Solar cell manufacturing method

In the present invention, a p-type silicon substrate is produced, a solution containing aluminum is misted, and the misted solution is sprayed onto the back surface of the p-type silicon substrate under non-vacuum to form a back surface passivation film made of the aluminum oxide film on the back surface of the p-type silicon substrate. Thereafter, a light irradiation processing in which an interface between the p-type silicon substrate and the back surface passivation film is irradiated with ultraviolet light is performed.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a solar cell, and more particularly, to a method for forming a passivation film on a silicon substrate.

BACKGROUND ART

In the field of a crystalline silicon solar cell, silicon substrates have been reduced in thickness in order to reduce the amount of silicon usage and to improve the conversion efficiency of silicon substrates. Unfortunately, thinner silicon substrates have remarkably lower conversion efficiency. The reason for this is, for example, that a large number of defects in the front surface of the silicon substrate having conductivity mainly cause the reduction in the lifetime of minority carriers (for example, electrons in a p-type substrate) generated by light irradiation. Thus, reducing the loss of minority carriers eventually improves the conversion efficiency of solar cells.

To regulate the reduction in the lifetime of carries, a passivation film is generally formed on the surface of a silicon substrate. An aluminum oxide film, one of a plurality of kinds of passivation films, has received attention because of its higher passivation effect (the function to regulate the reduction in lifetime) on the p-type silicon substrate.

The aluminum oxide films include negative fixed charges and are known to produce the passivation effect resulting from the field effect caused by the fixed charges. That is, when the passivation film made of the aluminum oxide film including negative fixed charges is formed on the front surface of the p-type silicon substrate the diffusion of electrons being minority carriers into the surface of the substrate can be regulated, resulting in prevention of the loss of carriers.

For example, Patent Document 1 discloses, as a method for manufacturing a solar cell, a method in which a misting method is employed as a method for forming the aluminum oxide film being the passivation film on the p-type silicon substrate. The manufacturing method achieves the effect of forming a passivation film, without inflicting damage to a silicon substrate, with a high degree of production efficiency at lower manufacturing cost by forming the passivation film by the misting method.

PRIOR ART DOCUMENTS

Patent Document

Patent Document 1: International Publication No. WO 2015/004767

SUMMARY OF INVENTION

Problems to be Solved by the Invention

However, when the passivation film is formed by the mist method, there has been a problem that film quality can be lower than that of the case in which the passivation film is formed by an ALD (Atomic Layer Deposition) method, a plasma CVD (Chemical Vapor Deposition) method or the like.

In the present invention, it is an object to solve a problem as described above and to provide a method for manufacturing a solar cell that allows for, without inflicting damage to a substrate, the formation of a passivation film of high film quality with a high degree of production efficiency at lower manufacturing cost.

Means for Solving the Problems

A method for manufacturing a solar cell in the present invention includes the steps of (a) producing a silicon substrate (4) having one main surface and the other main surface, (b) misting a solution (14) containing a metal clement, (c) spraying the misted solution onto one main surface of the silicon substrate under non-vacuum to form a passivation film (5) made of a metal oxide film on one main surface of the silicon substrate, (d) producing a solar cell structure using the silicon substrate having the passivation film formed thereon, and (e) performing a light irradiation processing in which an interface between the passivation film and the silicon substrate is irradiated with a given light (21).

Effects of the Invention

According to the method for manufacturing a solar cell in the present invention, a passivation film made of a metal oxide film is formed on one main surface of the silicon substrate by performing the steps (b) and (c), whereby a passivation film can be formed without inflicting damage to a silicon substrate, with a high degree of production efficiency at lower manufacturing cost.

Moreover, the present invention according to claim1allows for attaining a passivation film of high quality which improves a lifetime by the light irradiation processing according to the step (e).

These and other objects, features, aspects and advantages of the present invention will become more apparent from the following derailed description of the present invention when taken in conjunction with the accompanying drawings.

DESCRIPTION OF EMBODIMENTS

FIG. 1is a sectional view showing a solar cell structure manufactured by a method for manufacturing a solar cell according to the present embodiment (the embodiment 1, the embodiment 2).

As shown inFIG. 1, a silicon layer3(hereinafter referred to as “n-type silicon layer3”) of n-type conductivity is formed on the surface (the other main surface) of a silicon substrate4(hereinafter referred to as “p-type silicon substrate4”) of p-type conductivity. In addition, inFIG. 1, the solar cell is shown in a manner that the front surface of the p-type silicon substrate4is an upper surface and the back surface is a lower surface.

A front surface passivation film2being transparent is formed on the front surface of the n-type silicon layer3. As the front surface passivation film2, for example, a silicon oxide film, a silicon nitride film, an aluminum oxide film, or a laminated film including these films is conceivable. Then, a front surface electrode1penetrating through part of the front surface passivation film2is selectively formed on the front surface of the n-type silicon layer3, whereby the front surface electrode1is electrically connected to the n-type silicon layer3.

Moreover, a back surface passivation film5is formed on the back surface (one main surface) of the p-type silicon substrate4. As the back surface passivation film5, an aluminum oxide film, or a laminated film of an aluminum oxide film and a silicon. nitride film is employed. Further, a back surface electrode6penetrates part of the back surface passivation film5and is directly formed on the back surface of the p-type silicon substrate4and formed over the back surface of the back surface passivation film5. Accordingly, the back surface electrode6is electrically connected to the p-type silicon substrate4.

In the solar cell structure shown inFIG. 1, light incident from the front-surface-passivation-film-2side reaches a p-n junction between the n-type silicon layer3and the p-type silicon substrate4to generate carriers, so that electricity is produced, and then, the produced electricity is extracted through the electrodes1and6.

As described above, the passivation films2and5are formed in order to regulate the reduction in the lifetime of carriers. That is, a large number of defects (such as lattice defects) occur in the front surfaces of the n-type silicon layer3or the back surface of the p-type silicon substrate4, and thus, minority carriers generated by light irradiation through the defects are recombined. Therefore, the front surface passivation film2and the back surface passivation film5arc formed on the front surface of the n-type silicon layer3and the back surface of the p-type silicon substrate4, respectively, to regulate the recombination of carriers, whereby the lifetime of carriers can be improved.

The present invention pertains to an improvement of film quality of the back surface passivation film5formed on the back surface of the p-type silicon substrate4. Hereinafter, the present invention will be described in detail based on the drawings illustrating the embodiments thereof.

FIG. 2is an explanatory diagram showing a schematic configuration of a film forming apparatus for implementing a film forming method of a back surface passivation film5in the present embodiment (embodiment 1, embodiment 2).

As shown inFIG. 2, the film forming apparatus used in the film formation method of the present embodiment includes a reaction container11, a heater13for heating the reaction container11, a solution container15for containing a (material) solution14, and a mist forming apparatus16for misting the solution14in the solution container15.

In such a configuration, a solution14that has been misted by a mist forming apparatus16is sprayed onto the back surface of the p-type silicon substrate4in the reaction container11through a channel L1, whereby the back surface passivation film5made of the aluminum oxide film can be formed on the back surface of the p-type silicon substrate4. In this case, the p-type silicon substrate4is placed on the heater13in the reaction container11in a manner that the back surface is an upper surface and the front surface is a lower surface.

That is, while the p-type silicon substrate4is placed on the heater13, mist (the liquid solution14having a small particle diameter) is supplied into the reaction container11in the atmospheric pressure, and then, the back surface passivation film5is formed on the back surface of the p-type silicon substrate4as a result of a given reaction.

The heater13being, for example, a heating apparatus can heat the p-type silicon substrate4placed on the heater13. In the formation of film the heater13is heated through an external controller not shown until the heater reaches a temperature required to form the back surface passivation film5composed of the aluminum oxide film.

The solution container15is filled with the solution14serving as a raw material solution for forming the back surface passivation film5. The solution14contains aluminum (Al) elements as the metal source.

For example, an ultrasonic atomizing apparatus can be used as the mist forming apparatus16. The mist forming apparatus16being the ultrasonic atomizing apparatus applies ultrasonic waves to the solution14in the solution container15, thereby misting the solution14in the solution container15. The misted solution14passes through a channel L1to be supplied to the back surface (upper surface) of the p-type silicon substrate4in the reaction container11.

The misted solution14is supplied into the reaction container11, and then, the solution14undergoes a reaction on the back surface of the p-type silicon substrate4subjected to heating under the atmospheric pressure, whereby the back surface passivation film5is formed on the back surface of the p-type silicon substrate4. The solution14that is left unreacted in the reaction container11is discharged out of the reaction container11all the time (in a continuous manner) through a channel L2.

Next, a method for manufacturing a solar cell (particularly, a method for forming the back surface passivation film5(the aluminum oxide film)) of the embodiment 1 will be described.

Firstly, a given impurity is introduced to a silicon substrate using a crystalline silicon as a constituent material to produce a p-type silicon substrate4of p-type conductivity. Then, the p-type silicon substrate4is placed on the heater13in the reaction container11. In this time, the p-type silicon substrate4is placed on the heater13in a manner that the back surface is an upper surface and the front surface is a lower surface, and the inside of the reaction container11is set to the atmospheric pressure. In this way, the p-type silicon substrate4having a back surface and a front surface (one main surface and the other main surface) is produced.

The heater13heats the p-type silicon substrate4placed on the heater13until reaching a film formation temperature of the back surface passivation film5made of the aluminum oxide film, and maintains the p-type silicon substrate4at the film formation temperature.

Meanwhile, in the solution container15, the solution14is misted by the mist forming apparatus16. The misted solution14(the solution14having a small particle diameter) passes through the channel L1, undergoes the flow alignment, and is supplied into the reaction container11. The solution14contains aluminum as the metal source. In this way, the solution14(the raw material solution) containing aluminum being a metal element is misted.

The misted solution14having undergone the flow alignment is supplied to the back surface of the p-type silicon substrate4under application of heat under the atmospheric pressure. The misted solution14is sprayed onto the back surface of the p-type silicon substrate4under application of heat, and then, the back surface passivation film5made of the aluminum oxide film is formed on the back surface of the p-type silicon substrate4. As described above, the back surface passivation film5made of the aluminum oxide being a metal oxide film is formed on the back surface of the p-type silicon substrate4by spraying the misted solution14to the back surface of the p-type silicon substrate4in the atmospheric pressure (under non-vacuum).

After that, the solar cell structure shown inFIG. 1is produced using the p-type silicon substrate4including the back surface passivation film5(the aluminum oxide film) formed thereon. In general, the back surface passivation film5is formed after the formation of the front surface passivation film2and the n-type silicon layer3, and thereafter, the front surface electrode1and the back surface electrode6are formed. In addition, the order of film formation of the front surface passivation film2and the back surface passivation film5may be reversed.

FIG. 3is a sectional view showing a situation of a light irradiation processing by ultraviolet light21of the embodiment 1. As shown inFIG. 3, the light irradiation processing in which a solar cell is irradiated for 30 seconds with ultraviolet light21having a wavelength of 365 nm (given light) from above of the front surface of the solar cell structure (above the front surface of the p-type silicon substrate4) in a manner that the front surface of the p-type silicon substrate4is an upper surface, is performed. As described above the ultraviolet light21from above of the front surface of the solar cell structure is passed through a front surface passivation film2and an p-type silicon layer3, and a light irradiation processing in which an interface between the p-type silicon substrate4and the passivation film5is irradiated with the ultraviolet light21, is performed, whereby a solar cell of the embodiment 1 is completed.

As described above, the method for forming the back surface passivation film5(the aluminum oxide film) in the method for manufacturing a solar cell of the embodiment 1 employs the misting method (the method for forming a film by spraying the liquid solution14in the atmospheric pressure) to form the back surface passivation film5on the back surface of the p-type silicon substrate4.

The back surface passivation film5made of an aluminum oxide film is not formed by supplying the vaporized raw material to the p-type silicon substrate4in, for example, CVD or ALD. Alternatively, in the embodiment 1, the back surface passivation film5is formed by spraying the misted liquid solution14onto the p-type silicon substrate4. As described above, the solution14contains aluminum elements. Thus, the back surface passivation film5made of an aluminum oxide film can be formed on the back surface of the p-type silicon substrate4, using a material that is safe and easy to handle instead of using a material such as TMA (Tri-Methyl-Aluminum) that is expensive and difficult to handle.

Moreover, the embodiment 1, which is the film forming processing in the atmospheric pressure, eliminates the need for the vacuum processing and the like, thus enabling the reduction of manufacturing cost. In addition to this, in the embodiment 1, the misted solution14is sprayed onto the p-type silicon substrate4to perform the film forming processing. Therefore, in the film forming processing, the p-type silicon substrate4is not damaged by irradiation with plasma or the like.

The back surface passivation film5is formed by the misting method at a speed of 10 to 15 nm/min, which is five or more times as fast as the speed at which the aluminum oxide film is formed by, for example, ALD method. Accordingly, the employment of the method for forming a back surface passivation film5of the embodiment 1 can also improve the production efficiency.

In addition to this, in the embodiment 1, a light irradiation processing in which an interface between the back surface passivation film5and the p-type silicon substrate4is irradiated with ultraviolet light21is performed.

FIG. 4is a graph showing the effect of the above-mentioned light irradiation processing in the embodiment 1.FIG. 4shows the results of the following lifetime measurement. That is, a lifetime value after the formation of a film (before the light irradiation (processing)) of the back surface passivation film5is measured, and thereafter, the light irradiation processing in which a solar cell is irradiated for 30 seconds with ultraviolet light21having a wavelength of 365 nm, is performed and the lifetime is measured again.FIG. 4indicates an effective lifetime value which is determined by taking the lifetime value after the formation of a film (before the light irradiation) as a normalized value “1”.

As shown inFIG. 4, it is found that the effective lifetime value is raised to about “2.3” after the light irradiation. As described above, it is found that the light irradiation processing extensively improves a passivation effect (a function of regulating a reduction in lifetime) of the back surface passivation film5produced by the method for manufacturing a solar cell in the embodiment 1.

As described above, in the method for manufacturing a solar cell of the embodiment 1, the passivation film5made of the aluminum oxide film is formed by spraying the misted (material) solution14of aluminum onto the back surface of the p-type silicon substrate4, whereby the back surface passivation film5can be formed without inflicting damage to a substrate, with a high degree of production efficiency at lower manufacturing cost.

Moreover, a passivation film5of high film quality extensively improving an effective lifetime value can be obtained at a completion stage of a solar cell by the light irradiation processing by ultraviolet light21which is performed after the formation of the back surface passivation film5.

Further, it is possible to achieve an improvement of film quality of the passivation film5in a relatively short time (30 seconds in an example ofFIG. 4) of light irradiation by using the ultraviolet light21for the light irradiation processing.

In addition, while the example shown inFIG. 4indicates an example in which the p-type silicon substrate is irradiated for 30 seconds with the ultraviolet light21, a light irradiation time is preferably set to 1 second or more for surely achieving an improvement of the effective lifetime value of the back surface passivation film5.

A baking processing at the time of forming a front surface electrode1and a back surface electrode6is an essential processing in completing the solar cell. For example, when the front surface electrode1and the back surface electrode6are formed, an electrode material containing a metal as a principal component is applied, and then a baking processing is performed.

In the embodiment 1, there has been described a method of performing a light irradiation processing after the forming the back surface passivation film5without considering the presence or absence of the baking processing at the time of forming an electrode. That is, the embodiment 1 is a method for manufacturing a solar cell in which generally, the following steps (1) and (2) are perforated.

(1) A p-type silicon substrate4is produced, a solution14containing aluminum is misted, and the misted solution14is sprayed onto the back surface of the p-type silicon substrate4under non-vacuum to form a passivation film5made of the aluminum oxide film on the back surface of the p-type silicon substrate4.

(2) A light irradiation processing in which an interface between the back surface passivation film5and the p-type silicon substrate4is irradiated with ultraviolet light (21) is performed.

However, the above-mentioned baking processing is a substantially essential processing in forming the front surface electrode1and the back surface electrode6constituting a solar cell. That is, a process step of obtaining the solar cell structure shown inFIG. 1includes the step of forming the front surface electrode1(the other electrode) and the back surface electrode6(one electrode) on a front surface side and a back surface side of the p-type silicon substrate4, respectively, and this step includes the baking processing for baking the p-type silicon substrate4at a given baking temperature.

The embodiment 2 is a method for manufacturing a solar cell in which the light irradiation processing is performed in consideration of an influence of the baking processing at the time of forming the front surface electrode1and the back surface electrode6, and the embodiment 2 is generally performed by undergoing the following steps (1), (3) and (2)′.

(1) As with the embodiment 1, a p-type silicon substrate4is produced, a solution14containing aluminum is misted, and the misted solution14is sprayed onto the back surface of the p-type silicon substrate4under non-vacuum to form a passivation film5made of the aluminum oxide film on the back surface of the p-type silicon substrate4.

(3) The front surface electrode1and the back surface electrode6are formed to prepare a solar cell structure shown inFIG. 1. In doing so, the baking processing is performed at a baking temperature of 500° C. or higher in forming the front surface electrode1and the back surface electrode6.

(2)′ After the baking processing of the step (3), a light irradiation processing in which an interface between the p-type silicon substrate4and the back surface passivation film5is irradiated with ultraviolet light21, is performed.

FIG. 5is a graph showing the effect of a method for manufacturing a solar cell of the embodiment 2.FIG. 5shows the results of the following lifetime measurement. That is, a lifetime value after the formation of a film (before the light irradiation) of the back surface passivation film5is measured, and then, the baking processing being a heat treatment in the atmospheric pressure in which a baking temperature is 800° C. and a baking time 10 seconds; is performed and the lifetime value after the baking (processing) is measured. Thereafter, as with the embodiment 1, the light irradiation processing in which a solar cell is irradiated for 30 seconds with ultraviolet light21having a wavelength of 365 nm, is performed, and the lifetime value is measured.FIG. 5indicates an effective lifetime value which is determined by taking the lifetime value after the formation of a film (before the light irradiation) as a normalized value “1”.

As shown inFIG. 5, it is found that the effective lifetime value is raised to about “2.0” after the light irradiation. That is, the effective lifetime value is lowered from “1.0” to about “0.5” immediately after the baking processing, but by subsequent light irradiation processing, the effective lifetime value is increased to “2.0” extensively exceeding “1.0” exhibited before the baking processing.

As described above, in the method for manufacturing a solar cell of the embodiment 2, the effective lifetime value to indicate the film quality of the back surface passivation film5formed in the step (1) is lowered temporarily by performing the baking processing (step (3)) for forming the front surface electrode1and the back surface electrode6which is substantially essential in the manufacturing method of a solar cell, but by thereafter performing the light irradiation processing of the step (2)′, the film quality of the back surface passivation film5can be extensively improved more than the quality before performing the step (2)′.

In doing so, it is also possible to achieve an improvement of film quality of the back surface passivation film5even in the situation that baking processing is performed at a baking temperature of 800° C. being a baking temperature of 500° C. or higher.

In addition, an example of using ultraviolet light21as light (given light) to be used for the light irradiation processing is described in the embodiment 1 and the embodiment 2 described above; however, an another kind of light may be used. For example, it is allowed to use light whose photon energy is 1.1 eV or more (a wavelength is 1100 nm or less), that is, light which a crystalline silicon in the p-type silicon substrate4can absorb for implementing light irradiation to an interface between the p-type silicon substrate4and the back surface passivation film5in place of the ultraviolet light21.

It is also possible to achieve an improvement of film quality of the back surface passivation film5when as described above, the light irradiation processing using light which the crystalline silicon can absorb and whose photon energy is 1.1 eV or more (a wavelength is 1100 nm or less), is employed as the light irradiation processing in the method for manufacturing a solar cell of the embodiment 1 or the embodiment 2.

Further, it is possible to achieve an improvement of film quality of the back surface passivation film5at relatively low cost by using solar light or artificial solar light having AM of 1.5 as light to be used for the light irradiation processing in the method for manufacturing a solar cell of the embodiment 1 or the embodiment 2.

In addition, when an execution order of the baking processing (step (3)) and the light irradiation processing (step (2)′) is reversed in the method for manufacturing a solar cell of the embodiment 2, it cannot be expected to achieve an extensive improvement of film quality of the back surface passivation film5in contrast to the embodiment 2, but it is naturally possible to achieve the effect of improving film quality of the back surface passivation film5compared with a conventional manufacturing method in which the light irradiation processing is not performed.

While the present invention has been shown and described in detail the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations not exemplified can be devised without departing from the scope of the invention.