JP3659511B2 - Photovoltaic element - Google Patents

Description

【０１５６】
（比較例１）
ａ−Ｓｉ／ａ−ＳｉＧｅ／ａ−ＳｉＧｅトリプル型太陽電池のボトム層、ミドル層、トップ層のｉ層の膜厚を１８０ｎｍ、１４０ｎｍ、１００ｎｍとし、さらにボトム層、ミドル層のｉ層のゲルマニウム組成を０．３０とした以外は実施例１と同条件で従来型の太陽電池（ＳＣ比１）を作製した。
【０１５７】
図１の構成をしたトリプル型太陽電池のｉ層膜厚と光による加速劣化後の光電変換効率（ηｓ）の関係を調べた。
【０１５８】
予め初期光電変換効率を測定しておいた太陽電池を光による加速劣化装置に設置し、ＡＭ１．５スペクトル、照射光強度５０ＳＵＮ（５Ｗ／ｃｍ²）、基板表面温度１１０℃という劣化条件で太陽電池表面に光を照射した。光を照射し始めてから５００００秒経過したところで、この加速劣化装置から太陽電池を取り出した。次に、取り出した太陽電池を光電変換効率を測定する装置に設置し、ＡＭ１．５スペクトル、照射光強度１ＳＵＮ（１００ｍＷ／ｃｍ²）、基板表面温度２５℃の測定条件で光電変換効率を測定した。
【０１５９】
上記のような光による加速劣化と、加速劣化後の光電変換効率の測定という評価を太陽電池（ＳＣ実１）と太陽電池（ＳＣ比１）に対して行った。測定の結果、（ＳＣ実１）の加速劣化後の光電変換効率（ηｓ）は（ＳＣ比１）のそれよりも１．２３倍優れていることが分った。
【０１６０】
（実施例２）
ボトム層のｉ層（ｉ３層）を図３のようなグレーデッドバンドギャップ構造とした図１の構成をしたトリプル型太陽電池を作製し、実施例１と同様な評価を行った。
【０１６１】
ボトム層のｉ層のバンド構造が図３のようなバンドプロファイルになるようにＳｉＨ₄流量、ＧｅＨ₄流量を制御した以外は実施例１と同様なトリプル型太陽電池（ＳＣ実２）を作製し、実施例１と同様な評価を行ったところ、（ＳＣ実２）の光による加速劣化後の光電変換効率（ηｓ）は比較例１の従来型のトリプル型太陽電池（ＳＣ比１）のそれよりも１．２７倍優れていることが分った。
【０１６２】
（実施例３）
ｎ３層とｉ３層の間、ｉ３層とｐ３層の間、ｎ２層とｉ２層の間、ｉ２層とｐ２層の間にそれぞれＲＦＰＣＶＤ法で形成されａ−Ｓｉからなるバッファ層を積層する以外は実施例２と同様なトリプル型太陽電池（ＳＣ実３）を作製した。実施例１と同様な評価を行ったところ（ＳＣ実３）の光による加速劣化後の光電変換効率（ηｓ）は比較例１の従来型のトリプル型太陽電池（ＳＣ比１）のそれよりも１．３２倍優れていることが分った。
【０１６３】
（実施例４）
ｉ３層のバンド構造が図３のようなバンドプロファイルになるようにＳｉＨ₄流量、ＧｅＨ₄流量を制御し、さらにミドル層のｉ型層のゲルマニウム組成を０．６０とした以外は実施例３と同様なトリプル型太陽電池（ＳＣ実４）を作製した。実施例１と同様な評価を行ったところ（ＳＣ実４）の光による加速劣化後の光電変換効率（ηｓ）は比較例１の従来型のトリプル型太陽電池（ＳＣ比１）のそれよりも１．３５倍優れていることが分った。
【０１６４】
（実施例５）
ボトム層、ミドル層、トップ層のｉ層の膜厚を７５ｎｍ、７０ｎｍ、８５ｎｍとした以外は実施例４と同様なトリプル型太陽電池（ＳＣ実５）を作製した。実施例１と同様な評価を行ったところ（ＳＣ実５）の光による加速劣化後の光電変換効率（ηｓ）は実施例１の従来型のトリプル型太陽電池（ＳＣ比１）のそれよりも１．３７倍優れていることが分った。
【０１６５】
（実施例６）
ミドル層のｉ層のバンド構造が図３のようなバンドプロファイルになるようにＳｉＨ₄流量、ＧｅＨ₄流量を制御した以外は実施例３と同様なトリプル型太陽電池（ＳＣ実６）を作製し、実施例１と同様な評価を行ったところ、（ＳＣ実６）の光による加速劣化後の光電変換効率（ηｓ）は実施例１の従来型のトリプル型太陽電池（ＳＣ比１）のそれよりも１．４０倍優れていることが分った。
【０１６６】
（実施例７）
図４に示すロール・ツー・ロール方式を用いた製造装置を使用して、図１のトリプル型太陽電池を作製した。基板は長さ３００ｍ、幅３０ｃｍ、厚さ０．１ｍｍの両面を鏡面研磨した帯状ステンレスシートを用いた。ロール・ツー・ロール方式を用いたスパッタリング装置でこのステンレス基板表面上に基板温度３５０℃で層厚０．３μｍのＡｇからなる反射層を連続形成し、さらに基板温度３５０℃で層厚２．０μｍのＺｎＯからなる透明導電層を連続形成した。基板表面はテクスチャー化していることが分かった。基板表面の凹凸は、Ｒｍａｘで５００ｎｍであった。
【０１６７】
図４はロール・ツー・ロール方式を用いた半導体層の連続形成装置の概略図である。この装置は基板送り出し室４０１と、複数の堆積室４０２〜４１４と、基板巻き取り室４１５を順次配置し、それらの間を分離通路４１６で接続しており、帯状の基板４１７がこれらの中を通って、基板送り出し室から基板巻き取り室に絶え間なく移動することができ、かつ基板の移動と同時に各堆積室でそれぞれの半導体層を同時に形成することができる。
【０１６８】
４５０は堆積室を上から見た図で、それぞれの堆積室には原料ガスの入口４１９と原料ガスの排気口４２０があり、ＲＦ電極４２１、あるいはマイクロ波導入部４２２が必要に応じて取り付けられ、さらに基板を加熱するハロゲンランプヒーター４１８が内部に設置されている。また原料ガスの入口には不図示の原料ガス供給装置が接続されており、それぞれの原料ガスの排気口には油拡散ポンプ、メカニカルブースターポンプなどの真空排気ポンプが接続されている。それぞれのＲＦ電極にはＲＦ電源（不図示）が接続され、マイクロ波導入部にはマイクロ波（ＭＷ）電源が接続されている。堆積室に接続された分離通路には掃気ガスを流入させる入口４２４がある。基板送り出し室には送り出しロール４２５と基板に適度の張力を与えるためのガイドロール４２６があり、基板巻き取り室には巻き取りロール４２７とガイドロール４２８がある。
【０１６９】
まず、前記の反射層と透明導電層を形成した基板を送り出しロールに巻き付け、基板送り出し室にセットし、各堆積室内を通過させた後に基板の瑞を基板巻き取りロールに巻き付ける。装置全体を不図示の真空排気ポンプで真空排気し、各堆積室のランプヒーターを点灯させ、各堆積室内の基板温度が所定の温度になるように設定する。装置全体の圧力が１ｍＴｏｒｒ以下になったら各掃気ガスの入り口からＨ₂ガスを流入させ、基板を図の矢印の方向に移動させながら、巻き取りロールで巻き取っていく。実施例１と同様にして各堆積室に各半導体層を形成するための原料ガスを流入させる。この際、各堆積室に流入させる原料ガスが他の堆積室に拡散しないように各分離通路に流入させるＨ₂ガスの流量、各堆積室の圧力を調整する。
【０１７０】
次に、各堆積室にＲＦ電力、ＭＷ電力を導入してグロー放電を生起し、それぞれの半導体層を形成していく。
【０１７１】
基板上に堆積室４０２ではＲＦプラズマＣＶＤ法でｎ３層（ａ−Ｓｉ：Ｈ）を形成し、さらに堆積室４０３ではＲＦプラズマＣＶＤ法でｂ３１バッファ層（ａ−Ｓｉ：Ｈ）を、堆積室４０４ではＭＷプラズマＣＶＤ法でｉ３層（ａ−ＳｉＧｅ：Ｈ）を、堆積室４０５ではＲＦプラズマＣＶＤ法でｂ３２バッファ層（ａ−Ｓｉ：Ｈ）を、堆積室４０６ではＲＦプラズマＣＶＤ法でｐ３層（μｃ−Ｓｉ：Ｈ）を、堆積室４０７ではＲＦプラズマＣＶＤ法でｎ２層（ａ−Ｓｉ：Ｈ）を、堆積室４０８ではＲＦプラズマＣＶＤ法でｂ２１バッファ層（ａ−Ｓｉ：Ｈ）を、堆積室４０９ではＭＷプラズマＣＶＤ法でｉ２層（ａ−ＳｉＧｅ：Ｈ）を、堆積室４１０ではＲＦプラズマＣＶＤ法でｂ２２バッファ層（ａ−Ｓｉ：Ｈ）を、堆積室４１１ではＲＦプラズマＣＶＤ法でｐ２層（μｃ−Ｓｉ：Ｈ）を、堆積室４１２ではＲＦプラズマＣＶＤ法でｎ１層（μｃ−Ｓｉ：Ｈ）を、堆積室４１３ではＲＦプラズマＣＶＤ法でｉ３層（ａ−Ｓｉ：Ｈ）を、堆積室４１４ではＲＦプラズマＣＶＤ法でｐ１層（μｃ−Ｓｉ：Ｈ）を順次形成した。
【０１７２】
ｉ３層を形成する堆積室、ｉ２層を形成する堆積室の原料ガス入口４２９は複数に分かれており、それぞれの入口からＳｉＨ₄ガス、ＧｅＨ₄ガス、Ｈ₂ガスを流入させ、ｉ３層、ｉ２層のＧｅ組成が層厚方向に均一になり、バンドギャップが層厚方向に均一になるようにした。基板の搬送が終わったところでＭＷ電源、ＲＦ電源を切り、グロー放電を消滅させ、原料ガス、掃気ガスの流入を止めた。基板温度が室温まで下がったところで装置全体をリークし、巻き取られた基板を取り出した。詳細な作製条件は表２にまとめた。
【０１７３】
次にロール・ツー・ロール方式を用いたスパッタリング装置でｐ１層上に１７０℃で層厚７０ｎｍのＩＴＯからなる透明電極を連続形成した。
【０１７４】
次に、この基板の一部を５０ｍｍ×５０ｍｍの大きさに切断し、スクリーン印刷法で層厚５μｍ、線幅０．５ｍｍの銀ペーストを印刷し、集電電極を形成した。
【０１７５】
次に、定格電圧２５Ｖ、定格出力５０Ｗとなるように太陽電池同志の直列化、並列化を行った。
【０１７６】
次に、鋼材からなるバッキングプレート上に、ＥＶＡ（エチレンビニルアセテート）シート、ナイロンシート、ＥＶＡシート、クレーンガラス（ガラス繊維の不織布）、直列化・並列化を行った太陽電池、クレーンガラス、ＥＶＡシート、クレーンガラス、ＥＶＡシート、クレーンガラス、ＥＴＦＥ（エチレンテトラフロロエチレン）シートを乗せ、真空ラミネート（１５０℃、４０分）した。
【０１７７】
以上で、ロール・ツー・ロール方式を用いたトリプル型太陽電池モジュール（ＭＪ実７）の作製を終えた。
【０１７８】
【表２】

【０１７９】
（比較例７）
上記のロール・ツー・ロール方式の製造装置を用いて、ｉ１層、ｉ２層、ｉ３層の膜厚をそれぞれ、１００ｎｍ、１３０ｎｍ、１９０ｎｍとした従来のトリプル型太陽電池を作製した。膜厚を変える以外は実施例７と同様にしてトリプル型太陽電池（ＭＪ比７）を作製し、実験例１と同様な評価を行ったところ、実施例７の太陽電池モジュール（ＭＪ実７）の光による加速劣化後の光電変換効率（ηｓ）は比較例７の太陽電池モジュール（ＭＪ比７）のそれの１．２１倍であることが分かった。
【０１８０】
（実施例８）
実施例７において複数に分割された原料ガス入口４２９からＳｉＨ₄ガス、ＧｅＨ₄ガス、Ｈ₂ガスを流入させ、ｉ３層、ｉ２層を形成する際、ＧｅＨ₄ガスの流量を４３０のように調整し，ａ−ＳｉＧｅ：Ｈが図３のバンドギャップとなるようにする以外は実施例７と同様にして図１のトリプル型太陽電池モジュールを作製した。実験例１と同様な評価を行ったところ、光による加速劣化後の光電変換効率（ηｓ）は比較例７の太陽電池モジュール（ＭＪ比７）のそれの１．２３倍であることが分かった。
【０１８１】
（実施例９）
実施例７において堆積室４０５、堆積室４１０の一部にＨ₂ガスのみのプラズマを生起させる部分を設け、ｂ３２及びｂ２２層に水素プラズマ処理を施す以外は実施例７と同様にして図１のトリプル型太陽電池モジュール（ＭＪ実９）を作製した。実験例１と同様な評価を行ったところ、光による加速劣化後の光電変換効率（ηｓ）は比較例７の太陽電池モジュール（ＭＪ比７）のそれの１．２５倍であることが分かった。
【０１８２】
（実施例１０）
実施例７において堆積室４０４、堆積室４０９に流入させるＳｉＨ₄ガスの流量、ＧｅＨ₄ガスの流量を４／５にし、ＭＷ電力を４／５にする以外は実施例７と同様にして図１のトリプル型太陽電池モジュール（ＭＪ実１０）を作製した。実験例１と同様な評価を行ったところ、光による加速劣化後の光電変換効率（ηｓ）は比較例７の太陽電池モジュール（ＭＪ比７）のそれの１．２２倍であることが分かった。
【０１８３】
（実施例１１）
実施例７において堆積室４０４、堆積室４０９に流入させるＳｉＨ₄ガスの流量を４／５にし、ＧｅＨ₄ガスの流量を６／５にする以外は実施例７と同様にして図１のトリプル型太陽電池モジュール（ＭＪ実１１）を作製した。実験例１と同様な評価を行ったところ、光による加速劣化後の光電変換効率（ηｓ）は比較例７の太陽電池モジュール（ＭＪ比７）のそれの１．２６倍であることが分かった。
【０１８４】
（実施例１２）
実施例７において堆積室４０５内の基板温度を３５０℃、堆積室４１０内の基板温度を３３０℃にする以外は実施例７と同様にして図１のトリプル型太陽電池モジュール（ＭＪ実１２）を作製した。実験例１と同様な評価を行ったところ、光による加速劣化後の光電変換効率（ηｓ）は比較例７の太陽電池モジュール（ＭＪ比７）のそれの１．３１倍であることが分かった。
【０１８５】
（実施例１３）
実施例７においてｂ３２層、ｂ２２層形成時に新たにＣＨ₄ガスを１５ｓｃｃｍ、堆積室に流入させて形成したａ−ＳｉＣ：Ｈのバッファー層を用いたトリプル型太陽電池モジュール（ＭＪ実１３）を作製した。実験例１と同様な評価を行ったところ、光による加速劣化後の光電変換効率（ηｓ）は比較例７の太陽電池モジュール（ＭＪ比７）のそれの１．２９倍であることが分かった。
【０１８６】
（実施例１４）
実施例７において堆積室４０６、４１１、４１４に新たにマイクロ波導入部を設置し、ｐ１層、ｐ２層、ｐ３層形成時に新たにＣＨ₄ガスを１０ｓｃｃｍ、各堆積室に流入させＭＷＰＣＶＤ法で形成した微結晶ＳｉＣ：Ｈのｐ層を用いたトリプル型太陽電池モジュール（ＭＪ実１４）を作製した。実験例１と同様な評価を行ったところ、光による加速劣化後の光電変換効率（ηｓ）は比較例７の太陽電池モジュール（ＭＪ比７）のそれの１．３２倍であることが分かった。
【０１８７】
【発明の効果】
請求項１、２の発明により、即ち前記第二および第三のｐｉｎ接合のｉ層である非晶質シリコンゲルマニウムの膜厚を、従来好適と考えられてきた膜厚を外れて薄くすることによって、光照射によるｉ層中での局在準位の増加を抑制することができ、スタック型の光起電力素子の中でもさらに光劣化を抑制することができる。
【０１８８】
請求項３、４の発明により、即ち前記第ニおよび第三のｐｉｎ接合のｉ層である非晶質シリコンゲルマニウムのゲルマニウム組成を増大させることによって、非晶質シリコンゲルマニウムの膜厚を上述の如く薄くした場合においても、第一、第二及び第三のｐｉｎ接合で発生する電流のマッチングをとることができ、光劣化を抑制しながらなおかつ高い光電変換効率を維持することができる。
【０１８９】
請求項５〜７の発明によれば、非晶質シリコンゲルマニウム中のゲルマニウムの含有量を増大させた場合においても、光起電力素子の開放電圧（Ｖoc）と曲線因子（ＦＦ）を特に向上させることができ、光劣化を抑制しながらなおかつさらに高い変換効率を維持することができる。
【０１９０】
以上の構成の本発明のスタック型光起電力素子によって、前記第二、第三のｐｉｎ接合のｉ層である非晶質シリコンゲルマニウムの光劣化が抑制され、高い光電変換効率を維持しながら、光劣化を低下させ、光劣化後の変換効率を向上させることができる。それによって、実用に適した低いコストでありながら、信頼性が高くかつ光電変換効率の高い光起電力素子を提供することができる。
【０１９１】
また、本発明の光起電力素子の他の効果として、前記第二および第三のｐｉｎ接合のｉ層である非晶質シリコンゲルマニウムの膜厚を、従来好適と考えられてきた膜厚を外れて薄くすることによって、シリコンより少ない資源であるゲルマニウムの使用量を削減することができる。また、製造工程における使用エネルギーも削減することができる。さらに、光起電力素子全体の膜厚が薄くなることによって、積層した薄膜にかかる応力が減少し、基板を曲げたり反らせたりしたときの耐久力が増大し、堆積膜の基板からの膜はがれの問題を減少させることができる。それによって、製造行程の歩留まりが向上し、光起電力素子の利用形態の柔軟性が向上する。
【図面の簡単な説明】
【図１】 本発明の光起電力素子の一例を模式的に示す断面図である。
【図２】半導体層形成装置の一例を示す概念図である。
【図３】ｉ層のバンドギャップの膜厚方向の変化の好適な例を示すグラフである。
【図４】光起電力素子の連続形成装置の一例を示す概念図である。
【図５】光起電力素子の集電電極形成パターンの一例を示した概念図である。
【図６】コンスタントフォトカレントメソッドによる測定例を示すグラフである。
【符号の説明】
１０１、２０４、４１７ 基板、
１０２ 下部電極、
１０３ ｎ３層（ｐ３層）、
１０４ ｉ３層、
１０５ ｐ３層（ｎ３層）、
１０６ ｎ２層（ｐ２層）、
１０７ ｉ２層、
１０８ ｐ２層（ｎ２層）、
１０９ ｎ１層（ｐ１層）、
１１０ ｉ１層、
１１１ ｐ１層（ｎ１層）、
１１２ 透明電極、
１１３ 集電電極、
１１４ ボトム、
１１５ ミドル、
１１６ トップ、
２００ 堆積装置、
２０１ 堆積室、
２０２ 真空計、
２０３ バイアス（ＲＦ）電源、
２０５ ヒーター、
２０６ 導波管、
２０７ コンダクタンスバルブ、
２０８ バルブ、
２０９ 基板ホルダー、
２１０ バイアス（ＲＦ）電極、
２１１ ガス導入管、
２１２ マイクロ波導入部、
２１３ 誘電体窓、
２１５ シャッター、
２１９ マイクロ波電源、
２２０ 排気口、
４０１ 基板送り出し室、
４０２〜４１４ 堆積室、
４１５ 基板巻き取り室、
４１６ 分離通路、
４１８ ランプヒーター、
４１９ 原料ガス入口、
４２０ 排気口、
４２１ ＲＦ電極、
４２２ マイクロ波導入部、
４２４ 掃気ガス入口、
４２５ 送り出しロール、
４２６ ガイドロール、
４２７ 巻き取りロール、
４２８ ガイドロール、
４２９ 複数個のガス導入口、
４３０ ＧｅＨ_４の流量、
４３１ ＲＦバイアス電極、
４５０ 上から見た図、
５０１ 光起電力素子の光入射面、
５０２ 取り出し電極。[0001]
[Industrial application fields]
The present invention relates to a photovoltaic device, and more particularly to a photovoltaic device having high photoelectric conversion efficiency and high reliability.
[0002]
[Prior art]
Photovoltaic elements, which are photoelectric conversion elements that convert sunlight into electrical energy, are widely applied as power sources for consumer use such as calculators and wristwatches. In the future, so-called fossil fuels such as oil and coal are used. It is attracting attention as a technology that can be put to practical use as an alternative power.
[0003]
A photovoltaic device is a technology that uses the photovoltaic power of a pn junction of a semiconductor, and a semiconductor such as silicon absorbs sunlight and generates photocarriers of electrons and holes. It is drifted by an electric field and taken out to the outside. Such a photovoltaic element is manufactured by using a semiconductor process. Specifically, a silicon single crystal whose valence electrons are controlled to be p-type or n-type is produced by a crystal growth method such as CZ method, and the single crystal is sliced to produce a silicon wafer having a thickness of about 300 μm. Furthermore, a pn junction is formed by forming a layer of a different conductivity type by an appropriate means such as diffusion of a valence electron control agent so that the conductivity type is opposite to that of the wafer.
[0004]
By the way, single crystal silicon is currently used for photovoltaic devices that are mainly put into practical use from the viewpoint of reliability and conversion efficiency. Since it is used, the production cost is high.
[0005]
Another drawback of single crystal silicon photovoltaic devices is that single crystal silicon is an indirect transition and thus has a low light absorption coefficient, and single crystal photovoltaic devices are at least 50 microns thick to absorb incident sunlight. And the band gap is about 1.1 eV, which is narrower than 1.5 eV, which is suitable as a photovoltaic element, and the entire sunlight cannot be used effectively. Moreover, even if the production cost is reduced by using polycrystalline silicon, the problem of indirect transition remains, and the thickness of the photovoltaic element cannot be reduced. Polycrystalline silicon also has grain boundaries and other problems.
[0006]
Furthermore, since it is crystalline, a wafer with a large area cannot be manufactured and it is difficult to increase the area, and when taking out a large amount of power, wiring for serializing or paralleling the unit elements must be performed. Production costs per unit of power generation are not possible because of the need for expensive mounting to protect photovoltaic elements from mechanical damage caused by various weather conditions when used outdoors. There is a problem that it is expensive compared to the power generation method.
[0007]
Under these circumstances, in promoting the practical use of photovoltaic devices for power, cost reduction and large area are important technical issues, and various studies have been made, and low-cost materials and conversion There has been a search for materials such as highly efficient materials. As a material of such a photovoltaic element, tetrahedral amorphous semiconductors such as amorphous silicon, amorphous silicon germanium, and amorphous silicon carbide, CdS, Cu ₂ Examples include II-VI group compounds such as S and III-V group compound semiconductors such as GaAs and GaAlAs. In particular, a thin-film photovoltaic device using an amorphous semiconductor as a photovoltaic generation layer can produce a large-area film compared to a single-crystal photovoltaic device, and can be thin. It is promising because it has advantages such as being able to be deposited on other substrate materials.
[0008]
However, the photovoltaic element using the amorphous semiconductor has a problem in terms of improvement in photoelectric conversion efficiency and reliability as a power element.
[0009]
As a means for improving the photoelectric conversion efficiency of a photovoltaic element using an amorphous semiconductor, for example, the sensitivity to long-wavelength light is increased by narrowing the band gap. That is, since amorphous silicon has a band gap of about 1.7 eV, it cannot absorb light with a long wavelength of 700 nm or longer and cannot be used effectively. Therefore, it is sensitive to long wavelength light and has a narrow band gap. The use of is being considered. Examples of such a material include amorphous silicon germanium that can easily change the band gap from about 1.3 eV to about 1.7 eV easily by changing the ratio of the silicon source gas and the germanium source gas at the time of film formation.
[0010]
US Pat. No. 2,949,498 discloses the use of a so-called stack cell in which a plurality of photovoltaic elements having a unit element structure are stacked as another method for improving the conversion efficiency of a photovoltaic element. A pn junction crystal semiconductor was used for this stack cell, but the idea is common to both amorphous and crystalline materials, and the solar spectrum is efficiently absorbed by photovoltaic elements of different band gaps. The power generation efficiency is improved by increasing the open circuit voltage Voc.
[0011]
Stack cells improve the conversion efficiency by stacking elements with different band gaps and efficiently absorbing each part of the spectrum of sunlight. The so-called top layer band gap located on the light incident side of the stacked elements. Rather, the band gap of the so-called bottom layer located below the top layer is designed to be narrower. On the other hand, Hamakawa et al. Have reported a so-called cascade battery in which amorphous silicon having the same band gap is stacked in multiple layers without an insulating layer between photovoltaic elements to increase the Voc of the entire element. In this method, unit elements made of an amorphous silicon material having the same band gap are stacked.
[0012]
Amorphous silicon germanium has a narrow band gap and excellent sensitivity to light having a long wavelength, and therefore is used as a material suitable for the bottom layer of the stack cell described above.
[0013]
By the way, amorphous silicon germanium, which improves the long wavelength sensitivity in this way and can also be applied as the bottom layer of a stack cell, is generally inferior to amorphous silicon in terms of film quality, and thus produces a photovoltaic device. In this case, the conversion efficiency is low. This is because there are more localized levels in the band gap than amorphous silicon. For this reason, research and development has been carried out to improve the film quality by reducing the level in the band gap.For example, activated fluorine atoms are used to compensate for dangling bonds in amorphous silicon germanium. Amorphous silicon germanium with reduced localized level density is disclosed in Japanese Patent Publication No. 63-48197.
[0014]
On the other hand, improvement of characteristics of amorphous silicon germanium photovoltaic elements is also being studied by methods other than the essential improvement of film quality as described above. As an example, a method using a so-called buffer layer that gives a slope of a bandwidth at a junction interface between a p-type semiconductor and / or an n-type semiconductor and an i-type semiconductor layer is disclosed in US Pat. No. 4,254,429 and US Pat. No. 4,377,723. The purpose of the buffer layer is that there are many levels at the junction interface between a p-type semiconductor or n-type semiconductor made of amorphous silicon and an i-type semiconductor made of amorphous silicon germanium due to the difference in lattice constant. Therefore, by using amorphous silicon at the bonding interface, the level is eliminated, the bonding is improved, and Voc is improved so as not to impair the carrier runnability.
[0015]
Furthermore, as another method, a method of providing a so-called gradient layer that improves the characteristics by providing a distribution of composition in the intrinsic layer by changing the composition ratio of silicon and germanium is disclosed. For example, according to U.S. Pat. No. 4,816,082, the band gap of the i layer in the part in contact with the first valence-controlled semiconductor layer on the light incident side is increased, and gradually toward the center. A method is disclosed in which the band gap is narrowed and further the band gap is gradually increased toward the second valence-electron controlled semiconductor layer. According to this method, carriers generated by light can be separated efficiently by the action of an internal electric field, and the characteristics are improved.
[0016]
Incidentally, another problem of the photovoltaic element using an amorphous semiconductor is that the conversion efficiency is lowered by light irradiation. This is because amorphous silicon and amorphous silicon germanium are caused by the film quality being deteriorated by light irradiation, and therefore the carrier runnability is deteriorated. It is a phenomenon peculiar to quality semiconductors. Therefore, when it is used for electric power, the reliability is inferior and it is an obstacle to practical use. Research into improving the photodegradation of amorphous semiconductors has been conducted intensively, but studies have been made to prevent photodegradation by improving the film quality, while reducing the photodegradation by improving the cell structure has also been studied. Yes. As a specific example of such a method, use of the above-described stack cell structure is an effective means, and it has been confirmed that the deterioration of cell characteristics due to light irradiation is reduced.
[0017]
[Problems to be solved by the invention]
However, in the amorphous photovoltaic device, the film quality is inferior to that of the crystalline silicon photovoltaic device, so the conversion efficiency is not sufficient, and the power generation cost per watt is higher than that of existing nuclear power, thermal power, hydroelectric power generation, etc. It is expensive. In order for an amorphous silicon-based photovoltaic element to be used for electric power as compared with an existing power generation method, it is required to further improve the conversion efficiency.
[0018]
Furthermore, as described above, despite the elucidation of the mechanism of photodegradation and the study of preventing photodegradation, the problem of photodegradation has not been completely solved. The device is required to have further reliability.
[0019]
An object of the present invention is to solve the above-described problems, and to provide a photovoltaic device having low cost suitable for practical use, high reliability and high photoelectric conversion efficiency.
[0020]
[Means for Solving the Problems]
The inventors of the present invention have overcome the above-described problems, have made extensive studies on stack type photovoltaic devices with low photodegradation and high conversion efficiency, and have obtained the following knowledge from the experimental results described below. Obtained.
[0021]
That is,
(1) In order to reduce the light deterioration rate and improve the conversion efficiency after light deterioration while maintaining high photoelectric conversion efficiency, among the so-called stacked photovoltaic elements, the first is counted from the light incident side. A Si / SiGe / SiGe triple photovoltaic device using amorphous silicon as the i layer of the pin junction and amorphous silicon germanium as the i layer of the second and third pin junctions is suitable. about,
(2) Further, by reducing the film thickness of the amorphous silicon germanium, which is the i layer of the second or third pin junction, from the film thickness that has been considered to be suitable in the past, the stack type photovoltaic The ability to further suppress light degradation among the elements,
(3) Further, by increasing the germanium composition of amorphous silicon germanium which is the i layer of the second or third pin junction, high photoelectric conversion efficiency can be achieved even when the amorphous silicon germanium film thickness is reduced. That can be maintained,
(4) Furthermore, an amorphous silicon layer or an amorphous silicon carbon layer is disposed between the p layer of the second or third pin junction and the amorphous silicon germanium layer, and further, amorphous. By changing the germanium composition of the porous silicon germanium layer in the film thickness direction and measuring the position where the germanium composition is maximized from the p-layer side, the thickness is made less than one-fourth of the film thickness of the entire amorphous silicon germanium layer. The photoelectric conversion efficiency after photodegradation of the triple type photovoltaic device can be further increased,
(5) Furthermore, conventionally, it has been considered that amorphous silicon germanium increases the density of localized states and decreases the film quality when the germanium composition is increased. However, as a means for depositing amorphous silicon germanium, Triple-type photovoltaic device that suppresses localized state density and maintains good film quality even when the germanium composition is increased by controlling the plasma state at the initial stage of deposition using the wave CVD method Can further increase the photoelectric conversion efficiency after photodegradation of
It is.
[0022]
The present invention has been further researched based on the above knowledge, applied to photovoltaic devices, and has been completed.
[0023]
However, the gist of the present invention is a so-called stack type photovoltaic device in which a plurality of pin type semiconductor junctions formed by stacking a p type semiconductor layer, an i type semiconductor layer, and an n type semiconductor layer are stacked. Counting from the light incident side, using amorphous silicon as the i layer of the first pin junction, using amorphous silicon germanium as the i layer of the second and third pin junctions, and i of the second pin junction The thickness of the amorphous silicon germanium layer is 100 nm or less. about, In addition, the photovoltaic element is characterized in that the film thickness of amorphous silicon germanium which is the i layer of the third pin junction is 150 nm or less.
[0024]
Furthermore, the germanium composition (Ge / (Si + Ge)) of amorphous silicon germanium that is the i layer of the second pin junction is set to 0.35 or more, and the non-layer that is the i layer of the third pin junction. The germanium composition (Ge / (Si + Ge)) of crystalline silicon germanium is preferably 0.40 or more.
[0025]
Preferably, an amorphous silicon layer or an amorphous silicon carbon layer is disposed between the p layer of the second or / and third pin junction and the amorphous silicon germanium layer, and further amorphous silicon. It is preferable that the germanium composition of the germanium layer is changed in the film thickness direction, and the position where the germanium composition is maximized is measured from the p-layer side to be not more than a quarter of the film thickness of the entire amorphous silicon germanium layer.
[0026]
Further, when the local level density of amorphous silicon germanium, which is the i layer of the second and third pin junctions, is measured by a constant photocurrent method, 5 × 10 5 ¹⁶ / Cm ^Three It is preferable that:
[0027]
[Operation and embodiment examples]
Claim 1 And 2 According to the invention of No. Two By reducing the film thickness of amorphous silicon germanium, which is the i layer of the third pin junction, from the film thickness that has been considered suitable in the past, the localized level in the i layer by light irradiation is reduced. Of the stack type photovoltaic device can further suppress the light deterioration.
[0028]
Claim 3 and 4 According to the invention, when the amorphous silicon germanium film thickness is reduced as described above by increasing the germanium composition of the amorphous silicon germanium that is the i layer of the second or / and third pin junction. In this case, it is possible to match currents generated in the first, second and third pin junctions, and to maintain high photoelectric conversion efficiency while suppressing photodegradation.
[0029]
Claim 5-7 According to the invention, even when the content of germanium in the amorphous silicon germanium is increased, the open-circuit voltage (Voc) and the fill factor (FF) of the photovoltaic element can be particularly improved. Still higher photoelectric conversion efficiency can be maintained while suppressing deterioration.
[0030]
The stacked photovoltaic device of the present invention having the above configuration suppresses photodegradation of amorphous silicon germanium, which is the i layer of the second and third pin junctions, while maintaining high photoelectric conversion efficiency. The deterioration rate can be reduced and the conversion efficiency after light deterioration can be improved.
[0031]
Here, the effect | action of this invention is demonstrated in more detail with the example of an embodiment by showing the experiment which the present inventors conducted, and the knowledge obtained by the experiment.
[0032]
In order to further improve the conversion efficiency of an amorphous silicon-based photovoltaic element, to prevent light degradation, and to improve the reliability of the amorphous silicon-based photovoltaic element, the following The following examination was conducted.
[0033]
First, as described above, it has been known that a stack type configuration is effective as a photovoltaic device configuration as a method for reducing photodegradation. However, reduction of photodegradation is still insufficient. There wasn't. Thus, as a result of studying the configuration of the stack type photovoltaic device, the present inventors have obtained the following knowledge.
[0034]
That is, among the stacked photovoltaic elements, the triple type photovoltaic element having three pin junctions is more deteriorated than the double type photovoltaic element having two pin junctions. It can be reduced. This is because when the same type of semiconductor material is used, the triple type has less photocurrent generated at a single pin junction than the double type, and causes positive photodegradation of the amorphous semiconductor. This is thought to be because the recombination of holes and electrons decreases.
[0035]
Further, among triple type photovoltaic elements, amorphous silicon is used as the i layer of the first pin junction counted from the light incident side, and amorphous silicon is used as the i layer of the second and third pin junctions. The inventors have found that a Si / SiGe / SiGe triple type photovoltaic device using germanium is most suitable for reducing light degradation. This is because amorphous silicon germanium has a smaller band gap than amorphous silicon, so it can absorb long wavelength light that cannot be absorbed by amorphous silicon, and change the composition ratio of germanium. This is because of the advantage of controlling the size of the band gap. The band of the i-type semiconductor layer of the second pin junction is more in the Si / SiGe / SiGe configuration than the Si / Si / SiGe configuration using amorphous silicon as the i layer of the second pin junction. Since the gap is small, the thickness of the i-type semiconductor layer can be reduced, and as a result, light degradation can be further reduced.
[0036]
Accordingly, the present inventors have further investigated the Si / SiGe / SiGe triple type photovoltaic device. As a result, in order to maximize the photoelectric conversion efficiency of the Si / SiGe / SiGe triple photovoltaic element, the band gap of the amorphous silicon which is the i layer of the first pin junction is changed from 1.60 eV to 1. 85 eV, the band gap of amorphous silicon germanium that is the i layer of the second pin junction is changed from 1.5 eV to 1.6 eV, and the band gap of amorphous silicon germanium that is the i layer of the third pin junction is It has been found that a value of 1.45 eV to 1.55 eV is preferable. At this time, it turned out that the suitable film thickness of the amorphous silicon germanium which is i layer of the 2nd and 3rd pin junction is 100-200 nm and 200-350 nm, respectively. (Hereinafter, the first pin junction is abbreviated as top, the second pin junction is abbreviated as middle, and the third pin junction is abbreviated as bottom.)
However, the present inventors have further studied, considering both the photoelectric conversion efficiency and the photodegradation rate of the Si / SiGe / SiGe triple type photovoltaic device, and maintaining the high photoelectric conversion efficiency while reducing the photodegradation rate. Studies were made to reduce the maximum, that is, the photoelectric conversion efficiency (hereinafter referred to as stabilized conversion efficiency) after saturation of the decrease in photoelectric conversion efficiency due to photodegradation.
[0037]
First, when experiments were repeated focusing on the photodegradation rate of the Si / SiGe / SiGe-ripple type photovoltaic device, the middle and bottom photodegradation rates of the Si / SiGe / SiGe-ripple type photovoltaic device were repeated. However, it has been found that the light degradation rate of the entire device greatly affects. Then, it was found that it is desirable that the film thicknesses of amorphous silicon germanium which is the middle and bottom i layers are 100 nm or less and 150 nm or less, respectively, and more preferably 80 nm or less and 130 nm or less. In any conventional technique, such a thin semiconductor layer has never been used for the i-layer of the lower pin junction with respect to the light incident direction of the stacked photovoltaic element.
[0038]
On the other hand, since the stack type photovoltaic device has a configuration in which a plurality of pin junctions are connected in series, the current value generated at each pin junction is as large as possible and the current values are not close to each other. High photoelectric conversion efficiency cannot be obtained. This is called current matching of the stack cell. Therefore, if the film thickness of the amorphous silicon germanium that is the i layer of the middle and bottom of the Si / SiGe / SiGe triple type photovoltaic device is reduced to the above-mentioned value, it is generated at the middle and the bottom. It was expected that the photoelectric conversion efficiency would decrease because the current value would decrease.
[0039]
Therefore, as a result of further investigations and repeated experiments, the present inventors, even when the film thickness of the amorphous silicon germanium that is the middle and bottom i layer is reduced to the above-mentioned value, It was found that the current value generated at the middle and bottom can be maintained by reducing the band gap of amorphous silicon germanium. Specifically, the germanium composition (Ge / (Si + Ge)) of amorphous silicon germanium that is the middle i layer is changed from 0.35 to 0.80, and the germanium composition of amorphous silicon germanium that is the bottom i layer. (Ge / (Si + Ge)) is changed from 0.40 to 0.90, more preferably, the germanium composition of amorphous silicon germanium, which is the middle i layer, is changed from 0.40 to 0.70. It is desirable that the germanium composition of the amorphous silicon germanium that is the i layer is 0.45 to 0.80. It was found that by using such a germanium composition, the band gap of amorphous silicon germanium becomes small, and the current value generated at the middle and bottom can be maintained.
[0040]
Examples of experiments conducted for the above examination are shown in Experimental Examples 1-1 to 1-6.
[0041]
Thus, the present inventors were able to reduce the photodegradation rate, that is, to improve the stabilized conversion efficiency, while maintaining high photoelectric conversion efficiency.
[0042]
As a result of further study on the improvement of the stabilization conversion efficiency of the Si / SiGe / SiGe triple photovoltaic device, the present inventors have obtained the following knowledge.
[0043]
That is, by disposing an amorphous silicon layer or an amorphous silicon carbon layer between the middle and bottom p-layers and the amorphous silicon germanium layer, and further changing the germanium composition of the amorphous silicon germanium layer The position where the germanium composition is maximized is measured from the interface between the amorphous silicon layer or the amorphous silicon carbon layer and the amorphous silicon germanium layer, and the film thickness of the entire amorphous silicon germanium layer is 4 By making the ratio 1 / n or less, the photoelectric conversion efficiency after photodegradation of the triple photovoltaic element can be further increased. This is mainly because the open / close voltage (Voc) and fill factor (FF) of the Si / SiGe / SiGe triple photovoltaic element can be improved, and the photoelectric conversion efficiency of the element can be improved. It is considered a thing.
[0044]
Here, the effect of disposing an amorphous silicon layer or an amorphous silicon carbon layer between the middle and bottom p layers and the amorphous silicon germanium layer is mainly the open circuit voltage (Voc) of the photovoltaic device. It has a function to improve. This is presumably because the diffusion potential of the pin junction is increased by the amorphous silicon layer or the amorphous silicon carbon layer. It is also conceivable that an electron generated by light in amorphous silicon germanium forms a barrier that prevents the electrons from diffusing into the p layer. Further, by forming the amorphous silicon layer or the amorphous silicon carbon layer also between the n layer and the amorphous silicon germanium layer, the photoelectric conversion efficiency can be further improved.
[0045]
The film thickness of the amorphous silicon layer or the amorphous silicon carbon layer is preferably 3 nm to 50 nm, more preferably 5 nm to 40 nm. Further, even when an amorphous silicon layer or an amorphous silicon carbon layer having such a film thickness is disposed, the photodegradation rate of the photovoltaic element hardly increased.
[0046]
In addition, the germanium composition of the amorphous silicon germanium layer is changed in the film thickness direction, and the position where the germanium composition is maximized is measured from the interface between the amorphous silicon layer or the amorphous silicon carbon layer and the amorphous silicon germanium layer. As an effect of reducing the thickness of the amorphous silicon germanium layer to a quarter or less, it mainly serves to improve the open circuit voltage (Voc) and the fill factor (FF) of the photovoltaic element. . This is because in amorphous semiconductors, electrons and holes generated by light are considered to have a shorter diffusion length and rate limiting the overall characteristics, and the band gap of amorphous silicon germanium. Is changed in the film thickness direction, and the position where the band gap is minimized is arranged near the p layer, so that more photocarriers are distributed near the p layer than in the case of having a uniform band gap in the film thickness direction. In other words, it is considered that for the holes in the photocarrier, the short distance traveled in the i-type semiconductor leads to an improvement in fill factor (FF). In addition, by continuously changing the band gap of amorphous silicon germanium and increasing the valence band, diffusion of holes in the photocarrier is promoted, and the open circuit voltage (Voc) of the photovoltaic device is promoted. ) And fill factor (FF).
[0047]
Also, the preferred germanium composition when changing the germanium composition of the amorphous silicon germanium layer in the film thickness direction varies depending on how it is changed, but in the case where the germanium composition is the middle layer at the portion where germanium is maximum. 0.35 to 0.80, and preferably 0.40 to 0.90 in the case of the bottom layer.
[0048]
Examples of experiments conducted for the above examination are shown in Experimental Examples 2, 3, and 4.
[0049]
Furthermore, the present inventors have improved the photoelectric conversion efficiency of a photovoltaic device using amorphous silicon germanium in the i layer by suppressing the localized level of amorphous silicon germanium having a large germanium composition. The present inventors have obtained the knowledge that the light degradation rate can be reduced and the stabilization efficiency can be improved.
[0050]
Conventionally, a semiconductor material called amorphous silicon germanium generally has a low photoconductivity when the composition of germanium is increased. In particular, in the case of amorphous silicon germanium having a germanium composition of 0.35 or more, the photoconductivity is often significantly reduced. Therefore, it has been difficult to obtain high photoelectric conversion efficiency in a photovoltaic device using amorphous silicon germanium having a large germanium composition in the i layer. This is because the germanium composition increases, the localized level in the band gap increases, the level of the bottom state of the conduction band edge and the valence band edge (called tail state) increases, and the electrons and positive This is thought to be because the diffusion length of the holes is shortened.
[0051]
However, as a result of intensive studies, the inventors of the present invention formed amorphous silicon germanium by a microwave plasma CVD method, and further optimized the formation conditions to thereby form an amorphous silicon germanium having a germanium composition of 0.35 or more. In FIG. 5, the local level and tail state were reduced, and high-quality amorphous silicon germanium could be obtained. As a result, it was possible to improve the photoelectric conversion efficiency of the photovoltaic device using amorphous silicon germanium having a germanium composition of 0.35 or more for the i layer. Moreover, the light deterioration rate also decreased and the stabilization efficiency was able to be improved.
[0052]
This is because the amorphous silicon germanium is formed by the microwave plasma CVD method, whereas the amorphous silicon germanium is formed by using the RF plasma CVD method. Has been achieved. That is, when amorphous silicon germanium is formed using the RF plasma CVD method, it is difficult to reduce the pressure in the vacuum chamber for generating the plasma and decomposing the raw material gas to 100 mTorr or less. When amorphous silicon germanium is formed by the wave plasma CVD method, the pressure in the vacuum chamber for generating plasma and decomposing the source gas can be easily reduced to 50 mTorr or less. Therefore, the mean free path of the decomposed source gas molecules or atoms increases, and the molecules or atoms of the decomposed source gas in the gas phase react with each other to generate so-called clusters. It is considered that high-quality amorphous silicon germanium could be formed because of the increased mobility of the precursor on the growth surface of the thin film.
[0053]
Also, the substrate can be moved between multiple vacuum chambers by combining the so-called roll-to-roll method, which forms a thin film on the substrate surface while continuously transporting the belt-like substrate, and the above-mentioned microwave plasma CVD method. In comparison with the method of forming each layer by layer, the localized level in the vicinity of the interface of the plurality of semiconductor layers can be reduced, and the photoelectric conversion efficiency of the photovoltaic element can be improved.
[0054]
Examples of experiments conducted for the above examination are shown in Experimental Examples 5 and 6.
[0055]
Next, an evaluation method in the experimental example will be described.
[0056]
First, there is a method for evaluating photoelectric conversion efficiency (stabilization efficiency) after photodegradation of a photovoltaic device. In the case of evaluating photodegradation of a photovoltaic device that usually uses an amorphous silicon-based semiconductor as an i layer. The temperature of the element is kept at 50 ° C. and 1 SUN (100 mW / cm ² ) Is often measured for deterioration of photoelectric conversion efficiency. However, this method requires at least about 1000 hours of light irradiation to obtain the stabilization efficiency, and the problem is that the time required for evaluation is too long. Therefore, the present inventors consider that this light deterioration test is accelerated, and the intensity of the irradiated light is set to 50 SUN (5 W / cm ² The stabilization efficiency under each condition was measured by changing the temperature of the device under light irradiation from 50 ° C. to 130 ° C. As a result, 50 SUN (5 W / cm ² ), When accelerated deterioration at 110 ° C., the same stabilization efficiency as 1SUN light deterioration is obtained, and the light irradiation time required to obtain the stabilization efficiency is reduced to tens of thousands of seconds (10 to 20 hours). It was. Therefore, in the experimental examples and examples, 50 SUN (5 W / cm ² ), The light deterioration of the photovoltaic device was evaluated by accelerated deterioration at 110 ° C.
[0057]
In addition, the evaluation of the localized level density by the constant photocurrent method (CPM) was performed by a usual method. CPM obtains the intensity of monochromatic light capable of obtaining a constant photocurrent while changing the wavelength, and obtains the wavelength dependency of the absorption coefficient. As shown in FIG. 6, when the horizontal axis represents the light energy and the vertical axis represents the logarithm of the absorption coefficient, a portion where the logarithm of the absorption coefficient changes linearly with respect to the light energy appears. The inclination of this part is a characteristic value called Urbach energy. Then, the local level density in the band gap can be calculated from the area of the hatched portion in FIG. 6 between the portion deviating from the extension of the straight line portion and the straight line extension on the low energy side. The absolute value of the absorption coefficient can be obtained by fitting with an absorption coefficient obtained from optical measurement in the visible and near infrared regions.
[0058]
[Experimental example]
(Experimental Example 1-1)
The relationship between the i-layer thickness of the triple solar cell shown in FIG. 1 and the conversion efficiency (ηs) after accelerated deterioration by light was examined.
[0059]
A deposition apparatus 200 shown in FIG. 2 is a deposition apparatus suitable for manufacturing the triple solar cell of the present invention, and can perform a microwave plasma CVD (MWPCVD) method and an RF plasma CVD (RFPCVD) method. .
[0060]
First, in order to carry out the RFPCVD method, the substrate 204 is placed on the substrate holder 209 inside the deposition chamber 201, and the internal pressure is set to 1 with a vacuum exhaust device (not shown) (mechanical booster pump, oil diffusion pump, turbo molecular pump, etc.). × 10 ^-6 Evacuate until (Torr). Thereafter, the heater 205 is turned on so that the substrate temperature becomes constant at the set temperature, the layer forming source gas is introduced from the gas introduction pipe 211, and the opening is opened by the conductance valve 207 so that the predetermined pressure is obtained. adjust. RF power is supplied from the RF power source 203 to the RF electrode 210, RF plasma is generated, the shutter 215 is opened, and a desired deposited film is formed on the substrate. When the desired film thickness is formed, the shutter is closed, the supply of RF power is stopped, and the plasma is extinguished. After that, the supply of the source gas is stopped, and the internal pressure is again 1 × 10 ^-6 Evacuation is performed until (Torr) is reached, and formation of the next deposited film is started.
[0061]
In order to carry out the MWPCVD method, the substrate 204 is placed on the substrate holder 209 in the deposition chamber, and the internal pressure is 1 × 10 3 by a vacuum exhaust device (not shown) (mechanical booster pump, oil diffusion pump, turbo molecular pump, etc.). ^-6 Evacuate until (Torr). Thereafter, the heater 205 is turned on so that the substrate temperature becomes constant at the set temperature, the layer forming source gas is introduced from the gas introduction pipe 211, and the opening is opened by the conductance valve 207 so that the predetermined pressure is obtained. adjust. MW power is supplied from the MW power source 219 through the dielectric window 213 to generate microwave plasma. Further, RF power is supplied from the RF power source 203 to the RF electrode 210, the shutter 215 is opened, and a desired deposited film is formed on the substrate. When the desired film thickness is formed, the shutter is closed, the supply of MW power and RF power is stopped, and the plasma is extinguished. After that, the supply of the source gas is stopped, and the internal pressure is again 1 × 10 ^-6 Evacuation is performed until (Torr) is reached, and formation of the next deposited film is started.
[0062]
The triple solar cell of FIG. 1 was produced by the method as described above. Accelerated degradation device using conventional triple type solar cell in which germanium composition of i3 layer, i2 layer and i1 layer is 180 nm, 140 nm, 100 nm, i3 layer and i2 layer are 0.35 and 0.30, respectively, by light Installed in the AM1.5 spectrum, irradiation light intensity 50SUN (5W / cm ² ), The surface of the solar cell was irradiated with light under a deterioration condition of a substrate surface temperature of 110 ° C. When 50000 seconds had passed since the start of light irradiation, the solar cell was taken out from this accelerated deterioration device. Next, it installed in the apparatus which measures the photoelectric conversion efficiency of the taken-out solar cell, AM1.5 spectrum, irradiation light intensity 1SUN (100mW / cm ² ), The photoelectric conversion efficiency was measured under the measurement condition of the substrate surface temperature of 25 ° C.
[0063]
(Experimental example 1-2)
Evaluation of acceleration degradation due to light as described above and measurement of photoelectric conversion efficiency after accelerated degradation was performed on the triple solar cell of the present invention in which the film thicknesses of the i3 layer, i2 layer, and i1 layer were 140 nm, 140 nm, and 100 nm. I went against it. As a result of the measurement, it was found that the conversion efficiency (ηs) after accelerated deterioration was 1.22 times better than that of the conventional triple solar cell.
[0064]
(Experimental Example 1-3)
Similarly, evaluation of acceleration degradation due to light and measurement of photoelectric conversion efficiency after acceleration degradation is performed on the triple solar cell of the present invention in which the film thicknesses of the i3 layer, i2 layer, and i1 layer are 110 nm, 95 nm, and 90 nm. went. As a result of the measurement, it was found that the conversion efficiency (ηs) after accelerated deterioration was 1.31 times better than that of the conventional triple solar cell.
[0065]
(Experimental Example 1-4)
Similarly, evaluation of acceleration degradation due to light and measurement of photoelectric conversion efficiency after acceleration degradation was performed on the triple solar cell of the present invention in which the germanium composition in the i3 layer was 0.50 in Experimental Example 1-2. It was. As a result of the measurement, it was found that the conversion efficiency (ηs) after accelerated deterioration was 1.36 times better than that of the conventional triple solar cell.
[0066]
(Experimental Example 1-5)
Similarly, the evaluation of acceleration degradation due to light and the measurement of photoelectric conversion efficiency after acceleration degradation was performed on the triple solar cell of the present invention in which the germanium composition in the i2 layer was 0.40 in Experimental Example 1-3. went. As a result of the measurement, it was found that the conversion efficiency (ηs) after accelerated deterioration was 1.37 times better than that of the conventional triple solar cell.
[0067]
(Experimental example 1-6)
Further, in the same evaluation, the film thicknesses of the i3 layer, i2 layer, and i1 layer were 75 nm, 70 nm, and 85 nm, and the germanium composition in the i3 layer and i2 layer was 0.65 and 0.50, respectively. This was done for a triple solar cell. As a result of the measurement, it was found that the conversion efficiency (ηs) after accelerated deterioration was 1.39 times better than that of the conventional triple solar cell.
[0068]
(Experimental example 2)
The triple solar cell shown in FIG. 1 having the graded band gap structure of the i3 layer and the i2 layer was produced, and the same evaluation as in Experimental Example 1 was performed.
[0069]
SiH so that the band structure of the i3 layer and the i2 layer has a band profile as shown in FIG. _Four Flow rate, GeH _Four A triple solar cell similar to that of Experimental Example 1-2 was manufactured except that the flow rate was controlled, and evaluation similar to that of Experimental Example 1 was performed. As a result, the conversion efficiency (ηs ) Was found to be 1.27 times better than that of the conventional triple solar cell of Experimental Example 1.
[0070]
(Experimental example 3)
A buffer layer made of a-Si: H formed by RF plasma CVD between the i3 layer and the n3 layer, between the i3 layer and the p3 layer, between the i2 layer and the n2 layer, and between the i2 layer and the p2 layer. A triple solar cell was manufactured in the same manner as in Experimental Example 1-3, except that the layers were stacked. When the same evaluation as in Experimental Example 1 was performed, the conversion efficiency (ηs) after accelerated deterioration by light was 1.35 higher than that of the conventional triple solar cell of Experimental Example 1 compared to that of the conventional triple solar cell. It was found to be twice as good.
[0071]
(Experimental example 4)
The band structure of the i3 layer and the i2 layer of the triple solar cell fabricated in Experimental Example 3 is SiH so that the band profile is as shown in FIG. _Four Flow rate, GeH _Four A triple solar cell with a controlled flow rate was fabricated. When the same evaluation as in Experimental Example 1 was performed, the conversion efficiency (ηs) after accelerated deterioration by light was 1.39 higher than that of the conventional triple solar cell of Experimental Example 1 compared to the conventional triple solar cell. It was found to be twice as good.
[0072]
(Experimental example 5)
The relationship between the localized state density measured by the constant photocurrent method (CPM) and the conversion efficiency (ηs) after accelerated deterioration by light was investigated.
[0073]
A single cell using the middle layer of the triple solar cell produced in Experimental Example 1-3 (referred to as a middle cell) was newly produced in the same manner as in Experimental Example 1-3. Next, the current collector electrode of the middle cell and the stainless steel substrate were short-circuited using an external electric circuit, weak light (swept light) passing through the chopper was irradiated, and the current flowing through the external short-circuit was detected by a lock-in amplifier. The irradiation light intensity of the sweep light was changed so as to keep the detected current constant even when the wavelength of the sweep light was changed, and the absorption spectrum (750 nm to 1700 nm) of the i2 layer was measured. When the local level density was calculated from the dependence of the absorption spectrum on the swept light energy, the local level density of the i2 layer was 3 × 10. ¹⁶ (1 / cm ^Three )
[0074]
A single cell (referred to as the middle cell ratio) was newly produced without applying a bias (RF power to the RF electrode) when forming the i2 layer. When the local level density of the i2 layer having the middle cell ratio is calculated, the local level density of the i2 layer is 2 × 10. ¹⁷ (1 / cm ^Three )
[0075]
A triple solar cell was fabricated in the same manner as in Experimental Example 1-3, except that no bias (RF power applied to the RF electrode) was applied when the i2 layer was formed. Next, when the same evaluation as in Experimental Example 1 was performed, it was found that the conversion efficiency (ηs) after accelerated deterioration by light of this triple solar cell was 0.78 times that of Experimental Example 1-3. .
[0076]
(Experimental example 6)
Similarly, the relationship between the localized state density measured by the constant photocurrent method (CPM) and the conversion efficiency (ηs) after accelerated deterioration by light was investigated.
[0077]
First, a single cell using the bottom layer of the triple solar cell manufactured in Experimental Example 1-3 (referred to as a bottom cell) was newly manufactured in the same manner as in Experimental Example 1-3. Next, when the localized level density was calculated by the same method as in Experimental Example 5, the localized level density of the i3 layer was 4 × 10 4. ¹⁶ (1 / cm ^Three )
[0078]
A single cell (referred to as a bottom cell ratio) was newly produced without applying a bias (RF power to the RF electrode) when forming the i3 layer. When the localized level density of the i3 layer having the bottom cell ratio was calculated, the localized level density of the i3 layer was 3 × 10. ¹⁷ (1 / cm ^Three )
[0079]
A triple solar cell was fabricated in the same manner as in Experimental Example 1-3, except that no bias (RF power applied to the RF electrode) was applied when the i3 layer was formed. Next, when the same evaluation as in Experimental Example 1 was performed, it was found that the conversion efficiency (ηs) after accelerated deterioration by light of this triple solar cell was 0.81 times that of Experimental Example 1-3. .
[0080]
(Configuration of photovoltaic element)
Hereinafter, the configuration of the photovoltaic device of the present invention will be described in detail with reference to the drawings. However, the photovoltaic device of the present invention is not limited thereto.
[0081]
FIG. 1 schematically shows a cross section of a stacked photovoltaic element of the present invention. 1 has a structure in which three pin junctions are stacked, 116 is a first pin junction counted from the light incident side, and 115 is a second pin junction. , 114 is a third pin junction. These three pin junctions are stacked on the lower electrode 102 formed on the substrate 101, and a transparent electrode 112 and a current collecting electrode 113 are formed on the top of the three pin junctions to form a stack type. The photovoltaic element is formed. Each pin junction is composed of n-type semiconductor layers 103, 106, 109, i-type semiconductor layers 104, 107, 110, and p-type semiconductor layers 105, 108, 111. In the present invention, amorphous silicon is used as the first pin junction i-type semiconductor layer 110, and amorphous silicon germanium is used as the second and third semiconductor layers 107 and 104.
[0082]
Note that a configuration in which an n-type semiconductor layer and a p-type semiconductor layer in a pin junction are interchanged can be employed.
[0083]
(substrate)
Since the semiconductor layers 103 to 111 are thin films of about 1 μm at most, they are deposited on a suitable substrate. Such a substrate 101 may be monocrystalline or non-crystalline, and may be conductive or electrically insulating. Furthermore, they may be translucent or non-translucent, but preferably have a desired strength with little deformation and distortion.
[0084]
Specifically, metal such as Fe, Ni, Cr, Al, Mo, Au, Nb, Ta, V, Ti, Pt, and Pb, or an alloy thereof, for example, a thin plate such as brass or stainless steel and a composite thereof, and polyester , Polyethylene or polycarbonate, cellulose acetate, polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyamide, polyimide, film or sheet of heat-resistant synthetic resin such as epoxy or glass fiber, carbon fiber, boron fiber, metal fiber, etc. And thin films of these metals, metal thin films of different materials on the surface of resin sheets, etc. and / or SiO ₂ , Si _Three N _Four , Al ₂ O _Three , AlN and other insulating thin films subjected to surface coating treatment by sputtering, vapor deposition, plating, etc., and glass, ceramics and the like.
[0085]
When the substrate is used as a substrate for a photovoltaic element, when the strip substrate is electrically conductive such as metal, it may be used as an electrode for direct current extraction or may be electrically insulating such as synthetic resin. In some cases, Al, Ag, Pt, Au, Ni, Ti, Mo, W, Fe, V, Cr, Cu, stainless steel, brass, nichrome, SnO are formed on the surface on which the deposited film is formed. ₂ , In ₂ O _Three It is desirable to form a current extraction electrode by subjecting a so-called simple metal or alloy such as ZnO, ITO or the like and a transparent conductive oxide (TCO) to surface treatment in advance by a method such as plating, vapor deposition, sputtering, or the like. .
[0086]
Of course, even if the belt-like substrate is an electrically conductive material such as a metal, the reflectance of the long-wavelength light on the substrate surface is improved, or the interdiffusion of constituent elements between the substrate material and the deposited film is improved. For the purpose of preventing or the like, a different metal layer or the like may be provided on the side on which the deposited film on the substrate is formed. In addition, when the substrate is relatively transparent and a photovoltaic device having a layer structure in which light is incident from the substrate side, a conductive thin film such as the transparent conductive oxide or metal thin film is deposited in advance. It is desirable to form it.
[0087]
The surface property of the substrate may be a so-called smooth surface or a minute uneven surface.
[0088]
In the case of a minute uneven surface, the uneven shape is spherical, conical, pyramidal, etc., and the maximum height (Rmax) is preferably 0.05 μm to 2 μm, so that The light reflection becomes irregular reflection, resulting in an increase in the optical path length of the reflected light on the surface. The shape of the substrate can be a flat or uneven surface plate, long belt shape, cylindrical shape, etc., depending on the application, and its thickness can form a photovoltaic element as desired. Although it is determined as appropriate, when flexibility is required as the photovoltaic element, or when light is incident from the substrate side, the thickness is made as thin as possible within the range in which the function as the substrate is sufficiently exhibited. I can do it. However, it is usually set to 10 μm or more from the viewpoint of mechanical strength in manufacturing and handling the substrate.
[0089]
(Back electrode, light reflecting layer)
The back electrode used in the present invention is an electrode disposed on the back surface of the semiconductor layer with respect to the light incident direction. Therefore, when the substrate 101 is translucent and light is incident from the direction of the substrate, it is disposed at the position 112 in FIG. Examples of the material of the back electrode include metals such as gold, silver, copper, aluminum, nickel, iron, chromium, molybdenum, tungsten, titanium, cobalt, tantalum, niobium, and zirconium, and alloys such as stainless steel. Of these, metals having high reflectivity such as aluminum, copper, silver, and gold are particularly preferable. When a metal having a high reflectance is used, the back electrode can also serve as a light reflecting layer that reflects again the light that could not be absorbed by the semiconductor layer to the semiconductor layer.
[0090]
The shape of the back electrode may be flat, but it is more preferable to have an uneven shape that scatters light. By having a concavo-convex shape that scatters light, it scatters long-wavelength light that could not be absorbed by the semiconductor layer, thereby extending the optical path length in the semiconductor layer and improving the long-wavelength sensitivity of the photovoltaic device, thereby reducing the short-circuit current And the photoelectric conversion efficiency can be improved. The uneven shape that scatters light desirably has a difference in height between peaks and valleys of the unevenness of 0.2 μm to 2.0 μm in Rmax.
[0091]
However, when the substrate also serves as the back electrode, it may not be necessary to form the back electrode.
[0092]
Further, the back electrode is formed by vapor deposition, sputtering, plating, printing, or the like. When the back electrode is formed in an uneven shape that scatters light, the formed metal or alloy film is formed by dry etching, wet etching, sandblasting, or heating. . Moreover, the uneven | corrugated shape which scatters light can also be formed by vapor-depositing the above-mentioned metal or alloy, heating a board | substrate.
[0093]
Further, although not shown in the drawing, a diffusion prevention layer such as conductive zinc oxide may be provided between the back electrode 102 and the n-type semiconductor layer 103. The effect of the diffusion preventing layer is not only to prevent the metal element constituting the back electrode 102 from diffusing into the n-type semiconductor layer, but also to provide a slight resistance so that the back surface provided with the semiconductor layer interposed therebetween. The effect of preventing a short circuit caused by a defect such as a pinhole between the electrode 102 and the transparent electrode 112, and confining incident light in the photovoltaic element by generating multiple interference due to a thin film, etc. it can.
[0094]
(Semiconductor layer)
As a material of the semiconductor layer used in the present invention, a material using a group IV element such as Si, C, Ge or a material using a group IV alloy such as SiGe, SiC, SiSn is used.
[0095]
Among the above semiconductor materials, a semiconductor material particularly preferably used for the photovoltaic device of the present invention is a-Si: H (abbreviation for hydrogenated amorphous silicon, where a- is amorphous. A-Si: F, a-Si: H: F, a-SiGe: H, a-SiGe: F, a-SiGe: H: F, a-SiC: H, a-SiC: F , A-SiC: H: F group IV and group IV alloy amorphous semiconductor materials.
[0096]
In addition, the semiconductor layer can perform valence electron control and forbidden bandwidth control. Specifically, when forming a semiconductor layer, a raw material compound containing an element that becomes a valence electron controlling agent or a forbidden band width controlling agent is formed alone or mixed with the deposited film forming raw material gas or the dilution gas. What is necessary is just to introduce in the space.
[0097]
The semiconductor layer is at least partially doped with p-type and n-type by valence electron control to form at least a pair of pin junctions. By stacking a plurality of pin junctions, a so-called stack cell configuration is obtained.
[0098]
The semiconductor layer can be formed by various CVD methods such as microwave plasma CVD method, RF plasma CVD method, photo CVD method, thermal CVD method, MOCVD method, or EB deposition, MBE, ion plating, ion beam. It is formed by various vapor deposition methods such as a sputtering method, a sputtering method, a spray method, and a printing method. As a method adopted industrially, a plasma CVD method in which a source gas is decomposed with plasma and deposited on a substrate is preferably used. As the reaction apparatus, a batch type apparatus or a continuous film forming apparatus can be used as desired.
[0099]
Hereinafter, a semiconductor layer using a group IV and group IV alloy-based amorphous semiconductor material particularly suitable for the photovoltaic device of the present invention will be described in more detail.
[0100]
(1) i-type semiconductor layer (intrinsic semiconductor layer)
In particular, in a photovoltaic device using a group IV and group IV alloy-based amorphous semiconductor material, an i-type layer used for a pin junction is an important layer that generates and transports carriers with respect to irradiation light. As the i-type layer, only a p-type layer and a slight n-type layer can be used.
[0101]
As described above, group IV and group IV alloy amorphous semiconductor materials contain hydrogen atoms (H, D) or halogen atoms (X), which have an important function.
[0102]
Hydrogen atoms (H, D) or halogen atoms (X) contained in the i-type layer function to compensate for dangling bonds in the i-type layer, and the mobility of the carrier in the i-type layer. And improve the product of life. Also, it works to compensate the interface state of each interface of the p-type layer / i-type layer and n-type layer / i-type layer, and has the effect of improving the photovoltaic power, photocurrent and photoresponsiveness of the photovoltaic device. There is something. The optimum content of hydrogen atoms and / or halogen atoms contained in the i-type layer is 1 to 40 atm%. In particular, those having a large distribution of hydrogen atoms and / or halogen atoms on the interface side of each of the p-type layer / i-type layer and n-type layer / i-type layer can be mentioned as a preferred distribution form. The content of hydrogen atoms and / or halogen atoms in is preferably in the range of 1.1 to 2 times the content in the bulk. Furthermore, it is preferable that the content of hydrogen atoms and / or halogen atoms is changed corresponding to the content of silicon atoms.
[0103]
In the photovoltaic device of the present invention, amorphous silicon is used as a semiconductor material constituting the i-type semiconductor layer 110 of the first pin junction 116, and the i-type semiconductor layers of the second and third pin junctions are used. As a semiconductor material constituting the elements 107 and 104, amorphous silicon germanium is used.
[0104]
Amorphous silicon and amorphous silicon germanium are made of a-Si: H, a-Si: F, a-Si: H: F, a-SiGe: H, a-SiGe depending on elements for compensating dangling bonds. : F, a-SiGe: H: F.
[0105]
More specifically, for example, i-type hydrogenated amorphous silicon (a-Si: H) is used as the first pin junction i-type semiconductor layer 110 suitable for the photovoltaic device of the present invention. As the characteristics, the optical band gap (Eg) is 1.60 eV to 1.85 eV, the hydrogen atom content (C _H ) Is 1.0-25.0%, AM1.5, 100 mW / cm ² The photoelectric conductivity (σp) under simulated sunlight irradiation is 1.0 × 10 ^-Five S / cm or more, dark conductivity (σd) is 1.0 × 10 ^-9 S / cm or less, Urbach energy by constant photocurrent method (CPM) is 55 meV or less, and localized level density is 10 ¹⁷ / Cm ^Three The following are preferably used.
[0106]
The semiconductor material amorphous silicon germanium constituting the i-type semiconductor layers 107 and 104 of the second and third pin junctions of the photovoltaic device of the present invention is as described above.
[0107]
(2) p-type layer or n-type layer
The p-type layer or the n-type layer is also an important layer that determines the characteristics of the photovoltaic device of the present invention.
[0108]
Examples of the p-type layer or n-type layer amorphous material (denoted as a-) (of course, microcrystalline material (denoted as μc-) also falls into the category of amorphous materials), for example. a-Si: H, a-Si: HX, a-SiC: H, a-SiC: HX, a-SiGe: H, a-SiGeC: H, a-SiO: H, a-SiN: H, a- SiON: HX, a-SiOCN: HX, μc-Si: H, μc-SiC: H, μc-Si: HX, μc-SiC: HX, μc-SiGe: H, μc-SiO: H, μc-SiGeC: H, μc-SiN: H, μc-SiON: HX, μc-SiOCN: HX, etc. and p-type valence electron control agents (periodic group III atoms B, Al, Ga, In, Tl) and n-type A material in which a valence electron controlling agent (group V atom P, As, Sb, Bi) in a high concentration is added For example, poly-Si (H), poly-Si: HX, poly-SiC: H, poly-SiC: HX, poly-SiGe: H, poly-Si , Poly-SiC, poly-SiGe, etc., p-type valence electron control agent (periodic group III atoms B, Al, Ga, In, Tl) and n-type valence electron control agent (periodic table V group A material in which atoms P, As, Sb, Bi) are added at a high concentration can be mentioned. In particular, a crystalline semiconductor layer with little light absorption or an amorphous semiconductor layer with a wide band gap is suitable for the p-type layer or the n-type layer on the light incident side.
[0109]
Optimum amounts of addition of Group III atoms in the periodic table to the p-type layer and addition amounts of Group V atoms in the periodic table to the n-type layer are 0.1 to 50 atm%.
[0110]
Further, hydrogen atoms (H, D) or halogen atoms contained in the p-type layer or the n-type layer function to compensate dangling bonds of the p-type layer or the n-type layer, and the doping efficiency of the p-type layer or the n-type layer. Is to improve. The optimum amount of hydrogen atoms or halogen atoms added to the p-type layer or n-type layer is 0.1 to 40 atm%. In particular, when the p-type layer or the n-type layer is crystalline, the optimum amount of hydrogen atoms or halogen atoms is 0.1 to 8 atm%.
[0111]
Further, a preferable distribution form is one in which a large amount of hydrogen atoms and / or barogen atoms are distributed on each interface side of the p-type layer / i-type layer and n-type layer / i-type layer. The content of hydrogen atoms and / or halogen atoms is preferably in the range of 1.1 to 2 times the content in the bulk. Thus, by increasing the content of hydrogen atoms or halogen atoms in the vicinity of each interface of the p-type layer / i-type layer and n-type layer / i-type layer, the defect level and mechanical strain in the vicinity of the interface are reduced. The photovoltaic power and photocurrent of the photovoltaic device of the present invention can be increased.
[0112]
As the electrical characteristics of the p-type layer and the n-type layer of the photovoltaic device, those having an activation energy of 0.2 eV or less are preferable, and those having an activation energy of 0.1 eV or less are optimal. The specific resistance is preferably 100 Ωcm or less, and most preferably 1 Ωcm or less. Furthermore, the thickness of the p-type layer and the n-type layer is preferably 1 to 50 nm, and most preferably 3 to 10 nm.
[0113]
(3) Method for forming semiconductor layer
In order to form a group IV and group IV alloy-based amorphous semiconductor layer suitable as the semiconductor layer of the photovoltaic device of the present invention, the most preferable manufacturing method is a microwave plasma CVD method, and the next preferable The manufacturing method is an RF plasma CVD method.
[0114]
In the microwave plasma CVD method, a source gas, a dilution gas, or other material gas is introduced into a deposition chamber (vacuum chamber) that can be decompressed, and the vacuum chamber is evacuated while keeping the internal pressure of the deposition chamber constant. Microwaves oscillated by the substrate are guided by a waveguide, introduced into the deposition chamber through a dielectric window (alumina ceramics, etc.), and plasma of a material gas is generated and decomposed, and the substrate disposed in the deposition chamber This is a method for forming a desired deposited film thereon, and a deposited film applicable to a photovoltaic device can be formed under a wide range of deposition conditions.
[0115]
When the semiconductor layer for the photovoltaic device of the present invention is deposited by the microwave plasma CVD method, the substrate temperature in the deposition chamber is 100 to 450 ° C., the internal pressure is 0.5 to 30 mTorr, and the microwave power is 0.01 to 1 W. / Cm ^Three The microwave frequency is preferably in the range of 0.5 to 10 GHz.
[0116]
Further, when depositing by RF plasma CVD, the substrate temperature in the deposition chamber is 100 to 350 ° C., the internal pressure is 0.1 to 10 Torr, and the RF power is 0.01 to 5.0 W / cm. ² The deposition rate is preferably 0.01 to 3 nm / sec.
[0117]
Further, as a method for forming a deposited film suitable for forming a semiconductor layer of the photovoltaic device of the present invention, a roll-to-roll method described in US Pat. No. 4,400,409 is used. . In this deposited film forming method, a plurality of glow discharge regions are arranged along a path that sequentially penetrates, and a semiconductor substrate having a required conductivity type is deposited and formed in each glow discharge region, while a belt-like substrate is formed in the longitudinal direction. It is conveyed continuously in the direction. As a result, a photovoltaic element having a desired semiconductor junction can be formed continuously.
[0118]
The source gas suitable for the deposition of the group IV and group IV alloy-based amorphous semiconductor layer suitable for the photovoltaic device of the present invention is a gasifiable compound containing silicon atoms, and can be gasified containing germanium atoms. Examples thereof include a compound, a gasifiable compound containing a carbon atom, a gasifiable compound containing a nitrogen atom, a gasifiable compound containing an oxygen atom, and a mixed gas of the compound.
[0119]
Specifically, as a gasifiable compound containing a silicon atom, a chain or cyclic silane compound is used, for example, SiH. _Four , Si ₂ H ₆ , SiF _Four , SiFH _Three , SiF ₂ H ₂ , SiF _Three H, Si _Three H ₈ , SiD _Four , SiHD _Three , SiH ₂ D ₂ , SiH _Three D, SiFD _Three , SiF ₂ D ₂ , Si ₂ D _Three H _Three , (SiF ₂ ) _Five , (SiF ₂ ) ₆ , (SiF ₂ ) _Four , Si ₂ F ₆ , Si _Three F ₈ , Si ₂ H ₂ F _Four , Si ₂ H _Three F _Three , SiCl _Four , (SiCl ₂ ) _Five , SiBr _Four , (SiBr ₂ ) _Five , Si ₂ Cl ₆ , SiHCl _Three , SiH ₂ Br ₂ , SiH ₂ Cl ₂ , Si ₂ Cl _Three F _Three Or those that can be easily gasified.
[0120]
Specific examples of the gasifiable compound containing germanium atoms include GeH. _Four , GeD _Four , GeF _Four , GeFH _Three , GeF ₂ H ₂ , GeF _Three H, GeHD _Three , GeH ₂ D ₂ , GeH _Three D, Ge ₂ H ₆ , Ge ₂ D ₆ Etc. Specific examples of compounds that can be gasified containing carbon atoms include CH. _Four , CD _Four , C _n H _{2n + 2} (N is an integer), C _n H _2n (N is an integer), C ₂ H ₂ , C ₆ H ₆ , CO ₂ , CO and the like. N as the nitrogen-containing gas ₂ , NH _Three , ND _Three , NO, NO ₂ , N ₂ O is mentioned. O as an oxygen-containing gas ₂ , CO, CO ₂ , NO, NO ₂ , N ₂ O, CH _Three CH ₂ OH, CH _Three OH etc. are mentioned.
[0121]
Examples of the substance introduced into the p-type layer or the n-type layer for valence electron control include Group III atoms and Group V atoms in the periodic table. As a material effectively used as a starting material for introducing a group III atom, specifically, for introducing a boron atom, B ₂ H ₆ , B _Four H _Ten , B _Five H ₉ , B _Five H ₁₁ , B ₆ H _Ten , B ₆ H ₁₂ , B ₆ H ₁₄ Boron hydride such as BF _Three , BCl _Three , And the like. Besides this, AlCl _Three , GaCl _Three , InCl _Three , TlCl _Three Etc. can also be mentioned. Especially B ₂ H ₆ , BF _Three Is suitable.
[0122]
Specifically, it is effective to use as a starting material for introducing a group V atom, specifically, for introducing a phosphorus atom, PH. _Three , P ₂ H _Four Phosphorus hydrides such as PH _Four I, PF _Three , PF _Five , PCl _Three , PCl _Five , PBr _Three , PBr _Five , PI _Three Of the halogenated phosphorus. In addition, AsH _Three , AsF _Three , AsCl _Three , AsBr _Three , AsF _Five , SbH _Three , SbF _Three , SbF _Five , SbCl _Three , SbCl _Five , BiH _Three , BiCl _Three , BiBr _Three Etc. can also be mentioned. Especially PH _Three , PF _Three Is suitable.
[0123]
The gasatable compound is H ₂ , He, Ne, Ar, Xe, Kr, and the like may be diluted with a gas and introduced into the deposition chamber.
[0124]
In particular, when depositing a layer with small light absorption or wide band gap such as microcrystalline semiconductor or a-SiC: H, the source gas is diluted 2 to 100 times with hydrogen gas, and the microwave power or RF power is relatively high. It is preferable to introduce by power.
[0125]
(Transparent electrode)
In the present invention, the transparent electrode 112 is a light incident side electrode that transmits light, and also serves as an antireflection film by optimizing its film thickness. The transparent electrode 112 is required to have a high transmittance in the wavelength region that can be absorbed by the semiconductor layer and to have a low resistivity. Preferably, the transmittance at 550 nm is 80% or more, more preferably 85% or more. The resistivity is preferably 5 × 10. ^-3 Ωcm or less, more preferably 1 × 10 ^-3 It is desirable that it is Ωcm or less. As the material, In ₂ O _Three , SnO ₂ , ITO (In ₂ O _Three + SnO ₂ , ZnO, CdO, Cd ₂ SnO _Four , TiO ₂ , Ta ₂ O _Five , Bi ₂ O _Three , MoO _Three , Na _X WO _Three Conductive oxides such as these or a mixture thereof are preferably used. Moreover, you may add the element (dopant) which changes electrical conductivity to these compounds.
[0126]
As an element (dopant) for changing the conductivity, for example, when the transparent electrode 112 is ZnO, Al, In, B, Ga, Si, F, etc., and In ₂ O _Three In the case of Sn, F, Te, Ti, Sb, Pb, etc., SnO ₂ In this case, F, Sb, P, As, In, Tl, Te, W, Cl, Br, I, etc. are preferably used.
[0127]
As a method for forming the transparent electrode 112, a vapor deposition method, a CVD method, a spray method, a spin-on method, a dip method, or the like is preferably used.
[0128]
(Collector electrode)
In the present invention, the collecting electrode 113 is formed on a part of the transparent electrode 112 as necessary when the resistivity of the transparent electrode 112 cannot be sufficiently lowered, and the resistance of the electrode is lowered to reduce the series resistance of the photovoltaic element. It works to lower. The materials used are gold, silver, copper, aluminum, nickel, iron, chromium, molybdenum, tungsten, titanium, cobalt, tantalum, niobium, zirconium and other metals, alloys such as stainless steel, and conductive metals using powdered metals. A paste etc. are mentioned. The shape is formed in a branch shape as shown in FIG. 5, for example, so as not to block the incident light to the semiconductor layer as much as possible.
[0129]
Further, the area occupied by the current collecting electrode in the total area of the photovoltaic element is preferably 15% or less, more preferably 10% or less, and most preferably 5% or less.
[0130]
Further, a mask is used for forming the pattern of the collecting electrode, and as a forming method, an evaporation method, a sputtering method, a plating method, a printing method, or the like is used.
[0131]
When producing a photovoltaic device having a desired output voltage and output current using the photovoltaic device of the present invention, the photovoltaic devices of the present invention are connected in series or in parallel, and the front and back surfaces are connected. A protective layer is formed, and an output extraction electrode or the like is attached. Further, when the photovoltaic elements of the present invention are connected in series, a backflow preventing diode may be incorporated.
[0132]
【Example】
Hereinafter, although the photovoltaic element of this invention is demonstrated in detail by the solar cell which consists of a non-single-crystal silicon type semiconductor material, this invention is not limited to this.
[0133]
(Example 1)
A solar cell having the configuration shown in FIG. 1 was produced using the deposition apparatus shown in FIG. First, a substrate was produced. 0.5mm thickness, 50x50mm ² The stainless steel substrate was sufficiently ultrasonically washed with acetone and isopropanol, and then dried with warm air in a drying furnace maintained at 110 ° C. Next, an Ag light reflecting layer having a substrate temperature of 350 ° C. and a layer thickness of 0.45 μm is formed on the stainless steel substrate by sputtering, and ZnO having a substrate temperature of 350 ° C. and a layer thickness of 2.0 μm is formed thereon. The transparent conductive layer was formed, and the production of the substrate was completed. ZnO is a diffusion preventing layer.
[0134]
Next, each semiconductor layer was formed on the back reflective layer. The deposition apparatus of FIG. 2 can perform both MW plasma CVD (MWPCVD) and RF plasma CVD (RFPCVD) methods.
[0135]
A raw material gas supply device (not shown) is connected to the deposition apparatus 200 through a gas introduction pipe. All of the source gas cylinders are refined to ultra high purity, and SiH _Four Gas cylinder, GeH _Four Gas cylinder, Si ₂ H ₆ Gas cylinder, PH _Three / H ₂ (2%) Gas cylinder, BF _Three / H ₂ (1%) gas cylinder, SiH _Four / H ₂ (10%) Gas cylinder, H ₂ A gas cylinder and a He gas cylinder were connected. In addition, a vacuum exhaust pump (not shown) is connected to each deposition chamber.
[0136]
After the film formation preparation is completed, an n3 layer made of a-Si by RFPCVD method, an i3 layer made of a-SiGe by MWPCVD method, a p3 layer (above the bottom layer) made of μc-Si by RFPCVD method, and RFPCVD. N2 layer made of a-Si by the method, i2 layer made of a-SiGe by the MWPCVD method, p2 layer (more middle layer) made of μc-Si by the RFPCVD method, n1 layer made of a-Si by the RFPCVD method, RFPCVD method Then, the i1 layer made of a-Si and the p1 layer (more than the top layer) made of μc-Si by the RFPCVD method were sequentially laminated to produce an a-Si / a-SiGe / a-SiGe triple solar cell.
[0137]
To form an n3 layer made of a-Si, H ₂ A gas was introduced into the deposition chamber 201 and adjusted with a mass flow controller so that the flow rate was 50 sccm, and the pressure in the deposition chamber was adjusted to 1.3 Torr. Set the heater 205 so that the substrate temperature becomes 380 ° C. ₂ H ₆ Gas, PH _Three / H ₂ (2%) gas was introduced into the deposition chamber 201. At this time, Si ₂ H ₆ Gas flow rate is 1.0 sccm, H ₂ Gas flow rate is 50sccm, PH _Three / H ₂ (2%) The gas flow rate was adjusted to 0.5 sccm, and the pressure in the deposition chamber was adjusted to 1.0 Torr. The power of the RF power source was set to 2.0 W, RF power was applied to the RF electrode, glow discharge was generated, the shutter was opened, the n3 layer having a film thickness of 25 nm was formed, the shutter was closed, and the glow discharge was stopped. . Si into the deposition chamber ₂ H ₆ Gas, PH _Three / H ₂ (2%) Stop introducing gas and put H into the deposition chamber for 5 minutes. ₂ After continuing to flow gas, ₂ 1 × 10 in the deposition chamber and gas pipe ^-Five It was evacuated to Torr.
[0138]
The i3 layer made of a-SiGe was formed by the MWPCVD method. First, H ₂ Gas was introduced into the deposition chamber at 300 sccm so that the pressure was 7.0 mTorr and the substrate temperature was 400 ° C. When the substrate temperature is stable, SiH _Four Gas, GeH _Four Gas is introduced and SiH _Four Gas flow rate is 40sccm, GeH _Four Gas flow rate is 40sccm, H ₂ The gas flow rate was adjusted to 300 sccm and the pressure in the deposition chamber 201 was adjusted to 7.0 mTorr. Next, the power of the RF power source was set to 900 W and applied to the RF electrode. Thereafter, the power of the MW power source is set to 400 W, MW power is introduced into the deposition chamber through the dielectric window, glow discharge is generated, the shutter is opened, and the i3 layer having a thickness of 80 nm is formed on the n3 layer. Was closed to stop glow discharge. SiH into the deposition chamber _Four Gas, GeH _Four Stop the introduction of gas and enter the deposition chamber for 5 minutes. ₂ After continuing to flow gas, ₂ 1 × 10 in the deposition chamber and gas pipe ^-Five The vacuum was exhausted to Torr.
[0139]
To form a p3 layer made of μc-Si, H ₂ Gas was introduced into the deposition chamber 201 at 40 sccm, and the pressure in the deposition chamber was adjusted to 2.0 Torr. Set the heater so that the substrate temperature becomes 250 ° C, and when the substrate temperature is stable, SiH _Four / H ₂ (10%) Gas, BF _Three / H ₂ (1%) gas was introduced into the deposition chamber 201. At this time, SiH _Four / H ₂ (10%) Gas flow rate is 0.25 sccm, H ₂ Gas flow rate is 40sccm, BF _Three / H ₂ (1%) The gas flow rate was adjusted to 2.0 sccm, and the pressure in the deposition chamber was adjusted to 2.0 Torr. The power of the RF power source was set to 40 W, RF power was applied to the RF electrode to cause glow discharge, the shutter was opened, the shutter was closed when the p3 layer having a film thickness of 5 nm was formed, and the glow discharge was stopped. SiH into the deposition chamber _Four / H ₂ (10%) Gas, BF _Three / H ₂ (1%) Stop the gas introduction and enter the deposition chamber for 5 minutes. ₂ After continuing to flow gas, ₂ 1 × 10 in the deposition chamber and gas pipe ^-Five The vacuum was exhausted to Torr.
[0140]
This completes the formation of the bottom layer of the a-Si / a-SiGe / a-SiGe triple cell.
[0141]
Next, an n2 layer made of a-Si, an i2 layer made of a-SiGe, and a p2 layer made of μc-Si (middle layer) were formed on the bottom layer.
[0142]
To form an n2 layer made of a-Si, H ₂ A gas was introduced into the deposition chamber 201 and adjusted with a mass flow controller so that the flow rate was 50 sccm, and the pressure in the deposition chamber was adjusted to 1.3 Torr. Set the heater so that the substrate temperature is 220 ° C. ₂ H ₆ Gas, PH _Three / H ₂ (2%) gas was introduced into the deposition chamber 201. At this time, Si ₂ H ₆ Gas flow rate is 1.0 sccm, H ₂ Gas flow rate is 50sccm, PH _Three / H ₂ (2%) The gas flow rate was adjusted to 2.5 sccm, and the pressure in the deposition chamber was adjusted to 1.0 Torr. The power of the RF power source was set to 2.0 W, RF power was applied to the RF electrode, glow discharge was generated, the shutter was opened, the n2 layer having a thickness of 10 nm was formed, the shutter was closed, and the glow discharge was stopped. . Si into the deposition chamber ₂ H ₆ Gas, PH _Three / H ₂ (2%) Stop introducing gas and put H into the deposition chamber for 5 minutes. ₂ After continuing to flow gas, ₂ 1 × 10 in the deposition chamber and gas pipe ^-Five It was evacuated to Torr.
[0143]
The i2 layer made of a-SiGe was formed by the MWPCVD method. First, H ₂ Gas was introduced into the deposition chamber 201 at 300 sccm so that the pressure was 7.0 mTorr and the substrate temperature was 400 ° C. When the substrate temperature is stable, SiH _Four Gas, GeH _Four Gas introduced SiH _Four Gas flow rate is 40sccm, GeH4 gas flow rate is 40sccm, H ₂ The gas flow rate was adjusted to 300 sccm and the pressure in the deposition chamber 201 was adjusted to 7.0 mTorr. Next, the power of the RF power source was set to 900 W and applied to the RF electrode. Thereafter, the power of the MW power source is set to 400 W, MW power is introduced into the deposition chamber through the dielectric window, glow discharge is generated, the shutter is opened, and the shutter is opened when the i2 layer having a film thickness of 75 nm is formed on the n2 layer. Was closed to stop glow discharge. SiH into the deposition chamber _Four Gas, GeH _Four Stop the introduction of gas and enter the deposition chamber for 5 minutes. ₂ After continuing to flow gas, ₂ 1 × 10 in the deposition chamber and gas pipe ^-Five It was evacuated to Torr.
[0144]
To form a p2 layer made of μc-Si, H ₂ Gas was introduced into the deposition chamber 201 at 40 sccm, and the pressure in the deposition chamber was adjusted to 2.0 Torr. Set the heater so that the substrate temperature becomes 250 ° C, and when the substrate temperature is stable, SiH _Four / H ₂ (10%) Gas, BF _Three / H ₂ (1%) gas was introduced into the deposition chamber 201. At this time, SiH _Four / H ₂ (10%) Gas flow rate is 0.25 sccm, H ₂ Gas flow rate is 40sccm, BF _Three / H ₂ (1%) The gas flow rate was adjusted to 2.0 sccm, and the pressure in the deposition chamber was adjusted to 2.0 Torr. The power of the RF power supply was set to 40 W, RF power was applied to the RF electrode, glow discharge was generated, the shutter was opened, the p2 layer having a film thickness of 5 nm was formed, the shutter was closed, and the glow discharge was stopped. SiH into the deposition chamber _Four / H ₂ (10%) Gas, BF _Three / H ₂ (1%) Stop the gas introduction and enter the deposition chamber for 5 minutes. ₂ After continuing to flow gas, ₂ 1 × 10 in the deposition chamber and gas pipe ^-Five It was evacuated to Torr.
[0145]
This completes the formation of the middle layer in the a-Si / a-SiGe / a-SiGe triple cell.
[0146]
Next, an n1 layer made of a-Si, an i1 layer made of a-Si, and a p1 layer made of μc-Si (top layer) were formed.
[0147]
To form an n1 layer made of a-Si, H ₂ A gas was introduced into the deposition chamber 201 and adjusted with a mass flow controller so that the flow rate was 50 sccm, and the pressure in the deposition chamber was adjusted to 1.3 Torr. Set the heater so that the substrate temperature is 220 ° C. ₂ H ₆ Gas, PH _Three / H ₂ (2%) gas was introduced into the deposition chamber 201. At this time, Si ₂ H ₆ Gas flow rate is 1.0 sccm, H ₂ Gas flow rate is 50sccm, PH _Three / H ₂ (2%) The gas flow rate was adjusted to 2.5 sccm, and the pressure in the deposition chamber was adjusted to 1.0 Torr. The power of the RF power source was set to 2.0 W, RF power was applied to the RF electrode, glow discharge was generated, the shutter was opened, the n1 layer having a film thickness of 10 nm was formed, the shutter was closed, and the glow discharge was stopped. . Si into the deposition chamber ₂ H ₆ Gas, PH _Three / H ₂ (2%) Stop introducing gas and put H into the deposition chamber for 5 minutes. ₂ After continuing to flow gas, ₂ 1 × 10 in the deposition chamber and gas pipe ^-Five It was evacuated to Torr.
[0148]
In order to form an i1 layer made of a-Si, H ₂ Gas was introduced into the deposition chamber 201 and adjusted with a mass flow controller so that the flow rate was 100 sccm, and the pressure in the deposition chamber was adjusted to 0.5 Torr. Set the heater so that the substrate temperature is 250 ° C. ₂ H ₆ Gas was introduced into the deposition chamber. At this time, Si ₂ H ₆ Gas flow rate is 4.0 sccm, H ₂ The gas flow rate was adjusted to 100 sccm, and the pressure in the deposition chamber was adjusted to 0.5 Torr. The power of the RF power source was set to 3.0 W, RF power was applied to the RF electrode to cause glow discharge. When the shutter was opened to form an i1 layer having a film thickness of 85 nm, the shutter was closed to stop glow discharge. Si into the deposition chamber ₂ H ₆ Stop the introduction of gas and enter the deposition chamber for 5 minutes. ₂ After continuing to flow gas, ₂ 1 × 10 in the deposition chamber and gas pipe ^-Five It was evacuated to Torr.
[0149]
To form a p1 layer made of μc-Si, H ₂ Gas was introduced into the deposition chamber 201 at 40 sccm, and the pressure in the deposition chamber was adjusted to 2.0 Torr. Set the heater so that the substrate temperature becomes 250 ° C, and when the substrate temperature is stable, SiH _Four / H ₂ (10%), BF _Three / H ₂ (1%) gas was introduced into the deposition chamber. At this time, SiH _Four / H ₂ (10%) Gas flow rate is 0.25 sccm, H ₂ Gas flow rate is 40sccm, BF _Three / H ₂ (1%) The gas flow rate was adjusted to 2.0 sccm, and the pressure in the deposition chamber was adjusted to 2.0 Torr. The power of the RF power source was set to 40 W, RF power was applied to the RF electrode, glow discharge was generated, the shutter was opened, the p1 layer having a film thickness of 4 nm was formed, the shutter was closed, and glow discharge was stopped. SiH into the deposition chamber _Four / H ₂ (10%) Gas, BF _Three / H ₂ (1%) Stop the gas introduction and enter the deposition chamber for 5 minutes. ₂ After continuing to flow gas, ₂ 1 × 10 in the deposition chamber and gas pipe ^-Five It was evacuated to Torr.
[0150]
Thus, the formation of the top layer of the a-Si / a-SiGe / a-SiGe triple cell was completed, the leak valve was opened, the deposition chamber was leaked, and the triple solar cell was taken out.
[0151]
Next, ITO having a film thickness of 70 nm was vacuum-deposited on the p1 layer by a resistance heating vacuum deposition method as a transparent electrode.
[0152]
Next, a mask having a comb-shaped hole was placed on the transparent electrode, and a comb-shaped collector electrode made of Cr (20 nm) / Ag (800 nm) / Cr (20 nm) was vacuum-deposited by an electron beam vacuum deposition method. .
[0153]
This completes the production of the solar cell. This solar cell is referred to as (SC Ex 1), and the conditions for forming the bottom layer, middle layer, and top layer are shown in Table 1.
[0154]
In addition, the germanium composition of the i layer in the solar cell (SC Ex 1) was 0.60 for the bottom layer and 0.50 for the middle layer. The band gap of the i layer was 1.35 eV for the bottom layer, 1.40 eV for the middle layer, and 1.75 eV for the top layer.
[0155]
[Table 1]

[0156]
(Comparative Example 1)
In the a-Si / a-SiGe / a-SiGe triple-type solar cell, the bottom layer, middle layer, and top layer i layers have a thickness of 180 nm, 140 nm, and 100 nm, and the bottom layer and middle layer i layer germanium composition. A conventional solar cell (SC ratio: 1) was produced under the same conditions as in Example 1 except that was set to 0.30.
[0157]
The relationship between the i-layer thickness of the triple solar cell having the configuration shown in FIG. 1 and the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was examined.
[0158]
A solar cell whose initial photoelectric conversion efficiency has been measured in advance is installed in an accelerated deterioration apparatus using light, and an AM 1.5 spectrum, an irradiation light intensity of 50 SUN (5 W / cm ² ), The surface of the solar cell was irradiated with light under a deterioration condition of a substrate surface temperature of 110 ° C. When 50000 seconds had passed since the start of light irradiation, the solar cell was taken out from this accelerated deterioration device. Next, the taken-out solar cell was installed in the apparatus which measures photoelectric conversion efficiency, AM1.5 spectrum, irradiation light intensity 1SUN (100mW / cm ² ), The photoelectric conversion efficiency was measured under the measurement condition of the substrate surface temperature of 25 ° C.
[0159]
Evaluation of acceleration degradation due to light as described above and measurement of photoelectric conversion efficiency after acceleration degradation was performed on a solar cell (SC actual 1) and a solar cell (SC ratio 1). As a result of the measurement, it was found that the photoelectric conversion efficiency (ηs) after accelerated deterioration of (SC Ex 1) was 1.23 times better than that of (SC ratio 1).
[0160]
(Example 2)
A triple solar cell having the configuration of FIG. 1 having a graded band gap structure as shown in FIG. 3 as the bottom layer i layer (i3 layer) was produced, and the same evaluation as in Example 1 was performed.
[0161]
SiH so that the band structure of the i layer of the bottom layer has a band profile as shown in FIG. _Four Flow rate, GeH _Four A triple solar cell (SC Ex 2) similar to that in Example 1 was prepared except that the flow rate was controlled, and evaluation similar to that in Example 1 was performed. The conversion efficiency (ηs) was found to be 1.27 times better than that of the conventional triple solar cell of Comparative Example 1 (SC ratio 1).
[0162]
(Example 3)
Except for laminating a buffer layer made of RFPCVD between the n3 layer and the i3 layer, between the i3 layer and the p3 layer, between the n2 layer and the i2 layer, and between the i2 layer and the p2 layer. A triple solar cell (SC Ex 3) similar to that in Example 2 was produced. When the same evaluation as in Example 1 was performed (SC Ex. 3), the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was higher than that of the conventional triple solar cell of Comparative Example 1 (SC ratio 1). 1.32 times better.
[0163]
(Example 4)
SiH so that the band structure of the i3 layer has a band profile as shown in FIG. _Four Flow rate, GeH _Four A triple solar cell (SC Ex 4) was produced in the same manner as in Example 3 except that the flow rate was controlled and the germanium composition of the middle i-type layer was 0.60. When the same evaluation as in Example 1 was performed (SC Ex. 4), the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was higher than that of the conventional triple solar cell of Comparative Example 1 (SC ratio 1). It was found to be 1.35 times better.
[0164]
(Example 5)
A triple solar cell (SC Ex 5) was produced in the same manner as in Example 4 except that the film thicknesses of the bottom layer, middle layer, and top layer i layer were 75 nm, 70 nm, and 85 nm. When the same evaluation as in Example 1 was performed (SC Ex 5), the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was higher than that of the conventional triple solar cell of Example 1 (SC ratio 1). 1.37 times better.
[0165]
(Example 6)
SiH so that the band structure of the i layer of the middle layer has a band profile as shown in FIG. _Four Flow rate, GeH _Four A triple solar cell (SC Ex 6) similar to that in Example 3 was produced except that the flow rate was controlled, and evaluation similar to that in Example 1 was performed. The conversion efficiency (ηs) was found to be 1.40 times better than that of the conventional triple solar cell of Example 1 (SC ratio 1).
[0166]
(Example 7)
The triple solar cell of FIG. 1 was produced using the manufacturing apparatus using the roll-to-roll method shown in FIG. The substrate used was a strip-shaped stainless steel sheet having a mirror-polished surface with a length of 300 m, a width of 30 cm, and a thickness of 0.1 mm. A sputtering apparatus using a roll-to-roll method is used to continuously form a reflective layer made of Ag having a layer thickness of 0.3 μm at a substrate temperature of 350 ° C. on the surface of this stainless steel substrate, and further to a layer thickness of 2.0 μm at a substrate temperature of 350 ° C. A transparent conductive layer made of ZnO was continuously formed. It was found that the substrate surface was textured. The unevenness on the surface of the substrate was 500 nm in Rmax.
[0167]
FIG. 4 is a schematic view of an apparatus for continuously forming semiconductor layers using a roll-to-roll method. In this apparatus, a substrate delivery chamber 401, a plurality of deposition chambers 402 to 414, and a substrate take-up chamber 415 are sequentially arranged, and they are connected by a separation passage 416, and a belt-like substrate 417 is passed through them. Accordingly, the substrate can be continuously moved from the substrate delivery chamber to the substrate take-up chamber, and the respective semiconductor layers can be simultaneously formed in the respective deposition chambers simultaneously with the movement of the substrate.
[0168]
450 is a top view of the deposition chambers. Each deposition chamber has a source gas inlet 419 and a source gas exhaust port 420, and an RF electrode 421 or a microwave introduction portion 422 is attached as necessary. Further, a halogen lamp heater 418 for heating the substrate is installed inside. A raw material gas supply device (not shown) is connected to the raw material gas inlet, and a vacuum exhaust pump such as an oil diffusion pump or a mechanical booster pump is connected to each raw material gas exhaust port. An RF power source (not shown) is connected to each RF electrode, and a microwave (MW) power source is connected to the microwave introduction unit. The separation passage connected to the deposition chamber has an inlet 424 through which scavenging gas flows. The substrate delivery chamber has a delivery roll 425 and a guide roll 426 for applying appropriate tension to the substrate, and the substrate take-up chamber has a take-up roll 427 and a guide roll 428.
[0169]
First, the substrate on which the reflective layer and the transparent conductive layer are formed is wound around a delivery roll, set in a substrate delivery chamber, and after passing through each deposition chamber, the substrate is wound around a substrate take-up roll. The entire apparatus is evacuated by an unillustrated evacuation pump, the lamp heater in each deposition chamber is turned on, and the substrate temperature in each deposition chamber is set to a predetermined temperature. When the pressure of the whole apparatus becomes 1 mTorr or less, H from the inlet of each scavenging gas ₂ The gas is introduced, and the substrate is taken up by a take-up roll while moving the substrate in the direction of the arrow in the figure. In the same manner as in the first embodiment, a source gas for forming each semiconductor layer is caused to flow into each deposition chamber. At this time, H that is introduced into each separation passage so that the source gas flowing into each deposition chamber does not diffuse into other deposition chambers. ₂ Adjust the gas flow rate and the pressure in each deposition chamber.
[0170]
Next, RF power and MW power are introduced into each deposition chamber to cause glow discharge to form respective semiconductor layers.
[0171]
An n3 layer (a-Si: H) is formed by RF plasma CVD in the deposition chamber 402 on the substrate, and a b31 buffer layer (a-Si: H) is formed in the deposition chamber 403 by RF plasma CVD. In the deposition chamber 405, the b3 buffer layer (a-Si: H) is formed by the RF plasma CVD method. In the deposition chamber 406, the p3 layer is formed by the RF plasma CVD method (a-SiGe: H). μc-Si: H), n2 layer (a-Si: H) is deposited by RF plasma CVD in the deposition chamber 407, and b21 buffer layer (a-Si: H) is deposited by RF plasma CVD in the deposition chamber 408. In the chamber 409, the i2 layer (a-SiGe: H) is formed by the MW plasma CVD method, the b22 buffer layer (a-Si: H) is formed in the deposition chamber 410 by the RF plasma CVD method, and the RF is formed in the deposition chamber 411. The p2 layer (μc-Si: H) is formed by the plasma CVD method, the n1 layer (μc-Si: H) is formed by the RF plasma CVD method in the deposition chamber 412, and the i3 layer (a-Si) is formed by the RF plasma CVD method in the deposition chamber 413. : H), and in the deposition chamber 414, a p1 layer (μc-Si: H) was sequentially formed by RF plasma CVD.
[0172]
The source gas inlet 429 of the deposition chamber for forming the i3 layer and the deposition chamber for forming the i2 layer is divided into a plurality of portions, and SiH from each inlet. _Four Gas, GeH _Four Gas, H ₂ A gas was introduced so that the Ge composition of the i3 layer and the i2 layer was uniform in the layer thickness direction, and the band gap was uniform in the layer thickness direction. When the transfer of the substrate was completed, the MW power source and the RF power source were turned off, the glow discharge was extinguished, and the inflow of the source gas and the scavenging gas was stopped. When the substrate temperature dropped to room temperature, the entire apparatus was leaked, and the wound substrate was taken out. Detailed production conditions are summarized in Table 2.
[0173]
Next, the transparent electrode which consists of ITO with a layer thickness of 70 nm at 170 degreeC was continuously formed on p1 layer with the sputtering device using a roll-to-roll system.
[0174]
Next, a part of this substrate was cut into a size of 50 mm × 50 mm, and silver paste having a layer thickness of 5 μm and a line width of 0.5 mm was printed by a screen printing method to form a collecting electrode.
[0175]
Next, the solar cells were serialized and paralleled so that the rated voltage was 25 V and the rated output was 50 W.
[0176]
Next, on a steel backing plate, EVA (ethylene vinyl acetate) sheet, nylon sheet, EVA sheet, crane glass (glass fiber nonwoven fabric), serialized / parallel solar cell, crane glass, EVA sheet Crane glass, EVA sheet, crane glass, and ETFE (ethylene tetrafluoroethylene) sheet were placed and vacuum laminated (150 ° C., 40 minutes).
[0177]
This completes the production of the triple solar cell module (MJ real 7) using the roll-to-roll method.
[0178]
[Table 2]

[0179]
(Comparative Example 7)
Using the roll-to-roll manufacturing apparatus, a conventional triple solar cell in which the film thicknesses of the i1, i2, and i3 layers were 100 nm, 130 nm, and 190 nm, respectively, was produced. A triple solar cell (MJ ratio 7) was prepared in the same manner as in Example 7 except that the film thickness was changed, and the same evaluation as in Experimental Example 1 was performed. As a result, the solar cell module of Example 7 (MJ Actual 7) It was found that the photoelectric conversion efficiency (ηs) after accelerated degradation by light of 1.21 was 1.21 times that of the solar cell module of Comparative Example 7 (MJ ratio 7).
[0180]
(Example 8)
SiH from source gas inlet 429 divided into a plurality in Example 7 _Four Gas, GeH _Four Gas, H ₂ When gas is introduced to form the i3 layer and the i2 layer, GeH _Four The triple solar cell module of FIG. 1 was produced in the same manner as in Example 7 except that the gas flow rate was adjusted to 430 so that a-SiGe: H would be the band gap of FIG. When the same evaluation as in Experimental Example 1 was performed, it was found that the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was 1.23 times that of the solar cell module of Comparative Example 7 (MJ ratio 7). .
[0181]
Example 9
In Example 7, H is added to a part of the deposition chamber 405 and the deposition chamber 410. ₂ A triple solar cell module (MJ real 9) of FIG. 1 was produced in the same manner as in Example 7 except that a portion for generating gas-only plasma was provided and the hydrogen plasma treatment was performed on the b32 and b22 layers. When the same evaluation as in Experimental Example 1 was performed, it was found that the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was 1.25 times that of the solar cell module of Comparative Example 7 (MJ ratio 7). .
[0182]
(Example 10)
In Example 7, SiH to flow into the deposition chamber 404 and the deposition chamber 409 _Four Gas flow rate, GeH _Four A triple solar cell module (MJ actual 10) of FIG. 1 was produced in the same manner as in Example 7 except that the gas flow rate was changed to 4/5 and the MW power was changed to 4/5. When the same evaluation as in Experimental Example 1 was performed, it was found that the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was 1.22 times that of the solar cell module of Comparative Example 7 (MJ ratio 7). .
[0183]
(Example 11)
In Example 7, SiH to flow into the deposition chamber 404 and the deposition chamber 409 _Four The gas flow rate is 4/5 and GeH _Four A triple solar cell module (MJ11) of FIG. 1 was produced in the same manner as in Example 7 except that the gas flow rate was changed to 6/5. When the same evaluation as in Experimental Example 1 was performed, it was found that the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was 1.26 times that of the solar cell module of Comparative Example 7 (MJ ratio 7). .
[0184]
(Example 12)
1 in the same manner as in Example 7 except that the substrate temperature in the deposition chamber 405 is 350 ° C. and the substrate temperature in the deposition chamber 410 is 330 ° C. Produced. When the same evaluation as in Experimental Example 1 was performed, it was found that the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was 1.31 times that of the solar cell module of Comparative Example 7 (MJ ratio 7). .
[0185]
(Example 13)
In Example 7, a new CH32 layer and b22 layer were formed. _Four A triple solar cell module (MJ13) using an a-SiC: H buffer layer formed by flowing a gas into the deposition chamber at 15 sccm was produced. When the same evaluation as in Experimental Example 1 was performed, it was found that the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was 1.29 times that of the solar cell module of Comparative Example 7 (MJ ratio 7). .
[0186]
(Example 14)
In Example 7, a new microwave introduction unit was installed in the deposition chambers 406, 411, and 414, and a new CH was formed when the p1, p2, and p3 layers were formed. _Four A triple solar cell module (MJ real 14) using a p-layer of microcrystalline SiC: H formed by MWPCVD by flowing gas into each deposition chamber at 10 sccm was produced. When the same evaluation as in Experimental Example 1 was performed, it was found that the photoelectric conversion efficiency (ηs) after accelerated deterioration by light was 1.32 times that of the solar cell module of Comparative Example 7 (MJ ratio 7). .
[0187]
【The invention's effect】
Claim 1 2 In other words, the film thickness of the amorphous silicon germanium, which is the i layer of the second and third pin junctions, is reduced from the film thickness that has been considered to be suitable in the prior art. An increase in localized levels in the layer can be suppressed, and light degradation can be further suppressed in the stack type photovoltaic device.
[0188]
Claim 3, 4 Even when the amorphous silicon germanium film thickness is reduced as described above by increasing the germanium composition of the amorphous silicon germanium that is the i-layer of the second and third pin junctions. The current generated in the first, second, and third pin junctions can be matched, and high photoelectric conversion efficiency can be maintained while suppressing photodegradation.
[0189]
Claim 5-7 According to the invention, even when the content of germanium in the amorphous silicon germanium is increased, the open-circuit voltage (Voc) and the fill factor (FF) of the photovoltaic element can be particularly improved. It is possible to maintain higher conversion efficiency while suppressing deterioration.
[0190]
By the stacked photovoltaic element of the present invention having the above-described configuration, the photo-degradation of amorphous silicon germanium that is the i layer of the second and third pin junctions is suppressed, and while maintaining high photoelectric conversion efficiency, It is possible to reduce light deterioration and improve conversion efficiency after light deterioration. Accordingly, it is possible to provide a photovoltaic device with high reliability and high photoelectric conversion efficiency while having a low cost suitable for practical use.
[0191]
Further, as another effect of the photovoltaic device of the present invention, the film thickness of amorphous silicon germanium, which is the i layer of the second and third pin junctions, deviates from the film thickness that has conventionally been considered suitable. By using a thin film, the amount of germanium, which is a smaller resource than silicon, can be reduced. In addition, energy used in the manufacturing process can be reduced. Furthermore, the film thickness of the entire photovoltaic device is reduced, so that the stress applied to the laminated thin film is reduced, the durability when the substrate is bent or warped is increased, and the deposited film peels off from the substrate. The problem can be reduced. Thereby, the yield of the manufacturing process is improved, and the flexibility of the usage pattern of the photovoltaic device is improved.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically showing an example of a photovoltaic element of the present invention.
FIG. 2 is a conceptual diagram showing an example of a semiconductor layer forming apparatus.
FIG. 3 is a graph showing a preferred example of a change in the film thickness direction of the band gap of the i layer.
FIG. 4 is a conceptual diagram showing an example of a continuous device for forming photovoltaic elements.
FIG. 5 is a conceptual diagram showing an example of a collecting electrode formation pattern of a photovoltaic element.
FIG. 6 is a graph showing an example of measurement by a constant photocurrent method.
[Explanation of symbols]
101, 204, 417 substrate,
102 Lower electrode,
103 n3 layer (p3 layer),
104 i3 layer,
105 p3 layer (n3 layer),
106 n2 layer (p2 layer),
107 i2 layer,
108 p2 layer (n2 layer),
109 n1 layer (p1 layer),
110 i1 layer,
111 p1 layer (n1 layer),
112 transparent electrode,
113 current collecting electrode,
114 bottom,
115 middle,
116 Top,
200 deposition equipment,
201 Deposition chamber,
202 vacuum gauge,
203 bias (RF) power supply,
205 heater,
206 waveguide,
207 conductance valve,
208 valves,
209 substrate holder,
210 bias (RF) electrode,
211 gas introduction pipe,
212 microwave introduction part,
213 dielectric window,
215 shutter,
219 microwave power supply,
220 exhaust port,
401 substrate delivery chamber,
402-414 deposition chamber,
415 substrate take-up chamber,
416 separation passage,
418 lamp heater,
419 Raw material gas inlet,
420 exhaust port,
421 RF electrode,
422 microwave introduction part,
424 scavenging gas inlet,
425 delivery roll,
426 guide roll,
427 Winding roll,
428 guide roll,
429 multiple gas inlets;
430 GeH ₄ Flow rate,
431 RF bias electrode,
450 Viewed from above,
501 Light incident surface of the photovoltaic element,
502 Extraction electrode.

Claims (7)

  In a so-called stack type photovoltaic device in which a plurality of pin-type semiconductor junctions formed by stacking a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer are stacked, the first pin counted from the light incident side Amorphous silicon is used as the i layer of the junction, amorphous silicon germanium is used as the i layer of the second and third pin junctions, and the film thickness of the amorphous silicon germanium that is the i layer of the third pin junction A photovoltaic element characterized by having a thickness of 150 nm or less. 2. The photovoltaic element according to claim 1, wherein the film thickness of amorphous silicon germanium, which is the i layer of the second pin junction, is 100 nm or less. According to any one of claims 1-2, characterized in that the germanium content of the third amorphous silicon germanium is i layer of a pin junction of a (Ge / (Si + Ge) ) to 0.4 or more Photovoltaic element. According to any one of claims 1 to 3, characterized in that the said second germanium content of the amorphous silicon germanium is i layer of a pin junction (Ge / (Si + Ge) ) to 0.35 or higher Photovoltaic element. According to claim 1-4, characterized in that a amorphous silicon layer or amorphous silicon carbon layer between the second and / or third pin junction p-layer and the amorphous silicon germanium layer The photovoltaic element of any one of Claims. The germanium composition of the amorphous silicon germanium layer of the second or / and third pin junction is changed in the film thickness direction, and the position where the germanium composition is maximized is measured from the p-layer side to measure the amorphous silicon germanium layer The photovoltaic element according to any one of claims 1 to 5 , wherein the photovoltaic element is set to one quarter or less of the entire film thickness. The localized level density of amorphous silicon germanium, which is the i layer of the second and third pin junctions, is 5 × 10 ¹⁶ / cm 3 or less when measured by a constant photocurrent method. The photovoltaic device according to any one of claims 1 to 6 .

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