A Vibrational Sum Frequency Generation Study of the Interference Effect in a Thin Film of 4,4'-Bis(N-Carbazolyl)-1,10-Biphenyl (CBP) and the Interfacial Orientation.

Molecular organization of vapor-deposited organic molecules in the active layer of organic light emitting diodes (OLEDs) have been a matter of great interest as they directly influence 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 the free (air) and the buried (CaF2) interfaces. VSFG spectra were measured at four different polarization combinations for five different thicknesses of the CBP film. The spectral shift and the VSFG intensity change 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 the theoretical simulation reveal that the long molecular axis of CBP is oriented at an angle of ~580 (470 - 700) from the surface normal at the air/CBP interface, whereas at CBP/CaF2 interface, the angle is ~480 (430 - 520). Such a change in angle (~100) 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.