Polarization-resolved spectroscopy imaging of grain boundaries and optical excitations in crystalline organic thin films.

Z Pan,M Furis,I Cour,L Manning,N Rawat,R L Headrick

doi:10.1038/ncomms9201

Z Pan, M Furis + Show 4 more

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https://doi.org/10.1038/ncomms9201

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Abstract

Exploration of optical properties of organic crystalline semiconductors thin films is challenging due to submicron grain sizes and the presence of numerous structural defects, disorder and grain boundaries. Here we report on the results of combined linear dichroism (LD)/ polarization-resolved photoluminescence (PL) scanning microscopy experiments that simultaneously probe the excitonic radiative recombination and the molecular ordering in solution-processed metal-free phthalocyanine crystalline thin films with macroscopic grain sizes. LD/PL images reveal the relative orientation of the singlet exciton transition dipoles at the grain boundaries and the presence of a localized electronic state that acts like a barrier for exciton diffusion across the grain boundary. We also show how this energy barrier can be entirely eliminated through the optimization of deposition parameters that results in films with large grain sizes and small-angle boundaries. These studies open an avenue for exploring the influence of long-range order on exciton diffusion and carrier transport.

Highlights

Exploration of optical properties of organic crystalline semiconductors thin films is challenging due to submicron grain sizes and the presence of numerous structural defects, disorder and grain boundaries
While it is clear that the electronic properties of such films are highly tunable by chemical methods that modify their molecular building blocks or by physical methods through different fabrication techniques[5,6], there is a critical need for a deeper fundamental understanding of the influence of long-range ordering on collective phenomena such as exciton diffusion and recombination, or carrier transport
We report on a spectrally resolved PL/linear dichroism (LD) polarization microscopy study of excitonic states in individual crystalline grains of metal-free phthalocyanine (H2Pc) films fabricated with a novel hollow capillary pen-writing deposition technique[11,12,13]

Summary

Introduction

Exploration of optical properties of organic crystalline semiconductors thin films is challenging due to submicron grain sizes and the presence of numerous structural defects, disorder and grain boundaries. We show how this energy barrier can be entirely eliminated through the optimization of deposition parameters that results in films with large grain sizes and small-angle boundaries These studies open an avenue for exploring the influence of long-range order on exciton diffusion and carrier transport. An in-house built laser scanning polarizationresolved microscopy setup records 90 Â 90-mm-LD images, and probes the symmetry of optically allowed electronic states with a spatial resolution of B5 mm These images reveal how the relative orientation of exciton dipoles at the grain boundaries can be controlled in a marked manner through fine tuning of the deposition parameters (that is, pen-writing speed and solution concentration). We established that this localized state can be eliminated in films with large long grains where the relative angle between crystalline axes in adjacent grains is o5°

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