DEVELOPMENT OF ELECTROCONDUCTIVE COPPER PASTES FOR HETEROJUNCTION SOLAR CELL ELECTRODE MATERIALS

摘要

We developed copper pastes containing a low melting point alloy for heterojunction solar cell electrode wiring materials. Characteristics reside in that facts that 65 μm-wide line printing is possible and that a low melting point alloy is melted at 180°C or lower and alloys with copper, thereby possessing a low line resistance value and a low contact resistance value. We confirmed that our copper pastes have line resistance and contact resistance of the same degrees as those of Ag pastes under curing conditions of 180°C or lower for heterojunction solar cells. Also, we confirmed the state that a low melting point alloy is melted through SEM cross-sectional observation of the cross section. By achieving alloying, an improvement of humidity resistance is also contrived, and an increase ratio of the line resistance at 85°C and 85% RH for 1,000 hours is less than 30%. It is already reported that copper ions do not diffuse into a silicon wafer upon alloying. This suggests that the cell conversion efficiency does not become worse even with a lapse of time. Also, we report results of studies regarding the printability which has been of a problem so far. We contrived to renovate screen printing technologies and confirmed that 65 μm-wide lines can be printed. There was no problem in the continuous printability, and results in which a change of viscosity at every number of times of printing is small were revealed. Thus, we may consider that our developed copper pastes are on an industrially practically useful level.