chenjuan

A novel small-molecule acceptor, (2,2′-((5E,5′E)-5,5′-((5,5′-(4,4,9,9-tetrakis(5-hexylthiophen-2-yl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(4-(2-ethylhexyl)thiophene-5,2-diyl))bis(methanylylidene)) bis(3-hexyl-4-oxothiazolidine-5,2-diylidene))dimalononitrile (ITCN), end-capped with electron-deficient 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile groups, is designed, synthesized, and used as the third component in fullerene-free ternary polymer solar cells (PSCs). The cascaded energy-level structure enabled by the newly designed acceptor is beneficial to the carrier transport and separation. Meanwhile, the three materials show a complementary absorption in the visible region, resulting in efficient light harvesting. Hence, the PBDB-T:ITCN:IT-M ternary PSCs possess a high short-circuit current density (J_sc) under an optimal weight ratio of donors and acceptors. Moreover, the open-circuit voltage (V_oc) of the ternary PSCs is enhanced with an increase of the third acceptor ITCN content, which is attributed to the higher lowest unoccupied molecular orbital energy level of ITCN than that of IT-M, thus exhibits a higher V_oc in PBDB-T:ITCN binary system. Ultimately, the ternary PSCs achieve a power conversion efficiency of 12.16%, which is higher than the PBDB-T:ITM-based PSCs (10.89%) and PBDB-T:ITCN-based ones (2.21%). This work provides an effective strategy to improve the photovoltaic performance of PSCs.

Fullerene-free ternary polymer solar cells with a high efficiency of 12.16% are fabricated by adding a novel small-molecule acceptor to form a cascaded energy-level structure.

So far, most of the reported high-mobility conjugated polymers are p-type semiconductors. By contrast, the advances in high-mobility ambipolar polymers fall greatly behind those of p-type counterparts. Instead of unipolar p-type and n-type materials, ambipolar polymers, especially balanced ambipolar polymers, are potentially serviceable for easy-fabrication and low-cost complementary metal-oxide-semiconductor circuits. Therefore, it is a critical issue to develop high-mobility ambipolar polymers. Here, three isoindigo-based polymers, PIID-2FBT, P1FIID-2FBT, and P2FIID-2FBT are developed for high-performance ambipolar organic field-effect transistors. After the incorporation of fluorine atoms, the polymers exhibit enhanced coplanarity, lower energy levels, higher crystallinity, and thus increased µ_e. P2FIID-2FBT exhibits n-type dominant performance with a µ_e of 9.70 cm² V⁻¹ s⁻¹. Moreover, P1FIID-2FBT exhibits a highly balanced µ_h and µ_e of 6.41 and 6.76 cm² V⁻¹ s⁻¹, respectively, which are among the highest values for balanced ambipolar polymers. Moreover, a concept “effective mass” is introduced to further study the reasons for the high performance of the polymers. All the polymers have small effective masses, indicating good intramolecular charge transport. The results demonstrate that high-mobility ambipolar semiconductors can be obtained by designing polymers with fine-tuned energy levels, small effective masses, and high crystallinity.

Three isoindigo-based polymers, PIID-2FBT, P1FIID-2FBT, and P2FIID-2FBT are developed for high-performance ambipolar organic field-effect transistors. After the incorporation of fluorine atoms, the polymers show that an obvious mobility change from p-channel dominant to n-channel dominant transport characteristics. Especially, P1FIID-2FBT exhibits a highly balanced electron/hole mobility, resulting from the fine-tuned energy levels, high crystallinity, and relatively small effective mass.