Ring Defects Associated with Boron–Oxygen-Related Degradation in p-Type Silicon Heterojunction Solar Cells

Abstract

Silicon heterojunction (SHJ) cell architectures, which have dominated silicon single-junction efficiency records for the past 10 years, are processed at relatively low temperatures, on the order of ≈250 °C. Recombination-active oxygen complexes in crystalline silicon, formed from interstitial oxygen (O_i), typically require temperatures higher than this to form. Therefore, it is typically assumed that SHJ cells are immune to such defects. This contrasts with the high-temperature passivated emitter and rear cell (PERC) and tunneling oxide passivating contact (TOPCon) architectures, which can suffer from oxygen precipitates that are recombination active and difficult to predict. Herein, ring-like defects are observed in boron-doped p-type SHJ solar cells, which leads to a degradation of open-circuit voltage. It is shown that the spatial variation of this recombination activity is related to the boron–oxygen defect, the variation of which is likely due to the radial O_i distribution. Although boron-doped p-type wafers are no longer the industry standard, the defect engineering of wafers for SHJ production, using high-temperature processing, is gaining significant interest. Such wafers can have an increased susceptibility to ring-like defects. Therefore, spatially inhomogeneous defects causing recombination may become increasingly relevant for SHJ cells.