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 ( I sc ) densities in the range of 3−5 mA/cm 2, open circuit voltages ( V oc ) 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/cm 2 simulated AM1.5 illumination. The reference cells based on the nonchelating fullerene derivative [6,6]-phenyl-C 61 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.