Characteristics of triphenylamine-based dyes with multiple acceptors in application of dye-sensitized solar cells

We report the synthesis and photophysical/electrochemical properties of triphenylamine (TPA)-based multiple electron acceptor dyes (TPAR1, TPAR2, and TPAR3) as well as their applications in dye-sensitized solar cells (DSSCs). In these dyes, the TPA group and the rhodanine-3-acetic acid play the role of the basic electron donor unit and the electron acceptor, respectively. It was found that introduction of two rhodanine-3-acetic acid groups into the TPA unit (TPAR2) exhibited better photovoltaic performance due to the increase with a red shift and broadening of the absorption spectrum. The monolayer of these TPA-based dyes was adsorbed on the surface of nanocrystalline TiO₂ mesoporous electrode with the thickness of ∼6 μm, polyethylene oxide (PEO) used as the matrix of gel electrolyte, and 4-nm thick Pt used as a counter-electrode. Photovoltaic device can be realized in a single quasi-solid-state DSSC. TPAR2-based gel DSSC had an open circuit voltage and short circuit current density of about 541 and 10.7 mA cm⁻², respectively, at 1-sun.     

Synthesis and characterization of trivalent metal porphyrin with NCS ligand for application in dye-sensitized solar cells

Two novel trivalent metal porphyrin dyes, P_Mn-HT-SCN and P_Ga-HT-SCN, were designed, synthesized and firstly applied in dye-sensitized solar cells (DSSCs). These two dyes possess porphyrin donor modified with manganese (III) and gallium (III) as coordination metal and NCS as the second ligand, cyanoacrylic acid as electron-accepting moiety and 4-hexylthiophene as π-spacers. Each of the porphyrin showed different adsorption behavior and saturated coverage on the TiO₂ surface. Between the two dyes, the P_Mn-HT-SCN-based DSSCs afforded the best photovoltaic performance: a short-circuit photocurrent density (J_sc) of 4.32 mA/cm², an open-circuit photovoltage (V_oc) of 0.61 V and a fill factor (FF) of 0.58, corresponding to a solar-to-electricity conversion efficiency of 1.53% under 100 mW/cm² irradiation.     

Triphenylamine- and benzothiadiazole-based dyes with multiple acceptors for application in dye-sensitized solar cells

Two novel dyes TPAR3 and BTDR2 based on triphenylamine and benzothiadiazole, respectively, with multiple electron acceptors were synthesized and characterized by FT-IR, ¹H NMR, TGA and thermomechanical analysis (TMA). They carried terminal cyanoacrylic acid electron acceptors/anchoring moieties, which were connected with the central unit through a thiophene ring. The absorption bands of the dyes were extended up to ∼570 nm with long-wave absorption maximum at 425-455 nm and optical band gap of 2.10-2.17 eV. The dyes emitted yellow-orange light with photoluminescence maximum at 547-615 nm. We have investigated the photovoltaic properties of quasi solid state dye sensitized solar cells (DSSCs) based on these metal free organic dyes. It has been found that the power conversion efficiency of the DSSCs based on composite zinc titanium oxide (ZTO) nanocrystalline photoelectrode is higher than that for TiO₂ based DSSCs. This has been attributed to the longer electron lifetime and more negative conduction band edge of ZTO. The overall power conversion efficiency of the DSSCs based on TPAR3 and BTDR2 employing ZTO photoelectrode is 6.3% and 3.6%, respectively. These results indicate that both the acceptor moiety of metal free organic dyes and ZTO photoelectrode have an effect on the photovoltaic performance of DSSCs.     

Low band gap dyes based on 2-styryl-5-phenylazo-pyrrole: Synthesis and application for efficient dye-sensitized solar cells

A new series of low band gap dyes, C1, C2 and S, based on 2-styryl-5-phenylazo-pyrrole was synthesized. These dyes contain one carboxy, two carboxy and one sulfonic acid anchoring groups, respectively. They were soluble in common organic solvents, showed long-wavelength absorption maximum at ∼620 nm and optical band gap of 1.66-1.68 eV. The photophysical and electrochemical properties of these dyes were investigated and found to be suitable as photosensitizers for dye sensitized solar cells (DSSCs). The quasi solid state DSSCs with dye S showed a maximum monochromatic incident photon to current efficiency (IPCE) of 78% and an overall power conversion efficiency (PCE) of 4.17% under illumination intensity of 100 mW cm⁻² (1.5 AM), which is higher than the other dyes (3.26% for C2 and 2.59% for C1). Even though dye S contains one sulfonic acid anchoring group, the higher PCE for the DSSCs based on this dye has been attributed to the higher dye loading at the TiO₂ surface and enhanced electron lifetime in the device, as indicated by absorption spectra and electrochemical impedance spectra measurements. Finally, by increasing the molecular weight of poly(ethylene oxide) (PEO) in electrolyte, the PCE also increases up to 4.8% for the electrolyte with PEO molecular weight of 2.0 × 10⁶. This improvement has been attributed to the enhancement in iodide ions diffusion due to the increase in free volume of polymer gel electrolyte.     

Sensitizers containing donor cascade and rhodanine-3-acetic acid moieties for dye-sensitized solar cells

Three organic dyes with D-π-D-π-A structure based on triarylamine, dimethylarylamine, and rhodanine-3-acetic acid moieties are designed and synthesized. Incorporating thiophene moieties into the system affords sensitizers with high molar extinction coefficients. These dyes were applied into nanocrystalline TiO₂ dye-sensitized solar cells through standard operations. For a typical device the maximal monochromatic incident photon-to-current conversion efficiency (IPCE) can reach 73%, with a short-circuit photocurrent density (J_sc) of 7.3 mA/cm², an open-circuit voltage (V_oc) of 636 mV, and a fill factor (ff) of 0.61, corresponding to an overall conversion efficiency (η) of 2.86%.     

Algal buffer layers for enhancing the efficiency of anthocyanins extracted from rose petals for natural dye‐sensitized solar cell (DSSC)

Natural dye‐sensitized solar cells (DSSCs) are becoming promising candidates for replacing synthetic dyes. Anthocyanins, a flavonoid pigment which is responsible for the coloration in fruits and flowers, have shown productive results in employing them as natural dye for DSSC. But unfortunately, they exhibit low efficiency compared with synthetic dyes. Probing the reasons for the low efficiency of anthocyanin paves way for finding solution to increase the efficiency. This paper lists the important factors that are responsible for anthocyanin instability in DSSC. As a remedial measure, this paper introduces two buffer layer made of algal byproducts—sodium alginate and Spirulina. Rutile phase TiO₂ nanorods prepared by hydrothermal method were used as photoelectrode and are subsequently characterized by X ray diffraction, transmission electron microscopy, and optical studies. The use of sodium alginate above the photoelectrode has proved to improve the dye concentration in the film by introducing more hydroxyl groups on the surface of TiO₂. Anthocyanins extracted from rose petals using citric acid as solvent were used as dye for DSSC. Prior to the sensitization process with anthocyanin dye, the TiO₂ film (with sodium alginate) was sensitized with Spirulina. The chlorophylls, xanthophylls, phycocyanins, and amino acids present in Spirulina assist the anthocyanins to bond with TiO₂ efficiently. This helps in enhancing the efficiency of anthocyanins of rose dye from 0.99% to 1.47%.     

St. Lucie cherry,yellow jasmine,and madder berries as novel natural sensitizers for dye‐sensitized solar cells

Natural dyes extracted from fruits, vegetables, flowers, and leaves are considered as promising alternative sensitizers to replace synthetic dyes for dye‐sensitized solar cells (DSSCs). Generally, solar activity of natural dyes stem from anthocyanin pigment. Carbonyl, carboxyl, and hydroxyl groups present in the anthocyanin molecule improve the adsorption ability of dye on TiO₂ and therefore facilitate charge transfer. Here, for the first time, novel natural dyes extracted from St. Lucie cherry, yellow jasmine, and madder berries are reported to act as sensitizer in DSSCs. These novel natural dye extracts are prepared by dissolving related fruits in ethanol. The ingredient of the dyes is identified by FT‐IR spectroscopy. Accordingly, FT‐IR spectrum reveals that novel natural dye extracts exhibit all the characteristic peaks of anthocyanin pigment. Specifically, St. Lucie cherry consists of more distinct carbonyl group than other sources. Also, photoanodes composed of three TiO₂ layers are prepared by using a spin‐coating method. Then, they are immersed into natural dyes and analyzed by conducting UV‐Vis spectroscopy. Compared with bare TiO₂, natural dye–loaded photoanodes demonstrate far higher absorption ability in the visible region. After fabrication of devices with different novel natural dye sensitizers, current‐voltage characteristics and electrochemical impedance spectroscopy measurements are performed. The best power conversion efficiency (PCE) of 0.19% is obtained by sensitization of St. Lucie cherry with an open‐circuit voltage (V_oc) of 0.56 V, short‐circuit current density (J_sc) of 181 μA cm^?2, and fill factor (FF) of 0.55. Furthermore, St. Lucie cherry–sensitized devices show the lowest charge transfer and highest recombination resistances. This result can be attributed to the obvious carbonyl group exhibited by St. Lucie cherry.     

Effect of surface modification of TiO2 on the photovoltaic performance of the quasi solid state dye sensitized solar cells using a benzothiadiazole-based dye

We report the preparation of nanoporous TiO₂ electrode modified with an insulating material—BaCO₃ and used as electrode for quasi solid state dye sensitized solar cells (DSSCs), with a benzothiadiazole-based dye (BTDR2) as sensitizer and PEDOT:PSS coated FTO as counter electrode. We found that the BaCO₃ modification significantly increases the dye adsorption, resulting from the fact that the surface of BaCO₃ is more basic than TiO₂. The performance of the DSSCs with and without BaCO₃ modified TiO₂ electrodes were analyzed by cyclic voltammograms, optical absorption spectra, current-voltage characteristics in dark and under illumination and electrochemical impedance spectra. The photovoltaic performance has been significantly improved for the BaCO₃ modified electrode as compared to bare TiO₂ electrode having the same other components in the DSSCs. The value of the overall power conversion efficiency (η) improves from 2.42% to 4.38% under illumination intensity, when BaCO₃ modified electrode is used instead of bare TiO₂ electrode. The improvement in η has been attributed to the increased dye adsorption, reduction in recombination rate and enhancement in the electron lifetime when the modified TiO₂ electrode is used.     

Dye-sensitized nanocrystalline TiO2 solar cells based on novel coumarin dyes

We have developed dye-sensitized nanocrystalline TiO₂ solar cells (DSSCs) based on novel coumarin-dye photosensitizers. The absorption spectra of these novel dyes are red-shifted remarkably in the visible region relative to the spectrum of C343, a conventional coumarin dye. Introduction of a methine unit (–CH=CH–) connecting the cyano (–CN) and carboxyl (–COOH) groups into the coumarin framework expanded the π-conjugation in the dye and thus resulted in a wide absorption in the visible region. These novel dyes performed as efficient photosensitizers for DSSCs. A DSSC based on 2-cyano-5-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)-penta-2,4-dienoic acid (NKX-2311), produced a 6.0% solar energy-to-electricity conversion efficiency (η), the highest performance among DSSCs based on organic-dye photosensitizers, under AM 1.5 irradiation (100 mW cm^–2) with a short-circuit current density (J_sc) of 14.0 mA cm^–2, an open-circuit voltage (V_oc) of 0.60 V, and a fill factor of 0.71. Our results suggests that the structure of NKX-2311 whose carboxyl group is directly connected to the –CH=CH– unit, is advantageous for effective electron injection from the dye into the conduction band of TiO₂. In addition, the cyano group, owing to its strong electron-withdrawing ability, might play an important role in electron injection in addition to a red shift in the absorption region. On a long-term stability test under continuous irradiation with white light (80 mW cm^–2), stable performance was attained with a solar cell based on the NKX-2311 dye with a turnover number of 2.6×10⁷ per one molecule.     

Anthraquinone dyes as photosensitizers for dye-sensitized solar cells

Three anthraquinone dyes with carboxylic acid as anchoring group are designed and synthesized as sensitizers for dye-sensitized solar cells (DSSCs). Preliminary photophysical and photoelectrochemical measurements show that these anthraquinone dyes have very low performance on DSSC applications, although they have broad and intense absorption spectra in the visible region (up to 800 nm). Transient absorption kinetics, fluorescence lifetime measurements and density functional theory (DFT) calculations are conducted to investigate the cause of such low DSSC performance for these dyes. The results show that the strong electron-withdrawing character of the two carbonyl groups on anthraquinone framework may lie behind the low performance by suppressing the efficient electron injection from the dye to the conduction band of TiO₂.     

Novel iminocoumarin dyes as photosensitizers for dye-sensitized solar cell

Novel iminocoumarin dyes (2a-c and 3a-c) having carboxyl and hydroxyl anchoring groups onto the dyes skeletons have been designed and synthesized for the application of dye-sensitized nanocrystalline TiO₂ solar cells (DSSCs). The photophysical and electrochemical studies showed that these iminocoumarin dyes are suitable as light harvesting sensitizers in DSSC application. The dyes having carboxyl and hydroxyl anchoring groups (2a-c) showed better efficiency when compared to the dyes having carboxyl group (3a-c) alone. The cell consisted of dye 2a generated the highest solar-to-electricity conversion efficiency (η) of 0.767% (open circuit voltage (V_oc) = 0.491 V, short circuit photocurrent density (J_sc) = 2.461 mA cm⁻², fill factor (ff) = 0.635) under simulated AM 1.5 irradiation (1000 W m⁻²) with a total semiconductor area of 0.25 cm². The corresponding incident photon-to-current conversion efficiency (IPCE) of the above cell was 21.38%. The overall low efficiency of the dyes is ascribed to the lack of light harvesting ability at longer wavelength region.     

Effects of co-adsorbate and additive on the performance of dye-sensitized solar cells: A photophysical study

The effects of deoxycholic acid (DCA) in a dye solution as a co-adsorbate and guanidinium thiocyanate (GuSCN) in an electrolyte as an additive, on the photovoltaic performance of dye-sensitized solar cells (DSSCs) based on organic dye containing thiophene and fluorine segments (FL dye1) and black dye (BD) were investigated. The presence of DCA, up to 2 mM, increases both the photovoltage and photocurrent of the DSSC incorporating BD and further addition leads to decrease in the photocurrent. On the other hand, in the case of the DSSC containing FL dye1, the photocurrent decrease and photovoltage increase with the increase of DCA concentration. The pulsed laser measurement showed that electron lifetime (τ_e) of BD is higher than that of FL dye1 on a TiO₂ electrode and the electron diffusion coefficient (D_e) of the latter is higher than that of the former where there is little deviation for both τ_e and D_e with the addition of the DCA. The addition of GuSCN into electrolytes does enhance the photovoltage for DSSCs containing either FL dye1 or BD. This can be explained by the further adsorption of guanidinium cations following the dye adsorption that facilitates the self-assembly of dye molecules so as to either reduce the dark current or positively shift the conduction band edge of the TiO₂. The value of τ_e increases and the value of D_e decreases as a result of GuSCN addition.     

Efficient sensitization of nanocrystalline TiO2 films with cyanine and merocyanine organic dyes

Various kinds of cyanine and merocyanine organic dyes having short anchoring groups as sensitizers on nanocrystalline TiO₂ electrodes were investigated to promote the short-circuit photocurrent (J_sc) and the solar light-to-power conversion efficiency (η_sun). The J_sc and η_sun improved when the three different three dyes (yellow and red cyanine dyes, and blue squarylium cyanine dye) were adsorbed simultaneously on a TiO₂ electrode, as compared with the J_sc and η_sun of the TiO₂ electrodes adsorbed by each single dye. The maximum η_sun was 3.1% (AM-1.5, 100 mW/cm²). The J_sc and η_sun were influenced by the solvents for the dye adsorption on the TiO₂ electrode, and the efficiencies were improved by the addition of some cholic acids into the dye solution for adsorption. The electron transfer and/or the energy transfer from the red cyanine dye to the blue cyanine dye was observed on a SiO₂ film using emission spectroscopy, suggesting a strong interaction between two dyes. The J-like aggregates of the blue cyanine dyes hardly showed sensitization efficiency.     

Performance of the dye‐sensitized solar cells fabricated using natural dyes from Ixora coccinea flowers and Cymbopogon schoenanthus leaves as sensitizers

In this study, the chlorophyll and anthocyanin natural dyes were extracted from Cymbopogon schoenanthus leaves and Ixora coccinea flowers, respectively. Thereafter, these dyes were used as sensitizers in the TiO₂‐based dye‐sensitized solar cells (DSSCs). Ten solvents were used for solubilizing the dyes. Amongst the 10 solvents, the ethanol showed the highest absorption spectra for the anthocyanin and chlorophyll molecules. Temperature significantly affected the yield of the natural dyes. It was seen that an optimal extraction temperature of 70°C and 80°C results to higher anthocyanin and chlorophyll yields, respectively. However, an extraction temperature above 70°C and 80°C has shown a sharply decrease in the anthocyanin and chlorophyll concentrations, respectively. Also, the solution of acidic extraction, especially with a pH value of 4, increased the dyes concentrations. As seen in the results, the chlorophyll‐sensitized DSSCs had 0.23% conversion efficiency (?), short‐circuit current (I_sc) of 0.9 mA/cm^–2, open‐circuit voltage (V_oc) of 0.51 V, and 49.13% fill factor (FF). Meanwhile, the anthocyanin‐sensitized DSSCs showed 0.16% ?, 0.4 mA/cm^–2 I_sc, 0.53 V V_oc, and 75.93% FF.     

Dye-sensitized solar cells with a micro-porous TiO2 electrode and gel polymer electrolytes prepared by in situ cross-link reaction

The ionic conductivities and performances of dye-sensitized solar cells (DSSCs) of gel polymer electrolytes (GPEs) prepared by in situ cross-link reaction with different cross-linkers were investigated. The poly(imidazole-co-butylmethacrylate)-based GPE containing the 1,2,4,5-tetrakis(bromomethyl)benzene (B4Br) cross-linker showed a higher ionic conductivity than that containing cross-linkers with a linear structure, due to the formation of micro-phase separation that resulted in an increase in ion transport paths in the GPE. Moreover, the co-adsorbent ((4-pyridylthio) acetic acid, PAA) co-adsorbed with N3 dye on the TiO₂ electrode not only reduced dye aggregation, but also reacted with the cross-linkers in the GPE at the TiO₂/GPE interface. A decrease in the charge transport resistance at the TiO₂/GPE interface was noted after forming the gel; thus the value of J_SC significantly increased from 7.72 to 10.00 mA cm⁻². In addition, in order to reduce the ionic diffusion resistance within the TiO₂ electrode, incorporation of monodispersed PMMA in the TiO₂ paste was considered. With the optimal weight ratio of PMMA/TiO₂ (w/w=3.75), the TiO₂ electrode exhibited larger pores (ca. 350 nm) uniformly distributed after sintering at 500 °C, and the ionic diffusion resistance within the TiO₂ film could significantly be reduced. The cell conversion efficiency increased from 3.61% to 5.81% under illumination of 100 mW cm⁻², an improvement of ca. 55%.     

Pomegranate leaves and mulberry fruit as natural sensitizers for dye-sensitized solar cells

This study employs chlorophyll extract from pomegranate leaf and anthocyanin extract from mulberry fruit as the natural dyes for a dye-sensitized solar cell (DSSC). A self-developed nanofluid synthesis system is employed to prepare TiO₂ nanofluid with an average particle size of 25 nm. Electrophoresis deposition was performed to deposit TiO₂ nanoparticles on the indium tin oxide (ITO) conductive glass, forming a TiO₂ thin film with the thickness of 11 μm. Furthermore, this TiO₂ thin film was sintered at 450 °C to enhance the thin film compactness. Sputtering was used to prepare counter electrode by depositing Pt thin film on FTO glass at a thickness of 20 nm. The electrodes, electrolyte (), and dyes were assembled into a cell module and illuminated by a light source simulating AM 1.5 with a light strength of 100 mW/cm² to measure the photoelectric conversion efficiency of the prepared DSSCs. According to experimental results, the conversion efficiency of the DSSCs prepared by chlorophyll dyes from pomegranate leaf extract is 0.597%, with open-circuit voltage (V_OC) of 0.56 V, short-circuit current density (J_SC) of 2.05 mA/cm², and fill factor (FF) of 0.52. The conversion efficiency of the DSSCs prepared by anthocyanin dyes from mulberry extract is 0.548%, with V_OC of 0.555 V and J_SC of 1.89 mA/cm² and FF of 0.53. The conversion efficiency is 0.722% for chlorophyll and anthocyanin as the dye mixture, with V_OC of 0.53 V, J_SC of 2.8 mA/cm², and FF of 0.49.     

Cubic titanium dioxide photoanode for dye-sensitized solar cells

Following from the recently evolved concept of significantly improving the photovoltaic efficiency in dye-sensitized solar cells (DSSCs) by reducing the loss of electrons on the spherical surface of titanium dioxide, this study examines the synthesis of cubic TiO₂ with a special morphology to overcome this electron loss and investigates its application to DSSCs. Cubic TiO₂ is synthesized by an advanced rapid hydrothermal method, with the addition of an amine species additive. Transmission electron microscopy (TEM) images confirm the cubic shape of the TiO₂ particles with a diameter less than 5-10 nm. Using N719 dye under illumination with 100 mW cm⁻² simulated sunlight, the application of cubic TiO₂ to DSSCs affords an energy conversion efficiency of approximately 9.77% (4.0-μm thick TiO₂ film), which is considerably enhanced compared with that achieved using a commercial, spherical TiO₂. Electrostatic force microscopy (EFM) and impedance analyses reveal that the electrons are transferred more rapidly to the surface of a cubic TiO₂ film than on a spherical TiO₂ film.     

Influences of different TiO2 morphologies and solvents on the photovoltaic performance of dye-sensitized solar cells

The effects of TiO₂ photoelectrode's surface morphology and different solvents on the photovoltaic performance of dye-sensitized solar cells (DSSCs) were studied. By successive coating of TiO₂ suspension, composed of low and high molecular weight poly(ethylene)glycol (PEG) as a binder, double layered TiO₂ photoelectrodes with four different structures were obtained. Among the DSSCs with different TiO₂ electrodes, DSSC with P2P1 electrode (P2 and P1 correspond to PEG molecular weights of 20,000 and 200,000, respectively) showed higher performance under identical film thickness at a constant irradiation of 100 mW cm⁻², which may be correlated with large pore size and high surface area of the corresponding TiO₂ electrode. This was confirmed by electrochemical impedance spectroscopy (EIS) analysis of the DSSC and the transient photovoltage measurement of electrons in the TiO₂ electrode. Among the different solvents investigated here, the DSSC containing acetonitrile showed high conversion efficiency and the order of performance of the DSSCs with different solvents were AN > MPN > PC > GBL > DMA > DMF > DMSO. Better correlation was observed between the donor number of solvents and photoelectrochemical parameters of the DSSCs containing different solvents rather than the measured viscosity and dielectric constant of solvents. The reasons for the low performance of the DSSCs containing DMA, DMSO and DMF, respectively, were due to the negative shift of TiO₂ conduction band and the desorption of dye molecules from the TiO₂ photoelectrode by those solvents.     

Study on dye-sensitized solar cell using novel infrared dye

The performances of infrared-dye-sensitized solar cells fabricated using two novel cyanine dyes (light absorption edge: 900 nm) were investigated. The performance of the cell using NK6037 dye was superior to that using NK4432; hence, an intimate interaction between the dye and the TiO₂ photoelectrode via the functional group is essential for efficient electron injection. The efficiency reached 1.9% by optimizing the light-confining effect of the TiO₂ photoelectrode and TiO₂ film thickness, and reached 2.3% by adjusting the concentration of deoxycholic acid (DCA) in the dye solution. The roles of the dye and DCA were clarified by photochemical and electrochemical characterization.     

Design of new benzothiadiazole-based linear and star molecules with different functional groups as solar cells materials: A theoretical approach

The novel linear and star molecules based on benzothiadiazole as a central core, TPA as end groups and different functional groups as π-spacers were theoretically examined in the OSC applications. The geometry structures in ground and excited state, ionization potentials (IPs), electron affinities (EAs) and reorganization energy (E_λ⁺ and E_λ⁻) of linear and star molecules were studied by DFT methods. The absorption and the emission spectra were calculated by TD-BHandHLYP method. Star molecules based on ethynyl groups as π-spacer exhibit ameliorative IPs, EAs, E_λ⁺ and E_λ⁻, indicating they are more suitable for the formation and transfer of holes than the others. Moreover, the diethynyl groups improve the charge transfer character and extend the absorption wavelength towards the range with the maximum photon flux.