Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

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During the year 2008, we investigated further these degradation mechanisms within LPCVD
ZnO lay-
ers. The fitting of optical reflectance spectra in the Near Infra-Red (NIR) wavelength range of LP-CVD
ZnO with Drude-based theoretical curves allowed us to extract the electron mobility within the conical
grains that constitute the LPCVD ZnO layers. This “intra-grain mobility” (or “optical mobility”) was then
compared with the Hall mobility, which gives the mobility value of the electrons through the whole
layer. This comparison was show to allow a separation of the effect of grain boundaries from the effect
of bulk on the conduction mechanisms within the our ZnO films [Ste07].
Fig.5 shows that both mobility values don’t behave similarly after exposure
to damp-heat . Indeed,
whereas the Hall mobility is drastically reduced for increasing damp-heat time, whereas the optical
mobility is almost unaffected. This means that the degradation mechanisms within the LPCVD ZnO
layers occur mainly at grain boundaries. Therefore, we attributed the degradation mechanisms to an
increase of the potential barrier at the grain boundaries, which the electrons have to jump over. Moreover,
we could show that ZnO films that possess a higher doping level are more stable than lightly
doped ZnO films [Ste08]. The measurement of conductivity as a function of the temperature allowed
us to explain this effect by the fact that in highly doped ZnO films, the conduction mechanisms through
the grain boundaries are driven by tunneling transport rather than by thermo-ionic emission over the
potential barrier of the grain boundaries. This means that the increase of the potential barrier due to
degradation mechanisms has only a reduced influence on the electron mobility in highly doped ZnO
layers.
a) b)
Fig. 6: a) Current-voltage characteristics of an non-encapsulated a-Si:H solar cell fabricated with
standard doped LPCVD ZnO:B electrodes for various damp heat exposure (80°C, 100% humidity)
time
b) Same with highly doped LPCVD ZnO:B electrodes
Fig.6
shows the current-voltage characteristics of a-Si:H solar cells deposited with standard (b) and
heavily doped (a) LPCVD ZnO:B as electrodes for various damp heat exposure times (80°C, 100%
humidity, without encapsulation). After damp heat exposure, the solar cell deposited with the standard
LPCVD ZnO:B as electrodes shows electrical characteristics strongly degraded due to an increase of
the serial resistance of the cell. The fill factor (FF) of the cell decreases from 74 % to 28 % after 648
hours of damp heat exposure, leading in efficiency decreasing from 9.4 % to 2.8 %. The solar cell with
heavily doped LPCVD ZnO:B as electrodes shows slight changes in is current-voltage characteristics
after damp heat exposure. The FF varies from 76 % to 70 % and the efficiency slightly decreases from
8.2% to 7.4 % after 648 hours of damp heat exposure. These results demonstrate the possibility to
easily achieved more stable cells using heavily doped LPCVD ZnO:B as TCO, implying potential lower
requirements for the encapsulation of solar modules.
1.4
Other material developments
The development of the various doped
layers, absorber asymmetric intermediate reflectors, is docu-
mented in section 3.
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New processes and device structures for the fabrication of high efficiency thin film silicon photovoltaic modules, C. Ballif, University of Neuchâtel