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Imaging domain boundaries of rubrene thin crystallites by photoemission electron microscopy

Moha Naeimi, Katharina Engster, Waqas Pervez, Ingo Barke, Sylvia Speller

TL;DR

Problem addressed: how crystal morphology governs photoemission and, by extension, charge transport in rubrene organic crystals. Approach: wavelength-tuned photoemission imaging (PEEM) and time-of-flight spectroscopy on orthorhombic rubrene platelets on HOPG, including triclinic inclusions, under 3.1–6.2 eV excitation. Key findings: 2PPE localizes emission to domain boundaries implicating trap states, 1PPE yields uniform emission across domains, and intermediate photon energies produce non-integer photon orders with triclinic morphologies dominating; KPFM reveals morphology-dependent work-function differences. Significance: demonstrates a noninvasive, spectrally resolved method to assess crystalline quality and phase composition in organic semiconductor films, with implications for device performance and fabrication.

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.

Imaging domain boundaries of rubrene thin crystallites by photoemission electron microscopy

TL;DR

Problem addressed: how crystal morphology governs photoemission and, by extension, charge transport in rubrene organic crystals. Approach: wavelength-tuned photoemission imaging (PEEM) and time-of-flight spectroscopy on orthorhombic rubrene platelets on HOPG, including triclinic inclusions, under 3.1–6.2 eV excitation. Key findings: 2PPE localizes emission to domain boundaries implicating trap states, 1PPE yields uniform emission across domains, and intermediate photon energies produce non-integer photon orders with triclinic morphologies dominating; KPFM reveals morphology-dependent work-function differences. Significance: demonstrates a noninvasive, spectrally resolved method to assess crystalline quality and phase composition in organic semiconductor films, with implications for device performance and fabrication.

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 () 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.
Paper Structure (4 sections, 3 figures)

This paper contains 4 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Experimental section
  3. Results and discussion
  4. Conclusion

Figures (3)

  • Figure 1: Rubrene polycrystalline structures prepared on HOPG. The orthorhombic platelets with approximately circular shape consist of different domains as visible in polarised optical microscope. They are embedded in a colony of small triclinic crystals (e.g. as seen close to the borders of Fig.XXa), forming triclinic spherulites (not shown, see Naeimi2024). (a and b) Magnified POM images of two rubrene orthorhombic platelet prepared on HOPG. The platelet consists of domains that appear with different colour and brightness in POM. (scale: 100 $\mathrm{\mu m}$). (c and d) AFM and KPFM map of the area marked with a dashed frame in b (scale: 10 $\mathrm{\mu m}$). (e and f) AFM and KPFM map of an area with triclinic crystals. (Scale: 10 $\mathrm{\mu m}$)
  • Figure 2: 1PPE and 2PPE image and electron kinetic energy spectra of rubrene polycrystalline assemblies reveal a fundamental difference in internal photoemission properties. (a and b) PEEM images of the crystal shown in \ref{['fig:pol_kpfm']}b excited with 3.1 eV and 6.2 eV, respectively. (c) Electron spectra of rubrene orthorhombic platelet excited with different photon energies shown in a and b. (d) Energy diagram of rubrene orthorhombic platelet showing the 1PPE and 2PPE photoemission pathways. (scale: 50 $\mathrm{\mu m}$)
  • Figure 3: (a) Electron spectra (top left) and spectrally resolved photon order (bottom left), and corresponding PEEM image (right) of the rubrene polymorph shown in \ref{['fig:pol_kpfm']}.a, excited with photon energies equal to 3.1 eV and resolved for orthorhombic and triclinic crystal phases. (b and c) same as a for excitation photon energies equal to 3.4 eV and 6.2 eV. (scale: 50 $\mathrm{\mu m}$)