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Title: Light trapping structures for photovoltaics using silicon nanowires and silicon micro-pyramids
Author: Rahman, Tasmiat
ISNI:       0000 0004 5920 7479
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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The current photovoltaic industry is dominated by crystalline or poly-crystalline Si in a planar pn-junction configuration. The use of silicon nanowire arrays (SiNWA) within this industry has shown great promise due to its application as an anti-reflective layer, as well as benefits in charge carrier extraction. In this work, we use a metal assisted chemical etch process to fabricate SiNWAs onto a dense periodic array of pyramids that are formed using an alkaline etch masked with an oxide layer. The hybrid micro-nano structure acts as an anti-reflective coating with experimental reflectivity below 1% over the visible and near-infrared spectral regions. This represents an improvement of up to 11 and 14 times compared to the pyramid array and SiNWAs on bulk, respectively. In addition to the experimental work, we optically simulate the hybrid structure using the commercial Lumerical FDTD package. The results of the optical simulations support our experimental work, illustrating a reduced reflectivity in the hybrid structure. The nanowire array increases the absorbed carrier density within the pyramid by providing a guided transition of the refractive index along the light path from air into the silicon. Furthermore, electrical simulations which take into account surface and Auger recombination show an effi ciency increase for the hybrid structure of 56% over bulk, 11% over pyramid array and 8.5% over SiNWAs. Opto-electronic modelling was performed by establishing a tool flow to integrate the eff ective optical simulator Lumerical FDTD with the excellent fabrication and electrical simulation capability of Sentaurus TCAD. Interfacing between the two packages is achieved through tool command language and Matlab, off ering fast and accurate electro-optical characteristics of nano-structured PV devices.
Supervisor: Fobelets, Kristel Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral