Details

Autor(en) / Beteiligte

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

Titel

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

Ist Teil von

• Progress in photovoltaics, 2016-09, Vol.24 (9), p.1237-1250

Ort / Verlag

Bognor Regis: Blackwell Publishing Ltd

Erscheinungsjahr

2016

Quelle

Wiley-Blackwell Journals

Beschreibungen/Notizen

• 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. Copyright © 2016 John Wiley & Sons, Ltd. This work 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 EL images owing to high contact resistance. The firing reactions in Ag thick‐film contact formation are electrochemical in nature, establishing a potential difference between the Ag gridlines and Si emitter separated by molten glass. Formation of a short‐circuit spot between the Ag gridline and Si emitter during contact firing inhibits Ag crystallite formation at the contact interface along the gridline, leading to strong lateral variations in contact resistance across the cell area and a significant degradation of cell performance.