Linking Morphology to Radiative Decay Pathways by Using a Combined Experimental–Computational Study of CzDBA Emission Dynamics

Organic light-emitting diodes (OLEDs) have transformed modern displays, but exciton spin statistics limit their efficiency. When electrons and holes recombine, only 25% of excitons are emissive singlets, while 75% form non-radiative triplets. Phosphorescent OLEDs use heavy metals to harvest these triplets, but metal scarcity, toxicity, and poor blue stability drive the search for organic alternatives. Thermally Activated Delayed Fluorescence (TADF) materials overcome this by converting triplets to singlets through reverse intersystem crossing (RISC) when the singlet–triplet energy gap (ΔE_ST) is small. We show that molecular morphology critically influences the intrinsic fluorescence rate (k_f) of the TADF emitter CzDBA. Time-resolved photoluminescence reveals that amorphous thin films exhibit k_f values nearly two orders of magnitude higher than in dilute toluene, while delayed fluorescence shows non-exponential behavior consistent with a distribution of triplet energies. Combined molecular dynamics and TD-DFT calculations indicate that solid-state confinement narrows the frontier orbital density of states, suppressing dihedral motion and non-radiative pathways. These findings reveal that even amorphous morphology promotes radiative efficiency, highlighting morphology as a key parameter for optimizing TADF-based OLEDs.