Novel materials for solar energy harvest and storage [electronic resource]

Developing sustainable energy is one of the most pressing issue in modern society. Solar energy is clean and abundant. It is the champion of energy resources. However, we derive less than 1% of the total consumed energy from the sunlight. The huge gap between the solar energy we utilize every year and the tremendous undeveloped potential energy from sunlight presents significant challenges. To increase solar energy utilization, an efficient and convenient energy flow path is required. This dissertation presents an energy flow path for solar energy utilization. Solar energy is first converted into electricity using solar cell devices. Due to the intermittent nature of solar energy, the converted solar electricity needs to be integrated with energy storage systems. An electrochemical fuel is employed to store the converted solar electricity through a semi-flow battery. The electrochemical fuel can be transported and distributed to end users, and applied in electric vehicle applications through a refillable battery. The refillable battery can be "recharged" not only by solar electricity like conventional batteries, but also by refilling the electrochemical fuel. It offers a fast driving range extension method to effectively remove the main barriers in the broad adoption of electric vehicles, which in the meanwhile decrease energy dependence on finite fossil fuels in the transportation sector. In developing this solar energy flow path, we studied ultra-thin single-crystalline Si solar cells and transparent conducting electrodes in the solar to electricity conversion part. Recently, there is a strong trend to develop ultra-thin single-crystalline Si solar cells because of their lightweight and flexible form factor. Thin Si, however, absorbs less light, which means that an effective light trapping scheme must be used to improve light absorption in thin cells. Light trapping design using double-sided nanocone array surface textures were studied in this work, presenting very effective light absorption enhancement. Moreover, the processability of the ultra-thin Si in solar cell fabrication is also demonstrated. Besides crystalline Si technology, another major type of solar cell technology is thin film solar cells, where transparent conducting electrodes serve as essential components. In this work, we also demonstrate a hybrid concept for transparent conducting electrodes, which improves the performance of the existing technologies by several orders of magnitudes. In the solar electricity storage part, robust integrated system of solar module and Li/polysulfide semi-flow battery were studied, showing a high round-trip efficiency of 12%. The polysulfide material can be considered as an electrochemical fuel, charged by solar electricity, stored in external tanks, and then applied in a refillable battery to power electric vehicles. The low-lost refillable Li/polysulfide battery shows good rate capability and temperature performance, which are both very important factors for electric vehicle applications.

Keywords: Electrical Engineering.
Advisor: Cui Yi
Fan Shanhui
Harris J. S. (James Stewart)