High‐Performance Dipolar Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety in the π‐Conjugation Framework for Dye‐Sensitized Solar Cells

We report here the synthesis and electrochemical and photophysical properties of a series of easily prepared dipolar organic dyes and their application in dye‐sensitized solar cells (DSSCs). For the six organic dyes, the molecular structures comprised a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron‐deficient diphenylquinoxaline moiety integrated in the π‐conjugated spacer between the electron donor and acceptor moieties. The incorporation of the electron‐deficient diphenylquinoxaline moiety effectively reduces the energy gap of the dyes and broadly extends the spectral coverage. DSSCs based on dye 6 produced the best overall cell performance of 7.35 %, which translates to approximately 79 % of the intrinsic efficiency of the DSSCs based on the standard N719 dye under identical experimental conditions. The high performance of DSSCs based on dye 6 among the six dyes explored is attributed to the combined effects of high dye loading on a TiO₂ surface, rapid dye regeneration, and effective retardation of charge recombination.     

Structure–Performance Correlations of Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety for Dye‐Sensitized Solar Cells

The high performances of dye‐sensitized solar cells (DSSCs) based on seven new dyes are disclosed. Herein, the synthesis and electrochemical and photophysical properties of a series of intentionally designed dipolar organic dyes and their application in DSSCs are reported. The molecular structures of the seven organic dyes are composed of a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron‐deficient diphenylquinoxaline moiety integrated in the π‐conjugated spacer between the electron donor and acceptor moieties. The DSSCs based on the dye DJ104 gave the best overall cell performance of 8.06 %; the efficiency of the DSSC based on the standard N719 dye under the same experimental conditions was 8.82 %. The spectral coverage of incident photon‐to‐electron conversion efficiencies extends to the onset at the near‐infrared region due to strong internal charge‐transfer transition as well as the effect of electron‐deficient diphenylquinoxaline to lower the energy gap in these organic dyes. A combined tetraphenyl segment as a hydrophobic barrier in these organic dyes effectively slows down the charge recombination from TiO₂ to the electrolyte and boosts the photovoltage, comparable to their Ru^II counterparts. Detailed spectroscopic studies have revealed the dye structure–cell performance correlations, to allow future design of efficient light‐harvesting organic dyes.     

Synthesis and Characterization of (2E,2 E)‐3,3‐((9,9‐diocyl‐9H‐flourene‐2,7‐diyl)bis(4,1‐phenylene))bis(2‐cyanoacylic acid) as a Symmetrical Metal‐free Organic Dye

In this study a novel symmetrical metal‐free organic dye for applications in dye‐sensitized solar cells (DSSCs) was synthesized. This dye ( D ) was designed with A–π–D–π–A framework and synthesized with 9,9‐dioctylfluorene as electron donor, phenylene as π‐spacer and cyanoacetic acid as electron acceptor. The chemical structure of product was determined using UV‐Vis, FT‐IR, CNMR, HNMR spectroscopy techniques. The presence of a phenylene π‐bridge between the donor and the acceptor units and the di‐anchoring moieties in this structure led to enhancement of conjugation lengths and molar extinction coefficient (ε) that is promising for further improvement of the conversion efficiency of DSSCs.     

Bi‐anchoring Organic Dyes that Contain Benzimidazole Branches for Dye‐Sensitized Solar Cells: Effects of π Spacer and Peripheral Donor Groups

Benzimidazole‐branched bi‐anchoring organic dyes that contained triphenylamine/phenothiazine donors, 2‐cyanoacrylic acid acceptors, and various π linkers were synthesized and examined as sensitizers for dye‐sensitized solar cells. The structure–activity relationships in these dyes were systematically investigated by using absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The wavelength of the absorption peak was more‐heavily influenced by the nature of the π linker than by the nature of the donor. For a given donor, the absorption maximum (λ_max) was red‐shifted on changing the π linker from phenyl to 2,2′‐bithiophene, whilst the dyes that contained triphenylamine units displayed higher molar extinction coefficients (?) than their analogous phenothiazine‐based triphenylamine dyes, which led to good light‐harvesting properties in the triphenylamine‐based dyes. Electrochemical data for the dyes indicated that the triphenylamine‐based dyes possessed relatively low‐lying HOMOs, which could be beneficial for suppressing back electron transfer from the conduction band of TiO₂ to the oxidized dyes, owing to facile regeneration of the oxidized dye by the electrolyte. The best performance in the DSSCs was observed for a dye that possessed a triphenylamine donor and 2,2′‐bithiophene π linkers. Electron impedance spectroscopy (EIS) studies revealed that the use of triphenylamine as the donor and phenyl or 2,2′‐bithiophene as the π linkers was beneficial for disrupting the dark current and charge‐recombination kinetics, which led to a long electron lifetime of the injected electrons in the conduction band of TiO₂.     

High-Performance Dipolar Organic Dyes with an Electron-Deficient Diphenylquinoxaline Moiety in the π-Conjugation Framework for Dye-Sensitized Solar Cells

We report here the synthesis and electrochemical and photophysical properties of a series of easily prepared dipolar organic dyes and their application in dye-sensitized solar cells (DSSCs). For the six organic dyes, the molecular structures comprised a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron-deficient diphenylquinoxaline moiety integrated in the π-conjugated spacer between the electron donor and acceptor moieties. The incorporation of the electron-deficient diphenylquinoxaline moiety effectively reduces the energy gap of the dyes and broadly extends the spectral coverage. DSSCs based on dye 6 produced the best overall cell performance of 7.35?%, which translates to approximately 79?% of the intrinsic efficiency of the DSSCs based on the standard N719 dye under identical experimental conditions. The high performance of DSSCs based on dye 6 among the six dyes explored is attributed to the combined effects of high dye loading on a TiO(2) surface, rapid dye regeneration, and effective retardation of charge recombination.     

Substituent Effect on the π Linkers in Triphenylamine Dyes for Sensitized Solar Cells: A DFT/TDDFT Study

A series of metal‐free organic donor–π bridge–acceptor dyes are studied computationally using density functional theory (DFT) and time‐dependent DFT (TDDFT) approaches to explore their potential performances in dye‐sensitized solar cells (DSSCs). Taking triphenylamine (TPA) and cyanoacrylic acid moieties as donor and acceptor units, respectively, the effects of different substituents of the π linkers in the TPA‐based dyes on the energy conversion efficiency of the DSSCs are theoretically evaluated through optimized geometries, charge distributions, electronic structures, simulated absorption spectra, and free energies of injection. The results show that the molecular orbital energy levels and electron‐injection driving forces of the TPA dyes can be tuned by the introduction of substituents with different electron‐withdrawing or ‐donating abilities. The electron‐withdrawing substituent always lowers the energies of both frontier orbitals, while the electron‐donating one heightens them simultaneously. The efficiency trend of these TPA derivatives as sensitizers in DSSCs is also predicted by analyzing the light‐harvesting efficiencies and the free energies of injection. The following substituents are shown to increase the efficiency of the dye: OMe, OEt, OHe, and OH.     

Dipolar Compounds Containing Fluorene and a Heteroaromatic Ring as the Conjugating Bridge for High‐Performance Dye‐Sensitized Solar Cells

A novel series of dipolar organic dyes containing diarylamine as the electron donor, 2‐cyanoacrylic acid as the electron acceptor, and fluorene and a heteroaromatic ring as the conjugating bridge have been developed and characterized. These metal‐free dyes exhibited very high molar extinction coefficients in the electronic absorption spectra and have been successfully fabricated as efficient nanocrystalline TiO₂ dye‐sensitized solar cells (DSSCs). The solar‐energy‐to‐electricity conversion efficiencies of DSSCs ranged from 4.92 to 6.88 %, which reached 68–96 % of a standard device of N719 fabricated and measured under the same conditions. With a TiO₂ film thickness of 6 μm, DSSCs based on these dyes had photocurrents surpassing that of the N719‐based device. DFT computation results on these dyes also provide detailed structural information in connection with their high cell performance.     

Molecular Engineering of Pyrido[3,4‐b]pyrazine‐Based Donor–Acceptor–π‐Acceptor Organic Sensitizers: Effect of Auxiliary Acceptor in Cobalt‐ and Iodine‐Based Electrolytes

Due to the ease of tuning its redox potential, the cobalt‐based redox couple has been extensively applied for highly efficient dye‐sensitized solar cells (DSSCs) with extraordinarily high photovoltages. However, a cobalt electrolyte needs particular structural changes in the organic dye components to obtain such high photovoltages. To achieve high device performance, specific requirements in the molecular tailoring of organic sensitizers still need to be met. Besides the need for large electron donors, studies of the auxiliary acceptor segment of donor–acceptor–π‐acceptor (D‐A‐π‐A) organic sensitizers are still rare in molecular optimization in the context of cobalt electrolytes. In this work, two novel organic D‐A‐π‐A‐type sensitizers ( IQ13 and IQ17 ) have been developed and exploited in cobalt‐ and iodine‐based redox electrolyte DSSCs, specifically to provide insight into the effect of π‐bridge modification in different electrolytes. The investigation has been focused on the additional electron‐withdrawing acceptor capability with grafted long alkoxy chains. Optoelectronic transient measurements have indicated that IQ17 containing a pyrido[3,4‐b]pyrazine moiety bearing long alkoxyphenyl chains is more suitable for application in cobalt‐based DSSCs.     

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.     

Dye-sensitized solar cells based on donor-π-acceptor fluorescent dyes with a pyridine ring as an electron-withdrawing-injecting anchoring group

A new‐type of donor–acceptor π‐conjugated (D‐π‐A) fluorescent dyes NI3 – NI8 with a pyridine ring as electron‐withdrawing‐injecting anchoring group have been developed and their photovoltaic performances in dye‐sensitized solar cells (DSSCs) are investigated. The short‐circuit photocurrent densities and solar energy‐to‐electricity conversion yields of DSSCs based on NI3 – NI8 are greater than those for the conventional D‐π‐A dye sensitizers NI1 and NI2 with a carboxyl group as the electron‐withdrawing anchoring group. The IR spectra of NI3 – NI8 adsorbed on TiO₂ indicate the formation of coordinate bonds between the pyridine ring of dyes NI3 – NI8 and the Lewis acid sites (exposed Tiⁿ⁺ cations) of the TiO₂ surface. This work demonstrates that the pyridine rings of D‐π‐A dye sensitizers that form a coordinate bond with the Lewis acid site of a TiO₂ surface are promising candidates as not only electron‐withdrawing anchoring group but also electron‐injecting group, rather than the carboxyl groups of the conventional D‐π‐A dye sensitizers that form an ester linkage with the Brønsted acid sites of the TiO₂ surface.     

Computational studies on optoelectronic and charge transfer properties of some perylene‐based donor‐π‐acceptor systems for dye sensitized solar cell applications

We report DFT studies on some perylene‐based dyes for their electron transfer properties in solar cell applications. The study involves modeling of different donor‐π‐acceptor type sensitizers, with perylene as the donor, furan/pyrrole/thiophene as the π‐bridge and cyanoacrylic group as the acceptor. The effect of different π‐bridges and various substituents on the perylene donor was evaluated in terms of opto‐electronic and photovoltaic parameters such as HOMO‐LUMO energy gap, λ_max, light harvesting efficiency(LHE), electron injection efficiency (Ø_inject), excited state dye potential (E^dye*), reorganization energy(λ), and free energy of dye regeneration (). The effect of various substituents on the dye–I₂ interaction and hence recombination process was also evaluated. We found that the furan‐based dimethylamine derivative exhibits a better balance of the various optical and photovoltaic properties. Finally, we evaluated the overall opto‐electronic and transport parameters of the TiO₂‐dye assembly after anchoring the dyes on the model TiO₂ cluster assembly.     

New Bithiazole‐Based Sensitizers for Efficient and Stable Dye‐Sensitized Solar Cells

A series of new push–pull organic dyes ( BT‐I – VI ), incorporating electron‐withdrawing bithiazole with a thiophene, furan, benzene, or cyano moiety, as π spacer have been synthesized, characterized, and used as the sensitizers for dye‐sensitized solar cells (DSSCs). In comparison with the model compound T1 , these dyes containing a thiophene moiety between triphenylamine and bithiazole display enhanced spectral responses in the red portion of the solar spectrum. Electrochemical measurement data indicate that the HOMO and LUMO energy levels can be tuned by introducing different π spacers between the bithiazole moiety and cyanoacrylic acid acceptor. The incorporation of bithiazole substituted with two hexyl groups is highly beneficial to prevent close π–π aggregation, thus favorably suppressing charge recombination and intermolecular interaction. The overall conversion efficiencies of DSSCs based on bithiazole dyes are in the range of 3.58 to 7.51 %, in which BT‐I ‐based DSSCs showed the best photovoltaic performance: a maximum monochromatic incident photon‐to‐current conversion efficiency (IPCE) of 81.1 %, a short‐circuit photocurrent density (J_sc) of 15.69 mA cm^?2, an open‐circuit photovoltage (V_oc) of 778 mV, and a fill factor (ff) of 0.61, which correspond to an overall conversion efficiency of 7.51 % under standard global AM 1.5 solar light conditions. Most importantly, long‐term stability of the BT‐I – III ‐based DSSCs with ionic‐liquid electrolytes under 1000 h of light soaking was demonstrated and BT‐II with a furan moiety exhibited better photovoltaic performance of up to 5.75 % power conversion efficiency.     

Highly efficient organic indolocarbazole dye in different acceptor units for optoelectronic applications—a first principle study

A series of novel organic dyes (ICZA1, ICZA2, ICZA3, ICZA4) with D-π-A structural configuration incorporating indolo[3,2,1-jk]carbazole moiety as donor (D) unit, thiophene as π-linker and 2-cyanoacrylic acid as acceptor unit were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Indolo[3,2,1-jk]carbazole-based D-π-A dyes composed of different acceptor groups were designed. By modulating acceptor unit, the efficiency of D-π-A dye-based dye-sensitized solar cells (DSSCs) can be further improved. In the present work, four novel push-pull organic dyes only differing in electron acceptor, have been designed based on the experimental literature value of IC-2. In order to further improve the light harvesting capability of indolo[3,2,1-jk]carbazole dyes, the acceptor influence on the dye performance were examined. The NLO property of the designed dye molecules can be derived as polarizability and hyperpolarizability. The calculated value of ICZA2 dye is the best candidate for NLO properties. Furthermore, the designed organic dyes exhibit good photovoltaic performance of charge transfer characteristics, driving force of electron injection, dye regeneration, global reactivity, and light harvesting efficiency (LHE). From the calculated value of ICZA4 dye, it has been identified as a good candidate for DSSCs applications. Finally, it is concluded that the both ICZA2 and ICZA4 dyes theoretically agrees well with the experimental value of IC-2 dye. Hence, the dyes ICZA2 and ICZA4 can serve as an excellent electron withdrawing groups for NLO and DSSCs applications.     

High‐Performance Dye‐Sensitized Solar Cells Based on Phenothiazine Dyes Containing Double Anchors and Thiophene Spacers

A series of new push–pull phenothiazine‐based dyes ( HL1 , HL2 , HL3 , HL4 ) featuring various π spacers (thiophene, 3‐hexylthiophene, 4‐hexyl‐2,2′‐bithiophene) and double acceptors/anchors have been synthesized, characterized, and used as sensitizers for dye‐sensitized solar cells (DSSCs). Among them, the best conversion efficiency (7.31 %) reaches approximately 99 % of the N719‐based (7.38 %) DSSCs fabricated and measured under similar conditions. The dyes with two anchors have more efficient interfacial charge generation and transport compared with their congeners with only single anchor. Incorporation of hexyl chains into the π‐conjugated spacer of these double‐anchoring dyes can efficiently suppress dye aggregation and reduce charge recombination.     

1‐Alkyl‐1H‐imidazole‐Based Dipolar Organic Compounds for Dye‐Sensitized Solar Cells

A series of donor–π–acceptor‐type organic dyes based on 1‐alkyl‐1H‐imidazole spacers 1 , 2 , 3 , 4 , 5 have been developed and characterized. The two electron donors are at positions 4 and 5 of the imidazole, while the electron‐accepting cyanoacrylic acid is incorporated at position 2 by a spacer‐containing heteroaromatic rings, such as thiophene and thiazole. Detailed investigation on the relationship between the structure, spectral and electrochemical properties, and performance of DSSC is described here. Dye‐sensitized solar cells (DSSCs) using dyes as the sensitizers exhibit good efficiencies, ranging from 3.06 to 6.35 %, which reached 42–87 % with respect to that of N719‐based device (7.33 %) fabricated and measured under similar conditions. Time‐dependent density functional theory (TDDFT) calculations have been performed on the dyes, and the results show that both electron donors can contribute to electron injection upon photo‐excitation, either directly or indirectly by internal conversion to the lowest excited state.     

Organic Dyes Containing Pyrenylamine‐Based Cascade Donor Systems with Different Aromatic π Linkers for Dye‐Sensitized Solar Cells: Optical,Electrochemical, and Device Characteristics

New organic dyes containing pyrenylamine donors in a cascade arrangement and cyanoacrylic acid acceptors have been synthesized and characterized by optical, electrochemical, and theoretical studies. The dyes inherit a D ‐π¹‐D ‐π²‐A (D=donor, A=acceptor) molecular architecture where the π linkers π¹ are changed from phenyl to biphenyl and fluorene, whereas the π linker π² that connects the donor fragment with the acceptor is a phenyl unit. The conjugation pathway linking the two donor segments has been found to play a major role in the optical and electrochemical properties. Shorter π linkers such as phenyl groups facilitate the donor–acceptor interaction while the nonplanar biphenyl spacer decreases the electronic communication between the donors and enhances the oxidation propensity of the corresponding dye. All the dyes display an intense longer wavelength electronic transition,which is attributable to the amine‐to‐cyanoacrylic acid charge transfer. The extinction coefficient of this peak grows dramatically on increasing the conjugation pathway length between the two donor segments. The dyes were used as sensitizers in nanocrystalline TiO₂‐based dye‐sensitized solar cells (DSSCs) and the cascade donor system contributed to the enhancement in the device efficiency due to favorable absorption and redox properties.     

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.     

Dye‐Sensitized Solar Cells Based on (Donor‐π‐Acceptor)2 Dyes With Dithiafulvalene as the Donor

Dipolar metal‐free sensitizers (D‐π‐A; D=donor, π=conjugated bridge, A=acceptor) consisting of a dithiafulvalene (DTF) unit as the electron donor, a benzene, thiophene, or fluorene moiety as the conjugated spacer, and 2‐cyanoacrylic acid as the electron acceptor have been synthesized. Dimeric congeners of these dyes, (D‐π‐A)₂, were also synthesized through iodine‐induced dimerization of an appropriate DTF‐containing segment. Dye‐sensitized solar cells (DSSCs) with the new dyes as the sensitizers have cell efficiencies that range from 2.11 to 5.24 %. In addition to better light harvesting, more effective suppression of the dark current than the D‐π‐A dyes is possible with the (D‐π‐A)₂ dyes.     

Benzimidazole‐indole‐chalcone connected methacrylate‐based side chain D‐π‐A polymer and its application in organic photovoltaics

Herein, we report four metal‐free organic polymethacrylates (In‐In‐BzI)PMA , (Ac‐In‐BzI)PMA , (TPA‐In‐BzI)PMA , and (Py‐In‐BzI)PMA with pendant chromophores donor‐π‐conjugated‐acceptor (D‐π‐A) molecular framework as photosensitizers for dye‐sensitized solar cells (DSSCs). In which the donor‐acceptor units are attached by an indole‐chalcone extending side chain to inhibit back electron transfer and charge recombination; the π‐linker component contains varied chalcone‐based substituents to enhance the sunlight‐harvesting ability of the solar device. Photon‐current cells based on the DSSC format were fabricated using the polymers as sensitizers. The DSSC device assembled using (TPA‐In‐BzI)PMA exhibits a considerably better IPCE peak and J–V response, with an overall power conversion efficiency of 3.70% under the illumination of AM 1.5G (100 mW cm^–2). © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 997–1007     

Photophysical and Electrochemical Properties,and Molecular Structures of Organic Dyes for Dye‐Sensitized Solar Cells

Dye‐sensitized solar cells (DSSCs) based on organic dyes adsorbed on oxide semiconductor electrodes, such as TiO₂, ZnO, or NiO, which have emerged as a new generation of sustainable photovoltaic devices, have attracted much attention from chemists, physicists, and engineers because of enormous scientific interest in not only their construction and operational principles, but also in their high incident‐solar‐light‐to‐electricity conversion efficiency and low cost of production. To develop high‐performance DSSCs, it is important to create efficient organic dye sensitizers, which should be optimized for the photophysical and electrochemical properties of the dyes themselves, with molecular structures that provide good light‐harvesting features, good electron communication between the dye and semiconductor electrode and between the dye and electrolyte, and to control the molecular orientation and arrangement of the dyes on a semiconductor surface. The aim of this Review is not to make a list of a number of organic dye sensitizers developed so far, but to provide a new direction in the epoch‐making molecular design of organic dyes for high photovoltaic performance and long‐term stability of DSSCs, based on the accumulated knowledge of their photophysical and electrochemical properties, and molecular structures of the organic dye sensitizers developed so far.