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Abstract
Recent developments in solar photovoltaic cell technology have enabled significant cost
reductions so that in many markets it is now directly competitive with conventional energy
generation. To maintain downward pressure on module prices and to improve efficiencies,
continued developments of manufacturing processes are required. In this respect, laser
processing offers advantages in achieving various fabrication steps by providing spatially
precise and localised heating on short and controllable timescales, and offering continuous,
high throughput, in-line processing. In designing new laser-processing approaches and in
optimising existing ones, a detailed understanding of the resulting heat transfer, phase change,
and other relevant phenomena that occur during the process is arguably valuable. Since
experimental techniques to investigate these phenomena in detail would be very difficult and
unwieldy to use as an optimisation tool, this thesis advocates an approach based in numerical
modelling.
The thesis first focuses on development and validation of a numerical model of heat
transfer and phase change phenomena during laser-material interaction, and then implement
the numerical model to reveal these phenomena in three significant laser processes used in
the fabrication of solar cells: (1) laser based hydrogen passivation of defects in silicon wafers,
(2) laser annealing of the absorber layer in copper zinc tin sulphide (CZTS) based solar cells,
and (3) pulse laser-induced melting and solidification dynamics of the silicon wafers.
The numerical model is developed in OpenFOAM, an open-source computational fluid dynamics
toolbox written in C++. The developed OpenFOAM code is validated against several
analytical and experimental reference cases related to the simulation of laser-semiconductor
interaction, and an excellent agreement is observed between the model and the analytical and
experimental results.
In the first implementation of the model, the effect of continuous wave (CW) diode laserinduced
heat transfer phenomena on the hydrogen passivation of silicon wafers is modelled.
In the case of crystallographic defect passivation, it is demonstrated that an appropriate
combinations of parameters can be chosen to enable process characteristics in the same range
as those known to be optimal for conventional belt furnace or rapid thermal processing (RTP)
methods, which are used to enable hydrogen release and diffusion and to passivate these defects. It is observed that the optimal temperature regime for passivation of Boron-Oxygen
(B-O) defect complexes can also be obtained using different settings for the laser parameters.
In addition, by coupling the thermal model with a model of the B-O defect system reaction
rates, it is found that the passivated defect concentrations are significantly influenced by the
processing times and the temperature distributions within the depth of the wafer.
In the second implementation, the effect of CW diode laser-induced heat transfer phenomena
on the processing of CZTS thin film solar cells is demonstrated. The model is applied to
the situation of a CW diode laser beam annealing CZTS thin film deposited on a Molybdenum
(Mo) soda lime glass substrate. It is shown that the Mo remains isothermal, whereas
a temperature gradient can be observed in the CZTS thin film and the glass substrate. This
temperature gradient is demonstrated to increase with the CZTS absorber layer thicknesses,
which is expected to affect the absorber layer properties. Very thick absorber layers are
shown to generate high thermal stress, which is associated with risk of delamination. Finally,
appropriate settings of the laser-annealing parameters are determined that produce process
characteristics similar to those that result in a CZTS absorber layer with optimum properties
when processed via conventional methods such as the belt furnace and RTP.
In the final implementation, the dynamics of laser-induced melting and subsequent
resolidification of the silicon wafers are described. Silicon wafers are irradiated with a
number of widely used pulse shapes, Gate, Gaussian, Weibull, Asymmetric and Q-switched,
in the nanosecond regime to reveal the effect pulse shaping, i.e. the energy distribution
within a single pulse, on the thermal processes and the associated melting and solidification
dynamics. It is demonstrated that the transient behaviour of the heat transfer phenomena,
parameterised by the surface temperature, heating and cooling rates, is significantly influenced
by the variation of laser energy within the pulse. In turn, the heat transfer process controls
the melting and solidification dynamics. The results suggest that in achieving a long melt
duration with relatively low resolidification velocity and solid-phase thermal gradients, the
pulses that ramp up quickly but deliver energy more slowly in the latter ramp-down half of
the pulse would be beneficial, such as the Q-switched pulse.
In summary this thesis makes a contribution to understanding heat transfer and related
phenomena in key laser processing approaches used in solar cell manufacturing, providing
guidance as to the selection of processing parameters and hence improved processing outcomes.
It moreover demonstrates the utility of numerical models to provide this otherwise
lacking information, thus potentially opening many future avenues for development and
optimisation of laser processing methodologies.