Electrically tunable resistors realized in two terminal structures seem to be one of the most versatile innovations in the semiconductor industry with many possible applications such as logic circuitry or neuromorphic systems. In particular, inorganic resistive switching devices utilized as non-volatile memory are close to commercialization. Hysteretic current-voltage (IV) characteristics have also been observed from a huge manifold of organic devices employing different π-conjugated small molecules or polymers as well as dielectric materials like poly(methyl methacrylate) (PMMA). However, despite vital academic interest no consistent explanation of the working mechanism of resistive switching elements has been given to date. Various mechanisms are usually suggested, including charge-trapping and charge transfer mechanisms, filament formation, coulomb blockade or molecular conformation changes. Based on our most recent work[1], we here present a set of experiments (I/V characterization, photovoltaic measurements, impedance spectroscopy) to explain unipolar resistance switching: For the first time we are able to unambiguously rule out all charging based models which were held responsible for the switching in organic devices and show that the memory behaviour can be interpreted as the formation and rupture of a conductive pathway (‘filament’). We demonstrate that unipolar resistive switching is a universal and largely material independent (electrodes AND organics) property in electrode/organic/electrode thin-film structures. We also report on the fabrication of organic resistive switches using environmentally friendly inkjet-printing methods and their integration into fully functional hybrid crossbar array structures. Unipolar resistive switches can be integrated into memory arrays by utilizing a diode as selector device. In contrast to a planar transistor, a diode is itself realized as a 2-terminal device allowing for high density ‘4F²’ integration. The requirements to such a diode will be elucidated and a high-performance organic diode, fulfilling all needs, is demonstrated. We will further present novel and unique applications of organic resistive switching devices like flexible flat-panel image and x-ray detectors.
[1] S. Nau, S. Sax, E. J. W. List-Kratochvil, “Unravelling the nature of unipolar resistance switching by utilizing the photovoltaic effect”, Advanced Materials 2014, 26, 2508.
[2] S. Nau, C. Wolf, S. Sax, E. J. W. List-Kratochvil, “Organic Non-Volatile Resistive Photo-Switches for Flexible Image Detector Arrays“ Advanced Materials 2015 (DOI: 10.1002/adma.201403295R2)