Assessment of 2D materials and transition metal oxides as carrier selective contacts for silicon solar cells

Abstract

Although amorphous silicon as carrier selective contacts (CSCs) has been widely used for hetero-junction cell (p-aSi:H/i-aSi:H/c-Si/i-aSi:H/n-aSi:H) producing excellent device performance, the drawbacks like low doping efficiency and optical losses associated with a-Si are limitations that may be overcome by the alternate CSCs. In this thesis, the suitability of some alternative CSCs such as Graphene, Transition Metal Dichalcogenide (TMDs), Nickel Oxide (NiOx) and Vanadium Oxide (VOx) was investigated.

Extensive characterization of the fabricated graphene/Si solar cells was performed to establish a full heterojunction model. In comparison with the simple Schottky junction models, the heterojunction model allows for thorough investigation on the sensitivity of solar cell performance to graphene properties like doping level.

The full heterojunction model was also extended to simulate TMDs/Si heterojunction and investigate the origin of the widely reported ‘kink’ in the light J-V curves. Simulation results indicate that the kink is attributable to interface charge building up due to the TMD/Si band alignment, which leads to high recombination. More importantly, it was shown that selection of Si substrate type is critical for the function of TMD as a CSC. Results demonstrate that 2D materials (graphene and TMDs) /Si solar cells show immense potential in achieving over 20% efficiency.

Spin coated NiOx and VOx as CSCs for Si was also investigated with results showing an additional buffer layer is required to provide surface passivation. In this work, a-Si was used, but the anneal temperature of 350 °C required for NiOx makes it incompatible with a-Si. In contrast, VOx/a-Si showed excellent potential performance when applied to both p and n type substrate.

An alternate method for NiOx Pulsed Laser Deposition was investigated. Results for deposition on p Si under at temperatures of 300-900 °C indicate increasing crystallinity with temperature up to 500 °C. However, amorphous NiOx obtained for 700 °C and 900 °C gave better results both in terms of surface passivation and contact resistance. The results can help explain the relatively poor performance of the spin-coated NiOx, where greater crystallinity was seen, with these results suggesting amorphous material provide better CSCs for silicon.