  Solution-processed networks of semiconducting, single-walled carbon nanotubes
(SWCNTs) have attracted considerable attention as materials for next-generation
electronic devices and circuits. However, the impact of the SWCNT network
composition on charge transport on a microscopic level remains an open and
complex question. Here, we use charge-modulated absorption and
photoluminescence spectroscopy to probe exclusively the mobile charge carriers
in monochiral (6,5) and mixed SWCNT network field-effect transistors. Ground
state bleaching and charge-induced trion absorption features, as well as
exciton quenching are observed depending on applied voltage and modulation
frequency. Through correlation of the modulated mobile carrier density and the
optical response of the nanotubes, we find that charge transport in mixed SWCNT
networks depends strongly on the diameter and thus bandgap of the individual
species. Mobile charges are preferentially transported by small bandgap SWCNTs
especially at low gate voltages, whereas large bandgap species only start to
participate at higher carrier concentrations. Our results demonstrate the
excellent suitability of modulation spectroscopy to investigate charge
transport in nanotube network transistors and highlight the importance of SWCNT
network composition for their performance.
