Autodiffusion: A novel method for emitter formation in crystalline silicon thin-film solar cells

The in situ formation of an emitter in monocrystalline silicon thin-film solar cells by solid-state diffusion of dopants from the growth substrate during epitaxy is demonstrated. This approach, that we denote autodiffusion, combines the epitaxy and the diffusion into one single process. Layer-transfer with porous silicon (PSI process) is used to fabricate n-type silicon thin-film solar cells. The cells feature a boron emitter on the cell rear side that is formed by autodiffusion. The sheet resistance of this autodiffused emitter is 330 Ω/□- An independently confirmed conversion efficiency of (14.5 ± 0.4)% with a high short circuit current density of (33.3 ± 0.8) mA/cm² is achieved for a 2 × 2 cm² large cell with a thickness of (24 ± 1) μm. Transferred n-type silicon thin films made from the same run as the cells show effective carrier lifetimes exceeding 13 μs. From these samples a bulk diffusion ength L > 111 μm is deduced. Amorphous silicon is used to passivate the rear surface of these samples after the layer-transfer resulting in a surface recombination velocity lower than 38cm/s.