Note: in this version the excitation densities were computed using measured laser profiles instead of those calculated using the diffraction limited formula. AbstractWe compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination, over multiple orders of photo-excitation density. An analytic model is used to describe the excitation density dependence of PL intensity and extract the internal PL efficiency and multiple pertinent recombination parameters. A PL imaging technique is used to obtain carrier diffusion length without using a PL quencher, thus, free of unintended influence beyond pure diffusion. Our results show that perovskite samples tend to exhibit lower Shockley-Read-Hall (SRH) recombination rate in both bulk and surface, thus higher PL efficiency than the inorganic counterparts, particularly under low excitation density, even with no or preliminary surface passivation. PL lineshape and diffusion analysis indicate that there is considerable structural disordering in the perovskite materials, and thus photo-generated carriers are not in global thermal equilibrium, which in turn suppresses the nonradiative recombination. This study suggests that relatively low point-defect density, less detrimental surface recombination, and moderate structural disordering contribute to the high PV efficiency in the perovskite. This comparative photovoltaics study provides more insights into the fundamental material science and the search for optimal device designs by learning from different technologies.

We demonstrate the feasibility of hexacoordinate silicon complexes with dianionic pincer ligands as electron transport and electroluminescent components of organic electronic devices.

We demonstrate the first biosensing strategy that relies on quantum dot (QD) fluorescence blinking to report the presence of a target molecule. Unlike other biosensors that utilize QDs, our method does not require the analyte to induce any fluorescence intensity or color changes, making it readily applicable to a wide range of target species. Instead, our approach relies on the understanding that blinking, a single particle phenomenon, is obscured when several QDs lie within the detection volume of a confocal microscope. If QDs are engineered to aggregate when they encounter a particular target molecule, the observation of quasi-continuous emission should indicate its presence. As proof of concept, we programmed DNAs to drive rapid isothermal assembly of QDs in the presence of a target strand (oncogene K-ras). The assemblies, confirmed by various gel techniques, contained multiple QDs and were readily distinguished from free QDs by the absence of blinking.

Organic–inorganic hybrid perovskites display remarkable photovoltaic properties, but instability arises from the material’s surface and photoexcitation. The photodegradation process of methylammonium lead triiodide (MAPbI3) investigated with above bandgap excitation Raman spectroscopy displays two stages of structural change in polycrystalline films of varying domain sizes and a single crystal. Subtle surface changes occur below an excitation density threshold; PbI2 formation scales linearly above this threshold in thin films. However, the rate of change with excitation density increases with more grain surfaces. Whereas with a moderate level of heating into the cubic phase, accelerated photodegradation occurs in a single crystal. From experimental kinetic modeling, PbI2 nuclei on the surface grow until they meet in the polycrystalline films, proceeding one-dimensionally inward from the surface. These results suggest that the surface acts as a location for photogenerated carrier accumulation, leading to degradation and PbI2 formation.

We demonstrate an all optical approach that can surprisingly offer the possibility of yielding much more information than one would expect, pertinent to the carrier recombination dynamics via both radiative and nonradiative processes when only one dominant deep defect level is present in a semiconductor material. By applying a band-defect state coupling model that explicitly treats the inter-band radiative recombination and Shockley–Read–Hall (SRH) recombination via the deep defect states on an equal footing for any defect center occupation fraction, and analyzing photoluminescence (PL) as a function of excitation density over a wide range of the excitation density (e.g., 5–6 orders in magnitude), in conjunction with Raman measurements of the LO-phonon plasmon (LOPP) coupled mode, nearly all of the key parameters relevant to the recombination processes can be obtained. They include internal quantum efficiency (IQE), minority and majority carrier density, inter-band radiative recombination rate (Wr), minority carrier nonradiative recombination rate (Wnr), defect center occupation fraction (f), defect center density (Nt), and minority and majority carrier capture cross-sections (σt and σtM). While some of this information is thought to be obtainable optically, such as IQE and the Wr/Wnr ratio, most of the other parameters are generally considered to be attainable only through electrical techniques, such as current-voltage (I-V) characteristics and deep level transient spectroscopy (DLTS). Following a procedure developed herein, this approach has been successfully applied to three GaAs double-heterostructures that exhibit two distinctly different nonradiative recombination characteristics. The method greatly enhances the usefulness of the simple PL technique to an unprecedented level, facilitating comprehensive material and device characterization without the need for any device processing.