JP2004119491A - Method of manufacturing thin-film solar cell and thin-film solar cell manufactured by the method - Google Patents

Abstract

An object of the present invention is to provide a method of manufacturing a thin-film solar cell that exhibits a good light confinement effect and exhibits high photoelectric conversion efficiency.
A method for manufacturing a thin-film solar cell includes forming a first electrode layer on a substrate, forming a photoelectric conversion layer on the first electrode layer, and forming a second electrode layer on the photoelectric conversion layer. One of the step of forming the first electrode layer and the step of forming the second electrode layer includes forming a transparent conductive layer as an electrode layer, and forming irregularities on the surface of the formed transparent conductive layer. The etching process includes an etching process in which the surface of the transparent conductive layer is etched at least twice using an etching solution composed of an acidic or alkaline solution.
[Selection diagram] Fig. 1

Description

【００７８】
また、上述の実施例１〜７、並びに、比較例１に係る薄膜太陽電池のＡＭ１．５（１００ｍＷ／ｃｍ^２）照射条件下における電流−電圧特性を以下の表２に示す。
【００７９】
【表２】

【００８０】
ここで、上記の表１および表２を参照しつつ、実施例１〜６と比較例１とを比較・検討する。
表１に示されるように、実施例１〜６による薄膜太陽電池の透明導電層の表面には、エッチング処理が２回施されることによっていずれも複数の穴と、各穴の内面の微細な凹凸と、穴以外の表面における微細な凹凸が形成されている（図４参照）。
これに対し、比較例１では、実施例１〜６よりも穴の最大直径範囲が広がると共に穴の数が増加する一方で、穴の内面における微細な凹凸が小さくなり、またその数も減少している。
一方、Ｘ線回折測定による光電変換層の配向性Ｉ_２２０／Ｉ_１１１では、実施例１〜６および比較例１ともに５．０、もしくはそれ以上の値が得られたことから、いずれの薄膜太陽電池の光電変換層についても欠陥の少ない良好な膜が形成されていると考えられる。
【００８１】
さらに、表２に基づいて実施例１〜６、並びに、比較例１の薄膜太陽電池の電流−電圧特性を比較すると、いずれの薄膜太陽電池も同程度の開放電圧および形状因子が得られているにもかかわらず、比較例１の短絡電流が実施例１〜６に対して低くなっていることがわかる。つまり、比較例１に係る薄膜太陽電池は、光閉込効果が実施例１〜６ほど十分でないことがわかる。
これにより、透明導電層１１ｂの表面形状、すなわち実施例１〜６に係る薄膜太陽電池２０の透明導電層１１ｂに認められた複数の穴と各穴の内面の微細な凹凸が、光閉込効果に大きく寄与しているものと考えることができる。
具体的には、各穴の内面に微細な凹凸が形成されることにより、透明導電層１１ｂとシリコン薄膜との界面において、入射光に対して十分な光散乱を生じさるとともに、透明導電層とその上に堆積されるシリコン薄膜との界面における反射を低減し、光電変換層１７への効果的な光入射を促進することができたためと考えられる。この結果、良好な光閉込効果が得られたと考えられる。
【００８２】
また、実施例１〜６の中では、実施例２と実施例４が最も低い短絡電流値を示した。これは、２回目に施されたエッチング処理の条件が、１回目のエッチング処理の条件よりも穴が形成され易い条件であるため、穴の内面に微細な凹凸が十分に形成されなかったためと考えられる。
これにより、透明導電層１１ｂの表面に穴が形成されやすいエッチング処理を施した後に、微細な凹凸が形成されやすいエッチング処理を施すことが、透明導電層１１ｂの表面において十分な光散乱を生じさせ、光電変換層における十分な光閉込効果を得るうえで重要であり、ひいては高い光電変換効率を示す薄膜太陽電池を製造することにつながると考えることができる。
なお、実施例１〜６では、基板１１ａ上に形成された透明導電層１１ｂのみにエッチングによる表面処理を施したが、裏面電極１６側の裏面反射層１５として用いた透明導電層に同様の処理を施しても同様の光閉込効果が得られると考えられる。
【００８３】
実施例７
実施例７は、上述の実施形態で触れた図２に示されるサブストレート型構造で単接合型の薄膜太陽電池５０を製造したものである。
図２に示されるように、実施例７で製造した薄膜太陽電池５０は、太陽電池用基板４０の上に光電変換層４５、透明導電層４４が順に積層されて構成されている。
ここで、太陽電池用基板４０は、ステンレス基板４０ａの上に裏面電極４０ｂと、透明導電性材料からなる裏面反射層４０ｃが順に積層されて構成されている。
また、光電変換層４５は、ｎ型シリコン層４１、ｉ型シリコン層４２およびｐ型シリコン層４３が順に積層されて構成されている。
【００８４】
このようなサブストレート型構造で単接合型の薄膜太陽電池５０を、以下のような手順で製造した。
まず、電子ビーム蒸着法により、表面が平滑なステンレス基板４０ａ上に裏面電極４０ｂとして銀を厚さ５００ｎｍにわたって製膜し、次いで、マグネトロンスパッタリング法により、裏面電極４０ｂ上に裏面反射層４０ｃとして酸化亜鉛からなる透明導電層を厚さ５０ｎｍにわたって製膜し、太陽電池用基板４０を形成した。
【００８５】
続いて、得られた太陽電池用基板４０の上に厚さ３０ｎｍのｎ型シリコン層４１、厚さ２μｍのｉ型シリコン層４２、厚さ２０ｎｍのｐ型シリコン層４３を高周波プラズマＣＶＤ法により順に積層し、光電変換層４５を形成した。
なお、ｎ型シリコン層４１の製膜にあたっては、ＳｉＨ_４ガスを流量比で５０倍のＨ_２ガスにより希釈したものを原料ガスとし、この原料ガスにＰＨ_３ガスをＳｉＨ_４ガス流量に対して０．０１％添加した。ｉ型シリコン層１３の製膜にあたっては、ＳｉＨ_４ガスを流量比で５０倍のＨ_２ガスにより希釈したものを原料ガスとした。ｐ型シリコン層４３の製膜にあたっては、ＳｉＨ_４ガスを流量比で５０倍のＨ_２ガスにより希釈したものを原料ガスとし、この原料ガスにＢ_２Ｈ_６ガスをＳｉＨ_４ガス流量に対して０．１％添加した。また、製膜時の基板温度は各々の層において２００℃とした。
【００８６】
続いて、形成された光電変換層４５の上に、マグネトロンスパッタリング法により基板温度１５０℃、製膜圧力０．３ｍＴｏｒｒで、透明導電層４４として酸化亜鉛を厚さ１０００ｎｍにわたって製膜した。透明導電層４４には、１×１０^２１ｃｍ^−３程度のガリウムが添加されている。この結果、得られた透明導電層４４のシート抵抗は１０Ω／□であり、波長８００ｎｍの光に対する透過率は８０％であった。また、透明導電層４４に対してＸ線回折測定を行ったところ、ＺｎＯ（０００１）回折ピークの積分強度は、全回折ピークの積分強度の和に対して７５％であった。
【００８７】
続いて、透明導電層４４の表面に凹凸の形成を行った。
まず、エッチング液として０．３５重量％の塩酸水溶液（液温２５℃）を用意し、この塩酸水溶液に、先の工程で得られた作製中の薄膜太陽電池を１００秒浸漬した。次に、エッチング液として０．１重量％の塩酸水溶液（液温２５℃）を用意し、この塩酸水溶液に再び１００秒浸漬した後、表面を純水で十分に洗浄し、透明導電層４４側から光Ｌを入射するサブストレート型構造で単接合型の薄膜太陽電池５０を得た。
【００８８】
実施例１と同様に、透明導電層４４の表面を走査型電子顕微鏡で観察したところ、穴の形状は略円形で、その最大直径は３００〜１０００ｎｍの範囲内であり、個数密度は１個／μｍ^２程度であった。
また、実施例１と同様に、この穴の内面および周辺の表面を原子間力顕微鏡で更に詳細に観察したところ、図４に示す穴の深さ２１は８０〜９００ｎｍの範囲内であった。また、穴の内面にも微細な凹凸が形成されており、その微細な凹凸は、大きさ（凹凸の高低差）２３が１０〜１８０ｎｍの範囲内であって、間隔２４が８０〜６００ｎｍの範囲内であった。また、穴以外の透明導電層４４の表面にも微細な凹凸が形成されており、その微細な凹凸の大きさは３０〜１５０ｎｍの範囲内であった。
【００８９】
比較例２
比較例２は、透明導電層の表面に凹凸を形成する際に、１回のみのエッチング処理で凹凸を形成し、それ以外は実施例７と同様にしてサブストレート型構造で単接合型の薄膜太陽電池（図示せず）を製造したものである。
比較例１では、透明導電層の表面に凹凸を形成するにあたって、０．３５重量％の塩酸水溶液（液温２５℃）に受光面側の透明導電層まで順次積層された作製中の薄膜太陽電池を１２０秒浸漬した。
【００９０】
実施例１と同様に、透明導電層の表面を走査型電子顕微鏡で観察したところ、穴の形状は略円形で、図５に示す最大直径は１００〜１６００ｎｍの範囲内であり、密度はおよそ２．８個／μｍ^２程度であった。
また、実施例１と同様に、この穴の内面および周辺の表面を原子間力顕微鏡で更に詳細に観察したところ、図５に示す穴の深さ３１は３００〜９００ｎｍの範囲内であって、穴の内面には高低差３３が１０ｎｍ以下という僅かな形状の変化が認められるだけであった。
一方、穴以外の透明導電層の表面には、穴との区別が難しい程の大きな凹凸が形成されていた。
【００９１】
なお、比較例２において、穴の密度が２．８個／μｍ^２程度まで増加した理由としては、上述の比較例１と同様に、１回のみのエッチング処理で透明導電層を掘り下げたために微細な凹凸が形成されずらくなり、先の実施例７では穴以外の表面における微細な凹凸として分類されていたものが、比較例２では穴と分類されるほどまでそのサイズが大きくなったことが考えられる。
【００９２】
ここで、また、上述の実施例７および比較例２に係る薄膜太陽電池のＡＭ１．５（１００ｍＷ／ｃｍ^２）照射条件下における電流−電圧特性を以下の表３に示す。
【００９３】
【表３】

【００９４】
上記の表３に基づいて実施例７および比較例２に係る薄膜太陽電池の電流−電圧特性を比較すると、開放電圧および形状因子の値はほとんど変わらないにもかかわらず、短絡電流については実施例７の方が比較例２よりも高い値を示していることがわかる。
これにより、実施例７に係る薄膜太陽電池の透明導電層４４に認められた複数の穴と各穴の内面の微細な凹凸が、短絡電流の向上に大きく寄与しているものと考えることができる。
具体的には、各穴の内面に微細な凹凸が形成されることにより、透明導電層４４と外部の雰囲気との界面において入射光の反射が低減され、光電変換層４５に対する光入射が促進されたためためと考えられる。
【００９５】
なお、実施例７では、基板上に形成された透明導電層のみにエッチングによる表面処理を施したが、裏面電極側の裏面反射層として用いた透明導電層に同様の処理を施した場合には、裏面反射層と光電変換層の界面において十分な光散乱が生じ、光電変換層４５における良好な光閉込効果が得られると考えられる。
【００９６】
実施例８
実施例８は、上述の実施形態で触れた図３に示されるスーパーストレート構造で多接合型の薄膜太陽電池６０を製造したものである。
図３に示されるように、実施例８で製造した薄膜太陽電池６０は、基板１１ａとその上に形成された透明導電層１１ｂとからなる太陽電池用基板１１上に、第１光電変換層２７、透明導電材料からなる中間層１１ｃ、第２光電変換層３７、裏面反射層１５、裏面電極１６が順に積層されて構成されている。
【００９７】
第１光電変換層２７は、ｐ型非晶質シリコン層１２ａ、ｉ型非晶質シリコン層１３ａおよびｎ型非晶質シリコン層１４ａから構成されており、第２シリコン光電変換層３７は、ｐ型シリコン層１２ｂ、ｉ型シリコン層１３ｂおよびｎ型シリコン層１４ｂから構成されている。
【００９８】
このようなスーパーストレート構造で多接合型の薄膜太陽電池６０を、以下のような手順で製造した。
まず、実施例１と同様に、表面が平滑なガラス板を基板１１ａとして用い、この基板１１ａ上にマグネトロンスパッタリング法により基板温度１５０℃、製膜圧力０．３ｍＴｏｒｒで、透明導電層１１ｂとして酸化亜鉛を厚さ１２００ｎｍにわたって製膜し、太陽電池用基板１１を形成した。
続いて、透明導電層１１ｂ表面に実施例２と同様にして２回のエッチング処理を行い凹凸を形成した。
その後、実施例１と同様にして透明導電層の表面を走査型電子顕微鏡および原子間力顕微鏡で観察したところ、略円形の形状を有する複数の穴と穴の内面の微細な凹凸が確認された。
【００９９】
続いて、得られた太陽電池用基板１１の上に厚さ２０ｎｍのｐ型非晶質シリコン層１２ａ、厚さ３００ｎｍのｉ型非晶質シリコン層１３ａ、厚さ３０ｎｍのｎ型非晶質シリコン層１４ａを高周波プラズマＣＶＤ法により順に積層し、非晶質シリコンからなる第１光電変換層２７を形成した。
なお、ｐ型非晶質シリコン層１２ａの製膜にあたっては、ＳｉＨ_４ガスを流量比で５倍のＨ_２ガスにより希釈したものを原料ガスとし、この原料ガスにＢ_２Ｈ_６ガスをＳｉＨ_４ガス流量に対して０．１％添加した。ｉ型非晶質シリコン層１３ａの製膜にあたっては、ＳｉＨ_４ガスを流量比で５倍のＨ_２ガスにより希釈したものを原料ガスとした。ｎ型非晶質シリコン層１４ａの製膜にあたっては、ＳｉＨ_４ガスを流量比で５倍のＨ_２ガスにより希釈したものを原料ガスとし、この原料ガスにＰＨ_３ガスをＳｉＨ_４ガス流量に対して０．０１％添加した。また、製膜時の基板温度は、各々の層において２００℃とした。
【０１００】
その後、マグネトロンスパッタリング法により、第１光電変換層２７上に中間層１１ｃとして酸化亜鉛を透明導電層１１ｂと同条件で膜厚５ｎｍにわたって製膜した。
その後、中間層１１ｃ上に、実施例１の光電変換層１７と同条件で第２光電変換層３７を形成した。
第２光電変換層３７の形成後にＸ線回折測定を行ったところ、Ｓｉ（２２０）Ｘ線回折ピークの積分強度Ｉ_２２０とＳｉ（１１１）Ｘ線回折ピークの積分強度Ｉ_１１１の比Ｉ_２２０／Ｉ_１１１は５．５であり、実施例１との間に差は認められなかった。
その後、実施例１と同様にして、裏面反射層１５および裏面電極１６を形成し、基板１１ａから光Ｌを入射するスーパーストレート型構造で多接合型の薄膜太陽電池６０を得た。
【０１０１】
比較例３
比較例３は、太陽電池用基板を構成する透明導電層の表面に凹凸を形成する際に、１回のみのエッチング処理で凹凸を形成し、それ以外は実施例８と同様にしてスーパーストレート型構造で多接合型の薄膜太陽電池を製造したものである。
比較例３では、比較例１と同様に、透明導電層の表面に凹凸を形成するにあたって、２．５重量％の水酸化ナトリウム水溶液（液温２５℃）に太陽電池用基板を１２０秒浸漬した。
また、第２光電変換層３７の形成後、実施例８と同様にＸ線回折測定を行ったところ、Ｓｉ（２２０）Ｘ線回折ピークの積分強度Ｉ_２２０とＳｉ（１１１）Ｘ線回折ピークの積分強度Ｉ_１１１の比Ｉ_２２０／Ｉ_１１１は５．０であった。
【０１０２】
ここで、上述の実施例８および比較例３に係る薄膜太陽電池のＡＭ１．５（１００ｍＷ／ｃｍ^２）照射条件下における電流−電圧特性を表４に示す。
【０１０３】
【表４】

【０１０４】
上記の表４に基づいて実施例８および比較例３に係る薄膜太陽電池の電流−電圧特性を比較すると、開放電圧および形状因子の値はほとんど変わらないにもかかわらず、短絡電流については実施例８の方が比較例３よりも高い値を示していることがわかる。
これにより、実施例８に係る薄膜太陽電池の透明導電層１１ｂに認められた複数の穴と各穴の内面の微細な凹凸が、短絡電流の向上に大きく寄与しているものと考えることができる。
具体的には、各穴の内面に微細な凹凸が形成されることにより、透明導電層１１ｂと第１光電変換層２７との界面において入射光に対する十分な光散乱が生じ、光電変換層において良好な光閉込効果が得られたためと考えられる。
【０１０５】
なお、実施例８では、基板１１ａ上に形成された透明導電層１１ｂのみにエッチングによる表面処理を施したが、裏面電極１６側の裏面反射層１５や第１光電変換層２７と第２光電変換層３７との間に挿入される中間層１１ｃに同様の処理を施しても同様の効果が得られると考えられる。
【０１０６】
以上、実施例では酸化亜鉛からなる透明導電層１１ｂ、４４を有する薄膜太陽電池について説明したが、この発明は上述の実施例に限定されるものではない。上述の実施形態で説明した各種の透明導電層およびエッチング液以外でもこの発明と同様の効果を得ることができ、例えば、酸強度もしくはアルカリ強度の異なるエッチング液を用いた場合でも基本的にはこの発明と同様の効果を得ることができる。
【０１０７】
【発明の効果】
この発明によれば、酸性またはアルカリ性溶液からなるエッチング液を用いたエッチング処理が透明導電層の表面に少なくとも２回にわたって施されるので、１回目のエッチング処理によって透明導電層の表面に比較的大きな複数の凹部が形成され、２回目以降のエッチング処理によって各凹部の内面に微細な凹凸が形成され、結果として、光閉込効果を得るうえで理想的な凹凸が透明導電層の表面に形成され、高い光電変換効率を示す薄膜太陽電池を製造できる。
【図面の簡単な説明】
【図１】この発明の実施形態、並びに、実施例１〜６に係るスーパーストレート型構造で単接合型の薄膜太陽電池の構造を概略的に示す断面図である。
【図２】この発明の実施形態および実施例７に係るサブストレート型構造で単接合型の薄膜太陽電池の構造を概略的に示す断面図である。
【図３】この発明の実施形態および実施例８に係るスーパーストレート型構造で多接合型の薄膜太陽電池の構造を概略的に示す断面図である。
【図４】この発明の実施形態および実施例に係る薄膜太陽電池において、透明導電層の表面に形成された穴とその内面に形成された微細な凹凸、並びに穴周辺の表面形状を模式的に示す説明図である。
【図５】比較例に係る薄膜太陽電池において、透明導電層の表面に形成された穴およびその内面の形状、並びに穴周辺の表面形状を模式的に示す説明図である。
【符号の説明】
１１・・・太陽電池用基板
１１ａ・・・基板
１１ｂ・・・透明導電層
１２・・・ｐ型シリコン層
１３・・・ｉ型シリコン層
１４・・・ｎ型シリコン層
１５・・・裏面反射層
１６・・・裏面電極
１７・・・光電変換層
１８ａ，１８ｂ・・・電極
２０・・・薄膜太陽電池
Ｌ・・・光[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method of manufacturing a thin film solar cell and a thin film solar cell manufactured by the method, and more particularly, to a method of manufacturing a thin film solar cell having high photoelectric conversion efficiency and a thin film solar cell manufactured by the method.
[0002]
[Prior art]
Currently, fossil fuels such as petroleum, which are used as major energy sources, are concerned about future supply and demand, and have problems such as carbon dioxide emission which causes a global warming phenomenon. Therefore, in recent years, solar cells have attracted attention as an alternative energy source for fossil fuels such as petroleum.
As a solar cell, a silicon semiconductor having a pn junction is mainly used for converting light energy into electricity, and single crystal silicon is preferably used in terms of photoelectric conversion efficiency. However, a solar cell using single crystal silicon has problems such as a raw material supply, a large area, and a low cost.
[0003]
As one of means for solving these problems, some solar cells using a thin film amorphous silicon as a photoelectric conversion layer have been put into practical use.
Further, in order to obtain higher photoelectric conversion efficiency, introduction of a thin-film crystalline silicon into a photoelectric conversion layer instead of an amorphous material has been studied. This thin-film crystalline silicon can be formed by a chemical vapor deposition method (hereinafter, referred to as a CVD method) similarly to amorphous, and the film quality is a polycrystalline silicon thin film.
[0004]
By the way, in order to increase the photoelectric conversion efficiency in a thin-film solar cell, it is important to effectively use the light confinement effect.
Here, the light confinement effect means that the surface of the transparent conductive layer or the metal layer in contact with the photoelectric conversion layer is made uneven, and light is scattered at the interface to extend the optical path length, thereby reducing light absorption in the photoelectric conversion layer. It is the effect of increasing.
[0005]
As a method of forming irregularities on the surface of the transparent conductive layer, a method of doping boron when forming the transparent conductive layer by a CVD method is known (for example, see Patent Document 1).
Also, a method is known in which the surface of the transparent conductive layer is subjected to etching once using an acidic or alkaline solution (for example, see Patent Document 2).
[0006]
[Patent Document 1]
JP 2000-252504 A (Paragraph 0008)
[Patent Document 2]
JP-A-11-233800 (paragraphs 0015 to 0022)
[0007]
[Problems to be solved by the invention]
In the method of doping boron when forming a transparent conductive layer by the CVD method, special high-pressure gas equipment is required because diborane or the like is used as a boron source.
The method of performing an etching treatment using an acidic or alkaline solution is preferable in that it is simple. However, if the etching treatment is performed only once using an acidic or alkaline solution, relatively rough unevenness may occur on the transparent conductive layer. A good light confinement effect cannot be obtained only by forming the film almost uniformly over the entire surface.
[0008]
The present invention has been made in view of the above circumstances, and provides a method of manufacturing a thin-film solar cell exhibiting a good light confinement effect and exhibiting high photoelectric conversion efficiency, and a thin-film solar cell manufactured by the method. Is provided.
[0009]
[Means for Solving the Problems]
The present invention includes a step of forming a first electrode layer on a substrate, forming a photoelectric conversion layer on the first electrode layer, and forming a second electrode layer on the photoelectric conversion layer. One of the step of forming the second conductive layer and the step of forming the second electrode layer include an etching process of forming a transparent conductive layer as an electrode layer, and forming irregularities on the surface of the formed transparent conductive layer. And a method for manufacturing a thin-film solar cell including a process of etching the surface of a transparent conductive layer at least twice using an etching solution composed of an acidic or alkaline solution.
[0010]
According to such a manufacturing method, since the etching treatment using the etching solution composed of an acidic or alkaline solution is performed at least twice on the surface of the transparent conductive layer, the surface of the transparent conductive layer is subjected to the first etching treatment. A plurality of relatively large concave portions are formed, and fine irregularities are formed on the inner surface of each concave portion by the second and subsequent etching processes.
As a result, ideal concavities and convexities are formed on the surface of the transparent conductive layer for obtaining a good light confinement effect, and a thin-film solar cell exhibiting high photoelectric conversion efficiency can be manufactured.
The method for manufacturing a thin film solar cell according to the present invention can be applied regardless of the difference in the structure of the thin film solar cell such as a superstrate type structure or a substrate type structure.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
In the method for manufacturing a thin-film solar cell according to the present invention, it is preferable that the concentration of the etchant used in the second or subsequent etching is lower than that used in the first etching.
In the method for manufacturing a thin-film solar cell according to the present invention, it is preferable that the temperature of the etchant used in the second or subsequent etching is lower than that used in the first etching.
This is because, in surface treatment by wet etching using an etching solution composed of an acidic or alkaline solution, if the concentration or temperature of the etching solution is high, a relatively large hole-shaped concave portion tends to be formed as the processing time increases. On the other hand, when the concentration or temperature of the etching solution is low, fine irregularities are formed without being greatly affected by the processing time.
Further, by performing the treatment with the etching solution having a low concentration or temperature for the second time or later, there is an advantage that excessive enlargement of the concave portion formed in the first etching treatment and disappearance of the transparent conductive layer itself can be prevented. .
[0012]
In the method for manufacturing a thin-film solar cell according to the present invention, the etching solution is obtained by dissolving a solute composed of an acidic substance or an alkaline substance in a solvent, and is used for the first etching and the second and subsequent etchings. The etchant used is preferably the same solvent and solute.
This is because the use of the same solvent and solute can simplify the etching equipment in terms of chemical resistance and waste liquid treatment.
[0013]
In the method for manufacturing a thin film solar cell according to the present invention, the transparent conductive layer preferably has a zinc oxide thin film layer.
This is because the transparent conductive layer does not have a zinc oxide thin film₂In the case where the transparent conductive layer is formed only by the plasma CVD method, the transparent conductive layer is reduced by the hydrogen plasma contained in the process gas, the transmittance is reduced by blackening, and the characteristics of the thin film solar cell are reduced. This is because there is a danger of it getting lost. On the other hand, when the transparent conductive layer is provided with a zinc oxide thin film layer, the plasma resistance of the zinc oxide thin film layer prevents a decrease in the transmittance of the transparent conductive layer when the photoelectric conversion layer is deposited, thereby providing a thin film solar cell exhibiting high photoelectric conversion efficiency. Leads to
[0014]
Viewed from another viewpoint, the present invention is a thin-film solar cell manufactured by the above manufacturing method, wherein the unevenness is formed by a plurality of recesses formed on the surface of the transparent conductive layer in the first etching treatment, The present invention also provides a thin-film solar cell including fine irregularities formed on the inner surface of each concave portion by the etching process.
That is, in the thin-film solar cell manufactured by the above-described manufacturing method according to the present invention, the unevenness on the surface of the transparent conductive layer includes a plurality of concave portions formed on the surface of the transparent conductive layer in the first etching process, and the second and subsequent times. And the fine unevenness formed on the inner surface of each concave portion by the etching process, sufficient light scattering can be caused on the surface of the transparent conductive layer, and as a result, good light The photoelectric conversion efficiency is improved by the incorporation effect.
[0015]
In the thin film solar cell according to the present invention, each concave portion has a maximum diameter within a range of about 200 to 2000 nm and a depth within a range of about 50 to 1200 nm, and the fine irregularities have a height difference of about 10 to 300 nm. It is preferable that the
Here, each concave portion has a maximum diameter within a range of about 400 to 1200 nm and a depth within a range of about 100 to 800 nm, and the fine unevenness has a height difference within a range of about 20 to 200 nm. preferable.
[0016]
Further, in the thin-film solar cell according to the present invention, it is more preferable that fine unevenness is formed on the surface other than the concave portion in order to cause sufficient light scattering.
In this case, the height difference of the fine unevenness formed on the surface other than the concave portion is preferably in the range of about 10 to 300 nm, and more preferably in the range of about 20 to 200 nm.
[0017]
By setting the maximum diameter and depth of each concave portion, and the height difference of fine irregularities within the above range, not only light having a center wavelength of about 400 to 650 nm of the solar spectrum, but also light having a longer wavelength. Also, sufficient light scattering can be generated.
[0018]
Embodiment
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
As shown in FIG. 1, the thin-film solar cell 20 according to one embodiment of the present invention has a superstrate structure in which light L is incident from the substrate 11a side and photoelectric conversion is performed in the photoelectric conversion layer 17. .
The thin-film solar cell 20 is configured such that a photoelectric conversion layer 17, a back reflection layer 15, and a back electrode 16 are sequentially stacked on a solar cell substrate 11 including a substrate 11a and a transparent conductive layer 11b formed thereon. ing.
The photoelectric conversion layer 17 includes a p-type silicon layer 12, an i-type silicon layer 13, and an n-type silicon layer 14.
[0019]
In the thin-film solar cell 20, the electrodes 18a and 18b are extracted from the transparent conductive layer 11b and the back surface electrode 16, respectively, and the electric power generated by photoelectric conversion in the photoelectric conversion layer is extracted through these electrodes 18a and 18b.
[0020]
substrate
As the substrate 11a, for example, a plastic substrate such as glass or acrylic is used as the light-transmitting insulating substrate.
[0021]
Transparent conductive layer
The transparent conductive layer 11b is formed on the substrate 11a and is made of a zinc oxide (ZnO) thin film layer having high plasma resistance. However, the transparent conductive layer 11b is not limited to this, and the zinc oxide thin film layer and the SnO₂, In₂O₃, ITO or the like may be a composite layer. When a normal pressure CVD method or a plasma CVD method in which a film forming condition with low reducibility by hydrogen plasma is selected as a method for depositing the photoelectric conversion layer, the transparent conductive layer 11b is formed of SnO.₂, In₂O₃, ITO or the like or a composite layer obtained by laminating them.
[0022]
The transparent conductive layer 11b preferably has a thickness of about 1 nm to 2 μm and is formed by a sputtering method. This is because the sputtering method can easily control the transmittance and the resistivity of the transparent conductive layer 11b to those suitable for the crystalline silicon thin film solar cell. Further, the transparent conductive layer 11b may contain an impurity in order to reduce the resistivity. Examples of such impurities include Group III elements such as gallium and aluminum.²⁰~ 5 × 10²¹cm^-3It can be. In forming the transparent conductive layer 11b, a vacuum evaporation method, an EB evaporation method, a normal pressure CVD method, a low pressure CVD method, a sol-gel method, an electrodeposition method, or the like can be used other than the sputtering method.
[0023]
The surface of the transparent conductive layer 11b has a substantially hemispherical concave portion (hereinafter simply referred to as "hole" in this specification) that does not reach the substrate 11a by a plurality of etching processes using an acidic or alkaline etching solution. Many holes are formed, and finer irregularities are formed on the inner surface of the hole.
The term "a plurality of etching treatments" as used herein refers to two or more etching treatments, and practically two or three etching treatments are appropriate. That is, although the effects of the present invention can be obtained even if the processing is performed three or more times, if a suitable etching processing condition is selected, a plurality of holes and fine irregularities on the inner surface of each hole can be obtained by performing the etching processing about two to three times. Can be formed. Performing more etching processes than necessary only complicates the manufacturing process, and may cause a decrease in yield and an increase in manufacturing cost.
[0024]
Note that the plurality of holes formed on the surface of the transparent conductive layer 11b and the fine irregularities on the inner surface of each hole cause a favorable light scattering state with respect to the photoelectric conversion layer 17 and cause the light-emitting device to have a light scattering state on the transparent conductive layer 11b. The number, size, shape, depth, and the like are such that the defect density of the stacked photoelectric conversion layers 17 is reduced. The preferred setting of these holes and fine irregularities will be described in detail later.
[0025]
Photoelectric conversion layer
The photoelectric conversion layer 17 is formed on the transparent conductive layer 11b, and is formed by, for example, a pin junction. Here, the photoelectric conversion layer 17 may be a layer containing crystalline silicon or amorphous silicon. However, in the single-junction thin-film solar cell 20 shown in FIG. More preferably, it is a layer containing.
[0026]
The photoelectric conversion layer 17 can be composed of, for example, a p-type silicon layer 12, an i-type silicon layer 13, and an n-type silicon layer 14, and each of these layers can be formed of crystalline silicon. However, the present invention is not limited to this, and only one of the i-type silicon layer 13, the p-type silicon layer 12, and the n-type silicon layer 14 which is in contact with the transparent conductive layer 11b is made of crystalline silicon. Layer.
Further, the crystalline silicon referred to here is alloyed silicon, for example, Si to which carbon is added._xC_1-x, Germanium-doped Si_xGe_1-x, Or silicon to which other impurities or the like are added.
[0027]
p-type silicon layer
The p-type silicon layer 12 constituting the photoelectric conversion layer 17 is a layer containing a group III element (for example, boron, aluminum, indium, or the like). The p-type silicon layer 12 may be a single layer, or may be formed by a composite layer having a different group III element concentration or gradually changing for each of the stacked layers. The group III element concentration can be, for example, about 0.01 to 8 atomic%, and the thickness of the p-type silicon layer 12 can be, for example, about 1 to 200 nm.
[0028]
The p-type silicon layer 12 may be formed by any method as long as a silicon layer having a p-type conductivity can be formed, and a general method of forming a silicon layer is a CVD method. Here, the CVD method may be any one of normal pressure CVD, low pressure CVD, plasma CVD, ECR plasma CVD, high temperature CVD, low temperature CVD, etc., and a method using a high frequency in a frequency band from RF to VHF, ECR plasma It is preferable to form them by a CVD method or a combination thereof. For example, when using the plasma CVD method, the conditions are that the frequency is about 10 to 200 MHz, the power is about several W to several kW, the pressure in the chamber is about 0.1 to 20 Torr, and the substrate temperature is about room temperature to about 600 ° C. Can be.
[0029]
Examples of the silicon-containing gas used to form the silicon layer include SiH₄, Si₂H₆, SiF₄, SiH₂Cl₂, SiCl₄And the like. The silicon-containing gas is typically H₂, Ar, He, Ne, Xe, and other inert gases.₂Preferably, a gas is used. The mixing ratio of the silicon-containing gas and the diluent gas can be, for example, about 1: 1 to 1: 100 in terms of a volume ratio while being constant or changing.
[0030]
As the doping gas, a gas containing an arbitrary Group III element, for example, B₂H₆Etc. can be used. In this case, the mixing ratio between the silicon-containing gas and the gas containing the group III element can be adjusted as appropriate depending on the size of the film forming apparatus such as CVD, the desired concentration of the group III element, and the like. For example, the capacity ratio can be set to about 1: 0.001 to 1: 1. The doping of the group III element may be performed simultaneously with the formation of the crystalline silicon layer as described above, or after the formation of the silicon film, ion implantation, surface treatment of the crystalline silicon layer, solid phase diffusion, or the like. The method may be used. Further, a fluorine gas may be added to a silicon-containing gas or the like. As the fluorine-containing gas used here, for example, F₂, SiF₄, SiH₂F₂The amount of the fluorine-containing gas used in this case can be, for example, about 0.01 to 10 times the hydrogen gas.
[0031]
i-type silicon layer
The i-type silicon layer 13 is a layer that does not substantially exhibit the p-type and n-type conductivity types, but is a layer that exhibits a very weak p-type or n-type conductivity type that does not impair the photoelectric conversion function. You may. The i-type silicon layer 13 can be formed by, for example, plasma CVD, and its thickness can be, for example, about 0.1 to 10 μm. The i-type silicon layer 13 can be formed by a method substantially similar to that of the p-type silicon layer 12, except that the used gas does not include a group III element.
[0032]
n-type silicon layer
The n-type silicon layer 14 can be formed in the same manner as the p-type silicon layer 12 and the i-type silicon layer 13 except that a gas containing a group V element such as PH3 is used as a dopant gas. Can be, for example, about 10 to 100 nm. Examples of the impurity serving as a donor include phosphorus, arsenic, antimony, and the like.¹⁸-10²⁰cm^-3Degree.
If at least one active layer among the layers constituting the photoelectric conversion layer 17 is a layer made of crystalline silicon or a silicon alloy, long wavelength light of 700 nm or more which cannot be used for photoelectric conversion with amorphous silicon. Will be fully available.
This is due to the difference in light sensitivity between amorphous silicon having high short wavelength sensitivity and crystalline silicon having high long wavelength sensitivity.
[0033]
In particular, in the X-ray diffraction measurement, the integrated intensity I of the Si (220) X-ray diffraction peak of the active layer made of the above-mentioned crystalline silicon or silicon alloy is obtained.₂₂₀And integrated intensity I of Si (111) X-ray diffraction peak₁₁₁Ratio I₂₂₀/ I₁₁₁Is 5 or more, an active layer with very few defects is formed, and a high-quality photoelectric conversion layer 17 can be obtained.
[0034]
Back reflection layer
On the photoelectric conversion layer 17, a transparent conductive layer is formed as the back surface reflection layer 15 as necessary. This transparent conductive layer is made of, for example, SnO, similarly to the light incident side transparent conductive layer 11b.₂, In₂O₃, ZnO, ITO or the like, or a composite layer obtained by laminating them. Examples of the transparent conductive layer include a sputtering method, a vacuum evaporation method, an EB evaporation method, a normal-pressure CVD method, a low-pressure CVD method, a sol-gel method, and an electrodeposition method. In view of the advantage that the resistivity and the resistivity can be easily controlled to a value suitable for the crystalline silicon solar cell, it is preferable to form the film by using the sputtering method.
[0035]
Back electrode
On the photoelectric conversion layer 17 or the back reflection layer 15, as the back electrode 16, a single layer of a metal having a relatively high reflectance, for example, a metal such as silver or aluminum, or a composite layer obtained by laminating them is formed by a sputtering method. It is formed by a vacuum evaporation method, an EB evaporation method, a normal pressure CVD method, a low pressure CVD method, a sol-gel method, an electrodeposition method, or the like.
[0036]
Holes and fine irregularities
Here, the plurality of holes on the surface of the transparent conductive layer 11b and the fine unevenness formed on the inner surface of each hole will be described in detail.
Usually, a thin film of zinc oxide or the like used as the transparent conductive layer 11b is desirably low in resistance because it is used as an electrode. As the film quality, a film containing a large amount of crystalline is used. For this reason, a plurality of grain boundaries exist on the film surface and in the film, and when observed by a scanning electron microscope or measured by an X-ray diffractometer, a polycrystalline thin film having a particle size of several tens to several hundreds of nm is obtained. Is confirmed.
[0037]
When the transparent conductive layer 11b is oriented in a specific axis direction with respect to the substrate surface, the influence of plane orientation on processing such as etching is eliminated, and the plurality of transparent conductive layers 11b formed on the surface on the photoelectric conversion layer 17 side of the transparent conductive layer 11b are eliminated. The holes and fine irregularities formed on the inner surface of each hole can be formed uniformly.
Further, when the orientation is improved, the depth of the hole formed after the etching treatment is increased, so that long-wavelength light having a wavelength of 700 nm or more can be effectively absorbed.
[0038]
From the above, for example, when zinc oxide is used for the transparent conductive layer 11b, the integrated intensity of the ZnO (0001) diffraction peak obtained by X-ray diffraction measurement is 70% of the sum of the integrated intensity of all diffraction peaks. % Is preferable in terms of electrical and optical characteristics.
Then, in the present invention, a plurality of holes and fine irregularities on the inner surface of each hole are formed by performing a plurality of etching processes on the surface of the transparent conductive layer 11b as described above. A good light confinement effect is obtained.
[0039]
The reason why the etching process is performed a plurality of times is as follows.
That is, in general, when a transparent conductive layer such as zinc oxide, which is a polycrystalline thin film, is etched with an acidic or alkaline etching solution, only a specific surface of a crystal grain is dominant along the crystal grain boundary at the beginning of etching. Etching produces a plurality of fine holes. When the etching is further advanced, the etching proceeds as a whole, and the fine holes formed in the initial stage of the etching become larger as the etching time elapses, and eventually the transparent conductive layer itself disappears.
[0040]
Here, when the concentration or temperature of the etching solution is high, the hole diameter increases significantly as the etching time increases, and at the same time the hole diameter increases significantly. However, the etching tends to be dominant in the depth direction of the hole, and the way the hole spreads is not so large. However, in any case, only one etching process cannot form a plurality of holes and fine irregularities on the inner surface of each hole on the surface of the transparent conductive layer, which are sufficient for obtaining a good light confinement effect. .
[0041]
Therefore, in the method for manufacturing a thin-film solar cell according to the present invention, the surface etching treatment of the transparent conductive layer is divided into a plurality of times by using an acidic or alkaline etching solution, and the first etching treatment is performed in the second and subsequent etching treatments. A plurality of fine irregularities are formed on the inner surface of the hole without increasing the diameter of the hole formed by the method.
In order to form fine irregularities on the inner surface of the hole without increasing the diameter of the hole already formed in the second and subsequent etching processes, the etching conditions in the second and subsequent etching processes are increased. It is preferable to set the conditions such that the function of the anisotropic etching that proceeds in the direction is appropriately suppressed compared to the first etching process, and the inner surface of the hole formed in the first etching process is etched. . Specifically, the concentration of the etching solution used for the second and subsequent etching processes is set lower than the concentration of the acid or alkali of the etching solution used for the first etching process, or the concentration of the etching solution used for the first etching process is reduced. The temperature of the etchant used for the second and subsequent etching treatments may be set lower than the temperature of the etchant used.
[0042]
Here, the temperature range of the etching solution is preferably in the range of 5 to 80C. That is, when the temperature of the etching solution is higher than 80 ° C., it becomes difficult to control the concentration of the acid or alkali in the etching solution due to evaporation of the acid or the water as a solvent, and the desired unevenness is stably formed. This is because it becomes difficult. On the other hand, when the temperature of the etching solution is lower than 5 ° C., the etching rate may be reduced, and the production efficiency may be reduced.
[0043]
Examples of the acidic substance in the acidic etching solution include one or a mixture of two or more acids such as acetic acid, hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, and perchloric acid. Among them, acetic acid is preferable. As a solvent for these acidic substances, for example, water can be used. These acidic aqueous solutions are preferably used at a concentration of about 0.05 to 10% by weight.
On the other hand, examples of the alkaline substance in the alkaline etching solution include one or a mixture of two or more of sodium hydroxide, hydrazine, potassium hydroxide, and calcium hydroxide. Among these, sodium hydroxide is preferably used. Is preferred. Further, as a solvent for these alkaline substances, for example, water can be used. These alkaline aqueous solutions are preferably used at a concentration of about 1 to 10% by weight.
[0044]
For example, when the etching process is performed twice for the same time, and the solvent and solute of the etchant used for the first and second etching processes and the temperature are the same, the etching process used for the second etching process is performed. The concentration of the solution is lower than the concentration of the etching solution used in the first etching process.
It may be set to be lower by about 0.05 to 8% by weight.
When the etching process is performed twice for the same time, and the solvent and solute of the etchant used for the first and second etching processes and the concentration are the same, the etching process used for the second etching process is performed. The temperature of the solution is lower than the temperature of the etching solution used for the first etching process.
It may be set to be lower by about 5 to 50 ° C.
Of course, it is also possible to change both the concentration and the temperature in the first and second etching processes. In such a case, the concentration and the temperature may be appropriately changed with reference to the above-mentioned values.
[0045]
The maximum diameter of each hole formed on the surface of the transparent conductive layer 11b is in the range of about 200 to 2000 nm, and particularly preferably in the range of about 400 to 1200 nm. Further, the depth of the hole is in the range of about 50 to 1200 nm, and particularly preferably in the range of about 100 to 800 nm. Further, the height difference of the fine unevenness is in the range of about 10 to 300 nm, and particularly preferably in the range of about 20 to 200 nm.
Further, in order to generate a more favorable light scattering state, it is preferable that fine irregularities are formed on surfaces other than the holes formed on the surface of the transparent conductive layer 11b. As a result, the light scattering effect can be further enhanced, and a better light confinement effect can be obtained in the photoelectric conversion layer 17, and a thin-film solar cell exhibiting higher photoelectric conversion efficiency can be provided. In this case, the height difference of fine irregularities on the surface other than the holes is in the range of about 10 to 300 nm, and particularly preferably about 20 to 200 nm.
[0046]
By setting the size of each hole and the fine irregularities on the inner surface of each hole as described above, not only the light having a center wavelength of about 400 to 650 nm of the solar spectrum but also the light having a longer wavelength is sufficient. Light scattering can be caused. In addition, the light scattering, although there is a magnitude of the effect obtained by the uneven shape, it is not such that the effect is not obtained by the shape, the hole in the present invention, for example, a substantially cubic, rectangular parallelepiped, cylinder, cone, Any shape such as a sphere, a hemisphere, or a combination thereof may be used.
[0047]
The good light confinement effect leads to the effect that the thickness of each layer constituting the photoelectric conversion layer 17 can be reduced. In particular, in the case of an amorphous silicon solar cell, deterioration of the photoelectric conversion efficiency due to the Stabiler-Wronski effect can be suppressed. In addition, in the case of crystalline silicon solar cells that require a film thickness on the order of several μm, which is several times to ten and several times greater than amorphous silicon due to light absorption characteristics, the film formation time is greatly reduced. It leads to what you can do. Thus, a good light confinement effect contributes to improving all aspects of high efficiency, stabilization, and cost reduction, which are major issues in putting thin-film solar cells into practical use.
[0048]
Example of application of the present invention
When the present invention is applied to a thin film solar cell 20 having a superstrate type structure as shown in FIG. 1, fine irregularities on the inner surface of each hole in the surface of the transparent conductive layer 11b are deposited on the transparent conductive layer 11b and on the transparent conductive layer 11b. Acts as a thin layer having optical properties intermediate to that of the silicon thin film, reduces reflection at the interface between the transparent conductive layer 11b and the silicon thin film deposited thereon, and promotes light scattering, thereby promoting light scattering. 17 to improve the light confinement effect. As a result, a thin film solar cell 20 having a super straight type structure exhibiting high photoelectric conversion efficiency is provided.
[0049]
In addition, when the present invention is applied to a thin film solar cell 50 having a substrate type structure as shown in FIG. 2, fine irregularities on the inner surface of each hole on the surface of the transparent conductive layer 44 are caused by the transparent conductive layer 44 and an external Acts as a thin layer having optical characteristics intermediate to that of the atmosphere (which may be a protective film), reduces reflection at the interface between the transparent conductive layer 44 and the outside atmosphere, and allows light to enter the photoelectric conversion layer 45. To promote. As a result, a thin film solar cell 50 having a substrate type structure exhibiting high photoelectric conversion efficiency is provided.
[0050]
As described above, according to the present invention, in addition to the super-straight structure in which light enters from the substrate 11a side as shown in FIG. 1, the light L enters from the transparent conductive layer 44 side as shown in FIG. It can also be applied to a substrate type structure.
Further, since the above-described light confinement effect and the effect of reducing the reflection of incident light are not limited to polycrystalline silicon thin film solar cells, the present invention relates to amorphous silicon solar cells, CIGS solar cells, dye-sensitized solar cells, etc. Needless to say, it is also effective for thin film solar cells.
[0051]
Further, the application example of the present invention is not limited to the single-junction thin-film solar cells 20 and 50 as shown in FIGS. 1 and 2, and a multi-junction thin-film solar cell having a plurality of pin junctions as shown in FIG. The present invention is also applicable to the battery 60.
In the multi-junction thin-film solar cell 60 as shown in FIG. 3, all photoelectric conversion layers, that is, the first and second silicon photoelectric conversion layers 27 and 37 are unified to either crystalline silicon or amorphous silicon. However, for example, a layer containing crystalline silicon and a layer containing amorphous silicon may be stacked. Rather, in the multi-junction thin-film solar cell 60 shown in FIG. 3, it is preferable that a layer containing crystalline silicon and a layer containing amorphous silicon are mixed.
[0052]
When the first and second silicon photoelectric conversion layers 27 and 37 contain crystalline silicon, unless otherwise specified, not only polycrystals and single crystals but also all crystal states called so-called microcrystals or microcrystals Contains. These crystalline states may exist over the entirety of the first and second silicon photoelectric conversion layers 27 and 37, or may be entirely or partially present in each of the layers constituting the first and second silicon photoelectric conversion layers 27 and 37. May be present.
Examples of the method for manufacturing a thin film solar cell according to the present invention will be specifically described below.
[0053]
Example 1
In Example 1, the single-junction thin-film solar cell 20 having the superstrate type structure shown in FIG. 1 described in the above embodiment was manufactured.
[0054]
First, a glass plate having a smooth surface is used as the substrate 11a, and zinc oxide is formed as a transparent conductive layer 11b with a thickness of 1200 nm on the substrate 11a by a magnetron sputtering method at a substrate temperature of 150 ° C. and a film forming pressure of 0.3 mTorr. Then, a solar cell substrate 11 was formed.
The transparent conductive layer 11b has 1 × 10²¹cm^-3About gallium is added. As a result, the sheet resistance of the obtained transparent conductive layer 11b was 10Ω / □, and the transmittance for light having a wavelength of 800 nm was 80%. Further, the transparent conductive layer 11b was subjected to an X-ray diffraction measurement by a powder method using an X-ray diffractometer RINT {X-ray Diffactometer} manufactured by Rigaku Corporation. As a result, the integrated intensity of the ZnO (0001) diffraction peak was 75% with respect to the sum of the integrated intensity of all diffraction peaks.
[0055]
Subsequently, irregularities were formed on the surface of the transparent conductive layer 11b.
First, a 5.0% by weight aqueous solution of acetic acid (solution temperature: 25 ° C.) was prepared as an etching solution, and the solar cell substrate 11 was immersed in the aqueous solution of acetic acid for 150 seconds. Next, a 0.5% by weight aqueous solution of acetic acid (solution temperature: 25 ° C.) is prepared as an etching solution, and the solar cell substrate 11 is immersed in the acetic acid aqueous solution for 100 seconds. Was thoroughly washed. In the first embodiment, the first etching process and the second etching process are continuously performed, but a cleaning / drying process may be provided between them.
[0056]
Before the formation of the photoelectric conversion layer 17, the surface of the transparent conductive layer 11b was observed using a scanning electron microscope JSM-5310LV manufactured by JEOL Ltd., and the shape of the hole was substantially circular and the maximum diameter of the hole was 22 mm. (See FIG. 4) is within the range of 300 to 1200 nm, and the density is 1 piece / μm.²It was about.
In order to examine the inner surface and the peripheral surface of the hole in more detail, the surface of the transparent conductive layer 11b was observed using an atomic force microscope Nanoscope III manufactured by Toyo Corporation. FIG. 4 schematically shows the holes formed on the surface of the transparent conductive layer, the fine irregularities formed on the inner surface thereof, and the surface shape around the holes in the thin-film solar cell according to the embodiment of the present invention.
[0057]
As a result of the observation, the depth 21 of the hole shown in FIG. 4 was in the range of 100 to 800 nm. Fine irregularities are also formed on the inner surface of the hole, and the fine irregularities have a size 23 (difference in elevation) in the range of 20 to 150 nm and a gap 24 in the range of 80 to 350 nm. Was within. That is, it was found that the transparent conductive layer 11b had a structure in which finer irregularities were formed on the inner surface of the formed hole.
Fine irregularities were also formed on the surface of the transparent conductive layer 11b other than the holes, and the size of the fine irregularities was in the range of 50 to 150 nm.
[0058]
Subsequently, a 20 nm-thick p-type silicon layer 12, a 2 μm-thick i-type silicon layer 13, and a 30 nm-thick n-type silicon layer 14 are sequentially formed on the obtained solar cell substrate 11 by a high-frequency plasma CVD method. The layers were stacked to form a photoelectric conversion layer 17 made of crystalline silicon.
In forming the p-type silicon layer 12, SiH₄Gas is H at 50 times flow rate ratio₂The material diluted with the gas is used as a source gas, and₂H₆Gas is SiH₄0.1% was added to the gas flow rate. In forming the i-type silicon layer 13, SiH₄Gas is H at 50 times flow rate ratio₂The material diluted with the gas was used as a source gas. In forming the n-type silicon layer 14, SiH₄Gas is H at 50 times flow rate ratio₂The gas diluted with the gas is used as a source gas, and the₃Gas is SiH₄0.01% was added to the gas flow rate. The substrate temperature during film formation was 200 ° C. for each layer.
[0059]
After the formation of the photoelectric conversion layer 17, the solar cell substrate 11 was taken out of the plasma CVD apparatus, and the obtained photoelectric conversion layer 17 was subjected to X-ray diffraction measurement. As a result, the integrated intensity I of the Si (220) X-ray diffraction peak₂₂₀And integrated intensity I of Si (111) X-ray diffraction peak₁₁₁Ratio I₂₂₀/ I₁₁₁Was 5.5.
Here, the actually obtained X-ray diffraction peak is not information of the i-type silicon layer 13 alone in the photoelectric conversion layer 17, but the p-type silicon layer 12 and the n-type silicon Since the thickness of the layer 14 is very small, it is considered that the thickness reflects the crystal orientation of the i-type silicon layer 13.
[0060]
Thereafter, zinc oxide is formed as a back reflection layer 15 on the photoelectric conversion layer 17 by a magnetron sputtering method so as to have a thickness of about 50 nm, and then silver is formed as a back electrode 16 on the back reflection layer 15 by an electron beam evaporation method. A film was formed over a thickness of about 500 nm, and a single-junction thin-film solar cell 20 having a super straight type structure in which light L was incident from the substrate 11a side was obtained.
[0061]
Example 2
In the second embodiment, only the conditions of the etching process for forming irregularities on the surface of the transparent conductive layer are changed from those in the first embodiment, and the other conditions are the same as in the first embodiment, and the superstrate structure shown in FIG. This is a junction type thin-film solar cell 20 manufactured.
In Example 2, when forming irregularities on the surface of the transparent conductive layer 11b, first, an aqueous solution of acetic acid of 1.0% by weight (liquid temperature of 25 ° C.) was prepared as an etchant, and the solar cell substrate 11 was added to the aqueous solution of acetic acid. Was immersed for 250 seconds. Next, a 5.0% by weight acetic acid aqueous solution (solution temperature: 25 ° C.) was prepared as an etching solution, and the solar cell substrate 11 was immersed in the acetic acid aqueous solution for 30 seconds. Was thoroughly washed.
[0062]
When the surface of the transparent conductive layer 11b was observed with a scanning electron microscope in the same manner as in Example 1, the shape of the hole was substantially circular, the maximum diameter 22 shown in FIG. 4 was in the range of 200 to 1600 nm, and the density was 2 pieces / μm²It was about.
Further, as in Example 1, when the inner surface and the peripheral surface of the hole were observed in more detail with an atomic force microscope, the depth 21 of the hole shown in FIG. 4 was in the range of 200 to 1100 nm. Fine irregularities are also formed on the inner surface of the hole, and the fine irregularities have a size 23 (difference in height of irregularities) in a range of 10 to 50 nm and an interval 24 in a range of 300 to 1000 nm. Was within. Fine irregularities were also formed on the surface of the transparent conductive layer 11b other than the holes, and the size of the fine irregularities was in the range of 100 to 300 nm.
[0063]
Here, comparing the thin-film solar cells according to Examples 1 and 2, it can be seen that in Example 2, the interval 24 between the fine irregularities on the inner surface of the hole is wider than in Example 1.
This is because if the concentration of the etching solution used for the second etching process becomes higher than the concentration of the etching solution used for the first etching process, the hole is formed by combining the first etching process and the second etching process. This is considered to be due to the fact that it becomes difficult to form fine irregularities on the inner surface of the hole.
[0064]
Example 3
Example 3 differs from Examples 1 and 2 only in the conditions of the etching treatment for forming irregularities on the surface of the transparent conductive layer, and is the same as Example 1 except for the superstrate shown in FIG. A single-junction thin-film solar cell 20 having a mold structure is manufactured.
In Example 3, when forming irregularities on the surface of the transparent conductive layer 11b, first, a 1.5 wt% acetic acid aqueous solution (liquid temperature of 40 ° C.) was prepared as an etching solution, and the solar cell substrate 11 was added to the acetic acid aqueous solution. Was immersed for 50 seconds. Next, a 1.5% by weight aqueous acetic acid solution (solution temperature: 25 ° C.) was prepared as an etching solution, and the solar cell substrate 11 was immersed in the acetic acid aqueous solution for 60 seconds. Was thoroughly washed.
[0065]
As in Example 1, when the surface of the transparent conductive layer 11b was observed with a scanning electron microscope, the shape of the hole was substantially circular, the maximum diameter shown in FIG. 4 was in the range of 400 to 1200 nm, and the density was 1 Pieces / μm²It was about.
Further, as in Example 1, when the inner surface and the peripheral surface of the hole were observed in more detail with an atomic force microscope, the depth 21 of the hole shown in FIG. 4 was in the range of 100 to 900 nm. Fine irregularities are also formed on the inner surface of the hole, and the fine irregularities have a size 23 (difference in height of irregularities) in a range of 30 to 200 nm and a gap 24 in a range of 80 to 500 nm. Was within. Fine irregularities were also formed on the surface of the transparent conductive layer 11b other than the holes, and the size of the fine irregularities was in the range of 50 to 100 nm.
[0066]
Example 4
Example 4 differs from Examples 1 to 3 only in the conditions of the etching treatment for forming irregularities on the surface of the transparent conductive layer, and is otherwise the same as in Example 1 except that the superstrate shown in FIG. A single-junction thin-film solar cell 20 having a mold structure is manufactured.
In Example 4, when forming irregularities on the surface of the transparent conductive layer 11b, first, a 1.5% by weight acetic acid aqueous solution (liquid temperature: 25 ° C.) was prepared as an etching solution, and the acetic acid aqueous solution was added to the solar cell substrate 11 For 150 seconds. Next, a 1.5% by weight acetic acid aqueous solution (liquid temperature: 40 ° C.) was prepared as an etching solution, and the solar cell substrate 11 was immersed in the acetic acid aqueous solution for 30 seconds. Was thoroughly washed.
[0067]
As in Example 1, when the surface of the transparent conductive layer 11b was observed with a scanning electron microscope, the shape of the hole was substantially circular, the maximum diameter shown in FIG. 4 was in the range of 200 to 1800 nm, and the density was 1 Pieces / μm²It was about.
Further, as in Example 1, when the inner surface and the peripheral surface of the hole were observed in more detail with an atomic force microscope, the depth 21 of the hole shown in FIG. 4 was in the range of 300 to 1000 nm. Further, fine irregularities are also formed on the inner surface of the hole, and the fine irregularities have a size 23 (difference in height of irregularities) within a range of 20 to 80 nm, and an interval 24 within a range of 200 to 800 nm. Was within. Fine irregularities were also formed on the surface of the transparent conductive layer 11b other than the holes, and the size of the fine irregularities was in the range of 100 to 200 nm.
[0068]
Here, comparing the thin-film solar cells according to Examples 3 and 4, it can be seen that in Example 4, the interval 24 between the fine irregularities on the inner surface of the hole is wider than in Example 3.
This is because if the temperature of the etching solution used for the second etching process becomes higher than the temperature of the etching solution used for the first etching process, the hole is formed by combining the first etching process and the second etching process. This is considered to be due to the fact that it becomes difficult to form fine irregularities on the inner surface of the hole.
[0069]
Example 5
In the fifth embodiment, only the conditions of the etching process for forming irregularities on the surface of the transparent conductive layer are changed from those of the first to fourth embodiments, and otherwise, the superstrate shown in FIG. A single-junction thin-film solar cell 20 having a mold structure is manufactured.
In Example 5, when forming irregularities on the surface of the transparent conductive layer 11b, first, a 2.0% by weight aqueous solution of sodium hydroxide (solution temperature: 25 ° C.) was prepared as an etching solution, and The battery substrate 11 was immersed for 80 seconds. Next, a 0.5% by weight aqueous solution of sodium hydroxide (solution temperature: 25 ° C.) was prepared as an etchant, and the solar cell substrate 11 was immersed in the aqueous sodium hydroxide solution for 60 seconds. The surface was thoroughly washed with pure water.
[0070]
As in Example 1, when the surface of the transparent conductive layer 11b was observed with a scanning electron microscope, the shape of the hole was substantially circular, the maximum diameter shown in FIG. 4 was in the range of 300 to 1000 nm, and the density was 1 Pieces / μm²It was about.
Further, as in Example 1, when the inner surface and the peripheral surface of the hole were observed in more detail with an atomic force microscope, the depth 21 of the hole shown in FIG. 4 was in the range of 100 to 800 nm. Further, fine irregularities are also formed on the inner surface of the hole, and the fine irregularities have a size 23 (difference in elevation) within a range of 10 to 100 nm and an interval 24 within a range of 100 to 400 nm. Was within. Fine irregularities were also formed on the surface of the transparent conductive layer 11b other than the holes, and the size of the fine irregularities was in the range of 80 to 150 nm.
[0071]
Example 6
Example 6 differs from Examples 1 to 5 only in the etching conditions for forming irregularities on the surface of the transparent conductive layer in the same manner as in Example 1 except for the superstrate shown in FIG. A single-junction thin-film solar cell 20 having a mold structure is manufactured.
In the sixth embodiment, when forming irregularities on the surface of the transparent conductive layer 11b, first, a 1.0% by weight aqueous sodium hydroxide solution (liquid temperature of 40 ° C.) is prepared as an etching solution, and The battery substrate 11 was immersed for 80 seconds. Next, a 1.0% by weight aqueous solution of sodium hydroxide (solution temperature of 25 ° C.) was prepared as an etching solution, and the solar cell substrate 11 was immersed in the aqueous solution of sodium hydroxide for 30 seconds. The surface was thoroughly washed with pure water.
[0072]
As in Example 1, when the surface of the transparent conductive layer 11b was observed with a scanning electron microscope, the shape of the hole was substantially circular, the maximum diameter shown in FIG. 4 was in the range of 350 to 1200 nm, and the density was 1 Pieces / μm²It was about.
Further, as in Example 1, when the inner surface and the peripheral surface of the hole were observed in more detail with an atomic force microscope, the depth 21 of the hole shown in FIG. 4 was in the range of 150 to 1000 nm. Fine irregularities are also formed on the inner surface of the hole, and the fine irregularities have a size 23 (difference in height of irregularities) in a range of 20 to 180 nm and an interval 24 in a range of 120 to 460 nm. Was within. Fine irregularities were also formed on the surface of the transparent conductive layer 11b other than the holes, and the size of the fine irregularities was in the range of 100 to 200 nm.
[0073]
Comparative Example 1
In Comparative Example 1, when forming irregularities on the surface of the transparent conductive layer, the irregularities were formed by only one etching treatment, and otherwise the same as in Example 1 except that the thin film was of a super-straight type structure and a single-junction type. A solar cell (not shown) was manufactured.
In Comparative Example 1, when forming irregularities on the surface of the transparent conductive layer, the solar cell substrate was immersed in a 2.5% by weight aqueous sodium hydroxide solution (liquid temperature: 25 ° C.) for 120 seconds.
[0074]
When the surface of the transparent conductive layer was observed with a scanning electron microscope in the same manner as in Example 1, the shape of the hole was substantially circular, and the maximum diameter 32 (see FIG. 5) was in the range of 80 to 2000 nm. Is 2.5 pieces / μm²It was about.
Further, similarly to Example 1, the inner surface and the peripheral surface of the hole were observed in more detail with an atomic force microscope. FIG. 5 schematically shows the hole formed on the surface of the transparent conductive layer, the shape of the inner surface thereof, and the surface shape around the hole in the thin-film solar cell according to the comparative example. As shown in FIG. 5, the depth 31 of the hole formed in Comparative Example 1 was 300 to 1100 nm, and only a slight change in shape such as a height difference 33 of 10 nm or less was found on the inner surface of the hole. Was.
On the other hand, on the surface of the transparent conductive layer other than the holes, large irregularities were formed so as to be difficult to distinguish from the holes.
[0075]
In Comparative Example 1, the density of the holes was 2.5 holes / μm.²The reason for this is that the transparent conductive layer is dug down by only one etching process, so that fine irregularities are difficult to be formed. In Examples 1 to 6, the irregularities are classified as fine irregularities on the surface other than the holes. It is conceivable that the size was increased to such an extent that it was classified as a hole in Comparative Example 1.
[0076]
Here, the surface shape of the transparent conductive layer and the orientation of the photoelectric conversion layer of the thin film solar cells according to Examples 1 to 7 and Comparative Example 1 are shown in Table 1 below.
[0077]
[Table 1]

[0078]
In addition, the AM1.5 (100 mW / cm) of the thin-film solar cells according to Examples 1 to 7 and Comparative Example 1 described above.²Table 2 shows the current-voltage characteristics under the irradiation conditions.
[0079]
[Table 2]

[0080]
Here, Examples 1 to 6 and Comparative Example 1 will be compared and studied with reference to Tables 1 and 2 above.
As shown in Table 1, the surfaces of the transparent conductive layers of the thin-film solar cells according to Examples 1 to 6 were each subjected to a plurality of etching treatments so that a plurality of holes were formed, and the fine inner surface of each hole was formed. Irregularities and fine irregularities on the surface other than the holes are formed (see FIG. 4).
On the other hand, in Comparative Example 1, while the maximum diameter range of the hole is wider and the number of holes is larger than in Examples 1 to 6, fine irregularities on the inner surface of the hole are reduced, and the number is also reduced. ing.
On the other hand, the orientation I of the photoelectric conversion layer is determined by X-ray diffraction measurement.₂₂₀/ I₁₁₁In each of Examples 1 to 6 and Comparative Example 1, a value of 5.0 or more was obtained, so that a good film with few defects was formed for the photoelectric conversion layer of any thin-film solar cell. it is conceivable that.
[0081]
Furthermore, comparing the current-voltage characteristics of the thin-film solar cells of Examples 1 to 6 and Comparative Example 1 based on Table 2, all of the thin-film solar cells have similar open-circuit voltages and form factors. Nevertheless, it can be seen that the short-circuit current of Comparative Example 1 is lower than Examples 1 to 6. That is, the thin-film solar cell according to Comparative Example 1 has a light confinement effect that is not as sufficient as Examples 1 to 6.
Thereby, the surface shape of the transparent conductive layer 11b, that is, the plurality of holes observed in the transparent conductive layer 11b of the thin film solar cells 20 according to Examples 1 to 6 and the fine irregularities on the inner surface of each hole are reduced by the light confinement effect. Can be considered to have greatly contributed to
Specifically, by forming fine irregularities on the inner surface of each hole, sufficient light scattering for incident light is generated at the interface between the transparent conductive layer 11b and the silicon thin film, and the transparent conductive layer This is probably because reflection at the interface with the silicon thin film deposited thereon was reduced, and effective light incidence on the photoelectric conversion layer 17 was promoted. As a result, it is considered that a good light confinement effect was obtained.
[0082]
Further, among Examples 1 to 6, Examples 2 and 4 showed the lowest short-circuit current value. This is thought to be because the conditions of the second etching treatment were such that holes were more likely to be formed than the conditions of the first etching treatment, so that fine irregularities were not sufficiently formed on the inner surface of the holes. Can be
Thereby, after performing an etching process in which holes are easily formed on the surface of the transparent conductive layer 11b and then performing an etching process in which fine irregularities are easily formed, sufficient light scattering occurs on the surface of the transparent conductive layer 11b. This is important for obtaining a sufficient light confinement effect in the photoelectric conversion layer, and can be considered to lead to the production of a thin-film solar cell exhibiting high photoelectric conversion efficiency.
In Examples 1 to 6, only the transparent conductive layer 11b formed on the substrate 11a was subjected to the surface treatment by etching. However, the same processing was performed on the transparent conductive layer used as the back reflection layer 15 on the back electrode 16 side. It is considered that the same light confinement effect can be obtained even if the above method is applied.
[0083]
Example 7
In Example 7, a single-junction thin-film solar cell 50 having the substrate type structure shown in FIG. 2 mentioned in the above embodiment was manufactured.
As shown in FIG. 2, the thin-film solar cell 50 manufactured in Example 7 has a configuration in which a photoelectric conversion layer 45 and a transparent conductive layer 44 are sequentially stacked on a solar cell substrate 40.
Here, the solar cell substrate 40 is configured by sequentially laminating a back electrode 40b and a back reflection layer 40c made of a transparent conductive material on a stainless steel substrate 40a.
Further, the photoelectric conversion layer 45 is configured by sequentially stacking an n-type silicon layer 41, an i-type silicon layer 42, and a p-type silicon layer 43.
[0084]
The single-junction thin-film solar cell 50 having such a substrate-type structure was manufactured by the following procedure.
First, silver is formed in a thickness of 500 nm as a back electrode 40b on a stainless steel substrate 40a having a smooth surface by electron beam evaporation, and then zinc oxide is formed as a back reflection layer 40c on the back electrode 40b by magnetron sputtering. Was formed over a thickness of 50 nm to form a solar cell substrate 40.
[0085]
Subsequently, an n-type silicon layer 41 having a thickness of 30 nm, an i-type silicon layer 42 having a thickness of 2 μm, and a p-type silicon layer 43 having a thickness of 20 nm are sequentially formed on the obtained solar cell substrate 40 by a high-frequency plasma CVD method. The layers were stacked to form the photoelectric conversion layer 45.
In forming the n-type silicon layer 41, SiH₄Gas is H at 50 times flow rate ratio₂The gas diluted with the gas is used as a source gas, and the₃Gas is SiH₄0.01% was added to the gas flow rate. In forming the i-type silicon layer 13, SiH₄Gas is H at 50 times flow rate ratio₂The material diluted with the gas was used as a source gas. In forming the p-type silicon layer 43, SiH₄Gas is H at 50 times flow rate ratio₂The material diluted with the gas is used as a source gas, and₂H₆Gas is SiH₄0.1% was added to the gas flow rate. The substrate temperature during film formation was 200 ° C. for each layer.
[0086]
Subsequently, on the formed photoelectric conversion layer 45, zinc oxide was formed as a transparent conductive layer 44 to a thickness of 1000 nm as a transparent conductive layer 44 by a magnetron sputtering method at a substrate temperature of 150 ° C. and a film forming pressure of 0.3 mTorr. 1 × 10²¹cm^-3About gallium is added. As a result, the sheet resistance of the obtained transparent conductive layer 44 was 10 Ω / □, and the transmittance for light having a wavelength of 800 nm was 80%. When the X-ray diffraction measurement was performed on the transparent conductive layer 44, the integrated intensity of the ZnO (0001) diffraction peak was 75% with respect to the sum of the integrated intensity of all the diffraction peaks.
[0087]
Subsequently, irregularities were formed on the surface of the transparent conductive layer 44.
First, a 0.35% by weight aqueous solution of hydrochloric acid (solution temperature: 25 ° C.) was prepared as an etchant, and the thin film solar cell being manufactured in the previous step was immersed in the aqueous solution of hydrochloric acid for 100 seconds. Next, a 0.1% by weight aqueous solution of hydrochloric acid (solution temperature: 25 ° C.) was prepared as an etching solution, immersed again in this aqueous solution of hydrochloric acid for 100 seconds, and the surface was sufficiently washed with pure water. And a single-junction thin-film solar cell 50 having a substrate-type structure in which light L is incident.
[0088]
When the surface of the transparent conductive layer 44 was observed with a scanning electron microscope in the same manner as in Example 1, the shape of the hole was substantially circular, the maximum diameter was in the range of 300 to 1000 nm, and the number density was 1 / μm²It was about.
Further, as in Example 1, when the inner surface and the peripheral surface of the hole were observed in more detail with an atomic force microscope, the depth 21 of the hole shown in FIG. 4 was in the range of 80 to 900 nm. Fine irregularities are also formed on the inner surface of the hole, and the fine irregularities have a size (difference in height) 23 in the range of 10 to 180 nm and a gap 24 in the range of 80 to 600 nm. Was within. Fine irregularities were also formed on the surface of the transparent conductive layer 44 other than the holes, and the size of the fine irregularities was in the range of 30 to 150 nm.
[0089]
Comparative Example 2
In Comparative Example 2, when forming the irregularities on the surface of the transparent conductive layer, the irregularities were formed by only one etching process, and otherwise the same as in Example 7, the substrate type structure and the single-junction thin film. A solar cell (not shown) was manufactured.
In Comparative Example 1, in forming irregularities on the surface of the transparent conductive layer, a thin-film solar cell being manufactured in which a transparent conductive layer on the light-receiving surface side was sequentially laminated in a 0.35% by weight aqueous hydrochloric acid solution (liquid temperature: 25 ° C.). For 120 seconds.
[0090]
When the surface of the transparent conductive layer was observed with a scanning electron microscope in the same manner as in Example 1, the shape of the hole was substantially circular, the maximum diameter shown in FIG. 5 was in the range of 100 to 1600 nm, and the density was about 2 .8 pieces / μm²It was about.
Further, as in Example 1, when the inner surface and the peripheral surface of the hole were observed in more detail with an atomic force microscope, the depth 31 of the hole shown in FIG. 5 was in the range of 300 to 900 nm, Only a slight change in shape was observed on the inner surface of the hole with a height difference 33 of 10 nm or less.
On the other hand, on the surface of the transparent conductive layer other than the holes, large irregularities were formed so as to be difficult to distinguish from the holes.
[0091]
In Comparative Example 2, the density of the holes was 2.8 / μm.²The reason for the increase is that the transparent conductive layer is dug down by only one etching treatment as in Comparative Example 1 described above, so that fine irregularities are difficult to be formed. It is conceivable that the surface was classified as fine irregularities on the surface, but in Comparative Example 2, the size became large enough to be classified as a hole.
[0092]
Here, AM1.5 (100 mW / cm) of the thin-film solar cells according to Example 7 and Comparative Example 2 described above.²) The current-voltage characteristics under irradiation conditions are shown in Table 3 below.
[0093]
[Table 3]

[0094]
Comparing the current-voltage characteristics of the thin-film solar cells according to Example 7 and Comparative Example 2 based on Table 3 above, the short-circuit current was not changed even though the values of the open-circuit voltage and the form factor hardly changed. It can be seen that 7 shows a higher value than Comparative Example 2.
Accordingly, it can be considered that the plurality of holes and the fine irregularities on the inner surface of each hole, which are observed in the transparent conductive layer 44 of the thin-film solar cell according to Example 7, greatly contribute to the improvement of the short-circuit current. .
Specifically, by forming fine irregularities on the inner surface of each hole, reflection of incident light is reduced at the interface between the transparent conductive layer 44 and the outside atmosphere, and light incidence on the photoelectric conversion layer 45 is promoted. It is thought that it was accumulating.
[0095]
In Example 7, only the transparent conductive layer formed on the substrate was subjected to the surface treatment by etching. However, when the transparent conductive layer used as the back reflection layer on the back electrode side was subjected to the same treatment, It is considered that sufficient light scattering occurs at the interface between the back reflection layer and the photoelectric conversion layer, and a good light confinement effect in the photoelectric conversion layer 45 can be obtained.
[0096]
Example 8
In Example 8, a multi-junction thin-film solar cell 60 having the superstrate structure shown in FIG. 3 mentioned in the above embodiment was manufactured.
As shown in FIG. 3, the thin-film solar cell 60 manufactured in Example 8 includes a first photoelectric conversion layer 27 on a solar cell substrate 11 including a substrate 11a and a transparent conductive layer 11b formed thereon. , An intermediate layer 11c made of a transparent conductive material, a second photoelectric conversion layer 37, a back reflection layer 15, and a back electrode 16 are sequentially laminated.
[0097]
The first photoelectric conversion layer 27 is composed of a p-type amorphous silicon layer 12a, an i-type amorphous silicon layer 13a, and an n-type amorphous silicon layer 14a. It comprises a type silicon layer 12b, an i-type silicon layer 13b and an n-type silicon layer 14b.
[0098]
The multi-junction thin-film solar cell 60 having such a superstrate structure was manufactured by the following procedure.
First, as in Example 1, a glass plate having a smooth surface was used as the substrate 11a, and a zinc oxide was used as the transparent conductive layer 11b on the substrate 11a at a substrate temperature of 150 ° C., a film forming pressure of 0.3 mTorr by a magnetron sputtering method. Was formed over a thickness of 1200 nm to form a solar cell substrate 11.
Subsequently, the surface of the transparent conductive layer 11b was etched twice in the same manner as in Example 2 to form irregularities.
After that, when the surface of the transparent conductive layer was observed with a scanning electron microscope and an atomic force microscope in the same manner as in Example 1, a plurality of holes having a substantially circular shape and fine irregularities on the inner surface of the holes were confirmed. .
[0099]
Subsequently, on the obtained solar cell substrate 11, a 20-nm thick p-type amorphous silicon layer 12a, a 300-nm thick i-type amorphous silicon layer 13a, and a 30-nm thick n-type amorphous silicon The layers 14a were sequentially stacked by a high-frequency plasma CVD method to form a first photoelectric conversion layer 27 made of amorphous silicon.
In forming the p-type amorphous silicon layer 12a, SiH₄5 times the gas flow rate H₂The material diluted with the gas is used as a source gas, and₂H₆Gas is SiH₄0.1% was added to the gas flow rate. In forming the i-type amorphous silicon layer 13a, SiH₄5 times the gas flow rate H₂The material diluted with the gas was used as a source gas. In forming the n-type amorphous silicon layer 14a, SiH₄5 times the gas flow rate H₂The gas diluted with the gas is used as a source gas, and the₃Gas is SiH₄0.01% was added to the gas flow rate. The substrate temperature during film formation was 200 ° C. for each layer.
[0100]
Thereafter, zinc oxide was formed as the intermediate layer 11c on the first photoelectric conversion layer 27 over the film thickness of 5 nm under the same conditions as the transparent conductive layer 11b by magnetron sputtering.
Thereafter, a second photoelectric conversion layer 37 was formed on the intermediate layer 11c under the same conditions as the photoelectric conversion layer 17 of Example 1.
When the X-ray diffraction measurement was performed after the formation of the second photoelectric conversion layer 37, the integrated intensity I of the Si (220) X-ray diffraction peak was obtained.₂₂₀And integrated intensity I of Si (111) X-ray diffraction peak₁₁₁Ratio I₂₂₀/ I₁₁₁Was 5.5, and no difference from Example 1 was observed.
Thereafter, in the same manner as in Example 1, the back reflection layer 15 and the back electrode 16 were formed, and a multi-junction thin-film solar cell 60 having a super straight structure in which light L was incident from the substrate 11a was obtained.
[0101]
Comparative Example 3
In Comparative Example 3, when forming the irregularities on the surface of the transparent conductive layer constituting the solar cell substrate, the irregularities were formed by only one etching treatment. This is a multi-junction type thin-film solar cell with a structure.
In Comparative Example 3, as in Comparative Example 1, when forming irregularities on the surface of the transparent conductive layer, the solar cell substrate was immersed in a 2.5% by weight aqueous sodium hydroxide solution (at a liquid temperature of 25 ° C.) for 120 seconds. .
After the formation of the second photoelectric conversion layer 37, X-ray diffraction measurement was performed in the same manner as in Example 8, and the integrated intensity I of the Si (220) X-ray diffraction peak was obtained.₂₂₀And integrated intensity I of Si (111) X-ray diffraction peak₁₁₁Ratio I₂₂₀/ I₁₁₁Was 5.0.
[0102]
Here, the AM1.5 (100 mW / cm) of the thin film solar cells according to Example 8 and Comparative Example 3 described above.²Table 4 shows current-voltage characteristics under irradiation conditions.
[0103]
[Table 4]

[0104]
Comparing the current-voltage characteristics of the thin-film solar cells according to Example 8 and Comparative Example 3 based on Table 4 above, the short-circuit current was not changed even though the values of the open-circuit voltage and the form factor hardly changed. It can be seen that 8 shows a higher value than Comparative Example 3.
Accordingly, it can be considered that the plurality of holes and the fine irregularities on the inner surface of each hole, which are observed in the transparent conductive layer 11b of the thin-film solar cell according to Example 8, greatly contribute to the improvement of the short-circuit current. .
Specifically, by forming fine irregularities on the inner surface of each hole, sufficient light scattering with respect to incident light occurs at the interface between the transparent conductive layer 11b and the first photoelectric conversion layer 27, so that good photoelectric conversion layers are obtained. It is considered that a great light confinement effect was obtained.
[0105]
In Example 8, only the transparent conductive layer 11b formed on the substrate 11a was subjected to the surface treatment by etching. However, the back reflection layer 15, the first photoelectric conversion layer 27, and the second photoelectric conversion layer 27 on the back electrode 16 side. It is considered that the same effect can be obtained by performing the same treatment on the intermediate layer 11c inserted between the layer 37 and the intermediate layer 11c.
[0106]
In the above, the thin-film solar cell having the transparent conductive layers 11b and 44 made of zinc oxide has been described in the embodiment, but the present invention is not limited to the above-described embodiment. Other than the various transparent conductive layers and the etching solution described in the above embodiment, the same effects as those of the present invention can be obtained. For example, even when an etching solution having a different acid strength or alkali strength is used, this effect is basically obtained. The same effect as the invention can be obtained.
[0107]
【The invention's effect】
According to the present invention, since the etching process using the etching solution composed of an acidic or alkaline solution is performed at least twice on the surface of the transparent conductive layer, the surface of the transparent conductive layer is relatively large by the first etching process. A plurality of recesses are formed, and fine recesses and projections are formed on the inner surface of each recess by the second and subsequent etching processes. As a result, ideal recesses and protrusions for obtaining a light confinement effect are formed on the surface of the transparent conductive layer. A thin-film solar cell exhibiting high photoelectric conversion efficiency can be manufactured.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically showing a structure of a single-junction thin-film solar cell having a superstrate type structure and a single junction type according to an embodiment of the present invention and Examples 1 to 6.
FIG. 2 is a cross-sectional view schematically showing a structure of a single-junction thin-film solar cell having a substrate-type structure according to an embodiment of the present invention and Example 7.
FIG. 3 is a cross-sectional view schematically showing a structure of a multi-junction thin-film solar cell having a super straight structure according to an embodiment of the present invention and Example 8.
FIG. 4 schematically shows a hole formed on the surface of a transparent conductive layer, fine irregularities formed on the inner surface thereof, and a surface shape around the hole in the thin film solar cell according to the embodiment and the example of the present invention. FIG.
FIG. 5 is an explanatory view schematically showing a hole formed on the surface of a transparent conductive layer, a shape of an inner surface thereof, and a surface shape around the hole in a thin film solar cell according to a comparative example.
[Explanation of symbols]
11 ... Substrate for solar cell
11a ... substrate
11b ... Transparent conductive layer
12 ... p-type silicon layer
13 ... i-type silicon layer
14 ... n-type silicon layer
15 ... Back reflection layer
16 ・ ・ ・ Back electrode
17 ... photoelectric conversion layer
18a, 18b ... electrodes
20 ... Thin film solar cell
L: Light

Claims (7)

Forming a first electrode layer on a substrate, forming a photoelectric conversion layer on the first electrode layer, and forming a second electrode layer on the photoelectric conversion layer, forming the first electrode layer And one of the steps of forming the second electrode layer includes an etching process of forming a transparent conductive layer as an electrode layer, and forming irregularities on the surface of the formed transparent conductive layer, and the etching process is performed in an acidic or alkaline manner. A method for manufacturing a thin-film solar cell, comprising a step of etching a surface of a transparent conductive layer at least twice using an etching solution composed of a solution. The method for manufacturing a thin-film solar cell according to claim 1, wherein the concentration of the etchant used in the second or subsequent etching is lower than that used in the first etching. 3. The method for manufacturing a thin-film solar cell according to claim 1 or 2, wherein the temperature of the etchant used for the second or subsequent etching is lower than that used for the first etching. The etching solution is obtained by dissolving a solute composed of an acidic substance or an alkaline substance in a solvent, and the etching solution used for the first etching and the etching solution used for the second and subsequent etchings have the same solute and solvent. The method for manufacturing a thin-film solar cell according to claim 1. The method for manufacturing a thin-film solar cell according to claim 1, wherein the transparent conductive layer has a zinc oxide thin-film layer. A thin-film solar cell manufactured by the manufacturing method according to claim 1, wherein the unevenness is a plurality of recesses formed on a surface of the transparent conductive layer in a first etching process, and a second unevenness. A thin-film solar cell comprising fine irregularities formed on the inner surface of each concave portion in the subsequent etching process. The thin-film solar cell according to claim 6, wherein each concave portion has a maximum diameter in a range of 200 to 2000 nm and a depth in a range of 50 to 1200 nm, and the fine irregularities have a height difference in a range of 10 to 300 nm. .

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