| Abstract | 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. |
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