Growth of Large Crystalline Grains of Vanadyl‐Phthalocyanine without Epitaxy on Graphene

Abstract

and as the source-drain channel in organic thin fi lm transistors (OTFTs). [ 3 ] While these devices show promise for replacing their inorganic counterparts, performance optimization is needed.Performance improvements can come from adjusting the structure of the OSC molecules in the active layer: both the crystallography and the morphology of the OSC have a direct impact on the charge transport mobility. [ 7 ] For example, single crystals of organic molecules generally exhibit higher mobilities than polycrystalline fi lms, which suffer from charge scattering from grain boundaries and charge trapping. [ 8,9 ] The structure of OSC fi lms is dependent on the method of deposition, the deposition conditions, and the interactions between the substrate and OSC. Here we focus on organic molecular beam deposition (OMBD), [ 10 ] where the OSC is sublimed in ultrahigh vacuum (UHV) forming a molecular beam which condenses directly onto the growth substrate. OMBD has several advantages for studying OSC thin fi lm formation: the rate of deposition and thickness of fi lm can be controlled, sequential deposition can be used to build complex heterostructures, and control of the substrate temperature can be used to gain further control over the OSC fi lm morphology. [ 11,12 ] OMBD is appropriate for small molecule OSCs, such as the metallophthalocyanines (MPcs) [ 13 ] which have been widely studied as active materials in OTFTs [ 12 ] and OPVs. [ 6,14 ] Thin fi lm morphology depends on the MPc used: some such as copper phthalocyanine (CuPc) are planar molecules, whilst others such as vanadyl phthalocyanine (VOPc) are nonplanar due to the oxygen atom projecting out of the ligand plane. While much work has been done on the planar phthalocyanines, [15][16][17] less attention has been paid to the nonplanar type, [ 18 ] despite their promising optical absorption profi les and high performance in OPVs [ 19 ] and OTFTs. [ 20 ] VOPc is studied here as an archetype for these nonplanar phthalocyanines.There is considerable interest in understanding and controlling the crystallinity of MPc thin fi lms, and increasing the substrate temperature during OMBD can lead to more crystalline fi lms. [ 21 ] However, morphology and crystallinity are also substrate dependent; for example, templating layers such as CuI [ 11 ] The performance of organic semiconductor thin fi lms in electronic devices is related to their crystal structure and morphology, with charge transport mobility dependent on the degree of crystallinity and on the crystallographic orientation. Here organic molecular beam deposition of vanadyl phthalocyanine is studied on graphene and it is shown that crystalline grains up to several micrometers across can be formed at substrate temperatures of 155 °C, compared to room temperature grain sizes of ≈30 nm. Transmission electron microscopy confi rms the presence of long range order at elevated substrate temperatures and reveals that the molecules are stacked in an edge-on orientation, but are not epitaxially aligned to the graphene. The c...

Monolayer-to-thin-film transition in supramolecular assemblies: the role of topological protection

“… 20 , 21 Alternatively, graphene has been proposed as an electrode material in organic electronics, 22 with the ability to control molecular assembly to increase the crystallinity and define the orientation of the organic thin film, hence improving its electrical properties. 8 , 23 – 25 …”

2D–Organic Hybrid Heterostructures for Optoelectronic Applications

“…Following the synthesis of vdW 2D materials, organic molecules can be deposited via thermal evaporation, spin coating, dip coating, or physical vapor deposition (PVD) . Among these methods, PVD is advantageous for controlling the film thickness and/or morphology.…”

“…In particular, the PVD conditions (e.g., evaporation speed, temperature, and pressure) can be easily modulated to alter the structure of organic thin films from randomly oriented to highly ordered crystalline. This structural tunability would be beneficial for optimizing the structure–property–performance relationships . For example, for exciton transport in organic thin films, it is preferable to have a structure that is oriented normal to the π–π stacking direction to ensure optimized charge injection and/or extraction .…”

“…This structural tunability would be beneficial for optimizing the structure–property–performance relationships . For example, for exciton transport in organic thin films, it is preferable to have a structure that is oriented normal to the π–π stacking direction to ensure optimized charge injection and/or extraction . Similarly, organic FETs (OFETs) are required to have a standing‐up molecular orientation, as opposed to the lying‐down orientation in organic LEDs (OLEDs) or organic PVs (OPVs) .…”

“…Fortunately, highly ordered assembly of organic molecules on a predefined vdW 2D material is possible because of the weak intermolecular interactions. This approach enables spontaneous self‐assembly and molecular epitaxial growth and consequently lowers the resistance at the interface between the 2D material and the organic material . For example, molecular epitaxial growth of perylene‐3,4,9,10‐tetracarboxylic dianhydride (PTCDA) on graphene has been reported previously .…”