Beyond the Blackbox: Explaining the Machine Learning Pre-dictions of the Optical Response of Plasmon@Semiconductor Core-Shell Nanocylinders

Cross-section images and light absorption spectrum of Plasmon@Semiconductor Core-Shell Nanocylinders</p

Enhanced Harvesting of Red Photons in Nanowire Solar Cells: Evidence of Resonance Energy Transfer

Modern excitonic solar cells efficiently harvest photons in the 350−650 nm spectral range; however, device efficiencies are typically limited by poor quantum yields for red and near-infrared photons. Using Förster-type resonance energy transfer from zinc phthalocyanine donor molecules to ruthenium polypyridine complex acceptors, we demonstrate a four-fold increase in quantum yields for red photons in dye-sensitized nanowire array solar cells. The dissolved donor and surface anchored acceptor molecules are not tethered to each other, through either a direct chemical bond or a covalent linker layer. The spatial confinement of the electrolyte imposed by the wire-to-wire spacing of the close-packed nanowire array architecture ensures that the distances between a significant fraction of donors and acceptors are within a Förster radius. The critical distance for energy transfer from an isolated donor chromophore to a self-assembled monolayer of acceptors on a plane follows the inverse fourth power instead of the inverse sixth power relation. Consequently, we observe near quantitative energy transfer efficiencies in our devices. Our results represent a new design paradigm in excitonic solar cells and show it is possible to more closely match the spectral response of the device to the AM 1.5 solar spectrum through use of electronic energy transfer

Prediction of the Active Layer Nanomorphology in Polymer Solar Cells Using Molecular Dynamics Simulation

Active layer nanomorphology is a major factor that determines the efficiency of bulk heterojunction polymer solar cells (PSCs). Synthesizing diblock copolymers in which acceptor and donor materials are the constituent blocks is the most recent method to control the structure of the active layer. In the current work, a computational method is proposed to predict the nanomorphology of the active layer consisting of a diblock copolymer. Diblock copolymers have a tendency to self-organize and form well-defined nanostructures. The shape of the structure depends on the Flory–Huggins interaction parameter (i.e., χ), the total degree of polymerization (<i>N</i>) and volume fractions of the constituent blocks (φ_i). In this work, molecular dynamics (MD) simulations were used to calculate χ parameters for two different block copolymers used in PSCs: P3HT-<i>b</i>-poly­(S₈A₂)-C₆₀ and P3HT-<i>b</i>-poly­(n-butyl acrylate-<i>stat</i>-acrylate perylene) also known as P3HT-<i>b</i>-PPerAcr. Such calculations indicated strong segregation of blocks into cylindrical structure for P3HT-<i>b</i>-poly­(S₈A₂)-C₆₀ and intermediate segregation into cylindrical structure for P3HT-<i>b</i>-PPerAcr. Experimental results of P3HT-<i>b</i>-poly­(S₈A₂)-C₆₀ and P3HT-<i>b</i>-PTP4AP, a diblock copolymer having very similar structure to P3HT-<i>b</i>-PPerAcr, validate our predictions

Predicting Free Energies of Exfoliation and Solvation for Graphitic Carbon Nitrides Using Machine Learning

As a metal-free and visible-light-responsive photocatalyst, graphitic carbon nitride (g-C3N4) has emerged as a new research hotspot and has attracted broad attention in the field of solar energy conversion and thin-film transistors. Liquid-phase exfoliation (LPE) is the best-known method for the synthesis of 2D g-C3N4 nanosheets. In LPE, bulk g-C3N4 is exfoliated in a solvent via high-shear mixing or sonication in order to produce a stable suspension of individual nanosheets. Two parameters of importance in gauging the performance of a solvent in LPE are the free energy required to exfoliate a unit area of layered materials into individual sheets in the solvent (ΔGexf) and the solvation free energy per unit area of a nanosheet (ΔGsol). While approximations for the free energies exist, they are shown in our previous work to be inaccurate and incapable of capturing the experimentally observed efficacy of LPE. Molecular dynamics (MD) simulations can provide accurate free-energy calculations, but doing so for every single solvent is time- and resource-consuming. Herein, machine learning (ML) algorithms are used to predict ΔGexf and ΔGsol for g-C3N4. First, a database for ΔGexf and ΔGsol is created based on a series of MD simulations involving 49 different solvents with distinct chemical structures and properties. The data set also includes values of critical descriptors for the solvents, including density, surface tension, dielectric constant, etc. Different ML methods are compared, accompanied by descriptor selection, to develop the most accurate model for predicting ΔGexf and ΔGsol. The extra tree regressor is shown to be the best performer among the six ML methods studied. Experimental validation of the model is conducted by performing dispersibility tests in several solvents for which the free energies are predicted. Finally, the influence of the selected descriptors on the free energies is analyzed, and strategies for solvent selection in LPE are proposed

Anodic Growth of Large-Diameter Multipodal TiO₂ Nanotubes

We report on the formation of a new class of nanostructures, namely, multipodal hollow titania nanotubes possessing two or more legs, achieved during the electrochemical anodization of titanium in diethylene glycol (DEG)-based electrolytes. The unique multipodal porous structure is expected to extend and enhance the applications of TiO2 nanotube arrays. Multipodal nanotubes form by a process we term “nanotube combination”, which only occurs in viscous electrolytes at high anodization potentials in the presence of a low concentration of fluoride-bearing species. The mechanism of formation of multipodal nanotubes is considered, and the tube length at which nanotube combination occurs is predicted theoretically using a simplified analytical model. The results suggest that capillary forces strong enough to bend the TiO2 nanotubes by tens of degrees are generated during the imbibition of electrolyte into and out of the intertubular spaces between adjacent tapered nanotubes

Distinguishing between Deep Trapping Transients of Electrons and Holes in TiO₂ Nanotube Arrays Using Planar Microwave Resonator Sensor

A large signal direct current (DC) bias and a small signal microwave bias were simultaneously applied to TiO2 nanotube membranes mounted on a planar microwave resonator. The DC bias modulated the electron concentration in the TiO2 nanotubes and was varied between 0 and 120 V in this study. Transients immediately following the application and removal of DC bias were measured by monitoring the S-parameters of the resonator as a function of time. The DC bias stimulated Poole–Frenkel-type trap-mediated electrical injection of excess carriers into TiO2 nanotubes, which resulted in a near-constant resonant frequency but a pronounced decrease in the microwave amplitude due to free electron absorption. When ultraviolet illumination and DC bias were both present and then stepwise removed, the resonant frequency shifted due to trapping-mediated change in the dielectric constant of the nanotube membranes. Characteristic lifetimes of 60–80, 300–800, and ∼3000 s were present regardless of whether light or bias was applied and were also observed in the presence of a hole scavenger, which we attributed to oxygen adsorption and deep electron traps, whereas another characteristic lifetime >8000 s was only present when illumination was applied, and is attributed to the presence of hole traps

Dimension-Controlled Synthesis of Hybrid-Mixed Halide Perovskites for Solar Cell Application

Despite the rapid improvement of photovoltaic (PV) efficiency in hybrid organic–inorganic metal halide perovskites (HOIPs), the fabrication procedure of a compact thin film in a large-area application is still a tedious work. Apart from the quality of the thin film, the stability of the perovskite materials and the expensive organic hole transport layer (HTL) within the HOIP-based PV device are the major issues that need to be addressed prior to their commercialization. Herein, a unique glass rod-based facile fabrication technique for producing a compact and stable thin film utilizing a mixed-halide-based perovskite precursor solution is demonstrated. The fabricated devices deliver high photoconversion efficiency (PCE) without the use of any HTL and show an excellent stability under ambient conditions. By varying the organic CH3NH3I (MAI) and inorganic PbBr2 content, perovskite materials with different dimensions, i.e., 3D, 2D, and 1D, are synthesized to produce an active layer for PV devices. Although a 2D single-halide perovskite is reported earlier, herein two different mixed-halide 2D perovskites, i.e., MA2PbI2Br2 and MAPb2IBr4, are synthesized successfully, and their performance is compared in detail along with that of 1D and 3D mixed-halide perovskites. The facile synthesized mixed-halide 2D-based MA2PbI2Br2 perovskite shows a PCE of 10.14% with a high stability of 92% after 100 days without encapsulation, which is much superior as compared to that of the mixed-halide 3D MAPbIBr2. The semiconducting behavior as well as the nature of the bandgap of the synthesized compounds is examined by pursuing density functional theory calculations. Specifically, the role of iodine doping to modify the electronic band structure is investigated, and introduction of iodine is found to reduce the effective masses of both electrons and holes in the perovskite material

High Breakdown Strength Schottky Diodes Made from Electrodeposited ZnO for Power Electronics Applications

The synthesis of ZnO films by optimized electrodeposition led to the achievement of a critical electric field of 800 kV/cm. This value, which is 2–3 times higher than in monocrystalline silicon, was derived from a vertical Schottky diode application of columnar-structured ZnO films electrodeposited on platinum. The device exhibited a free carrier concentration of 2.5 × 1015 cm–3, a rectification ratio of 3 × 108, and an ideality factor of 1.10, a value uncommonly obtained in solution-processed ZnO. High breakdown strength and high thickness capability make this environment-friendly process a serious option for power electronics and energy harvesting

Multinuclear Magnetic Resonance Tracking of Hydro, Thermal, and Hydrothermal Decomposition of CH₃NH₃PbI₃

An NMR investigation of methylammonium lead iodide, the leading member of the hybrid organic–inorganic perovskite class of materials, and of its putative decomposition products as a result of exposure to heat and humidity, has been undertaken. We show that the ²⁰⁷Pb NMR spectra of the compound of interest and of the proposed lead-containing decomposition products, CH₃NH₃PbI₃·H₂O, (CH₃NH₃)₄PbI₆·2H₂O, and PbI₂, have distinctive chemical shifts spanning over 1400 ppm, making ²⁰⁷Pb NMR an ideal tool for investigating this material; further information may be gained from ¹³C and ¹H NMR spectra. As reported in many investigations of CH₃NH₃PbI₃ on films, the bulk material hydrates in the presence of high relative humidity (approximately 80%), yielding the monohydrated perovskite CH₃NH₃PbI₃·H₂O. This reaction is reversible by heating the sample to 341 K. We show that neither (CH₃NH₃)₄PbI₆·2H₂O nor PbI₂ is observed as a decomposition product and that, in contrast to many studies on CH₃NH₃PbI₃ films, the bulk material does not decompose or degrade beyond CH₃NH₃PbI₃·H₂O upon prolonged exposure to humidity at ambient temperature. However, exposing CH₃NH₃PbI₃ concurrently to heat and humidity, or directly exposing it to liquid water, leads to the irreversible formation of PbI₂. In spite of its absence among the decomposition products, the response of (CH₃NH₃)₄PbI₆·2H₂O to heat was also investigated. It is stable at temperatures below 336 K but then rapidly dehydrates, first to CH₃NH₃PbI₃·H₂O and then to CH₃NH₃PbI₃. The higher stability of the bulk material as reported here is a promising advance, since stability is a major concern in the development of commercial applications for this material

Vertically Aligned Single Crystal TiO₂ Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis Details and Applications

Single-crystal one-dimensional (1D) semiconductor architectures are important in materials-based applications requiring a large surface area, morphological control, and superior charge transport. Titania has widespread utility in applications including photocatalysis, photochromism, photovoltaics, and gas sensors. While considerable efforts have focused on the preparation of 1D TiO2, no methods have been available to grow crystalline nanowire arrays directly onto transparent conducting oxide (TCO) substrates, greatly limiting the performance of TiO2 photoelectrochemical devices. Herein, we present a straightforward low temperature method to prepare single crystal rutile TiO2 nanowire arrays up to 5 μm long on TCO glass via a non-polar solvent/hydrophilic substrate interfacial reaction under mild hydrothermal conditions. The as-prepared densely packed nanowires grow vertically oriented from the TCO glass substrate along the (110) crystal plane with a preferred (001) orientation. In a dye sensitized solar cell, N719 dye, using TiO2 nanowire arrays 2−3 μm long we achieve an AM 1.5 photoconversion efficiency of 5.02%