Screen-printed copper for front- and back-side metallization of single- and multi-crystalline silicon solar cells

The metallization of multi-crystalline and single-crystalline Si (m- and sc-Si, respectively) photovoltaic (PV) solar cells was investigated by using copper screen print pastes designed for electronics packaging applications. For this purpose, Cu features were printed on m-Si and sc-Si silicon substrates using low pressure chemically vapor deposited (LPCVD) tungsten layers as adhesion promoters and barriers against Cu diffusion, and self-assembled monolayers (SAMs) of (3-mercaptopropyl) trimethoxysilane (MPTMS) as adhesion promoters. It was found that pastes may be annealed at temperatures of the order of 600 °C, in primary vacuum and under reducing ambient to give acceptable Cu/Si contacts that pass the Scotch test and standard metallic strips may be soldered on them. Contacts of the form Cu/MPTMS/m-Si were proved of limited lifetime; their study has shown that the Schottky barrier height was of the order of 0.45 eV. Screen-printed Cu/LPCVD W/Si (substrate) contacts proved stable in time for sc-Si but were found unstable for m-Si. Cu was also printed on the Al layer used for the creation of the back-surface field in industrial m-Si PV cells by using SAMs of 11-mercaptodouncyl phosphorus acid (MDTA) as adhesion promoters. The corresponding contacts did not affect the performance of cells and proved stable for a time period of the order of 1 month.