Efficiency Enhancement of All-Solution-Processed Inverted-Structure Green Quantum Dot Light-Emitting Diodes Via Partial Ligand Exchange with Thiophenol Derivatives Having Negative Dipole Moment

Thiophenol derivatives having a negative dipole moment such as thiophenol (TP), 4-methylthiophenol (4-MTP), and 4-(dimethylamino)thiophenol (4-DMATP) are incorporated into quantum dots (QDs) and the efficiency of all-solution-processed inverted-structure quantum dot light-emitting diodes (QLEDs) is enhanced. The negative dipole moments of TP, 4-MTP, and 4-DMATP are attributed to the enhancement of the efficiency of the QLEDs. The valence band maximum (VBM) of the QDs is upshifted, and the energy gap between the VBM and the highest molecular orbital level of the hole transport layer is reduced to -0.16 from 0.46 eV by the negative dipole moment of thiophenol derivatives. The electron and hole transport of the QDs are accelerated, and charge balance is achieved at 5 V with a reduction of 1.25 V by the thiophenol derivatives. The QLEDs with 4-DMATP exhibit a maximum luminance of 106 400 cd m(-2), a maximum current efficiency of 98.2 cd A(-1), and an external quantum efficiency (EQE) of 24.8%. The EQE of 24.8% is the highest value reported thus far for all-solution-processed inverted-structure single-device QLEDs, to the best of knowledge. Improvement by factors of 2.99, 1.57, and 1.54 are achieved in the maximum luminescence, maximum current efficiency, and EQE respectively, compared with a QLED with OA ligands.