David Moerman

Poly(3-alkylthiophene) with tuneable regioregularity: synthesis and self-assembling properties

This work reports a synthetic strategy to generate poly(3-alkylthiophene)s (P3ATs) with joint-simultaneous control of the molar mass and the regioregularity. A series of chiral P3ATs with different regioregularities is synthesized using a Pd(RuPhos)-catalyzed chain-growth polymerisation. All polymers have molar masses and polydispersities (PDI) that lie within a narrow region. Furthermore, it is shown that the Pd-catalyst forms all kinds of couplings [head-to-tail (HT), tail-to-tail (TT) and head-to-head (HH)] to a similar extent, which allows insertion of predictable amounts of regio-irregularities into the polymer chain. This enables a thorough study of the influence of the regioregularity on the properties of P3AT, which was performed using UV-vis and circular dichroism (CD) spectroscopy, differential scanning calorimetry (DSC) and atomic force microscopy (AFM) measurements. Unexpectedly, it is found that under “kinetic” conditions the highest crystallinity, π-stacking, supramolecular organisation and chiral expression are not obtained for fully regioregular P3AT with 100% HT couplings, but that a small amount of regio-irregularity increases these properties and the chiral expression. Under “thermodynamic” conditions (after annealing, very slow solvent evaporation or very slow cooling from the melt), this effect is less pronounced or not found. This behaviour can be explained by a higher degree of motional freedom within the non-perfect polymer chains due to the increased steric repulsion from the HH-couplings, which leads to a more easy stacking under “kinetic” conditions.

Synthesis of poly[(4,4′-(dihexyl)dithieno(3,2-b;2′,3′-d)silole)] and copolymerization with 3-hexylthiophene: new semiconducting materials with extended optical absorption

The synthesis of poly[(4,4′-(dihexyl)dithieno(3,2-b;2′,3′-d)silole)] has been carried out through Kumada catalyst transfer polycondensation and its copolymerization with 3-hexylthiophene (3HT) leads to well-defined diblock copolymers in terms of molecular characteristics. These copolymers show a wider absorption window than P3HT, which is of potential interest for photovoltaic applications.

Efficient bulk heterojunction photovoltaic cells with a pre-organized poly(3- hexylthiophene) phase

This paper presents the controlled generation of nanostructured poly(3-hexylthiophene) (P3HT) from solution to fully fibrillate solid-state films. The fibrillate morphology of P3HT is further maintained when the deposition is carried out from a mixed solution with the acceptor material ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM). This enables the fabrication of active layers for bulk heterojunction organic photovoltaic devices with a controlled morphology. The contribution of these nanostructures to the photovoltaic performances is evidenced. High-resolution electrical characterization with conductive atomic force microscopy (C-AFM) confirms an improved charge transport throughout the fibrillate P3HT matrix. A local hole mobility value of 0.9 × 10−4 cm2/V s is derived from the C-AFM measurements.

Direct visualization of microphase separation in block copoly(3-alkylthiophene)s

A poly(3-octylthiophene)-block-poly(3-butylthiophene) block copolymer was synthesized in a one-pot block copolymerization reaction, starting from a functional o-tolyl initiator in order to maximize A–B diblock copolymer formation. First, the composition of this block copolymer is extensively studied using gel permeation chromatography (GPC) and 1H NMR measurements. A complete block copolymer formation is obtained with almost equal block length; B–A–B block copolymer contamination is shown to be very limited. In a second part, the self-assembly was analysed through differential scanning calorimetry (DSC), atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) measurements, focusing on the microphase separation. A direct visualization of the different microphases can be obtained with STM.

Influence of the regioregularity on the chiral supramolecular organization of poly(3-alkylsulfanylthiophene)s

The manuscript investigates the influence of the regioregularity (RR) of poly(3-alkylsulfanylthiophene)s (P3AST) on their properties. Therefore, a series of P3ASTs (P1–P5) with different RR were synthesized using a combination of a “reversed McCullough method” and the GRIM method. The degree of RR was determined by 1H NMR spectroscopy. A detailed chiroptical study in good solvent, poor solvent and film was performed, which revealed that the tendency to form chiral supramolecular aggregates clearly depends on the RR, but that the relation is not simply continuously increasing. Instead, the strongest effects were observed in P3ASTs with high, but not 100% RR. As a consequence, a similar behaviour can be expected for a regioregular and a particular irregular P3AST. This hypothesis was tested on a regioregular and an irregular P3AST that proved to have similar chiroptical properties in a previous study. Their properties in the solid state were investigated in detail using STM and AFM.

Multimodal noncontact atomic force microscopy and Kelvin probe force microscopy investigations of organolead tribromide perovskite single crystals

In this work, methylammonium lead tribromide (MAPbBr3) single crystals are studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). We demonstrate that the surface photovoltage and crystal photostriction can be simultaneously investigated by implementing a specific protocol based on the acquisition of the tip height and surface potential during illumination sequences. The obtained data confirm the existence of lattice expansion under illumination in MAPbBr3 and that negative photocarriers accumulate near the crystal surface due to band bending effects. Time-dependent changes of the surface potential occurring under illumination on the scale of a few seconds reveal the existence of slow ion-migration mechanisms. Lastly, photopotential decay at the sub-millisecond time scale related to the photocarrier lifetime is quantified by performing KPFM measurements under frequency-modulated illumination. Our multimodal approach provides a unique way to investigate the interplay between the charges and ionic species, the photocarrier-lattice coupling and the photocarrier dynamics in hybrid perovskites.