Atmospheric plasma surface modification for continuous processing of solar cells
faces similarly to vacuum plasma treatment groups chosen to react within the plasma. Cleaning and functionalizing the surface of
processes—the surface energy of treated ma- The bombardment of the polymer flexible base films and foils in a continu-
terials increases substantially, correspond- surface with energetic particles and ous process prior to panel fabrication to
ing to enhancements in surface cleanliness, radiation of plasma produces the ablation improve thin film adhesions and output
wettability, printability and adhesion and micro-etching effects. The bombard- efficiencies can be critical in achieving
properties. The APT process consists of ment by plasma species is able to create commercialization. Moreover, as Table 1
exposing a polymer to a low-temperature, a nano-roughness on a polymeric film, outlines, avoiding the use of wet chemical
high-density glow discharge (i.e., plasma). for example, that does not modify the cleaning solutions in favor of ‘green’ pro-
The resulting plasma is a partially ionized mechanical bulk properties of the film cess techniques that do not generate VOCs
gas consisting of a mixture of neutral mol- but removes low molecular weight surface or waste effluents can also significantly
ecules, electrons, ions, excited atomic and organics and thereby strongly increases improve commercial returns.
free radical species. Excitation of the gas surface adhesion (Figure 3). Where bond Given the process benefits of APT
molecules is accomplished by subjecting strength is required, atmospheric plasma’s above, an experimental was performed em-
the gas to an electric field, typically at high highly reactive species significantly increase ploying this continuous process. Referring
frequency. Free electrons gain energy from the creation of polar groups on the surface to Table 2, a microcosm of solar cell base
the imposed high frequency electric field of materials so that strong covalent bond- materials were exposed to an APT process
colliding with neutral gas molecules and ing between the substrate and its immedi- for the specified treatment purposes for op-
transferring energy, dissociating the mol- ate interface (i.e., coatings, adhesives) takes timizing interfacial adhesion and improv-
ecules to form numerous reactive species. place. ing solar cell output efficiency.
Interaction of electrons, UV radiation and Surface cleaning via atmospheric The treatment protocols identify the
excited species with solid surfaces placed plasma techniques reduces organic con- base plasma inert gas chemistry, assisted by
in opposition to the plasma results in the tamination on the surface in the form of a reactive oxygen component, which was
chemical and physical modification of the residues, anti-oxidants, carbon residues determined to optimize treatment results
material surface. and other organic compounds. Oxygen- relative to the solar cell application. For
The effect of plasma on a given material based atmospheric plasmas in particular example, specific peel adhesion bench-
is determined by the chemistry of the reac- are effective in removing organics whereby marks were targeted for PVC adhesion
tions between the surface and the reactive mono-atomic oxygen (O+, O-) reacts with to a solvent-base adhesive. Relative to
species present in the plasma. At the low organic species resulting in plasma volatil- cleanliness benchmarks, pre-specified low
exposure energies typically used for surface ization and removal (Figure 4). level organic particle contamination con-
treatment, the plasma surface interactions Solar cell processes transferrable to centrations were established to optimize
only change the surface of the material; the atmospheric pressure plasma processes are lamination adhesions. The required power
effects are confined to a region only several therefore dry etching, surface cleaning, densities applied to each protocol were pre-
molecular layers deep and do not change etching and activation. Layer reductions determined relative to the required surface
the bulk properties of the substrate. The using hydrogen-based atmospheric glow effect by laboratory trials on commercial
surface is subjected to ablation and activa- discharge plasmas is also therefore an roll-to-roll and tangential atmospheric
tion processes (Figure 2). Activation is a employable aspect of the technology. plasma surface treatment systems at the
process where surface functional groups are Enercon Industries pilot facility.
replaced with different atoms or chemical experimentals One specific experimental employed
As described above there are a significant polyimide film which was surface treated
number of solar technology platforms, by APT at a power density of 20W/ft
2
/
many of which are undergoing cost reduc- in using an argon/oxygen plasma. Surface
tions and efficiency improvements to tension was raised from its inherent level
enable or extend their commercial viability. of 40 dynes/cm to water wettability, or
Figure 1. Corona discharge compared with
!
!
atmospheric plasma between planar electrodes.
!
Figure 2. Plasma activation of polymer surface by creation of free radicals through substitution.
Figure 3. Atmospheric plasma micro-etching effect Figure 4. Micrograph of PET film (a) untreated with low molecular weight organic contamination, (b) after
of PE film, 30,000 SEM magnification. corona discharge cleaning, and (c) after oxygen-based atmospheric plasma cleaning.
www.globalsolartechnology.com Global Solar Technology – September/October 2008 – 9
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