Nanostructured silicon thin film solar cell

Abstract

In this dissertation, a series of experimental and simulation studies were carried out in an effort to fabricate and characterize nanostructured silicon thin film solar cells. The nanostructures were designed for the purposes of enhancing light trapping within the active layer and improving the electrical performances of functional devices. Silicon nanostructured thin film solar cell is believed to be important because its utilization of light trapping technology which enhances light absorption within the thin film material. In this dissertation, processes in fabricating silicon nanostructures were developed. We firstly summarized an electroless chemical etching technique using polystyrene nanospheres as self-assembled mask to fabricate size-controlled, periodic silicon nanopillars (SiNPs) and subsequent silicon nanocones (SiNCs) arrays. The nanocones were obtained based on the nanopillar structure using ammonia-related etching chemistry. With solution-based processes, the cost of the procedures is limited. The diameter, height, and period of the nanopillars and cones are systematically controlled. Furthermore, we transferred this technology onto silicon thin film and demonstrated the accomplishment of silicon thin film nanostructures. SiNPs and SiNCs on glass were obtained by conventional reactive ion etching technique. The etching was masked by PS nanospheres again, and different morphologies (diameter and periods ranging from 200nm to 800nm) of patterned arrays were obtained with different masking condition and etching parameters. Optical properties such as reflectance and absorption of these patterned silicon-on-glass nanostructures were characterized. With the integration of SiNPs and SiNCs, a significant reduction in reflectance and increase in absorption were observed. Meanwhile, simulation efforts were also made in analyzing the optical effects of our nanostructures. Satisfactory results were achieved as simulations agreed in-principle with experimental results, thus serving as a theoretical support of our work. The simulation was performed using FDTD commercial software Lumerical. Furthermore, solar cells were fabricated subsequently. Due to the enhanced properties in optical absorption, the external quantum efficiency and short circuit current of cells based on these nanostructures demonstrated a remarkable increase. The EQE for nanostructured sample can reach over 90% at wavelength of 600nm against only 60% of control sample. The cell efficiency is also increased from 1.6% for flat thin film cell to 3.8% for nanostructured sample. The impact of various nanostructures and dimensions were investigated ranging from 200 nm to 800 nm. In summary, this dissertation firstly addresses the development and characterization of nanostructured silicon on both wafer and glass substrates, followed by further influences on subsequent nanostructured thin film photovoltaic (PV) devices. The work done will offer opportunities and approaches to further improve performances of PV device for potential implementation in…