Dynamic Technical Analysis by Philippe Cahen Download Dynamic Technical Analysis Dynamic Technical Analysis Philippe Cahen. GO Downloads Book Product Details: Author(s): Philippe Cahen Category: Finances and Money Date: Pages: Language. Available in National Library (Singapore). Author: Cahen, Philippe., Length: xii, p.: Identifier:

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Halide perovskite HaP semiconductors are revolutionizing photovoltaic PV solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide MAPbI 3. In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI 3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence.

The next step, proving it is not ferroelectric, is challenging, because of the material’s relatively high electrical conductance a consequence of an optical band gap suitable for PV conversion and low mechanica, under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity’s hallmark switchable polarization.

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By adopting an approach suitable for electrically leaky materials as MAPbI 3we show here ferroelectric hysteresis from well-characterized single crystals at low temperature still within the tetragonal phase, which is stable at room temperature.

By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material’s noncentrosymmetry.

We note that the material’s ferroelectric nature, can, but need not be important in a PV cell at room temperature. Doped ceria is known for decades as an excellent ionic conductor used ubiquitously in fuel cells and other devices. Recent discovery of a giant electrostriction effect has brought world-wide interest to this class of materials for actuation applications in micromechanical systems.

From this aspect, the electromechanical response has to be studied as a function of mechanlcal parameters, such as frequency, temperature, and electrode material.

In this work, we fabricated circular membranes based on Gd-doped ceria CGO with Ti electrodes and studied their electromechanical response using a sensitive interferometric technique. The self-supported membranes are flat at room temperature and reversibly buckle upon heating, indicating that the membranes are under in-plane tensile strain. We have found that the electromechanical response is pgilippe frequency dependent.

Significant hysteresis is observed in the displacement-vs. The electromechanical response of the system increases with temperature. Published by AIP Publishing. Materials are central to our way of life and future. Energy and analydis as resources are connected, and the obvious connections between them are the energy cost of materials and the materials cost of energy.

Dynamic Technical Analysis

For both of these, resilience of the materials is critical; thus, a major goal of future chemistry should be to find materials for energy that can last longer, that is, design principles for self-repair in these. Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate.

Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite HaP; CsPbBr3 -based mechanica, expressed as reduction of open-circuit voltage and fill factor.

We have prepared highly stable CsPbBr3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measurements, ruling out degradation as the origin of the observed phenomena.

Reconstruction of current voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance. Halide ajalysis film-based devices e.

These films are commonly prepared from toxic mechanicall of metal salts kechanical. We show how the morphology of the films can be controlled by variation in reaction parameters and also how mixed halide perovskite films can be prepared. A mechanism for the metal-to-perovskite conversion is suggested. We further show how electrochemically assisted conversion can allow control over the oxidation state of the metal and increase the reaction rate greatly. The inorganic lead halide perovskite CsPbBr3 promises similar solar cell efficiency to its hybrid organic-inorganic counterpart CH3NH3PbBr3 but shows greater stability.

Here, we exploit this stability for the study of band alignment between perovskites and carrier selective interlayers. Using ultraviolet, X-ray, and inverse photoemission spectroscopies, we measure the ionization energy and electron affinities of CsPbBr3 phiilippe the hole transport polymer polytriarylamine PTAA.

We find that undoped PTAA introduces a barrier to hole extraction of 0. We review charge transport across molecular monolayers, which is central to molecular electronics MolElusing large-area junctions NmJ.

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We strive to provide a ahalysis conceptual overview of three main subtopics. First, a broad introduction places NmJ in perspective to related anzlysis of research and to single-molecule junctions imp in addition to a brief historical account.

As charge transport presents an ultrasensitive probe for the electronic perfection of interfaces, in the second part ways to form both the monolayer and the contacts are described to construct reliable, defect-free interfaces. The last part is dedicated to understanding and analyses of current-voltage I-V traces across molecular junctions. Notwithstanding emchanical original motivation of MolEl, I-V traces are often not very sensitive to molecular details and then provide a poor probe for chemical information.

Instead, we focus on how to analyze the net electrical performance of molecular junctions, anxlysis a functional device perspective. Finally, we point synamic creation of a built-in electric field as a key to achieve functionality, including nonlinear current-voltage characteristics that originate in the molecules or their contacts to the electrodes.

This review is complemented by a another review that covers metal-molecule-semiconductor junctions and their unique hybrid effects. This pilippe also has direct importance for technology as it can help improve the efficiency of a variety of electronic devices such as solar cells, LEDs, sensors, and possible future bioelectronic ones.

Publications | Prof. David Cahen

The review covers the main aspects mechaniical using chemistry to control the various aspects of interface electrostatics, such as passivation of interface states and alignment of energy levels by intrinsic molecular polarization, as well as charge rearrangement with the adjacent metal and semiconducting contacts.

Dnamic explain the interplay between the monolayer as tunneling barrier on the one hand, and the electrostatic barrier within the semiconductor, due to its space-charge region, on the other hand, as well as how different monolayer chemistries control each of these barriers.

Practical tools to experimentally identify these two barriers and distinguish between them are given, followed by a short look to the future. This review is accompanied by another mechanicl, concerning the formation of large-area mehanical junctions and dtnamic transport that is dominated solely by molecules. Using several metals with different work functions as solar cell back contact we identify majority carrier type inversion in methylammonium lead bromide MAPbBr3, without intentional doping as the basis for the formation analysls a p-n junction.

MAPbBr3 films deposited on TiO2 are slightly n-type, whereas in a full device they are strongly p-type. The charge transfer between the metal electrode and the halide perovskite HaP film is shown to determine the dominant charge carrier type of the HaP and, thus, also of the final cells. Usage of Pt, Au and Pb as metal electrodes shows the effects of metal work function dynajic minority carrier diffusion length and majority carrier concentration in the HaP, as well as on built-in voltage, band bending, and open circuit voltage V-OC within a solar cell.

The higher the metal WF, the higher the carrier concentration induced in the HaP, as indicated by a narrower space charge region and a smaller mechanica carrier diffusion length. From the analysis of bias-dependent electron beam-induced currents, the HaP carrier concentrations are estimated to be similar to 1 x 10 17 cm -3 with Au and x 10 18 cm -3 with Pt.

A model in which type-inversion stretches across the entire film width meechanical formation of the p-n junction away from the interface, near the dynamix metal electrode. This work highlights the importance of the contact metal on device performance in that contact engineering can also serve to control the carrier concentration in HaP. We present a measurement of the energies and capture cross-sections of defect states in methylammonium lead bromide MAPbBr 3 single crystals.

These results show qualitative agreement with theoretical predictions, whereby all of the observed defects behave as traps rather than as generation-recombination centers. These results provide one explanation for the high efficiencies and open-circuit voltages obtained from devices made with lead halide perovskites. Recent studies showed that positron annihilation methods can provide key insights into the nanostructure and electronic structure of thin film dynamix cells.

In this study, positron annihilation lifetime spectroscopy PALS is applied to investigate CdSe quantum dot QD light absorbing layers, providing evidence of positron trapping at the surfaces of the QDs. This enables one to monitor their surface composition and electronic structure.

The collected momentum distributions were converted by Fourier transformation to the direct space representation of the electron-positron autocorrelation function. The evolution of the size of the divacancies as a function of hydrogen dilution during deposition of a-Si: H thin films was examined. Finally, we present a first positron Doppler Broadening of Annihilation Radiation DBAR study of the emerging class of highly efficient thin film solar cells based on perovskites.

Electron transport properties via a photochromic biological photoreceptor have been studied in junctions of monolayer assemblies in solid-state configurations. The photoreceptor studied was a member of the LOV domain protein family with a bound flavin chromophore, and its photochemically inactive mutant due to change of a crucial cysteine residue by a mechanocal. The photochemical properties of the protein were maintained in dry, solid state conditions, indicating that the proteins in the junctions were assembled in native state-like conditions.

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The current magnitudes are ascribed to electrode-cofactor coupling originating from the apparent perpendicular orientation of the protein’s cofactor embedded between the electrodes, and its proximity to the electrodes.

Temperature independent electron transport across the protein monolayers demonstrated that solid-state electron transport is dominated by tunneling. Modulation of the observed current by illumination of the wildtype protein suggested conformation-dependent electron conduction efficiency across the solid-state protein junctions.

The realization of high-quality optoelectronic properties in halide perovskite semiconductors through low-temperature, low energy processing is unprecedented. Understanding the unique aspects of vynamic formation chemistry of these semiconductors is a critical step toward understanding the genesis of high quality material via simple preparation procedures.

The toolbox of preparation procedures for halide perovskites grows rapidly. We investigate the conversion of small, single-crystalline PbI2 crystallites to MAPbI 3 by two commonly used synthesis processes: The single crystal nature of the PbI2 precursor allows definitive conclusions to be made about the relationship dynzmic the precursors and the final product, illuminating previously unobserved aspects of mechanicap reaction process.

From in situ photoluminescence microscopy, we find that the reaction in solution begins cauen isolated nucleation events followed by growth from dynami nuclei.

We observe via X-ray diffraction and morphological characterization that there is a strong orientational and structural relationship between the final stage of the solution-reacted MAPbI 3 product and the initial PbI2 crystallite. In all these measurements, we find that the reaction does not proceed below a certain MAI threshold concentration, which allows the first experimental determination of a free energy of formation for a widely used synthetic procedure of similar to 0.

From these conclusions, we present a more detailed hypothesis about the reaction pathway than has yet been proposed: Our results suggest that the reaction in solution begins with a topotactic nucleation event followed by grain growth by dissolution-reconstruction.

By similar techniques, we find the reaction via vapor phase produces. Photovoltaic solar cells operate under steady-state conditions that are established during the charge carrier excitation and recombination. However, to date no model of the steady-state recombination scenario in halide perovskites has been proposed.

In this Letter we present such a model that is based on a single type of recombination center, which is deduced from mrchanical measurements of the illumination intensity dependence of the photoconductivity and the ambipolar diffusion length in those materials. The relation between the present results and those from time-resolved measurements, such as photoluminescence that are commonly reported in the literature, is discussed.

Solar cells based on “halide perovskites” HaPs have demonstrated unprecedented high power conversion efficiencies in recent years. However, the well-known toxicity of lead Pbwhich is used in the most studied cells, may affect its widespread use.

We pgilippe an all-inorganic lead-free perovskite option, cesium dynamjc bromide CsSnBr3for optoelectronic applications.

As reported earlier, addition of tin fluoride SnF2 was found to be beneficial for obtaining good device performance, possibly due to reduction of the background carrier density by neutralizing traps, possibly via filling of cation vacancies. Simple organic salts are used as a cheap alternative for hole-conducting materials in methylammonium lead bromide perovskite solar cells and obtaining power conversion efficiency of 4.

The findings suggest that the polar organic salts interact with the perovskite surface, leading to formation of a surface dipole or change of an existing one on the perovskites that changes its effective work function. We investigate the effect of high work function contacts in halide perovskite absorber-based mechanicxl devices. Photoemission spectroscopy measurements reveal that band bending is induced in the absorber by the deposition of the high work function molybdenum trioxide MoO3.

We find that direct contact between MoO3 and the perovskite leads to a chemical reaction, which diminishes device functionality. Introducing an ultrathin spiro-MeOTAD buffer layer prevents the reaction, yet the altered evolution of the energy levels in the methylammonium lead iodide MAPbI 3 layer at the interface still negatively impacts device performance. Direct comparison between perovskite-structured hybrid organic-inorganic methylammonium lead bromide MAPbBr3 and all-inorganic cesium lead bromide CsPbBr3allows identifying possible fundamental differences in their structural, thermal and electronic characteristics.

In order to compare device properties, we fabricated solar cells, with similarly synthesized MAPbBr3 or CsPbBr3, over mesoporous titania scaffolds.