Since the pioneer work of the Neuchâtel group in the early
90’s,2) hydrogenated nanocrystalline silicon (nc-Si:H), also called
microcrystalline silicon (μc-Si:H), has been widely studied and
used as absorber layer in the bottom cells in multi-junction solar
cells. The advantages of nc-Si:H over a-SiGe:H are low
light-induced degradation and high absorption coefficients in the
red and infrared wavelength region. The higher current capability
and better stability of a-Si:H and nc-Si:H based multi-junction
solar cells imply higher stable solar cell and module efficiencies
than a-Si:H and a-SiGe:H based multi-junction solar cells. With the
great efforts of the community, the a-Si:H and nc-Si:H
multi-junction solar cell efficiency has exceeded the previous
records attained using a-Si:H/a-SiGe:H/a-SiGe:H triple-junction
solar cells. The main disadvantage of nc-Si:H as the absorber layer
in solar cell is the low absorption in the short and middle
wavelength region such that a thick nc-Si:H intrinsic layer is
needed. Therefore, a high deposition rate is essential for cost
effective nc-Si:H solar cell manufacturing. Very high frequency
(VHF) glow discharge has been proven an effective method to
increase nc-Si:H deposition rate, while the reduced wavelength of
VHF signal compared to radio frequency (RF) excitation leads to
more technical challenges in large area uniform deposition.
Therefore, high efficiency, high rate disposition, and large-area
uniformity are still the major tasks for the thin film silicon PV
community. We have focused on these three aspects. In this paper,
we mainly review the issues related to efficiency improvements
and the progresses made in United Solar.