This paper reports the development of copper screen printing pastes for silicon heterojunction solar cells. Nanoparticle copper paste formulations with a varying amount of copper (percentage by weight) were evaluated in terms of printability, line resistance, and contact formation to Indium-Tin Oxide (ITO) transparent conductive oxide layers. The screen-printed Cu samples were cured under vacuum conditions (< 300 ppm O-2) at temperatures between 200 degrees C and 400 degrees C for 30 min. Scanning electron microscopy was used to investigate Cu nanoparticle sintering at the microstructural level and determine optimal curing conditions for the pastes. The optimized Cu paste formulation yielded consistent finger widths between 53 and 60 tun and finger heights above 20 mu m. The average specific contact resistivity of the Cu-ITO contact for the best-performing paste formulation under optimal curing conditions was 0.4 m Omega.cm(2). The resistivity of printed Cu lines after curing at 400 degrees C for 30 min was 27 pacm. In terms of printability and contact resistance to ITO, the paste formulations developed in this study are suitable for application to silicon heterojunction cells. Steps to further improve the resistivity of the printed Cu lines are discussed. Insights from this study revealed the critical influence of Cu paste composition, rheology, screen printing parameters, and curing conditions on the properties of printed electrodes.