Surface and contact passivation

Impact of metal work-function on surface recombination

Understanding the impact of metal contacts on recombination within a passivated crystalline silicon wafer is crucial for the optimisation of various photovoltaic devices, such as passivating-contact-based solar cells. In this type of device, the metal contacts are offset from the silicon wafer surface by additional layers. The latter (1) minimise recombination losses—either by chemically reducing the density of defects at the wafer surface or by increasing the imbalance between the majority and minority carrier density near the surface—and (2) are selective for one type of carrier. An asymmetric population of electrons and holes near the c-Si surface can be obtained by applying a contact layer with a different work function than that of silicon. Therefore, we expect that the presence of a metal contact forms an extremely thin accumulation or inversion layer close to the wafer surface. This imbalance in concentration of the two carrier types will change the recombination statistics at the passivated surface. To date, it has been challenging to assess the impact of the metal work function on the obtained passivation as photoconductance-based lifetime measurements cannot be used for metallised devices. Using our novel photolumnscence lifetime tester, we investigate for the first time the impact of work function. In the figure to the right we compare the obtained surface passivation for aluminium oxide deposited samples with and without metalisation.