PhD thesis Andreas Bentzen

Abstract

The task of increasing the efficiency of multicrystalline silicon solar cells relies on the understanding and optimization of each individual processing step, as well as of the interplay between the material properties and the processing conditions. In particular, as typically being the processing step of highest temperature during solar cell manufacturing, understanding the mechanisms and material correlations of the emitter diffusion is of utmost importance.

This thesis considers diffusion of phosphorus in silicon, with particular devotion to formation of the n-p junction in multicrystalline silicon solar cells. The thesis is divided in two parts; the first part treats various aspects of phosphorus diffusion for emitter formation in silicon solar cell devices, while the second part deals with the material properties of multicrystalline silicon and the corresponding response to high concentration phosphorus diffusion.

For phosphorus emitters diffused from a high concentration spray-on source, the influence of processing conditions on the emitter sheet resistance as well as the cor- respondence between sheet resistance and emitter profile are studied in detail in this thesis. Furthermore, the physical mechanisms relevant for such high concentration in-diffusion are investigated, and an integrated diffusion model is adapted and ana- lyzed. Based on these findings, a unified model for high concentration in-diffusion of phosphorus in silicon is developed, allowing accurate simulation of emitter profiles using only three free parameters; the diffusion temperature, the diffusion time, and the phosphorus concentration near the surface of the sample.

Multicrystalline silicon wafers typically exhibit large inhomogeneities in the material properties, both laterally within a single wafer as well as longitudinally de- pending on the position of the wafer in the column from which it is sawed. These inhomogeneities are investigated in this thesis with particular attention to the charge carrier recombination lifetime, as well as to the gettering response from a phosphorus emitter diffusion and the bulk passivation ability of hydrogen released from an over- lying hydrogenated silicon nitride film. Particular attention is devoted to gettering of selected transition metal impurities and the interplay with material microstructure, as well as to the influence of emitter diffusion parameters on the gettering behavior of multicrystalline silicon wafers.

Based on the learnings from these investigations, the influence of the emitter processing conditions on the performance of screen-printed multicrystalline silicon solar cells is studied in greater detail. Different diffusion profiles exhibiting the same emitter sheet resistance are employed in fully processed solar cells, and are investigated in order to increase the understanding of the abilities and limitations of emitter diffusion in multicrystalline silicon solar cell processing.