The role of the emitter on the diffusion current inn ⁺ p-junctions

Abstract

On three different samples of crystallinep-siliconn ⁺ p-junctions have been prepared by coating the surface with a phosphorous glass and a subsequent diffusion at 915°C for 30 min. From the measurement of the short circuit current density and the open circuit voltage under high illumination conditions the diffusion current of the solar cells has been determined as a function of the temperature between 77K and 300 K. The contribution of the emitter and the base region to the diffusion current was examined. Under the assumption of Auger recombination governing the hole lifetime in the emitter the narrowing of the mobility bandgap due to the high density of states within the bandgap of the emitter was determined. A shrinkage as high as 0.18 eV was observed in the case of a 0.2 μcm sample of lower purity. For high purity 6 μcm silicon the reduction of the mobility bandgap was determined as 0.10 eV. A polycrystalline sample exhibits a shrinkage of 0.15 eV. The reduction of the mobility bandgap was found to determine the lower limit of the current losses due to carrier diffusion in our solar cells at current densities around 1×10⁻⁷ Am⁻².