Al-alloyed local contacts for industrial PERC cells by local printing

In this paper, a detailed investigation of the Al-alloyed local rear contacts for industrial PERC cells is presented. Three types of voids and their influences to PERC cells are evaluated with 2D numerical device simulations. By a detailed study of the formation mechanism of local contacts, an effective method of two-step metallization is proposed to suppress the generation of voids. In step 1, the Al paste is locally printed to limit the lateral diffusion of Al into Si during the firing process. In step 2, a full-area metallization with low temperature firing is applied to connect all the rear local Al contacts. With this method, a clear decrease of the void density after an industrial firing process is demonstrated. Nearly 0% voids rate can be achieved if the design width of the Al contact is small enough. This can prevent the recombination of minority carriers at the rear side, and contribute to V_oc over 666 mV. Average cell efficiency of 20.26% and best efficiency of 20.50% are achieved in batch run in a pilot line. With advanced module technologies, a best module power of 326.3 W_p was achieved for a 60-cell-based module and independently confirmed.