(Abstract)

This work has focused on strain-balanced quantum well and stepped quantum well solar cells. The unique points of this work from previous reports and publications are as followed:

1.Stepped quantum well structure is an outstanding point of this work, since at the present time all of publications related to normal quantum well solar cell. Through the research work in this thesis, we know there is some other choice, allowing us to improve solar cell performance better.

2.Not only solar cell performances like IV and QE were measured, but also carrier escape kinetics and recombination losses were investigated by time-resolved PL, temperature-dependent PL and bias-dependent PL. It is beneficial to provide a clear way to understand how to optimize the structure in order to improve efficiency.

The experiments in this thesis have been carried out step-by-step from the investigation of strained InGaAs/GaAsP quantum well solar cell, strain-balanced (SB) InGaAs/GaAsP quantum well solar cell, strain-balanced InGaAs/GaAs/GaAsP stepped quantum well solar cell and deep stepped quantum well solar cell.

In chapter 3, it has related to the epilayer growth of InGaAs/GaAsP MQWs by metal-organic vapor-phase epitaxy (MOVPE). In the absence of lattice matched l.2 eV material, strained InGaAs was used as the quantum well material. However strained quantum wells introduce misfit dislocations into the structure that dominate the recombination of the cell. The associated loss in short-circuit-current and open-circuit-voltage is shown to be too large to allow strained GaAs/InGaAs QW cells to realistically match GaAs in terms of power conversion efficiency. Strain-compensation (SC) is exploited to surmount the limitations imposed by strain. GaAsP tensile strain compensation layers were introduced into QW solar cell to compensate compressive strain from InGaAs quantum wells. Actually the more strain accumulated in the QWs, the worse crystal quality, which can be characterized by ex-situ XRD and photoluminescence.

Furthermore, strained and strain-compensated MQW solar cells have been fabricated to check out the contribution of MQW. As strained QW and strain-compensated QW show such significant difference in crystal quality, we fabricated the strained QW solar cell and the strain-compensated (SC) QW solar cell by MOVPE to make a comparison. Under simulated 1 sun AM1.5 conditions, IV and QE results have demonstrated the effectiveness of strain-compensation for improving the performance of quantum well solar cells. The SB InGaAs/GaAsP QW solar cell shows an increase both in short-circuit-current and open-circuit-voltage compared with the strained InGaAs/GaAs solar cell owing to reduced minority carrier recombination. The increased Voc can be related to a reduction in recombination current through the dependence of Voc on both Jsc and dark current.

In chapter 4, stepped MQW structure was designed to lessen large mismatch at the interface and promote carrier escape outside the QWs. The fundamental principle of a quantum well solar cell is that MQWs are inserted in the undoped region of p-i-n cell so that it can absorb photons with energy below host material's bandgap. In this way, the short-circuit current can be increased depending on the well material with lower bandgap. However, there is still a risk of trapping carriers inside QWs and promoting radiative and non-radiative recombination which leads to an increase in dark current and a reduction in open circuit voltage, although it is possible to minimize the negative effect by strain-compensated technology. With the aim of promoting carrier escape from quantum wells, we introduce In0.16Ga0.84As/GaAs/GaAs0.79P0.21 strain-compensated multiple stepped quantum wells (SC-MSQWs). We describe and contrast QWSCs with and without GaAs step layer between the InGaAs well and the GaAsP barrier. The effect of the GaAs step layer was evaluated by atomic force microscopy (AFM) and XRD. The MSQW cell shows a smoother surface with smaller RMS measured by AFM. The total net strain in the QW structure seems to be reduced by inserting GaAs strain-reducing layer. The MSQW cell exhibits higher Jsc (19.29 mA/cm2) than the normal MQW cell (16.85 mA/cm2). The enhanced FF for the MSQW cell suggests that carrier transport was improved through the emitter and intrinsic region.

In chapter 5, we investigate the relationship between strain distribution and the GaAs step layer thickness. The accumulated strain decreases inside the QWs with the increasing GaAs step layer, the main reason improving the crystal quality. Furthermore, we also studied the absorptance spectra, sub-GaAs-bandgap quantum efficiencies, IV curves, and their relationship with the GaAs step layer thickness. As the InGaAs absorber layer thickness keeps the same, the absorption doesn't show much difference. Sub-GaAs-bandgap increases with the increasing GaAs step layer, and it is related to the increasing collection efficiency, rather than the absorption. However, no matter carrier escape rate or collection efficiency, they become saturated when the GaAs step layer reaches 8 nm. The conclusion is the optimal GaAs step layer thickness is 8 nm.

In previous chapters, shallow wells (16% Indium) have been shown to increase the short circuit current of strain-balanced quantum well solar cells (SB-QWSCs) with a comparatively small loss in open circuit voltage and are thus able to enhance efficiency relative to comparable conventional cells. These shallow well solar cells extend the photon absorption edge to 960 nm in the solar spectrum. An improvement in three-junction solar cell efficiency is expected for deeper wells with a band edge beyond 1000 nm. However, fabrication of abrupt heterointerface in InGaAs/GaAsP systems is difficult by metal organic vapor phase epitaxy (MOVPE). This is mainly due to Indium diffusion and surface segregation, especially in the case of high Indium content. In chapter 6, deep stepped quantum well solar cell with the double step layer InGaAs/GaAs was developed. The diffusion of indium between InGaAs/GaAsP interfaces and InGaAs/GaAs step interfaces was investigated in MQW solar cell. The absorptance spectra in the wavelength region of 900nm to 1000nm have been measured to check how much light is absorbed by the effective absorber layer InGaAs. The Indium diffusion between InGaAs/GaAsP interfaces can kill photon absorption. It will also degrade sub-GaAs bandgap QE of solar cells. The sub-GaAs bandgap QE for the deep MSQW cell is higher than that of the normal MQW cell.

This thesis has clearly demonstrated the potential and capability of stepped quantum well structure. The strong absorption and carrier collection ability has been successfully realized.

本論文は,"An Investigation on Strain-balanced Stepped-potential Quantum Well Solar Cells for Higher Efficiency (高効率化に向けた歪み補償階段ポテンシャル量子井戸太陽電池に関する研究)"と題し,多接合太陽電池のミドルセルとしての量子井戸太陽電池について,キャリアの取り出し効率を高める独自構造を提案し,その設計,試作,特性評価解析を行った成果を英文で纏めたもので,7章より構成されている.

第1章は序論であって,研究の背景,動機,目的と,論文の構成が述べられている.化石燃料の枯渇や二酸化炭素排出問題が懸念される中で,新たなエネルギー創出手段として太陽電池への期待が益々高まっていること,太陽電池の動作原理,単接合太陽電池の変換効率限界,量子井戸太陽電池の動向に関し,記述している.

第2章は"Solar cell processing and characterization"と題し,本論文内で共通に用いられる太陽電池の製造技術,評価技術に関し論じている.表面のフォトリソグラフィーによるパターニングと電極形成,キャップ層と素子分離のエッチング,裏面電極堆積技術の詳細について述べた後,X線回折(XRD),フォトルミネッセンス(PL),紫外~可視域分光測定,透過型電子顕微鏡(TEM)観察などの材料評価技術について述べ,最後に太陽電池素子としての電流電圧特性および量子効率の測定評価方法を記述している.

第3章は"Strained and strain-balanced InGaAs/GaAsP MQW solar cells"と題し,本研究で狙う多接合太陽電池のミドルセルとしての量子井戸電池において,不可避的に導入される格子歪みにどのように対処するかが論じられている.まず,GaAs基板上のInGaAs/GaAsP量子井戸システムにおける歪み補償条件を,連続弾性体モデルと高精度XRDによって決定したことについて述べている.次に,異なる格子歪みを有するInGaAs/GaAsP多重量子井戸を有機金属気相エピタキシャル成長(MOVPE)を用いて成長し,フォトルミネッセンス,XRD,TEMにより評価解析した結果について述べている.さらに,格子歪みが太陽電池としての電圧電流特性と量子効率特性にどのように影響するかが比較検討されている.

第4章は"Strain-balanced stepped quantum well solar cell"と題し,量子井戸太陽電池のキャリア取り出し効率を高める独自の「階段ポテンシャル構造」を提案するとともに,その特性を理論,実験両面で論じている.まずInGaAs/GaAsP量子井戸にGaAs階段層を導入してキャリア取り出しを向上する考え方について述べ,GaAs階段層入りの量子井戸構造を実際に成長して,それが結晶品質に与える影響をXRDと原子間力顕微鏡(AFM)で調べている.GaAs階段層によるキャリア離脱の促進効果が時間分解PL,温度依存PL,バイアス依存PLにより評価されている.さらに,歪み補償量子井戸と階段ポテンシャル量子井戸太陽電池の電圧電流特性,量子効率を測定評価している.

第5章は"Effect of GaAs step layer thickness on the optical characteristic of quantum well solar cell"と題し,前章の階段層の層厚依存性を論じている.井戸内での再結合寿命および再結合損失を,時間分解PLとバイアス依存PLで調べた後,井戸内再結合と階段層厚との関係を求めている.その結果,階段ポテンシャル量子井戸内の再結合損失は階段層が厚くなるとともに減少することがわかり,階段ポテンシャル量子井戸が短絡電流の増加に効果的であることを明らかにしている.

第6章は"Deep MSQW solar cell"と題し,階段層をInGaAsとGaAsの二段階で構成することにより,井戸層にさらにインジウム組成の高い,従ってより狭禁制帯幅のInGaAs層を用いることができることを提案し,実験で有効性を検証したことについて論じている.InGaAs/GaAsP量子井戸におけるインジウムの拡散は,障壁層をInGaAsP化し,特性劣化に繋がることが明らかになった.これを防ぐためにも,階段層にGaAsを挿入する必要のあることが示された.

第7章は結論であって,得られた成果を総括するとともに将来展望について述べている.

以上のように本論文は,III-V族化合物半導体単結晶多接合太陽電池のミドルセルとしての量子井戸太陽電池に着目し,InGaAs井戸層とGaAsP障壁層の間に中間層を挿入する階段ポテンシャル構造を提案し,その光電変換特性の解析を通じて,短絡電流とフィルファクターを同時に増加させることのできることを明らかにしたもので,多接合太陽電池のさらなる高効率化に寄与するものであり,先端学際工学分野への貢献が大きい.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.