A PROCESS SEQUENCE FOR SILICON SOLAR CELLS WITH LOCAL BORON DIFFUSED BACK SURFACE FIELD

摘要

Boron diffusion is considered to be a sophisticated technological process. With a few exceptions, a transfer of this technology into an industrial production environment has not yet been performed, although it is used in nearly all laboratory high efficiency silicon solar cell concepts for back surface field or emitter formation, respectively. We arrange a simple and reliable technology for producing silicon solar cells with boron back-surface field. A special combination of high- and low-temperature process steps is established, avoiding the effect of boron depletion and maintaining the boron surface concentration practically at the initial value after diffusion. We use silicon nitride, deposited by low-temperature plasma-enhanced chemical vapor deposition, as a capping layer for the boron diffused regions, thus acting as diffusion barrier during a subsequent phosphorous diffusion for emitter formation. Solar cells on p-type silicon wafers utilizing the described combination of boron diffusion, silicon nitride deposition, and phosphorous diffusion achieve energy conversion efficiencies of up to 21.6 %. Additionally, we prepare back-contacted n-type solar cells using the proposed process sequence to generate the boron emitter and the phosphorous back-surface field. With this approach we achieve efficiencies up to 19.6 %.