Grain Boundaries in Multicrystalline Silicon

Abstract

Directionally solidified (DS) silicon is typically multicrystalline (mc), i.e., it contains per definition grain boundaries. Even so-called quasi-mono silicon is not free of grain boundaries. The crystallographic arrangement of neighboring grains is used for a definition of the certain types of grain boundaries by the so-called coincidence site lattice parameter Σ. It turns out that the predominant types of grain boundaries are twin (Σ = 3), small angle (Σ ~ 1), and large angle (“random”) grain boundaries. For the solar cell application, it is of great relevance that the nontwin boundaries are often accompanied by dislocation defects. These dislocations, especially their clusters, are well known to reduce the minority charge carrier lifetime and hence the efficiency of solar cells. Therefore, the corresponding characterization methods for the types of grain boundaries, their length, spatial distribution, and grain size will be presented in this chapter.

The main part of the chapter presents a detailed treatment of the occurrence of the various types of grain boundaries and the related dislocations structures for different variants of the DS method. The most important DS variants differ from each other mainly by the seeding and nucleation processes which result in different sizes of the grains and also different prevailing grain boundaries. The so-called classic mc, dendritic mc, and quasi-mono Si material have relatively large average grain sizes ranging from mm up to cm. The solar cell performance of this material is mainly limited by the occurrence of dislocation structures which can easily spread in the relatively large grains. This problem seems to be decreased in a recently developed fine grained material (micro-meter up to mm scale). The variety of nucleation concepts to achieve a fine grained structure reaches from seeding with small Si feed or non-Si particles to specially structured profiles of the crucible bottom. The resulting higher performance of solar cells is promising for the future and gave reason to call the material high performance mc Si (HPM).

The whole chapter includes results of recent worldwide research and development activities but provides also its proving under production-like conditions. All results are illustrated by corresponding figures and allocated to important references.