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 degrees 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/LPCVDW/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. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim