Interfacial rearrangements and strain evolution in the thin film growth of ZnPc on glass

Thomas L. Derrien,Pascal Kaienburg,Chris Nicklin,Moritz Riede,Ellis Hancox,Andreas E. Lauritzen

doi:10.1103/physrevmaterials.6.033401

Thomas L. Derrien, Pascal Kaienburg + Show 4 more

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https://doi.org/10.1103/physrevmaterials.6.033401

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Abstract

We report on the characterization of the growth of vacuum-deposited zinc phthalocyanine (ZnPc) thin films on glass through a combination of in situ grazing incidence x-ray scattering, x-ray reflectivity, and atomic force microscopy. We found that the growth at room temperature proceeds via the formation of two structurally unique substrate-induced interfacial layers, followed by the growth of the $\ensuremath{\gamma}$-ZnPc polymorph thereafter (thickness $\ensuremath{\approx}1.0$ nm). As the growth of the bulk $\ensuremath{\gamma}$-ZnPc progresses, a substantial out-of-plane lattice strain ($\ensuremath{\approx}15%$ relative to $\ensuremath{\gamma}$-ZnPc powder) is continually relaxed during the thin film growth. The rate of strain relaxation was slowed after a thickness of $\ensuremath{\approx}13$ nm, corresponding to the transition from layer growth to island growth. The findings reveal the real-time microstructural evolution of ZnPc and highlight the importance of substrate-induced strain on thin film growth.

Highlights

Organic electronics, based on vacuum-deposited small molecule organic semiconductors (OSCs) have been used to successfully build devices such as organic field-effect transistors (OFETs) [1,2], organic light emitting diodes (OLEDs) [3], and organic photovolataics (OPVs) [4–6]
Device microstructure is critical for OPV and OFET performance and controlling the microstructure has even been found to improve OLED properties [12,13]
zinc phthalocyanine (ZnPc) evaporation in MINERVA was achieved with a low-temperature thermal evaporation source at a rate of 0.26 ± 0.03 Å/s as monitored by a water cooled quartz crystal microbalance (QCM), previously calibrated using ellipsometry

Summary

Introduction

Organic electronics, based on vacuum-deposited small molecule organic semiconductors (OSCs) have been used to successfully build devices such as organic field-effect transistors (OFETs) [1,2], organic light emitting diodes (OLEDs) [3], and organic photovolataics (OPVs) [4–6]. While OLEDs have met commercial success due to their high efficiencies, sufficient lifetimes, and the scalability of vacuum deposition, the performance of OFETs and OPVs still lags behind those of inorganic devices. This is partially due to the complex mechanisms by which thin film OSCs develop, where elaborate. Of particular importance is the substrate on which these films are deposited, which can serve as a templating layer to influence the thin film morphology [14]. As the morphology of the active layer can have profound effects on device performance, it is critical to develop a thorough understanding of the parameters controlling the morphology of OSCs in vacuum-deposition [18,19]

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