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

Rapid and potentially low-cost process techniques are analyzed and successf ully applied toward the fabrication of high-efficiency monocrystalline Si s olar 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 dif fusion under tungsten halogen lamps in both beltline and rapid thermal proc essing (RTP) systems (instead of in a conventional infrared furnace). Third , a combination of SP aluminum and RTP is used to form an excellent back su rface field (BSF) in 2 min to achieve an effective back surface recombinati on velocity (S-eff) of 200 cm/s on 2.3 Omega-cm Si. Next, a novel dielectri c passivation scheme (formed by stacking a plasma silicon nitride film on t op of a rapid thermal oxide layer) is developed that reduces the surface re combination velocity (S) to approximately 10 cm/s on the 1.3 Omega-cm p-Si surface. The essential feature of the stack passivation scheme is its abili ty to withstand short 700-850 degrees C anneal treatments (like the ones us ed to fire SP contacts) without degradation in S. The stack also lowers the emitter saturation current density (J(oe)) of 40 and 90 Omega/sq emitters by a factor of three and ten, respectively, compared to no passivation. Fin ally, the above individual processes are integrated to achieve 1) > 19% eff icient salar cells with emitter and Al-BSF formed by RTP and contacts forme d by vacuum evaporation and lift-off, 2) 17% efficient manufacturable cells with emitter and Al-BSF formed in a beltline furnace and contacts formed b y SP, and 3) 17% efficient gridded-back contact (bifacial) cells with surfa ce passivation accomplished by the stack and gridded front and back contact s formed by SP and cofiring.