p-n interface stabilization of planar heterojunction organic photovoltaics by an ethyleneoxy side chain of methanofullerenes

• Fullerene derivatives with ethyleneoxy side chains were prepared.
• The fullerenes formed stable interface with copper phthalocyanine.
• Energy conversion efficiency of the photovoltaic cell was improved.

Methanofullerenes bearing both an ethyleneoxy side chain, −(CH2CH2O)n-, n = 1 or 2, and a 5-ethyl-2-thienyl group were prepared. These methanofullerenes were much more polar than PCBM and C60, and solubility in chlorobenzene (23–24 g/L) was comparable to that of PCBM. Planar heterojunction photovoltaic cells employing the methanofullerenes as an electron acceptor and copper phthalocyanine (CuPc) as an electron donor were fabricated. The short-circuit current density of the photovoltaic cell (4.61–4.66 mA/cm2) was larger than that of the photovoltaic cell prepared from the standard fullerene acceptor, PCBM, (4.08 mA/cm2), and consequently the power conversion efficiency was improved. Comparisons of contact angles of water and hexadecane on fullerene films indicated that the fullerenes with an ethyleneoxy side chain formed more stable interface with CuPc than PCBM. These results suggest that side chain engineering aiming at stable p-n interface is an attractive strategy for designing efficient organic photovoltaic cells.

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