Organometal halide perovskites are highly promising materials for photovoltaic applications, yet their rapid degradation remains a significant challenge. Here, the light-induced structural degradation mechanism of methylammonium lead iodide (MAPbI3) perovskite films and devices is studied in low humidity environment using X-Ray Diffraction, Ultraviolet-Visible (UV-Vis) absorption spectroscopy, Extended X-ray Absorption Fine Structure spectroscopy, Fourier Transform Infrared spectroscopy, and device measurements. Under dry conditions, the perovskite film degrades only in the presence of both light and oxygen, which together induce the formation of halide anions through donation of electrons to the surrounding oxygen. The halide anions generate free radicals that deprotonate the methylammonium cation and form the highly volatile CH3NH2 molecules that escape and leave pure PbI2 behind. The device findings show that changes in the local structure at the TiO2 mesoporous layer occur with light, even in the absence of oxygen, and yet such changes can be prevented by the application of UV blocking layer on the cells. Our results indicate that the stability of mp-TiO2-MAPbI3 photovoltaics can be dramatically improved with effective encapsulation that protects the device from UV light, oxygen, and moisture.

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A Luminescent Solar Concentrator (LSC) greenhouse and an identical control greenhouse were constructed with photovoltaic (PV) cells attached to the roof panels of both structures. The placement and types of PV cells used in the LSC panels were varied for performance comparisons. Solar power generation was monitored continuously for one year, with leading LSC panels exhibiting a 37% increase in power production compared to the reference. The 22.3 m2 greenhouse was projected to generate a total of 1342 kWh per year, or 57.4 kWh/m2 if it were composed solely of the leading panel of Criss Cross panel design. The LSC panels showed no signs of degradation throughout the trial demonstrating the material's robustness in field conditions.

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The numerous electronic and optoelectronic applications that rely on semiconductors require tuning their properties through doping. Germanium quantum dots (Ge QDs) were successfully doped with bismuth up to 1.5 mol %, which is not achievable in the bulk Ge system. The structures of oleylamine-and dodecanethiol-capped Ge QDs were probed with EXAFS, and the results are consistent with Bi dopants occupying surface lattice sites. Increasing the amount of Bi dopant from 0.50 to 1.5 mol % results in increasing disorder. In particular, the nearestneighbor Bi−Ge bond length is much longer than the Ge−Ge bond length in Ge QDs. Oleylamine to dodecanethiol ligand exchange was shown to partially restore order in doped QDs. Transport measurements of the Bidoped Ge QD thin films revealed that Bi doping leads to a significant increase in dark current and photocurrent. These results indicate that doping can provide a pathway for improving the performance of group IV quantum dots for energy conversion applications including photodiodes and photovoltaic cells.

Doped and alloyed germanium nanocrystals (Ge NCs) are potential candidates for a variety of applications such as photovoltaics and near IR detectors. Recently, bismuth (Bi) as an n-type group 15 element was shown to be successfully and kinetically doped into Ge NCs through a microwave-assisted solution-based synthesis, although Bi is thermodynamically insoluble in bulk crystalline Ge. To expand the composition manipulation of Ge NCs, another more common n-type group 15 element for semiconductors, antimony (Sb), is investigated. Oleylamine (OAm)- and OAm/trioctylphosphine (TOP)-capped Sb-doped Ge NCs have been synthesized by the microwave-assisted solution reaction of GeI2 with SbI3. Passivating the Ge surface with a binary ligand system of OAm/TOP results in formation of consistently larger NCs compared to OAm alone. The TOP coordination on the Ge surface is confirmed by 31P NMR and SEM-EDS. The lattice parameter of Ge NCs increases with increasing Sb concentration (0.00–2.0 mol %), consistent with incorporation of Sb. An increase in the NC diameter with higher content of SbI3 in the reaction is shown by TEM. XPS and EDS confirm the presence of Sb before and after removal of surface ligands with hydrazine and recapping the Ge NC surface with dodecanethiol (DDT). EXAFS analysis suggests that Sb resides within the NCs on highly distorted sites next to a Ge vacancy as well as on the crystallite surface. High Urbach energies obtained from photothermal deflection spectroscopy (PDS) of the films prepared from pristine Ge NC and Sb-doped Ge NCs indicate high levels of disorder, in agreement with EXAFS data. Electrical measurements on TiO2–NC electron- and hole-only devices show an increase in hole conduction, suggesting that the Sb-vacancy defects are behaving as a p-type dopant in the Ge NCs, consistent with the vacancy model derived from the EXAFS results.

Amorphous selenium (a-Se) is a glass-former capable of deposition at high rates by thermal evaporation over a large area. It was chosen as a direct conversion material due to its appealing properties for imaging in both low and high X-ray energy ranges (<30 keV and <30 keV, respectively). It has a bandgap of 2.2 eV and can achieve high photodetection efficiency at short wavelengths less than 400 nm which makes it appealing for indirect conversion detectors. The integration of a-Se with readout integrated circuits started with thin-film transistors for digital flat panel X-ray detectors. With increasing applications in life science, biomedical imaging, X-ray imaging, high energy physics, and industrial imaging that require high spatial resolution, the integration of a-Se and CMOS is one direct way to improve the high-contrast visualization and high-frequency response. Over the past decade, significant improvements in a-Se/CMOS technologies have been achieved with improvements to modulation transfer function and detective quantum efficiency. We summarize recent advances in integrating and photon-counting detectors based on a-Se coupled with CMOS readout and discuss some of the shortcomings in the detector structure, such as low charge conversion efficiency at low electric field and high dark current at high electric field. Different pixel architectures and their performance will be highlighted.

Since its first application as a substrate for graphene field effect transistors (FETs), hexagonal boron nitride (hBN) has become a prominent component in two-dimensional (2D) material devices. In addition, hBN has been shown to host defects that can be manipulated to change the electronic properties of adjacent 2D materials. Despite the wide use of such defect manipulations, no focused efforts have been made to further the understanding of defect excitations and their influence in graphene/hBN FETs. In this study, we explore the effect of high electric fields (∼10V/nm) on graphene/hBN FETs and find that persistent and reversible shifts in graphene's charge neutrality point (CNP) occur. By increasing the applied electric field and temperature of our device, we find that this CNP shift is enhanced. With this insight, we propose a mechanism that explains these observations based on Poole–Frenkel emissions from defects in hBN. Finally, we show that such an effect may be suppressed by using graphite as a backgate, thus preventing unintended changes in the electrical properties of graphene/hBN FETs.