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

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.