Optimizing through-space charge transfer in thermally activated delayed fluorescence emitters for enhanced OLED efficiency

Abstract

The intramolecular through-space charge transfer (TSCT) excited state has been utilized to develop thermally activated delayed fluorescence (TADF) emitters. However, excessive TSCT can lead to complete electron–hole separation, which diminishes the transition dipole moment, resulting in non-radiative losses and reduced device efficiency. In this study, three TADF emitters (2TPA, 3TPA and 2PhTPA) were synthesized by tuning donor–acceptor spatial configurations and conjugation lengths to modulate TSCT. Strong TSCT in 2TPA and 3TPA induced severe non-radiative decay, yielding OLEDs with low external quantum efficiencies (EQE_max < 8%). In contrast, 2PhTPA optimized exciton dynamics via moderate TSCT and multi-channel reverse intersystem crossing enabled by extended donor conjugation, suppressing non-radiative losses. This design conferred 2PhTPA a high photoluminescent quantum yield, reduced ΔE_ST, and superior EQE_max of 17.9%. The work underscores TSCT regulation as pivotal for balancing radiative and non-radiative pathways in TADF systems. By structurally controlling TSCT intensity to mitigate exciton separation, this strategy advances OLED efficiency, demonstrating molecular engineering's critical role in enhancing optoelectronic device performance.