Microstructural comparison of silicon solar cells’ front-side Ag contact and the evolution of current conduction mechanisms

Abstract

Microstructures of front-side Ag contact of crystalline Si solar cells fired at temperatures from below to above optimal were systematically investigated using advanced electron microscopy. Ag pastes studied included commercial pastes and an experimental paste containing nano-sized metallic Zn additive. Microstructures of optimally fired cells determined from cross-sectional and top-view images were found to be consistent with a primary tunneling mechanism for current flow (a “nano-Ag colloids assisted tunneling” model) due to the lack of Ag crystallites connecting the silver conductor line to the silicon emitter. We mapped the evolution of the interfacial microstructures across the firing temperature window and correlated it with cell performance. Our result sheds light on the relative importance of the two current transport models as the peak firing temperature was swept from below to above optimal.

Influence of Oxygen Micro Atmosphere During Contact Formation

“…For example Li et al showed an improvement in cell characteristics with the addition of nano -ZnO to the glass containing silver powder [5] . Another possibility is vary the oxygen activity of the glass forming melt.…”

Influence of random pyramid surface texture on silver screen-printed contact formation for monocrystalline silicon wafer solar cells

“…These guidelines are developed for the case when current conduction through Ag crystallites is prevalent at the Ag/n þ Si interface. In some previous studies, Ag screen-printed contact formation to n þ Si was observed presumably entirely through nano-Ag colloids dispersed in the interfacial glass layer (as no Ag crystallites were observed at the contact interface) [5,6]. For such cases our guidelines are not applicable.…”

“…Tunnelling through the interfacial glass may be enhanced by dispersed Ag in the glass layer [3,4]. Some authors have recently stressed the importance of nano-Ag colloids dispersed in the interfacial glass layer for contact formation and suggested that Ag crystallites may not be necessary for contact formation [5,6]. The nano-Ag colloids dispersed in glass are assumed to aid a multi-step tunnel contact through the glass layer.…”

Insights into metastability of photovoltaic materials at the mesoscale through massiveI–Vanalytics

“…29,30 The geometry, morphology, and components of the conductive lines have been studied to minimize shading of the cells while maintaining good conductance. [31][32][33] Research on the conductive lines has typically focused on the conductor line microstructure throughout the firing process 34 and the interface between the screen-printed conductive lines and the emitter as a function of precursor constituents to optimize the initial electrical performance. 30,[34][35][36][37][38] The microscopic mechanisms that result in the formation of the low resistance contact between the silver conductive lines and the solar cell are still being uncovered, 28 but two prevailing models are generally accepted.…”

“…[31][32][33] Research on the conductive lines has typically focused on the conductor line microstructure throughout the firing process 34 and the interface between the screen-printed conductive lines and the emitter as a function of precursor constituents to optimize the initial electrical performance. 30,[34][35][36][37][38] The microscopic mechanisms that result in the formation of the low resistance contact between the silver conductive lines and the solar cell are still being uncovered, 28 but two prevailing models are generally accepted. One hypothesis suggests that the Ag becomes dissolved within the molten glass frit, and upon cooling down, the supersaturated solution allows large grain growth at the Si interface causing a Schottky-barrierlike boundary that acts more Ohmic as the emitter doping concentration is increased to n þþ levels.…”