Nanomechanical Testing of Printed Nanolayers for Application on Flexible Organic Electronics Devices

Flexible organic printed electronics devices (FOPEs) such as Organic Photovoltaics (OPVs) and Organic Light Emitting Diodes are paving the way to new advanced low-cost and large area products for solar energy harvesting and lighting, respectively. FOPE devices are comprised by sequential functional nanolayers with a thickness ranging from 30 nm to 500 nm, which are printed the one on top of the other using different solution, printing and curing conditions. The mechanical properties of the individual layers, the adhesion among these nanolayers and their interfaces play critical role to the FOPEs performance and service life.<br/>In this work, we focus on the OPVs devices and we extensively test the mechanical properties of the OPV nanolayers such as the Transparent Conductive Oxide (TCO), the Electron transport layer (ETL), and the active nanolayer with the target to enable understanding on the nanomechanical testing of soft and flexible materials and hybrid (inorganic / organic) interfaces a, and contribute to the performance and durability optimization of new next generation FOPEs. Therefore, we have developed a protocol for the nanomechanical testing of the OPVs’ nanolayers based on the nanoindentation (NI) to probe the mechanical properties of the individual layers as well as the OPV device as a whole and to provides quantitative results at the nanoscale level. The Continuous Stiffness Measurements NI set-up was used to study the mechanical behavior of the nanolayers and their interfaces in the OPV stack in relation to the thickness and the surface roughness of the nanolayers, with the aim of accurately extracting the Elastic Modulus (E) and the Hardness (H) of the nanolayers. Atomic Force Microscopy and Spectroscopic Ellipsometry (SE) was also used to gain information about the surface and the optical properties, respectively, while SE was also used measure the thickness of the printed nanolayers layers.<br/>Two different TCOs were tested, ITO and IMI, grown on top of flexible PET substrate. Both samples showed elastic / plastic deformation. For ITO/PET the E and H values were calculated to 26 GPa and 4.5 GPa, while for the IMI/PET the E=15 GPa and H=3.2 GPa. In both cases, the E and H values were affected by the compliant PET substrate. Both TCO did not de-adhere from the substrate after NI testing. Tin Oxide (SnO) nanolayer was used as the ETL functional nanolayer and it was printed be slot die coating on top of TCO. The NI testing showed that the printed SnO nanolayers did not de-adhere from the flexible TCO/PET substrate, while their E and H values were 9.2 GPa and 0.58 GPa, respectively.