Transparent conducting aluminium doped zinc oxide for silicon quantum dot solar cell devices in third generation photovoltaics

Silicon quantum dots (QDs), a subset of Si nanocrystals (NCs), in dielectric matrices with bandgap tunability are promising thin film materials for third generation photovoltaics, which aim to cost effectively exceed the Shockley-Queisser limit of efficiency. The Si QDs investigated in this thesis were fabricated by magnetron sputtered thin film superlattice bilayers of silicon rich oxide (SRO) and silicon dioxide (SiO2) followed by annealing at 1100 °C. The annealing causes solid-state nucleation and subsequent formation of Si NCs in the SRO layers. However, the main issue with this type of Si NC material is its poor carrier transport and material quality. Solar cell devices in the past have experienced heavy current crowding and high series resistance despite their reasonable open-circuit voltages up to 492 mV. Aluminium doped zinc oxide (AZO) is a promising transparent conducting oxide (TCO) which is often used in thin-film solar cells as transparent contacts. From the literature surveyed, AZO has never been used to make Si NC solar cell devices before. The key advantage of AZO is its high melting point of 1975 °C, which is much higher than other common TCOs. This allows AZO to be annealed at 1100 °C together with the SRO/SiO2 bilayers, although the structural, electrical and optical properties of the AZO thin films change after annealing. The main issue is the heavy decrease in conductivity and cross-diffusion of elements.For the first time, nucleation of Si NCs in SRO/SiO2 bilayers was observed in real-time via an aberration-corrected high resolution transmission electron microscope with in situ heating up to 600 °C. This particular experiment showed that nucleation of Si NCs begins at an unexpectedly low temperature (450 °C) which suggests that ex situ annealing at 1100 °C may not be necessary. However, through external furnace annealing temperature dependence experiments later it was shown that the higher the annealing temperature, the better the extent of crystallisation of the Si NCs. The advantages of high Si content SRO/SiO2 bilayer superlattices with boron and phosphorus doping were also investigated. It was also shown that AZO forms a good ohmic contact to both the high Si content B and P doped SRO/SiO2 bilayer superlattices. Finally, together with utilising AZO as a transparent contact, a prototype superstrate configured Si NC solar cell device is demonstrated