Electrochemical nature of contact firing reactions for screen-printed silicon solar cells: Origin of "gray finger" phenomenon

Hee Soo Kim, Sung Bin Cho, Heesan Kim, Donghwan Kim, Miki Dovrat, Guy Eytan, Joo Youl Huh

Research output: Contribution to journalArticlepeer-review

15 Citations (Scopus)


Fire-through Ag thick-film metallization of crystalline Si (c-Si) solar cells often yields macroscopically non-uniform contact quality over the cell area, degrading the cell performance and causing cell-to-cell variations of the conversion efficiency in a cell production line. This study analyzes the root cause of the "gray finger" phenomenon, in which part of the fire-through Ag contact gridlines of a c-Si solar cell appears in gray or dark contrast in the electroluminescence images owing to high contact resistance. Few Ag crystallites were formed on the corrugated emitter surface at the contact interfaces underneath the gray fingers. The present results revealed that the gray finger phenomenon was caused by a short-circuit spot that formed between the Ag gridlines and underlying Si emitter during contact firing. The electrochemical reactions involved in fire-through Ag contact formation established a potential difference between the sintered Ag gridlines and Si emitter separated by molten glass. The molten glass acted as an electrolyte containing mobile Ag+ and O2- ions during contact firing. Therefore, the short-circuiting between the sintered Ag gridlines and Si emitter produced a galvanic cell during contact firing, which inhibited Ag crystallite formation at the contact interface along the gridlines in a short circuit and produced the gray fingers. The firing reactions in Ag thick-film contact formation could be interpreted in terms of the mixed potential theory of corrosion. The degradation of cell performance because of the gray finger phenomenon was also evaluated for 6-in. screen-printed c-Si solar cells.

Original languageEnglish
JournalProgress in Photovoltaics: Research and Applications
Publication statusAccepted/In press - 2016


  • Ag crystallite
  • Contact firing
  • Crystalline Si solar cell
  • Electrochemical reaction
  • Mixed potential theory
  • Screen-printed Ag metallization

ASJC Scopus subject areas

  • Renewable Energy, Sustainability and the Environment
  • Electrical and Electronic Engineering
  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics


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