Boron-Doped Silicon Surfaces From B$_{\bf 2}$H $_{\bf 6}$ Passivated by ALD Al$_{\bf 2}$O$_{\bf 3}$ for Solar Cells

Abstract

A p+ - doping method for silicon solar cells is presented whereby boron atoms from a pure boron (PureB) layer deposited by chemical vapor deposition using B2H6 as precursor were thermally diffused into silicon. The applicability of this doping process for the doped surfaces of silicon solar cells was evaluated in terms of surface morphology after thermal diffusion, the boron dopant profiles, and sheet resistances, as well as the recombination parameter $J_{0{\rm p+}}$ , when the doped layers were passivated by Al2O3 films prepared by atomic layer deposition. Adequate surface passivation could be achieved with a surface recombination contribution to $J_{0{\rm p+}}$ of <20 fA/cm2 for most samples. However, when a boron-rich layer (BRL) was present at the Si surface, a much higher recombination current density was observed, proving that a BRL was detrimental to the high-quality surface passivation of boron-diffused surfaces. It was found that sufficient O2 in the furnace during the thermal diffusion process could eliminate any potential residual BRL, while excessive O2 concentration results in boron depletion and a higher sheet resistance. Therefore, in addition to optimizing the initial PureB layer thickness and thermal budget to control the dopant profiles, the O2 concentration during the diffusion must also be well controlled.