Ruangchai Tarsang

Blue light-emitting and hole-transporting materials based on 9,9-bis(4-diphenylaminophenyl)fluorenes for efficient electroluminescent devices

New bifunctional materials namely BTTF, TPTF and BPTF having 9,9-bis(4-diphenylaminophenyl)fluorene as a molecular platform were synthesized and characterized. These molecules showed strong blue emission in both solution and solid state with a solution fluorescence quantum efficiency of up to 74% and were thermally stable amorphous materials with glass transition temperature well above 170 °C. The abilities of these materials as blue light-emitting materials for blue OLEDs and hole-transporting materials for green OLEDs in terms of device performance and thermal property were superior to a commonly used N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB). Efficient non-doped blue and Alq3-based green OLEDs with maximum luminance efficiencies and CIE coordinates of 2.06 cd A−1 and (0.15, 0.13), and 4.94 cd A−1 and (0.29, 0.52) were achieved, respectively, with BPTF having two pyrene substituents as the emitting layer and the hole-transporting layer, respectively.

Pyrene-functionalized carbazole derivatives as non-doped blue emitters for highly efficient blue organic light-emitting diodes

A series of pyrene-functionalized carbazole derivatives, namely N-dodecyl-3,6-di(pyren-1-yl)carbazole (CP2), N-dodecyl-1,3,6-tri(pyren-1-yl)carbazole (CP3) and N-dodecyl-1,3,6,8-tetra(pyren-1-yl)carbazole (CP4), are synthesized and characterized as simple non-doped solution processed blue emitters for OLEDs. By multiple substitution of pyrene on the carbazole ring, we are able to retain the high blue emissive ability of pyrene in the solid as well as improve the amorphous stability and solubility of the material. These materials show high solution fluorescence quantum efficiencies (up to 94%) and can form morphologically stable amorphous thin films with Tg as high as 170 °C. Solution processed double-layer OLEDs using these materials as non-doped blue emitters exhibit good device performance with luminance efficiencies up to 2.53 cd A−1. Their multi-layer devices exhibit outstanding performances. The CP2-based device shows a low turn-on voltage (down to 2.6 V) with an excellent blue emission color (λem = 436 nm) and CIE coordinates of (0.16, 0.14), while the CP3 and CP4-based blue devices also display low turn-on voltages (down to 2.7 V) with high luminance (up to 24479 cd m−2) and luminance efficiencies (up to 6.92 cd A−1).

Synthesis and characterization of carbazole dendronized coumarin derivatives as solution-processed non-doped emitters and hole-transporters for electroluminescent devices

A series of carbazole dendronized coumarin derivatives (CTmGn, m = 0–2, n = 1–3) containing oligothiophenyl coumarins as cores and carbazole dendrons up to the third generation as substituents were synthesized and characterized. Their optical, thermal, electrochemical, and electroluminescent properties as non-doped solution-processed light-emitters and hole-transporters for OLEDs were investigated. They exhibited a bright light blue to yellow fluorescence with solution fluorescence quantum yields (ΦF) of about 0.10–0.40 and morphologically stable amorphous thin films with glass transition temperatures as high as 285 °C. As emissive layers, solution-processed OLEDs with the structure of ITO/PEDOT:PSS/CTmGn/BCP/LiF:Al emitted light blue to yellow colours with a maximum luminance efficiency as high as 7.92 cd A−1 and a low turn-on voltage of 3.6 V. As hole-transporting layers, solution-processed OLEDs with the structure of ITO/PEDOT:PSS/CGn/Alq3/LiF:Al showed a bright green emission (λEL = 518 nm, CIE = 0.27, 0.53) with a maximum luminance efficiency as high as 6.54 cd A−1 and a low turn-on voltage of 2.5 V.

Modification of D-A-π-A configuration toward a high-performance triphenylamine-based sensitizer for dye-sensitized solar cells: A theoretical investigation

In an attempt to shed light on how the addition of a benzothiadiazole (BTD) moiety influences the properties of dyes, a series of newly designed triphenylamine-based sensitizers incorporating a BTD unit as an additional electron-withdrawing group in a specific donor-acceptor-π-acceptor architecture has been investigated. We found that different positions of the BTD unit provided significantly different responses for light absorption. Among these, it was established that the further the BTD unit is away from the donor part, the broader the absorption spectra, which is an observation that can be applied to improve light-harvesting ability. However, when the BTD unit is connected to the anchoring group a faster, unfavorable charge recombination takes place; therefore, a thiophene unit was inserted between these two acceptors, providing redshifted absorption spectra as well as blocking unfavorable charge recombination. The results of our calculations provide valuable information and illustrate the potential benefits of using computation-aided sensitizer design prior to further experimental synthesis.

Triple bond-modified anthracene sensitizers for dye-sensitized solar cells: a computational study

We performed a theoretical investigation on a series of organic dyes incorporating an anthracene moiety between a carbazole donor group and a cyanoacrylic acid acceptor, in which a triple bond (TB)-modified moiety acts as a π-conjugated linker. Density functional theory (DFT) and time-dependent DFT (TD-DFT) were applied to understand the electronic, photophysical, and electron injection properties of the dyes. We found that optimized anthracene structures lay almost perpendicular to the plane of the adjacent substituents. The introduction of a modified TB moiety significantly decreases the dihedral angle and results in a planar structure, which extends the length of the π-conjugated system to provide a broader absorption spectrum, the An4 dye exhibited the greatest red shift of the maximum absorption wavelength. Introduction of a TB moiety into the dye structure facilitates electron transfer from the donor and acceptor. The TB-modified dye structure has a significant effect on electron injection from the dye sensitizer to the TiO2 surface. Our results demonstrate that use of computational design can to help the experimentalist for looking out for future developments to identify TB-modified anthracene sensitizers for highly efficient solar cells.

Theoretical studies on electronic structures and photophysical properties of anthracene derivatives as hole-transporting materials for OLEDs

The electronic structures and photophysical properties of anthracene derivatives as hole-transporting materials (HTM) in OLEDs have been studied by DFT and TD-DFT methods. Thiophene and triphenylamine (TPA) moieties are used as substituents in anthracene based HTMs providing FATn and FAPn compounds (n=1-2), respectively. The calculated electronic levels by B3LYP show proper energy matching of FAPn and hole-injecting layer (HIL), indicating that the hole-transports of the FAPn compounds are better than the FATn compounds. The photophysical properties calculated by TD-B3LYP elucidate that TPA in FAPn compounds acts as electron donating group and induces charge transfer character in the absorptions. Furthermore, the calculated ionization potential (IP), electron affinity (EA) and reorganization energies also revealed that the extended FAP2 compound has the highest charge-transporting ability among the studied compounds. The calculated results are consistent to our experimental observations showing that FAP2 exhibits bright fluorescence with highest quantum yield in electroluminescent devices. Understanding of these properties is useful for further design of new HTMs of desired properties, such as high efficiency and stability.

New D–D–π–A type organic dyes having carbazol-N-yl phenothiazine moiety as a donor (D–D) unit for efficient dye-sensitized solar cells: experimental and theoretical studies

A series of novel organic dyes (CPhTnPA, n = 0–2) with the D–D–π–A structural configuration incorporating carbazol-N-yl phenothiazine moiety as a donor (D–D) unit, phenyl oligothiophenes as a π-linker and cyanoacrylic acid as both electron-acceptor and anchoring group were synthesized and characterized for dye sensitized solar cells. Detailed investigation on the relationship between the structure, spectral and electrochemical properties, and performance of DSSC was described here. Time dependent density functional theory (TDDFT) calculations have been performed on the dyes, and the results showed that both electron donors (D–D) can contribute to electron injection upon photo-excitation, either directly or indirectly by internal conversion to the lowest excited state. Dye-sensitized solar cells (DSSCs) using dyes as the sensitizers exhibited good efficiencies. In virtue of co-sensitization, the CPhT2PA based solar cell can achieve photovoltaic efficiency as high as 7.78% (JSC = 15.22 mA cm−2, VOC = 0.74 V and FF = 0.69) which reached 95% with respect to that of the reference N719-based device (8.20%) in parallel investigations.

Significant enhancement in the performance of porphyrin for dye-sensitized solar cells: aggregation control using chenodeoxycholic acid

In this work, a series of porphyrin-based sensitizers, coded as ZnTEP, ZnTTEP, and ZnTTTD in which different kinds of thienyl groups are used as the π-linker, have been designed and successfully synthesized. Their synthesis, photophysical and electrochemical properties, and theoretical DFT calculations, as well as their applications in dye-sensitized solar cells (DSSCs) are reported. The results showed that the power conversion efficiency (η) increases in the order of 1.14, 2.15 and 3.51% for ZnTEP, ZnTTEP and ZnTTTD, respectively. A significant improvement in the η value was achieved with ZnTTTD-based solar cells, which was approximately three times the magnitude when compared to the lowest ZnTEP-based solar cells. Moreover, the effects of chenodeoxycholic acid (CDCA) in a dye solution as a co-adsorbent on the photovoltaic performance of DSSCs were investigated. It was found that the coadsorption of CDCA can hinder the formation of dye aggregates and the improve electron injection yield and thus Jsc. The η value for ZnTTTD with the CDCA co-sensitizer dramatically increases up to 6.51%. This understanding of π-linker and co-sensitizer effects on the performance of porphyrins will serve as a guideline for the design of future dye sensitizers for DSSCs.

Synthesis and properties of fluorene-oligothiophenes perylenediimide triads and their electropolymerizations

A series of novel donor–acceptor–donor triads, namely PFTn (n = 0–4), consisting of fluorene-oligothiophenes (FTn) as a donor and perylenediimide (PDI) as an acceptor were synthesized and characterized. The chemically bonded electron donor and acceptor chromophores in the triads were found to be independent. These triads showed a strong intramolecular photo-induced electron transfer (PET) between both chromophores, which gave rise to a quasi-quantitative fluorescence quenching of both PDI and FTn moieties. Distinctively, a reverse energy transfer from the acceptor PDI to the donor FTn induced a strong fluorescence emission from the FTn moieties in the triads PFT3 and PFT4. The PFTn (n = 1–3) triads were proven to undergo an electrochemical oxidative coupling reaction and become electropolymerizable materials to form polymer plastic films (poly(PFTnn)), while PFT4 proved to be an electrochemically stable molecule.