TWI868936B - Tandem solar cell and method of manufacturing the same - Google Patents

Abstract

A tandem solar cell and a method of manufacturing the same are provided. The tandem solar cell includes a bottom solar cell, a silicon oxide (SiO _x, x＜2) thin film disposed on the bottom solar cell, a transparent conductive thin film disposed on the silicon oxide thin film, and a top solar cell disposed on the transparent conductive thin film and series connected to the bottom solar cell. The silicon oxide thin film has a refractive index of 2.0 to 3.5 for visible light with wavelength of 700 nm to 750 nm, and the transparent conductive thin film has a refractive index of 1.7 to 2.1 for visible light with wavelength of 700 nm to 750 nm. A better optical matching can be achieved by using the silicon oxide thin film and the transparent conductive thin film, and a conversion efficiency of the tandem solar cell can be further increased.

Description

Stacked solar cell and manufacturing method thereof

The present invention relates to a stacked solar cell and a manufacturing method thereof, and in particular to a stacked solar cell with an intermediate layer and a manufacturing method thereof.

For decades, silicon-based solar cells have been the mainstream of the solar market (currently accounting for about 95% of the market), due to their advantages such as high-efficiency assembly, abundant materials, non-toxic elements and long service life. In recent years, the conversion efficiency of silicon-based solar cells has been continuously improved due to the emergence of Passivated Emitter Rear Cell (PERC) and Tunnel Oxide Passivated Contact (TOPCon) components.

Despite this, the conversion efficiency of a single solar cell cannot meet the application requirements. Therefore, stacking two or more solar cells to make a stacked solar cell has the opportunity to further improve the conversion efficiency. The stacked solar cell can be a four-terminal (4T) stacked structure or a two-terminal (2T) stacked structure. The 4T stacked solar cell is an independent connection between the upper solar cell and the lower solar cell, while the 2T stacked solar cell is a series connection between the upper solar cell and the lower solar cell. 4T stacked solar cells do not need to consider the current matching between the upper solar cell and the lower solar cell, so they are easier to implement, but 2T stacked solar cells only require one transparent electrode, so the cost is lower.

However, the 2T stacked solar cell must use an interlayer to connect the upper solar cell and the lower solar cell to ensure the recombination of electrons and holes inside the cell, so the interlayer must have good carrier transport capabilities. Since the upper solar cell and the lower solar cell must maintain good ohmic contact, but there is usually a huge difference in optical refractive index, which will cause photocurrent loss in the lower solar cell, the interlayer must have a specific refractive index. Therefore, the optical matching of the interlayer is one of the key factors in the conversion efficiency of the 2T stacked solar cell.

In view of this, there is an urgent need to provide a stacked solar cell that uses a low-oxide silicon film and a transparent conductive film as an intermediate layer to achieve good light matching, thereby improving the conversion efficiency of the solar cell.

One aspect of the present invention is to provide a laminated solar cell comprising a low-oxide silicon film with a specific refractive index and a transparent conductive film to improve the conversion efficiency of the solar cell.

Another aspect of the present invention is to provide a method for manufacturing a stacked solar cell, which is used to manufacture the stacked solar cell of the above aspect.

According to one aspect of the present invention, a stacked solar cell is provided, which includes a lower solar cell, a low-oxide silicon film disposed on the lower solar cell, a transparent conductive film disposed on the low-oxide silicon film, and an upper solar cell disposed on the transparent conductive film and connected in series with the lower solar cell. The low-oxide silicon film has a refractive index of 2.0 to 3.5 for visible light with a wavelength of 700 nm to 750 nm, and the transparent conductive film has a refractive index of 1.7 to 2.1 for visible light with a wavelength of 700 nm to 750 nm. The upper solar cell faces an incident light source.

According to one embodiment of the present invention, the lower solar cell is a silicon-based solar cell, and the upper solar cell is a calcium-titanium solar cell.

According to an embodiment of the present invention, the thickness of the silicon suboxide film is 10 nm to 50 nm, and the sheet resistance of the silicon suboxide film is 1.0×10 ^-3 Ω-cm to 1.0×10 ^-2 Ω-cm.

According to one embodiment of the present invention, the optical band gap of the transparent conductive film is 3.5 to 4.9 eV.

According to an embodiment of the present invention, the thickness of the transparent conductive film is 15 nm to 45 nm, and the sheet resistance of the transparent conductive film is 1.0×10 ^-4 Ω-cm to 1.0×10 ^-3 Ω-cm.

According to one embodiment of the present invention, the transparent conductive film comprises gallium oxide, indium oxide, indium tin oxide, indium gallium oxide or a combination thereof.

According to an embodiment of the present invention, for the visible light with a wavelength of 700 nm to 750 nm, the refractive index of the lower solar cell is greater than the refractive index of the silicon oxide film, and the refractive index of the transparent conductive film is greater than the refractive index of the upper solar cell.

According to another aspect of the present invention, a method for manufacturing a stacked solar cell is provided, which is used to manufacture the stacked solar cell of the above aspect, wherein the low oxide silicon film of the stacked solar cell is formed by plasma assisted chemical vapor deposition, and the transparent conductive film of the stacked solar cell is formed by plasma assisted atomic layer deposition.

According to an embodiment of the present invention, the plasma-assisted chemical vapor deposition method is performed using a mixed gas of silane and nitrous oxide or a mixed gas of silane and carbon dioxide.

According to an embodiment of the present invention, the plasma-assisted atomic layer deposition method is performed using at least one of trimethyl gallium, trimethyl indium and tetrakis(dimethylamino)tin.

The stacked solar cell and the manufacturing method thereof of the present invention are applied to utilize a low-oxide silicon film and a transparent conductive film with a specific refractive index as the middle layer between the upper solar cell and the lower solar cell to achieve better light matching, thereby improving the conversion efficiency of the stacked solar cell.

The following disclosure provides many different embodiments or examples to implement different features of the invention. The specific examples of components and configurations described below are intended to simplify the disclosure. These are of course only examples and are not intended to be limiting. For example, a description of a first feature formed on or above a second feature includes embodiments in which the first feature and the second feature are in direct contact, and also includes embodiments in which other features are formed between the first feature and the second feature, so that the first feature and the second feature are not in direct contact. In addition, the disclosure repeats component symbols and/or letters in various specific examples. The purpose of this repetition is to simplify and clarify the description and does not represent a relationship between the various discussed embodiments and/or configurations.

Furthermore, spatially relative terms, such as "beneath," "below," "lower," "above," "upper," etc., are used to facilitate description of the relationship between a part or feature shown in a drawing and other parts or features. Spatially relative terms include different orientations of an element when in use or operation in addition to the orientation depicted in the drawing. A device may be oriented in other ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used in the present disclosure may be interpreted in this manner.

As used in the present invention, "around", "about", "approximately" or "substantially" generally means within 20%, within 10%, or within 5% of the stated value or range.

As described above, the present invention provides a stacked solar cell and a manufacturing method thereof, which utilizes a low-oxide silicon film and a transparent conductive film with a specific refractive index as the middle layer between the upper solar cell and the lower solar cell to achieve better light matching and good carrier transmission capability, thereby effectively improving the conversion efficiency of the stacked solar cell.

Please refer to FIG. 1, which is a schematic diagram of a stacked solar cell 100 according to some embodiments of the present invention. The schematic diagram of the stacked solar cell 100 includes a lower solar cell 110, a middle layer 120, and an upper solar cell 130. In some embodiments, the stacked solar cell 100 is a two-terminal (2T) stacked solar cell, so the stacked solar cell 100 further includes an electrode 101 and a transparent electrode 103 to conduct the current after the light energy is converted.

In some embodiments, the lower solar cell 110 is a silicon-based solar cell. In some specific examples, the lower solar cell 110 may be a Passivated Emitter Rear Cell (PERC) solar cell, a Tunnel Oxide Passivated Contact (TOPCon) solar cell, or a Heterojunction with Intrinsic Thin Layer (HJT) solar cell. In some embodiments, the lower solar cell 110 has a refractive index of about 3.6 to about 4.0 for visible light with a wavelength of 700 nm to 750 nm.

In some embodiments, the upper solar cell 130 is a perovskite solar cell. In some embodiments, the upper solar cell 130 has a refractive index of about 1.5 to about 2.0 for visible light with a wavelength of 700 nm to 750 nm. The upper solar cell 130 and the lower solar cell 110 are connected in series.

The middle layer 120 includes a low-oxide silicon film 122 and a transparent conductive film 124. In some embodiments, the low-oxide silicon film 122 has a refractive index of about 2.0 to about 3.5 for visible light with a wavelength of 700 nm to 750 nm. In some embodiments, the refractive index of the low-oxide silicon film 122 is less than the refractive index of the lower solar cell 110 for visible light with a wavelength of 700 nm to 750 nm, so that the lower solar cell 110 and the upper solar cell 130 have better optical matching.

The low oxide silicon film 122 is composed of p-type doped or n-type doped low oxide silicon (SiO _x , where x is less than 2). In some embodiments, the thickness of the low oxide silicon film 122 is about 10 nm to about 50 nm, and the sheet resistance of the low oxide silicon film 122 is about 1.0×10 ^-3 Ω-cm to about 1.0×10 ^-2 Ω-cm. The low oxide silicon film 122 having the thickness and/or sheet resistance within the aforementioned range may have better carrier transport capability.

In some embodiments, the low silicon oxide film 122 may be formed by plasma enhanced chemical vapor deposition (PECVD). In the aforementioned embodiment, the plasma frequency is about 40 MHz to about 60 MHz, for example, 40.68 MHz, and the plasma power is about 60 watts to about 300 watts. The plasma frequency and the plasma power are controlled within the aforementioned ranges to ensure that the deposited film has better quality.

In some embodiments, the low silicon oxide film 122 can be formed using a mixed gas of silane and nitrous oxide or a mixed gas of silane and carbon dioxide. In some embodiments, the mixed gas may optionally contain a small amount of hydrogen, for example, the content ratio of silane/hydrogen is 1/2 to 1/4. If the hydrogen content is not within the aforementioned range, the deposition rate of the low silicon oxide film 122 may be slowed down. In the embodiment where the low silicon oxide film 122 is formed by a mixed gas of silane and carbon dioxide, based on the volume of carbon dioxide being 100%, the volume of silane is about 15% to about 40%. The low silicon oxide film 122 formed with the aforementioned mixed gas volume ratio can produce a low silicon oxide layer having a specific refractive index. If PECVD is performed using only silane, suboxide silicon cannot be formed and the refractive index of the resulting film does not meet the requirements.

After the plasma-assisted chemical vapor deposition, a high temperature annealing is used to transform the amorphous silicon suboxide layer into a crystallized silicon suboxide film 122. In some embodiments, the grain size of the silicon suboxide film 122 is about 2 nm to about 30 nm. The silicon suboxide film 122 having a grain size in the aforementioned range may have fewer lattice defects and a suitable refractive index. In some embodiments, the high temperature annealing is performed at a temperature of about 700° C. to about 950° C. for about 1 hour to about 3 hours to repair the silicon suboxide film 122 and enhance the passivation characteristics.

In some embodiments, the transparent conductive film 124 has a refractive index of about 1.7 to about 2.1 for visible light with a wavelength of 700 nm to 750 nm. In some embodiments, the refractive index of the transparent conductive film 124 is greater than the refractive index of the upper solar cell 130 for visible light with a wavelength of 700 nm to 750 nm, so that the lower solar cell 110 and the upper solar cell 130 have better optical matching. In some embodiments, the refractive index of the low silicon oxide film 122 is greater than the refractive index of the transparent conductive film 124 for visible light with a wavelength of 700 nm to 750 nm, so that the middle layer 120 has better optical matching. By combining the refractive indices between the above layers, the conversion efficiency of the stacked solar cell 100 can be further improved.

In some embodiments, the transparent conductive film 124 has a thickness of about 15 nm to about 45 nm, and a sheet resistance of about 1.0×10 ⁻⁴ Ω-cm to about 1.0×10 ⁻³ Ω-cm. The transparent conductive film 124 having the thickness and/or sheet resistance within the aforementioned range may have better carrier transport capability.

In some embodiments, the transparent conductive film 124 may be formed by plasma enhanced atomic layer deposition (PEALD). In the aforementioned embodiments, the plasma frequency is about 40 MHz to about 60 MHz, for example, 40.68 MHz, and the plasma power is about 100 watts to about 300 watts to ensure that the deposited film has good quality. Since the low-oxide silicon film 122 formed by plasma-assisted chemical deposition may have a relatively uneven surface, the present invention uses plasma-assisted atomic layer deposition to form the transparent conductive film 124, which can have better gap filling ability, better surface coverage and better uniformity than the conventional physical vapor deposition (PVD). Furthermore, compared with the conventional physical vapor deposition method, which may damage the lower solar cell 110 or other intermediate layers disposed on the lower solar cell 110, the present invention uses plasma-assisted atomic layer deposition to avoid damage by controlling the plasma.

In some embodiments, the transparent conductive film 124 includes gallium oxide, indium oxide, indium tin oxide (ITO), indium gallium oxide (IGO) or a combination thereof. The transparent conductive film 124 is preferably indium gallium oxide, because indium gallium oxide has a larger optical band gap, which allows more light to enter the lower solar cell 110, thereby improving the conversion efficiency of the stacked solar cell 100.

In some embodiments, when the transparent conductive film 124 is formed by plasma-assisted atomic layer deposition, at least one of trimethyl gallium, trimethyl indium and tetrakis (dimethylamino) tin can be used as a precursor, and high concentrations of oxygen and inert gas are introduced, wherein oxygen is used to generate bonds to form oxides, and inert gas can be used to purge unreacted residual gas. It should be understood that the selection of the precursor is determined according to the film material to be formed. In the embodiment where the transparent conductive film 124 is indium gallium oxide, trimethyl gallium and trimethyl indium are used as precursors, and the gas ratio is about 1:4 to about 1:34. By using this gas ratio, the oxygen vacancies in the film material can be controlled, so that the produced transparent conductive film 124 has better conductivity (ie, lower resistivity) and can have a suitable refractive index.

The upper solar cell 130 is arranged to face the direction of the incident light 150. The lower solar cell 110 is disposed on the electrode 101, and the transparent electrode 103 is disposed on the upper solar cell 130, that is, the lower solar cell 110 and the upper solar cell 130 are disposed between the electrode 101 and the transparent electrode 103. Therefore, the incident light 150 enters the stacked solar cell 100 and passes through the transparent electrode 103, the upper solar cell 130, the intermediate layer 120, the lower solar cell 110 and the electrode 101 in sequence.

Several examples are used below to illustrate the application of the present invention, but they are not intended to limit the present invention. Those with ordinary knowledge in the technical field of the present invention can make various changes and modifications without departing from the spirit and scope of the present invention. Experimental Example 1

Experimental Example 1 is to deposit a low-oxide silicon layer using a mixed gas of silane and carbon dioxide in different ratios, and then perform high-temperature annealing at a temperature of 800°C for 1 hour to form a low-oxide silicon film. The refractive index, extinction coefficient and grain size of the formed low-oxide silicon film are detected. Figure 2A is an analysis diagram of the refractive index and extinction coefficient of the low-oxide silicon film formed by different mixed gas ratios for 700 nm visible light; and Figure 2B is an analysis diagram of different mixed gas ratios and grain size.

As shown in Figure 2A, without adding carbon dioxide, the refractive index of the film formed by silane deposition alone is too high to meet the requirements. After adding a specific proportion of carbon dioxide, a low-oxide silicon film can be formed, so the refractive index can fall within the range of application requirements. Furthermore, after adding a specific proportion of carbon dioxide, the extinction coefficient of the low-oxide silicon film can be reduced to avoid absorbing incident light and reducing the conversion efficiency of the stacked solar cell.

As shown in FIG2B , after high temperature annealing, the amorphous low-oxide silicon layer can be transformed into a crystalline low-oxide silicon film, and the specific gas ratio can have a suitable grain size, and will not grow into overly large grains to cause defects. Experimental Example 2

Experimental Example 2 is a transparent conductive film of indium-gallium oxide formed by different ratios of trimethyl indium and trimethyl gallium. The refractive index, extinction coefficient and sheet resistance of the formed transparent conductive film are tested. Figure 3A is an analysis diagram of the refractive index and extinction coefficient of the transparent conductive film formed by different ratios of indium/gallium for 700 nm visible light; and Figure 3B is an analysis diagram of the transparent conductive film and sheet resistance formed by different ratios of indium/gallium.

As shown in Figure 3A, when the ratio of indium to gallium is in the range of 9 to 24, the refractive index of the transparent conductive film is between 1.76 and 1.86, which meets the application requirements. The extinction coefficient of the transparent conductive film gradually decreases as the ratio of indium to gallium increases (in the range of 9 to 24), which can reduce the absorption of incident light and improve the conversion efficiency of the stacked solar cell.

As shown in FIG3B , when the ratio of Indium to Gallium increases from 9 to 19, the sheet resistance of the transparent conductive film gradually decreases. This is because when the ratio of Indium to Gallium is low, the oxygen atoms are insufficient to saturate the -OH bond, which reduces the oxygen vacancy and increases the impedance. When the ratio of Indium to Gallium increases from 19 to 24, the sheet resistance of the transparent conductive film gradually increases. Therefore, the ratio of Indium to Gallium should not be too high, otherwise the sheet resistance may be too high and does not meet the requirements.

According to the above embodiments, the present invention provides a stacked solar cell and a manufacturing method thereof, which utilizes a low-oxide silicon film with a specific refractive index and a transparent conductive film as an intermediate layer between an upper solar cell and a lower solar cell, thereby helping to achieve better light matching and having good carrier transmission capability, thereby effectively improving the conversion efficiency of the stacked solar cell.

Although the present invention has been disclosed as above with several embodiments, they are not intended to limit the present invention. Anyone with ordinary knowledge in the technical field to which the present invention belongs can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be defined by the scope of the attached patent application.

100: stacked solar cell 101: electrode 103: transparent electrode 110: lower solar cell 120: middle layer 122: low oxide silicon film 124: transparent conductive film 130: upper solar cell 150: incident light

The following detailed description and the accompanying drawings provide a better understanding of the disclosed aspects. It should be noted that, as is standard practice in the industry, many features are not drawn to scale. In fact, for the sake of clarity of discussion, the sizes of many features can be arbitrarily scaled. [FIG. 1] is a schematic diagram of a stacked solar cell according to some embodiments of the present invention. [FIG. 2A] is an analysis of the refractive index and extinction coefficient of a low-oxide silicon film formed with different mixed gas ratios for 700 nm visible light according to Embodiment 1. [FIG. 2B] is an analysis of the low-oxide silicon film and grain size formed with different mixed gas ratios according to Embodiment 1. [FIG. 3A] is an analysis diagram showing the refractive index and extinction coefficient of the transparent conductive film formed with different indium/gallium ratios according to Example 2 for 700 nm visible light. [FIG. 3B] is an analysis diagram showing the transparent conductive film and sheet resistance formed with different indium/gallium ratios according to Example 2.

Domestic storage information (please note in the order of storage institution, date, and number) None Foreign storage information (please note in the order of storage country, institution, date, and number) None

100: stacked solar cells

101:Electrode

103: Transparent electrode

110: Install solar battery

120: Middle layer

122: Low oxide silicon film

124: Transparent conductive film

130: Solar battery

150: Incident light

Claims (10)

A stacked solar cell comprises: a lower solar cell; a low-oxide silicon film disposed on the lower solar cell, wherein the low-oxide silicon film has a refractive index of 2.0 to 3.5 for visible light with a wavelength of 700nm to 750nm; a transparent conductive film disposed on the low-oxide silicon film, wherein the transparent conductive film has a refractive index of 1.7 to 2.1 for the visible light with a wavelength of 700nm to 750nm; and an upper solar cell disposed on the transparent conductive film and connected in series with the lower solar cell, wherein the upper solar cell faces an incident light source, and the refractive index of the transparent conductive film is greater than a refractive index of the upper solar cell for the visible light with a wavelength of 700nm to 750nm. The stacked solar cell as described in claim 1, wherein the lower solar cell is a silicon-based solar cell and the upper solar cell is a calcium-titanium solar cell. The stacked solar cell as claimed in claim 1, wherein a thickness of the low oxide silicon film is 10 nm to 50 nm, and a film resistance of the low oxide silicon film is 1.0×10 ^-3 Ω-cm to 1.0×10 ^-2 Ω-cm. The stacked solar cell as described in claim 1, wherein the optical band gap of the transparent conductive film is 3.5 eV to 4.9 eV. The stacked solar cell as claimed in claim 1, wherein a thickness of the transparent conductive film is 15 nm to 45 nm, and a sheet resistance of the transparent conductive film is 1.0×10 ^-4 Ω-cm to 1.0×10 ^-3 Ω-cm. The stacked solar cell as described in claim 1, wherein the transparent conductive film comprises gallium oxide, indium oxide, indium tin oxide, indium gallium oxide or a combination thereof. The stacked solar cell as described in claim 1, wherein for the visible light with a wavelength of 700nm to 750nm, a refractive index of the lower solar cell is greater than the refractive index of the low oxide silicon film. A method for manufacturing a stacked solar cell is used to manufacture the stacked solar cell described in any one of claims 1 to 7, wherein a low-oxide silicon film of the stacked solar cell is formed by a plasma-assisted chemical vapor deposition method, and a transparent conductive film of the stacked solar cell is formed by a plasma-assisted atomic layer deposition method. A method for manufacturing a stacked solar cell as described in claim 8, wherein the plasma-assisted chemical vapor deposition method is performed using a mixed gas of silane and nitrous oxide or a mixed gas of silane and carbon dioxide. A method for manufacturing a stacked solar cell as described in claim 8, wherein the plasma-assisted atomic layer deposition method is performed using at least one of trimethyl gallium, trimethyl indium and tetrakis(dimethylamino)tin.