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

• Published in: Solar energy materials and solar cells 2019-03, Vol.191, p.117-122
• Main Authors: Shanmugam, Vinodh, Chen, Ning, Yan, Xia, Khanna, Ankit, Nagarajan, Balaji, Rodriguez, John, Nandakumar, Naomi, Knauss, Holger, Haverkamp, Helge, Aberle, Armin, Duttagupta, Shubham
• Format: Article
• Language: English
• Subjects: Bias; Bifacial; Boron; Breakdown; Cytology; Diffusion; Electroluminescence; Emitters; Fabrication; Flow; High efficiency; I-V characteristics; Impact analysis; Masking; Masks; N-type silicon; nFAB solar cell; Organic chemistry; Phosphorus; Photovoltaic cells; Plasma enhanced chemical vapor deposition; Reverse bias; Silicon; Silicon nitride; Silicon oxynitride; Silicon wafer solar cell; Silicon wafers; Solar cells
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2018.11.014

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. •Investigation of n-FAB (Front and Back contact) silicon wafer solar cells with a diffused boron front emitter and phosphorus back surface field.•Three different process flows are evaluated for the fabrication of n-FAB solar cells.•All groups have an identical batch average efficiency of 21.3%. However the reverse I-V characteristics were observed to be remarkably different.•Investigates breakdown sites in n-type cells by studying the impact of the manufacturing process flow, specifically with regards to the use of PECVD dielectrics as diffusion masks.