Through Positional Isomerism: Impact of Molecular Composition on Enhanced Triplet Harvest for Solution-Processed OLED Efficiency Improvement

Abstract

We present a strategy for developing an in-depth understanding of the synergy between triplet energy (ET) and structural composition of molecular hosts for organic light-emitting diodes (OLEDs) initially through virtual screening of molecules with the help of density functional theory (DFT) followed by experimental validation. Four solution-processable hosts belonging to two groups of positional isomers based on a similar constitutional core have been theoretically screened and employed in OLED devices. Remarkably, substitutional change of the cyano moiety from para- to meta-position restricts the orbital overlap between the cyano group and other parts of the molecule, which enhances the ET of the compound, leading to better device performance. An excellent increment in the power efficacy of the OLED device has been observed when a para-cyano (ET = 2.59 eV)-based host is replaced with its meta-counterparts (ET = 2.72 eV). The constitutional factors of the molecules also highly influenced the efficacy as we observed around 1.5 times increment in the power efficacy when one phenyl unit of the host AT110 is replaced by one carbazole functionality in AT111 although both hosts possess similar ET (2.72 eV). The solution-processed fabricated best green device exhibited a maximum power efficacy of 72.3 lm W–1 and current efficacy of 69.9 cd A–1 with driving voltage of 2.8 V.