The development of etch-back processes for industrial silicon solar cells

Abstract

The exactly controlled wet-chemical etch-back of diffusion profiles can shift the efficiency limit of the widely used screen printing process to higher values. In conjunction with an inkjet or screen printing masking step, the cost effective production of selective emitter solar cells is possible. We present the development of our etch-back process using Cz-Si and mc-Si wafers. Emitter saturation current densities of etch-back emitters are lower compared to directly POCl3-diffused samples with the same sheet resistance. Negative effects of the etch-back process on reflection properties are negligible if less than 90 nm are removed from the surface. Grain boundaries of mc-Si wafers may be etched faster than the rest of the wafer. A screen printed 5 inch Cz-Si selective emitter solar cell with full area Al BSF reached an efficiency of 19.0%. The same process was applied to n-type Cz-Si wafers with a selective front surface field (FSF) and a full area screen printed Al emitter. An efficiency of 18.5% on 6 inch n-type Cz solar cells could be reached in a first test. This emphasizes the highly effective FSF and makes the process attractive for possible industrial production, since no boron-oxygen degradation occurs in n-type silicon.