Understanding and implementation of rapid thermal technologies for high-efficiency silicon solar cells

Rapid and potentially low-cost process techniques are analyzed and successfully applied toward the fabrication of high-efficiency monocrystalline Si solar cells. First, a methodology for achieving high-quality screen-printed (SP) contacts is developed to achieve fill factors (FF´s) of 0.785-0.795 on monocrystalline Si. Second, rapid emitter formation is accomplished by diffusion under tungsten halogen lamps in both beltline and rapid thermal processing (RTP) systems (instead of in a conventional infrared furnace). Third, a combination of SP aluminum and RTP is used to form an excellent back surface field (BSF) in 2 min to achieve an effective back surface recombination velocity (S_eff) of 200 cm/s on 2.3 Ω-cm Si. Next, a novel dielectric passivation scheme (formed by stacking a plasma silicon nitride film on top of a rapid thermal oxide layer) is developed that reduces the surface recombination velocity (S) to approximately 10 cm/s on the 1.3 Ω-cm p-Si surface. The essential feature of the stack passivation scheme is its ability to withstand short 700-850°C anneal treatments (like the ones used to fire SP contacts) without degradation in S. The stack also lowers the emitter saturation current density (J_oe) of 40 and 90 Ω/sq emitters by a factor of three and ten, respectively, compared to no passivation. Finally, the above individual processes are integrated to achieve (1) >19% efficient solar cells with emitter and Al-BSF formed by RTP and contacts formed by vacuum evaporation and lift-off, (2) 17% efficient manufacturable cells with emitter and Al-BSF formed in a beltline furnace and contacts formed by SP, and (3) 17% efficient gridded-back contact (bifacial) cells with surface passivation accomplished by the stack and gridded front and back contacts formed by SP and cofiring