Polymorphism and structure formation in copper phthalocyanine thin films.

Berthold Reisz,Alexander Gerlach,Valentina Belova,Linus Pithan,Martin Hodas,Frank Schreiber,Clemens Zeiser,Peter Šiffalovič,Jakub Hagara,Takuya Hosokai,Giuliano Duva,Alexander Hinderhofer

doi:10.1107/s1600576720015472

Abstract

Many polymorphic crystal structures of copper phthalocyanine (CuPc) have been reported over the past few decades, but despite its manifold applicability, the structure of the frequently mentioned α polymorph remained unclear. The base-centered unit cell (space group C2/c) suggested in 1966 was ruled out in 2003 and was replaced by a primitive triclinic unit cell (space group P 1). This study proves unequivocally that both α structures coexist in vacuum-deposited CuPc thin films on native silicon oxide by reciprocal space mapping using synchrotron radiation in grazing incidence. The unit-cell parameters and the space group were determined by kinematic scattering theory and provide possible molecular arrangements within the unit cell of the C2/c structure by excluded-volume considerations. In situ X-ray diffraction experiments and ex situ atomic force microscopy complement the experimental data further and provide insight into the formation of a smooth thin film by a temperature-driven downward diffusion of CuPc molecules during growth.

Highlights

Together with the possibility of tuning growth parameters such as substrate temperature and deposition rate during organic molecular beam deposition (Forrest, 1997), the application of organic compounds in electronic devices has attracted increasing attention (Witte & Woll, 2004)
This study proves unequivocally that both structures coexist in vacuum-deposited crystal structures of copper phthalocyanine (CuPc) thin films on native silicon oxide by reciprocal space mapping using synchrotron radiation in grazing incidence
This study demonstrates that both polymorphs of CuPc, the C2/c and the P1 structures, coexist in vacuum-deposited CuPc thin films on native silicon oxide

Summary

Introduction

Together with the possibility of tuning growth parameters such as substrate temperature and deposition rate during organic molecular beam deposition (Forrest, 1997), the application of organic compounds in electronic devices has attracted increasing attention (Witte & Woll, 2004). Copper phthalocyanine (CuPc) is an important compound for application in electronic and optoelectronic devices and serves as a model for the entire family of phthalocyanines. The functionality and efficiency of these devices crucially depend on their thin-film structure (Opitz et al, 2010) and the molecular orientation of CuPc, which has been investigated several times (Suito et al, 1962; Nonaka et al, 1995; Nakamura et al, 1996; Hiesgen et al, 2000; Berger et al, 2000). In 1935, Robertson had already prepared needle-like single crystals of CuPc by low-pressure sublimation and determined the unit-cell parameters, the space group P21/a and the molecular orientation by X-ray diffraction (Robertson, 1935).

Results

Discussion

Conclusion