Passivated contacts for high efficiency monocrystalline silicon solar cells

Global energy demands have been increasing and the ability of fossil fuels to meet these demands is limited. Due to the associated climate change concerns, most of the current new energy installations have been based on renewable energy resources such as wind and solar. However, to further develop solar energy as a renewable energy resource, improvements in silicon-based solar cells, which represent more than 90% of the current photovoltaics market, is critical. In this thesis work we explore strategies for more efficient and cheaper solar cells. Efficiency improvements are enabled via passivated contacts, which serve both as a contact layer and a passivation layer for the crystalline silicon (c-Si) surface, and are a potential candidate for next-generation industrial c Si solar cells. In this thesis work, we identify a few salient features of passivated contacts comprising of a polycrystalline Si (poly-Si) deposited on top of ultrathin, 1.5–2.2 nm thick SiOx layers forming a metal/poly-Si/SiOx/c-Si contact stack. Poly-Si/SiOx contact passivation and conduction depends on both the SiOx thickness and contact annealing temperature. Depending on the processing conditions, two different scenarios for conduction through the SiOx layer are observed: uniform tunneling conduction or locally enhanced conduction. The locally enhanced conduction occurs through 10s of nanometer size regions with either no SiOx layer, or a thinned-down tunneling SiOx layer. The performance of the poly-Si/SiOx contacts on a pyramidal textured Si surface, which is critical for light-trapping, is also studied. The poorer passivation on a textured surface is related to the surface morphology: both the pyramidal morphology and nanoscale roughness over the pyramidal shape, causing SiOx related nonuniformities. Both the pyramidal morphology and nanoscale roughness can be modified using wet-chemical etching via HF:HNO3 solution. Such a morphological change improves surface passivation, but deteriorates the light trapping properties of the Si surface. We also explored strategies to replace current solar cell metallization processes based on the expensive Ag metal with a cheaper Cu metal, which necessitates a conductive Cu diffusion barrier interlayer between Cu and Si. The superior Cu diffusion barrier properties and thermal stability of a Cu/NiSi/Si stack over a Cu/Ni/Si stack is demonstrated.