A Photoconductive Thienothiophene‐Based Covalent Organic Framework Showing Charge Transfer Towards Included Fullerene

Abstract

Organic bulk heterojunctions combining electron donor and acceptor phases are of great interest for designing organic photovoltaic devices. While impressive advances have been achieved with these systems, so far a deterministic control of their nanoscale morphology has been elusive. It would be a major breakthrough to be able to create model systems with periodic, interpenetrating networks of electron donor and acceptor phases providing maximum control over all structural and electronic features. Herein we report a significant step towards this goal on the basis of the recently discovered class of crystalline covalent organic frameworks (COFs) which are created by condensation of molecular building blocks. Specifically, the stacked layers of two-dimensional COFs permit charge migration through the framework, and several semiconducting structures with high carrier mobilities have been described. We have created a COF containing stacked thieno[2,3-b]thiophene-based building blocks serving as electron donors (TT-COF), with high surface area and a 3 nm open pore system. This open framework takes up the wellknown fullerene electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), thus forming a novel structurally ordered donor–acceptor network. Spectroscopic results demonstrate light-induced charge transfer from the photoconductive TT-COF donor network to the encapsulated PCBM phase in the pore system. Moreover, we have created the first working COF-based photovoltaic device with the above components. The organization of the molecular building blocks into a crystalline framework with defined conduction paths provides a promising model system for ordered and interpenetrated networks of donors and acceptors at the nanoscale. The most prominent hole-conducting material used in organic solar cells is poly(3-hexylthiophene) (P3HT), a thiophene-containing polymer with high charge-carrier mobilities. The soluble fullerene derivative PCBM is often used as an electron acceptor in organic photovoltaics. Because of the lack of structural order in the respective bulk heterojunctions it is very difficult to assess the impact of molecular building blocks, bonding motifs, and energy levels on the microscopic processes involving light-induced exciton formation, charge separation, and transport in such systems. Hence ordered charge-transporting networks with a periodicity of several nanometers are of great interest to understand the mechanistic details of the light-induced processes and ultimately to obtain design rules for the creation of efficient and stable organic photovoltaic devices. The new TT-COF was synthesized under solvothermal conditions by co-condensation of thieno[3,2-b]thiophene-2,5diyldiboronic acid (TTBA) and the polyol 2,3,6,7,10,11hexahydroxytriphenylene (HHTP; Figure 1a). Reaction parameters are described in the Supporting Information. As described in the following, the thienothiophene-based COF forms stacks in an AA arrangement, as confirmed by N2 sorption and powder X-ray diffraction. Powder X-ray diffraction (PXRD) confirms the formation of a highly crystalline COF. Identification of the new structure was conducted by comparison of structures modeled with MS Studio (see Figures S1–S5 in the Supporting Information). Corresponding powder patterns were simulated and compared to the experimentally obtained data. For previous COF structures different stacking types of the hexagonal planar sheets were reported. Hence calculations were carried out simulating an eclipsed AA arrangement and a staggered AB arrangement. The experimental PXRD pattern for TT-COF agrees very well with the simulated pattern for an eclipsed AA arrangement (Figure 1b) with a hexagonal P6m symmetry. Moreover, unit-cell parameters determined from the experimental X-ray patterns match very well with those obtained from the structure simulations (peak broadening included). FT-IR spectroscopy can confirm the presence of the newly formed boronate ester functionality. As previously reported, the attenuation of the OH stretching band resulting from the ester formation is apparent, and furthermore the most characteristic modes of the C-B and C-O functionalities can be assigned to the bands at 1395 cm 1 and 1353 cm 1 (see Figure S8 in the Supporting Information). The B MAS NMR spectrum (see Figure S9 in the Supporting Information) shows a trigonal-planar boron atom with a chemical shift of d= 21 ppm, which can be distinguished from the starting material (TTBA: d= 15 ppm). Transmission electron microscopy (TEM) images show the nanoscale morphology of the crystals. A slightly tilted side view shows the long ordered channels with distinct pore sizes (see Figure S12 in the Supporting Information). A top view [*] Dr. M. Dogru, M. Handloser, F. Auras, Dr. T. Kunz, Dr. D. Medina, Prof. Dr. A. Hartschuh, Prof. Dr. P. Knochel, Prof. Dr. T. Bein Department of Chemistry and Center for NanoScience (CeNS), Ludwig-Maximilians-Universit t Munich (LMU) Butenandtstrase 5–13 (E), 81377 Munich (Germany) E-mail: knoch@cup.uni-muenchen.de bein@lmu.de Homepage: http://www.bein.cup.uni-muenchen.de