Characterization of Defects in N-type Cz Silicon for Solar Cells

Abstract

With increasing attention to high efficiency solar cells in the photovoltaic industry, ntype Cz silicon has become increasingly popular due to its potential for high-end devices. As the metal content in Cz silicon is reduced and n-type silicon is much less sensitive to most metal impurities than p-type silicon, the grown-in defects becomes the dominating features controlling the cell efficiency. In this thesis, the aim is to develop techniques to characterize grown-in defects in n-type Cz silicon.

Cz silicon crystal growth always starts in the silicon interstitial mode, some transition regions will be inherited from the crystallization process before the growth changes to the vacancy mode. Several transition regions are very harmful to the performance of conventional solar cells, because they are enriched with oxide particles and significant precipitation can happen in the solar cell process. It is therefore very important to define defect transitions in order to better control silicon growth. Various techniques have been applied to characterize defect patterns in Cz silicon. In this work, three different techniques are studied and compared. Preferential etching on Cu-decorated wafer was found to be the most effective method to delineate all types of defects in Cz silicon and defect patterns can be seen even with the naked eye. Secco etching directly applied on polished as-grown wafer can also determine the position of defect boundaries by quantification of flow pattern defects which corresponded to voids in the sample. By mapping the lifetime of a silicon wafer, a P-band was shown as a low-lifetime region between the interstitial defect region and the vacancy defect region.

Although preferential etching is effective to delineate defect patterns in Cu-decorated Cz silicon, corrosive and toxic chemicals are always included in the etching process. A fast and etch-free method was developed to characterize Cu-decorated slabs using carrier density imaging. The validity of the new method was confirmed by preferential etching. The mapping results of carrier density imaging were investigated and the mechanisms behind the results were analyzed.

Minority carrier trapping leads to abnormally high lifetime at low injection level during lifetime measurement by the photoconductance decay technique. Trapping has been widely studied by quasi-steady-state photoconductance decay in multicrystalline silicon and p-type Cz silicon but few work has been performed in n-type silicon material. In this work, minority carrier trapping was investigated in n-type Cz ingot using transient photoconductance decay. A simple model was developed for transient-PCD to fit the lifetime results and calculate the trap density. It was found that the calculated trap density was strongly correlated to the concentration of thermal donors and interstitial oxygen. Further annealing experiments was done to investigate the effect of thermal donors on minority carrier trapping. Additionally, the detrapping process was investigated and the detrapping constant was measured.

Thermal donors, believed to be oxygen clusters, are known to affect carrier transport properties and resistivity in silicon materials, and interstitial oxygen atoms act as precursors in the formation of active oxygen-related complexes during further annealing process. The distribution of both thermal donors and interstitial oxygen are thus of great importance to better understand and control the macroscopic properties of Si wafers and solar cells. A new method was developed to map the concentration of thermal donors and interstitial oxygen using carrier density imaging. The concentration of thermal donors was first imaged from the free carrier density image, and the concentration of interstitial oxygen was then calculated according to an experienced model. The results were confirmed by the resistivity method validating the new method. By mapping the concentration of thermal donors in an as-grown n-type Cz silicon slab, a strong correlation was found between the recombination rate and thermal donor concentration.