Zone-sectored organic crystals with spatially resolved exciton dynamics

TL;DR

This study introduces a novel zone-sectored rubrene microcrystal with distinct exciton dynamics and photoluminescence properties, characterized by advanced microscopy techniques, revealing insights into exciton behavior and potential photonic applications.

Contribution

The paper reports the creation of zone-sectored orthorhombic rubrene microcrystals with spatially resolved exciton dynamics and photoluminescence, providing a new approach for photonic device design.

Findings

Distinct photoluminescence spectra in different sectors

Polarization-dependent emission linked to transition dipole moments

Quantitative modeling of exciton fusion processes

Abstract

Among the organic semiconductors, rubrene stands out in terms of hole mobility, luminescence yield and exciton migration distance. A novel type of rubrene microcrystal is prepared in the orthorhombic phase, exhibiting zone-sectored tabular domains with distinct photoluminescence (PL) characteristics. These sectors exhibit distinct PL spectra and time-evolution, arising from differences in the in-plane orientation of the orthorhombic unit cell relative to the crystal surface. A combination of polarised optical microscopy, fluorescence lifetime imaging microscopy (FLIM), and atomic force microscopy (AFM) is used to characterise the samples in terms of crystal orientation, fluorescence lifetime, and photoluminescence spectra. Spatially resolved PL spectroscopy reveals that the redshifted 650 nm emission band has polarisation along the transition dipole moment and is associated with high photon absorption due to the alignment of excitation polarisation and transition dipole moment and selectively localized within specific sectors of the crystal. The detected photon originates from direct emission of a geminate coherent triplet pair, or from its fusion. This band exhibits pure mono-exponential dynamics with 3.7 ns lifetime. The triplet fusion behaviour in the succeeding time regimes can be treated in the framework of power law scaling and random walk. The emission kinetics are modelled using rate equations describing geminate and non-geminate exciton fusion processes, enabling a quantitative interpretation of the spatially resolved PL kinetics. These findings introduce a material-based strategy, opening novel routes for photonic applications and light harvesting.