Geoffrey E. Purdum

An all-conjugated gradient copolymer approach for morphological control of polymer solar cells

This work introduces fully π-conjugated gradient copolymers as promising materials to control and stabilize the nanoscale morphology of polymer:fullerene solar cells. Gradient and block sequence copolymers of 3-hexylselenophene (3HS) and 3-hexylthiophene (3HT) are utilized as the donors (D) in bulk-heterojunction (BHJ) solar cells with phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor (A). We show that for the same overall copolymer composition, the ordering of molecular constituents along the copolymer chain (copolymer sequence) significantly influences the nanoscale morphology and phase separation behavior of π-conjugated copolymer:fullerene devices. In addition, energy-filtered transmission electron microscopy (EFTEM) of the blends revealed that relative to the block copolymer:PCBM, the gradient copolymer:PCBM sample formed a more uniform, continuous and interconnected network of polymer fibrils within the acceptor-rich phase, associated with a large D/A interface. Charge extraction of photogenerated carriers by linearly increasing voltage (photo-CELIV) shows that the gradient copolymer:PCBM device possesses the highest initial carrier density, n(0) = (3.92 ± 0.3) × 1018 cm−3, consistent with a larger D/A interfacial area suggested by the observed morphology, albeit at the expense of increased carrier recombination rate. Accelerated degradation studies show that the gradient copolymer:PCBM system maintains the highest efficiency over prolonged heat treatment.

Solution-processable, crystalline material for quantitative singlet fission

Amorphous nanoparticles of the singlet fission chromophore 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) are fully crystallized through co-precipitation with a chemical additive. Time-resolved measurements indicate that singlet fission in the crystalline nanoparticles is quantitative, or lossless, whereas losses are evident in the amorphous nanoparticles as a result of frustrated triplet pair separation. Because triplet pairs form rapidly and separate slowly in amorphous material, mixed-phase samples are unable to compensate for these losses.

Presence of Short Intermolecular Contacts Screens for Kinetic Stability in Packing Polymorphs

Polymorphism is pervasive in molecular solids. While computational predictions of the molecular polymorphic landscape have improved significantly, identifying which polymorphs are preferentially accessed and experimentally stable remains a challenge. We report a framework that correlates short intermolecular contacts with polymorphic stability. The presence of short contacts between neighboring molecules prevents structural rearrangement and stabilizes the packing arrangement, even when the stabilized polymorph is not enthalpically favored. In the absence of such intermolecular short contacts, the molecules have added degrees of freedom for structural rearrangement, and solid-solid polymorphic transformations occur readily. Starting with a series of core-halogenated naphthalene tetracarboxylic diimides, we establish this framework with the packing polymorphs of more than 20 compounds, ranging from molecular semiconductors to pharmaceutics and biological building blocks. This framework, widely applicable across molecular solids, can help refine computational predictions by identifying the polymorphs that are kinetically stable.

CCDC 1481034: Experimental Crystal Structure Determination

Related Article: Thilanga Liyanage, Anna Hailey, Geoffrey Purdum, Luisa Whittaker, Anthony Petty, Sean Parkin, Lynn Loo, John E Anthony|2017|CSD Communication|||