Felicia A. Bokel

Real-time X-ray scattering studies of film evolution in high performing small-molecule–fullerene organic solar cells

We have studied the influence of the formulation additive 1,8-diiodooctane (DIO) on the structural evolution of bulk heterojunction (BHJ) films based the small molecule donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-5-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) and phenyl-C71-butyric-acid-methyl ester ([70]PCBM). Real-time, in situ, grazing-incidence X-ray scattering experiments allow us to characterize the development of crystalline order via diffraction and phase separation via small angle scattering. The performance of p-DTS(FBTTh2)2 based solar cells exhibits a distinct optimum with respect to volume fraction of DIO in the coating solution, unlike many polymer–fullerene systems that exhibit plateaus in performance above a certain additive volume fraction. Increasing the DIO volume fraction increases the crystallinity of p-DTS(FBTTh2)2 and dramatically increases the phase separation length scale even at small DIO amounts. These results suggest that the existence of an optimal DIO amount is a consequence of the phase separation length scale and its relationship to the optimal length for exciton dissociation. The effects of DIO on the time evolution of the drying films indicates that it acts as both a solvent and a plasticizer for p-DTS(FBTTh2)2, controlling its nucleation density and promoting its crystal growth.

Reversible, Self Cross-linking Nanowires from Thiol-Functionalized Polythiophene Diblock Copolymers

Poly(3-hexylthiophene)-block-poly(3-(3-thioacetylpropyl) oxymethylthiophene) (P3HT)-b-(P3TT) diblock copolymers were synthesized and manipulated by solvent-induced crystallization to afford reversibly cross-linked semiconductor nanowires. To cross-link the nanowires, we deprotected the thioacetate groups to thiols and they subsequently oxidized to disulfides. Cross-linked nanowires maintained their structural integrity in solvents that normally dissolve the polymers. These robust nanowires could be reduced to the fully solvated polymer, representing a novel, reversible cross-linking procedure for functional P3HT-based nanowire fibrils. Field-effect transistor measurements were carried out to determine the charge transport properties of these nanostructures.

Robust polythiophene nanowires cross-linked with functional fullerenes

Poly(3-hexyl thiophene) (P3HT)-block-poly(3-(3-aminopropyl)oxymethyl thiophene) (P3AmT) diblock copolymers were synthesized and assembled into nanowires by solvent-induced crystallization. Bis(4-[1,6-hexyldiisocyanate]benzylpyrrolidine)-C60 was synthesized and used to covalently cross-link the structures, affording robust p-type/n-type nanowires. These cross-linked nanowires proved stable to solvents and temperatures that would disrupt conventional P3HT-nanowires, as characterized by transmission electron microscopy (TEM) and ultraviolet-visible (UV-Vis) spectroscopy. Photoluminescence spectroscopy showed quenching of the PL signal of the fullerene-crosslinked material, suggesting electronic communication between the polymer and fullerene in these novel donor/acceptor assemblies. Grazing incidence X-ray diffraction (GIXD) showed a similar crystal structure for nanowires before and after cross-linking, while field effect transistor transfer measurements of the cross-linked nanowires showed hole and electron mobilities of 3.5 × 10−5 cm2 V−1 s−1 and 4.6 × 10−5 cm2 V−1 s−1, respectively.