Adhesion Promoting Polymer Interlayers for Ag Layers Deposited in OLED Processing

Abstract

One of the main drivers for organic electronic research are the backplanes for flexible active-matrix displays using organic light-emitting diodes (AM-OLEDs). In our work, we processed organic thin-film transistors (OTFTs) at low temperatures (<160°C) on poly(ethylene naphthalate) (PEN) foils. There are several challenges in the integration of OLEDs on top of OTFT backplanes on a foil. One of them is the interlayer between the OTFT backplane and the OLEDs. For this layer, low-temperature crosslinkable, chemically resistant polymers are required, which, on the one hand, provide proper electrical insulation to OTFT and OLED devices and, on the other hand, can be processed reliably and provide good adhesion to the OLED anode. From the point of reflectivity and, therefore, light emission efficiency, Ag would be a preferred option. The challenge from the processing point of view is the poor adhesion of evaporated or e-beam deposited Ag on most surfaces. Commonly used microelectronic approaches such as sputtered metal or Ar pre-sputtering cannot be applied because of resulting surface leakage paths on the organic dielectric caused by dangling bonds. To address this issue, we tested a variety of low-temperature crosslinkable polymers (e.g., SU-8, parylene) regarding adhesion, roughness and processability and we measured the adhesion of the Ag deposited at different thicknesses using the Scotch tape test (ASTM D 3359). The best adhesion properties were obtained with parylene N allowing Ag layers with thicknesses up to 200 nm and surface roughness around 7–8 nm.