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

Abstract

Organic−metal 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.

Elucidation of the bonding of a near infrared dye to hollow gold nanospheres – a chalcogen tripod

“…This approach is similar to recent SFG studies of the C C stretching mode of polythiophene films for determining interfacial ring orientation. 50 – 52 The molecular frame used for the pyrylium core follows standard convention with the molecular c -axis lying along the direction of the net transition dipole (which falls in the average plane of the core), the a -axis also is in the plane of the pyrylium backbone and is oriented along the two aromatic units, and the b -axis is orthogonal to a and c . It is not expected that the selenophene substituents will induce a meaningful distortion to this symmetry and are left out of the SFG orientation calculation for the 1600 cm –1 mode for simplicity.…”

Polymer Solar Cells—Interfacial Processes Related to Performance Issues

“…Finally, non-linear optical spectroscopic techniques such as second-harmonic generation (SHG) and sum-frequency generation vibrational spectroscopy (SFG) have a good potential to investigate interfaces in organic electronics (Motti et al, 2014; Maia and Miranda, 2015), but have not been extensively used so far. Since they rely on broken inversion symmetry, they could, in principle, probe the vertical phase segregation in a photovoltaic blend, since in a homogeneous blend there is inversion symmetry (on the scale of optical wavelengths), while at an interface this symmetry is broken.…”

Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces

“…The results also showed that spontaneous charge transfer between metal and CP was affected by the polymer molecular arrangements (Figure 10c). 100 A study of the epoxy/copper interface correlated the interfacial molecular structures of the epoxy and the adhesion promoters with the mechanical adhesion strength of such interfaces. 101…”

“…99 In another study, a similar sandwiched geometry was used to investigate the molecular ordering of conjugated polymers (CPs) at metal interfaces. 100 It was found that polymer orientations were different at two types of CP/metal interfaces: metal evaporated on the CP, and CP spin coated on the metal. The results also showed that spontaneous charge transfer between metal and CP was affected by the polymer molecular arrangements (Figure 10c).…”

“…The results also showed that spontaneous charge transfer between metal and CP was affected by the polymer molecular arrangements (Figure 10c). 100 A study of the On the left, polymer films with different thicknesses were deposited on the metal surfaces. On the right, polymer films were sandwiched between a metal film and a silica substrate.…”