From molecular design to device-relevant behavior: Computational understanding of TADF mechanism

Abstract

Achieving high-performance organic light-emitting diodes remains a significant challenge due to complex interplay between charge transport, exciton dynamics, and light-emission processes. Thermally activated delayed fluorescence (TADF) exploits intramolecular charge-transfer states to harvest triplet excitons in organic light-emitting diodes (OLEDs) through the twisted configuration of the donor and acceptor units. In this investigation, we establish a structure-property relationships in donor (phenyl carbazole)-acceptor (triphenyl amine)-based organic emitters by systematically varying the rigidity of donor and acceptor units. Specifically, we model four different types of combinations: (i) non-rigid donor / non-rigid acceptor, (ii) rigid donor / non-rigid acceptor, (iii) non-rigid donor / rigid acceptor, and (iv) rigid donor / rigid acceptor. Using density functional theory and time-dependent density functional theory methods, we systematically evaluated how these different rigidity patterns influence the geometrical, excited-state, and photophysical properties of organic emitters. By comparing photophysical properties such as intersystem crossing, reverse intersystem crossing, and rates of radiative and non-radiative decay in the above-mentioned classes of molecules, we elucidate the individual and combined roles of rigidity of donor and acceptors governing emission phenomenon. Our results also shed light on which structural component, donor or acceptor, plays a more dominant role in improving the TADF properties and reducing the energy loss pathways. Overall, the insights obtained from our study will provide guidelines to experimentalists for rational design of high performing organic emitters.