Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

“…External quantum efficiencies (EQEs) of 30% have been reported recently [1][2][3][4][5][6] ; this is considered to be the light-outcoupling efficiency limit of phosphorescent OLEDs under isotropically oriented transition dipole moments 4 . It has long been recognized that orienting the transition dipole moment of an emitter along the horizontal direction (parallel to the substrate) can enhance the outcoupling efficiency beyond that achieved under isotropic orientation, as demonstrated in polymer-based and vacuumevaporated fluorescent molecule-based OLEDs [7][8][9][10][11][12][13] . Nonetheless, the orientation of the transition dipole moments of iridium complexes used as phosphorescent emitters in efficient OLEDs is typically considered to be isotropic because they are near-globular and small enough to have configurational diversity in their orientational states.…”

Horizontal molecular orientation in solution-processed organic light-emitting diodes

“…4,5 To achieve high electroluminescent (EL) efficiency in OLEDs, a few works have also demonstrated the importance of the horizontal orientation of the transition dipole moments of the light-emitting molecules on the light out-coupling efficiency and the overall performance of the devices. [6][7][8][9][10][11][12][13][14][15][16] For instance, a large variety of vapor-deposited glassy fluorescent molecules in organic thin films have confirmed to show a preferential horizontal molecular orientation. 6,7 The results demonstrated that linear and planar molecular shapes are much more favorable to achieve high molecular orientational order in the plane parallel to the substrate.…”

“…[6][7][8][9][10][11][12][13][14][15][16] For instance, a large variety of vapor-deposited glassy fluorescent molecules in organic thin films have confirmed to show a preferential horizontal molecular orientation. 6,7 The results demonstrated that linear and planar molecular shapes are much more favorable to achieve high molecular orientational order in the plane parallel to the substrate. Recently, phosphorescent OLEDs with horizontally oriented emitters were found to show an external EL quantum efficiency (EQE) higher than 30%.…”

“…17,18 Thus, the observed optical anisotropy indicates that the heptafluorene molecules tend to be slightly oriented parallel to the substrate surface. To quantify the degree of molecular orientation, 6,15,19 we calculated the orientation order parameter (S) using the following equation: S ¼ (k e -k 0 )/(k e þ 2k 0 ) and taking the extinction coefficient values at the peak wavelength. As explained in a previous study, 15 S ¼ À0.5 if the molecules are parallel to the substrate surface, S ¼ 0 if the molecules are randomly oriented and S ¼ 1 if the molecules are aligned perpendicular to the substrate surface.…”

“…The results suggest that the CBP host molecules play an important role on the orientation of the heptafluorene guest molecules during the formation of the organic film by spin-coating. In vapor-deposited glassy lightemitting films, 6,7 horizontal molecular orientation strongly depend on the weak van der Walls interactions between the molecules and the substrate and on intermolecular interactions. In case of spin-coated glassy films, solvent-molecules interactions should also play a key-role on the orientational processes.…”