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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
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 OSTI2024

Investigation of properties of CdSeTe/Sb and Sb/CdSeTe stacks

The effect of Sb inclusion in CdSeTe thin films was examined. CdSeTe alloys were grown by co-evaporation, either on 5 nm thick electron-beam evaporated Sb layers (Sb/CdSeTe) or coated with Sb post-deposition (CdSeTe/Sb). Sb inclusion improved CdSeTe grain structure, a critical factor for device performance, with average grain size increasing from 0.6 to 1.5 μm. All samples crystallized on annealing to the cubic structure, with a change in film orientation from (111) to random or (220). The presence of Sb in CdSeTe/Sb or Sb/CdSeTe samples, however, had a limited effect on the carrier concentration compared to Sb-free samples. Time resolved photoluminescence showed Sb inclusion increased decay lifetimes, with τ1 = 0.8 ns for and τ2 = 6.9 ns for the Sb/CdSeTe sample.

14 SOLAR ENERGY↗
DOE OSTI2022

Alternative Rear Contacts for Ultrathin CdSexTe1-x Solar Cells

The back contact is a key component for further developments of CdSeTe solar cells. In this contribution, we investigate several materials and designs that could help the development of highly-reflective, ohmic back contacts for conventional (3.45 um) and thin (800 nm) CdSeTe solar cells. We also explore numerically, nanostructured metal back contacts for light-trapping in ultrathin (200 nm) CdTe solar cells.

back contact↗
DOE OSTI2022

Electrical Potential Investigation of Reversible Metastability and Irreversible Degradation of CdTe Solar Cells

In this paper, we report on the stability of CdTe devices with a structure of TCO/MZO/CdSeTe/CdTe/back-contact. The device showed reversible transitions between the light-soak state (LSS) with the best device efficiency and the dark-soak state (DSS) with an inferior efficiency. However, it showed an irreversible degradation state (DgS) driven by long-hour light soaking at an elevated temperature. We have investigated transitions between these three states from the perspective of the electric field by nm-resolution potential imaging across the devices using Kelvin probe force microscopy (KPFM). The results exhibit different anomalous electric field profiles. At the LSS, the electric field exhibits a main peak inside the CdSeTe layer instead of the MZO/CdSeTe heterointerface, illustrating a buried homojunction (BHJ) of the device. At the DSS, a large electric field peak at the MZO/CdSeTe interface was measured, which probably resulted in the inferior fill factor at the DSS. At the DgS, the electric field peak at the MZO/CdSeTe interface increased further and a third electric field was measured at the back contact of the device. Device modeling using COMSOL software, in alignment with both the electric field and device current-voltage curves, elucidates that a low n-doped CdSeTe in the region near the MZO/CdSeTe interface caused the BHJ in the LSS and a loss of MZO doping and/or increase of the conduction band offset spike due to long-term stress caused the increased electric field near the MZO/CdSeTe interface at the DgS.

CdTe solar cell↗
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 OSTI2021

Colossal Grains for a New Structural Paradigm in Thin-Film Photovoltaics

II-VI based thin-film photovoltaics (PV) have experienced substantial commercial success on a foundation of fast large-scale deposition and processing. This type of processing, regardless of material type, nearly always results in small grain (nm to several um diameters) polycrystalline material that, if not for effective grain boundary (GB) and defect passivation, would severely limit PV performance. In the case of CdTe based PV technology, the nearly ubiquitous CdCl2 treatment is used, in part, to passivate the polycrystalline p-type absorber even though Cl is known to (and used to) compensate p-type II-VI materials. Given that VOC is still the most limited performance metric and it can only be improved through higher p-type absorber concentrations, CdCl2 treatments may be holding the technology back. In this talk, we will discuss results related to a process by which small grain CdSeTe material is rapidly transformed into very large grain material (100-1000um diameters, Fig. 1a). This explosive recrystallization, which we have termed colossal grain growth (CGG), occurs by a Cl free process with thin films (<10um) deposited using the same fast deposition techniques the industry has relied on for more than a decade. Our modeling suggests that GB recombination becomes significantly less important at these grain sizes and thus may eliminate the need for a CdCl2 treatment. This result is complimented by previous results of epitaxial growth at high rates on single crystal seed layers. Thin CdSeTe CGG seed layers can be used for rapid epitaxy of CdTe to complete a standard device stack (Fig.1b). This is an optimistic view and the current limitations and challenges related to this process and these materials will also be discussed, including alloy and substrate roughness sensitivity, intragrain material quality, and complex defect chemistry. These challenges are not insignificant but overcoming them may enable a new structural paradigm for thin-film PV.

CdTe↗
DOE OSTI2022

Metastability and Degradation of CdTe Solar Cells Investigated by nm-Scale Electrical Potential Imaging

We report on investigations of reversible metastability and irreversible degradation of MZO/CdSeTe/CdTe devices from the perspective of electric field across the device using Kelvin probe force microscopy (KPFM). The device showed reversible transitions between the light-soak state (LSS) with the best device efficiency and the dark-soak state (DSS) with an inferior efficiency. However, it showed an irreversible degradation state (DgS) driven by long-hour light soaking at an elevated temperature. The nm-scale KPFM electric field imaging on cross-sections of the devices revealed different anomalous electric field profiles. The electric field at the LSS exhibits a main peak inside the CdSeTe layer but not at the MZO/CdSeTe heterointerface, demonstrating that working junction of the device is a buried homojunction (BHJ). At the DSS, a second electric field peak was observed at the MZO/CdSeTe interface with a similar strength to the main BHJ, which probably caused the decrease in fill factor at the DSS. At the DgS, the electric field peak at the MZO/CdSeTe interface increased significantly and a third electric field was measured at the back contact of the device. Device modeling using COMSOL and in alignment with both the electric field and device current-voltage curves suggest that a slightly low n-doped CdSeTe in the region near the MZO/CdSeTe interface caused the BHJ, and that either a loss of MZO doping or increase of the conduction band offset spike due to long-term stress caused the fill factor-dominated degradation and the increased electric field near the MZO/CdSeTe interface at the DgS. The former, MZO doping decrease, is more plausible by referring the related literature.

BHJ↗
DOE OSTI2021

Exceeding 200 ns Lifetimes in Polycrystalline CdTe Solar Cells

CdTe photovoltaics has achieved one of the lowest levelized costs of electricity among all energy sources. However, for decades, carrier lifetimes have been inferior to those of other prevalent solar cell materials. This quality has inhibited common methods to improve solar cell efficiency such as back-surface fields, electron reflectors, or bifacial solar cells. In this work, a significant increase in carrier lifetime to values exceeding 200 ns in fully functional CdTe solar cells is demonstrated. The increased lifetime is achieved by large CdSeTe grains at the absorber/emitter interface, intragrain passivation in the absorber layer, and chemical passivation by forming nanoscale oxidized tellurium species at the transparent conducting oxide interface. The carrier lifetime is correlated to the open-circuit voltage and enables paths for back-surface manipulation and novel cell architectures to further improve CdTe photovoltaic performance.

14 SOLAR ENERGY↗
DOE OSTI2020

Chapter 8: Time-Resolved Photoluminescence Characterization of Polycrystalline Thin-Film Solar Cells

This chapter describes some photoluminescence (PL) approaches to characterize thin film solar cells with emphasis on time-resolved methods. Spectral PL analysis is complementary and was recently reviewed. Since TRPL is not very commonly used in PV characterization, we consider interface and bulk recombination in the test structures and in devices, briefly describe charge-carrier transport and recombination microscopy, and finally, summarize and compare some CdTe, CdSeTe, CIGS, kesterite, and perovskite EO characteristics.

bulk recombination↗
DOE OSTI2020

Se diffusion in CdTe thin films for photovoltaics

Manipulating CdSeTe bandgrading to enhance photocurrent and carrier lifetime is an essential part of high-performance CdTe photovoltaics (PVs). Here, we examine Se diffusion kinetics in single-crystal and polycrystalline CdTe during deposition, thermal annealing, and CdCl 2 treatments. Se distributions are determined by dynamic secondary-ion-mass spectroscopy and Auger electron spectroscopy depth profiling and coupled with electron backscatter diffraction images of the crystalline structure. Effective bulk and grain boundary diffusion coefficients are determined by analytical models and discussed in the context of processing and film morphology. Se is found to diffuse in CdTe at much higher rates during CdCl 2 treatments than with thermal processing alone. GB diffusion also occurs at a significantly faster rate than bulk diffusion. As a result of these two effects, the near interface bulk and GB Se diffusion during CdCl 2 treatments dominates the bandgrading profiles in CdTe PVs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Increased Voltage in CdSe Solar Cells by Mitigation of Charge Carrier Trapping Due to Se Vacancies

Cadmium selenide (CdSe), with a 1.7 eV bandgap, is a promising high-bandgap semiconductor for tandem solar cells, yet device efficiencies are hindered by rapid minority carrier recombination. Here, in this study, polycrystalline CdSe solar cells are investigated using radiative emission spectroscopy, time-resolved photoluminescence, and density functional theory, revealing fast (sub-nanosecond) minority carrier trapping by selenium vacancy-related defect states with densities of (5–50) × 10 17 cm −3 , limiting carrier mobility and increasing recombination. By reducing absorber thickness to ≈0.5 µm, trapping effects are mitigated, achieving a record open-circuit voltage of 917 mV, a 165 mV improvement over prior reports. These findings clarify the role of Se vacancies in limiting CdSe solar cell performance and provide insights applicable to CdSe and CdSeTe thin-film photovoltaics. This work advances understanding of defect-mediated losses in II–VI semiconductors and suggests pathways for improving solar cell performance through defect control.

14 SOLAR ENERGY↗
DOE OSTI2021

Identification of Recombination Losses in CdSe/CdTe Solar Cells from Spectroscopic and Microscopic Time-Resolved Photoluminescence

Due to the lowest-cost and best reliability, CdTe solar cells are the leading thin-film photovoltaic technology. Increasing open-circuit voltage by reducing recombination represents the most promising path toward further improvements. Analysis is needed to identify limitations that cause efficiency losses. To achieve this goal for Cu-doped CdSe/CdTe solar cells, time-resolved spectroscopy and microscopy are developed and applied. Recombination lifetimes and radiative efficiency identify that defect-mediated recombination is the dominant voltage loss mechanism. When carrier lifetimes are averaged over many crystalline grains, they increase from 180 to 430 ns when Al2O3 is applied to the back contact. The quasi-Fermi-level splitting correspondingly increases from 880–905 to 906–931 mV, indicating a pathway to overcome the long-standing 900 mV voltage limitation. However, the dominant recombination losses are attributed to the absorber bulk. From microscopic carrier lifetime measurements, it is identified that space charge fields due to charged grain boundaries (GBs) lead to recombination in the CdTe absorber bulk. At high injection, GB space charge fields are screened, but that occurs above 1 Sun excitation conditions. Alloying with selenium in the near-interface CdSeTe absorber region reduces GB losses and is identified as one of the factors leading to high radiative and power conversion efficiency.

14 SOLAR ENERGY↗
DOE OSTI2023

Charge Carrier Lifetime Determination in Graded Absorber Solar Cells Using Time‐Resolved Photoluminescence Simulations and Measurements

Thin‐film photovoltaic device efficiencies are limited by carrier recombination, thus understanding recombination mechanisms is critical for performance improvements. Bulk minority carrier lifetime ( τ bulk ) is a critical parameter for solar cells but is difficult to determine in P–N junction devices, especially for high doping. As doping ≥10 16 cm −3 is required for efficient drift‐charge‐carrier‐collection devices, a method for τ bulk determination in doped P–N junction devices is necessary. This work utilizes time‐resolved photoluminescence (TRPL) simulations to quantify bulk and interface recombination properties in highly doped, graded absorber CdSeTe structures. The two methods developed here for τ bulk determination include utilization of an instantaneous lifetime representation to guide TRPL fitting and direct comparison between measured and simulated decays. Simulations verified that both methods are valid for state‐of‐the‐art device architectures which include graded bandgap absorbers, graded doping, and graded lifetimes. Shifts in the dominant recombination mechanism are identified for sufficiently long τ bulk , where front and back interface quality plays a more prominent role. Evaluation of surface recombination velocities and conduction band offset illustrate electro‐optical advantages of a positive conduction band offset and highlight the necessity of improved interfaces as bulk quality in photovoltaic devices improves.

14 SOLAR ENERGY↗
DOE OSTI2025

First principles investigation of dopants and defect complexes in CdSe$_x$Te$_{1-x}$

Se alloying is a common approach to improve the performance of CdTe solar cells by tuning the bandgap, defect levels, and carrier density. A fundamental understanding of these improvements, specifically the effect of Se alloying on the behavior of defects and dopants in CdTe, remains unclear. Here, in this work, we present a density functional theory (DFT) study of point defect energetics in CdTe and CdSe x Te 1-x with x = 0.25, leading to a comparison of how native defects, dopants (As and Cu), impurities (Cl and O), and related defect complexes behave in CdTe vs CdSe x Te 1-x . Our calculations, performed by combining semi-local and nonlocal hybrid functionals, show a general lowering of the formation energies of native defects as well as substitutional defects formed by As and Cl upon Se addition. For successful p-type doping with As, destabilizing Cl-based defects in the CdSeTe lattice would be essential. We find evidence for some low-energy defect complexes of As, Cl, and O in CdSe 0.25 Te 0.75 . The computed defect formation energies further enable estimates of temperature-dependent defect concentrations and self-consistent Fermi levels. A comparison of defect energetics with the energies of impurity phases reveals that As, Cu, Cl, and O overwhelmingly prefer being segregated to unwanted As 2 O 5 , AsCl 3 , Cd 2 AsCl 2 , and CuO x phases rather than remain at defect sites, but such segregation is less likely to happen in CdSe 0.25 Te 0.75 than in CdTe. Overall, our work presents a list of likely defects and complexes in CdTe and Se-incorporated CdTe, paving the way to explain and mitigate limited dopant activation in experimental observations.

CdTe↗
DOE OSTI2026

Eutectic Processing of Semiconductor Colloidal Nanocrystals for Energy Applications

Colloidal semiconductor nanocrystals (NCs) offer a costeffective platform for light-energy conversion in X-ray scintillators, photovoltaics, lasers, and display technologies. Yet, device-relevant NCs often require complex heterostructured compositions, where lattice imperfections compromise the efficiency and stability of photoconversion processes. Here, we show that a simple synthetic detour through a eutectic state of II−VI semiconductor NCs (e.g., CdSe, ZnSe) with halide salts (e.g., CdCl 2 , ZnCl 2 ) overcomes this limitation by melting and reconstructing NC lattices into defect-free alloyed and core/shell architectures. Applied to ternary CdSeTe NCs, this process produces downconverters with record brightness and minimal line widths, delivering a 3-fold increase in film-side external quantum efficiency of commercial CdTe photovoltaic modules (First Solar Inc.). Meanwhile, eutectic processing of CdSe-based core/shell emitters yields an 8-fold enhancement in their photoluminescence stability under backlight operation, addressing the reliability bottleneck for display technologies. Together, these findings establish eutectic NC processing as a scalable route to efficient, durable photoconversion materials for energy applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
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 OSTI2021

Colossal grain growth in Cd(Se,Te) thin films and their subsequent use in CdTe epitaxy by close-spaced sublimation

Many technologies deposit thin films on inexpensive substrates, resulting in small grains due to classic nucleation and grain growth theory. For example, state-of-the-art solar cells are made by depositing CdSeTe and CdTe layers on inexpensive glass coated with nanocrystalline transparent conducting oxides (TCOs), like SnO 2 . Characteristically, the grain size of these films is on the order of the film thickness, i.e. a few microns. CdTe small-grain films have poor electro-optical properties and require CdCl 2 passivation which fails to fully passivate grain boundaries, causes carrier compensation, and prevents implementing other II–VI alloys and materials to improve performance. Here, we present a method to increase grain size to 1 mm in CdSe x Te 1-x thin films deposited on glass/TCO substrates without CdCl 2 treatment. The colossal grain growth is driven by mechanisms distinct from classic nucleation, grain growth, and Ostwald ripening and only occurs at low selenium content (x ~ 0.1). We also demonstrate how these films can serve as templates for subsequent large-grain epitaxy of other compositions like CdTe, again without exposure to CdCl 2 . The results open new paths for thin film solar cell technology, and thin film devices in general.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Synthesis of CdSeCdSexTe1-x/CdTe for Graded Solar Cells

Here, we have developed CdSe x Te 1-x /CdTe bilayer deposition for graded solar cell technology. Smoothly graded Se profiles with > 16% efficiency with Jsc=28 m A/cm 2 and 20-ns bulk lifetime has been observed observed. The CdSeTe reveals longer carrier lifetimes than CdTe within the same device.

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