5-Phenyl-iminostilbene based organic dyes for efficient dye-sensitized solar cells

Four new 5-phenyl-iminostilbene dyes (ISB-3–6) containing electron-withdrawing benzo-[c][1,2,5]thiadiazole have been designed and synthesized for use as DSSCs. Their absorption properties and electrochemical and photovoltaic performances have been investigated systematically. Among these dyes, DSSCs based on a dye containing benzo-[c][1,2,5]thiadiazole and benzene moieties (ISB-4) showed the best performance: a short-circuit photocurrent density (J_sc) of 13.69 mA cm⁻², an open-circuit photovoltage (V_oc) of 722 mV, and a fill factor (FF) of 0.71, which corresponds to a power conversion efficiency (PCE) of 6.71%, under optimized conditions. Additionally, long-term stability of the ISB-4 based DSSCs with ionic-liquid electrolytes was demonstrated under 1000 h of light soaking, the photovoltaic performance is up to 5.75%. The results suggest that 5-phenyl-iminostilbene containing dyes are promising candidates for application in DSSCs.     

Improvement of dye-sensitized solar cells' performance via co-sensitization of new azo thiazole organic dyes with ruthenium (II) based N-719 dye

Three novel azo thiazole organic dyes, NA-1–3, have been synthesized and utilized as co-sensitizers in dye-sensitized solar cells (DSSCs). These co-sensitizers were designed with a thiazole ring π-bridge that mediates between the diazo (–N = N–) functional group and carboxylic acid anchoring unit. They possess a rod-like molecular structure and exhibit strong UV–vis absorption near 600 nm. Co-sensitization studies were also conducted with the ruthenium complex N719. The co-sensitized DSSCs showed enhanced short-circuit and open-circuit photocurrents (Jsc) and voltages (Voc), resulting in more efficient photovoltaic performance compared to N719 (PCE 7.25%). Electrochemical impedance spectroscopy (EIS) and incident photon to current efficiency (IPCE) were employed to investigate the underlying reasons for these improvements. It was found that co-sensitization effectively reduced electron recombination, resulting in a higher Voc without compromising photocurrent loss.     

Synthesis and applications of novel acceptor-donor-acceptor organic dyes with dithienopyrrole- and fluorene-cores for dye-sensitized solar cells

Four novel symmetrical organic dyes (S1-S4) configured with acceptor-donor-acceptor (A-D-A) structures containing electron donating fluorene (S1 and S2) and N-alkyl dithieno[3,2-b:2′,3′-d]pyrrole (DTP) (S3 and S4) cores terminated with two anchoring cyanoacrylic acids (as electron acceptors) were synthesized and applied to dye-sensitized solar cells (DSSCs). The DSSC device based on S2 dye showed the best photovoltaic performance among S1-S4 dyes: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 76%, a short circuit current (J_SC) of 12.27 mA/cm², an open circuit voltage (V_OC) of 0.61 V, a fill factor (FF) of 0.63, and an overall power conversion efficiency (η) of 4.73%. Besides, the utilization of chenodoxycholic acid (CDCA) as a co-adsorbent in the DSSC device based on S3 dye showed a significant improvement in its η value (from 3.70% to 4.31%), which is attributed to the suppression of dye aggregation on TiO₂ surface and thus to increase the J_SC value eventually.     

New thieno[3,2-b]indole conjugates with 5-(methylene)rhodanine-3-acetic acid in dye-sensitized solar cells

Five new dyes with D–π–A structure bearing 5-(methylene)-rhodanine-3-acetic acid as an acceptor-anchoring part and thieno[3,2-b]indole or benzo[g]thieno[3,2-b]indole as an electron-donating part were synthesized and applied as photosensitizers for dye-sensitized solar cells (DSSCs). In addition, thermal stability, optical and electrochemical properties of these dyes were investigated. The highest PCE value of 1.09% (J_sc = 3.01 mA cm^–2, V_oc = 0.53 V, FF = 0.69) was achieved for DSSC based on benzo[g]thieno[3,2-b]indole dye under AM 1.5G irradiation.     

Raman spectroscopic study of dye adsorption on TiO2 electrodes of dye-sensitized solar cells

The state of dye adsorption on TiO₂ electrodes in dye-sensitized solar-cell (DSSC) systems is important for its power-conversion efficiency (PCE). We propose a non-destructive and quantitative method to evaluate the amount of adsorbed dye on TiO₂ electrodes by using micro-Raman spectroscopy. The Raman peak intensity ratio of adsorbed dye to TiO₂, I_d/I_t, is defined as a dye adsorption parameter. Based on a comparison between I_d/I_t and the amount of dye evaluated from UV–vis absorption, the quantitativity and reproducibility of our method are verified.We investigated the change of I_d/I_t spatial distribution of TiO₂ electrodes immersed in a dye solution for different time scales. The statistical analysis of I_d/I_t distribution suggests that dyes adsorbed on TiO₂ electrodes with chemical coordination increase at first, and after their saturation, dye aggregations are formed over the chemisorption layer. We also describe the effect of the I_d/I_t distribution on PCE. From a comparison of PCE and I_d/I_t distribution obtained from various immersion processes, it was considered that the PCE of DSSCs can be optimized by minimizing the I_d/I_t dispersion.     

Synthesis and photovoltaic properties of new [1,2,5]thiadiazolo[3,4-c]pyridine-based organic Broadly absorbing sensitizers for dye-sensitized solar cells

In this paper, by introducing [1,2,5]thiadiazolo[3,4-c]pyridine (PT) as an auxiliary acceptor into the molecular design of organic sensitizers, we have synthesized four new dyes (PT1–PT4) for dye-sensitized solar cells (DSSCs) with triphenylamine or N,N-diphenylthiophen-2-amine as the donor units and thiophene or benzene as the π-bridges, respectively. All the structures, optical and electrochemical properties were fully characterized. Nanocrystalline TiO₂ dye-sensitized solar cells were also fabricated using these dyes. Among them, PT2-based DSSCs showed the highest overall conversion efficiency of 6.11% with V_oc=668 mV, J_sc=12.61 mA cm⁻² and a fill factor (FF)=0.74 after a chenodeoxycholic acid (CDCA) treatment under standard illumination condition (100 mW cm⁻² simulated AM 1.5 solar light).     

Performance of dye-sensitized solar cells based on various sensitizers applied on TiO2-Nb2O5 core/shell photoanode structure

The present investigation described the performance of dye-sensitized solar cells (DSSCs) based on various sensitizers applied on TiO₂-Nb₂O₅ core/shell photoanode film. The novel photoanodes were prepared using composite of TiO₂ nanoparticles (TNPs) and TiO₂ nanorods (TNRs) as core (TNPRs) layer with Nb₂O₅ shell coating. As well, tantalum pentoxide (Ta₂O₅), a blocking layer applied over the core/shell film. The DSSCs were fabricated based on various sensitizers namely zinc phthalocyanine, indoline, indigo carmine, zinc porphyrin, N719, coumarin NKX-2700, polymer dye, quantum dots (QDs), perylene and squaraine. The I–V characteristics of the DSSCs, photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF), and photoconversion efficiency (PCE) were determined under illumination of AM 1.5 G. Electrochemical impedance spectroscopy (EIS) analysis is carried out to study the charge transport and life-time of charge carriers at photoanode/dye/electrolyte interface of the DSSCs. The I–V and EIS results explicated that the core/shell with blocking layers were able to alleviate the electron transport and suppressed charge recombination at photoanode/dye/electrolyte interface of the DSSCs. Concerning the sensitizers, PCE of the DSSCs exemplify the order N719 > zinc porphyrin > coumarin NKX-2700 > indoline > squaraine > QDs > zinc phthalocyanine > perylene > polymer dye > indigo carmine dye. The results of the present work demonstrated that among the sensitizers studied, N719 showed the highest PCE and fill factor. Besides, the metal-free organic sensitizers (coumarin NKX-2700 and indoline) exhibited comparable PCE as compared to N719.     

应用于染料敏化太阳能电池的基于染料R6的含有不同吸电子基团的有机染料的理论研究

Dye-sensitized solar cells (DSSCs) are the most promising alternatives to traditional fossil energy because of their advantages of low production cost, facile structure, relatively low environmental impact, relatively high photoelectronic absorption efficiency, and overall high efficiency. In addition, several studies on sensitizers as vital components have been conducted over the last three decades. Compared to metal dyes, metal-free organic dyes have been considered as promising candidates because of their simple fabrication, multiple structures, high molar absorption coefficients, easily tunable properties, and environmental friendliness. In this study, we systematically investigated the optoelectronic properties of six metal-free organic donor-acceptor dyes (RD1–6) derived from the known dye R6 by using the density functional theory (DFT) and time-dependent DFT methods. Cell performance parameters were discussed, including the geometrical and electronic structures, absorption spectrum, adsorption energy, light harvesting efficiency (LHE) curve, predictive short circuit current density (J_sc^Pred.), predictive open circuit voltage (V_oc^Pred.), and theoretical power conversion efficiency (PCE). Results revealed that all the designed dyes exhibited high theoretical PCE. In particular, dyes RD1, 2, and 4–6 showed greater conjugations, and dyes RD1–3 had smaller energy gaps than those of the reference dye. In addition, dyes RD1–3, 5, and 6 exhibited better light harvesting capacities that covered the entire visible region and extended to the near-infrared region with obviously red-shift maximum absorption wavelengths (λ_max), wider LHE curves, and higher J_sc^Pred. as compared to the reference dye. It was critical that dyes RD1 and 2 not only have greater conjugations and narrow band gaps but also good light harvesting capacities with more than 56-nm red-shift maximum absorption wavelengths and broadened LHE curves than those of the reference dye. Notably, mainly because of an average increment of 12.0% of J_sc^Pred., a remarkable increment of the theoretical power conversion efficiency was observed from 12.6% for dye R6 to 14.1% for dyes RD1 and 2. Thus, dyes RD1 and 2 exhibited superior cell performances and could be promising sensitizer candidates for highly efficient DSSCs. These results could be used to guide effective synthetic efforts in the discovery of efficient metal-free organic dye sensitizers in DSSCs.     

Thieno[2,3-a]carbazole-based donor–π–acceptor organic dyes for efficient dye-sensitized solar cells

New donor–π–acceptor organic dyes K-1 and K-2 containing thieno[2,3-a]carbazole as an electron donor were designed and synthesized for dye-sensitized solar cells (DSCs). Photophysical and electrochemical properties of K-dyes were investigated. DSCs based on K-dyes showed a high conversion efficiency of 6.6–6.7% with a J_sc of 12.40–12.49 mA cm⁻² and a V_oc of 0.70–0.71 V. The molecular geometry calculation indicated that the existence of thienocarbazole donor in K-dyes enhanced the molecular planarity compared to the carbazole analogue dye MK-3. As a result, DSCs based on K-dyes showed high IPCEs, perhaps due to efficient intramolecular charge transfer and electron injection from excited dye to TiO₂ conduction band.     

Molecular design of novel indacenodithiophene-based organic dyes for efficient dye-sensitized solar cells applications

Indacenodithiophene (IDT)-based high-efficiency photovoltaics have received increasing attention recently. This paper reports a density functional theory investigation of the electronic and optical properties of three IDT-based organic dyes together with the dye/(TiO₂)₄₆ interface. In order to enhance the photoelectric properties of IDT dyes, this paper considers two methods for the structure modification of the experimentally reported dye DPInDT (J. Org. Chem. 2011, 76, 8977): the extension of the conjugation length by dithienothiophene as well as the heteroatom substitution of the bridging atoms by electron-rich nitrogen atoms. Our calculations show that both methods obviously affect the distributions of the molecular orbitals and notably red shift the absorption peaks of around 20 nm, with the former method demonstrating enhanced light harvesting efficiency. The structure modifications proposed also enhance the emission spectrum properties for IDT-based organic dyes. The calculated ultrafast injection time of electrons from the excited state of IDT dyes to the (TiO₂)₄₆ belongs to the femtosecond order of magnitude, and is ideal for efficient photoelectric conversion process in dye-sensitized solar cells (DSSCs) applications. The IDT dyes designed in this paper have good electronic and spectroscopic properties. This study is expected to provide useful guidance for the development of novel IDT dyes for applications in DSSCs.     

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 of double D–A branched organic dyes employing indole and phenoxazine as donors for efficient DSSCs

Four novel metal-free organic sensitizers bearing double donor–acceptor (D–A) branches with indole and phenoxazine units as donors (SDD1–4) and cyanoacrylic acid as electron acceptor were synthesized and characterized for dye-sensitized solar cells (DSSCs). Dyes SDD1–3 were designed with indole as a donor and 1,4-phenylenebis(methylene), 1,4-butylene and 1,6-hexylene as a linker, respectively, while the dye SDD4 was designed with phenoxazine as a core donor. Their photophysical, electrochemical, and DSSCs characteristics were investigated. The results show that the architecture structure of the linkage affects the performance of the cells slightly. The DSSCs based on SDD4 shows much higher η than the DSSCs based on SDD1–3, which indicated that phenoxazine is a better donor than indole. Under standard global AM 1.5 solar condition (100 mW cm⁻²), the SDD4 dye-sensitized cell gave the highest η of 4.33% with chenodeoxycholic acid as a coadsorbent, reaching 82% of N719-based DSSCs.     

Molecular design of organic dyes with diketopyrrolopyrrole for dye‐sensitized solar cell: A theoretical approach

Three designed metal‐free dyes based on 3‐(10‐butyl‐8‐(methylthio)‐10H‐phenothiazin‐3‐yl)‐2‐cyanoacrylic acid (V5) are investigated by density functional theory (DFT) and time‐dependent DFT to improve the efficiency of V5‐based solar cell devices. We have studied the geometrical structures, excitations, electronic structures, and conduction band shift caused by dye adsorption. The results indicate that the designed dyes have several merits compared with V5 including: (i) smaller energy band gaps and the LUMO closer to conduction band of TiO₂; (ii) wider absorption spectra and higher oscillator strength; (iii) larger dipole moment that lead to higher V_oc value. Our work suggests that the modification of π‐bridge with diketopyrrolopyrrole unit is very effective for designing novel metal‐free dyes with improved performance for dye‐sensitized solar cells (DSSCs). These findings are expected to provide a bright way to design new efficient metal‐free organic DSSCs. © 2014 Wiley Periodicals, Inc.     

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.     

Metal-free organic dyes with benzothiadiazole as an internal acceptor for dye-sensitized solar cells

We synthesized three metal-free organic dyes (H11–H13) consisting of a 3,6-disubstituted carbazole, benzothiadiazole, and cyanoacrylic acid. All the dyes exhibited high molar extinction coefficients and suitable energy levels for electron transfer from the electrolyte to the TiO₂ nanoparticles. Under standard AM 1.5G solar irradiation, the device using dye H13 with co-adsorbed chenodeoxycholic acid (CDCA) displayed the best performance: an open-circuit voltage (V_oc) of 0.71 V, a short-circuit current density (J_sc) of 12.69 mA cm⁻², a fill factor (FF) of 0.71, and a power conversion efficiency (PCE) of 6.32%. The PCE was ∼79% of that for commercially available N719 cells (8.02%) under the same conditions.     

Second-generation artificial hydrogenases based on the biotin–avidin technology: Improving selectivity and organic solvent tolerance by introduction of an (R)-proline spacer

We report on our efforts to create efficient artificial metalloenzymes for the enantioselective hydrogenation of N-protected dehydroamino acids using streptavidin as host protein. Introduction of an (R)-proline spacer between the biotin anchor and the diphosphine moiety affords a versatile ligand Biot-(R)-Pro-1 which displays good (S)-selectivities in the presence of streptavidin (91% ee). The resulting artificial metalloenzyme [Rh(Biot-(R)-Pro-1)(COD)]⁺ ⊂ WT-Sav displays increased stability against organic solvents.     

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.     

An efficient stereocontrolled synthesis of (−)-10-epi-5β,11-dihydroxyeudesmane and (−)-4,10-epi-5β,11-dihydroxyeudesmane

A concise and efficient synthesis of (−)-10-epi-5β,11-dihydroxyeudesmane 1 and (−)-4,10-epi-5β,11-dihydroxyeudesmane 2, via (−)-10-epi-γ-eudesmol 5, was accomplished starting from (+)-dihydrocarvone. The salient feature of our synthesis is the utilization of substrate-directable epoxidation and homogeneous hydrogenation to control the stereochemistry at the C-4 and C-5 positions of the title compounds.     

Efficient organic sensitizers containing benzo[cd]indole: Effect of molecular isomerization for photovoltaic properties

Two isomeric organic dyes, JK-51 and JK-52 containing benzo[cd]indole were synthesized. Under standard global AM 1.5 solar condition, the JK-51 sensitized cell gave a short circuit photocurrent density of 17.43 mA/cm², open circuit voltage of 0.680 V, and a fill factor of 0.71, corresponding to an overall conversion efficiency η of 8.42%. On the other hand, a solar-to-electric conversion efficiency of 6.88% was achieved with the isomeric dye JK-52. We found that the power conversion efficiency was shown to be quite sensitive to the isomeric configurations.     

Carbenoid induced irreversible ring opening of naphthopyrans

Contrary to expectation rhodium carbenoids do not undergo cycloaddition to the 2H-pyran unit of the isomeric naphthopyrans 3 and 5. With 3, a naphtho[2,1-b]pyran-8-ylacetate, 4 is formed and a novel merocyanine dye 6 results from a cycloaddition across the C-5-C-6 double bond of the naphtho[1,2-b]pyran 5. Tethering the carbenoid to the naphtho[1,2-b]pyran system 5, as in 10, results in a similar mode of addition and affords the intensely coloured tetracycle 11.