Energy transfer processes in hyperfluorescent organic light-emitting diodes

Hyperfluorescent organic light-emitting diodes (OLEDs) are based on a combination of molecules displaying thermally activated delayed fluorescence (TADF) and of fluorescent emitters embedded into a host matrix; excitons formed on the TADF molecules are expected to transfer to the fluorescent emitters. As a result, device performance strongly depends on the efficiency of the relevant energy transfer processes. Here, we investigate the morphology, excited-state properties, and energy-transfer processes in a ternary TBRb:4CzIPN:mCBP blend by using complementary molecular dynamics simulations and density functional theory calculations. The results indicate that the rate constants for singlet exciton energy transfer from 4CzIPN (TADF) molecules to TBRb fluorescent emitters are about three orders of magnitude larger than both the intersystem crossing (ISC) and radiative decay rate constants of 4CzIPN; thus, the vast majority of the singlet 4CzIPN excitons can efficiently transfer to the emitters. In contrast, the transfer of triplet excitons from 4CzIPN to the emitters is limited due to a fast reverse ISC (RISC) transition. Also, it is found that singlet and triplet energy transfer from mCBP to 4CzIPN and TBRb is very efficient. As a result of quasi resonance between the emissive first excited state and the second triplet state of TBRb, not all triplet excitons that reach TBRb are lost since part of them can convert into singlet excitons via a RISC process.