A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials

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.

Organic-inorganic hybrid material for hole transport in inverted perovskite solar cells

Hole mobility is critical to the power conversion efficiencies of perovskite solar cells (PSCs). Organic small-molecule hole-transporting materials (HTMs) have attracted considerable interest in PSCs due to their structural flexibility and operational durability, but they suffer from modest hole mobility. On the other hand, inorganic HTMs with good hole mobility are inflexible in structural variation and exhibit unsatisfactory cell efficiency. In this study, a ligand BT28 and its zinc-based coordination complex BTZ30 were synthesized, characterized, and investigated as HTMs for PSC applications. The mixed-halide perovskites can be grown uniformly with large crystalline grains on both HTMs, which exhibit similar optical and electrochemical properties. However, it was discovered that the BTZ30-based solar cell exhibited an open-circuit voltage of 1.0817 V and a high short-circuit current density of 23.1392 mA cm-2 with a champion power conversion efficiency of close to 20 %. The performance difference between the two HTMs can be attributed to the difference in their hole mobilities, which is 63.31 % higher for BTZ30 than BT28. The comparison of non-metal and metal HTMs revealed the importance of considering hybrid structures to overcome some shortcomings associated with organic and inorganic HTMs and achieve high-performance PSCs.

Influence of the Lithium-Ion Concentration in Electrolytes on the Performance of Dye-Sensitized Photorechargeable Batteries

Dye-sensitized photorechargeable batteries (DSPBs) have recently gained attention for realizing energy recycling systems under dim light conditions. However, their performance under high storage efficiency (i.e., the capacity charged within a limited time) for practical application remains to be evaluated. Herein, we varied the lithium (Li)-ion concentration, which plays a dual role as energy charging and storage components, to obtain the optimized energy density of DSPBs. Electrochemical studies showed that the Li-ion concentration strongly affected the resistance characteristics of DSPBs. In particular, increasing the Li-ion concentration improved the output capacity and decreased the output voltage. Consequently, the energy density of the finely optimized DSPB improved from 8.73 to 12.64 mWh/cm3 when irradiated by a 1000-lx indoor light-emitting-diode lamp. These findings on the effects of Li-ion concentrations in electrolytes on the performance of DSPBs represent a step forward in realizing the practical application of DSPBs.

Push-pull photochromic dyes for semi-transparent solar cells with light-adjustable optical properties and high color-rendering index

We report the design, synthesis and characterization of push-pull photochromic naphthopyran dyes, incorporating different carbazole moieties as the electron-donor group for use in dye-sensitized solar cells. Compared to a reference dye incorporating a diphenylamine-type donor moiety, the introduction of functionalized carbazoles allows for a hypsochromic shift of the absorption of the coloured isomers of the dyes in the visible region and a better tuning of their spectra to the photopic response of the human eye. Under illumination, the molecules exhibit a broad absorption with a maximum comprised between 546 nm and 571 nm in solution and they reveal relatively fast discoloration kinetics. By using these dyes to fabricate photochromic solar cells whose optical and photovoltaic properties vary with the light exposure, we have achieved a PCE of up to 3% in opaque cells. Using these molecules in semi-transparent solar cells with different electrolytes, a PCE of 2.3% was achieved. We also produced a semi-transparent mini-module with an average visible transmittance varying between 66% and 50% and a colour rendering index around 95 in both the uncoloured and coloured states.

Spin Crossover and Fluorine-Specific Interactions in Metal Complexes of Terpyridines with Polyfluorocarbon Tails

In coordination chemistry and materials science, terpyridine ligands are of great interest, due to their ability to form stable complexes with a broad range of transition metal ions. We report three terpyridine ligands containing different perfluorocarbon (PFC) tails on the backbone and the corresponding FeII and CoII complexes. The CoII complexes display spin crossover close to ambient temperature, and the nature of this spin transition is influenced by the length of the PFC tail on the ligand backbone. The electrochemical properties of the metal complexes were investigated with cyclic voltammetry revealing one oxidation and several reduction processes. The fluorine-specific interactions were investigated by EPR measurements. Analysis of the EPR spectra of the complexes as microcrystalline powders and in solution reveals exchange-narrowed spectra without resolved hyperfine splittings arising from the 59 Co nucleus; this suggests complex aggregation in solution mediated by interactions of the PFC tails. Interestingly, addition of perfluoro-octanol in different ratios to the acetonitrile solution of the sample resulted in the disruption of the F ⋯ ${\cdots }$ F interactions of the tails. To the best of our knowledge, this is the first investigation of fluorine-specific interactions in metal complexes through EPR spectroscopy, as exemplified by exchange narrowing.

Progress of Photocapacitors

In response to the current trend of miniaturization of electronic devices and sensors, the complementary coupling of high-efficiency energy conversion and low-loss energy storage technologies has given rise to the development of photocapacitors (PCs), which combine energy conversion and storage in a single device. Photovoltaic systems integrated with supercapacitors offer unique light conversion and storage capabilities, resulting in improved overall efficiency over the past decade. Consequently, researchers have explored a wide range of device combinations, materials, and characterization techniques. This review provides a comprehensive overview of photocapacitors, including their configurations, operating mechanisms, manufacturing techniques, and materials, with a focus on emerging applications in small wireless devices, Internet of Things (IoT), and Internet of Everything (IoE). Furthermore, we highlight the importance of cutting-edge materials such as metal-organic frameworks (MOFs) and organic materials for supercapacitors, as well as novel materials in photovoltaics, in advancing PCs for a carbon-free, sustainable society. We also evaluate the potential development, prospects, and application scenarios of this emerging area of research.

Redox Shuttle-Based Electrolytes for Dye-Sensitized Solar Cells: Comprehensive Guidance, Recent Progress, and Future Perspective

A redox electrolyte is a crucial part of dye-sensitized solar cells (DSSCs), which plays a significant role in the photovoltage and photocurrent of the DSSCs through efficient dye regeneration and minimization of charge recombination. An I^-/I₃ ^- redox shuttle has been mostly utilized, but it limits the open-circuit voltage (V _oc) to 0.7-0.8 V. To improve the V _oc value, an alternative redox shuttle with more positive redox potential is required. Thus, by utilizing cobalt complexes with polypyridyl ligands, a significant power conversion efficiency (PCE) of above 14% with a high V _oc of up to 1 V under 1-sun illumination was achieved. Recently, the V _oc of a DSSC has exceeded 1 V with a PCE of around 15% by using Cu-complex-based redox shuttles. The PCE of over 34% in DSSCs under ambient light by using these Cu-complex-based redox shuttles also proves the potential for the commercialization of DSSCs in indoor applications. However, most of the developed highly efficient porphyrin and organic dyes cannot be used for the Cu-complex-based redox shuttles due to their higher positive redox potentials. Therefore, the replacement of suitable ligands in Cu complexes or an alternative redox shuttle with a redox potential of 0.45-0.65 V has been required to utilize the highly efficient porphyrin and organic dyes. As a consequence, for the first time, the proposed strategy for a PCE enhancement of over 16% in DSSCs with a suitable redox shuttle is made by finding a superior counter electrode to enhance the fill factor and a suitable near-infrared (NIR)-absorbing dye for cosensitization with the existing dyes to further broaden the light absorption and enhance the short-circuit current density (J _sc) value. This review comprehensively analyzes the redox shuttles and redox-shuttle-based liquid electrolytes for DSSCs and gives recent progress and perspectives.

Stable Cobalt-Mediated Monolithic Dye-Sensitized Solar Cells by Full Glass Encapsulation

Dye-sensitized solar cells (DSSCs) emerged in the market as one of the most promising indoor photovoltaic technologies to address the need for wireless powering of low-consuming electronics and sensor nodes of the internet of things (IoT). The monolithic design structure of the cell (M-DSSCs) makes the devices simpler and cheaper, and it is straightforward for constructing in-series modules. The most efficient DSSCs reported so far are Co(III/II)-mediated liquid junction cells with acetonitrile electrolytes; however, they are mostly unstable. This study reports on highly stable cobalt-mediated M-DSSCs, passing thermal cycling tests up to 85 °C according to ISOS standard protocols. Under 1000 h of aging in the dark and under simulated solar and artificial light soaking, all tested cells improved or retained their initial power conversion efficiency. Advanced long-term stability was achieved by eliminating the extrinsic factors of degradation, such as the interaction of the cell components with the environment and electrolyte leakage. This was obtained by encapsulation of the devices using a glass-frit sealant, including the holes for filling up the liquid components of the cells. The hermeticity of the encapsulation complies with the MIL-STD-883 standard fine helium gas leakage test, and its hermeticity remained unchanged after humidity-freeze cycles according to IEC 61646. The elimination of extrinsic degradation factors allowed reliable assessment of inner factors accountable for aging. The impact of the ISOS-protocol test conditions on the intrinsic device stability and long-term photovoltaic history of the M-DSSCs is discussed.

A Review on Gel Polymer Electrolytes for Dye-Sensitized Solar Cells

Significant growth has been observed in the research domain of dye-sensitized solar cells (DSSCs) due to the simplicity in its manufacturing, low cost, and high-energy conversion efficiency. The electrolytes in DSSCs play an important role in determining the photovoltaic performance of the DSSCs, e.g., volatile liquid electrolytes suffer from poor thermal stability. Although low volatility liquid electrolytes and solid polymer electrolytes circumvent the stability issues, gel polymer electrolytes with high ionic conductivity and enduring stability are stimulating substitutes for liquid electrolytes in DSSC. In this review paper, the advantages of gel polymer electrolytes (GPEs) are discussed along with other types of electrolytes, e.g., solid polymer electrolytes and p-type semiconductor-based electrolytes. The benefits of incorporating ionic liquids into GPEs are highlighted in conjunction with the factors that affect the ionic conductivity of GPEs. The strategies on the improvement of the properties of DSSCs based on GPE are also presented.

Quantitative Assessment of Ligand Substituent Effects on σ‐ and π‐Contributions to Fe−N Bonds in Spin Crossover Fe II Complexes

The effect of para‐substituent X on the electronic structure of sixteen tridentate 4‐X‐(2,6‐di(pyrazol‐1‐yl))‐pyridine (bpp^X) ligands and the corresponding solution spin crossover [Fe^II(bpp^X)₂]²⁺ complexes is analysed further, to supply quantitative insights into the effect of X on the σ‐donor and π‐acceptor character of the Fe‐N_A(pyridine) bonds. EDA‐NOCV on the sixteen LS complexes revealed that neither ΔE_orb,σ+π (R²=0.48) nor ΔE_orb,π (R²=0.31) correlated with the experimental solution T_1/2 values (which are expected to reflect the ligand field imposed on the iron centre), but that ΔE_orb,σ correlates well (R²=0.82) and implies that as X changes from EDG→EWG (Electron Donating to Withdrawing Group), the ligand becomes a better σ‐donor. This counter‐intuitive result was further probed by Mulliken analysis of the N_A atomic orbitals: N_A(p_x) involved in the Fe−N σ‐bond vs. the perpendicular N_A(p_z) employed in the ligand aromatic π‐system. As X changes EDG→EWG, the electron population on N_A(p_z) decreases, making it a better π‐acceptor, whilst that in N_A(p_x) increases, making it a better σ‐bond donor; both increase ligand field, and T_1/2 as observed. In 2016, Halcrow, Deeth and co‐workers proposed an intuitively reasonable explanation of the effect of the para‐X substituents on the

T_1/2 values in this family of complexes, consistent with the calculated MO energy levels, that M→L π‐backdonation dominates in these M−L bonds. Here the quantitative EDA‐NOCV analysis of the M−L bond contributions provides a more complete, coherent and detailed picture of the relative impact of M−L σ‐versus π‐bonding in determining the observed T_1/2, refining the earlier interpretation and revealing the importance of the σ‐bonding. Furthermore, our results are in perfect agreement with the ΔE(HS‐LS) vs. σ_p⁺(X) correlation reported in their work.

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.

Characterizing Interfacial Structures of Dye-Sensitized Solar Cell Working Electrodes

In this feature article, we discuss the fundamental use of materials-characterization methods that directly determine structural information on the dye···TiO₂ interface in dye-sensitized solar cells (DSCs). This interface is usually buried within the DSC and submerged in solvent and electrolyte, which renders such metrological work nontrivial. We will show how ex-situ X-ray reflectometry (XRR), atomic-force microscopy (AFM), grazing-incidence X-ray scattering (GIXS), pair-distribution-function analysis of X-ray diffraction data (gaPDF), and in-situ neutron reflectometry (NR) can be used to deliver specific structural information on the dye···TiO₂ interface regarding dye anchoring, dye aggregation, molecular dye orientation, intermolecular spacing between dye molecules, interactions between the dye molecules and the TiO₂ surface, and interactions between the dye molecules and the electrolyte components and precursors. Some of these materials-characterization techniques have been developed specifically for this purpose. We will demonstrate how the direct acquisition of such information from materials-characterization experiments is crucial for assembling a holistic structural picture of this interface, which in turn can be used to develop DSC design guidelines. Moreover, we will show how these methodologies can be used in the experimental-validation process of "design-to-device" pipelines for big-data- and machine-learning-based materials discovery. We conclude with an outlook on further developments of this design-to-device approach as well as the materials characterization of more dye···TiO₂ interfacial structures that involve known DSC dyes using the methods described herein. In addition, we propose to combine these formally disparate metrologies so that their complementary merits can be exploited simultaneously. New metrologies of this kind could serve as a "one-stop-shop" for the materials characterization of surfaces, interfaces, and bulk structures in DSCs and other devices with layered architectures.

Bio-inspired strategies for next-generation perovskite solar mobile power sources

Smart electronic devices are becoming ubiquitous due to many appealing attributes including portability, long operational time, rechargeability and compatibility with the user-desired form factor. Integration of mobile power sources (MPS) based on photovoltaic technologies with smart electronics will continue to drive improved sustainability and independence. With high efficiency, low cost, flexibility and lightweight features, halide perovskite photovoltaics have become promising candidates for MPS. Realization of these photovoltaic MPS (PV-MPS) with unconventionally extraordinary attributes requires new 'out-of-box' designs. Natural materials have provided promising designing solutions to engineer properties under a broad range of boundary conditions, ranging from molecules, proteins, cells, tissues, apparatus to systems in animals, plants, and humans optimized through billions of years of evolution. Applying bio-inspired strategies in PV-MPS could be biomolecular modification on crystallization at the atomic/meso-scale, bio-structural duplication at the device/system level and bio-mimicking at the functional level to render efficient charge delivery, energy transport/utilization, as well as stronger resistance against environmental stimuli (e.g., self-healing and self-cleaning). In this review, we discuss the bio-inspired/-mimetic structures, experimental models, and working principles, with the goal of revealing physics and bio-microstructures relevant for PV-MPS. Here the emphasis is on identifying the strategies and material designs towards improvement of the performance of emerging halide perovskite PVs and strategizing their bridge to future MPS.

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.

Synthesis and Physico-Chemical Properties of Homoleptic Copper(I) Complexes with Asymmetric Ligands as a DSSC Dye

To develop low-cost and efficient dye-sensitized solar cells (DSSCs), we designed and prepared three homoleptic Cu(I) complexes with asymmetric ligands, M1, M2, and Y3, which have the advantages of heteroleptic-type complexes and compensate for their synthetic challenges. The three copper(I) complexes were characterized by elemental analysis, UV-vis absorption spectroscopy, and electrochemical measurements. Their absorption spectra and orbital energies were evaluated and are discussed in the context of TD-DFT calculations. The complexes have high V_OC values (0.48, 0.60, and 0.66 V for M1, M2, and Y3, respectively) which are similar to previously reported copper(I) dyes with symmetric ligands, although their energy conversion efficiencies are relatively low (0.17, 0.64, and 2.66%, respectively).

Fluorenyl Indoline as an Efficient Electron Donor for Concerted Companion Dyes: Enhanced Light-Harvesting and Photocurrent

Concerted companion dyes (CC dyes) like XW61 have been demonstrated to be an effective platform for developing efficient DSSCs. However, the moderated phenothiazine-based electron donor in XW61 results in unsatisfactory J_sc. To address this problem, a stronger fluorenyl indoline-based electron donor has been used to construct porphyrin dye XW68 and organic dyes Y1-Y2. The stronger electron-donating character of the fluorenyl indoline unit leads to an enhanced J_sc value (20.48 mA·cm^-2) for the individual dye XW68. On this basis, CC dyes XW69-XW70-C8 have been designed and synthesized by combining the frameworks of Y1 and Y2 with XW68. The complementary absorption characters of the porphyrin and the organic dye moieties lead to panchromatic absorption with a strong light-harvesting capability from 350 to 700 nm and the onset wavelength extended to ca. 840 nm in the IPCE curves. As a result, excellent J_sc values have been achieved (>22 mA·cm^-2). In addition to the advantages of high J_sc, bulky octyl groups have been introduced into the donor of XW70-C8 to reduce dye aggregation and suppress charge recombination. Finally, a highest PCE of 11.1% with a satisfactory J_sc (22.25 mA·cm^-2) and an enhanced V_oc (750 mV) has been achieved upon coadsorption of XW70-C8 with CDCA. In addition, the CC dye XW70-C8-based solar cells exhibit excellent long-term photostability. These results provide an effective method for rationally improving the photovoltaic behavior, especially the J_sc of CC dyes, by introducing strong electron donor moieties with suitable substituents.

Starburst-Shaped D-π-A Chromophores Possessing a Hexaethynylbenzene Core for Dye-Sensitized Solar Cells

Two starburst-shaped organic chromophores, incorporating a hexaethynylbenzene core modified by five donor branches (D-branches) of (p-dioctylaminostyryl)benzene and one acceptor/anchoring branch (A-branch) of either carboxylic acid-terminated phenylethynylbenzene (SB-07) or cyanoacrylic acid-terminated diketopyrrolopyrrole (DPP)-thiophene (SB-08), were synthesized and applied to dye-sensitized solar cells (DSSCs). In these chromophores, the common donor moiety, five (p-dioctylaminostyryl)phenyl groups, exhibits excellent optical absorption in the visible region (molar absorption coefficient ε > 10⁵ M^-1 cm^-1 below 500 nm). The A-branch of SB-07 does not possess strong electron-accepting properties; however, the A-branch of SB-08, the DPP-thiophene moiety, serves as a strong electron acceptor site. Furthermore, the intramolecular charge-transfer (ICT) transition between the thiophene and DPP moieties extends the optical absorption range to the near-infrared region (∼800 nm). Optimized DSSC devices using SB-08 with coadsorption of chenodeoxycholic acid, in conjunction with iodide/triiodide-based electrolytes, exhibited incident photon-to-current conversion efficiency (IPCE) exceeding 70% in the 370-700 nm range and over 20% even at 800 nm, with a short-circuit photocurrent density (J_sc) of 19.3 mA cm^-2 and a power conversion efficiency (PCE) of 6.4% under AM 1.5G illumination (100 mW cm^-2). These results are considerably better than those of SB-07 (J_sc = 7.0 mA cm^-2, PCE = 3.3%). The starburst-shaped architecture presented here can be used as a novel structural motif for metal-free organic sensitizers because it enables flexible modification of the multiple D-branches that enhance light-harvesting ability and the A-branch that serves as an excited electron transport pathway.

Transparent and Colorless Dye-Sensitized Solar Cells Exceeding 75% Average Visible Transmittance

Most photovoltaic (PV) technologies are opaque to maximize visible light absorption. However, see-through solar cells open additional perspectives for PV integration. Looking beyond maximizing visible light harvesting, this work considers the human eye photopic response to optimize a selective near-infrared sensitizer based on a polymethine cyanine structure (VG20-C x ) to render dye-sensitized solar cells (DSSCs) fully transparent and colorless. This peculiarity was achieved by conferring to the dye the ability to strongly and sharply absorb beyond 800 nm (S0-S1 transition) while rejecting the upper S0-S n contributions far in the blue where the human retina is poorly sensitive. When associated with an aggregation-free anatase TiO2 photoanode, the selective NIR-DSSC can display 3.1% power conversion efficiency, up to 76% average visible transmittance (AVT), a value approaching the 78% AVT value of a standard double glazing window while reaching a color rendering index (CRI) of 92.1%. The ultrafast and fast charge transfer processes are herein discussed, clarifying the different relaxation channels from the dye monomer excited states and highlighting the limiting steps to provide future directions to enhance the performances of this nonintrusive NIR-DSSC technology.

The impact of insufficient time resolution on dye regeneration lifetime determined using transient absorption spectroscopy

Dye regeneration lifetimes of a combination of dyes and redox mediators were determined by two transient absorption (TA) spectrometers with 0.5 ns (sub-ns) and 6 ns (ns) time resolutions to elucidate the impact of insufficient time resolution on the measurements of dye regeneration kinetics in dye-sensitised semiconductor electrodes. Due to the disordered nature of the dye-sensitised electrodes, the dye regeneration lifetime is often characterised by half-decay time (τ1/2) of the initial signal magnitude. Alternatively, τ1/2,S is calculated from stretched-exponential lifetime (τww) and the distribution of lifetimes characterised by the stretch parameter (β). Stretched-exponential functions were numerically modelled, showing that to keep the error in τ1/2 ≤ 10%, τww needs to be at least 20 times longer than the time resolution in case of non-dispersive transients (β = 0.9) but at least 870 times longer when dispersive (β = 0.5). To test the predictions, TA decays of a combination of organic and porhyrin dyes and three cobalt-complex mediators are analysed, spanning a range of τww and β. These examples show that a 262% error in τ1/2 is possible if the time resolution of the TA setup is only 13 times faster than τww and smaller β results in larger error when τww is similar. Determining τ1/2,S by stretched-exponential fitting generally reduces the error compared to that determined directly from the graph. However, if the stretched-exponential function does not correctly describe the early signal transient, even a larger error by stretched-exponetial fitting is introduced. The key requirement for accurate measurement is to have a fast-enough TA setup to resolve the initial plateau of the TA signal. To demonstrate the impact of the measured errors, the measured regeneration lifetimes are plotted versus the driving force of the reaction and modelled using Marcus theory. Erroneous regeneration rates lead to an underestimated electronic coupling term by 2.2 times in case of a series of porphyrin dyes matched with Co complex electrolytes, a significant impact when the interpretation of factors affecting electron transfer at dye-sensitised semiconductor/electrolyte interface is discussed.

All-in-One, Solid-State, Solar-Powered Electrochemical Cell

Solar-powered electrochemical cells (SPECs) have been perceived as a potential strategy for coping with the intermittent nature of solar power. Most of the SPECs reported so far use corrosive/toxic liquid electrolyte and/or need very careful packaging, which is restricted by the scenario of implementation and arises the fabrication cost. Here, we demonstrate an all-in-one, solid-state SPEC with solar-to-output energy conversion efficiency of ca. 2.8% under AM 1.5 G irradiation. In this SPEC, a LiBr/polyacrylamide (PAM) hydrogel serves both as electrolyte, cathode-active mediator, and separator, which is sandwiched between an FTO/BiVO₄ photoanode and an FTO/Prussian blue (PB) anode. The use of solid-state PAM hydrogel promotes the charge-transfer dynamics at the interface of the photoanode and suppressed the undesired side reactions of electrolyte decomposition, representing an effective strategy by interfacial engineering toward the development of high-performance SPECs.

Fine-Tuning by Triple Bond of Carbazole Derivative Dyes to Obtain High Efficiency for Dye-Sensitized Solar Cells with Copper Electrolyte

Three novel dyes consisting of a 5,8,15-tris(2-ethylhexyl)-8,15-dihydro-5H-benzo[1,2-b:3,4-b':6,5-b″]tricarbazole (BTC) electron-donating group and a 4,7-bis(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (BTBT) π-bridge with an anchoring group of phenyl carboxyl acid were synthesized and applied in dye-sensitized solar cells (DSCs).The AJ202 did not contain any triple bonds, the AJ201's ethynyl group was inserted between the BTC and BTBT units, and the AJ206's ethynyl group was introduced between the BTBT moiety and the anchor group. The inclusion and position of the ethynyl linkage in the sensitizer molecules significantly altered the electrochemical properties of these dyes, which can fine-tune the energy levels of the dyes. The best performing devices contained AJ206 as a sensitizer and a Cu(I/II) redox couple, which resulted in a power conversion efficiency (PCE) up to 10.8% under the standard AM 1.5 G illumination, which obtained PCEs higher than those from the devices that contained AJ201 (9.2%) and AJ202 (9.7%) under the same conditions. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the sensitizers were tuned to be well-suited for the Cu(I/II) redox potential and the Fermi level of TiO₂. The innovative synthesis of a tricarbazole-based donor moiety in a sensitizer used in combination with a Cu(I/II) redox couple has resulted in relatively high PCEs.

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

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

A 25 mA cm-2 dye-sensitized solar cell based on a near-infrared-absorbing organic dye and application of the device in SSM-DSCs

A blue organic near infrared (NIR) absorbing sensitizer, AP25, is investigated for use in broadly absorbing dye-sensitized solar cells (DSCs). AP25 shows solar-to-electric conversion with an onset near 900 nm in DSC devices and a photocurrent near 25 mA cm-2 when co-sensitized. An all-organic SSM-DSC device reaches 10.3% PCE.

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

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

Alkyl-Group-Wrapped Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells: Branched Alkyl Chains Modulate the Aggregation of Dyes and Charge Recombination Processes

Electron transfer processes at the interfaces dictate the factors that improve the photovoltaic parameters, such as open-circuit voltage (V_oc) and short-circuit current (J_sc), of a dye-sensitized solar cell device, besides selection of a set of suitable anode, dye, electrolyte, and cathode materials. An inefficient charge injection process at the dye-TiO₂ interface and charge recombination at the TiO₂-dye/electrolyte interface have detrimental effects on improving both J_sc and V_oc. Hence, tailoring the factors that govern the improvement of J_sc and V_oc will be an ideal approach to get the desired sensitizers with good device efficiencies. Squaraines are far-red-active zwitterionic dyes and have a high molar extinction coefficient along with unique aggregation properties due to the large dipole moment associated with them. Here, we report a series of unsymmetrical squaraine dyes, SQS1 to SQS6, with systematic variation of alkyl groups at the sp³-C and N-atoms of the indoline unit that is away from the anchoring group to control the dye-dye interactions on the TiO₂ surface. The branched alkyl groups help in modulating the self-assembly of sensitizers on the TiO₂ surface, besides passivating the surface that helps avoid the charge recombination processes. Light harvesting efficiency and cyclic voltammetry studies of dye-sensitized TiO₂ electrodes indicate that the aggregation and charge hopping process between the dye molecules can be modulated, respectively, by systematically increasing the number of carbon atoms in the alkyl groups. Such a variation in the branched alkyl group helps enhance V_oc from 672 (SQS1) to 718 mV (SQS6) and J_sc from 7.95 (SQS1) to 12.22 mA/cm² (SQS6), with the device efficiency ranging from 3.82% to 6.23% without any coadsorbent. Dye SQS4 achieves the highest efficiency of 7.1% (V_oc = 715 mV, J_sc = 13.05 mA/cm²) with coadsorbent chenodeoxycholic acid (CDCA) using an iodine (I^-/I₃^-) electrolyte compared to its analogues. An analysis of the incident photon-to-current efficiency profiles indicates that the major contribution to photocurrent generation is from the aggregated squaraine dyes on TiO₂.

Quantum dot embedded N-doped functionalized multiwall carbon nanotubes boost the short-circuit current of Ru(ii) based dye-sensitized solar cells

Here, we report zinc sulfide quantum dots, ZnS(QDs), moored on N-doped functionalized multiwall carbon nanotubes (MWCNTs) wrapped with reduced graphene oxide (rGO). The MWCNTs have a tangled network, a particular surface area, and a distinctive hollow structure that may be suitable for use as a counter electrode (CE) material. A ZnS@N.f-MWCNTs@rGO composite as the CE on a fluorine-doped tin oxide substrate in a dye-sensitized solar cell (DSSC) was fabricated using a doctor blade technique. The electrochemical performance showed that at the electrolyte/CE interface, the ZnS(QDs) and N-doped functionalized MWCNTs wrapped with rGO (ZnS@N.f-MWCNTs@rGO) electrode has a lower transfer charge resistance (R_ct) and a greater catalytic capacity than naked ZnS(QDs). A power conversion efficiency (PCE) of 9.4% was attained for this DSSC gadget, which is higher than that of a DSSC gadget utilizing ZnS(QDs), ZnS@N.f-MWCNTs, ZnS@rGO and Pt. Also, the DSSC device using ZnS@N.f-MWCNTs@rGO had a fill factor (FF) that was better than the other counter electrodes. The cyclic voltammetry and electrochemical impedance spectra (EIS) electron transfer measurements showed that ZnS@N.f-MWCNTs@rGO films can provide fast electron transfer from the electrolyte to the CE and great electrocatalytic activity to reduce triiodide to a CE based on ZnS@N.f-MWCNTs@rGO in the DSSC.

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.

Precious metal-free solar-to-fuel generation: SSM-DSCs powering water splitting with NanoCOT and NiMoZn electrocatalysts

A precious metal-free sequential series multijunction dye-sensitized solar cell (SSM-DSC)-powered water electrolysis system is demonstrated using NanoCOT and NiMoZn electrodes.

Solid-State Solar Energy Conversion from WO3 Nano and Microstructures with Charge Transportation and Light-Scattering Characteristics

Solar energy conversion devices composed of highly crystalline gel polymers with disk-WO₃ nanostructure and plate-WO₃ microstructures (D-WO₃ and P-WO₃, respectively) exhibited higher power conversion efficiency than those with a gel electrolyte. In this study, D-WO₃ and P-WO₃ were prepared using a hydrothermal process and their structural and morphological features were investigated for application in solar energy conversion devices. The P-WO₃ solid-state electrolyte significantly enhanced the cell performance owing to its charge transportation and light-scattering characteristics. The P-WO₃ solid-state electrolyte showed a power conversion efficiency of 6.3%, which is higher than those of the gel (4.2%) and D-WO₃ solid-state (5.5%) electrolytes. The electro-chemical impedance spectroscopy (EIS), intensity-modulated voltage spectroscopy (IMVS), diffuse reflectance, and incident photon-to-current conversion efficiency (IPCE) analysis results showed that the P-WO₃ solid-state electrolyte showed improved charge transportation and light scattering, and hence enhanced the cell performance.

Effect of Ligand Structures of Copper Redox Shuttles on Photovoltaic Performance of Dye-Sensitized Solar Cells

In recent years, copper(I/II) complexes have emerged as alternative redox shuttles in dye-sensitized solar cells (DSSCs), exhibiting more positive redox potential than iodine- and cobalt-based redox shuttles. In particular, copper(I/II) complexes with 1,10-phenanthroline- or 2,2'-bipyridyl-based ligands attained moderate to high power conversion efficiencies (6-11%) with a high open-circuit voltage (V_OC) over 1.0 V due to the positive potentials. Although copper(I/II) complexes with 1,10-phenanthroline-based ligands with 2,9-substituents have been developed, the effect of their ligand structures on the photovoltaic performance of DSSCs have not been fully addressed due to limited synthetic access to 1,10-phenanthroline derivatives. In this study, we designed and synthesized a series of copper(I/II) complexes with 1,10-phenanthroline ligands with different substituents at the 2,9-positions: bis(2-n-butyl-1,10-phenanthroline)copper(I/II) ([Cu(bp)₂]^1+/2+), bis(2-ethyl-9-methyl-1,10-phenanthroline)copper(I/II) ([Cu(emp)₂]^1+/2+), bis(2,9-diethyl-1,10-phenanthroline)copper(I/II) ([Cu(dep)₂]^1+/2+), and bis(2,9-diphenyl-1,10-phenanthroline)copper(I/II) ([Cu(dpp)₂]^1+/2+). The more positive redox potentials of [Cu(emp)₂]^1+/2+ and [Cu(dep)₂]^1+/2+, compared to that of bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu(dmp)₂]^1+/2+), originate from the larger steric hindrance of the ethyl group instead of the methyl group, whereas the redox potential of [Cu(bp)₂]^1+/2+ is significantly shifted to the negative direction because of the lower steric hindrance of the 2-monosubstituted 1,10-phenanthroline ligands. The efficiency of the DSSC with [Cu(bp)₂]^1+/2+ (5.90%) is almost comparable to the DSSC with [Cu(dmp)₂]^1+/2+ (6.29%). In contrast, the DSSCs with [Cu(emp)₂]^1+/2+ (3.25%), [Cu(dep)₂]^1+/2+ (2.56%), and [Cu(dpp)₂]^1+/2+ (2.21%) exhibited lower efficiencies than those with [Cu(dmp)₂]^1+/2+ and [Cu(bp)₂]^1+/2+. The difference can be rationalized by the electron collection efficiencies. Considering the similar photovoltaic properties of the DSSCs with [Cu(bp)₂]^1+/2+ and [Cu(dmp)₂]^1+/2+, the use of copper(I/II) complexes with 2-monosubstituted 1,10-phenanthroline ligands as the redox shuttle may be useful to improve the short-circuit current density while retaining the rather high V_OC value when dyes with a smaller bandgap (i.e., better light harvesting) are developed.

Methodologies in Spectral Tuning of DSSC Chromophores through Rational Design and Chemical-Structure Engineering

The investigation of new photosensitizers for Grätzel-type organic dye-sensitized solar cells (DSSCs) remains a topic of interest for researchers of alternative solar cell materials. Over the past 20 years, considerable and increasing research efforts have been devoted to the design and synthesis of new materials, based on "donor, π-conjugated bridge, acceptor" (D-π-A) organic dye photosensitizers. In this paper, the computational chemistry methods are outlined and the design of organic sensitizers (compounds, dyes) is discussed. With reference to recent literature reports, rational molecular design is demonstrated as an effective process to study structure-property relationships. Examples from established organic dye sensitizer structures, such as TA-St-CA, Carbz-PAHTDDT (S9), and metalloporphyrin (PZn-EDOT), are used as reference structures for an examination of this concept applied to generate systematically modified structural derivatives and hence new photosensitizers (i.e., dyes). Using computer-aided rational design (CARD), the in silico design of new chromophores targeted an improvement in spectral properties via the tuning of electronic structures by substitution of molecular fragments, as evaluated by the calculation of absorption profiles. This mini review provides important rational design strategies for engineering new organic light-absorbing compounds towards improved spectral absorption and related optoelectronic properties of chromophores for photovoltaic applications, including the dye-sensitized solar cell (DSSC).