Impact of Molecular Weight on Microstructure and Carrier Mobility in Poly(dithienofluorene-<i>alt</i>-dithienodiketopyrrolopyrrole)

The synthesis of three poly(dithienofluorene-alt-dithienodiketopyrrolopyrrole) polymers, PDTFDPP-H with an average molecular weight (Mn) of 77 kg mol–1, PDTFDPP-M with an Mn of 28 kg mol–1, and PDTFDPP-L with an Mn of 11 kg mol–1, has been accomplished. UV–vis spectroscopy reveals a shoulder at 780 nm for PDTFDPP-H, indicating potential polymer aggregation. Solvent- and temperature-variable UV–vis absorption spectra, coupled with DFT calculations, support this observation. Hole mobility (μh) follows the sequence PDTFDPP-H (0.305 cm2 V–1 s–1) > PDTFDPP-M (1.88 × 10–3 cm2 V–1 s–1) > PDTFDPP-L (5.19 × 10–4 cm2 V–1 s–1). Grazing incidence X-ray scattering reveals that PDTFDPP-L and PDTFDPP-M primarily exhibit edge-on crystallites, while PDTFDPP-H displays both edge-on and face-on crystallites, all demonstrating short-range π-order. The formation of edge-on or face-on crystallites seems to correlate with the polymer molecular weight. Transmission electron microscopy analysis of PDTFDPP-H reveals chain entanglement. Overall, the results suggest that the PDTFDPP-H aggregates are interconnected with the help of one-dimensional polymer chains, achieving a μh higher than 0.1 cm2 V–1 s–1.