Characteristics of Bi-based glass frit having similar mean size and morphology to those of silver powders at high firing temperatures

doi:10.1016/j.jallcom.2010.03.022

Journal of Alloys and Compounds

Volume 497, Issues 1–2, 14 May 2010, Pages 259-266

https://doi.org/10.1016/j.jallcom.2010.03.022 Get rights and content

Abstract

Bi₂O₃–ZnO–SiO₂–B₂O₃–Al₂O₃–CaO glass frits with glass transition temperature of 442 °C are prepared by spray pyrolysis. The mean size of the glass frits prepared at 1400 °C is 0.87 μm. The grain size of the silver conducting film formed from the Ag paste increases with increasing addition amount of glass frit. The glass frit improves the adhesion strength of the silver conducting film to the Si wafer substrate. Glass material is well distributed inside the grains of the silver conducting films fired at 600 °C, in which segregation of the glass material did not occur. Segregation of glass material in grain boundaries of the silver conducting films increases with increasing firing temperatures. The specific resistances of the silver conducting films with 3 and 5 wt.% glass material of silver component are 2.6 and 2.8 μΩ cm at a firing temperature of 700 °C.

Introduction

During the preparation of single-crystalline or polycrystalline Si solar cells, a silver electrode is formed on the side with an anti-reflecting preventing film. The silver electrode is generally fabricated by the firing of a printed layer formed by a screen printing method using silver paste containing a silver powder, a glass frit, and a resin binder [1], [2], [3], [4], [5]. Although glass frit holds the minimum weight percentage in the pastes (1–5 wt.%), it plays the important role during Ag/Si metal–semiconductor contact formation by etching through the antireflection coating (ARC) and ensuring a stable mechanical contact. The glass also serves as a transport medium for Ag to recrystallize on the Si emitter surface and guarantees the formation of nearly perfect Ag/Si ohmic contacts at the temperatures below the Ag/Si eutectic point [2]. The glass powders act as a permanent binder and also promote sintering of metal powders during firing process and enable binding of the functional film to the substrate [6], [7], [8], [9]. Therefore, the glass frit in Ag thick-film pastes is the main factor that determines the Si surface etching, contact resistance, and ultimately the overall Ag grid surface performances [2], [3].

Pb- and Bi-based glass frits with appropriate glass transition temperature are generally used as an inorganic binder. Silver paste with glass frit having an appropriate glass transition temperature can be sintered at 600–800 °C, wetted appropriately, and bonded appropriately to a silicon substrate. In this study, the characteristics of Bi-based glass frit with similar mean size and shape to those of silver powders as an inorganic binder for Si solar cell applications were investigated. The effects of Bi-based glass frit on the sintering characteristics of the silver conducting film at temperatures between 600 and 900 °C were investigated. The segregation characteristics of glass material in the silver conducting films according to the firing temperatures were also investigated.

Section snippets

Experimental procedure

Glass frits were prepared by spray pyrolysis from droplets with several micron sizes. Glass frit with a composition of Bi₂O₃–ZnO–SiO₂–B₂O₃–Al₂O₃–CaO was directly prepared by spray pyrolysis. The spray pyrolysis equipment used consisted of six ultrasonic spray generators that operated at 1.7 MHz, a 1000-mm-long tubular alumina reactor of 50-mm ID, and a bag filter. The preparation temperature of glass frits was changed from 1100 to 1400 °C. The flow rate of the carrier gas was fixed at 20 L/min.

Results and discussion

In the spray pyrolysis, glass frits were formed from the droplets containing precursors of glass components by drying, decomposition, melting and quenching processes. The decomposition characteristics of precursors of glass components are affected by the preparation conditions. The formation conditions of glass material by melting and quenching processes is affected by the glass composition. Therefore, the optimum preparation conditions of the glass frit in the spray pyrolysis are affected by

Conclusions

The characteristics of Bi₂O₃–ZnO–SiO₂–B₂O₃–Al₂O₃–CaO glass frit with spherical shape and submicron size as an inorganic binder for silver conducting film are investigated. The optimum preparation temperature of glass frit with dense structure is 1400 °C. The glass frits improve the sintering characteristics of the silver powders at 700 °C to form the silver conducting film with dense structure and high adhesion strength to the Si substrate. Segregation of glass material in grain boundaries of the

Acknowledgement

This study was supported by a grant (M2009010025) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea. This study was supported by Seoul R & BD Program (WR090671). This study was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. 2009-0074023).

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