Tuning Emission Colors from Blue to Green in Polymeric Light-Emitting Diodes Fabricated using Polyfluorene Blends

All-Solution Processed Single-Layer WOLEDs Using [Pt(salicylidenes)] as Guests in a PFO Matrix

Herein, we report the synthesis and characterization of two Pt(II) coordination compounds, the new platinum(II)[N,N'-bis(salicylidene)-3,4-diaminobenzophenone)] ([Pt(sal-3,4-ben)]) and the already well-known platinum(II)[N,N'-bis(salicylidene)-o-phenylenediamine] ([Pt(salophen)]), along with their application as guests in a poly [9,9-dioctylfluorenyl-2,7-diyl] (PFO) conjugated polymer in all-solution processed single-layer white organic light-emitting diodes. Completely different performances were achieved: 2.2% and 15.3% of external quantum efficiencies; 2.8 cd A-1 and 12.1 cd A-1 of current efficiencies; and 3103 cd m-2 and 6224 cd m-2 of luminance for the [Pt(salophen)] and [Pt(sal-3,4-ben)] complexes, respectively. The Commission Internationale de l'Eclairage (CIE 1931) chromaticity color coordinates are (0.33, 0.33) for both 0.1% mol/mol Pt(II):PFO composites at between approximately 3.2 and 8 V. The optoelectronic properties of doped and neat PFO films have been investigated, using steady-state and time-resolved photoluminescence. Theoretical calculations at the level of relativistic density functional theory explained these results, based on the presence of the Pt(II) central ion's phosphorescence emission, considering spin-orbit coupling relationships. The overall results are explained, taking into account the active layer morphological properties, along with the device's electric balance and the emitter's efficiencies, according to deep-trap space-charge models. Considering the very simple structure of the device and the ease of synthesis of such compounds, the developed framework can offer a good trade-off for solution-deposited white organic light-emitting diodes (WOLEDs), with further applications in the field of lighting and signage.

Electronic and Optical Properties of Polythiophene Molecules and Derivatives

The electronic and optical properties of polythiophene (PT) for polymer light-emitting diodes (PLEDs) were calculated using density functional theory (DFT) and time-dependent DFT. We calculated the electronic and optical properties of thiophene and PT polymers with degrees of polymerization (DP) from 2 to 30 monomers (T1–T30) and their derivatives. The associated highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy, band gaps, electron orbitals, and molecular structures were determined. As the DP increased, the LUMO energy gradually decreased, and the HOMO energy gradually increased. The band gap of PT approached 2 eV as the DP of the PT polymer increased from 1 to 30. The calculations and exchange–correlation functional were verified against values in the literature and experimental data from cyclic voltammetry (redox potential) and ultraviolet-visible, photoluminescence, and ultraviolet photoelectron spectra. The color of PT PLEDs can be adjusted by controlling the DP of the polymer and the substituents.

Influence of SiO₂/TiO₂ Nanocomposite on the Optoelectronic Properties of PFO/MEH-PPV-Based OLED Devices

The influence of SiO₂/TiO₂ nanocomposites on the performance of organic light-emitting diodes (OLEDs) based on poly(9,9'-di-n-octylfluorenyl-2,7-diyl) (PFO) and various amounts of poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) was investigated. Prior to the fabrication of the OLEDs on indium-tin oxide (ITO) substrates, the hybrids of PFO/MEH-PPV, in the presence and absence of the SiO₂/TiO₂ nanocomposites, were prepared via the solution blending technique. Improvement of the performances of the devices in the presence of the SiO₂/TiO₂ nanocomposites was detected. The existence of the SiO₂/TiO₂ nanocomposites led to better charge carrier injection and, thus, a significant reduction in the turn-on voltage of the devices. The enhancement of MEH-PPV electroluminescence peaks in the hybrids in the presence of SiO₂/TiO₂ nanocomposites is not only a result of the Förster resonance energy transfer, but also of hole-electron recombination, which is of greater significance. Moreover, the existence of the SiO₂/TiO₂ nanocomposites led to a shift of the CIE chromaticity coordinates of the devices.