Addressing optical, recombination and resistive losses in crystalline silicon solar cells
Date
2017
Authors
Allen, Thomas Gerald
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Abstract
The performance of any photovoltaic device is determined by its
ability to mitigate optical, recombination, and resistive energy
losses. This thesis investigates new materials and nascent
technologies to address these energy loss mechanisms in
crystalline silicon solar cells.
Optical losses, specifically the suppression of energy losses
resulting from front surface reflection, are first analysed. The
use of reactive ion etched black silicon texturing, a nano-scale
surface texture, is assessed with respect to the two conventional
texturing processes: isotexture and random pyramids. While
nano-scale surface textures offer a means of almost eliminating
front surface reflection, relatively poor internal optical
properties (i.e. light trapping) compared to both conventional
textures can compromise any optical gains realised on the front
surface. It is also shown that enhanced recombination losses
remains a barrier to the application of black silicon texturing
to further improve high performance devices, though this will
likely have less of an impact on multi-crystalline silicon cells
where bulk recombination dominates.
The suppression of recombination losses at surface defects by
gallium oxide (Ga2O3), an alternative to aluminium oxide (Al2O3),
is also investigated. It is demonstrated that, as in Al2O3, thin
films of amorphous Ga2O3 can passivate surface defects through a
direct reduction of recombination active defects and via the
establishment of a high negative charge density. Further
investigations demonstrate that Ga2O3 is applicable to random
pyramid surfaces textures, and is compatible with plasma enhanced
chemical vapour deposited silicon nitride (SiNx) capping for
anti-reflection purposes. Indeed, the Ga2O3 / SiNx stack is shown
to result in enhanced thermal stability and surface passivation
properties comparable to state-of-the-art Al2O3 films. In
addition, it is also shown that Ga2O3 can act as a Ga source in a
laser doping process, as demonstrated by a proof-of-concept
p-type laser doped partial rear contact solar cell with an
efficiency of 19.2%.
Finally, the resistive losses associated with metal / silicon
contacts are addressed. It is demonstrated that a significant
asymmetry in the work function of the electron and hole contact
materials is sufficient to induce carrier selectivity without the
need for heavy doping. This had recently been demonstrated for
hole contacts with the high work function material molybdenum
oxide. In this thesis specific attention is given to finding a
suitable low work function material for the electron contact.
Calcium, a common low work function electrode in organic
electronic devices, is shown to act as a low resistance Ohmic
contact to crystalline silicon without the need for heavy doping.
Fabrication of n-type solar cells with partial rear calcium
contacts resulted in a device efficiency of 20.3%, limited
largely by recombination at the Ca / Si interface. This
limitation to device efficiency is shown to be partially
alleviated by the application of a passivating titania (TiOx)
interlayer into the cell structure, resulting in an increase in
device efficiency to 21.8% -- the highest reported efficiency for
a TiOx-based heterojunction solar cell to date.
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photovoltaics, solar cells, silicon, gallium oxide, passivated contacts, titanium dioxide, heterojunction, calcium, work function
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