  Thermally activated delayed fluorescence (TADF) emitters consisting of donor
and acceptor molecules are potentially highly interesting for
electroluminescence (EL) applications. Their strong fluorescence emission is
considered to be due to reverse intersystem crossing (RISC), in which
energetically close triplet and singlet charge transfer (CT) states, also
called exciplex states, are involved. In order to distinguish between different
mechanisms and excited states involved, temperature-dependent spin-sensitive
measurements on organic light-emitting diodes (OLEDs) and thin films are
essential. In our work we apply continuous wave (cw) and time-resolved (tr)
photoluminescence (PL) spectroscopy as well as spin-sensitive EL and PL
detected magnetic resonance to films and OLED devices made of three different
donor:acceptor combinations. Our results clearly show that triplet exciplex
states are formed and contribute to delayed fluorescence (DF) via RISC in both
electrically driven OLEDs and optically excited films. In the same sample set
we also found molecular triplet excitons, which occurred only in PL experiments
under optical excitation and for some material systems only at low
temperatures. We conclude that in all investigated molecular systems exciplex
states formed at the donor:acceptor interface are responsible for TADF in OLEDs
with distinct activation energies. Molecular (local) triplet exciton states are
also detectable, but only under optical excitation, while they are not found in
OLEDs when excited states are generated electrically. We believe that the
weakly bound emissive exciplex states and the strongly bound non-emissive
molecular triplet excited states coexist in the TADF emitters, and it is
imperative to distinguish between optical and electrical generation paths as
they may involve different intermediate excited states.
