The Role of Electron–Hole Separation in Thermally Activated Delayed Fluorescence in Donor–Acceptor Blends

Publication Type:

Journal Article

Source:

J. Phys. Chem. C, ACS Publications, Volume 119, Issue 45, p.25591-25597 (2015)

Keywords:

2015

Abstract:

Thermally activated delayed fluorescence (TADF) is becoming an increasingly important OLED technology that extracts light from
nonemissive triplet states via reverse intersystem crossing (RISC) to the
bright singlet state. Here we present the rather surprising finding that
in TADF materials that contain a mixture of donor and acceptor molecules
the electron–hole separation fluctuates as a function of time. By
performing time-resolved photoluminescence experiments, both with and
without a magnetic field, we observe that at short times the TADF dynamics
are insensitive to magnetic field, but a large magnetic field effect (MFE)
occurs at longer times. We explain these observations by constructing a
quantum mechanical rate model in which the electron and hole cycle between
a near-neighbor exciplex state that shows no MFE and a separated
polaron-pair state that is not emissive but does show magnetic field
dependent dynamics. Interestingly, the model suggests that only a portion
of TADF in these blends comes from direct RISC from triplet to singlet
exciplex. A substantial contribution comes from an indirect path, where
the electron and hole separate, undergo RISC from hyperfine interactions,
and then recombine to form a bright singlet exciplex. These observations
have a significant impact on the design rules for TADF materials, as they
imply a separate set of electronic parameters that can influence
efficiency.