Investigation of electronic and structural properties of silicon solar cell contacts

Abstract

Screen-printing technique is used since 1970’s, but the understanding of screen printed contacts is quite poor. Several different models of contact formation exist. Two mechanisms of current transport from bulk semiconductor to bulk metal are proposed but not experimentally proved. Metal/semiconductor (Ag/Si) interface is the most important part of screen-printed contact and reduction of series resistance between the bulk Ag and the Si wafer has a large potential for solar cell performance improvement. Composition of the silver paste together with firing parameters is the most important players in formation of well conducting metal/semiconductor interface. Commercially available silver pastes which are used for front contacts consist of silver powder, lead-glass frit powder and an organic vehicle. A crucial condition which determines the quality of the contact is firing temperature. During firing, the glass frit plays a critical role as it forms a glassy layer between the Si wafer and the bulk Ag, works as a medium for diffusion of Ag and Pb particles and is responsible for etching the SiN layer. In order to enhance solar cells’ performance it is important to explain how contact formation depends on firing temperature and to improve understanding of electron transport between the Si wafer and the silver contact. In this work, an Ag-Pb based paste was used and peak firing temperature (PFT) was varied in order to get optimally fired (best performance), under fired and over fired (lower performance) cells. Electronic properties of the cells were investigated using a solar simulator and a series resistance mapping technique. Transmission Electron Microscopy (TEM) imaging, Selected Area Diffraction (SAD) and Energy Dispersive Spectroscopy (EDS) techniques were used to study the structure and chemistry of the metal/semiconductor interfaces. The results show that in under fired cells the PFT was too low to etch away the SiN layer which works as an insulating layer and impedes current collection. In optimally fired cells the PFT was high enough to partly etch away the SiN layer. In areas with no SiN, glassy layer with many Ag and Pb precipitates is in direct contact with the Si wafer. Distances between precipitates are <3 nm and current is efficiently collected through tunneling. In over fired cells SiN is completely etched away. Larger Ag crystallites are found on the Si interface. In the vicinity of these crystallites, the number of Ag and Pb precipitates in the glassy phase decreases and particles increase in size. Distances between precipitates become too large for electrons to tunnel and series resistance increases.