Atmospheric Plasma Processing of Functional Films

We want to acquire fundamental understanding of the atmospheric discharge and the plasma enhanced chemical vapor deposition process (AP-PECVD). Although the discharge is characterized by a spatio-temporal non-uniform appearance, extremely smooth films with excellent moisture barrier properties can be achieved. To obtain better control over the thin film properties and particularly the nano-porous structure of the films we aim for fundamental understanding of the plasma physics and chemistry, the plasma surface interactions and the film formation mechanisms. The research will lead to an ultimate control of the film morphology on different length scales, i.e. from the sub nanometer of the pore size to the kilometer length scale of film rolls.

The current performance of the moisture barriers as deposited under atmospheric pressure conditions is comparable or better than barrier films deposited in traditional vacuum PECVD equipment. In the synthesis of moisture barrier films in roll-to-roll atmospheric pressure plasma we are globally the leading research group.


Schematic drawing of the cylindrical DBD set-up

High Current Dielectric Barrier Discharge

We have particular interest in understanding the physics and chemistry of the discharge in the perspective of novel power supply matching schemes and different reactor geometries. To obtain better understanding of the process we investigate the spatio-temporal discharge behavior in industrially relevant electrode geometry’s by reactive species density measurements using advanced diagnostic tools. Here the research focus is on the 2D spatio-temporal discharge evolution in relation to the precursor dissociation mechanism and gas flow dynamics under varying external plasma parameters as well as on the interaction of the discharge with the surface of the polymer.


Experimental roll-to-roll AP-PECVD set-up

Surface Growth Front Evolution

The physical mechanisms involved in the surface growth front development are not well understood. We study the thin film growth mechanisms using atomic force microscopy (AFM) in the spatio-temporal plasma discharge under atmospheric conditions and we investigate the film nucleation on low density polymer surface under non-uniform discharge conditions. In addition the role of the cylindrical electrode geometry on thin film growth is studied. Surprisingly smooth silica layers can be deposited in atmospheric plasma with no roughness development up to several hundreds of nm. The exceptional smoothening phenomenon observed in the surface growth front evolution of the AP-PECVD process appears to be independent of external plasma parameters and type of precursor which suggests more a physical than a chemical smoothening mechanism.


AFM micrograph of the pristine polymer
surface morphology 


AFM micrograph of the surface morphology
with 200 nm silica


AFM micrograph of the surface morphology
with 5 nm silica

Gas Permeation Mechanisms in Thin Silica-like Films

Here we study the mechanisms of gas permeation in porous and dense silica layers by analysis of the film nucleation and evolution of the microstructure of the film as synthesized by the high current dielectric barrier discharge at atmospheric pressure. A key aspect to achieve low oxygen and moisture permeation rates in silica layers is to control the plasma polymer interactions. Of special interest is the formation of the interphase layer between the low density polymer and the inorganic film, how this interphase layer affects the adhesion and the nucleation of potential diffusion pathways through the barrier film. Effective water vapor transmission rates, expressed in grams per square meter per day, that can be achieved are in the range of 10-3 ~ 10-5 g/m2.day.

Thin film applications

Encapsulation foils for thin film solar cells

Commercial breakthrough of low cost flexible photovoltaic technologies will rely on the advancements in thin film synthesis. Emerging thin film photovoltaic (PV) technologies, such as the dye sensitized solar cell (DSSC), perovskite and cadmium indium gallium selenide (CIGS) solar cell, require low cost availability of a robust, transparent and flexible encapsulation foil to secure a prolonged device life time.


Basic structure of the laminate thin film photovoltaic device 

Membranes

Another important application area of functional thin films is in the field of large scale membrane synthesis. Membranes can separate more energy efficient than traditional technologies as used in the chemical industry but also membranes play a pivotal role in photo electro-chemical conversion, fuel cells, blue energy, CO2 separation as well as in energy saving technologies for industrial waste water cleaning, water desalination, etc.