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ADVANCES OF CADMIUM TELLURIDE (CDTE) THIN FILM PHOTOVOLTAIC CELLS


The U.S. Department of Energy (DOE) Solar Office supports innovative research focused on overcoming the current technological and commercial barriers for cadmium telluride (CdTe) solar cells. Below are a list of current projects, summary of the benefits, and discussion of the production and manufacturing techniques used for this solar technology.


CdTe solar cells are the second most common photovoltaic (PV) technology in the world marketplace after mono-crystalline silicon cell currently representing 5% of the world market. CdTe thin-film solar cells can be manufactured quickly and inexpensively, providing an alternative to conventional silicon-based technologies. The record efficiency for a laboratory CdTe solar cell is 22.1% by First Solar. First Solar also reported its average commercial module efficiency to be approximately 18% at the end of 2020. 
Thin-film photovoltaic (PV) devices based on CdTe absorbers represent one of the fastest growing segments of all PV technologies; it is even more remarkable that most of this impact has occurred within the past few years. Much of the reason for this rapid development can be traced to two facts:  (1)thin-film PV modules have been designed specifically to embody production advantages over historic PV products (i.e., wafer-based technologies) and  (2)CdTe PV modules and/or production processes presently embody some advantages over other thin-film technologies.

One of the first-noted uses of CdTe was for gamma and X-ray detectors, and it was through research related to these applications that much of the early material understanding was established. Additionally, because the photoelectric absorption coefficient of CdTe is similar to that of normal skin tissue, several uses have been investigated for use in nuclear medicine. The first notable investigations of CdTe as a solar absorber were published by Vodakov and Naumov in 1960 and 1961, Although few details are cited in these papers, devices with reported efficiencies up to 6% were fabricated by diffusing semi-transparent surface coatings into crystalline n-type-doped CdTe to form Cu2−xTe/CdTe devices. Both thin-film and crystalline n-CdTe layers were used, and the process involved dipping the CdTe into a solution containing Copper ions, Results of these studies provided considerable insight relating to the formation of the p-type Cu2−xTe layer during cell processing and indicated that the Cu2−xTe/CdTe device performs more like a heterojunction than a CdTe homojunction. This study also revealed several problems related to CdTe PV devices that remain today. Noteworthy of these were a close relationship between effects occurring at the CdTe contact electrodes and junction characteristics and the observation that the best II–VI devices almost always involved the use of Cu.
In the same year, Nicoll became one of the first to report on the use of close-space vapour transport (CSVT) for the deposition of CdTe. During the next several years, measurements of the CdTe minority-carrier lifetimes indicated that the maximum lifetime for n-CdTe is about 10 ns, with even shorter lifetimes observed in p-CdTe. Subsequent modelling indicated that efficiencies >10% would not be achieved for CdTe homojunction devices unless the minority-carrier lifetimes were longer than 100 ns.

This benchmark lifetime was an order of magnitude longer than that observed for the lowest doped, crystalline CdTe. Because the homojunction emitter would have to be heavily doped to reduce series resistance effects, most research efforts during about the last 40 years have been directed toward development of CdTe heterojunction devices. Recently, however, reports have emerged suggesting that crystalline homojunction CdTe-based devices may demonstrate a significant opportunity for multijunction concentrator PV devices. In the CdTe heterojunction configuration, the choice for an appropriate material is limited by several considerations. For example, because of the near-optimum bandgap of CdTe, the heterojunction should be designed so that the majority of the absorption occurs within the CdTe bulk. Thus, the heteroface partner must act as a highly transparent, low-resistance window layer and not be responsible for carrier generation. Because all known wide-band gap, low-resistivity window materials are n-type, the CdTe layer must be p-type. The choice of window layer is further constrained in that it is preferable for it to have a small lattice mismatch with CdTe to avoid excessive interface recombination. Some of the other window materials may eventually demonstrate certain advantages over CdS. However, the most efficient cells to date have been produced using the CdS/CdTe configuration. Additionally, because the CdS/CdTe configuration represents a three-element system (rather than four-element or more), it may produce fewer potential native defects and ultimately present fewer unforeseen industrial (production) problems and/or problems due to inter-diffusion.


In the United States, scientists and manufacturers are working to expand production of cadmium telluride solar technology. Cadmium telluride is a type of “thin film” solar cell, and, as that name suggests, it’s much thinner than a traditional silicon cell. Today, panels using cadmium telluride supply about 40% of the U.S utility-scale market, and about 5% of the global solar market. And they stand to benefit from the headwinds facing the broader solar industry.
“It’s a very volatile time, especially for the crystalline silicon supply chain in general,” said Kelsey Goss, a solar research analyst for the energy consultancy group Wood Mackenzie. “There’s great potential for cadmium telluride manufacturers to take more market share in the coming year.” Especially, she noted, since the cadmium telluride sector is already scaling up.

In June, the solar manufacturer First Solar said it would invest $680 million in a third cadmium telluride solar factory in northwest Ohio. When the facility is finished, in 2025, the company will be able to make 6 gigawatts’ worth of solar panels in the area. That’s enough to power roughly 1 million American homes. Another Ohio-based solar firm, Toledo Solar, recently entered the market and is making cadmium telluride panels for residential rooftops. And in June, the U.S. Department of Energy and its National Renewable Energy Laboratory, or NREL, launched a $20 million program to accelerate research and grow the supply chain for cadmium telluride. One of the goals of the program is to help insulate the U.S. solar market from global supply constraints.
Researchers at NREL and First Solar, previously called Solar Cell Inc., have worked together since the early 1990s to develop cadmium telluride technology. Cadmium and telluride are by-products of smelting zinc ores and refining copper, respectively. Whereas silicon wafers are wired together to make cells, cadmium and telluride are applied as a thin layer — about one-tenth of the diameter of a human hair — to a pane of glass, along with other electricity-conducting materials. First Solar, now the world’s largest thin film manufacturer, has supplied panels for solar installations in 45 countries.
Emily Pontecorvo stated that the technology has certain advantages over crystalline silicon, said NREL scientist Lorelle Mansfield. For instance, the thin film process requires fewer materials than the wafer-based approach. Thin film technology is also well-suited for use in flexible panels, like ones that cover backpacks or drones or are integrated into building façades and windows. Importantly, the thin film panels perform better in hot temperatures, while silicon panels can overheat and become less efficient at generating electricity, she said.

But crystalline silicon has the upper hand in other areas, such as their average efficiency — meaning the percentage of sunlight that panels absorb and convert into electricity. Historically, silicon panels have had higher efficiencies than cadmium telluride technology; though the gap is narrowing.
Today’s industrially produced silicon panels can achieve efficiencies of 18% to 22%, while First Solar has reported an average efficiency of 18% for its newest commercial panels. 
Still, the main reason silicon has dominated the global market is relatively simple. “It all comes down to the cost,” Goss said. “The solar market tends to be highly driven by the cheapest technology.”
Crystalline silicon costs about $0.24 to $0.25 to produce each watt of solar power, which is less than other contenders, she said. First Solar said it no longer reports the cost-per-watt to produce its cadmium telluride panels, only that costs have “declined significantly” since 2015 — when the company reported costs of $0.46 per watt — and continue to drop every year. There are a few reasons for silicon’s relative cheapness. The raw material poly-silicon, which is also used in computers and smartphones, is more widely available and inexpensive than supplies of cadmium and telluride. As factories for silicon panels and related components have scaled up, the overall costs of making and installing the technology have declined. The Chinese government has also heavily supported and subsidized the country’s silicon solar sector — so much so that about 80% of the world’s solar manufacturing supply chain now runs through China.

Falling panel costs have driven the global solar boom. Over the last decade, the world’s total installed solar capacity has seen a nearly tenfold increase, from about 74,000 megawatts in 2011 to nearly 714,000 megawatts in 2020, according to the International Renewable Energy Agency. The United States accounts for about one-seventh of the world’s total, and solar is now one of the largest sources of new electricity capacity installed in the U.S. every year.

The cost per watt of cadmium telluride and other thin film technologies is similarly expected to shrink as manufacturing expands. (First Solar says that when its new Ohio facility opens, the company will deliver the lowest cost per watt on the entire solar market.) But cost isn’t the only metric that matters, as the industry’s current supply chain issues and labor concerns make clear.
Mark Widmar, CEO of First Solar, said the company’s planned $680 million expansion is part of a larger effort to build a self-sufficient supply chain and “decouple” the U.S. solar industry from China. Although cadmium telluride panels don’t use any poly-silicon, First Solar has felt other challenges facing the industry, like pandemic-induced backlogs in the maritime shipping industry. In April, First Solar told investors that congestion at American ports was holding up panel shipments from its facilities in Asia. Increasing U.S. production will allow the company to use roads and railways to ship its panels, not cargo ships, Widmar said. And the company’s existing recycling program for its solar panels allows it to reuse materials many times over, further reducing its reliance on foreign supply chains and raw materials.

As First Solar churns out panels, scientists at both the company and NREL continue to test and improve cadmium telluride technology. In 2019, the partners developed a new approach that involves “doping” the thin film materials with copper and chlorine to achieve even higher efficiencies. Earlier this month, NREL announced the results of a 25-year field test at its outdoor facility in Golden, Colorado. A 12-panel array of cadmium telluride panels was operating at 88 % original efficiency, a strong result for a panel that’s sat outside for over two decades. The degradation “is in line with what silicon systems do,” according to the NREL release.

Mansfield, the NREL scientist, said the goal isn’t to replace crystalline silicon with cadmium telluride or establish one technology as superior to the other. “I think there’s a place for all of them in the market, and they each have their applications,” she said. “We want all energy to go to renewable sources, so we really need all of these different types of technology to meet that challenge.”

CdTe solar cells are the second most common photovoltaic (PV) technology in the world marketplace after crystalline silicon, currently representing 5% of the world market. CdTe thin-film solar cells can be manufactured quickly and inexpensively, providing an alternative to conventional silicon-based technologies. The record efficiency for a laboratory CdTe solar cell is 22.1% by First Solar. First Solar also reported its average commercial module efficiency to be approximately 18% at the end of 2020. 

RESEARCH DIRECTIONS
Current projects seek higher cell efficiencies by increasing crystal quality, improving doping control, and increasing the minority carrier lifetime. Manufacturers are also working to improve materials reuse and recycling as a way to mitigate concerns on toxicity and materials scarcity.
3.3 The Cadmium Telluride (CdTe) Photovoltaic (PV) Accelerator: The Cadmium Telluride (CdTe) Photovoltaic (PV) Accelerator program is intended to enhance U.S. technology leadership and competitiveness in CdTe PV. By 2030, the program aims to increase domestic CdTe PV material and module production, achieve cell efficiencies above 26%, and decrease module costs to below $0.15/watt. 
The Cadmium Telluride (CdTe) Photovoltaics (PV) Accelerator was announced by the U.S. Department of Energy (DOE) Solar Energy Technologies Office (SETO) on March 25, 2021. NREL released a competitive solicitation on June 7, 2021, to select leadership for the CdTe technology development consortium. Applications are due on July 19, 2021, at 6:00 p.m.
The CdTe PV Accelerator program, announced by the U.S. Department of Energy on March 25, 2021, is designed to:
Support the planning and operations of a technology development consortium to enhance U.S. technology leadership and competitiveness in CdTe PV
Enable cell efficiencies above 24% and module costs below $0.20/W by 2025
Enable cell efficiencies above 26% and module costs below $0.15/W by 2030
Maintain or increase domestic CdTe PV material and module production through 2030.
The CdTe PV Accelerator solicitation will formalize the leadership to identify the technology development priorities of a consortium consisting of leading companies and research institutes that are able to impact the entire domestic CdTe supply chain.
The solicitation was released on June 7. Proposals were due by July 19, 2021.
Solution for Predictive Physical Modelling in CdTe and Other Thin-Film PV
his project aims to develop a software tool to enable a more accurate interpretation of cadmium telluride (CdTe) thin-film photovoltaic (PV) device performance and material properties, with the goal of enabling predictive device design. The software includes a modeling tool that accounts for atomic diffusion and drift as well as electronic behavior and device performance. This will allow researchers to simulate recombination losses over time in II-VI absorber materials under specified process and stress conditions.
The Arizonal State University research team is working to eliminate the ambiguity between the observed device performance and the physical root cause. They will develop several models that account for diffusion and drift at the atomistic level coupled to the electronic subsystem responsible for a PV device’s function. More advanced physical models will describe the capture, emission, and recombination phenomena relevant to multivalent dopants, amphoteric centers, and donor-acceptor pairs. These models will be implemented in a self-contained simulation tool that will drastically reduce interpretation ambiguity and, for the first time, allow for predictive design of thin-film PV devices.

The software tool developed by this project will help to explain and predict performance of thin film PV devices built in a II-VI material system. The software tool will include reaction-diffusion models to accommodate the time and position dependency of II-VI semiconductor alloy stoichiometry and corresponding dependencies of the properties of point defects and complexes. It will also include rate models to describe ionization reactions of electrically active centres in II-VI semiconductors and interactions at interfaces with a high degree of disorder. The software will be able to efficiently solve large systems of diffusion-reaction equations.

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