N-type Semiconducting Polymers Based on Dicyano Naphthobisthiadiazole: High Electron Mobility with Unfavorable Backbone Twist

Abstract

Development of n-type semiconducting polymers is essential for organic electronic devices. In this work, we synthesize a new strong acceptor unit by converting the fluorine groups on the naphthobisthiadiazole (NTz) moiety into cyano groups via a nucleophilic aromatic substitution reaction. As a result, the p-type character observed for polymers based on NTz and difluorinated NTz is switched into an ambipolar and/or n-type character in polymers based on dicyano NTz because of the significantly improved electron affinity and thus lowered lowest unoccupied molecular orbital levels. Interestingly, although the cyano groups cause an unfavorable sterical twist in the backbone, the polymers show high electron mobilities in organic field-effect transistors. We highlight that the dicyano NTz-based polymers form a high crystalline structure similar to the noncyano NTz-based polymers and that there seems to be short contacts between the cyano group and the NTz core in the adjacent polymer chain. These structural features would account for the high mobilities despite the backbone twisting. We propose that the introduction of cyano groups into a large π-system with an electron-poor nature is crucial for the development of n-type semiconducting polymers.

Acceptor Modulation Strategies for Improving the Electron Transport in High‐Performance Organic Field‐Effect Transistors

“…A series of copolymers (P40a−P40d) based on CN‐naphthobisthiadiazole (CNNTz) and bithiophene with different F atoms (0, 2F, and 4F) were synthesized by Osaka et.al. [ 128 ] Due to the introduction of the electron‐withdrawing CN groups in the NTz moiety, the HOMO/LUMO energy level of P40a was −5.49/−3.66 eV, which was lower than that of the analogues based on NTz and F‐NTz. [ 129,130 ] In addition, increasing the F atoms on the 2T moieties gradually lowered the HOMO and LUMO energy levels from P40a to P40d.…”

Engineering of the alkyl chain branching point on a lactone polymer donor yields 17.81% efficiency

“…[15][16][17] Nowadays, many high-performance CPs in OSCs and OFETs contain branched alkyl chains. [18][19][20][21][22][23][24][25][26][27][28] Some studies indicate that changing the branching point of alkyl chains can vary the packing of CPs, leading to different lm morphology and device performance. 29,30 Manipulating the alkyl chain branching point has become an effective strategy to improve the performance of CPs in OSCs and OFETs.…”

Naphthobispyrazine Bisimide: A Strong Acceptor Unit for Conjugated Polymers Enabling Highly Coplanar Backbone, Short π–π Stacking, and High Electron Transport

“…Naphthobischalcogenadiazole (NXz) units, such as naphtho­[1,2- c :5,6- c ′]­bis­[1,2,5]­thiadiazole (NTz), − naphtho­[1,2- c :5,6- c ′]­bis­[1,2,5]­oxadiazole (NOz), , and their derivatives, − are emerging acceptor units for high-performance π-conjugated polymers (Figure a). Due to the large π-electron systems that can induce strong π–π intermolecular interaction, π-conjugated polymers based on these building units can give highly ordered packing structures and thus high charge carrier mobilities in OFETs. ,,,, Interestingly, whereas polymers based on NTz showed p-type characteristics, those based on NOz showed the ambipolar behavior due to the enhanced electron deficiency in NOz relative to that in NTz . We have recently reported a series of polymers based on dicyano NTz (CN-NTz), which also exhibits enhanced electron deficiency compared to NTz, and found that the polymers have ambipolar to n-type semiconducting characteristics depending on the co-monomer unit in the backbone .…”

“…Due to the large π-electron systems that can induce strong π–π intermolecular interaction, π-conjugated polymers based on these building units can give highly ordered packing structures and thus high charge carrier mobilities in OFETs. ,,,, Interestingly, whereas polymers based on NTz showed p-type characteristics, those based on NOz showed the ambipolar behavior due to the enhanced electron deficiency in NOz relative to that in NTz . We have recently reported a series of polymers based on dicyano NTz (CN-NTz), which also exhibits enhanced electron deficiency compared to NTz, and found that the polymers have ambipolar to n-type semiconducting characteristics depending on the co-monomer unit in the backbone . We have also focused on π-conjugated polymers based on naphtho­[1,2- b :5,6- b ′]­bispyrazine (NPz), which is an analogue of NXz (Figure a), , whereas other groups have reported NPz-based polymers as p-type semiconductors for OPVs. − An important feature of NPz is that it allows the introduction of four substituents at 2-, 3-, 8-, and 9-positions, in contrast to NTz and NOz.…”