Search NASA⌕ Search

Engineering topics

Pandey, Ramesh

Publications and source records attributed to Pandey, Ramesh.

11 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Electro-Optical Characterization of Arsenic-Doped CdSeTe and CdTe Solar Cell Absorbers Doped in-situ During Close Space Sublimation

Most contemporary device models predict that an acceptor concentration of at least 10^16 cm^-3 is required to reach an open circuit voltage of 1 V in polycrystalline CdTe-based solar cells. While copper has traditionally been used as the de facto p-type dopant in polycrystalline cadmium telluride (CdTe) and cadmium selenide telluride (CdSeTe), reaching high acceptor concentrations has proved to be challenging in such devices due to significant dopant compensation. The acceptor concentration in copper-doped CdTe and CdSeTe typically ranges from 10^13 to 10^15 cm^-3 and routinely exhibit low external radiative efficiencies below 0.01%, limiting their implied voltage (i.e., quasi-Fermi level splitting) to approximately 900 mV. As an alternative to copper, this work explores the use of arsenic as a p-type dopant for CdTe and CdSeTe. Using a novel technique in which a thin layer of arsenic-containing material is deposited and used as a reservoir for arsenic to diffuse into a front layer of previously undoped material, this contribution demonstrates that high external radiative efficiencies are achievable, a direct result of combined high acceptor concentrations and long minority-carrier lifetimes in the absorber. This leads to improved implied voltages, and indicates that As-doping represents a promising pathway towards improving the external voltage of CdSeTe/CdTe solar cells.

cadmium telluride↗
DOE OSTI2022

Te/CdTe and Al/CdTe Interfacial Energy Band Alignment by Atomistic Modeling

A synergistic approach that incorporates first-principles atomistic modeling with numerical device simulations is used to systematically evaluate the role of heterointerfaces within metal-chalcogenide-based photovoltaic technologies. Two interfaces involving either a tellurium back contact or aluminum back electrode combined with a cadmium telluride absorber layer within cadmium-telluride-based thin-film solar cells are investigated on an atomic scale to determine the mechanisms contributing to variations in device performance. Electronic structures and predicted charge transport behavior with respect to cadmium and tellurium termination of the absorber layer are studied along the polar oriented CdTe{111} facets. The computational methodology reveals a noticeable contrast between the Schottky barrier forming Al/CdTe interface versus the Type I Te/CdTe heterojunction. Greater band bending features are exhibited by the cadmium termination as opposed to the tellurium termination for each interface case. Subsequent device modeling suggests that 3.6% higher photovoltaic conversion efficiency is achievable for the cadmium termination relative to the tellurium termination of the Te/CdTe interface. Based strictly on an idealistic representation, both interface models show the importance of atomic-scale interfacial properties for cadmium telluride solar cell device performance with their bulk properties being validated in comparison to published experimental data. Here, the synergistic approach offers a suitable method to analyze solar cell interfaces through a predictive computational framework for the engineering and optimization of metal-chalcogenide-based thin-film photovoltaic technologies.

Green’s function↗
DOE OSTI2021

Understanding the Role of CdTe in Polycrystalline CdSe x Te 1– x /CdTe-Graded Bilayer Photovoltaic Devices

Grading of bandgap by alloying CdTe with selenium to form a CdSe x Te 1–x /CdTe-graded bilayer device has led to a device efficiency over 19%. A CdSe x Te 1–x absorber would increase the short-circuit current due to its lower bandgap but at the expense of open-circuit voltage. It has been demonstrated that adding a CdTe layer at the back of such a CdSe x Te 1–x film reduces the voltage deficit caused by the lower bandgap of absorber from selenium alloying while maintaining the higher short-circuit current. This leads to a photovoltaic device that draws advantage from both materials with an efficiency greater than either of them. In this report there is a detailed account using device data, ultraviolet photoelectron spectroscopy, electron microscopy, and first-principles density functional theory modeling is provided, which shows that CdTe acts as an electron reflector for CdSe x Te 1–x .

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2021

Mitigation of J–V distortion in CdTe solar cells by Ga-doping of MgZnO emitter

Distortions in the current–voltage curves of CdTe-based thin-film solar cells with MgZnO emitter layers are not uncommon. Even when such distortions are very subtle with standard sunlight, they can be greatly accentuated by the removal of short-wavelength photons with optical filters, and the dark-curve turn-on voltage can be noticeably increased. The strong suggestion is that good J–V curves with full-spectrum light often rely on photogeneration of electrons from trap states in the MgZnO emitter. When the carrier concentration of the CdTe absorber is increased with external doping, there is a corresponding need for a larger MgZnO carrier concentration. Based on the assumption that the emitter carrier concentration needs to be comparable to that of the absorber, Ga-doping of the MgZnO was shown to very nearly eliminate the large filtered-light distortions and is arguably superior to MgZnO that relies on photogeneration. In addition, it was shown that photogeneration and recovery in MgZnO is relatively slow, so that without Ga-doping, the cell efficiency improves gradually for most of an hour in the light, and the turn-on voltage of the dark curve increases on a similar time scale when the light is removed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Transparent MgO for Back-Contact Passivation of CdTe-Based Solar Cells

The passivating effects of MgO have been studied by integrating into an existing superstrate CdTe thin film solar cell device architecture. When implemented as an emitter to replace the typical MZO, the device performance was below par, but improved PL and TRPL was detected with significant carrier lifetimes in comparison to the MZO devices. However, when integrated at the back, the PL signal was detected upon the illumination of excitation laser from the back. This has opened the possibility of probing the back of the CdTe thin film solar cells with transparent back contact with improved back passivation and a bifacial device structure. Further, the TRPL measurements suggests that the MgO shows passivating effects to a CdTe surface irrespective of method of deposition implemented.

bifacial↗
DOE OSTI2021

CdTe‐Based Solar Cells with Variations in Mg Concentration in the MgZnO Emitter

The optimal fraction of Mg incorporation in sputter-deposited Mg X Zn 1−X O (MZO) emitters for thin-film CdTe-based solar cells is evaluated by varying it over a range of x from 0 to 0.35. This range allows a variation in the conduction band offset from −0.1 eV (cliff like) to +0.32 eV (spike like). Here, a maximum efficiency of 18.5% for cells with the bilayer CdSeTe/CdTe absorber occurs at x = 0.15, which corresponds to a spike-like band offset near 0.2 eV, as confirmed by X-ray photoelectron spectroscopy. In addition, good cell performance is seen over a fairly broad range of x extending from 0.1 to 0.25. The MZO optical bandgap increases with the Mg fraction, consistent with an increasing conduction band offset. Temperature-dependent current−voltage measurements and time-resolved photoluminescence show improvement in the emitter/absorber interface with the incorporation of Mg. Capacitance−voltage measurements show that the depletion region extends further into the absorber with more Mg, and X-ray diffraction confirms a change from a hexagonal-dominant crystal structure toward zinc blende at x = 0.35.

14 SOLAR ENERGY↗
DOE OSTI2020

Impact of Carrier Concentration and Carrier Lifetime on MgZnO/CdSeTe/CdTe Solar Cells

Here, the performance impact of multiple parameters related to the use of MgZnO (MZO) as the emitter for bilayer CdSeTe/CdTe solar cells has been investigated in detail through numerical simulations. Such a comprehensive study is particularly important, because while cell fabrication using MZO has been highly successful in some cases, it has been less so in others, and it has not been clear which combinations of parameter values are most effective. The parameters considered here include the recombination velocity at the emitter/absorber interface, bulk recombination lifetime, and the carrier concentrations of the emitter and absorber. The ranges chosen for the simulation parameters are those most likely to be found experimentally. The primary finding is that independent of the interfacial recombination velocity and bulk recombination lifetime, the MZO carrier density should be > 1017 cm-3 and in any case greater than that of the absorber to reduce interface recombination. At the same time, a shallow dopant the order of 50 meV or less should reduce SRH recombination in the bulk and enable VOC > 1V.

14 SOLAR ENERGY↗
DOE OSTI2020

Arsenic Doping of Polycrystalline CdSeTe Devices for Microsecond Life-times with High Carrier Concentrations

We report seminal advances in fabrication and understanding of group V (As) doped thin-film polycrystalline CdTe-based solar cells. The devices are fabricated using a novel approach, by sublimating layers of CdSeTe and CdSeTe:As. This new method allowed us to achieve minority carrier lifetime of over 1 μs, carrier concentration of more than 5x1015 cc-1 and external radiative efficiency of over 2% in a device configuration. We find an increase in open-circuit voltage when comparing As-doped, Cu-doped and undoped devices. The choice of CdSeTe instead of a CdTe-only absorber has been explained using first-principle density functional theory model. A SCAPS device model is used to analyze the potential causes for lower open-circuit voltage.

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 OSTI2020

Arsenic Doping of Polycrystalline CdSeTe Devices for Microsecond Life-times with High Carrier Concentrations

We report seminal advances in fabrication and understanding of group V (As) doped thin-film polycrystalline CdTe-based solar cells. The devices are fabricated using a novel approach, by sublimating layers of CdSeTe and CdSeTe:As. This new method allowed us to achieve minority carrier lifetime of over 1 μs, carrier concentration of more than 5×l0 15 cc -1 and external radiative efficiency of over 2 % in a device configuration. We find an increase in open-circuit voltage when comparing As-doped, Cu-doped and undoped devices. The choice of CdSeTe instead of a CdTe-only absorber has been explained using first-principle density functional theory model. A SCAPS device model is used to analyze the potential causes for lower open-circuit voltage.

Munshi, Amit H.↗