Supramolecular Association of Pyrrolidinofullerenes Bearing Chelating Pyridyl Groups and Zinc Phthalocyanine for Organic Solar Cells

We investigated donor−acceptor bilayer heterojunctions formed by deposition of solution-processed pyrrolidinofullerenes bearing chelating pyridyl groups (PyFs) on vacuum-evaporated films of zinc phthalocyanine (ZnPc). It is shown that coordination complexes are formed at the interface between these donor and acceptor components; such association facilitates photoinduced charge separation and results in improved performance of the photovoltaic devices. Thus, the bilayer photovoltaic cells fabricated from different pyrrolidinofullerenes and ZnPc exhibit short circuit current (Isc) densities in the range of 3−5 mA/cm2, open circuit voltages (Voc) of 400−600 mV, and fill factors (FF) of 40−50% that correspond to power conversion efficiencies (η) of up to 1.5% under 100 mW/cm2 simulated AM1.5 illumination. The reference cells based on the nonchelating fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor component yield lower power conversion efficiencies (0.4−0.6%); the performance of such devices can be increased significantly by mixing PCBM with a small amount (4% w/w) of PyF in the acceptor layer. A novel multicomponent organic solar cell architecture is suggested in order to expand the active layer absorption by formal combination of the solution-processed bulk heterojunction polymer/fullerene cells with evaporated bilayer ZnPc/fullerene devices. For this purpose, a blend of the fullerene derivatives (PCBM and PyF mixed in different ratios) with the polyconjugated polymer poly((2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene) vinylene (MDMO−PPV) is spin-coated on the ZnPc film sublimed on an indium tin oxide (ITO) substrate. Evaporation of the top aluminum electrodes yields photovoltaic devices that demonstrate power conversion efficiencies of up to 2% and efficient photocurrent generation in the full range from 350 to 820 nm.