Reversible Shift from Excitonic to Excimer Emission in Fluorescent Organic Light-Emitting Diodes: Dependence on Deposition Parameters and Electrical Bias.

Organic light-emitting diodes (OLEDs), in general, require multilayer devices and microcavity structures for emission tuning, which increases the complexity and cost of production. Hence, it is imperative to develop techniques for spectral tuning, which employ simplified device structures. In this study, we have selected a tris(8-hydroxyquinolinato)aluminum (Alq): 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1,5,11-(1)benzopyropyrano (6,7-8-,)quinolizin-11-one (C545T)-based OLED and investigated the dependence of the OLED emission on various deposition parameters and the electrical bias. The concentration of the dopant in the emissive layer (EML) was varied from 3 to 50%, and the single dopant emitter as a limiting case was also studied along with studies on the varied deposition rates and EML thickness. By varying the deposition parameters, the emission was observed to change from excitonic green to excimeric yellow. With increased doping concentration, reduction in pure exciton emission with an increase in excimer emission was observed, resulting in electroluminescent spectral red shift. Similarly, electroluminescence spectra have shown different levels of broadening, depending on the deposition rate and thickness of the EML. These effects could be reversed with increasing applied electric field. Thus, it is indicated that, by suitably optimizing the deposition parameters of the dopant material, spectral tuning can easily be obtained, which may form the basis of simplified and cost-effective device structures.