Samsung Advanced Institute of Technology

In thermally activated delayed fluorescence (TADF) materials, the fluorescence efficiency is enhanced through reverse intersystem crossing (RISC) from triplet to singlet excitons. The performance of organic light emitting devices (OLEDs) based on TADF materials highly depend on the thermal conversion efficiency of the lowest triplet exciton T₁ into the lowest singlet exciton S₁, which relates to the energy difference, delta E (S1-T1). Here we have investigated the RISC process in two TADF compounds having vastly different   using magneto-optical spectroscopies that include magneto-photoinduced absorption (MPA) in films, and magneto-electroluminescence in OLEDs. The photoinduced absorption spectrum of the fast RISC material clearly shows that both singlet and triplet excitons coexist under steady state conditions. We also found that the magnetic field in films and OLEDs does not substantially influence the RISC rate in these compounds.  However the external field substantially influences the injected polaron pair species before they decay into S₁ and T₁ excitonic states in the emissive TADF layer.