Here the effect of solvent quality on the microstructure and organic field-effect transistor (OFET) performance of thin films of the high mobility naphthalene-diimide-thiophene based n-type semiconducting copolymer P(NDI2OD-T2) is investigated. A strong correlation between OFET mobility and solvent quality is observed with average electron mobility increasing from 0.21 cm 2 /Vs for samples prepared from tolerably-good solvents to 0.56 cm 2 /Vs for samples prepared from poor solvents, with a maximum electron mobility of 1.5 cm 2 /Vs observed for transistors processed from toluene. The variation in transistor performance with solvent quality is linked to the nature and extent of the solution aggregation of P(NDI2OD-T2) chains. Small angle X-ray scattering measurements reveal elongated rod-like aggregates up to 300 nm in length in solutions prepared using poor solvents, in contrast to more coil-like chains with radius of gyration of ~ 10 to 15 nm for solutions based on good to tolerably-poor solvents. Thin films produced from decreasing solvent quality show an increase in the extent of correlated ordering of backbones and the degree edge-on orientation of polymer chains at the air/film interface. This work establishes the important link between solution-phase chain aggregation behavior, thin-film microstructure and transistor performance in the P(DNI2OD-T2) system.

The charge transport and microstructural properties of five different molecular weight (MW) batches of the naphthalenediimide-thiophene copolymer P(NDI2OD-T2) are investigated. In particular, the field-effect transistor (FET) performance and thin-film microstructure of samples with MW varying from M n = 10 to 41 kDa are studied. Unlike conventional semiconducting polymers such as poly(3-hexylthiophene) where FET mobility dramatically drops with decreasing molecular weight, the FET mobility of P(NDI2OD-T2)-based transistors processed from 1,2-dichlorobenzene is found to increase with decreasing MW. Using a combination of grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, atomic force microscopy, and resonant soft X-ray scattering, the increase in FET mobility with decreasing MW is attributed to the pronounced increase in the orientational correlation length (OCL) with decreasing MW. In particular, the OCL is observed to systematically increase from <100 nm for the highest MW samples to ≈1 µm for the lowest MW samples. The improvement in OCL and hence mobility for low MW samples is attributed to the lack of aggregation of low MW chains in solution promoting backbone ordering, with the pre-aggregation of chains in 1,2-dichlorobenzene found to suppress longer-range liquid crystalline order.

Polymer analogous thionation of the n-type conjugated polymer PNDIT2 is investigated using Lawesson's reagent (LR). Detailed high-temperature NMR spectroscopic investigations show that due to the copolymer structure, two out of the four available carbonyl groups present in the naphthalene diimide (NDI) comonomer are sterically less hindered and react preferentially. This leads to regioselective thionation in the trans-configuration even for a large excess of LR. For high degrees of O/S conversion, signals of minor intensity show up in addition pointing to undesired side reactions. These signals could not be eliminated despite further optimized reaction conditions including different aromatic solvents and reaction temperatures. Compared to PNDIT2, the resulting 2S-trans-PNDIT2 features strong aggregation, lower solubility, an 80 nm bathochromic shift of the charge-transfer band, a by 0.22 eV lower LUMO energy level, a lower thermal stability, and higher melting temperatures (T m ). As the combination of the lower thermal stability and higher melting points renders the characterization of thermal transitions challenging, fast scanning calorimetry (flash-DSC) is successfully used to determine T m . With increasing O/S conversion, T m first increases but then decreases, which is ascribed to a combined effect of stronger main chain interactions and increasing chemical defects. Microstructural order and field-effect electron mobilities decrease with increasing O/S conversion compared to PNDIT2.

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