Abstract

Optically and electrically detected magnetic resonance studies of luminescent π-conjugated thin films and organic light-emitting devices (OLEDs) have provided striking insight into their various excitations. The salient feature in the photoluminescence (PL)-detected magnetic resonance (PLDMR) of luminescent π-conjugated polymers and small molecules is the positive (PL-enhancing) spin 1/2 polaron resonance. Although its nature is still debated, there is very strong evidence that it results from reduced quenching of the luminescent singlet excitons (SEs) by reduced populations of polarons and triplet excitons (TEs). The reduction in these populations is due, in turn, to the well-known strongly spin-dependent quenching of TEs by polarons. This is also plausible due to the simple observation that, under typical steady-state conditions, the populations of these species overwhelms the SE population (by > 100 fold under photoexcitation, by > 10⁴ fold under carrier injection). The positive (PL-enhancing) spin 1 TE resonances are similarly due to reduced quenching of SEs by TEs. These scenarios lead to the conclusion that the internal quantum efficiency of fluorescent (as opposed to phosphorescent) OLEDs is limited to 25%. The negative (PL-, electroluminescence (EL)- and electrical current (E)-quenching) resonances observed under short wavelength photoexcitation or carrier injection are due to enhanced formation of bipolarons or trions at specific sites, and their significance is also discussed.

© 2010 Optical Society of America