Dye-sensitized solar cells strike back Muñoz-García, Ana Belén; Benesperi, Iacopo; Boschloo, Gerrit ...
Chemical Society reviews,
11/2021, Letnik:
5, Številka:
22
Journal Article
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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.
Dye-sensitized solar cells (DSCs) are celebrating their 30
th
birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. Righteous font designed by Astigmatic and licensed under the Open Font License.
A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review ...highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).
The scientific community now agrees that the rise in atmospheric CO2, the most abundant green house gas, comes from anthropogenic sources such as the burning of fossil fuels. This atmospheric rise in ...CO2 results in global climate change. Therefore methods for photochemically transforming CO2 into a source of fuel could offer an attractive way to decrease atmospheric concentrations. One way to accomplish this conversion is through the light-driven reduction of carbon dioxide to methane (CH4(g)) or methanol (CH3OH(l)) with electrons and protons derived from water. Existing infrastructure already supports the delivery of natural gas and liquid fuels, which makes these possible CO2 reduction products particularly appealing. This Account focuses on molecular approaches to photochemical CO2 reduction in homogeneous solution. The reduction of CO2 by one electron to form CO2 •− is highly unfavorable, having a formal reduction potential of −2.14 V vs SCE. Rapid reduction requires an overpotential of up to 0.6 V, due at least in part to the kinetic restrictions imposed by the structural difference between linear CO2 and bent CO2 •−. An alternative and more favorable pathway is to reduce CO2 though proton-assisted multiple-electron transfer. The development of catalysts, redox mediators, or both that efficiently drive these reactions remains an important and active area of research. We divide these reactions into two class types. In Type I photocatalysis, a molecular light absorber and a transition metal catalyst work in concert. We also consider a special case of Type 1 photocatalysis, where a saturated hydrocarbon links the catalyst and the light absorber in a supramolecular compound. In Type II photocatalysis, the light absorber and the catalyst are the same molecule. In these reactions, transition-metal coordination compounds often serve as catalysts because they can absorb a significant portion of the solar spectrum and can promote activation of small molecules. This Account discusses four classes of transition-metal catalysts: (A) metal tetraaza-macrocyclic compounds; (B) supramolecular complexes; (C) metalloporphyrins and related metallomacrocycles; (D) Re(CO)3(bpy)X-based compounds where bpy = 2,2′-bipyridine. Carbon monoxide and formate are the primary CO2 reduction products, and we also propose bicarbonate/carbonate production. For comprehensiveness, we briefly discuss hydrogen formation, a common side reaction that occurs concurrently with CO2 reduction, though the details of that process are beyond the scope of this Account. It is our hope that drawing attention both to current mechanistic hypotheses and to the areas that are poorly understood will stimulate research that could one day provide an efficient solution to this global problem.
The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore–catalyst ...assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO2 by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO2 reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO2 reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.
A central theme in introductory and advanced chemical education courses pertains directly to the transfer of electrons between atoms, ions, or molecules. This article presents theoretical treatments ...of electron transfer with specific attention toward applying these principles to experiment. The goal is to revitalize teaching electron transfer at the undergraduate and graduate levels. Central theoretical aspects are presented through the construction of Gibbs free (potential) energy surfaces with definitions and semiquantitative descriptions of the three key parameters necessary to compute electron transfer rate constants with Marcus theory: (1) the Gibbs free energy change, ΔG°; (2) the reorganization energy, λ; and (3) the electronic coupling between D and A wave functions, H DA. A simplified description of this theory is presented with classical free energy surfaces for the electron donor and acceptor wherein the force constant in Hooke’s Law is replaced by λ. Variation of ΔG° results in a Gaussian distribution of activation energies that give rise to Marcus normal, activationless, and inverted kinetic behaviors. Classical and contemporary experimental examples that have tested and utilized Marcus theory are described, including the first validation of the inverted kinetic region. It is shown that, as the donor–acceptor coupling increases, adiabatic electron transfer may result where it becomes more difficult to decouple the Marcus parameters through experiment. The trials and tribulations of doing so are described that provide context and enable readers to understand the prior electron transfer literature and use the pedagogical foundations presented herein for their own learning and pleasure.
Enantioselective catalysis of excited-state photoreactions remains a substantial challenge in synthetic chemistry, and intermolecular photoreactions have proven especially difficult to conduct in a ...stereocontrolled fashion. Herein, we report a highly enantioselective intermolecular 2 + 2 cycloaddition of 3-alkoxyquinolones catalyzed by a chiral hydrogen-bonding iridium photosensitizer. Enantioselectivities as high as 99% ee were measured in reactions with a range of maleimides and other electron-deficient alkene reaction partners. An array of kinetic, spectroscopic, and computational studies supports a mechanism in which the photocatalyst and quinolone form a hydrogen-bonded complex to control selectivity, yet upon photoexcitation of this complex, energy transfer sensitization of maleimide is preferred. The sensitized maleimide then reacts with the hydrogen-bonded quinolone–photocatalyst complex to afford a highly enantioenriched cycloadduct. This finding contradicts a long-standing tenet of enantioselective photochemistry that held that stereoselective photoreactions require strong preassociation to the sensitized substrate in order to overcome the short lifetimes of electronically excited organic molecules. This system therefore suggests that a broader range of alternate design strategies for asymmetric photocatalysis might be possible.
Chloride oxidation has tremendous utility in the burgeoning field of chlorine-mediated C–H activation, yet it remains a challenging process to initiate with light because of the exceedingly positive ...one-electron reduction potential, E° (Cl•/–), beyond most common transition-metal photooxidants. Herein, two photocatalytic chloride oxidation pathways that involve either one- or consecutive two-photon excitation of N-phenylphenothiazine (PTH) are presented. The one-photon pathway generates PTH• + by oxidative quenching that subsequently disproportionates to yield PTH2+ that oxidizes chloride; this pathway is also accessed by the electrochemical oxidation of PTH. The two-photon pathway, which proceeded through the radical cation excited state, 2PTH• +*, was of particular interest as this super-photooxidant was capable of directly oxidizing chloride to chlorine atoms. Laser flash photolysis revealed that the photooxidation by the doublet excited state proceeded on a subnanosecond timescale through a static quenching mechanism with an ion-pairing equilibrium constant of 0.36 M–1. The PTH photoredox chemistry was quantified spectroscopically on nanosecond and longer time scales, and chloride oxidation chemistry was revealed by reactivity studies with model organic substrates. One- and two-photon excitation of PTH enabled chlorination of unactivated C(sp3)–H bonds of organic compounds such as cyclohexane with substantial yield enhancement observed from inclusion of the second excitation wavelength. This study provides new mechanistic insights into chloride oxidation catalyzed by an inexpensive and commercially available organic photooxidant.
The total reorganization energy, λ, for interfacial electron transfer, ET, from a conductive electrode to redox-active molecules at fixed positions within the electric double layer, EDL, has been ...determined experimentally. Conductive indium–tin-oxide (ITO, In2O3:Sn) mesoporous films were functionalized with 4-N,N-di(p-tolyl)-aminobenzylphosphonic acid (TPA) and/or RuII(bpy)2(4,4′-(PO3H2)2-bpy)2+ (RuP), where bpy is 2,2′-bipyridine. The small inner-sphere reorganizations, λi, for RuIII/IIP and TPA+/0 make them excellent probes of outer-sphere reorganization energy, λo, as λi ≪ λo such that λ = λi + λo ≈ λo. Consecutive layer-by-layer addition of ZrIV-bridged methylenediphosphonic acid enabled positioning at distances from 4 to 27 Å from the ITO. Excited-state injection into the ITO by RuP* generated ITO(e–)|RuIIIP. For ITO cofunctionalized with TPA and RuP, subnanosecond lateral ET yielded ITO(e–)|TPA+. The kinetics for ET from ITO to RuIIIP or TPA+ were quantified spectroscopically as a function of applied potential (E app) and hence driving force, −ΔG°. Marcus–Gerischer analysis of this data provided λ. Significantly, λo was near zero at close electrode proximity, λ = 0.11 eV at a distance of ∼4 Å, as manifest by kinetics largely insensitive to E app. In agreement with dielectric continuum theory, λ increased to values expected in CH3CN solution when the molecule was positioned at a distance of ∼27 Å (λ = 0.94 eV). The data reveal small intrinsic barriers for electron transfer proximate to conductive interfaces, which is an exploitable behavior in solar energy conversion and other applications that utilize transparent conductive oxides to accept or deliver electrons.
The 2010 Millennium Technology Grand Prize was awarded to Michael Grätzel for his ground-breaking research that has led to the practical application of dye-sensitized solar cells. Although Grätzel ...began his research well before nanotechnology had the “buzz” that it does today, the mesoscopic thin films he has developed have paved the way for generations of scientists to exploit the nanoscale for energy conversion. In addition to practical application, his research has led to a deeper understanding of photoinitiated charge-transfer processes at semiconductor interfaces. Here, the key scientific developments that guided early progress in dye-sensitized solar cells are summarized, with emphasis on fundamental advances that have enabled practical application.
Three iridium photosensitizers, Ir(dCF3ppy)2(N–N)+, where N–N is 1,4,5,8-tetraazaphenanthrene (TAP), pyrazino2,3-aphenazine (pzph), or benzoapyrazino2,3-hphenazine (bpph) and dCF3ppy is ...2-(3,5-bis(trifluoromethyl-phenyl)pyridine), were found to be remarkably strong photo-oxidants with enhanced light absorption in the visible region. In particular, judicious ligand design provided access to Ir-bpph, with a molar absorption coefficient, ε = 9800 M–1 cm–1, at 450 nm and an excited-state reduction potential, E(Ir+*/0) = 1.76 V vs NHE. These complexes were successful in performing light-driven charge separation and energy storage, where all complexes photo-oxidized seven different electron donors with rate constants (0.089−3.06) × 1010 M–1 s–1. A Marcus analysis provided a total reorganization energy of 0.7 ± 0.1 eV for excited-state electron transfer.
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