Ashlee A. Jahnke

Controlling Phase Separation and Optical Properties in Conjugated Polymers through Selenophene-Thiophene Copolymerization

Selenophene-thiophene block copolymers were synthesized and studied. The properties of these novel block copolymers are distinct from those of statistical copolymers prepared from the same monomers with a similar composition. Specifically, the block copolymers exhibit broad and red-shifted absorbance features and phase-separated domains in the solid state. Scanning transmission electron microscopy and topographic elemental mapping confirmed that the domains are either rich in selenophene or thiophene, indicating that the blocks of distinct heterocycles preferentially associate with one another in the solid state. This preference is surprising in view of the chemical similarities between repeat units. The overall results demonstrate a phase separation that is controlled by elemental differences. As a result of this phase separation, these novel conjugated block copolymers should find utility in a variety of studies and optoelectronics uses.

Poly(3-alkyltellurophene)s Are Solution-Processable Polyheterocycles

The synthesis and characterization of a series of poly(3-alkyltellurophene)s are described. Polymers are prepared by both electrochemical and Kumada catalyst transfer polymerization methods. These polymers have reasonably high molecular weights (M(n) = 5.4-11.3 kDa) and can be processed in a manner analogous to that of their lighter atom analogues. All examples exhibit red-shifted optical absorption, as well as solid-state organization, as evidenced by absorption spectroscopy and atomic force microscopy. Overall, the synthesis and characterization of these materials open up a wide range of future studies involving tellurium-based polyheterocycles.

Exciton delocalization drives rapid singlet fission in nanoparticles of acene derivatives.

We compare the singlet fission dynamics of five pentacene derivatives precipitated to form nanoparticles. Two nanoparticle types were distinguished by differences in their solid-state order and kinetics of triplet formation. Nanoparticles that comprise primarily weakly coupled chromophores lack the bulk structural order of the single crystal and exhibit nonexponential triplet formation kinetics (Type I), while nanoparticles that comprise primarily more strongly coupled chromophores exhibit order resembling that of the bulk crystal and triplet formation kinetics associated with the intrinsic singlet fission rates (Type II). In the highly ordered nanoparticles, singlet fission occurs most rapidly. We relate the molecular packing arrangement derived from the crystal structure of the pentacene derivatives to their singlet fission dynamics and find that slip stacking leads to rapid, subpicosecond singlet fission. We present evidence that exciton delocalization, coincident with an increased relative admixture of charge-transfer configurations in the description of the exciton wave function, facilitates rapid triplet pair formation in the case of single-step singlet fission. We extend the study to include two hexacene derivatives and find that these conclusions are generally applicable. This work highlights acene derivatives as versatile singlet fission chromophores and shows how chemical functionalization affects both solid-state order and exciton interactions and how these attributes in turn affect the rate of singlet fission.

Tellurophenes with Delocalized π-Systems and Their Extended Valence Adducts

The π-conjugated 2,5-substituted tellurophene compounds 2,5-bis(2-(9,9-dihexylfluorene))tellurophene (1) and 2,5-diphenyltellurophene (3) were synthesized through ring closing reactions of 1,4-substituted butadiyne. The oxidative addition of Br(2) to tellurophene compounds 1 and 3 was studied through absorption spectroscopy, NMR, electrochemistry, X-ray crystallography, and density functional theory (DFT) calculations. When Br(2) adds to the tellurium center the absorption spectrum shifts to a lower energy. From electrochemistry and DFT calculations we show that this is caused by lowering the lowest unoccupied orbital. The highest occupied orbital is also lowered, but to a lesser extent. This shift in absorption spectrum and lowering of the oxidation potential can provide a method to modify tellurophene containing materials. The two-electron oxidative addition is promising for catalyzing energy storage reactions.

Polyselenophenes with distinct crystallization properties

The polythiophene derivative poly(3-hexyl)thiophene (P3HT) has become one of the most well studied organic materials due to its interesting and important chemical and physical properties. Two different crystal structures have been observed for P3HT, type-1 and type-2, however a pure type-2 structure has never been obtained. Herein, we investigate the crystal structure of polyselenophene analogs (P3HS), and discover that a pure type-2 phase is formed in low molecular weight P3HS (Mn = 5.9 kg mol−1). Wide-angle X-ray scattering shows that the type-2 phase is readily formed and stable at room temperature, which is very distinct from what is observed in control experiments with P3HT. Absorption spectra of P3HS films with the pure type-2 phase lack the typical shoulder peaks indicating that π–π stacking is relatively poor in the type-2 phase. Scanning transmission electron microscopy (STEM) images, however, show that large nanofibers are formed by type-2 crystallization thereby demonstrating the potential of P3HS to drive unique types of self-assembled structures through crystallization, and should motivate continued efforts on selenophene analogs of P3HT for a variety of studies and uses.

Evidence for the Rapid Conversion of Primary Photoexcitations to Triplet States in Seleno- and Telluro- Analogues of Poly(3-hexylthiophene)

Broadband pump-probe spectroscopy is used to examine the ultrafast photophysics of the π-conjugated polymers poly(3-hexylselenophene) (P3HS) and poly(3-hexyltellurophene) (P3HTe) in solution. An excited-state absorption feature that we attribute to a transition in the triplet manifold appears on the picosecond time scale. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations support this assignment. The formation of triplets is consistent with significant fluorescence quenching observed in solutions of the neat polymers. Triplet formation occurs in ~26 and ~1.8 ps (upper limit) for P3HS and P3HTe, respectively. The successive decrease in fluorescence quantum efficiency and triplet formation time are consistent with intersystem crossing facilitated by the heavier selenium and tellurium atoms. These results strongly suggest that primary photoexcitations are rapidly converted into triplet states in P3HS and P3HTe.

Molecular weight and end capping effects on the optoelectronic properties of structurally related ‘heavy atom’ donor–acceptor polymers

Two donor–acceptor polymers containing either Si or Ge in the donor and Se in the acceptor, poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzoselenadiazole)-4,7-diyl] and poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]germole)-2,6-diyl-alt-(2,1,3-benzoselena diazole)-4,7-diyl], were synthesized by microwave assisted polymerization. These polymer structures are attractive because they combine the red light absorption characteristics of the Se acceptor, with high charge carrier mobility inherent to the Si- or Ge-containing donor. Here we study the effects of molecular weight and end capping on the photophysical, morphological, and photovoltaic properties. The solution and film absorption profiles and solution onset are dictated by molecular weight, whereas the subtler heteroatom effect dictates the absorption onset in the polymer films. Molecular weight appears to affect polymer absorption to the greatest degree in a medium molecular weight regime and these effects have a significant aggregation component. Highlighting the red-light absorption of the Se-acceptor monomer, both Si-donor and Ge-donor polymer devices display improved photon harvesting beyond 850 nm relative to their S-acceptor analogues. Higher hole mobility relative to the C-donor/Se-acceptor polymer analogue indicates successful integration of heavy atom donor properties with the 2,1,3-benzoselenadiazole acceptor. Molecular weight invokes the greatest change on polymer/fullerene blend morphology, followed by phenyl end capping, and finally by the Si or Ge heteroatom.

Influence of selenophene–thiophene phase separation on solar cell performance

We compare the morphology and solar cell device performance of selenophene–thiophene copolymers that have the same degree of polymerization and composition, and differ only in their sequence (statistical vs. block copolymers). P3HS-b-P3HT spontaneously undergoes phase separation and P3HS-s-P3HT does not. P3HS-b-P3HT performs best when the intrinsic self-assembled nanostructure is the most perturbed. P3HS-s-P3HT does not undergo intrinsic phase separation, and vapor annealing can be used to optimize the polymer:fullerene morphology, where better nanostructure is well correlated with the best device. While the block structure provides the best stability, the statistical structure is a valuable method to balance the advantage of different monomers while precluding large-scale polymer self-assembly that is a strong intrinsic property of block structure.

Polytellurophenes provide imaging contrast towards unravelling the structure-property-function relationships in semiconductor:insulator polymer blends†

Polymer blends are broadly important in chemical science and chemical engineering and have led to a wide range of commercial products, however their precise structure and phase morphology is often not well understood. Here we show for the first time that π-conjugated polytellurophenes and high-density polyethylene form blends that can serve as active layers in field-effect transistor devices and can be characterized by a variety of element-specific imaging techniques such as STEM and EDX. Changing the hydrocarbon content and degree of branching on the polytellurophene side-chain leads to a variety of blend structures, and these variations can be readily visualized. Characterization by electron microscopy is complemented by topographic and X-ray methods to establish a nano- to micro-scale picture of these systems. We find that blends that possess microscale networks function best as electronic devices; however, contrary to previous notions a strong correlation between nanofiber formation and electrical performance is not observed. Our work demonstrates that use of organometallic polymers assists in clarifying relevant structure–property–function relationships in multicomponent systems such as semiconductor:insulator blends and sheds light on the structure development in polymer:polymer blends including crystallization, phase separation, and formation of supramolecular arrangements.