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

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Introduction / Project Goals
The focus of this project is on up-scaling the deposition processes of flexible Cu(In,Ga)Se2, called
CIGS, solar cells on polyimide foils with emphasis on improving the performance, process reproducibility
and large area deposition capabilities.
We have been developing deposition equipment for large area deposition of CIGS and CdS layers
using our own designs. These in-house assembled equipments can accommodate substrate size up
to 30 x 30 cm 2 , but the area of useful layers in terms of thickness and composition uniformity suitable
for solar cell processing will be smaller because of the size of the deposition chambers. Several components
of the CIGS deposition system, for example the crucially important linear evaporation sources
and substrate heating and moving mechanisms, have been developed by our group through a project
partly supported by the Swiss Federal Office of Energy (SFOE): Project Number 100964/151131
(FLEXCIM).
One objective of the proposed project is to further improve the CIGS vacuum deposition equipment
as well as the CdS chemical bath deposition equipment to develop solar cells on large area substrates
and optimise a process for high efficiency and reliability. It is difficult to predict the efficiency
potential of the in-house developed equipment but our endeavour will be towards 12% efficiency cells
on in-line moving polyimide foils. This efficiency target is somewhat lower than our own world record of
14.1% achieved on a “small static deposition” equipment, but the 12% efficiency target is still higher
than the current state of the art of worldwide competing groups on this topic.
A second objective is to develop high efficiency flexible CIGS solar cells on polyimide foils
coated with alternative electrical back contacts for CIGS instead of the well established Mo layers.
There is three-fold motivation behind the development of flexible CIGS solar cells using alternative
materials such as transparent conducting oxides as back contact:
i) to overcome the performance instability problem associated with the oxidation of conventional Mo
layer;
ii) to provide multi-functionality in solar cell and for futuristic multi-junction (tandem) solar cells;
iii) to reduce the thickness of the CIGS absorber by applying back contacts with higher reflectivity
than of Mo layer.
Successful development of highly efficient CIGS solar cells may solve the problems of degradation of
the back contact in moisture, micro-cracking and de-lamination, especially in flexible cells, and open
further possibilities for device engineering.
Short description of the project
The paragraph gives an overview of the equipment used for this project. Conventional vacuum in-line
processing of large area solar cells requires mainly two kind of deposition equipment:
� Sputter coating of front and back contact with inline moving mechanism.
Sputtering technology is well established since many years and widely used for thin film deposition
of metal and ceramic layers on any kind of substrates. For this project a refurbished production
equipment from MRC (material research corporation), equipped with 3 linear magnetron targets
for back contact deposition (Molybdenum, Indium-tin-oxide) and front contact (ZnO:Al) with a
moving substrate carrier was tested for its homogeneity on 30x30cm 2 and used for processing,
see Figure 1.
� Thermal evaporator for the CIGS absorber layer with an inline sample moving mechanism. This
kind of equipment is not yet commercially available as a packet. One of the most important aspects
is the large area deposition uniformity achieved with the evaporators. Due to the growing interest
in thin film photovoltaics evaporation source suppliers were attracted and start to develop
solutions for this field. ETH has already developed large area sources for this application. Inline
process control is further of crucial importance for reliable processing, See figure 1.
Large Area Flexible CIGS, D. Brémaud, ETHZ
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