Molecular Ordering of Conjugated Polymers at Metallic Interfaces Probed by SFG Vibrational Spectroscopy

Organicmetal interfaces play a fundamental role in charge injection to organic electronic devices, and their performance significantly affects the device efficiency. Therefore, these interfaces have been studied by numerous experimental techniques, but most of them are either not strictly interface-specific or require special samples, with ultrathin metal or molecular coatings, which may not be representative of real devices. Here we use sum-frequency generation vibrational spectroscopy (SFG-VS), a surface-specific tool based on nonlinear optics, to probe the molecular arrangement of conjugated polymers (CPs) at metallic interfaces with architectures that are typical of organic devices. The CPs investigated were the widely used poly(3-hexylthiophene) (P3HT) and poly(octylfluorene) (PF8), in contact with either gold (Au) or aluminum (Al). On the basis of a simple model for the optical nonlinearity of polymer chains, we were able to quantitatively determine the orientation of CP chains at metallic interfaces. The results show that the polymer orientation is different in the two types of CP-metal interfaces (CP spin-coated on metal or metal thermally evaporated on the CP film), but in all of them the polymer backbone is not planar, on average, having a preferential torsion either toward or away from the interface. For PF8/Au interfaces, SFG spectra are reported for the first time and indicate spontaneous charge transfer. For the interfaces between PF8 and Al, the polymer chains are nearly parallel to the substrate, with the conjugated rings lying almost flat on the surface. Although charge transfer to/from the metal would be favored by this molecular configuration, the SFG spectra suggest the absence of polymer doping, which may be explained by the coincidence of Fermi levels for Al and PF8. In the case of P3HT, the chains are either quite parallel to the interface for polymer-on-metal samples, with thiophene rings nearly perpendicular to the metal substrates (edge-on orientation), or quite tilted from the interface, mainly exposing their ends to the metal, for thermally evaporated metal-on-polymer samples. For both P3HT/Al and P3HT/Au interfaces, SFG spectra indicate that spontaneous charge transfer is hindered by these molecular arrangements. These results may shed light on the mechanisms for charge injection across organic/metal interfaces and could also guide the design of novel organic devices with molecularly tailored interfaces to improve their efficiency.