Abstract
Excitons dominate the optoelectronic properties of organic devices. Normal organic light-emitting devices (OLEDs) generally linearly use exciton to generate electroluminescence (EL), in which one photon is produced from per exciton. Eliminating bi-excitons quenching is of great concern for efficient devices. Here, we have theoretically and experimentally investigated a non-linear EL process mediated by triplet-triplet annihilation (TTA) in rubrene/C-60-based OLEDs. This non-linear EL process realizes the electrically pumped up-conversion by promoting two low-energy triplet excitons into one high-energy exciton, thus extremely lowering the working voltage of resulting OLEDs. It is clearly seen that this up-conversion EL intensity essentially exhibits two distinct regimes at different current densities, i.e. a quadratic dependence at low current density where mono-triplet decay is dominant, and a linear dependence at high current density where bi-triplet decay becomes dominant. These results further our understanding of the non-linear optoelectronic process. In particular, our results demonstrate that energetically utilizing the TTA provides possibility of fabricating low-driving voltage, high efficiency OLEDs via non-linear EL process. (C) 2017 Elsevier B.V. All rights reserved.