Role of intrinsic molecular dipole in energy level alignment at organic interfaces

Abstract

The energy level alignment in metal-organic and organic-organic junctions of the widely used materials tris-(8-hydroxyquinoline)aluminum (Alq3) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) is investigated. The measured alignment schemes for single and bilayer films of Alq3 and NTCDA are interpreted with the integer charge transfer (ICT) model. Single layer films of Alq3 feature a constant vacuum level shift of ∼0.2–0.4 eV in the absence of charge transfer across the interface. This finding is attributed to the intrinsic dipole of the Alq3 molecule and (partial) ordering of the molecules at the interfaces. The vacuum level shift changes the onset of Fermi level pinning, as it changes the energy needed for equilibrium charge transfer across the interface.

Regular Energetics at Conjugated Electrolyte/Electrode Modifier for Organic Electronics and their Implications on Design Rules

“…The findings are surprisingly similar to recent results on interfaces featuring molecules with intrinsic dipoles [35] and for molecule-doped conjugated polymer/electrode interfaces. [36] In the former case, a potential step is induced from (partial) ordering of the molecular dipoles.…”

“…We explore this hypothesis using photoemission spectroscopy, with the aim to propose a general model for energy level alignment involving CE interlayers. As the work function modification is strongly dependent on the ionic functionality, [34] we use a CE series with various ionic groups, including cationic poly( Table 1.The findings are surprisingly similar to recent results on interfaces featuring molecules with intrinsic dipoles [35] and for molecule-doped conjugated polymer/electrode interfaces. [36] In the former case, a potential step is induced from (partial) ordering of the molecular dipoles.…”

“…The P(NSO3)2 does not feature ionic species of differing mobility, but a negative potential step at the P(NSO3)2 interfaces attributed to electrostatic realignment of the dipolar zwitterionic side chains instead is expected, [21] and is indeed observed in our results and in analogy with the results on molecules with intrinsic dipoles. [35] The results thus enable us to propose a general model for describing the energetics of CE/electrode interfaces:…”

“…The P(NSO3)2 does not feature ionic species of differing mobility, but a negative potential step at the P(NSO3)2 interfaces attributed to electrostatic realignment of the dipolar zwitterionic side chains instead is expected, [21] and is indeed observed in our results and in analogy with the results on molecules with intrinsic dipoles. [35] The results thus enable us to propose a general model for describing the energetics of CE/electrode interfaces:(i) Equilibration of the Fermi level due to oxidation/reduction of -delocalized backbone at interface as per the ICT model Additionally (ii) a constant interfacial potential step induced by ion migration or side chain electrostatic realignment.The effect of the image charge induced double dipole step D is illustrated in Fig. 4.…”

Negative polarity of phenyl-C61 butyric acid methyl ester adjacent to donor macromolecule domains

“…4,[7][8][9] The relative position of poly(3-hexylthiophene) (P3HT) and phenyl-C 61 butyric acid methyl ester (PCBM) molecular orbitals-measured by UPS and IPES of bilayers-has been seen to depend on whether or not P3HT and PCBM are in contact. To explain this, vacuum level alignment at the donor-acceptor interface is dismissed in favor of an interfacial dipole.…”