High
structural quality of crystalline organic semiconductors is the basis
of their superior electrical performance. Recent progress in quasi
two-dimensional (2D) organic semiconductor films challenges bulk single
crystals because both demonstrate competing charge-carrier mobilities.
As the thinnest molecular semiconductors, monolayers offer numerous
advantages such as unmatched flexibility and light transparency as
well they are an excellent platform for sensing. Oligothiophene-based
materials are among the most promising ones for light-emitting applications
because of the combination of efficient luminescence and decent charge-carrier
mobility. Here, we demonstrate single-crystal monolayers of unprecedented
structural order grown from four alkyl-substituted thiophene and thiophene–phenylene
oligomers. The monolayer crystals with lateral dimensions up to 3
mm were grown from the solution on substrates with various surface
energies and roughness by drop or spin-casting with subsequent slow
solvent evaporation. Our data indicate that 2D crystallization resulting
in single-crystal monolayers occurs at the receding gas–solution–substrate
contact line. The structural properties of the monolayers were studied
by grazing-incidence X-ray diffraction/reflectivity, atomic force
and differential interference contrast microscopies, and imaging spectroscopic
ellipsometry. These highly ordered monolayers demonstrated an excellent
performance in organic field-effect transistors approaching the best
values reported for the thiophene or thiophene–phenylene oligomers.
Our findings pave the way for efficient monolayer organic electronics
highlighting the high potential of simple solution-processing techniques
for the growth of large-size single-crystal monolayers with excellent
structural order and electrical performance competing against bulk
single crystals.
Abstract2D organic semiconductor single crystals comprising one or a few molecular layers of macroscopic lateral sizes are ideal materials for ultrathin, flexible, and transparent field‐effect devices—a platform for transistors and sensors. In recent years, these 2D materials have demonstrated high performance not inferior to their 3D counterparts. However, light emissive properties of 2D organic semiconductor single crystals have not yet been reported, and a combination of high charge‐carrier mobility and bright luminescence in one material is still a challenge for 2D organic optoelectronics. Emissive high‐mobility 2D organic semiconductor based on a [1]benzothieno[3,2‐b]benzothiophene (BTBT)‐derivative, 2,7‐bis(4‐decylphenyl)[1]benzothieno[3,2‐b][1]benzothiophene (DPBTBT), is presented here. DPBTBT molecules self‐organize in large‐area ultrathin single‐crystalline films consisting of one or a few molecular layers. These 2D single crystals perfectly suit as an active layer of organic field‐effect transistors in full accordance with Shockley's model and uniquely combine the high charge‐carrier mobility reaching 7.5 cm2 V–1 s–1 with prominent light emissive properties, which allow a demonstration of the first 2D organic light‐emitting transistor. The high charge‐carrier mobility and thermal stability of the crystalline phases, pronounced luminescence, and good shelf‐life stability suggest that emissive BTBT‐type molecules are a promising avenue for 2D organic optoelectronics.
Recently developed
ultrathin two-dimensional (2D) organic semiconductor
crystals are a promising platform for advanced organic electronic
devices. Remarkable quality of such crystals results in charge-carrier
mobilities comparable to those of bulk crystals, but their structure
and orientation are hard to study because of their extremely small
thickness. Here, we applied surface-enhanced Raman spectroscopy (SERS)
to investigate the structure of the thinnest 2D single crystalsmonolayers,
which are based on thiophene-phenylene co-oligomers: 1,4-bis(5′-decyl-2,2′-bithiene-5-yl)benzene
and 1,4-bis(5′-hexyl-2,2′-bithiene-5-yl)benzene. Their
Raman spectra were calculated as a function of the molecule orientation
and SERS microscopy maps were acquired. High sensitivity of SERS allowed
us to study monolayer single-crystal domains with the optical spatial
resolution. Raman anisotropy was used to probe the orientations of
single-crystal domains and the molecule orientation within them. Notably,
the SERS microscopy detected the presence of a submonolayeramorphous
material between the crystalline domains, which is practically inaccessible
to optical or conventional atomic force microscopies (AFMs). The submonolayer
was also studied by lateral-force AFM, which showed notably higher
friction and adhesion. We found that the measured Raman anisotropy
significantly reduced by the metal-covered substrate still allowing
us to distinguish orientations of molecules in the 2D crystals and
in the submonolayer. Anisotropy-sensitive SERS was shown to be promising
for studying 2D organic semiconductor crystals.
Two-dimensional (2D) organic semiconductor macroscopic single crystals have recently demonstrated their excellent charge-transport properties competing with their 3D counterparts. However, the combination of efficient charge transport and prominent luminescent properties...
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