Morphological, Optical, and Electronic Consequences of Coexisting Crystal Orientations in β-Copper Phthalocyanine Thin Films

Abstract

The crystal structure of the β phase of copper phthalocyanine (CuPc) is a monoclinic herringbone, which is commonly created in thin films by either thermal annealing of α-CuPc at ∼300 °C or deposition on heated substrates. Of the several known CuPc crystal polymorphs, the β phase is of particular interest due to its thermodynamic stability. We observe three coexisting crystal orientations for thin films of β-CuPc to be (101), (1̅05), and (5̅02) using grazing incidence wide-angle X-ray scattering. Each of the three crystal orientations have distinct surface roughness, as measured by atomic force microscopy, but are electrostatically similar by Kelvin probe force microscopy. However, local optical properties, as measured by micro-UV–vis spectroscopy, are very different in the different domains.

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“…FTIR and Raman spectra of CuPc have been extensively explored to determine the orientation of thin films and the polymorphic transitions [22][23][24]. UV-VIS spectrum has been proved to be suitable to study the crystal structures of CuPc, especially as shown in the recent report of Microscopic UV-VIS absorption measurements [25].…”

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“…With exposure to FBBB(B) there are negligible changes to the UV–vis absorbance spectra of the unsubstituted Pcs (Figure S4, Supporting Information) but, following addition of THC, CoPc has a sharp increase in the intensity of its second Q band at 683 nm, suggesting coordination of the FBBB(B) + THC conjugate to the central cobalt, in addition to an increase in relative number of β‐crystallites. [ 46,56 ] Upon exposure to both FBBB(B) and THC, CuPc also has an increase in its second Q band (695 nm), albeit much lower in relative intensity to that of CoPc, further suggesting the absorbance peak intensity shifts are a result of strong FBBB(B) + THC conjugate interactions. In our previous work, [ 13 ] H 2 Pc was found to uniquely coordinate with the FBBB(B) + THC conjugate in solution.…”

“…Though highly crystalline, the β-form is generally poorer performing or non-semiconducting in OTFTs, with mobilities on the order of 10 −5 cm 2 V −1 s −1 due to the increased face to face orientation of the Pcs. [45,46] H 2 Pc and F 16 -CuPc undergo rapid changes, gaining peak intensity within the first 10 min treatment, and continuing to gain peak intensity at each time point, becoming highly α-crystalline after 24 h of exposure to toluene vapor. While no phase angle shift occurs in the XRD spectra, large micrometer scale crystals are again observed in what were previously featureless films.…”

“…These rearrangements are detrimental to device performance, with varied crystal types and domains resulting in increased grain boundaries and limiting the number of viable charge transport pathways. [46,51] A balance must then be struck between re-arrangement effects and coordinating effects as enhanced sensitivity can limit selectivity.…”

“…[11,45,48,49] Additionally, both αand β-crystals frequently coexist within the same film but charge transfer between different crystal domains can be limited, reducing viable charge transport pathways within the film. [46] Finally, deformations of the macrocyclic ring can occur from strong metal-ligand coordination [47] or as a result of larger central metals, promoting unique packing domains. [50] As charge transfer involves discrete particles moving between the π-electron clouds of individual molecules, molecular packing orientation has profound implications on the physical paths of charge transport, enabling or inhibiting different transport regimes, and altering measured electrical properties.…”