Growing Ultra-flat Organic Films on Graphene with a Face-on Stacking via Moderate Molecule-Substrate Interaction

Abstract

The electronic properties of small molecule organic crystals depend heavily on the molecular orientation. For multi-layer organic photovoltaics, it is desirable for the molecules to have a face-on orientation in order to enhance the out-of-plane transport properties. However, it is challenging to grow well-ordered and smooth films with a face-on stacking on conventional substrates such as metals and oxides. In this work, metal-phthalocyanine molecules is used as a model system to demonstrate that two-dimensional crystals such as graphene can serve as a template for growing high quality, ultra-flat organic films with a face-on orientation. Furthermore, the molecule-substrate interaction is varied systematically from strong to weak interaction regime with the interaction strength characterized by ultrafast electron transfer measurements. We find that in order to achieve the optimum orientation and morphology, the molecule-substrate interaction needs to be strong enough to ensure a face-on stacking while it needs to be weak enough to avoid film roughening.

Spontaneous Exciton Dissociation at Organic Semiconductor Interfaces Facilitated by the Orientation of the Delocalized Electron–Hole Wavefunction

“…We will first discuss the results obtained from the ZnPc/F 8 ZnPc interface in which the spontaneous dissociation of CT excitons is observed. ZnPc and F 8 ZnPc molecules are grown on a highly ordered pyrolytic graphite (HOPG) substrate on which they are expected to have a face‐on orientation 46. Because face‐on and edge‐on molecules show different ionization potentials (IP),47,48 we can verify the face‐on orientation of the ZnPc and F 8 ZnPc molecules by measuring their IP (Section S1, Supporting Information).…”

“…These molecules were deposited on a highly ordered pyrolytic graphite (HOPG) crystal. ZnPc molecules can be deposited on HOPG with a face‐on orientation in a layer‐by‐layer fashion 46. Prior to the deposition, the outer surface of the HOPG substrate was peeled off by using a scotch tape.…”

Reliable Work Function Determination of Multicomponent Surfaces and Interfaces: The Role of Electrostatic Potentials in Ultraviolet Photoelectron Spectroscopy

“…There are, however, unresolved issues regarding the reliability and fundamental understanding of Φ determination with UPS, particularly with regard to samples with inhomogeneous surfaces and nanostructured materials. For example, a shift of Φ upon variation of surface composition could be interpreted as due to band bending, and there are instances when more than one Φ value could be derived from UPS spectra . In this contribution, we attend to these issues and provide experimental and theoretical insight that will allow an improved use of Φ as important material and sample parameter.…”

“…For instance, most studies only consider one SECO corresponding to one area‐averaged (or macroscopic) work function value . However, UPS measurements on heterogeneous surfaces often reveal more than one SECO or broad spectral distribution . Recently, Sharma et al studied heterogeneous indium‐tin‐oxide/Au and nanoroughened, sol–gel‐derived, polycrystalline ZnO surfaces by UPS.…”

Solution Processable Transition Metal Phthalocyanine Nanofibres

“…Their robustness under chemical and thermal experimental conditions is also desirable for diverse applications. MPcs belong to a class of organic semiconductors, which exhibit structure‐dependent anisotropic properties due to their molecular arrangement . This has been exploited for various applications such as field effect transistors and gas sensors, organic photovoltaics, organic light‐emitting diodes (OLEDs), etc .…”