Reduction of Oxygen Impurity in Multicrystalline Silicon Production

Bing Gao,Koichi Kakimoto,Satoshi Nakano

doi:10.1155/2013/908786

Abstract

Effective control of oxygen impurity in multicrystalline silicon is required for the production of a high-quality crystal. The basic principle and some techniques for reducing oxygen impurity in multicrystalline silicon during the unidirectional solidification process are described in this paper. The oxygen impurity in multicrystalline silicon mainly originates from the silica crucible. To effectively reduce the oxygen impurity, it is essential to reduce the oxygen generation and enhance oxygen evaporation. For reduction of oxygen generation, it is necessary to prevent or weaken any chemical reaction with the crucible, and for the enhancement of oxygen evaporation, it is necessary to control convection direction of the melt and strengthen gas flow above the melt. Global numerical simulation, which includes heat transfer in global furnace, argon gas convection inside furnace, and impurity transport in both melt and gas regions, has been implemented to validate the above methods.

Highlights

Multicrystalline silicon has become the main material in the photovoltaic market because of its low production cost and because of the high conversion efficiency of solar cells made from this material
Oxygen can form a variety of inhomogeneous defects: such as thermal donors [11, 12] due to clusters of a few oxygen atoms [13], and some other donors due to SiO2 precipitates [11, 12]. Except for those inhomogeneous defects, there is another kind of uniform distributed defect: boron-oxygen complexes, which is responsible for an asymptotic degradation of solar cell performance by up to 10% relative when the time scale of illumination is close to hours [14, 15]
Incorporation of oxygen impurity into multicrystalline silicon occurs during the global unidirectional solidification process

Summary

Introduction

Multicrystalline silicon has become the main material in the photovoltaic market because of its low production cost and because of the high conversion efficiency of solar cells made from this material. Oxygen can form a variety of inhomogeneous defects: such as thermal donors [11, 12] due to clusters of a few oxygen atoms [13], and some other donors due to SiO2 precipitates [11, 12] Except for those inhomogeneous defects, there is another kind of uniform distributed defect: boron-oxygen complexes, which is responsible for an asymptotic degradation of solar cell performance by up to 10% relative when the time scale of illumination is close to hours [14, 15]. This paper provides a basic framework for oxygen reduction in multicrystalline silicon during the unidirectional solidification process from the view of control of chemical reaction, melt convection, and gas flow. The control of melt convection for oxygen reduction is first reported by the present paper

Mechanism of Oxygen Incorporation

Techniques for Oxygen Reduction and Numerical Models

Techniques for Oxygen Reduction

Findings

Conclusions