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.

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.

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