Magnetic field-assisted formation of charge transfer complex in π-conjugated polymer and fullerene-blended films

Blended films of poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV)/[6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) were annealed at 70°C under rotational magnetic field (RMF) (∼350 G with 600 RPM) for 10 min. At low temperatures, the films prepared with RMF contained new spectral features. The photoluminescence (PL) spectra of films prepared under RMF had two peaks (1.59 and 1.73 eV), whereas films prepared without RMF had a single PL band at 1.75 eV. Furthermore, the photoinduced absorption (PIA) spectra of films prepared with RMF showed an additional PIA band at 1.15 eV, along with the well-known PIA band at 1.35 eV due to excited state absorption by polarons. We conclude that the spectral features in MEH-PPV/PCBM films were due to charge transfer complexes (CTC), the formation of which is promoted by RMF. Our results suggested that applying proper magnetic field while preparing polymer/fullerene film could improve the formation of CTC. We discuss the relevance to solar cell applications.