High-efficiency back-junction silicon solar cell with an in-line evaporated aluminum front grid

We present n-type silicon PERT (passivated emitter and rear totally diffused) solar cells with an Al<inf>2</inf>O<inf>3</inf>/SiN<inf>x</inf>-passivated BBr<inf>3</inf>-diffused back-junction emitter. The aluminum front side grid is evaporated in an industrial-type in-line evaporation system. The efficiency of the best 4 cm² solar cell is 19.6%, the fill factor is 79%, the open-circuit voltage is 660 mV and the short-circuit current density is 37.6 mA/cm². We use (700±10) μm-thick silicon shadow masks for the in-line evaporation of the aluminum front grid. The masks are fabricated by a laser process and subsequent anisotropic etching. The resulting finger width of the in-line evaporated fingers is (60±20) μm. The dynamic aluminum deposition rate is 5 μm×m/min for a carrier speed of 0.33 m/min. During in-line evaporation the linear thermal expansion of the mask is limited to (15±5) μm. Our n⁺np⁺-solar cell structure uses 2 high temperature diffusion processes: A P diffusion and a subsequent B diffusion. An essential element of our cell process is the order of these diffusions: We redistribute the phosphorus atoms of the front surface field during the subsequent high-temperature boron diffusion on the rear. The resulting phosphorus surface concentration is (7±1)×10¹⁹ cm³ and the corresponding sheet resistance is (80±10) Ω/□. A reference cell with a front grid statically evaporated in a laboratory-type reactor through a metal mask shows an efficiency of 20.4%. We thus confirm the high quality of the passivated boron back-junction emitter and of the efficiency potential of our solar cell structure.