Quadruple-Junction Thin-Film Silicon-Based Solar Cells
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Abstract
The direct utilization of sunlight is a critical energy source in a sustainable future. One of the options is to convert the solar energy into electricity using thin-film silicon-based solar cells (TFSSCs). Solar cells in a triple-junction configuration have exhibited the highest energy conversion efficiencies within the thin-film silicon photovoltaic technology. Going further from the state-of-the-art device structures, this thesis works on the concept of quadruple-junction TFSSCs, and explores the potential and feasibility of such configuration.
The initial experimental realization of quadruple-junction TFSSCs is demonstrated in Chapter 2. The fabricated thin-film a-SiOx:H/a-Si:H/nc-Si:H/nc-Si:H solar cells showed favorable fill factors (FF) and exceptionally high open-circuit voltages (VOC) up to 2.91 V, suggesting a high quality of the material depositions and of the process control. Optical simulations were used in the design of the device structure, to precisely control the thickness and optical absorption in the layers. This preliminary experiment indicated how improvements can be made by better light management.
The spectral response of the component subcells is important information for the study of multi-junction solar cells, and the accurate measurement of such properties turns out to be challenging. Chapter 3 analyzes the mechanism of the spectral response measurement of multi-junction solar cells, by means of modeling the optoelectrical response of the subcells and their internal interactions. The formation of measurement artifacts, and their dependence on cell properties and measurement conditions, are elucidated. The analyses lead to comprehensive guidelines on how to conduct a trustworthy measurement and sensible data interpretation.
Absorbing semiconductor materials with different bandgaps are desirable for multi-junction solar cells. Thin-film a-SiGex:H cells have been developed to accommodate an absorber material with an intermediate bandgap between that of a-Si:H and nc-Si:H. Chapter 4 reports the development of a-SiGex:H cells using mixed-phase SiOx:H materials in the doped layers. Bearing the band alignment in mind, the optimization of p- and n-type SiOx:H layers resulted in satisfying device performance. The use of SiOx:H p- and n-layers offers great flexibility when integrating the cell in a multi-junction solar cell.
Chapter 5 describes the development of quadruple-junction TFSSCs using four different absorber materials. The thin-film wide-gap a-Si:H/narrow-gap a-Si:H/a-SiGex:H/nc-Si:H solar cells promotes reasonable spectral utilization because of the descending bandgap along the direction of light incidence. The tunnel recombination junctions between the subcells have been optimized to ensure effective interconnections thus the proper functioning of the multi-junction device. Advanced light management, which involved the use of modulated surface textured front electrode, was arranged for enhancing the optical performance. These investigations reveal the potential of quadruple-junction TFSSCs.
Chapter 6 evaluates the benefit of multi-junction solar cells with different number of subcells. The gains and losses inherent in adding more subcells have been critically assessed from the optical and electrical points of view. The effects of optical reflection, parasitic absorption, tunnel recombination junctions, and filtered illumination in multi-junction cells on the performance were investigated. In general, all types of losses increase with the number of subcells. Among them, the filtered illumination in the subcells can play a significant role in case of a large number of subcells. These results show that such comprehensive analysis helps to judge whether it is reasonable to develop a multi-junction solar cell with a certain structure.