Metal Coordination Complexes as Redox Mediators in Regenerative Dye-Sensitized Solar Cells

Dual-Emissive Rectangular Supramolecular Pt(II)-p-Biphenyl with 4,4'-Bipyridine Derivative Metallacycles: Stepwise Synthesis and Photophysical Properties

Mixed-ligand tetranuclear supramolecular coordination complexes (SCCs) of Pt(II)-p-biphenyl and bridging ligands derivatives of 4,4'-bypiridine (8)-(10), were synthesized and characterized. The SCCs were synthesized stepwise, starting from the Pt-p-biphenyl -Pt core. The crystal structure of complex {[Pt(2,2'-bpy)]4(μ-bph)2(μ-(4,4'-bpy)2}{PF6}4 (2,2'-bpy = 2,2'-bipyridine, bph = p-biphenyl and 4,4'-bpy = 4,4' bipyridine), was determined using single-crystal diffraction methods. The emission profile of the tetranuclear complexes (8)-(10) was influenced by the length of the bridging ligands and was found to depend on solvent polarity. Dual-emission patterns in methanol-water mixtures were observed only in the cases of complexes (9) and (10), attributed to aggregation-induced emission phenomena.

Exploring DSSC Efficiency Enhancement: SQI-F and SQI-Cl Dyes with Iodolyte Electrolytes and CDCA Optimization

This investigation delves into the potential use of halogen bonding to enhance both the short-circuit current (JSC) and overall efficiency of dye-sensitized solar cells (DSSCs). Specifically, we synthesized two distinct dyes, SQI-F and SQI-Cl, and characterized them using FT-IR, 1HNMR, 13C NMR, and mass spectroscopy. These dyes are based on the concept of incorporating halogen atoms within unsymmetrical squaraine structures with a donor-acceptor-donor (D-A-D) configuration. This strategic design aims to achieve optimal performance within DSSCs. We conducted comprehensive assessments using DSSC devices and integrated these synthesized dyes with iodolyte electrolytes, denoted as Z-50 and Z-100. Further enhancements were achieved through the addition of CDCA. Remarkably, in the absence of CDCA, both SQI-F and SQI-Cl dyes displayed distinct photovoltaic characteristics. However, through sensitization with three equivalents of CDCA, a significant improvement in performance became evident. The peak of performance was reached with the SQI-F dye, sensitized with three equivalents of CDCA, and paired with iodolyte Z-100. This combination yielded an impressive DSSC device efficiency of 6.74%, an open-circuit voltage (VOC) of 0.694 V, and a current density (JSC) of 13.67 mA/cm2. This substantial improvement in performance can primarily be attributed to the presence of a σ-hole, which facilitates a robust interaction between the electrolyte and the dyes anchored on the TiO2 substrate. This interaction optimizes the critical dye regeneration process within the DSSCs, ultimately leading to the observed enhancement in efficiency.

Recent progress in organic waste recycling materials for solar cell applications

Organic waste-derived solar cells (OWSC) are a classification of third-generation photovoltaic cells in which one or more constituents are fabricated from organic waste material. They are an inspirational complement to the conventional third-generation solar cell with the potential of revolutionizing our future approach to solar cell manufacture. This article provides a study and summary of solar cells that fall under the category of OWSC. OWSC own their merit to low cost of manufacturing and environmental friendliness. This review article reveals different organic waste raw materials, preparation-to-assembly methodologies, and novel approaches to solar cell manufacturing. Ideas for the optimization of the performance of OWSC are presented. The assembly configurations and photovoltaic parameters of reported OWSC are compared in detail. An overview of the trends in the research regarding OWSC in the past decade is given. Also, the advantages and disadvantages of the different solar cell technologies are discussed, and possible trends are proposed. Industrial organic waste raw materials such as paper, coal, and plastics are among the least explored and yet most attractive for solar cell fabrication. The power conversion efficiencies for the cited works are mentioned while emphasizing the products and functions of the organic waste raw materials used.

Artificial Photosynthesis: Current Advancements and Future Prospects

Artificial photosynthesis is a technology with immense potential that aims to emulate the natural photosynthetic process. The process of natural photosynthesis involves the conversion of solar energy into chemical energy, which is stored in organic compounds. Catalysis is an essential aspect of artificial photosynthesis, as it facilitates the reactions that convert solar energy into chemical energy. In this review, we aim to provide an extensive overview of recent developments in the field of artificial photosynthesis by catalysis. We will discuss the various catalyst types used in artificial photosynthesis, including homogeneous catalysts, heterogeneous catalysts, and biocatalysts. Additionally, we will explore the different strategies employed to enhance the efficiency and selectivity of catalytic reactions, such as the utilization of nanomaterials, photoelectrochemical cells, and molecular engineering. Lastly, we will examine the challenges and opportunities of this technology as well as its potential applications in areas such as renewable energy, carbon capture and utilization, and sustainable agriculture. This review aims to provide a comprehensive and critical analysis of state-of-the-art methods in artificial photosynthesis by catalysis, as well as to identify key research directions for future advancements in this field.

Copper(ii) and cobalt(iii) Schiff base complexes with hydroxy anchors as sensitizers in dye-sensitized solar cells (DSSCs)

Two Schiff base complexes of copper(ii) and cobalt(iii) having the formulae [CuL₂] (Cu-Sal) and [CoL₃] (Co-Sal) (HL = 2-(((2-hydroxyethyl)imino)methyl)phenol) have been synthesized and characterized microanalytically, spectroscopically and in the case of Cu-Sal using single crystal X-ray diffraction technique. The single crystal X-ray analysis reveals a square planar geometry around Cu(ii) satisfied by phenoxide oxygen and imine nitrogen of the L^- ligand to generate a six membered chelate ring. The solid state structure of Cu-Sal is satisfied by varied intermolecular non-covalent interactions. The nature of these interactions has been addressed with the aid of Hirshfeld surface analysis. Both compounds have been used as sensitizers in TiO₂ based dye sensitized solar cells (DSSCs) and the DSSC experiments revealed that Co-Sal offers better photovoltaic performance in comparison to Cu-Sal. The Co-Sal exhibited a J _sc of 9.75 mA cm^-2 with a V _oc of -0.648 V, incident photon to current conversion efficiency (IPCE) of 57% and η of 3.84%. The relatively better photovoltaic performance of Co-Sal could be attributed to better light absorption and dye loading than that of Cu-Sal.

Redox Data of Tris(polypyridine)manganese(II) Complexes

Very little cyclic voltammetry data for tris(polypyridine)manganese(II) complexes, [MnII(N^N)3]2+, where N^N is bipyridine (bpy), phenanthroline (phen) or substituted bpy or phen ligands, respectively; are available in the literature. Cyclic voltammograms were found for tris(4,7-diphenyl-1,10-phenanthroline)manganese(II) perchlorate only. In addition to our recently published related research article, the data presented here provides cyclic voltammograms and corresponding voltage-current data obtained during electrochemical oxidation and the reduction of four [MnII(N^N)3]2+ complexes, using different scan rates and analyte concentrations. The results show increased concentration and scan rates resulting in higher Mn(II/III) peak oxidation potentials and increased peak current-voltage separations of the irreversible Mn(II/III) redox event. The average peak oxidation and peak reduction potentials of the Mn(II/III) redox events stayed constant within 0.01 V. Similarly, the average of the peak oxidation and reduction potentials of the ligand-based reduction events of [MnII(N^N)3]2+ were constant within 0.01 V.

Exploring Structure-Property Relationships in a Family of Ferrocene-Containing, Triphenylamine-Based Hybrid Organic Dyes

In this work, a new family of triphenylamine-based dyes equipped with ferrocene (Fc)-containing donors have been synthesized, characterized, and studied within dye-sensitized solar cells (DSSC). With the objective of designing a redox-robust iron-containing dye that effectively sensitizes titania, a family of five structurally related dyes containing ferrocene were prepared. The physicochemical properties of all dyestuffs were studied using UV-Vis and electrochemical measurements. Different cross-coupling strategies resulted in the ability to modestly tune the Fc/Fc+ redox potentials of the dye. Despite the Fc-containing dyes having optoelectronic properties consistent with the non-ferrocene parent dye, the performance of the dyes in devices was dismal and decreased with the number of appended ferrocenes. While this finding was consistent with previous attempts to explore Fc-TPA DSSC dyes, our spectroelectrochemical data supports the hypothesis that the ferrocene component of the dye is oxidized by the electrolyte, ultimately decreasing the dye’s ability to be a suitable sensitizer. While these dyes are not suitable for DSSC applications, they might find applications in other photo-induced integrated devices where charge recombination is minimal.

PEDOT-Carbon Nanotube Counter Electrodes and Bipyridine Cobalt (II/III) Mediators as Universally Compatible Components in Bio-Sensitized Solar Cells Using Photosystem I and Bacteriorhodopsin

In nature, solar energy is captured by different types of light harvesting protein-pigment complexes. Two of these photoactivatable proteins are bacteriorhodopsin (bR), which utilizes a retinal moiety to function as a proton pump, and photosystem I (PSI), which uses a chlorophyll antenna to catalyze unidirectional electron transfer. Both PSI and bR are well characterized biochemically and have been integrated into solar photovoltaic (PV) devices built from sustainable materials. Both PSI and bR are some of the best performing photosensitizers in the bio-sensitized PV field, yet relatively little attention has been devoted to the development of more sustainable, biocompatible alternative counter electrodes and electrolytes for bio-sensitized solar cells. Careful selection of the electrolyte and counter electrode components is critical to designing bio-sensitized solar cells with more sustainable materials and improved device performance. This work explores the use of poly (3,4-ethylenedioxythiophene) (PEDOT) modified with multi-walled carbon nanotubes (PEDOT/CNT) as counter electrodes and aqueous-soluble bipyridine cobaltII/III complexes as direct redox mediators for both PSI and bR devices. We report a unique counter electrode and redox mediator system that can perform remarkably well for both bio-photosensitizers that have independently evolved over millions of years. The compatibility of disparate proteins with common mediators and counter electrodes may further the improvement of bio-sensitized PV design in a way that is more universally biocompatible for device outputs and longevity.

A Simple Method to Obtain Protective Film against Acid Rain

Acid rain is a major problem for animals, plants, buildings, and also for the top glass of photovoltaic (PV) solar panels and greenhouses. Air pollutants such as NOx, NH3, and H2S can mix with water in the atmosphere to form acid rain. It was discovered that atmospheric water vapor adsorbed on the surface of glass can also lead to corrosion of the glass surface. The purpose of this work is to obtain a protective film for glasses used in different domains such as solar cells, windows, stained glass windows from historical buildings, etc. Thin film deposited on glass must be protective against acid rain, transparent in the visible domain with a band gap up to 3.2 eV, and have a vitreous structure (glass). Electron beam (e-gun) technology is a deposition technique for producing high-purity and dense coatings in a short time. It is well known that Ta2O5 is an oxide with anticorrosive properties, but it is expensive and cannot form glass by itself. ZnO is an oxide known as a glass former, exhibiting good optical properties. In this paper, a thin film obtained by the deposition of ZnO and Ta2O5 on a glass substrate using e-gun technology are studied. The simulated acid rain effect on the structure, morphology, and optical properties of thin films are studied after a 65% nitric acid attack on the surface. The X-ray diffraction (XRD) pattern shows the vitreous state of the thin film with a composition 50%ZnO 50%Ta2O5 before and after the acid attack. The morphology, composition, and thickness of the film are investigated using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and profilometry.

Photophysics and Electrochemistry of Biomimetic Pyranoflavyliums: What Can Bioinspiration from Red Wines Offer?

Natural dyes and pigments offer incomparable diversity of structures and functionalities, making them an excellent source of inspiration for the design and development of synthetic chromophores with a myriad of emerging properties. Formed during maturation of red wines, pyranoanthocyanins are electron-deficient cationic pyranoflavylium dyes with broad absorption in the visible spectral region and pronounced chemical and photostability. Herein, we survey the optical and electrochemical properties of synthetic pyranoflavylium dyes functionalized with different electron-donating and electron-withdrawing groups, which vary their reduction potentials over a range of about 400 mV. Despite their highly electron-deficient cores, the exploration of pyranoflavyliums as photosensitizers has been limited to the "classical" n-type dye-sensitized solar cells (DSSCs) where they act as electron donors. In light of their electrochemical and spectroscopic properties, however, these biomimetic synthetic dyes should prove to be immensely beneficial as chromophores in p-type DSSCs, where their ability to act as photooxidants, along with their pronounced photostability, can benefit key advances in solar-energy science and engineering.

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.

Electronic structure and interfacial features of triphenylamine- and phenothiazine-based hole transport materials for methylammonium lead iodide perovskite solar cells

Recently, great research efforts have been devoted to perovskite solar cells (PSCs) leading to sunlight-to-power conversion efficiencies above 25%. However, several barriers still hinder the full deployment of these devices....

Recent Investigations on Thiocyanate-Free Ruthenium(II) 2,2'-Bipyridyl Complexes for Dye-Sensitized Solar Cells

Three decades ago, dye-sensitized solar cells (DSSCs) emerged as a method for harnessing the energy of the sun and for converting it into electricity. Since then, a lot of work has been devoted to create better global photovoltaic efficiencies and long term stability. Among photosensitizers for DSSCs, thiocyanate-free ruthenium(II) complexes have gained increasing interest due to their better stability compared to conventional thiocyanate-based complexes, such as benchmark dyes N719 and Z907. In this mini-review, two classes of thiocyanate-free Ru(II) complexes are presented: (a) bis-bipyridyl compounds bearing an ancillary cyclometalating bidentate ligand; (b) bipyridyl compounds bearing non-cyclometalating ancillary ligands. The coverage, mainly from 2014 up to now, is not exhaustive, but illustrates the most recent design strategies and photovoltaic properties of these two families of ruthenium(II) dyes.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Photoinduced Ligand Exchange Dynamics of a Polypyridyl Ruthenium Complex in Aqueous Solution

The understanding of photoinduced ligand exchange mechanisms in polypyridyl ruthenium(II) complexes operating in aqueous solution is of crucial importance to rationalize their photoreactivity. Herein, we demonstrate that a synergetic use of ab initio molecular dynamics simulations and static calculations, both conducted at the DFT level, can provide a full understanding of photosubstitution mechanisms of a monodentate ligand by a solvent water molecule in archetypal ruthenium complexes in explicit water. The simulations show that the photoinduced loss of a monodentate ligand generates an unreactive 16-electron species in a hitherto undescribed pentacoordinated triplet excited state that converts, via an easily accessible crossing point, to a reactive 16-electron singlet ground state, which combines with a solvent water molecule to yield the experimentally observed aqua complex in less than 10 ps. This work paves the way for the rational design of novel photoactive metal complexes relevant for biological applications.

Advanced research trends in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) are an efficient photovoltaic technology for powering electronic applications such as wireless sensors with indoor light. Their low cost and abundant materials, as well as their capability to be manufactured as thin and light-weight flexible solar modules highlight their potential for economic indoor photovoltaics. However, their fabrication methods must be scaled to industrial manufacturing with high photovoltaic efficiency and performance stability under typical indoor conditions. This paper reviews the recent progress in DSSC research towards this goal through the development of new device structures, alternative redox shuttles, solid-state hole conductors, TiO2 photoelectrodes, catalyst materials, and sealing techniques. We discuss how each functional component of a DSSC has been improved with these new materials and fabrication techniques. In addition, we propose a scalable cell fabrication process that integrates these developments to a new monolithic cell design based on several features including inkjet and screen printing of the dye, a solid state hole conductor, PEDOT contact, compact TiO2, mesoporous TiO2, carbon nanotubes counter electrode, epoxy encapsulation layers and silver conductors. Finally, we discuss the need to design new stability testing protocols to assess the probable deployment of DSSCs in portable electronics and internet-of-things devices.

Aqueous-soluble bipyridine cobalt(ii/iii) complexes act as direct redox mediators in photosystem I-based biophotovoltaic devices

Sustainable energy production is critical for meeting growing worldwide energy demands. Due to its stability and reduction potential, photosystem I (PSI) is attractive as the photosensitizer in biophotovoltaic devices. Herein, we characterize aqueous and organic solvent soluble synthetic bipyridine-based cobalt complexes as redox mediators for PSI-based biophotovoltaics applications. Cobalt-based complexes are not destructive to protein and have appropriate midpoint potentials for electron donation to PSI. We report on PSI stability in organic solvents commonly used in biophotovoltaics. We also show the effects of a mixed organic solvent phase on PSI reduction kinetics, slowing reduction rates approximately 8–38 fold as compared to fully aqueous systems, with implications for dye regeneration rates in PSI-based biophotovoltaics. Further, we show evidence of direct electron transfer from cobalt complexes to PSI. Finally, we report on photocurrent generation from Co mediator-PSI biophotovoltaic devices. Taken together, we discuss the development of novel Co complexes and our ability to fine-tune their characteristics via functional groups and counteranion choice to drive interaction with a biological electron acceptor on multiple levels from redox midpoints, spectral overlap, and solvent requirements, among others. This work suggests that fine-tuning of redox active species for interaction with a biological partner is possible for the creation and improvement of low cost, carbon-neutral energy production in the future.

Sustainable energy production is critical for meeting growing worldwide energy demands.

Copper Complexes as Alternative Redox Mediators in Dye-Sensitized Solar Cells

Thirty years ago, dye-sensitized solar cells (DSSCs) emerged as a method for harnessing the sun's energy and converting it into electricity. Since then, a lot of work has been dedicated to improving their global photovoltaic efficiency and their eco-sustainability. Recently, various articles showed the great potential of copper complexes as a convenient and cheap alternative to the traditional ruthenium dyes. In addition, copper complexes demonstrate that they can act as redox mediators for DSSCs, thus being an answer to the problems related to the I₃^-/I^- redox couple. The aim of this review is to report on the most recent impact made by copper complexes as alternative redox mediators. The coverage, mainly from 2016 up to now, is not exhaustive, but allows us to understand the great role played by copper complexes in the design of eco-sustainable DSSCs.

Synthesis and characterization of Ni-doped anatase TiO2 loaded on magnetic activated carbon for rapidly removing triphenylmethane dyes

In this work, we employed the in situ synthesis method to implant Fe₃O₄ into activated carbon (AC), in which the synthesis of the magnetic AC (MAC) was realized. Thence, Ni-doped anatase TiO₂ (NATiO₂) were anchored on different addition amount of MAC to synthesize the series of Ni-TiO₂/MAC photocatalysts. The chemical compositions and physical properties of these nanocomposites were analyzed by various characterization technologies. The photocatalytic capabilities of as-produced materials were then investigated via adsorption and photodegradation of triphenylmethane dyes (TPMs) as crystal violet (CV), basic fuchsine (BF), and malachite green (MG) solution. The results revealed that the removal of Ni-TiO₂/AC, Ni-TiO₂/2MAC, Ni-TiO₂/4MAC, and Ni-TiO₂/8MAC on TPMs is a very fast process and the removal efficiency can almost reach to about 90% in 10 min, and the catalyst has good cycle stability and is easy to be reused. This work provides a novel, low-cost, and effective way to rationally design and synthesize TiO₂-based photocatalysts for effective removal of TPMs.

Methoxy-substituted copper complexes as possible redox mediators in dye-sensitized solar cells

Methoxy-substituted aromatic diimines and corresponding homoleptic copper(i) and copper(ii) complexes as possible redox mediators in dye-sensitized solar cells.

Well-dispersed Te-doped mesoporous carbons as Pt-free counter electrodes for high-performance dye-sensitized solar cells

A tellurium-doped carbon nanomaterial (Te-MC(P)) was newly developed by the soft-templated carbonization of the PAN-b-PBA copolymer with poly(3-hexyltellurophene). Te-MC(P) was characterized with various characterization methods, including the nitrogen sorption isotherm measurement (BET), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS), which reveal that the Te atoms are homogeneously dispersed in the three-dimensional hierarchical, graphite-like mesoporous carbon matrix with a Te doping level of 0.27 atom %. Based on the characterization results, the electrocatalytic ability of Te-MC(P) was evaluated by using a symmetrical dummy cell test with both Co(bpy)32+/3+ (bpy = 2,2'-bipyridine) and I-/I3- redox electrolytes as counter electrodes (CEs). The Te-MC(P) CEs showed remarkably lower charge-transfer resistance (Rct) values by approximately 10 times in the electrochemical impedance spectroscopy (EIS) measurement, compared to the counterpart platinum (Pt) and the tellurium-based material (Te-MC(A)), prepared with a telluric acid precursor that has a lower Te doping level of 0.15 at%. As a result, the excellent electrocatalytic ability of Te-MC(P) resulted in the improvement of photovoltaic performance. The power conversion efficiencies (PCEs) of Te-MC(P)-based dye-sensitized solar cells (DSSCs) were 12.69% for the Co(bpy)32+/3+ redox electrolyte with the SGT-021 porphyrin dye and 9.73% for the I-/I3- redox electrolyte with the N719 ruthenium dye. Furthermore, Te- MC(P) CEs exhibited remarkable electrochemical stability in the two redox electrolytes. These results could suggest that the Te-MC(P) CE is one of the best promising alternatives to Pt CEs as a low-cost, highly stable and efficient electrocatalytic CE for practical applications.

Bis(2,2'-bipyridil)Copper(II) Chloride Complex: Tyrosinase Biomimetic Catalyst or Redox Mediator?

In this article, construction of amperometric sensor(s) based on screen-printed carbon electrodes covered by thin layers of two types of carbon nanomaterials serving as amplifiers, and containing [Cu(bipy)2Cl]Cl∙5H2O complex is reported. Their performance and biomimetic activity towards two selected neurotransmitters (dopamine and serotonin) was studied mainly using flow injection analysis (FIA). The important parameters of FIA such as working potential, flow rate, and pH were optimized. The mechanism of the catalytic activity is explained and experimentally confirmed. It reveals that presence of hydrogen peroxide plays a crucial role which leads to answer the title question: can presented complex really be considered as a tyrosinase biomimetic catalyst or only as a redox mediator?

Application of Metal-Organic Frameworks and Covalent Organic Frameworks as (Photo)Active Material in Hybrid Photovoltaic Technologies

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are two innovative classes of porous coordination polymers. MOFs are three-dimensional materials made up of secondary building blocks comprised of metal ions/clusters and organic ligands whereas COFs are 2D or 3D highly porous organic solids made up by light elements (i.e., H, B, C, N, O). Both MOFs and COFs, being highly conjugated scaffolds, are very promising as photoactive materials for applications in photocatalysis and artificial photosynthesis because of their tunable electronic properties, high surface area, remarkable light and thermal stability, easy and relative low-cost synthesis, and structural versatility. These properties make them perfectly suitable for photovoltaic application: throughout this review, we summarize recent advances in the employment of both MOFs and COFs in emerging photovoltaics, namely dye-sensitized solar cells (DSSCs) organic photovoltaic (OPV) and perovskite solar cells (PSCs). MOFs are successfully implemented in DSSCs as photoanodic material or solid-state sensitizers and in PSCs mainly as hole or electron transporting materials. An innovative paradigm, in which the porous conductive polymer acts as standing-alone sensitized photoanode, is exploited too. Conversely, COFs are mostly implemented as photoactive material or as hole transporting material in PSCs.

The Hagfeldt Donor and Use of Next-Generation Bulky Donor Designs in Dye-Sensitized Solar Cells

"The Hagfeldt donor" is a bulky triarylamine building block with four alkyl chains in a 3-dimensional arrangement that is used with organic dyes in dye-sensitized solar cells (DSCs) in over 140 publications. Many of the highest performing DSC devices in literature make use of this group due to exceptional TiO₂ surface protection properties, which slows recombination of electrons in TiO₂ with the electrolyte. Importantly, record-setting cobalt and copper redox shuttle-based DSCs require exceptional surface protection to slow a facile recombination of electrons to these positively charged redox shuttles. Several syntheses have emerged for the Hagfeldt donor due to the need for iterative aryl-halide cross- coupling reactions complicating a straightforward route. Six synthetic strategies found in literature are described along with the challenges of each route. A recent method that has been put forward in the literature as a scalable, regioisomerically pure route is highlighted.

Copper Coordination Complexes for Energy-Relevant Applications

Copper coordination complexes have emerged as a group of transition metal complexes that play important roles in solar energy conversion, utilization and storage, and have the potential to replace the quintessential commonly used transition metals, like Co, Pt, Ir and Ru as light sensitizers, redox mediators, electron donors and catalytic centers. The applications of copper coordination compounds in chemistry and energy related technologies are many and demonstrate their rightful place as sustainable, low toxicity and Earth-abundant alternative materials. In this perspective we show the most recent impact made by copper coordination complexes in dye-sensitized solar cells and other energy relevant applications.

Copper-based redox shuttles supported by preorganized tetradentate ligands for dye-sensitized solar cells

Three copper redox shuttles ([Cu(1)]2+/1+, [Cu(2)]2+/1+, and [Cu(3)]2+/1+) featuring tetradentate ligands were synthesized and evaluated computationally, electrochemically, and in dye-sensitized solar cell (DSC) devices using a benchmark organic dye, Y123. Neutral polyaromatic ligands with limited flexibility were targeted as a strategy to improve solar-to-electrical energy conversion by reducing voltage losses associated with redox shuttle electron transfer events. Inner-sphere electron transfer reorganization energies (λ) were computed quantum chemically and compared to the commonly used [Co(bpy)3]3+/2+ redox shuttle which has a reported λ value of 0.61 eV. The geometrically constrained biphenyl-based Cu redox shuttles investigated here have lower reorganization energies (0.34-0.53 eV) and thus can potentially operate with lower driving forces for dye regeneration (ΔGreg) in DSC devices when compared to [Co(bpy)3]3+/2+-based devices. The rigid tetradentate ligand design promotes more efficient electron transfer reactions leading to an improved JSC (14.1 mA cm-2), higher stability due to the chelate effect, and a decrease in VlossOC for one of the copper redox shuttle-based devices.

There Is a Future for N-Heterocyclic Carbene Iron(II) Dyes in Dye-Sensitized Solar Cells: Improving Performance through Changes in the Electrolyte

By systematic tuning of the components of the electrolyte, the performances of dye-sensitized solar cells (DSCs) with an N-heterocyclic carbene iron(II) dye have been significantly improved. The beneficial effects of an increased Li+ ion concentration in the electrolyte lead to photoconversion efficiencies (PCEs) up to 0.66% for fully masked cells (representing 11.8% relative to 100% set for N719) and an external quantum efficiency maximum (EQEmax) up to approximately 25% due to an increased short-circuit current density (JSC). A study of the effects of varying the length of the alkyl chain in 1-alkyl-3-methylimidazolium iodide ionic liquids (ILs) shows that a longer chain results in an increase in JSC with an overall efficiency up to 0.61% (10.9% relative to N719 set at 100%) on going from n-methyl to n-butyl chain, although an n-hexyl chain leads to no further gain in PCE. The results of electrochemical impedance spectroscopy (EIS) support the trends in JSC and open-circuit voltage (VOC) parameters. A change in the counterion from I- to [BF4]- for 1-propyl-3-methylimidazolium iodide ionic liquid leads to DSCs with a remarkably high JSC value for an N-heterocyclic carbene iron(II) dye of 4.90 mA cm-2, but a low VOC of 244 mV. Our investigations have shown that an increased concentration of Li+ in combination with an optimized alkyl chain length in the 1-alkyl-3-methylimidazolium iodide IL in the electrolyte leads to iron(II)-sensitized DSC performances comparable with those of containing some copper(I)-based dyes.

Effect of Auxiliary Donors on 3,8-Phenothiazine Dyes for Dye-Sensitized Solar Cells

Phenothiazines are one of the more common dye scaffolds for dye-sensitized solar cells. However, these sensitizers are exclusively based on a 3,7-substitution pattern. Herein, we have synthesized and characterized novel 3,8-substituted phenothiazine dyes in order to evaluate the effect of auxiliary donor groups on the performance of this new dye class. The power conversion efficiency increased by 7%–10% upon insertion of an auxiliary donor in position 8 of the phenothiazine, but the structure of the auxiliary donor (phenyl, naphthyl, pyrene) had a low impact when electrodes were stained with chenodeoxycholic acid (CDCA) additive. In the absence of CDCA, the highest power conversion efficiency was seen for the phenyl-based sensitizer attributed to a higher quality dye-monolayer. By comparing the novel dyes to their previously reported 3,7- analogues, only subtle differences were seen in photophysical, electrochemical, and performance measurements. The most notable difference between the two geometries is a lowering of the oxidation potentials of the 3,8-dyes by 40–50 mV compared to the 3,7-analogues. The best auxiliary donor for the 3,8-phenothiazine dyes was found to be pyrenyl, with the best device delivering a power conversion efficiency of 6.23% (99 mW cm⁻², 10 eq. CDCA, J_SC = 10.20 mA cm⁻², V_OC = 791 mV, and FF = 0.765).

Effect of furan π-spacer and triethylene oxide methyl ether substituents on performance of phenothiazine sensitizers in dye-sensitized solar cells

Synthesis and characterization of three phenothiazine dyes inspired by literature dye EO3 improving absorption properties and photovoltaic performance in DSSCs.