Spatially, Temporally and Polarization-Resolved Photoluminescence Exploration of Excitons in Crystalline Phthalocyanine Thin Films

TL;DR

This study employs advanced microscopy techniques to explore exciton behavior in crystalline phthalocyanine thin films, revealing a delocalized exciton along the high mobility axis and its temperature-dependent properties.

Contribution

It introduces a novel spatially, temporally, and polarization-resolved microscopy approach to investigate excitons in crystalline organic thin films, uncovering their delocalization and structural phase transition effects.

Findings

Discovery of a delocalized singlet exciton polarized along the high mobility axis

Observation of a structural phase transition above 100 K affecting exciton visibility

Correlation between molecular stacking and exciton radiative recombination rules

Abstract

The lack of long range order in organic semiconductor thin films prevents the unveiling of the complete nature of excitons in optical experiments, because the diffraction limited beam diameters in the bandgap region far exceed typical crystalline grain sizes. Here we present spatially-, temporally- and polarization-resolved dual photoluminescence/linear dichroism microscopy experiments that investigate exciton states within a single crystalline grain in solution-processed phthalocyanine thin films. These experiments reveal the existence of a delocalized singlet exciton, polarized along the high mobility axis in this quasi-1D electronic system. The strong delocalized {\pi} orbitals overlap controlled by the molecular stacking along the high mobility axis is responsible for breaking the radiative recombination selection rules. Using our linear dichroism scanning microscopy setup we further established a rotation of molecules (i.e. a structural phase transition) that occurs above 100 K prevents the observation of this exciton at room temperature.