Quantum Optoelectronics

Abstract:

A method for controlling the optical properties of a solid-state film over a broad wavelength range is highly desirable and could have significant commercial impact. One such application is smart glazing technology where near-infrared solar radiation is harvested in the winter and reflected it in the summer─an impossibility for materials with fixed thermal and optical properties. Here, we experimentally demonstrate the first spectrally tunable, low-emissivity coating using a chalcogenide-based phase-change material (Ge20Te80), which can modulate the solar heat gain of a window while maintaining neutral-coloration and constant transmission of light at visible wavelengths. We additionally demonstrate the controlled transfer of absorbed near-infrared energy to far-infrared radiation, which can be used to heat a building’s interior and show fast, sub-millisecond switching using transparent electrical heaters integrated on glass substrates. These combined properties result in a smart window that is efficient and aesthetically pleasing─crucial for successful adoption of green technology.

Abstract:

The photocatalytic overall water splitting (POWS) reaction using particulate catalysts is considered as an ideal approach for capturing solar energy and storing it in the form of hydrogen, however, current POWS systems are hindered by the slow separation but fast recombination of the photo-generated charge carriers, hence giving unsatisfactory performances. Here we report a dramatically improved POWS system for a Au-supported Fe₃O₄/N-TiO₂ superparamagnetic photocatalyst promoted by local magnetic field effects. Strong local magnetic flux was induced by a weak external magnetic field of 180 mT, which then resulted in a quantum efficiency of 88.7% at 437 nm at 270 °C without any sacrificial reagent. The mechanism of the magnetic field effects was explored systematically and quantitatively by time-resolved spectroscopic technique and first-principles calculations, which suggested such enhancement was due to the greatly prolonged excitonic lifetime, originating from both the Lorentz force and spin-polarisation effects. By controllable manipulation of both features using local magnetic field, an unprecedented solar-to-hydrogen conversion efficiency of 11.9 ± 0.5% and an overall energy efficiency of 1.16 ± 0.05% were achieved in a particulate POWS system under AM 1.5G simulated solar illumination, which exceeds the STH goal of 10% for practical applications of POWS systems imposed by the United States Department of Energy.

Abstract:

We have designed a method of harvesting electrical energy using plasmon-enhanced light pressure. A device was fabricated as a cut cone structure that optimizes light collection so that the weak incident light pressure can be sufficiently enhanced inside the cut cone to generate electrical energy. An increase in the device's current output is a strong indication that the pressure of incident light has been enhanced by the surface plasmons on a platinum layer inside the cut cone. The electrical energy harvested in a few minutes by irradiating pulsed laser light on a single micro device was possible to illuminate a blue LED.

Abstract:

Covalent triazine frameworks (CTFs) synthesized through nucleophilic substitution of 4,4’ bipyridine on the carbon atoms of cyanuric chloride were studied as fluorescent sensors. The band gap of the materials was calculated to be 2.95 eV from diffuse reflectance measurements, while from the adsorption in aqueous dispersions, we obtained the value of 3.7 eV. A partial exfoliation of the layered CTFs in water or tetrahydrofuran led to different morphologies, increased emission lifetime and fluorescence quantum yield. The pattern of their light emission properties in combination with their redox states was defined with the addition of a series of acidic and basic analytes. Another unique aspect of these semiconducting materials is the induced aggregation and the subsequent enhancement of emission under ultraviolet illumination.

Abstract:

Transient optical spectroscopy is used to quantify the temperature-dependence of charge separation and recombination dynamics in P3TEA:SF-PDI2 and PM6:Y6, two non-fullerene organic photovoltaic (OPV) systems with a negligible driving force and high photocurrent quantum yields. By tracking the intensity of the transient electroabsorption response that arises upon interfacial charge separation in P3TEA:SF-PDI2, a free charge generation rate constant of ≈2.4 × 1010 s−1 is observed at room temperature, with an average energy of ≈230 meV stored between the interfacial charge pairs. Thermally activated charge separation is also observed in PM6:Y6, and a faster charge separation rate of ≈5.5 × 1010 s−1 is estimated at room temperature, which is consistent with the higher device efficiency. When both blends are cooled down to cryogenic temperature, the reduced charge separation rate leads to increasing charge recombination either directly at the donor-acceptor interface or via the emissive singlet exciton state. A kinetic model is used to rationalize the results, showing that although photogenerated charges have to overcome a significant Coulomb potential to generate free carriers, OPV blends can achieve high photocurrent generation yields given that the thermal dissociation rate of charges outcompetes the recombination rate.