Effect of Fluorination on Molecular Orientation of Conjugated Polymers in High Performance Field-Effect Transistors

Abstract

Fluorinated conjugated polymers have been widely used in high performance polymer solar cells, but they showed limited application in field-effect transistors (FETs). In this paper, we focus on the influence of fluorine atoms upon charge transport of conjugated polymers in FET devices. Two series of conjugated polymers without or with fluorine atoms were designed and applied into FETs. Nonfluorinated conjugated polymers show high hole mobilties up to 11.16 cm2 V–1 s–1, while fluorinated polymers exhibit low hole mobilities below 1.80 cm2 V–1 s–1. Further investigation by differential scanning calorimetry (DSC) and 2D grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) reveal that fluorinated conjugated polymers show low crystallinity and “face-on” orientation in thin films, explaining their poor hole mobilities in FET devices. Our results clearly show how the chemical structures influence the charge transport properties, which can be used to design new conjugated polymers toward high performance FETs.

Challenge and Solution of Characterizing Glass Transition Temperature for Conjugated Polymers by Differential Scanning Calorimetry

“…DSC measurements for PDPP‐T3 and PNDI‐C0, which have highly planar and rigid backbone structures, are presented in Figure (d,e), the step change in c p , thus the glass transition, cannot be readily seen from the DSC scans. A linear curve is observed and is expected based on various previous reports . PDPP‐T3 shows a small curvature at the vicinity of 0 °C.…”

“…( T g discussed herein refers to backbone T g .) For example, it has been shown that for diketopyrrolopyrrole‐based (DPP) polymers, neither conventional DSC nor Flash DSC exhibits a clear T g …”

Glass Transition Phenomenon for Conjugated Polymers

“…In addition, it has been commented that the regular semicrystalline polymers often exhibit a broad glass transition, which leads to the difficulty of T g determination . We believe this is also true for the conjugated polymers, for example, the diketopyrrolopyrrole (DPP) based polymers whose T g cannot be readily obtained from DSC . Third, indeed ideally the first heating scan should remove the thermal history, that is, melts the crystalline fraction, if many conjugated polymers could still crystallize under the cooling rate accessible by standard DSC, the crystallization could still take place on cooling.…”

Controlling Ambipolar Charge Transport in Isoindigo‐Based Conjugated Polymers by Altering Fluorine Substitution Position for High‐Performance Organic Field‐Effect Transistors

“…Thus, PIIG‐oFT2 has the smallest d 010 and L h 00 among the three polymers, resulting in an enhanced tendency to aggregate in solution state owing to the more planar backbone, thereby lowering the solubility of the polymer. It is assumed that polymer chains can not have enough time to self‐assemble, limiting the growth of large‐size polymer crystals …”