The first solvent-free crystal structure of PCBM, an organic semiconductor widely used in solvent-free nanocrystalline films in plastic solar cells, is reported and its relevance to structure-property relationships discussed. The PCBM structure, obtained from o-dichlorobenzene solvates by solvent abstraction, was solved using powder diffraction, demonstrating this possibility for functionalized fullerenes.

The fullerene derivative PCBM ([6,6]phenyl-C61-butyric acid methyl ester) is one of the best electron acceptors used so far in solution-processed organic photovoltaic devices. The reasons for this success depend partly on its favourable electronic properties, partly on its solubility in common organic solvents and plausibly also on the possibility to optimize its structure and morphology by postdeposition treatments (solvent or thermal annealing). The latter feature is still largely a matter of speculation, as experimentally validated structural models of PCBM molecular organization within the devices are still unavailable. This structural characterization is non-trivial, given that poorly ordered\ud PCBM nanocrystals and amorphous domains appear to often coexist in bulk-heterojunction films based on this system. Here we address some of these issues using molecular dynamics (MD)\ud simulations. Our starting points are the only two published PCBM crystal structures, which were obtained by crystallization from oDCB (ortho-dichlorobenzene) and MCB (monochlorobenzene). Both contain guest molecules of the specific solvent. We simulated their thermal behavior, from room temperature up to their apparent melting points. Additional MD simulations involved model crystals obtained by removing solvent molecules from these co-crystal structures. Models that can apply to the\ud amorphous phase or to nanocrystalline samples have been obtained by cooling molten PCBM, after removing the solvent at different stages in the simulation. Their densities are close to the experimental values and they present a well interconnected network of fullerene moieties, where each of them has an\ud average of seven close neighbours available for charge hopping. Pre- and post-melting structural features such as intermolecular pair distribution functions are discussed in the framework of organic solar cell production and host–guest system dynamics

Organic mixed conductors find use in batteries, bioelectronics technologies, neuromorphic computing, and sensing. While great progress has been achieved, polymer‐based mixed conductors frequently experience significant volumetric changes during ion uptake/rejection, i.e., during doping/de‐doping and charging/discharging. Although ion dynamics may be enhanced in expanded networks, these volumetric changes can have undesirable consequences, e.g., negatively affecting hole/electron conduction and severely shortening device lifetime. Here, the authors present a new material poly[3‐(6‐hydroxy)hexylthiophene] (P3HHT) that is able to transport ions and electrons/holes, as tested in electrochemical absorption spectroscopy and organic electrochemical transistors, and that exhibits low swelling, attributed to the hydroxylated alkyl side‐chain functionalization. P3HHT displays a thickness change upon passive swelling of only +2.5%, compared to +90% observed for the ubiquitous poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate, and +10 to +15% for polymers such as poly(2‐(3,3′‐bis(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)‐[2,2′‐bithiophen]‐5‐yl)thieno[3,2‐b]thiophene) (p[g2T‐TT]). Applying a bias pulse during swelling, this discrepancy becomes even more pronounced, with the thickness of P3HHT films changing by <10% while that of p(g2T‐TT) structures increases by +75 to +80%. Importantly, the initial P3HHT film thickness is essentially restored after de‐doping while p(g2T‐TT) remains substantially swollen. The authors, thus, expand the materials‐design toolbox for the creation of low‐swelling soft mixed conductors with tailored properties and applications in bioelectronics and beyond.

Two crystal polymorphs of a thiophene-phenylene hexamer with bulky terminal substituents are characterized by different molecular conformations and parallel versus herringbone packing. Irrespective of their similar emissive spectra and common H-aggregate features, evidenced by crystal structure analysis and confirmed by solid-phase and excited-state first-principles calculations, their luminescence is relatively high and, for one form, nearly double than that for the other. Interaromatic packing energy contributions are established by quantum chemical calculations and can be compared quantitatively as the same species in different crystal environments is examined. The different luminescence efficiency of the two phases highlights the crucial role of the interaromatic packing for the luminescence properties of polyaromatic oligomers.

Atomistic MD simulation allows following continuously the experimentally observed transition between form I and form II poly(3-hexylthiophene) and poly(3-butylthiophene), evidencing unexpected reorganization.

Organic mixed ionic electronic conductors (OMIECs) have the potential to enable diverse new technologies, ranging from biosensors to flexible energy storage devices and neuromorphic computing platforms. However, a study of these materials in their operating state, which convolves both passive and potential-driven solvent, cation, and anion ingress, is extremely difficult, inhibiting rational material design. In this report, we present a novel approach to the in situ studies of the electrochemical switching of a prototypical OMIEC based on oligoethylene glycol (oEG) substitution of semicrystalline regioregular polythiophene via grazing-incidence X-ray scattering. By studying the crystal lattice both dry and in contact with the electrolyte while maintaining potential control, we can directly observe the evolution of the crystalline domains and their relationship to film performance in an electrochemically gated transistor. Despite the oEG side-chain enabling bulk electrolyte uptake, we find that the crystalline regions are relatively hydrophobic, exhibiting little (less than one water per thiophene) swelling of the undoped polymer, suggesting that the amorphous regions dominate the reported passive swelling behavior. With applied potential, we observe that the π–π separation in the crystals contracts while the lamella spacing increases in a balanced fashion, resulting in a negligible change in the crystal volume. The potential-induced changes in the crystal structure do not clearly correlate to the electrical performance of the film as an organic electrochemical transistor, suggesting that the transistor performance is strongly influenced by the amorphous regions of the film.

In situ UV–vis–NIR spectroelectrochemistry has been intensively used to evaluate the electronic transitions during the charging/discharging process of π-conjugated polymers. However, the type of charge carrier and the mechanisms of their transport, remains still a point of discussion. Herein, the coupling between UV–vis–NIR spectroscopy and in situ electrochemical-conductance measurements is proposed to compare the doping process of three different thiophene-based conducting polymers. The simultaneous monitoring of electrical and absorption properties, associated with low energy electronic transitions characteristic for polarons and bipolarons, was achieved. In addition, this method allows evaluating the reversible charge trapping mechanism of poly-3,4-o-xylendioxythiophene (PXDOT), caused by the formation of σ-dimers, making it a very useful tool to determine relevant physicochemical properties of conductive materials.

OECTs soaked in an electrolyte for up to 40 days produced with a low swelling mixed conductor, poly[3-(6-hydroxy)hexylthiophene] (P3HHT), showed enhanced stability in their electrochemical performance in comparison to PEDOT:PSS-based OECT.