Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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Fig. 1: Refractive index n and absorption coefficient � of doped silicon oxides deposited on glass substrates
as a function of the CO2/SiH4 ratio.
We observe that layers deposited with higher CO2 /SiH4 ratios result in better optical properties, i.e.
lower refractive indexes and lower absorption coefficient. The electrical conductivity however decreases
which, beneath a certain threshold, is detrimental to the serial resistance of the tandem solar
cell. To counter this effect, the ratio of doping source gas (PH3) to SiH4 is increased to maintain a sufficient
conductivity (approximately >10 -10 S/cm, measured in plane).
Incorporation of doped silicon oxides in tandem solar cells
A series of layers with intermediate reflectors of different thicknesses (Fig. 2a) shows the increase of
current in the top amorphous cell. With further optimization of the layer in the micromorph device, initial
efficiency of 12.2% is reached (Fig. 2b). The cell has a thin amorphous i-layer thickness (270 nm),
guaranteeing a low degradation, and an microcrystalline i-layer thickness of 1.8 µm [PB07].
a) b)
Fig. 2: a) External quantum efficiency (EQE) of a micromorph solar cell deposited in the same run in a
large area deposition system from OC Oerlikon with intermediate reflectors (SOIRs) of increasing
thicknesses. The EQE curves show that a part of the light of the bottom cell (without intermediate reflector)
is transferred to the top (with intermediate reflectors). The thicker is the intermediate mirror, the
higher is the current in the top solar cell. b) J-V curve of a 1.2 cm 2 micromorph solar cell incorporating
an intermediate reflector made of boron doped silicon oxide deposited in the same reactor as the
doped layers of the solar cell. The top and bottom current-densities are calculated from the measurement
of the external quantum efficiency curve using AM1.5g solar spectrum.
Thin film silicon solar cells: advanced processing and characterization, C. Ballif, IMT Seite 31 von 288