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Organic multilayer systems, which are stacked layers of different organic materials, are used in various organic electronic devices such as organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). In particular, OFETs are promising as key components in flexible electronic devices. In this study, we investigated how the inclusion of an insulating tetratetracontane (TTC) interlayer in ambipolar indigo-based OFETs can be used to alter the crystallinity and electrical properties of the indigo charge transport layer. We find that the inclusion of a 20-nm-thick TTC film thermally annealed at a low temperature of 70 °C acts to significantly increase the ambipolar electrical transport of the indigo layer. X-ray diffraction, atomic force microscopy, and vibrational sum frequency generation measurements showed that annealing the TTC film significantly improved its ordering. The electronic sum-frequency generation spectra of TTC/indigo bilayers show that this improved ordering of TTC films promotes the growth of crystalline indigo films that exhibit charge mobilities in OFET that are nearly an order of magnitude larger than those measured for devices grown on unannealed TTC layers. Furthermore, using vibrational sum-frequency generation spectroscopy, we found that pre-annealing the TTC layer prior to indigo deposition can suppress the formation of defects within the TTC layer during indigo film growth, which also contributes to enhanced charge transport. Our results highlight the importance of controlling the molecular ordering within the interlayer contacts in OFET structures to achieve an enhanced performance. [ABSTRACT FROM AUTHOR]

Research was conducted to investigate the electronic structures of evaporated films of five organic-light emitting and carrier-injecting materials for organic electroluminescent devices. Ultraviolet photoemission spectroscopy was used in studying light-emitting materials tris(8-hydroxyquinolino) aluminum an 1,2,3,4,5-pentaphenylcyclopentadiene, a hold injecting material and electron-injecting materials. Results reveal the correlation between the ionization threshold energies.

Operando analysis of organic devices is crucial for understanding the device operation mechanism and intrinsic degradation mechanism. Buried organic layers in OLEDs during operation have been investigated using electric‐field‐induced doubly resonant sum‐frequency generation (EFI‐DR‐SFG) and time‐resolved EFI‐DR‐SFG experiments. The time‐resolved EFI‐SFG technique allows us to observe the carrier transportation directly and non‐destructively. DC electric field leads to a significant increase in the SFG intensity produced by the charged carriers. Electric‐field‐induced SFG results clearly show that charge accumulation occurs at the interface between the light emitting layer and the electron‐blocking layer in the blue OLED and that this charge accumulation strongly contributes for the light emission. Particularly in the TTA process, high charge accumulation is effective in generating triplet excitons and improving the luminescence efficiency, but at the same time, it reduces the lifetime of the device. [ABSTRACT FROM AUTHOR]

Theoretical simulations of the angle-resolved ultraviolet photoemission spectra (ARUPS) for the oligomer of poly(tetrafluoroethylene) [(CF2)n ; PTFE] were performed using the independent-atomic-center approximation combined with ab initio molecular orbital calculations. Previously observed normal-emission spectra for the end-on oriented sample (with long-chain axis perpendicular to the surface) showed the incident photon-energy (hv) dependence due to the intramolecular energy-band dispersion along the one-dimensional chain, and the present simulations successfully reproduced this hv dependence of the observed spectra. We employed the experimentally observed helical structure for PTFE oligomers for the simulations. We also calculated the density of states (DOS) for the planar zigzag structure, and examined the changes in the electronic structure due to the difference in the molecular structure by comparing the DOS for the helical and planar zigzag structures. Only a small change in the DOS was found between these structures, showing little change of the electronic structure between these conformations. We also evaluated the inner potential V0 , which is the parameter defining the energy origin of the free-electron-like final state, and checked the validity of the value of 10 Rev estimated in our previous study using the experimentally observed hv dependence of the peak intensity. The estimation of V0 was performed by pursuing the best agreement between the energy-band dispersion [E = E(k)] relation along the chain direction obtained from the simulated spectra and the experimentally deduced one. An excellent agreement in the topmost band was achieved when the assumed inner potential V0 was set at about zero. This value of V0 is much different from the value of V0 = -10 eV in the previous study, suggesting the invalidity of the previous assumption at the estimation of V0 from the peak intensity variation with hv. Using the presently obtained V0, we could derive more reliable E = E(k) dispersion relation from the observed ARUPS spectra. The comparison of this newly derived relation gave good agreement with theoretically calculated E = E(k) relations, in contrast to the poor agreement for the previous results with V0 = -10 eV. [ABSTRACT FROM AUTHOR]

The infrared-visible sum frequency generation (SFG) spectroscopy was used to study the air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethane-sulfonate ([bmim]OTf). The results of the line width and the shift of the S[O.sub.3]-ss peak revealed that [[bmim].sup.+] cations and OTf anions interact in a specific way at the surface as compared to the bulk.

Reports that the ultraviolet photoelectron spectra (UPS) were measured using synchroton radiation for thin films of poly(1,10-phenanthroline-3,8-diyl) (PPhen) and its potassium-doped state. UPS spectra of neutral poly(1,10-phenanthroline-3,8-diyl); UPS spectra of potassium-doped PPhen; Simulation of photoemission spectra of PPhen and potassium-doped states.

Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π-conjugated polymers, poly(pyridine-2,5-diyl) (PPy) and poly(2,2′-bipyridine-5,5′-diyl) (PBPy) which exhibit n-type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π-conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K-doped PBPy. While K-doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K-doped PBPy. The difference of the dependence on dopant concentration between K-doped PPy and K-doped PBPy is discussed based on the conformational difference between these polymers. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper we report on the angle-resolved ultraviolet photoelectron spectroscopy on ultrathin films of bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole) (BTQBT) deposited on a MoS2 surface with synchrotron radiation, and describe the quantitative analysis of the angular distribution of photoelectron from the highest occupied state. The observed angular distributions were better explained by those calculated with the single-scattering approximation combined with molecular orbital calculation considering intramolecularly scattered waves than the previously used independent-atomic-center approximation combined with molecular orbital calculation. Further, the low-energy-electron diffraction measurements were performed on the film. From the low-energy-electron diffraction, the two-dimensional lattice of the ultrathin films on the MoS2 was found to be MoS2(0001)-(13×13, R=±13.9°)-BTQBT, and from the analysis of the photoelectron angular distributions, it was found that at the lattice points the molecules lie flat with azimuthal orientations of 19° and 47° with respect to each surface crystal axis of MoS2. On the basis of these results, the full structure of the thin film, the two-dimensional lattice, and full molecular orientations at the lattice points, is proposed. © 1997 American Institute of Physics. [ABSTRACT FROM AUTHOR]