Studies of silicon oxide ageing and titanium oxide passivation of silicon for use in solar cells

Abstract

In this thesis the dielectrics silicon oxide (SiO{u2082}) and titanium oxide (TiO{u2082}) are investigated as to their ability to passivate surfaces of silicon (Si) solar cells. Ageing is identified as a degradation mechanism that affects all oxide-passivated surfaces, which in some cases would have a drastic impact on the performance of a solar cell. Also, the addition of hydrogen to TiO{u2082} is explored to improve the passivation of TiO{u2082}/SiO{u2082}-Si anti-reflection stacks. Furthermore, we show for the first time that TiO{u2082}, without SiO{u2082} (TiO{u2082}-Si), can be deposited in a manner that enables it to passivate silicon surfaces with a negative charge. These studies improve the understanding of SiO{u2082}-Si interface passivation and investigate methods for applying passivating AR coatings. These measures could improve the efficiency and reduce the cost of a solar cell, ultimately reducing the cost of renewable energy. The investigations of surface passivation were performed on lifetime test structures, where the surfaces were passivated with the aforementioned dielectrics with and without surface-dopant diffusions. Transient injection-dependent lifetime spectroscopy or photoconductance was used to determine the surface recombination, which is a measure of surface passivation. This was performed after sample fabrication, ageing and annealing. The effect of ageing oxide-passivated silicon is a major focus of this thesis. Oxide ageing creates defects at the SiO{u2082}-Si interface that are more recombining for electrons rather than holes. These defects are created in the dark, at low temperatures (22{u00B0} to 100{u00B0} C), and cause up to a 45-fold or 2-fold increase in recombination for boron-or phosphorus-diffused surfaces. The oxide ageing reaction takes many months to saturate and occurs readily in dry ambients. Oxide ageing will reduce the performance and prevent the certification of oxide-passivated devices. We show amorphous silicon nitride (SiNx) applied atop of the SiO{u2082} mitigates the effect of oxide ageing. The studies of TiO{u2082}/SiO{u2082}-Si passivation are combined with measurements of TiO{u2082}/SiO{u2082}-Si ageing. Deposition of TiO{u2082} degrades the SiO{u2082}-Si interface. For planar silicon this is reversible with a FGA, but it is not completely reversible for textured silicon samples. This, combined with our finding that TiO{u2082} does not prevent oxide ageing, indicates the passivating performance of SiNx/SiO{u2082}-Si structures is better than that of TiO{u2082}/SiO{u2082}-Si after fabrication and ageing. In an attempt to improve TiO{u2082}/SiO{u2082}-Si passivation through the addition of hydrogen to the TiO{u2082} deposition reaction, significant concentrations of hydrogen was found in TiO{u2082}. This hydrogen was not effective in passivating the SiO{u2082}-Si interface. Finally, we show for the first time, TiO{u2082} can act as a cheap passivating coating for boron-diffused and lightly doped surfaces. The passivation afforded by TiO{u2082} is enhanced by light. It is apparent that light soaking causes negative charging in or at the Ti0{u2082} surface. We propose a mechanism for the passivation afforded by TiO{u2082} by considering TiO{u2082} surface photo reactions. TiO{u2082} was compared with as-fabricated and aged SiO{u2082} passivation, where it is as good or better than aged oxide-passivated surfaces for non-and boron-diffused silicon. The best emitter saturation current density achieved thus far, for a boron-diffused surface was 90 fA/cm{u00B2} which would limit the open circuit voltage of a device using TiO{u2082} passivation to 685 mV.