Triphenylamine Dyes Research Articles

Efficient dye regeneration at low driving force achieved in triphenylamine dye LEG4 and TEMPO redox mediator based dye-sensitized solar cells.

Minimizing the driving force required for the regeneration of oxidized dyes using redox mediators in an electrolyte is essential to further improve the open-circuit voltage and efficiency of dye-sensitized solar cells (DSSCs). Appropriate combinations of redox mediators and dye molecules should be explored to achieve this goal. Herein, we present a triphenylamine dye, LEG4, in combination with a TEMPO-based electrolyte in acetonitrile (E(0) = 0.89 V vs. NHE), reaching an efficiency of up to 5.4% under one sun illumination and 40% performance improvement compared to the previously and widely used indoline dye D149. The origin of this improvement was found to be the increased dye regeneration efficiency of LEG4 using the TEMPO redox mediator, which regenerated more than 80% of the oxidized dye with a driving force of only ∼0.2 eV. Detailed mechanistic studies further revealed that in addition to electron recombination to oxidized dyes, recombination of electrons from the conducting substrate and the mesoporous TiO2 film to the TEMPO(+) redox species in the electrolyte accounts for the reduced short circuit current, compared to the state-of-the-art cobalt tris(bipyridine) electrolyte system. The diffusion length of the TEMPO-electrolyte based DSSCs was determined to be ∼0.5 μm, which is smaller than the ∼2.8 μm found for cobalt-electrolyte based DSSCs. These results show the advantages of using LEG4 as a sensitizer, compared to previously record indoline dyes, in combination with a TEMPO-based electrolyte. The low driving force for efficient dye regeneration presented by these results shows the potential to further improve the power conversion efficiency (PCE) of DSSCs by utilizing redox couples and dyes with a minimal need of driving force for high regeneration yields.

Can elongation of the π-system in triarylamine derived sensitizers with either benzothiadiazole and/or ortho-fluorophenyl moieties enrich their light harvesting efficiency? – a theoretical study

This study established that elongation of the π-system in a series of hypothetical triphenylamine dyes by the judicious placement either a fluorophenyl or benzothiadiazole group, or a combination of both groups, results in improved light harvesting efficiency in DSSCs.

A promising candidate with D-A-A-A architecture as an efficient sensitizer for dye-sensitized solar cells.

A series of metal-free organic dyes with electron-rich (D) and electron-deficient units (A) as π linkers have been studied theoretically by means of density functional theory (DFT) and time-dependent DFT calculations to explore the effects of π spacers on the optical and electronic properties of triphenylamine dyes. The results show that Dye 1 with a structure of D-A-A-A is superior to the typical C218 dye in various key aspects, including the maximum absorption (λmax =511 nm), the charge-transfer characteristics (D/Δq/t is 5.49 Å/0.818 e(-) /4.41 Å), the driving force for charge-carrier injection (ΔGinject =1.35 eV)/dye regeneration (ΔGregen =0.27 eV), and the lifetime of the first excited state (τ=3.1 ns). It is thus proposed to be a promising candidate in dye-sensitized solar cell applications.

Can 2-pyrone derivative act as an effective π-linker for dye-sensitized solar cells: a theoretical study?

Thiophene and derivatives have been broadly used in metal-free organic dyes as π-bridge in the past 10 years. However, the relatively sharp and narrow visible absorption bands of these organic dyes not only severely attenuated the light capture capability but also restrict the efficiency. In this contribution, to design efficient sensitizers for dye-sensitized solar cells, a series of triphenylamine (TPA) dyes with 2-pyrone as the π-bridge are investigated using the density functional theory and time-dependent density functional theory approaches. The results show that the designed dyes have smaller gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, the absorption bands are greatly bathochromic-shifted by at least 39 nm and the light-harvesting efficiencies are improved compared to the experimentally efficient sensitizer T with thiophene as the π-bridge. The calculated values of free energy change ΔG inject for all the designed dyes are very negative, which favors electron injection from the excited-state dye to the TiO2 conduction band edge. Our simulations show that the sensitizers studied here are strongly adsorbed to the TiO2 cluster. During light excitation, electrons are transferred from the TPA group through the π-spacer to the surface-bound cyanoacrylate, facilitating electron injection to the TiO2 nanoclusters. Our calculations indicate that the newly designed dyes will be promising candidates for the future solar cell applications.

Probing the regeneration process of triphenylamine-based organic dyes in dye-sensitized solar cells

The regeneration processes of triphenylamine (TPA)-based dyes with cobalt redox mediator in dye-sensitized solar cells (DSSCs) have been investigated using density functional theory combined with the Marcus theory of electron transfer. Our results show that with the extension or rigidification of the oligothiophene conjugation linker the absorption spectra of TPA dyes exhibit observable red-shift in the maximum absorbance that favors light-harvesting, while the electron transfer rates for dye regeneration decrease in some degrees due to the increased activation free energies and the reduced electronic coupling energies which hampers the dye regeneration. Importantly, the undesirable influences on dye regeneration by extending the linker moiety are more significant than that by the way of rigidification. Thus, the rigidification is a better choice than the extension of the conjugated moiety for the design of D-π-A type dyes based on the properties of light-harvesting and the kinetics of dye regeneration.

Multiple-Anchoring Triphenylamine Dyes for Dye-Sensitized Solar Cell Application

A series of triphenylamine-based dyes (TPACR1, TPACR2, and TPACR3) with multiple corhodanine derivatives as acceptors were prepared and examined as sensitizers for dye-sensitized solar cells. The overall conversion efficiencies of DSSCs based on these dyes were in the range of 2.63 to 5.31%, in which TPACR2-based DSSC showed the best photovoltaic performance: a short-circuit current (Jsc) of 15.03 mA·cm–2, an open-circuit voltage (Voc) of 552 mV, and a fill factor (FF) of 0.64, corresponding to an overall efficiency of 5.31% under simulated AM 1.5 solar irradiation (100 mW·cm–2). Compared with monoanchoring TPACR1 dye and trianchoring TPACR3 dye, dianchoring TPACR2 dye had a relatively slower charge recombination rate between injected electrons and triphenylamine dye cations, which was indicated by transient absorption kinetics Furthermore, the suppression of the charge recombination between injected electron and the I3– in the electrolyte lead to the longer electron lifetime observed with the DSSC based ...

Rational design of triphenylamine dyes for highly efficient p-type dye sensitized solar cells

Two new sensitizers based on triphenylamine-dicyanovinylene have been synthesized and used for p-type dye-sensitized solar cells. The best performance amongst solar cells was achieved by the dye with a ter-thiophene bridge ligand between carboxylic acid group and the triphenylamine part (with power conversion efficiency of 0.19%, short circuit current of 4.01 mA cm−2, open circuit voltage at 144 mV, and fill factor of 0.33). Results indicate that the ter-thiophene groups in the dyes strongly affects both charge recombination and hole injection in the photoelectrode. In addition, the hexyl chains on the bridged thiophene rings also help to avoid dye aggregation on the nickel oxide film and block I− in electrolyte from approaching the surface of nickel oxide, which leads to a reduction in the charge recombination between nickel oxide semiconductor and electrolyte. This study suggested that modification of the bridge moiety between triphenylamine and the carboxylic group by increasing thiophene units is a promising way for preventing charge recombination and increasing the power conversion efficiency.

Effect of different acceptors in di-anchoring triphenylamine dyes on the performance of dye-sensitized solar cells

Three di-anchoring triphenylamine dyes, which were coded as TPAC1, TPAR2 and TPACR2, were designed and synthesized for dye-sensitized solar cell application. The structural modification effect of different anchoring groups on photophysical, electrochemical and photovoltaic properties of the related DSSCs was extensively investigated. With the variation from cyanoacetic acid via rhodanine-3-acetic acid to co-rhodanine units, the molar extinction coefficients of the maximum absorption wavelength for the three dyes gradually increase due to the extension of π system. In comparison with that for TPAC1 and TPAR2, DSSC based on dye TPACR2 with double co-rhodanine groups shows the best overall conversion efficiency of 4.64% with simultaneous enhancement of photocurrent and photovoltage, which is attributed to the higher molar extinction coefficient (6.5 × 104 M−1 cm−1), broader absorption spectra, broader IPCE spectra and longer electron lifetime.

Promising ZnO-based DSSC performance using HMP molecular dyes of high extinction coefficients.

Employing newly synthesized di-substituted tri-phenyl amine (HMP-9) and carbazole (HMP-11) dyes (with limited acidic carboxyl anchor groups), a power conversion efficiency as high as 7.03% in ZnO nanocrystallite (NC)-based dye-sensitized solar cells (DSSCs) is achieved. The specific molecular designs of HMP-09 and HMP-11 consisting of with and without hexyloxy spacer groups, and added tri-phenyl amine or 9-phenyl-9H-carbazole donor groups, respectively, attached on the ancillary ligands are advantageous, evidenced from electrochemical impedance spectroscopy measurements, for ZnO NC-based DSSCs.

A benzothiazole–cyclopentadithiophene bridged D–A–π–A sensitizer with enhanced light absorption for high efficiency dye-sensitized solar cells

We demonstrate an efficient D-A-π-A sensitizer with a benzothiadiazole-cyclopentadithiophene [corrected] moiety as the spacer in a triphenylamine organic dye for dye-sensitized solar cells. The dye has a broad visible light absorption range up to 800 nm. A power conversion efficiency >9% has been achieved using a [Co(bpy)3](2+/3+)-based electrolyte.

Tuning the electron donating ability in the triphenylamine-based D-π-A architecture for highly efficient dye-sensitized solar cells

A new series of triphenylamine-based organic dye sensitizers, namely TPA1-TPA6 with introduced auxiliary donor groups including carbazole and diphenylamine moieties to triphenylamine core, and with inserted fluorene moiety between triphenylamine and auxiliary donor, was designed and theoretically investigated. The starburst triphenylamine dyes acting as electron donor groups capped with mono- and di-substituted auxiliary donors coded as D-D-π-A and 2D-D-π-A respectively, were studied for the propose of comparison with only one triphenylamine moiety as donor, namely TPA, in the D-π-A system. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were conducted to investigate the molecular structures, frontier molecular orbitals, absorption spectra as well as dye-(TiO2)38 complexes of these dyes as implemented in the Gaussian 09 program. Among three dye architectures, the results suggested that the 2D-D-π-A system showed the largest absorption range. We found that different types of auxiliary donors provided different light-harvesting ability; the diphenylamine auxiliary donor can improve properties of red-shifted absorption region. In addition, the optimized geometries showed that adding of diphenylamine auxiliary donor provided smaller external dihedral angles (EDA) leading to wider absorption range with strong charge-transfer character compared to other dyes. Our results are suggested to be the possible reasons for the enhancement of conversion efficiency in dye-sensitized solar cells.

Effects of the acceptors in triphenylamine-based D–A′–π–A dyes on photophysical, electrochemical, and photovoltaic properties

Three D–A′–π–A structural triphenylamine dyes (Dye 1, Dye 2, and Dye 3) with cyanoacetic acid or malononitrile as an electron acceptor (A) and fluorenone or 9-(dicyanomethylene)fluorene as an additional acceptor (A′) have been designed and synthesized. Results revealed that both A and A′ have great influence on the optical, electrochemical and photovoltaic properties. In comparison with Dye 3, the photovoltaic performance of Dye 1 is remarkably improved by replacing 9-(dicyanomethylene)fluorene with fluorenone as the additional acceptor. The power conversion efficiency of dye-sensitized solar cell based on the Dye 1 (5.33%) is four times higher than that of Dye 3 (1.25%). The low efficiency of cell based on Dye 3 is mainly because the delocalization of the excited state is broken between the strong electron-withdrawing additional acceptor (9-(dicyanomethylene)fluorene) unit and cyanoacetic acid, which affects the efficient electron injection from Dye 3 to the conduction band of TiO2.

Novel organic dye employing dithiafulvenyl-substituted arylamine hybrid donor unit for dye-sensitized solar cells

In order to increase electron-donating ability of the donor part of the organic dye, two dithiafulvenyl (DTF) units were introduced into a triphenylamine unit to form dithiafulvenyl-substituted triphenylamine hybrid donor for dye-sensitized solar cells (DSSCs) for the first time. Novel donor–acceptor organic dye WD-10 containing this hybrid donor and 2-cyanoacetic acid acceptor has been designed, synthesized and applied in DSSCs. The influence of the substituent unit DTF in the dye on the device performance has been investigated. It was found that the dye with dithiafulvenyl-substituted triphenylamine hybrid donor gave higher photocurrent, open-circuit voltage, and efficiency value. The DSSC based on organic dye WD-10 displayed a short-circuit current (Jsc) of 9.58mAcm−2, an open-circuit voltage (Voc) of 648mV, and a fill factor (ff) of 0.71, corresponding to an overall conversion efficiency of 4.41%. An increase in η of about 79% was obtained from simple triphenylamine dye L0 to WD-10. The different photovoltaic behaviors of the solar cells based on the organic dyes were further elucidated by the electrochemical impedance spectroscopy. This work identifies that the introduction of DTF unit into the simple triphenylamine dye could significant improve the photovoltaic performance.

Functional Assessment for Predicting Charge-Transfer Excitations of Dyes in Complexed State: A Study of Triphenylamine–Donor Dyes on Titania for Dye-Sensitized Solar Cells

Time-dependent density functional theory (TD-DFT) was employed to calculate the UV/vis spectra for three of the triphenylamine (TPA)-donor dyes, TC1, L1, and LJ1, in isolation as well as when complexed with a titania nanoparticle. TPA-donor dyes are a class of promising organic dyes for use in dye-sensitized solar cells (DSSCs). The three dyes studied here are among the smallest of these molecules and provide important insight into the entire series of TPA dyes that are being explored as possible sensitizers in titania-based DSSCs. An attempt to calculate the optical spectra for these dyes within the B3LYP approximation to the exchange correlation functional produces erroneous results. However, Coulomb attenuated approximation (CAM-B3LYP) captures the correct photophysics of the dyes and produces more accurate charge-transfer excitation energies of their complexes with titania. This work shows that the extent to which a given approximation fails or succeeds to correctly predict the charge-transfer excitation energies in the isolated dyes is propagated in that it fails (or succeeds) to correctly predict the values of the excitation energies for the complexes. It is, therefore, important to determine the most appropriate functional for a dye before considering it in more complicated structures such as dye-titania complexes.

Influence of thiophene and benzene unit in triphenylamine dyes on the performance of dye-sensitized solar cells

Five triphenylamine dyes (WR1-5) featuring thiophene and benzene unit as the π-conjugated spacers or the substitutes attached to the triphenylamine core were synthesized for investigating the influence of thiophene and benzene unit on the performance of dye-sensitized solar cells (DSSCs). Apart from the alteration of the thiophene and benzene, the position of thiophene or benzene in triphenylamine dyes also has a major impact on some important electro-optical properties such as molar extinction coefficient, maximum absorption peaks and the ground-state oxidation behavior of these dyes. How the thiophene and benzene unit affect the charge recombination is discussed in terms of dye surface blocking and intermolecular interactions between dyes and electrolyte acceptor species. Among these dyes, WR2 containing thiophene unit as the π-conjugated spacer and thiophene donor group attached to the triphenylamine core yield the best photovoltaic performance: a short-circuit photocurrent density (JSC) of 10.8mAcm−2, an open-circuit photovoltage (VOC) of 860mV, and a fill factor (FF) of 0.66, corresponding to an overall conversion efficiency of 6.13% under standard global AM1.5 solar light conditions.

Red emitting solid state fluorescent triphenylamine dyes: Synthesis, photo-physical property and DFT study

In this work, four red-emitting push–pull chromophoric dyes containing triphenylamine are reported. The synthesis and solvatochromic behavior of novel triphenylamine based Y-shaped dyes obtained by the condensation of 4-(diphenylamino) benzaldehyde and 4,4′-diformyltriphenylamine with (1-phenylethylidene) propanedinitrile and ethyl-2-cyano-3-phenyl-2-butenoate are reported. The UV–Visible absorption and fluorescence emission spectra of these dyes were studied in solvents of different polarities. The photophysical behavior and the relation between structure and properties of the chromophores were investigated experimentally. These dyes exhibited positive solvatochromism and solvatofluorism in solution. The dyes were characterized by FT-IR, 1 H NMR and Mass spectral analysis. Thermal analysis showed that, these dyes are thermally stable up to 300 °C. Density Functional Theory [B3LYP/6-31G(d)] computations have been used to have more understanding of structural, molecular, electronic and photophysical parameters of push–pull dyes. The absorption wavelength values are found to be in good agreement with the experimental results. Non-linear properties ( β o ) were calculated by theoretical method and found in the range of 196.45 × 10 −30 to 222.18 × 10 −30 e.s.u. ► Synthesis of push–pull triphenylamine based solid state red-emitting fluorescent dyes. ► A good agreement between the experimental and theoretical methods (DFT) results. ► Electronic spectra are highly sensitive to the polarity of solvent medium. ► Also, the quantum yields of these dyes were responsive to solvent polarity. ► Synthesized dyes are thermally stable up to 300 °C.

Theoretical Study of Triphenylamine and Indoline-Based Dyes with Dual Donor and Dual Acceptor for Dye-Sensitized Solar Cells

New triphenylamine and indoline-based dyes with dual donor and dual acceptor were designed and their electronic and optical properties were investigated by using density functional theory and time-dependent density functional theory calculations. The results showed that the homoleptic and heteroleptic dual acceptors of the dyes (C-TPA-In-C, R-TPA-In-C) intensified each absorption band between 450 and 600 nm, compared to the single acceptor of the reference dye (TPA-In-C). Considering overall properties, it is expected that the R-TPA-In-C dye with dual electron acceptor would show highest performance of the dyes in conversion efficiency for dye-sensitized solar cells.

Bibridged bianchoring metal-free dyes based on phenoxazine and triphenyl amine as donors for dye-sensitized solar cell applications

The design and synthesis of two new metal-free bibridged bianchoring ambipolar dyes (D–(π–A)2) by linking electron-donating triphenyl amine or phenoxazine and electron-accepting cyanoacetic acid through a cyanovinyl thiophene π-bridge for fabrication of metal-free, dye-sensitized solar cells (DSSCs). The structures of the synthesized dyes were confirmed by NMR, mass spectrometry and elemental analysis. Experimental and theoretical (DFT) approaches revealed that the phenoxazine (POX-th-CN) dye exhibits a lower band gap (2.03 eV) compared to the triphenyl amine (TPA-th-CN) dye (2.19 eV). Due to this lower band gap, the electronic absorption of the phenoxazine dye becomes red-shifted, and, in addition, the fabricated dye-sensitized solar cell device based on this dye shows higher current conversion efficiencies (η = 1.78%) compared to the TPA-th-CN dye.

Comprehensive investigation of the excited-state dynamics of push–pull triphenylamine dyes as models for photonic applications

A series of emitting push-pull triarylamine derivatives, models of their widely used homologues in photonics and organic electronics, was investigated by steady-state and time-resolved spectroscopy. Their structural originality stems from the sole change of the electron-withdrawing substituent X (-H: 1, -CN: 2, -NO2: 3, -CHC(CN)2: 4), giving rise to efficient emission tuning from blue to red upon increasing the X electron-withdrawing character. All compounds are highly fluorescent in alkanes. The more polar compounds 2-4 undergo considerable Stokes shift and emission quenching in polar solvents. Femtosecond transient absorption data allowed us to identify the nature of the emissive state which varies as a function of the compound and surrounding polarity. A long-lived ππ* excited state with weak charge transfer character was found for 1. This excited state evolves into a long-lived ICT state with red-shifted emission for 2 in polar solvents. For 3 and 4, the ICT state is directly populated in all solvents. Long-lived and emissive in n-hexane, it relaxes in toluene to a new ICT' conformation with stronger charge transfer character and enhanced Stokes shift. In more polar THF, ethanol, and nitrile solvents, ICT relaxes to a dark excited state ICT'' with viscosity-dependent kinetics (<10 ps). The ICT'' state lifetime drops with increasing solvent polarity (150 ps for 3 in THF, 8.5 ps in butyronitrile, 1.9 ps in acetonitrile), denoting an efficient radiationless deactivation to the ground state (back charge transfer). This result reveals a very small S0-S1 energy gap at the relaxed ICT'' geometry, with a possible close-lying S0-S1 conical intersection, which suggests that the ICT → ICT'' process results from a structural change involving a large-amplitude molecular distortion. This fast structural change can account for the strong fluorescence quenching observed for 3 and 4 in polar solvents. Finally, the magnitude of intersystem crossing between the singlet and triplet excited states largely depends on the electron-deficient X unit and the solvent itself. These observations help one conclude on the prevailing role played by the electron-withdrawing groups and the surrounding polarity in the photophysical performances of triphenylamine derivatives, largely employed in numerous emissive solid-state devices.

Comparative study on electronic structures and optical properties of indoline and triphenylamine dye sensitizers for solar cells

The computations of the geometries, electronic structures, dipole moments and polarizabilities for indoline and triphenylamine (TPA) based dye sensitizers, including D102, D131, D149, D205, TPAR1, TPAR2, TPAR4, and TPAR5, were performed using density functional theory, and the electronic absorption properties were investigated via time-dependent density functional theory with polarizable continuum model for solvent effects. The population analysis indicates that the donating electron capability of TPA is better than that of indoline group. The reduction driving forces for the oxidized D131 and TPAR1 are slightly larger than that of other dyes because of their lower highest occupied molecular orbital level. The absorption properties and molecular orbital analysis suggest that the TPA and 4-(2,2diphenylethenyl)phenyl substituent indoline groups are effective chromophores in intramolecular charge transfer (IMCT), and they play an important role in sensitization of dye-sensitized solar cells (DSCs). The better performance of D205 in DSCs results from more IMCT excited states with larger oscillator strength and higher light harvesting efficiency. While for TPA dyes, the longer conjugate bridges generate the larger oscillator strength and light harvesting efficiency, and the TPAR1 and TPAR4 have larger free energy change for electron injection and dye regeneration.