Influence of the Aluminum Paste Surface Density on the Electrical Parameters of Silicon Solar Cells

The industrial process of silicon solar cells is mainly based on the aluminum back surface field (Al-BSF) performed by the conventional screen printing and firing in a belt furnace. The goal of this paper is to present the analysis of the influence of the amount of the Al paste on the electrical parameters and on the minority carrier diffusion length. The silicon solar cells were processed in p-type Czochralski solar grade wafers. The amount of Al paste deposited to form the BSF, denominated of Al paste surface density, was ranged from 2.8 mg/cm2 to 8.8 mg/cm2. The peak firing temperature for each amount of Al paste was optimized and the depth of the Al-BSF was estimated. The best results were found for the Al paste surface density of 3.5 mg/cm2 and the peak firing temperature of 840 °C, resulting in the efficiency of the solar cells of (15.0 ± 0.1) %. In this case, the depth of the Al-BSF was (5 ± 1) μm. Taking into account the peak firing temperature obtained for each Al paste surface density, we observed that the short-circuit current increased up to the average Al paste amount of 3.5 mg/cm2. On the other hand, the fill factor decreased with the increasing of the Al paste surface density. The open circuit voltage was slightly affected by the Al paste amount. The minority carrier lifetime rose from 30 μs to 120 μs after the phosphorus diffusion. A strong improvement in the minority carrier diffusion length was observed after the firing process and depends on the Al paste amount. The measured average values were 500 μm, 1280 μm and 780 μm for the Al paste surface density of about 2.8 mg/cm2, 3.5 mg/cm2 and 8.8 mg/cm2, respectively.