Impact of structural imperfections on the energy-level alignment in organic films

Abstract

This paper reports that structural imperfection in an organic thin film modulates the electronic structure to result in a serious band bending and change in the energy-level alignment (ELA) at the organic-conductor interface. Ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) were adopted to investigate thickness dependences of the electronic structure of polar phthalocyanine (chlorogallium phthalocyanine) thin films grown on graphite with respect to the film structure. We observed a large bandbending-like shift of occupied molecular-orbital bands toward the Fermi level and a continuous increase in the vacuum level for the as-grown film, whereas these phenomena were considerably suppressed by annealing the film. Both the as-grown and annealed films were characterized as essentially the same stacked bilayer film structure; however, high-resolution UPS and MAES measurements evidenced that there are structural defects in the as-grown film but not clearly in the annealed film, indicating that the defects are the origin of the modulation of the ELA and the band bending. Controlling the structural imperfections is a key issue for the desired ELA in organic devices.

Control of the dipole layer of polar organic molecules adsorbed on metal surfaces via different charge-transfer channels

“…From these results, key factors that determine the orientation of the ClAlPc dipole layer are isolated. In contrast to previous results of ClGaPc on HOPG and ClAlPc on Au(111), 13,22 we find it feasible to grow a ML of either Cl-up or Cl-down configuration on Ag(111), leading to a significant difference in charge transfer at the interface. Further growth of ClAlPc on top of these two different ML templates reveals considerable differences in ELA, up to ~0.4 eV, demonstrating our ability to control the interfacial molecular configuration and thus the system properties by adjusting the growing conditions.…”

“…This is in contrast to prior published results for 1-ML ClAlPc films on Au(111), 22 where a large downward VL shift of −0.45 eV was observed for the AG films, while post annealing led to even larger negative VL shifts of −0.89 eV. On the other hand, in the case for AN ClGaPc on HOPG, 13 positive VL shifts at 1 ML were observed and attributed to the ClGaPc electric dipole of the Cl-up configuration. Both ClAlPc on Au(111) and ClGaPc on HOPG 13,22 are known to host Cl-up and -down orientations for the AG films, where the electric dipole contributions from the two orientations mutually cancel.…”

“…12 Dependence of the ELA on the polar-molecule orientation and packing density has also been reported [13][14][15][16][17][18][19][20][21][22] , and connection to device applications has been demonstrated. [23][24][25] Hosokai et al, who compared the as-grown (AG) and annealed (AN) chlorogallium-phthalocyanine (ClGaPc) films grown on highly oriented pyrolytic graphite (HOPG), reported large band bending of the HOMO state for the AG films with a stacked bilayer structure, but this band bending could be suppressed in the AN films because of the formation of a uniform Cl-up configuration after the thermal treatment; 13 the substrate, HOPG, was however fairly inert in this case, and charge transfer at the organic/metal interface was expected to be minimal.…”

“…On the other hand, in the case for AN ClGaPc on HOPG, 13 positive VL shifts at 1 ML were observed and attributed to the ClGaPc electric dipole of the Cl-up configuration. Both ClAlPc on Au(111) and ClGaPc on HOPG 13,22 are known to host Cl-up and -down orientations for the AG films, where the electric dipole contributions from the two orientations mutually cancel.…”

Accessing Surface Brillouin Zone and Band Structure of Picene Single Crystals

“…higher quality have a lower density of gap states (charge trapping states), which may shift the E F closer to the HOMO [12,16,17]. SC #2 did not show any clear density of occupied states in the energy gap [ Fig.…”