Spectroscopic Features of Thin Films of Thiophene/Phenylene Co-oligomers with Vertical Molecular Alignment

Hotta, Shu; Ichino, Yoshiro; Yoshida, and Yuji; Yoshida, Masaru

doi:10.1021/jp002166s

Cited by 51 publications

(42 citation statements)

References 28 publications

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“…These tilt angles are comparable with those observed in the well-defined layers in bulk sexithiophene crystals. [16] The suggested increase of the tilt angle with molecular length agrees with the trend reported for unsubstituted oligothiophenes [17] but is contrary to the trend deduced from PXRD of hexylphenylene oligothiophenes. [18] The tilt angles calculated here for 1±3, however, do not account for possible interdigitation of adjacent layers or the occurrence of ªsynº conformations within the oligothiophene backbone, factors that would affect the angles estimated from PXRD.…”

Section: Resultssupporting

confidence: 65%

Organic Thin‐Film Transistors Based on Tolyl‐Substituted Oligothiophenes

Mohapatra

Holmes

Newman

et al. 2004

Adv Funct Materials

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Thin films based on the tolyl‐substituted oligothiophenes 5,5′′‐bis(4‐methylphenyl)‐2,2′:5′,2′′‐terthiophene (1), 5,5′′′‐bis(4‐methylphenyl)‐2,2′:5′,2′′:5′′,2′′′‐quaterthiophene (2) and 5,5′′′′‐bis(4‐methylphenyl)‐2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′‐quinqethiophene (3) exhibit hole‐transport behavior in a thin‐film transistor (TFT) configuration, with reasonable mobilities and high current on/off (Ion/Ioff) ratios. Powder X‐ray diffraction (PXRD) reveals that these films, grown by vacuum deposition onto the thermally grown silicon oxide surface of a TFT, are highly crystalline, a characteristic that can be attributed to the general tendency of phenyl groups to promote crystallinity. Atomic force microscopy (AFM) reveals that the films grow layer by layer to form large domains, with some basal domain areas approaching 1000 μm2. The PXRD and AFM data are consistent with an “end‐on” orientation of the molecules on the oxide substrate. Variable‐temperature current–voltage (I–V) measurements identified the activation regime for hole transport and revealed shallow level traps in thin films of 1 and 2, and both shallow and deep level traps in thin films of 3. The activation energies for thin films of 1, 2, and 3 were similar, with values of Ea = 121, 100, and 109 meV, respectively. The corresponding trap densities were Ntrap/Nv = 0.012, 0.023, and 0.094, where Ntrap is the number of trap states and Nv is the number of conduction states. The hole mobilities for the three compounds were similar (μ ≃ 0.03 cm2 V–1 s–1), and the Ion/Ioff ratios were comparable with the highest values reported for organic TFTs, with films of 2 approaching Ion/Ioff = 109 at room temperature.

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Section: Resultssupporting

confidence: 65%

Organic Thin‐Film Transistors Based on Tolyl‐Substituted Oligothiophenes

Mohapatra

Holmes

Newman

et al. 2004

Adv Funct Materials

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“…Again, the intensity increases with f. The emission spectra exhibit fine structures, reflecting vibronic coupling. [20] In conclusion, the results in the present study demonstrate that applying AC gate voltages is effective in producing strong emissions in OLEFETs. The device constitution is virtually unaltered, and stable metals such as gold and platinum can be used in the electrodes, to inject both holes and electrons into the organic layer.…”

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confidence: 60%

Organic Light‐Emitting Field‐Effect Transistors Operated by Alternating‐Current Gate Voltages

et al. 2008

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Organic light-emitting field-effect transistors (OLEFETs) are currently under intensive study owing to their multifunctionality, including current modulation and light emission.[1] This multifunctionality is associated with their potential applicability to display technology. Organic light-emitting diodes (OLEDs) constitute another class of organic lightemitting devices, which require transistors controlling their luminance. In OLEFETs, on the other hand, the luminance can be modulated by changing only gate voltages, without any additional devices. Thus, display panels using OLEFETs have the great advantage of largely reducing both the number of devices and the circuit complexity. The device performance of OLEFETs is based upon the simultaneous carrier injection of both holes and electrons into an organic semiconductor layer, and their subsequent recombination. So far, OLEFETs [2][3][4][5][6][7][8][9][10][11] have been operated by applying direct-current (DC) voltages to the gate contact and between the source and drain electrodes. Within this framework several attempts were made to promote the injection and recombination of the carriers. Examples include the use of heterogeneous metal electrodes [6,8,10,11] and composite organic semiconductors.[3] These attempts, however, made the device structure complicated, and the improvement was yet insufficient. Thus, the difficulty in injecting electrons and holes simultaneously into the organic layer imposed serious restrictions on the structure and steady operation of such devices, and hence hampered the versatility of OLEFETs. This has urged us to renovate the device operation method. In this communication, we propose a novel method of operation for the devices, characterized by the application of alternating-current (AC) voltages to the gate electrode. This prompts the carrier injection from both the drain and source electrodes into an organic layer. Moreover, stable metals such as gold and platinum can be used in those electrodes, to inject both holes and electrons. The organic layer can be constructed in any desired form, e.g., as a thin film or a crystal. As a result, we have been successful in effectively producing strong emissions without altering the device constitution. The devices are operated under AC gate frequencies of 2-20,000 Hz of appropriate gate voltage amplitudes; stronger emissions take place with increasing gate frequencies. Detailed analysis of the electrical data allows us to distinguish two different regimes in the emission processes. This provides a powerful tool in studying the carrier injection and transport mechanisms in organic semiconducting materials. Amongst organic semiconducting materials, thiophene/ phenylene co-oligomers are considered promising because of their unique optoelectronic properties. [5,8,12,13] Hence we have chosen for the present study one of the co-oligomers, 2,5-bis(4-biphenylyl)thiophene (BP1T) [14] (see structural formula in Fig. 1a). The carrier mobility of the BP1T thin-film device is estimated at $10 À3 cm 2 V À1 ...

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“…15 This means that the crystals consist of a molecular layered structure with the ''molecular long axis'' exhibiting vertical alignment against the substrate plane. 15 ͑vide infra͒. Under weaker incident intensities the emission spectra exhibit a broad feature with relatively fine modes superimposed on it.…”

mentioning

confidence: 99%

Emission gain narrowing from single crystals of a thiophene/phenylene co-oligomer

Nagawa

Hibino

Hotta

et al. 2002

Applied Physics Letters

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Emission gain narrowing has been observed for single crystals of a thiophene/phenylene co-oligomer. The hexagon flake crystals were placed on a quartz substrate with the crystals' face in close contact with the substrate plane. These crystals were irradiated with a N 2 laser with a 337.1 nm wavelength at a repetition rate 10 Hz that tuned its intensity to 100-1150 J/cm 2 . The emission gain narrowing takes place at 21490 ͑465.4 nm͒ and 20220 cm Ϫ1 ͑494.5 nm͒ with increased intensities, with their half width at half maxima reaching ϳ50 cm Ϫ1 . On the basis of the nonlinear relationship between the emission peak intensities and the laser light intensity, the gain narrowing has been attributed to the amplified spontaneous emission. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1435797͔ Conjugated semiconducting polymers and oligomers are currently attracting great attention as potentially useful materials for optoelectronic devices such as thin-film transistors and light-emitting diodes. 1 Solid-state lasing and amplified spontaneous emission ͑ASE͒ have been observed in these materials. 2 Since excitation through photopumping is a facile way to attain lasing and ASE, this method has long been used. [3][4][5] The earliest examples can be found for anthracene molecules embedded in host matrices of, e.g., fluorene. 6 Later this approach was more widely applied to dye-doped systems and polymer thin films. 7 Very recently solid-state injection lasing was achieved using a tetracene single crystal, vouching for the high practicality of organic semiconductors. 8 In view of current injection, crystals of high quality are more advantageous than any other systems that involve defects which may well act as carrier traps. Reported observation of lasing or ASE of the crystals, however, remains scarce even in the case of photopumping. This is partly because the photoluminescent efficiency of chromophores is decreased by higher concentrations, 9 even though their efficiency is high at dilution. Yet a typical examples for the ASE haves been verified by Fichou et al. 10 and by Horowitz et al. 11 using single crystals of octithiophene and sexithiophene, respectively.Yanagi and Morikawa 12 showed that self-waveguided emission takes place along the crystals' long axis of needleshaped crystals of p-sexiphenyl that are epitaxially grown on top of a single crystal substrate of potassium chloride. The molecular axes of p-sexiphenyl align perpendicular to the crystals' needle axis so that the propagation of the polarized emission can be enhanced with the transverse electric mode along that needle. The uniaxially aligned transition dipoles in the p-sexiphenyl crystals are responsible for the selfwaveguided emission. Recent progress demonstrated that this is analogous to the case of a newly emerging class of semiconducting molecular crystals, thiophene/phenylene co-oligomers. 13 These materials were developed by Hotta and co-workers 14 and are characterized by a variety of extensions of conjugation along the backbone. The conjugation ...

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Spectroscopic Features of Thin Films of Thiophene/Phenylene Co-oligomers with Vertical Molecular Alignment

Cited by 51 publications

References 28 publications

Organic Thin‐Film Transistors Based on Tolyl‐Substituted Oligothiophenes

Organic Thin‐Film Transistors Based on Tolyl‐Substituted Oligothiophenes

Organic Light‐Emitting Field‐Effect Transistors Operated by Alternating‐Current Gate Voltages

Emission gain narrowing from single crystals of a thiophene/phenylene co-oligomer

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