To improve the passivation effect at a-Si:H/c-Si interface in heterojunction (HJ) solar cells, ultrathin SiOx layers with a thickness of approximately 2 nm were pre-formed on c-Si surfaces in chemical solutions. It was demonstrated that the SiOx layers pre-formed in hot de-ionized water and hydrochloric acid solutions improve effective carrier lifetime, and it is further enhanced through a post annealing process. When the thin SiOx layers were applied to HJ solar cells, increase in both Voc and Jsc was achieved, implying the improved interface quality for these HJ solar cells, as compared with the reference without the SiOx layer.

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The hydrogen content (C H ) and microstructure of a hydrogenated amorphous-silicon (a-Si:H) layer fabricated by plasma-enhanced chemical vapor deposition (PECVD) were analyzed to determine the effect of surface passivation on crystalline silicon (c-Si). The ratio of radio-frequency power to gas pressure (C pp in W&Pa %1 ) of the PECVD system is defined as a characterization parameter. It was found that C H and the passivation of a-Si:H layers were sensitively affected by C pp . However, C H remained almost constant at the same C pp even though the PECVD power and pressure were widely varied. We determined that an optimal region exists in the range of 0.75 < C pp < 1.25, where a high implied open-circuit voltage of 732 mV was obtained from a passivated a-Si:H/c-Si/a-Si:H structure, indicating that C pp was a useful parameter for characterizing the surface passivation effect of a a-Si:H layer on c-Si.