Photoinduced charge transfer between tetracyano-anthraquino-dimethane derivatives and conjugated polymers for photovoltaics

The photoinduced charge transfer between tetracyano-anthraquino-dimethane ( TCAQ) derivatives and poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) has been studied by photoinduced absorption (PIA) spec troscopy in the VIS and IR spectral region and by light induced electron sp in resonance (LESR). Studies on three different TCAQ derivatives with diffe rent side chain substitutions are reported. Among them a supermolecule with TCAQ attached to a fullerene have been synthesized to serve as tandem elec tron acceptor. The photoinduced absorption in the Vis/near-IR range shows a broad plateau around 1.8 eV followed by two peaks at 1.35 and 1.24 eV with an additional broad feature below 0.5 eV for all three accepters in compos ite with MDMO-PPV. All PW features have a power law excitation intensity de pendence with an exponent close to 0.5 as expected for bimolecular kinetics . LESR studies show one absorption at a g-factor of 2.0028 with a width (pe ak-to-peak) of 3.5 G, originating from TCAQ anion radical and the polymer c ation radical. Evidence for the formation of a C-60 anion was found in the case of electron transfer from the MDMO-PPV polymer to the TCAQ-C-60 accept or dyade from these LESR studies. Finally bulk-heterojunction type thin fil m structures of MDMO-PPV as donor and the TCAQ derivatives as acceptor have been fabricated and tested as photovoltaic devices.