiment 2, the solar cell which has high light utilization efficiency and a high open circuit voltage, and was excellent in photoelectric conversion efficiency is implement | achieved.

In the above embodiment, Sn ₅₇ Bi ₁ Ag is used. However, the composition of the lead-free solder such as Sn—Bi—Ag solder mainly composed of Sn ₅₇ Bi ₁ Ag can be changed as appropriate. is there. Here, Bi contributes to lowering the melting point, and Ag is considered to contribute to increase creep properties, and it is effective to replace it with other components as appropriate. It seems desirable to use a material having a low melting point, a high creep property, and a material whose creep property does not change significantly in the operating temperature range.

DESCRIPTION OF SYMBOLS 1 Solar cell 1a Light-receiving surface side 1b Back surface side 2 Lead-free solder 3, 3a, 3b Conductive member 4 Three-dimensional laser displacement meter 101a Light-receiving surface side 101b Back surface side 102 i-type amorphous silicon layer 103 p-type amorphous silicon layer 104 p-side Electrode 105 i-type amorphous silicon layer 106 n-type amorphous silicon layer 107 n-side electrode T texture 201 silicon substrate 202 second conductivity type layer 203 first conductivity type layer 204 insulating layer 205 back side electrode

Claims (11)

A thin-film solar cell using a semiconductor thin film;
On the surface of the solar cell,
A solar cell comprising: a conductive member joined using Sn-Bi-Ag lead-free solder. 2. The solar cell according to claim 1, wherein the Sn—Bi—Ag-based lead-free solder is a solder mainly composed of Sn ₅₇ Bi ₁ Ag. The semiconductor thin film is formed by a CVD method to constitute a heterostructure solar cell,
The solar battery according to claim 1, wherein at least one of the electrodes of the solar battery cell is joined to the conductive member.   The solar cell according to claim 3, wherein the conductive member is a wiring material made of Cu or a Cu alloy.   The solar cell according to claim 4, wherein the lead-free solder has a thickness of 10 μm to 40 μm.   The solar cell according to claim 5, wherein the solar cell has a thickness of 100 μm to 300 μm.   The solar cell according to claim 6, wherein the conductive member has a thickness of 20 μm to 400 μm.   The solar cell according to claim 6, wherein the conductive member has a thickness of 200 μm to 400 μm.   The solar cell according to claim 6, wherein the conductive member has a thickness of 20 μm to 50 μm. Forming a semiconductor thin film on a substrate and forming a solar cell;
A method of manufacturing a solar cell comprising a step of connecting a conductive member for external connection to the semiconductor thin film,
The connecting step includes
Forming a Sn-Bi-Ag lead-free solder on the solar cell;
Arranging and heating the conductive member on the lead-free solder,
The manufacturing method of the solar cell characterized by including the process of joining the said photovoltaic cell and the said electrically-conductive member. The connecting step includes
Forming a solder composed mainly of Sn ₅₇ Bi ₁ Ag on the solar cell;
Arranging and heating a conductive member made of Cu and Cu alloy on the lead-free solder,
The method for manufacturing a solar cell according to claim 10, comprising a step of joining the solar cell and the conductive member.

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