A wide variety of solar cells are recently researched and brought to the market. To realize the high-performance solar cells, the "evaluation" of the solar cells is crucial process because the information obtained by the evaluation is beneficial for the fabrication process and leads to the improvement of the conversion efficiency. The evaluation of the solar cells is classified into three phases: the modules, the cells and the materials. In this study, we focus attention on the evaluation of the materials used in the solar cells, for which the method of the microscopic evaluation is needed. On the other hand, scanning probe microscopy (SPM) is powerful tool for evaluating sample surface, and is used in many different scientific disciplines including surface science, materials science, surface chemistry, electrochemistry, biology, metrology, etc. There are many kinds of microscopy such as scanning tunneling microscopy (STM), atomic force microscopy (AFM), magnetic force microscopy (MFM), Kelvin probe force microscopy (KFM) and so on. Such a SPM technique, where the interactive force between the tip and sample surface are used, enables us to obtain various significant information of the sample surface with high spatial resolution. The study aims to develop the novel evaluation methods using Kelvin probe force microscopy (KFM), which is a kind of SPM and characterize the solar cells of the multicrystalline silicon and CuInSe2 thin film materials by those methods to improve the solar cell efficiency.

The KFM is based on atomic force microscopy (AFM), and allows us to obtain potential information of sample surface with high spatial resolution by detecting an electrostatic force between a tip and a sample. On the other hand, we originally proposed Photoassisted Kelvin probe force microscopy (P-KFM), which consists of KFM and laser system, and it enables us to obtain photovoltage as well as potential information with high spatial resolution. For accurate photovoltage measurements in our P-KFM, we make all kinds of efforts. For instance, we use a piezoresistive cantilever suited for photovoltage measurements and adopt the new feedback method of potential determination. Furthermore, we have developed the methods by P-KFM for obtaining diffusion length, lifetime and mobility of minority carriers.

First, we adopted these P-KFM methods to multicrystalline silicon solar cells to investigate the local characteristics on these materials. Multicrystalline silicon solar cells exceed single-crystal ones in production volume mainly because of their great advantage in terms of fabrication cost, but are inferior to single-crystal silicon solar cells in terms of conversion efficiency. The influence of grains and their boundaries (GBs) in such multicrystalline materials is a possible cause of the low conversion efficiency.

In photovoltage measurements, we performed photovoltage mapping around the GB on a certain area including GB. Consequently, we confirmed photovoltage drop around the GB and photovoltage difference between the different grains. We also found that this site dependence of photovoltage can be well explained from the intrinsic potential distribution obtained in the dark condition, where the potential became high at the GB. When the intrinsic potential for electrons is low or high, the electron is attracted or repulsed, respectively, and therefore the photogenerated electrons accumulate in the low potential region rather than the high potential region. Since the photovoltage depends on the number of electrons accumulating in the surface n-type layer, the photovoltage in the high potential region should be lower than that in the low potential region, which is very consistent with the experimental results obtained by our P-KFM method.

In our method for evaluating minority carrier diffusion length, the basic principle used to evaluate the diffusion length is same as that used in the conventional SPV method, where laser light at various wavelengths was used to illuminate the sample surface and the photovoltage as a function of the wavelength of the illuminating laser light was measured to estimate the diffusion length. To investigate the diffusion length around the GB, we mapped the diffusion length distribution. As a result, we observed a reduction of the diffusion length in the vicinity of the GB. Additionally, differences in the diffusion lengths between the grains existed even when the influence of lateral diffusion was considered.

In minority carrier lifetime measurements by P-KFM, a sample surface is illuminated by a modulated light and the minority carrier lifetime is extracted from a temporally averaged photovoltage at various modulation frequencies. This method enables us to evaluate a bulk recombination lifetime without independently of surface recombination lifetime without any surface passivation process as well as to investigate a spatial distribution of at high spatial resolution. From the lifetime measurements, we found the lifetime decreasing in the vicinity of the GB in the multicrystalline silicon solar cell material. These results of photovoltage, diffusion length and lifetime measurements indicate that the GB degrades the solar cell performance by acting as a carrier recombination site.

By Einstein relationship, the minority carrier mobility was also numerically obtained from the diffusion coefficient, which was derived from the diffusion length and lifetime values. The mobility difference was observed between the grains, and this result indicates that the crystal quality differs between the grains.

Second, we have applied the P-KFM method to CuInSe2 (CIS) materials such as Cu(InGa)Se2 [CIGS] and Cu(InAl)Se2 [CIAS] for purpose of the microscopic characterization of local properties around the GBs in these materials. Thin film solar cells based on CIS materials have attracted a growing interest in recent years because of their prospective high performance. The band gap of CIS materials is variable by the addition of Ga and Al. Most of the CIS materials used in the solar cells present multicrystalline structures, but the behavior of their grain boundaries GBs to the solar cell is not fully understood yet. In this study, we used CIGS and CIAS materials. The CIGS material is widely used for solar cell devices and very recently, CIAS materials have also been regarded as another candidate owing to wide tunability of their band gap.

We investigated the surface potential distribution of the as-grown CIGS and CIAS films in the dark condition. As a result, we observed the abrupt drops of the potential at the GBs on both CIGS and CIAS films. One possible cause of this potential distribution is a change in the material composition near the GBs due to the segregation within the single grains. The observed potential distribution implies that the photogenerated electrons in the CIGS and CIAS layers tend to accumulate near the GBs rather than to stay on the grains.

To investigate the photovoltaic properties in the solar cells on CIGS or CIAS materials, we measured the surface potentials in the dark condition again and under the light illumination by P-KFM. The surface potentials of both samples under the dark condition showed the similar features to those on the as-grown surfaces of CIGS and CIAS films, where the abrupt drops of the potential clearly appeared near the GBs. This fact suggests that the surface potential distribution on the solar cell structures were still dominated by the underlying CIGS or CIAS layers even after forming some additional layers. The observed potential distribution means that the photogenerated electrons will gather near the GBs as mentioned above. Actually, photovoltage enhancement at GBs was observed. This behavior of the GB was quite different from that in the multicrystalline Si materials where the photovoltage, the diffusion length and the lifetime of the minority carriers were apparently degraded near the GBs. Therefore, we can expect that the GBs in CIS materials do not act as a carrier recombination center and this inactiveness of the GBs is a big advantage for the solar cell application.

To characterize the GBs in more detail, we investigated the dependence of the potential drop at GBs on a ratio of Ga to (In+Ga) in the CIGS layer and defined the potential drop around the GB as the built-in potential. As a result, we found that the built-in potential monotonically decreases as an increase of Ga ratio. Moreover, we performed photovoltage mapping on two CIGS solar cells at the Ga ratio of 0.2 and 0.5, and compared the photovoltage distributions between two samples. The results indicate that the photovoltage distribution becomes flat as an increase of Ga ratio. These results of the dependence of the built-in potential and photovoltage distribution on Ga ratio imply that the material composition becomes uniform inside the single grain of CIGS layer. We also found that the whole photovoltage generated in the sample at the Ga ratio of 0.5 is higer than that in the sample at the Ga ratio of 0.2. This increase in photovoltage is simply attributable to the difference in the band gap of the CIGS material.

We also performed scanning tunneling spectroscopy (STS) measurement, which uses a STM to probe the local density of electronic states (LDOS) and band gap of surfaces and materials on surfaces at the atomic scale. We investigated the difference of the band gap of CIGS at between the grain and the GB. As a result, we found that the band gap at the GB is 0.4 eV higher than that at the grain. This result indicates that the material composition differs between the grain and the GB. Furthermore, we consider that the carrier recombination at the GB is less likely to occur, owing to the band diagram on CIGS layer, where the electrons is easy to accumulate and the holes are repulsed at the GB.

本論文は,「"Local Characterization on Solar Cells by Photoassisted Kelvin Probe Force Microscopy"(光ケルビンプローブフォース顕微鏡による太陽電池の局所的物性評価)」と題し,ケルビンプローブフォース顕微鏡(KFM)を光照射下で動作させることによって実現した光KFMを用いた新しい評価手法の開拓とそれによる太陽電池材料の局所的評価を行った結果について述べたものであり,全6章から成り,英文で書かれている.

第1章は「Introduction(はじめに)」であり,本研究の背景を解説している.太陽電池の変換効率向上のためには,それらの材料レベルでの評価が大切であることを指摘し,また,そこから得られた知見を太陽電池の高効率化への取り組みにフィードバックすることの重要性について述べている.特に,多結晶や微結晶材料系の太陽電池を評価するためにはナノ領域における局所的評価が必要であり,その目的には走査プローブ顕微鏡が有効であることを示している.さらに,太陽電池や走査プローブ顕微鏡の原理など,実験によって得られたデータを精査するために必要な基本的事項に言及するとともに,本論文の構成を述べている.

第2章は「Photoassisted Kelvin Probe Force Microscopy(光ケルビンプローブフォース顕微鏡)」と題し,一般的なKFMの動作原理を述べた後,本論文で新しい評価手法として提案している光KFMの概略や新しいポテンシャル決定法,光KFMで用いるピエゾ抵抗カンチレバーなどについて解説を加えた上で,局所的な光起電力計測系の具体的な構成について述べている.まず,一般的なカンチレバーの変位検出法である光てこ方式では,試料表面への漏れ光の影響で,光起電力測定の正確さに懸念が生じることを指摘し,正確な光起電力測定を実現するためには,変位検出に光を必要としないピエゾ抵抗カンチレバーの使用が有効であることを述べている.その上で,同カンチレバーによるポテンシャル測定では,カンチレバー内部のピエゾ抵抗素子部と試料の間の容量性結合による変位電流信号成分が外乱となるために通常のポテンシャル決定法では正しいポテンシャル測定が行えないことを指摘し.この問題点の解決法として,交流である変位検出系の信号の位相成分に着目したフィードバック制御法と,変位電流信号成分の大部分を擬似的な交流電圧信号によって相殺する方法を新たに提案し,それらの方法を導入した光KFM実験系の構成と光起電力決定の手順等を示している.

第3章は「Development of Novel Methods Using P-KFM and Characterization on Multicrystalline Silicon Solar Cells(光KFMを用いた新しい評価手法の開拓と多結晶シリコン太陽電池評価)」と題し,光KFMによる光起電力の計測法や少数キャリアの拡散長,ライフタイム,移動度を評価する手法の原理を示し,それらの測定手法を多結晶シリコン太陽電池に適用して得られた結果と考察を述べている.まず,光KFMによる光起電力測定では,太陽電池に照射する光強度の増大に伴って光起電力が増加しつつ飽和する傾向が得られており,本手法による光起電力測定の妥当性が確認されている.また,多結晶シリコン太陽電池の結晶粒界で光起電力が低下する結果が得られており,この光起電力の低下は暗状態でのポテンシャル分布から予測される光励起キャリアの再分布の状態によって説明できることを示している.次いで,少数キャリアの拡散長測定では,一般的な手法であるSPV法の原理を光KFMに適用できることを示し,さらにSPV法よりも高空間分解能な測定を実現できることが示されている.また,結晶粒界近傍で拡散長の低下が観測されたことも示されている.一方,少数キャリアのライフタイムは,オン・オフ変調した光を試料に照射した際の時間平均光起電力を測定し,その周波数依存性を元にして光起電力減衰の時定数として,数値的に導出できることを示している.また本手法は,一般的な手法であるμ-PCD法と比べて,十分な空間分解能を持っていること,バルク内再結合ライフタイムと表面再結合ライフタイムの切り分けが可能であると考えられることから測定前のパッシベーション処理が不要であること,を指摘している.実際のライフタイム測定の結果,結晶粒界近傍でライフタイムの低下が観測されることが示されている.このように,結晶粒界近傍で,光起電力,拡散長,ライフタイムといった諸特性が劣化しているという結果は,結晶粒界がキャリアのリークパスや再結合サイトとして働いてしまい,それらが太陽電池素子の特性に悪影響を及ぼしていることを示している.また,拡散長とライフタイムの値から計算的に少数キャリアの移動度を求められることや,異なる結晶粒では,拡散長,ライフタイム,移動度などの諸特性が異っており,結晶粒毎に結晶の品質の差異があることも指摘している.

第4章は「Characterization on CuInSe2 Solar Cells by P-KFM(光KFMによるCuInSe2系太陽電池評価)」と題し,CuInSe2系材料であるCuInGaSe2 (CIGS)とCuInAlSe2 (CIAS)材料の薄膜と太陽電池構造に対して,光KFMにてポテンシャル及び光起電力分布測定を行った結果,ならびに走査トンネル分光法(STS)によってバンドギャップを推定した結果を述べるとともに,それらから総合的に推定されたCIGS薄膜の結晶粒界近傍におけるバンドプロファイルを示し,それと光起電力の空間分布との関連性について議論している.まず,光KFMでのポテンシャル測定および光起電力測定によって,CIGSやCIASの薄膜および太陽電池構造では,結晶粒界付近でポテンシャルが低下するとともに光起電力が増大するという,多結晶シリコン太陽電池の結晶粒界とは対照的な振る舞いが見られることを明らかにしている.この結晶粒界近傍でのポテンシャル低下分(ビルトインポテンシャル)と光起電力分布についてCIGS材料中Ga濃度に対する依存性を実験的に調べた結果,Ga濃度の上昇に伴ってビルトインポテンシャルが小さくなると同時に光起電力分布は均一化することが示されている.一方,結晶粒中央部と粒界近傍にてSTS測定を行い,コンダクタンスがほぼ零となるバイアス電圧幅からバンドギャップ幅を推定した結果,結晶粒界近傍ではバンドギャップ幅が増加することを見出している.先のポテンシャル分布と合わせてバンドプロファイルを描くことにより,結晶粒界付近では,強い内蔵電界によって光キャリアの空間分離効果が増大し,実効的な再結合確率が減少している可能性があることを指摘している.

第5章は「Conclusion(結論)」であり,本論文全体の研究成果をまとめて要約するとともにその意義を述べている.

第6章は「Future Works(今後の課題)」であり,多結晶シリコン材料でのライフタイム測定と同様の手法により,CIS系太陽電池におけるキャリア再結合の時定数を測定することの意義などを述べている.

以上これを要するに,本論文は,ピエゾ抵抗カンチレバーと新しいポテンシャル決定手法を導入して構築した光KFM測定系を利用して,多結晶シリコン太陽電池とCIS系薄膜太陽電池の結晶粒界近傍におけるポテンシャルや光起電力分布,あるいは,少数キャリアの拡散長,ライフタイム,移動度などの物性パラメータを局所的に計測することによって,両材料系の結晶粒界近傍でのバンドプロファイルを推定するともに,それらと光起電力特性や光キャリアの振舞いとの関連性を明らかにしたものであり,電子工学上,寄与するところが少なくない.

よって本論文は,博士(工学)の学位請求論文として合格と認められる.