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

Anion Effect on the CuII-Neocuproine Mediator and Its Electrocatalysts for Dye-Sensitized Solar Cells: Polymeric Chalcogenides of PEDOT-PEDTT and [Ag2(SePh)2]n

The synthetical methodology for the [Cu(dmp)2]2+/1+ (dmp = 2,9-dimethyl-1,10-phenanthroline; neocuproine) complexes has been systematically investigated by using various copper precursors, including CuCl2, Cu(NO3)2, and Cu(ClO4)2. After an anion exchange to trifluoromethanesulfonimide (TFSI), the tetra-coordinated CuII(dmp)2(TFSI)2-Cu(ClO4)2 (7.43%) outperformed the penta-coordinated CuII(dmp)2(TFSI)(NO3)-Cu(NO3)2 (4.30%) and CuII(dmp)2(TFSI)(Cl)-CuCl2. Polymeric chalcogenides, including a conducting copolymeric electrode of PEDOT-PEDTT [PEDOT = poly(3,4-ethylenedioxythiophene); PEDTT = poly(3,4-ethylenedithiothiophene)] and a coordination polymeric electrode of silver bezeneselenolate ([Ag2(SePh)2]n; mithrene), are introduced as the electrocatalysts for [Cu(dmp)2]2+/1+ for the first time. After optimization, dye-sensitized solar cells (DSSCs) based on carbon cloth (CC)/AgSePh-30 (10.18%) showed superior electrocatalytic ability compared to the benchmark CC/Pt (7.43%) due to numerous active sites provided by electron-donating Se atoms, high film roughness, and bottom-up 2D charge transfer routes. The DSSC based on CC/PEDTT-50 (10.38%) also outperformed CC/Pt due to numerous active sites provided by electron-donating S atoms and proper energy band structure. This work sheds light on the future design and synthesis in Cu-complex mediators and functional polymeric chalcogenides for high-performance DSSCs.

Open-Cage Copper Complexes Modulate Coordination and Charge Transfer

This study presents a novel copper-based redox shuttle that employs the PY5 pentadentate polypyridyl ligand in a dye-sensitized solar cell (DSSC). The [Cu(PY5)]2+ complex exhibits a unique five-coordinate square pyramidal geometry, characterized by a strategically labile axial position, to facilitate efficient dye regeneration while minimizing electron recombination, thereby enhancing DSSC performance. Notably, the inclusion of 4-tert-butylpyridine (TBP) as an additive is shown to significantly modulate the electrochemical and photophysical properties of the copper complexes, attributed to its coordination to the vacant axial site. This interaction leads to an improved open-circuit voltage and overall device efficiency, with the complexes achieving promising efficiencies under standard solar irradiance. The findings underscore the potential of utilizing copper-based redox shuttles with designed ligand geometries to overcome the limitations of current DSSC materials, opening new avenues for the design and optimization of solar energy conversion devices. This work not only contributes to the fundamental understanding of the behavior of copper complexes in DSSCs but also paves the way for future research aimed at exploiting the full potential of such geometrical and electronic configurations for the development of more robust and efficient solar energy solutions.

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.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

Stable Dye-Sensitized Solar Cells Based on Copper(II/I) Redox Mediators Bearing a Pentadentate Ligand

In recent years, copper redox mediators have attracted growing interest in dye-sensitized solar cells (DSCs). However, experiments revealed that ubiquitously used Lewis-base additives in the electrolytes coordinate to the Cu^II species, which restricts further enhancement of device performance and stability. We report the application of copper complexes endowed with diamine-tripyridine pentadentate ligands, [Cu(tpe)]^2+/+ (tpe=N-benzyl-N,N',N'-tris(pyridin-2-ylmethyl)ethylenediamine) and [Cu(tme)]^2+/+ (tme=N-benzyl-N,N',N'-tris(6-methylpyridin-2-ylmethyl)ethylenediamine), as redox mediators in DSCs. Experimental measurements demonstrate that the coordination environment of Cu(II) complexes with pentadentate ligands remains unchanged in the presence of TBP, which is in stark contrast to the state-of-the-art bipyridyl counterpart. DSCs based on [Cu(tme)]^2+/+ complexes exhibit an excellent long-term stability and maintain more than 90 % of the initial efficiency after 400 h under continuous illumination, which outperform the reference devices incorporating the bipyridyl counterpart (less than 80 %) under identical conditions.

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.

Phenanthrene-Fused-Quinoxaline as a Key Building Block for Highly Efficient and Stable Sensitizers in Copper-Electrolyte-Based Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) based on Cu^II/I bipyridyl or phenanthroline complexes as redox shuttles have achieved very high open-circuit voltages (V_OC , more than 1 V). However, their short-circuit photocurrent density (J_SC ) has remained modest. Increasing the J_SC is expected to extend the spectral response of sensitizers to the red or NIR region while maintaining efficient electron injection in the mesoscopic TiO₂ film and fast regeneration by the Cu^I complex. Herein, we report two new D-A-π-A-featured sensitizers termed HY63 and HY64, which employ benzothiadiazole (BT) or phenanthrene-fused-quinoxaline (PFQ), respectively, as the auxiliary electron-withdrawing acceptor moiety. Despite their very similar energy levels and absorption onsets, HY64-based DSSCs outperform their HY63 counterparts, achieving a power conversion efficiency (PCE) of 12.5 %. PFQ is superior to BT in reducing charge recombination resulting in the near-quantitative collection of photogenerated charge carriers.

Copper-based homogeneous and heterogeneous catalysts for electrochemical water oxidation

Water oxidation is currently believed to be the bottleneck in the field of electrochemical water splitting and artificial photosynthesis. Enormous efforts have been devoted toward the exploration of water oxidation catalysts (WOCs), including homogeneous and heterogeneous catalysts. Recently, Cu-based WOCs have been widely developed because of their high abundance, low cost, and biological relevance. However, to the best of our knowledge, no review has been made so far on such types of catalysts. Thus, we have summarized the recent progress made in the development of homogeneous and heterogeneous Cu-based WOCs for electrochemical catalysis. Furthermore, the evaluations of catalytic activity, stability, and mechanism of these catalysts are carefully concluded and highlighted. We believe that this review can summarize the current progress in the field of Cu-based electrochemical WOCs and help in the design of more efficient and stable WOCs.

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.

Chlorine-Doped Perovskite Oxide: A Platinum-Free Cathode for Dye-Sensitized Solar Cells

Triiodide/iodide (I₃^-/I^-) redox couple-mediated solar cells, batteries, and electrochromic devices require highly efficient and stable electrocatalysts for I₃^- reduction reaction (IRR) to overcome performance limitations, whereas the widely used platinum (Pt) cathode for IRR has limitations of high price and unfavorable durability. In this work, we present a halogen element (chlorine) doping strategy to design low-cost perovskite-type electrocatalysts with enhanced IRR activity and stability. The dye-sensitized solar cell (DSSC) assembled by the LaFeO_2.965-δCl_0.035 cathode delivers an attractive power conversion efficiency (PCE) of 11.4% with a remarkable PCE enhancement factor of 23% compared with Pt, which is higher than most of the reported non-Pt DSSC cathodes. Attractively, LaFeO_2.965-δCl_0.035 displays superior IRR activity/stability and structural stability in the I₃^-/I^--based electrolyte compared to pristine LaFeO₃ because chlorine doping facilitates the creation of oxygen vacancies (active sites) and enhances surface acidity simultaneously. This study provides a new way for designing outstanding IRR electrocatalysts, which could be applied to many redox couple-mediated photo/electrochemical devices.

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

Dye-sensitized solar cells (DSSCs) have attracted a substantial interest in the last 30 years for the conversion of solar power to electricity. An important component is the redox mediator effecting the transport of charge between the photoelectrode and the dark counter electrode (CE). Among the possible mediators, metal coordination complexes play a prominent role and at present are incorporated in several types of devices with a power conversion efficiency exceeding 10%. The present review, after a brief introduction to the operation of DSSCs, discusses at first the requirements for a successful mediator. Subsequently, the properties of various classes of inorganic coordination complexes functioning as mediators relevant to DSSC operation are presented and the operational characteristics of DSSC devices analyzed. Particular emphasis is paid to the two main classes of efficient redox mediators, the coordination complexes of cobalt and copper; however other less efficient but promising classes of mediators, notably complexes of iron, nickel, manganese and vanadium, are also presented.

CoSe2/porous carbon shell composites as high-performance catalysts toward tri-iodide reduction in dye-sensitized solar cells

CoSe₂/carbon shell composites with many active sites were developed as catalysts for I₃⁻ reduction in dye-sensitized solar cells with efficiency and stability exceeding those of Pt.