Quantitative Cathodoluminescence Mapping: A CdMgSeTe Thin-Film Case Study

Full-spectrum cathodoluminescence (CL) mapping provides a point-by-point spatial measurement of the apparent band gap of a semiconductor thin film. In most studies, analysis of the electrical film properties from CL is presented as color mapping images. We have developed a spectra data analysis algorithm to functionalize, analyze, and generate statistical measurements of the luminescence data to provide additional insights. This algorithm was coded in the R language program, and a set of CdMgSeTe films were studied as an application case study. CL maps were measured for samples with different luminescent responses. A quantitative measure of the heterogeneity of the films was generated by statistical analysis of luminescent intensity and wavelength, spectra type curves, frequency distributions of peak wavelength, and relative intensity maps. The final CL analysis facilitates the investigation of the CdMgSeTe films and has potential applications for many semiconductor films.

The effect of hydrochloric acid (HCl) on permanent molars: A scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) study

It is well established that acid disposal is a potentially effective method used by criminal syndicates to hinder the identification of victims. This study documents the effects of continuous immersion in hydrochloric acid (HCl, 37%) on molars using macroscopic analysis, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The goal of this study is to aid in distinguishing visually unrecognizable fragments of dental remains when drastic changes in morphology have occurred as a result of acid exposure. Macroscopic, SEM, and EDS analysis were conducted on seven maxillary molars before and after HCl treatment. Molars reduced in weight relative to the length of time immersed in HCl and the dissolution time was over 40 hours longer than reported in previous studies, at just over 66 hours. SEM and EDS analysis showed acid-treated teeth exhibited morphological patterns such as cracking and layering visible at high magnification. Calcium/phosphorous ratios fell within the expected range of 1.6-2.5, indicating that HCl-treated teeth are still identifiable as osseous or dental tissue even when not visually identifiable as teeth. This is the first study to present SEM images of molar cementum before and after immersion in HCl and to present EDS results. This information can assist researchers and investigators in determining the presence of dental tissue in a forensic context associated with acid disposal.

Multiexciton Solar Cells of CuInSe2 Nanocrystals

Peak external quantum efficiencies (EQEs) of just over 120% were observed in photovoltaic (PV) devices of CuInSe2 nanocrystals prepared with a photonic curing process. The extraction of more than one electron/hole pair as a result of the absorption of a single photon can occur if multiple excitons are generated and extracted. Multiexciton generation (MEG) in the nanocrystal films was substantiated by transient absorption spectroscopy. We propose that photonic curing leads to sufficient electronic coupling between nanocrystals to enable multiexciton extraction under typical solar illumination conditions. Under low light conditions, however, the EQE drops significantly, indicating that photonic curing-induced ligand desorption creates a significant amount of traps in the film that limit the overall power conversion efficiency of the device.

Copper indium gallium selenide (CIGS) photovoltaic devices made using multistep selenization of nanocrystal films

The power conversion efficiency of photovoltaic devices made with ink-deposited Cu(InxGa1-x)Se2 (CIGS) nanocrystal layers can be enhanced by sintering the nanocrystals with a high temperature selenization process. This process, however, can be challenging to control. Here, we report that ink deposition followed by annealing under inert gas and then selenization can provide better control over CIGS nanocrystal sintering and yield generally improved device efficiency. Annealing under argon at 525 °C removes organic ligands and diffuses sodium from the underlying soda lime glass into the Mo back contact to improve the rate and quality of nanocrystal sintering during selenization at 500 °C. Shorter selenization time alleviates excessive MoSe2 formation at the Mo back contact that leads to film delamination, which in turn enables multiple cycles of nanocrystal deposition and selenization to create thicker, more uniform absorber films. Devices with power conversion efficiency greater than 7% are fabricated using the multiple step nanocrystal deposition and sintering process.

CuInSe2 Quantum Dot Solar Cells with High Open-Circuit Voltage

CuInSe2 (CISe) quantum dots (QDs) were synthesized with tunable size from less than 2 to 7 nm diameter. Nanocrystals were made using a secondary phosphine selenide as the Se source, which, compared to tertiary phosphine selenide precursors, was found to provide higher product yields and smaller nanocrystals that elicit quantum confinement with a size-dependent optical gap. Photovoltaic devices fabricated from spray-cast CISe QD films exhibited large, size-dependent, open-circuit voltages, up to 849 mV for absorber films with a 1.46 eV optical gap, suggesting that midgap trapping does not dominate the performance of these CISe QD solar cells.

Influence of composition on the performance of sintered Cu(In,Ga)Se2 nanocrystal thin-film photovoltaic devices

Thin-film photovoltaic devices (PVs) were prepared by selenization using oleylamine-capped Cu(In,Ga)Se2 (CIGS) nanocrystals sintered at a high temperature (>500 °C) under Se vapor. The device performance varied significantly with [Ga]/[In+Ga] content in the nanocrystals. The highest power conversion efficiency (PCE) observed in the devices studied was 5.1 % under air mass 1.5 global (AM 1.5 G) illumination, obtained with [Ga]/[In+Ga]=0.32. The variation in PCE with composition is partly a result of bandgap tuning and optimization, but the main influence of nanocrystal composition appeared to be on the quality of the sintered films. The [Cu]/[In+Ga] content was found to be strongly influenced by the [Ga]/[In+Ga] concentration, which appears to be correlated with the morphology of the sintered film. For this reason, only small changes in the [Ga]/[In+Ga] content resulted in significant variations in device efficiency.

Pyrite Nanocrystal Solar Cells: Promising, or Fool's Gold?

Pyrite-phase iron sulfide (FeS2) nanocrystals were synthesized to form solvent-based dispersions, or "solar paint," to fabricate photovoltaic devices (PVs). Nanocrystals were sprayed onto substrates as absorber layers in devices with several different architectures, including Schottky barrier, heterojunction, and organic/inorganic hybrid architectures, to explore their viability as a PV material. None of the devices exhibited PV response. XRD and Raman spectroscopy confirmed the pyrite composition and phase purity of the nanocrystals. The electrical conductivity of the nanocrystal films was about 4 to 5 S/cm, more typical of metal nanocrystal films than semiconductor nanocrystal films, and the lack of PV response appears to derive from the highly conductive surface-related defects in pyrite that have been proposed.

Comparison of the photovoltaic response of oleylamine and inorganic ligand-capped CuInSe2 nanocrystals

Thin film photovoltaic devices (PVs) were fabricated with CuInSe(2) (CIS) nanocrystals capped with either oleylamine, inorganic metal chalcogenide-hydrazinium complexes (MCC), or S(2-), HS(-), and OH(-). A CIS nanocrystal layer deposited from solvent-based inks without high temperature processing served as the active light-absorbing material in the devices. The MCC ligand-capped CIS nanocrystal PVs exhibited power conversion efficiency under AM1.5 illumination (1.7%) comparable to the oleylamine-capped CIS nanocrystals (1.6%), but with significantly thinner absorber layers. S(2-)-capped CIS nanocrystals could be deposited from aqueous dispersions, but exhibited lower photovoltaic performance.