1,1'-Bis-(diphenylphosphino)ferrocene appended d8- and d10-configuration based thiosquarates: the molecular and electronic configurational insights into their sensitization and co-sensitization properties for dye sensitized solar cells

Three new d8- and d10-configuration based 1,1'-bis-(diphenylphosphino)ferrocene (dppf) appended thiosquarates complexes with general composition [M(mtsq)2dppf] (M = Ni2+ (NiL2); Zn2+ (ZnL2) and Cd2+ (CdL2)) (mtsq = 3-ethoxycyclobutenedione-4-thiolate) have been synthesized and characterized spectroscopically as well as in case of NiL2 by single crystal X-ray diffraction technique. The single crystal X-ray analysis reveals square planar geometry around Ni(II) in NiL2, where Ni(II) coordinates with two sulfur centres of two mtsq ligands in monodentate fashion and two phosphorus of a dppf ligand in chelating mode. The supramolecular architecture of NiL2 is sustained by intermolecular C-H⋯O interactions to form one-dimensional chain. Further, the application of these newly synthesized complexes as sensitizers and co-sensitizers/co-absorbents with ruthenium based N719 sensitizer in dye-sensitized solar cells (DSSCs) have been explored. The DSSC set-up based on NiL2 offers best photovoltaic performance with photovoltaic efficiency (η) 5.12%, short-circuit current (Jsc) 11.60 mA cm-2, open circuit potential (Voc) 0.690 V and incident photon to current conversion efficiency (IPCE) 63%. In co-sensitized DSSC set-up, ZnL2 along with state-of-the-art N719 dye displays best photovoltaic performance with η 6.65%, Jsc 14.47 mA cm-2, Voc 0.729 V and IPCE 69%, thereby showing an improvement by 15.25% in photovoltaic efficiency in comparison to the photovoltaic efficiency of N719 sensitized DSSC set-up. Variation in co-sensitization behaviour have been ascribed to the differences in the excited state energy level of co-sensitizers. The ZnL2 and CdL2 have a higher energy level position than N719 dye, allowing efficient electron transfer to N719 during light irradiation, while excited state of NiL2 is lower than N719 dye, preventing photoexcited electron transfer to N719, resulting in its lowest overall efficiency among the three co-sensitized DSSC setups.

A Review of Transition Metal Sulfides as Counter Electrodes for Dye-Sensitized and Quantum Dot-Sensitized Solar Cells

Third-generation solar cells, including dye-sensitized solar cells (DSSCs) and quantum dot-sensitized solar cells (QDSSCs), have been associated with low-cost material requirements, simple fabrication processes, and mechanical robustness. Hence, counter electrodes (CEs) are a critical component for the functionality of these solar cells. Although platinum (Pt)-based CEs have been dominant in CE fabrication, they are costly and have limited market availability. Therefore, it is important to find alternative materials to overcome these issues. Transition metal chalcogenides (TMCs) and transition metal dichalcogenides (TMDs) have demonstrated capabilities as a more cost-effective alternative to Pt materials. This advantage has been attributed to their strong electrocatalytic activity, excellent thermal stability, tunability of bandgap energies, and variable crystalline morphologies. In this study, a comprehensive review of the major components and working principles of the DSSC and QDSSC are presented. In developing CEs for DSSCs and QDSSCs, various TMS materials synthesized through several techniques are thoroughly reviewed. The performance efficiencies of DSSCs and QDSSCs resulting from TMS-based CEs are subjected to in-depth comparative analysis with Pt-based CEs. Thus, the power conversion efficiency (PCE), fill factor (FF), short circuit current density (Jsc) and open circuit voltage (Voc) are investigated. Based on this review, the PCEs for DSSCs and QDSSCs are found to range from 5.37 to 9.80% (I-/I3- redox couple electrolyte) and 1.62 to 6.70% (S-2/Sx- electrolyte). This review seeks to navigate the future direction of TMS-based CEs towards the performance efficiency improvement of DSSCs and QDSSCs in the most cost-effective and environmentally friendly manner.

Efficient dye-sensitized solar cell fabricated using a less toxic alternative to electrolyte and charge collector

The photovoltaic investigation of novel and efficient dye-sensitized solar cells is discussed in this paper. Ruthenium-based synthetic dye (N3) is used as a sensitizer. A less toxic alternative is suggested for toxic indium-based glass substrates by using aluminum-doped zinc oxide (AZO) and fluorine-doped tin oxide (FTO) as charge collectors. Moreover, the electrolyte used is a mixture of polymer (polyaniline) and an iodide-triiodide couple to go for the approach involving a lower amount of iodine. In the paper study, on the extent of light, absorption of dye is done by ultraviolet-visible (UV-vis) spectroscopy. The morphological study of sheets is done using scanning electron microscopic (SEM) images to understand the binding of titania on photoanode. Photovoltaic characteristics (I-V) and induced photon to current efficiency (IPCE) measurements, and light harvesting efficiency (LHE) are also investigated. The highest power conversion efficiency of 6.18% is observed in the suggested fabricated green solar cell. Hence, more efficient, indium-free, and novel cell is fabricated by the usage of different charge collector substrates and quasi solid-state electrolytes.

Solar energy conversion using first row d-block metal coordination compound sensitizers and redox mediators

The use of renewable energy is essential for the future of the Earth, and solar photons are the ultimate source of energy to satisfy the ever-increasing global energy demands. Photoconversion using dye-sensitized solar cells (DSCs) is becoming an established technology to contribute to the sustainable energy market, and among state-of-the art DSCs are those which rely on ruthenium(ii) sensitizers and the triiodide/iodide (I3 -/I-) redox mediator. Ruthenium is a critical raw material, and in this review, we focus on the use of coordination complexes of the more abundant first row d-block metals, in particular copper, iron and zinc, as dyes in DSCs. A major challenge in these DSCs is an enhancement of their photoconversion efficiencies (PCEs) which currently lag significantly behind those containing ruthenium-based dyes. The redox mediator in a DSC is responsible for regenerating the ground state of the dye. Although the I3 -/I- couple has become an established redox shuttle, it has disadvantages: its redox potential limits the values of the open-circuit voltage (V OC) in the DSC and its use creates a corrosive chemical environment within the DSC which impacts upon the long-term stability of the cells. First row d-block metal coordination compounds, especially those containing cobalt, and copper, have come to the fore in the development of alternative redox mediators and we detail the progress in this field over the last decade, with particular attention to Cu2+/Cu+ redox mediators which, when coupled with appropriate dyes, have achieved V OC values in excess of 1000 mV. We also draw attention to aspects of the recyclability of DSCs.

Ferrocene Derivatives Functionalized with Donor/Acceptor (Hetero)Aromatic Substituents: Tuning of Redox Properties

A series of functionalized ferrocene derivatives carrying electron-donor and electron-withdrawing (hetero)aromatic substituents has been designed as potential alternative electrolyte redox couples for dye-sensitized solar cells (DSSC). The compounds have been synthesized and fully characterized in their optical and electrochemical properties. A general synthetic approach that implies the use of a microwave assisted Suzuki coupling has been developed to access a significative number of compounds. The presence of different electron-rich and electron-poor substituents provided fine tuning of optical properties and energy levels. HOMO and LUMO energy values showed that the substitution of one or two cyclopentadienyl rings of ferrocene can be successfully exploited to increase the maximum attainable voltage from a standard DSSC device using TiO2 as a semiconductor, opening the way to highly efficient, non-toxic, and cheap redox shuttles to be employed in solar energy technologies.

tert-Butylpyridine Coordination with [Cu(dmp)2]2+/+ Redox Couple and Its Connection to the Stability of the Dye-Sensitized Solar Cell

Cu(I)/(II) complex redox couples in dye-sensitized solar cell (DSSC) are of particular interest because of their low reorganization energy between Cu(I) and Cu(II), which minimizes the potential loss during sensitizer regeneration, thus allowing the open-circuit voltage of the device to go over 1.0 V. However, Cu(I)/(II)-based redox couples are reported to coordinate with 4-tert-butylpyridine (TBP), which is a standard additive in the electrolyte, and this is believed to account for the poor durability of a Cu(I)/(II)-based DSSCs. Despite TBP coordination on Cu(I)/(II) complexes are confirmed in the literature, its detailed mechanism is yet to be directly proven. In addition, how TBP coordination with Cu(I)/(II) complexes affects the stability of the device is never reported. Here, we choose bis(2,9-dimethyl-1,10-phenanthroline) copper(I)/(II) ([Cu(dmp)₂^2+/+]) as the modeling redox couple to investigate its interaction with TBP. It is found that [Cu(dmp)₂⁺] is resistive to TBP coordination but could form three new TBP-coordinated compounds. Moreover, it is also confirmed their electrochemical activity on Pt catalyst and mass transfer capability are both demoted significantly. As a result, serious fill factor loss is observed on the stability trail while short-circuit current density and open-circuit voltage are relatively unaffected. This unique degradation pattern may resemble a feature of Cu(I)/(II)-based redox couple after TBP poisoning.

The molecular engineering, synthesis and photovoltaic studies of a novel highly efficient Ru(ii) complex incorporating a bulky TPA ancillary ligand for DSSCs: donor versus π-spacer effects

The IA-7 complex was applied in DSSCs.

High efficiency dye-sensitized solar cells with VOC–JSC trade off eradication by interfacial engineering of the photoanode|electrolyte interface

Interfacial modification of the photoanode|electrolyte interface using oleic acid (OA) is thoroughly investigated in this present study. The overall photoconversion efficiency of 11.8% was achieved under the illumination of 100 mW cm⁻² with an optical filter of AM 1.5 G. OA molecules were meant to be adsorbed on to the vacant areas of the TiO₂ and the OA moieties leached out the aggregated C106 dye molecules from the TiO₂ surface. There was a strong spectral overlap between the absorption spectrum of donor (OA) and the emission spectrum of acceptor (C106), leading to effective Förster Resonance Energy Transfer (FRET) between OA and C106 and suggested an excellent opportunity to improve the photovoltaic performances of DSSCs. UV-vis DRS and UPS analysis revealed that OA molecules created new surface (mid-gap energy) states (SS) in TiO₂ and these SS played a major role in the electron transport kinetics. Mott–Schottky analysis of DSSCs under dark conditions was carried out to find the shift in the flat band potential of TiO₂ upon OA modification. Surprisingly, no trade off between V_OC and J_SC was observed after interfacial modification with OA. The dynamics of charge recombination and electron transport at the photoanode|electrolyte interface were studied in detail using electrochemical impedance spectroscopy.

V_OC–J_SC trade off is eliminated. Newly created surface states by OA in TiO₂ facilitated the charge transfer kinetics.

Interfacial Modification of Photoanode|Electrolyte Interface Using Oleic Acid Enhancing the Efficiency of Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are useful devices in converting renewable solar energy into electrical energy. In DSSCs, the triiodide reduction at the surface of TiO₂ is one of the detrimental processes that limit the realization of high efficiencies of the device. To alleviate the active sites available on the semiconductor surface for this detrimental process, the interfacial modification of the dye-adsorbed TiO₂|electrolyte interface has been attempted by coadsorption of oleic acid (OA) over the TiO₂ surface. Thus, the modified cell exhibited a higher efficiency (η) of 12.9% under one sun illumination when compared with that of the unmodified cell (η = 11.1%). To provide an insight into the OA anchoring and dynamics of electron transport at the photoanode|electrolyte interface, molecular spectroscopic and electrochemical impedance spectroscopic analyses were carried out. A red shift in the optical absorption spectrum was observed after the addition of OA to dye-adsorbed TiO₂. The binding of OA to TiO₂ surface was found to be through bridging bidentate type. Mott-Schottky analyses of the DSSCs under dark conditions were made to probe the shift in the Fermi level of TiO₂ upon OA modification. In addition, the Förster resonance energy transfer (FRET) has been found between OA and N719 dye. Thus, the red shift in the optical absorption, enhanced electron-transfer kinetics, and FRET contributes to the observed enhancement in the efficiency of the device containing OA-modified photoanode.

Efficient and Stable Dye-Sensitized Solar Cells Based on a Tetradentate Copper(II/I) Redox Mediator

The identification of an efficient and stable redox mediator is of paramount importance for commercialization of dye-sensitized solar cells (DSCs). Herein, we report a new class of copper complexes containing diamine-dipyridine tetradentate ligands (L1 = N, N'-dibenzyl- N, N'-bis(pyridin-2-ylmethyl)ethylenediamine; L2 = N, N'-dibenzyl- N, N'-bis(6-methylpyridin-2-ylmethyl)ethylenediamine) as redox mediators in DSCs. Devices constructed with [Cu(L2)]^2+/+ redox couple afford an impressive power conversion efficiency (PCE) of 9.2% measured under simulated one sun irradiation (100 mW cm^-2, AM 1.5G), which is among the top efficiencies reported thus far for DSCs with copper complex-based redox mediators. Remarkably, the excellent air, photo, and electrochemical stability of the [Cu(L2)]^2+/+ complexes renders an outstanding long-term stability of the whole DSC device, maintaining ∼90% of the initial efficiency over 500 h under continuous full sun irradiation. This work unfolds a new platform for developing highly efficient and stable redox mediators for large-scale application of DSCs.

BODIPYs for Dye-Sensitized Solar Cells

BODIPY, abbreviation of boron-dipyrromethene, is one class of robust organic molecules that has been used widely in bioimaging, sensing, and logic gate design. Recently, BODIPY dyes have been explored for dye-sensitized solar cells (DSCs). Studies demonstrate their potential as light absorbers for the conversion of solar energy to electricity. However, their photovoltaic performance is inferior to many other dyes, including porphyrin dyes. In this review, several synthetic strategies of BODIPY dyes for DSCs and their further functionalization are described. The photophysical properties of dye molecules and their photovoltaic performances in DSCs are summarized. We aim to provide readers a clear picture of the field and expect to shed light on the next generation of BODIPY dyes for their applications in solar energy conversion.

Iodine-Pseudohalogen Ionic Liquid-Based Electrolytes for Quasi-Solid-State Dye-Sensitized Solar Cells

In the current work, novel symmetrically alkyl-substituted imidazolium-based ionic liquids have been synthesized featuring either iodide (I^-) or selenocyanate (SeCN^-) as counteranions. Physicochemical assays based on spectroscopy and electrochemistry techniques have been performed to identify the best ionic liquid for application as electrolytes in quasi-solid-state dye-sensitized solar cells (qssDSSC). The latter were mixed with additives such as 4-tert-butylpyridine (4tbpy) and guanidinium thiocyanate (GuSCN) to optimize electrode surface coverage, ionic diffusion, and dye regeneration. In addition, we demonstrate that electrolytes containing a mixture of I₂ and (SeCN)₂ enhance the open-circuit voltage of the final quasi-solid-state device by up to 70 mV. As such, iodine-pseudohalogen electrolytes reveal in qssDSSCs a good balance between dye regeneration and hole transport and, in turn, enhance the overall solar energy conversion efficiency by 70% with respect to reference qssDSSCs with iodine-based electrolytes. Finally, devices with the iodine-pseudohalogen electrolyte show a 1000 h stable efficiency of 7-8% under outdoor temperature operation conditions and 1 sun illumination.

Two-dimensional transition metal dichalcogenide-based counter electrodes for dye-sensitized solar cells

Dye-sensitized solar cell using counter electrode based on transition metal dichalcogenides.

Supramolecular Hemicage Cobalt Mediators for Dye-Sensitized Solar Cells

A new class of dye-sensitized solar cells (DSSCs) using the hemicage cobalt-based mediator [Co(ttb)]2+/3+ with the highly preorganized hexadentate ligand 5,5'',5''''-((2,4,6-triethyl benzene-1,3,5-triyl)tris(ethane-2,1-diyl))tri-2,2'-bipyridine (ttb) has been fully investigated. The performances of DSSCs sensitized with organic D-π-A dyes utilizing either [Co(ttb)]2+/3+ or the conventional [Co(bpy)3 ]2+/3+ (bpy=2,2'-bipyridine) redox mediator are comparable under 1000 W m-2 AM 1.5 G illumination. However, the hemicage complexes exhibit exceptional stability under thermal and light stress. In particular, a 120-hour continuous light illumination stability test for DSSCs using [Co(ttb)]2+/3+ resulted in a 10 % increase in the performance, whereas a 40 % decrease in performance was found for [Co(bpy)3 ]2+/3+ electrolyte-based DSSCs under the same conditions. These results demonstrate the great promise of [Co(ttb)]2+/3+ complexes as redox mediators for efficient, cost-effective, large-scale DSSC devices.

Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage

Redox mediators play a major role determining the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the reduction of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO₂ and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu^(II/I)(dmby)₂TFSI_2/1 (0.97 V vs SHE, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu^(II/I)(tmby)₂TFSI_2/1 (0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu^(II/I)(dmp)₂TFSI_2/1 (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 μs) was observed for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu^(II/I)(tmby)₂TFSI_2/1 as evidenced by longer lifetimes. The solar-to-electrical power conversion efficiencies for [Cu(tmby)₂]^2+/1+, [Cu(dmby)₂]^2+/1+, and [Cu(dmp)₂]^2+/1+ based electrolytes were 10.3%, 10.0%, and 10.3%, respectively, using the organic Y123 dye under 1000 W m^-2 AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.

Achievement of over 1.4 V photovoltage in a dye-sensitized solar cell by the application of a silyl-anchor coumarin dye

A dye-sensitized solar cell (DSSC) fabricated by using a novel silyl-anchor coumarin dye with alkyl-chain substitutes, a Br₃⁻/Br⁻ redox electrolyte solution containing water, and a Mg²⁺-doped anatase-TiO₂ electrode with twofold surface modification by MgO and Al₂O₃ exhibited an open-circuit photovoltage over 1.4 V, demonstrating the possibility of DSSCs as practical photovoltaic devices.

Bi-anchoring Organic Dyes that Contain Benzimidazole Branches for Dye-Sensitized Solar Cells: Effects of π Spacer and Peripheral Donor Groups

Benzimidazole-branched bi-anchoring organic dyes that contained triphenylamine/phenothiazine donors, 2-cyanoacrylic acid acceptors, and various π linkers were synthesized and examined as sensitizers for dye-sensitized solar cells. The structure-activity relationships in these dyes were systematically investigated by using absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The wavelength of the absorption peak was more-heavily influenced by the nature of the π linker than by the nature of the donor. For a given donor, the absorption maximum (λmax ) was red-shifted on changing the π linker from phenyl to 2,2'-bithiophene, whilst the dyes that contained triphenylamine units displayed higher molar extinction coefficients (ϵ) than their analogous phenothiazine-based triphenylamine dyes, which led to good light-harvesting properties in the triphenylamine-based dyes. Electrochemical data for the dyes indicated that the triphenylamine-based dyes possessed relatively low-lying HOMOs, which could be beneficial for suppressing back electron transfer from the conduction band of TiO2 to the oxidized dyes, owing to facile regeneration of the oxidized dye by the electrolyte. The best performance in the DSSCs was observed for a dye that possessed a triphenylamine donor and 2,2'-bithiophene π linkers. Electron impedance spectroscopy (EIS) studies revealed that the use of triphenylamine as the donor and phenyl or 2,2'-bithiophene as the π linkers was beneficial for disrupting the dark current and charge-recombination kinetics, which led to a long electron lifetime of the injected electrons in the conduction band of TiO2 .

Ionic Liquid Crystals: Versatile Materials

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Highly efficient dye-sensitized solar cells based on low concentration organic thiolate/disulfide redox couples

An organic thiolate/disulfide (BMIT/BMIDT) redox couple was synthesized and applied in DSSCs with very low concentration.

Redox Active Compounds in Controlled Radical Polymerization and Dye-Sensitized Solar Cells: Mutual Solutions to Disparate Problems

Controlled radical polymerization (CRP) and dye-sensitized solar cells (DSSCs) are two fields of research that at an initial glance appear to have little in common. However, despite their obvious differences, both in application and in scientific nature, a closer look reveals a striking similarity between many of the compounds widely used as control agents in radical polymerization and as redox couples in dye-sensitized solar cells. Herein, we review the various redox active compounds used and examine the characteristics that give them the ability to perform this dual function. In addition we explore the advances in the understanding of the structural features that enhance their activity in both CRP and DSSCs. It is hoped that such a comparison will be conducive to improving process performance in both fields.

Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes

In dye-sensitized solar cells co-photosensitized with an alkoxysilyl-anchor dye ADEKA-1 and a carboxy-anchor organic dye LEG4, LEG4 was revealed to work collaboratively by enhancing the electron injection from the light-excited dyes to the TiO2 electrodes, and the cells exhibited a high conversion efficiency of over 14% under one sun illumination.

Recent Advances of Cobalt(II/III) Redox Couples for Dye-Sensitized Solar Cell Applications

In recent years dye-sensitized solar cells (DSSCs) have emerged as one of the alternatives for the global energy crisis. DSSCs have achieved a certified efficiency of >11% by using the I(-) /I3 (-) redox couple. In order to commercialize the technology almost all components of the device have to be improved. Among the various components of DSSCs, the redox couple that regenerates the oxidized sensitizer plays a crucial role in achieving high efficiency and durability of the cell. However, the I(-) /I3 (-) redox couple has certain limitations such as the absorption of triiodide up to 430 nm and the volatile nature of iodine, which also corrodes the silver-based current collectors. These limitations are obstructing the commercialization of this technology. For this reason, one has to identify alternative redox couples. In this regard, the Co(II/III) redox couple is found to be the best alternative to the existing I(-) /I3 (-) redox couple. Recently, DSSC test cell efficiency has risen up to 13% by using the cobalt redox couple. This review emphasizes the recent development of Co(II/III) redox couples for DSSC applications.

Synthesis and characterization of substituted Schiff-base ligands and their d10 metal complexes: structure-induced luminescence tuning behaviors and applications in co-sensitized solar cells

The structure-induced luminescence tuning behaviors of Schiff-base ligand based d¹⁰ metal complexes and their applications in co-sensitized solar cells.

Transition metal complex redox shuttles for dye-sensitized solar cells

This review provides an in-depth investigation into exciting alternative electrolyte shuttles in DSSCs and the various advantages that they provide, such as high conversion efficiency and non-corrosive properties.

Nitrogen-doped graphene as a cathode material for dye-sensitized solar cells: effects of hydrothermal reaction and annealing on electrocatalytic performance

Nitrogen doped graphene prepared via an inhomogeneous hydrothermal reaction was applied to DSCs as a cathode material, yielding a cell efficiency of 8.2%.

Effect of TiO₂ microbead pore size on the performance of DSSCs with a cobalt based electrolyte

Mesoporous TiO2 microbeads with well-defined intra-bead pore sizes (14 nm, 23 nm or 33 nm) were employed to investigate the effect of pore size on the performance of dye-sensitized solar cells constructed with an organic dye (MK2) and a [Co(bpy)3](2+/3+) (bpy = 2,2'-bipyridine)-based electrolyte. The TiCl4 post treatment and film thickness were optimized for the TiO2 electrodes made from beads with 33 nm intra-bead pores, and an overall energy conversion efficiency of 8.5% was achieved for a device with a 6.5 μm thick TiO2 film treated with a 20 mM TiCl4 solution. Although beads with larger pores had a smaller specific surface area, devices derived from these beads produced better photovoltaic performance. This is attributed to the improved diffusion of cobalt species inside the working electrode, as evidenced by a higher electron lifetime and dye regeneration rate recorded on these solar cells.

Characterization of the TiO2/dye/electrolyte interfaces in dye-sensitized solar cells by means of a titania-binding nitroxide

Dye-sensitized solar cells (DSSCs) have been characterized in several literature examples by using relatively complex methods and/or modified DSSC conditions with respect to the usual working ones. In this study, we propose a method for the investigation of the interfaces TiO2/dye/electrolyte in a DSSC at its usual working conditions. This method implies the use of a computer-aided analysis of the electron paramagnetic resonance (EPR) spectra of the spin probe 4-carboxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4-carboxy-TEMPO, indicated as 4-cT). This probe well-mimics the dyes in their interactions with TiO2 surface, but does not perturb dye adsorption onto TiO2 surface, as verified by UV-vis measurements. First, we investigated the interacting ability toward 4-cT of commercially available TiO2 used for assembling the DSSC. It was found that interactions are modulated by the different distribution of interacting sites at the solid surface and powder aggregation. Further, experiments on 4-cT were carried out in the presence of a series of other molecules coded as N3, N719, and D149, which are commonly used as dyes in DSSCs. Then, the effect of solutions added to the electrodes was investigated. On the basis of the interactions occurring at the TiO2/dye/electrolyte interfaces, we selected the ingredients of the DSSCs. Electrical and EPR characterizations of these DSSCs miniaturized to enter the EPR cavity, together with time-dependent laser-light on-off experiments, were carried out, which demonstrated the ability of the EPR analysis to monitor the types and strengths of the interactions occurring at the cell's different interfaces. This method using the standard continuous wave EPR technique at room temperature may be profitably used to characterize the quality and performances of a DSSC.

Thiocyanate-free ruthenium(II) sensitizers for dye-sensitized solar cells based on the cobalt redox couple

Two thiocyanate-free ruthenium(II) sensitizers, TFRS-41 and TFRS-42, with distinctive dialkoxyphenyl thienyl substituents were successfully prepared and tested for potential applications in making dye-sensitized solar cells (DSCs). Subsequent device fabrication was conducted by using a [Co(bpy)3 ](2+/3+) -based (bpy=2,2'-bipyridine) electrolyte, for which the best performance data, namely, JSC =13.11 mA cm(-2) , VOC =862 mV, fill factor=0.771, and η=8.71%, were recorded for the sensitizer TFRS-42 with a 2,6-dialkoxyphenyl substituent under AM 1.5G irradiation. The markedly higher Voc value was confirmed by the longer electron lifetime revealed in transient photovoltage (TPV) measurements versus the TFRS-1 sensitizer. In addition, DFT calculation and detailed first-principles computational analysis were conducted to provide a rationale for the observed trends in their photovoltaic performances and electron lifetimes, with reference to different performances exhibited by three thiocyanate-free sensitizers, TFRS-1, TFRS-41 and TFRS-42, versus Z907 reference. Through the proper control of peripheral substituents, the thiocyanate-free ruthenium(II)-based DSC sensitizers can positively influence the performances of DSCs, with better light-harvesting capability and suppressed charge recombination, for DSC cells fabricated by using a [Co(bpy)3 ](2+/3+) -based electrolyte.

Carbazole-based hole-transport materials for efficient solid-state dye-sensitized solar cells and perovskite solar cells

Two carbazole-based small molecule hole-transport materials (HTMs) are synthesized and investigated in solid-state dye-sensitized solar cells (ssDSCs) and perovskite solar cells (PSCs). The HTM X51-based devices exhibit high power conversion efficiencies (PCEs) of 6.0% and 9.8% in ssDSCs and PSCs, respectively. These results are superior or comparable to those of 5.5% and 10.2%, respectively, obtained for the analogous cells using the state-of-the-art HTM Spiro-OMeTAD.

Efficient organic sensitizers with pyridine-N-oxide as an anchor group for dye-sensitized solar cells

Five organic dyes with pyridine-N-oxide as the anchor group and electron acceptor have been synthesized and applied in dye-sensitized solar cells (DSSCs). Benzothiadiazole was introduced in the conjugation system to increase the electron withdrawing properties, FTIR spectra showed that the coordination was between the pyridine-N-oxide and the Brønsted acid site on the TiO2 surface. The relationship between different dye structures and the performance of the DSSCs was investigated systematically. The location of the thiophene unit was studied, and the direct linkage of benzothiadiazole with pyridine-N-oxide was beneficial to broaden the absorption. The donor-acceptor-acceptor-configured dye WL307, which has 2-ethylhexyloxy chains in the donor part, showed the best efficiency of 6.08% under 100 mW cm(-2) light illumination. The dye series showed a fairly good stability during the one month test period.

Phenothiazinedioxide-conjugated sensitizers and a dual-TEMPO/iodide redox mediator for dye-sensitized solar cells

Metal-free dyes containing a phenothiazinedioxide entity in the spacer were synthesized. The best conversion efficiency (7.47%) of the dye-sensitized solar cell (DSSC) by using new sensitizers with chenodeoxycholic acid as a co-adsorbent and the I(-) /I3 (-) electrolyte reached over 90% of that of the standard N719-based cell (8.10%). A new type of ionic liquid containing the nitroxide radical (N-O(.) ) and iodide was successfully synthesized and applied to the DSSCs. If the I(-) /I3 (-) electrolyte was replaced with a dual redox electrolyte, that is, a TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) derivative with a dangling imidazolium iodide entity, the cell exhibited a high open-circuit voltage of 0.85 V and a cell efficiency of 8.36%.

Universal low-temperature MWCNT-COOH-based counter electrode and a new thiolate/disulfide electrolyte system for dye-sensitized solar cells

A new thiolate/disulfide organic-based electrolyte system composed of the tetrabutylammonium salt of 2-methyl-5-trifluoromethyl-2H-[1,2,4]triazole-3-thiol (S(-)) and its oxidized form 3,3'-dithiobis(2-methyl-5-trifluoromethyl-2H-[1,2,4]triazole) (DS) has been formulated and used in dye-sensitized solar cells (DSSCs). The electrocatalytic activity of different counter electrodes (CEs) has been evaluated by means of measuring J-V curves, cyclic voltammetry, Tafel plots, and electrochemical impedance spectroscopy. A stable and low-temperature CE based on acid-functionalized multiwalled carbon nanotubes (MWCNT-COOH) was investigated with our S(-)/DS, I(-)/I3(-), T(-)/T2, and Co(II/III)-based electrolyte systems. The proposed CE showed superb electrocatalytic activity toward the regeneration of the different electrolytes. In addition, good stability of solar cell devices based on the reported electrolyte and CE was shown.