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

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The Project aims to develop a manufacturing-oriented process for all different scribing steps, which
consists in the choice of appropriate light sources and beam displacement for high-speed processing
with exact positioning, and in more detail:
1. Identification of cost effective laser and experimental strategies for scribing of constituent layers
of CIGS solar cells.
2. Laser scribed Mo metal layers on flexible substrate (thin polyimide and metal foils). Evaluation of
processing speed and costs.
3. Laser scribed CIGS layers on Mo coated flexible foil. Evaluation of processing speed and costs.
4. Laser scribing of transparent conducting oxide (ZnO:Al) front contact layers on CIGS/Mo coated
flexible substrates. Evaluation of processing speed and costs.
5. Monolithically integrated CIGS solar modules on flexible substrates
6. Proof of concept for an automated laser scribing set-up suitable for implementation in roll-to-roll
manufacturing of monolithic solar modules.
Work performed and results obtained
The above-mentioned criteria led to the choice of an pulsed laser-system, delivering ultrashort light
pulses at high pulse energies and high repetition rates, thus enabling high-speed processing. Important
criteria included the flexibility of the system concerning available wavelength and customizable
repetition rates.
Different options are also available for beam displacement for positioning of the laser spot on the surface
of the solar module. The need for high-speed processing led to the choice of a scanner system
allowing for line-scribing speeds of the order of meters per second and positioning accuracy of several
micrometers.
The system components have been purchased and the full system has been installed offering maximum
flexibility concerning laser wavelength, spot size and sample size/geometry. The installed system
was first employed for the identification of process windows for scribing of individual layers. As mentioned
earlier, the limits were given by electrical parameters (scribes P1, P3 should separate
back/front contact whereas P2 must leave the conducting back contact untouched). Apart from electric
characterisation, optical and electron microscopy are used to characterize scribe quality. For the
P3 scribe, chemical analysis was employed to identify process parameters limiting the transformation
of the CIGS absorber into a conducting phase, which would short-circuit the solar module. Possible
process windows were identified for all three scribing steps.
In a second step, the analysis was concentrated on the characterisation of the full scribing and deposition
process chain with main focus on the electrical properties and their influence on the performance
of monolithical integrated modules. More in detail, analysis includes the intended functionality of the
single scribes (good electrical conductivity of front contact deposited on CIGS-covered P1 scribe, absence
of short-circuits after deposition of full stack for P1 and P3 scribe, low resistance of the contact
zone of front and back contact for P2). Several optimizations were needed to limit detrimental effects
on module performance. Some of these optimizations are still under investigation. Figure 2 shows
SEM pictures of a P2 and a P3 scribe after deposition of the full layer stack.
In parallel, the setup was equipped for repositioning of single scribing steps, allowing for module
preparation and characterisation with minimized distance between scribes. This was done by installation
of an imaging system to position scribes with respect to detected scribes of a preceding step. The
performance of monolithically integrated solar modules on polymer foils is currently under optimization,
as is the processing width.
The available imaging system also would allow for an automated repositioning of the scribe lines,
based on image recognition. The image quality is currently assessed to allow for this application
Picture of a monolithically interconnected CIGS solar module on polymer film is shown in figure 3.
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Laser patterning of Cu(In,Ga)Se2 solar cells on flexible foils for monolithic integration, A. N. Tiwari, ETHZ