Enhancing Phosphorescence through Rigidifying the Conformation to Achieve High-Efficiency OLEDs by Modified PEDOT

Bis(diphenylphosphinomethyl)phenylphosphine (dpmp)-supported Pt2Au heterotrinuclear complexes [Pt2Au(dpmp)2(C≡CPh)4](ClO4) (1), [Pt2Au(dpmp)2(DEBf)(C≡CPh)2](ClO4) (2), and [Pt2Au(dpmp)2(DECz)(C≡CPh)2](ClO4) (3) were prepared and used in organic light-emitting diodes (OLEDs) as a new class of light emitters, where DEBf = dibenzofuran-4,6-diacetylide and DECz = 3,6-di-tert-butylcarbazole-1,8-diacetylide. Although the flexible structure of Pt2Au complex 1 (λem = 503 nm, Φem < 0.1%) results in weak photoluminescence in fluid CH2Cl2, complexes 2 (λem = 585 nm, Φem = 4.9%) and 3 (λem = 589 nm, Φem = 3.2%) with a rigid conformation give a much stronger phosphorescence. The displacement of two σ-bonded phenylacetylide ligands with a diacetylide ligand such as DEBf and DECz to fasten Pt2Au structures facilitates greatly luminescent emission so that the emissive quantum yield in doping film is as high as 89% for 2 and 93% for 3. As revealed by a theoretical study, the severe structural distortion of diacetylide-linked Pt2Au complexes 2 (λem = 585 nm) and 3 (λem = 589 nm) in a triplet excited state gives rise to significant red shifts of phosphorescent emission spectra relative to that of complex 1 (λem = 503 nm). By means of Pt2Au complexes as phosphorescent emitters, solution-processed OLEDs achieved a relatively low external quantum efficiency (EQE < 9.5%) when commercial poly(ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) was used as the hole-injection layer (HIL). In contrast, the peak EQE was increased to 18.3% with a dramatic increase of efficiency by the use of modified HILs composed of PEDOT:PSS and PSS–Na, which provide a higher work function and a better film morphology.