Experimental Determination of the Excited-State Polarizability and Dipole Moment in a Thin Organic Semiconductor Film

Abstract

We report the evolution of the electronic structure of excited (sub)monolayer films of vanadyl naphthalocyanine (VONc) at the interface with highly oriented pyrolytic graphite (HOPG). Using two-photon photoemission spectroscopy, an unoccupied state corresponding to the lowest unoccupied molecular orbital of VONc is observed. The energy of this state shows a significant dependence on coverage, interpreted in the context of the electrostatic environment at the interface. On the basis of a simple electrostatic model, we were able to determine the excited-state polarizability and dipole moment of VONc at the interface with HOPG. The results suggest that local electric fields may have a major influence on interfacial energy level alignment in the excited-state manifold, with direct consequences for interfacial charge-transfer dynamics.

Field-dependent exciton dissociation in organic heterojunction solar cells

“…47,50,51 This deviation did not contradict the validity of our fit because the obtained estimate only represented an upper limit for the polarizability α in our model. Additionally, the above-mentioned experimental values were not obtained from solar-cell-like structures but from sub-monolayer samples, 47 from solutions in toluene, 51 and from the gas phase.…”

“…47,50,51 This deviation did not contradict the validity of our fit because the obtained estimate only represented an upper limit for the polarizability α in our model. Additionally, the above-mentioned experimental values were not obtained from solar-cell-like structures but from sub-monolayer samples, 47 from solutions in toluene, 51 and from the gas phase. 50 Taking into account that not only the molecular structure 52 but also the polarity of the molecules' surroundings influences the polarizability, 53 the agreement between the experimental values and the estimate based on our fit parameters was rather good.…”

Theoretical investigation of molecular excited states in polar organic monolayers via an efficient embedding approach

“…We will consider the case in which only a single molecule is excited, whereas all the other molecules are frozen in their ground state. Such a description is well suited to model optical experiments in molecular films and crystals in which the population of excited molecules is very low [28,29].…”

“…In fact, we are not aiming to model the excitation energies of a molecular crystal or slab, whose excited states need to described in term of excitonic and/or charge-transfer bands [30][31][32][33]. Our approach is instead relevant to compute directly the polarization term of the excitonic band-structure [34,35], to describe the excited-state depolarization in thin organic films [29], to study the dependence of single-molecule excited states on the coverage [36], and to describe the modifications of the photoluminescence of single molecule in a molecular crystal [34,37].…”

“…1. VONc has been largely investigated from an experimental point view [29,36,38,42] due to its large dipole moment perpendicular to a large macrocyle; mercaptobiphenyl has been the subject of many theoretical studies as a model system of SAM/metal interfaces [10,16,39,40]; AlQ 3 is a molecule often employed in the realization of optoelectronic devices [43][44][45][46][47].…”