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 similar to 58 degrees (47-70 degrees) from the surface normal at the air/CBP interface, whereas at the CBP/CaF2 interface, the angle is similar to 48 degrees (43-52 degrees). Such a change in the angle (similar to 10 degrees) suggests that the CBP molecule tends to orient more vertically (edge-on) at the buried CaF2 interface, which may be attributed to the intermolecular pi-pi - stacking interaction between adjacent CBP molecules.