Cadmium Telluride Photovoltaic Research Articles

Lead Telluride Nanowires for Surface Passivation in Cadmium Telluride Photovoltaics

Cadmium telluride (CdTe) is one of the leading technologies in thin-film solar cells; however, its open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ) is limited by defects associated with polycrystalline nature, carrier concentration, and interfaces. To overcome the deep work function of the CdTe absorber and create a passivating back contact interface, a lattice-matched and highly doped p-type material is needed to function as a buffer layer between the CdTe and a highly conductive electrode. Here, we report that lead telluride (PbTe) nanowires (NWs) serve to passivate the CdTe back contact, enhancing the optoelectronic properties and device performance. The steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements showed improved PL intensity and minority carrier lifetime for devices utilizing a PbTe NW back buffer, consistent with surface passivation. Additionally, the devices employing PbTe NWs showed improved V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> and fill factor (FF). For instance, implementation of PbTe NWs as a passivating layer to CdS/CdTe solar cells yielded a photoconversion efficiency of 14.8% (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> = 855 mV and FF = 77.3%), whereas the control device reached 14.0% (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> = 840 mV and FF = 75.9%). This improvement in V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> and FF is consistent with the reduction in both back-barrier height and recombination losses observed for devices with the PbTe NW back buffer.

Quantification of the Impact of Fine Particulate Matter on Solar Energy Resources and Energy Performance of Different Photovoltaic Technologies.

Ambient fine particulate matter (PM2.5) could be a potential environmental risk for decreasing the available solar energy resources and solar photovoltaic (PV) power generation. This study quantifies the attenuation effects of PM2.5 on surface solar irradiance and system performance of different solar PV technologies in Hong Kong. The analysis based on observational irradiation data shows that the global horizontal irradiance decreased by more than 5% in most months under the conditions of PM2.5 concentration exceeding 33.5 μg/m3. During the experiment, the average PM2.5-related losses in the energy output of crystalline silicon and thin-film PV systems could be up to 7.00 and 9.73%, respectively. The measured energy outputs of the experimental PV modules suggest that PM2.5 affects the energy performance of thin-film solar cells with a larger band gap more significantly than that of crystalline silicon PV modules. Moreover, an increasing trend in the performance ratio of monocrystalline silicon, polycrystalline silicon, and copper indium gallium selenide PV systems with the increase of PM2.5 concentration is observed. In contrast, the amorphous silicon and cadmium telluride PV systems with a narrower spectral response range show a decreasing trend in the performance ratio over the experiment. Results indicate that the losses in the available solar energy resources and PV energy potential are expected to increase in areas where heavier PM2.5 pollution exists.

Integration of raw materials indicators of energy technologies into energy system models

Raw materials and their related environmental impacts will play a key role in the implementation of renewable energy infrastructures for decarbonization. Despite the growing amount of data quantifying raw materials for energy production technologies, few examples of these data sources are being included in current energy system models. Accordingly, this paper introduces possible pathways for integrating material-specific life cycle assessment outputs and material metabolism indicators into energy system models so that raw material requirements, and their associated impacts, can be accounted for. The paper discusses the availability of life cycle inventories, impact assessment methods and important output indicators. The material metabolism indicators most relevant to the current policy debate surrounding the European Green Deal–namely, material supply risk and contribution of recycled materials to total supply–are also discussed alongside the value of adding this information to energy system models. A methodology for using data from both approaches is offered and operationalised using four sub-technologies of both wind turbines and solar photovoltaic panels as case studies. The results show that considerable variation exists between and within the two groups for all indicators. The technologies with the lowest global warming potential, cumulative energy demand and supply risk are turbines with gearbox double-fed induction generators and cadmium telluride photovoltaics. Furthermore, wind turbines exhibit significantly higher recycling rates than photovoltaics. Ultimately, the integration of such methodologies into energy system models could greatly increase the awareness of raw material issues and guide policies that maximise compatibilities between resource availability and cleaner energy systems.

Investigation of life cycle CO2 emissions of the polycrystalline and cadmium telluride PV panels

The importance of energy increases in human life with the development of technology. However, most of the world’s energy needs are still met by fossil fuels. The interest in renewable energy is increasing due to the increase in demand for fossil fuels, their limitedness, and environmental damage. Solar energy, which is an unlimited and clean energy source, is among the most popular renewable energy sources. Solar energy is used by processes such as heating and electricity generation. In this study, the emission amount of polycrystalline and cadmium telluride (CdTe) photovoltaic (PV) panels to the environment during the life cycle were compared. During the life cycle, the amount of emission released to the environment during the production, recycling, and electricity generation of the panel was determined. In addition, energy payback times of these two PV types were calculated. The emission amount for both of 1 m2 polycrystalline and CdTe PV panel throughout the life cycle was determined to be 201.4 and 115.04 kg−CO2, respectively. Besides, the average energy payback period of the polycrystalline panel is 0.92 years, while it is 0.57 years for the CdTe panel. In addition, the emission amount of the solar panel and the natural gas-powered thermal power plant known as a clean energy source were compared. As a result, a huge difference was observed between the emission amount released by the PV panel to the environment and the emission released as a result of the energy produced by the thermal power plant under the same conditions during its 17-year lifetime. PV panel saves 1.72 tons of CO2 emissions compared to the thermal power plant, and so PV panels appear to be 9.52 times more environmentally friendly.

Assessing the Regulatory Requirements of Lead-Based Perovskite Photovoltaics

Nicole Moody, Samuel Sesena, Dane W. deQuilettes, Benjia Dak Dou, Richard Swartwout, Anna Johnson, Udochukwu Eze, Roberto Brenes, Matthew Johnston, Vladimir Bulovic, and Moungi G. Bawendi are members of Tata-MIT GridEdge Solar, an interdisciplinary research program at the Massachusetts Institute of Technology working toward scalable design and manufacturing of lightweight, flexible solar cells. Joseph T. Buchman and Christy L. Haynes are part of the Center for Sustainable Nanotechnology, a multi-institutional partnership aimed at understanding the fundamental chemical and physical processes that govern the transformations and interactions of nanoparticles in the environment.

Wet chemical etching of cadmium telluride photovoltaics for enhanced open-circuit voltage, fill factor, and power conversion efficiency

Abstract

Solution-processed Nanocrystal Based Thin Films as Hole Transport Materials in Cadmium Telluride Photovoltaics

The cadmium telluride (CdTe) photovoltaic (PV) comprise an efficient and cost-effective technology for harvesting solar energy. However, device efficiency remains limited in part by low-open circuit voltage (VOC) and fill factor (FF) due to inefficient transport of photo-generated charge carriers. Given the deep valence band of CdTe, the use of copper/gold (Cu/Au) as a back contact serves primarily to narrow the width of the inherent Schottky junction evident in CdTe solar cells (in our laboratory, Cu/Au has been used as a standard back contact). For efficient transport of carriers to and into the back contact, a hole transport layer (HTL) is desired with valence band edge comparable to that of CdTe (∼ -5.9 eV). Here, we report solution-processed nanocrystal (NCs) based thin films as HTLs in CdTe solar cells. The earth abundant materials we discuss include iron pyrite (FeS2), nickel-alloyed iron pyrite (NixFe1-xS2), zinc copper sulfide (ZnxCu1-xS) nanocomposites, and perovskite-based films. The FeS2 and NixFe1-xS2 NCs are synthesized by a hot-injection route, and thin films are fabricated by drop-casting, and spin-coating techniques using colloidal NCs. ZnxCu1-xS thin films are fabricated by chemical bath deposition. These NC-based thin films are applied and studied as the HTLs in CdTe devices. On using these materials, the device performance can be increased up to 10% compared to the standard Cu/Au back contact. Here, we discuss the benefits, challenges, and opportunities for these back contact materials in CdTe photovoltaics.

Impact of average photon-energy coefficient of solar spectrum on the short circuit current of photovoltaic modules

The output energy of photovoltaic (PV) modules is influenced by the spectral irradiance distribution of the solar spectrum under outdoor conditions. To rate the precise output energy of PV modules, the correction of short circuit current (ISC) based on actual environmental conditions is needed, because ISC significantly depends on the shape of the spectral irradiance distribution. The average photon energy (APE) is a zero-dimensional index for spectral irradiance distribution, and APE value uniquely describes the shape of a solar spectrum. Thus, APE has an impact on ISC of PV modules. In this contribution, the relationship between APE coefficient and ISC of the multi-crystalline silicon, single-crystalline silicon, heterojunction intrinsic thin-layer, back contact, copper indium selenide and cadmium telluride PV modules has explored. It is revealed that APE value changes the ISC of PV modules which appeared to have immense possibilities of ISC correction using APE coefficient. This new approach can be very effective for precise rating the output energy of PV modules under actual outdoor conditions.

Properties of Nitrogen-Doped Zinc Telluride Films for Back Contact to Cadmium Telluride Photovoltaics

Zinc telluride (ZnTe) films have been deposited onto uncoated glass superstrates by reactive radiofrequency (RF) sputtering with different amounts of nitrogen introduced into the process gas, and the structural and electronic transport properties of the resulting nitrogen-doped ZnTe (ZnTe:N) films characterized. Based on transmission and x-ray diffraction measurements, it was observed that the crystalline quality of the ZnTe:N films decreased with increasing nitrogen in the deposition process. The bulk carrier concentration of the ZnTe:N films determined from Hall-effect measurements showed a slight decrease at 4% nitrogen flow rate. The effect of ZnTe:N films as back contact to cadmium telluride (CdTe) solar cells was also investigated. ZnTe:N films were deposited before or after CdCl2 passivation on CdTe/CdS samples. Small-area devices were characterized for their electronic properties. Glancing-angle x-ray diffraction measurements and energy-dispersive spectroscopy analysis confirmed substantial loss of zinc from the samples where CdCl2 passivation was carried out after ZnTe:N film deposition.

Review of the Life Cycle Greenhouse Gas Emissions from Different Photovoltaic and Concentrating Solar Power Electricity Generation Systems

This paper contains an extensive review of life cycle assessment (LCA) studies on greenhouse gas emissions (GHG) from different material-based photovoltaic (PV) and working mechanism-based concentrating solar power (CSP) electricity generation systems. Statistical evaluation of the life cycle GHG emissions is conducted to assess the role of different PVs and CSPs in reducing GHG emissions. The widely-used parabolic trough and central receiver CSP electricity generation systems emitted approximately 50% more GHGs than the paraboloidal dish, solar chimney, and solar pond CSP electricity generation systems. The cadmium telluride PVs and solar pond CSPs contributed to minimum life cycle GHGs. Thin-film PVs are also suitable for wider implementation, due to their lower Energy Pay-Back Time (EPBT) periods, in addition to lower GHG emission, in comparison with c-Si PVs.

Inkjet and laser hybrid processing for series interconnection of thin film photovoltaics

Inkjet deposition can be a complementary technology to laser ablation to enable new processes. One such process is the One step interconnect for thin film photovoltaics, which is an improved method for series interconnection. The standard series interconnection process consists of three laser scribes between the deposition of the three key cell layers; transparent front contact, absorber layer and the metallic back contact. The one step interconnect allows the series interconnection to occur after the deposition of all layers significantly simplifying the manufacturing process. This is achieved by inkjet printing of conductive and insulative materials concurrently with depth selective laser scribes. The one step interconnect process has been shown to make effective interconnects on cadmium telluride photovoltaics with fill factors >60%. The benefits are many and include the reduction of capital equipment costs, reduced panel wastage and potentially improved material performance. The process is fully scalable and production ready.

Analysis of spectral effects on the energy yield of different PV (photovoltaic) technologies: The case of four specific sites

This work is aimed at looking into the impact of the solar spectrum distribution on the energy yield of some PV (photovoltaic) materials on both monthly and annual time scales. The relative spectral responses of eight different PV solar cells – representative of each considered technology – have been selected. Modeling solar spectra in four sites located in the north hemisphere together with a 12-month experimental campaign in which spectral irradiances were periodically recorded in two of these sites located in Spain have provided highly interesting results. Regardless of the site, the considered amorphous silicon and cadmium telluride PV cells experience the most noticeable modeled and experimental variations of their monthly spectral gains, whilst flatter seasonal ones are identified in the remaining considered PV technologies. Thus, the maximum monthly variations of these spectral gains take place in the a-Si PV cell, ranging from −16% (winter) to 4% (summer) in Stuttgart. Anyway, the monthly spread of the spectral effects decreases as the latitude of the site does. Last, the impact of the solar spectrum distribution is remarkably reduced when the period of integration is enlarged up to a year. In fact, annual spectral gains keep below 2.2% for all the studied technologies and sites.

Increased conversion efficiency in cadmium telluride photovoltaics by luminescent downshifting with quantum dot/poly(methyl methacrylate) films

AbstractCommercially available quantum dots have been encapsulated in a poly(methyl methacrylate) film and used as a luminescent downshifting layer on cadmium sulfide/cadmium telluride photovoltaic devices. Application of these films has resulted in a relative improvement to the short‐circuit current of over 4% by I–V measurement, with a significant increase in the contribution of short‐wavelength light resulting in 25% of the current available in this part of the spectrum being captured. The films have been shown to be highly scattering and the associated difficulties this provides to external quantum efficiency measurements have been discussed. A range of optical characterisation techniques, particularly laser beam induced current, have been used to probe the effect the films have on a cadmium sulfide/cadmium telluride device. An alternate methodology for performing external quantum efficiency measurements with the quantum dot films has been proposed. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Changes in Cadmium Telluride Photovoltaic System Performance due to Spectrum

Seasonal and short-term weather-related changes in the solar spectrum can induce shifts in the performance of photovoltaic (PV) systems that affect both annual energy predictions and system characterization. The spectral shift factor, which is a metric indicative of how much the performance of a PV system will vary from nameplate due to deviations from the ASTM G173 spectrum (air mass of 1.5), is predicted using TMY3 data and the simple model of the atmospheric radiative transfer for sunshine (SMARTS) model and is correlated with cadmium telluride (CdTe) PV system performance in four different climates. The predicted spectral shift factors for CdTe systems show improved performance in the late summer and early fall and diminished performance in the winter. These intraannual variations can be as large as ±3%, but annual spectral shift factors are typically within ± 1% of nameplate. The spectral shift factor of CdTe systems was found to be most sensitive to the precipitable water content of the atmosphere. Consequently, a parameterization of CdTe spectral shift factor as an exponential function of precipitable water is derived using the outputs of the SMARTS model in 11 locations. This parameterization is shown to predict observed monthly and daily fluctuations in CdTe PV performance. Future efforts will incorporate this methodology into energy predictions that will reduce uncertainty.

Life Cycle Water Usage in CdTe Photovoltaics

Life cycle water withdrawal for cadmium telluride photovoltaics (CdTe PV) ranges from approximately 382-425 L/MWh, with only ~12% from direct on-site usage. The remainder is related to indirect water withdrawal from the use of grid electricity and raw materials throughout the product life cycle. Approximately half of life cycle water withdrawal is associated with module manufacturing, one-third from balance of systems (BOS), and the remainder from takeback and recycling. Primary contributors to life cycle water withdrawal are the use of grid electricity, glass, and on-site water during manufacturing; steel, copper, inverters, and on-site water in the BOS; and electricity, chemical use, and transport during takeback and recycling. During manufacturing, water consumption is approximately one quarter of withdrawal and is due to cooling tower water evaporation and site irrigation. When deployed in the U.S. Southwest, a CdTe PV array can provide net displacement of life cycle water withdrawal of over 1700-5600 L/MWh relative to grid electricity.

Future recycling flows of tellurium from cadmium telluride photovoltaic waste

According to the European Photovoltaic Industry Association, photovoltaic energy has the potential to contribute up to 13% to the global electricity supply by 2040. A part of this electricity production will come from thin-film photovoltaic technologies. From various thin-film technologies available on the market today, low-cost cadmium telluride photovoltaics (CdTe-PV) can be considered the market leader with a market share of 5% at annual production. There are however two major concerns about this technology: first, the potential negative environmental impacts of cadmium contamination from CdTe-PV; and second, the possible shortage of the metal tellurium in the future. Because of these concerns, the recycling of production scrap and end-of-life PV modules is essential. In this paper we estimate how much tellurium will be recovered from PV scrap to substitute for primary tellurium. In order to estimate global tellurium flows until 2040, we have created a dynamic material flow model for the life-cycle of CdTe-PV modules. Three scenarios, which describe different market developments and technology trajectories, show how material efficiency measures – higher material utilization in production, decrease of material content in PV modules, and recycling of production scrap and end-of-life modules – will affect demand, waste flows, and recycling flows of semiconductor grade tellurium. The results depict that efficiency measures at process and cell level will reduce the specific tellurium demand per watt peak such that total tellurium demand starts to decline after 2020 despite further market growth. Thus, the CdTe-PV industry has the potential to fully rely on tellurium from recycled end-of-life modules by 2038. However, in order to achieve this goal, material efficiency must be substantially improved and efficient collection and recycling systems have to be built up.

Fate and transport evaluation of potential leaching risks from cadmium telluride photovoltaics

Fate and transport analysis has been performed to evaluate potential exposures to cadmium (Cd) from cadmium telluride (CdTe) photovoltaics (PV) for rainwater leaching from broken modules in a commercial building scenario. Leaching from broken modules is modeled using the worst-case scenario of total release of Cd, and residential screening levels are used to evaluate potential health impacts to on-site workers and off-site residents. A rooftop installation was considered rather than a ground-mount installation because rainwater runoff is concentrated via building downspouts in a rooftop installation rather than being dispersed across large areas in a ground-mount installation. Fate and transport of Cd from leachate to soil are modeled using equilibrium soil/soil-water partitioning. Subsequent migration to ambient air as windblown dust is evaluated with a screening Gaussian plume dispersion model, and migration to groundwater is evaluated with a dilution-attenuation factor approach. Exposure point concentrations in soil, air, and groundwater are one to six orders of magnitude below conservative (residential soil, residential air, drinking water) human health screening levels in both a California and southern Germany (Baden-Württemberg) exposure scenario. Potential exposures to Cd from rainwater leaching of broken modules in a commercial building scenario are highly unlikely to pose a potential health risk to on-site workers or off-site residents.

Trend for Thermoelectric Materials and Their Earth Abundance

The low crustal abundance of materials such as tellurium (Te) (0.001 ppm by weight), antimony (Sb) (0.2 ppm), and germanium (Ge) (1.4 ppm) contributes to their price volatility as applications (competing with thermoelectrics) continue to grow, for example, cadmium telluride photovoltaics, antimony-lead alloy for batteries, and Ge for fiber optics and infrared optical technologies. Previous consideration of material scarcity has focused on Te-based thermoelectrics. Here, we broaden the analysis to include recent high-figure-of-merit (ZT) materials such as skutterudites, Zintl phase compounds, and clathrates that employ Sb, ytterbium (2.8 ppm), and Ge. The maximum demonstrated ZT for each particular alloy exhibits an empirical dependence on the crustal abundance, A, such that ZT = A−b, where b is in the range from 0.05 to 0.10. This analysis shows that no material with crustal abundance of 30 ppm (~4 × 1018 metric tons) has ZT greater than 0.8.

Potential Cd emissions from end-of-life CdTe PV

Cadmium telluride photovoltaics (CdTe PV) have grown considerably in the last few years and are now a mainstream energy technology. Concern has been voiced regarding the potential impact caused by the dispersal of the Cd contained in the modules after they are decommissioned. This study presents a new comprehensive analysis of the end-of-life of CdTe PV and reports on the associated Cd emissions to air and water. Three end-of-life scenarios were considered for CdTe PV. In the first one, 100% of the modules are collected and sent to recycling; in the other two, 85% of the modules are recycled, and the rest are assumed to be either treated as normal municipal solid waste or pre-selected and sent to landfills. All input and output data for module decommissioning and recycling were based on the information directly provided by the world-leading CdTe PV manufacturer (First Solar). The inventory modelling was performed with the GaBi life cycle analysis software package in conjunction with the Ecoinvent v.2 database. In all scenarios, end-of-life Cd emissions from CdTe PV were found to be relatively low, for instance when compared to those from NiCd batteries, when expressed per kilogram of Cd content. The on-going growth of CdTe PV is unlikely to produce a worrisome increase in the overall Cd emissions to the environment; principally thanks to the expected stringent control of the related Cd-containing waste flows.

The Impact of Tellurium Supply on Cadmium Telluride Photovoltaics

For decades, the material associated with photovoltaic (PV) cells has been silicon. However, after many years of development, cadmium telluride (CdTe) PV modules have become the lowest-cost producer of solar electricity, despite working at lower efficiency than crystalline silicon cells. CdTe sales are growing rapidly, but there is concern about projecting hundredfold increases in power production relative to current production with CdTe PV modules. One reason is that Te, a humble nonmetal that is actually abundant in the universe, is as rare as many of the precious metals recovered from Earth's crust ( 1 ). Furthermore, current technology now uses Te at rates that are substantial fractions of its supply. Here, I argue that the long-term potential for CdTe PV modules need not be bleak, given realistic developments in module technology and Te recovery.