Cobalt Redox Mediator Research Articles

Chemically synthesized poly(3,4-ethylenedioxythiophene) conducting polymer as a robust electrocatalyst for highly efficient dye-sensitized solar cells.

Chemically synthesized PEDOT (poly(3,4-ethylenedioxythiophene)) nanomaterials, with various nanostructured morphologies as well as different intrinsic electrical conductivities and crystallinities, were compared as electrocatalysts for Co(III) reduction in dye-sensitized solar cells (DSSCs). Electrochemical parameters, charge transfer resistance toward the electrode/electrolyte interface, catalytic activity for Co(III)-reduction, and diffusion of cobalt redox species greatly depend on the morphology, crystallinity, and intrinsic electrical conductivity of the chemically synthesized PEDOTs and optimization of the fabrication procedure for counter electrodes. The PEDOT counter electrode, fabricated by spin coating a DMSO-dispersed PEDOT solution with an ordered 1D structure and nanosized fibers averaging 70 nm in diameter and an electrical conductivity of ∼16 S cm-1, exhibits the lowest charge transfer resistance, highest diffusion for a cobalt redox mediator and superior electrocatalytic performance compared to a traditional Pt-counter electrode. The photovoltaic performance of the DSSC using chemically synthesized PEDOT exceeds that of a Pt-electrode device because of the enhanced current density, which is directly related to the superior electrocatalytic ability of PEDOT for Co(III)-reduction. This simple spin-coated counter electrode prepared using cheap and scalable chemically synthesized PEDOT can be a potential alternative to the expensive Pt-counter electrode for cobalt and other redox electrolytes in DSSCs and various flexible electronic devices.

Investigating mass transport and recombination as a function of structural variation in dye-sensitized solar cells employing indole fused heterocyclic organic sensitizers and cobalt electrolytes

Alternate cobalt redox mediator based dye-sensitized solar cells (DSCs) are getting widespread attention taking advantage of their one-electron transfer mechanism compared to the conventional iodide/triiodide electrolyte. In the present study, we used indole fused heterocyclic organic sensitizers having indolo[3,2-b]indole as donor with three different π-spacers [(benzene (IID-1), thiophene (IID-2) and furan (IID-3)] along with cobalt bipyridine derivatives as redox mediators having different peripheral substituents {[Co(bpy)3]3+/2+, [Co(Me2bpy)3]3+/2+, and [Co(t-Bu2bpy)3]3+/2+}. A detailed investigation was carried out to understand the fundamental charge transfer processes and loss mechanism happening at the various interfaces as a function of structural variations in the present dye-electrolyte combinations. Among the investigated systems, higher performance was obtained for the association of furan substituted dye (IID-3) with [Co(t-Bu2bpy)3]3+/2+ electrolyte. The importance of choosing the right combination of sensitizer and electrolyte is critical to realize higher performance in dye-sensitized solar cells particularly while employing organic dyes and alternate metal complex redox electrolytes which was systematically investigated in the present manuscript.

Metal‐free Organic Dyes Featuring an Azobenzene Bridge for Photochromic Dye‐Sensitized Solar Cells

AbstractWe report the facile synthesis of photochromic donor‐π‐acceptor (D‐π‐A) dyes featuring an azobenzene spacer with diphenylamine and dimethylamine as donor residues (DPAC and DMAC). We also investigate the spectroscopic and electrochemical properties of these dyes and their application to dye‐sensitized solar cells (DSSCs). The trans‐cis isomerization of the azobenzene bridge permits regulation of the photovoltaic behaviour in the DSSCs containing iodine‐ and cobalt‐based redox electrolytes. The iodidie/triiodidie redox shuttle is suitable for DMAC‐DSSCs, whereas the cobalt redox mediator is optimal for DPAC‐DSSCs. DMAC‐ and DPAC‐DSSCs yield power conversion efficiencies (PCEs) up to 1.99 % and 1.08 % under one sun illumination, when the solar cells are at the photostationary (PS) state.

Minimizing the Reorganization Energy of Cobalt Redox Mediators Maximizes Charge Transfer Rates from Quantum Dots

AbstractLight‐induced charge separation is at the very heart of many solar harvesting technologies. The reduction of energetic barriers to charge separation and transfer increases the rate of separation and the overall efficiency of these technologies. Here we report that the internal reorganization energy of the redox acceptor, the movement of the atoms with changing charge, has a profound effect on the charge transfer rates from donor quantum dots. We experimentally studied and modelled with Marcus Theory charge transfer to cobalt complexes that have similar redox potentials covering 350 mV, but vastly different reorganization energies spanning 2 eV. While the driving force does influence the electron transfer rates, the reorganization energies had a far more profound effect, increasing charge transfer rates by several orders of magnitude. Our studies suggest that careful design of redox mediators to minimize reorganization energy is an untapped route to drastically increase the efficiency of quantum dot applications that feature charge transfer.

Minimizing the Reorganization Energy of Cobalt Redox Mediators Maximizes Charge Transfer Rates from Quantum Dots.

Light-induced charge separation is at the very heart of many solar harvesting technologies. The reduction of energetic barriers to charge separation and transfer increases the rate of separation and the overall efficiency of these technologies. Here we report that the internal reorganization energy of the redox acceptor, the movement of the atoms with changing charge, has a profound effect on the charge transfer rates from donor quantum dots. We experimentally studied and modelled with Marcus Theory charge transfer to cobalt complexes that have similar redox potentials covering 350 mV, but vastly different reorganization energies spanning 2 eV. While the driving force does influence the electron transfer rates, the reorganization energies had a far more profound effect, increasing charge transfer rates by several orders of magnitude. Our studies suggest that careful design of redox mediators to minimize reorganization energy is an untapped route to drastically increase the efficiency of quantum dot applications that feature charge transfer.

Role of alkyl groups regulating recombination and mass transport at cobalt electrolyte-dye interface in dye sensitized solar cells

Cobalt-based one electron outer-sphere redox mediators are considered as promising alternatives to conventional I3−/I− redox electrolyte in dye sensitized solar cells (DSSC). So far, [Co(bpy)3]3+/2+ is one among the most extensively studied alternate cobalt electrolyte used in DSSC. The performance of these alternate cobalt redox shuttles varies extensively with respect to the organic sensitizers. Hence, cobalt redox mediators' selectivity as a function of structural variations in dyes needs to be studied to understand the intermolecular interactions and their effect on various charge transfer processes. In the present work, we systematically investigated the structure-property correlations of two cobalt redox mediators, [Co(bpy)3]3+/2+ and [Co(tbpy)3]3+/2+ with variable alkyl functionality (with and without t-butyl substituent) at the periphery with two carbazole based D-π-A sensitizers, CCN and TCCN having similar peripheral substituents and its role in regulating recombination and mass transport. Photovoltaic measurements reveal that even with mass transport limitation, bulky t-butyl groups effectively suppressed recombination between TiO2 and redox mediator, leading to a longer lifetime and higher open-circuit potential. Our results provide insight into the need for custom dye design, which extends the possibility of using non-conventional cobalt electrolytes like [Co(tbpy)3]3+/2+ offering better charge transfer properties.

Self-Assembled Multinuclear Complexes for Cobalt(II/III) Mediated Sensitized Solar Cells

In this work, we designed a tetranuclear self-assembled dye 4 (2Z907-Ag+-(Ru(TMAM))) exploiting a combination of the antenna effect and positively-charged groups designed to repel the oxidized form of cationic cobalt redox mediators, in order to reduce recombination and increase the efficiency of dye sensitized solar cells (DSSCs). Charge transfer and excited dynamics were probed by photoelectrochemical and photophysical measurements. The sensitized cell performance, recorded with a [Co(bpy)3]3+/2+ redox mediator and PEDOT counter electrode, showed an improvement when passing from Z907 to the multinuclear systems. The enhancement of the efficiency compared to Z907 resulted mainly from a superior steric and electrostatic shielding determined by the simultaneous presence of long alkyl chains and quaternary ammonia ion units in the architecture of 4.

Building Block Design for Rapid Mass Transport in Quasi-Solid-State Solar Cells

Mesoscopic solar cells have rapidly emerged as a promising alternative to conventional silicon photovoltaics due to the combined benefits of low-cost, simple fabrication and competitive solar energy conversion efficiency. Recent success in low light absorption cobalt (II/III) polypyridyl complexes has led to encouraging solar energy conversion efficiencies in excess of 13%. Moreover, as one-electron redox mediators, cobalt (II/III) complexes based DSC systems also show a lower overpotential for dye regeneration and better compatibility with flexible metal substrates than iodide/tri-iodide. Such efforts open up great possibilities towards commercial applications.In contrast to the conventional organic liquid electrolytes that commonly have challenges durable encapsulation of the devices due to solvent evaporation, gel electrolytes have exhibited an impressive long-term stability, especially for outdoor applications where temperature of the DSC system can reach 80-85 oC under sunlight. Improvement on mass transport of the considerably large sized cobalt (II/III) complexes, particularly in high viscosity electrolyte, is crucial for solid-state dye-sensitized solar cells (SS-DSCs) to reach a reliable high efficiency toward practical applications.In this work, titania nanorod aggregates (TNA) with a great specific surface area and well-developed crystalline network were synthesized through a solvothermal approach and utilized as photoanode building blocks for application in cobalt (II/III) tris(2′2-bipyridine) complexes based solid-state electrolyte. An initial efficiency excess 7.1% and a long term stable efficiency of approximately 8.0% were achieved at full sun illumination by freshly assembled and aged TNA SS-DSCs, respectively. This represents dramatic enhancements of nearly 35% and 100% against the SS-DSCs prepared from two standard TiO2 nanoparticle samples – CCIC (approx. 5.9%) and Degussa P25 (approx. 4.0%), correspondingly. The TNA is characterised by a combination of mesopores within each aggregate and macro inter-aggregates voids, a high specific surface area and a great light scattering ability; such features make the aggregates superior photoanode building blocks for the application in bulky cobalt redox mediator based SS-DSCs system.

Influence of Diffusion Coefficient of Cobalt Redox Mediator Using Triphenylamine Dyes with Various Number of Anchoring Groups: Photovoltaic Performance of DSSCs

The development of electrolytes containing a novel redox shuttle is essential for achieving highly efficient and stable dye-sensitized solar cells (DSSCs). Here, the effect of the diffusion coefficient of redox shuttles in the electrolyte on the photovoltaic performance of DSSCs using triphenylamine dyes by a different number of anchoring groups was investigated. Two different types of cobalt (II)/(III) polypyridine complexes, namely, [Co(bpy)3]2+/3+ and [Co(dtb)3]2+/3+ (where bpy = (2,2′-bipyridine) and dtb = (4,4′-di-tert-butyl-2,2′-bipyridine)), are applied to the hole transporting redox shuttles in DSSCs. Their diffusion coefficients are estimated by linear sweep voltammetry (LSV) at various scan rates. The diffusion coefficient of [Co(bpy)3]2+ and [Co(dtb)3]2+ in the prepared electrolyte are 1.91 × 10−7 and 2.27 × 10−8 cm2/s, respectively. The high diffusion coefficient of [Co(bpy)3]2+ leads to a high ionic conductivity of the electrolyte that is nearly two times higher than the [Co(dtb)3]2+/3+-based electrolyte. This [Co(bpy)3]2+/3+-based electrolyte leads to an increase in the DSSCs photovoltaic performance with an increase in electronic coupling with the organic dye and TiO2. This is due to the photovoltaic performance is not limited by the mass-transportation process.

Development of Graphene Nano‐Platelet Ink for High Voltage Flexible Dye Sensitized Solar Cells with Cobalt Complex Electrolytes

Graphene nanoparticles have been subject to intensive investigation as a replacement for platinum as the catalyst in dye sensitized solar cells (DSCs), but few of these investigations examine the application for flexible cells with deposition processes suitable for industrial fabrication. This work introduces a transparent water based graphene ink that can be dried rapidly at less than 110 ºC making it particularly suitable for roll to roll deposition on plastic substrates. This ink has application as a catalyst for dye sensitized solar cells utilizing cobalt complex electrolytes with efficiency of over 6 % and a Voc of 0.89 V at 1 sun demonstrated. A flexible DSC with a printed catalyst and cobalt redox mediator is reported, with efficiency of over 6.0 % and Voc of 0.6 V at 800 lux.

Effective suppression of interfacial charge recombination by a 12-crown-4 substituent on a double-anchored organic sensitizer and rotating disk electrochemical evidence

A crown-ether-substituted double-anchored organic dye suppressed charge recombination with iodide and cobalt redox mediators to reach the efficiencies of 10.12% (under 1 sun) and 11.17% (under 0.1 sun).

Investigation of cobalt redox mediators and effects of TiO2 film topology in dye-sensitized solar cells

Cobalt complexes represent interesting alternative redox mediators in dye-sensitized solar cells, with weak visible light absorption and a wide variety in redox potentials. Its diffusion in mesoporous TiO2 may, however, limit its performance.

Exploring the regeneration process of ruthenium(II) dyes by cobalt mediator in dye-sensitized solar cells from first-principle calculations

The regeneration of ruthenium(II) dyes by cobalt redox mediator in dye-sensitized solar cells (DSSCs) has been investigated using density functional theory combined with the Marcus theory of electron transfer. Our results show that the regeneration reaction rates gradually increase with the growth of the dye protonation degree. By comparing the natures of protonated and deprotonated states, we reveal that design of charge-neutral dye is the best choice to obtain fast dye regeneration and suppress the undesired recombination reaction of the injected photoelectrons. Furthermore, the dye-cobalt mediator associated configuration is found to influence the regeneration reaction rates remarkably by varying the electronic coupling energy. The electronic coupling energy largely depends on the frontier orbitals distribution of the reactants, so the matching of the electronic configuration of the dyes and the cobalt mediators should be judiciously conceived.

Efficient dye-sensitized solar cell with a pure thin film of a hybrid polyoxometalate covalently attached organic dye as a working electrode in a cobalt redox mediator system

A Keggin-type hybrid polyoxometalate is used as a pure semiconductor thin film in the working electrode of a dye-sensitized solar cell fabricated with a D35 organic dye and a cobalt redox mediator system, giving a maximum IPCE of about 91% .

Probing the regeneration process of triphenylamine-based organic dyes in dye-sensitized solar cells

The regeneration processes of triphenylamine (TPA)-based dyes with cobalt redox mediator in dye-sensitized solar cells (DSSCs) have been investigated using density functional theory combined with the Marcus theory of electron transfer. Our results show that with the extension or rigidification of the oligothiophene conjugation linker the absorption spectra of TPA dyes exhibit observable red-shift in the maximum absorbance that favors light-harvesting, while the electron transfer rates for dye regeneration decrease in some degrees due to the increased activation free energies and the reduced electronic coupling energies which hampers the dye regeneration. Importantly, the undesirable influences on dye regeneration by extending the linker moiety are more significant than that by the way of rigidification. Thus, the rigidification is a better choice than the extension of the conjugated moiety for the design of D-π-A type dyes based on the properties of light-harvesting and the kinetics of dye regeneration.

Enhanced Photovoltaic Properties and Long-Term Stability in Plasmonic Dye-Sensitized Solar Cells via Noncorrosive Redox Mediator

We demonstrate the localized surface plasmon resonance (LSPR) effect, which can enhance the photovoltaic properties of dye-sensitized solar cells (DSSCs), and the long-term stability of size-controlled plasmonic structures using a noncorrosive redox mediator. Gold nanoparticles (Au NPs) were synthesized with a phase transfer method based on ligand exchange. This synthetic method is advantageous because the uniformly sized Au NPs, can be mass produced and easily applied to DSSC photoanodes. The plasmonic DSSCs showed an 11% improvement of power conversion efficiency due to the incorporation of 0.07 wt % Au NPs, compared to the reference DSSCs without Au NPs. The improved efficiency was primarily due to the enhanced photocurrent generation by LSPR effect. With the cobalt redox mediator, the long-term stability of the plasmonic structures also significantly increased. The plasmonic DSSCs with cobalt(II/III) tris(2,2'-bipyridine) ([Co(bpy)3](2+/3+)) redox mediator maintained the LSPR effect with stable photovoltaic performance for 1000 h. This is, to our knowledge, the first demonstration of the long-term stability of plasmonic nanostructures in plasmonic DSSCs based on liquid electrolytes. As a result, the enhanced long-term stability of plasmonic NPs via a noncorrosive redox mediator will increase the feasibility of plasmonic DSSCs.

Mesoporous TiO2 Beads Offer Improved Mass Transport for Cobalt‐Based Redox Couples Leading to High Efficiency Dye‐Sensitized Solar Cells

Overcoming ionic diffusion limitations is essential for the development of high‐efficiency dye‐sensitized solar cells based on cobalt redox mediators. Here, improved mass transport is reported for photoanodes composed of mesoporous TiO2 beads of varying pore sizes and porosities in combination with the high extinction YD2‐o‐C8 porphyrin dye. Compared to a photoanode made of 20 nm‐sized TiO2 particles, electrolyte diffusion through these films is greatly improved due to the large interstitial pores between the TiO2 beads, resulting in up to 70% increase in diffusion‐limited current. Simultaneously, transient photocurrent measurements reveal no mass transport limitations for films of up to 10 μm thickness. In contrast, standard photoanodes made of 20 nm‐sized TiO2 particles show non‐linear behavior in photocurrent under 1 sun illumination for a film thickness as low as 7 μm. By including a transparent thin mesoporous TiO2 underlayer in order to reduce optical losses at the fluorine‐doped tin oxide (FTO)‐TiO2 interface, an efficiency of 11.4% under AM1.5G 1 sun illumination is achieved. The combination of high surface area, strong scattering behavior, and high porosity makes these mesoporous TiO2 beads particularly suitable for dye‐sensitized solar cells using bulky redox couples and/or viscous electrolytes.

Highly electrocatalytic hybrid silver nanowire-graphene counter electrode for Co3+/2+ redox mediator based dye-sensitized solar cells

Solution processed hybrid silver nanowire (AgNW)-graphene nanoplatelets (GNP) films were prepared and the electrocatalytic activity for the cobalt redox electrolyte in dye-sensitized solar cells (DSSCs) was examined. Cyclic voltammograms and Tafel polarization measurements confirmed the superior electrocatalytic activity of AgNW-GNP electrode than that of standard Pt counter electrode (CE). Electrochemical impedance spectroscopic (EIS) study also revealed the lower charge transfer resistance (Rct) of AgNW-GNP compared to Pt based CE. However, the resistance of the electrolyte diffusion was higher for the AgNW-GNP based CE resulted from the porous structure of the hybrid CE. The photoconversion efficiencies of the DSSC prepared with AgNW-GNP based CE was found to be 2.44%, which was comparable to that of Pt based DSSC, 2.55%. This study proposed an alternative low-cost cathode material for the cobalt redox mediator based DSSCs.

A quasi-liquid polymer-based cobalt redox mediator electrolyte for dye-sensitized solar cells

Recently, cobalt redox electrolyte mediators have emerged as a promising alternative to the commonly used iodide/triiodide redox shuttle in dye-sensitized solar cells (DSCs). Here, we report the successful use of a new quasi-liquid, polymer-based electrolyte containing the Co(3+)/Co(2+) redox mediator in 3-methoxy propionitrile solvent in order to overcome the limitations of high cell resistance, low diffusion coefficient and rapid recombination losses. The performance of the solar cells containing the polymer based electrolytes increased by a factor of 1.2 with respect to an analogous electrolyte without the polymer. The performances of the fabricated DSCs have been investigated in detail by photovoltaic, transient electron measurements, EIS, Raman and UV-vis spectroscopy. This approach offers an effective way to make high-performance and long-lasting DSCs.

Cobalt Electrolyte/Dye Interactions in Dye-Sensitized Solar Cells: A Combined Computational and Experimental Study

We report a combined experimental and computational investigation to understand the nature of the interactions between cobalt redox mediators and TiO(2) surfaces sensitized by ruthenium and organic dyes, and their impact on the performance of the corresponding dye-sensitized solar cells (DSSCs). We focus on different ruthenium dyes and fully organic dyes, to understand the dramatic loss of efficiency observed for the prototype Ru(II) N719 dye in conjunction with cobalt electrolytes. Both N719- and Z907-based DSSCs showed an increased lifetime in iodine-based electrolyte compared to the cobalt-based redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with long alkoxy chains, show no significant change in the electron lifetime regardless of employed electrolyte and deliver a high photovoltaic efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular dynamics simulations show the formation of a complex between the cobalt electrolyte and the surface-adsorbed ruthenium dye, which brings the [Co(bpy)(3)](3+) species into contact with the TiO(2) surface. This translates into a high probability of intercepting TiO(2)-injected electrons by the oxidized [Co(bpy)(3)](3+) species, lying close to the N719-sensitized TiO(2) surface. Investigation of the dye regeneration mechanism by the cobalt electrolyte in the Marcus theory framework led to substantially different reorganization energies for the high-spin (HS) and low-spin (LS) reaction pathways. Our calculated reorganization energies for the LS pathways are in excellent agreement with recent data for a series of cobalt complexes, lending support to the proposed regeneration pathway. Finally, we systematically investigate a series of Co(II)/Co(III) complexes to gauge the impact of ligand substitution and of metal coordination (tris-bidentate vs bis-tridentate) on the HS/LS energy difference and reorganization energies. Our results allow us to trace structure/property relations required for further development of cobalt electrolytes for DSSCs.