Stark Effect of Hybrid Charge Transfer States at Planar ZnO/Organic Interfaces

TL;DR

This study examines how the emission energy of hybrid charge transfer states at ZnO/organic interfaces shifts with bias, revealing electrostatic Stark effects influenced by internal electric fields and molecular polarizability.

Contribution

It introduces a simple electrostatic model explaining bias-induced spectral shifts, challenging existing continuum models that overestimate hole delocalization.

Findings

Bias-dependent emission peaks increase linearly with voltage

Spectral shifts are due to internal electric fields, not state filling

Proposed electrostatic model accounts for Stark effects at interfaces

Abstract

We investigate the bias-dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photo-doping allow us to confirm that this bias-induced spectral shifts relate to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image.