US8512463: Thick-Film Pastes Containing Lead- and Tellurium-oxides, and Their Use
in the Manufacture of Semiconductor Devices
This patent documents the invention of an electroconductive thick film paste that allows printing on the front-side of a solar cell with one or more insulating layers. The thick film paste is composed of both an electrically conductive metal, and a bismuth-tellurium-oxide which is diffused into an organic medium. The respective bismuth-tellurium-oxide powder batch materials were loaded into a furnace and heated at 900° C. in an O2 environment to melt the mixture. After this, a layer of phosphorus is diffused onto the front-surface of a silicon substrate, and an insulating and anti-reflection layer is added. Then, an Ag paste for the front electrode is printed on the silicon substrate, while Ag/Al are printed onto the backside. After drying, the materials are fired in an infrared (IR) belt furnace between the approximate temperatures of 750° to 8500° C., for range of time between several seconds to several tens of minutes. This temperature firing profile allows for the burn-off of organic binder materials present in the dried thick film paste, and multiple temperature zones in belt furnaces can support the preferred temperature profile. This process provides a method for manufacturing solar cells with good efficiency.