Characterisation of local Al-BSF formation for PERC solar cell structures

Silicon wafers still represent a significant part of the costs of current solar modules. Passivated Emitter and Rear Cells (PERC) are a very promising technology in comparison to conventional cells. In this study PERC point like contacts at the back surface of p-type CZ silicon wafers realized with an industrial feasible technique are investigated. Since the formation of the Back Surface Field (BSF) is a critical step for the performance of solar cells, cross sections of the local contacts have been analyzed at the Scanning Electron Microscope (SEM). In order to investigate how to control the formation of the BSF, two different pastes consisting of pure aluminium particles were screen printed and different thermal profiles were applied. As a function of the width of the contacts, three major profiles can be observed: triangular, trapezoidal and with rounded edges at the sides. An explanation of the origin of these profiles is offered here. Concerning the results, one of the pastes allows achieving thicker and more homogenous BSF layers for widths of the contacts in the range of 120 to 190 Bm. This investigation shows that the present optical characterization is helpful to characterize the thickness of the BSF and, hence, its synergic use with electrical investigations can bring about enhancements in the performance of the cells