《An optimized rapid aluminum back surface field technique for silicon》.pdf

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 7, JULY 1999 1363 An Optimized Rapid Aluminum Back Surface Field Technique for Silicon Solar Cells Shreesh Narasimha, Ajeet Rohatgi, Fellow, IEEE, and A. W. Weeber Abstract— Screen-printing and rapid thermal annealing have A robust structure capable of reducing is the back surface been combined to achieve an aluminum-alloyed back surface field (BSF), or high-low junction. This region acts to transform field (Al-BSF) that lowers the effective back surface recombi- the true into an effective recombination velocity nation velocity (SSSe ) to approximately 200 cm/s for solar cells formed on 2.3 -cm Si. Analysis and characterization of the at the BSF junction edge [1]–[8]. Commonly implemented BSF structures show that this formation process satisfies the two on p-type substrates is the aluminum-alloyed BSF (Al-BSF). main requirements for achieving low SSSe : 1) deep p+ regions The Al-BSF is attractive because the p region is formed by and 2) uniform junctions. Screen-printing is ideally suited for metal-Si alloying instead of dopant diffusion. As a result, BSF fast deposition of thick Al films which, upon alloying, result in formation can be accomplished very quickly (within seconds deep BSF regions. Use of a rapid alloying treatment is shown to significantly improve the BSF junction uniformity and reduce or minutes) and at moderate temperatures (900 C). This SSSe . The Al-BSF’s formed by screen-printing and rapid alloying provides a distinct advantage over,