Effect of methyl-substitution on carbazole/oxadiazole donor-acceptor (D-A) type host materials for efficient solution-processed green organic light-emitting diodes

Two new bipolar hosts methyl-substituted carbazole/oxadiazole donor-acceptor (D-A) type compounds of 2,5-bis(2-(3,6-dimethyl-9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (3,6-MeCzOXD) and 2,5-bis(2-(2,7-dimethyl-9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (2,7-MeCzOXD) are reported. The thermostability and solubility were greatly improved by attaching two methyl groups on carbazole in contrast to that of parent o-CzOXD without any substituents. The different positions of termination between the methyl substituents and the carbazole unite have almost no impact on thermal stability, whereas methyl-substituted carbazole at 3,6-positions shows more effective π-conjugation and significantly better electrochemical stability than 2,7-positions. The triplet energies of 3,6-MeCzOXD and 2,7-MeCzOXD were measured to be 2.56 and 2.64 eV. Solution-processed green PhOLEDs based on the two new o-CzOXD derivatives as hosts were achieved with CE_max of 32 cd A⁻¹ and PE_max of 26 lm W⁻¹. Besides, green TADF devices employing 3,6-MeCzOXD as host material maintain the values with CE_max of 32.4 cd A⁻¹ and PE_max of 17.2 lm W⁻¹.