Influence of molecular structure and microstructure on device performance of polycrystalline pentacene thin-film transistors

Abstract

The authors have fabricated the pentacene thin films on polymethylmethacrylate ͑PMMA͒ and on silicon dioxide dielectric surfaces featuring similar surface energy and surface roughness. On both surfaces the pentacene films displayed high crystal quality from x-ray diffraction scans, although the film on PMMA had significantly smaller grain size. The pentacene transistors with PMMA exhibited excellent electrical characteristics, including high mobility of above 1.1 cm 2 / V s, on/off ratio above 10 6 , and sharp subthreshold slope below 1 V / decade. The analysis of molecular microstructure of the pentacene films provided a reasonable explanation for the high performance using resonance micro-Raman spectroscopy.The performance of organic thin-film transistors ͑OT-FTs͒ has greatly improved lately. In particular, we can see performance improvements in conjugated oligoacenes, i.e., pentacene. 1-3 Pentacene, deposited on dielectric surfaces with surface-induced structure, forms a polycrystalline thin film with grain morphology. 1 General assumptions suggest that mobility properties appear to be dominated by grain boundary effects that occur due to the large amount of charged trapping states at the boundary. 1,2,4 Most researchers utilize crystal structure and grain size to interpret the performance of OTFTs. However, some reported data are not consistent with the observations presented above. 3,5,6 Whether in the intragrain or in grain boundary of organic film, the structures still have weak van der Waals forces, unlike the inorganic polycrystalline media. However, little is known about the importance of molecular structure and microstructure within a polycrystalline organic film on OTFT performance.Surface properties of a dielectric layer, i.e., surface energy ͑␥ s ͒ and surface roughness ͑R rms ͒, are the distinctive factors that determine potential improvements in electric characteristics of OTFT. 7 Polymeric insulators have been considered as a preferable surface modification layer and/or gate dielectric materials due to their numerous advantages over inorganic materials. Therefore, in addition to studying the ␥ s and R rms , it is also interesting to explore the influence of polar groups of polymer insulators on the structure of organic semiconductors and the corresponding performance of OTFTs.In this letter, we have investigated the crystal structure, thin-film morphology, and molecular structure and microstructure of pentacene films grown on the polymeric and inorganic dielectric surfaces with similar ␥ s and R rms values. Hence, the impact of different ␥ s and R rms values could be distinguished. The study includes silicon dioxide ͑SiO 2 , ␥ s = 49.8 mJ/ m 2 , R rms = 4.8 Å͒ and polymethylmethacrylate ͑PMMA͒ ͑␥ s = 49.1 mJ/ m 2 , R rms = 5.0 Å͒ surfaces. We should note that the pentacene film, grown on PMMA surface with a very small grain size, allows the production of OTFTs with very good performance characteristics. Here, we used x-ray diffraction ͑XRD͒, atomic force microscope ͑AFM͒, and resonance micro-Rama...

Spin-pump-induced spin transport in a thermally evaporated pentacene film

“…11 The carrier mobility for pentacene film is above 1 cm 2 /Vs, which is the highest value for any OFET with molecular films fabricated by thermal evaporation and high enough for simple electronic devices. 11 This means that numerous active carriers are present in the pentacene film (in this case, holes) to propagate the spin angular momentum. Moreover, pentacene shows photo-conductivity for visible light, 12 where the spin transport properties of 4 pentacene can be controlled through light irradiation.…”

“…Secondly, pentacene has good crystallinity, even in films formed by thermal evaporation, and shows relatively high electrical conductivity without any dopants. 11 Pentacene, which is a p-type semiconductor, is well known as an active-layer material in organic field-effect transistors (OFETs). 11 The carrier mobility for pentacene film is above 1 cm 2 /Vs, which is the highest value for any OFET with molecular films fabricated by thermal evaporation and high enough for simple electronic devices.…”

“…11 Pentacene, which is a p-type semiconductor, is well known as an active-layer material in organic field-effect transistors (OFETs). 11 The carrier mobility for pentacene film is above 1 cm 2 /Vs, which is the highest value for any OFET with molecular films fabricated by thermal evaporation and high enough for simple electronic devices. 11 This means that numerous active carriers are present in the pentacene film (in this case, holes) to propagate the spin angular momentum.…”

Spin current relaxation time in thermally evaporated pentacene films

“…[4][5][6][7][8][9] We have focused on pentacene molecular films prepared by thermal evaporation as one candidate material for the spin transport, in terms of the good crystallinity even in the thermally evaporated films, and the high electrical conductivity without any dopants. 10 Moreover, pentacene shows the photo-conductivity for visible light, 11 where the spin transport properties of pentacene can be controlled through visible light irradiation. Recently, the spin transport in thermally evaporated pentacene films was achieved at room temperature (RT) with a pure spin current induced by spin-pumping, and the spin diffusion length (λ) in pentacene films was estimated to be about 42±10 nm.…”

Franck-Condon processes in pentacene monolayers revealed in resonance Raman scattering

“…The frequencies of the two components are almost the same in the two films studied. The lower energy mode at ∼1177cm −1 =0.146eV is a symmetric (A g ) mode from in-plane bending of C-H bonds along the sides of pentacene molecules 10,19,20 (shown in the inset on the left of Fig. 2(a)).…”