Fulleropyrrolidine interlayers: Tailoring electrodes 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 cathode. We found that in single-junction polymer solar cells, this problem can be surmounted by solution-based incorporation of fulleropyrrolidines with amine (C-60-N) or zwitterionic (C-60-SB) substituents as cathode-independent buffer layers. Specifically, a thin layer of C-60-N reduced the effective work function of Ag, Cu, and Au electrodes to 3.65 electron volts. Power conversion efficiency 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 C-60-N and C-60-SB layers ranging from 5 to 55 nanometers performed with high efficiency.