N-type multicrystalline silicon: material for solar cell processes with high efficiency potential

We present the characterisation of directionally solidified n-type Si ingots. Three ingots with a range of bulk resistivities and different n-type doping elements (Sb, P and As) were studied. We show from Hall measurements that the mc-Si material has excellent electrical transport properties. The mobilities are close to the theoretical limit, which is given mainly by scattering at acoustical phonons. Mobilities so close to the theoretical value have, to our knowledge, not been demonstrated for comparable p-type mc-Si wafers. Additional measurements on high quality p-type mc-Si material support this statement. This means that other scattering mechanisms reduce the mobility in p-type mc-Si material, but are not present in n-type silicon. Lifetime measurements were conducted by μW-PCD using an iodine-ethanol surface passivation. This passivation was used preferably to SiN_x, as in some experiments the hydrogen from the PECVD SiN seemed to passivate the bulk at deposition temperatures. Average values in excess of 120 μs over large areas were measured. In order to exploit the good material properties of n-type mc-Si, solar cell concepts must be developed and the processes optimised. B-diffusion is the most problematic step as it is considered to be both destructive to material quality and energy consuming. In this paper, we show that a BBr₃-emitter diffusion is possible at moderate temperatures without degrading the carrier lifetime of the mc-Si material. An additional contribution from Libal et al. on solar cell processing is included in this conference.