Rapid thermal processing for high efficiency silicon solar cells
The formation of pn junctions and surface passivation layers by rapid thermal processing is opening new possibilities for a low-cost and environmentally safe silicon solar cell production. As the main process mechanisms like the diffusion of different dopants and the oxidation of the silicon surface are enhanced, the total process time at high temperature can be kept in the magnitude of one minute, for the realization of emitter, back surface field (BSF) and surface passivation. The surface concentration of the diffused junction is revealed as one of the major parameters to be controlled during the diffusion process, in order to obtain suitable front surface recombination velocities. Simultaneous diffusion of phosphorus and aluminum is used to realize the emitter and back surface field in a single high-temperature step, with optimized gettering effect. During this step the gettering efficiency is given by the formation of a silicon-aluminum alloy, opening the possibility to apply different phosphorus sources to control the emitter surface concentration. Reduced surface recombination velocities and controlled bulk diffusion lengths are observed. Controlling the mentioned parameters on industrial 1 cm CZ material led to 17.5% efficient solar cells on a surface of 25 cm2 .