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

1.2. Comparison of ZnO and SiOx intermediate reflectors on various TCO’s [Dom08,Dom08b]
Silicon oxide intermediate reflectors were inserted at the recombination junction of micromorph tandem
cells in p-i-n configuration deposited by very-high frequency plasma enhanced chemical vapour
deposition (VHF-PECVD). All the layers were deposited in a laboratory scale dual chamber deposition
system. For comparison, intermediate reflectors made out of ZnO were also processed.
In [Dom08, Dom08B], the influence of the nature of the intermediate reflector (SiOx or ZnO) on the
current matching and cell performance was investigated in detail. It was shown (Fig.4a) that both reflectors
acts very similarly, whereas and increased grain size could lead up to 1.8 mA/cm2 in full micromorph
devices. The effectiveness of the intermediate reflector was shown to be higher for small
grain TCO (Fig.4b).
a) b)
Fig.4: a) EQEs of micromorph cells with 150 nm thick SOIR layers deposited on type-A (small size)
and type-C (large size) front ZnO.
b) current in top and bottom cells for various roughness of the front TCO.
1.3 LP-CVD Transparent conducting zinc oxide (ZnO) and TCO’s [Ste08]
The in-house LPCVD ZnO developed at IMT is particularly well suited as electrodes in thin film silicon
amorphous and micromorph solar cells because, in addition to good transparency and conductivity
properties, it possesses an as-grown rough surface texture that efficiently scatters the light. Commercially
available solar modules, in which LPCVD ZnO is already used as electrodes, are submitted to
standard stability test like damp heat exposure. ZnO is known to have its electrical conductivity degraded
by a humid environment. Even if encapsulated modules using LPCVD ZnO:B layers have been
shown by Oerlikon Solar [Kro06] to successfully pass the standard damp heat test (exposure to 85%
humidity at 85°C during 1000 hours), it is important to understand the stability behavior of LPCVD
ZnO:B films in a humid environment as these properties will dictate some of the requirements on the
encapsulation.
a) b)
Fig.5: a) Optical and Hall mobility of a 2 µm thick standard doped LPCVD ZnO:B film in function of
the damp heat exposure (40°C, 100% humidity) time
b) Carrier concentration detected optically and measured by the hall effect [Stei08]
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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
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