Photoinduced Charge Transfer in Covalent Organic Frameworks

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Invited Speaker, Thomas Bein, presentation 308
Publication date: 5th February 2015

Organic building blocks can be linked together through covalent bonds to form periodic systems called Covalent Organic Frameworks (COFs). These nanoscale frameworks offer the opportunity to spatially integrate photoactive building blocks into a crystalline lattice, thus serving as models for organic bulk heterojunctions. We will present different strategies based on the COF structural paradigm aimed at creating electroactive networks capable of light-induced charge transfer. For example, we have developed a COF containing stacked thieno[2,3-b]thiophene-based building blocks acting as electron donors (TT-COF) with a 3 nm open pore system, which showed light-induced charge transfer to an intercalated fullerene acceptor.^[1] Expanding on this concept, we discuss the synthesis of a new COF structure based on benzodithiophene (BDT) building blocksthatserve as electron donors in the covalent organic framework.^[2] We succeeded in growing oriented thin (BDT)COF films on indium-doped tin oxide (ITO) substrates with and without a hole-selective blocking layer such as NiO. Different fullerene-based electron-acceptor molecules ([60]PCBM and [70]PCBM) were infiltrated into the porous thin BDT-COF films to obtain an interpenetrated electron-donor/acceptor system. These films were also incorporated into photovoltaic devices, showing light-induced charge separation and -collection. Following a complementary approach, we have recently designed a COF integrated heterojunction consisting of alternating columns of stacked donor and acceptor molecules. This triphenylene-porphyrin COF features an intrinsic type II heterojunction that promotes the photo-induced generation of mobile charge carriers inside the COF network. This donor-acceptor COF can serve as the photoactive junction of an organic photovoltaic device, as well as a photoconductor under reverse bias. The newly developed triphenylene-porphyrin COF was grown as an oriented thin film with the donor and acceptor units forming one-dimensional stacks that extend along the substrate normal, thus providing a favorable geometry for charge carrier transport. Summarizing, due to the great structural diversity and the large degree of morphological precision that can be achieved with COFs, these materials are viewed as intriguing model systems for organic heterojunctions.

[1] M. Dogru, M. Handloser, F. Auras, T. Kunz, D. Medina, A. Hartschuh, P. Knochel, T. Bein, Angew. Chem. Int. Ed. 52 (2013) 2920. [2] D. D. Medina, V. Werner, F. Auras, R. Tautz, M. Dogru, J. Schuster, S. Linke, M. Döblinger, J. Feldmann, P. Knochel, T. Bein, ACS Nano 8 (2014), 4042. [3] M. Calik, F. Auras, L. M. Salonen, K. Bader, I. Grill, M. Handloser, D. D. Medina, M. Dogru, F. Löbermann, D. Trauner, A. Hartschuh, T. Bein, J. Am. Chem. Soc. 136 (2014) 17802.

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