No abstract

Silicon phthalocyanines (SiPcs) have shown great potential as n-type or ambipolar organic semiconductors in organic thin-film transistors (OTFTs) and organic photovoltaics. Although properly designed SiPcs rival current state-of-the-art n-type organic semiconducting materials, relatively few structure–property relationships have been established to determine the impact of axial substituents on OTFT performance, hindering the intelligent design of the next generation of SiPcs. To address this omission, we have developed structure–property relationships for vapor-deposited SiPcs with phenoxy axial substituents. In addition to thorough electrical characterization of bottom-gate top-contact OTFTs, we extensively investigated SiPc thin films using X-ray diffraction, atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and density functional theory (DFT) modeling. OTFT performance, including relative electron mobility (μe) of materials, was in general agreement with values obtained through DFT modeling including reorganization energy. Another significant trend observed from device performance was that increasing the electron-withdrawing character of the axial pendant groups led to a reduction in threshold voltage (V T) from 47.9 to 21.1 V. This was corroborated by DFT modeling, which predicted that V T decreases with the square of the dipole induced at the interface between the SiPc pendant and substrate. Discrepancies between modeling predictions and experimental results can be explained through analysis of thin-film morphology and orientation by AFM and GIWAXS. Our results demonstrate that a combination of DFT modeling to select prospective candidate materials, combined with appropriate processing conditions to deposit molecules with a favorable thin-film morphology in an “edge-on” orientation relative to the substrate, yields high-performance n-type SiPc-based OTFTs.

Conjugated polymers have proven to be an important class of materials for flexible and stretchable electronics. To ensure long-term thermal and mechanical stability of associated devices, there is a need to determine the origin of the polymer ductility and toughness. In this work, we investigate a variety of high-performance conjugated polymers and relate their thermomechanical behavior to film toughness. Dynamic mechanical analysis (DMA) is used to probe thermomechanical relaxations of the conjugated polymers. Film ductility is measured as a function of temperature to determine the temperature that corresponds to a significant loss in film toughness. We systematically study polymers with changes to the side-chain structure, backbone structure, and crystallinity. We also compare polymers that have a clear glass transition (T g ) to those that do not. It is found that secondary thermal relaxations (sub-T g ) play a critical role in film toughness. This sub-T g is found to be a local molecular relaxation that appears to relate to side-chain and backbone mobility. We also find that many of the polymers considered continue to show moderate ductility below their sub-T g , which is attributed to crystallites or aggregates that have active slip systems. These results provide new insights into how conjugated polymer structure and related thermal relaxations influence film toughness that will assist in realizing mechanically robust devices.

Ultrathin organic films (typically < 10 nm) attracted great attention due to their (semi)transparency and unique optoelectronic properties that benefit applications such as sensors and flexible electronics. At the core of that, achieving high mobility in an ultrathin film is essential for the efficient operation of relevant electronic devices. While the state‐of‐the‐art material systems, e.g., P(NDI2OD‐T2) also known as N2200 can achieve high mobility in a thin film (typically > 20 nm), multitudinous challenges remain in processing an ultrathin film exhibiting desired charge transport morphology within a preferred thickness limit. Here, high electron mobility (a tenfold increase compared to annealed spin‐coated films) is reported in both the top and bottom‐gate configuration organic field‐effect transistors comprising ultrathin N2200 films produced with a water‐floating film transfer method. A range of characterization techniques are used to investigate these ultrathin films and their microstructure, and conclude that favorable edge‐on polymer orientation at the top as well as throughout the ultrathin film thickness and the quality of π–π ordering as captured by the largest coherences length resulted in this high mobility in N2200 ultrathin films, in stark contrast to the commonly observed microstructural gradient in spin‐coated thin films. The results provide new insight into the electronic and microstructural properties of thin films of organic semiconductors.

Understanding excited-state reorganization energies, exciton diffusion lengths and non-radiative (NR) recombination and the overall optoelectronic responses of nonfullerene small molecule acceptors (NF-SMAs) is important in order to rationally design new...