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京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24010号 / エネ博第446号 / 新制||エネ||84(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー応用科学専攻 / (主査)教授 平藤 哲司, 教授 土井 俊哉, 教授 藤本 仁 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

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With looming energy crisis across the globe, achieving high efficiency and low cost solar cells have long been the key objective for photovoltaic researchers. III-V compound semiconductor based multijunction solar cells have been the dominant choice for space power due to their superior performance compared to any other existing solar cell technologies. In spite of unmatched performance of III-V solar cells, Si cells have dominated the terrestrial market due to their lower cost. Most of the current III-V solar cells are grown on Ge or GaAs substrates, which are not only smaller in diameter, but are also more expensive than Si substrate. Direct integration of high efficiency III-V solar cells on larger diameter, cheaper and readily available Si substrate is highly desirable for increased density, low-cost and lightweight photovoltaics. However, the polar-on-nonpolar epitaxy, the thermal mismatch and the 4% lattice mismatch makes the direct growth of GaAs on Si challenging, rendering the metamorphic cell sensitive to dislocations. The focus of this work is to investigate and correlate the impact of threading dislocation density on the performance of lattice-mismatched single-junction (1J) GaAs and dual-junction (2J) InGaP/GaAs solar cells on Si substrate. Utilizing our calibrated dislocation-assisted modeling process, we present the design methodology to optimize the structure of 2J InGaP/GaAs solar cell on Si substrate. Our modeling results suggest an optimistic future for integrating III-V solar cell technology on Si substrate and will be useful for future design and prediction of metamorphic III-V solar cell performance on Si substrate. / Master of Science

GaInN has the potential to revolutionize the solar cell industry, enabling higher efficiency solar cells with its wide bandgap range spanning the entire solar spectrum. However, material quality issues stemming from the large lattice mismatch between its binary endpoints and questionable range of p-type doping has thus far prevented realization of high efficiency solar cells. Nonetheless, amorphous and multi-crystalline forms of GaInN have been theorized to exhibit a defect-free bandgap, enabling GaInN alloys at any indium composition to be realized. But the range of possible p-type doping has not yet been determined and no device quality material has been demonstrated thus far. Nonetheless, a Schottky barrier design (to bypass the p-type doping issue) on single-crystal GaInN can be used to provide some insight into the future of amorphous and micro-crystalline GaInN Schottky barrier solar cells. Through demonstration of a functional single crystalline GaInN Schottky barrier solar cell and comparison of the results to the best published reports for more conventional p-i-n GaInN solar cells, this work aims to establish the feasibility of amorphous and multi-crystalline GaInN solar cells. / Ph. D.

Realization of low-resistance electrical contacts on both sides of a solar cell is essential for obtaining the best possible performance. A key component of a solar cell is a metal contact on the illuminated side of the cell which should efficiently collect carriers. These contacts can be formed using an opaque metal grid/finger pattern. The metal electrode may be used alone or in combination with a broad-area transparent conductive film. This work aims at investigating the impact of the electrical contacting scheme employed in InGaN/GaN Schottky barrier solar cells on their performance. InGaN is a III-V compound semiconductor and has a tunable direct band-gap (0.7 eV to 3.4 eV) which spans most of the solar spectrum; this fact, along with other beneficial material properties, motivates the study of InGaN photovoltaic devices. A number of groups have recently investigated InGaN-based homo-junction and hetero-junction p-i-n solar cells. However, very few groups have worked on InGaN Schottky solar cells. Compared to p-n junctions, Schottky barrier solar cells are cheaper to grow and fabricate; they are also expected to improve the spectral response because of near surface depletion regions in the shorter wavelength regions. In this particular work on InGaN based solar cells, a Schottky diode structure was used to avoid the issue of highly resistive p-type InGaN. In this study, platinum (Pt) is used to form a Schottky barrier with an InGaN/GaN absorber region. Electrical and optical properties of platinum films are investigated as a function of their thickness. InGaN/GaN Schottky solar cells with platinum as the transparent conductive film are reported and their performance is evaluated as a function of the metal thickness. / Master of Science

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