Degradation and morphological disorder in organic light-emitting diodes

Date of Completion

January 2004


Engineering, Electronics and Electrical|Physics, Condensed Matter|Engineering, Materials Science




In this study, the intrinsic oxidative degradation present in organic light emitting diodes (OLEDs), wherein aluminum (III) 8-hydroxyquinoline (Alq 3) is utilized as an electron transporting and light emitting layer is analyzed as a function of the relative hole current in the device. The extent to which holes are allowed into the Alq3 layer in the typical hole transport layer/electron transport layer sandwich structure in OLEDs is controlled by means of the variable conductivity of a salt containing polymer layer (henceforth referred to as the oxidized transport layer or OTL) acting as a hole injection layer. The device lifetime and quantum efficiency were observed to attain an optimum value for injected hole current. Based on these results, the hypothesis that hole current in excess of that required for light emission results in the intrinsic failure of OLEDs is proved. ^ Further improvement in OLED lifetimes was attempted by minimizing thin film morphological instabilities. Co-evaporation of Alg3 with its close adduct aluminum (III) 4-methyl, 8-hydroxyquinoline (4m-Alq3) was utilized to provide entropically driven resistance to crystallization. The device quantum efficiencies thus obtained were the highest relative to 4m-Alq3 or Alq3 OLEDs made without co-evaporation, but the measured lifetimes were not extended over that of Alga based OLEDs. This is due to the poorer chemical stability of the 4m-Alq3 molecule as well as the energetic disorder introduced into the film by virtue of the blending process. ^ The observation of this effect of energetic disorder led to the study of the same present in Alq3 itself. The utilization of 27Al solid-state magic angle spinning NMR enabled the detection of molecular scale disorder in thermally evaporated films of Alq3. Based on variations in the dipole correlation effects resulting from molecular scale disorder, the evolution of the density of states for Alq3 as a function of evaporation rate was estimated. This study clearly proved the existence of traps inherent to organometallic systems comprised of octahedrally complexed asymmetric bidendate ligands and has not been taken into consideration thus far. The implications of these studies in research geared towards realizing more durable organic electron devices is discussed. ^