Tightly Bound Double‐Caged [60]Fullerene Derivatives with Enhanced Solubility: Structural Features and Application in Solar Cells

A series of novel highly soluble double‐caged [60]fullerene derivatives were prepared by means of lithium‐salt‐assisted [2+3] cycloaddition. The bispheric molecules feature rigid linking of the fullerene spheres through a four‐membered cycle and a pyrrolizidine bridge with an ester function CO2R (R=n‐decyl, n‐octadecyl, benzyl, and n‐butyl; compounds 1 a–d, respectively), as demonstrated by NMR spectroscopy and X‐ray diffraction. Cyclic voltammetry studies revealed three closely overlapping pairs of reversible peaks owing to consecutive one‐electron reductions of fullerene cages, as well as an irreversible oxidation peak attributed to ion of an electron from the nitrogen lone‐electron pair. Owing to charge delocalization over both carbon cages, compounds 1 a–d are characterized by upshifted energies of frontier molecular orbitals, a narrowed bandgap, and reduced electron‐transfer reorganization energy relative to pristine C60. Neat thin films of the n‐decyl compound 1 a demonstrated electron mobility of (1.3±0.4)×10−3 cm2 V−1 s−1, which was comparable to phenyl‐C61‐butyric acid methyl ester (PCBM) and thus potentially advantageous for organic solar cells (OSC). Application of 1 in OSC allowed a twofold increase in the power conversion efficiencies of as‐cast poly(3‐hexylthiophene‐2,5‐diyl) (P3HT)/1 devices relative to the as‐cast P3HT/PCBM ones. This is attributed to the good solubility of 1 and their enhanced charge‐transport properties — both intramolecular, owing to tightly linked fullerene cages, and intermolecular, owing to the large number of close contacts between the neighboring double‐caged molecules. Test P3HT/1 OSCs demonstrated power‐conversion efficiencies up to 2.6 % (1 a). Surprisingly low optimal content of double‐caged fullerene acceptor 1 in the photoactive layer (≈30 wt %) favored better light harvesting and carrier transport owing to the greater content of P3HT and its higher degree of crystallinity. Acting cagey: Highly soluble double‐caged fullerene derivatives 1 were prepared and characterized (see figure). Test organic solar cells involving 1 demonstrated a twofold increase in power conversion efficiency relative to that with phenyl‐C61‐butyric acid methyl ester (PCBM).