The Aggregation of Poly(3-hexylthiophene) into Nanowires: With and without Chemical Doping

Abstract

Non-doped and chemically p-doped poly(3-hexylthiophene) (P3HT) nanowires are investigated by optical spectroscopy and atomic force microscopy (AFM) to reveal the effects of doping on their aggregation processes and nanoscale morphologies. The AFM studies on the non-doped P3HT nanowires formed in the early aggregation stage demonstrate that P3HT molecules will probably go through either packing of high-aspect ratio multi-chain/single-chain aggregates or packing of solubilized individual chains depending on experimental conditions. High-resolution AFM images also show the connecting segments between ordered domains of P3HT. For pdoping of P3HT in the solution phase, the addition of 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (F 4 -TCNQ), a p-dopant, will greatly accelerate the aggregation rate of 1D P3HT nanostructures. The doped P3HT nanowires are comprised of linearly connected domains that are 40− 60 nm wide and 5−10 nm high. Compared with the non-doped P3HT nanowires, doped nanowires show smoother edges and less protruding segments, likely due to a different aggregation mechanism. Furthermore, the doped nanowires tend to agglomerate into disordered bundles and clusters because of the presence of F 4 -TCNQ counterions and complexity resulting from Coulomb interactions and other doping-induced growth defects.

The impact of aggregation on the p-doping kinetics of poly(3-hexylthiophene)

“…The p-doping of s-P3HT appears to enhance the aggregation peaks, which has been focused in a separate study. 38 The p-doping of a 30 μg/mL P3HT aggregated solution by F 4 -TCNQ also yields F 4 -TCNQ anion and P3HT polaron, as evidenced by the emergence of identical absorbance bands in the NIR range shown in Figure 2(b). An interesting finding here is that the p-doping reaction of the aggregated solution is dramatically faster than that of s-P3HT.…”

The optical signatures of molecular-doping induced polarons in poly(3-hexylthiophene-2,5-diyl): individual polymer chainsversusaggregates

“…The red shift of the absorption peak is ascribed to more exciton delocalization that accompanies planarization of P3HT chains once formed into ordered aggregates, which results in decreasing its energy. 19,[30][31][32] The absorption fine structure results from the coupling between the electronic and vibrational transitions in crystalline P3HT, as well as the interchain coupling. 1,30 Increasing the temperature of re-P3HT solution in poor solvents increases solubility, and the re-P3HT aggregates transform to individual chains.…”

“…[14][15][16][17] The abundance of re-P3HT aggregates in solution was reported to increase upon addition of molecular p-type dopants, with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) being the one most frequently used, and was ascribed to decreased solubility of the corresponding ionized species after ion pair (IPA) formation (doping involving integer electron transfer). 18,19 It was shown that aggregation occurs more readily for solution-mixed doping as compared to sequential doping of re-P3HT thin films. 20 Doping efficiency (fraction of dopants that become ionized), mechanism, and kinetics of solution-mixed doped P3HT are influenced by the regioregularity and molecular-scale order of P3HT in solution before doping, and accordingly, on the relative presence of ordered aggregates and solvated individual chains.…”

Poly(3‐hexylthiophene) aggregation at solvent–solvent interfaces

“…The presence of a larger amount of good solvent results in more favorable solvent–polymer interactions, which minimizes the extent of the initial nucleation processes and subsequent fractionation effects associated with secondary growth. McFarland et al describe single‐chain and multi‐chain aggregation processes that generate, respectively, well‐ordered and disordered aggregates, and these processes can be rationalized in terms of parallel thermodynamic and kinetic factors. Roehling et al posited a loosened chain conformation mechanism, wherein slower kinetics of the P3HT assembly facilitates the formation of aggregates with higher structural order because it allows the P3HT chains to search for the lowest energy arrangement during aggregation .…”