Nanoparticle and 2-D materials are useful for printed electronics as they can be incorporated in all kind of inks which are compatible to various printing processes. Metallic nanoparticles based inks are already well established for conducting layers, and now the concept is being tried with oxide nanoparticels and 2-D materials, etc. Avoiding agglomeration of nanoparticles and low temperature annealing are two key challenges in this effort. This presentation discusses results on two such inks based on ZnO nanoparticles and graphene. Our interest is to develop these inks for fabricating all solution processed polymer light emitting diodes (PLEDs).
Solution processed ZnO layer is being developed as an alternative to LiF which acts as an electron injection layer and is thermally evaporated in our standard PLED stack (ITO/PEDOT (40-60nm)/MEH- PPV(60-80nm)/LiF/Al. Many applications use zinc oxide film prepared by sol-gel technique using 2-methoxyethanol and ethanolamine. This technique requires annealing at temperatures above 250o C which can damage organic layers in PLEDs. In order to reduce the annealing temperature, we have adopted the approach to first synthesize nanoparticles of ZnO which can be annealed at low temperatures. We varied temperature during synthesis from 25, 40 and 60o C, TEM image analysis shows nanoparticle size changes from 12, 6 and 4nm, respectively. Dynamic light scattering measurements show size in 100s of nm indicating aggregation and poor film formation. To suppress aggregation repeated purification (up to three times) process led to narrow particle size distribution and reduced particle size. This colloidal solution was used to make thin films on glass using spin coating. Film morphology and structure is studied using AFM and XRD and thickness is measured using profilometer.
Graphene is an excellent candidate for transparent conducting electrodes because of properties such as excellent optical transparency, high electrical conductivity together with flexibility. For this purpose, optimization in ink formulation will be reported to form films of graphene and PEDOT:PSS.