Direct comparison of the electrical properties of multicrystalline silicon materials for solar cells: Conventional p-type, n-type and high performance p-type

We compare the recombination properties of three important types of multicrystalline silicon wafers for solar cells, namely conventionally-solidified p-type and n-type multicrystalline wafers, and also the recently developed 'high performance' p-type multicrystalline wafers. Three distinct regions of the wafers are examined in detail. These are the intra-grain regions, the grain boundaries, and the dislocation networks. The response of these regions to phosphorous gettering and hydrogenation are also characterised and compared. The electrical properties of intra-grain regions are assessed based on both the minority carrier lifetime and diffusion length. The recombination activities of grain boundaries are compared quantitatively in terms of their effective surface recombination velocities. The recombination behaviour of dislocations is evaluated qualitatively based on photoluminescence images. Our results show that the main performance limiting factors are likely to be recombination at crystal defects. Overall, grain boundaries in conventional p-type samples are more recombination active than those in high performance p-type and conventional n-type samples. The benefits of hydrogenation are most visible on n-type samples, leading to significant deactivation of most of the grain boundaries. As-grown grain boundaries and dislocations in high performance multicrystalline silicon tend not to be recombination active, and only become active after either gettering or hydrogenation.