Scrutinizing Design Principles toward Efficient, Long-Term Stable Green Light-Emitting Electrochemical Cells

Enhancing the efficiency and lifetime of light emitting electrochemical cells (LEC) is the most important challenge on the way to energy efficient lighting devices of the future. To avail this, emissive Ir(III) complexes with fluoro-substituted cyclometallated ligands and electron donating groups (methyl and tert-butyl)-substituted diimine ancillary ((NN)-N-boolean AND) ligands and their associated LEC devices are studied. Four different complexes of general composition [Ir(4ppy)(2)((NN)-N-boolean AND)][PF6] (4Fppy = 2-(4-fluorophenyl)pyridine) with the (NN)-N-boolean AND ligand being either 2,2'-bipyridine (1), 4.4'-dimethyl-2,2'-bipyridine (2), 5.5'-dimethyl-2,2'-bipyridine (3), or 4.4'-di-tert-butyl-2,2'-bipyridine (4) are synthesized and characterized. All complexes emit in the green region of light with emission maxima of 529-547 nm and photoluminescence quantum yields in the range of 50.6%-59.9%. LECs for electroluminescence studies are fabricated based on these complexes. The LEC based on (1) driven under pulsed current mode demonstrated the best performance, reaching a maximum luminance of 1605 cd m(-2) resulting in 16 cd A(-1) and 8.6 lm W-1 for current and power efficiency, respectively, and device lifetime of 668 h. Compared to this, LECs based on (3) and (4) perform lower, with luminance and lifetime of 1314 cd m(-2), 45.7 h and 1193 cd m(-2), 54.9 h, respectively. Interestingly, in contrast to common belief, the fluorine content of the Ir-iTMCs does not adversely affect the LEC performance, but rather electron donating substituents on the (NN)-N-boolean AND ligands are found to dramatically reduce both performance and stability of the green LECs. In light of this, design concepts for green light emitting electrochemical devices have to be reconsidered.