Yuan Jay Chang

Phenothiazine derivatives as organic sensitizers for highly efficient dye-sensitized solar cells

A series of organic dyes containing a phenothiazine central unit were synthesized and were used effectively in the fabrication of dye-sensitized solar cells (DSSCs). A cyanoacrylate moiety was added at the C(3) position of the phenothiazine as an electron acceptor, and a triarylamine moiety was attached at the C(7) position as an electron donor. The DSSCs made with these dyes displayed remarkable quantum efficiency, ranging from 4.2–6.2% under an AM 1.5 solar condition (100 mW cm−2). A variety of substituents, i.e., methyl, hexyl and triphenylamino groups, were added at the N(10) of phenothiazine in order to optimize the incident photon-to-current conversion efficiency. Along the main chromophore a thiophenylene group was inserted at different positions to examine its influence on the properties of devices. The best performance was found in compound NSPt-C6, in which a hexyl group was attached at the N(10) of phenothiazine and a thiophenylene at the C(7) position. It displayed a short-circuit current (Jsc) of 14.42 mA cm−2, an open-circuit voltage (Voc) of 0.69 V, and a fill factor (ff) of 0.63, corresponding to an overall conversion efficiency of 6.22%. Their photophysical properties were analyzed with the aid of a time-dependent density functional theory (TDDFT) model with the B3LYP functional. Their photovoltaic behavior was further elucidated by the electrochemical impedance spectroscopy.

Highly efficient triarylene conjugated dyes for sensitized solar cells

A new series of organic dyes containing a triarylamine donor group, a triarylene-linked bridging moiety, and a cyanoacrylic acid acceptor group were synthesized through a simple procedure in high yields. A selected set of substituents were added onto the phenyl group ortho to the cyanoacrylic acid in order to examine their influences on the performance of dye-sensitized solar cells (DSSCs). Their photochemical behaviors were examined under AM1.5 solar condition (100 mW cm−2). A typical device made with a compound containing a -CF3 substituent (PSP-CF3) displayed a short-circuit current (Jsc) 15.16 mA cm−2, an open-circuit voltage (Voc) 0.68 V, a fill factor (ff) 0.68, corresponding to an overall conversion efficiency of 7.0% and a maximal monochromatic incident photon-to-current conversion efficiency (IPCE) 78%. Their photophysical and electrochemical properties were analyzed with the aid of a time-dependent density functional theory (TDDFT) model with the B3LYP functional. Their HOMO and LUMO energy levels are verified by both electrochemical measurements and theoretical calculations.

Organic dyes containing oligo-phenothiazine for dye-sensitized solar cells

A series of organic dyes containing oligo-phenothiazine were synthesized and used effectively on the fabrication of dye-sensitized solar cells (DSSCs). In these compounds the phenothiazine moiety functions both as an electron donor and as a π-bridge. These materials exhibit considerably high values of open-circuit voltage (Voc) ranging from 0.78–0.83 V under an AM1.5 solar condition (100 mW cm−2). Two kinds of substituents, i.e., hexyl and hexyloxyphenyl groups, were added onto the N(10) of phenothiazine for comparison. The best device displayed a short-circuit current (Jsc) of 14.3 mA cm−2, an open-circuit voltage (Voc) of 0.83 V, a fill factor (FF) of 0.65, corresponding to an overall conversion efficiency of 7.78%. Their photophysical properties were analyzed with the aid of a time-dependent density functional theory (TDDFT) model with the B3LYP functional. The electronic nature of the devices was further elucidated by using electrochemical impedance spectroscopy.

Highly efficient red fluorescent dyes for organic light-emitting diodes

Two red color luminescent dyes, 4-(dicyanomethylidene)-2-tert-butyl-6-(2-(1-(4-tert-butylphenyl)-1,2,3,4-tetrahydroquinolin-6-yl)vinyl)-4H-pyran (OJ1) and 4-(dicyanomethylidene)-2-tert-butyl-6-(2-(1-(4-tert-butylphenyl)-2,3-dihydro-2-methylindolin-5-yl)vinyl)-4H-pyran (OJ2), were synthesized readily in high yields and were used in the fabrication of organic light-emitting diodes (OLEDs). The physical properties of these materials are fully investigated in this report. Their structures are analogous to the well-known red dye, 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), yet with better electrostability. A typical device made with OJ2 doped 1% in aluminium tris(8-hydroxyquinolinate) (Alq3) displayed a red light with a narrow bandwidth (fwhm 72 nm) at λmax 615 nm corresponding to a CIE coordinate (0.62, 0.38). The device can be turned on at 3.5 V, and reaches a luminance intensity of 43 400 cd m at ca. 20 V. It shows a luminance efficiency of 6.46 cd A−1, a power efficiency of 2.04 lm W−1, and an external quantum efficiency of 3.27%. The performances of devices are tunable either by adjusting the doping concentrations or by changing the device configurations.

High-Performance Organic Materials for Dye-Sensitized Solar Cells: Triarylene-Linked Dyads with a 4-tert-Butylphenylamine Donor

A series of organic dyes were prepared that displayed remarkable solar-to-energy conversion efficiencies in dye-sensitized solar cells (DSSCs). These dyes are composed of a 4-tert-butylphenylamine donor group (D), a cyanoacrylic-acid acceptor group (A), and a phenylene-thiophene-phenylene (PSP) spacer group, forming a D-π-A system. A dye containing a bulky tert-butylphenylene-substituted carbazole (CB) donor group showed the highest performance, with an overall conversion efficiency of 6.70%. The performance of the device was correlated to the structural features of the donor groups; that is, the presence of a tert-butyl group can not only enhance the electron-donating ability of the donor, but can also suppress intermolecular aggregation. A typical device made with the CB-PSP dye afforded a maximum photon-to-current conversion efficiency (IPCE) of 80% in the region 400-480 nm, a short-circuit photocurrent density J(sc) =14.63 mA cm(-2), an open-circuit photovoltage V(oc) =0.685 V, and a fill factor FF=0.67. When chenodeoxycholic acid (CDCA) was used as a co-absorbent, the open-circuit voltage of CB-PSP was elevated significantly, yet the overall performance decreased by 16-18%. This result indicated that the presence of 4-tert-butylphenyl substituents can effectively inhibit self-aggregation, even without CDCA.

Hole-Transporting Materials Based on Twisted Bimesitylenes for Stable Perovskite Solar Cells with High Efficiency

A new class of hole-transport materials (HTMs) based on the bimesitylene core designed for mesoporous perovskite solar cells is introduced. Devices fabricated using two of these derivatives yield higher open-circuit voltage values than the commonly used spiro-OMeTAD. Power conversion efficiency (PCE) values of up to 12.11% are obtained in perovskite-based devices using these new HTMs. The stability of the device made using the highest performing HTM (P1) is improved compared with spiro-OMeTAD as evidenced through long-term stability tests over 1000 h.

Solution-Processed Optoelectronic Properties of Functionalized Anthradifuran

Acenes are promising molecules for their applications in optoelectronic devices such as organic field-effect transistors (OFET), organic light-emitting diodes (OLED), and organic photovoltaic devices (OPV), and so forth. Heteroacenes containing a fused heterocyclic aromatic ring are more versatile than the corresponding parent acenes owing to their higher thermalas well as photochemical stability. A wide variety of fused heteroacenes have been reported in the literature, such as selenophene, pyridine, pyrazine, and 2,1,3benzothiaziazole, and so forth. Among them, thiophenecontaining heteroacenes have attracted the most attention, such as aceno ACHTUNGTRENNUNG[2,3-b]thiophene,[3] 2,2’-binaphtho ACHTUNGTRENNUNG[2,3-b]thiophene, thienoACHTUNGTRENNUNG[3,2-b]thiophene[5] derivatives, and benzo[2a,6] , naphtho, and anthra[2,3-b :6,7-b’]dithiophene (ADT), for their applications in OFETs. In the family of chalcogen elements, oxygen is the smallest with the highest electronegativity. Acenes that contain furan rings are known to show a lower degree of aromaticity compared to thiophene and selenophene. With respect to the electronic properties, a lower degree of aromaticity generally leads to a lower potential energy of the HOMO (highest occupied molecular orbital). This behavior of furanoacene could influence the OFET characteristics. Recently, a few examples of furan-containing heteroacenes have been reported as components for OFET devices, for example, naphtho[2,1-b ;6,5b’]difuran derivatives have good hole mobilities, and naphtho[2,3-b:7,6-b’]difuran displayed not only a good hole mobility but also significant OPV properties. The molecular scaffold of benzofuran has been proven to be useful in the design of optoelectronic materials, such as in OLEDs and OLETs. The syntheses of the anti-form of anthra[2,3b :6,7-b’]-difuran (anti-ADF) and its diphenyl derivatives have been previously reported in the literature. Although the application of nonsubstituted anti-ADF in an OFET was unsuccessful, its diphenyl-substituted derivative showed a good hole transporting property. In this study, two new ADF derivatives have been synthesized. Two alkylethynylsilyl substituents were added to the ring moieties of the ADF molecule to improve their stability and solubility in organic solvents. Their physical properties and their applications in both OFET and OPV are discussed. The synthetic procedure for the alkylethynylsilyl-functionalized ADF is shown in Scheme 1. An aldol-type reaction of

Pyridomethene–BF2 complex/phenothiazine hybrid sensitizer with high molar extinction coefficient for efficient, sensitized solar cells

A series of new pyridomethene–BF2/phenothiazine hybrid metal-free organic sensitizers K1–K8 containing different π-spacers were synthesized and applied in dye-sensitized solar cells (DSSC). The introduction of the pyridomethene–BF2 complex unit to the phenothiazine chromophore resulted in a high molar extinction coefficient in favor of light harvesting. Quantum chemical calculations were performed using the density functional theory (DFT) at the B3LYP/6-31G(d,p) level to investigate the structural properties and electron density distributions of these dyes. The effect of dyes K1–K8 on the performance of DSSC was investigated systematically with comparisons to the plain phenothiazine dyes R1 and R2. Upon co-adsorption with deoxycholic acid, the dye K3, with a thiophene unit between the phenothiazine and pyridomethene–BF2 units, exhibited the best photovoltaic performance. The short-circuit current density (Jsc) was 15.43 mA cm−2 with an open-circuit voltage (Voc) of 0.69 V and a fill factor (FF) of 0.62, which correspond to a power-conversion efficiency (η) of 6.58% under AM 1.5G irradiation (100 mW cm−2). The n-hexyl chain attached to the thiophene in K4–K5 improved the Voc value. The presence of the phenyl pyridomethene–BF2 moiety at the N(10) atom of phenothiazine in K6–K8 reduced π–π aggregation. These results reveal the advantage of incorporating a pyridomethene–BF2 group in the dyes for high-performance DSSC cells.

A soluble precursor of hexacene and its application in thin film transistors

A soluble precursor of hexacene is prepared from a cycloaddition of hexacene and diethyl ketomalonate in high yield. It can be used to make hexacene thin-films through spin-coating for the fabrication of organic field effect transistors.

Impact of alkoxy chain length on carbazole-based, visible light-driven, dye sensitized photocatalytic hydrogen production

Alkoxyphenyl-substituted carbazole-based metal-free organic dyes were synthesized and effectively used for dye-sensitized, visible-light-driven, photocatalytic hydrogen production. Photocatalytic hydrogen production was investigated using a TiO2/dye/Pt structure with triethanolamine as the sacrificial reagent. The dye-loaded TiO2 photocatalyst exhibited a high yield of hydrogen production when the length of the alkoxy chain was long enough to sufficiently improve the hydrophobicity at the interface between the dye-loaded TiO2 and the water medium. In the alkoxyphenyl-substituted carbazole dyes, the dye with the longest alkoxy chain (C22) exhibited the best hydrogen production performance, but it had a yield only slightly better than that of the dye with the second longest chain length (C16). The dye C22 displayed a turnover number (TON) of 3094 after 24 h of visible light irradiation (>420 nm). However, the compound with no hydrophobic substituent (C1), exhibited the lowest hydrogen production performance with a TON of 1497. Thus, a 207% increase in the hydrogen production yield was observed when hydrophobic substituents were present. Analysis of time-resolved absorption spectra, impedance spectra and incident photon conversion efficiency spectra revealed that the alkoxy chain has a hydrophobic effect at the interface between the dye-loaded TiO2 and the water. Specifically, the hydrophobicity of the dye improved the charge-recombination lifetime for electron injection from the dye into the TiO2 surface in the water for hydrogen production.