Iodide-free ionic liquid with dual redox couples for dye-sensitized solar cells with high open-circuit voltage

Choline chloride-based deep eutectic solvents as effective electrolytes for dye-sensitized solar cells

Electrolytes for dye-sensitized solar cells remain a challenge for large-scale production and commercialization, hindering the wide application of solar cells. We have developed two new electrolyte-based deep eutectic solvents using a mixture of choline chloride with urea and with ethylene glycol for dye-sensitized solar cells. The prominent features of the two deep eutectic solvent electrolytes are simple preparation for large-scale production with inexpensive, available, and nontoxic starting materials and biodegradability. The solar cell devices proceeded in a safe manner as the two deep eutectic solvents afforded low-cost technology and comparative conversion efficiency to a popular ionic liquid, namely 1-ethyl-3-methylimidazolium tetracyanoborate. Results showed that devices with choline chloride and urea electrolyte exhibited improved open circuit voltage values (V_OC), while the ones with choline chloride and ethylene glycol showed an increase in the short circuit current (I_sc). Characterization of the devices by electrochemical impedance spectroscopy helped explain the effects of their molecular structures on the enhancement of either V_OC or I_sc values. These new solvents expand the electrolyte choices for designing dye-sensitized solar cells, especially for the purpose of using low-cost and eco-friendly materials for massive production.

Electrolytes for dye-sensitized solar cells remain a challenge for large-scale production and commercialization, hindering the wide application of solar cells.

Polyradical PROXYL/TEMPO Conjugates Connected by Ester/Amide Bridges: Synthesis, Physicochemical Studies, and DFT Calculations

A series of di-/trinitroxide esters and amides featuring PROXYL and/or TEMPO radicals connected with alicyclic bridges were prepared in 61-92 % yields and their properties were analysed by using multiple experimental techniques. The examination of EPR spectra of radicals in organic solvents augmented with DFT calculations brought valuable information on the conformational dynamics and spin exchange mechanisms. Cyclic voltammetry investigations revealed (quasi)reversible electrochemical behaviour of studied nitroxides with their half-wave potentials ranging from -51 to -17 mV. SQUID measurements of selected radicals revealed that the magnetism of di- and trinitroxides is significantly different, since antiferromagnetic coupling in biradicals is notably larger than in triradicals. The single-crystal X-ray analysis of selected biradicals revealed the existence of 3D supramolecular networks of molecules linked through hydrogen-bonding interactions. These polynitroxide radicals can serve as promising bridging or chelating ligands in the synthesis of transition-metal-based molecular magnets.

Electroactive and Sustainable Cu-MOF/PEDOT Composite Electrocatalysts for Multiple Redox Mediators and for High-Performance Dye-Sensitized Solar Cells

An electrically conductive Cu-MOF, {[Cu₂(6-mercaptonicotinic acid)(6-mercaptonicotinate)]·NH₄}_n, was successfully electrodeposited on the conductive substrates via using poly(3,4-ethylenedioxythiophene) (PEDOT) as the binder. Multiple functionalities of the Cu-MOF microparticle within the Cu-MOF/PEDOT composite electrode were systematically vindicated as (1) releasing the cohesive strength among the PEDOT matrix, thus enhancing the film adhesion to substrate, (2) providing excellent intrinsic heterogeneous rate constant via lowering the reaction active energy, (3) supplying numerous active sites at the center or edges on its (-Cu-S-)_n honeycomb-like planes, (4) facilitating the electron transfer through its two-dimensional (-Cu-S-)_n plains, and (5) benefiting the penetration of the redox mediators through its porous frameworks. In multiple redox mediators (i.e., I^-/I₃^-, cobalt(II/III)-complex, and copper(I/II)-complex), the composite Cu-MOF/PEDOT electrode exhibited superior electrocatalyst activity and kept almost 100% of its initial redox peak currents after continuous cyclic voltammetric scanning for 300 cycles. As a high-performance electrocatalyst for the counter electrode in dye-sensitized solar cells (DSSCs), the composite Cu-MOF/PEDOT electrode rendered its cell a decent solar-to-electricity conversion efficiency of up to 9.45% at 1 sun and 22.80% at room light illumination. Compared to the traditional platinum electrode (7.67%), the low-cost Cu-MOF/PEDOT composite electrode has great possibility to be used for various electrochemical devices and the Internet-of-things applications.

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).

Stable Radical Materials for Energy Applications

Although less studied than their closed-shell counterparts, materials containing stable open-shell chemistries have played a key role in many energy storage and energy conversion devices. In particular, the oxidation-reduction (redox) properties of these stable radicals have made them a substantial contributor to the progress of organic batteries. Moreover, the use of radical-based materials in photovoltaic devices and thermoelectric systems has allowed for these emerging molecules to have impacts in the energy conversion realm. Additionally, the unique doublet states of radical-based materials provide access to otherwise inaccessible spin states in optoelectronic devices, offering many new opportunities for efficient usage of energy in light-emitting devices. Here, we review the current state of the art regarding the molecular design, synthesis, and application of stable radicals in these energy-related applications. Finally, we point to fundamental and applied arenas of future promise for these designer open-shell molecules, which have only just begun to be evaluated in full.

1-Alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids: novel and low-viscosity ionic liquid electrolytes for dye-sensitized solar cells

Dye-sensitized Solar Cells (DSCs) based on ruthenium complex N719 as sensitizer have received much attention due to their affordability and high efficiency. However, their best performance is only achieved when using volatile organic solvents as electrolyte solutions, which are unstable under prolonged thermal stress. Thus, we developed a new series of 1-alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids used as robust DSC electrolytes. These ionic liquids exhibit low viscosity, high conductivity, and thermal stability. The implementation of 1-but-3-enyl-3-methyl-imidazolium trifluoromethanesulfonate, [ButMIm]OTf, into DSCs gave the best photovoltaic performance. The results are fairly comparable to those reports for other popular ionic liquid electrolytes currently used in DSC field. An insightful discussion on the relationship between the structure of these new ionic liquids and the J–V characterization as well as electrochemical impedance measurement of DSCs will give more interesting information. The results are useful for large-scale outdoor application of DSCs.

A new series of 1-alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids was prepared under microwave irradiation for application in DSC electrolytes.

Metal Selenides as Efficient Counter Electrodes for Dye-Sensitized Solar Cells

Solar energy is the most abundant renewable energy available to the earth and can meet the energy needs of humankind, but efficient conversion of solar energy to electricity is an urgent issue of scientific research. As the third-generation photovoltaic technology, dye-sensitized solar cells (DSSCs) have gained great attention since the landmark efficiency of ∼7% reported by O'Regan and Grätzel. The most attractive features of DSSCs include low cost, simple manufacturing processes, medium-purity materials, and theoretically high power conversion efficiencies. As one of the key materials in DSSCs, the counter electrode (CE) plays a crucial role in completing the electric circuit by catalyzing the reduction of the oxidized state to the reduced state for a redox couple (e.g., I₃^-/I^-) in the electrolyte at the CE-electrolyte interface. To lower the cost caused by the typically used Pt CE, which restricts the large-scale application because of its low reserves and high price, great effort has been made to develop new CE materials alternative to Pt. A lot of Pt-free electrocatalysts, such as carbon materials, inorganic compounds, conductive polymers, and their composites with good electrocatalytic activity, have been applied as CEs in DSSCs in the past years. Metal selenides have been widely used as electrocatalysts for the oxygen reduction reaction and light-harvesting materials for solar cells. Our group first expanded their applications to the DSSC field by using in situ-grown Co_0.85Se nanosheet and Ni_0.85Se nanoparticle films as CEs. This finding has inspired extensive studies on developing new metal selenides in order to seek more efficient CE materials for low-cost DSSCs, and a lot of meaningful results have been achieved in the past years. In this Account, we summarize recent advances in binary and mutinary metal selenides applied as CEs in DSSCs. The synthetic methods for metal selenides with various morphologies and stoichiometric ratios and deposition methods for CE films are described. We emphasize that the in situ growth method exhibits advantages over other methods for fabricating stable and efficient CEs. We focus on the effect of morphology on the electocatalytic and photovoltaic performance. Application of transparent metal selenide CEs in bifacial DSSCs and the superiority of in situ-grown metal selenide nanosheet fiber CEs used for fiber DSSCs are presented. In addition, we show that metal selenides with a hollow sphere structure can function not only as an efficient electrocatalyst but also as a light-scattering layer. Finally, we present our views on the current challenges and future development of metal selenide CE materials.

Brønsted acidic ionic liquid-promoted direct C3-acylation of N-unsubstituted indoles with acid anhydrides under microwave irradiation

A green and efficient method for the synthesis of 3-acylindoles using a Brønsted acidic ionic liquid under microwave irradiation has been developed.

Multifunctional Iodide-Free Polymeric Ionic Liquid for Quasi-Solid-State Dye-Sensitized Solar Cells with a High Open-Circuit Voltage

A polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was newly synthesized and used for a multifunctional gel electrolyte in a quasi-solid-state dye-sensitized solar cell (QSS-DSSC). POEI-IS has several functions: (a) acts as a gelling agent for the electrolyte of the DSSC, (b) possesses a redox mediator of SeCN(-), which is aimed to form a SeCN(-)/(SeCN)3(-) redox couple with a more positive redox potential than that of traditional I(-)/I3(-), (c) chelates the potassium cations through the lone pair electrons of the oxygen atoms of its poly(oxyethylene)-imide-imidazolium (POEI-I) segments, and (d) obstructs the recombination of photoinjected electrons with (SeCN)3(-) ions in the electrolyte through its POEI-I segments. Thus, the POEI-IS renders a high open-circuit voltage (VOC) to the QSS-DSSC due to its functions of b-d and prolongs the stability of the cell due to its function of a. The QSS-DSSC with the gel electrolyte containing 30 wt % of the POEI-IS in liquid selenocyanate electrolyte exhibited a high VOC of 825.50 ± 3.51 mV and a high power conversion efficiency (η) of 8.18 ± 0.02%. The QSS-DSSC with 30 wt % POEI-IS retained up to 95% of its initial η after an at-rest stability test with the period of more than 1,000 h.

Substituted ferrocenes and iodine as synergistic thermoelectrochemical heat harvesting redox couples in ionic liquids

Combining ferrocene and iodine results in enhanced thermoelectrochemical (or thermogalvanic) waste heat harvesting abilities, for both the Seebeck coefficient and the overall power output. All systems displayed a mixture of ferrocene, ferrocenium, iodine and triiodide. The observed enhancement correlates with lower electron-density on the ferrocene; the synergistic improvement observed for mixtures of substituted ferrocenes and iodine is attributed to the formation of charge-transfer complexes. Combining dibutanoylferrocene and iodine resulted in the highest Seebeck coefficient of 1.67 mV K(-1).

Plasmon-Induced Broadband Light-Harvesting for Dye-Sensitized Solar Cells Using a Mixture of Gold Nanocrystals

The efficiency of dye-sensitized solar cells (DSSCs) is generally limited by the mismatch between the absorption spectrum of the photosensitizer and the solar irradiation spectrum. This work describes the use of a mixture that containing proper proportions of SiO2 coated Au nanospheres (AuNSs@SiO2 ) and Au nanorods (AuNRs@SiO2 ) (the mixture was denoted as AuNCs@SiO2 ) to enhance the sunlight utility in DSSCs. The incorporation of AuNCs@SiO2 into the TiO2 photoanode induced broadband light-harvesting at both low- and long- wavelengths and thus enhanced the photocurrent compared to that of plasmonic solar cells based on either AuNSs@SiO2 or AuNRs@SiO2 . Upon the doping of AuNCs@SiO2 , the overall power conversion efficiency (PCE) increased from 7.39 to 9.12 % for DSSCs based on organic liquid electrolytes.

Newly Elaborated Multipurpose Polymer Electrolyte Encompassing RTILs for Smart Energy-Efficient Devices

Profoundly ion-conducting, self-standing, and tack-free ethylene oxide-based polymer electrolytes encompassing a room-temperature ionic liquid (RTIL) with specific amounts of lithium salt are successfully prepared via a rapid and easily upscalable process including a UV irradiation step. All prepared materials are thoroughly characterized in terms of their physical, chemical, and morphological properties and eventually galvanostatically cycled in lab-scale lithium batteries (LIBs) exploiting a novel direct polymerization procedure to get intimate electrode/electrolyte interfacial characteristics. The promising multipurpose characteristics of the newly elaborated materials are demonstrated by testing them in dye-sensitized solar cells (DSSCs), where the introduction of the iodine/iodide-based redox mediator in the polymer matrix assured the functioning of a lab-scale test cell with conversion efficiency exceeding 6% at 1 sun. The reported results enlighten the promising prospects of the material to be successfully implemented as stable, durable, and efficient electrolyte in next-generation energy conversion and storage devices.