Interfacial Reaction of Fulleropyrrolidines Affecting Organic Photovoltaic Performance

Fulleropyrrolidine derivatives are intrinsically basic owing to the amino group within the pyrrolidine structure. It can be predicted that the basicity of fulleropyrrolidine may affect the photovoltaic devices containing an acidic layer (e.g.

PSS). To clarify the effect of basic fulleropyrrolidine derivatives, we synthesized compounds with an N-benzyl substituent group and fabricated organic photovoltaic (OPV) cells using this N-benzyl fulleropyrrolidine. A device structure with the ITO/PEDOT:PSS/organic layer (PTB7:fulleropyrrolidine)/Ca/Al showed high series resistance, short-circuit current density (J_sc), and low fill factor (FF) values. However, OPV cells having an inverted structure, without the PEDOT:PSS layer, contributed good device performance. We were able to reproduce the high series resistance in a model experiment using aqueous ammonia vapor to treat the PEDOT:PSS layer. Our results indicated that the activity of the PEDOT:PSS layer was affected by the basicity of the fulleropyrrolidines. These results also explain why this phenomenon does not occur at the interface of OPV devices when conventional [6,6]-phenyl C₆₁ butyric acid methyl ester is used as an acceptor material. This finding would contribute to enhancing the OPV device performances from a chemical view point of designing a new compound.