Observation of room temperature negative differential resistance (NDR) in organic light-emitting diode with inorganic dopant Original Research Article

Room temperature negative differential resistance (NDR) current–voltage (I–V) curves with high peak-to-valley (PTV) current ratio (>5) were observed first in organic light electroluminescence diodes (OLEDs). The OLED has the configuration of Al/Alq3 (aluminum tris-(8-hydroxygninoline))/TPD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,l′-bipheny-4,4′-diamine)/indium tin oxide (ITO) with TDP doped with iodine (I2) or nitrogen (N2) as holes transport layer (HTL). The high PTV current ratio was achieved by evaporation of TPD under nitrogen gas ambient pressure 1×10−4 Torr and iodine powder in weight ratio 10/1. Under low bias, the recombination between the low-energy holes in the dopant-induced guest-hopping sites and the electrons in lowest unoccupied molecular orbital (LUMO) only generates low-energy phonons, which scatter the carriers, thus leading to the NDR phenomenon. However, for high voltage, the recombination generates high-energy photons, thus enhancing the OLEDs output luminance. The doping of iodine and nitrogen promotes output luminance of OLED to 480%/200% and 200%/165% in magnitude under bias of 7 V/10 V, respectively.