The performance of C60‐based organic vertical field‐effect transistors (VFETs) is investigated as a function of key geometrical parameters to attain a better understanding of their operation mechanism and eventually to enhance their output current for maximal driving capability. To this end, a 2D device simulation is performed and compared with experimental results. The results reveal that the output current scales mostly with the width of its drain electrode, which is in essence equivalent to the channel width in conventional lateral‐channel transistors, but that of the source electrode and the thickness of C60 layers underneath the source electrode also play subtle but important roles mainly due to the source contact‐limited behavior of the organic VFETs under study. With design strategies acquired from this study, a VFET with an on/off ratio of 5.5 × 105 and on‐current corresponding to a channel length of near 1 μm in a conventional lateral‐channel organic field‐effect transistor (FET) is demonstrated, while the drain width of the VFET and the channel width of the lateral‐channel organic FET are the same.
Organic vapor‐jet printing (OVJP) is an ink‐free printing method for organic thin films wherein a thermally generated vapor of organic small molecules is jetted through a nozzle by inert carrier gas. OVJP holds a great promise because it uses a field‐proven small‐molecular technology as it yet keeps the benefits of jet printing methods such as being mask‐free, easily scalable, and frugal in material usage. However, fabrication on plastic substrates with OVJPs is challenging due to the excessive heat from the nozzle, preventing its use for flexible electronics. Thermal damage issues in the OVJP are particularly of problem because its nozzle should be close to a substrate for high‐resolution patterning. Here, a simple yet highly effective method is proposed to reduce the radiant heat transfer from the nozzle and thereby enable fabrication of flexible organic devices. Upon application of an electroplated gold layer on the nozzle as a low‐emissivity coating, the temperature of a substrate underneath the nozzle gets significantly reduced below its glass‐transition temperature. A flexible organic light‐emitting diode signage device and an active‐matrix tactile sensor patch are demonstrated on plastic substrates with the proposed approach, demonstrating its feasibility in unlocking the full potential of OVJP methods.
Graphene can be synthesized from polyacrylonitrile (PAN) polymer through pyrolysis. A metal catalyst such as nickel (Ni) is required for the conversion of the polymer to graphene. The metal catalysts can be placed either atop or underneath the polymer precursor. We observed that spatially non-uniform and disconnected graphene was fabricated when PAN film coated with a Ni layer was pyrolyzed, resulting in flake-like graphene. Formation of the flake-like graphene is attributed to the dewetting of the Ni layer coated on the PAN film. Dewetting phenomenon can be reduced by decreasing the pyrolysis temperature, and hence, more uniform graphene could be prepared. The effects of Ni coating thickness and the pyrolysis temperature on the fabricated graphene have been experimentally analyzed.
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