Akshaya K. Palai

Symmetrically functionalized diketopyrrolopyrrole with alkylated thiophene moiety: from synthesis to electronic devices applications

A symmetrically conjugated molecular semiconductor derived from diketopyrrolopyrrole (DPP), i.e., 2,5-dihexadecyl-3,6-bis(5-(3-hexylthiophen-2-yl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione [DPP(3HT)2], has been synthesized using the Stille coupling reaction. The optical band gap of DPP(3HT)2 is found to be 1.65xa0eV with HOMO energy level of −5.13xa0eV. DPP(3HT)2 is thermally stable up to 348xa0°C exhibiting only 5xa0% weight loss. X-ray diffraction analysis revealed the DPP(3HT)2 to be polycrystalline with a high degree of crystallinity. It showed highly narrow and strong diffraction peaks with a d-spacing of ca. 30.18xa0Å (at 2θxa0=xa02.09°, full width at half maximum: 2θxa0=xa00.126°) indicating an end-to-end packing distance with the long alkyl chains (along the a-axis) being slightly tilted and partially interdigitated for effective packing. Field-effect transistors and complementary inverters employing DPP(3HT)2 as a p-channel semiconductor have been fabricated and characterized.Graphical Abstract

Facile synthesis of arylthiophenyl-functionalized diketopyrrolopyrrole derivatives via direct C–H arylation: characterization and utilization in organic electronic devices

A series of symmetrically arylthiophenyl-functionalized diketopyrrolopyrrole (DPP) derivatives were synthesized via palladium-catalyzed direct C–H arylation to develop an active material for organic electronic devices. The properties of DPPs could be tuned by simple variation of the end groups such as t-butylphenyl, cyanophenyl and methoxynaphthyl. The effect of the substituent on the optical, electrochemical, and thermal properties of DPPs was evaluated using UV-visible spectroscopy, cyclic voltammetry, and thermogravimetric analysis. The morphology and molecular packing of thin-films of DPPs were analyzed by atomic force microscopy (AFM), density functional theory (DFT) calculations, and two-dimensional grazing incidence X-ray diffraction (2D-GIXD) experiments. Utilization of the synthesized DPPs as channel materials in organic field-effect transistors was demonstrated.

Polyalkylthiophene-containing electron donor and acceptor heteroaromatic bicycles: synthesis, photo-physical, and electroluminescent properties

The synthesis and characterization of a series of polyalkylthiophenes-containing electron-rich thienothiophene (donor heteroaromatic bicycle) and electron-deficient benzothiadiazole (acceptor heteroaromatic bicycle) block have been reported. The polymers are synthesized by Stille cross-coupling reaction and are found to be having high molecular weight with number-average molecular weight in the range of 7.1xa0×xa0104–5.7xa0×xa0104. The photo-physical, electro-chemical, and electroluminescent (EL) properties of the polymers are investigated in detail. The optical band gap of the polymers is found to be in the range of 1.53–1.54xa0eV. These new polymers are luminescent in nature and showed red photoluminescence in chloroform solution (722–740xa0nm) as well as in thin film (781–786xa0nm). Ionization potential for these polymers is calculated and falling in the range of 5.23–5.33xa0eV. Polymer light emitting diodes with configuration ITO/PDOT:PSS/polymer/BCP/Alq3/LiF/Al have been fabricated, and a deep red emission is observed. The EL maxima of polymers are found to be in the range of 750–760xa0nm with threshold voltages around 4.0–5.5xa0V. The fabricated devices show luminescence around 40xa0cd/m2 at current density of 100xa0mA/cm2 with maximum value of 580–810xa0cd/m2 at 11xa0V.Graphical abstract

n-Type small aromatic core diimides flanked with electron donating thienylethyl moieties and electrical responses in organic devices

We report n-type small aromatic core diimide derivatives functionalized with electron donating heteroaromatic ring bearing moieties, N,N′-bis[2-(2-thienyl)ethyl]-benzene-1,2,4,5-tetracarboxylic diimide and N,N′-bis[2-(2-thienyl)ethyl]-napthalene-1,4,5,8-tetracarboxylic diimide. The synthesis, and characterization of the newly synthesized diimide based organic semiconductors and electrical responses in electronic devices are investigated in detail. The structure, and electrochemical and optical properties of the diimide derivatives were determined using 1H NMR, 13C NMR, mass spectrometry, UV-visible spectroscopy and cyclic voltammetry. The electron density distribution of the materials was also studied using density functional theory calculations to relate the structure and devices characteristics. The materials were employed for the preparation of organic thin-film transistors with inorganic and polymeric gate dielectrics at various substrate temperatures, and the devices electrical characteristics have been analyzed in detail. We also further explore its applicability in advanced electronic devices by fabricating a resistance load-type organic inverter, and the dynamic response behavior was investigated.

Fluorene-based conjugated poly(arylene ethynylene)s containing heteroaromatic bicycles: preparation and electro-optical properties

A series of high molecular weight fluorene-based soluble poly(arylene ethynylene)s (PAEs) have been prepared and characterized. The polymers consist of 2,5-bis(3-tetradecylthiophen-2-yl)-3a,6a-dihydrothieno[3,2-b]thiophene, 2,5-bis(3-tetradecylthiophen-2-yl)-3a,6a-dihydrothiazolo[5,4-d]thiazole, or 4,7-bis(3-tetradecylthiophen-2-yl)benzo[c] [1, 2, 5] thiadiazole unit with an electron donor 9,9-bis(2-ethylhexyl)-9H-fluorene unit connected via electron accepting ethynylene linkage. The molecular weights (Mw) of the polymers were found to be in the range of 103600–179000xa0g/mol with polydispersity index (PDI) of 3.9–5.0. Optical and redox properties have been investigated by UV–visible, fluorescence spectroscopy, and cyclic voltammetry (CV) measurements. Combination of experimental and density functional theory (DFT) calculations indicated that the benzothiadiazole unit incorporated polymer has lowest band gap with most stable lowest unoccupied molecular orbital (LUMO) energy level. Polymer light emitting diode properties have been investigated for the polymer having highest molecular weight with device configuration ITO/PEDOT:PSS/Polymer/LiF/Al. Well-behaved diode characteristics with EL maxima at 600xa0nm were observed.Graphical Abstract

An organic microcrystal array-embedded layer: highly directional alternating p- and n-channels for ambipolar transistors and inverters

An organic microcrystal array-embedded active layer for highly directional alternate p- and n-channels is prepared for fabricating a high-performance ambipolar organic field-effect transistor (OFET) and complementary inverter by successive deposition of p- and n-channel organic semiconductors on a polymeric gate dielectric. First, a microcrystal array of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene, p-channel) is directly grown over the gate dielectric, and then a copper hexadecafluorophthalocyanine (F16CuPc) layer is formed by a thermal evaporation method, resulting in the formation of an organic microcrystal array-embedded active layer with highly directional alternating n- and p-channels between the source and drain electrodes. Devices based on this active layer exhibited clear ambipolar charge transport characteristics with high ambipolarity (>90%). The ambipolar charge transport mechanism is discussed in detail. In addition, air-stable complementary inverters comprising two ambipolar OFETs are also demonstrated.

Synthesis and structural analysis of dimethylaminophenyl-end-capped diketopyrrolopyrrole for highly stable electronic devices with polymeric gate dielectric

Herein, we report the synthesis and structural analysis of 3,6-bis(5-(4-(dimethylamino)phenyl)thiophen-2-yl)-2,5-dihexadecylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione [DPP(PhNMe2)2], a stable diketopyrrolopyrrole derivative end-capped with a strongly electron-donating dimethylaminophenyl moiety. Optical and electrochemical characterization determined the band gap, HOMO, and LUMO energies of the above compound as 1.63, −4.65, and −3.02 eV, respectively. The prepared DPP(PhNMe2)2 was fabricated into thin films to construct field-effect transistors and resistance load-type inverters, the responses of which to static/dynamic electrical stimuli were analyzed in detail. Specifically, field-effect transistors demonstrated stable output/transfer characteristics during 100 sweeping cycles and showed excellent performance stability (over 300 days) under ambient conditions, with the corresponding dynamic switching characteristics being well maintained during 500 on–off cycles. The above inverters also showed good performance under ambient conditions. To clarify the origin of this performance enhancement, the above thin films were subjected to structural analysis by atomic force microscopy, density function theory calculations, and two-dimensional grazing incidence X-ray diffraction, which revealed that DPP(PhNMe2)2 molecules were stacked over the surface of the gate dielectric via their NMe2 groups. Based on the obtained results, the improved device performance was ascribed to the end-on orientation and close packing of DPP(PhNMe2)2 molecules along the π–π stacking direction.

Bluish-Green Light Emitting Zinc Complex for OLED Application

Zinc (II) was complexed with 8-hydroxyquinoline-5-sulphonic acid (L) to give an ZnL2 complex (1:2 molar ratio). The synthesized complex was characterized by infrared spectra, differential thermal analysis and thermogravimetry. The UV-absorption and photoluminescence (PL) spectra show peaks at 377 nm and 502 nm respectively. The OLED device has been fabricated by depositing the thin films of different materials on ITO, hole transport layer was obtained by depositing a thin film of 4,4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), Bis(8-hydroxyquinolinato-5-sulphonic acid)zinc(ZnL2) as emissive layer, BCP as hole blocking layer, thin film of Alq3 as electron transport layer and LiF/Al as electron injecting electrode. The EL device was characterized for electroluminescence and the peak was reported at 503 nm. The EL spectrum of complexes ZnL2 is found that there is a considerable blue shift in the EL spectra from its un-substituted counter complex when electron-withdrawing group is introduced at 5th position in quinoline ring.

Rubidium carbonate modified gold electrodes for efficient electron injection in n-type organic field-effect transistors

We report on the performance enhancement of n-type organic field-effect transistors (OFETs) through the use of gold source and drain electrodes that are both modified with rubidium carbonate (Rb2CO3) reducing the electron injection barrier. Devices are fabricated using n-channel N, N?-ditridecyl-3,4,9,10-perylenetetracarboxylicdiimide (PTCDI-C13) and a polymeric gate dielectric with various thicknesses of Rb2CO3, and the dependence of devices electrical performance on Rb2CO3 thickness is investigated. The device with 10?? Rb2CO3 exhibits the best performance, and its mobility is five times higher than that of the device without Rb2CO3. UV?visible, x-ray and ultraviolet photoemission spectroscopy are used to investigate the interface between Rb2CO3 and PTCDI-C13, and we find that charge transfer from Rb2CO3 to PTCDI-C13 occurs, resulting in the reduction of the electron charge injection barrier from the gold electrode. The charge injection mechanism and OFET performance enhancement with Rb2CO3 are discussed in detail.