Pieter Verstappen

Unraveling the Fluorescence Features of Individual Corrole NH Tautomers

The fluorescence spectra of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole have been studied in the temperature range from 4.2 to 332 K. For the first time, the individual fluorescence profiles of the two corrole NH tautomers have been assigned over the whole temperature range. The pronounced temperature dependence of the fluorescence spectra of the meso-pyrimidinylcorrole under study was found to originate from switching between the fluorescence emissions of the two tautomers due to a reduced NH tautomerization rate with decreasing temperature. As a result, the long wavelength tautomer dominates the total emission spectrum at room temperature, whereas at low temperatures, the majority of the emission comes from the short wavelength tautomer. Energy level diagrams (involving the two NH tautomers) explaining the excitation energy deactivation channels in the meso-pyrimidinylcorrole at room temperature and below are presented. A significant H/D isotope effect on the NH tautomerization rate has been observed, resulting in an enhanced contribution of the short wavelength tautomer to the total fluorescence spectrum at the expense of that of the long wavelength tautomer. Substantially different fluorescence quantum yields have been determined for the individual NH tautomers, leading to a pronounced temperature dependence of the overall fluorescence quantum yield. The obtained results allow the unambiguous statement that the two NH tautomers of corroles coexist in fluid and solid solutions in a wide range of temperatures, with the proportion depending on the corrole substitution pattern. Moreover, this study shows that the (future) interpretation of the fluorescence properties of meso-pyrimidinylcorroles and all other corrole materials should be done (more) carefully, taking into account the coexistence of NH tautomers with individual spectral signatures.

Continuous Flow Polymer Synthesis toward Reproducible Large-Scale Production for Efficient Bulk Heterojunction Organic Solar Cells

Organic photovoltaics (OPV) have attracted great interest as a solar cell technology with appealing mechanical, aesthetical, and economies-of-scale features. To drive OPV toward economic viability, low-cost, large-scale module production has to be realized in combination with increased top-quality material availability and minimal batch-to-batch variation. To this extent, continuous flow chemistry can serve as a powerful tool. In this contribution, a flow protocol is optimized for the high performance benzodithiophene-thienopyrroledione copolymer PBDTTPD and the material quality is probed through systematic solar-cell evaluation. A stepwise approach is adopted to turn the batch process into a reproducible and scalable continuous flow procedure. Solar cell devices fabricated using the obtained polymer batches deliver an average power conversion efficiency of 7.2 %. Upon incorporation of an ionic polythiophene-based cathodic interlayer, the photovoltaic performance could be enhanced to a maximum efficiency of 9.1 %.

Enhanced open-circuit voltage in polymer solar cells by dithieno[3,2-b:2′,3′-d]pyrrole N-acylation

A series of low bandgap copolymers composed of N-acyl-substituted dithieno[3,2-b:2′,3′-d]pyrroles (DTPs) as the electron rich donor constituents (with various alkyl side chain patterns) combined with different electron deficient acceptor building blocks are developed for polymer solar cell applications. Due to the introduction of the N-acyl substituents, the HOMO energy levels of the push–pull copolymers decrease as compared to the N-alkyl-DTP analogues, resulting in an increased open-circuit voltage (Voc) and hence solar cell performance. For an N-acyl-DTP-alt-thieno[3,4-c]pyrrole-4,6-dione (PDTP-TPD) copolymer a bulk heterojunction device with a Voc up to 0.80 V and a power conversion efficiency of 4.0% is obtained, the highest value for DTP-based polymer materials to date. Moreover, by implementation of a conjugated polyelectrolyte cathode interlayer the short-circuit current noticeably increases, enhancing the solar cell efficiency to 5.8%.

Fluorination as an effective tool to increase the open-circuit voltage and charge carrier mobility of organic solar cells based on poly(cyclopenta[2,1-b:3,4-b′]dithiophene-alt-quinoxaline) copolymers

The effect of fluorination on the optoelectronic properties and the polymer : fullerene solar cell characteristics of PCPDTQx-type (poly{4-(2′-ethylhexyl)-4-octyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-alt-2,3-bis[5′-(2′′-ethylhexyl)thiophen-2′-yl]quinoxaline}) low bandgap copolymers is reported. The introduction of fluorine atoms on the quinoxaline constituents is an effective way to lower the HOMO and LUMO energy levels of the alternating copolymers, resulting in an enhanced open-circuit voltage for the devices based on the fluorinated polymers (∼0.1 V per F added). Furthermore, fluorination also improves the charge carrier mobility in the bulk heterojunction blends. Despite the formation of unfavorable photoactive layer morphologies, the best solar cell performance is obtained for the copolymer prepared from the difluorinated quinoxaline monomer, affording a power conversion efficiency of 5.26% under AM 1.5G irradiation, with an open-circuit voltage of 0.83 V, a short-circuit current density of 11.58 mA cm−2 and a fill factor of 55%.

Dye-sensitization of boron-doped diamond foam: champion photoelectrochemical performance of diamond electrodes under solar light illumination

Diamond foams composed of hollow spheres of polycrystalline boron-doped diamond are chemically modified with two donor–acceptor type molecular dyes, BT-Rho and CPDT-Fur, and tested as electrode materials for p-type dye-sensitized solar cells with an aqueous electrolyte solution containing methyl viologen as a redox mediator. Reference experiments with flat polycrystalline diamond electrodes evidence full blocking of the methyl viologen redox reaction by these dyes, whereas only partial blocking is observed for the diamond foams. This is ascribed to sp2-carbon impurities in the foam, viz. trans-polyacetylene and graphite-like carbon. Cathodic photocurrents under solar light illumination are about 3 times larger on foam electrodes compared to flat diamond. Long-term (1–2 days) illumination of the sensitized foam electrodes with chopped light at 1 sun intensity causes an increase of the cathodic photocurrent density to ca. 15–22 μA cm−2. These photocurrent densities represent the largest values reported so far for dye-sensitized diamond electrodes. The photoelectrochemical activation of the sensitized diamond electrodes is accompanied with characteristic changes of the dark voltammogram of the MV2+/MV+ redox couple and with gradual changes of the IPCE spectra.

A direct arylation approach towards efficient small molecule organic solar cells

Three extended molecular chromophores, differing in their central acceptor moiety and specifically designed as electron donor components for small molecule organic solar cells, are synthesized via a two-fold C–H arylation protocol. Upon removal of the side products inherent to the applied direct (hetero)arylation procedure, a record power conversion efficiency of 5.1% is achieved.

Impact of structure and homo-coupling of the central donor unit of small molecule organic semiconductors on solar cell performance

Currently, both low bandgap conjugated polymers and small molecule analogues are employed as electron donor components in state of the art bulk heterojunction organic photovoltaics, providing similar record efficiencies (∼10%). However, to evaluate molecular structure-device performance relations and (in particular) the effect of material purity, small molecule chromophores can be considered to be more versatile probes. In the present study, we have synthesized three small molecule donor materials with a varying central electron-rich building block, inspired by the well-known high-performance small molecule p-DTS(FBTTh2)2. The influence of this structural modification on the physicochemical material properties, electro-optical characteristics and solar cell performance is analysed. Most importantly, it is shown that the presence of homo-coupled side products generated during Stille cross-coupling reactions – which can be very hard to remove, even for small molecule semiconductors – is detrimental to solar cell performance, with a noticeable effect on the open-circuit voltage.

Branched and linear A2–D–A1–D–A2 isoindigo-based solution-processable small molecules for organic field-effect transistors and solar cells

To establish a correlation between the molecular structure, physicochemical properties, thin film morphology, charge carrier mobility and photovoltaic performance of isoindigo-based electron donor type molecular semiconductors, a series of branched and linear A2–D–A1–D–A2 small molecules (A = acceptor, D = donor) are synthesized. The extended π-conjugated molecular chromophores have an electron-accepting isoindigo core, a bridging oligothiophene electron donor part and terminal octyl cyanoacrylate acceptor moieties. Their photophysical, thermal and electrochemical properties are analysed and the materials are applied in organic field-effect transistors and bulk heterojunction organic solar cells. Compared to an analogous benzothiadiazole-based small molecule, the isoindigo core deepens the HOMO energy level, enabling higher open-circuit voltages in organic solar cells. The linear isoindigo-based small molecule shows an enhanced hole mobility compared to the branched derivatives. The best power conversion efficiency of the investigated set is also obtained for the solar cell based on the linear (CA-3T-IID-3T-CA-l) donor molecule in combination with PC71BM.

Synthesis of a multifunctional poly(p-phenylene ethynylene) scaffold with clickable azide-containing side chains for (bio)sensor applications

Poly(p-phenylene ethynylene) (PPE) copolymers with alternating alkoxy and azide-functionalized alkoxy side chains were efficiently synthesized via Sonogashira polymerization and the materials were fully characterized. Among the different synthetic protocols investigated, the best results were obtained when employing a pre-polymerization functionalization approach and upon implementing the azide-functionalized side chains on the diiodophenyl building block. End-capping of the conjugated polymer chains is shown to be essential for preventing side reactions. As a proof of principle, phenylacetylene is clicked in a nearly quantitative way on the polymer backbone. Furthermore, successful click-immobilization of randomly alkynylated protein A onto an azide-functionalized PPE film illustrates the potential use of these materials in a variety of (bio)sensor applications.

Synthesis of N,N’-dialkyl-6,6’-dibromoisoindigo derivatives by continuous flow

In this work, the synthesis of N,N′-dialkyl-6,6′-dibromoisoindigo derivatives by continuous-flow chemistry is explored as a means to enhance material availability and structural diversity, in particular toward the application of isoindigo-based semiconductors in high-performance organic photovoltaic devices. The individual steps in the conventional batch synthesis protocol are evaluated and, when needed, adapted to flow reactors. To overcome the low solubility of nonalkylated 6,6′-dibromoisoindigo in common organic solvents, the flow condensation reaction between the 6-bromo-isatin and 6-bromo-oxindole precursors is evaluated in polar aprotic solvents. Dialkylation of 6,6′-dibromoisoindigo is readily performed in flow using a solid-phase reactor packed with potassium carbonate. In an alternative strategy, solubility is ensured by first introducing the N-alkyl side chains on 6-bromo-isatin and 6-bromo-oxindole (accessible via a highyielding flow reduction of alkylated 6-bromo-isatin), followed by condensation using the conventional method in acetic-hydrochloric acid medium. The N,N′-dialkylated 6,6′-dibromoisoindigo derivatives indeed show enhanced solubility in the hot reaction mixture compared to the non-alkylated material but eventually precipitate when the reaction mixture is cooled down. Nevertheless, the condensation between both alkylated starting materials is achieved in flow without any blockages by keeping the outlet from the reactor heated and as short as possible. The latter strategy allows the preparation of both symmetrically and asymmetrically N-substituted isoindigo compounds.