Triphenylamine-based Organic Sensitizers Research Articles

Introducing thiophene and benzothiadiazole groups in triphenylamine-based organic dyes with rigidly fused π-bridge to design high-efficiency solar cells: A theoretical investigation

The influence of different types of π-bridges in triphenylamine-based organic sensitizers on their photophysical and photovoltaic properties is investigated by using sophisticated first-principles calculations and reliable theoretical models. Two well-designed strategies, namely the cyclization between separate benzothiadiazole and bithiophene as well as the insertion of electron-rich/deficient groups between the rigidly fused π-bridge and anchor moieties are proposed. It is found that the calculated photophysical and photovoltaic parameters of the dye with an unfused π-bridge reproduce very well the available experimental data, ensuring the rationality of present theoretical methods and models. The designed dye featuring a rigidly fused π-bridge shows a remarkable decrease of power conversion efficiency (PCE), with respect to the dye with an unfused π-spacer (6.36% vs. 11.18%), which implies that simply cyclizing separate aromatic heterocyclic groups is unfavorable for improving the photoelectrical performance. However, the PCE can be increased to 14.20% when an electron-rich thiophene group is introduced between the fused π-bridge and the anchor units. More importantly, replacing the thiophene moiety with an electron-deficient benzothiadiazole group can further improve the photovoltaic performance, and consequently a PCE as high as 23.81% is achieved, which is a significant theoretical improvement for triphenylamine-based organic dyes. To sum up, this study theoretically elucidates the effect of unfused and fused π-bridges as well as the introduction of electron-rich/deficient groups in organic dyes on their photophysical and photoelectric properties and provides a useful guidance for further designing highly-efficient dye sensitizers by molecular engineering.

Theoretical study of triphenylamine-based organic dyes with mono-, di-, and tri-anchoring groups for dye-sensitized solar cells

Abstract In this report, DFT and TDDFT calculations used to explain the noticeable difference in the power conversion efficiencies (PCEs) of three reported triphenylamine-based organic dyes with mono-, di-, and tri-cyanoacrylic acid units as anchoring groups, and thiophene unit as π-bridges used as potential dyes in dye-sensitized solar cell applications. These dyes were showed the superiority of the multi-anchoring systems over the mono-anchoring system. The light was shed on the effect of various functionals, e.g., B3LYP, CAM-B3LYP, ωB 97 XD , M06, PBE0 and BHandHLYP on absorption spectra. The CAM-B3LYP functional was depicted the best among numerous adopted functionals which reproduced the experimental absorption data. The geometries, electronic structures and UV-Vis absorption spectra of the isolated dyes and their adsorbed dye@TiO2 complexes were calculated. The associated physical parameters that control the effectiveness of the cell, such as the light harvesting efficiency ( L H E ), hole and electron reorganization energies ( λ h o l e , and λ e l e ), and the driving force for electron injection ( Δ G i n j ) and dye regeneration ( Δ G r e g ) were computed and analyzed in order to determine their contributions to the open-circuit photovoltage ( V o c ) or the short-circuit photocurrent density ( J s c ), and correlate the structure with property. Also, charge transfer descriptors, such as the charge transfer distance ( D C T ), the amount of charge transferred ( q C T ), and dipole moment ( μ C T ) were investigated. The results reveal that the increasing of anchoring groups made significant influence on the PCE of the cells. The effect of mono-, di-, and tri-substituted anchored cyanoacrylic acid group(s) on the triphenylamine-TiO2 complexes has been explored on the frontier molecular orbitals, binding energies and absorption wavelengths. Among all the studied derivatives, electron injection energy barrier for TPA3T3A has been found the smallest which revealed that electron injection in this compound might be superior to other counterparts. Low-lying lowest unoccupied molecular orbital of TPA3T3A would favor it to stabilize thermodynamically. Moreover, TPA3T3A@TiO2 cluster was observed the most stable adsorption complex especially anchoring of TPA3T3A sensitizer at TiO2 cluster from B-side. Comprehensible intramolecular charge transfer has been observed from occupied to unoccupied molecular orbitals. The red shift in the absorption spectra has been perceived by increasing the number of anchoring groups, i.e., cyanoacrylic acid. The type-II band gap alignment between triphenylamine-based organic sensitizers and TiO2 is revealing that photovoltaic efficiency in these studied derivatives would be enhanced. Our quantum chemical analysis explored that these TPA derivatives especially tri-anchored dye would be proficient to be used in photovoltaic devices which would enhance the solar cell efficiency.

Triphenylamine-based organic sensitizers with π-spacer structural engineering for dye-sensitized solar cells: Synthesis, theoretical calculations, molecular spectroscopy and structure-property-performance relationships

The heteroaromatic units play a key role in the D-π-A organic sensitizers for dye-sensitized solar cells (DSSCs), not only as π-spacers for tuning the optical properties and energy levels, but also as π-bridges for efficient electron transfer from donor to acceptor unit. The enhancement of π-spacer donating and accepting abilities is one of essential requirements to improve the molar absorption coefficient and red-shift the absorption band in the D-π-A organic sensitizer, which is beneficial to light harvesting ability. In this study, the effect of π-spacers in the triphenylamine (TPA)-based sensitizers, such as phenyl, pyridine, thiophene, thienothiophene and benzothiadiazole-thiophene, on photovoltaic performances was systematically investigated. Time-dependent density functional theoretical calculations were also performed to unravel the nature of the absorption induced electronic excitations. Lippert-Mataga analysis shows the characteristics of intramolecular charge transfer on the change of dipole moment to provide a further understanding of charge separation in the organic sensitizer. As the electron richness of π-spacer is increased, the absorption band and photocurrent density are enhanced in the order of the π-spacer capability. The DSSC based on TPA-based sensitizer containing thienothiophene exhibits the better power conversion efficiency of 7.40%, Jsc of 14.38 mA cm−2, Voc of 694 mV and fill factor of 0.74. These established structure-property-performance relationships give the deep insight on the impact of various π-spacers in organic sensitizers on device performances and can give molecular design criteria on D-π-A structured organic sensitizers for high efficiency DSSCs.

Second order hyperpolarizability of triphenylamine based organic sensitizers: a first principle theoretical study

Designed metal-free dyes have been investigated by Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) to evaluate the ground state and excited state geometries of triphenylamine-based organic sensitizers.

Effect of donor-donor-π-acceptor architecture of triphenylamine-based organic sensitizers over TiO2 photocatalysts for visible-light-driven hydrogen production

In this paper, we report a series of triphenylamine (TPA) donor based dyes in sensitized hydrogen production over TiO2–Pt catalysts under visible light. Among the selected dyes, 4-(diphenylamino) phenylcyanoacrylic acid (DN-F01), 5-[4-(diphenylamino)phenyl]thiophene-2-cyanoacrylic acid (DN-F02), 3-(5-(4-(diphenylamino)styryl) thiophen-2-yl)-2-cyanoacrylic acid (DN-F03), (E)-3-(5-(4-(bis(2′,4′-dibutoxy-[1,1′-biphenyl]-4-yl)amino)phenyl) thiophen-2-yl)-2-cyanoacrylic acid (DN-F04) and 3-(6-(4-(bis(2′,4′-dibutoxy-[1,1′-biphenyl]-4-yl)amino)phenyl)-4,4-dihexyl-4H-cyclopenta[1,2-b:5,4-b′] dithiophen-2-yl)-2-cyanoacrylic acid (DN-F05) were adsorbed on Pt–TiO2 and tested for photocatalytic hydrogen production under visible-light in presence of triethanolamine (TEOA) as sacrificial electron donor (SED). The enhanced light absorption and effective interfacial charge transfer from excited dye to TiO2 is significant in DN-F05-TiO2. The DN-F05 sensitized Pt–TiO2 (DN5@T) photocatalyst exhibited higher turnover number (TON 1864) and apparent quantum efficiency (AQE ∼44%) when compared to their corresponding simple architecture dye molecules. Moreover, the additional donating group di-butoxyphenyl and dihexyl-cyclopentyl π-conjugated bridge units provide an excellent surface protection through steric hindrance leading to a good photocatalytic performance. Electrochemical and computational studies suggest the better photocatalytic activity of DN5@T. The photocatalyst shows long term stability and reproducibility at neutral pH.

TDDFT studies of electronic spectra and excited states of the triphenylamine-based organic sensitizers and organic sensitizer–titanium dioxide cluster complexes

The absorption spectra and excited states of four triphenylamine-based organic dye molecules (TC1, TC2, TC3 and TC4) for dye sensitized solar cells (DSSC) have been theoretically investigated with density function theory (DFT) and time-dependent density function theory (TDDFT). In order to simulate the optical properties of the real DSSCs, sensitizer/titanium dioxide cluster interface systems TC4-(TiO2)n (n = 1–8) are also modeled. It is found that different adsorption sites and sizes of the clusters will exert critical effects on the molecular orbital energy levels of the complexes, and ultimately on their light-harvesting capabilities. In all the tested sensitizer/titanium dioxide cluster interface systems, the binding of the organic dye molecule to the 3-fold-coordinated titanium of the cluster (TiO2)6 could most benefit the widening of the spectrum response region and the interactions between the sensitizer and the cluster. From the intuitive physical pictures given by the three-dimensional (3D) real-space analysis methods of transition and charge difference densities, one reveals the photo-induced charge transfer microcosmic process of the sampled organic sensitizer/titanium dioxide cluster system TC4-(TiO2)6.

Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) with cobalt-based mediators with efficiencies surpassing the record for DSCs with iodide-free electrolytes were developed by selecting a suitable combination of a cobalt polypyridine complex and an organic sensitizer. The effect of the steric properties of two triphenylamine-based organic sensitizers and a series of cobalt polypyridine redox mediators on the overall device performance in DSCs as well as on transport and recombination processes in these devices was compared. The recombination and mass-transport limitations that, previously, have been found to limit the performance of these mediators were avoided by matching the properties of the dye and the cobalt redox mediator. Organic dyes with higher extinction coefficients than the standard ruthenium sensitizers were employed in DSCs in combination with outer-sphere redox mediators, enabling thinner TiO(2) films to be used. Recombination was reduced further by introducing insulating butoxyl chains on the dye rather than on the cobalt redox mediator, enabling redox couples with higher diffusion coefficients and more suitable redox potential to be used, simultaneously improving the photocurrent and photovoltage of the device. Optimization of DSCs sensitized with a triphenylamine-based organic dye in combination with tris(2,2'-bipyridyl)cobalt(II/III) yielded solar cells with overall conversion efficiencies of 6.7% and open-circuit potentials of more than 0.9 V under 1000 W m(-2) AM1.5 G illumination. Excellent performance was also found under low light intensity indoor conditions.

Triphenylamine-Based Ionic Dyes with Simple Structures: Broad Photoresponse and Limitations on Open-Circuit Voltage in Dye-Sensitized Solar Cells

Metal-free ionic dyes possess attractive near-infrared light-capturing abilities but show generally low open-circuit voltage (VOC) in dye-sensitized solar cells (DSCs). As part of our systematic development of triphenylamine-based organic sensitizers, two ionic dyes (TI dyes) featuring a 1,1-diphenylvinyl auxiliary electron donor and N-alkyl indolium carboxylic acid acceptors have been engineered. With a very simple π-conjugated bridge (a C═C double bond), one of them has achieved broad photoresponse ranging from 400 to 800 nm. Characteristic of ionic dyes, TI dyes show much lower VOC values than those of nonionic triphenylamine dyes, suffering from serious interfacial charge recombination and downward shift of the TiO2 conduction band. Through photovoltaic measurements, electrochemical impedance investigation, recombination reaction monitoring, electrostatic potential analysis, and dipole moment calculation, we conclude that the electronic distribution over ionic dye molecules is intrinsically not ideal ...

Correlating Dye Adsorption Behavior with the Open-Circuit Voltage of Triphenylamine-Based Dye-Sensitized Solar Cells

To study the relationship between dye adsorption behavior and open-circuit voltage (VOC) of dye-sensitized solar cells (DSCs), four triphenylamine-based organic sensitizers with closely related molecular structures but with two different adsorption orientations were engineered and compared. The origin of VOC was investigated in terms of band-edge movement of the TiO2 conduction band (CB) and interfacial charge recombination, with the latter found to be the governing factor. The two dyes with cyanoacetic acid as an anchoring group (TC dyes) adopt a standing adsorption mode and exert a larger surface dipole potential on TiO2 than their counterparts bearing rhodanine-3-acetic acid (TR dyes), which lie along the surface. TR dyes exhibit a greater extent of charge recombination than TC dyes because of the low surface-blocking efficiency of the dye layer and the intimacy between the I3−-bound dyes and the TiO2. The differences in both CB movement and charge recombination between TR and TC dyes amplify with the ...

Structural Modification of Organic Dyes for Efficient Coadsorbent-Free Dye-Sensitized Solar Cells

Three triphenylamine-based organic sensitizers with different electron-donating substituents (butoxyl chains or dimethylamine groups) were examined to investigate the effect of bulky alkoxy donor s ...