Exciton dissociation in polymer field-effect transistors studied using terahertz spectroscopy

Abstract

We have used terahertz time-domain spectroscopy to investigate photoinduced charge generation in conjugated polymer field-effect transistors. Our measurements show that excitons dissociate in the accumulation layer under the application of a gate voltage, with a quantum efficiency of $\ensuremath{\sim}0.1$ for an average gate field of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{8}\phantom{\rule{0.3em}{0ex}}\mathrm{V}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$. The transistor history is found to affect the exciton dissociation efficiency, which decreases as holes are increasingly trapped in the accumulation layer. The quantum efficiency of charge formation from excitons is compared with the two contrasting models proposed by Onsager and Arkhipov based on the assumption that field-induced exciton dissociation is assisted by the Brownian diffusive motion or an initial excess energy supplied by excited vibrational modes, respectively.