Verlag des Forschungszentrums Jülich
JUEL-4254
Mai, Yaohua
Microcrystalline silicon layers for thin film solar cells prepared with Hot Wire Chemical Vapour Deposition and Plasma Enhanced Chemical Vapour Deposition
5, 144 S., 2007
High rate growth process, material quality and related solar cell performance of hydrogenated
microcrystalline silicon (μ-Si:H) were investigated in this work. High deposition
rate (RD) was achieved by very high frequency (VHF) plasma-enhanced chemical vapor
deposition (PECVD) working at high pressure and high power (hphP). Compared to the
μ-Si:H material deposited with conventional low pressure, low power (lplP), the hphP
films showed equivalent optical and electrical properties, indicating their abilities as absorbers
in thin film silicon solar cells. The influences of the deposition parameters on the
solar cell deposition rate and performance were systematically investigated in this thesis.
It was found that optimum cells were always found close to the transition from highly
microcrystalline to the amorphous growth and with medium crystallinity. Variations of
many deposition parameters can tune the crystallinity. Among them, varying silane concentration
(SC) is the most easy and straightforward way. Under optimized conditions,
high efficiency of 9.8% was obtained at RD over 1 nm/s for a single junction p-i-n solar
cell. Efforts were also made to find out the correlation between the material properties
and solar cell performance. The Raman structure depth profile method revealed that hphP
solar cells consisted of a more amorphous incubation layer at the p/i interface, which was
found to reduce the short wavelength light response of the solar cells.
Besides PECVD, Hot Wire (HW) CVD is an alternative method for μ-Si:H deposition.
It was found that HWCVD μ-Si H cells showed higher VOC and FF than the PECVD cells
in a wide range of i-layer crystallinity. This was attributed to the better p/i interface
quality in the HWCVD cells. Inserting an intrinsic microcrystalline p/i interface layer
deposited by HWCVD into PECVD cells nearly eliminated the above differences. Raman
structure depth profile, transmission electron microscopy and selective area electron
diffraction were applied to investigate the structure properties of the solar cells. However,
differences could hardly be found in the already homogeneous i-layers of PECVD and
HWCVD cells. Thus the positive effect of the HW-buffer for facilitating nucleation was
not observed. An amorphous HW-buffer layer in PECVD cells resulted in a more amorphous
p/i interface and an increasing crystallinity along the growth axis. However, such
amorphous interface layer still enhanced the VOC and FF of the resulting cells. Therefore,
it was concluded that structure homogeneity was not the reason for the better performance
of the HWCVD cells. Applying the HW-buffer concept to the PECVD hphP cells, we obtained
a high efficiency of 10.3% at a high RD of 1.1 nm/s. This is the highest efficiency
reported so far for the single junction μ-Si:H solar cells in p-i-n configuration.
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