n-Type silicon - enabling efficiencies > 20% in industrial production

In the first part of this paper we estimate the efficiency potential of crystalline silicon solar cells on conventionally pulled p-type boron-doped Czochralski-grown silicon with typical oxygen concentrations. Taking into account an industrial high-efficiency cell structure featuring fine-line metallization, shallow and well-passivated emitter and a rear surface structure with dielectric passivation and local laser-fired point contacts, the maximum achievable efficiency is around 20%. The main limitation of such a cell is due to the rather low bulk lifetime after light-induced degradation. Even when avoiding the metastable boron-oxygen defect by using Gallium-doped or magnetic Cz-silicon, it has to be kept in mind that the detrimental impact of metal contaminations on p-type silicon is greater than on n-type silicon. A potential strategy to reduce this loss is the use of n-type silicon. Therefore, the second part of the paper discusses different architectures for solar cells on n-type silicon substrates and shows the latest results achieved at Fraunhofer ISE in this field.