Identifying and investigating spatial features in InGaAs solar cells by hyperspectral luminescence imaging

Hyperspectral luminescence imaging adds high-resolution spectral data to the electroluminescence and photoluminescence images of photovoltaic materials and devices. This enables absolute calibration across a range of spectra and, subsequently, enhances the information that can be gained from such measurements. We present a temperature-dependent luminescence hyperspectral imaging study of dilute InGaAs solar cells. We are able to identify the cause of dark spots on the device as local areas with increased defect-related recombination and identify a likely candidate for the type of defect. Hyperspectral images also reveal a device-wide pattern in low-energy-tail luminescence and In alloy fraction, which corresponds with increased non-radiative recombination. This pattern would not be identifiable with conventional imaging methods. Detailed information on such features is useful as, paired with the knowledge of fabrication processes and device design features, it can help identify the ways to reduce associated non-radiative recombination and improve device performance

Improvement of GaAsP/SiGe tandem solar cell on silicon by optical characterisation, modelling, analysis, and design

The GaAsP/SiGe tandem solar cell grown directly on Si has the potential for high efficiency at relatively low cost. However, achieving high efficiency in any photovoltaic device requires careful design based on both optical and electrical material parameters. The III-V alloys selected for lattice- and current- matching in this device have been little studied, with parameters mostly absent from the literature. This work uses optical characterisation, modelling, and analysis to determine these parameters and thereby improve the tandem efficiency. Complex indicies of refraction of previously unstudied III-V alloys are extracted. These enable optical modelling of the device, through which unexpected collection of carriers generated in the window layer is discovered. A double-layer antireflection coating (DARC) is designed and it is demonstrated that co-optimisation of the window layer results in low broadband reflection even in current-matched situations, enabling the use of a simpler structure than commonly considered necessary. The top cell current is increased to >20 mA/cm^2 by experimental application of the DARC. Extracted constants have the potential to improve interpolation of values for additional alloy fractions. The optical constants are also applied to the extraction of previously unmeasured electrical properties through the direct fitting of modelled IQE curves to experimental data on a variety of devices. The results enable the identification of the best materials independent of variations in layer thickness, the determination of collection profiles with varying layer thickness, and the evaluation of design changes to increase top cell current by up to 1 mA/cm^2 and maximise efficiency in tandem structures. New device designs are optimised for performance under concentration. For the first time, the suitability of Si wet etching processes to the presence of III-V materials is considered. Rear surface processes developed on SiGe-on-Si single junction cells are adapted for tandem fabrication. The first outdoor measurements of this device are reported, achieving 17.0% 2T tandem efficiency with fully isolated devices under AM1.5-D. With the material parameters extracted and optimisations performed in this work, the ability of the next generation of devices to exceed 26% efficiency at one sun, and 32% under concentration, is clearly demonstrated

Illumination-Dependent Requirements for Heterojunctions and Carrier-Selective Contacts on Silicon

High efficiency silicon solar cells generally feature heterojunction or carrier-selective contact architectures, for which there is current interest in developing structures using a wide range of materials. The electrical and optical requirements that these layers must fulfill have been investigated previously for standard test conditions. Here, we investigate how the required work functions and layer thickness differ under other illumination conditions. The differences will be important for the optimization of tandem device subcells, and for devices which are intended for use in low-light conditions or under low-level concentration. Heterojunction cells are fabricated and the effect of reduced contact thickness and doping at different illumination levels is experimentally demonstrated. Simulations of a-Si/c-Si heterojunctions and ideal metal-semiconductor junctions reveal a logarithmic variation with illumination level of 0.1-10 suns in the electrode work function, and the heterojunction contact layer work function and thickness required to minimize efficiency losses

Lessons in a Box Make a Difference for Head Start Youth

This article examines the health education implications of targeted nutrition lesson plans at Head Start programs in south central Minnesota. The Head Start program in Mankato and the University of Minnesota Extension collaborated to deliver and evaluate a nutrition education program directed at preschool children and their families. Nine lesson plans on various nutrition topics were developed and delivered to Head Start preschoolers. The program goals were to increase nutrition-related knowledge among children, improve healthy eating patterns/preferences, and increase physical activity. The evaluation intended to determine program effects and participant changes towards healthy eating habits

Elucidating the role of TiCl₄post-treatment on percolation of TiO₂electron transport layer in perovskite solar cells

The ideal electron transport layer of a high performance perovskite solar cell should have good optical transparency, high electron mobility, and an energy level alignment well-matched with the perovskite material. In this work, we investigate the role of TiCl4 post-treatment of the mesoporous TiO2 electron transport layer by varying the concentration of TiCl4 and characterizing optical and electrical properties, charge carrier dynamics, and photovoltaic performance of mesoscopic CH3NH3PbI3 solar cells. It is found that the TiCl4 treatment provides an additional interconnection between the TiO2 particles, leading to better percolation as evident from high resolution cross-section images and chemical maps. This enhances effective electron mobility in the material as well as significantly reduces average sub-bandgap absorption due to defects and electronic disorder determined by photothermal deflection spectroscopy. Moreover, improvement of interfacial contact due to a smoother surface contributes to more efficient charge extraction and suppressed charge recombination and reduced hysteresis. As a result, the optimized device based on TiCl4 post-treated mesoporous TiO2 achieved the highest conversion efficiency of 17.4% compared with 14.1% for the device with pristine mesoporous TiO2.</p

Towards Quantitative Interpretation of Fourier-Transform Photocurrent Spectroscopy on Thin-Film Solar Cells

The method of detecting deep defects in photovoltaic materials by Fourier-Transform Photocurrent Spectroscopy has gone through continuous development during the last two decades. Still, giving quantitative predictions of photovoltaic device performance is a challenging task. As new materials appear, a prediction of potentially achievable open-circuit voltage with respect to bandgap is highly desirable. From thermodynamics, a prediction can be made based on the radiative limit, neglecting non-radiative recombination and carrier transport effects. Beyond this, more accurate analysis has to be done. First, the absolute defect density has to be calculated, taking into account optical effects, such as absorption enhancement, due to scattering. Secondly, the electrical effect of thickness variation has to be addressed. We analyzed a series of state-of-the-art hydrogenated amorphous silicon solar cells of different thicknesses at different states of light soaking degradation. Based on a combination of empirical results with optical, electrical and thermodynamic simulations, we provide a predictive model of the open-circuit voltage of a device with a given defect density and absorber thickness. We observed that, rather than the defect density or thickness alone, it is their product or the total number of defects, that matters. Alternatively, including defect absorption into the thermodynamic radiative limit gives close upper bounds to the open-circuit voltage with the advantage of a much easier evaluation

Lead Halide Residue as a Source of Light-Induced Reversible Defects in Hybrid Perovskite Layers and Solar Cells

Advanced characterization methods avoiding transient effects in combination with solar cell performance monitoring reveal details of reversible light-induced perovskite degradation under vacuum. A clear signature of related deep defects in at least the 1 ppm range is observed by low absorptance photocurrent spectroscopy. An efficiency drop, together with deep defects, appears after minutes-long blue illumination and disappears after 1 h or more in the dark. Systematic comparison of perovskite materials prepared by different methods indicates that this behavior is caused by the lead halide residual phase inherently present in material prepared by the two-step method. X-ray photoelectron spectroscopy confirms that lead halide when illuminated decomposes into metallic lead and mobile iodine, which diffuses into the perovskite phase, likely producing interstitial defects. Single-step preparation, as well as preventing lead halide illumination, eliminates this effect

<b>Role and Perspective of Certified Diabetes Care and Education Specialists in the Development of the 4T Program</b>

The Diabetes Control and Complications Trial (DCCT) clearly delineated the benefits of intensive diabetes management in preventing long-term complications in people with insulin-dependent diabetes (1). Despite the data from the DCCT, a majority of youth with type 1 diabetes do not meet glycemic targets. One aspect of the DCCT intervention was frequent insulin dose adjustments by a care team member. Although this was a landmark clinical trial, translation of its findings into clinical practice has been challenging because of barriers in implementing glucose data-sharing technology and clinical time constraints. There is also a shortage of diabetes care team members (2) to review glucose data and communicate insulin dosing advice and provide diabetes self-management education and support (DSMES). In particular, there is a nationwide shortage of certified diabetes care and education specialists (CDCESs) (2).</p