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.     

Triphenylamine-based organic dye containing the diphenylvinyl and rhodanine-3-acetic acid moieties for efficient dye-sensitized solar cells

In this paper, we report on the synthesis, characterization of a new triphenylamine (TPA) derivative (TPAR14) with a “secondary electron transferring” structure as well as its application to dye-sensitized solar cells (DSCs). The introduction of diphenylvinyl to the adjacent phenyl ring of the TPA core could not only increase the extinction coefficient and λ_max by the extension of π-conjugation, but also form three-dimensional obstacle for dye aggregation and triiodide that is in favor of higher open-circuit photovoltage (V_oc). An overall light-to-electricity conversion efficiency of 6.27% is obtained for the DSC with the as-synthesized dye under AM 1.5 irradiation (100 mW cm⁻²), which is the highest value among the TPA-rhodanine-3-acetic acid-based DSCs. The long-term stability test of TPAR14 under heat shows its character of high stability. The present work indicates that the as-synthesized TPAR14 is promising in the application of DSCs.     

Influence of acceptor moiety in triphenylamine-based dyes on the properties of dye-sensitized solar cells

Triphenylamine-based organic dyes with different acceptor parts have been synthesized in an attempt to investigate the effect of the acceptor moiety on the properties of dye-sensitized solar cells. The light-to-electricity conversion efficiencies of 4.67% and 5.05% were obtained for the DSCs based on 2-(4-oxo-5-(4-(phenyl(4-styrylphenyl)amino)benzylidene)-2-thioxothiazolidin-3-yl)acetic acid (TPAR11) and 2-cyano-3-(4-(phenyl(4-styrylphenyl)amino)phenyl)acrylic acid (TC12), respectively. A molecular-orbital calculation shows that the delocalization of the excited state for TPAR11 is broken between the 4-oxo-2-thioxothiazolidine ring and the acetic acid, which affects the electron movement from dye molecule to the semiconductor film. The effects of chenodeoxycholic acid (CDCA) as the coadsorbent on the photovoltaic performance of the DSCs based on TPAR11 and TC12 have been also studied. It has been found that the addition of 1 mM CDCA coadsorbent improves the photocurrent for TPAR11 and the photovoltage for TC12, owing to the suppression of the quenching processes of the excited electrons between dye molecules or/and a more negative conductive band edge of TiO₂ film. With the addition of 1 mM CDCA, the light-to-electricity conversion efficiencies of the DSCs based on TPAR11 and TC12 were 5.46% and 5.96%, respectively. This result indicates that both the acceptor moiety of metal-free organic dyes and the coadsorbent added in the electrode preparation have the effect on the photovoltaic performance of DSCs.     

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.     

Influence of polymer concentration and dyes on photovoltaic performance of dye-sensitized solar cell with P(VdF-HFP)-based gel polymer electrolyte

A series of gel polymer electrolytes (GPEs) is synthesized using Poly(vinylidenefluoride-hexafluoropropylene) P(VdF-HFP) as the host matrix and propylene carbonate (PC)–diethyl carbonate (DEC) as plasticizers to fabricate dye-sensitized solar cells. Equal amounts of PC and DEC are used to comprehend high dielectric constant and low viscosity of the electrolyte. The as-prepared GPEs are characterized by XRD, FTIR and SEM. Their thermal properties and ionic conductivities are investigated by TGA/DSC analyses and AC impedance measurements, respectively. The optimized gel polymer electrolyte gives a maximum ionic conductivity of 5.25 × 10⁻³ S cm⁻¹ at room temperature. The formation of porous structure in the electrolyte film supports the entrapment of large volumes of liquid electrolyte inside its cavities. The role of N3 and N719 dyes are also investigated for better photovoltaic performance of DSSC. The overall light-to-electrical-energy conversion efficiencies of 3.95% and 4.41% are obtained for N3 and N719 dyes, respectively, under 100 mW cm⁻² irradiation, which are comparable to those obtained from the corresponding liquid electrolyte cell.     

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%.     

Exploring the role of the spacers and acceptors on the triphenylamine-based dyes for dye-sensitized solar cells

Six triphenylamine-based dyes were explored for their application in dye-sensitized solar cells (DSSCs). Dyes 1–3 and dyes 4–6 possess cyanoacrylic acid (C-acceptor) and rhodanine-3-acetic acid (R-acceptor), respectively. Stilbene (in dyes 2 and 5) and bis(styryl)benzene (in dyes 3 and 6) were used as π-spacers. There is no π-spacer in the dye 1 and 4. To elucidate the role of π-spacers, optical, electrochemical, and photovoltaic properties of the dyes were studied. Among C-acceptor dyes, dye 2 exhibits the highest light-to-electricity conversion efficiency of 4.45%, followed by dye 3 (4.16%). Similarly, among R-acceptor dyes, dye 5 is the best. These results indicate that stilbene is a better π-spacer over bis(styryl)benzene. Although bis(styryl)benzene could extend the light absorption range (in dye-adsorbed TiO₂ film), its tendency to promote intermolecular π-π stacking is possibly the reason for its poor performance in DSSCs. Furthermore, the conjugation break in the R-acceptor moiety attached to the TiO₂ surface limits the electron injection of R-acceptor dyes poorer than C-acceptor dyes. Density functional theory calculations were performed for the dye-(TiO₂)₈ cluster, assuming a bidentate chelation of a carboxylic acid group with Ti⁴⁺ of TiO₂ anatase. The natural bond orbital (NBO) analysis indicated relatively more electron-accepting ability of cyanoacrylic acid over rhodanine-3-acetic acid.     

Synthesis of new low band gap dyes with BF2-azopyrrole complex and their use for dye-sensitized solar cells

The diazonium salt derived from 4-aminobenzoic acid, 4-aminophenol or 2-aminophenol reacted with half equivalent of pyrrole to afford symmetrical 2,5-bisazopyrroles. They reacted subsequently with boron trifluoride in the presence of triethylamine to afford the corresponding BF₂-azopyrrole complexes D1, D2 and D3 respectively. They were soluble and stable in nonprotic solvents such as chloroform, dichloromethane and tetrahydrofuran but unstable in protic solvents such as ethanol. Their absorption spectra were broad with optical band gap of 1.49-1.70 eV. Among these dyes D2 displayed the broader absorption spectrum with low band gap of 1.49 eV. We have utilized these complexes as photosensitizers for quasi solid state dye-sensitized solar cells (DSSCs) and achieved power conversion efficiency in the range of 4.0-6.0%. We have also found that the co-adsorption of citric acid hindered the formation of dye aggregates and might improve the electron injection efficiency leading to an enhancement in short circuit photocurrent. This work suggests that metal-free dyes based on BF₂-azopyrrole complex are promising candidates for improvement of the DSSC performance.     

Synthesis and photovoltaic properties of biindene-C70 monoadduct as acceptor in polymer solar cells

A new fullerene derivative, biindene-C₇₀ monoadduct (BC₇₀MA), was synthesized by [4+2] cycloaddition reaction between 1,1′-biindene and C₇₀, for the application as acceptor in polymer solar cells (PSCs). BC₇₀MA is soluble in common organic solvents such as tetrahydrofuran, chloroform, toluene, o-dichlorobenzene, etc., and shows stronger absorption in the visible region and a slightly up-shifted lowest unoccupied molecular orbital (LUMO) energy level than that of PCBM. PSCs were fabricated with BC₇₀MA as acceptor and poly(3-hexylthiophene) (P3HT) as donor for investigating the photovoltaic properties of BC₇₀MA. The power conversion efficiency of the PSC based on P3HT/BC₇₀MA (1:1, w/w) with the additive of 3% octane-1,8-dithiol and thermal annealing at 110 °C for 10 min reached 3.44% with open circuit voltage of 0.64 V, short circuit current of 8.02 mA/cm² and fill-factor of 0.67, under the illumination of AM1.5, 100 mW/cm².     

A dye-sensitized photo-supercapacitor based on PProDOT-Et2 thick films

A photo-rechargeable supercapacitor (photo-supercapacitor, or PSC) is studied using a N3-dye adsorbed TiO₂ photoelectrode and PProDOT-Et₂ poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) polymer films as supercapacitor materials for electron storage. The PSC device, comprising a dye-sensitized solar cell (DSSC) and a supercapacitor (SC), can store the photo-to-electric energy. The PProDOT-Et₂ films are potentiostatically electropolymerized to form thick films (ca. 0.5 mm) with a specific capacitance of ca. 6.5 F cm⁻². A symmetrical (p/p) supercapacitor, with PProDOT-Et₂ coated on both electrodes, is also characterized before fabricating the three-electrode PSC. The PSC is tested under light illumination of 100 mW cm⁻², and attaining a photocharged voltage of 0.75 V and a discharged energy density of 21.3 μWh cm⁻².     

On the photophysical and electrochemical studies of dye-sensitized solar cells with the new dye CYC-B1

In this study, the photoelectrochemical characteristics of a ruthenium photosensitizer with an alkyl bithiophene group, designated as CYC-B1, are studied. The effect of mesoporous TiO₂ film thickness on the photovoltaic performance of CYC-B1 and N3 dye-sensitized solar cells was investigated. The performance of the dye-sensitized nanocrystalline TiO₂ solar cells (DSSC) fabricated using CYC-B1 dye-anchored TiO₂ photoelectrode showed a convincing enhancement in cell efficiency when the TiO₂ film thickness was increased from 3 μm (eff.=5.41%) to 6 μm (eff.=7.19%). The efficiency of the CYC-B1-sensitized DSSC was maximum at 6 μm of the TiO₂ film thickness, reached its limiting value and remained constant up to 53 μm, although a similar trend was also observed for N3 dye-sensitized DSSC, however, the maximum efficiency achieved was only at 27 μm thickness (eff.=6.75%). As expected, the photocurrent density generated in the DSSC modified by CYC-B1 dye is larger than that from N3 dye. The effect of guanidinium thiocyanate (GuSCN) (additive) addition to the electrolyte on the photovoltaic performance of DSSCs based on CYC-B1 was also investigated. Furthermore, the electrochemical impedance spectroscopy (EIS) technique and photo-transient laser method have been employed to analyze the charge transfer resistances (R_ct) and the lifetime of the injected electrons on the TiO₂ containing different thicknesses.     

Novel thixotropic gel electrolytes based on dicationic bis-imidazolium salts for quasi-solid-state dye-sensitized solar cells

Novel thixotropic gel electrolytes have been successfully prepared by utilizing oligomeric poly(ethylene oxide) (PEO)-based bis-imidazolium diiodide salts and hydrophilic silica nanoparticles for application in quasi-solid-state dye-sensitized solar cells (DSSCs). The thixotropic gel-state of the ionic liquid-based composite electrolytes is confirmed by observing the typical hysteresis loop and temporary hydrogen bonding. On using the PEO-based composite electrolyte, a quasi-solid-state DSSC exhibited highly improved properties such as easy penetration of the electrolyte into the cell without leakage, long-term stability, high open-circuit voltage without the use of 4-tert-butylpyridine, and a high energy-conversion efficiency of 5.25% under AM 1.5 illumination (100 mW cm⁻²).     

Mordant dyes as sensitisers in dye-sensitised solar cells

Many mordant dyes commonly used in the textile industry form coordination complexes at the surface of nanocrystalline TiO₂. Dyes having a salicylate chelating group are particularly effective. Forty-nine commercial mordant dyes were studied as sensitisers in a non-optimised dye-sensitised solar cell (DSSC) and their performance compared to the N3 ruthenium complex. Although N3 produced the highest output, six mordant dyes produced photocurrents >0.2 mA. UV–visible spectra of the dye-complexed photoanodes suggest that some mordant dyes are more strongly bound to the TiO₂ surface than N3. Photocatalytic oxidation of these dyes does not appear to occur in a DSSC environment.     

Incorporating carbon nanotube in a low-temperature fabrication process for dye-sensitized TiO2 solar cells

Dye-sensitized solar cells (DSSCs) incorporating TiO₂ porous films, prepared at a low temperature (150 °C), along with multi-wall carbon nanotubes (MWCNTs) were studied using two different electrolytes, namely LiI and THI. Electrochemical impedance spectroscopy (EIS) was employed to quantify the charge transport resistance and electron lifetime (τ_e) under different levels (wt%) of MWCNTs and electrolytes. The charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) increased as a function of the MWCNT concentration, which ranged 0.1-0.5 wt%, due to a decrease in the surface area and decreased dye adsorption. The characteristic peak shifted to a lower frequency at 0.1 wt% of MWCNT, indicating a longer electron lifetime. The DSSC with the TiO₂ electrode containing 0.1 wt% of MWCNT resulted in a higher short-circuited current density (J_SC) of 9.08 mA/cm², an open-circuit voltage (V_OC) of 0.781 V, and a cell conversion efficiency of 5.02%. EIS was also conducted under dark conditions. The large value at a middle frequency represented electron transport at the TiO₂/dye/electrolyte interface (R_rec). The R_rec for 0.1 wt% MWCNT/TiO₂ was found to be 114 Ω, and for those with 0.3 and 0.5 wt% were 35 and 30 Ω, respectively. The significantly higher value of R_rec suggested that the charge recombination between injected electrons and electron acceptors in the redox electrolyte, I₃⁻, was remarkably retarded. Finally, electrolytes with LiI and THI were used to compare the cell conversion performance under the same conditions. It was found that more electrons were injected in the TiO₂ electrode and the electron recombination reaction was faster in the DSSC with THI than that with LiI.     

Unique TiO2 paste for high efficiency dye-sensitized solar cells

A novel titanium oxide paste based on Pechini sol-gel method and nanocrystalline titanium oxide powder have been successfully developed. Titanium oxide layers possess high inner surface area assuring high dye loading and well-connected nanocrystalline grains assuring good electron transport within the layer. The dye-sensitized layers have been used to assemble dye-sensitized solar cells with acetonitrile- and ionic liquid-based electrolyte. Overall conversion efficiencies of dye-sensitized solar cells (DSSCs) determined under standard test conditions (100 mW/cm², 25 °C and AM 1.5 G) are 10.2% for acetonitrile and 7.3% for ionic liquid-based electrolyte.     

Synthesis of TiO2 submicro-rings and their application in dye-sensitized solar cell

In this paper, novel TiO₂ submicro-rings were synthesized via potentiostatic anodization of titanium powder coated on transparent conducting oxide glass. The TiO₂ submicro-rings film was characterized by SEM, XPS and 3D optical profiling. Accordingly, a possible growth mechanism of submicro-rings was discussed. The TiO₂ submicro-rings based dye-sensitized solar cell (DSSC) with the film thickness of ca. 3.1 μm was assembled and a conversion efficiency of 1.36% was achieved under AM 1.5 illumination. The photoelectron transport properties of TiO₂ submicro-rings based DSSC were also discussed according to the electron impedance spectroscopy.     

Enhanced performance of a quasi-solid-state dye-sensitized solar cell with aluminum nitride in its gel polymer electrolyte

The effects of incorporation of aluminum nitride (AlN) in the gel polymer electrolyte (GPE) of a quasi-solid-state dye-sensitized solar cell (DSSC) were studied in terms of performance of the cell. The electrolyte, consisting of lithium iodide (LiI), iodine (I₂), and 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile (MPN), was solidified with poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP). The 0.05, 0.1, 0.3, and 0.5 wt% of AlN were added to the electrolyte for this study. XRD analysis showed a reduction of crystallinity in the polymer PVDF-HFP for all the additions of AlN. The DSSC fabricated with a GPE containing 0.1 wt% AlN showed a short-circuit current density (J_SC) and power-conversion efficiency (η) of 12.92±0.54 mA/cm² and 5.27±0.23%, respectively, at 100 mW/cm² illumination, in contrast to the corresponding values of 11.52±0.21 mA/cm² and 4.75±0.08% for a cell without AlN. The increases both in J_SC and in η of the promoted DSSC are attributed to the higher apparent diffusion coefficient of I⁻ in its electrolyte (3.52×10⁻⁶ cm²/s), compared to that in the electrolyte without AlN of a DSSC (2.97×10⁻⁶ cm²/s). At-rest stability of the quasi-solid-state DSSC with 0.1 wt% of AlN was found to decrease hardly by 5% and 7% at room temperature and at 40 °C, respectively, after 1000 h duration. The DSSC with a liquid electrolyte showed a decrease of about 40% at room temperature, while it virtually lost its performance in about 150 h at 40 °C. Explanations are further substantiated by means of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and by porosity measurements.     

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.     

Electrochemical performance of LiCoO2 cathodes by surface modification using lanthanum aluminum garnet

LiCoO₂ particles were coated with various wt.% of lanthanum aluminum garnets (3LaAlO₃:Al₂O₃) by an in situ sol–gel process, followed by calcination at 1123 K for 12 h in air. X-ray diffraction (XRD) patterns confirmed the formation of a 3LaAlO₃:Al₂O₃ compound and the in situ sol–gel process synthesized 3LaAlO₃:Al₂O₃-coated LiCoO₂ was a single-phase hexagonal α-NaFeO₂-type structure of the core material without any modification. Scanning electron microscope (SEM) images revealed a modification of the surface of the cathode particles. Transmission electron microscope (TEM) images exposed that the surface of the core material was coated with a uniform compact layer of 3LaAlO₃:Al₂O₃, which had an average thickness of 40 nm. Galvanostatic cycling studies demonstrated that the 1.0 wt.% 3LaAlO₃:Al₂O₃-coated LiCoO₂ cathode showed excellent cycle stability of 182 cycles, which was much higher than the 38 cycles sustained by the pristine LiCoO₂ cathode material when it was charged at 4.4 V.     

Novel Al2O3-based compressive seals for IT-SOFC applications

Novel compressive Al₂O₃-based seals were developed and characterized under simulated intermediate temperature solid oxide fuel cell (IT-SOFC) environment. The seals were prepared by tape casting, mainly composed of fine Al₂O₃ powder with various contents of fine Al powder addition. The leakage rates were determined at 800 °C under 0.14–0.69 MPa compressive stresses, and the stabilities were evaluated at 750 °C under constant 0.35 MPa compressive stress. The leakage rates at 800 °C were in range of 0.2–0.01 sccm cm⁻¹, decreasing with increasing the compressive stress and Al content; Al addition significantly improved the stability, the leakage rate with 20 wt% Al addition was as low as 0.025 sccm cm⁻¹ at 800 °C under 0.35 MPa compressive stress with a gauge pressure of 6.9 kPa, and exhibited good stability at 750 °C. Single cell test also confirmed the effectiveness of the tape cast Al₂O₃-based seal for planar IT-SOFC applications.