Abstract:
Multi-junction III–V solar cells for terrestrial and space applications have attracted
increasing attention in recent years for their very high conversion efficiencies. However,
increasing the efficiency while maintaining the cost within certain limits, still remains a
big challenge for present day researchers. Much work has been done for solar cells
under ideal conditions and concentrated sun, but a detailed study considering non
idealities such as recombination effects is still missing. Currently, state-of-the-art high
efficiency III–V solar cells utilize a three-junction design that includes a Ge bottom
junction formed in the Ge substrate in conjunction with lattice-matched Ga0.5In0.5P and
GaAs top junctions. Such a structure is limited by the lattice matching constraint and
hence does not offer many choices of materials. One way around is to use a
mechanically stacked configuration which will remove the problem for lattice matching,
at the same time reducing cost. In this thesis, the compound semiconductor SixGe1-x is
used for the first time as the bottom cell material and its composition is varied to obtain
maximum efficiency for a particular composition. Also non ideal effects such as surface
recombination, Shockley Reed Hall recombination and Auger recombination are all
considered in the study. To start off Si0.11Ge0.89 was selected for bottom cell and for this,
the I-V characteristics were studied. Later, the thicknesses of the layers were varied and
its effects were seen on the overall short circuit current and efficiency. It was observed
that as the top cell thickness was increased, the top cell short circuit current also
increased, but the middle cell short circuit current significantly decreased due to
shadowing effect. Again, with the increase in middle cell thickness, the middle cell
short circuit current was found to increase. Thus an optimum thickness was chosen for
the best possible efficiency. For Si0.65Ge0.35 bottom cell composition, top cell thickness
of 0.4μm and middle cell thickness of 4μm, highest efficiency of 32.2% was obtained.
This structure is then further optimized by changing the thickness of the additional
layers that are used to minimize losses and enhance device performance, such as the
window layer, back surface field (BSF) layer. For a window layer thickness of 0.01μm
and BSF layer thickness of 0.02μm, maximum efficiency of 34.85% was obtained.
Finally for this optimized structure, an efficiency of 41.34% was achieved under ideal
conditions.
