Organic Resonant Tunnelling Diodes ORTD may lead to improved printed memory, analog oscillators and RTD based circuits. Some may even double as optoelectronic devices as well. Tianhang Zheng, Wallace Choy and Yuxiu Sun of the University of Hong Kong have recently produced versions enhanced by a self-organising silver nanoparticle AgNPs layer inserted onto the indium tin oxide ITO electrode in such devices with the organic material on top. Here we have both an electronic component and a display in one device with parameters that may now become good enough for commercial products.
Improved parameters
In particular, a better Peak to Valley Current Ratio PVCR was sought. The structure used in this work achieved such a better PVCR with repeatable figures of up to 4.6 vs around 2 in previous work at room temperature. Line width was desirably narrow at about 1.4 V and the resonant peak was nearly symmetrical. The mechanism is thought to be based on a strong charge trapping effect in the nanosilver.
Traditionally, ORTDs relying on the phenomenon of Negative Differential Resistance NDR, are constructed in the sequence Organic, Nanoparticle, Organic ONO. This new structure was ITO/Ag NPs/N,N0-diphenyl-N,N0-bis(1-naphthyl)-(1,10-biphenyl)-4,40-diamine (NPB) (40 nm)/tris(8-hydroxyquinoline) aluminum (Alq3)(50 nm)/LiF(1 nm)/Al(100 nm) with different Ag concentrations.
These demonstrated the familiar ORTD at low bias voltage and primitive Organic Light Emitting Diode OLED characteristics at higher voltages. The physics was as follows. The energy levels below give the value of Highest Occupied Molecule Orbital (HOMO) level and the Lowest Unoccupied Molecule Orbital (LUMO) level for the used organic materials employed.
Schematic diagram of the operating mechanisms in hybrid NP/organic devices
a) Charging mode, b) emission mode, and c) energy level alignment.
Source: Adv. Funct. Mater. 2009, 19, 1-6
Substituting the archetypal organic conductor/ semiconductor PEDOT:PSS
(poly(ethylenedioxythiphene):polystyrenesulphonic acid) gave inferior performance.
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