Imaging domain boundaries of rubrene thin crystallites by photoemission electron microscopy

TL;DR

This study uses photoemission electron microscopy and spectroscopy to analyze the domain boundaries and defect states in rubrene thin crystals, revealing how morphology influences photoemission mechanisms and material quality.

Contribution

It introduces a wavelength-dependent PEEM approach to distinguish crystal morphologies and defect states in organic semiconductor thin films, advancing characterization techniques.

Findings

PEEM images show domain boundary emission indicating trap states.

Different photoemission mechanisms are observed at 2PPE and 1PPE.

Nonlinear photon order reveals morphology-specific emission contributions.

Abstract

The progress of designing organic semiconductors is extensively dependent on the quality of prepared organic molecular assemblies, since the charge transport mechanism is strongly efficient in highly ordered crystals compared to amorphous domains. Here we present a comprehensive photoemission electron microscopy (PEEM) and time-of-flight (TOF) spectroscopic study of rubrene ($\mathrm{C}_{48}\mathrm{H}_{24}$) thin crystals focusing on recently developed orthorhombic crystalline morphologies applied in organic electronic devices. Using femtosecond pulsed lasers with photon energies between 3-6 eV, we explore the interplay between photoemission processes, crystal morphology, and defect states. In a 2-photon photoemission process (2PPE), the PEEM images reveal dominant emission localized at domain boundaries, indicating strong contributions from trap states. In contrast, in 1PPE nm excitation uniform emission across the crystal surface is observed, highlighting a fundamental difference in photoemission mechanisms. Furthermore, in the intermediate photon energy range, we identify a nonlinear, non-integer photon order, where mostly the triclinic morphology contributes to the emission, distinguishing it from the orthorhombic phase. These findings provide a new framework for assessing the quality and internal structure of organic semiconductor thin films via wavelength-dependent photoemission imaging and spectroscopy.