Covalently Doped and Highly Oriented Covalent Organic Framework Thin Films

Strict control of both crystallographic orientation and band structure is crucial in realizing future high performing semiconducting microelectronic devices based on 2D covalent organic frameworks (COFs). Due to the insoluble nature from extensive aromaticity, processing of these materials into well-ordered, highly crystalline thin films presents a great challenge. In this work, a strategy to enable controlled covalent doping of imine COF thin films with thiophene linkers is presented. By incorporating different aldehyde ratios of terephthalaldehyde (PDA) and 2,5-thiophenedicarboxaldehyde (TDA) with 1,3,5-tris(4-aminophenyl)benzene (TAPB) in a liquid–solid synthesis approach, a series of highly crystalline and uniformly oriented TAPB-PDA-TDA COF thin films with varying percentages of TDA linkers incorporated into the framework were obtained. In this case, incorporation of thiophene linkers up to 20% resulted in minimal disruption of the long-range crystallographic ordering. Moreover, a small amount of thiophene molecules covalently doped into the highly ordered structure results in a small reduction in the band gap and a corresponding increase in the work function and decrease in the valence band maximum, effectively behaving like a p-type dopant in conventional semiconductors. The covalently doped thiophene unit is shown to increase the π-conjugation through enhanced crystallinity in the framework, improving the electron delocalization in the structure.