Ahmed M. El-Zohry

Isomerization and Aggregation of the Solar Cell Dye D149

D149, a metal-free indoline dye, is one of the most promising sensitizers for dye-sensitized solar cells (DSSCs) and has shown very high solar energy conversion efficiencies of 9%. Effective electron injection from the excited state is a prerequisite for high efficiencies and is lowered by competing deactivation pathways. Previous investigations have shown surprisingly short-lived excited states for this dye, with maximum lifetime components of 100–720 ps in different solvents and less than 120 ps for surface-adsorbed D149. Using steady-state and time-resolved fluorescence, we have investigated the photochemical properties of D149 in nonpolar and polar solvents, polymer matrices, and adsorbed on ZrO2, partially including a coadsorbent. In solution, excitation to the S2 state yields a product that is identified as a photoisomer. The reaction is reversible, and the involved double-bond is identified by NMR spectroscopy. Our results further show that lifetimes of 100–330 ps in the solvents used are increased to more than 2 ns for D149 in polymer matrices and on ZrO2. This is in part attributed to blocked internal motion due to steric constraint. Conversely, concentration-dependent aggregation leads to a dramatic reduction in lifetimes that can affect solar cell performance. Our results explain the unexpectedly short lifetimes observed previously. We also show that photochemical properties such as lifetimes determined in solution are different from the ones determined on semiconductor surfaces used in solar cells. The obtained mechanistic understanding should help develop design strategies for further improvement of solar cell dyes.

Improved Morphology Control Using a Modified Two-Step Method for Efficient Perovskite Solar Cells

A two-step wet chemical synthesis method for methylammonium lead(II) triiodide (CH3NH3PbI3) perovskite is further developed for the preparation of highly reproducible solar cells, with the following structure: fluorine-doped tin oxide (FTO)/TiO2 (compact)/TiO2 (mesoporous)/CH3NH3PbI3/spiro-OMeTAD/Ag. The morphology of the perovskite layer could be controlled by careful variation of the processing conditions. Specifically, by modifying the drying process and inclusion of a dichloromethane treatment, more uniform films could be prepared, with longer emission lifetime in the perovskite material and longer electron lifetime in solar cell devices, as well as faster electron transport and enhanced charge collection at the selective contacts. Solar cell efficiencies up to 13.5% were obtained.

Photoisomerization of the cyanoacrylic acid acceptor group – a potential problem for organic dyes in solar cells

Organic solar cell dyes containing the most common anchoring group, cyanoacrylic acid, are shown to be photolabile and undergo photoisomerization. This may have significant consequences for dye-sensitized solar cells, as isomerisation competes with electron injection and leads to modifications of the dye and surface arrangement.

Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO2 as a Reversible Electron Acceptor in a TiO2–Re Catalyst System for CO2 Photoreduction

Attaching the phosphonated molecular catalyst [ReIBr(bpy)(CO)3]0 to the wide-bandgap semiconductor TiO2 strongly enhances the rate of visible-light-driven reduction of CO2 to CO in dimethylformamide with triethanolamine (TEOA) as sacrificial electron donor. Herein, we show by transient mid-IR spectroscopy that the mechanism of catalyst photoreduction is initiated by ultrafast electron injection into TiO2, followed by rapid (ps-ns) and sequential two-electron oxidation of TEOA that is coordinated to the Re center. The injected electrons can be stored in the conduction band of TiO2 on an ms-s time scale, and we propose that they lead to further reduction of the Re catalyst and completion of the catalytic cycle. Thus, the excited Re catalyst gives away one electron and would eventually get three electrons back. The function of an electron reservoir would represent a role for TiO2 in photocatalytic CO2 reduction that has previously not been considered. We propose that the increase in photocatalytic activity upon heterogenization of the catalyst to TiO2 is due to the slow charge recombination and the high oxidative power of the ReII species after electron injection as compared to the excited MLCT state of the unbound Re catalyst or when immobilized on ZrO2, which results in a more efficient reaction with TEOA.

Double Charged Surface Layers in Lead Halide Perovskite Crystals

Understanding defect chemistry, particularly ion migration, and its significant effect on the surfaces optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

Environmental Method to Determine Dopamine and Ascorbic Acid Simultaneously via Derivative Spectrophotometry

−1 and 0.352–5.28 mg L −1 , respectively. These obtained methods could be used to de termine both reagents in real and synthesized samples.

The Role of Site-Specific Hydrogen Bonding Interactions in the Solvation Dynamics of N‑Acetyltryptophanamide

Measurements of the ultrafast broadband UV fluorescence of N-acetyltryptophanamide (NATA) provide detailed information on its relaxation patterns in three different solvents: methanol (MeOH), water and acetonitrile (ACN). Several processes leading to excited state solvation and cooling are found to occur on different characteristic time scales and are thoroughly analyzed. Comparison between protic MeOH and aprotic ACN allows one to single out a 12 ps component in the former, which is attributed to the rearrangement of H-bonds existing between the protic solvent and excited NATA. This significantly stabilizes the excited state and provides the molecule with an efficient cooling mechanism. The corresponding dynamics in water are much faster (<1.5 ps). Comparison with static spectroscopic properties stresses the role of site-specific H-bonding in controlling the fluorescence quantum yield of NATA in protic solvents. These findings are consistent with existing models that describe tryptophan quenching as a result of charge transfer from the indole to the amide assisted by H-bonding at the carbonyl site.

Environmental method for spectrophotometric determination of copper(II)

Abstract A highly stable ternary complex of copper(II) with 5-(4-nitrophenylazo) salicylic acid (NPAS) and 2,2′-Dipyridyl (Dp) in ethanol medium at room temperature gave a maximum absorption at 520 nm with a molar absorptivity 2.60×104 L mol-1 cm-1. This method was characterized by safety to the environment as there are no uses of hazardous solvents, which may harm our environment. The linear range for copper determination was 2.2–6.3 mg L-1. The method was sensitive, accurate, and tolerant to many foreign ions. All the reagents were stable under the working conditions. Moreover, the method was easy to perform for the determination of copper in pharmaceutical, biological, and water samples.

Facile Spectrophotometric Determination of Aluminum(III) and Copper(II) Simultaneously with Alizarin Yellow R in Aluminum-Copper Alloys

A facile, highly sensitive, and selective spectrophotometric method has been developed for the determination of aluminum and copper in different alloys. The method is based on the formation of the binary complexes of aluminum and copper with Alizarin yellow R (AYR) [5-(4-nitrophenylazo) salicylic acid] at pH 2.0 and 6.2, respectively, at wavelengths 422 and 445 nm with molar absorbtivity 1.1 × 104 l mol−1 cm−1 for each one. Aluminum and copper were thus determined in the ranges 0.4–2.0 μg ml−1 and 1.9–3.6 μg ml−1, respectively, in the presence of one to another. The detection limits were found to be 1.4 ng ml−1 of aluminum and 0.015 μg ml−1 for copper. The relative standard deviations were in all instances less than 1.3%. The proposed method was successfully applied for the simultaneous determination of aluminum and copper in certified reference aluminum-copper alloys.

Light-Induced Interfacial Dynamics Dramatically Improve the Photocurrent in Dye-Sensitized Solar Cells: An Electrolyte Effect

A significant increase in the photocurrent generation during light soaking for solar cells sensitized by the triphenylamine-based D-π-A organic dyes (PD2 and LEG1) and mediated by cobalt bipyridine redox complexes has been observed and investigated. The crucial role of the electrolyte has been identified in the performance improvement. Control experiments based on a pre-treatment strategy reveals TBP as the origin. The increase in the current and IPCE has been interpreted by the interfacial charge-transfer kinetics studies. A slow component in the injection kinetics was exposed for this system. This change explains the increase in the electron lifetime and collection efficiency. Photoelectron spectroscopic measurements show energy shifts at the dye/TiO2 interface, leading us to formulate a hypothesis with respect to an electrolyte-induced dye reorganization at the surface.