Influence of Pendant Group on Mobility of Organic Thin Film Transistor in Correlation with Reorganization Energy of Molecules

Abstract Charge transport properties of common donor copolymers in organic photovoltaics, poly({4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl}{3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl}) (PTB7) and poly([2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene]{3‐fluoro‐2[(2‐ethylhexyl) carbonyl]thieno[3,4‐b]thiophenediyl}) (PTB7‐Th), with molecular structures differing only in the pendant group, are studied. This is the first report of field‐effect transistor mobility ( µ FET ) of PTB7‐Th (0.14 cm 2 V −1 s −1 ) and the highest µ FET for PTB7 (0.01 cm 2 V −1 s −1 ). µ FET of PTB7‐Th is found to be almost one order of magnitude higher than PTB7. To understand the influence of molecular structure on charge transport, hole reorganization energy (λ h ) is calculated from first‐principles. λ h of PTB7‐Th (≈150 meV) is found to be lower than PTB7 (≈346 meV). Further, the ratio of hopping rate versus square of charge transfer integral calculated from Marcus theory using λ h for these systems is found to indicate a higher rate of hole transfer across dimers or homojunction interface for PTB7‐Th. These results are supplemented by experimentally determined λ using bulk‐heterojunction organic solar cells, where λ PTB7‐Th ≈200 meV and λ PTB7 ≈310 meV follow a similar trend. The effective hole‐mobility estimation from BHJ devices correlates well with these λ values. This study provides understanding of charge transport properties via reorganization energy, as a function of pendant group without altering the backbone of the chains.