Relationship between electroluminescence and current transport in organic heterojunction light-emitting devices

TLDR

The study examines how the intrinsic link between electroluminescence and current transport can guide OLED efficiency and voltage optimization. The authors measured current–voltage and electroluminescence of single‑heterojunction vacuum‑deposited OLEDs across diverse materials, temperatures, and currents. The current is limited by a dense exponential trap distribution below the LUMO with a 0.15 eV depth, and in metal‑quinolate devices electroluminescence originates from Frenkel exciton recombination driven by Coulombic relaxation of trapped electrons and injected holes, a behavior that semi‑empirical modeling attributes to conformationally induced exponential state distributions below the LUMO.

Abstract

We measure the current–voltage and electroluminescence characteristics of single-heterojunction, vacuum-deposited organic light-emitting devices (OLEDs) over a wide range of materials, temperatures, and currents. We find that the current is limited by a large density of traps with an exponential energy distribution below the lowest unoccupied molecular orbital. The characteristic trap depth is 0.15 eV. Furthermore, in metal–quinolate-based devices, electroluminescence originates from recombination of Frenkel excitons, and its temperature dependence is consistent with the excitons being formed by Coulombic relaxation of the trapped electrons with holes injected from the counter electrode. By semiempirical molecular orbital modeling, we find that the trap distribution obtained from the current–voltage characteristics is consistent with a distribution in the metal–quinolate molecular conformations which result in a continuous, exponential distribution of allowed states below the lowest unoccupied molecular orbital. We discuss the implications of the intrinsic relationship between electroluminescence and current transport in OLEDs for the optimization of efficiency and operating voltage in these devices.