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Electroactive Donor-Acceptor Covalent Organic Frameworks

Final Report Summary - ECOF (Electroactive Donor-Acceptor Covalent Organic Frameworks)

Common organic semiconducting polymers are usually either amorphous or contain small crystalline domains separated by amorphous regions, making the interpretation and prediction of their physical properties rather difficult. Recently, a new class of crystalline two-dimensional layered (2D) and three-dimensional polymers (covalent organic frameworks) has been discovered that promises to offer much more structural control and greater understanding of their physical properties. The fundamental idea of the ECOF project is to develop such molecule-based periodic covalent organic frameworks (COFs) that can form semiconducting phases, including interpenetrating phases with electron-donor and electron-acceptor properties. For example, these novel frameworks could ultimately serve as periodic bulk heterojunctions that can harvest light, generate excitons, and separate the latter into charges that are collected at electrodes or that are converted into chemical bonds. Gaining access to such novel ordered heterojunctions allows us to deterministically control the degree of electronic coupling between building blocks, tunnel barriers, band alignments, and mutual spatial disposition of the building blocks – all factors that are very difficult to control in “classical” disordered bulk heterojunctions. Moreover, the high degree of order in such systems also provides a platform for a deep understanding of the charge carrier dynamics and for allowing us to create more efficient solar cells and other optoelectronic devices.

To reach these ambitious goals, we have separated the challenges into workpackages. These range from the synthesis of (i) multifunctional molecular building blocks with the desired HOMO-LUMO gap and energy level positions to (ii) the synthesis and structural characterization of highly crystalline COFs, (iii) the development of thin film growth strategies including oriented films on conducting substrates, (iv) device construction, and (v) detailed characterization of the optoelectronic and dynamic behavior of the new systems.

During the course of the project, we have discovered many new structures and phenomena that we could not even foresee during the early stages of the project. Numerous novel organic linkers were successfully synthesized that allowed us to (i) control molecular packing in COFs, obtaining highly ordered structures, (ii) tune the pore size and geometry in the COFs, and (iii) extend the optical absorption (light harvesting) across the entire visible spectrum and even into the near infrared. Many new and highly ordered COF structures have been synthesized with these building blocks, which were investigated, for example, regarding molecular packing, enhanced light-harvesting, charge transfer states, J-type intermolecular interactions in the COF framework, or solvatochromic behavior that could be utilized in ultrafast optical vapor sensing. Moreover, we have pioneered the development of several novel film growth strategies for COFs, including direct growth of oriented COF layers on transparent conducting substrates, vapor-assisted conversion of thin layers of precursor materials, and electrophoretic deposition of COF nanoparticles. The direct film growth method often resulted in highly oriented channel systems on surfaces, with the open channels oriented vertically on the substrate thus ensuring rapid diffusion of guest molecules into the channels. These films allowed for detailed studies regarding exciton and charge carrier dynamics over wide time scales ranging from femtoseconds to microseconds, as well as conductivity and mobility studies. Finally, we have demonstrated several novel types of COF-based optoelectronic devices, namely, different types of ordered periodic heterojunctions for photovoltaics, the first photoelectrochemical devices for solar water splitting, and ultrafast solvatochromic optical sensors for vapor sensing.

This work opens up new opportunities to generate precise models and predictions on the relationship between the spatial and electronic structure of organic semiconductors and their optoelectronic behavior. We expect these insights to be of fundamental importance for the future design of efficient devices based on organic semiconductors, such as organic solar cells and organic light-emitting diodes.