Self n-doped [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)-(dimethyl)ammonium) ethyl ester diiodides as a cathode interlayer for inverted polymer solar cells

Abstract

A series of self n-doped fullerene ammonium derivatives have been synthesized and confirmed with electron paramagnetic resonance and conductivity measurements. The existence of stable C₆₀O₂˙⁻ anion radical in these materials resulted in intrinsically high conductivities between 1.05 × 10⁻² and 1.98 × 10⁻² S cm⁻¹. Among fullerenes with different numbers of ammonium and counter anions, [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)-(dimethyl)ammonium)-ethyl ester diiodides (PCBDANI) showed the best solvent resistance, which was confirmed by the measurement of film thickness and corresponding UV-vis absorption before and after rinsing with dichlorobenzene. Most importantly, the inverted polymer solar cells with the structure of ITO/PCBDANI/P3HT:PCBM/MoO₃/Ag retained reasonably high power conversion efficiency even at a thickness of 82 nm of the PCBDANI film as the cathode interlayer. Thus large-area devices via printing this interlayer or printing on this interlayer could become feasible.