We show that rational functionalization of the naphthodithiophene core in copolymers based on naphthodithiophene and naphthobisthiadiazole improves the solubility without an alteration of the electronic structure. Surprisingly, the introduction of linear alkyl chains brings about a drastic change in polymer orientation into the face-on motif, which is beneficial for the charge transport in solar cells. As a result, the present polymers exhibit high power conversion efficiencies of up to ~8.2% in conventional single-junction solar cells.

Selective functionalization protocols of naphtho[1,2-b;5,6-b']dithiophene (NDT3) by combining protection of the thiophene α-positions and direct borylation on the naphthalene core are described, which allows synthesizing a number of new NDT3-based building blocks with various substituents and isomeric NDT3-based polymers with different main chain structures. The same protocol is applicable to other isomeric naphthodithiophenes (NDTs), which affords a set of key building blocks for the development of elaborated functional π-materials.

Semiconducting polymers with alkylated naphtho [1,2b:5,6-b′]dithiophene (NDT3) and naphtho[2,1-b:6,5-b′]dithiophene (NDT4) are synthesized and characterized. The solubility of the present polymers is significantly improved as compared to the nonalkylated counterparts with preserving the good charge transport properties. Interestingly, the effect of alkylation is found to be quite distinct between the NDT3 and NDT4 cores. In the NDT3-based polymers, alkylation leads to the more ordered backbone structure and thus the increased crystalline order in the thin film. On the other hand, in the NDT4-based polymers, alkylation is detriment to the backbone ordering, which gives rise to the face-on orientation or amorphous like film structure. This difference can be qualitatively explained by the different alkyl placement; all the neighboring alkyl groups are in the anti arrangement in the NDT3-based polymers, whereas the arrangement is a mixture of anti and syn in the NDT4-based polymers, which likely causes steric impact on the backbone. These observations make us better understood how the alkylation affect the ordering structures, which would be an important guideline for the design of superior semiconducting polymers.

To investigate the effect of the branching position of the alkyl groups on the side chain of semiconducting polymers, we synthesized two series of semiconducting polymers based on thienothiophene-2,5-dione (PTTD4Ts) and quinacridone (PQA2Ts). 2-Decyltetradecyl, 3-decylpentadecyl, 4-decylhexadecyl, and 5-decylheptadecyl groups were used, and the branching position was systematically varied from the second carbon from the backbone to the fifth carbon. These branched side chains are introduced into the thiophene ring for PTTD4Ts and the quinacridone unit for PQA2Ts. The polymer thin films exhibited small but clear differences in their optical absorption spectra, suggesting that the intermolecular interaction in the solid state varied based on the branching position. The grazing incident X-ray diffraction study revealed that the π-π stacking d-spacing of both polymers decreased when the branching position was moved away from the backbones, indicating that the intermolecular interaction was enhanced. Therefore, regardless of the core where the alkyl groups were introduced, the branching position effectively improved the ordering structure of the polymers, which was most likely due to suppressed steric hindrance. Although PTTD4Ts did not exhibit a clear correlation between the branching position and charge carrier mobility, the mobility of PQA2Ts gradually increased as the branching position moved away from the backbone.