Verteilung lebensdauerlimitierender Defekte in kristallinem Silizium für Solarzellen

Abstract

In this work the spatial distribution of defects in multicrystalline silicon for solar cells and their influence on the minority carrier lifetime and thus the efficiency of the finished cells is analysed. Each defect class (point defects, dislocations, grain boundaries and extended defects) shows specific recombination mechanisms. They are predominantly characterised by the Shockley-Read-Hall-formalism for carrier recombination. The most important defect species exhibit a variety of recombination mechanisms and a non-uniform spatial distribution. Thus the spatially resolved carrier lifetime is the essential parameter focused on in this work.For quantifying recombination patterns, existing models for the evaluation of point defects and grain boundaries based on lifetime measurements have been adapted for wafers with passivated surfaces. Furtheron a model for the influence of dislocations, evenly spread as dislocation lines perpendicular to the wafer surfaces, has been developed. One- and twodimensional topographies of carrier densities in wafers have been simulated based on measurements of the dislocation density and grain boundary distribution applying assumptions for the point defect distribution and influence of surfaces.The defect distribution in multicrystalline block silicon and its resulting lifetime profile is determined by the incorporation of defects into the crystal during crystallization and the subsequent cooling down phase. Horizontally and vertically cut wafers of p-type and n-type material have been analysed for the evaluation of height dependent lifetime profiles. Analysis and simulations for the block bottom of p-type material point to an indiffusion of metallic impurities from the crucible and crucible lining. The resulting lifetime profiles superimpose the effect of other defects, most probably caused by oxygen precipitates at crystal defects forming strong recombination centers. In the upper part of the block, the lifetime pattern is dominated by a backdiffusion of metallic impurities from a zone near the block cap with a high density of precipitates. In n-type silicon lifetime values up to a factor of ten higher than in p-type material are found. Assuming a strong lifetime limitation by metallic impurities, this can be explained by the significant asymmetry of capture cross sections for electrons and holes for these elements. Analysis of EFG-material does not give any hints on lateral diffusion of impurities after crystallization. The lifetime distribution is pinned directly after solidification by the crystal structure.After diffusion processes and phosphorous-aluminium-diffusion as gettering step, an increase of lifetime due to a reduction of distributed defects could be found together with a strong lifetime decrease in areas with small grains. Analysis of oxidation processes reveal a small direct influence of recombination at dislocations in the middle of grains on the lifetime pattern. The dominating recombination channels were recombination at grain boundaries and distributed point defects in the vicinity of grain boundaries. The influence of defect types on the lifetime distribution was investigated by the evaluation of measured and simulated one-dimensional lifetime profiles. Whereas the lifetime profile in the starting material is governed by recombination at grain boundaries and dislocations, the reduction after high temperature steps is predominantly due to the dissolution of impurities from precipitates at grain boundaries and their partial reagglomeration after cool down. The fitting of the simulated patterns to the lifetime distribution does not count as proof for the effect of different defect species, but enables the study of the effect of relevant defects and recombination channels. Thus an assessment of the influence of different defect types on the lifetime distribution and thus solar cell parameters could be made.