An N-Annulated Perylene Butyl Tetra Ester for Use in Organic Field-Effect Transistors as the Active Material Layer

Faced with exponential growth and an increasing dependence on technology, the world requires practical materials to contribute to the Internet of Things. Organic electronic materials are emerging as an alternative to silicon-based devices as they are low in cost, can exhibit a low environmental impact in regards to their synthesis and processing, are recyclable, and can produce large scale thin films on a variety of substrates. Furthermore, soluble small molecule organic semiconducting (OSC) materials can be made into thin films at low temperatures with minimal energy input via solution processing. However, while solution processing is desired, the typical solvents used are halogenated and thus are detrimental to both human health and the environment. Therefore, it is necessary to explore new OSC materials that are green solvent processable, in which a green solvent can be described as exhibiting a minimal negative impact on both human health and the environment in terms of production, usage, and disposal. While organic light-emitting diodes (OLEDs) have had success to the point of commercialization, other organic electronic materials for use in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs) have lagged in their commercialization due to a lack of device performance caused by low charge carrier mobilities. Of the three different types of OSC materials (n-type, p-type, and ambipolar), p-type OSCs are considered superior due to their air-stability and high charge carrier mobilities with several reportings of over 1 cm²/Vs.<br/>One class of semiconducting materials for OPV and OFET devices are perylene derivatives, the most popular of which are perylene diimides. However, perylene diimides are traditionally n-type OSCs due to their strongly electron withdrawing imide groups, making them unstable in air. By replacing these imide groups with four butyl ester substituents the electron withdrawing effects are lessened, resulting in an increase in the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). By doing so, this perylene butyl tetra ester becomes more suitable as a p-type OSC material. Additionally, N-annulation on the perylene core results in further destabilization of the HOMO and LUMO energies, as this pyrrolic nitrogen donates electron density into the π-orbitals οn the perylene core. In the past, N-annulated perylene tetra esters have gained attention for their potential use in OLEDs, however, despite being a semiconducting material they have not been considered for usage in OFET devices.^1,2 This is likely due to their strong emissive properties making them attractive for use in OLED devices.^1,2 However, it has been discovered that the N-annulated perylene butyl tetra ester is natively soluble in 1-butanol, a green solvent that has a favourable vapour pressure for thin-film processing. This property of green solvent processability prompted further investigation of the compound, specifically for application in OFET devices. Initial OFET devices with the N-annulated perylene butyl tetra ester processed with 1-butanol have provided hole mobilities as high as 4.5 x 10^-5 cm²/Vs. The hole mobilities are expected to be improved upon by blending with 1-butanol soluble polymers that also act as dopants to improve the morphology of the thin-films as well as decrease the Fermi level of the material for improved charge generation.<br/><u>References</u><br/>1. <i>Chem. Phys. Chem.</i> <b>2016</b>, <i>17</i>, 859 – 872.<br/>2. <i>Soft Matter</i>, <b>2015</b>, <i>11</i>, 3629 – 3636.