The power conversion efficiency of amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells has exceeded 26% recently.¹⁾ Such high efficiency is essentially achievable thanks to the notable passivation capability of the several-nm-thick amorphous silicon layer deposited on the c-Si surfaces. For further improvement of the performance, it is necessary to understand and to control the structural properties of the a-Si:H/c-Si heterointerface.

Recently, we have found that there is a systematic correlation between the microvoid size determined by the positron annihilation spectroscopy (PAS) and the optical constant of a-Si:H by spectroscopic ellipsometry (SE).²⁾ However, it has been still not clear that this correlation is universally consistent for any a-Si:H thin-films that are fabricated under various preparation conditions using any different apparatus.

In this study, we prepared undoped a-Si:H films on H-terminated FZ Si(111) substrates under various chemical vapor deposition conditions including different source gases (SiH₄ or Si₂H₆) and deposition systems. The average diameter of microvoids (D_void) and the dielectric function (ε = ε₁ - iε₂) in these a-Si:H films were estimated via PAS and SE measurements, respectively. The peak values of ε₂, ε₂^peak, were extracted from the dielectric function determined based on the Tauc-Lorentz dispersion model.³⁾ It is notable that the D_void vs. plots are aligned on a straight line; this result indicates that D_void in undoped nm-thin a-Si:H can be estimated readily from SE measurement results using the found D_voidーε₂^peaklinear correlation. Details in this method and results will be discussed in the presentation.

References
1. Yoshikawa, H. Kawasaki, W. Yoshida, T. Irie, K. Konishi, K. Nakano, T. Uto, D. Adachi, M. Kanematsu, H. Uzu and K. Yamamoto, Nature Energy 2, 17032 (2017).

2. N. Matsuki, A. Uedono, B. E.O’Rourke and N. Oshima., 8th Intl. Conf. on Plasma-Nano Technol. and Sci., 3-O-03 (2015).

3. S. Kageyama, M. Akagawa, and H. Fujiwara, Phys Rev. B 83, 195205 (2011).