Impact of the manufacturing process on the reverse-bias characteristics of high-efficiency n-type bifacial silicon wafer solar cells

Abstract

In this paper, bifacial n-type silicon wafer solar cells with a front boron-diffused emitter and a rear phosphorus-diffused back surface field are investigated. The cell structure is abbreviated as nFAB (n-type Front and Back Contact). Three different process flows are evaluated for the fabrication of nFAB cells: (1) process flow with a silicon nitride diffusion mask deposited by plasma-enhanced chemical vapour deposition (PECVD), (2) process flow with a silicon oxynitride diffusion mask also deposited by PECVD, and (3) process flow using single-sided atmospheric pressure chemical vapour deposition (APCVD), without diffusion masks. An average batch efficiency of 21.3% was measured (front side illumination only) for all three nFAB process flows. However, the reverse bias I-V characteristics are observed to be remarkably different and dependent on the process flow, especially with regards to the masking processes. For process flow 1 that uses a silicon nitride PECVD mask, the reverse bias I-V characteristics are poor with a severe breakdown occurring at − 5 V and a reverse current of 18 A at −10 V. Process flow 2 with a silicon oxynitride PECVD mask leads to significantly improved reverse characteristics with a reverse current of 1 A at −10 V. For the APCVD process flow without diffusion masks, excellent reverse bias characteristics are observed with a reverse current of only 3 mA at −10 V. Reverse-biased electroluminescence imaging is used to determine the breakdown regions of the cells for the different process flows and to analyse the impact of the diffusion masking step on the cells’ breakdown characteristics.