JP2006196798A - Method of inspecting thin film solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of inspecting a thin film solar cell capable of executing total inspection on a p-type semiconductor thin film such as a CIS thin film and CIGS thin film. <P>SOLUTION: A conductive film, p-type compound semiconductor thin film as a light absorbing layer, n-type semiconductor thin film, and a transparent conductive film are sequentially formed on a substrate to manufacture a thin film solar. After forming the p-type compound semiconductor thin film in that process, the surface area along a surface contour of the arbitrary region is measured, which is compared to the area of a flat surface specified by the profile of the arbitrary region. The p-type compound semiconductor thin film is determined to be a good product when the area ratio of the surface area against the flat surface is larger than 1 while equal to or less than a specified value, for example ≤1.3. So, the p-type compound semiconductor thin film is evaluated in total number by nondestructive manner in a short time, to predict the solar cell characteristics of a thin-film solar cell formed like this. Whether a higher conversion efficiency is available or not is determined at high accuracy, for improved yield. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for inspecting a thin film solar cell, and more particularly to a method for inspecting a thin film solar cell using a chalcopyrite type compound semiconductor thin film as a light absorption layer.

A thin-film solar cell using, as a light-absorbing layer, a conventional thin-film solar cell using an I-III-VI ₂ type chalcopyrite compound semiconductor thin film composed of Group I, Group III, and Group VI elements, specifically CuInSe ₂ ( There is a thin film solar cell using a light absorption layer of CIS) or Cu (In, Ga) Se ₂ (referred to as CIGS).

FIG. 6 shows a manufacturing process for forming a typical structure of a CuInSe ₂ (CIS) thin film solar cell. A Mo electrode film as a conductive film 2 is formed on a glass substrate 1, and a CIS semiconductor thin film (hereinafter referred to as a CIS thin film) 3 as a p-type compound semiconductor thin film (hereinafter referred to as a p-type semiconductor thin film) 3 is formed thereon, n A ZnO / CdS film as the type semiconductor thin film 4 and an ITO transparent conductive film as the transparent conductive film 5 are formed in this order. When light enters the laminated structure from the transparent conductive film 5 side, the light is absorbed mainly by the p-type semiconductor thin film 3, and carriers are generated by the energy of the absorbed light, and a photovoltaic power is generated by the electric field of the pn junction. Therefore, this electric power is taken out from the conductive film 2 and the transparent conductive film 5.

A method for producing a CIS thin film solar cell will be described in detail with reference to Patent Document 1 with reference to the flowchart of FIG.
The glass substrate 1 is cleaned (step S11), and a Mo electrode film as the conductive film 2 is formed thereon by a sputtering method so as to have a thickness of 0.5 μm (step S12).

  A CIS thin film as a p-type semiconductor thin film 3 is formed on the conductive film 2 so as to have a thickness of 1 to 4 μm (step S13). This CIS thin film is formed, for example, by sputtering or multi-source co-evaporation as described in Non-Patent Document 1, but the composition of Cu, In, and Se, which are constituent elements, is used in any method. It is known that control is important, and this composition is a factor determining solar cell characteristics. For example, when the composition ratio Cu / In between Cu (Group I element) and In (Group III element) is 1 or more, a Cu—Se phase is deposited on the CIS thin film. Since the Cu—Se phase has a low resistance, a shunt occurs, and in most cases, the conversion efficiency of the solar cell characteristics is less than 0.1%. Therefore, the film forming process is controlled so that Cu / In is less than 1.

  After the film formation is completed, the CIS thin film is inspected (step S14). Generally, the composition measurement of a CIS thin film is performed. However, in Patent Document 1, photoluminescence measurement is performed using one of a plurality of CIS thin films formed as a test piece, and a non-defective product or a defective product is determined based on the intensity of the emission peak. ing.

A ZnO / CdS film as the n-type semiconductor thin film 4 is formed on the CIS thin film determined to be non-defective in the inspection so as to have a thickness of several hundred nm (step S15). In that case, after the CdS film is formed by the CBD method, the ZnO film is formed by the sputtering method. Finally, an ITO transparent conductive film as the transparent conductive film 5 is formed by sputtering so as to have a thickness of 500 nm (step S16).
Japanese Patent No. 2984114 Edited by Makoto Konagai, “Fundamentals and Applications of Thin-film Solar Cells: New Developments in Environmentally Friendly Solar Power Generation”, Ohmsha, March 20, 2001 (Chapter 5, p175-212)

  Although it is described also in patent document 1, even if it is judged that it is a non-defective product by the composition measurement in the inspection process of the CIS thin film, the conversion efficiency is less than 0.1% in the solar cell that has been completed up to the final process. is there. Depending on the composition measurement, the solar cell characteristics of the CIS thin film cannot be fully evaluated. According to the photoluminescence measurement proposed in Patent Document 1, it was possible to distinguish between a CIS thin film having a conversion efficiency of about 10% and a conversion efficiency of 0.1%.

  However, a CIS thin film with a conversion efficiency of 0.1% is rarely formed in an actual mass production process, and the distinction by photoluminescence measurement is not sufficient for predicting whether or not a CIS thin film solar cell is a good product. Furthermore, a CIGS thin film solar cell using a CIGS thin film as a light absorption layer has higher conversion efficiency than a CIS thin film solar cell, and the conversion efficiency is 8% or more in the mass production process, and the conversion efficiency is less than 1%. There is almost no. In order to improve the yield rate of CIGS thin film solar cells, it is necessary to predict with high probability whether or not the conversion efficiency will be 8% or more in the inspection process of the CIGS thin film. It is not appropriate to apply photoluminescence measurement as the method.

Further, in the inspection process by photoluminescence measurement, as described above, one of the plurality of CIS thin films formed is inspected as a test piece. This is due to the following reason. In order to obtain a photoluminescence spectrum that more accurately shows the state of the defect level in the band gap, the object is immersed in liquid nitrogen and cooled to 77K or reduced to liquid helium for the purpose of reducing the influence of heat. It is necessary to immerse and cool to 4.2K. When the specimen is cooled to such an extremely low temperature and then returned to room temperature, internal destruction occurs due to shrinkage and expansion, and the subsequent process may not function as a solar cell. In the case where the subject is 10 cm ² or more used as a solar cell panel, it is difficult to cool to a very low temperature. However, because it is an inspection with a test piece in this way, that is, it is not a total inspection, even if it is judged as a good product at this stage, sufficient solar cell characteristics may not be obtained with a thin film solar cell as a finished product. Not enough to improve the rate.

  In view of the above problems, an object of the present invention is to provide a thin-film solar cell inspection method capable of predicting solar cell characteristics by inspecting all p-type semiconductor thin films such as CIS thin films.

  In order to achieve the above object, a method for inspecting a thin film solar cell according to the present invention comprises sequentially forming a conductive film, a p-type compound semiconductor thin film as a light absorption layer, an n-type semiconductor thin film, and a transparent conductive film on a substrate. A flat surface defined by the outer shape of the arbitrary region by measuring the surface area along the surface irregularity shape of the arbitrary region after the formation of the p-type compound semiconductor thin film. When the area ratio of the surface area to the flat surface is greater than 1 and less than or equal to a predetermined value, the p-type compound semiconductor thin film is determined to be non-defective. According to this, the p-type compound semiconductor thin film is non-destructive in a short time, and therefore the total number of products to be inspected is evaluated, and the solar cell characteristics of the thin film solar cell formed using the compound semiconductor thin film are highly accurate. Is predictable.

By using 1.3 as the predetermined value, it is possible to obtain a thin film solar cell having a conversion efficiency of 10% or more.
The measurement of the surface area can be suitably performed with an atomic force microscope. The scanning interval of the atomic force microscope is preferably 10 ⁻³ μm or more and 10 ⁻¹ μm or less. This is because sufficiently accurate surface area measurement is possible if the scanning interval is 10 ⁻¹ μm or less, and if the scanning interval is less than 10 ⁻³ μm, the measurement time becomes too long.

The flat area of the arbitrary region is preferably 10 μm ² or more and 10 ⁶ μm ² or less. If it is 10 μm ² or more, since there are at least several crystal particles (generally several hundred nm to several μm) of the CIGS thin film in the region, the CIGS thin film can be sufficiently represented, while 10 ⁶ μm ² This is because the measurement time becomes too long when the value exceeds.

  According to the method for inspecting a thin film solar cell of the present invention, after forming a p-type compound semiconductor thin film to be used as a light absorption layer, the surface area of the arbitrary region is measured, so that the thin film solar can be obtained without performing the subsequent steps. The conversion efficiency of the battery can be predicted with high accuracy. Since the surface area can be measured in a non-destructive manner, 100% inspection is possible. If it is determined that the product is defective, it can be eliminated at that stage, so that the yield rate of thin film solar cells can be improved.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Since the thin film solar cell for carrying out the inspection method of the present invention has the same structure as the conventional one described above with reference to FIG. 6, it will be described with reference to FIG. Where p-type semiconductor thin film 3 functioning as a light absorbing layer is intended to form the _{Cu (In, Ga) Se 2} (CIGS) thin film.

  In a CIGS thin film solar cell, a Mo electrode film as a conductive film 2 is formed on a glass substrate 1, and a CIGS semiconductor thin film (hereinafter referred to as a CIGS thin film) as a p-type semiconductor thin film 3 and an n-type semiconductor thin film are formed thereon. The ZnO / CdS film as 4 and the ITO transparent conductive film as the transparent conductive film 5 are formed in this order.

The manufacturing method of a CIGS thin film solar cell will be described in detail with reference to the flowchart of FIG.
After cleaning the glass substrate 1 (step S1), a Mo electrode film as a conductive film 2 is formed thereon by a sputtering method so as to have a thickness of 0.5 μm (step S2), and a p-type semiconductor is further formed thereon. A CIGS thin film as the thin film 3 is formed to have a film thickness of about 3 μm by the multi-source simultaneous vapor deposition method (step S3).

  The formed CIGS thin film is inspected as described later (step S4). A ZnO / CdS film as the n-type semiconductor thin film 4 is formed so as to have a thickness of several hundred nm only on the CIGS thin film determined to be non-defective by the inspection (step S5). In that case, after the CdS film is formed by the CBD method, the ZnO film is formed by the sputtering method. Finally, an ITO transparent conductive film as the transparent conductive film 5 is formed by sputtering so as to have a thickness of 500 nm (step S6).

A method for inspecting a CIGS thin film will be described.
First, the chemical composition of the CIGS thin film is measured nondestructively using a fluorescent X-ray analyzer.
Here, for the CIGS thin film, the composition ratio Cu / (In + Ga) = 0.90 to 0.98 of the group I element Cu and the group III element In and Ga was confirmed to be 1 or less. If Cu / (In + Ga) is greater than 1 at this stage, a Cu—Se phase is formed mainly on the surface of the CIGS thin film as a different phase in addition to the CIGS phase. Since the Cu—Se phase has a low resistance, it causes a shunt, and in most cases, the CIGS thin film solar cell including the CIGS thin film with Cu / (In + Ga)> 1 in which the Cu—Se phase is present has a conversion efficiency of 0.1%. Less than. For this reason, the CIGS thin film with Cu / (In + Ga)> 1 is determined as a defective product, and the subsequent steps are not performed.

Next, the surface area of the CIGS thin film is measured using an atomic force microscope.
Here, an atomic force microscope (manufactured by JEOL, JSPM-5700) was used, and the surface area was measured by scanning the measurement region 10 μm × 10 μm with 512 lines. This scanning interval corresponds to about 0.02 μm (10 μm / 512 lines≈0.02 μm). The “surface area” is the area of the upper surface of the CIGS thin film, that is, the surface on which the ZnO / CdS film is laminated in the next step. The surface closely attached to the Mo electrode film is not included.

The principle of surface area measurement with an atomic force microscope is as follows.
As shown in FIG. 2, an arbitrary measurement region a (μm) × b (μm) on the upper surface of the CIGS thin film 3a is scanned by m lines, and height data is acquired at n positions with a predetermined interval for each line. . In this case, there are m × n measurement points P in a grid shape in the measurement region.

The coordinates of an arbitrary measurement point P _{i, j} are (x _{i, j} , y _{i, j} , z _{i, j} ) (where i = 1 to m−1, j = 1 to n−1) and are adjacent to each other. Sa _{i, j} is the area of a triangle with three points (P _{i, j} , P _{i + 1, j} , P _{i, j + 1} ) and three points (P _{i + 1, j} , P _{i, j + 1} , P _{i + 1, j + 1} ) as vertices, respectively. , Sb _{i, j} , the surface area Ss of the measurement region is expressed by the following equation (1).

表面積測定が終了したら、測定領域の「表面積」に対する「平坦面積」の比を算出する。ここで「平坦面積」とは凹凸の全く無い理想的な平面の面積を言い、測定領域が上記した寸法の長方形であれば、平坦面積Ｓｆ＝ａ×ｂ（μｍ^２）で表される。

When the surface area measurement is completed, the ratio of the “flat area” to the “surface area” of the measurement region is calculated. Here, “flat area” means an area of an ideal plane having no irregularities, and is expressed by a flat area Sf = a × b (μm ² ) if the measurement region is a rectangle having the dimensions described above.

  Thereafter, a non-defective product and a defective product are determined based on the calculated “surface area / flat area ratio”, that is, Ss / Sf. When the “surface area / flat area ratio” is greater than 1 and less than or equal to a predetermined value, the CIGS thin film is determined to be non-defective.

FIG. 3 shows the reverse saturation current density J ₀ obtained from the measurement results of IV characteristics for CIGS thin film solar cells prepared by performing surface area measurement of a plurality of CIGS thin films and performing subsequent steps on each CIGS thin film. The correlation with the “surface area / flat area ratio” obtained from the surface area measurement results is shown. FIG. 4 shows the “surface area / flat area ratio” and the conversion efficiency Eff. The correlation is shown.

A method for obtaining the reverse saturation current density J ₀ (A / cm ² ) will be described.
In the equivalent circuit of the thin film solar cell shown in FIG. 5, the relationship between the current density J and the voltage V measured at both terminals of the solar cell can be expressed by the following equation (2).

［式中、Ｊ_scは短絡電流密度、Ｒ_sは直列抵抗、Ｒ_shは並列抵抗、ｑは電子電荷（Ｃ）、ｎはダイオード因子、ｋはボルツマン定数1.38×10Ｅ^-23，Ｔは絶対温度（Ｋ）である］
電流Ｉと電流密度Ｊの関係は、太陽電池の受光面積をＳとすると、Ｉ／Ｓ＝Ｊとなる。したがって、Ｊ_０値は、ＣＩＧＳ薄膜太陽電池のＩ−Ｖ測定を行い、得られたＩ−Ｖカーブを式（２）にフィッティングすることで求めることができる。

[ _Where J _sc is the short-circuit current density, R _s is the series resistance, R _sh is the parallel resistance, q is the electronic charge (C), n is the diode factor, k is the Boltzmann constant 1.38 × 10E ⁻²³ , T is the absolute temperature (K)]
The relationship between the current I and the current density J is I / S = J, where S is the light receiving area of the solar cell. Thus, _{J 0} value performs I-V measurements of the CIGS thin film solar cell, the I-V curve obtained can be determined by fitting to the equation (2).

The relationship between the solar cell characteristics such as the reverse saturation current density J ₀ and the “surface area / flat area ratio” will be described.
In the CIGS thin film solar cell, a pn junction is formed at the interface between the p-type CIGS thin film and the n-type ZnO / CdS film. The pn junction is the most important element for separating the carriers generated by the irradiation light in the solar cell and generating photovoltaic power. If there is a defect at the interface of the pn junction, carrier recombination occurs and a reverse saturation current flows. When the reverse saturation current value increases, the solar cell characteristics deteriorate.

  The junction area (area of the interface) of the pn junction corresponds to the surface area of the CIGS thin film. If the amount of defects per unit bonding area is uniform, the larger the bonding area (surface area), the larger the amount of defects and the larger the reverse saturation current value. However, when the size of the crystal particles of the CIGS thin film is small, the surface area is generally large, and the CIGS thin film formed under an inappropriate condition such that the size of the crystal particles is small is considered to increase the amount of defects per unit surface area. It is done. That is, in the CIGS thin film having a large surface area, the amount of defects per unit surface area also increases, so the reverse saturation current value increases synergistically and the solar cell characteristics deteriorate.

Referring to FIGS. 3 and 4 again, in FIG. 3, the reverse saturation current density J ₀ increases as the “surface area / flat area ratio” increases. In FIG. 4, as the “surface area / flat area ratio” increases, the conversion efficiency Eff. Is decreasing. 3 and 4, the “surface area / flat area ratio” corresponds to the junction area, and the smaller the _value is, the smaller the reverse saturation current density J ₀ becomes. As a result, the conversion efficiency Eff. It is thought that became higher. Therefore, by measuring the surface area, it is possible to predict the solar cell characteristics of a thin film solar cell manufactured by performing the subsequent steps.

  According to FIG. 4, in order to obtain a highly efficient thin film solar cell with a conversion efficiency of 14% or more, the “surface area / flat area ratio” needs to be 1.2 or less. In order to obtain a thin film solar cell having a conversion efficiency of 10% or more, the “surface area / flat area ratio” may be 1.3 or less. If the “surface area / flat area ratio” is greater than 1.3, the conversion efficiency is less than 10%, and sufficient power can be obtained to connect to a solar cell having a conversion efficiency of 10% or more. It cannot be used as a solar cell.

  Thus, by performing the inspection to measure the surface area after forming the CIGS thin film, preferably by performing the inspection on the total number of CIGS thin films, the conversion of the CIGS thin film solar cell produced by performing the subsequent steps The efficiency can be predicted with a high probability. For example, it is possible to determine whether the CIGS thin film has a possibility of obtaining a CIGS thin film solar cell of 14% or more.

  By taking measures such as not performing subsequent processes based on the judgment results, waste due to process loss can be eliminated, feedback to the CIGS thin film deposition process is possible, and the yield rate is dramatically improved. It is possible.

  The produced CIGS thin film solar cell can be dissolved and removed from the ZnO / CdS film and the ITO transparent conductive film laminated on the CIGS thin film by being immersed in an acidic aqueous solution. When the surface area of the CIGS thin film thus removed was measured, it was almost the same as the surface area measured before forming the ZnO / CdS film, and the variation was ± 2%. This result shows that the inspection accuracy equivalent to the inspection of the finished product of the CIGS thin film solar cell can be realized by inspecting after the CIGS thin film is formed (before the ZnO / CdS film is formed). .

In the above-described embodiment, as a surface area measurement condition of the CIGS thin film using an atomic force microscope, the measurement region is 10 μm × 10 μm = 100 μm ² and scanning is performed with 512 lines (scanning interval is about 0.02 μm). This is because the crystal particle size of the CIGS thin film is generally several hundred nm to several μm, so that at least several crystal particles are present in the measurement region and the CIGS thin film can be sufficiently represented.

However, the present invention is not limited to this. For example, a measurement area of ² μm × 5 μm = 10 μm ² may be scanned by 20 lines, and the scanning interval may be set to 0.1 μm (2 μm ÷ 20 lines = 0.1 μm). Measurement is possible.

In the above-described embodiment, the multi-source co-evaporation method is exemplified as the CIGS thin film forming method, but the inspection method of the present invention can also be applied when a selenization method, a sputtering method, a spray method, an electrodeposition method, or the like is used. Needless to say. Further, the present invention is not limited to CIGS thin films, but is also applied to thin film solar cells using other I-III-VI group compound semiconductor thin films as light absorption layers such as CIS thin films and Cu (In, Ga) (S, Se) ₂ thin films. The inspection method of the invention can be applied.

In the above-described embodiment, a laminated film made of CdS and ZnO is exemplified as the n-type semiconductor thin film, but ZnMgO, ZnS, ZnSe, or the like may be used. Although the ITO transparent conductive film is exemplified as the transparent conductive film, ZnO: Al, ZnO: B, SnO ₂ or the like may be used.

  The method for inspecting a thin film solar cell of the present invention is useful for inspecting a I-III-VI group compound semiconductor thin film such as a CIGS thin film used for a light absorption layer, and can realize a high quality thin film solar cell.

The flowchart of the thin film solar cell manufacturing process which enforces the test | inspection method of the thin film solar cell in one Embodiment of this invention The schematic diagram which shows the principle of the surface area measurement by the atomic force microscope used with the inspection method of the thin film solar cell of this invention Correlation diagram between surface area / flat area ratio and reverse saturation current density obtained by inspection method of thin film solar cell of the present invention Correlation diagram between surface area / flat area ratio required by thin film solar cell inspection method of the present invention and conversion efficiency of finished thin film solar cell Equivalent circuit diagram for explaining how to determine reverse saturation current density of thin film solar cell Manufacturing process sectional drawing of this invention and the conventional thin film solar cell Flow chart of thin film solar cell manufacturing process for implementing a conventional thin film solar cell inspection method

Explanation of symbols

1 glass substrate 2 conductive film 3 p-type compound semiconductor thin film 4 n-type semiconductor thin film 5 transparent conductive film

Claims (5)

An inspection method for manufacturing a thin film solar cell by sequentially forming a conductive film, a p-type compound semiconductor thin film as a light absorption layer, an n-type semiconductor thin film, and a transparent conductive film on a substrate,
After the formation of the p-type compound semiconductor thin film, the surface area along the surface irregularity shape of the arbitrary region is measured, and compared with the area of the flat surface assumed inside the arbitrary region, the area ratio of the surface area to the flat surface A method for inspecting a thin-film solar cell, wherein the p-type compound semiconductor thin film is determined to be a non-defective product when is greater than 1 and less than or equal to a predetermined value.   The method for inspecting a thin film solar cell according to claim 1, wherein the predetermined value is 1.3.   The method for inspecting a thin film solar cell according to claim 1, wherein the surface area is measured by an atomic force microscope. The method for inspecting a thin film solar cell according to claim ³ , wherein the scanning interval of the atomic force microscope is 10 ^-3 µm or more and 10 ^-1 µm or less. The thin film solar cell inspection method according to claim 1, wherein a flat area of the arbitrary region is 10 μm ² or more and 10 ⁶ μm ² or less.

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