Strong molecular weight effects of gate-insulating memory polymers in low-voltage organic nonvolatile memory transistors with outstanding retention characteristics

Organic nonvolatile memory transistors, featuring low-voltage operation (⩽5 V) and high retention characteristics (>10 000 cycles), are demonstrated by introducing high molecular weight poly(vinyl alcohol) (PVA) as a gate insulating layer. PVA polymers with four different molecular weights (9.5–166 kDa) are examined for organic memory devices with poly(3-hexylthiophene) channel layers. All devices show excellent p-type transistor behavior and strong hysteresis in the transfer curves, but the lower molecular weight PVA delivers the higher hole mobility and the wider memory window. This has been attributed to the higher ratio of hydroxyl group dipoles that align in the out-of-plane direction of the PVA layers, as supported by impedance spectroscopy (dielectric constants), polarized Fourier transform-infrared spectroscopy and synchrotron radiation grazing incidence X-ray diffraction measurements. However, outstanding retention characteristics (<4% current variation after 10 000 cycles) have been achieved with the higher molecular weight PVA (166 kDa) rather than the lower molecular weight PVA (9.5 kDa). Researchers have created an organic transistor that retains memory states for over 10,000 cycles with minimal, sub-5-volt power requirements. Flexible memory devices made from semiconducting plastics are significantly easier to fabricate than silicon-based materials. However, the high voltages usually needed to switch the on/off state of organic transistors are detrimental to device longevity. An international team led by Youngkyoo Kim from Kyungpook National University in Korea used insulating poly(vinyl alcohol) (PVA) films with high molecular weights to reduce the voltage demands of plastic circuits. The researchers inserted PVA films between a transparent electrode and a semiconducting polymer, and then fabricated the stack into a source/drain/gate transistor setup. The most stable data retention characteristics were achieved with the heaviest PVA films — a relationship the researchers attribute to hydroxyl dipole orientations at interfacial boundaries. Low-voltage (<5 V) organic memory transistors with outstanding retention stability (>10 000 cycles), which were achieved by employing both the P3HT channel layer and the PVA gate-insulating memory layer, were strongly influenced by the PVA molecular weights due to the different orientation of hydroxyl dipoles.