Dye Regeneration Process Research Articles

Structural and optical properties of Purpurin for dye-sensitized solar cells

In this work, we reported a combined experimental and theoretical study on molecular structure, vibrational spectra and Homo–Lumo analysis of Purpurin and TiO2/Purpurin. The geometries, electronic structures, molecular orbital analysis of natural dye sensitizer Purpurin were studied based on density functional theory (DFT) using the hybrid functional B3LYP. Fourier transform infrared (FT-IR) and FT-Raman spectra have been recorded and extensive spectroscopic investigations have been carried out on Purpurin. The optimized geometries, wave number and intensity of the vibrational bands of Purpurin have been calculated using density functional level of theory (DFT/B3LYP) employing 6-311G (d,p) basis set. Based on the comparison between calculated and experimental results, assignments of the fundamental vibrational modes are examined. Features of the electronic absorption spectrum in the visible and near-UV regions were assigned based on TD-DFT calculations. The calculated results suggest that the three excited states with the lowest excited energies in 1,2,4, trihydroxy 9-10 anthraquinone was due to photo-induced electron transfer processes. Frontier molecular orbitals (FMO), LUMO, HOMO, and energy gap, of these dyes have been analyzed to show their effect on the process of electron injection and dye regeneration. Interaction between HOMO and LUMO of Purpurin are investigated to understand the recombination process and charge transfer process involving these dyes. We also performed analysis of I–V characteristics to investigate the role of charge transfer and the stability of the dye molecule.

Modulation of photovoltaic behavior of dye-sensitized solar cells by electron donors of porphyrin dyes and cosensitization

Abstract Porphyrin dyes have received great attention due to their excellent photovoltaic performance in dye-sensitized solar cells (DSSCs). In this work, dyes XC1 – XC3 were synthesized by introducing various numbers of bis(4-methoxyphenyl)amino and p -hexyloxyphenyl groups to porphyrin meso -positions. The XC1 molecule contains two p -hexyloxyphenyl groups, and its DSSCs showed the power conversion efficiency of 4.81%. For XC2 and XC3 , the replacement of p -hexyloxyphenyl with diphenylamino groups can effectively enhance the light harvesting around 500 nm. However, the highest occupied molecular orbitals (HOMOs) were elevated too much, which suppressed the dye regeneration processes, leading to low cell efficiencies of 2.51% and 1.27% for XC2 , and XC3 , respectively. To further improve the cell performance, an anthracene derivative C1 was used as the cosensitizer for XC1 , which increased both the J sc and V oc values, with an improved efficiency of 5.75%.

Comparative study on photovoltaic properties of imidazole-based dyes containing varying electron acceptors in dye-sensitized solar cells

In this paper, a comparative study on the photophysical, electrochemical properties and photovoltaic performances of simple imidazole-based dyes containing varying electron acceptors is done. Two organic dyes containing 4,5-diphenyl-1H-imidazole (DPIM) unit as electron donor and cyanoacetic acid (for DB-1) or rhodanine-3-acetic acid (for DB-2) as electron acceptor are designed and synthesized. The LUMO and HOMO energy levels of the two dyes can be ensuring positive effect on the process of electron injection and dye regeneration. The overall conversion efficiencies of 1.63% and 0.51% were obtained for DSSCs based on DB-1 and DB-2, respectively. In comparison with DB-1, DB-2 shows the lower JSC, VOC, and efficiency, which is attributed to the lower excited electron injection efficiency and faster charge recombination rate.

INVESTIGATION OF DYE-REGENERATION KINETICS AT DYE-SENSITIZED p-TYPE CuCrO2 FILM/ELECTROLYTES INTERFACE WITH SCANNING ELECTROCHEMICAL MICROSCOPY

The power conversion efficiency of p-type dye-sensitized solar cells (DSSC) is determined by the kinetics of hole injection and dye-regeneration reaction at the dye/electrolyte interface. In this work, the photochemical regeneration kinetics of dye adsorbed on CuCrO 2 mesoporous film was investigated by using scanning electrochemical microscopy with feedback mode. Organic P1 and C343 sensitizers in combination with iodide-based and thiolate-based electrolytes were selected to understand the effect of sensitizers and redox shuttles on dye-regeneration process. A fast regeneration kinetic rate constant was confirmed in thiolate-based sample compared with iodide-based electrolyte, indicating that the organic redox shuttle was an efficient mediator to optimize the performance of p-type DSSC.

Benzotriazole-based dyes containing a low band gap for dye-sensitised solar cells: a theoretical study

In this study, we investigated a series of metal-free benzotriazole-based organic dyes. The geometries, electronic properties, light harvesting efficiency, and electronic absorption spectra of these dyes were studied using the density functional theory and time-dependent density functional theory. The optimised geometries indicate that these dyes are non-planar and thereby effectively inhibit close intermolecular π–π aggregation. The band gap of these dyes ensures a positive effect on the process of electron injection and dye regeneration. The band gap trend corroborates well with the predicted spectra data. Our theoretical calculations reveal that the designed metal-free organic dyes can be used as potential sensitisers for solar cells compared to the best known organic sensitiser (Y123) to date.

Theoretical design of thiazolothiazole-based organic dyes with different electron donors for dye-sensitized solar cells

In this study, we have designed four novel organic donor–π–acceptor dyes (D1, D2, D3, D4), used for dye sensitized solar cells (DSSCs). The electron acceptor (anchoring) group was 2-cyanoacrylic for all dyes whereas the electron donor unit varied (coumarin, indoline, carbazole, triphenylamine) and the influence was investigated. These dyes, based on thiazolothiazole as π-spacer, were studied by density functional theory (DFT) and its extensible time dependant DFT (TDDFT) approaches to shed light on how the π-conjugation order influence the performance of the dyes in the DSSCs. The theoretical results have shown that the LUMO and HOMO energy levels of these dyes can be ensuring positive effect on the process of electron injection and dye regeneration. The trend of the calculated HOMO–LUMO gaps nicely compares with the spectral data. Key parameters in close connection with the short-circuit current density (Jsc), including light harvesting efficiency (LHE), injection driving force (ΔGinject.) and total reorganization energy (λtotal), were discussed. The calculated results of these dyes reveal that dye D2, with indoline as electron donor group, can be used as a potential sensitizer for TiO2 nanocrystalline solar cells due to its best electronic and optical properties and good photovoltaic parameters.

Investigation of Dye Regeneration Kinetics in Sensitized Solar Cells by Scanning Electrochemical Microscopy

Sensitizers are responsible for the light harvesting and the charge injection in dye-sensitized solar cells (DSSCs). A fast dye-regeneration process is necessary to obtain highly efficient DSSC devices. Herein, dye-regeneration rates of two DSSC device types, that is, the reduction of immediately formed photo-oxidized sensitizers (ruthenium complex C106TBA and porphyrin LD14, k(ox)') by iodide ions (I(-)) and [Co(bpy)3](2+), and the oxidation of formed photo-reduced sensitizers (organic dye P1, k(re)') by triiodide ions (I3(-)) and the disulfide dimer (T2) are investigated by scanning electrochemical microscopy (SECM). We provide a thorough experimental verification of the feedback mode to compare the kinetics for dye-regeneration by using the above mentioned mediators. The charge recombination at the dye/semiconductor/electrolyte interface is further investigated by SECM. A theoretical model is applied to interpret the current response at the tip under short-circuit conditions, providing important information on factors that govern the dynamics of dye-regeneration onto the dye-sensitized heterojunction.

Theoretical Studies on the Interaction of Ruthenium Sensitizers and Redox Couple in Different Deprotonation Situations

We report a DFT study of interaction between the Ru complex sensitizer [Ru(dcbpy)2(NCS)2: dcbpy = 4,4'-dicarboxy-2,2'-bipyridyl] (N3) and iodide ion under the influence of different deprotonation situations. There are two kinds of interaction mechanisms: iodide ion interacts with metal-center Ru atom or carboxyl, derived from the natural charge distribution analysis. The calculation indicated that there were several stable intermedium forms in different deprotonation degree. The stability of these intermedium forms would be perturbed gradually while the number of eliminated protons increased. It can be predicted that in the initial period of absorption and injection as well as the dissolve process where the deprotonation was demanded, the dye will not attacked by the iodide ion in solution extensively. Additionally, dye with more carboxyls will reduce the activity of redox reaction and more obstacles are required to be overcome before or during the redox reaction. The comparison of natural charge between isolate N3 and N3 with iodide ion intermedium (N3I(-)) showed the iodide ion attacking made the charge contribute on N3 molecule more negative, nevertheless the N3I(-) still has ability to attract another iodide ion. The attacking of iodide ion will also influence the electronic transition and absorption properties through the analysis of the frontier molecular orbitals and the densities of states. The results reported in this paper give us the guidance to carry out the further investigations about the dye regeneration process.

Comparison of TiO2and ZnO Solar Cells Sensitized with an Indoline Dye: Time-Resolved Laser Spectroscopy Studies of Partial Charge Separation Processes

Time-resolved laser spectroscopy techniques in the time range from femtoseconds to seconds were applied to investigate the charge separation processes in complete dye-sensitized solar cells (DSC) made with iodide/iodine liquid electrolyte and indoline dye D149 interacting with TiO2 or ZnO nanoparticles. The aim of the studies was to explain the differences in the photocurrents of the cells (3-4 times higher for TiO2 than for ZnO ones). Electrochemical impedance spectroscopy and nanosecond flash photolysis studies revealed that the better performance of TiO2 samples is not due to the charge collection and dye regeneration processes. Femtosecond transient absorption results indicated that after first 100 ps the number of photoinduced electrons in the semiconductor is 3 times higher for TiO2 than for ZnO solar cells. Picosecond emission studies showed that the lifetime of the D149 excited state is about 3 times longer for ZnO than for TiO2 samples. Therefore, the results indicate that lower performance of ZnO solar cells is likely due to slower electron injection. The studies show how to correlate the laser spectroscopy methodology with global parameters of the solar cells and should help in better understanding of the behavior of alternative materials for porous electrodes for DSC and related devices.

Theoretical investigation of new thiazolothiazole-based D-π-A organic dyes for efficient dye-sensitized solar cell

Novel ten organic donor-π-acceptor dyes (D-π-A), used for dye-sensitized solar cells (DSSCs), based on thiazolothiazole were studied by density functional theory (DFT) and time dependant DFT (TDDFT) approaches to shed light on how the π-conjugation order influence the performance of the dyes. The electron acceptor (anchoring) group was 2-cyanoacrylic for all dyes whereas the electron-donor unit varied and the influence was investigated. The theoretical results have shown that TDDFT calculations using the Coulomb attenuating method CAM-B3LYP with the polarized split-valence 6-31G (d,p) basis sets and the polarizable continuum model (PCM) were reasonably capable of predicting the excitation energies, the absorption and the emission spectra of the molecules. The LUMO and HOMO energy levels of these dyes can ensure a positive effect on the process of electron injection and dye regeneration. The trend of the calculated HOMO–LUMO gaps nicely compares with the spectral data. Key parameters in close connection with the short-circuit current density (Jsc), including light-harvesting efficiency (LHE), injection driving force (ΔGinject) and total reorganization energy (λtotal), were discussed. In addition, the estimated values of open-circuit photovoltage (Voc) for these dyes were presented. The calculated results of these dyes reveal that the D6 dye can be used as a potential sensitizer for TiO2 nanocrystalline solar cells due to its best electronic and optical properties and good photovoltaic parameters.

Novel ruthenium sensitizers with a dianionic tridentate ligand for dye-sensitized solar cells: the relationship between the solar cell performances and the electron-withdrawing ability of substituents on the ligand

Five novel ruthenium sensitizers (TUS sensitizers) with a dianionic tridentate ligand (pyridine-2,6-dicarboxyamidato and its derivatives) have been synthesized for application to dye-sensitized solar cells (DSCs). These TUS sensitizers have much larger molar absorption coefficients in the wavelength range below 600 nm compared with those of Black dye which is a structural analog and a highly efficient ruthenium sensitizer. The energy levels of HOMOs and LUMOs of TUS sensitizers shifted to the positive direction with increasing the electron-withdrawing ability of the substituents on the dianionic tridentate ligand. The energy levels of HOMO and LUMO showed linear correlation with respect to the Hammett constant of the substituents. The DSCs with TUS sensitizers showed much lower performances than that of Black dye. Both inferior adsorptivity on the TiO2 surface and unfavorable energy levels of HOMOs and LUMOs for the effective electron transfer reactions in the DSCs are considered to be the main reasons for the much lower performances of TUS sensitizers. The conversion efficiency of the DSC with a TUS sensitizer increased with increasing the electron-withdrawing ability of the substituents on the dianionic tridentate ligand. The observed linear relationship between the conversion efficiency and the driving force of the reduction process of the oxidized form of dyes by I(-) suggests that the dye regeneration process is a rate-determining step in the DSCs with TUS sensitizers.

Pyran-Squaraine as Photosensitizers for Dye-Sensitized Solar Cells: DFT/TDDFT Study of the Electronic Structures and Absorption Properties

In an effort to provide, assess, and evaluate a theoretical approach which enables designing efficient donor-acceptor dye systems, the electronic structure and optical properties of pyran-squaraine as donor-acceptor dyes used in dye-sensitized solar cells were investigated. Ground state properties have been computed at the B3LYP/6-31+G**level of theory. The long-range corrected density functionals CAM-B3LYP, PBEPBE, PBE1PBE (PBE0), and TPSSH with 6-311++G**were employed to examine absorption properties of the studied dyes. In an extensive comparison between experimental results and ab initio benchmark calculations, the TPSSH functional with 6-311++G**basis set was found to be the most appropriate in describing the electronic properties for the studied pyran and squaraine dyes. Natural transition orbitals (NTO), frontier molecular orbitals (FMO), LUMO, HOMO, and energy gaps, of these dyes, have been analyzed to show their effect on the process of electron injection and dye regeneration. Interaction between HOMO and LUMO of pyran and squaraine dyes was investigated to understand the recombination process and charge-transfer process involving these dyes. Additionally, we performed natural bond orbital (NBO) analysis to investigate the role of charge delocalization and hyperconjugative interactions in the stability of the molecule.

Theoretical investigation of phenothiazine–triphenylamine-based organic dyes with different π spacers for dye-sensitized solar cells

Three phenothiazine–triphenylamine-based organic dyes (CD-1, CD-2 and CD-3) are designed based on the dye WD-8. The geometries, electronic structures, and electronic absorption spectra of these dyes before and after binding to TiO2 are studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The calculated geometries indicate that these dyes show good steric hindrance effect which is advantage to inhibit the close intermolecular π–π aggregation effectively. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of these dyes could ensure positive effect on the process of electron injection and dye regeneration. The simulated spectra of CD-1∼3 show better absorption than that of WD-8 in the low energy zone. All the calculated results demonstrate that these dyes could be used as potential sensitizers for DSSCs and show better performances than WD-8.

Zinc-Porphyrin Based Dyes for Dye-Sensitized Solar Cells

We have designed seven efficient sensitizers based on the zinc-porphyrin structure for dye sensitized solar cells (DSSCs). The geometries, electronic properties, light harvesting efficiency (LHE), and electronic absorption spectra of these sensitizers are studied using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. We found that the designed sensitizers have smaller HOMO-LUMO energy with broadened and red-shifted absorption bands (300-1100 nm) having high molar extinction coefficient compared to the so far known best sensitizer (YD2-o-C8). The position of HOMO-LUMO energy level of these sensitizers ensures a positive effect on the process of electron injection and dye regeneration. Our theoretical calculations reveal that the new sensitizer can be used as a potential sensitizer for DSSCs compared to YD2-o-C8.

Julolidine dyes with different acceptors and thiophene-conjugation bridge: Design, synthesis and their application in dye-sensitized solar cells

Four novel donor-π conjugated bridge-acceptor (D-π-A) metal-free organic dyes (J1, J2, J3 and J4) with a julolidine moiety as the electron donor for dye-sensitized solar cells (DSSCs) are synthesized. Their absorption spectra, electrochemical and photovoltaic properties were extensively investigated. Transient absorption spectra measurements show that insertion of one thiophene unit and replacing rhodanine-3-acetic acid by cyanoacetic acid can increase the dye regeneration process. Among these dyes, J3 gives the best photoelectric conversion efficiency of 1.52% with Jsc=7.53mAcm–2, Voc=353mV, FF=0.57. Compared with those of J1, the Jsc and the conversion efficiency of solar cell based on J3 are significantly increased by the insertion of a thiophene unit.

Solid state dye solar cell modules

We fabricated the first solid state dye solar cell (SDSC) module using poly(3-hexilthiophene) (P3HT) as Hole Transport Material for the dye regeneration process. Integrated interconnections were obtained following the “Monolithic” architecture for series connections. The fabricated SDSC module has a conversion efficiency of 2.0% on active area. These results represent a promising starting point for an effective up-scaling of SDSC devices.

Organic sensitizers incorporating 3,4-ethylenedioxythiophene as the conjugated bridge: Joint photophysical and electrochemical analysis of photovoltaic performance

Two novel D–π–A organic dyes containing either an indoline donor or a triphenylamine donor with each containing the 3,4-ethylenedioxythiophene unit as the conjugated bridge, were developed for dye-sensitized solar cells. Incorporating 3,4-ethylenedioxythiophene as the conjugated bridge brings several characteristics, such as broadening absorption range and increasing molar extinction coefficients, and improving electron injection with enhancement of the short-circuit photocurrent. The indoline containing dye shows a more negative oxidation potential and a bathochromic shift in absorption spectra than the triphenylamine substituted dye, indicative of the more powerful electron-donating capability of the indoline unit. With coadsorption of chenodeoxycholic acid, the indoline dye based dye-sensitized solar cell exhibited promising conversion efficiency of 6.05%, with a short-circuit photocurrent of 13.23 mA cm−2, open circuit voltage of 642 mV, and a fill factor of 0.711. In the indoline dye system, the dye-sensitized solar cell is workable with the driving force of 150 mV for the dye regeneration process, paving a road toward minimizing energy losses in the dye regeneration process.

Recent advances in alternative cathode materials for iodine-free dye-sensitized solar cells

Over the last decades, iodine-free redox couples have been widely investigated as alternatives for the ubiquitous triiodide/iodide redox shuttle in dye-sensitized solar cells (DSCs). This is mainly motivated by desires to overcome the disadvantages associated with the latter, such as large energy loss in the dye regeneration process, visible light absorption and corrosiveness towards current-collecting metal grids. However, conventional Pt cathodes show poor catalytic activity towards those iodine-free redox couples, resulting in poor fill factors and relatively moderate power conversion efficiencies. The ultimate solution to address this challenge is to develop alternative economical Pt-free catalysts. This review selectively discusses and summarizes the recent advances in novel cathode materials for electrolytes free of iodine-based redox couples, mainly including inorganic transition metal compounds, organic conductive polymers and carbonaceous materials (particularly carbon nanotubes and graphene). Most of these advances have been realized by judiciously controlling the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, this review highlights the importance of matchup between catalysts and the redox couples, and points to crucial issues that should be addressed in the pursuit of developing low-cost and high-catalytic cathodes for DSCs.

Effect of porphyrin loading on performance of dye sensitized solar cells based on iodide/tri-iodide and cobalt electrolytes

Two zinc–porphyrin sensitizers, 1a and 1b, bearing triphenylamine donor groups, were synthesized and their efficiencies measured in nanocrystalline TiO2 dye sensitized solar cells employing iodide/tri-iodide and tris(1,10-phenanthroline) cobalt electrolytes. Optimized sensitization time for the TiO2 photoanode was found to depend on the electrolyte employed: devices based on iodide/tri-iodide showed better efficiencies with shorter sensitization times (1.5 hours) whereas those based on tris(1,10-phenanthroline) cobalt showed better efficiencies with longer sensitization times (6 hours). From UV-Vis absorption spectra it is estimated that there is roughly twice as much dye loaded onto the TiO2 film sensitized for 6 hours compared to the 1.5 hour film. Interfacial processes were probed using transient photovoltage, transient absorption and fluorescence lifetime measurements. The results indicate that sensitization time does not affect either dye regeneration or interfacial recombination processes in the presence of either electrolyte. However, sensitization time does have a considerable impact on device photocurrent, and moreover, the effect is different for the two electrolytes studied. This work demonstrates how device preparation must be tailored carefully depending on the electrolyte red/ox couple used.

Effect of Cation on Dye Regeneration Kinetics of N719-Sensitized TiO2 Films in Acetonitrile-Based and Ionic-Liquid-Based Electrolytes Investigated by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) in the feedback mode has been used to investigate the kinetics of dye regeneration on cis-di(thiocyanato)-bis(2,2′-bipyridyl-4,4″-dicarboxylate) ruthenium(II)-sensitized TiO2 photoelectrochemical electrodes in contact with room temperature ionic liquids electrolytes. Acetonitrile based electrolytes were used for comparison. The data are analyzed with the help of a revised model for relating steady state approach curves in the SECM feedback mode to an effective rate constant for the dye regeneration process. This effective rate constant summarizes the effect of electron transfer kinetics, mass transport of the mediator inside the porous electrode, and recombination processes. We found a remarkable dependence of the dye regeneration on the nature and concentration of deliberately added cations.