Reducing charge recombination losses in solid state dye sensitized solar cells: the use of donor-acceptor sensitizer dyes

Herein we report the application of supramolecular dyes to control charge recombination between photo-injected electrons and oxidized hole-transporting material, resulting in an enhancement in the performance of dye sensitized solar cell devices based upon such dyes.     

Electronic excited state energy transfer,trapping by dimers and fluorescence quenching in concentrated dye solutions: Picosecond transient grating exp

Picosecond transient grating experiments are used to examine electronic excited state dynamics in concentrated dye solutions. A model based on radiationless excited state transport and trapping by dimers describes the phenomena responsible for fluorescence quenching. The trapping rate constant is found to have a cubic concentration dependence. Rhodamine 6G dimer lifetimes in glycerol and ethanol are 830 ps and <50 ps respectively. The difference arises due to the viscosity dependence of the dimer radiationless relaxation rate.     

Efficient dye-sensitized solar cells based on concerted companion dyes: Systematic optimization of thiophene units in the organic dye components

To develop efficient concerted companion (CC) dyes for fabricating high-performance DSSCs, three organic dyes XL1-XL3 have been designed by varying the position and number of the β-hexylthiophene (HT) bridges, and these organic dye units are covalently linked with our previously reported porphyrin dye XW10 to construct the corresponding CC dyes XW74-XW76. Among the organic dyes, XL3 contains two β-hexylthiophene units at both the donor and acceptor parts and thus possesses stronger light-harvesting capability in the green light region. Because of the most complementary absorption between XL3 and XW10 as well as the excellent photovoltaic behavior of the individual XL3 dye, the corresponding CC dye XW76 affords the best PCE (10.78%) among all the CC dyes. Upon coadsorption with CDCA, XW76 affords a highest PCE of 11.35%, which outperforms the previous cosensitization system of XW10+WS-5. This work provides an approach for developing efficient DSSCs based on CC dyes composed of an organic dye unit with suitable π spacers inserted at appropriate positions.     

Stepped light-induced transient measurements of photocurrent and voltage in dye-sensitized solar cells: application for highly viscous electrolyte systems

To measure electron diffusion coefficients (D) and electron lifetimes (tau) of dye-sensitized solar cells (DSC), we introduced stepped light-induced transient measurements of photocurrent and voltage (SLIM-PCV), which can simplify the optical setup and reduce measurement time in comparison to conventional time-of-flight and frequency-modulated measurements. The method was applied to investigate the influence of the viscosity of a thermally stable high-boiling-point solvent on the energy conversion efficiency of DSCs. By systematic study of the influence of the viscosity, the species of cations as the counter charge of I(-)/I(3)(-), and the concentrations of electrolytes, we concluded that a lower dye cation reduction rate due to slower iodine diffusion is a limiting factor for a highly viscous electrolyte system. On the other hand, comparable values of D and increased values of tau were observed in a highly viscous electrolyte. By employing 0.5 M TBAI and 0.05 M I(2) in propylene carbonate, the efficiency of the DSC became comparable to that of a DSC using conventional electrolytes consisting of LiI, imidazolium iodide, and 4-tert-butylpyridine in methoxyacetonitrile. The simultaneous evaluation of D and tau through the appropriately simple measurement realizes fast optimization of the efficient and reliable DSC composed of thermally stable but often viscous electrolytes.     

Molecular engineering and theoretical investigation of organic sensitizers based on indoline dyes for quasi-solid state dye-sensitized solar cells

Novel indoline dyes, I-1-I-4, with structural modification of π-linker group in the D-π-A system have been synthesized and fully characterized. Molecular engineering through expanding the π-linker segment has been performed. The ground and excited state properties of the dyes have been studied by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). Larger π-conjugation linkers would lead to broader spectral response and higher molar extinction coefficient but would decrease dye-loaded amount on TiO(2) electrode and LUMO level. While applied in DSSCs, the variation trends in short-circuit current density (J(sc)) and open-circuit voltage (V(oc)) were observed to be opposite to each other. The internal reasons were studied by experimental data and theoretical calculations in detail. Notably, I-2 showed comparable photocurrent values with liquid and quasi-solid state electrolyte, which suggested through molecular engineering of organic sensitizers the dilemma between optical absorption and charge diffusion lengths can be balanced well. Through studies of photophysical, electrochemical, and theoretical calculation results, the internal relations between chemical structure and efficiency have been revealed, which serve to enhance our knowledge regarding design and optimization of new sensitizers for quasi-solid state DSSCs, providing a powerful strategy for prediction of photovoltaic performances.     

Photocurrents of solar cells sensitized by aggregate-forming polyenes: Enhancement due to suppression of singlet–triplet annihilation by lowering of dye concentration or light intensity

Titania-based Grätzel-type solar cells were fabricated by the use of polyene dyes with various transition dipole moments. In the dye having the largest transition dipole among the samples, an aggregate was readily formed through dispersive interaction, and the photocurrent was increased when the dye concentration or the light intensity was lowered. This observation was ascribed to the suppression of the singlet–triplet annihilation reaction. In the dye having the smallest transition dipole, there was no sign of aggregate formation, and the photocurrent was decreased when the dye concentration or the light intensity was lowered.     

Dye-sensitized solar cells based on oriented TiO2 nanotube arrays: transport, trapping, and transfer of electrons

Dye-sensitized solar cells fabricated using ordered arrays of titania nanotubes (tube lengths 5, 10, and 20 microm) grown on titanium have been characterized by a range of experimental methods. The collection efficiency for photoinjected electrons in the cells is close to 100% under short circuit conditions, even for a 20 microm thick nanotube array. Transport, trapping, and back transfer of electrons in the nanotube cells have been studied in detail by a range of complementary experimental techniques. Analysis of the experimental results has shown that the electron diffusion length (which depends on the diffusion coefficient and lifetime of the photoinjected electrons) is of the order of 100 microm in the titania nanotube cells. This is consistent with the observation that the collection efficiency for electrons is close to 100%, even for the thickest (20 microm) nanotube films used in the study. The study revealed a substantial discrepancy between the shapes of the electron trap distributions measured experimentally using charge extraction techniques and those inferred indirectly from transient current and voltage measurements. The discrepancy is resolved by introduction of a numerical factor to account for non-ideal thermodynamic behavior of free electrons in the nanostructured titania.     

The influence of Yb, B, and Ga-doped Er3+:Y3Al5O12 on solar light photocatalytic activity of TiO2 in degradation of organic dyes

Five up-conversion luminescence agents (Er³⁺:Y₃Al₅O₁₂, Er³⁺:Yb _n Y_{3 ? n} Al₅O₁₂, Er³⁺:Y₃B _a Al_{5 ? a} O₁₂, Er³⁺:Y₃Ga _b Al_{5 ? b} O₁₂, and Er³⁺:Yb _n Y_{3 ? n} B _a Ga _b Al_{5 ? a ? b} O₁₂) were synthesized using sol-gel method and then the corresponding coated composites (Er³⁺:Y₃Al₅O₁₂/TiO₂, Er³⁺:Yb _n Y_3?n Al₅O₁₂/TiO₂, Er³⁺:Y₃B _a Al_{5 ? a} O₁₂/TiO₂, Er³⁺:Y₃Ga _b Al_{5 ? b} O₁₂/TiO₂, and Er³⁺:Yb _n Y_{3 ? n} B _a Ga _b Al_{5 ? a ? b} O₁₂/TiO₂) as photocatalysts were prepared by sol-gel coating process. The XRD and SEM were used to confirm the crystalline phase and surface morphology. The UV-vis absorption and fluorescence-emission spectra were used to research the effect of doping category and amount on the up-conversion emission ability. The photocatalytic activities were detected through the degradation of Acid Red B dye in aqueous solution. Some key parameters of catalyst amount and initial concentration of organic dye on solar light photocatalytic degradation were also examined. The extensive feasibility of prepared photocatalysts in solar light degradation was detected by other organic dyes. The results suggest that the photocatalysts can be widely used in sewage treatment.     

Challenges in the simulation of dye-sensitized ZnO solar cells: quantum confinement, alignment of energy levels and excited state nature at the dye/semiconductor interface

We report a first principles density functional theory/time-dependent density functional theory (DFT/TDDFT) computational investigation on a prototypical perylene dye anchored to realistic ZnO nanostructures, approaching the size of the ZnO nanowires used in dye-sensitized solar cells devices. DFT calculations were performed on (ZnO)(n) clusters of increasing size, with n up to 222, of 1.3 × 1.5 × 3.4 nm dimensions, and for the related dye-sensitized models. We show that quantum confinement in the ZnO nanostructures substantially affects the dye/semiconductor alignment of energy levels, with smaller ZnO models providing unfavourable electron injection. An increasing broadening of the dye LUMO is found moving to larger substrates, substantially contributing to the interfacial electronic coupling. TDDFT excited state calculations for the investigated dye@(ZnO)(222) system are fully consistent with experimental data, quantitatively reproducing the red-shift and broadening of the visible absorption spectrum observed for the ZnO-anchored dye compared to the dye in solution. TDDFT calculations on the fully interacting system also introduce a contribution to the dye/semiconductor admixture, due to configurational excited state mixing. Our results highlight the importance of quantum confinement in dye-sensitized ZnO interfaces, and provide the fundamental insight lying at the heart of the associated DSC devices.     

Recent advances in intermixed phase of organic solar cells: Characterization,regulating strategies and device applications

Organic solar cells (OSCs) have unique advantages of low-cost solution processing, light weight, flexibility, and semitransparency, which is a promising photovoltaic technology. The intermixed phase plays a key role in determining the power conversion efficiencies (PCE) of OSCs. The intermixed phase is an amorphous region, where the donor and acceptor mix at the molecular level. Great efforts have been devoted to optimize the content and the composition of the intermixed phase. This perspective focuses on the functions of intermixed phase and elaborates the relationship between intermixed phase behavior and photophysical process, in particular, the exciton dissociation and charge transport. Then the characterization methods, including quantitative and qualitative characterizations, for the content and composition of intermixed phases are introduced. Meanwhile, this review also introduces the strategies to control the intermixed phase behavior, such as adjusting the miscibility between donor and acceptor, changing the ratio of donor to acceptor, regulating the crystallinity and so on. Moreover, representative examples are given and discussed to understand the key parameters on tuning the intermixed phase behavior. Finally, a future controlling and development of intermixed phase behavior is briefly outlooked, which may help to achieve high PCE of OSCs.     

2,7-Diaminofluorene-based organic dyes for dye-sensitized solar cells: effect of auxiliary donor on optical and electrochemical properties

New organic dyes containing a diarylaminofluorene unit as an electron donor and cyanoacrylic acid as acceptor and anchoring group in a donor-π-donor-π-acceptor architecture have been synthesized and characterized as sensitizers for nanocrystalline TiO(2)-based dye-sensitized solar cells. They have shown three major electronic absorptions originating from the π-π* and charge-transfer transitions covering the broad visible range (250-550 nm) in solution. The charge-transfer transition of the dyes exhibited negative solvatochromism, suggesting a polarized ground state. They have also displayed acidochromism in solution owing to the presence of a protonation-deprotonation equilibrium. On comparison with the triphenylamine and carbazole-based parent dyes (E)-2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid and (E)-2-cyano-3-(9-ethyl-9H-carbazol-3-yl)acrylic acid they exhibited longer wavelength absorptions and facile oxidation, indicating the stronger electron-donating ability of the auxiliary chromophores. In addition, they exhibited nearly two times larger light-to-electron conversion efficiency under simulated AM 1.5 G irradiation (100 mW cm(-2)) with an aperture mask when compared to the parent dyes. Among the new dyes, the one containing the naphthylphenylamine segment showed better device characteristics attributable to the higher HOMO energy level which probably facilitates the regeneration of the dye and effective suppression of the back reaction of the injected electrons with the I(3)(-) in the electrolyte. The optical properties of the dyes were modeled using TDDFT simulations employing different theoretical models (B3LYP, CAM-B3LYP, and MPW1K), and the best correlations with the observed parameters have been found for CAM-B3LYP and MPW1K calculations. The electron lifetimes extracted from the electrochemical impedance measurements of the dye-sensitized solar cells were used to interpret the solar cell efficiency alternations.     

Bridge effect on the charge transfer and optoelectronic properties of triphenylamine-based organic dye sensitized solar cells: theoretical approach

Research on Chemical Intermediates - In this work, a series of six organic dyes-sensitized solar cells (DSSCs) combining various π-bridges with a fixed donor (triphenylamine) and a fixed...     

Glass-encapsulated light harvesters: more efficient dye-sensitized solar cells by deposition of self-aligned, conformal, and self-limited silica layers

A major loss mechanism in dye-sensitized solar cells (DSCs) is recombination at the TiO(2)/electrolyte interface. Here we report a method to reduce greatly this loss mechanism. We deposit insulating and transparent silica (SiO(2)) onto the open areas of a nanoparticulate TiO(2) surface while avoiding any deposition of SiO(2) over or under the organic dye molecules. The SiO(2) coating covers the highly convoluted surface of the TiO(2) conformally and with a uniform thickness throughout the thousands of layers of nanoparticles. DSCs incorporating these selective and self-aligned SiO(2) layers achieved a 36% increase in relative efficiency versus control uncoated cells.     

Kinetic investigations on the UV-induced photopolymerization of a diacrylate by time-resolved FTIR spectroscopy: the influence of photoinitiator concentration, light intensity and temperature

Time-resolved FTIR-ATR spectroscopy was used to study the kinetics of the photopolymerization of a diacrylate using a morpholino ketone as photoinitiator. The curing reaction was induced by monochromatic UV radiation with a wavelength of 313 nm. The influence of photoinitiator concentration [PI], light intensity I₀, and temperature on the polymerization rate R_p and the double bond conversion was investigated. The dependence of R_p on [PI] and I₀, respectively, was found to fit with theoretical predictions very well. In contrast, an increase of the temperature was found to have no effect on R_p.     

Photophysical studies of dipolar organic dyes that feature a 1,3-cyclohexadiene conjugated linkage: the implication of a twisted intramolecular charge-transfer state on the efficiency of dye-sensitized solar cells

A detailed study of the synthesis and photophysical properties of a new series of dipolar organic photosensitizers that feature a 1,3‐cyclohexadiene moiety integrated into the π‐conjugated structural backbone has been carried out. Dye‐sensitized solar cells (DSSCs) based on these structurally simple dyes have shown appreciable photo‐to‐electrical energy conversion efficiency, with the highest one up to 4.03 %. Solvent‐dependent fluorescence studies along with the observation of dual emission on dye 4 b and single emission on dyes 4 a and 32 suggest that dye 4 b possesses a highly polar emissive excited state located at a lower‐energy position than at the normal emissive excited state. A detailed photophysical investigation in conjunction with computational studies confirmed the twisted intramolecular charge‐transfer (TICT) state to be the lowest emissive excited state for dye 4 b in polar solvents. The relaxation from higher‐charge‐injection excited states to the lowest TICT state renders the back‐electron transfer process a forbidden one and significantly retards the charge recombination to boost the photocurrent. The electrochemical impedance under illumination and transient photovoltage decay studies showed smaller charge resistance and longer electron lifetime in 4 b ‐based DSSC compared to the DSSCs with reference dyes 4 a and 32 , which further illustrates the positive influence of the TICT state on the performance of DSSCs.     

Conjugate spacer effect on molecular structures and absorption spectra of triphenylamine dyes for sensitized solar cells: density functional theory calculations

The molecular structures and absorption spectra of triphenylamine dyes containing variable thiophene units as the spacers (TPA1-TPA3) were investigated by density functional theory (DFT) and time-dependent DFT. The calculated results indicate that the strong conjugation is formed in the dyes and the length of conjugate bridge increases gradually with the increased thiophene spacers. The interfacial charge transfer between the TiO2 electrode and TPA1-TPA3 are electron injection processes from the excited dyes to the semiconductor conduction band. The simulated absorption bands are assigned to π→π* transitions, which exhibit appreciable red-shift with respect to the experimental bands due to the lack of direct solute-solvent interaction and the inherent approximations in TD-DFT. The effect of thiophene spacers on the molecular structures, absorption spectra and photovoltaic performance were comparatively discussed and points out that the choice of appropriate conjugate bridge is very important for the design of new dyes with improved performance.     

Mg(OOCCH(3))(2) as an electrolyte additive for quasi-solid dye-sensitized solar cells: with the purpose of enhancing both the photovoltage and photocurrent by modifying the TiO(2)/dye/electrolyte interfaces

The interface modification effect within quasi-solid dye-sensitized solar cells and the photovoltaic performance were investigated after the introduction of Mg(OOCCH(3))(2) as an additive into a polymer gel electrolyte. Electrochemical impedance spectroscopy showed that the addition of Mg(OOCCH(3))(2) into the polymer gel electrolyte can efficiently retard charge recombination at the TiO(2)/electrolyte interface. Mg(OOCCH(3))(2) in the electrolyte can also contribute to the enhancement of the incident photon-to-electron conversion efficiency by modifying the dye molecules. This results in an improvement in the photovoltage and photocurrent due to a barrier layer at the TiO(2)/electrolyte interface and the promotion of charge injection at the dye/TiO(2) interface, respectively. Photovoltaic measurements reveal that a conversion efficiency enhancement from 4.05% to 4.96% under 100 mW cm(-2) is obtained after the amount of Mg(OOCCH(3))(2) added was optimized.     

Preparation of Er3+:YAlO3/Fe-doped ZnO composite and investigation on solar light photocatalytic degradation of organic dyes

The Er³⁺:YAlO₃/Fe-doped ZnO composite, a new photocatalyst which could effectively utilize visible light, was prepared. In succession, the Er³⁺:YAlO₃/Fe-doped ZnO was characterized by XRD and SEM, respectively. Acid Red B dyes, was degraded under solar light irradiation to evaluate the photocatalytic activity of the Er³⁺:YAlO₃/Fe-doped ZnO. In addition, the effects of Er³⁺:YAlO₃ content, heat-treatment temperature and time on the photocatalytic activity of Er³⁺:YAlO₃/Fe-doped ZnO were reviewed. Otherwise, the effect of initial dye concentration, Er³⁺:YAlO₃/Fe-doped ZnO amount and solar light irradiation time on the photocatalytic degradation of Acid Red B were also investigated. It was found that the photocatalytic activity of Er³⁺:YAlO₃/Fe-doped ZnO is much higher than that of Fe-doped ZnO and pure ZnO for the similar system. Perhaps, the use of the Er³⁺:YAlO₃/Fe-doped ZnO may provide a new way to take advantage of ZnO in sewage treatment aspects using solar energy.     

Synthesis, structure, and properties of [Pt(II)(diimine)(dithiolate)] dyes with 3,3'-, 4,4'-, and 5,5'-disubstituted bipyridyl: applications in dye-sensitized solar cells

A family of [Pt(II)(diimine)(dithiolate)] complexes of general formula [Pt{X,X'(CO(2)R)(2)-2,2'-bipyridyl}(maleonitriledithiolate)] (where X = 3, 4, or 5 and R = H or Et) have been synthesized, spectroscopically and electrochemically characterized, and attached to a TiO(2) substrate to be tested as solar cell sensitizers. A single-crystal X-ray structure showing a large torsion angle between the bipyridyl rings was determined for [Pt{3,3'(CO(2)Et)(2)-2,2'-bipyridyl}(maleonitriledithiolate)].MeCN. The effect of changing the position of the bipyridyl substituents from 3,3' to 4,4' and 5,5' is discussed with reference to structural and electronic changes seen within the different members of the family of molecules. The first UV/vis/NIR spectroelectrochemical study of complexes of this general formula is discussed. All three complexes (where R = H) were tested as solar cell sensitizers, with the 3,3'-disubstituted bipyridyl complex giving an intermediate dye loading value but superior photovoltaic performance to those of the other two. The performance of this sensitizer is then compared with that of a well-known Ru polypyridyl sensitizer, the ditetrabutylammonium salt of [RuL(2)(NCS)(2)] (L = 2,2'-bipyridyl-4,4'-dicarboxylato), commonly called N719.