Impact of molecular quadrupole moments on the energy levels at organic heterojunctions

Abstract

The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the π-π-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor−acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.

Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells

“…Our calculations showed that this energy offset is determined by the different orientations of the molecular quadrupoles in each side of the grain boundary, which dictate a step in the electrostatic potential across the interface (Fig. 4b, c) that affects occupied and unoccupied levels 54 . As sketched in Fig.…”

Barrierless Free Charge Generation in the High‐Performance PM6:Y6 Bulk Heterojunction Non‐Fullerene Solar Cell

“…To understand this, we first point out that because of the phase‐separated structure the electrostatic potential at a D/A interface can have a pronounced bend due to a molecular concentration gradient . This concentration gradient modulates the solid‐state crystal field around the charge . In other words, a hole away from the interface interacts with a smaller density of acceptors dispersed in the donor phase.…”

On the Physical Origins of Charge Separation at Donor–Acceptor Interfaces in Organic Solar Cells: Energy Bending versus Energy Disorder

“…have shown that, depending on the relative D–A dielectric constant values or on the presence of local (interfacial) electric fields stemming from D→A charge transfer, energy bending with appropriate or inappropriate evolution can be obtained. Schwarze et al . highlight the impact of molecular quadrupole moments and the layer thickness on the ionization energies.…”