Electronic processes in end-functionalized oligomeric poly(diphenylvinylene)s

Abstract

The photoluminescence and photoelectrical properties of soluble poly(diphenylvinylene)s (PDV) of the mean number-average degree of polymerization ranging from 7 to 9 with main chains end-capped with: (i) diphenylmethylene (PDV-P), (ii) phenylcarbonyl (PDV-C), and (iii) fluorene-9,9-diyl (PDV-F) groups are reported. The introduced groups were found to affect the decay path of photogenerated excitons significantly: PDV-P shows the medium-intensity photoluminescence in the blue spectral region and poor photoconductivity decreasing about one order of magnitude when going from the excitation wavelength λ exc of 254–366 nm; PDV-C exhibits fluorescence quenching but the photoconductivity 10 times as high as that of PDV-P, which, however, decreases about almost two orders of magnitude when changing λ exc from 254 to 366 nm; PDV-F shows strong photoluminescence and high photoconductivity which does not decrease significantly if λ exc is changed from 254 to 366 nm. The observed behavior of PDV-F shows that not always a strong photoluminescence should be accompanied with a weak photoconductivity, which is explained by a formation of aggregates in PDV-F, into which excitons are effectively transferred. This trapping results in a prolonged lifetime of excitons and, consequently, in an increase in the charge carrier formation at high electric field. On the other hand, the aggregates also act as traps hindering the charge carrier transport in PDV-F. Since PDV-C also shows a reasonable photoelectrical sensitivity at low electric fields, it was tested as an active material for photovoltaic cells. The obtained parameters of the ITO/PDV-C/Al sandwich cell (illumination with white light of the intensity ca. 10 mW cm −2): short-circuit current I sc = 2 × 10 −10 A, open circuit voltage U oc = 0.5 V and fill factor FF = 0.5 provided a rather discouraging value of the power efficiency of about 10 −7.