Addressing optical, recombination and resistive losses in crystalline silicon solar cells
Creators: name: Allen, Thomas Gerald
Publisher: name: The Australian National University
Year: 2017
Subjects: photovoltaics
solar cells
silicon
gallium oxide
passivated contacts
titanium dioxide
heterojunction
calcium
work function

description: 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.