Linear Dichroism and Photoluminescence Microscopy Imaging of Grain Boundaries in Crystalline Metal-Free Phthalocyanine Thin Films*

We employed a combination of linear dichroism and photoluminescence microscopy with spatial resolution of 5$\mu $m to study the excitonic properties of solution-processed metal-free phthalocyanine (H2Pc) crystalline thin films with millimeter-sized grains. We observe a highly-localized, sharp, monomer-like emission at the high angle grain boundaries, in contrast to samples with more uniform grain orientation where no such feature has been observed. The energy difference between the grain boundary luminescence and the HOMO-LUMO singlet exciton recombination of the crystalline H2Pc is measured to be 160meV. Our systematic survey of grain boundaries indicates this localized state is never present at low angle boundaries where the $\pi $-orbital overlap between adjacent grains is significant. It supports recent results which associated a decrease in carrier mobility with the presence of large angle boundaries in similar crystalline pentacene films.