Graded Gap Research Articles

Chemical Weathering Processes of Granitic Residual Soils and Gap Grading, Soil Compaction Properties

Chemical Weathering Processes of Granitic Residual Soils and Gap Grading, Soil Compaction Properties

Fabricating Cu2Zn(SnxGe1-x)Se4 thin-film solar cells with back surface Ge grading by magnetron sputtering

In this paper, an approach of forming a band gap gradient by depositing a layer of Ge between the Mo layers by magnetron sputtering is proposed. The diffusion depth of the Ge into Cu2ZnSnSe4 (CZTSe) thin film can be effectively controlled by the Mo transition layer, resulting in the formation of Cu2Zn(SnxGe1-x)Se4 (CZTGSe) thin film with a band gap grading along the back surface. To validate the formation of the Ge gradient in the absorber, energy dispersive spectrometer line scanning and grazing incidence X-ray diffraction were performed. Furthermore, the impact of Ge layer on the morphologies of CZTSe was assessed using scanning electron microscopy. As a result, the introduced Ge layer formed a Ge grading along the back surface and improved the crystal quality of the absorber. Compared to CZTSe solar cells, the power conversion efficiency (PCE) of CZTGSe solar cells with a Ge gradient improved from 5.79 % to 9.28 %.

Revealing Stereotypes: Evidence from Immigrants in Schools

We study how people change their behavior after being made aware of bias. Teachers in Italian schools give lower grades to immigrant students relative to natives of comparable ability. In two experiments, we reveal to teachers their own stereotypes, measured by an Implicit Association Test (IAT). In the first, we find that learning one’s IAT before assigning grades reduces the native-immigrant grade gap. In the second, IAT disclosure and generic debiasing have similar average effects, but there is heterogeneity: teachers with stronger negative stereotypes do not respond to generic debiasing but change their behavior when informed about their own IAT. (JEL D91, I24, J15, J45)

Stereotyping and ethnicity gaps in teacher assigned grades

We explore the contribution of stereotyping to attainment gaps between pupils from different ethnic groups when grades are assigned by teachers. We exploit a change in assessment methods in England to compare grades based on teacher predictions to grades received through blindly marked examinations. When grades are assigned by teachers, ethnic minority pupils receive higher grades in maths and lower grades in English relative to White British pupils and compared to when grades are assigned through exams marked by external assessors. We use an extension of the Gelbach decomposition (Gelbach, 2016) to investigate whether the effects can be accounted for by differences in the levels of, or returns to, observed characteristics between years. Accounting for these differences partially reduces the grade gap changes in maths but roughly doubles the magnitude of the grade gap changes in English. Grade gap changes are also not driven by time trends or ceiling effects. We conclude that group-specific stereotyping is a convincing explanation of the results.

Influence Volumetric Characteristic on Skid Number with Difference Aggregate Gradation

The purpose of this document is to present an investigation into the impact of volumetric characteristics, specifically the composition and distribution of aggregates, on the skid resistance performance of road surfaces. The study focuses on the variations in aggregate gradations and their influence on the volumetric properties of asphalt mixtures. Two distinct gradation types, continuous and gap grading, are analyzed to assess their effects on skid number values. VFA value does not have a significant effect on skid number value. The volumetric value with continuous gradation under standard conditions shows that the VMA value has the most significant influence on the skid value, and the VIM value gap gradation has the most significant influence on the skid value. While in immersion conditions, for volumetrics that affect the skid number value in continuous gradation and gaps, the VMA value has the most significant influence on the performance of skid resistance. The linear regression results in volumetrics and skid values on continuous and gap gradation in standard condition is y = 1.507 x VMA + 12.841 x VIM – 174.097. with an influence on the skid value of 83.7% and y = -5.576 x VMA + 6.233 x VFA + 33.345 x VIM – 461.402. with an influence on the skid value of 99.7%. The linear regression results at volumetric and skid values on continuous and gap gradation in immersion conditions are y = 5.242x VMA + 0.779 x VIM – 51.695. with an influence on the skid value of 45.8% and y = -3.462 x VMA – 0.454 x VFA -2.088 x VIM + 153.820. with an influence on the skid value of 98.3%. The effect of skid resistance performance on temperature, it can be concluded that a consistent increase in temperature causes a decrease in the skid resistance value.

STEM Faculty Instructional Beliefs Regarding Assessment, Grading, and Diversity are Linked to Racial Equity Grade Gaps

STEM Faculty Instructional Beliefs Regarding Assessment, Grading, and Diversity are Linked to Racial Equity Grade Gaps

Exploring the deposition pathway in the notch region of double-graded bandgap ACIGS solar cells

The bandgap widening of Ag-alloyed Cu(In,Ga)Se2 (ACIGS) thin films poses a significant challenge in enhancing their photocurrent. This study demonstrates correlations between element diffusion behavior and notch-point formation in ACIGS films, which ultimately influence the resulting current circuit voltage and fill factor. Our findings delineate a novel approach for fabricating a suitably tailored band gap grading in ACIGS, which serves as a bottom subcell in tandem configuration. This was achieved by modulating a wide range of process temperatures from 340 to 470 °C, which were measured by pyrothermal, to assess the GGI profile during deposition. Experiments were undertaken to variate the second-stage conditions, striving to sustain the photocurrent, mitigate defects, and enhance crystallinity, achieving an impressive performance of 17.7 % without applying post-deposition treatments or anti-reflection coatings. The efficiencies surpassing 8 % were achieved as we measured these cells under a 715 nm long-pass filter, which corresponds to the bandgap of a typical top cell in tandem applications (1.73 eV photon energy). These results emphasize the significance of understanding the deposition temperature and its impact on the properties of ACIGS films, paving the way for further advancements in solar-cell technology.

Improved PCE in stable lead-free perovskite solar cells based on band engineering of ETL and absorber

In this paper, a unique structure of graded band gap (GBG) electron transport layer (ETL) based lead-free Perovskite solar cell (PSC) is proposed and numerically investigated. The structure consists of cesium Formamidinium tin iodide (FACsSnI3) with band-gap grading aspect sandwiched between electron and hole transport layers. Various ETLs such as Zn(S,O), ZnO, ZnMgO and C60 have been considered and investigated with and without GBG aspect. It is revealed that the use of band engineering based on band gap grading ETL and absorber films allows the dual-advantages of enhanced photogeneration and carrier extraction mechanisms. The merits of the proposed PSC structure are compared to other published results in terms of improved photovoltaic parameters. The obtained results show improvements in photoconversion properties of the PSCs when GBG-Zn(S,O) is considered as an ETL. The effective conduction band alignment of GBG-Zn(S,O)/GBG-FACsSnI3 heterostructure interface leads to an improved carrier collection efficiency and subsequently improved efficiency. The optimized PSC structure presents power conversion efficiency as high as 21.67 %, with an open circuit voltage of 0.96 V, short-circuit current of 30.75 mA/cm2, and fill factor of 72.88%. The obtained photovoltaic parameters of the proposed GBG-ETL/GBG-FACsSnI3 heterostructure can provide new perspectives for the realization of efficient cost-effective lead-free PSCs.

Study on macro and micro shear strength of continuously graded and gap-graded sand

The particle gradation of sand has a significant influence on its shear strength, yet the similarities and differences between the effects of continuous and gap grading have yet to be fully explored. In this study, the discrete element method (DEM) was used to simulate biaxial tests on granular samples that were both continuously graded and gap-graded. The macroscopic analysis revealed that the shear strength of continuously graded sands increases initially and then decreases as the uniformity of particle size distribution decreases. On the other hand, the lack of medium particles in gap-graded sands amplifies the difference in particle size between coarse and fine particles, leading to a decrease in shear strength. Microscopically, both continuous and gap gradings affect the internal packing structure of the particle assembly, which consequently affects particle stress distributions, contact forces, coordination numbers, stress-induced anisotropies, and contact force networks, thus having an impact on the macroscopic shear strength. The global uniformity of particle size distribution was unidirectionally affected by continuous grading, while gap grading had a locally bidirectional influence. These findings provide a better understanding of the effects of particle grading on the macroscopic shear strength of sands.

Bridging the Grade Gap: Reducing Assessment Bias in a Multi-Grader Class

AbstractMany large survey courses rely on multiple professors or teaching assistants to judge student responses to open-ended questions. Even following best practices, students with similar levels of conceptual understanding can receive widely varying assessments from different graders. We detail how this can occur and argue that it is an example of differential item functioning (or interpersonal incomparability), where graders interpret the same possible grading range differently. Using both actual assessment data from a large survey course in Comparative Politics and simulation methods, we show that the bias can be corrected by a small number of “bridging” observations across graders. We conclude by offering best practices for fair assessment in large survey courses.

Reduced Graphene Oxide/MXene-Derived TiO2 Hybrid Interface Layer for the Improvement of Zinc Oxide Nanorod Growth and Their Applications in Glutathione Sensing

Photoelectrochemical (PEC) sensor is an important type of biosensor widely used in glutathione (GSH) sensing. The PEC properties of the photoanode present in the sensor are critical to its sensing performance. Zinc oxide (ZnO) is an excellent semiconductor with a suitable band gap and light absorption ability for photoanode applications. Meanwhile, the interfacial layer is also important in the separation and transportation process of the excitons. In this work, high-quality ZnO nanorods were grown on the indium tin oxide (ITO) substrates. An interfacial layer consisting of reduced graphene oxide (RGO) or MXene (a two-dimensional transition metal carbide)-derived TiO2 was introduced. Our results show that the introduction of the RGO/TiO2 hybrid interfacial layer can promote both the high-quality growth of ZnO nanorods and also provides suitable band gap grading for efficient excitons separation and transportation. The GSH sensing performance of the PEC sensor based on the ZnO nanorods grown on the RGO/TiO2 hybrid layer-coated ITO photoanode can dramatically improve the photocurrent strength and linearity.

Improving the absorption spectrum and performance of CIGS solar cells by optimizing the stepped band gap profile of the multilayer absorber

In this paper, we investigate a way to improve the performance of thin films CIGS-based solar cells by optimizing their spectral responses. Band gap profile grading, aroused this last decade as a very promising strategy to achieve higher efficiency. This work was performed on a CIGS-based thin-film solar cell model, using cadmium sulphide CdS as a buffer layer, with zinc oxide ZnO as window layer. Focusing on electrical properties such as band gap and electron susceptibility, we modified gallium content of the absorber layer of a reference cell. Using aforementioned band gap grading strategy, we stacked three CIGS layers with different gallium contents, and we modelled a stepped band gap profile using SCAPS-1D. The comparative analysis of the results obtained through numerical simulation, with those of six different band gap profiles of the absorber layer, supports the statement that, stepped band gap profile is a promising approach for engineering improvement of high-efficiency CIGS-based solar cells. In fact, the absorption spectrum of the optimized cell has improved compared to the other band gap profiles simulated in this work; with a peak external quantum efficiency (EQE) of 94%. Efficiencies losses dues to recombination were reduced and the generated photocurrent is higher than that of classical profiles. Record efficiency of 28.03% was achieved with a fill factor of 86.42%. We believe absorber layer stacked in a stepped band gap profile, as a complementary approach to improve performances, is definitely a must-have technology for all future generations high efficiency thin films solar cells.

Controlling the Anionic Ratio and Gradient in Kesterite Technology.

Accurate anionic control during the formation of chalcogenide solid solutions is fundamental for tuning the physicochemical properties of this class of materials. Compositional grading is the key aspect of band gap engineering and is especially valuable at the device interfaces for an optimum band alignment, for controlling interface defects and recombination and for optimizing the formation of carrier-selective contacts. However, a simple and reliable technique that allows standardizing anionic compositional profiles is currently missing for kesterites and the feasibility of achieving a compositional gradient remains a challenging task. This work aims at addressing these issues by a simple and innovative technique. It basically consists of first preparing a pure sulfide absorber with a specific thickness followed by the synthesis of a pure selenide part of complementary thickness on top of it. Specifically, the technique is applied to the synthesis of Cu2ZnSn(S,Se)4 and Cu2ZnGe(S,Se)4 kesterite absorbers, and a series of characterizations are performed to understand the anionic redistribution within the absorbers. For identical processing conditions, different Se incorporation dynamics is identified for Sn- and Ge-based kesterites, leading to a homogeneous or graded composition in depth. It is first demonstrated that for Sn-based kesterite the anionic composition can be perfectly controlled through the thicknesses ratio of the sulfide and selenide absorber parts. Then, it is demonstrated that for Ge-based kesterite an anionic (Se–S) gradient is obtained and that by adjusting the processing conditions the composition at the back side can be finely tuned. This technique represents an innovative approach that will help to improve the compositional reproducibility and determine a band gap grading strategy pathway for kesterites. Furthermore, due to its simplicity and reliability, the proposed methodology could be extended to other chalcogenide materials.

Signal Control Strategy for Supersaturated Traffic at Urban Road Intersections with a Large Technical Grade Gap

To reduce the risk of queuing overflow on the urban minor road at the intersection under supersaturation where the capacity of the arterial and minor roads shows extreme disparity, reduce the adverse effects caused by long queues of vehicles on the minor road, and comprehensively balance the multiobjective requirements such as priority of the main road, queuing restrictions, and delay on the minor road, the minor road queue model at the end of red, a road remaining capacity model, and multiparameter coordinated signal control model were established, and a multiobjective genetic algorithm was used to optimize this solution. As an example, the multiparameter coordinated control strategy decreased the delay per vehicle by approximately 17% and the queue length by approximately 30%–50% on the minor road and slightly increased the delay per vehicle at the intersection and the length on the main road queue. This control strategy can make full use of the capacity of the main road to control the queue length on the minor road, effectively reduce the risk of minor road queue overflow blocking local road network traffic operation involved, and comprehensively balance the traffic demand between arterial and minor roads. It provides a reference control method for coping with the transfer of the main traffic contradiction under the oversaturated state of the road intersection with large disparity.

Theoretical impacts of single band gap grading of perovskite and valence band offset of perovskite/hole transport layer interface on its solar cell performances

Single bandgap (Eg)-grading in CH3NH3(SnxPb1−x)(I1−yBry)3 perovskites and valence band offset (VBO) of a perovskite/hole transport layer (HTL) interface of perovskite solar cells (PSCs) are theoretically scrutinized. Eg values at front and back is defined as Eg values of perovskite at light incident side and back side, respectively, which are changed from 1.10 eV to 1.80 eV based on Sn and Br compositions for different Eg-grading. It is disclosed that optimized single Eg-grading (1.47 / 1.33 eV at front/back) results in enhancement of cell performances because of effective utilization of broader solar spectrum and formation of electric field in direction of promoting charge separation. Furthermore, optimized carrier diffusion length of 2.5 μm and perovskite thickness of 0.75 μm yields the highest power conversion efficiency of 26.0%. CH3NH3(Sn0.07Pb0.93)I3 at front for front Eg of 1.47 eV and CH3NH3(Sn0.38Pb0.62)I3 at back for back Eg of 1.33 eV are thus suggested for optimized single Eg-grading. Moreover, optimized VBO is in a range from − 0.1 eV to + 0.2 eV regardless of Eg-grading to improve cell performances. The results indicated that PEDOT:PSS, Spiro-OMeTAD, and PTAA as HTL are ultimately suitable for CH3NH3 (SnxPb1−x)I3 based PSCs, whereas NiOx and CuOx as HTL is appropriate for CH3NH3 (SnxPb1−x)I3 and CH3NH3Pb(I1−yBry)3 based PSCs.

Development of arsenic doped Cd(Se,Te) absorbers by MOCVD for thin film solar cells

Recent developments in CdTe solar cell technology have included the incorporation of ternary alloy Cd(Se,Te) in the devices. CdTe absorber band gap grading due to Se alloying contributes to current density enhancement and can result in device performance improvement. Here we report Cd(Se,Te) polycrystalline thin films grown by a chamberless inline atmospheric pressure metal organic chemical vapour deposition technique, with subsequent incorporation in CdTe solar cells. The compositional dependence of the crystal structure and optical properties of Cd(Se,Te) are examined. Selenium graded Cd(Se,Te)/CdTe absorber structure in devices are demonstrated using either a single CdSe layer or CdSe/Cd(Se,Te) bilayer (with or without As doping in the Cd(Se,Te) layer). Cross-sectional TEM/EDS, photoluminescence spectra and secondary ion mass spectroscopy analysis confirmed the formation of a graded Se profile toward the back contact with a diffusion length of ~1.5 μm and revealed back-diffusion of Group V (As) dopants from the CdTe layer into Cd(Se,Te) grains. Due to the strong Se/Te interdiffusion, CdSe in the Se bilayer configuration was unable to form an n-type emitter layer in processed devices. In situ As doping of the Cd(Se,Te) layer benefited the device junction quality with current density reaching 28.3 mA/cm 2 . The results provide useful insights for the optimisation of Cd(Se,Te)/CdTe solar cells. Compositional control of optical band gap in Cd(Se,Te) thin films (0 ≤ x ≤ 1) (left) and the Se profile in finished Cd(Se,Te) devices (right) fabricated by MOCVD. • Cd(Se,Te) thin films (0 ≤ x ≤ 1) deposited with linear compositional control using a chamber-less MOCVD process. • CdSe-only and CdSe/CdSe 0.2 Te 0.8 bilayers utilised for Cd(Se,Te) grading in devices. • In-situ and ex-situ group V (arsenic) doping of the graded Cd(Se,Te) obtained. • Device performance improved for thinner CdSe layers and in-situ doped bilayers. • Good control of Se profile achieved via CdSe and CdSe/CdSe 0.2 Te 0.8 properties.

Preparing Cu2ZnSn(SxSe1−x)4 thin-film solar cells with front band gap grading by plasma sulfurization

In this paper, Cu2ZnSn(SxSe1−x)4(CZTSSe) thin-films with a front-graded band gap were prepared by plasma sulfurization. The method of selenization followed by low-temperature plasma sulfurization was adopted to effectively control the uneven diffusion of S in the absorption layer. By investigating and analyzing the grazing incidence X-ray diffraction, Raman spectroscopy, and PL spectra, the CZTSSe absorption layer with an S-rich top and S-poor bottom layer could be obtained by plasma sulfurization. The inhomogeneous distribution of S led to the formation of band gap gradients. Based on that, the band gap near the top of the absorption layer was determined to be 1.13 eV and the bottom band gap was 1.03 eV. Approximately 300 nm thick sulfur-rich CZTSSe films were found near the surface favoring an increase in the band gap at the interface, which could be attributed to a high open circuit voltage (Voc). The establishment of the band gap gradient formed a barrier towards the motion of holes towards the interface and increased the effective interfacial recombination band gap, both of which significantly reduced the interfacial recombination. Finally, the CZTSSe thin-film solar cells with a band gap gradient were prepared by plasma sulfurization exhibited a power conversion efficiency (PCE) of 8.11%. Compared with CZTSSe thin-film solar cells without post plasma sulfurization, which had a PCE of 6.98%, the Voc of the CZTSSe solar cell improved considerably from 471.37 mV to 513.93 mV; however, it had little effect on short-circuit current density (Jsc).

Bandgap engineered Cu2ZnGexSn1−xS4 solar cells using an adhesive TiN back contact layer

Kesterite-based solar cells are mainly restricted by their lower than expected open-circuit voltage (Voc) due to non-radiative recombination. Therefore, an approach to reduce bulk and interface recombination through band gap grading to induce a back surface field is attempted. This contribution presents the challenges in the formation of compositional grading of the wide bandgap material Cu2ZnGexSn1−xS4 (CZGTS) and successful fabrication of solar cells with an additional adhesive TiN interlayer. It is observed that the TiN interlayer improves adhesion between CZGTS and the back contact. The microstructure of the Cu2ZnSnS4 (CZTS) film is significantly affected by the concentration of Ge, and the existence of a Ge concentration gradient is strongly correlated to the formation of smaller Ge-rich and larger Sn-rich grains. The bandgap grading is exploited with a moderate Ge concentration of up to (Ge/(Ge+Sn) = 0.25) in CZTS. As the Ge profile stretched all the way to the front interface, the cliff-like band alignment at the front interface of the absorber could negate the beneficial effect of Ge inclusion in the bulk and back interface of the absorber. Ordering the absorber can introduce an additional downward shift in the valence band. In one of the samples, the increased ordering and high concentration of Ge in CZTS are suggested to enhance the hole barrier at the back interface. It is concluded that the effect of the bandgap grading with Ge can only be realized with optimization of interface band alignment and back contact formation.

The effect of gender on student self-assessment skills in operative preclinical dentistry.

Self-assessment is a fundamental skill for dentists and other health care providers. It enables these professionals' ability to critically evaluate the quality of their clinical work and improve through self-directed learning. Researchers have investigated how gender affects self-assessment skills and have shown that male students tend to overestimate their performance while female students tend to underestimate theirs as compared to peer or faculty assessment. The goal of this study was to evaluate how the self-assessment skills of dental students differ by gender in operative preclinical dentistry. Third-year dental students (N=208, Class of 2016-2021) self-assessed their work using the same rubrics as faculty on four operative dentistry competency examination procedures: Class II amalgam preparation and restoration and Class III resin-composite preparation and restoration. Two calibrated full-time faculty graded all procedures independently. The Student self-assessment-Faculty grade (S-F) gap scores were calculated and statistically analyzed. Overall, both genders overestimated their self-assessment for all four procedures as compared to actual scores given by faculty. Males overestimated more significantly than females for Class II Amalgam preparation, but not for the other three procedures. However, when the S-F gap for all procedures was combined and analyzed together, male students significantly overestimated their self-assessments compared to female students. When female and male students grading scores were stratified into quartiles, there were significantly larger differences between males and females in the lower quartiles, which suggest that difference in self-assessment abilities was more prominent among the lower performing students. The results of this study demonstrate that gender may affect the accuracy of self-assessment in operative preclinical dentistry.

A new method for CdSexTe1-x band grading for high efficiency thin-absorber CdTe solar cells

In recent years the efficiency of CdTe based solar cells has been significantly improved, mainly by the increase of the short circuit current value. This has been achieved by removing the CdS window layer and by adding a CdSexTe1-x layer, grading the band gap of the CdTe. On the other hand, ultra-thin absorbers allow to reduce the materials impact in terms of availability, cost and environmental impact.In this paper we present and study the fabrication of an ultra-thin CdTe based device with band gap grading. Moreover, we present a novel method to generate CdSexTe1-x compounds: by annealing a thin part of the absorber layer in a selenium atmosphere (selenization). This technique allows to narrow the absorber band gap to 1.41 eV, to obtain the desired selenium profile and to considerably increase the external quantum efficiency at long wavelengths.We observe a large increase in the spectral response and an explicit improvement in the short circuit current for CdTe cells with an absorber thickness of 2 μm and without CdS. Current density values of 26 mA/cm2 have been achieved, which are the highest reported for thin CdTe absorbers.