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

Publication Type:

Journal Article

Source:

J. Appl. Phys., AIP Publishing, Volume 79, Issue 10, p.7991-8006 (1996)

ISBN:

0021-8979

Keywords:

1996, 2013 and earlier

Abstract:

We measure the current–voltage and electroluminescencecharacteristics 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.