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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

In 2 O 3 -Ga 2 O 3 Alloys as Potential Buffer Layers in CdTe Thin-Film Solar Cells

The efficiency of state-of-the-art Cd Te solar cells remains limited by the relatively low open-circuit voltage (V OC ). Improving the front interface is key towards realizing a higher V OC after achieving the necessary bulk carrier density and lifetime. Recent efforts in identifying buffer layers beyond Cd S have focused on Mg x Zn 1-x O, which offers tunability of the band offsets, but often suffers from high interfacial defect densities. Ga 2 O 3 -based buffer layers demonstrate tremendous improvements in interfacial defect passivation in crystalline silicon and dye-sensitized solar cells, leading to record high V OC , yet remain largely unexplored in Cd Te-based devices. Here, we perform hybrid density-functional-theory calculations to investigate pure Ga 2 O 3 and InGaO 3 alloys as a window layer in Cd Te photovoltaics. We report calculated band offsets for several pairs of solid-solid interfaces comprising transparent conducting oxide (TCO) and Cd Te heterojunctions. The results support a large conduction band offset spike of 0.67 eV for the Cd Te/Ga 2 O 3 (100) interface, while the offset is reduced to 0.18 eV for the InGaO 3 alloy and matches closely with the preferred optimum value of 0.2 eV. Device-level modeling tests of Cd Te solar cells integrating our results indicate that the highest efficiency is achieved with InGaO 3 acting both as a buffer layer and TCO. Finally, our results suggest that alloys of In 2 O 3 and Ga 2 O 3 may be attractive alternatives to Mg x Zn 1-x O for tailoring optimal conduction-band offsets of the buffer and TCO layers in high-efficiency Cd Te thin-film solar cells.

14 SOLAR ENERGY↗
DOE OSTI2022

Significant improvement of conversion efficiency by passivation of low-angle grain boundaries in flexible low-cost single-crystal-like GaAs thin-film solar cells directly deposited on metal tape

We develop an easily implementable yet effective process to achieve high power conversion efficiency in newly-developed flexible single-crystal-like GaAs thin-film photovoltaic solar cells on cheap metal tape. We study various techniques on the fabricated devices to passivate the defect states associated with dangling bonds at low-angle grain boundaries (LA-GBs) existing in the single-crystal-like GaAs material. We report that sulfur by trioctylphosphine sulfide (TOP:S) solution treatment can partially but effectively passivate the defect states at the LA-GBs to increase all the solar cell performance parameters of open-circuit voltage VOC (33.2%), short-circuit current JSC (19.5%), and fill factor FF (25.8%), resulting in the improvement of the power conversion efficiency more than double. The material and device characterizations and numerical modeling are performed to propose a mechanism, also suggesting that the power conversion efficiency higher than 20%, which is an important target milestone in emerging solar cell technologies, can be achieved. This work presents the first effective passivation of the bulk defects in GaAs at the device level and can also offer a solution for other photonic and electronic devices using non-single-crystalline GaAs films for flexible electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2021

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 um 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.

14 SOLAR ENERGY↗
DOE OSTI2025

Bias‐Dependent Quantum Efficiency Reveals Recombination Pathways in Thin Film Solar Cells

Identifying where recombination predominantly occurs—whether at the front interface, back interface, or throughout the bulk—is crucial for optimizing CdSeTe solar cells and many other photovoltaic device architectures. Here, a simple and effective diagnostic is demonstrated: measuring external quantum efficiency (QE) under varying forward biases. The drift–diffusion simulations reveal that each recombination pathway leaves a distinct bias-induced signature in the normalized QE: a progressive drop at long wavelengths for back-limited devices, a short-wavelength decline for front-limited devices, and a relatively uniform decrease across all wavelengths for bulk-limited devices. These predictions are validated with experiments on As-doped and Cu-doped CdSeTe devices, with and without passivation layers or different front buffers. In each case, the observed bias-dependent QE spectral changes align with the simulated recombination map. Because this method uses standard QE instrumentation, it offers a broadly accessible and cost-efficient means to diagnose recombination pathways—extending beyond CdSeTe to other thin-film and emerging solar cell technologies. By pinpointing recombination bottlenecks, researchers and manufacturers can strategically refine doping profiles, passivation schemes, and interface designs to further improve device performance.

CdTe↗
DOE OSTI2022

Large-Area (Ag,Cu)(In,Ga)Se 2 Thin-Film Solar Cells with Increased Bandgap and Reduced Voltage Losses Realized with Bulk Defect Reduction and Front-Grading of the Absorber Bandgap

The 1.24 eV bandgap, 18.8% power conversion efficiency Ag-alloyed chalcopyrite (Ag,Cu)(In,Ga)Se 2 (ACIGS) solar cells are characterized to relate voltage and efficiency improvements to electro-optical (EO) characteristics. Shockley-Read-Hall recombination center defect density, identified and characterized through deep level transient spectroscopy and time-resolved photoluminescence (TRPL), is reduced through potassium and copper treatment optimization. Concomitantly, longer minority carrier lifetimes are achieved, which increases open-circuit voltage (V OC ). Near-conduction band defects associated in earlier studies with light-induced current instability are also mitigated. Analysis of charge-carrier dynamics after single- and two-photon excitation is used to separate recombination at the front interface and in the absorber bulk. From TRPL decay simulations, the authors estimate ranges of key solar cell material characteristics: bulk carrier lifetime T bulk = 110-210 ns, charge-carrier mobility u = 110-160 cm 2 V -1 s -1 , and front interface recombination velocity S front = 700-1050 cm s -1 . This lowest-reported S front for ACIGS absorbers originates from the notched conduction band grading, which also makes the impact of the back interface recombination negligible. It is suggested in the results that solar cell performance enhancements can be made most readily with two distinct strategies: improving device architecture and reducing semiconductor defect densities. Using these approaches, power conversion efficiency in large-area solar cells is improved by 1.1% absolute.

14 SOLAR ENERGY↗
DOE OSTI2020

Thin-Film Solar Cells with 19% Efficiency by Thermal Evaporation of CdSe and CdTe

CdTe-based solar cells exhibiting 19% power conversion efficiency were produced using widely available thermal evaporation deposition of the absorber layers on SnO2-coated glass with or without a transparent MgZnO buffer layer. Evaporating CdSe and CdTe sequentially by thermal evaporation and subsequent CdCl2 annealing establishes efffective CdSeTe band grading as well as dense, large-grain films. These results show that high-performance II-VI photovoltaics can be made by inexpensive, commercially available evaporation systems without the need to build customized equipment, enabling CdTe photovoltaics research and manufacturing to be more accessible to the broader photovoltaics community.

14 SOLAR ENERGY↗
DOE OSTI2020

A Density Functional Theory (DFT) Study on the Effect of Chlorine in the Magnesium Zinc Oxide (MZO) Front Interface for Cadmium Telluride (CdTe) Thin Film Solar Cells

Cadmium telluride photovoltaics has benefitted greatly from the cadmium chloride treatment which causes a remarkable improvement in cell efficiencies. This work investigates the effect of elemental chlorine at the interface between the magnesium zinc oxide front contact and the cadmium telluride absorber in such a device. Band alignment including band bending and interface states and its role in potential charge carrier transport is the key feature studied. It is seen that a small amount of chlorine is leads to a more desirable band alignment. Higher concentrations of chlorine result in unfavorable band characteristics.

14 SOLAR ENERGY↗
DOE OSTI2020

Influence of Process Parameters and Absorber Thickness on Efficiency of Polycrystalline CdSeTe/CdTe Thin Film Solar Cells

Graded absorbers devices with with CdSe 0.4 Te 0.6 (molar basis) and CdTe were fabricated. CdCl 2 treatment time, post-deposition CdCl 2 anneal time and thicknesses of CdSeTe and CdTe layers were varied. Photoluminescence and electrical measurements were performed on the fabricated devices. Results revealed that the individual thicknesses of CdSeTe and CdTe is critical to overall efficiency of the devices. Device fabricated on substrate with 0.5 μm CdSeTe, 3 μm CdTe with rest of process parameters kept unchanged, produced an efficiency of 20.14%. We report highest device efficiency among academia and research institutions.

Shimpi, Tushar↗
DOE OSTI2023

12.3% Efficient Lifted-Off and Reconstructed As-Doped CdTe Thin Film Solar Cell

CdTe:As devices grown by MOCVD in the superstrate configuration have been reconfigured to a substrate structure, by employing a cleaving and reconstruction technique. Indium and aluminium doped Zinc oxide (IZO and AZO) layers were used as the transparent front interface without a metal grid. A substrate efficiency of 12.3% was realized with AZO front contact, retaining 80% of the superstrate device efficiency and an impressive approximately 99% of the open-circuit voltage. The IZO-based substrate device on the other hand performed at approximately 69 % retained efficiency. The crucial role of front interfaces is highlighted by the two contacts used here. The results in this work show promise for the development of highly efficient substrate CdTe:As devices.

II-VI semiconductor materials↗
DOE OSTI2022

A New Low-Temperature Approach for Efficient and Low-Cost Group V Doping in CdTe Thin Film Solar Cells (Final Technical Report)

The goal of this project is to develop a highly efficient low-temperature ex-situ Group V diffusion doping approach in the CdCl 2 treated polycrystalline CdTe film using the group V chlorides (i.e., PCl 3 , AsCl 3 , SbCl 3 , and BiCl 3 ) as dopants source. For the first time, our innovative doping strategies through the low-temperature ex-situ Group V doping process in polycrystalline CdSeTe successfully demonstrated a promising power conversion efficiency of ~19% compared to the Cu-doped counterparts. Our newly developed low-temperature ex-situ group V doping allows dedicated control of group V diffusion amount, generates a desirable depth profile, and is completely compatible with commercial CdTe module manufacturing. We built the partnership with the largest CdTe solar module manufacturing in the US, e.g., First Solar, Inc to further develop these low-temperature diffusion doped CdSeTe solar cells. Our technology was granted US Patent No. US11257977B2. In this project, we systematically investigate and characterize the new ex-situ group V doping mechanisms, defect chemistry, microstructure, and device physics in CdTe solar cells by combining experimental and theoretical means to pave the way to achieve lower cost toward $\$$0.02/kWh and higher power conversion efficiency (PCE) toward 25% in polycrystalline CdTe solar technology. This project will benefit the public by providing more affordable solar electricity and provide an effective way to address climate change.

14 SOLAR ENERGY↗
DOE OSTI2024

Monolithic Bifacial Halide Perovskite-Cadmium Selenide Telluride (CST) Tandem Thin-Film Solar Cells (Final Technical Report)

In this project, we develop a bifacial tandem design for monolithic metal halide perovskite-cadmium selenide telluride (CST) tandem solar cells. The bifacial tandem design not only enables the monolithic integration of perovskite-CST tandems but also delivers high efficiency by harvesting the albedo light. It also allows better current match and thus enables higher power output of the tandem devices than conventional tandem cells. We worked on understanding the critical factors limiting bifacial CST cells and developing strategies to improve their bifaciality. We also developed efficient wide-bandgap perovskite subcells and interconnecting layers that enable the monolithic perovskite-CST tandem solar cells. We successfully demonstrated prototype monolithic bifacial perovskite-CST tandem solar cells with bifacial equivalent efficiencies of more than 25% under an albedo of 0.4. Additionally, we optimized the wide-bandgap perovskite to realize four-terminal perovskite-CST tandem devices with efficiencies of 25.5% under monofacial illumination. The team disseminated these results to stakeholders in the PV academic and industry and discussed the challenges and major technical barriers for their market entry. The technologies developed in the project are promising but are not yet ready for immediate commercialization. Future fundamental study and R&D work are needed to advance these promising technologies.

14 SOLAR ENERGY↗
DOE OSTI2020

Light-Induced Metastabilities in Highly Efficient MZO/CdSeTe/CdTe Thin Film Solar Cells

Metastability effects have been investigated in high efficiency MZO/CdSeTe/CdTe solar cells. Two different preconditioning procedures have been studied to recover the performance of the devices. These preconditioning procedures include light soaking, annealing and cooling down the device under different atmospheric conditions. J-V curves before any preconditioning showed a `S` shaped behavior and disappears after 15 minutes of light soaking under 1000 Wm-2. Both Atmospheric and Vacuum preconditioning showed similar recovery of the device performance. However, this recovery only lasts for 3 days when the devices were kept under vacuum in the dark.

14 SOLAR ENERGY↗