Molecular-weight-dependent mobilities in regioregular poly„3-hexyl-thiophene... diodes

We have investigated the transport properties in the direction perpendicular to the substrate of regioregular polys3-hexyl-thiophened of different molecular weights sMWd in a diode geometry. In these devices, which exhibit space-charge-limited behavior, we find that the mobility values at room temperature increase from 1.33310−5 cm2/V s to 3.30310−4 cm2/V s as the MW is increased from 2.9 to 31.1 kg/mol. The mobility is found to be field independent for high MW films, but field dependent for the low MW films. The current–voltage characteristics of the diodes are also studied as a function of temperature from 160 K to 300 K. The activation energy for carrier transport, extracted from the Arrhenius plot, is found to decrease gradually from 143 meV to 126 meV as the MW is increased. © 2005 American Institute of Physics. fDOI: 10.1063/1.1891301g