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.

The progress over the last decade in the applications of first row d-block metal, especially iron, cobalt, copper and zinc, coordination compounds in redox shuttles and sensitizers in dye-sensitized solar cells is reviewed.     

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.     

Controlling Interfacial Recombination in Aqueous Dye‐Sensitized Solar Cells by Octadecyltrichlorosilane Surface Treatment

A general and convenient strategy is proposed for enhancing photovoltaic performance of aqueous dye‐sensitized solar cells (DSCs) through the surface modification of titania using an organic alkyl silane. Introduction of octadecyltrichlorosilane on the surface of dyed titania photoanode as an organic barrier layer leads to the efficient suppression of electron recombination with oxidized cobalt species by restricting access of the cobalt redox couple to the titania surface. The champion ODTS‐treated aqueous DSCs (0.25 mM ODTS in hexane for 5 min) exhibit a V_oc of 821±4 mV and J_sc of 10.17±0.21 mA cm^?2, yielding a record PCE of 5.64±0.10 %. This surface treatment thus serves as a promising post‐dye strategy for improving the photovoltaic performance of other aqueous DSCs.     

Synthesis of new truxene based organic sensitizers for iodine-free dye-sensitized solar cells

Developing arylamine photosensitizers with high extinction coefficients, proper electronic structures, and steric properties is warranted for the dye-sensitized solar cells (DSCs) employing iodine-free redox shuttles. Two new organic sensitizers (M21 and M22) featuring unsymmetrical truxene-based triarylamine donor have been synthesized and compared to its reference sensitizer M4. The effects of unsymmetrical truxene-based triarylamine donors were investigated by their absorption spectra, electrochemical and photovoltaic properties. The incorporation of strong electron donor unit (i.e., dipropylfluorene and 4-methoxybiphenyl) has resulted in an improved light harvesting capacity, and thus photocurrent as well as efficiency of cells. M22 sensitized DSCs employing the Co(II/III)tris(1,10-phenanthroline)-based redox electrolyte affords a short circuit photocurrent of 13.1 mA cm⁻², an open circuit voltage of 861 mV, and a fill factor of 0.70, corresponding to an overall conversion efficiency of 7.89% under standard AM 1.5 sunlight.     

Redox couple related influences of bulky electron donor as well as spacer in organic dye-sensitized mesoscopic solar cells

For dye-sensitized solar cells (DSCs), it is of great significance to understand the structure–performance relationship of photosensitizers. Herein, we scrutinize the influences of the arylamine electron donors as well as the fused thiophene spacer on the optoelectronic features of thin-film DSCs employing the tris(1,10-phenanthroline)cobalt(II/III) or the iodide/triiodide redox couple. Interestingly, the incorporation of bulky dihexyloxybenzene-substituted triphenylamine (DHOB-TPA) electron donor does not resulted in an improved electron lifetime, which is sharp contrast with the conventional concept on bulky electron donor. On the other hand, the introduction of the DHOB-TPA electron donor or the dithienopyrrole spacer significantly increases the molar absorption coefficients of dyes, which govern the performance of thin-film DSCs. This work demonstrates how organic dyes must be tailored carefully depending on the electrolyte red/ox couple used.     

The effects of polymer gel electrolyte composition on performance of quasi-solid-state dye-sensitized solar cells

Polymer gel electrolytes based on poly(acrylic acid)-poly(ethylene glycol) (PAA–PEG) hybrid have been prepared and applied to developed quasi-solid-state dye-sensitized solar cells (DSCs). PAA–PEG hybrid was synthesized by polymerization reaction. Quasi-solid-state DSCs were fabricated with synthesized PAA–PEG electrolyte. The effects of alkali iodides LiI, KI, and I₂ concentrations on liquid electrolyte absorbency and ionic conductivity of PAA–PEG were investigated. The evolution of the solar cell parameters with polymer gel electrolyte compositions was revealed. DSCs based on PAA–PEG with optimized KI/I₂ concentrations showed better performances than those with optimized LiI/I₂ concentrations. The electrochemical impedance spectroscopy technique was employed to examine the electron lifetime in the TiO₂ electrode and quantify charge transfer resistances at the TiO₂/dye/electrolyte interface and the counter electrode in the solar cells based on the PAA–PEG hybrid gels. A maximum conversion efficiency of 4.96% was obtained for DSCs using KI based quasi-solid electrolyte under 100 mW cm⁻². Our work suggests that KI can be the promising alkali metal iodide for improving the performance of PAA–PEG hybrid gel DSCs.     

TCO-free dye solar cells based on Ti back contact electrode by facile printing method

The back contact dye solar cells (BCDSCs), in which the TCO(Transparent Conductive oxide) is omitted, have a potential for use of intact low-cost general substrates such as glass, metal foil and papers. Herein, we introduce a facile manufacturing method of a Ti back contact electrode (Ti BCE) for the BCDSCs. We found that the polylinkers such as poly(butyl titanate) have a strong binding property to make Ti particles connect one with another. A porous Ti film, which consists of Ti particles of ≤ 10? size connected by a small amount of polylinkers, has an excellent low sheet resistance of 10 Ω sq^-1 for an efficient electron collection for DSCs. This Ti BCE can be prepared by using a facile printing method under normal ambient conditions. Conjugating the new back contact electrode technology with the traditional monolithic structure using the carbon counter electrode, we fabricated TCO-less DSCs. These four-layer structurered DSCs consist of a dye-adsorbed nanocrystalline TiO₂ film on a glass substrate, a porous Ti back contact layer, a ZrO₂ spacer layer and a carbon counter electrode in a layered structure. Under AM 1.5 G and 100 mWcm^?2 simulated sunlight illumination, the four-layer structurered DSCs with N719 dyes and I^-/I₃^-redox electrolytes achieved PCEs up to 5.21 %.     

Photovoltaic Performance of Triphenylamine Dyes-sensitized Solar Cells Employing Cobalt Redox Shuttle and Influence of π-conjugated Spacers

Developing photosensitizers suitable for the cobalt electrolyte and understanding the structure-property relationship of organic dyes is warranted for the dye-sensitized solar cells (DSSCs). The DSSCs incorporating tris(1,10-phenanthroline)cobalt(II/III)-based redox elec-trolyte and four synthesized organic dyes as photosensitizers are described. The photovoltaic performance of these dyes-sensitized solar cells employing the cobalt redox shuttle and the influences of the π-conjugated spacers of organic dyes upon the photovoltage and photocur-rent of mesoscopic titania solar cells are investigated. It is found that organic dyes with thiophene derivates as linkers are suitable for DSSCs employing cobalt electrolytes. DSSCs sensitized with the as-synthesized dyes in combination with the cobalt redox shuttle yield an overall power conversion efficiency of 6.1% under 100 mW/cm² AM1.5 G illumination.     

Ultrafast Photodynamics of the Indoline Dye D149 Adsorbed to Porous ZnO in Dye‐Sensitized Solar Cells

We investigate the ultrafast dynamics of the photoinduced electron transfer between surface‐adsorbed indoline D149 dye and porous ZnO as used in the working electrodes of dye‐sensitized solar cells. Transient absorption spectroscopy was conducted on the dye in solution, on solid state samples and for the latter in contact to a I^?/I₃^? redox electrolyte typical for dye‐sensitized solar cells to elucidate the effect of each component in the observed dynamics. D149 in a solution of 1:1 acetonitrile and tert‐butyl alcohol shows excited‐state lifetimes of 300±50 ps. This signature is severely quenched when D149 is adsorbed to ZnO, with the fastest component of the decay trace measured at 150±20 fs due to the charge‐transfer mechanism. Absorption bands of the oxidized dye molecule were investigated to determine regeneration times which are in excess of 1 ns. The addition of the redox electrolyte to the system results in faster regeneration times, of the order of 1 ns.     

Strongly Coupled Cyclometalated Ruthenium Triarylamine Chromophores as Sensitizers for DSSCs

A series of anchor‐functionalized cyclometalated bis(tridentate) ruthenium(II) triarylamine hybrids [Ru(dbp‐X)(tctpy)]^2? [ 2 a ]^2?–[ 2 c ]^2? (H₃tctpy=2,2′;6′,2′′‐terpyridine‐4,4′,4′′‐tricarboxylic acid; dpbH=1,3‐dipyridylbenzene; X=N(4‐C₆H₄OMe)₂ ([ 2 a ]^2?), NPh₂ ([ 2 b ]^2?), N‐carbazolyl [ 2 c ]^2?) was synthesized and characterized. All complexes show broad absorption bands in the range 300–700 nm with a maximum at about 545 nm. Methyl esters [Ru(Me₃tctpy)(dpb‐X)]⁺ [ 1 a ]⁺–[ 1 c ]⁺ are oxidized to the strongly coupled mixed‐valent species [ 1 a ]²⁺–[ 1 c ]²⁺ and the Ru^III(aminium) complexes [ 1 a ]³⁺–[ 1 c ]³⁺ at comparably low oxidation potentials. Theoretical calculations suggest an increasing spin delocalization between the metal center and the triarylamine unit in the order [ 1 a ]²⁺<[ 1 b ]²⁺<[ 1 c ]²⁺. Solar cells were prepared with the saponified complexes [ 2 a ]^2?–[ 2 c ]^2? and the reference dye N719 as sensitizers using the I₃^?/I^? couple and [Co(bpy)₃]^3+/2+ and [Co(ddpd)₂]^3+/2+ couples as [B(C₆F₅)₄]^? salts as electrolytes (bpy=2,2′‐bipyridine; ddpd=N,N′‐dimethyl‐N,N′‐dipyridin‐2‐yl‐pyridine‐2,6‐diamine). Cells with [ 2 c ]^2? and I₃^?/I^? electrolyte perform similarly to cells with N719 . In the presence of cobalt electrolytes, all efficiencies are reduced, yet under these conditions [ 2 c ]^2? outperforms N719 .     

SnS‐Quantum Dot Solar Cells Using Novel TiC Counter Electrode and Organic Redox Couples

Low‐cost quantum‐dot sensitized solar cells (QDSSCs) were fabricated by using the earth‐abundant element SnS quantum dot, novel TiC counter electrodes, and the organic disulfide/thiolate (T₂/T^?) redox couple, and reached an efficiency of 1.03 %. QDSSCs based on I^?/I₃^?, T₂/T^?, and S^2?/S_x^2? redox couples were assembled to study the role of the redox couples in the regeneration of sensitizers. Charge‐extraction results reveal the reasons for the difference in J_SC in three QDSSCs based on I^?/I₃^?, T₂/T^?, and S^2?/S_x^2? redox couples. The catalytic selectivity of TiC and Pt towards T₂/T^? and I^?/I₃^? redox couples was investigated using Tafel polarization and electrochemical impedance analysis. These results indicated that Pt and TiC show a similar catalytic selectivity for I^?/I₃^?. However, TiC possesses better catalytic activity for T₂/T^? than for I^?/I₃^?. These results indicate the great potential of transition metal carbide materials and organic redox couples used in QDSSCs.     

High-surface-area microporous carbon as the efficient photocathode of dye-sensitized solar cells

This paper reports on the application of cornstalks-derived high-surface-area microporous carbon (MC) as the efficient photocathode of dye-sensitized solar cells (DSCs). The photocathode, which contains MC active material, Vulcan XC–72 carbon black conductive agent, and TiO₂ binder, was obtained by a doctor blade method. Electronic impedance spectroscopy (EIS) of the MC film uniformly coated on fluorine doped SnO₂ (FTO) glass displayed a low charge-transfer resistance of 1.32 Ω cm². Cyclic voltammetry (CV) analysis of the as-prepared MC film exhibited excellent catalytic activity for I₃^?/I^? redox reactions. The DSCs assembled with the MC film photocathode presented a short-circuit photocurrent density (J_sc) of 14.8 mA cm^?2, an open-circuit photovoltage (V_oc) of 798 mV, and a fill factor (FF) of 62.3%, corresponding to an overall conversion efficiency of 7.36% under AM 1.5 irradiation (100 mW cm^?2), which is comparable to that of DSCs with Pt photocathode obtained by conventional thermal decomposition.     

Enhanced Electrocatalytic Performance of a Porous g‐C3N4/Graphene Composite as a Counter Electrode for Dye‐Sensitized Solar Cells

A porous graphitic carbon nitride (g‐C₃N₄)/graphene composite was prepared by a simple hydrothermal method and explored as the counter electrode of dye‐sensitized solar cells (DSCs). The obtained g‐C₃N₄/graphene composite was characterized by XRD, SEM, TEM, FTIR spectroscopy, and X‐ray photoelectron spectroscopy. The results show that incorporating graphene nanosheets into g‐C₃N₄ forms a three‐dimensional architecture with a high surface area, porous structure, efficient electron‐transport network, and fast charge‐transfer kinetics at the g‐C₃N₄/graphene interfaces. These properties result in more electrocatalytic active sites and facilitate electrolyte diffusion and electron transport in the porous framework. As a result, the as‐prepared porous g‐C₃N₄/graphene composite exhibits an excellent electrocatalytic activity. In I^?/I₃^? redox electrolyte, the charge‐transfer resistance of the porous g‐C₃N₄/graphene composite electrode is 1.8 Ω cm², which is much lower than those of individual g‐C₃N₄ (70.1 Ω cm²) and graphene (32.4 Ω cm²) electrodes. This enhanced electrocatalytic performance is beneficial for improving the photovoltaic performance of DSCs. By employing the porous g‐C₃N₄/graphene composite as the counter electrode, the DSC achieves a conversion efficiency of 7.13 %. This efficiency is comparable to 7.37 % for a cell with a platinum counter electrode.     

Effects of water-based gel electrolyte on the charge recombination and performance of dye-sensitized solar cells

A new water-based solution of ion-conductive polymeric gel electrolyte composed of polyethylene glycol and polyvinylpyrrolidone as gel-forming substances, I^?/I₃ ^? as reversible redox couple, and various ratios of acetonitrile/water solvents was prepared and used in the fabrication of dye-sensitized solar cells. The effects of water on the electrochemical behavior of the prepared electrolyte solutions were examined by the cyclic voltammetry and electrochemical impedance spectroscopy techniques. Electrochemical impedance spectroscopy was employed to quantify the charge-transfer resistance and the electron lifetime at the TiO₂ conduction band. The characteristic peak shifted to a lower frequency in the Bode phase plot, which is an indication of a longer electron lifetime for the cell containing more water content. Photovoltaic performance of the cells prepared by the new water-based gel electrolyte was studied. Changes in the current density–voltage (J–V) characteristics can be explained based on the effect of water on the energetics and kinetics of charge transport and charge recombination in the dye-sensitized solar cells (DSSCs). It was observed that the increase in open-circuit voltage (V _oc) and fill factor and decrease in J _SC were noticeable for cells containing water-based gel electrolyte. It was indicated that the charge recombination between injected electrons and electron acceptors (polyiodide) in the redox electrolyte was remarkably inhibited by the increase of water. The photovoltaic performance stability of the DSSC containing gel electrolyte solution including 50 wt% of water was examined, and it was shown that it is more stable than conventional cells considerably for 168 h. Energy conversion efficiency of 2.30 % was achieved, under illumination with a simulated solar light of 100 mW cm^?2.     

Synthesis of sensitizers containing donor cascade of triarylamine and dimethylarylamine moieties for dye-sensitized solar cells

Four triarylamine derivatives (XS6-9) containing N,N-dimethylaryl amine units as secondary electron-donating groups are designed and synthesized. 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 93%, with a short-circuit photocurrent density (J_sc) 10.8 mA cm⁻², an open-circuit photovoltage (V_oc) 690 mV, and fill factor (FF) 0.61, which corresponds to an overall conversion efficiency of 4.54%.     

Influence of amylopectin in dimethylsulfoxide on the improved performance of dye-sensitized solar cells

A highly viscous liquid electrolyte is prepared by adding a small amount of amylopectin into dimethylsulfoxide solvent. By using this viscous electrolyte, a dye-sensitized solar cell (DSSC) enhances the short-circuit photocurrent density and solar-to-electricity conversion efficiency by 22% and 8.4%, respectively, compared to those obtained with the reference cell without amylopectin. Furthermore, the stability of the DSSC is enormously improved by the addition of amylopectin. Polarization curves indicate that amylopectin is a reasonable corrosion inhibitor for silver metal in the electrolyte containing I₃^?/I^? couple.     

Preparation,Characterization, and Electrocatalytic Properties of Cobalt Oxide and Cobalt Hexacyanoferrate Hybrid Films

Polynuclear mixed‐valent films of cobalt oxide and cobalt hexacyanoferrate (CoOCoHCF) have been deposited on electrode surfaces from a solution of Co²⁺ and Fe(CN)₆^3? ions by repetitive potential cycling method. Simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance measurements demonstrate the steady growth of modified film. The effect of type of monovalent cations as well as acidity of the supporting electrolyte on film growth and redox behavior of resulting film was investigated. In pure supporting electrolyte, electrochemical responses of modified electrode resemble with that of a surface immobilized redox couple. The hybrid film electrodes showed electrocatalytic activity toward oxidation of NADH, hydrazine and hydroxylamine. The feasibility of using our modified electrodes for analytical application was also explored.     

MnWO4/生物质衍生碳纳米复合催化剂的制备及催化性能

通过共沉淀法合成了双金属氧化物MnWO₄镶嵌生物质衍生碳（MnWO₄/BC）纳米复合催化剂,并将其作为对电极（counter electrode,CE）催化剂组装了染料敏化太阳能电池（dye-sensitized solar cell,DSSC）,探究了MnWO₄/BC在非碘体系中的催化性能和光伏性能。结果表明：在铜氧化还原（Cu²⁺/Cu⁺）电对DSSC中获得的光电能量转换效率（power conversion efficiency,PCE）为3.57%（D35）和1.59%（Y123）,高于Pt电极的PCE（3.12%,1.16%）;50次连续循环伏安测试表明,MnWO₄/BC催化剂具有较好的电化学稳定性。     

Synthesis and Characterization of Donor–π‐Acceptor‐Based Porphyrin Sensitizers: Potential Application of Dye‐Sensitized Solar Cells

New porphyrin sensitizers based on donor–π‐acceptor (D‐π‐A) approach have been designed, synthesized, characterized by various spectroscopic techniques and their photovoltaic properties explored. N,N′‐Diphenylamine acts as donor, the porphyrin is the π‐spacer, and either carboxylic acid or cyanoacryclic acid acts as acceptor. All compounds were characterized by using ¹H NMR spectroscopy, ESI‐MS, UV–visible emission spectroscopies as well as electrochemical methods. The presence of aromatic groups between porphyrin π‐plane and acceptor group push the absorption of both Soret and Q‐bands of porphyrin towards the red region. The electrochemical properties suggests that LUMO of these sensitizers above the TiO₂ conduction band. Finally, the device was fabricated using liquid redox electrolyte (I^?/I₃^?) and its efficiency was compared with that of a leading sensitizer.     

Competition between dissolution and redox reactions in the electrochemistry ofn-GaP

The lowering of the photocorrosion ofn-GaP in 1M KOH in the presence of several redox systems was investigated using neutron activation analysis. After thermal neutron irradiation of GaP and annihilation of irradiation induced defects by annealing processes the photodissolution was investigated by measuring the activity of the corresponding electrolyte solutions. The dependence of the stabilization of the photoelectrode on concentration and redox potential of the reduced form of the redox couples I^–, Fe (CN) ₆ ^4– and SO ₃ ^2– was measured. It was found that the stabilization is growing with growing concentration and lowering of the redox potential of the corresponding redox couple.