KR101233206B1 - Photovoltaic Device Including Buffer Layer And Fabrication Method thereof - Google Patents

Abstract

The present invention relates to a high-efficiency silicon thin-film photovoltaic conversion device through the growth of a new buffer layer, wherein a buffer layer containing tin (Sn) whose composition varies from high concentration to low concentration according to a concentration gradient is formed between the transparent conductive film and the semiconductor layer. do.

According to the present invention, it is possible to obtain a high efficiency and high reliability photovoltaic conversion device by eliminating the defect of the transparent conductive film generated by the hydrogen plasma and the defect caused by the contact resistance of the transparent conductive film and the semiconductor layer.

Photovoltaic conversion element, buffer layer, silicon, transparent conductive film, semiconductor layer, hydrogen plasma

Description

Silicon thin film type photovoltaic device comprising a buffer layer and a method of manufacturing the same {Photovoltaic Device Including Buffer Layer And Fabrication Method

1 is a cross-sectional view showing a structure of a silicon solar cell according to an embodiment of the prior art.

2 is a cross-sectional view illustrating a structure of a silicon thin film solar cell including a buffer layer according to an exemplary embodiment of the present invention.

3 is a schematic diagram showing a process of forming a buffer layer according to an embodiment of the present invention on a transparent conductive film in one (3a) or two (3b) layers.

4 is a cross-sectional view of a solar cell showing a step of forming a buffer layer on a transparent conductive film according to an embodiment of the present invention.

｝ Explanation of symbols for main part of drawing

100,200: glass substrate 101,201,301,401: transparent conductive film

102,202,302 p-type semiconductor layer 103,203 i-type semiconductor layer

104,204 n-type semiconductor layer 105,205 transparent conductive film

106,206: metal electrode layer 210,310,410: buffer layer

312,412 Initial buffer layer 311,411 Late buffer layer

The present invention relates to a highly efficient silicon thin film photovoltaic device and a method of manufacturing the same by growing a buffer layer, and more particularly, a fluorine-containing buffer layer containing tin (Sn) whose composition varies from a high concentration to a low concentration according to a concentration gradient. The present invention provides a photovoltaic device having a high efficiency and high reliability by removing defects between the transparent conductive film and the semiconductor layer by forming between the transparent conductive film made of a doped material such as SnO ₂ and the p-type semiconductor layer.

Photovoltaic power generation is actively being researched as a next-generation clean energy source due to the advantage that renewable energy can be obtained anywhere without causing environmental damage.

Currently, silicon single crystal solar cells, which are widely commercialized for photovoltaic generation, are limited in their use due to high manufacturing costs due to the use of expensive wafers.

Various attempts have been proposed and studied to solve the problem and to develop a thin film solar cell which can achieve high efficiency and high reliability while significantly reducing the cost of raw materials.

Thin film solar cells using amorphous, microcrystalline, and polycrystalline silicon, which are currently manufactured, have a cross-sectional structure as shown in Fig. 1.

As shown in FIG. 1, the structure of a conventional solar cell includes a transparent conductive film 101, a pin-type silicon semiconductor layer 102, 103, 104, a transparent conductive film 105, and a metal electrode layer on a glass substrate 100. 106) is laminated in order.

As a method for obtaining higher efficiency in conventional solar cells, a structure of a tandem cell that connects these solar cell modules having different optical band gaps in series has been proposed.

In the structure of the solar cell, the p-type semiconductor layer has a high optical bandgap and an excellent interfacial property with the transparent conductive film. The amorphous or microcrystalline hydrogenated silicon (Si) having a thickness of several tens of nanometers (nm) is required. Layers and SiC layers are typically used in silicon thin film solar cells for obtaining high efficiency.

However, under the hydrogen plasma environment required for the formation of the p-type semiconductor layer, the stability of the transparent conductive film made of fluorine-doped SnO ₂ is lowered, resulting in the reduction of oxygen on the surface of the conductive film. A problem arises in which a large amount of defects including dislocations and the generation of an uncontrollable metal tin (Sn) layer are unavoidable.

As a result, a large amount of defects exist at the interface between the transparent conductive film and the p-type semiconductor layer, which promotes recombination of carriers and reduces photoelectric conversion efficiency.

To avoid the generation of such defects, a buffer layer made of very thin hydrogenated Si or SiC is generally inserted at the interface between the transparent conductive film and the p-type semiconductor layer.

Attempts have been made to insert these buffer layers to reduce the contact resistance between the transparent conductive film and the p-type semiconductor layer and to protect the transparent electrode film from the high plasma density required for the formation of the p layer. One example is the insertion of hydrogenated microcrystalline Si into a buffer layer when hydrogenated SiC is used as a p-type semiconductor. However, this structure is also inevitable due to the application of the hydrogen plasma for the generation of the buffer layer, and the generation of the oxygen defective tin (Sn) layer. In addition, the formation of a buffer layer as a heterogeneous material on the polycrystalline transparent conductive film cannot remove contact resistance due to mismatches of phase or lattice. In addition, there is a problem that deformation due to bulk diffusion of hydrogen in the transparent conductive film in a hydrogen plasma environment is inevitable.

An object of the present invention is to solve the problems related to defects and contact resistance between the transparent conductive film and the semiconductor layer of the conventional solar cell structure, the transparent conductive film is protected even under a hydrogen plasma environment and suppressed defect generation The present invention provides a solar cell having a structure in which contact resistance between a transparent conductive film and a semiconductor layer is removed.

In addition, the present invention can solve the problem of the conventional solar cell simply and efficiently simply by generating a buffer layer containing tin intentionally gradient composition without adding any other process to the conventional solar cell manufacturing process. The present invention provides a method for manufacturing a solar cell.

In order to achieve the above object, the photovoltaic conversion device of the present invention has a composition gradient of intentionally generated concentration between a transparent conductive film and q semiconductor layer present in a structure of a single junction silicon-based solar cell or a tandem silicon-based solar cell. A single layer or multilayer buffer layer containing tin (Sn) having a structure is inserted. That is, a buffer layer including tin (Sn) whose composition varies from high concentration to low concentration according to the concentration gradient may be provided as at least one layer between the transparent conductive film and the semiconductor layer.

In the present invention, the buffer layer may be made of at least one material selected from hydrogenated SiSn, hydrogenated SiSnO, hydrogenated SiSnC, hydrogenated SiSnCO and the like.

When the buffer layer is provided in multiple layers, the multilayer buffer layer includes an initial buffer layer made of a material such as hydrogenated SiSn, hydrogenated SiSnO, hydrogenated SiSnC, or hydrogenated SiSnCO, and a later layer made of a material such as hydrogenated Si or hydrogenated SiC. It may be a buffer layer.

The crystal structure of the buffer layer may have an amorphous, fine crystalline (microcrystalline) or polycrystalline (polycrystalline) structure and may be doped with p-type or without doping.

In the present invention, the transparent conductive film may include, in particular, tin oxide (SnO ₂ ) doped with fluorine (F) or undoped tin oxide (SnO ₂ ), and the semiconductor layer is particularly hydrogenated Si or hydrogenated SiC. It may be a p-type semiconductor layer constituted.

In order to achieve the above object, a method of manufacturing a photovoltaic conversion device according to the present invention includes forming a buffer layer including tin (Sn) whose composition varies from a high concentration to a low concentration according to a concentration gradient on a transparent conductive film and a semiconductor layer on the buffer layer. Forming a step.

When the buffer layer is formed in multiple layers, the forming of the buffer layer may be continuously grown from the initial buffer layer to the late formation buffer layer.

The initial buffer layer is a layer of hydrogenated SiSn, hydrogenated SiSnO under a gas atmosphere such as hydrogenated Si, hydrogenated C, hydrogenated SiH ₄ , and hydrogenated CH ₄ after formation of a Sn layer or an oxygen-deficient SnO layer. Layer, hydrogenated SiSnC layer, hydrogenated SiSnCO layer and the like can be formed by the process of conversion. As the conversion proceeds, the tin (Sn) concentration may have a high concentration at the interface in contact with the transparent conductive film, and may have an inclined structure having a concentration gradient gradually toward the upper portion.

The thickness of the initial buffer layer may be controlled by the amount of hydrogen (H ₂ ) injected when the Sn layer or the oxygen-deficient SnO layer is formed, and the thickness thereof is in the range of several tens of nanometers, preferably 1 nm to 90 nm.

In the present invention, the late buffer layer may be composed of hydrogenated Si or hydrogenated SiC in which the initial buffer layer continues to grow and Sn is not present.

In general, serious contact resistance is generated at the interface between the transparent reflective film and the p-type semiconductor layer by nucleation of heterogeneous materials. However, in the present invention, the interface contact resistance can be reduced by continuously changing the composition of the buffer layer containing Sn. Can be.

Since 3D nucleation of heterogeneous materials does not proceed at the upper and lower interfaces of the buffer layer, there is an excellent contact between the buffer layer, the transparent conductive film, and the buffer layer and the p-type semiconductor layer at these interfaces.

Gradual changes in the composition of SnO ₂ to SiSn or SiSnC at the interface between the transparent conductive film and the lower buffer layer adjacent thereto may cancel misfit at the interface and thus suppress the generation of defects.

In addition, the conversion from Si or SiC, which is an upper buffer layer, to a p-type semiconductor layer, which is a homogeneous material, can also proceed without defects at the interface.

And by adding a change in the relative concentration of Sn and C in the SiSnC compound, the lattice size can be adjusted to a value very close to that of SiC or Si.

The buffer layer may be grown amorphous, microcrystalline or polycrystalline depending on the growth conditions. Sn composition of about 2% or more in SiSn and Sn composition of more than 2.5% in SiSnC induces a change from microcrystalline to crystalline through nucleation, resulting in low plasma power, low plasma frequency, low growth temperature, Even at low hydrogen ratios, growth of microcrystalline or polycrystalline buffer layers can be achieved. Damage by plasma can be suppressed in the growth conditions of these buffer layers.

The buffer layer may induce microcrystallization or polycrystallization of the p-type semiconductor layer at a low plasma density that can significantly reduce defects caused by plasma. In the case of an amorphous buffer layer, by lowering the Sn concentration to 2% or less in SiSn, it is possible to maintain the amorphous state without nucleation by reducing the Sn concentration in SiSnC or increasing the relative concentration of C to Sn.

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

In adding reference numerals to the components of the following drawings, the same components, even if displayed on the other drawings to have the same reference numerals as much as possible and known functions that are determined to unnecessarily obscure the subject matter of the invention Detailed description of the configuration will be omitted.

2 is a cross-sectional view schematically illustrating a stacked structure of a photovoltaic device according to an embodiment of the present invention. Referring to this, the transparent conductive film 201, the buffer layer 210, and the p-type semiconductor are formed on the glass substrate 200. The layer 202, the i-type semiconductor layer 203, the n-type semiconductor layer 204, the transparent conductive film 205, and the metal electrode layer 206 are sequentially formed, but this is only one embodiment. This is not limited to this.

3 is a process of forming one (FIG. 3A) or two (FIG. 3B) layers between the transparent conductive film 301 and the p-type semiconductor layer 302 according to an embodiment of the present invention. It is a schematic diagram. As shown in FIG. 3, by generating a buffer layer having a single layer or a multilayer structure in which a Sn-containing composition varies with thickness, it is generated by misfit at the interface between the transparent conductive film and the buffer layer and at the interface between the buffer layer and the p-type semiconductor layer. It is a structure that can increase efficiency by reducing drastically.

3A illustrates that one buffer layer 310 is formed on the transparent conductive film 301. According to an embodiment of the present invention, the transparent conductive film may be formed of SnO ₂ doped with fluorine (F) and hydrogenated. Si-C gas or silicon hydride (SiH ₄ ) or methane (CH ₄ ) gas with or without hydrogen reacts under low plasma energy, low frequency, low substrate temperature, low gas flow rate to form a buffer layer can do. At this time, the buffer layer is hydrogenated SiSnC and the concentration of tin (Sn) is high at the interface portion of the transparent conductive film and gradually decreases upward.

After the formation of the buffer layer 310, silicon hydride (SiH ₄ ) or silicon hydride (SiH ₄ ) and methane (CH ₄ ) gas with or without hydrogen, higher plasma energy, higher frequency, higher The p-type semiconductor layer can be continuously formed by reacting at a substrate temperature and a high gas flow rate. In this case, the p-type semiconductor layer may be formed of amorphous, microcrystalline, or polycrystalline hydrogenated silicon (Si) or SiC.

FIG. 3B shows two buffer layers formed on the transparent conductive film 301, and the late buffer layer 311 is formed in succession to the initially formed initial buffer layer 312.

In the solar cell of the present invention according to FIG. 3B, as in the formation of a single buffer layer, the transparent conductive film may be composed of SnO ₂ doped with fluorine (F), and hydrogenated Si-C gas or silicon hydride (SiH ₄ ) or methane. The initial buffer layer 312 can be formed by reacting (CH ₄ ) gas with or without hydrogen, under low plasma energy, low frequency, low substrate temperature, and low gas flow rate.

At this time, the initial buffer layer is hydrogenated SiSnC and the concentration of tin (Sn) is increased at the interface portion of the transparent conductive film and forms a concentration gradient gradually decreasing upward.

After the initial buffer layer 312 is formed, silicon hydride (SiH ₄ ) or silicon hydride (SiH ₄ ) and methane (CH ₄ ) gases are injected with or without hydrogen and have higher plasma energy and higher than when the buffer layer is formed. Reaction under high frequency, high substrate temperature, and high gas flow rate results in the formation of hydrogenated SiC layers that do not react with thinned tin (Sn) before formation of the p-type semiconductor layer, which is a late buffer layer. 311 may be formed.

After the formation of the late buffer layer, a p-type semiconductor layer made of amorphous, microcrystalline or polycrystalline hydrogenated silicon (Si) or SiC is formed, so that the contact resistance is greatly reduced because it is provided in the same line as the material of the late buffer layer. The occurrence of defects is significantly reduced.

As can be seen in FIGS. 3A and 3B, the initial growth of the buffer layer is performed by using Sn plasma having a very thin thickness relaxed by a desorption of oxygen at the surface of the transparent conductive film using low plasma energy. It begins with the formation of a layer or an O-deficient SnO layer. The Sn layer should have a thickness of up to several tens of nanometers or less, which can be fully converted to SiSnC (or SiSn) compounds by reaction of gaseous Si with C (or Si) in subsequent process steps. Low plasma density is obtained by applying low plasma energy and / or low plasma frequency and / or low growth temperature. In order to prevent the rapid reduction of oxygen due to the large amount of H during the formation of the Sn layer, H ₂ injected into the reactor is limited to a small amount.

The thickness of the buffer layer decreases as the amount of H ₂ injected decreases, thereby reducing the oxygen reduction reaction and consequently restricting the generation of the Sn layer. If the growth conditions are not controlled as described above in this initial growth stage, high density defects are generated on the surface by rapid oxygen reduction as shown in the prior art, and a Sn layer having a thickness that cannot be removed by a subsequent process is generated. do.

Due to the reaction between the Sn layer or the oxygen-deficient Sn layer and Si and / or C elements transferred from the gas phase, an initial buffer layer is formed and a concentration gradient of Sn is achieved. That is, a buffer layer composed of SiSn or SiSnC is generated by diffusion through the Sn layer (injection bulk diffusion of Si and C into the Sn layer or diffusion of Sn outflow from the Sn layer), whereby the Sn layer forms an upper p-type semiconductor layer. Almost disappear at the interface. The thickness of the initial buffer layer is observed as tens of nanometers below 1 nm.

The growth temperature is maintained near 232 ° C., the melting point of metal tin, to aid diffusion in the Sn layer. This growth temperature is a sufficiently low value that does not affect the resistance value of the transparent conductive film made of fluorine-doped SnO ₂ .

In order to reduce defects and lower interfacial contact resistance at the interface between the transparent conductive film and the initial buffer layer, the concentration of Sn in the initial buffer layer gradually decreases with thickness and the Si / Sn / C composition ratio decreases with the thickness by precisely controlling the diffusion. Create an initial buffer layer with varying composition gradients. At the interface between the transparent conductive film and the initial buffer layer, a large amount of Sn is present, so that a gradual change in the composition from SnO ₂ to SiSn or SiSnC occurs, rather than a sudden change between heterogeneous materials.

This is precisely controlled using plasma energy and frequency, the flow rate of silicon hydride (SiH ₄ ) / methane (CH ₄ ) / hydrogen (H ₂ ), and substrate growth temperature to reach the reaction surface from the gas phase. It can be obtained by keeping the amounts of Si and C in a small amount.

In the present invention, methods for continuously increasing the amount of injected gas, changing the injection timing of each gas, or increasing the plasma energy gradually are proposed.

Gases containing SI-H or Si-CH may be used as the injection gas, and mixed gases of H ₂ -SiH ₄ -CH ₄ , H ₂ -SiH ₄ , H ₂ -CH ₄ , and SiH ₄ -CH ₄ are excited. Included in The initial buffer layer may include oxygen in a concentration that cannot be ignored by diffusion of oxygen depending on growth conditions. In this case, the composition of the buffer layer is hydrogenated SiSnO or hydrogenated SiSnCO.

After the growth of the initial buffer layer was stopped due to the increase in thickness, the supply of Sn through diffusion on the reaction surface was stopped, and then the growth of the late buffer layer having the composition of hydrogenated Si or hydrogenated SiC without Sn was continuously performed. Proceed. In addition, the conversion of the growth from the buffer layer to the p-type semiconductor layer is performed by changing the composition of the injection gas, flow rate, plasma energy, plasma frequency, and substrate growth temperature. 4 shows the progression from the growth of the initial buffer layer to the growth of the late buffer layer during the fabrication of a solar cell having a multilayer buffer layer.

Referring to Figure 4 it can be seen that in the starting material of the buffer layer Sn is externally diffused to the surface is combined and converted under an external gas atmosphere such as Si or SiC.

As described above, the present invention has been described with reference to a preferred embodiment of the present invention, but those skilled in the art can vary the present invention without departing from the spirit and scope of the present invention as set forth in the claims below. It will be appreciated that modifications and variations can be made.

As described above, according to the present invention, it is possible to economically and efficiently manufacture a solar cell element having a high photoelectric conversion efficiency. When the present invention is commercialized, it will contribute to the preservation of the global environment as a next-generation clean energy source, and can be applied directly to public facilities, private facilities, military facilities, etc., and can generate enormous economic value.

Claims (15)

A photovoltaic device comprising: a buffer layer comprising tin (Sn) whose composition varies from high concentration to low concentration according to a concentration gradient, provided with at least one layer between the transparent conductive film and the semiconductor layer. The photovoltaic device of claim 1, wherein the buffer layer is formed of at least one material selected from the group consisting of hydrogenated SiSn, hydrogenated SiSnO, hydrogenated SiSnC, and hydrogenated SiSnCO. The method of claim 1, wherein the buffer layer comprises a multilayer buffer layer, The multilayer buffer layer includes an initial buffer layer comprising at least one material selected from the group consisting of hydrogenated SiSn, hydrogenated SiSnO, hydrogenated SiSnC, and hydrogenated SiSnCO, and at least one selected from the group consisting of hydrogenated Si and hydrogenated SiC. A photovoltaic device, characterized in that the late buffer layer made of a material. The photovoltaic device according to any one of claims 1 to 3, wherein the crystal structure of the buffer layer is any one of an amorphous, a microcrystalline, and a polycrystalline structure. The photovoltaic device of claim 1, wherein the transparent conductive film comprises tin oxide (SnO ₂ ) doped with fluorine (F) or undoped tin oxide (SnO ₂ ). The photovoltaic device according to claim 1, wherein the semiconductor layer is a p-type semiconductor layer composed of hydrogenated Si or hydrogenated SiC. Forming a buffer layer including tin (Sn) on the transparent conductive film, the composition of which is varied from high concentration to low concentration according to a concentration gradient; And A method of manufacturing a photovoltaic device comprising the step of forming a semiconductor layer on the buffer layer. The photovoltaic device of claim 7, wherein the buffer layer is formed of a single layer and is formed of at least one material selected from the group consisting of hydrogenated SiSn, hydrogenated SiSnO, hydrogenated SiSnC, and hydrogenated SiSnCO. Way. The method of claim 7, wherein the buffer layer is continuously grown from an initial buffer layer to a late formation buffer layer. The method of claim 9, wherein the initial buffer layer, Forming a Sn layer or an oxygen-deficient SnO layer, and The Sn layer or oxygen-deficient SnO layer is hydrogenated SiSn layer, hydrogenated SiSnO layer, hydrogenated under one or more gas atmospheres selected from the group consisting of hydrogenated Si, hydrogenated C, hydrogenated SiH ₄ , and hydrogenated CH ₄ Method of manufacturing a photovoltaic device, characterized in that formed by the step of converting into at least one layer selected from the group consisting of SiSnC layer, hydrogenated SiSnCO layer. The method of claim 10, wherein the thickness of the initial buffer layer is controlled by the amount of hydrogen (H ₂ ) injected when the Sn layer or the oxygen-deficient SnO layer is formed. The method of claim 11, wherein the initial buffer layer has a thickness of about 1 nm to about 90 nm. 10. The method of claim 9, wherein the late buffer layer is formed of hydrogenated Si or hydrogenated SiC in which the initial buffer layer is continuously grown and Sn is not present. The method of claim 7, wherein the method for manufacturing a photovoltaic conversion element characterized by including the transparent conductive film is fluorine (F) doped tin oxide (SnO ₂₎ or non-doped tin oxide (SnO _2). The method of manufacturing a photovoltaic device according to claim 7, wherein the semiconductor layer is a p-type semiconductor layer composed of hydrogenated Si or hydrogenated SiC.

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