Enabling dielectric rear side passivation for industrial mass production by developing lean printing-based solar cell processes

Abstract

AI2O3 rear-passivated large-area silicon solar cells with screen-printed metallization are demonstrated for the first time. An industrially feasible solar cell process is described that is based on printing steps to contact base and emitter of large area solar cells with dielectric rear side passivation. The base of the cell is contacted at the rear by a full area screen-printed aluminum layer on an inkjet-structured AI2O3/SiNx-layer stack. The Al rear contacts are co-fired with the screen-printed silver front contacts. The firing temperature is reduced to limit deterioration of the passivation ability of the aluminum oxide layer. Synergies are exploited by combining the structuring steps for the formation of openings in the rear side dielectric by hydrofluoric acid with the selective emitter formation on the front side. Investigations on lifetime samples show a 2.5-fold increase in effective lifetime for surfaces passivated by an AI2O3/SiNx stack compared to fully metalized AI-BSF rear sides. This low surface recombination velocity is combined with a low contact resistance. On 125 × 125 mm² boron-doped Czochralski wafers with resistivity of 3 Ωcm an efficiency of 18.6% is achieved, that is a gain of 0.7% absolute compared to the efficiency of 17.9% of the best reference cells with a full area AI-BSF. An increase in the infrared spectrum of the internal quantum efficiency is determined as the source of this gain. Also, a higher reflectance at the rear side is measured that originates most probably from the Si/AI2O3 interface. The quality of the rear side passivation is assessed for the metalized and non-metalized area qualitatively and quantitatively. The local rear contacts are examined via scanning electron microscopy (SEM). A contact passivation mechanism based on a local BSF formation is found that is dependent on firing conditions.