The effects of charge transport traps on organic light-emitting diode performance

The role of charge carrier trapping on the efficiency of single-layer organic light-emitting diodes (OLEDs) is investigated both theoretically, by incorporating trapping effects into the OLED device model of Davids et al., and experimentally, via the fabrication and characterization of polymeric and oligomeric OLEDs. Carrier trapping directly modifies charge carrier injection and charge carrier mobility and therefore indirectly modifies carrier spatial profiles and carrier recombination rates. Computer simulations of prototypical devices are used to demonstrate the influences of trapping on device efficiency through its effects on charge balance, mobility balance, recombination rate, and the location of the recombination region when quenching at metallic contacts is important. These calculations show that energetic disorder resulting from traps can either improve or degrade device performance depending on a variety of material and device design parameters. The device models in the literature generally neglect contact quenching and treat the organic material as trap free. The results presented in this work emphasize the importance of incorporating trapping and quenching effects in device modeling while demonstrating potentially surprising methods for improving device efficiency. Results of computations on model devices are found to be in good agreement with the experimental studies of Menon et al., in which introduction of energetic disorder using composites of oligomeric and polymeric derivatives of poly(p-phenylenevinylene) (PPV) resulted in significant reduction of OLED efficiency. The primary cause of reduced efficiency was found to be redistribution of the spatial profile of recombination to reduce excited state quenching near the metallic cathode. Based on the model device studies, strategies are suggested for improving efficiency of single layer OLEDs by engineering energetic disorder in the active layer or by compensating for intrinsic disorder by selection of contacts. Guided by the theory, OLEDs based on oligomers and polymers of Oxadiazole PPV were fabricated that exhibit an increase in device efficiency upon deliberate addition of traps via an increase in polymer chain length distribution.