Investigation of carrier lifetime instabilities in Cz-grown silicon

In the literature it is well known that the low-injection bulk carrier lifetime of boron-doped Cz-grown silicon is not a constant material property but, depending on previous thermal treatments and light exposure, varies between two states corresponding to a high and a low lifetime value. The upper state is obtained by means of low-temperature annealing, while illumination degrades the lifetime towards the value of the lower state. In order to improve the understanding of this phenomenon, we performed comprehensive carrier lifetime measurements on solar- and electronic-grade boron, gallium, and phosphorus doped Cz wafers obtained from different manufacturers. Based on the experimental results, a new model is introduced which attributes the disappointingly low stable lifetimes of illuminated boron-doped Cz silicon with resistivity around 1 Ωcm to boron-oxygen pairs. From this model, simple recipes are derived which might lead to an improvement of the efficiency of commercial Cz silicon solar cells