Si heterojunction solar cells with a-Si passivation films formed from liquid Si

We fabricated Si heterojunction (SHJ) solar cells with a-Si passivation films formed from liquid silicon, and confirmed the actual operation of SHJ solar cells with liquid-Si-based a-Si films for the first time. Cyclopentasilane (Si
₅
H
₁₀
, CPS) was used as a source material and intrinsic a-Si (i-a-Si) films were formed from CPS vapor through thermal chemical vapor deposition (CVD) at an atmospheric pressure in nitrogen atmosphere. We refer to this novel deposition technique as liquid-source vapor deposition (LVD). Interestingly, LVD can realize the simultaneous deposition of a-Si films on both sides of a c-Si wafer. i-a-Si films with a thickness of about 15 nm were formed on (100)-oriented mirror-polished n-type c-Si wafers by LVD at substrate temperatures of 360-400 °C. Their effective minority carrier lifetimes (r
_eff
) of the c-Si wafers with LVD i-a-Si films evaluated by microwave photoconductivity decay (μ-PCD) were more than 300 μs. The LVD a-Si passivation films showed high thermal stability against post-annealing at 360 °C or more. This property is particularly important to form additional films on the LVD i-aSi films by using liquid Si which requires annealing at high temperatures of around 400 °C. We also confirmed that LVD a-Si passivation films have high stability against air exposure and light soaking. The SHJ solar cells fabricated in this study consisted of LVD i-a-Si passivation layers, p- and n-type a-Si films formed by catalytic chemical vapor deposition (Cat-CVD), sputtered ITO films, and comb-shaped Ag electrodes. The SHJ cells fabricated showed actual photovoltaic property with an open-circuit voltage (Voc) of more than 0.6 V under 1-sun illumination. This result indicates that SHJ solar cells can be fabricated by using liquid Si, which does not require the usage of high-cost vacuum systems.