Large Modulation Of Chains Alignment In Donor-acceptor Conjugated Polymers

D-A type conjugated polymers have received considerable attentions due to their solution processability and flexibility for use in low-cost and large-area optoelectronic devices such as light-emitting diodes(LEDs) and field effect transistors(FETs). Due to the anisotropy of charge transport in conjugated polymers, modulation on the molecular and crystalline alignment of conjugated polymers is crucial to the improvement of device performances. The typical high-mobility D-A type conjugated polymer we synthesized is poly[2,5-bis(2-octyldodecyl)pyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-2,2′:5′,2′′:5′′,2′′′-quaterthiophene](PDQT), we have demonstrated various widths of PDQT nanostrip arrays can be readily fabricated by nanotransfer printing. Furthermore, we studied the alignment of PDQT chains in nanocrystalline lamellae within the nanostructures, and the role of confinement dimension in the modulation of crystalline alignment and the subsequent carrier mobilities in FETs. This study mainly comprises three parts:(1)Synthesis of PDQT and preparation of PDQT nanostrip arrays. We have successfully synthesized the D-A conjugated polymer PDQT with Stille coupling polymerization, and demonstrated large-area PDQT nanostrip arrays with various widths can be readily fabricated by nanotransfer printing, which was characterized by atomic force microscopy(AFM).(2)Modulation of the alignment of PDQT chains. We characterized the orientation of polymer chains in PDQT films and nanostructures by means of grazing incidence wide-angle X-ray diffraction(GIWAXD) and selected area electron diffraction(SAED), and compared the orientation degree of PDQT crystals varying the width of nanostrips with the assistance of cross-polarizer optical miscropy(CPOM). We demonstrated edge-on orientation with random alignment of b- and c-axis in plane is adopted in unprocessed thin films which are composed of nanocrystalline lamellae out of order in the plane. After nanotransfer printing, arrays of nanostrips which are composted of aligned nanocrystalline lamellae can be obtained. Importantly, crystallographic b-axis of nanocrystalline lamellae is aligned essentially parallel to the long axis of nanostrips remaining edge-on packing due to the confinement effect of mold, and therefore c-axis is perpendicular to the long axis of nanostrips. Moreover, orientation degree of b-axis was demonstrated dependent on the width of nanostrips supported by CPOM results that the orientation degree of nanocrystalline p-stacking lamellae increases with the decreasing width of nanotrips.(3)Modulation of PDQT nanocrystalline alignment on carrier mobilities of FETs. Using FET channels placed on the PDQT nanostrip arrays, we have measured the macroscopic charge transport mobility in the b-c plane arising from the alignment of b轴. The mobility along the nanostrip arrays can be increased to be one order higher than homogeneous thin films. We attribute this observation to the anisotropic charge transport in the b-c plane, in which the b-axis provides a more effective way for achieving faster charge transport. Additionally, using this method, we comprehensively tuned the FET mobility, which helps us understand the high performance of transistors based on D-A type semiconducting polymers. The presented method for obtaining well-oriented polymer thin films and the tunning of charge transport mobility both show great potential for the future application of solubble p-conjugated polymers in organic electronic devices.