Molecular-weight-dependent mobilities in regioregular poly(3-hexyl-thiophene) diodes

Abstract

We have investigated the transport properties in the direction perpendicular to the substrate of regioregular poly(3-hexyl-thiophene) of different molecular weights (MW) in a diode geometry. In these devices, which exhibit space-charge-limited behavior, we find that the mobility values at room temperature increase from 1.33 × 10 − 5 cm 2 ∕ V s to 3.30 × 10 − 4 cm 2 ∕ 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.