Assessment of Normal force testing to measure adhesion at organic-inorganic interfaces in organic optoelectronic devices

Abstract

Organic photovoltaic (OPV) devices are emerging as a reliable source of energy due to their combination of unique features. Though desired for their flexibility, low cost, light weight, large area fabrication compatibility and eco-friendly nature, these devices face numerous challenges in achieving high performance and stability. The organic-electrode interface specifically plays a key role in controlling device stability. Recent studies have revealed that the stability is heavily affected by the adhesion of the organic-electrode interface. Measurement of adhesion at these interfaces, however, is a challenging task. In this study, Normal Force Adhesion testing was assessed to determine its suitability for organic devices. In this approach, force is applied perpendicular to the substrate, over the entire surface area of one device (9 mm2) until delamination occurs. In addition to the extracted force-distance curves, images of the interfaces before and after each experiment and a real-time, in-situ video taken from a lateral perspective were examined. All three of these critical pieces of information are necessary to obtain a complete picture of the success of a Normal test. A statistical assessment has been made of the testing apparatus, using many samples (> 50) of one metal - organic combination, Al-Alq3 - an archetypal combination for organic electronics. In addition, five other metal-organic combinations widely used in organic electronic devices, have been chosen to assess the Normal force approach. Due to the ease of testing a large number of samples, Normal force testing does appear to be a viable approach to examining interfacial adhesion, though care must be taken in the experimental design to avoid common experimental failures. Based on the results, a few recommendations have been made to improve the utility of the adhesion testing system for rapid quality testing of organic devices.