Dry cleaning and activation of flexible glass using nonthermal plasma before PEDOT:PSS coating
|Fakulta / Pracoviště MU
|Substrate conditioning is an important part of coating processes, and it varies with the materials involved. Coating of ultra-thin flexible glass (UTFG) is specific due to its low weight and high fragility, while its advantageous chemical and optical properties predestine UTFG as a substrate, barrier and packaging material for various optoelectronic applications. Coating techniques with conductive inks are often applied in flexible electronics manufacturing. UTFG has lower surface energy than a common float glass, which normally hinders the coating process, and to improve the coating quality, it is advisable to increase the substrate's surface energy. The present study compares standard wet cleaning method and dry atmospheric-pressure plasma treatment applied prior to deposition of conductive polymer PEDOT:PSS layers. Glass substrates for PEDOT:PSS layers are usually pre-cleaned in ultrasonic bath using three liquids. Plasma generated with dielectric barrier discharges (DBDs) in ambient air was tested as a potential alternative viable for UTFG cleaning. A 30 µm thick UTFG was used as the substrate for two different deposition techniques of various PEDOT:PSS dispersions—spin and spray coating. Two different DBD geometries (volume and coplanar) were compared to study the additional effect of substrate activation induced by plasma. DBDs are promising plasma sources for direct exposure of UTFG due to their nonthermal character. Volume DBD (VDBD) plasma source is the most industrially known configuration for modifying flexible materials. The comparison with coplanar DBD geometry was realized by applying Diffuse Coplanar Surface Barrier Discharge (DCSBD). Both plasma sources were tested in curved implementation, which can be employed in roll-to-roll systems. Plasma was generated in ambient air at atmospheric pressure to meet the ecologic and economic aspects of modern industrial production. Before coatings, surface analyses of UTFG targeted its hydrophilicity, chemical composition, and morphological changes, and were carried out with WCA measurement, XPS analysis and AFM, respectively. The quality of PEDOT:PSS layers was studied with a focus on their uniformity, thickness and electrical conductivity. SEM imaging, profilometry and four-point probe measurements were carried out to explore these properties. Both applied coating techniques showed similarly significant improvement of PEDOT:PSS layers deposited on pre-conditioned UTFG. Plasma treatment exhibited the same cleaning efficiency as ultrasonic cleaning but in a substantially shorter time—in just one second—and therefore proved efficient for PEDOT:PSS coating without any additional requirements for cleaning. Moreover, rapid exposure of UTFG to DBD plasma did not cause any damage or deterioration of its extraordinarily smooth surface. Simultaneously, the gentle cleaning effect of plasma exposure induced the activation of the UTFG surface due to an increase in the present oxygen-based functional groups. The electric properties of PEDOT:PSS layers were in concordance with the increased surface energy of plasma-activated UTFG before coating. Compared with common wet cleaning, the use of VDBD plasma-modified UTFG resulted in a significantly lower sheet resistance of consequently deposited PEDOT:PSS layers.