(472g) Modeling the Horizontal Ribbon Growth of Solar Silicon Crystals

The development of low-cost and reliable solar-to-electric conversion methods has been a bottleneck in establishing photovoltaic (PV) technology as a viable source of renewable energy. Present-day PV technology is predominantly based on the fabrication of solar cells from silicon. The cost and efficiency of silicon-based solar cells depend primarily on the growth rate and degree of single-crystallinity attained by the growth process. Traditional silicon crystal growth methods are either very expensive or compromise on efficiency. The horizontal ribbon growth (HRG) process is a promising crystal growth technology that has the potential to overcome these limitations. The idea of the HRG process was first conceived by William Shockley in late 1950's with subsequent efforts by Kudo in Japan in late 1970's and by Energy Materials Corporation in US in early 1980's. After encouraging initial development, the efforts stalled owing to the technical difficulties encountered in its implementation.

We are applying a comprehensive thermal-capillary model to study the coupled phenomena of heat transfer and interfacial phenomena (solidification and capillarity) in the HRG process. This model accounts for i) heat transfer in the melt-crystal-crucible domains with radiative heat loss from high temperature surfaces, ii) melt convection due to buoyancy and surface-tension forces, and iii) determination of melt-crystal, melt-ambient and crystal-ambient interface shapes. The model is solved numerically by the Galerkin finite element method, with elliptic mesh generation techniques to handle the moving boundary nature of the problem.

We first discuss the application of this model to vertical edge-defined film-fed (EFG) systems to study the effect of pulling at an inclination. Implementing the thermal-capillary model for the HRG system will promote extensive parametric sensitivity studies to identify the variables that can have a direct impact on the operation of the process. The effect of pull rate, pulling angle, contact angles of the meniscus with the crystal and crucible wall, crucible geometry, and furnace heat transfer are investigated. Transient simulations are performed to assess the dynamics of the growth process and to understand system stability.