Investigation of the behaviour of codeposited pentacene:perfluoropentacene blends with different mixing ratios by TEM techniques

Abstract

Organic semiconductors (OSCs) have gained a great interest in the last years since they are a promising alternative to conventional inorganic semiconductors. There have been successful demonstrations of their applications in OFETs [1,2] , OLEDs [3,4] and photovoltaic devices [5,6]. Among other OSCs, pentacene (PEN, C 22 H 14 ) and perfluoropentancene (PFP, C 22 F 14 ) blends attract a special attention since they can form donor/acceptor systems (p‐n junctions) and are expected to be structurally compatible due to their similar molecular geometry. However, for good optical and structural properties of these junctions, a coupling between PEN and PFP at a molecular level is needed. For this reason, the understanding of the behaviour of this PEN:PFP intermixture is of prime interest. Previous studies by global characterization methods (XRD, GIXRD, RSM and PL) have demonstrated the intermolecular coupling of codeposited PEN:PFP [7‐11]. Moreover, Hinderhofer et al. [9] reported that the evidence for coupling in all PEN:PFP blends (with independence of mixing ratio) is based on the formation of a new “mixed crystal” phase with exclusively equimolecular concentrations of these compounds. Non‐equimolecular blends lead to a phase separation between the new “mixed crystal” and the respective pure phases. Until now PEN:PFP blends have only been studied by averaging characterization methods, whose spatial resolution is no high enough to get a microstructural information on the films. In consequence, local methods are also demanded to give information about the local crystallinity and crystallographic phases. The most commonly used technique to elucidate the structure at nanoscale dimension is TEM. Here, we present a study of codeposited PEN:PFP grown on SiO 2 using different mixing ratios: [1:1] equimolecular PEN:PFP (Fig. 1), [2:1] with excess of PEN (Fig. 2) and [1:2] with excess of PFP (Fig. 3). The SiO 2 is an amorphous substrate which minimizes the molecule‐substrate interactions. The characterization of the structure and morphology of these blends were performed by AFM and TEM techniques (using SAED patterns, conventional dark field (DF) and bright field (BF) pictures and STEM analyses). The SAED patterns taken from large length scales show polycrystalline character and diffraction rings of a “common phase” that systematically appears for all blend ratios used. This phase can be assigned to the new “mixed crystal” previously observed [9]. In addition, SAED measurements performed at small length scales (Fig. 4), display the possible monocrystalline diffraction pattern of this new “mixed phase” for the first time. This SAED pattern is rather similar to pure PEN in the [001] expected orientation normal to SiO 2 substrates, suggesting that the crystalline structures of both (the “mixed phase” and pure PEN) should be very similar. Additional DF‐TEM pictures reveal that no single domains at large scale (in the range of µm) can be found for the different phases (“mixed phase” and respective pure phase) in the case of non‐equimolecular mixed blends. However, a grainy structure is visible (order of 100 ‐200 nm), suggesting that a large and homogeneous crystal for this “mixed phase” is not formed. TEM characterization manifests as an useful tool to understand local and extended crystal orientation by a combination of imaging and diffraction techniques. In this study, three different blends of PEN and PFP are compared using TEM tools. Hence, diffraction techniques are used to obtain information on the arrangement of the not yet well‐understood phase (new “mixed phase”) formed as consequence of the favorable coupling between the PEN and PFP.