Solution-processed small molecule-based solar cells have demonstrated high power conversion efficiencies in recent years. However, several challenges have yet to be overcome, including achieving of low cost and excellent long-term stability of donor small molecules. Therefore, development of stable blocks to design organic semiconductors with optimal properties remains an actual problem. We report an alkyl-free star-shaped donor–acceptor (D–A) molecule, N(Ph-2T-DCV-PhF)3, containing p-fluorophenyldicyanovinyl (FPh-DCV) electron-withdrawing groups, triphenylamine as the donor core, and 2,2′-bithiophenes as the π-bridges between them. The study of thermal, optical, and electrochemical properties of the molecule in comparison to the direct analog with phenyldicyanovinyl groups, N(Ph-2T-DCV-Ph)3, made it possible to demonstrate the effect of the fluorine substituent on such key parameters as solubility, bandgap, lowest unoccupied molecular orbital energy level, phase behavior, thermal stability, and wettability. This work suggests that usage of the FPh-DCV block is an effective and simple tool to tune physical and physicochemical properties of stable D–A small molecules.
Thiophene-phenylene co-oligomers (TPCO) single crystals are promising materials for organic light-emitting devices, e.g., light-emitting transistors (OLETs), due to their ability to combine high luminescence and efficient charge transport. However, optical confinement in platy single crystals strongly decreases light emission from their top surface degrading the device performance. To avoid optical waveguiding, single crystals thinner than 100 nm would be beneficial. Herein, we report on solution-processed ultrathin single crystals of TPCO and study their charge transport properties. As materials we used 1,4-bis(5'-hexyl-2,2'-bithiophene-5-yl)benzene (DH-TTPTT) and 1,4-bis(5'-decyl-2,2'-bithiophene-5-yl)benzene (DD-TTPTT). The ultrathin single crystals were studied by optical polarization, atomic-force, and transmission electron microscopies, and as active layers in organic field effect transistors (OFET). The OFET hole mobility was increased tenfold for the oligomer with longer alkyl substituents (DD-TTPTT) reaching 0.2 cm2/Vs. Our studies of crystal growth indicate that if the substrate is wetted, it has no significant effect on the crystal growth. We conclude that solution-processed ultrathin TPCO single crystals are a promising platform for organic optoelectronic field-effect devices.
A series of star-shaped donor-acceptor oligomers having alkyldicyanovinyl fragments as electron-withdrawing groups, 2,2′-bithiophene as a conjugated π-bridge and either triphenylamine or its modified analogs (9-phenyl-9H-carbazole derivatives or tris(2-methoxyphenyl)amine as electron-donating cores was designed, synthesized and investigated. Variation of the chemical nature of the donor core allowed elucidating the structure-properties relationships for their solubility, absorption spectra, electrochemistry, phase behavior as well as photovoltaic performance in bulk heterojunction organic solar cells as donor materials in their mixtures with PC70BM. The star-shaped molecules based on triphenylamine and tris(2-methoxyphenyl)amine cores have reduced bandgaps, which leads to more efficient absorption of the sun light and better photovoltaic performance.
Оrganic field-effect transistors (OFET) can combine photodetection and light amplification and, for example, work as phototransistors. Such organic phototransistors can be used in light-controlled switches and amplifiers, detection circuits, and sensors of ultrasensitive images. In this work, we present photophysical characterization of well-defined ultrathin organic field-effect devices with a semiconductive channel based on Langmuir-Blodgett monolayer film. We observe clear generation of photocurrent under illumination with a modulated laser at 405 nm. The increase of photocurrent with the optical modulation frequency indicates the presence of defect states serving as traps for photogenerated carriers and/or the saturation of charge concentration in the thin active layer. We also propose a simple one-dimensional numerical model of a photosensitive OFET. The model is based on the Poisson, current continuity and drift-diffusion equations allows future evaluation of the photocurrent generation mechanism in the studied systems.
A novel fast, easily processible and highly reproducible approach to thiophene-based monolayer OFETs fabrication by Langmuir-Blodgett or Langmuir-Schaefer techniques was developed and successfully applied. It is based on selfassembly of organosilicon derivatives of oligothiophenes or benzothienobenzothiophene on the water-air interface. Influence of the conjugation length and the anchor group chemistry of the self-assembling molecules on the monolayer structure and electric performance of monolayer OFETs was systematically investigated. The efficient monolayer OFETs with the charge carrier mobilities up to 0.01 cm2/Vs and on/off ratio up to 106 were fabricated, and their functionality in integrated circuits under normal air conditions was demonstrated.
Light scattering and absorption in the visible and near-infrared ranges were studied in films of donor-acceptor blends of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with 2,4,7-trinitrofluorenone (TNF) or 1,5-dinitroantraquinone (DNAQ). Charge-transfer interaction in the electronic ground state of MEH-PPV/TNF and MEHPPV/DNAQ blends results in optical absorption in the bandgap of MEH-PPV. At the same time, because of donor-acceptor phase separation, light scattering can grow with increasing the acceptor content in the blends contributing to their optical transmission. To evaluate the amount of absorbed and scattered light separately, we have elaborated a simple method in which the optical transmission of the films is measured at several laser wavelengths using a wide aperture photodetector with or without a dump blocking the straight laser beam. Making some assumptions about the scattering indicatrix, the scattering and absorption coefficients of the blends can be evaluated. We have found that the amount of scattered light in MEH-PPV/TNF blends does not depend smoothly on the acceptor content. At the molar acceptor concentration MEH-PPV:TNF below 1:0.33, the light scattering is low and depends on the TNF content only weakly, whereas at higher ratios, the scattered light begins to grow dramatically. We assign this growth to phase separation in the blend owing to excess acceptor molecules that are not involved in charge-transfer interaction with the donor. In MEH-PPV/DNAQ blends, the phase separated DNAQ forms large species, which weakly contribute to the observed light scattering.
We apply a transient pump-probe photo deflection technique to measure the profiles of subnanosecond sound photoexcitation. We show that the photoexcited electron-hole plasma expands with a supersonic velocity in GaAs and a subsonic velocity in Si.
Dipole-forbidden states of trans-nanopolyacetylene in the near infra-red are probed by a photothermal method at room temperature. We found that light absorption at approximately 1 micrometers is four orders weaker than that corresponding to the main dipole-allowed transition.
We measured small Faraday polarization rotation in nanopolyacetylene in a magnetic field approximately 150 Oe. We found that approximately 1% polyacetylene nanoparticles increase up to 50% the polarization rotation angle of the transparent matrix containing them when probed near and at the NPA absorption band. The rotation angles were in the range 1 - 15 (mu) rad.
We present a highly sensitive method of pump-probe polarimetry for measurements of small pump-induced polarization rotation and ellipticity. The sensitivity of the method is analyzed for technical and shot-noise probe power fluctuations. The optimal polarimeter configuration is found. We show that it is possible to enhance the sensitivity to the pump-induced ellipticity by setting a small probe ellipticity inside the polarimeter with the help of a small shift wave plate. We applied the method in cw and ps measurements of photoinduced polarization parameters in a nanopolyacetylene film. The typical measured angles of polarization rotation were approximately 10-5 and approximately 10-8 rad in the cw and ps experiments, respectively.
A highly sensitive photoinduced polarimetry method was applied to probe optical anisotropy of the photoexcited states in non-oriented films of nano-polyacetylene. The photoinduced absorption anisotropy was observed in both cw and transient polarization experiments. The photoinduced dichroism parameter of the nano-polyacetylene films, (mu) equals (forula available in paper) was measured on the sub-nanosecond time scale ((mu) approximately equals 1) and on the millisecond time scale ((mu) approximately equals (formula available in paper) are the absorption indexes of the linear polarized probe polarized along and across the linear polarized pump, correspondingly. We discuss delocalization of (pi) electrons in the photoexcited states of trans-nano-polyacetylene chains.
The evolution of off-resonant and resonant Raman scattering spectra of cis and trans nanopolyacetylene (NPA) with change of frequency and intensity of incident laser light was studied. It was found that laser irradiation with wavelength 514.5 nm in process of Raman scattering spectra recording is accompanied by effective cis trans isomerization of approximately 50% cis - 50% trans NPA-PVB blend. The results obtained allow assuming that NPA in the initial compositions is a mixture of two types of nanoparticles.
We present the results of steady-state photoinduced and electroabsorption spectroscopy studies in nonoriented films of highly stable soluble trans-polyacetylene. Using Kramers- Kronig analysis of the electroabsorption spectra the nonlinear third-order non-resonant susceptibility (chi) (3) ((omega) ;(omega) ,0,0) of trans-polyacetylene was estimated to be in the range of 10-9-8 esu.
We present the simple and very effective radio frequency lock-in technique for precise measurements with cw-pumped mode-locked lasers. The technique is applied for the photodeflection transient spectroscopy that allowed to reach the sensitivity to surface displacement about one femtometer. We use the method for probing hypersound generation and propagation in a semiconductor plate.
For the first time depolarization degree of the diode-pumped monolithic ring Nd3+:YAG laser has been measured. The obtained value of depolarization degree equals 3 (DOT) 10-3. It has been shown that the dominant mechanism of the intracavity losses and the output light depolarization is diffraction on random inhomogeneities of birefringence in the laser media.
Non-steady-state polarization response of photoexcited GaAs has been investigated with the specular micropolarimetry particularly in crystals with laser induced damages of near surface layer. Physical models of the polarization response in the excited GaAs due to electron-hole plasma photogeneration are discussed: the nonlocal electron-hole plasma model the optically induced piezogyrotropy the electrooptic effect in the Dember field. 1.
High degrees of linear polarization of laser radiation and sensitivity to its variation are achieved by applying frequency converters using nonlinear crystals as polarizers and analyzers. The factors leading to distortion of linear polarization of light in such devices are identified. A KD*P polarimeter with extinction ratio of 5 (DOT) 10-9 is developed on the basis of a quasi-CW picosecond YAG laser.
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