Interchain interaction, i.e., pi-pi stacking, can benefit the carrier transport in conjugated regio-regular poly(3-hexylthiophene) (P3HT) thin films. However, the existence of the insulating side hexyl chains in the surface region may be detrimental to the charge transfer between the polymer backbone and overlayer molecules. The control of the molecular orientation in the surface region is expected to alter the distribution of the pi electron density at the surface to solve such problems, which can be achieved by controlling the solvent removal rate during solidification. The evidence that the pi-electron density distribution at the outermost surface can be controlled is demonstrated by the investigation using the powerful combination of near edge X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy, and the most surface-sensitive technique: Penning ionization electron spectroscopy. From the spectroscopic studies, it can be deduced that the slower removal rate of the solvent makes the polymer chains even at the surface have sufficient time to adopt a more nearly equilibrium structure with edge-on conformation. Thus, the side hexyl chains extend outside the surface, which buries the pi-electron density contributed from the polymer backbone. Contrarily, the quench of obtaining a thermo-equilibrium structure in the surface region due to the faster removal of the solvent residual can lead to the surface chain conformation without persisting to the strong bulk orientation preference. Therefore, the face-on conformation of the polymer chain at the surface of thin films coated with high spin coating speed facilitate the electron density of the polymer backbone exposed outside the surface. Finally, thickness dependence of the surface electronic structure of P3HT thin films is also discussed.

A systematic density functional theory study of the electronic structure of bulk and (001) surface of transitionmetals carbides Penning ionization electron spectroscopy ͑PIES͒ was adopted to examine surface electronic property of conjugated poly͑3-hexylthiophene͒ ͑P3HT͒ aiming to detect the electronic density tailing outside a polymer surface. The electronic wave function of the highest occupied molecular orbital ͑HOMO͒ state is shielded by side hexyl chain for a P3HT film with edge-on conformation and was not detected by PIES, while it is tailing outside the polymer surface for face-on conformation and was observed clearly by PIES. The presence of HOMO electronic wave function outside the polymer surface makes it possible to form HOMO-HOMO overlapping with overlayer materials, and therefore more efficient charge transfer is expected in a heterojunction structure for device application.

The orientation of vacuum-sublimated thymine molecules on graphite was studied by ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). MAES gave pronounced band intensity enhancement for π molecular orbitals, which confirms the nearly lying-down orientation of the molecules proposed by Uchihashi et al. (Appl. Surf. Sci. 2000, 157, 244). In UPS, a lowering of the vacuum level was observed upon monolayer formation and attributed to inclination of the molecular dipoles from the surface. The tilt angle of the molecular plane was estimated to be 2.1-4.4°by applying the Helmholtz equation to the vacuum level shift.