Probing the interfacial molecular packing in TIPS-pentacene organic semiconductors by surface enhanced Raman scattering

In organic thin film transistors (OTFTs), the molecular structure of the first few monolayers at the semiconductor-dielectric interface is crucial to the device performance. The assumption of homogeneous molecular packing throughout the thickness of the film is not always valid considering interfacial effects. However, it remains challenging to unambiguously determine the molecular packing at both the top surface and the buried bottom interface, due to the lack of a nanoscopic tool. Here we show that a combination of Raman spectroscopy and surface enhanced Raman scattering (SERS) provides a means for effective characterization of the interfacial packing in 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) films. We observed that the TIPS-pentacene crystal lattices assume a non-equilibrium packing state near the substrate interface, which gradually relaxes towards equilibrium packing near the top interface. Our investigation suggests the existence of non-equilibrium molecular packing for TIPS-pentacene.