Tuning the colour and efficiency in OLEDs by using amorphous or polycrystalline emitting layers

We investigated the nature of the emissive states in newly synthesized cyclometallated Pt complexes containing a chelating 2-pyridyl tetrazolate (2-PTZ) ligand, namely Pt(ppy)(2-PTZ), and Pt(F2ppy)(2-PTZ) as a solid-state phosphor, by examining their structural properties versus their phosphorescence (PH) and electroluminescence (EL) characteristics. It is found that the observed tuning of both PH and EL spectra, their red shift and shortening decay with increasing concentration in the complex blends are due to the competition between three emissive states: monomer, excimer and dimer. The pure dimer emission appeared in neat films, reaching a high PH quantum yield of about 75% and external EL efficiency approaching 10% for the OLED based on the neat Pt(F2ppy)(2-PTZ) complex film as an emitting layer (EML). X-ray diffraction proved the high structural order of the latter thin film. These findings have a direct impact on the design of a new OLED generation based on single phosphor multi-emission controlled by the structural order degree of the EML.