Current Transport Through Lead–Borosilicate Interfacial Glass Layers at the Screen–Printed Silver-Silicon Front Contact

During the firing of screen-printed silver contacts in industrial crystalline silicon solar cells, an interfacial glass layer is formed between the silver bulk contact and the silicon substrate. By means of numerical modeling, we investigate conduction mechanisms across the interfacial glass layer, which consists of lead-borosilicate glass enclosing silver colloids. As there is a considerable lack of parameters for the interfacial glass layer, we revisit properties of similar materials such as as-grown crystalline and polycrystalline lead monoxide. We revise characteristics of applications that use similar glasses, such as photodetectors, thick-film transistors, and surface passivation. We propose that current transport via the interfacial glass is similar to the one in thick-film resistors based on ruthenium-doped lead-borosilicate glass. Device simulations are performed using direct tunneling, trap-assisted tunneling, and a conducting insulator model for current transport across the interfacial glass layer. The tunneling mechanisms are only effective for thin interfacial glass layers up to 2 nm. A combination of direct tunneling and the conducting insulator modeling is well suited to model interfacial glass layers between 0.5 and 1000 nm. Applications to specific contact resistance measurements show that the resistivity of the interfacial glass is in the range of thick-film resistors of similar lead monoxide content.