Multiscale modeling and operation of PECVD of thin film solar cells

This work proposes a multiscale modeling and operation framework for plasma-enhanced chemical vapor deposition (PECVD) of thin film silicon solar cells with uniform thickness and film surface microstructure that optimizes light trapping. Specifically, we focus on a single-wafer parallel-electrode PECVD process with showerhead arrangement and develop a multiscale model capturing both the gas-phase reaction and transport phenomena that lead to the deposition of the thin film across the wafer as well as multiple microscopic models that describe the evolution of the thin film surface microstructure at equispaced, discrete spatial locations across the wafer. While the modeling of chemical reactions and transport-phenomena (both diffusion and convection) in the gas-phase adopts the continuum hypothesis and is based on two-dimensional in space partial differential equations, a novel microscopic model is developed for the a-Si:H thin film surface evolution, which accounts for four microscopic processes: physisorption, surface migration, hydrogen abstraction and chemisorption. A hybrid kinetic Monte Carlo (kMC) algorithm is utilized to reduce computational requirements without compromising the accuracy of established chemical models that account for interactions amongst physisorbed radicals, and the microscopic model fidelity is established though calibration with experimentally obtained growth rates and surface morphology data. The results of the multiscale process model indicate that in order to produce a thin film with a diameter of 20 cm and a uniform thickness with surface microstructure that optimizes light trapping: (a) a sinusoidally grated wafer surface should be used in which the grating period and depth should correspond to values that lead to film surface roughness and height-height correlation length that are on the order of visible light wavelength range, and (b) the substrate temperature should be adjusted, along several concentric zones across the substrate, to compensate for a radially non-uniform deposition rate of the nil on the wafer owing to gas-phase transport phenomena. Due to the dependence of film growth rate on substrate temperature, the wafer surface is separated into four concentric zones, each with an independent heating element. Extensive simulations demonstrate that the use of appropriate sinusoidal wafer grating and the regulation of substrate temperature provide a viable and effective way for the PECVD of thin film silicon solar cells with uniform thickness and film surface microstructure that optimizes light trapping. (C) 2015 Elsevier Ltd. All rights reserved.