Sustainable Recycling of Selenium-Based Optoelectronic Devices.

Selenium (Se), the world's oldest optoelectronic material, has been widely applied in various optoelectronic devices such as commercial X-ray flat-panel detectors and photovoltaics. However, despite the rare and widely-dispersed nature of Se element, a sustainable recycling of Se and other valuable materials from spent Se-based devices has not been developed so far. Here a sustainable strategy is reported that makes use of the significantly higher vapor pressure of volatile Se compared to other functional layers to recycle all of them from end-of-life Se-based devices through a closed-space evaporation process, utilizing Se photovoltaic devices as a case study. This strategy results in high recycling yields of ≈ 98% for Se and 100% for other functional materials including valuable gold electrodes and glass/FTO/TiO2 substrates. The refabricated photovoltaic devices based on these recycled materials achieve an efficiency of 12.33% under 1000-lux indoor illumination, comparable to devices fabricated using commercially sourced materials and surpassing the current indoor photovoltaic industry standard of amorphous silicon cells.

Structure of Amorphous Selenium: Small Ring, Big Controversy.

Selenium (Se) discovered in 1817 belongs to the family of chalcogens. Surprisingly, despite the long history of over two centuries and the chemical simplicity of Se, the structure of amorphous Se (a-Se) remains controversial to date regarding the dominance of chains versus rings. Here, we find that vapor-deposited a-Se is composed of disordered rings rather than chains in melt-quenched a-Se. We further reveal that the main origin of this controversy is the facile transition of rings to chains arising from the inherent instability of rings. This transition can be inadvertently triggered by certain characterization techniques themselves containing above-bandgap illumination (above 2.1 eV) or heating (above 50 °C). We finally build a roadmap for obtaining accurate Raman spectra by using above-bandgap excitation lasers with low photon flux (below 1017 phs m-2 s-1) and below-bandgap excitation lasers measured at low temperatures (below -40 °C) to minimize the photoexcitation- and heat-induced ring-to-chain transitions.

Indoor photovoltaics awaken the world's first solar cells.

Selenium (Se) solar cells were the world's first solid-state photovoltaics reported in 1883, opening the modern photovoltaics. However, its wide bandgap (~1.9 eV) limits sunlight harvesting. Here, we revisit the world's oldest but long-ignored photovoltaic material with the emergence of indoor photovoltaics (IPVs); the absorption spectrum of Se perfectly matches the emission spectra of commonly used indoor light sources in the 400 to 700 nm range. We find that the widely used Te adhesion layer also passivates defects at the nonbonded Se/TiO2 interface. By optimizing the Te coverage from 6.9 to 70.4%, the resulting Se cells exhibit an efficiency of 15.1% under 1000 lux indoor illumination and show no efficiency loss after 1000 hours of continuous indoor illumination without encapsulation, outperforming the present IPV industry standard of amorphous silicon cells in both efficiency and stability. We further fabricate Se modules (6.75 cm2) that produce 232.6 µW output power under indoor illumination, powering a radio-frequency identification-based localization tag.

Solution-processed Ge(ii)-based chalcogenide thin films with tunable bandgaps for photovoltaics.

Solution processes have been widely used to construct chalcogenide-based thin-film optoelectronic and electronic devices that combine high performance with low-cost manufacturing. However, Ge(ii)-based chalcogenide thin films possessing great potential for optoelectronic devices have not been reported using solution-based processes; this is mainly attributed to the easy oxidation of intermediate Ge(ii) to Ge(iv) in the precursor solution. Here we report solution-processed deposition of Ge(ii)-based chalcogenide thin films in the case of GeSe and GeS films by introducing hypophosphorous acid as a suitable reducing agent and strong acid. This enables the generation of Ge(ii) from low-cost and stable GeO2 powders while suppressing the oxidation of Ge(ii) to Ge(iv) in the precursor solution. We further show that such solution processes can also be used to deposit GeSe1-x S x alloy films with continuously tunable bandgaps ranging from 1.71 eV (GeS) to 1.14 eV (GeSe) by adjusting the atomic ratio of S- to Se-precursors in solution, thus allowing the realization of optimal-bandgap single-junction photovoltaic devices and multi-junction devices.

Surface-Defect States in Photovoltaic Absorber GeSe.

GeSe is an emerging promising light-harvesting material for photovoltaics due to its excellent optoelectronic properties, nontoxic and earth-abundant constituents, and high stability. In particular, perovskite-like antibonding states at the valence band maximum arising from Ge-4s and Se-4p coupling enable the bulk-defect-tolerant properties in GeSe. However, a fundamental understanding of surface-defect states in GeSe, another important factor for high-performance photovoltaics, is still lacking. Here, we investigate the surface-defect properties of GeSe through first-principle calculations. We find that different from common semiconductors possessing numerous surface dangling bonds, some GeSe surfaces are prone to reconstruction, thus eliminating the dangling bonds. The rearranged armchair edges exhibit unexpected benign defect properties, similar to those of bulk GeSe, arising from the formation of bulk-like [GeSe3] tetrahedrons. We further show that the stable exposed (111) surfaces are hard to reconstruct due to the stiff structure but are effectively passivated by the addition of H.

Investigation of the sublimation mechanism of GeSe and GeS.

GeSe and GeS have emerged as promising light-harvesting materials for photovoltaics due to their attractive optoelectronic properties, non-toxic and earth-abundant constituents, and excellent stability. Here we unveil the diatomic molecule sublimation mechanism of GeSe and GeS that directly guides the optimization of GeSe and GeS solar-cell fabricated via the close-space sublimation method.

Delamination and Engineered Interlayers of Ti3C2 MXenes using Phosphorous Vapor toward Flame-Retardant Epoxy Nanocomposites.

As recently created inorganic nanosheet materials, more and more light has been shed on MXenes, which have emerged as a hotspot of intensive investigations. The simple exfoliation method for MXenes attracts numerous studies to pay efforts on. Compared with the extensive research about ultrasonication and mechanical milling, gas-assisted exfoliation has never been carried out for MXenes. Meanwhile, MXene-based nanocomposites are always prepared after exfoliation step by step. In this work, a facile way to fabricate a few-layered Ti3C2 MXene delaminated using phosphorous vapor evolved from commercial red phosphorous (RP) is put forward. The vapor deposits on the surface of Ti3C2 and also partially intercalates into the interlayers to obtain a novel two-dimensional RP/Ti3C2 nanocomposite directly. The P element strongly connects with the substrate by a covalent bond that improves the safety problems for RP during storage and usage. Due to the versatile feature of MXenes, the nanocomposite has the potential to be applied in a variety of fields. Herein, it is employed as a flame retardant for epoxide resin and effectively reduces fire disaster. The one-step exfoliation plus nanocomposite fabrication provides a more feasible way for the practical application of MXenes.

Applicability of boosting techniques in calibrating safety performance functions for freeways.

Safety performance functions (SPFs) are indispensable analytical tools that usually play a crucial role in estimating crash frequencies, identifying hotspots, analyzing crash contributing factors, and assessing the effectiveness of safety countermeasures. Due to the limited availability of safety data, municipalities tended to adopt SPFs from Highway Safety Manual or other neighboring jurisdictions. Recently, boosting algorithms have been frequently exploited for data analysis and statistical regression modeling problems. This research, therefore, aims to examine the efficiency of boosting calibration techniques to transfer the SPF using the limited region data in an international context. To this end, AdaBoost.R2, an adaptive boosting algorithm, Two-stage TrAdaBoost.R2, an instance-based transfer learning algorithm, and Gradient Boosting algorithm were employed to investigate their efficiencies in acquiring knowledge from the available source domain data to predict crashes in the target domain. As a comparison, the calibration factor method was adopted to transfer the traditional negative binomial (NB) regression model. Two training dataset groups were developed to train the four calibration techniques. The first group was used to examine the adaptability of the employed calibration techniques to the limited target region data. While the second group was utilized to further investigate the influence of larger vital information on the performance of transferred models. This study was conducted between two U.S. states, Florida and New York, and two Chinese cities, Shanghai and Suzhou. According to the goodness-of-fit results, boosting calibration techniques showed better prediction accuracy than the calibrated NB-based model using the limited target region data. In addition, the amount and distribution of the training dataset were considered the two significant factors that influence the proficiency of the boosting calibration techniques.

Interfacial Strain Engineering in Wide-Bandgap GeS Thin Films for Photovoltaics.

Wide-bandgap semiconductors exhibiting a bandgap of ∼1.7-1.9 eV have generated great interest recently due to their important applications in tandem solar cells as top cells and emerging indoor photovoltaics. However, concerns about the stability and toxicity especially in indoor application limit the choice of these materials. Here we report a new member of this family, germanium monosulfide (GeS); this material displays a wide bandgap of 1.7 eV, nontoxic and earth-abundant constituents, and high stability. We find that the little success of GeS solar cells to date is primarily attributed to the challenge in fabricating high-quality polycrystalline GeS films, wherein the high thermal expansion coefficient (α = 3.1 × 10-5 K-1) combined with high crystallization temperature (375 °C) of GeS induces large tensile strain in the GeS film that peels off GeS from the substrate. By introducing a high-α buffer layer between GeS and substrate, we achieve a high-quality polycrystalline GeS thin film that compactly adheres to substrate with no voids. Solar cells fabricated by these GeS films show a power conversion efficiency of 1.36% under AM 1.5G illumination (100 mW cm-2). The unencapsulated devices are stable when stored in ambient atmosphere for 1500 h. Their efficiencies further increase to 3.6% under indoor illumination of 1000 lux.

Virtual simulation of otolith movement for the diagnosis and treatment of benign paroxysmal positional vertigo.

Benign paroxysmal positional vertigo (BPPV) is a clinical condition. The existing diagnostic methods cannot determine the specific location of otolith on the short or long brachial sides. Thus, visual and quantitative evaluation of the existing clinical standard diagnostic modality Dix-Hallpike test is needed to improve medical efficiency. Our goal was to develop a real-time virtual simulation system to assess a BPPV treatment manipulation. In this study, we used the proposed simulation system to observe otolith movement during a posterior semicircular canal BPPV diagnostic test, and to analyze the diagnostic mechanisms and strategies. Through visual cluster analysis of otolith position and analysis of otolith movement time in the standard Dix-Hallpike test, we can find that the positions of otoliths are relatively scattered, especially on the z-axis (z 1 = 10.67 ± 3.98), and the fall time of otoliths at different positions has relatively large changes (t 1 = 22.21 ± 1.40). But in the modified experiment z 2 = 4.93 ± 0.32 and t 2 = 26.21 ± 0.28. The experimental results show that the simulation system could track the state and the movement of otolith in real-time, which is of great significance for understanding the diagnostic mechanisms of BPPV evaluations and improving the diagnostic method.

Transferability of safety performance functions and hotspot identification for freeways of the United States and China.

Safety performance function (SPF) has been a vital tool in traffic safety evaluation including finding contributing factors to crashes, identifying hotspots, and assessing safety effects of countermeasures. In the United States (U.S.), the Highway Safety Manual provides a number of SPFs for a variety of road facilities. Due to the limited availability of traffic data in many regions, the transferability of SPFs has been an important topic in traffic safety analysis and has been evaluated by several studies. Nevertheless, the international transferability of freeway SPFs and the applicability of transferred SPFs on hotspot identification has been rarely investigated. Based on data from two Chinese cities, Shanghai and Suzhou, and three U.S. states, Texas, New York and Florida, this study analyzes the transferability of freeway SPFs between Chinese and U.S. regions. These SPFs are then transferred to the other country and their performance on hotspot identification is investigated. SPFs were developed in the frameworks of Poisson, Poisson-lognormal and negative binomial regressions for the five localities separately, and were calibrated using the calibration functions before being transferred. Without calibration, the poor model transferability was found between the two countries, while after calibration, the transferred SPFs between Shanghai/Suzhou and Texas/New York showed satisfactory performance on both model fitting and hotspot identification. However, the transferability of SPFs between Florida and the Chinese cities turned out to be unsatisfactory regardless of whether being calibrated or not, which was attributable to the considerable difference in traffic flow. The findings of this study are expected to be a good reference for researchers and practitioners who want to understand the transferability and applicability of SPFs in the international context.

Freeway single and multi-vehicle crash safety analysis: Influencing factors and hotspots.

Single-vehicle (SV) and multi-vehicle (MV) crashes have been recognized as differing in spatial distribution and influencing factors, but little consideration has been given to these differences as related to hotspot identification. For the purpose of better hotspot identification, this study aims to analyze influencing factors of SV and MV crashes and to explore the consistency between SV and MV hotspots. Crash data, roadway geometric design features, and traffic characteristics were collected along the two directions of a 45-km freeway section in Shanghai, China. Univariate negative binomial conditional autoregressive (NB-CAR) and bivariate negative binomial spatial conditional autoregressive (BNB-CAR) models were developed to analyze the influencing factors and specifically address (1) site correlation between SV and MV crashes within the same freeway segment, and (2) spatial correlation among different freeway segments within the same direction. The modeling results showed substantial differences in the significant factors that influence SV and MV crashes, including both roadway geometric features and traffic operational factors. A non-negligible site correlation was found between SV and MV crashes. Taking into account the site correlation, the BNB-CAR model outperformed the NB-CAR model in terms of parameter estimation and model fitting. For hotspot identification, potential for safety improvement based on the empirical Bayes method was adopted to handle the crash fluctuation problem. Substantial inconsistency was found between SV and MV hotspots despite the site correlation: in the top ten hotspots, no hotspot was shared by the two crash types. This result highlights the importance of differentiating SV and MV crashes when identifying hotspots, providing insight into freeway safety analysis.