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JP2007335792A - Thin-film solar cell - Google Patents

Thin-film solar cell Download PDF

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JP2007335792A
JP2007335792A JP2006168570A JP2006168570A JP2007335792A JP 2007335792 A JP2007335792 A JP 2007335792A JP 2006168570 A JP2006168570 A JP 2006168570A JP 2006168570 A JP2006168570 A JP 2006168570A JP 2007335792 A JP2007335792 A JP 2007335792A
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semiconductor layer
layer
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band gap
solar cell
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JP4919710B2 (en
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Takayuki Negami
卓之 根上
Takuya Sato
琢也 佐藤
Shigeo Hayashi
茂生 林
Yasuhiro Hashimoto
泰宏 橋本
Shinichi Shimakawa
伸一 島川
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Panasonic Holdings Corp
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin-film solar cell having a band structure ideal for improving the energy conversion efficiency. <P>SOLUTION: The thin-film solar cell 10 is provided with light absorbing layers 13a, 13b each made of a chalcopyrite structure semiconductor. The light-absorbing layers 13a, 13b include a first semiconductor layer 13a, made of CIGS and a second semiconductor layer 13b made of CIAS. In the first semiconductor layer 13a, a band gap becomes smaller, the closer it comes to the second semiconductor layer 13b. The second semiconductor layer 13b has a band gap larger than the smallest band gap in the first semiconductor layer 13a. A double grated band gap is formed of the first and second semiconductor layers 13a, 13b. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

本発明は、エネルギー変換効率が高い薄膜太陽電池に関する。   The present invention relates to a thin film solar cell having high energy conversion efficiency.

１１族、１３族および１６族元素からなる化合物半導体（いわゆるカルコパイライト構造半導体）の代表例であるＣｕ（Ｉｎ，Ｇａ）Ｓｅ2（ＣＩＧＳ）は、ＩｎとＧａの組成比によりバンドギャップを制御することができる。ＣＩＧＳ膜の深さ方向でＩｎとＧａの組成比を変化させてグレーデッドバンドギャップを形成することにより、ＣＩＧＳ膜を用いた薄膜太陽電池の高効率化が行われている。 Cu (In, Ga) Se 2 (CIGS), which is a representative example of a compound semiconductor (so-called chalcopyrite structure semiconductor) composed of Group 11, Group 13 and Group 16 elements, controls the band gap by the composition ratio of In and Ga. be able to. The efficiency of a thin film solar cell using a CIGS film is increased by changing the composition ratio of In and Ga in the depth direction of the CIGS film to form a graded band gap.

特許文献１で開示されているように、ｐｎ接合側の表面（光入射側の主面）から裏面に向けて、ＧａとＩｎの組成比であるＧａ／（Ｉｎ＋Ｇａ）が徐々に増加する分布によれば、バンドギャップが表面から裏面に向けて拡大するグレーデッドバンドギャップを形成できる。バンドギャップの変化によって、ＣＩＧＳ膜内部に電界が生じ、その電界により、光励起されたキャリアがＣＩＧＳ膜の表面に形成されるｐｎ接合へと輸送されるため変換効率が向上する。   As disclosed in Patent Document 1, Ga / (In + Ga), which is a composition ratio of Ga and In, gradually increases from the surface on the pn junction side (main surface on the light incident side) to the back surface. According to this, a graded band gap in which the band gap expands from the front surface to the back surface can be formed. Due to the change in the band gap, an electric field is generated in the CIGS film, and the photoexcited carriers are transported to the pn junction formed on the surface of the CIGS film by the electric field, so that the conversion efficiency is improved.

また、非特許文献２に開示されているように、グレーデッドバンドギャップに加えて、ＣＩＧＳ膜の光入射側の表層部にＧａ濃度の高い層を形成することにより、ｐｎ接合界面でのバンドギャップを拡大し、開放端電圧を向上させるダブルグレーデッドバンドギャップを形成できる。ダブルグレーデッドバンドギャップによれば、より高い変換効率を達成できる。   Further, as disclosed in Non-Patent Document 2, in addition to the graded band gap, a band gap at the pn junction interface is formed by forming a layer with a high Ga concentration in the surface layer portion on the light incident side of the CIGS film. And a double graded band gap that improves the open circuit voltage can be formed. According to the double graded band gap, higher conversion efficiency can be achieved.

１１族、１３族および１６族元素からなる化合物半導体薄膜のバンドギャップの制御は、ＩｎとＧａだけでなく、例えば、１３族元素のＡｌとＩｎの組成比や１６族元素のＳｅとＳの組成比でも行うことができる。ダブルグレーデッドバンドギャップを形成するために、ＡｌとＩｎの組成比を変化させる方法が特許文献３に、ＳｅとＳの組成比を変化させる方法が特許文献４や特許文献５に開示されている。
特開平１１−１６３３７６号公報 M. A. Contreras， J. Tuttle， A. Gabor， A. Tarrant， K. Ramanathan， S. Asher， A. Franz， J. Kean， L. Wang， R. Noufi， "High efficiency graded bandgap thin-film polycrystalline Cu（In，Ga） Se2-based solar cells"， Solar Energy Materials & Solar Cells， vol. 41／42， p.231 （1996）. 特開平１１−２７４５２６号公報 特開平９−２１３９７７号公報 特開平１０−１３５４９８号公報 特表平１０−５１３６０６号公報 特開平１０−１３５４９５号公報 The band gap of the compound semiconductor thin film composed of the 11th, 13th and 16th group elements is controlled not only by In and Ga but also by, for example, the composition ratio of the 13th group element Al and In or the 16th group element Se and S Ratio can also be done. In order to form a double graded band gap, a method of changing the composition ratio of Al and In is disclosed in Patent Document 3, and a method of changing the composition ratio of Se and S is disclosed in Patent Document 4 and Patent Document 5. .
JP-A-11-163376 MA Contreras, J. Tuttle, A. Gabor, A. Tarrant, K. Ramanathan, S. Asher, A. Franz, J. Kean, L. Wang, R. Noufi, "High efficiency graded bandgap thin-film coated Cu ( In, Ga) Se2-based solar cells ", Solar Energy Materials & Solar Cells, vol. 41/42, p.231 (1996). JP-A-11-274526 Japanese Patent Laid-Open No. 9-213977 JP-A-10-135498 Japanese National Patent Publication No. 10-513606 JP-A-10-135495

ダブルグレーデッドバンドギャップは、カルコパイライト構造半導体を用いた薄膜太陽電池の高効率化に有効である。非特許文献２で開示されているように、好適なダブルグレーデッドバンドギャップを形成するには、Ｇａ濃度が高いバンドギャップ拡大層の厚さをなるべく小さくことが望ましい。例えば、特許文献６で開示されているように、初めの第１段階でＩｎとＧａとＳｅを蒸着し、（Ｉｎ，Ｇａ）2Ｓｅ3膜を形成した後に、次の第２段階でＣｕとＳｅを蒸着してＣｕ過剰のＣＩＧＳ膜の形成し、最後の第３段階でさらにＩｎとＧａとＳｅを蒸着してＣｕ不足のＣＩＧＳ膜を形成する方法によれば、第１段階と第３段階のＩｎとＧａの蒸着量の制御でバンドギャップ拡大層を形成することができる。 The double graded band gap is effective for increasing the efficiency of a thin film solar cell using a chalcopyrite structure semiconductor. As disclosed in Non-Patent Document 2, in order to form a suitable double graded band gap, it is desirable to reduce the thickness of the band gap expanding layer having a high Ga concentration as much as possible. For example, as disclosed in Patent Document 6, after vapor-depositing In, Ga, and Se in the first first stage to form an (In, Ga) 2 Se 3 film, Cu and in the next second stage. According to the method of depositing Se to form a Cu-excess CIGS film, and further depositing In, Ga and Se in the final third stage to form a Cu-deficient CIGS film, the first stage and the third stage The band gap expansion layer can be formed by controlling the deposition amounts of In and Ga.

しかしながら、上記方法によれば、第３段階でＣｕ過剰のＣＩＧＳ膜にＩｎとＧａを拡散させるため、Ｇａ濃度が高いバンドギャップ拡大層の厚さ（形成深さ）を常に制御することは容易ではない。特に、太陽電池の生産量を増加するためにＣＩＧＳ膜の成膜速度を高速化する場合は、バンドギャップ拡大層の厚さを制御することは困難である。一例として、約１μｍ／分で作製したＣＩＧＳ膜の深さ方向に関するＧａ／（Ｉｎ＋Ｇａ）比の分布を図１に示す。図１のＣＩＧＳ膜は、特許文献６で開示された方法とは異なるが、ＩｎとＧａの蒸着量を変化させてＧａ／（Ｉｎ＋Ｇａ）比を変化させている。図１から明らかなように、表面からＧａ／（Ｉｎ＋Ｇａ）比が徐々に低下し、深さ約１μｍでＧａ／（Ｉｎ＋Ｇａ）比が最小となっている。そして、１μｍ以上の深さから裏面に向けてＧａ／（Ｉｎ＋Ｇａ）比が増加する分布となっている。このように、図１に示すＣＩＧＳ膜では、バンドギャップ拡大層の厚さは約１μｍにも達し、蒸着法によるＣＩＧＳ膜の高速形成では、表面のごく近傍にのみバンドギャップ拡大層を設けることが困難であることがわかる。   However, according to the above method, since In and Ga are diffused in the Cu-excess CIGS film in the third stage, it is not easy to always control the thickness (formation depth) of the band gap expansion layer having a high Ga concentration. Absent. In particular, when increasing the deposition rate of the CIGS film in order to increase the production amount of the solar cell, it is difficult to control the thickness of the band gap widening layer. As an example, FIG. 1 shows a Ga / (In + Ga) ratio distribution in the depth direction of a CIGS film manufactured at about 1 μm / min. The CIGS film in FIG. 1 is different from the method disclosed in Patent Document 6, but the Ga / (In + Ga) ratio is changed by changing the deposition amounts of In and Ga. As is clear from FIG. 1, the Ga / (In + Ga) ratio gradually decreases from the surface, and the Ga / (In + Ga) ratio is minimum at a depth of about 1 μm. And it has distribution which Ga / (In + Ga) ratio increases toward the back surface from the depth of 1 micrometer or more. As described above, in the CIGS film shown in FIG. 1, the thickness of the band gap expansion layer reaches about 1 μm, and in the high-speed formation of the CIGS film by the vapor deposition method, the band gap expansion layer can be provided only in the very vicinity of the surface. It turns out to be difficult.

一方、特許文献７は、ダブルグレーデッドバンドギャップを有するＣｕ（Ｉｎ，Ｇａ）（Ｓｅ，Ｓ）2膜の形成方法について開示している。この方法では、まず、ＣｕとＩｎとＧａからなる金属膜をＨ2Ｓｅガスで焼成することによりＣｕ（Ｉｎ，Ｇａ）Ｓｅ2膜を形成する。得られたＣｕ（Ｉｎ，Ｇａ）Ｓｅ2膜をＨ2Ｓガスでさらに焼成すれば、表面にＳの混入したバンドギャップの広いＣｕ（Ｉｎ，Ｇａ）（Ｓｅ，Ｓ）2層を形成できる。ただし、この方法は、焼成後の冷却過程でＨ2Ｓ雰囲気が残留しているため、Ｃｕ（Ｉｎ，Ｇａ）（Ｓｅ，Ｓ）2の意図しない形成反応が進み、好適なダブルグレーデッドバンドギャップの再現性が悪い。 On the other hand, Patent Document 7 discloses a method for forming a Cu (In, Ga) (Se, S) 2 film having a double graded band gap. In this method, first, a Cu (In, Ga) Se 2 film is formed by baking a metal film made of Cu, In, and Ga with H 2 Se gas. If the obtained Cu (In, Ga) Se 2 film is further baked with H 2 S gas, a Cu (In, Ga) (Se, S) 2 layer having a wide band gap mixed with S on the surface can be formed. However, in this method, since the H 2 S atmosphere remains in the cooling process after firing, an unintended formation reaction of Cu (In, Ga) (Se, S) 2 proceeds, and a suitable double graded band gap is achieved. The reproducibility of is poor.

また、特許文献３は、ダブルグレーデッドバンドギャップを有するＣｕ（Ｉｎ，Ａｌ）Ｓｅ2膜（ＣＩＡＳ膜）の形成方法について開示している。特許文献３によれば、初めにＡｌＳｅ膜を形成し、そのＡｌＳｅ膜上にＩｎＳｅ膜を蒸着により形成した後、Ｃｕを蒸着して加熱しＣｕ過剰のＣＩＡＳ膜を形成する。その後さらに、ＡｌＳｅ膜を蒸着により形成して熱処理することによりＣｕ不足のＣＩＡＳ膜を作製する。ＡｌはＧａに比べ拡散速度が遅いことから、表層部にＡｌ濃度の高いバンドギャップ拡大層を形成できると記述している。しかしながら、この方法によれば、光入射側のみならず裏面側でもＡｌの偏析が起こってしまうので、ＣＩＡＳ膜内部にはＡｌがほとんど拡散せず、実際には、ほぼ均一なバンドギャップ分布となる。従って、バンドギャップが半導体薄膜内で連続的、かつなだらかに変化するグレーデッドバンドギャップ分布による光励起されたキャリアの輸送効果はほとんど得られない。 Patent Document 3 discloses a method for forming a Cu (In, Al) Se 2 film (CIAS film) having a double graded band gap. According to Patent Document 3, an AlSe film is first formed, an InSe film is formed on the AlSe film by vapor deposition, and then Cu is vapor deposited and heated to form a Cu excess CIAS film. Thereafter, an AlSe film is further formed by vapor deposition and heat treatment is performed to produce a Cu-deficient CIAS film. Since Al has a slower diffusion rate than Ga, it is described that a bandgap expansion layer having a high Al concentration can be formed in the surface layer portion. However, according to this method, since Al segregation occurs not only on the light incident side but also on the back surface side, Al hardly diffuses inside the CIAS film, and in practice, a substantially uniform band gap distribution is obtained. . Therefore, the effect of transporting the photoexcited carriers by the graded band gap distribution in which the band gap changes continuously and gently in the semiconductor thin film is hardly obtained.

また、以上に説明した例において、好適なダブルグレーデッドバンドギャップを形成できる元素の濃度や組成比は、必ずしも明らかとなっていない。   Moreover, in the example demonstrated above, the density | concentration and composition ratio of an element which can form a suitable double graded band gap are not necessarily clear.

こうした事情に鑑み、本発明は、エネルギー変換効率の向上に理想的なバンド構造を有する薄膜太陽電池を提供することを目的とする。   In view of such circumstances, an object of the present invention is to provide a thin-film solar cell having an ideal band structure for improving energy conversion efficiency.

すなわち、本発明は、
ＧａおよびＩｎからなる群より選ばれる少なくとも１つの元素と、Ｓ、ＳｅおよびＴｅからなる群より選ばれる少なくとも１つの元素と、Ｃｕと、Ａｌとを含みカルコパイライト構造である半導体薄膜を光吸収層として備え、
半導体薄膜は、第１の半導体層と、第１の半導体層よりも光入射側に配置された第２の半導体層とを含み、
第１の半導体層は、第２の半導体層に近づくにつれてバンドギャップが小さくなっており、
第２の半導体層は、半導体薄膜内においてＡｌ濃度の最も大きい領域を含むとともに、第１の半導体層における最小のバンドギャップよりも大きいバンドギャップを有する、薄膜太陽電池を提供する。 That is, the present invention
A semiconductor thin film having a chalcopyrite structure including at least one element selected from the group consisting of Ga and In, at least one element selected from the group consisting of S, Se, and Te, Cu, and Al. Prepared as
The semiconductor thin film includes a first semiconductor layer and a second semiconductor layer disposed closer to the light incident side than the first semiconductor layer,
The first semiconductor layer has a smaller band gap as it approaches the second semiconductor layer,
The second semiconductor layer provides a thin film solar cell including a region having the highest Al concentration in the semiconductor thin film and having a band gap larger than a minimum band gap in the first semiconductor layer.

上記本発明によれば、第２の半導体層がバンドギャップ拡大層として機能し、第１の半導体層のバンドギャップが第２の半導体層に近づくにつれて小さくなっている。つまり、第１の半導体層と第２の半導体層とにより、グレーデッドバンドギャップが形成されている。さらに、第２の半導体層は、Ａｌ濃度の最も大きい領域を含むとともに、第１の半導体層における最小のバンドギャップよりも大きいバンドギャップを有する。こうして、理想的なダブルグレーデッドバンドギャップが形成され、ｐｎ接合へのキャリアの輸送効果が高まる。   According to the present invention, the second semiconductor layer functions as a band gap expanding layer, and the band gap of the first semiconductor layer becomes smaller as it approaches the second semiconductor layer. That is, a graded band gap is formed by the first semiconductor layer and the second semiconductor layer. Further, the second semiconductor layer includes a region having the highest Al concentration and has a band gap larger than the minimum band gap in the first semiconductor layer. Thus, an ideal double graded band gap is formed, and the effect of transporting carriers to the pn junction is enhanced.

以下、１６族元素としてＳｅを用いたＣｕ（Ｉｎ，Ｇａ，Ａｌ）Ｓｅ2膜（ＣＩＧＡＳ膜）を例に、発明の効果をさらに説明する。なお、１６族元素として、ＳやＴｅあるいはＳとＳｅの２元素を用いても同様な効果が得られる。 Hereinafter, the effect of the invention will be further described by taking a Cu (In, Ga, Al) Se 2 film (CIGAS film) using Se as a group 16 element as an example. Similar effects can be obtained by using S, Te, or two elements of S and Se as group 16 elements.

本発明の薄膜太陽電池によれば、光吸収層の表層部がＡｌを含有する。Ｃｕ−Ａｌ−Ｓｅ、Ｃｕ−Ｇａ−Ｓｅ、Ｃｕ−Ｉｎ−Ｓｅの系では、二元化合物、例えばＣｕ2Ｓｅ、Ａｌ2Ｓｅ3、Ｇａ2Ｓｅ3およびＩｎ2Ｓｅ3と、三元化合物であるＣｕＡｌＳｅ2、ＣｕＧａＳｅ2およびＣｕＩｎＳｅ2が生成される。ここで、二元化合物であるＡｌ2Ｓｅ3、Ｇａ2Ｓｅ3およびＩｎ2Ｓｅ3と、三元化合物であるＣｕＡｌＳｅ2、ＣｕＧａＳｅ2およびＣｕＩｎＳｅ2との生成エンタルピーを比較する。 According to the thin film solar cell of the present invention, the surface layer portion of the light absorption layer contains Al. In the system of Cu—Al—Se, Cu—Ga—Se, Cu—In—Se, binary compounds such as Cu 2 Se, Al 2 Se 3 , Ga 2 Se 3 and In 2 Se 3 and ternary compounds Certain CuAlSe 2 , CuGaSe 2 and CuInSe 2 are produced. Here, the formation enthalpies of the binary compounds Al 2 Se 3 , Ga 2 Se 3 and In 2 Se 3 and the ternary compounds CuAlSe 2 , CuGaSe 2 and CuInSe 2 are compared.

Ｃｕ−Ａｌ−Ｓｅ系では、Ａｌ2Ｓｅ3とＣｕＡｌＳｅ2のＡｌ１モルを標準とした生成エンタルピーは、それぞれ、２８２ｋＪ／ｍｏｌと２３２ｋＪ／ｍｏｌである。Ｃｕ−Ｇａ−Ｓｅ系では、Ｇａ2Ｓｅ3とＣｕＧａＳｅ2のＧａ１モルを標準とした生成エンタルピーは、それぞれ、２１９ｋＪ／ｍｏｌと２７５ｋＪ／ｍｏｌである。Ｃｕ−Ｉｎ−Ｓｅ系では、Ｉｎ2Ｓｅ3とＣｕＩｎＳｅ2のＩｎ１モルを標準とした生成エンタルピーは、それぞれ、１７１ｋＪ／ｍｏｌと２８０ｋＪ／ｍｏｌである。 In the Cu—Al—Se system, the generation enthalpies of Al 2 Se 3 and CuAlSe 2 based on Al 1 mol as standard are 282 kJ / mol and 232 kJ / mol, respectively. In the Cu—Ga—Se system, the generation enthalpies with Ga 2 Se 3 and CuGaSe 2 as standard are 219 kJ / mol and 275 kJ / mol, respectively. In the Cu—In—Se system, the enthalpies of formation based on In1Se of In 2 Se 3 and CuInSe 2 are 171 kJ / mol and 280 kJ / mol, respectively.

従って、Ｃｕ−Ａｌ−Ｓｅ系では、二元化合物Ａｌ2Ｓｅ3の方が生成されやすくなる。また、Ｃｕ−Ｇａ−Ｓｅ系とＣｕ−Ｉｎ−Ｓｅ系では三元化合物が生成されやすいが、エンタルピーの差からＣｕＩｎＳｅ2がより生成されやすい。従って、Ｃｕ−Ｉｎ−Ｇａ−Ａｌ−Ｓｅの系の反応では、Ａｌ2Ｓｅ3とＧａ2Ｓｅ3とＣｕＧａＳｅ2とＣｕＩｎＳｅ2が生成され、その後、Ｃｕの拡散でＡｌ2Ｓｅ3とＣｕ（Ｉｎ，Ｇａ）Ｓｅ2に反応が進み、最後にＣｕ（Ｉｎ，Ｇａ，Ａｌ）Ｓｅ2の混晶化合物になると考えられる。最後にＡｌ2Ｓｅ3とＣｕ（Ｉｎ，Ｇａ）Ｓｅ2が反応するため、Ａｌの偏析した層（第２の半導体層：バンドギャップ拡大層）が形成される。 Therefore, in the Cu—Al—Se system, the binary compound Al 2 Se 3 is more likely to be generated. Although easy ternary compound is produced in the Cu-Ga-Se system and the Cu-In-Se system, the CuInSe 2 from the difference in enthalpy is more generated easily. Therefore, in the reaction of the Cu—In—Ga—Al—Se system, Al 2 Se 3 , Ga 2 Se 3 , CuGaSe 2 and CuInSe 2 are generated, and then Al 2 Se 3 and Cu (In , Ga) Se 2 is considered to be a mixed crystal compound of Cu (In, Ga, Al) Se 2 . Finally, Al 2 Se 3 and Cu (In, Ga) Se 2 react to form an Al segregated layer (second semiconductor layer: bandgap expansion layer).

例えば、ＣｕＡｌＳｅ2のバンドギャップは２．７ｅＶであり、ＣｕＧａＳｅ2のバンドギャップ１．６ｅＶやＣｕＩｎＳｅ2のバンドギャップ１．０１ｅＶより広いので、上記した３つの化合物の混晶系において、薄膜太陽電池に好適なバンドギャップ拡大層を光入射側に形成することができる。 For example, the band gap of CuAlSe 2 is 2.7 eV, since wider than the band gap 1.01eV bandgap 1.6eV and CuInSe 2 of CuGaSe 2, the mixed crystal of the three compounds described above, the thin-film solar cell A suitable band gap expansion layer can be formed on the light incident side.

ＣＩＧＡＳ膜の表面から深さ方向の組成比分布の一例を図２に示す。本発明の薄膜太陽電池の構成によれば、ＣＩＧＡＳ膜上にＣＩＧＡＳ膜とは導電型の異なる半導体層を積層してｐｎ接合を形成することができる。ｐｎ接合に隣接してバンドギャップの大きいＣＩＡＳ層が形成されることにより、ｐｎ接合での漏れ電流あるいは逆飽和電流が低減し、太陽電池の開放端電圧が増加する。   An example of the composition ratio distribution in the depth direction from the surface of the CIGAS film is shown in FIG. According to the configuration of the thin-film solar cell of the present invention, a pn junction can be formed by stacking a semiconductor layer having a conductivity type different from that of the CIGAS film on the CIGAS film. By forming a CIAS layer having a large band gap adjacent to the pn junction, leakage current or reverse saturation current at the pn junction is reduced, and the open-circuit voltage of the solar cell is increased.

以下、本発明の実施の形態について図面を参照しながら説明する。   Hereinafter, embodiments of the present invention will be described with reference to the drawings.

図３は、本発明にかかる薄膜太陽電池の一例の断面模式図である。薄膜太陽電池１０は、基板１１、裏面電極層１２、第１の半導体層１３ａ、第２の半導体層１３ｂ、窓層１４、透明導電膜１５および取り出し電極１６を備えている。各要素は、第１の半導体層１３ａが裏面電極層１２に隣接し、第２の半導体層１３ｂが窓層１４に隣接するように、上記の順番で積層されている。第１の半導体層１３ａおよび第２の半導体層１３ｂは、１１族元素であるＣｕと、１３族元素であるＡｌ、ＧａおよびＩｎと、Ｓ、ＳｅおよびＴｅからなる１６族元素群より選ばれる少なくとも１種とからなるカルコパイライト構造半導体薄膜からなり、光吸収層として機能する。第１の半導体層１３ａと第２の半導体層１３ｂとは直接積層されており、これら半導体層１３ａ，１３ｂの導電型は、いずれもｐ型である。窓層１４は、光吸収層とは反対の導電型であるｎ型半導体薄膜からなる。窓層１４と第２の半導体層１３ｂとの界面にｐｎ接合が形成される。   FIG. 3 is a schematic cross-sectional view of an example of a thin film solar cell according to the present invention. The thin film solar cell 10 includes a substrate 11, a back electrode layer 12, a first semiconductor layer 13 a, a second semiconductor layer 13 b, a window layer 14, a transparent conductive film 15, and an extraction electrode 16. The elements are stacked in the above order so that the first semiconductor layer 13 a is adjacent to the back electrode layer 12 and the second semiconductor layer 13 b is adjacent to the window layer 14. The first semiconductor layer 13a and the second semiconductor layer 13b are at least selected from the group 11 element group consisting of Cu which is a group 11 element, Al, Ga and In which are group 13 elements, and S, Se and Te. It consists of a chalcopyrite structure semiconductor thin film composed of one kind and functions as a light absorption layer. The first semiconductor layer 13a and the second semiconductor layer 13b are directly stacked, and the conductivity types of these semiconductor layers 13a and 13b are both p-type. The window layer 14 is made of an n-type semiconductor thin film having a conductivity type opposite to that of the light absorption layer. A pn junction is formed at the interface between the window layer 14 and the second semiconductor layer 13b.

第１の半導体層１３ａは、光入射側とは反対側の裏面から第２の半導体層１３ｂに近づくにつれてＧａが濃化することにより、グレーデッドバンドギャップが形成されている。第２の半導体層１３ｂのＡｌ濃度は、第２の半導体層１３ｂのバンドギャップが、第１の半導体層１３ａの表面（光入射側の主面）におけるバンドギャップよりも大きくなるように調整されている。すなわち、第１の半導体層１３ａと第２の半導体層１３ｂとにより、いわゆるダブルグレーデッドバンドギャップが形成されている。言い換えれば、薄膜太陽電池１０の光吸収層は、Ａｌ濃度の最も大きい領域を含むＡｌ濃化層（第２の半導体層１３ｂ）を光入射側の表層部に有し、かつ光入射側とは反対からＡｌ濃化層（第２の半導体層１３ｂ）に近づくにつれてバンドギャップが小さくなっている。   The first semiconductor layer 13a has a graded band gap as Ga is concentrated from the back surface opposite to the light incident side toward the second semiconductor layer 13b. The Al concentration of the second semiconductor layer 13b is adjusted so that the band gap of the second semiconductor layer 13b is larger than the band gap on the surface (main surface on the light incident side) of the first semiconductor layer 13a. Yes. That is, a so-called double graded band gap is formed by the first semiconductor layer 13a and the second semiconductor layer 13b. In other words, the light absorption layer of the thin-film solar cell 10 has an Al concentrated layer (second semiconductor layer 13b) including a region with the highest Al concentration in the surface layer portion on the light incident side, and what is the light incident side? From the opposite side, the band gap becomes smaller as it approaches the Al concentrated layer (second semiconductor layer 13b).

基板１１としては、例えば、ガラスや金属板やポリイミド樹脂を用いることができる。金属板としては、ステンレス、Ｔｉ、Ｃｒ等を用いることができる。また、ステンレス、Ｔｉ等の金属板の上にガラス等の絶縁体を被覆した基板を用いることもできる。   As the substrate 11, for example, glass, a metal plate, or a polyimide resin can be used. As the metal plate, stainless steel, Ti, Cr or the like can be used. A substrate in which an insulator such as glass is coated on a metal plate such as stainless steel or Ti can also be used.

裏面電極層１２としては、金属膜を用いることができ、例えば、Ｍｏ膜を用いることができる。また、例えば、タンデム太陽電池の上部太陽電池のように、光吸収層となる半導体層で吸収されない長波長の太陽光を透過する必要がある場合は、透光性の導電層を用いてもよい。例えば、後述する透明導電膜１５に好適な透明導電体を用いることができる。   As the back electrode layer 12, a metal film can be used, for example, a Mo film can be used. In addition, for example, when it is necessary to transmit sunlight having a long wavelength that is not absorbed by the semiconductor layer serving as the light absorption layer, such as an upper solar cell of a tandem solar cell, a light-transmitting conductive layer may be used. . For example, a transparent conductor suitable for the transparent conductive film 15 described later can be used.

第１の半導体層１３ａとしては、実質的にＡｌを含有しないカルコパイライト構造半導体、例えば、ＣｕＩｎＳｅ2（ＣＩＳ）、ＣＩＳのＩｎをＧａで一部置換したＣｕ（Ｉｎ，Ｇａ）Ｓｅ2（ＣＩＧＳ）、ＣＩＧＳのＳｅをＳで一部置換したＣｕＩｎ（Ｓｅ，Ｓ）2（ＣＩＳＳ）、Ｃｕ（Ｉｎ，Ｇａ）（Ｓｅ，Ｓ）2（ＣＩＧＳＳ）またはＣｕＩｎＳ2を用いることができる。また、例えば、ＣＩＧＳにおいては、裏面電極層１２の近傍でＧａ／（Ｉｎ＋Ｇａ）比が高く、第２の半導体層１３ｂに向かうにつれてＧａ／（Ｉｎ＋Ｇａ）比が徐々に小さくなる傾斜組成であることが好ましい。同様に、ＣＩＳＳにおいては、裏面電極層１２の近傍でＳ／（Ｓｅ＋Ｓ）比が高く、第２の半導体層１３ｂに向かうにつれてＳ／（Ｓｅ＋Ｓ）比が徐々に小さくなる傾斜組成であることが好ましい。 As the first semiconductor layer 13a, a chalcopyrite structure semiconductor that does not substantially contain Al, for example, CuInSe 2 (CIS), Cu (In, Ga) Se 2 (CIGS) in which CIS is partially substituted with Ga. In addition, CuIn (Se, S) 2 (CISS), Cu (In, Ga) (Se, S) 2 (CIGSS), or CuInS 2 in which CIGS Se is partially substituted with S can be used. For example, in CIGS, the Ga / (In + Ga) ratio is high in the vicinity of the back electrode layer 12, and the Ga / (In + Ga) ratio gradually decreases as it goes toward the second semiconductor layer 13b. preferable. Similarly, in the CISS, it is preferable that the S / (Se + S) ratio is high in the vicinity of the back electrode layer 12 and the S / (Se + S) ratio is gradually decreased toward the second semiconductor layer 13b. .

すなわち、第１の半導体層１３ａは、下記条件（１）および／または（２）を満足することが好ましい。そのような構成によれば、Ａｌ濃化によりバンドギャップが拡大された第２の半導体層１３ｂと相俟って、好適なダブルグレーデッドバンドギャップを形成できるため、薄膜太陽電池の漏れ電流が低減し、ひいては変換効率が向上する。
（１）ＧａとＩｎを必須成分とし、かつ、第２の半導体層１３ｂとの界面において、ＧａとＩｎの組成比であるＧａ／（Ｉｎ＋Ｇａ）が最小となる。
（２）ＳとＳｅを必須成分とし、かつ、第２の半導体層１３ｂとの界面において、ＳとＳｅの組成比であるＳ／（Ｓｅ＋Ｓ）が最小となる。 That is, the first semiconductor layer 13a preferably satisfies the following conditions (1) and / or (2). According to such a configuration, a suitable double graded band gap can be formed in combination with the second semiconductor layer 13b whose band gap has been expanded by Al concentration, so that the leakage current of the thin film solar cell is reduced. As a result, the conversion efficiency is improved.
(1) Ga / In is an essential component, and Ga / (In + Ga), which is the composition ratio of Ga and In, is minimized at the interface with the second semiconductor layer 13b.
(2) S / Se is an essential component, and S / (Se + S), which is the composition ratio of S and Se, is minimized at the interface with the second semiconductor layer 13b.

また、第１の半導体層１３ａは、第２の半導体層１３ｂとの界面をなす表面から裏面に向けてＧａ／（Ｉｎ＋Ｇａ）および／またはＳ／（Ｓｅ＋Ｓ）が連続的に増加していることが好ましい。もちろん、Ｇａ／（Ｉｎ＋Ｇａ）および／またはＳ／（Ｓｅ＋Ｓ）を段階的に変化させる構成としてもよいが、連続的な組成の変化により、好適なグレーデッドバンドギャップが形成される。   In the first semiconductor layer 13a, Ga / (In + Ga) and / or S / (Se + S) continuously increase from the front surface forming the interface with the second semiconductor layer 13b toward the back surface. preferable. Of course, Ga / (In + Ga) and / or S / (Se + S) may be changed stepwise, but a suitable graded band gap is formed by a continuous composition change.

なお、実質的に含有しないとは、バンドギャップの変化に寄与しない含有率であることを意味する。例えば、Ａｌの含有率１原子％以下あるいはＡｌ／（Ｉｎ＋Ｇａ＋Ａｌ）比が０.０５以下であれば、Ａｌを含有していないＣＩＧＳとＡｌをわずかに含有したＣＩＧＳのバンドギャップはほぼ同一である。   In addition, that it does not contain substantially means that it is the content rate which does not contribute to the change of a band gap. For example, when the Al content is 1 atomic% or less or the Al / (In + Ga + Al) ratio is 0.05 or less, the band gaps of CIGS not containing Al and CIGS containing a little Al are almost the same.

光入射側に位置する第２の半導体層１３ｂとしては、例えば、Ｃｕ（Ｉｎ，Ａｌ）Ｓｅ2（ＣＩＡＳ）、Ｃｕ（Ｉｎ，Ｇａ，Ａｌ）Ｓｅ2（ＣＩＧＡＳ）、Ｃｕ（Ｉｎ，Ａｌ）Ｓ2、Ｃｕ（Ｉｎ，Ｇａ，Ａｌ）Ｓ2、Ｃｕ（Ｉｎ，Ａｌ）（Ｓｅ，Ｓ）2（ＣＩＡＳＳ）またはＣｕ（Ｉｎ，Ｇａ，Ａｌ）（Ｓｅ，Ｓ）2（ＣＩＧＡＳＳ）を用いることができる。 As the second semiconductor layer 13b positioned on the light incident side, for example, Cu (In, Al) Se 2 (CIAS), Cu (In, Ga, Al) Se 2 (CIGAS), Cu (In, Al) S are used. 2, Cu (In, Ga, Al) S 2, Cu (In, Al) (Se, S) 2 (CIASS) or Cu (In, Ga, Al) (Se, S) be used 2 (CIGASS) it can.

第２の半導体層１３ｂの厚さは０．２μｍ以下が好適である（より好ましくは０．０５μｍ以上０．２μｍ以下）。カルコパイライト構造半導体を用いた薄膜太陽電池において、光吸収層に生ずる空乏層領域は、通常、ｐｎ接合から０．２μｍの範囲内である。従って、バンドギャップ拡大層である第２の半導体層１３ｂの厚さをそれ以下とすることが、ｐｎ接合での漏れ電流または逆飽和電流の低減に有効である。   The thickness of the second semiconductor layer 13b is preferably 0.2 μm or less (more preferably 0.05 μm or more and 0.2 μm or less). In a thin film solar cell using a chalcopyrite structure semiconductor, a depletion layer region generated in a light absorption layer is usually within a range of 0.2 μm from a pn junction. Therefore, it is effective to reduce the leakage current or reverse saturation current at the pn junction to make the thickness of the second semiconductor layer 13b, which is the band gap expansion layer, smaller than that.

窓層１４は、光吸収層である第１および第２の半導体層１３ａ，１３ｂとは異なる導電型の半導体、つまり、ｎ型半導体からなる。具体的には、例えば、ＣｄＳ、ＺｎＯ、Ｚｎ（Ｏ，Ｓ）、Ｚｎ1-xＭｇxＯ（０＜ｘ＜１）等を用いることができる。また、これらの半導体薄膜の積層膜、例えば、ＣｄＳの上にＺｎＯを積層した積層膜を窓層１４に用いてもよい。 The window layer 14 is made of a semiconductor having a different conductivity type from the first and second semiconductor layers 13a and 13b, which are light absorption layers, that is, an n-type semiconductor. Specifically, for example, CdS, ZnO, Zn (O, S), Zn 1-x Mg x O (0 <x <1), or the like can be used. Further, a laminated film of these semiconductor thin films, for example, a laminated film in which ZnO is laminated on CdS may be used for the window layer 14.

太陽光を透過し、励起されたキャリアを収集する透明導電膜１５としては、近紫外域〜近赤外域で透光性を有し、かつ導電性を有する材料からなる薄膜を用いることができる。具体的には、例えば、透光性ＩＸＯ（Ｘ添加Ｉｎ2Ｏ3，Ｘとして、Ｓｎ，Ｍｎ，Ｍｏ，Ｔｉ，Ｚｎ）、Ｆ添加ＳｎＯ2、Ａｌ添加ＺｎＯ、Ｇａ添加ＺｎＯ等を用いることができる。また、上記材料の複層膜を用いてもよい。 As the transparent conductive film 15 that transmits sunlight and collects excited carriers, a thin film made of a material having translucency in the near ultraviolet region to the near infrared region and having conductivity can be used. Specifically, for example, translucent IXO (X-added In 2 O 3 , X is Sn, Mn, Mo, Ti, Zn), F-added SnO 2 , Al-added ZnO, Ga-added ZnO, or the like is used. it can. Alternatively, a multilayer film of the above material may be used.

取り出し電極１６としては、例えば、Ａｌ、Ａｇ、Ａｕを用いることができる。また、透明導電膜１５との密着性を向上させるために、ＡｌとＣｒ、ＡｌとＮｉ、ＡｌとＮｉＣｒ等の複層の金属膜を用いてもよい。   As the extraction electrode 16, for example, Al, Ag, or Au can be used. In order to improve the adhesion to the transparent conductive film 15, a multilayer metal film such as Al and Cr, Al and Ni, or Al and NiCr may be used.

一例として、基板１１としてガラスを用い、裏面電極層１２としてＭｏを用い、第１の半導体層１３ａとしてＣＩＧＳを用い、第２の半導体層１３ｂとしてＣＩＡＳを用い、窓層１４としてＣｄＳとＺｎＯの積層膜を用い、透明導電膜１５としてＡｌ添加ＺｎＯを用いた薄膜太陽電池の特性の数値シミュレーションを行った。第１の半導体層１３ａをなすＣＩＧＳ層は、図２に示すように、裏面電極層１２の近傍のＧａ／（Ｉｎ＋Ｇａ）比が高く、裏面側から第２の半導体層１３ｂに向けてＧａ／（Ｉｎ＋Ｇａ）比が傾き一定で単調減少する傾斜組成を有している。また、ＣＩＡＳ層中（第２の半導体層１３ｂ）でのＡｌ／（Ｉｎ＋Ａｌ）比は表面からの深さによらず一定であると仮定した。実際の製造では、Ａｌが偏析するので、Ａｌの濃度は第１の半導体層と第２の半導体層との境界で急峻に変化する。従って、Ａｌ濃度が第２の半導体層中で一定とするモデルには妥当性がある。   As an example, glass is used as the substrate 11, Mo is used as the back electrode layer 12, CIGS is used as the first semiconductor layer 13a, CIAS is used as the second semiconductor layer 13b, and CdS and ZnO are stacked as the window layer 14. The film was used, and the numerical simulation of the characteristics of the thin film solar cell using Al-added ZnO as the transparent conductive film 15 was performed. As shown in FIG. 2, the CIGS layer forming the first semiconductor layer 13a has a high Ga / (In + Ga) ratio in the vicinity of the back electrode layer 12, and the Ga / (Ga / (from the back side toward the second semiconductor layer 13b). (In + Ga) ratio has a gradient composition in which the gradient is constant and monotonously decreases. In addition, the Al / (In + Al) ratio in the CIAS layer (second semiconductor layer 13b) was assumed to be constant regardless of the depth from the surface. In actual manufacturing, since Al is segregated, the concentration of Al changes sharply at the boundary between the first semiconductor layer and the second semiconductor layer. Therefore, a model in which the Al concentration is constant in the second semiconductor layer is valid.

図４は、ＣＩＡＳ層の表面（光入射側の面）におけるＡｌ／（Ｉｎ＋Ａｌ）比とＣＩＧＳ層の裏面（裏面電極層１２側の面）におけるＧａ／（Ｉｎ＋Ｇａ）比に対する変換効率の等高線図である。本明細書では、ＣＩＡＳ層の表面におけるＡｌ／（Ｉｎ＋Ａｌ）比を表面Ａｌ／（Ｉｎ＋Ａｌ）比または単にＡｌ／（Ｉｎ＋Ａｌ）比という。また、ＣＩＧＳ層の裏面におけるＧａ／（Ｉｎ＋Ｇａ）比を裏面Ｇａ／（Ｉｎ＋Ｇａ）比または単にＧａ／（Ｉｎ＋Ｇａ）比という。   FIG. 4 is a contour map of the conversion efficiency with respect to the Al / (In + Al) ratio on the surface of the CIAS layer (surface on the light incident side) and the Ga / (In + Ga) ratio on the back surface of the CIGS layer (surface on the back electrode layer 12 side). is there. In this specification, the Al / (In + Al) ratio on the surface of the CIAS layer is referred to as a surface Al / (In + Al) ratio or simply an Al / (In + Al) ratio. The Ga / (In + Ga) ratio on the back surface of the CIGS layer is referred to as a back surface Ga / (In + Ga) ratio or simply a Ga / (In + Ga) ratio.

薄膜太陽電池の特性は、図２において、ＣＩＡＳ層の厚さを０．１μｍ、ＣＩＧＳ層で最小となるＧａ／（Ｉｎ＋Ｇａ）比を０．２に固定して計算した。図４中に示す直線は、下記（式１）を満足する。
１．０７ｘ＋０．６２ｘ2＝０．４２ｙ＋０．２４ｙ2 ・・・（式１） The characteristics of the thin film solar cell were calculated by fixing the CIAS layer thickness to 0.1 μm and the minimum Ga / (In + Ga) ratio to 0.2 in the CIGS layer in FIG. The straight line shown in FIG. 4 satisfies the following (Formula 1).
1.07x + 0.62x 2 = 0.42y + 0.24y 2 (Formula 1)

ここで、ｘとｙは、それぞれ、ＣＩＡＳ層の表面Ａｌ／（Ｉｎ＋Ａｌ）比とＣＩＧＳ層の裏面Ｇａ／（Ｉｎ＋Ｇａ）比を表す。図４から明らかなように、直線より上側の領域で高い変換効率が得られる。すなわち、組成比が上記（式１）の（左辺）≦（右辺）を満足し、かつ裏面電極層１２の近傍でのＣＩＧＳ層のバンドギャップがＣＩＡＳ層のバンドギャップより大きくなる場合に高い変換効率が得られる。具体的には、ＣＩＧＳ層の裏面でのバンドギャップがＣＩＡＳ層の表面でのバンドギャップよりも大きい場合を示せる。   Here, x and y represent the surface Al / (In + Al) ratio of the CIAS layer and the back surface Ga / (In + Ga) ratio of the CIGS layer, respectively. As is clear from FIG. 4, high conversion efficiency can be obtained in the region above the straight line. That is, when the composition ratio satisfies (left side) ≦ (right side) of the above (Formula 1) and the band gap of the CIGS layer in the vicinity of the back electrode layer 12 is larger than the band gap of the CIAS layer, high conversion efficiency Is obtained. Specifically, the case where the band gap on the back surface of the CIGS layer is larger than the band gap on the surface of the CIAS layer can be shown.

ＣＩＡＳ層のバンドギャップがＣＩＧＳ層に比べて大きいと、ＣＩＧＳ層で光励起されてｐｎ接合側のＣＩＡＳ層へと輸送されるキャリアにとってＣＩＡＳ層が障壁となる。これに対し、本実施形態の構成によれば、ＣＩＧＳ層で光励起されたキャリアのエネルギーは、ＣＩＡＳ層のバンドギャップより大きくなるため、ＣＩＡＳ層が障壁とならず、光励起されたキャリアのエネルギーを、ｐｎ接合を介して光電流として取り出すことができる。従って、高変換効率の薄膜太陽電池を実現できる。   When the band gap of the CIAS layer is larger than that of the CIGS layer, the CIAS layer becomes a barrier for carriers that are photoexcited in the CIGS layer and transported to the CIAS layer on the pn junction side. On the other hand, according to the configuration of the present embodiment, the energy of the carrier photoexcited in the CIGS layer is larger than the band gap of the CIAS layer, so the CIAS layer does not become a barrier, and the energy of the photoexcited carrier is It can be taken out as a photocurrent through a pn junction. Therefore, a thin film solar cell with high conversion efficiency can be realized.

次に、図２において、ＣＩＡＳ層の厚さを０．１μｍ、ＣＩＧＳ層の裏面Ｇａ／（Ｉｎ＋Ｇａ）比を０．３５に固定して、ＣＩＡＳ層の表面Ａｌ／（Ｉｎ＋Ａｌ）比とＣＩＧＳ層の中で最小となるＧａ／（Ｉｎ＋Ｇａ）比に対する変換効率の関係を計算した。本明細書では、ＣＩＧＳ層の中で変化しているＧａ／（Ｉｎ＋Ｇａ）比の最小値を、最小Ｇａ／（Ｉｎ＋Ｇａ）比ともいう。変換効率の等高線図を図５に示す。図５の中の直線は、下記（式２）を満足する。
１．０７ｘ＋０．６２ｘ2＝０．４２ｚ＋０．２４ｚ2 ・・・（式２） Next, in FIG. 2, the thickness of the CIAS layer is fixed to 0.1 μm, the back surface Ga / (In + Ga) ratio of the CIGS layer is fixed to 0.35, and the surface Al / (In + Al) ratio of the CIAS layer and the CIGS layer The relationship of the conversion efficiency with respect to the Ga / (In + Ga) ratio which becomes the minimum among them was calculated. In the present specification, the minimum value of the Ga / (In + Ga) ratio changing in the CIGS layer is also referred to as the minimum Ga / (In + Ga) ratio. A contour map of the conversion efficiency is shown in FIG. The straight line in FIG. 5 satisfies the following (Formula 2).
1.07x + 0.62x 2 = 0.42z + 0.24z 2 (Formula 2)

ここで、ｘとｚは、それぞれ、ＣＩＡＳ層の表面Ａｌ／（Ｉｎ＋Ａｌ）比とＣＩＧＳ層の最小Ｇａ／（Ｉｎ＋Ｇａ）比を表す。図５から明らかなように、直線より下側の領域で高い変換効率が得られる。すなわち、組成比が上記（式２）の（左辺）≧（右辺）を満足し、ＣＩＡＳ層のバンドギャップがＣＩＧＳ層のＧａ／（Ｉｎ＋Ｇａ）比が最小となる領域のバンドギャップより大きくなる場合に高い変換効率が得られる。これは、バンドギャップの大きなＣＩＡＳ層をｐｎ接合に隣接して形成することにより、ｐｎ接合での漏れ電流あるいは逆飽和電流が低減し、薄膜太陽電池の開放端電圧が増加するためである。   Here, x and z represent the surface Al / (In + Al) ratio of the CIAS layer and the minimum Ga / (In + Ga) ratio of the CIGS layer, respectively. As is clear from FIG. 5, high conversion efficiency is obtained in the region below the straight line. That is, when the composition ratio satisfies (left side) ≧ (right side) of (Equation 2) and the band gap of the CIAS layer is larger than the band gap of the region where the Ga / (In + Ga) ratio of the CIGS layer is minimum. High conversion efficiency can be obtained. This is because by forming a CIAS layer having a large band gap adjacent to the pn junction, leakage current or reverse saturation current at the pn junction is reduced, and the open circuit voltage of the thin film solar cell is increased.

次に、ＣＩＧＳ層の裏面Ｇａ／（Ｉｎ＋Ｇａ）比を０．３５に固定して、ＣＩＡＳ層の厚さを０．２μｍ、０．３μｍとした場合のＣＩＡＳ層の表面Ａｌ／（Ｉｎ＋Ａｌ）比とＣＩＧＳ層の最小Ｇａ／（Ｉｎ＋Ｇａ）比に対する変換効率の関係を計算した結果を図６と図７にそれぞれ示す。図５と比較すれば分かるように、ＣＩＡＳ層の厚さを大きくすると、効率２０％が得られる組成領域が狭くなり、ＣＩＡＳ層の厚さ０．３μｍ（図７）では、上記（式２）を満たす条件でも効率１５％の領域が現れてくる。このことからも、ＣＩＡＳ層の厚さは０．２μｍ以下が薄膜太陽電池の変換効率向上に好適であることがわかる。   Next, the surface Al / (In + Al) ratio of the CIAS layer when the back surface Ga / (In + Ga) ratio of the CIGS layer is fixed to 0.35 and the thickness of the CIAS layer is 0.2 μm and 0.3 μm, The calculation results of the relationship of the conversion efficiency with respect to the minimum Ga / (In + Ga) ratio of the CIGS layer are shown in FIGS. 6 and 7, respectively. As can be seen from comparison with FIG. 5, when the thickness of the CIAS layer is increased, the composition region in which the efficiency of 20% can be obtained is narrowed, and when the thickness of the CIAS layer is 0.3 μm (FIG. 7), A region with an efficiency of 15% appears even under conditions that satisfy the above conditions. This also shows that the thickness of the CIAS layer is preferably 0.2 μm or less for improving the conversion efficiency of the thin film solar cell.

一般に、カルコパイライト構造半導体を用いた薄膜太陽電池で採用されるダブルグレーデッドバンドギャップは、バンドギャップが最小となる領域でキャリアの再結合確率が高くなり、変換効率の向上を阻害するといわれている。本実施形態でいうと、ＣＩＡＳ層とＣＩＧＳ層との界面では、バンドギャップが最小となることに基づくエネルギー障壁により、励起キャリアが蓄積される。空乏層外にエネルギー障壁が存在する場合は、内部電界が小さいため、エネルギー障壁によりキャリアが蓄積し、再結合確率が高くなる。   In general, double graded band gaps used in thin film solar cells using chalcopyrite structure semiconductors are said to increase the recombination probability of carriers in the region where the band gap is minimized, and hinder the improvement of conversion efficiency. . In this embodiment, excited carriers are accumulated at the interface between the CIAS layer and the CIGS layer due to the energy barrier based on the minimum band gap. When an energy barrier exists outside the depletion layer, the internal electric field is small, so that carriers accumulate due to the energy barrier and the recombination probability increases.

これに対し、バンドギャップが大きいＣＩＡＳ層の全部が空乏層内に収まっている場合、つまりＣＩＧＳ層中でバンドギャップが最小となる領域が空乏層内にある場合は、空乏層内の内部電界が大きいため、キャリアは窓層１４へと効率よく輸送される。これにより、バンドギャップの最小点での励起キャリアの蓄積による再結合を抑制することができ、変換効率のさらなる向上を図ることができる。   On the other hand, when the entire CIAS layer having a large band gap is contained in the depletion layer, that is, when the region where the band gap is minimum in the CIGS layer is in the depletion layer, the internal electric field in the depletion layer is reduced. Since it is large, the carrier is efficiently transported to the window layer 14. As a result, recombination due to accumulation of excited carriers at the minimum point of the band gap can be suppressed, and conversion efficiency can be further improved.

以上に記載したように、本発明によれば、エネルギー変換効率の高い薄膜太陽電池を安定に提供できる。   As described above, according to the present invention, a thin-film solar cell with high energy conversion efficiency can be provided stably.

（実施例１）
図３に示す構造の薄膜太陽電池を以下に示す方法で製造した。
基板１１として、ソーダライムガラスを用いた。ガラス基板１１上に、スパッタ法により、裏面電極層１２としてのＭｏ膜を約０．４μｍの厚さで堆積させた。スパッタは、Ｍｏをターゲットとして、Ａｒガス雰囲気中でＤＣ１ｋＷを印加することにより行った。 Example 1
A thin film solar cell having the structure shown in FIG. 3 was manufactured by the method shown below.
As the substrate 11, soda lime glass was used. A Mo film as the back electrode layer 12 was deposited on the glass substrate 11 to a thickness of about 0.4 μm by sputtering. Sputtering was performed by applying DC 1 kW in an Ar gas atmosphere using Mo as a target.

光吸収層となる第１の半導体層１３ａとして、ＣＩＧＳ膜を蒸着法により作製した。初めに、Ｇａ／（Ｉｎ＋Ｇａ）比が約０．３５、Ｃｕ／（Ｉｎ＋Ｇａ）比が約０．８になるように、Ｃｕ、Ｉｎ、Ｇａ、Ｓｅの各蒸発源からの蒸着レートを制御して、基板温度５００℃でＣｕ不足のＣＩＧＳ膜を作製した。次に、基板温度は一定とし、表面のＧａ／（Ｉｎ＋Ｇａ）比が約０．１５、膜全体のＣｕ／（Ｉｎ＋Ｇａ）比が約１．２になるように、Ｃｕ、Ｉｎ、Ｇａ、Ｓｅの各蒸発源からの蒸着レートを制御して、Ｃｕ過剰のＣＩＧＳ膜を形成した。これは、Ｃｕ過剰とすることにより、Ｃｕ−Ｓｅ液相を析出させ、ＣＩＧＳ膜の結晶成長を促進するためである。最後に、基板温度を５５０℃に設定して、ＩｎとＳｅの蒸気を供給して、Ｃｕ不足のＣＩＧＳ膜を形成した。その後、基板温度５５０℃に設定して、Ａｌを蒸着し拡散させることにより、第２の半導体層１３ｂとしてのＣｕ（Ｉｎ，Ａｌ）Ｓｅ2層を形成した。 As the first semiconductor layer 13a serving as the light absorption layer, a CIGS film was formed by an evaporation method. First, the deposition rate from each evaporation source of Cu, In, Ga, and Se is controlled so that the Ga / (In + Ga) ratio is about 0.35 and the Cu / (In + Ga) ratio is about 0.8. A CI-deficient CIGS film was prepared at a substrate temperature of 500 ° C. Next, the substrate temperature is constant, the surface Ga / (In + Ga) ratio is about 0.15, and the Cu / (In + Ga) ratio of the entire film is about 1.2. The deposition rate from each evaporation source was controlled to form a Cu-excess CIGS film. This is because the Cu—Se liquid phase is precipitated by increasing the Cu content and the crystal growth of the CIGS film is promoted. Finally, the substrate temperature was set to 550 ° C. and In and Se vapors were supplied to form a CI-deficient CIGS film. Thereafter, the substrate temperature was set to 550 ° C., and Al was deposited and diffused to form a Cu (In, Al) Se 2 layer as the second semiconductor layer 13b.

次に、窓層１４として、複層の半導体膜を形成した。まず、約７０ｎｍの厚さのＣｄＳ膜を化学析出法により堆積した。化学析出法は、硝酸Ｃｄ、チオ尿素およびアンモニアを含む水溶液を約８０℃に温め、表層部にＣＩＡＳ層が形成されたＣＩＧＳ膜（要するにＣＩＧＡＳ膜）を上記水溶液に浸漬することにより行った。さらに、ＣｄＳ膜の上に約８０ｎｍの厚さのＺｎＯ膜をスパッタ法で形成した。スパッタ法は、ＺｎＯ焼結体をターゲットとして、Ａｒガス雰囲気中でＲＦ４００Ｗを印加することにより行った。   Next, a multilayer semiconductor film was formed as the window layer 14. First, a CdS film having a thickness of about 70 nm was deposited by chemical precipitation. The chemical precipitation method was performed by warming an aqueous solution containing Cd nitrate, thiourea and ammonia to about 80 ° C. and immersing a CIGS film (in short, a CIGAS film) having a CIAS layer formed on the surface layer portion in the aqueous solution. Further, a ZnO film having a thickness of about 80 nm was formed on the CdS film by sputtering. The sputtering method was performed by applying RF 400 W in an Ar gas atmosphere using a ZnO sintered body as a target.

次に、スパッタ法により、透明導電膜１５として、約２００ｎｍの厚さのＡｌ添加ＺｎＯ膜を堆積した。スパッタは、Ａｌ2Ｏ3を２質量％含有したＡｌ添加ＺｎＯ焼結体をターゲットに用い、Ａｒの雰囲気中でＲＦ４００Ｗを印加することにより行った。 Next, an Al-added ZnO film having a thickness of about 200 nm was deposited as the transparent conductive film 15 by sputtering. Sputtering was performed by applying RF 400 W in an Ar atmosphere using an Al-added ZnO sintered body containing 2% by mass of Al 2 O 3 as a target.

最後に、取り出し電極１６として、ＮｉＣｒとＡｕの積層膜を電子ビーム蒸着法で形成した。ＮｉＣｒとＡｕの膜厚は各々５０ｎｍと３００ｎｍであった。   Finally, a NiCr and Au laminated film was formed as the extraction electrode 16 by an electron beam evaporation method. The film thicknesses of NiCr and Au were 50 nm and 300 nm, respectively.

以上の製造方法によれば、光吸収層としてＣＩＧＡＳ膜を用いた図３に示す構造の薄膜太陽電池を製造できる。   According to the above manufacturing method, the thin film solar cell having the structure shown in FIG. 3 using the CIGAS film as the light absorption layer can be manufactured.

（実施例２）
薄膜太陽電池の製造方法のその他の一例について述べる。
基板１１、裏面電極層１２、窓層１４、透明導電膜１５、取り出し電極１６の材料と製造方法は実施例１と同じである。重複を避けるために、光吸収層となる第１の半導体層１３ａと第２の半導体層１３ｂの作製方法についてのみ記載する。 (Example 2)
Another example of the method for manufacturing a thin film solar cell will be described.
The materials and manufacturing methods of the substrate 11, the back electrode layer 12, the window layer 14, the transparent conductive film 15, and the extraction electrode 16 are the same as those in the first embodiment. In order to avoid duplication, only a method for manufacturing the first semiconductor layer 13a and the second semiconductor layer 13b to be the light absorption layer will be described.

裏面電極層１２の上に、スパッタ法によりＣｕ−Ｇａ合金とＩｎ金属をターゲットして、Ｃｕ−Ｇａ膜とＩｎ膜がこの順番で積層された積層膜を形成した。ここで、Ｃｕ−Ｇａ合金のＧａ含有率は２５原子％である。スパッタ法は、Ａｒガス雰囲気中でＤＣ１ｋＷを印加することにより行った。Ｃｕ−Ｇａ膜とＩｎ膜の膜厚はそれぞれ約０．３μｍと０.５μｍであった。なお、Ｃｕ−Ｇａ膜とＩｎ膜の積層順序は上記と逆でもよいが、上記順序のほうが、Ｍｏ膜とＣＩＧＳ膜との密着性が高くなるので好ましい。   On the back electrode layer 12, a Cu—Ga alloy and In metal were targeted by a sputtering method to form a laminated film in which a Cu—Ga film and an In film were laminated in this order. Here, the Ga content of the Cu—Ga alloy is 25 atomic%. The sputtering method was performed by applying DC 1 kW in an Ar gas atmosphere. The film thicknesses of the Cu—Ga film and In film were about 0.3 μm and 0.5 μm, respectively. Note that the stacking order of the Cu—Ga film and the In film may be reversed, but the above order is preferable because the adhesion between the Mo film and the CIGS film becomes higher.

次に、このＣｕ−Ｇａ膜とＩｎ膜を積層した基板を電気炉の中に入れて、１体積％のＨ2Ｓｅガスを含むＮ2ガスの６０Ｐａの雰囲気中で約５２０℃に加熱して熱処理を行い、ＣＩＧＳ膜を作製した。先に説明したように、Ｃｕ−Ｉｎ−Ｇａ−Ｓｅの系では、二元化合物のＧａ2Ｓｅ3と、三元化合物のＣｕＧａＳｅ2およびＣｕＩｎＳｅ2とが生成されやすい。従って、上記熱処理を行うとＧａ2Ｓｅ3が裏面電極１２の近傍に析出する。この結果、裏面電極１２の近傍のＧａ／（Ｉｎ＋Ｇａ）比が高く、膜表面に向けてＧａ／（Ｉｎ＋Ｇａ）比が徐々に小さくなる傾斜組成となる。 Next, the substrate on which the Cu—Ga film and the In film are laminated is placed in an electric furnace and heated to about 520 ° C. in an atmosphere of N 2 gas containing 1% by volume of H 2 Se gas. Heat treatment was performed to produce a CIGS film. As described above, in the Cu—In—Ga—Se system, the binary compound Ga 2 Se 3 and the ternary compounds CuGaSe 2 and CuInSe 2 are likely to be generated. Therefore, when the heat treatment is performed, Ga 2 Se 3 is deposited in the vicinity of the back electrode 12. As a result, a gradient composition is obtained in which the Ga / (In + Ga) ratio in the vicinity of the back electrode 12 is high and the Ga / (In + Ga) ratio gradually decreases toward the film surface.

このＣＩＧＳ膜の上にＡｌ膜をスパッタで形成し、さらにＳｅ膜を蒸着にて形成した。Ａｌ膜のスパッタは、Ａｌをターゲットして、Ａｒガス雰囲気中でＤＣ１ｋＷを印加することにより行った。また、Ｓｅの蒸着は、金属Ｓｅを蒸発源として、蒸着るつぼの温度を２００〜３００℃の範囲内で設定し、Ｓｅを蒸発させて行った。ＡｌとＳｅの膜厚はそれぞれ約５０ｎｍと３００ｎｍであった。   An Al film was formed on the CIGS film by sputtering, and an Se film was formed by vapor deposition. The Al film was sputtered by targeting Al and applying DC 1 kW in an Ar gas atmosphere. Further, the deposition of Se was performed by setting the temperature of the deposition crucible within a range of 200 to 300 ° C. using metal Se as an evaporation source and evaporating Se. The film thicknesses of Al and Se were about 50 nm and 300 nm, respectively.

次に、ＣＩＧＳ膜とＡｌ膜とＳｅ膜の積層膜をＮ2ガス雰囲気中、５５０℃で約１分の急速熱処理を行い、第２の半導体層１３ｂとしてのＣＩＡＳ層を形成した。先に説明したように、Ｃｕ−Ｉｎ−Ｇａ−Ａｌ−Ｓｅの系では、Ａｌは偏析しやすく、拡散しにくいため、上記急速熱処理を行うと表面近傍にのみＣＩＡＳ層を形成できる。ＣＩＡＳ層を除いた残部は、実質的にＡｌを含有しないＣＩＧＳ層（第１の半導体層１３ａ）となる。 Next, the laminated film of the CIGS film, the Al film, and the Se film was subjected to rapid heat treatment at 550 ° C. for about 1 minute in an N 2 gas atmosphere to form a CIAS layer as the second semiconductor layer 13b. As described above, in the Cu—In—Ga—Al—Se system, Al is easily segregated and difficult to diffuse. Therefore, the CIAS layer can be formed only in the vicinity of the surface when the rapid thermal treatment is performed. The remainder excluding the CIAS layer becomes a CIGS layer (first semiconductor layer 13a) that does not substantially contain Al.

なお、本実施例では、ＣＩＡＳ層を形成するために、ＡｌとＳｅの積層膜を作製し、熱処理しているが、Ａｌ膜をＣＩＧＳ膜の上に堆積した後、Ｈ2Ｓｅガスを含む雰囲気中で再度熱処理しても同様な構造の薄膜太陽電池を製造できる。 In this embodiment, in order to form a CIAS layer, a laminated film of Al and Se is produced and heat-treated. After the Al film is deposited on the CIGS film, an atmosphere containing H 2 Se gas is used. A thin-film solar cell having the same structure can be manufactured even if it is heat-treated again.

（実施例３）
図３に示す薄膜太陽電池の製造方法のその他の一例について述べる。
基板１１、裏面電極層１２、窓層となる半導体層１４、透明導電膜１５、取り出し電極１６の材料と製造方法は実施例１と同じである。光吸収層となる第１の半導体層１３ａと第２の半導体層１３ｂの作製方法についてのみ記載する。 (Example 3)
Another example of the manufacturing method of the thin film solar cell shown in FIG. 3 will be described.
The materials and manufacturing methods of the substrate 11, the back electrode layer 12, the semiconductor layer 14 serving as the window layer, the transparent conductive film 15, and the extraction electrode 16 are the same as those in the first embodiment. Only a method for manufacturing the first semiconductor layer 13a and the second semiconductor layer 13b to be the light absorption layer will be described.

裏面電極層１２の上に、スパッタ法によりＣｕ−Ｇａ合金とＩｎ金属とＡｌ金属をターゲットして、Ｃｕ−Ｇａ膜、Ｉｎ膜およびＡｌ膜がこの順番で積層された積層膜を形成した。ここで、Ｃｕ−Ｇａ合金のＧａ含有率は２５原子％である。スパッタ法は、Ａｒガス雰囲気中でＤＣ１ｋＷを印加することにより行った。Ｃｕ−Ｇａ膜、Ｉｎ膜とＡｌ膜の膜厚はそれぞれ約０．３μｍ、０．５μｍ、５０ｎｍであった。なお、Ｃｕ−Ｇａ膜とＩｎ膜の積層順序は上記と逆でもよいが、上記順序のほうが、Ｍｏ膜とＣＩＧＳ膜との密着性が高くなるので好ましい。また、Ａｌ膜については最表面にくることが必要である。   On the back electrode layer 12, a Cu—Ga alloy, In metal, and Al metal were targeted by a sputtering method to form a laminated film in which a Cu—Ga film, an In film, and an Al film were laminated in this order. Here, the Ga content of the Cu—Ga alloy is 25 atomic%. The sputtering method was performed by applying DC 1 kW in an Ar gas atmosphere. The film thicknesses of the Cu—Ga film, In film and Al film were about 0.3 μm, 0.5 μm and 50 nm, respectively. Note that the stacking order of the Cu—Ga film and the In film may be reversed, but the above order is preferable because the adhesion between the Mo film and the CIGS film becomes higher. Further, the Al film needs to come to the outermost surface.

次に、このＣｕ−Ｇａ膜とＩｎ膜とＡｌ膜を積層した基板を電気炉の中に入れて熱処理を行い、第１および第２の半導体層１３ａ，１３ｂとしてのＣＩＧＡＳ膜を作製した。電気炉内の雰囲気は、１体積％のＨ2Ｓｅガスを含むＮ2ガスの５００ｍＴｏｒｒ、５００℃〜５３０℃とした。先に説明したように、Ｃｕ−Ｉｎ−Ｇａ−Ａｌ−Ｓｅの系では、Ａｌは偏析しやすく、拡散しにくいため、上記熱処理を行うと表面近傍にのみＣＩＡＳ層を形成できる。また、Ｇａ2Ｓｅ3が裏面電極層１２の近傍に析出するため、裏面電極層１２の近傍でＧａ／（Ｉｎ＋Ｇａ）比が高く、膜表面に向けてＧａ／（Ｉｎ＋Ｇａ）比が徐々に小さくなる傾斜組成となる。 Next, the substrate on which the Cu—Ga film, the In film, and the Al film were laminated was placed in an electric furnace and subjected to heat treatment, and CIGAS films as the first and second semiconductor layers 13a and 13b were manufactured. The atmosphere in the electric furnace was set to 500 mTorr and 500 ° C. to 530 ° C. of N 2 gas containing 1% by volume of H 2 Se gas. As described above, in the Cu—In—Ga—Al—Se system, Al is easily segregated and hardly diffuses. Therefore, when the heat treatment is performed, a CIAS layer can be formed only in the vicinity of the surface. Further, since Ga 2 Se 3 is deposited in the vicinity of the back electrode layer 12, the Ga / (In + Ga) ratio is high in the vicinity of the back electrode layer 12, and the Ga / (In + Ga) ratio is gradually reduced toward the film surface. It becomes a gradient composition.

なお、本実施例では、ＣＩＡＳ層を形成するために、Ｈ2Ｓｅガス雰囲気中で熱処理を行っているが、Ｃｕ−Ｇａ膜とＩｎ膜とＡｌ膜とＳｅ膜の積層膜を作製し、Ｎ2雰囲気中温度５５０℃で約１分の急速熱処理を行っても同様な構造の薄膜太陽電池を製造できる。 In this embodiment, in order to form the CIAS layer, heat treatment is performed in an H 2 Se gas atmosphere. However, a stacked film of a Cu—Ga film, an In film, an Al film, and a Se film is formed, and N 2 A thin-film solar cell having the same structure can be manufactured even if rapid thermal processing is performed for about 1 minute at a temperature of 550 ° C. in an atmosphere.

従来のＣＩＧＳ膜の深さ方向に関するＧａ／（Ｉｎ＋Ｇａ）比の分布図。The distribution map of Ga / (In + Ga) ratio regarding the depth direction of the conventional CIGS film | membrane. 本発明にかかる薄膜太陽電池の光吸収層であるＣＩＧＡＳ膜の深さ方向に関するＡｌ／（Ｉｎ＋Ａｌ）比とＧａ／（Ｉｎ＋Ｇａ）比の分布を示す模式図。The schematic diagram which shows distribution of Al / (In + Al) ratio and Ga / (In + Ga) ratio regarding the depth direction of the CIGAS film | membrane which is a light absorption layer of the thin film solar cell concerning this invention. 本発明の薄膜太陽電池の一例を示す断面模式図。The cross-sectional schematic diagram which shows an example of the thin film solar cell of this invention. ＣＩＡＳ層の表面Ａｌ／（Ｉｎ＋Ａｌ）比とＣＩＧＳ層の裏面Ｇａ／（Ｉｎ＋Ｇａ）比に対する変換効率の等高線図。The contour map of the conversion efficiency with respect to the surface Al / (In + Al) ratio of a CIAS layer, and the back surface Ga / (In + Ga) ratio of a CIGS layer. ＣＩＡＳ層の厚さを０．１μｍとした場合の表面Ａｌ／（Ｉｎ＋Ａｌ）比とＣＩＧＳ層の最小Ｇａ／（Ｉｎ＋Ｇａ）比に対する変換効率の等高線図。The contour map of the conversion efficiency with respect to the surface Al / (In + Al) ratio and CIGS layer minimum Ga / (In + Ga) ratio when the thickness of the CIAS layer is 0.1 μm. ＣＩＡＳ層の厚さを０．２μｍとした場合の表面Ａｌ／（Ｉｎ＋Ａｌ）比とＣＩＧＳ層の最小Ｇａ／（Ｉｎ＋Ｇａ）比に対する変換効率の等高線図。The contour map of the conversion efficiency with respect to the surface Al / (In + Al) ratio and the minimum Ga / (In + Ga) ratio of the CIGS layer when the thickness of the CIAS layer is 0.2 μm. ＣＩＡＳ層の厚さを０．３μｍとした場合の表面Ａｌ／（Ｉｎ＋Ａｌ）比とＣＩＧＡ層の最小Ｇａ／（Ｉｎ＋Ｇａ）比に対する変換効率の等高線図。The contour map of the conversion efficiency with respect to the surface Al / (In + Al) ratio when the thickness of the CIAS layer is 0.3 μm and the minimum Ga / (In + Ga) ratio of the CIGA layer.

符号の説明Explanation of symbols

１０ 薄膜太陽電池
１１ 基板
１２ 裏面電極層（Ｍｏ膜）
１３ａ 第１の半導体層（Ｃｕ（Ｉｎ，Ｇａ）Ｓｅ2層）
１３ｂ 第２の半導体層（Ｃｕ（Ｉｎ，Ａｌ）Ｓｅ2層）
１４ 窓層（ＣｄＳ／ＺｎＯ積層膜）
１５ 透明導電膜（ＩＴＯ膜）
１６ 電極（ＮｉＣｒ／Ａｕ積層膜） 10 Thin Film Solar Cell 11 Substrate 12 Back Electrode Layer (Mo Film)
13a First semiconductor layer (Cu (In, Ga) Se 2 layer)
13b Second semiconductor layer (Cu (In, Al) Se 2 layer)
14 Window layer (CdS / ZnO laminated film)
15 Transparent conductive film (ITO film)
16 electrode (NiCr / Au laminated film)

Claims (8)

ＧａおよびＩｎからなる群より選ばれる少なくとも１つの元素と、Ｓ、ＳｅおよびＴｅからなる群より選ばれる少なくとも１つの元素と、Ｃｕと、Ａｌとを含みカルコパイライト構造である半導体薄膜を光吸収層として備え、
前記半導体薄膜は、第１の半導体層と、前記第１の半導体層よりも光入射側に配置された第２の半導体層とを含み、
前記第１の半導体層は、前記第２の半導体層に近づくにつれてバンドギャップが小さくなっており、
前記第２の半導体層は、前記半導体薄膜内においてＡｌ濃度の最も大きい領域を含むとともに、前記第１の半導体層における最小のバンドギャップよりも大きいバンドギャップを有する、薄膜太陽電池。 A semiconductor thin film having a chalcopyrite structure including at least one element selected from the group consisting of Ga and In, at least one element selected from the group consisting of S, Se, and Te, Cu, and Al. Prepared as
The semiconductor thin film includes a first semiconductor layer, and a second semiconductor layer disposed on the light incident side of the first semiconductor layer,
The first semiconductor layer has a smaller band gap as it approaches the second semiconductor layer,
The second semiconductor layer includes a region having the highest Al concentration in the semiconductor thin film, and has a band gap larger than a minimum band gap in the first semiconductor layer. 前記第１の半導体層と前記第２の半導体層とが直接積層され、
前記第１の半導体層は、ＧａとＩｎとを含み、かつ、前記第２の半導体層との界面において、組成比Ｇａ／（Ｉｎ＋Ｇａ）が最小となる、請求項１記載の薄膜太陽電池。 The first semiconductor layer and the second semiconductor layer are directly stacked,
2. The thin film solar cell according to claim 1, wherein the first semiconductor layer contains Ga and In, and a composition ratio Ga / (In + Ga) is minimized at an interface between the first semiconductor layer and the second semiconductor layer. 前記第１の半導体層は、実質的にＡｌを含有しない層である、請求項１または請求項２記載の薄膜太陽電池。   The thin film solar cell according to claim 1, wherein the first semiconductor layer is a layer that does not substantially contain Al. 前記第２の半導体層の厚さが０．２μｍ以下である、請求項３記載の薄膜太陽電池。   The thin film solar cell according to claim 3, wherein the thickness of the second semiconductor layer is 0.2 μm or less. 前記第２の半導体層がＩｎを含み、
前記第２の半導体層の光入射側の表面における組成比Ａｌ／（Ｉｎ＋Ａｌ）をｘとし、前記第１の半導体層の裏面における組成比Ｇａ／（Ｉｎ＋Ｇａ）をｙと定義したとき、ｘとｙが下記（式１）の関係を満足する、請求項２記載の薄膜太陽電池。
１.０７ｘ＋０.６２ｘ2≦０.４２ｙ＋０.２４ｙ2 ・・・（式１） The second semiconductor layer includes In;
When the composition ratio Al / (In + Al) on the light incident side surface of the second semiconductor layer is defined as x and the composition ratio Ga / (In + Ga) on the back surface of the first semiconductor layer is defined as y, x and y The thin film solar cell of Claim 2 which satisfies the relationship of following (Formula 1).
1.07x + 0.62x 2 ≦ 0.42y + 0.24y 2 (Formula 1) 前記第２の半導体層がＩｎを含み、
前記第２の半導体層の光入射側の表面における組成比Ａｌ／（Ｉｎ＋Ａｌ）をｘとし、前記第１の半導体層中において変化している組成比Ｇａ／（Ｉｎ＋Ｇａ）の最小値をｚと定義したとき、ｘとｚが下記（式２）の関係を満足する、請求項２記載の薄膜太陽電池。
１.０７ｘ＋０.６２ｘ2≧０.４２ｚ＋０.２４ｚ2 ・・・（式２） The second semiconductor layer includes In;
The composition ratio Al / (In + Al) on the light incident side surface of the second semiconductor layer is defined as x, and the minimum value of the composition ratio Ga / (In + Ga) changing in the first semiconductor layer is defined as z. The thin film solar cell according to claim 2, wherein x and z satisfy the following relationship (Formula 2).
1.07x + 0.62x 2 ≧ 0.42z + 0.24z 2 (Formula 2) 前記第１の半導体層は、前記第２の半導体層との界面をなす表面から裏面に向けて組成比Ｇａ／（Ｉｎ＋Ｇａ）が連続的に増加している、請求項２記載の薄膜太陽電池。   The thin film solar cell according to claim 2, wherein the first semiconductor layer has a composition ratio Ga / (In + Ga) continuously increasing from the front surface forming the interface with the second semiconductor layer toward the back surface. 基板と、
裏面電極層と、
前記第１および前記第２の半導体層とは反対の導電型の半導体からなる窓層と、
透明導電膜とをさらに備えた、請求項１記載の薄膜太陽電池。 A substrate,
A back electrode layer;
A window layer made of a semiconductor of a conductivity type opposite to the first and second semiconductor layers;
The thin film solar cell according to claim 1, further comprising a transparent conductive film.
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