Efficient green organic light-emitting diodes with stericly hindered coumarin dopants

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Abstract
We have discovered two stericly hindered green coumarin derivatives which showed significantly better thermal stability and overall electroluminescence(EL) performance than those of the corresponding 3-(2-benzothiazolyl)-7-(diethylamino)- 2H -1-benzopyran-2-one (C-6) and 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl- 2,3,6,7-tetrahydro- 1H,5H,11H- benzo [l] pyrano [6,7, 8-ij] quinolizin-11-one (C-545T). The strategic incorporation of bulky spacer t-butyl groups in these dopants considerably delays the onset of concentration quenching. In particular, 10-[2 (5,7- di - t - butyl ) benzothiazolyl ]-1,1,7,7- tetramethyl -2,3,6,7- tetrahydro -1H,5H,11H- benzo -[l] pyrano [6,7,8-ij] quinolizin -11-one (C-545TB) with its superb thermal stability (T g =142 ° C ; T d =327 ° C ), high luminance efficiency (∼13 cd/A), good Commission Interationale d’Eclairage (CIE) coordinates (x=0.30; y=0.64) and a nearly flat EL efficiency dependence on drive current stands out as one of the best choice of green dopants for use in the passive-matrix organic light-emitting diode displays.