Diarylfluorene-modified fulleropyrrolidine acceptors to tune aggregate morphology for solution-processable polymer/fullerene bulk-heterojunction solar cells

A series of n-type fulleropyrrolidine derivatives as the acceptors, including Th-C60, PFTh-C60, and OPFTh-C60, have been synthesized via the key step of the typical Prato reaction to investigate the steric hindrance effect of various phenylfluorenyl moieties on the electronic structures, aggregate morphologies, and device performances of solar cells. Conjugation-interrupted linkage obviously does not change the energy bandgaps and lowest unoccupied molecular orbital (LUMO) energy levels in PFTh-C60 and OPFTh-C60 models with respect to that of precursor Th-C60 according to UV-vis spectra and cyclic voltammetry. In contrast, dramatically different phase separation behaviors in the bulk heterojunction (BHJ) film blending with poly(3-hexylthiophene) (P3HT) were observed by atomic force microscopy. A prototype OPFTh-C60-based BHJ polymer solar cell (PSC) with the configuration of ITO/PEDOT:PSS/P3HT:OPFTh-C60 (1:1) (200 nm)/Ca/Al has the performance with the short-circuit current (I _sc) of 8.68 mA/cm ², open-circuit voltage (V _oc) of 0.63 V, fill factor of 0.51, and power conversion efficiency of 2.80%, better than that in PFTh-C60 or Th-C60-based counterpart devices. Our results indicate that high-performance solar cells can be achieved by the morphology control of active thin films. Diarylfluorene-modified C60 derivatives are promising n-type organic semiconductors for their applications in BHJ PSCs.