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Title: Processing and characterisation of silicon micro-rod solar cells
Author: Oates, Andrew
ISNI:       0000 0004 7224 7846
Awarding Body: London South Bank University
Current Institution: London South Bank University
Date of Award: 2017
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Silicon based solar cells are the dominant photovoltaic (PV) technology worldwide, but like any technology they must be subject to regular development to maintain this position. The refinement of raw silicon is an expensive process and significant cost reductions are achievable by reducing the bulk material usage. An indirect band-gap semiconductor, silicon is an inefficient absorber of light; therefore light trapping and absorption enhancing schemes are necessary to permit effective reduction in material usage. This is described as making the material optically thick but physically thin. A common approach to this problem is the fabrication of nano and micro-scale rod-like structures on the surface of thin silicon devices which decouple the optical absorption length from the electronic carrier collection distance. This has the added benefit of reducing the material quality requirement which is typically difficult to maintain for thin silicon as it would likely be deposited as a polycrystalline film rather than utilising conventional single crystal or multicrystalline wafers. This project investigates the fabrication and performance of rod-like PV structures which have diameters on the low micron scale (1 μm and 10 μm). The design and fabrication of the structures is described together with results on their optical properties. These demonstrate an average reduction in reflection, compared to planar silicon, of 40% for the 1 μm diameter features and 10-20% for the 10 μm diameter features. Various rod configurations were modelled optically by finite difference time domain (FDTD) simulations and comparisons of the modelled and measured reflection are presented. The results demonstrate good correlation and lend confidence to the use of modelling to inform the design of future structuring schemes. This was believed to be the first systematic study of identical geometry modelled and fabricated devices, particularly on the micron scale. Proximity rapid thermal diffusion (PRTD) is developed as a doping technique and applied for emitter formation. It is believed that this is the first time that this process has been used in conjunction with structured devices for PV purposes. This approach permitted the formation of n-type emitters as shallow as ≈ 200 nm, with a diffusion time of less than three minutes and without the use of toxic process gases or diffusion specific hardware. Work undertaken to optimise the emitter formation process is described and results are presented which support the premise that whilst good absorption is clearly important in a solar cell, without an effective emitter, good efficiencies will remain out of reach. Devices featuring 1 μm diameter rods confirmed this, proving challenging to form effective emitter layers on and were limited to matching planar device performance (conversion efficiency of 5.63% vs 5.64% for the planar control). Whilst subsequent refinement of the emitter diffusion process demonstrated the potential to exceed planar performance, it reiterated the challenging nature of fabricating effective electronic devices involving features on this scale. Conversely, devices with 10 μm diameter rods, whilst exhibiting more modest absorption improvements over equivalent planar devices, ultimately achieved peak efficiencies of 7.68%, a slightly greater than 2% absolute increase over their planar counterparts. The open circuit voltage (Voc) of all devices with length-diameter aspect ratios of 1:1 was found to be in the region 10-20 mV higher than that of a planar control device, whilst devices with 2:1 aspect ratio exhibited Voc values which were broadly comparable to the control. This was generally contradictory to the literature which commonly reports Voc reductions of 10-50 mV for devices with increased surface area compared to planar. The development of various cell contacting schemes is discussed with a particular focus on aluminium doped zinc oxide (AZO), a transparent conductive oxide (TCO). In addition to the expected electronic properties, the sputter deposited films were found to possess useful antireflective performance. Results are presented for conformal coatings of AZO applied to rod structures, with 50-60% reduction in reflection demonstrated over planar silicon for 10 μm diameter rods and over 70% for 1 μm diameter features.
Supervisor: Reehal, Hari ; Bao, Yuqing Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral