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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Solar@Scale: A Local Government Guidebook for Improving Large-Scale Solar Development Outcomes (Third Edition)

The purpose of the Solar@Scale guidebook is to help local government practitioners—including planners, economic development professionals, local government managers, and elected and appointed officials—make decisions that improve large-scale solar development outcomes. Given the variation between state and regional contexts and priorities in the United States, the guidebook avoids a “one-size-fits-all” approach. Rather, users will need to apply the guidebook’s key points to the social, political, environmental, and economic situations in their community or the communities they serve. This guidebook contains eight related, but semi-independent, modules and each concludes with key takeaways and a list of references and links to supplemental resources.

14 SOLAR ENERGY↗
DOE OSTI2016

Toward a High-Efficient Utilization of Solar Radiation by Quad-Band Solar Spectral Splitting

The promising quad-band solar spectral splitter incorporates the properties of the optical filter and the spectrally selective solar thermal absorber can direct PV band to PV modules and absorb thermal band energy for thermal process with low thermal losses. Finally, it provides a new strategy for spectral splitting and offers potential ways for hybrid PVT system design.

14 SOLAR ENERGY↗
DOE OSTI2017

Understanding Film Formation Morphology and Orientation in High Member 2D Ruddlesden-Popper Perovskites for High-Efficiency Solar Cells

We report that 2D Ruddlesden–Popper (RP) perovskites have recently emerged as promising candidates for hybrid perovskite photovoltaic cells, realizing power-conversion efficiencies (PCEs) of over 10% with technologically relevant stability. To achieve solar cell performance comparable to the state-of-the-art 3D perovskite cells, it is highly desirable to increase the conductivity and lower the optical bandgap for enhanced near-IR region absorption by increasing the perovskite slab thickness. Here, the use of the 2D higher member (n = 5) RP perovskite (n-butyl-NH 3 ) 2 (MeNH 3 ) 4 Pb 5 I 16 in depositing highly oriented thin films from dimethylformamide/dimethylsulfoxide mixtures using the hot-casting method is reported. In addition, they exhibit superior environmental stability over thin films of their 3D counterpart. These films are assembled into high-efficiency solar cells with an open-circuit voltage of ≈1 V and PCE of up to 10%. This is achieved by fine-tuning the solvent ratio, crystal growth orientation, and grain size in the thin films. Finally, the enhanced performance of the optimized devices is ascribed to the growth of micrometer-sized grains as opposed to more typically obtained nanometer grain size and highly crystalline, densely packed microstructures with the majority of the inorganic slabs preferentially aligned out of plane to the substrate, as confirmed by X-ray diffraction and grazing-incidence wide-angle X-ray scattering mapping.

14 SOLAR ENERGY↗
DOE OSTI2020

Too Much of a Good Thing? Global Trends in the Curtailment of Solar PV

Solar photovoltaic (PV) systems generate electricity with no marginal costs or emissions. As a result, PV output is almost always prioritized over other fuel sources and delivered to the electric grid. However, PV curtailment is increasing as PV composes greater shares of grid capacity. In this paper, we present a novel synthesis of curtailment in four key countries: Chile, China, Germany, and the United States. We find that about 6.5 million MWh of PV output was curtailed in these countries in 2018. We find that: Policy and grid planning practices influence where, when, and how much PV is curtailed; Some PV curtailment is attributable to limited transmission capacity connecting remote solar resources to load centers; PV curtailment peaks in the spring and fall, when PV output is relatively high but electricity demand is relatively low. We discuss available measures to reduce PV curtailment as well as increasing PV curtailment in the contexts of evolving grids and energy technologies.

41 EE - Solar Energy Technologies Office (EE-4S)↗
DOE OSTI2022

Use of Physics to Improve Solar Forecast: Part II, Machine Learning and Model Interpretability

Machine learning (ML) models have been applied to forecast solar energy; however, they often lack clarity of interpretability and underlying physics. This work addresses such challenges by developing a hierarchy of ML models that gradually introduce predictors to improve the forecast accuracy based on a physics-based framework. Three ML models (ARIMA, LSTM, and XGBoost) are examined and compared with four physics-informed persistence models reported in Part I and the simple persistence model to assess the improvement of different models. The 7-year measurements at the U.S. Department of Energy's Atmospheric Radiation Measurement's Southern Great Plains Central Facility site are used for forecasts and evaluations. The results reveal that the step-by-step introduction of predictors leads to different improvements for models at different hierarchical levels. Comparison of the ML models with persistence models shows that LSTM and XGBoost outperform all the persistence models, with LSTM having the overall best performance; however, ARIMA underperforms the four physics-informed persistence models. This study demonstrates the importance and utility of incorporating physics into ML models in improving forecast accuracy by introducing a hierarchy of physics-based predictors, distinguishing predictor contributions, and enhancing the ML interpretability. The combined use of Global Horizontal Irradiance (GHI) and Direct Normal Irradiance (DNI) significantly improves the forecast accuracy compared to using individual irradiances alone because the pair contains more information on cloud-radiation interactions.

interpretability↗
DOE OSTI2022

Carbazole-Based Hole Transport Polymer for Methylammonium-Free Tin-Lead Perovskite Solar Cells with Enhanced Efficiency and Stability

As the most commonly used hole transport material (HTM) in tin-lead (Sn-Pb) perovskite solar cells (PSCs), poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) limits the power conversion efficiency (PCE) and stability of the PSCs due to its acidic characteristics. Herein, an easily synthesized polymer HTM poly[(phenyl)imino[9-(2-ethylhexyl)carbazole]-2,7-diyl] (CzAn) with a shallow highest occupied molecular orbital (HOMO) level of -4.95 eV is used in a p-i-n structure, methylammonium-free, Sn-Pb PSC to replace PEDOT:PSS. Upon optimization using doping and surface engineering, high quality Sn-Pb PSCs could be successfully fabricated, boosting the PCE to 22.6% (stabilized PCE of 21.3%) compared with 21.2% for PEDOT:PSS. The perovskite films prepared on the modified CzAn HTM possess improved crystallinity, reduced trap-state density, and larger carrier mobility resulting in PSCs with greatly improved stability.

carrier mobility↗
DOE OSTI2024

Barrier reinforcement for enhanced perovskite solar cell stability under reverse bias

Stability of perovskite solar cells (PSCs) under light, heat, humidity and their combinations have been notably improved recently. However, PSCs have poor reverse-bias stability that limits their real-world application. Here we report a systematic study on the degradation mechanisms of p-i-n structure PSCs under reverse bias. The oxidation of iodide by injected holes at the cathode side initialize the reverse-bias-induced degradation, then the generated neutral iodine oxidizes metal electrode such as copper, followed by drift of Cu+ into perovskites and its reduction by injected electrons, resulting in localized metallic filaments and thus device breakdown. A reinforced barrier with combined lithium fluoride, tin oxide and indium tin oxide at the cathode side reduces device dark current and avoids the corrosion of Cu0. It dramatically increases breakdown voltage to above -20 V and improved the T90 lifetime of PSCs to ~1,000 h under -1.6 V. The modified minimodule also maintained over 90% of its initial performance after 720 h of shadow tests.

degradation mechanisms↗
DOE OSTI2016

In situ investigation of the formation and metastability of formamidinium lead tri-iodide perovskite solar cells

Organic–inorganic perovskites have emerged as an important class of next generation solar cells due to their remarkably low cost, band gap, and sub-900 nm absorption onset. Here, we show a series of in situ observations inside electron microscopes and X-ray diffractometers under device-relevant synthesis conditions focused on revealing the crystallization process of the formamidinium lead-triiodide perovskite at the optimum temperature of 175 °C. Direct in situ observations of the structure and chemistry over relevant spatial, temporal, and temperature scales enabled identification of key perovskite formation and degradation mechanisms related to grain evolution and interface chemistry. The lead composition was observed to fluctuate at grain boundaries, indicating a mobile lead-containing species, a process found to be partially reversible at a key temperature of 175 °C. Using low energy electron microscopy and valence electron energy loss spectroscopy, lead is found to be bonded in the grain interior with iodine in a tetrahedral configuration. At the grain boundaries, the binding energy associated with lead is consequently shifted by nearly 2 eV and a doublet peak is resolved due presumably to a greater degree of hybridization and the potential for several different bonding configurations. At the grain boundaries there is adsorption of hydrogen and OH¯ ions as a result of residual water vapor trapped as a non-crystalline material during formation. Lastly, insights into the relevant formation and decomposition reactions of formamidinium lead iodide at low to high temperatures, observed metastabilities, and relationship with the photovoltaic performance were obtained and used to optimize device processing resulting in conversion efficiencies of up to 17.09% within the stability period of the devices.

14 SOLAR ENERGY↗
DOE OSTI2014

14%-efficient flexible CdTe solar cells on ultra-thin glass substrates

Flexible glass enables high-temperature, roll-to-roll processing of superstrate devices with higher photocurrents than flexible polymer foils because of its higher optical transmission. Using flexible glass in our high-temperature CdTe process, we achieved a certified record conversion efficiency of 14.05% for a flexible CdTe solar cell. Little has been reported on the flexibility of CdTe devices, so we investigated the effects of three different static bending conditions on device performance. We observed a consistent trend of increased short-circuit current and fill factor, whereas the open-circuit voltage consistently dropped. The quantum efficiency under the same static bend condition showed no change in the response. After storage in a flexed state for 24 h, there was very little change in device efficiency relative to its unflexed state. This indicates that flexible glass is a suitable replacement for rigid glass substrates, and that CdTe solar cells can tolerate bending without a decrease in device performance.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Band tail recombination in polymer:fullerene organic solar cells

Recombination through band tail localized states is studied analytically and by measurement of the forward-bias dark current as a function of temperature in three different organic bulk-heterojunction solar cells. The Shockley-Read-Hall mechanism is analyzed for the specific case of recombination between mobile carriers and an exponential distribution of localized band tail states. The analysis gives a simple relation between the dark current ideality factor and the band tail slope. Assumptions of the model are verified by numerical drift-diffusion modeling. Diode current-voltage measurements give good agreement with the analytical model, confirming that the band tail recombination mechanism applies to at least some organic solar cells. Deep traps provide a secondary recombination channel in some devices.

14 SOLAR ENERGY↗
DOE OSTI2016

Electron and hole drift mobility measurements on methylammonium lead iodide perovskite solar cells

We report nanosecond domain time-of-flight measurements of electron and hole photocarriers in methylammonium lead iodide perovskite solar cells. The mobilities ranged from 0.06 to 1.4 cm{sup 2}/Vs at room temperature, but there is little systematic difference between the two carriers. We also find that the drift mobilities are dispersive (time-dependent). The dispersion parameters are in the range of 0.4–0.7, and they imply that terahertz domain mobilities will be much larger than nanosecond domain mobilities. The temperature-dependences of the dispersion parameters are consistent with confinement of electron and hole transport to fractal-like spatial networks within nanoseconds of their photogeneration.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Emitter/absorber interface of CdTe solar cells

The performance of CdTe solar cells can be very sensitive to the emitter/absorber interface, especially for high-efficiency cells with high bulk lifetime. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. Numerical simulations show that the emitter/absorber band alignment, the emitter doping and thickness, and the defect properties of the interface (i.e., defect density, defect type, and defect energy) can all play significant roles in the interface recombination. In particular, a type I heterojunction with small conduction-band offset (0.1 eV ≤ ΔE{sub C} ≤ 0.3 eV) can help maintain good cell efficiency in spite of high interface defect density, much like with Cu(In,Ga)Se{sub 2} (CIGS) cells. The basic principle is that positive ΔE{sub C}, often referred to as a “spike,” creates an absorber inversion and hence a large hole barrier adjacent to the interface. As a result, the electron-hole recombination is suppressed due to an insufficient hole supply at the interface. A large spike (ΔE{sub C} ≥ 0.4 eV), however, can impede electron transport and lead to a reduction of photocurrent and fill-factor. In contrast to the spike, a “cliff” (ΔE{sub C} < 0 eV) allows high hole concentration in the vicinity of the interface, which will assist interface recombination and result in a reduced open-circuit voltage. Another way to mitigate performance losses due to interface defects is to use a thin and highly doped emitter, which can invert the absorber and form a large hole barrier at the interface. CdS is the most common emitter material used in CdTe solar cells, but the CdS/CdTe interface is in the cliff category and is not favorable from the band-offset perspective. The ΔE{sub C} of other n-type emitter choices, such as (Mg,Zn)O, Cd(S,O), or (Cd,Mg)Te, can be tuned by varying the elemental ratio for an optimal positive value of ΔE{sub C}. These materials are predicted to yield higher voltages and would therefore be better candidates for the CdTe-cell emitter.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance

Amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers—inserted between substrate and (front or rear) contacts—since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap silicon oxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. As a consequence, device implementation of such films as window layers—without degraded carrier collection—demands electron collection at the front and hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Beneficial effect of post-deposition treatment in high-efficiency Cu(In,Ga)Se{sub 2} solar cells through reduced potential fluctuations

World-record power conversion efficiencies for Cu(In,Ga)Se{sub 2} (CIGS) solar cells have been achieved via a post-deposition treatment with alkaline metals, which increases the open-circuit voltage and fill factor. We explore the role of the potassium fluoride (KF) post-deposition treatment in CIGS by employing energy- and time-resolved photoluminescence spectroscopy and electrical characterization combined with numerical modeling. The bulk carrier lifetime is found to increase with post-deposition treatment from 255 ns to 388 ns, which is the longest charge carrier lifetime reported for CIGS, and within ∼40% of the radiative limit. We find evidence that the post-deposition treatment causes a decrease in the electronic potential fluctuations. These potential fluctuations have previously been shown to reduce the open-circuit voltage and the device efficiency in CIGS. Additionally, numerical simulations based on the measured carrier lifetimes and mobilities show a diffusion length of ∼10 μm, which is ∼4 times larger than the film thickness. Thus, carrier collection in the bulk is not a limiting factor for device efficiency. By considering differences in doping, bandgap, and potential fluctuations, we present a possible explanation for the voltage difference between KF-treated and untreated samples.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Power Conversion Efficiencies of Perovskite and Dye-Sensitized Solar Cells under Various Solar Radiation Intensities

We present the results of a comparative study on behaviour of the photovoltaic parameters in perovskite (PSC), dye-sensitized (DSC) and crystalline silicon (c-Si) solar cells under various intensities of solar radiation (10–1000 W/m{sup 2}). It was found that unlike c-Si, the power conversion efficiencies of PSC and DSC under low radiation intensities are comparable with the corresponding values observed under standard light intensity of 1000 W/m{sup 2} (AM1.5G). We have shown that high performance of the PSC and DSC at low and diffuse lighting conditions can be explained by the properties of the nanostructured TiO{sub 2}-based photoelectrodes depending on the structure, morphology and the thickness of the TiO{sub 2} layers.

14 SOLAR ENERGY↗