Development of ZnTe as a back contact material for thin film cadmium telluride solar cells

Cadmium telluride (CdTe) is high-efficiency commercialised thin film photovoltaic technology. However, developing a stable low-resistivity back contact to the CdTe solar cells is still an issue. High work function and low level of doping of this material don't allow to create an ohmic contact with metals directly. Copper is commonly used to lower the back contact barrier in CdTe solar cells, but an excessive amount of copper diffusing through the cell is harmful for the device performance and stability. In this work a copper-doped ZnTe (ZnTe:Cu) buffer layer was incorporated in between CdTe and gold metal contact by high-rate pulsed DC magnetron sputtering. The back contact was then activated by rapid thermal processing (RTP) resulting in spectacular improvement in key device performance indicators, open circuit voltage (VOC) and fill factor (FF)

Structural and chemical characterization of the back contact region in high efficiency CdTe solar cells

Cadmium telluride (CdTe) is the leading commercialized thin-film photovoltaic technology. Copper is commonly used in back contacts to obtain high efficiency, but has also been implicated as a harmful factor for device stability. T hus it is critical to understand its composition and distribution within complete devices. In this work the composition and structure of the back contact region was examined in high efficiency devices (-16%) contacted using a ZnTe:Cu buffer layer followed by gold metallization. T he microstructure was examined in the asdeposited state and after rapid thermal processing (RTP) using high resolution transmission electron microscopy and EDX chemical mapping. After RTP the ZnTe exhibits a bilayer structure with polycrystalline, twinned grains adjacent to Au and an amorphous region adjacent to CdTe characterized by extensive Cd-Zn interdiffusion. T he copper that is co-deposited uniformly within ZnTe is found to segregate dramatically after RTP activation, either collecting near the ZnTe/Au interface or forming CUxTe clusters in CdTe at defects or grain boundaries near the interface with ZnTe. Chlorine, present throughout CdTe and concentrated at grain boundaries, does not penetrate significantly into the back contact region during RTP activation

Synthesis of Stoichiometric FeS₂ through Plasma-Assisted Sulfurization of Fe₂O₃ Nanorods

Pyrite (FeS₂) thin films were synthesized using a H₂S plasma to sulfurize hematite (Fe₂O₃) nanorods deposited by chemical bath deposition. The high S activity within the plasma enabled a direct solid-state transformation between the two materials, bypassing S-deficient contaminant phases (Fe_1–<i>x</i>S). The application of plasma dramatically enhanced both the rate of conversion and the quality of the resulting material; stoichiometric FeS₂ was obtained at a moderate temperature of 400 °C using a chalcogen partial pressure <6 × 10^–5 atm. As the S:Fe atomic ratio increased from 0 to 2.0, the apparent optical band gap dropped from 2.2 (hematite) to ∼1 eV (pyrite), with completely converted layers exhibiting absorption coefficients >10⁵ cm^–1 in the visible range. Room-temperature conductivity of FeS₂ films was on the order of 10^–4 S cm^–1 and approximately doubled under calibrated solar illumination

Low-Temperature Synthesis of <i>n</i>‑Type WS₂ Thin Films via H₂S Plasma Sulfurization of WO₃

Thin tungsten disulfide (WS₂) films were prepared on SnO₂:F (FTO)-coated glass substrates by H₂S plasma sulfurization of sputtered WO₃. The reactive environment provided by the plasma enabled the complete transformation of a 75 nm oxide film to stoichiometric WS₂ within 1 h at 500 °C. An apparent activation energy of 63.6 ± 1.9 kJ/mol was calculated for the plasma conversion process, which is less than half the barrier reported for the reaction of WO₃ with H₂S. The conversion followed Deal–Grove behavior, with the growing WS₂ overlayer hindering diffusion to and from the reactive interface. The calibrated light absorption and relative intensity of the second-order Raman 2LA­(M) peak were identified as two additional methods for progressively monitoring the thickness of the WS₂ layer. The semiconducting WS₂ layers exhibited <i>n</i>-type behavior with an indirect band gap at 1.4 eV and an absorption coefficient of ∼5 × 10⁴ cm^–1. Preliminary electrochemical measurements showed that the presence of WS₂ reduced the overpotential required for the hydrogen evolution reaction by 360 mV relative to FTO while displaying good stability

Copper-induced recrystallization and interdiffusion of CdTe/ZnTe thin films

© 2018 Author(s). ZnTe is commonly employed as a buffer layer between CdTe and the metallization layer at the back contact of state-of-the-art CdTe solar cells. Here, the critical role of Cu in catalyzing recrystallization and interdiffusion between CdTe and ZnTe layers during back contact activation is presented. Several CdTe/ZnTe:Cu thin-film samples were prepared with varying levels of copper loading and annealed as a function of temperature and time. The samples were characterized by x-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and energy dispersive x-ray spectroscopy. The results show that stress is present in the as-deposited bilayers and that negligible interdiffusion occurs in the absence of Cu. The presence of Cu facilitates rapid interdiffusion, predominantly via Cd migration into the ZnTe phase. Zn migration into CdTe is limited to areas around defects and grain boundaries. Ternary Cd x Zn 1-x Te interlayers are formed, and the extent of alloy formation ranges from 0.08 < x < 0.5 throughout the whole ZnTe layer. The level of Cu loading controls the composition of the Cu x Te clusters observed, while their size and migration is a function of annealing conditions

Scalable Synthesis of Selenide Solid-State Electrolytes for Sodium-Ion Batteries

Solid-state sodium-ion batteries employing superionic solid-state electrolytes (SSEs) offer low manufacturing costs and improved safety and are considered to be a promising alternative to current Li-ion batteries. Solid-state electrolytes must have high chemical/electrochemical stability and superior ionic conductivity. In this work, we employed precursor and solvent engineering to design scalable and cost-efficient solution routes to produce air-stable sodium selenoantimonate (Na3SbSe4). First, a simple metathesis route is demonstrated for the production of the Sb2Se3 precursor that is subsequently used to form ternary Na3SbSe4 through two different routes: alcohol-mediated redox and alkahest amine-thiol approaches. In the former, the electrolyte was successfully synthesized in EtOH by using a similar redox solution coupled with Sb2Se3, Se, and NaOH as a basic reagent. In the alkahest approach, an amine-thiol solvent mixture is utilized for the dissolution of elemental Se and Na and further reaction with the binary precursor to obtain Na3SbSe4. Both routes produced electrolytes with room temperature ionic conductivity (∼0.2 mS cm–1) on par with reported performance from other conventional thermo-mechanical routes. These novel solution-phase approaches showcase the diversity and application of wet chemistry in producing selenide-based electrolytes for all-solid-state sodium batteries

Structural and chemical evolution of the CdS:O window layer during individual CdTe solar cell processing steps

© 2017 Elsevier Ltd Oxygenated cadmium sulfide (CdS:O) is often used as the n-type window layer in high-performance CdTe heterojunction solar cells. The as-deposited layer prepared by reactive sputtering is XRD amorphous, with a bulk composition of CdS 0.8 O 1.2 . Recently it was shown that this layer undergoes significant transformation during device fabrication, but the roles of the individual high temperature processing steps was unclear. In this work high resolution transmission electron microscopy coupled to elemental analysis was used to understand the evolution of the heterojunction region through the individual high temperature fabrication steps of CdTe de position, CdCl 2 activation, and back contact activation. It is found that during CdTe deposition by close spaced sublimation at 600 °C the CdS:O film undergoes recrystallization, accompanied by a significant (∼30%) reduction in thickness. It is observed that oxygen segregates during this step, forming a bi-layer morphology consisting of nanocrystalline CdS adjacent to the tin oxide contact and an oxygen-rich layer adjacent to the CdTe absorber. This bilayer structure is then lost during the 400 °C CdCl 2 treatment where the film transforms into a heterogeneous structure with cadmium sulfate clusters distributed randomly throughout the window layer. The thickness of window layer remains essentially unchanged after CdCl 2 treatment, but a ∼25 nm graded interfacial layer between CdTe and the window region is formed. Finally, the rapid thermal processing step used to activate the back contact was found to have a negligible impact on the structure or composition of the heterojunction region

Experimental and Theoretical Insights into the Potential of V₂O₃ Surface Coatings for Hydrogen Permeable Vanadium Membranes

A grand challenge of vanadium-based H₂ permeable membranes is the development of effective, cheap, and stable catalysts to facilitate H₂ dissociation and recombination. This work investigates a facile air treatment to form catalytically active vanadium oxide on the surfaces of dense vanadium foils. The treatment consisted of short air exposure followed by H₂ reduction at 823 K, which produced a well-faceted and nanocrystalline V₂O₃ layer on the foil surfaces. The resulting membranes displayed a stable H₂ permeability of 2 ± 0.25 × 10^–8 mol·m^–1·s^–1·Pa^–0.5, but transient declines in permeation were observed when operated at both elevated and reduced temperatures. DFT calculations revealed that V₂O₃ (0001) surfaces display barriers and adsorption energies for H₂ dissociation/recombination that are comparable to those of known H₂ activation catalysts. It was found that H₂ dissociation is expected to proceed spontaneously on metal-terminated V₂O₃, with recombinative-desorption anticipated as the rate limiting step

The roles of ZnTe buffer layers on CdTe solar cell performance

The use of ZnTe buffer layers at the back contact of CdTe solar cells has been credited with contributing to recent improvements in both champion cell efficiency and module stability. To better understand the controlling physical and chemical phenomena, high resolution transmission electron microscopy (HR-TEM) and atom probe tomography (APT) were used to study the evolution of the back contact region during rapid thermal processing (RTP) of this layer. After activation the ZnTe layer, initially nanocrystalline and homogenous, transforms into a bilayer structure consisting of a disordered region in contact with CdTe characterized by significant Cd-Zn interdiffusion, and a nanocrystalline layer that shows evidence of grain growth and twin formation. Copper, co-evaporated uniformly within ZnTe, is found to dramatically segregate and aggregate after RTP, either collecting near the ZnTe|Au interface or forming CuxTe clusters in the CdTe layer at defects or grain boundaries near the interface. Analysis of TEM images revealed that Zn accumulates at the edge of these clusters, and three-dimensional APT images confirmed that these are core-shell nanostructures consisting of Cu1.4Te clusters encased in Zn. These changes in morphology and composition are related to cell performance and stability