Flexible photovoltaics have been and will be increasingly demanded in various applications in todays and future society. The search for an ideal flexible photovoltaic technology that can perfectly meet these expanding demands has long been an active branch of photovoltaic research. Monograin layer technology (MGL) is one possible path to develop the lightweight, flexible, and semi-transparent solar cells. The major innovations in the MGL solar cell technology are the light absorbing layer made of high quality micro-crystalline semiconductor powder enabling theoretically higher efficiencies and the low cost and easily up-scalable roll-to-roll PV module production process. In recent years, the main research focus of the monograin technology has been on the understanding of the synthesis and optoelectronic properties of Cu2ZnSnS4 absorber materials. The highest power conversion efficiency of this type of devices is 12.06% with output parameters as follows: VOC =0.745 V, JSC=28.36 mA/cm2 and FF = 57.10 %. In this study temperature dependence (T = 20- 320K) of current-voltage (J-V) characteristics of record efficiency Cu2ZnSnS4 MGL solar cell were investigated to clarify the main losses in CZTS, which are still not fully understood. The light J−V curve analysis was used to evaluate the quality of the p−n junction and losses related to resistive components of the device. In this study, the single exponential diode equation was employed to analyze the light J−V data. It turned out that at lower temperatures (T < 180 K) a second blocking diode appears and it is related to back contact barrier. We believe that this back contact barrier has an effect also at higher temperatures causing relative low values of FF. At T=300 K the diode ideality factor n has a value 2.58 and the series resistance Rs =2.6 Ωcm2.
We show that trapping impacts charge carrier dynamics in undoped CdSeTe, undoped CdSeTe/CdTe bilayers, and in As-doped CdSeTe/CdTe solar cells. Trapping has much smaller influence in CdTe-only (no Se, no As) films. Electrostatic potential fluctuation model (amplitude γ > 30 meV) applies to As-doped CdSeTe/CdTe, and defect model (defect activation energy Ea = 0.2 eV) applies to undoped CdSeTe. Unusually, undoped CdSeTe with trap states can have high radiative efficiencies and TRPL lifetimes (10-15 μs). As a result, thermodynamic CdTe solar cell analysis (radiative voltage, implied voltage, external radiative efficiency metrics) needs to be used with caution. As a metric for trap states, PL emission spectroscopy and charge carrier lifetimes at low temperatures provide distinct trap signatures. Trapping impact on devices can be evaluated in modeling and assessed in measurements outlined in this paper, perhaps providing a verifiable hypothesis when we search how to overcome voltage bottleneck in CdTe solar cells.
The high cell efficiency of ~ 22% and cost-effective commercially available modules make thin-film polycrystalline CdTe-based solar cells promising alternatives to conventional energy sources. The photoconversion efficiency of CdTe-based photovoltaics can be further improved by Se doping in the absorber layer. The superior performance of CdSe1-xTex (CdSeTe)-based solar cells is attributed to a lower band gap of the CdSeTe layer in the absorber, which leads to increased carrier lifetime, and simultaneous passivation of grain interiors and grain boundaries. Se doping in the absorber layer also suppresses non-radiative recombination due to the reduction of mid-gap defect states. Despite conclusive evidence of enhanced device performance upon Se doping in the absorber layer, direct atomic-scale observations of changes in the bulk, defect, and electronic structures for CdSeTe are still lacking. Moreover, the diffusion of Se through the absorber layer, especially at low temperatures, is not well understood. Here, we have used atomic-resolution scanning transmission electron microscopy and first-principles density functional theory calculations to understand the role of Se doping on the atomic and electronic structure of CdSeTe-based photovoltaic devices. We observe that the smaller grains (< 1μm) in the CdSeTe layer are accompanied by an increased density of dislocations and dislocation cores. We find that the Se concentration profile changes across the absorber layer upon aging for 100' of hours at operational temperatures. We show that the presence of planar defects in the absorber layer leads to heterogeneities in the chemical distribution of Se. Overall, our findings provide atomic-scale insights into the role of Se doping, which can be further leveraged to improve the photovoltaic efficiency of CdSeTe-based solar cells.
Estimating glare is becoming an increasingly important step in the planning stage of PV installations. This is especially true for building-integrated photovoltaics (BIPV), where atypical orientations and tilt angles can cause reflections in unusual locations. Currently, simulation tools perform simplified ray-tracing calculations to determine specular reflections from PV installations in their environment, often based on reflectance fit functions. In this work, gonioreflectometric measurements are used to determine the bi-directional reflectance distribution function (BRDF) of PV mini-modules with different surface glasses. They show significant differences between smooth and structured glass surfaces, with satinated glass showing overall lowest reflectance. At medium to high incidence angles, however, reflections are not only higher than at normal incidence, but may also peak at different view angles than the specular reflection angles. This indicates, that the common methodology of simulating only specular reflections may underestimate glare. It furthermore suggests, that a low number of reflectance fit functions may be insufficient to describe possible glare from structured surfaces, as they can exhibit vastly different BRDFs, despite possibly similar specular reflectance fit functions. In addition, calculations based on retinal irradiance threshold values for eye damage show that even for PV modules with flat glass, temporary retinal damage in form of flash blindness is highly unlikely. In any case, retinal irradiance values are far below values required for retinal burn damage, limiting reflections from PV installations to disability or discomfort glare levels. Even for the lowest reflecting surfaces, i.e. satinated glass, reflectances at incidence angles beyond 50° are high enough to possibly cause uncomfortable reflections, requiring ray-tracing simulations for accurate estimations of glare.
PV modules of the same make and model are often assembled with different bills of materials (BOMs). We describe two case studies of utility-scale silicon PV systems in which these differing BOMs were associated with faster-than-expected degradation. In one of the sites we found that different metallization paste had been used for grid lines in some cells leading to loss of contact to the cell and severe series resistance degradation. In another site, we found that two different types of cell had been used, one of which suffered from light and elevated temperature induced degradation (LeTID). Our results from both sites underscore that variations in BOM, even among modules of the same make and model can lead to reliability challenges.
Co-extruded backsheets based on polypropylene (PP) are an interesting alternative to laminated backsheets containing polyester films (PET). Backsheet cracking has become a frequent failure mode in the last years, causing not only safety issues but on the long term also reducing the lifetime of PV modules. In this work the crack susceptibility of three different backsheets was investigated using solder bump coupon specimens: two co-extruded backsheets based on polyamide (P A) and PP, together with one laminated backsheet containing a PET core layer and polyvinyl fluoride (PVF) outer layers. The solder bump coupons were exposed to test sequence of exposure to UV light followed by thermal cycles. Overall, the PP as well as the PVF-PE T backsheet showed excellent stability and no susceptibility to material embrittlement or cracking. By comparison, the PA based backsheets showed next to significant discoloration also strong cracking after a few test cycles. Overall, the study confirms that co-extruded PP backsheets show great potential to be a valid replacement of standard PET based backsheets in PV modules.
Underperforming cells in a photovoltaic (PV) module or the modules in a PV string are typically detected and mapped using electroluminescence (EL) infrared (IR) imaging, and current voltage (IV) curve techniques. In the current work, a non-contact electrostatic voltmeter (ESV) technique is presented to detect and map the underperforming spots in a cell and the cells in a module. The ESV technique relies on the voltage mapping of the charged surface of the superstrate glass. The voltage values obtained using ESV at various good and poor performing spots of the cells have been validated using the voltage values obtained in EL analysis. The difference between EL-derived voltage and ESV- measured voltage is determined to be less than 2%. In this work, we combine the strengths of two complementary techniques of ESV (strength: quantitative) and EL (strength: spatial mapping) to obtain a quantitative spatial mapping of defects. This work is further extendable to detect poor performing modules in PV power plants.
We developed a back surface scattering technique for inverted III-V solar cells by texturing GaInP with HCl and PH3 in situ within a hydride vapor phase epitaxy (HVPE) reactor. Back surface texturing is a promising light management method for ultrathin solar cells to enhance photocurrent collection, however most methods demonstrated to date are expensive. This fully in situ method is a potentially low-cost and high-throughput technique, in which the rear GaInP contact layer is textured via vapor phase etching and redeposition of Ga-rich GaInP. We demonstrate single junction GaAs solar cells with a relative 5% boost in short circuit current density for in situ back surface textured devices without any loss in open circuit voltage or fill factor. This work supports the development of low-cost ultrathin III-V photovoltaics.
We conducted research on TiOx to create an electron selective contact structure that does not require a high-temperature heat-treatment process. Titanium metal was deposited by thermal evaporator and an additional oxidation process was conducted to form titanium oxide. The chemical composition and phase of the titanium dioxide layers were analyzed using X-ray diffraction and X-ray photoelectron spectroscopy. Passivation effects of each titanium oxide layer were measured by quasi-steady-state photoconductance. Electron selectivity of titanium oxide layers and band alignment of TiOx/Si was demonstrated by UV photoelectron spectroscopy (UPS) and UV-vis spectroscopy analyses. With this oxidized titanium oxide on the silicon surface, band offsets of the conduction and valence bands were analyzed to confirm the selectivity of the layers.
The intentional removal of one or more photovoltaic modules from a string, thus shortening the length of the string relative to others within the array, may occur for a variety of reasons. The result is a mismatch in string length which our previous work has shown to impact the operation of the array by 1) shifting the ideal maximum power point of the array, and 2) inducing reverse currents in the shortened strings at VOC, a condition experienced by arrays under normal operation and during some maintenance activities. This work takes the experimentally verified simulation results of our previous small-scale studies and expands the simulations to elucidate behaviors at commercial and utility scales.
Performance of a thin film Cu(In,Ga)(S,Se)2 [CIGSSe] solar cell has been well improved by CsF-treatment. In this study, we have investigated the effect of the CsF -treatment by photothermal atomic force microscopy [PT -AFM] through examination of non-radiative recombination properties. The used samples were CdS/CIGSSe materials with or without the CsF -treatment, and PT-AFM were performed under three excitation conditions: (1) standard condition, (2) high-photon-energy condition, where a light with high photon energy was used for shallow excitation near the surface by shortening a penetration depth of light, and (3) high-modulation-frequency condition, where an incident light was modulated at a high frequency to detect the fast thermal expansion caused by non-radiative recombination near the surface. Under conditions (2) and (3), the PT signals are considered to be dominated by the heat generation near the surface. Topographic and PT signal images were taken by PT-AFM on two samples under three excitation conditions, and we have compared the intensities and distributions of the PT signals among them to discuss the effects of the CsF -treatment. First, we found that the PT signal intensity on the CsF -treated sample was weaker than that on the as-grown sample under any excitation condition, indicating that the non-radiative recombination was suppressed by the CsF -treatment. Especially on the CsF -treated sample, the PT signals were weakened under conditions (2) and (3), compared with those under condition (1). Therefore, we can consider that the CsF -treatment effectively passivates the recombination centers near the surface. Moreover, the reduction of the PT signal was very apparent around GBs. From those results, we conclude that the non-radiative recombination centers distributed at the CdS/CIGSSe interface and along GB were effectively passivated by the CsF-treatment.
An improved epitaxial lift-off (ELO) technique is developed and monocrystalline CdTe/MgCdTe double-heterostructure (DH) thin films on conductive flexible superstrates are demonstrated. The post-ELO DH shows strong photoluminescence and uniform surface morphology comparable to that prior to the lift-off process. Solar cells fabricated with the post-ELO thin films exhibit an open circuit voltage of 0.79 V and an efficiency of 9.8%, showing that the ELO technique is practical for fabrication of flexible, light-weight monocrystalline CdTe solar cells for space and terrestrial applications.
The NREL Python Panel-Segmentation package is a toolkit that automates the process of extracting accurate and valuable metadata related to solar array installations, using publicly available Google Maps satellite imagery. Our previously published work includes automated azimuth estimation for individual solar installations in satellite images. Our continued research focuses on automated detection and classification of solar installation mounting configuration (tracking, fixed-tilt) and type (rooftop, ground, carport). Specifically, a Faster-RCNN Resnet-50 feature pyramid network (FPN) model was trained and validated on over 770 manually labeled satellite images. This model was used to perform object detection on satellite imagery, locating and classifying individual solar installations' mounting configuration and type. Preliminary model results showed a combined mean average precision score (mAP) score across classes of 49.87% using an Intersection over Union (IoU) threshold of 0.5. We intend to release the complete image data set with labels on the NREL DuraMAT DataHub, to encourage further research in this area. Additionally, a pipeline for automated metadata extraction, including detection of mounting configuration and type as well as azimuth estimation, will be released via the NREL Panel-Segmentation package for public use.
Cadmium selenide (CdSe) plays a vital role to achieving the high short-circuit current density (JSC) and passivating the defects in the absorber layer for CdTe photovoltaics necessary to reach high efficiency. Incorporation of CdSe into devices can be done either by fabricating a CdSe/CdTe bilayer or directly depositing the CdSexTe1-x (CST). While the bilayer results in better device performance, the intrinsic properties of the CST suggest it should be the better absorber material. Here, we fabricated and investigated the structural and opto-electronic properties of fixed composition CST films for varying Se concentrations and report device parameters. The films were produced by leveraging our multisource evaporation chamber, allowing a wide range of Se compositions to be investigated without modification to the system. For fixed compositions CST absorber layers, the minority carrier lifetime is improved with higher Se content though the grain sizes are slightly smaller for higher Se content. Note that all these samples (pure CdTe and CST) have undergone same CdCl2 treatment. The device efficiency for fixed composition CST absorber layer observed is as high as 12.2% while for pure CdTe device (no Se) is 7%. The short circuit current density is high (28 mAcm−2), but CST devices suffer from low open circuit voltage (Voc) and fill factor (FF). For comparison, CdSe/CdTe bilayer devices also fabricated using this system were able to reach efficiency up to 17.7% (Voc 839 mV, Jsc 29.0 m Acm−2, FF 72.6%), indicating the system produces good material. We will discuss the material properties of CST and correlate these values to the device performance.
Increasing population and industrialization haveled to an uptick in energy requirements. Many traditional energy sources are not anymore attractive due to climate change, instead, the interest has turned to power generation from renewable sources, such as wind energy, hydro-power, and solar energy. The wide availability of sunlight and simplicity in converting sunlight to electricity has led to the search for synthesized semiconductors that give high efficiency in this conversion. A family of such semiconductors attains the perovskite structure, the most established being Methyl Ammonium Lead Iodide. The shortcomings of this compound include lead poisoning, motivating the search for perovskite structures that have low electron band-gap and are stable. A family of such perovskite structures is compounds that attain an $\mathbf{A}_{2}\mathbf{XY}_{6}$ type structure. This paper demonstrates some methods that can be used to calculate the electron band-gap of such compounds. The metrics found from Support Vector Machine Regression and Random Forest Regression are compared and analyzed to propose a scalable model for predicting electron band-gap.
for a specific site and provides solutions to those challenges. The paper also presents the controller/power-hardware-in-the-loop evaluation platform built to suit the needs of the microgrid as well as the IEEE 2030.8 consistent test cases used to evaluate the proper operation of the microgrid controller. Finally, the paper presents the results from a subset of the experiments performed to evaluate the microgrid controller. a microgrid controller for a site because of the unique requirements presented by the controllable and uncontrollable elements in the system. There are also challenges to testing the performance of the microgrid controller for such unique microgrids. The research presented here discusses the challenges to evaluating a microgrid controller for a specific site and provides solutions to those challenges. The paper also presents the controller/power-hardware-in-the-loop evaluation platform built to suit the needs of the microgrid as well as the IEEE 2030.8 consistent test cases used to evaluate the proper operation of the microgrid controller. Finally, the paper presents the results from a subset of the experiments performed to evaluate the microgrid controller.
Photovoltaic (PV) modules work best in the sunniest environments. Unfortunately., often the sunniest places also have substantial amounts of airborne “dust” that deposits on the front surface of the modules and blocks the sunlight; reducing energy output. In fact, natural soiling has reduced the energy output of PV systems since the technology was first used, and viable mitigation strategies have remained elusive ever since. With the ever-increasing deployments around the world, especially in dusty environments, soiling is becoming a billion-dollar problem, worldwide. While substantial work has been done to examine and resolve some of the issues with PV soiling, often mitigation comes down to physically cleaning the modules. However, a more systematic evaluation of the different module properties correlations to soiling mitigation needs to be done. In many instances, the causal connections between module properties and soiling are simply not known. This lack of knowledge results in a substantial increase in time and effort to evaluate and qualify appropriate soiling mitigation protocols based on site specific issues and the intrinsic module properties that are typically not optimized for mitigating soiling in a given environment. Thus, module property protocols and/or standards are needed to more quickly help identify appropriate module and site-specific mitigation. Thus, in this paper, we will present a review of the different issues between module properties and their relationship to soiling mitigation, and then outline a roadmap of the issues that still need to be resolved with additional research and development. Issues from frameless modules to anti-fungal glass compositions will be discussed.
This work reports on the use of a front side SiON/TCO bilayer in double side poly-Si/SiOX-based passivated contacts solar cells. This approach presents the advantage of a low indium consumption either by reducing the indium-based transparent conductive oxide (TCO) thickness or by enhancing its substitution with a Zinc-based TCO, such as AZO (Aluminum-doped Zinc Oxide). An electrical study with a TCO thickness reduced to 20 nm on textured surfaces has shown excellent responses for SiON/AZO stacks, especially regarding the contact resistivity. The developed SiON/TCO bilayers were finally integrated in complete solar cells. Interestingly, the substitution of the standard 70 nm-thick ITO:H layer by a 20 nm-thick ITO:H film covered by SiON led to an efficiency gain of +0.5% abs. Regarding AZO, the replacement of the standard 70 nm-thick AZO layer by a 20 nm-thick AZO film covered by SiON conducted to a JSC gain of +0.8 mA/cm2. These gains in performances could be raised with further post-treatments still under investigation. However, the current results already confirm the possibility to optimize thin-poly-Si based passivated contacts solar cells towards In-free fabrication processes.
Sodium induced shunting continues to be a challenging issue in crystalline Si solar modules. Potential-Induced Degradation of the Shunting type (PID-s) has been linked to Na, but the source is unclear. In this paper we evaluate the ion migration kinetics in encapsulant material under operational conditions. Analysis of Na migration profiles reveal the diffusivity constant and activation energy of Na in EVA. Implementing these results in breakthrough time simulations indicates that Na migration from the glass through EVA is too slow to account for experimentally observed PID-s degradation indicating contamination during production as the most likely source.
