Michael L. Blumenfeld

Image states at the interface with a dipolar organic semiconductor

Image states of the dipolar organic semiconductor vanadyl naphthalocyanine on highly oriented pyrolytic graphite are investigated in the submonolayer to few monolayer regime. The presence of a significant molecular dipole in the organized thin films leads to a strong modification of the image states with coverage. In the 0-1 ML regime, we observe successive stabilization of the image state with increasing coverage. Above 1 ML, a new image state develops, corresponding to the screened interaction at the organic semiconductor/substrate interface. We show that the evolution of the observed image states can be understood on the basis of resonance-enhanced anion formation in the presence of strong electric fields. These data represent a step toward understanding the influence of electrostatic fields on electronic structure at organic semiconductor interfaces.

Toward single molecule interfacial charge transfer dynamics in a dye-sensitized solar cell model

Using confocal fluorescence microscopy under ultrahigh vacuum conditions, we investigate the heterogeneous interactions between a perylene bisimide fluorophore and single crystalline Al2O3 (0001) at the single molecule level. We find that the dye molecules undergo reversible transitions to long-lived dark states, with bright and dark periods lasting from several hundred milliseconds to many tens of seconds. These periods are power-law distributed and point towards charge tunneling processes from the molecule to the substrate. The fluorescence intensity levels show a bimodal distribution, indicating different classes of adsorption sites on the sapphire surface. This study is aimed at obtaining a better understanding of interfacial structure and dynamics in order to address ultimately both the growth of organic semiconductor films on inorganic surfaces and the heterogeneous nature of charge transfer in excitonic solar cells.

Interfacial structure and dynamics in molecular solar cells

A novel approach to studying interfacial processes in dye-sensitized solar cells is presented. In order to reduce the complexities of heterogeneity at the heterojunction in such cells, charge transfer is investigated from single fluorescent molecules (alkyl-perylene bisimide) to a highly defined single-crystalline wide-bandgap semiconductor (GaN) using confocal fluorescence microscopy under ultrahigh vacuum conditions. We report detailed studies on the energy level alignment between the perylene bisimide and GaN, characterize the nature of the surfaces involved and demonstrate confocal fluorescence microscopy in an ultrahigh vacuum set-up. The results reported here indicate that the excited state in the chromophore lies at 0 ± 100 meV with respect to the bulk conduction band minimum of GaN.