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A Study on the Epitaxial Germanium Thin-Film for Solar Cell Applications
- Alternative Title
- A Study on the Epitaxial Germanium Thin-Film for Solar Cell Applications
- Author(s)
- 김강호
- Alternative Author(s)
- Kangho Kim
- Advisor
- 이재진, 강호관
- Department
- 일반대학원 전자공학과
- Publisher
- The Graduate School, Ajou University
- Publication Year
- 2018-02
- Language
- eng
- Keyword
- metalorganic chemical vapor deposition (MOCVD); Solar cells; photovoltaics; multi-junction; tandem; power conversion efficiency (PCE); III-V compound semiconductors; germanium; isobutylgermane (IBuGe); thin-film; epitaxial lift-off (ELO); flexible; epitaxial growth
- Alternative Abstract
- Currently, the focus of solar cell R&D is to develop high-efficiency and low-cost devices to meet grid parity. III-V compound semiconductor multi-junction solar cells promise super high efficiencies due to the efficient use of sunlight by the tandem structure consisting of monolithically stacked two or more subcells with different band gap. Despite its high efficiency, the III-V based solar cells have been used for limited applications such as space and concentrated photovoltaic (CPV) system due to the high cost. The most expensive cost driver is the substrate for the growth of the solar cell epi layers. Manufacturing cost of III-V based solar cells can be reduced by epitaxial lift-off (ELO) processes that separates epi layer from the substrate through chemical etching process. The separated substrate can be reused for growth of the high efficiency III-V based solar cells. Additionally, it is possible to fabricate thin-film solar cells with high-efficiency and low cost. However, the most widely used InGaP/InGaAs/Ge triple-junction solar cell. Since the p-n junction of the Ge bottom subcell is realized by atomic diffusion into a p-type Ge substrate, the most commonly used InGaP/InGaAs/Ge triple-junction solar cell is difficult to reduce manufacturing cost by reusing the Ge substrate. The primary goal of this dissertation is to fabricate high-quality epitaxial Ge thin-film solar cells on flexible substrates for high-efficiency and low cost thin-film InGaP/InGaAs/Ge tandem solar cells. In order to achieve the final goals, this research has been focused on three parts: high-quality epitaxial Ge grown by metalorganic chemical vapor deposition (MOCVD) on GaAs substrate and transferred to flexible substrate using ELO processes, realization of inverted epitaxial Ge p-n junction and fabrication of epitaxial Ge thin-film solar cell, and the optimization of inverted epitaxial Ge thin-film solar cell structures. First, the high crystal quality epitaxial Ge thin-films have been grown on n-type GaAs (001) substrates with AlAs sacrificial layer by MOCVD using an isobutylgermane (IBuGe) metalorganic source which is a novel Ge precursor. The full width at half maximum values of 14 arcsec is achieved by growth of the epitaxial Ge thin-films on GaAs substrates, comparable to the value of 11.6 arcsec from the reference Ge substrate and coincident with value expected in a perfect crystal. And As grown Ge epi layer is separated from the GaAs substrate and transferred to flexible substrates by the ELO processes. The high-quality epitaxial Ge thin-films have been demonstrated on flexible substrates with a surface root mean square roughness of 1.1 nm. Second, the inverted epitaxial Ge p-n junctions have been grown by MOCVD using IBuGe without any growth techniques. The n-type epitaxial Ge layer has been grown without additional injection of n-type dopants, while the p-type epitaxial Ge layer has been grown with Ga n-type dopants. N-type doping concentration of around 1.4×1019 cm-3 and p-type doping concentration of over 1×1019 cm-3 are investigated in the undoped n-type epitaxial Ge layer and Ga continuous doped p-type Ge epitaxial layer, respectively. The first demonstrated inverted epitaxial Ge thin-film solar cell structures with extremely high doped Ge p-n junction have been fabricated by ELO processes and thin-film solar cell fabrication methods. As a result, although it achieved the low power conversion efficiency (PCE) of 3.39 %, the Ge thin-film solar cells are successfully demonstrated on flexible substrate. In addition, it is identified that n-type Ge emitter layer thickness is increased due to the internal diffusion of n-type dopants such as arsenic (As) and phosphorus (P) from the GaAs window layer and InGaP etch stop layer during the growth of the thick p-type epitaxial Ge layer. Finally, the inverted epitaxial Ge thin-film solar cells with various n-type Ge emitter layers and p-type Ge base layers have been grown to improve the PCE. Lowered doping concentrations around 3×1018 cm-3 have been obtained from the epitaxially grown p-type Ge layers by increasing the Ga injection ratio on the dicontinuous doping technique without post-annealing. The best PCE of 4.82 % which is improved over 42 % is achieved from the inverted epitaxial Ge thin-film solar cell with 50 nm-thick emitter layer and 2000 nm-thick base layer. This device has been packaged by printed circuit board and wire bonding processes and achieved the PCE of 5.46 % with Voc of 0.21 V, Jsc of 42.80 mA/cm2, and FF of 0.6082. Although the PCE of 5.46 % is lower than that of 6.57 % of the upright epitaxial Ge solar cell, the high-quality epitaxial Ge thin-film on the flexible substrate is expected to be very useful for various devices. Especially, the inverted Ge epitaxial growth technique could be applied for III-V multi-junction solar cells with excellent stability.
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- Graduate School of Ajou University > Department of Electronic Engineering > 4. Theses(Ph.D)
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