The future of our energy supply cannot continue to depend on the use of exhaustible fossil fuels. The successful transition to a society powered by renewable energy sources is one of the major challenges our current generation faces. Renewable systems will only fully establish themselves if high conversion efficiencies can be obtained at a reasonable cost, in order to compete with conventional, carbon-based sources. With this in mind, photovoltaic solar energy can be a promising solution.
This study focuses on the investigation of the potential efficiency of a thin-film tandem solar cell, combining amorphous silicon and microcrystalline silicon for a high performance device. With the use of flexible, light substrates, inexpensive modules can be manufactured in a roll-to-roll configuration to produce high efficiency products for a large scale implementation in society. The semiconductor modelling software ASA was used to optimise the structure of the tandem, identifying the absorber layer thicknesses that maximise the efficiency. Moreover, the impact of possible optical variations in the design of the device on the performance were analysed.
To start, model parameters of single-junction amorphous silicon and microcrystalline silicon cells were calibrated to recreate the performance of such devices based on experimental data. By conducting a sensitivity analysis of the model parameters, this was effectively achieved. The single-junction models were combined in a tandem structure to forecast the operation of the multi-junction device.
It was found that the performance of the tandem was highly sensitive to the choice of refractive indices of the layers at the junction between the two subcells. A high mismatch
between the refractive indices of these layers and the absorber layers of the subcells results in an increased reflection of light to the top cell. This is beneficial for the current production of this subcell, but is too detrimental for the operation of the bottom cell. By bringing the refractive indices closer together, an efficiency of 13.0% was predicted. The inclusion of an encapsulation at the front of the cell boosts this efficiency to 13.7%, at a top/bottom absorber layer thickness combination of 160 nm/0.6 µm.
The possibilities of enhancing the efficiency with an intermediate reflector were examined. The optical conditions for the ideal light distribution with such a reflector were
computed to provide estimations of the potential increase in performance. For the best performing device, an increase of the initial efficiency of 13.0% to 13.2% was foreseen, at slightly thinner top cells and similar bottom cell thicknesses.
All in all, the predicted efficiencies of the amorphous silicon/microcrystalline silicon tandem solar cell evidences the potential of these devices for the production of cheap,
highly efficient modules. The developed analysis of the ideal intermediate reflector can be generalised to other multi-junction solar cells to establish the potential efficiency increase such a layer brings along. This can help in the decision of whether the increase in performance justifies the added manufacturing complexity and costs. ... Read more

The accurate approximation of the surface tension force is paramount for continuum surface models in the field of computational fluid dynamics for multiphase flow where surface tension is relevant. This involves being able to accurately calculate the curvature at the interface. This study focuses on the use of convolution in smoothing the VOF colour field in order to obtain better approximations of the curvature. Given the sudden jump in values of the VOF colour field, the calculation of its derivative for the curvature is sensitive to errors, given the large values of high order terms that determine the truncation error. To deal with this problem, convolution of this abruptly varying field can be used to create a smoother transition. The curvature approximation of a circular interface improved as the support of the convolution was increased.
It was proven analytically that, for these interfaces, the original curvature is retrieved from the convoluted field. Interfaces along which the curvature varies were also considered, and it was found that there is a critical convolution support that minimizes the error in the curvature, given that the choice of the support length can modify the curvature that is estimated.
An algorithm was implemented in OpenFOAM that calculates the convolution of the VOF colour field. The resulting smoothed field was then used to calculate the curvature, which is needed for the surface tension force of the system. The simulations of a two-dimensional rising bubble resulted in more accurate results for the circularity and the rising velocity, when compared to the original OpenFOAM implementation with no smoothing. With the convolution algorithm, the terminal velocity deviated only 0.01% from a well-accepted benchmark case, a great improvement when compared to the 4.2% difference when no smoothing was used. However, simulations of a static bubble in zero-gravity rapidly resulted in unphysical flow, manifested as a wavy interface, when a convolution support larger than 2 cells was chosen. An improvement of the estimation of the surface tension force direction may be needed for this behaviour to disappear. ... Read more