INVESTIGATION OF SCREEN-PRINTED METAL CONTACTS TO n-DOPED MULTI-CRYSTALLINE Si SUBSTRATE

Abstract

Ohmic metal contact formation to n and p-doped surfaces of a solar cell plays a central role in determining its series and shunt resistances. Rapid thermal annealing (RTA) based on conveyor-belt translation of the screen-printed wafers at elevated temperatures is the standard industrial practice for metal contact formation. In an industrial process, screen-printed wafer travels through multiple high temperature zones at controllable speeds to form low resistance ohmic contacts. In this study, inexpensive alternate RTA methods aimed at screen-printed electrical contact optimization as a function of the peak RTA temperature at fixed transit time have been investigated. Three different RTA systems were investigated: an industrial 6-zone conveyer belt furnace, a custom designed 3-zone quartz 5”-diameter tube furnace, and a table-top 3”-diameter quartz tube system with computer-controlled bank of quartz lamps. Starting material was p doped wafers with the bulk resistivity in ~ 1-3 ohm-cm range. Emitter formation was carried out using inexpensive, non-toxic spin-on phosphorous acid and conventional POCl₃ diffusion processes. The contact resistivity measurements were based on transmission line method (TLM). The lowest contact resistivities measured were: (a) ~ 2.0mΩ cm² for Ag and 412 mΩ cm² for Al in industrial conveyor belt furnace at peak firing temperature of 860 ^oC, (b) ~ 4.1 mΩ.cm2 for Ag and 77.7 mΩ.cm2 for Al at the peak temperature of 925 ^o C in quartz tube furnace, and (c) ~ 1.2 m Ω.cm² for Ag and 63.5 mΩ.cm² for Al at the peak firing temperature of 780 ^oC in table-top RTA system. For Ag, the lowest contact resistance was achieved in the table-top RTA system, while for Al, the highest contact resistance was observed in the industrial conveyor belt six zone furnace.