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

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Fig.12: IV curves for an aSi:H/aSi:H tandem (i-layer thicknesses 150/380 nm) in initial state and after
1000h of light-soaking in standard test conditions. The relative degradation measured is equal
to 15%.
3.3. Solar cells in n-i-p configuration (processes compatible with plastic substrates) [Hau08,
Sod08, Sod08b]
During the reporting period, the efforts have been focussed on the development of tandem cells which
in turn requires the development of certain aspects of the component cells. Therefore, a part of the
studies once more concentrated on µc-Si:H single cells, now with the particular interest of achieving
high current densities for current matching in the tandems. Further, the degradation behaviour of the
amorphous top cells was studied. In particular, this work addressed the relation between i-layer thickness
and light induced degradation for cells on a given substrate structure. Finally, a new class of
intermediate reflector layers was investigated; normally only thin films with conformal coverage are
considered for this layer in order to achieve selective reflection by interference effects. For n-i-p cells it
turned out that optically thick layers are more advantageous, particularly when they develop their own
substrate structure which is the case for thick intermediate reflectors made form LPCVD-ZnO [Sod09].
3.3.1. Microcrystalline silicon
In order to assess the potential in terms of current, a thickness series of single junction microcrystalline
cells on the 2D grating substrate was studied. Figure 13 shows the dependence of the photocurrent
on the i-layer thickness. Initially the current increases rapidly with the thickness of the absorber
layer but it saturates at approximately 25 mA/cm 2 . Once the i-layer thickness exceeds 2.5 µm, further
increment is marginal. Similar results have been observed on glass substrates with hot silver back
contact. Based on this result, a short circuit current density of about 12.5 mA/cm 2 can be expected
realistically in a tandem with ideal current matching.
EQE [%]
1.0
0.8
0.6
0.4
0.2
0.0
1.1 �m
1.5 �m
2.0 �m
2.5 �m
3.0 �m
400 500 600 700 800 900 1000 1100
wavelength [nm]
Photocurrent [mA/cm 2 ]
25.5
25.0
24.5
24.0
23.5
23.0
22.5
1.0 1.5 2.0 2.5 3.0
Absorber layer thickness [�m]
Fig.13: External quantum efficiency of microcrystalline single junction cells on the 2D grating (left).
The right panel shows the dependence of the photocurrent on the absorber layer thickness.
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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