Rearrangement of the molecular orientation of Alq3 in organic light-emitting diodes under constant current aging investigated using sum frequency generation spectroscopy

Spontaneous formation of metastable orientation with well-organized permanent dipole moment in organic glassy films

The performance of organic optoelectronic and energy-harvesting devices is largely determined by the molecular orientation and resultant permanent dipole moment, yet this property is difficult to control during film preparation. Here, we demonstrate the active control of dipole direction-that is, vector direction and magnitude-in organic glassy films by physical vapour deposition. An organic glassy film with metastable permanent dipole moment orientation can be obtained by utilizing the small surface free energy of a trifluoromethyl unit and intramolecular permanent dipole moment induced by functional groups. The proposed molecular design rule could pave a way toward the formation of spontaneously polarized organic glassy films, leading to improvement in the performance of organic molecular devices.

Vibrational Sum Frequency Generation Study of the Interference Effect on a Thin Film of 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl (CBP) and Its Interfacial Orientation

Molecular organization of vapor-deposited organic molecules in the active layer of organic light-emitting diodes (OLEDs) has been a matter of great interest as it directly influences various optoelectronic properties and the overall performance of the devices. Contrary to the general assumption of isotropic molecular orientation in vacuum-deposited thin-film OLEDs, it is possible to achieve an anisotropic molecular distribution at or near the surface under controlled experimental conditions. In this study, we have used interface-specific vibrational sum frequency generation (VSFG) spectroscopy to determine the orientation of a low-molecular weight OLED material, 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP), at free (air) and buried (CaF₂) interfaces. VSFG spectra were measured at four different polarization combinations for five different thicknesses of the CBP film. The spectral shift and VSFG intensity changes with the film thickness can be accurately modeled by considering the optical interference effect of the signals coming from the CBP/air and CBP/CaF₂ interfaces. A global fitting of the experimental spectra for all thicknesses along with theoretical simulations reveal that the long molecular axis of CBP is oriented at an angle of ∼58° (47-70°) from the surface normal at the air/CBP interface, whereas at the CBP/CaF₂ interface, the angle is ∼48° (43-52°). Such a change in the angle (∼10°) suggests that the CBP molecule tends to orient more vertically (edge-on) at the buried CaF₂ interface, which may be attributed to the intermolecular π-π stacking interaction between adjacent CBP molecules.