MECHANICAL PROPTERTIES AND FAILURE OF AG NANOWIRES AND TRANSPARENT ELECTRODES STUDIED BY MEANS OF IN SITU TENSILE TESTING

摘要

Organic (opto)electronics have undergone a rapid development in recent years. Applications are for instance organic solar cells (OSCs), flexible displays or "smart clothing", which means clothing with integrated electronics. For such applications it is decisive that the active layers as well as the electrodes withstand mechanical loading. Indium tin oxide (ITO), which is still a common material as electrode, behaves brittle under deformation and is relatively expensive. Metallic nanowire networks, especially silver nanowires (AgNWs), are highly promising alternatives. They fulfill the requirement of a low sheet resistance combined with high transmittance. On a macroscopic scale bending tests as well as tensile tests revealed the excellent performance of Ag NW films, since the increase of resistance is small compared to ITO films. In order to understand failure mechanisms and prospectively optimize the deformation behavior of AgNW electrodes in situ mechanical testing in the transmission electron microscope (TEM) is conducted. In situ tensile tests of single Ag NWs were performed with a Hysitron PI95 TEM Picoindenter™. Moreover, tensile tests of AgNW networks on different flexible substrates such as PEDOT:PSS and PET, which are both used for flexible OSCs, were performed with a Gatan TEM straining holder 654. For these straining tests a dogbone-shaped sample of the AgNW polymer composite is cut in a Focused Ion Beam (Fig. 1(a)). The Ag NW network is prepared by doctor blading. On a nanometer scale tensile tests of single 5-fold twinned Ag NWs show a ductile behavior and a size effect of the strength. The pristine NWs are almost defect free except for the twin boundaries. In situ testing reveals nucleation and propagation of partial dislocations during straining. Tensile tests of Ag NW networks in the TEM provide direct insights in the behavior of the network as well as the response of the whole AgNW polymer composite. A map of the local strain was determined via digital image correlation and depicts a homogeneous strain distribution. The fracture behavior of the Ag NWs is ductile, whereas the PEDOT:PSS film fracture appears to be rather brittle (Fig 1. (b,c)). Since the interaction of mechanical and electrical properties is crucial for applications in (opto)electronics, we are currently working on combining in situ mechanical and electrical measurements in the TEM.