Poly(5,6-dithiooctylisothianaphtene), a new low band gap polymer: spectroscopy and solar cell construction

To enhance the efficiency of polymer photovoltaics, much effort is put into synthesis of novel compounds which show a better harvesting of solar light. In this respect, a new low band gap polymer, namely, poly(5,6-dithiooctylisothianaphtene), was synthesised. This work focusses on the spectroscopic characterisation of the material. The dynamics of the photoexcitations were studied by monitoring the dependence of the photoinduced absorption band on the laser modulation frequency and indicated dispersive recombination kinetics in this material. An appropriate model was used to describe the observed behaviour. To investigate the nature of the photogenerated species more profound, photoinduced absorption spectroscopy in the infrared was performed, showing an infrared active vibrational pattern (IRAV). Solar cells were constructed with an active layer consisting of the pristine material or a mixture with an electron accepting moiety. For this purpose, (6,6)-phenyl-C61-butyric-acid (PCBM) in 1:1 a (w/w) ratio with respect to the polymer was used. A clear improvement of the diode behaviour was observed going from the pristine material to the mixture. Photocurrent action spectra of the solar cells with the polymer:PCBM-mixture indicated an active contribution of the polymer to the photocurrent.