Diketopyrrolopyrrole (DPP)-based copolymers, including poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-(E)-1,2-di(2,2’-bithiophen-5-yl)ethene (PDDBE) and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-(E)-1,2-bis(6-hexylthieno[3,2-b]thiophene-2-yl)ethene (PDTTE), were synthesized by alternating a DPP-derivative acceptor (A) block with different donor (D) blocks, such as (E)-1,2-di(2,2’-bithiophen-5-yl)ethene and (E)-1,2-bis(6-hexylthieno[3,2-b]thiophene-2-yl)ethene. These copolymers had drastically different crystal structures on polymer-grafted SiO2 dielectrics. Multiple-layered crystallites of PDDBE had a long-range, π-conjugated extension but a wide conjugated distance, d(010), of 3.90 Å. Nanorod-percolated agglomerates of PDTTE had a much shorter d(010) of 3.71 Å, originating from the alternating structures of the DPP derivative with slightly different D blocks. The corresponding crystallites yielded a wide range of mobilities from 0.01 to 1.40 cm2V-1s-1 in the OFETs.
Solution-processible thiophene/phenylene -based oligomer derivatives with different end substituents are presented as p-type
semiconducting materials in OFETs. These films were deposited on OTS-treated SiO2/Si or polymeric
bivinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB)/ITO/quartz substrates, via drop-casting and spin-casting.
Synchrotron-based grazing-incidence X-ray diffraction and atomic force microscopy reveals that both drop- and spin-cast
films have highly crystalline structures with edge-on molecules and parallel π-π stacking conjugated planes with
respect to the substrate. In particular, temperature-dependent solubility of these materials can give a strategy for highly
ordered crystalline structure in spin-cast films grown on cooler substrates, when compared to warmed solutions. Field-effect
mobilities of these spin-cast films in a top-contacted electrode OFETs with BCB dielectrics are reached as high as
~0.01 cm2/Vs.
It has been shown that high charge mobility in solution-processible organic semiconductor-based field effect transistors
is due in part to a highly parallel π-π stacking plane orientation of the semiconductors with respect to gate-dielectric. Fast
solvent evaporation methods, generally, exacerbate kinetically random crystal orientations in the films deposited,
specifically, from good solvents. We have investigated solubility-driven thin film structures of thiophene derivative
polymers via spin- and drop-casting with volatile solvents of a low boiling point. Among volatile solvents examined,
marginal solvents, which have temperature-dependent solubility for the semiconductors (e.g. methylene chloride for
regioregular poly(3-alkylthiophene)s), can be used to direct the favorable crystal orientation regardless of solvent drying
time, when the temperature of gate-dielectrics is held to relatively cooler than the warm solution. Grazing-incidence X-ray
diffraction and atomic force microscopy strongly support that significant control of crystal orientation and mesoscale
morphology using a "cold" substrate holds true for both drop and spin casting. The effects of physiochemical post-modificaiton
on film crystal structures and morphologies of poly(9,9-dioctylfluorene-co-bithiophene) have also been
investigated.
We have investigated crystalline structures in organic semiconducting thin films with regioregular poly(3-alkyl
thiophene)s, pentacene, and oligofluorene-thiophene derivatives, using grazing-incidence X-ray diffraction and atomic
force microscopy. We found that crystalline morphologies and molecular orientation in these thin films strongly affect
electric characteristics of these film-based organic thin-thin transistors (OTFTs). Specifically, in solution- and vacuumdeposited
films, preferentially perpendicular orientation of π-conjugated crystalline planes with respect to dielectric
substrates require results in high-performance charge mobility in the OTFT devices.
We studied how the underlying grain boundary morphology in the first pentacene monolayer affects the "macroscopic" mobility of the 40 nm thick OTFT film. Through manipulating surface properties of a SiO2 dielectric layer using self assembled monolayers (SAMs), we controlled the crystalline domain morphology of pentacene films that have thicknesses ranging from sub-monolayer to 40 nm. Atomic force microscopy (AFM) and two-dimensional grazing incidence X-ray diffraction are employed to characterize the morphology and crystalline structure of pentane films. In addition, the spatial variation of charge carriers in the first few layers was investigated using conducting AFM (C-AFM). In particular, faceted or dendritic island morphology has been observed in the first pentacene layer mainly depending on surface morphology of hydrophobic SAMs, and C-AFM supported that the faceted islands showed larger current flow than the dendritic islands. This C-AFM current tendency correlates with the "macroscopic" charge mobility in OTFT. Because the faceted morphology should represent the single crystal-like pentacene island, faceted islands have fewer internal crystal defects and the higher current flow than the dendritic islands.
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