Towards a graphene transparent conducting electrode for perovskite/silicon tandem solar cells

Indium-based transparent conducting electrodes (TCEs) are a major limiting factor in perovskite/silicon tandem cell scalability, while also limiting maximum cell efficiencies. In this work, we propose a novel TCE based on electrostatically doped graphene monolayers to circumvent these challenges. The electrode is enabled by a thin film dielectric that is charged and interfaced to a graphene film, optimally exploiting electrostatic doping. The field effect mechanism allows the modulation of charge carriers in monolayer graphene as a function of charge concentration in the dielectric thin film. Electrostatic charge was deposited on SiO2 membranes, and graphene transferred onto them exhibited a reduction in sheet resistance because of the induced charge carriers. We show a reduction in sheet resistance of graphene by 60% in just 3 min of dielectric charging, without impacting the transmission of light through the film stack. Hall effect measurements indicated that the mobility of the films was not significantly degraded. The deposition of negative electrostatic charge reversed this effect, allowing for precise tunability of charge concentration from n- to p-type. We develop a model to determine the required sheet resistance of a graphene TCE with 97% transmittance in a perovskite/silicon tandem cell. As the technique here reported does not impact transmittance, a graphene TCE with a sheet resistance below 50 Ω/□ could enable efficiencies up to 44%, presenting a promising alternative to indium-based TCEs

Microstructural evolution of mechanically deformed polycrystalline silicon for kerfless photovoltaics

Silicon wafers for photovoltaics could be produced without kerf loss by rolling, provided sufficient control of defects such as dislocations can be achieved. A study using mainly high resolution electron backscatter diffraction (HR‐EBSD) of the microstructural evolution of Siemens polycrystalline silicon feedstock during a series of processes designed to mimic high temperature rolling is reported here. The starting material is heavily textured and annealing at 1400 °C results in 90% recrystallization and a reduction in average geometrically necessary dislocation (GND) density from >1014 to 1013 m−2. Subsequent compression at 1150 °C – analogous to rolling – produce sub‐grain boundaries seen as continuous curved high GND content linear features spanning grain interiors. Post‐deformation annealing at 1400 °C facilitates a secondary recrystallization process, resulting in large grains typically of 100 μm diameter. HR‐EBSD gives the final average GND density in as 3.2 × 1012 m−2. This value is considerably higher than the dislocation density of 5 × 1010 m−2 from etch pit counting, so the discrepancy is investigated by direct comparison of GND maps and etch pit patterns. The GND map from HR‐EBSD gives erroneously high values at the method's noise floor (≈1012 m−2) in regions with low dislocation densities

Investigating the link between LeTID and hydrogen induced contact resistance in PERC devices

In recent years academia and industry have made significant efforts to mitigate the problem of LeTID in silicon solar cells, particularly in p-type multi crystalline PERC cells. Many of these approaches involve a post-firing thermal anneal between 300-500 °C after metal contact firing. This paper investigates observed increases in the front contact resistance of PERC cells in this temperature range. Changes in contact resistance have been primarily attributed to a hydrogen passivation effect, which might then be used to observe and study hydrogen kinetics. A new sample structure is developed to allow a more direct measurement of the current-voltage characteristics of the front contacts, without the contribution from resistance elsewhere in the cell. A careful analysis of such measurements leads to three key findings: (i) It is experimentally shown that there is no increase in resistance for any region of the device other than the front contact. (ii) It is shown that the contact resistance change is affected by the frequency of in-situ measurements and becomes highly unstable once resistance change reaches a high value, and (iii) Annealing prior I-V measurements can act to significantly increase the rate at which contact resistance changes, likely through an increase in the mobile hydrogen concentration throughout the cell. These findings are crucial to the understanding and future study of hydrogen in silicon and its relation to degradation in solar cells

Comparison of three device generations of the StepWatch Activity Monitor: analysis of model version agreement in pediatric and adult independent ambulators

PurposeDevices such as the StepWatch Activity Monitor (SAM) have been available for 20 years and have been shown to accurately measure ambulatory activity. This study aimed to evaluate the agreement among the three generations of the StepWatch Activity Monitor (SW3, SW4, and SW5) with respect to stride count.MethodsA total of 36 participants (age range, 6–55 years) participated in this institutional review board-approved study. The participants concurrently wore three different SAM model devices on the same leg and performed a 6-min walk test (6MWT). A research staff member of the laboratory manually counted the number of strides for the first 2 min of the test (2MWT). Agreement among the device models was evaluated by calculating ANOVAs and interclass correlation coefficients (ICCs) and creating Bland–Altman plots.ResultsThere was no significant difference among the model versions during the 6MWT and 2MWT (p > 0.05). The ICC for the total stride count was 0.993 (95% CI = 0.988–0.996) during the 2MWT and 0.992 (95% CI = 0.986–0.996) during the 6MWT. There was a near-perfect agreement (ICC ≥ 0.990) of each model version to the manually counted strides during the 2MWT. The systematic bias of all three SAM model versions was <1 step.ConclusionsThe results from the present study demonstrate that the stride counts among all three devices are comparable and relative to the manual stride count. All three SAM model versions had an ICC of >0.90. Researchers can safely incorporate historical data from previous SAM model versions with newer data collected with the latest SAM model version

To what extent is behaviour a problem in English schools?:Exploring the scale and prevalence of deficits in classroom climate

The working atmosphere in the classroom is an important variable in the process of education in schools, with several studies suggesting that classroom climate is an important influence on pupil attainment. There are wide differences in the extent to which classroom climate is considered to be a problem in English schools. Some ‘official’ reports suggest that behaviour in schools is ‘satisfactory or better’ in the vast majority of schools; other sources have pointed to behaviour being a serious and widespread problem. The paper details four studies conducted over the past decade which aimed to explore these disparities. The aim of the research was to gain a more accurate insight into the extent to which deficits in classroom climate limit educational attainment and equality of educational opportunity in English schools. The findings question the suggestion that behaviour is satisfactory or better in 99.7% of English schools and the concluding section suggests ways in which deficits in classroom climate might be addressed. Although the study is limited to classrooms in England, OECD studies suggest that deficits in the working atmosphere in classrooms occur in many countries. The study therefore has potential relevance for education systems in other countries

Electrically tunable Si-based THz photomodulator using dielectric/polymer surface gating

Silicon-based terahertz (THz) photomodulators suffer from a modulation speed limited by the lifetime of the charge carriers photoexcited in the silicon. We report a silicon-based THz photomodulator scheme offering real-time reconfiguration of the switching behavior by manipulation of effective charge carrier lifetime. Atomic layer deposition was used to coat silicon samples with dielectric layers to passivate the surfaces with a conductive polymer (PEDOT:PSS) subsequently deposited to enable electrical gating over the whole surface. The resulting gated photomodulators are characterized using photoconductance decay and photoluminescence imaging. A gated photomodulator with HfO2 passivation is then implemented into a THz time domain spectroscopy setup to demonstrate the potential for live photomodulation optimization during a single-pixel imaging experiment. We use the device to achieve a real-time improvement of the signal-to-noise ratio of the images by a factor of 8

Specimen preparation methods for elemental characterisation of grain boundaries and isolated dislocations in multicrystalline silicon using atom probe tomography

Multicrystalline silicon (mc-Si) is a cost effective feedstock for solar photovoltaic devices but is limited by the presence of defects and impurities. Imaging impurities segregated to nanometre-scale dislocations and grain boundaries is a challenge that few materials characterisation techniques can achieve. Atom Probe Tomography (APT) is a 3-dimensional time-of-flight microscopy technique that can image the distribution of elements at the atomic scale, however one of the most challenging factors when using APT is the complexity of specimen preparation for specific regions of interest. Atom probe specimen preparation methods have been developed in a dual FIB/SEM system that enable a specific extended defect such as an isolated dislocation or a section of a grain boundary to be selected for APT analysis. The methods were used to fabricate APT specimens from an isolated dislocation and a grain boundary in mc-Si samples. Complementary TEM images confirm the presence of the defects in both specimens, whilst APT analyses also reveal segregation of impurities to the defects

Development and characterisation of a large diameter decellularised vascular allograft

The aims of this study were to develop a biological large diameter vascular graft by decellularisation of native human aorta to remove the immunogenic cells whilst retaining the essential biomechanical, and biochemical properties for the ultimate benefit of patients with infected synthetic grafts. Donor aortas (n = 6) were subjected to an adaptation of a propriety decellularisation process to remove the cells and acellularity assessed by histological analysis and extraction and quantification of total DNA. The biocompatibility of the acellular aortas was determined using standard contact cytotoxicity tests. Collagen and denatured collagen content of aortas was determined and immunohistochemistry was used to determine the presence of specific extracellular matrix proteins. Donor aortas (n = 6) were divided into two, with one half subject to decellularisation and the other half retained as native tissue. The native and decellularised aorta sections were then subject to uniaxial tensile testing to failure [axial and circumferential directions] and suture retention testing. The data was compared using a paired t-test. Histological evaluation showed an absence of cells in the treated aortas and retention of histoarchitecture including elastin content. The decellularised aortas had less than 15 ng mg¯¹ total DNA per dry weight (mean 94% reduction) and were biocompatible as determined by in vitro contact cytotoxicity tests. There were no gross changes in the histoarchitecture [elastin and collagen matrix] of the acellular aortas compared to native controls. The decellularisation process also reduced calcium deposits within the tissue. The uniaxial tensile and suture retention testing revealed no significant differences in the material properties (p > 0.05) of decellularised aorta. The decellularisation procedure resulted in minimal changes to the biological and biomechanical properties of the donor aortas. Acellular donor aorta has excellent potential for use as a large diameter vascular graft