The effects of device structure on the performances of distyrylbiphenyl compounds based organic light emitting diodes

The effects of device structure and applied current density on the performances of blue organic light emitting diodes based on unsymmetrical distyrylbiphenyl compounds were investigated. According to the attached functional groups on the unsymmetrical distyrylbiphenyls, two kinds of compounds were synthesized and evaluated: (Z)-4-(2-p-tolylvinyl)-4′-[1-phenyl-2-(4-tert-butylphenyl)vinyl]biphenyl (U-DSB_tBM) and (Z)-4-[2-(4-tert-butylphenyl)vinyl]-4′-[1-phenyl-2-(4-tert-butylphenyl)vinyl]biphenyl (U-DSB_tBtB). As the emission layers of U-DSB_tBM or U-DSB_tBtB thickened, the operation voltage increased and the emitted light became more blue. After inserting a hole blocking layer, the values of the Commission Internationale de l’Eclairage (CIE) chromaticity coordinates were (0.14–0.21, 0.15–0.28). As the applied current increased, the y coordinate of the sample using U-DSB_tBM increased; however, that of the sample using U-DSB_tBtB decreased regardless of the device structure. The X-ray diffraction data and the atomic force microscopy imaging results suggest that the hole mobility of U-DSB_tBM is greater than that of U-DSB_tBtB, indicating that the recombination region is located in U-DSB_tBM at a low current, and that it expands to the electron transport layer at a high current. Therefore, the device structure and mobility affect device performance, and both factors should be considered to optimize the device properties.

► Two kinds of compounds were synthesized (U-DSA_tBM and U-DSA_tBtB).
► As the applied current increased, the y coordinate of the sample using U-DSA_tBM increased.
► As the applied current increased, the y coordinate of the sample using U-DSB_tBtB decreased.
► The hole mobility of U-DSB_tBM is greater than that of U-DSB_tBtB.
► The recombination region is changed with current density.