Excited-State Dynamics of Organic Dyes in Solar Cells

Organic dyes are promising candidates for wide applications in solar cells, due to their controlled environmental impact, and low-cost. However, their performances in several solar cell architectures are not high enough to compete with the traditional semiconductor based solar cells. Therefore, several efforts should be gathered to improve the efficiency of these organic dyes. Herein, we discuss several deactivation processes recently found in several organic dyes using optical spectroscopic techniques. These processes are believed to be mostly detrimental for the performance of organic dyes in solar cells. These processes include deactivation phenomena such as isomerization, twisting, and chemical interactions with redox couple. Thus, based on similar studies, more optimized synthetic procedures for organic dyes could be implemented in the near future for high efficient solar cells based on organic dyes

Approaches to increase the resiliency of Egyptian agriculture to climate change

Climate change is expected to affect agricultural production in direct and indirect pathways. The increase in mean temperatures directly accelerates crop development, the change in seasonal precipitation amounts together with increasing evaporative demand can indirectly lead to more drought stress for crops. In Egypt, the agricultural sector is highly vulnerable to climate change due to its dependence on the Nile River for irrigation, increasing soil salinity by sea water intrusion and soil deterioration as a result of decomposition of its organic contents. In this article, previous research carried out in Egypt on climate change assessments on water resources (the Nile River and rainfall on the north coast of Egypt), crop evapotranspiration, crop water requirements, crop yield, agricultural soils and national cultivated area are reviewed. Furthermore, the implemented actions to increase crop resilience to climate change were discussed. Additionally, the procedures used to reduce greenhouse gases emission were also reviewed. Keywords: Water resources, soil resources, climate-resilient crops, greenhouse gases emissions, carbon sequestration, biogas productio

Elucidation of the Intersystem Crossing Mechanism in a Helical BODIPY for Low-Dose Photodynamic Therapy

Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co-existence of efficient ISC and long triplet excited lifetime in a heavy atomfree bodipy helicene molecule. Via theoretical computation and time-resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (epsilon = 1.76 x 10(5)M(-1) cm(-1) at 630 nm), satisfactory triplet quantum yield (Phi(T)= 52 %), and long-lived triplet state (T-T = 492 mu s), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT-mediated antitumor immunity amplification with an ultra-low dose (0.25 mu g kg(-1)), which is several hundred times lower than that of the existing PDT reagents

An Overview of Common Infrared Techniques for Detecting CO Intermediates on Metal Surfaces for Hydrocarbon Products

Detection of intermediates during the catalytic process by infrared techniques has been widely implemented for many important reactions. For the reduction of CO2 into hydrocarbons on metal surfaces, CO molecule is one of the most important transient species to be followed due to its involvement in several products’ pathways, and its distinct vibrational features. Herein, basic understandings behind these utilized infrared techniques are illustrated aiming for highlighting the potential of each infrared technique and its advantages over the other ones for detecting CO molecules on metal surfaces

Understanding Charge Recombination of TiO2 using Ultrafast mid-Infrared Spectroscopy: The Effect of Photogenerated Holes

We utilize mid-infrared probe to explore the mechanism for the dramatic charge recombination process of the photogenerated charges within the band gap of TiO2. Using the low-energy photons probes the free electrons in the conduction band of TiO2 and upon trapping to shallow-trap states. We found that > 70% of the photogenerated charges disappear from the conduction band in the first few nanoseconds, due to electron trapping or charge recombination. We compare the behavior of free electron dynamics within the bandgap of TiO2 and upon generating them across the interface with adsorbing organic dyes. This comparison shows that the main driving force of dramatic charge recombination of photogenerated charges is the presence of hole within the band-gap of TiO2 or close to the interface of TiO2. Once, the hole is far from the TiO2 surface, the electron trapping process is hindered and almost 100% of photogenerated charges can live up till nanoseconds. This paper allows for further understandings of the charge trapping and charge recombination processes in TiO2 and in other semiconductors

Exploring Organic Dyes for Grätzel Cells Using Time-Resolved Spectroscopy

Grätzel cells or Dye-Sensitized Solar Cells (DSSCs) are considered one of the most promising methods to convert the sun's energy into electricity due to their low cost and simple technology of production. The Grätzel cell is based on a photosensitizer adsorbed on a low band gap semiconductor. The photosensitizer can be a metal complex or an organic dye. Organic dyes can be produced on a large scale resulting in cheaper dyes than complexes based on rare elements. However, the performance of Grätzel cells based on metal-free, organic dyes is not high enough yet. The dye's performance depends primarily on the electron dynamics. The electron dynamics in Grätzel cells includes electron injection, recombination, and regeneration. Different deactivation processes affect the electron dynamics and the cells’ performance. In this thesis, the electron dynamics was explored by various time-resolved spectroscopic techniques, namely time-correlated single photon counting, streak camera, and femtosecond transient absorption. Using these techniques, new deactivation processes for organic dyes used in DSSCs were uncovered. These processes include photoisomerization, and quenching through complexation with the electrolyte. These deactivation processes affect the performance of organic dyes in Grätzel cells, and should be avoided. For instance, the photoisomerization can compete with the electron injection and produce isomers with unknown performance. Photoisomerization as a general phenomenon in DSSC dyes has not been shown before, but is shown to occur in several organic dyes, among them D149, D102, L0 and L0Br. In addition, D149 forms ground state complexes with the standard iodide/triiodide electrolyte, which directly affect the electron dynamics on TiO2. Also, new dyes were designed with the aim of using ferrocene(s) as intramolecular regenerators, and their dynamics was studied by transient absorption. This thesis provides deeper insights into some deactivation processes of organic dyes used in DSSCs. New rules for the design of organic dyes, based on these insights, can further improve the efficiency of DSSCs.

Complex Formation with the Iodide Electrolyte Influences Electron Dynamics in Dye-Sensitized Solar Cells

Ground state complexes between the components of the iodide/triiodide redox couple and D149, a wellknown organic dye used in dye sensitized solar cells, have been detected in acetonitrile and on semiconductor surfaces. Generally, in acetonitrile, these complexes have high formation constants in the case of the donor moiety of D149, D149ester and D149. These complexes adsorb on semiconductor surfaces and show different electron dynamics on ZrO2 and TiO2 in comparison to D149 itself. Such complexes on semiconductor surfaces can certainly limit the efficiency of a working cell based on similar organic dyes

Exploring Organic Dyes for Grätzel Cells Using Time-Resolved Spectroscopy

Grätzel cells or Dye-Sensitized Solar Cells (DSSCs) are considered one of the most promising methods to convert the sun's energy into electricity due to their low cost and simple technology of production. The Grätzel cell is based on a photosensitizer adsorbed on a low band gap semiconductor. The photosensitizer can be a metal complex or an organic dye. Organic dyes can be produced on a large scale resulting in cheaper dyes than complexes based on rare elements. However, the performance of Grätzel cells based on metal-free, organic dyes is not high enough yet. The dye's performance depends primarily on the electron dynamics. The electron dynamics in Grätzel cells includes electron injection, recombination, and regeneration. Different deactivation processes affect the electron dynamics and the cells’ performance. In this thesis, the electron dynamics was explored by various time-resolved spectroscopic techniques, namely time-correlated single photon counting, streak camera, and femtosecond transient absorption. Using these techniques, new deactivation processes for organic dyes used in DSSCs were uncovered. These processes include photoisomerization, and quenching through complexation with the electrolyte. These deactivation processes affect the performance of organic dyes in Grätzel cells, and should be avoided. For instance, the photoisomerization can compete with the electron injection and produce isomers with unknown performance. Photoisomerization as a general phenomenon in DSSC dyes has not been shown before, but is shown to occur in several organic dyes, among them D149, D102, L0 and L0Br. In addition, D149 forms ground state complexes with the standard iodide/triiodide electrolyte, which directly affect the electron dynamics on TiO2. Also, new dyes were designed with the aim of using ferrocene(s) as intramolecular regenerators, and their dynamics was studied by transient absorption. This thesis provides deeper insights into some deactivation processes of organic dyes used in DSSCs. New rules for the design of organic dyes, based on these insights, can further improve the efficiency of DSSCs.

Water Oil Emulsions Studied by Vibrational Sum Frequency Generation

Interfacial interactions between water and oil phases are present in various fields, in which other additives, such as surfactants, are utilized to minimize the surface stress between these phases to form an emulsion. However, the consequence of adding additives is not always easy to understand or to control, due to the plethora of parameters that control the production of an emulsion phase. There are several macroscopic techniques to study the properties of emulsions. Nevertheless, these techniques cannot describe the mechanistic steps at molecular level for interfaces with nanometer thicknesses. Among surface sensitive techniques, VSFG (vibrational sum frequency generation) has been utilized to study various parameters that control the formation of surfactant monolayers at water-oil interfaces. In this brief-review, basics about utilization of VSFG along with recent advances for studying the water-oil interfaces will be presented, aiming at familiarizing other scientists with the current understanding of the water-oil interfaces as studied by VSF