Morphology Associated Positive Correlation between Carrier Mobility and Carrier Density in Highly Doped Donor–Acceptor Conjugated Polymers

Abstract

AbstractMolecular doping in conjugated polymers (CPs) has recently received intensive attention for its potential to achieve high electrical conductivity in organic thermoelectric materials. In particular, it affects not only the carrier density n but also the carrier mobility µ because high degree of molecular doping changes the morphological properties. Herein, the effect of molecular doping in CP thin films on the pathways and mechanisms of charge transport is investigated, which govern the µ‐n relationship. Two representative donor–acceptor type CPs with similar µ but different molecular assembly in an undoped state, that is poly[2,5‐(2‐octyldodecyl)‐3,6‐diketopyrrolopyrrole‐alt‐5,5‐(2,5‐di(thien‐2‐yl)thieno[3,2‐b]thiophene)] (DPPDTT) and indacenodithiophene‐co‐benzothiadiazole (IDTBT), are prepared. Heavy doping with iron chloride (FeCl3) induced DPPDTT with highly crystalline edge‐on orientation to increase its µ up to 19.6 cm2 V−1 s−1, whereas IDTBT with irregular intermolecular stacking showed little change in µ. It is revealed that this different µ‐n relationship is highly attributed to the initial molecular ordering of CP films. The charge transport mechanism also becomes significantly different: both coherent and incoherent transports are observed in the doped DPPDTT, whereas incoherent transport is only found in the doped IDTBT. This study suggests guidelines for enhancing charge transport of CPs under doping in terms of structural disorder.