Evidence of environmental strains on charge injection in silole based organic light emitting diodes

TL;DR

This study combines DFT calculations and experimental techniques to reveal how environmental strains influence charge injection in silole-based OLEDs, showing temperature-dependent behavior linked to molecular relaxation and electronic affinity changes.

Contribution

It provides new insights into how environmental strains affect molecular structure and charge injection in silole-based OLEDs, supported by both theoretical and experimental evidence.

Findings

Structural relaxation affects LUMO levels significantly.

Temperature influences electronic affinity and charge injection efficiency.

Environmental strains induce notable skeletal relaxation in silole molecules.

Abstract

Using d. functional theory (DFT) computations, the authors demonstrated a substantial skeletal relaxation when the structure of 2,5-bis-[4-anthracene-9-yl-phenyl]-1,1-dimethyl-3,4-diphenyl-silole (BAS) is optimized in the gas-phase comparing with the mol. structure detd. from monocrystal x-ray diffraction. The origin of such a relaxation is explained by a strong environmental strains induced by the presence of anthracene entities. Also, the estn. of the frontier orbital levels showed that this structural relaxation affects mainly the LUMO that is lowered of 190 meV in the gas phase. To check if these theor. findings would be confirmed for thin films of BAS, the authors turned to UV photoemission spectroscopy and/or inverse photoemission spectroscopy and electrooptical measurements. The study of the c.d. or voltage and luminance or voltage characteristics of an ITO/PEDOT/BAS/Au device clearly demonstrated a very unusual temp.-dependent behavior. Using a thermally assisted tunnel transfer model, this behavior likely originated from the variation of the electronic affinity of the silole deriv. with the temp. The thermal agitation relaxes the mol. strains in thin films as it is shown when passing from the cryst. to the gas phase. The relaxation of the intramol. thus induces an increase of the electronic affinity and, as a consequence, the more efficient electron injection in org. light-emitting diodes.