Fulleropyrrolidine interlayers lower cathode work function to raise organic solar cell efficiency

A major challenge in organic solar cell design is the trade-off between oxidative stability and work function of the metal used as a cathode. Here we report that solution-based incorporation of fulleropyrrolidines with amine (\textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-N}) or zwitterionic (\textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-SB}) substituents as cathode-independent buffer layers conveniently surmounts this barrier in single junction polymer solar cells. Specifically, a thin layer of \textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-N} reduced the effective work function of Ag, Cu, and Au electrodes to 3.65 eV. Power conversion efficiency (PCE) values exceeding 8.5{\%} were obtained for organic photovoltaics independent of the cathode selection (Al, Ag, Cu or Au). Such high efficiencies did not require precise control over interlayer thickness, as devices prepared with \textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-N} and \textbf{C}$_{\mathrm{\mathbf{60}}}$\textbf{-SB} layers ranging from 5 to 55 nm functioned with high efficiency.