Dana D. Medina

A photoconductive thienothiophene-based covalent organic framework showing charge transfer towards included fullerene

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

Extraction of photogenerated electrons and holes from a covalent organic framework integrated heterojunction.

Covalent organic frameworks (COFs) offer a strategy to position molecular semiconductors within a rigid network in a highly controlled and predictable manner. The π-stacked columns of layered two-dimensional COFs enable electronic interactions between the COF sheets, thereby providing a path for exciton and charge carrier migration. Frameworks comprising two electronically separated subunits can form highly defined interdigitated donor–acceptor heterojunctions, which can drive the photogeneration of free charge carriers. Here we report the first example of a photovoltaic device that utilizes exclusively a crystalline organic framework with an inherent type II heterojunction as the active layer. 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 an optimal geometry for charge carrier transport. As a result of the degree of morphological precision that can be achieved with COFs and the enormous diversity of functional molecular building blocks that can be used to construct the frameworks, these materials show great potential as model systems for organic heterojunctions and might ultimately provide an alternative to the current disordered bulk heterojunctions.

Isoreticular Two-Dimensional Covalent Organic Frameworks Synthesized by On-Surface Condensation of Diboronic Acids

On-surface self-condensation of 1,4-benzenediboronic acid was previously shown to yield extended surface-supported, long-range-ordered two-dimensional covalent organic frameworks (2D COFs). The most important prerequisite for obtaining high structural quality is that the polycondensation (dehydration) reaction is carried out under slightly reversible reaction conditions, i.e., in the presence of water. Only then can the subtle balance between kinetic and thermodynamic control of the polycondensation be favorably influenced, and defects that are unavoidable during growth can be corrected. In the present study we extend the previously developed straightforward preparation protocol to a variety of para-diboronic acid building blocks with the aim to tune lattice parameters and pore sizes of 2D COFs. Scanning tunneling microscopy is employed for structural characterization of the covalent networks and of noncovalently self-assembled structures that form on the surface prior to the thermally activated polycondensation reaction.

Oriented Thin Films of a Benzodithiophene Covalent Organic Framework

A mesoporous electron-donor covalent organic framework based on a benzodithiophene core, BDT-COF, was obtained through condensation of a benzodithiophene-containing diboronic acid and hexahydroxytriphenylene (HHTP). BDT-COF is a highly porous, crystalline, and thermally stable material, which can be handled in air. Highly porous, crystalline oriented thin BDT-COF films were synthesized from solution on different polycrystalline surfaces, indicating the generality of the synthetic strategy. The favorable orientation, crystallinity, porosity, and the growth mode of the thin BDT-COF films were studied by means of X-ray diffraction (XRD), 2D grazing incidence diffraction (GID), transmission and scanning electron microscopy (TEM, SEM), and krypton sorption. The highly porous thin BDT-COF films were infiltrated with soluble fullerene derivatives, such as [6,6]-phenyl C61 butyric acid methyl ester (PCBM), to obtain an interpenetrated electron-donor/acceptor host–guest system. Light-induced charge transfer from the BDT-framework to PCBM acceptor molecules was indicated by efficient photoluminescence quenching. Moreover, we monitored the dynamics of photogenerated hole-polarons via transient absorption spectroscopy. This work represents a combined study of the structural and optical properties of highly oriented mesoporous thin COF films serving as host for the generation of periodic interpenetrated electron-donor and electron-acceptor systems.

Room Temperature Synthesis of Covalent–Organic Framework Films through Vapor-Assisted Conversion

We describe the facile synthesis of several two-dimensional covalent–organic frameworks (2D COFs) as films by vapor-assisted conversion at room temperature. High-quality films of benzodithiophene-containing BDT-COF and COF-5 with tunable thickness were synthesized under different conditions on various substrates. BDT-COF films of several micrometer thickness exhibit mesoporosity as well as textural porosity, whereas thinner BDT-COF films materialize as a cohesive dense layer. In addition, we studied the formation of COF-5 films with different solvent mixture compositions serving as vapor source. Room temperature vapor-assisted conversion is an excellent method to form COF films of fragile precursors and on sensitive substrates.

Oriented Films of Conjugated 2D Covalent Organic Frameworks as Photocathodes for Water Splitting

Light-driven water electrolysis at a semiconductor surface is a promising way to generate hydrogen from sustainable energy sources, but its efficiency is limited by the performance of available photoabsorbers. Here we report the first time investigation of covalent organic frameworks (COFs) as a new class of photoelectrodes. The presented 2D-COF structure is assembled from aromatic amine-functionalized tetraphenylethylene and thiophene-based dialdehyde building blocks to form conjugated polyimine sheets, which π-stack in the third dimension to create photoactive porous frameworks. Highly oriented COF films absorb light in the visible range to generate photoexcited electrons that diffuse to the surface and are transferred to the electrolyte, resulting in proton reduction and hydrogen evolution. The observed photoelectrochemical activity of the 2D-COF films and their photocorrosion stability in water pave the way for a novel class of photoabsorber materials with versatile optical and electronic properties that are tunable through the selection of appropriate building blocks and their three-dimensional stacking.

Directional Charge-Carrier Transport in Oriented Benzodithiophene Covalent Organic Framework Thin Films

Charge-carrier transport in oriented COF thin films is an important factor for realizing COF-based optoelectronic devices. We describe how highly oriented electron-donating benzodithiophene BDT-COF thin films serve as a model system for a directed charge-transport study. Oriented BDT-COF films were deposited on different electrodes with excellent control over film roughness and topology, allowing for high-quality electrode–COF interfaces suitable for device fabrication. Hole-only devices were constructed to study the columnar hole mobility of the BDT-COF films. The transport measurements reveal a clear dependency of the measured hole mobilities on the BDT-COF film thickness, where thinner films showed about two orders of magnitude higher mobilities than thicker ones. Transport measurements under illumination yielded an order of magnitude higher mobility than in the dark. In-plane electrical conductivity values of up to 5 × 10–7 S cm–1 were obtained for the oriented films. Impedance measurements of the hole-only devices provided further electrical description of the oriented BDT-COF films in terms of capacitance, recombination resistance, and dielectric constant. An exceptionally low dielectric constant value of approximately 1.7 was estimated for the BDT-COF films, a further indication of their highly porous nature. DFT and molecular-dynamics simulations were carried out to gain further insights into the relationships between the COF layer interactions, electronic structure, and the potential device performance.

A supramolecular strategy based on molecular dipole moments for high-quality covalent organic frameworks

A supramolecular strategy based on strong molecular dipole moments is presented to gain access to covalent organic framework structures with high crystallinity and porosity. Antiparallel alignment of the molecules within the pore walls is proposed to lead to reinforced columnar stacking, thus affording a high-quality material. As a proof of principle, a novel pyrene dione building block was prepared and reacted with hexahydroxytriphenylene to form a boronic ester-linked covalent organic framework. We anticipate the strategy presented herein to be valuable for producing highly defined COF structures.

Passivation of PbS Quantum Dot Surface with l-Glutathione in Solid-State Quantum-Dot-Sensitized Solar Cells

Surface oxidation of quantum dots (QDs) is one of the biggest challenges in quantum dot-sensitized solar cells (QDSCs), because it introduces surface states that enhance electron-hole recombination and degrade device performance. Protection of QDs from surface oxidation by passivating the surface with organic or inorganic layers can be one way to overcome this issue. In this study, solid-state QDSCs with a PbS QD absorber layer were prepared from thin mesoporous TiO2 layers by the successive ionic layer adsorption/reaction (SILAR) method. Spiro-OMeTAD was used as the organic p-type hole transporting material (HTM). The effects on the solar cell performance of passivating the surface of the PbS QDs with the tripeptide l-glutathione (GSH) were investigated. Current-voltage characteristics and external quantum efficiency measurements of the solar cell devices showed that GSH-treatment of the QD-sensitized TiO2 electrodes more than doubled the short circuit current and conversion efficiency. Impedance spectroscopy, intensity-modulated photovoltage and photocurrent spectroscopy analysis of the devices revealed that the enhancement in solar cell performance of the GSH-treated cells originates from improved charge injection from PbS QDs into the conduction band of TiO2. Time-resolved photoluminescence decay measurements show that passivation of the surface of QDs with GSH ligands increases the exciton lifetime in the QDs.

From benzodithiophene to diethoxy-benzodithiophene covalent organic frameworks – structural investigations

The incorporation of side groups into a covalent organic framework (COF) backbone can be of significant importance for developing new frameworks with enhanced properties. Here we present a novel π-stacked thiophene-based COF containing a benzodithiophene building unit modified with ethoxy side chains. The resulting BDT-OEt COF is a crystalline mesoporous material featuring high surface area and accessible hexagonal pores. We further synthesized a series of COFs containing both BDT and BDT-OEt building units at different ratios. The impact of the gradual incorporation of the BDT-OEt building units into the COF backbone on the crystallinity and porosity was investigated. Furthermore, molecular dynamic simulations shed light on the possible processes governing the COF assembly from molecular building blocks.