Novel fulleropyrrolidines bearing π-conjugated thiophene derivatives as compatibilizing group for developing highly stable polymer solar cells

• Fulleropyrrolidines bearing terthiophene/dithienylthienothiophene were synthesized.
• These novel fulleropyrrolidine derivatives used as a compatibilizing agent.
• Ethylhexyl side chain prevents large C60 clusters and increase D/A interface.
• Both 3T-EH-C60 & 2TTT-EH-C60 exhibit stable morphology against long-term aging.

A series of new fulleropyrrolidines, which comprise one terthiophene (3T) or dithienyl thienothiophene (2TTT) conjugated segment along with two hexyl (H) or ethylhexyl (EH) chains, abbreviated as 3T-H-C60, 3T-EH-C60, 2TTT-H-C60 and 2TTT-EH-C60, were designed and synthesized to improve their structural compatibility with poly(3-hexylthiophene) (P3HT). These [60]fullerene monoadducts exhibit a characteristic UV–Vis absorption band, associated with the conjugated substituent, and similar optical bandgaps and lowest unoccupied molecular orbital (LUMO) levels of approximately 1.71 eV and −3.72 eV, respectively. The influence of the structure of the substituent in the fulleropyrrolidine on its intermolecular packing within a polymer matrix was investigated. Photoluminescence measurements of the blended films of such C60 derivatives with P3HT reveal that the interfacial area between the two materials is larger in the 3T-based system than in the 2TTT-based system; moreover, the branched ethylhexyl chain is more effective than the linear hexyl group in reducing the size of the phase domains. Transmission electron microscopic images further confirm that relatively serious phase segregation occurs in the 2TTT-C60/P3HT films. Accordingly, solar cells with 3T-C60/P3HT as the active layer have an increased short-circuit current density owing to improved exciton dissociation efficiency, but 2TTT-C60/P3HT devices have a boosted fill factor, owing to the formation of less obstructed pathways for the transport of charge carriers. More interestingly, both 3T-EH-C60/P3HT and 2TTT-EH-C60/P3HT exhibit highly stable blend morphology and the solar cells that are derived from them have extremely steady power conversion efficiency against long-term aging at high temperature.

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