The Balance between Charge Mobility and Efficiency in All-Solution-Processed Organic Light-Emitting Diodes of Zn(II) Coordination Compounds/PFO Composites

Zn(II) coordination compounds with asymmetric salicylidene ligands in poly[9,9-dioctylfluorenyl-2,7-diyl] (PFO) composites were used for all-solution-processable white organic light-emitting diode (WOLED) purposes. These Zn(II) complexes were strategically synthesized to optimize their electronic structures, starting from the widespread known salophen core. Vertical excitation energies and natural transition orbital (NTO) densities of Zn(salicylidenes) were evaluated using density functional theory/time-dependent density functional theory (DFT/TD-DFT) at the PBE0/6–311++G(d,p) level of theory, presenting good accuracy to their experimental optical properties. Diodes were assembled with Zn(salicylidenes) dispersed into a PFO-conducting polymer matrix at 0.1, 0.5, 1.0, and 2.5 % mol·mol-1 with the architecture: ITO|PEDOT:PSS|PVK|PFO:Zn(salicylidene)|Ca|Al. The electrical properties of the diodes are dependent on the relative concentration of the coordination compounds in the polymer matrix, where an optimized performance was obtained using diluted samples. The modified Zn(salophen) derivatives (II and III) also presented greater performance compared to the precursor Zn(salophen) (I). In addition, interesting color tunability was obtained depending on the device composition, and for Zn(BTS) with 0.1 and 0.5%, we obtained coordinated color emission very similar to white (0.27, 0.36) and (0.29, 0.41), respectively. The observed color was a composition of the polymeric blue emission and green emission from the coordination compounds. The discussion of the efficiency was based on the determination of the carrier mobilities using the trap-charge limited current (TCLC) model. According to these values, the best performance of the diodes with Zn(BTS) is due to a pronounced increase of the charge mobility compared to the polymer itself and other coordination compounds.