Identifying structure formation in semicrystalline conjugated polymers is the fundamental basis to understand electronic processes in these materials. Although correlations between physical properties, structure formation, and device parameters of regioregular, semicrystalline poly(3-hexylthiophene) (P3HT) have been established, it has remained difficult to disentangle the influence of regioregularity, polydispersity, and molecular weight. Here we show that the most commonly used synthetic protocol for the synthesis of P3HT, the living Kumada catalyst transfer polycondensation (KCTP) with Ni(dppp)Cl(2) as the catalyst, leads to regioregular chains with one single tail-to-tail (TT) defect distributed over the whole chain, in contrast to the hitherto assumed exclusive location at the chain end. NMR end-group analysis and simulations are used to quantify this effect. A series of entirely defect-free P3HT materials with different molecular weights is synthesized via new, soluble nickel initiators. Data on structure formation in defect-free P3HT, as elucidated by various calorimetric and scattering experiments, allow the development of a simple model for estimating the degree of crystallinity. We find very good agreement for predicted and experimentally determined degrees of crystallinities as high as ∼70%. For Ni(dppp)Cl(2)-initiated chains comprising one distributed TT unit, the comparison of simulated crystallinities with calorimetric and optical measurements strongly suggests incorporation of the TT unit into the crystal lattice, which is accompanied by an increase in backbone torsion. Polydispersity is identified as a major parameter determining crystallinity within the molecular weight range investigated. We believe that the presented approach and results not only contribute to understanding structure formation in P3HT but are generally applicable to other semicrystalline conjugated polymers as well.

cells today. While the majority of donoracceptor polymers are hole-conducting (p-type), [3][4][5] important progress has been achieved in the development of high performance, n-type polymeric semiconductors in recent years. [6][7][8] In 2009, Facchetti and co-workers introduced a novel n-type, donor-acceptor polymer, poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} P(NDI2OD-T2), which exhibits excellent electron mobilities as high as 0.85 cm 2 V −1 s −1 in top gate transistor devices under ambient conditions; [ 6 ] bulk-mobilities were found in the range of 5 × 10 −3 cm 2 V −1 s −1 for timeof-fl ight and electron-only current measurements. [ 9 ] Promising results have been further shown for all-polymer solar cells based on poly(3-hexylthiophene)s (P3HT) and P(NDI2OD-T2) as donor and acceptor, respectively, reaching power conversion effi ciencies of 1.4%. [ 10 ] Publications on P(NDI2OD-T2) initially focused on the chemistry, [ 7 ] charge transport and injection in multiple devices, [ 9,11,12 ] whereas little was reported about the structure of the semiconductor layer. While at fi rst it was assumed that P(NDI2OD-T2) forms mainly amorphous layers, [ 6 ] X-ray diffraction analysis and transmission electron microscopy (TEM) revealed the semicrystalline character of P(NDI2OD-T2) thin fi lms in recent years. [13][14][15][16][17] Rivnay et al. were the fi rst to show a remarkable degree of in-plane order in as-cast fi lms with an unconventional face-on texture in the bulk. [ 13 ] A striking texture change was observed upon melt-annealing, when the polymer chains undergo a transition from mainly face-on to edge-on. [ 15,18 ] A very recent study by Schuettfort et al. reports on a preferential edge-on texture at the top surface both for as-cast and melt-annealed layers. [ 16 ] Using mainly spectroscopic measurements, Steyrleuthner et al. demonstrated the strong tendency for aggregation not only in thin fi lms but also in solution, thereby identifying two different kinds of aggregates. [ 19 ] The precise stacking mode of the naphthalene diimide (NDI) and bithiophene (T2) units within the crystalline lattice was investigated via TEM by Brinkmann and coworkers [ 17 ] and Heeger and coworkers. [ 20 ] Highly oriented thin fi lms of P(NDI2OD-T2) were prepared by directional epitaxial crystallization (DEC) on 1,3,5-trichlorobenzene (TCB) and epitaxy on aligned fi lms of poly(tetrafl uoroethylene) (PTFE). Two polymorphs were identifi ed: i) in form I, the NDI and T2 which is up to 10 times higher than those perpendicular to the polymer chain.

A "walking" process of Ni catalysts during Kumada catalyst-transfer polycondensation along polymerizing poly(3-hexylthiophene), P3HT, chains was investigated. To simplify polymer end group identifications, a compound Br-C(6)H(4)-Ni(dppe)-Br was prepared and used as an externally addable initiator. Normally, aryl moieties present in initiators incorporate into the structure of the resulting P3HT as the starting groups. We demonstrate that due to the presence of the C-Br group located in the para-position to the Ni substituent of the initiator, two different polymeric products are formed. One of them is the "normal" product, that is, P3HT with a para-bromophenyl end group, whereas another one has the phenyl ring inside the P3HT chain. The content of the product with the internal phenyl ring increases with the increase of the polymerization degree. Control experiments demonstrated that no intermolecular catalyst transfer takes place in the conditions used. Such results suggest that catalytic Ni(0) species are able to walk along the polymerizing chain containing many tens of thienyl rings up to the opposite end and can initiate polymerization there. Numerical analysis of a random hopping model was undertaken, which revealed that a combination of a random catalyst walking along the chain and a "sticking effect" at the end groups is operative in Kumada catalyst-transfer polycondensation.

Herein, we present a new paradigm in the engineering of nanostructured hybrids between conjugated polymer and inorganic materials via a chain-growth surface-initiated Kumada catalyst-transfer polycondensation (SI-KCTP) from particles. Poly(3-hexylthiophene), P3HT, a benchmark material for organic electronics, was selectively grown by SI-KCTP from (nano)particles bearing surface-immobilized Ni catalysts supported by bidentate phosphorus ligands, that resulted in hairy (nano)particles with end-tethered P3HT chains. Densely grafted P3HT chains exhibit strongly altered optical properties compared to the untethered counterparts (red shift and vibronic fine structure in absorption and fluorescence spectra), as a result of efficient planarization and chain-aggregation. These effects are observed in solvents that are normally recognized as good solvents for P3HT (e.g., tetrahydrofurane). We attribute this to strong interchain interactions within densely grafted P3HT chains, which can be tuned by changing the surface curvature (or size) of the supporting particle. The hairy P3HT nanoparticles were successfully applied in bulk heterojunction solar cells.