Graphene, a two-dimensional thin membrane, has unique electrical and mechanical properties. It has the potential to replace the conventional electrode materials used for flexible optoelectronic and photovoltaic devices. Furthermore, it has also been shown to be impermeable to the smallest gas molecules, and hence it can also provide additional functionalities in device architectures. Graphene Oxide (GO) or oxidised graphite, provides a chemical route to synthesize graphene derivative known as reduced graphene oxide (rGO). rGO has been researched extensively for applications similar to that of graphene, as it retains its superlative properties. In addition to this, solution processability of rGO assists in its scalable fabrication on substrates of any given topography. This work is focused on the application of rGO/GO for stretchable strain sensors and stable optoelectronic devices. In this talk, two such applications will be presented, which are briefly discussed as following.
In the first work, we consider graphene oxide in the electrically conducting reduced form. The reliability aspect of rGO films under mechanical deformation, for application as an electrode in flexible devices, was studied. In this regard, transport measurements were performed on the solution processed rGO films on PDMS substrates, under variable uniaxial strain. Investigation by optical microscopy and scanning electron microscopy (SEM) revealed that quasi-periodic cracks are formed in the rGO films upon straining, which result in the observed material failure. It is demonstrated that for an optimal thickness (3-coat), the films behave as strain-resistant, while for all other values it becomes strain-responsive, attributed to a favourable combination of crack density and width. This study of the film thickness dependent response and the crack propagation mechanism under strain is useful for rationalizing the application of layered graphene-like systems for flexible optoelectronic and strain sensing applications. When the thickness is tuned for enhanced extent of crack propagation, strain-sensors with gauge factors between 500 – 1500 are realized with the same material. When thickness is chosen to suppress the crack propagation, strain-resistive flexible photoconductor is realized.
In the second work, electrically non-conducting F-GO (Functionalized GO) is used as a moisture barrier film to protect methyl ammonium lead triiodide (MAPbI3) perovskite films from moisture induced degradation. MAPbI3, though being at the focus as a next generation material for
photovoltaic/optoelectronic devices, currently faces several challenges, one of them importantly being the moisture induced degradation. In an attempt to solve this issue, F-GO–polymer nanocomposite is used as a moisture barrier in the device.