Effects of Facet Growth and Nucleation on Microcrystalline Silicon by Numerical Model
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A9.29.1
Effects of Facet Growth and Nucleation on Microcrystalline Silicon by Numerical Model Yasuyuki Kobayashi and Koji Satake Advanced Technology Research Center, Mitsubishi Heavy Industries, Ltd. 1-8-1, Sachiura, Kanazawa-ku, Yokohama 236-8515, Japan. ABSTRACT We have presented a model of microcrystalline silicon (µc-Si) growth based on the Van der Drift model. The model needs growth velocities of the facets (100) and (111), an amorphous silicon growth velocity and a grain nucleation rate. The growth velocity ratio of the facets, (100) and (111), determines the preferred orientation and the morphology of the µc-Si film, especially oriented to (110). As the grain nucleation rate increases, the ratio of the living grain number to the total grain number decreases and the crystallinity increases, so the grain nucleation rate governs the trade-off relation of the µc-Si cells between decreasing the open circuit voltage and increasing the short circuit current as the crystallinity increases.
INTRODUCTION Microcrystalline Silicon (µc-Si) is desirable for thin film solar cells. The µc-Si film is often deposited by plasma enhanced chemical vapor deposition (PECVD) with silane and hydrogen, and µc-Si consists of a complicated structure of crystalline silicon (c-Si) grains and amorphous silicon (a-Si) tissue [1]. It is important to understand the relation among the growth conditions, the film structure, and the electrical properties of the µc-Si film. Recently the kinetic Monte Carlo (KMC) method has been applied to the µc-Si films [2]. However, in low density species system such as PECVD, all the facets have the same chemical potential and the surface reaction governs the facet growth. So the µc-Si system can be approximated with facet growth velocities. We applied the polycrystalline growth model by Van der Drift [3, 4] to the µc-Si growth model [5]. The Van der Drift model needs growth velocities of the facets on the grain, and the model is easy to calculate. In this paper, we simulated the µc-Si film growth of grains with two facets, (100) and (111), whose growth velocity ratio determines the preferred orientation and the morphology of the µc-Si film. We also took into account an a-Si growth velocity, a grain nucleation rate in the µc-Si film growth. The results suggest the trade-off relation of µc-Si cells between open circuit voltage (Voc) and short circuit current (Jsc) follows the relation with a grain nucleation rate between the living grain number to the total grain number and the crystallinity.
A9.29.2
THEORY The Van der Drift model [3] of the polycrystalline film growth has the procedure as follows: (i) grain nuclei are placed and oriented randomly, (ii) the grains without collisions grow with keeping similar shapes called growth forms determined by the growth condition, (iii) finally the strongest grains in the growth competition are those whose maximum growth velocities (Vmaxs) from the centers to the corners in the grains’ growth forms are normal to the substrate, and the Vmax directions in the growth fo
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