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

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High efficiency solar cell fabrication on plastic
So far cell fabrication has been performed mostly on plastic foils. Table I reports the best laboratory
cell results obtained for n-i-p a-Si:H, µc-Si:H and micromorph structures, deposited on PEN substrates,
whereas part of the development for the a-Si and µc-Si cells has been performed in the frame
of another project (OFEN 101191). The current values are obtained from EQE measurements. More
details are given by T. Söderström et al [5]. The high Jsc values for the �c-Si:H cells (close to
24 mA/cm 2 ) indicates that the nanostructured substrates fully plays its role of light scatterer. Noticeably
no light induced degradation is observed for the µc-Si cell which reaches an efficiency of 8.6% and
is therefore suited for integration into micromorph devices. A remarkable initial efficiency of 10.9% is
obtained for a tandem micromorph cell with a i-layer thickness of 270 nm.
Table I: Best n-i-p laboratory cells obtained at UniNe by VHF-PECVD on plastic foils. The a-Si and
µc-Si cells were redevelopped for incorporation into the tandem micromorph cell.
Jsc Voc FF �
(mA/cm 2 ) (mV) % %
�c-Si:H 23.85 510 70 8.6
a-Si:H 14.3 888 70 8.8
a-Si:H/µc-Si 11.3 1350 71.5 10.9
Figure 3 gives an example of results obtained when comparing the internal quantum efficiency (IQE)
and external quantum efficiency (EQE) of µc-Si:H solar cells on textured and flat plastic substrates in
the spectral range from 350 to 1000nm under short circuit conditions. The thickness of the �c-Si:H n-i-p
stack amounts to 1.55 �m. When compared to cells on non-textured substrate coated with Ag and
ZnO (80 nm), the light conversion of textured cells is enhanced in the spectral region above 700 nm.
We find current densities (Jsc) of 17.9 mA/cm 2 and 22.8 mA/cm 2 , respectively, revealing a gain of 27%.
Figure 3 also includes the IQE’s where the external data have been corrected by data from reflection
measurements of the different solar cells. It is observed that the textured cell suffers from some loss in
the spectral region between 650 and 800 nm. A possible origin could be an increased absorption in
the reflective back contact; Indeed, surface textures have been reported to increase the absorptive
losses in silver and other metallic layers [6].
3) 4)
Fig. 3: External and internal quantum efficiency (IQE) of 1.55 µm thick µc-Si:H cells on textured PEN
(full) and flat reference (dashed lines).
Fig. 4: EQE of a tandem micromorph solar with (dashed lines) and without intermediate reflector (full
lines). The µc-Si intrinsic layer thickness is only 1.2 µm in this example (check ref. [5] for details).
The Jsc of the micromorph tandem cells (11.3mA/cm 2 ) will be further increased by introducing additional
optical features such as an intermediate reflector (IR) between the a-Si:H and µc-Si:H cells.
Preliminary tests of intermediate reflector have been performed. When introducing an SiOx based IR, a
Jsc gain around 1mA/cm 2 is found in the top a-Si:H cell, as shown in Figure 4. The IR is prepared by
PECVD and can potentially be integrated into a production line. The last developments on SiO-based
intermediate reflectors are reported by Buehlmann et al. [7].
Flexcellence, C. Ballif, IMT
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