Download files
Abstract
The majority of commercial solar cells are fabricated on crystalline silicon wafers with diffused homojunctions. This is forecast to continue into the near future. This thesis explores how improving the carrier selectivity of homojunction solar cells can achieve higher conversion efficiencies, by reducing the losses at the contacted and non-contacted surfaces.
The carrier conductivity at the metal-silicon contact was investigated for both heavily doped n+ and p+ silicon. For diffused n+ electron collectors, it was shown that thin layers (<5 nm) of aluminium oxide (AlOx) deposited between the silicon nitride (SiNx) and screen-printed silver paste can improve contact resistivity, whereas thicker layers increased the contact resistivity. Through simulation and cell fabrication, this effect was shown to be beneficial for solar cells limited by contact resistance, whereas the adverse effects of thicker layers were mitigated by changing the paste formulation. For diffused p+ hole collectors, a method of electroless plating nickel seed layers to boron diffused silicon was developed which demonstrated contact resistivity’s < 1 mΩ.cm2 for lightly diffused p+ silicon. However, it was found that the contact recombination and contact resistivity was higher for the electroless nickel plated contacts in comparison to aluminium evaporated references.
Regarding the non metallised, heavily doped silicon surfaces, improved understanding of the recombination rate at diffused inverted surfaces was achieved through modelling of the injection level dependent lifetime behaviour with technology computer-aided-design (TCAD) simulations. This resulted in the development of a novel contactless method of extracting the interface charge (Q) and surface recombination velocity parameters from passivated and diffused silicon surfaces. This addresses a current limitation of existing interface characterisation techniques, and allows the study of the electronic properties of these highly relevant surfaces. This method was demonstrated on a range of dielectric passivated n+ and p+ surfaces, and the extracted Q values were comparable to results obtained using conventional techniques.
Finally, the findings were implemented in TCAD simulations of state-of-the-art homojunction interdigitated back contact (IBC) solar cells, demonstrating the technology potential. This work highlights novel technologies and methods to improve the next generation of diffused silicon wafer solar cells.