In-situ observations of glass frit related effects during the front side paste contact formation

Abstract

A large number of solar cells is metallized by printing and firing glass containing silver pastes. However, the contact formation is not fully understood so far. There is still a lack of understanding the role of the glass phase in the complex contact formation scenario because single effects could not been seperatly observed and evaluated up to now. To overcome this, an in-situ method to observe the contact formation via a contact resistance measurement was introduced. A special measuring device was applied to characterize two typical front side pastes, featuring a PbO-containing as well as a PbO-free glass frit during firing. The viscosity of the paste glass showed decisive influence for the etching of the anti-reflection coating (ARC). The ARC was opened immediately after entering the softening range of the respective glass, regardless of large differences in glass chemistry. Furthermore, the viscosity-temperature behaviour of the paste glass determines the intensity of the redox-reaction and related silver precipitation at the interface, which takes part between ARC opening and glass resolidification. The cooling slope was confirmed to have decisive influence on the final interface conductivity, because a crucial part of silver colloids can be formed here

Electrochemical nature of contact firing reactions for screen‐printed silicon solar cells: origin of “gray finger” phenomenon

“…Because the electrical properties of fire‐through Ag contacts are strongly correlated with the microstructure of the contact interface, clear understanding of the reactions involved in fire‐through contact formation is of great importance and has also been studied extensively . Although glass frit (typically a lead borosilicate glass) is added to the Ag paste in a minimal amount (<5 wt.%), it has long been known to play a vital role in fire‐through contact formation.…”

Silver dissolution and precipitation in an Na2O–ZnO–B2O3 metallization paste glass

“…The chemical compositions of NZB, Ag0.5NZB, and Ag5NZB were analyzed by inductively coupled plasmaoptical emission spectrometry (ICP-OES 5100, Agilent Technologies, Waldbronn, Germany) on the basis of DIN 51086-2. 25 The analyzed glass composition (mol%) of NZB amounts to 19 silver precipitates were visible. Their chemical analyses were not representative due to the formation of fuliginous depositions during the digestion of the samples.…”

“…In the light of the previous description, the present paper is focused on silver dissolution and precipitation in 19Na 2 O–28ZnO–53B 2 O 3 glasses. The experiments were designed to keep the heat treatments shorter than necessary for approaching equilibrium states, because dwell times during technical applications using rapid thermal processing (RTP) are rather short, that is, in the range of seconds 17,19 …”

“…The experiments were designed to keep the heat treatments shorter than necessary for approaching equilibrium states, because dwell times during technical applications using rapid thermal processing (RTP) are rather short, that is, in the range of seconds. 17,19 As well known from silicate glasses, a higher concentration of silver in the glass can be achieved by ion exchange, because the silver solubility is limited using conventional melting technique. 20 However, to incorporate the maximum silver ion concentration is not possible due to the process conditions starting with metallic silver and using short RTP times.…”

Printing technologies for silicon solar cell metallization: A comprehensive review

“…Based on manifold studies, the contact formation process and the properties of the metal-semiconductor interface of fire-through silver paste on n-type silicon emitters have been deeply investigated. Today, it is mostly accepted that several current paths contribute to the current flow between the silicon semiconductor and the printed and fired silver grid, 38,[184][185][186][187][188][189] namely, local direct contacts between silver bulk and the silicon surface, 16,145 silver crystallites formed on the silicon surface during the contact formation process, 36 and tunneling of charge carriers through glass layers enriched with dissolved silver and silver colloids [190][191][192][193][194][195] as shown in Figure 3. A further important finding discovered the crucial role of electrons during the firing process.…”