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

4.7 Summary and perspectives for work-package 4
This first year's work brought a handful collection of encouraging results and, in a short time, the
knowledge and competence of our group has made a significant step forward in back end technology.
Nevertheless, further efforts are needed considering that:
� harsh weathering techniques are numerous and, very often, there is no clear relation between
these accelerated ageing techniques and real world weathering,
� the number of available embedding solutions for the solar industry increases constantly,
� sophisticated techniques are often needed to characterize the quality of a laminate.
During the next year, our efforts will, among others, focus on establishing a reliable contacting and
wiring scheme and on developing a small footprint laminator suited for the small sized cells made in
Neuchâtel.
5. INFRASTRUCTURE IMPROVEMENT
5.1 Generalities
In 2008, IMT was granted a special credit of the State of Neuchâtel, which provided the necessary
base founding to proceed to several changes in infrastructure at IMT.
A sputtering tools (3 targets, RF and DC sputtering on 30x30 cm2) for dielectric (TCO) and metallic
layer deposition was purchased and taken into operation. Also, two new fully automated cluster systems
for PECVD systems from are now being installed by the company Indeotec SA. The various layers
produced by the IMT systems can also now be characterized by a new spectroscopic ellipsometry
laser UVISEL 2 of Horiba Jobin Yvon with a spectral range of 210-2100 nm.
In parallel, a set of equipment for module lamination, measurement and reliability testing is now installed
and functional at IMT. A modernization of the PDS system is also underway.
Finally new clean room facilities have been prepared and lab space have been granted to the PV-Lab,
which allowed a significant gain in space for experiments.
In terms of characterization/processes two specifics subjects are presented in more detail here below.
5.2. Degradation batch
The objectives were to assess the effects of operating conditions of thin film solar cell onto its degradation
kinetics. A preliminary experimental set-up was therefore developed to allow the degradation of
cells at open-circuit, Maximum Power Point (MPP) and even short circuit conditions. The preliminary
set-up integrates series potentiometers between the electrodes of the cell under test. The MPP is
tracked by a “Perturb and Observe” (P/O) method by reading out both voltage V and current and by
changing the resistance value to get maximum power P (at dP/dV = 0). Different alternatives will be
studied in the future to permit a fully automated maximum power point tracking and performance
measurement. The P/O method will be first automated and the possibility to use a dedicated integrated
circuit tracking the maximum power point will also be looked at.
Measurements were carried out on two different single junction a-Si:H solar cells each with a thickness
of 300 nm. The two samples were degraded at open circuit and MPP, the second sample being also
degraded at short circuit condition. Results are shown, in Fig.19 for the first 300 hours of degradation
(still under way). The first sample exhibits higher relative degradation (down to 22 %) because of the
use of a carbide (a-SiC:H) buffer layer to improve the cell initial VOC. Different kinetics are observed
for both samples depending on the operating conditions of the solar cell. The slowest degradation is
observed for cells placed at short circuit, while cells operated at MPP have lower degradation than
cells operated at open circuit. Degradation of a-Si:H solar cells performance is due to the increase of
defect density, itself induced by recombination phenomena in the cell. Operation at Voc should hence
lead to higher recombination rates of generated free carriers than at MPP as no current is driven out of
the cell, explaining the different degradations observed with time. The exact kinetics should be confirmed
and detailed by further experiments with the fully automated set-up. The a-Si:H/a-Si:H tandem
degradation (at Voc) is shown for comparison.
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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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