Photoinduced Electron Transfer Dynamics of Cyclometalated Ruthenium (II)–Naphthalenediimide Dyad at NiO Photocathode

Both forward and backward electron transfer kinetics at the sensitizer/NiO interface is critical for p-type dye-sensitized photocathodic device. In this article, we report the photoinduced electron transfer kinetics of a Ru­(II) chromophore–acceptor dyad sensitized NiO photocathode. The dyad (O26) is based on a cyclometalated Ru­(N^∧C^∧N)­(N^∧N^∧N) (Ru­[II]) chromophore and a naphthalenediimide (NDI) acceptor, where N^∧C^∧N represents 2,2′-(4,6-dimethyl-phenylene)-bispyridine and N^∧N^∧N represents 2,2′,6′,6″-terpyridine ligand. When the dyad is dissolved in a CH₃CN solution, electron transfer to form the Ru­(III)–NDI^– occurs with a rate constant <i>k</i>_f = 1.1 × 10¹⁰ s^–1 (τ_f = 91 ps), and electron–hole pair recombines to regenerate ground state with a rate constant <i>k</i>_b = 4.1 × 10⁹ s^–1 (τ_b = 241 ps). When the dyad is adsorbed on a NiO film by covalent attachment through the carboxylic acid group, hole injection takes place first within our instrument response time (∼180 fs) followed by the subsequent electron shift onto the NDI to produce the interfacial charge-separated state [NiO­(h⁺)–Ru­(II)–NDI^–] with a rate constant <i>k</i>_f = 9.1 × 10¹¹ s^–1 (τ_f = 1.1 ps). The recovery of the ground state occurs with a multiexponential rate constant <i>k</i>_b = 2.3 × 10⁹ s^–1 (τ_b = 426 ps). The charge recombination rate constant is slightly slower than a reference cyclometalated ruthenium compound (O25) with no NDI group (τ_b = 371 ps). The fast formation of interfacial charge separated state is a result of ultrafast hole injection resulting in the reduced form of sensitizer, which provides a larger driving force for NDI reduction. The kinetic study suggests that Ru­(II) chromophore–acceptor dyads are promising sensitizers for the NiO photocathode devices

Electrochromic Graphene Molecules

Polyclic aromatic hydrocarbons also called Graphene Molecules (GMs), with chemical composition C₁₃₂H₃₆(COOH)₂ were synthesized <i>in situ</i> on the surface of transparent nanocrystalline indium tin oxide (<i>nc</i>-ITO) electrodes and their electronic structure was studied electrochemically and spectro-electrochemically. Variations in the potential applied onto the <i>nc</i>-ITO/GM electrodes induce only small changes in the observed current, but they produce dramatic changes in the absorption of the GMs, which are associated with their oxidation and reduction. Analysis of the absorption changes using a modified Nernst equation is used to determine standard potentials associated with the individual charge transfer processes. For the GMs prepared here, these were found to be <i>E</i>_1,ox⁰ = 0.77 ± 0.01 V and <i>E</i>_2,ox⁰ = 1.24 ± 0.02 V <i>vs</i> NHE for the first and second oxidation and <i>E</i>_1,red⁰ = −1.50 ± 0.04 V for the first reduction. The charge transfer processes are found to be nonideal. The nonideality factors associated with the oxidation and reduction processes are attributed to strong interactions between the GM redox centers. Under the conditions of potential cycling, GMs show rapid (seconds) color change with high contrast and stability. An electrochromic application is demonstrated wherein the GMs are used as the optically active component

p-Type Dye-Sensitized Solar Cells Based on Delafossite CuGaO₂ Nanoplates with Saturation Photovoltages Exceeding 460 mV

Exploring new p-type semiconductor nanoparticles alternative to the commonly used NiO is crucial for p-type dye-sensitized solar cells (p-DSSCs) to achieve higher open-circuit voltages (<i>V</i>_oc). Here we report the first application of delafossite CuGaO₂ nanoplates for p-DSSCs with high photovoltages. In contrast to the dark color of NiO, our CuGaO₂ nanoplates are white. Therefore, the porous films made of these nanoplates barely compete with the dye sensitizers for visible light absorption. This presents an attractive advantage over the NiO films commonly used in p-DSSCs. We have measured the dependence of <i>V</i>_oc on the illumination intensity to estimate the maximum obtainable <i>V</i>_oc from the CuGaO₂-based p-DSSCs. Excitingly, a saturation photovoltage of 464 mV has been observed when a polypyridyl Co^3+/2+(dtb-bpy) electrolyte was used. Under 1 Sun AM 1.5 illumination, a <i>V</i>_oc of 357 mV has been achieved. These are among the highest values that have been reported for p-DSSCs

Probing the Low Fill Factor of NiO p‑Type Dye-Sensitized Solar Cells

p-Type dye-sensitized solar cells (<i>p</i>-DSCs) have attracted increasing attention recently, but they suffer from low fill factors (FFs) and unsatisfactory efficiencies. A full comprehension of the hole transport and recombination processes in the NiO <i>p</i>-DSC is of paramount importance for both the fundamental study and the practical device optimization. In this article, NiO <i>p</i>-DSCs were systematically probed under various bias and illumination conditions using electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectroscopy (IMPS), and intensity modulated photovoltage spectroscopy (IMVS). Under the constant 1 sun illumination, the recombination resistance (<i>R</i>_rec) of the cell deviates from an exponential relationship with the potential and saturates at ∼130 Ω cm² under the short circuit condition, which is ascribed to the overwhelming recombination with the reduced dye anions. Such a small <i>R</i>_rec results in the small dc resistance, which decreases the “flatness” of the <i>J–V</i> curve. The quantitative analysis demonstrates that the FF value is largely attenuated by the recombination of holes in NiO with the reduced dyes. Our analysis also shows that if this recombination can be eliminated, then an FF value of 0.6 can be reached, which agrees with the theoretical calculation with a <i>V</i>_oc of 160 mV

Synthesis, Photophysics, and Photovoltaic Studies of Ruthenium Cyclometalated Complexes as Sensitizers for p‑Type NiO Dye-Sensitized Solar Cells

We report the first application of cyclometalated ruthenium complexes of the type Ru­[(N^∧N)₂(C^∧N)]⁺ as sensitizers for p-type NiO dye-sensitized solar cells (NiO p-DSCs). These dyes exhibit broad absorption in the visible region. The carboxylic anchoring group is attached to the phenylpyridine ligand, which results in efficient hole injection. Moreover, the distance between the Ru­[(N^∧N)₂(C^∧N)]⁺ core and the carboxylic anchoring group is systematically varied by inserting rigid phenylene linkers. Femtosecond transient absorption (TA) studies reveal that the interfacial charge recombination rate between reduced sensitizers and holes in the valence band of NiO decreases as the number of phenylene linkers increases across the series. As a result, the solar cell made of the dye with the longest spacer (O12) exhibits the highest efficiency with both increased short-circuit current (<i>J</i>_sc) and open-circuit voltage (<i>V</i>_oc). The incident photon-to-current conversion efficiency (IPCE) spectra match well with the absorption spectra of sensitizers, suggesting the observed cathodic current is generated from the dye sensitization. In addition, the absorbed photon-to-current conversion efficiencies (APCEs) display an increment across the series. We further studied the interfacial charge recombination of our solar cells by electrochemical impedance spectroscopy (EIS). The results reveal an enhanced hole lifetime as the number of phenylene linkers increases. This study opens up opportunities of using cyclometalated Ru complexes for p-DSCs

In Situ Synthesis of Graphene Molecules on TiO₂: Application in Sensitized Solar Cells

We present a method for preparation of graphene molecules (GMs), whereby a polyphenylene precursor functionalized with surface anchoring groups, preadsorbed on surface of TiO₂, is oxidatively dehydrogenated in situ via a Scholl reaction. The reaction, performed at ambient conditions, yields surface adsorbed GMs structurally and electronically equivalent to those synthesized in solution. The new synthetic approach reduces the challenges associated with the tendency of GMs to aggregate and provides a convenient path for integration of GMs into optoelectronic applications. The surface synthesized GMs can be effectively reduced or oxidized via an interfacial charge transfer and can also function as sensitizers for metal oxides in light harvesting applications. Sensitized solar cells (SSCs) prepared from mesoscopic TiO₂/GM films and an iodide-based liquid electrolyte show photocurrents of ∼2.5 mA/cm², an open circuit voltage of ∼0.55 V and fill factor of ∼0.65 under AM 1.5 illumination. The observed power conversion efficiency of η = 0.87% is the highest reported efficiency for the GM sensitized solar cell. The performance of the devices was reproducible and stable for a period of at least 3 weeks. We also report first external and internal quantum efficiency measurements for GM SSCs, which point to possible paths for further performance improvements

Supplementary document for Evaluation of the surface topography quality of large-area diamonds by image processing and mathematical modeling - 6007737.pdf

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