Mixed Phases as a Route to Self-Passivation: Effect of π‐Stacking Backbone on the Physical and Electrical Properties of Naphthalenediimide Derivatives

Solution processable pand n-type organic semiconductors are candidates for low-cost, large-area, and roll-to-roll printing of inexpensive mass-production electronics. In these organic semiconductors, it is the π-conjugated backbone that plays the major role in chargecarrier transport across the channel. In order to achieve better device performance, it is required to have better packing/crystallinity to minimize defects and avoid deep traps, so that effective transfer of charge carriers can take place in the solid state. Excellent results have been reported by blending crystal-forming organic semiconductors with amorphous polymers that serve as binders or passivating agents. We show that, for some molecular structures and processing conditions, mixtures of stereoisomers can separate and self-arrange into a thin amorphous layer covered by a polycrystalline layer. In this work, we focus on two families of constitutional isomers that differ only in the position of the pyridine groups on the π-skeleton and study the effect of the structure on the physical and electrical properties using absorption spectroscopy, AFM, X-ray, and organic field-effect transistor current−voltage response. ■ INTRODUCTION Organic semiconductors have been studied for many years due to their potential applications in many areas, such as lightemitting diodes, organic lasers, field-effect transistors (FETs), sensors, and photovoltaic cells. In these, one of the most researched areas is organic field-effect transistors (OFETs), due to their potential low production costs, large area compatibility, and potential flexibility, a desired property in flexible and wearable electronics. OFETs can be either p-type (hole conductors) or n-type (electron conductors), which nowadays are both relatively easy to make. However, highperformance OFETs, especially n-type, often require surface passivation of the gate dielectric. For example, in the case of silicon oxide surface, the hydroxyl groups act as traps for electrons, leading to poor OFET performance. Solution processable organic semiconductors are of great interest for roll-to-roll deposition and low-cost production. Solution processability of small molecules is normally achieved by attaching bulky side groups to their skeleton, and the effect of various bulky groups has been widely studied. For longer oligomers, it is observed that solution processability and crystallinity of the final film are a function of the interplay between the flatness of the π-conjugated backbone and the nature of the solubilizing groups. In both cases, solubilizing groups may hinder the π−π-stacking and thus reduce crystallinity. Recently, strategies involving the blending of amorphous polymers with crystalline small molecules resulted in devices having high mobility values. Here we report that small molecules may self-segregate into an amorphous layer covered by a polycrystalline one. In this study, we try to minimize, as much as possible, the differences in the chemical structure so as to emphasize the role of molecular stacking and crystallization. We have used two almost identical π-backbones (constitutional isomers) having the same solubilizing groups and very similar conjugated backbones, differing only in the position where the pyridine groups are attached to the π-backbone (Figure 1a). As a result, in one system the pyridine groups are attached at a position that prevents planarization of the π-system (NNF), while in the other family (Figure 1b), the pyridine groups are connected in a way that does not cause significant steric hindrance, resulting in a flatter π-skeleton (NF). We tested thin films made of NNF and NF following annealing at different temperatures inside a vacuum oven. The films made of the nonflat (NNF) molecule showed striking differences when annealed at temperatures close to the melting point. We observed a vertical phase separation where the amorphous phase passivates the SiO2 dielectric and on top of it crystals start to grow. It was also found that if the SiO2 was prepassivated by octadecyltrichlorosilane (OTS), the vertical Received: June 18, 2016 Revised: September 26, 2016 Published: September 27, 2016 Article