Zinc(ii)-heteroligand compounds for wet processing OLEDs: a study on balancing charge carrier transport and energy transfer

Abstract

Organic light-emitting diodes (OLEDs) are one of the most studied and utilized optoelectronic components in display technology. However, their application in lighting remains limited due to materials costs and a guaranteed feasible deposition technique. To address this challenge, we explored the use of easily synthesized organic molecules capable of complexation with abundant transition metals to enhance their optoelectronic properties, coupled with low-cost wet processing protocols. Four zinc(II) coordination compounds were synthesized and the impact of incorporating two different ligands into a metal center was evaluated in terms of their optoelectronic properties. A photophysical investigation was made, encompassing emission and absorption analyses in both solid-state and thin film configurations. Förster resonance energy transfer (FRET) processes were performed using polyfluorene (PFO) and zinc(II) compounds in a host–guest system, revealing FRET efficiencies ranging from 10 to 68%, depending on the concentration of zinc(II) compounds in the PFO matrix. Subsequently, solution-processed OLEDs were fabricated using PFO:zinc(II) homo (ZnL11 and ZnL22) and heteroligand (ZnL13 and ZnL23) compounds as the emissive layer at a concentration of 1%, following a straightforward architecture, ITO|PEDOT:PSS|PVK|PFO:Zn(II)-compounds|TmPyPB|Ca|Al. The OLEDs achieved external quantum efficiencies (EQE) close to the theoretical limit of these active layers, ranging from 1.2% to 1.8%, with an applicable brightness value (L > 100 cd m⁻²), coupled with low roll-off in EQE values. Notably, the heteroligand coordination compounds exhibited superior device performance, attributed to their high electrical charge-carrier mobilities, trap-state profiles, and density of free carriers, as elucidated by space-charge shallow- (SCLC) and deep-trap (TCLC) transport models.