Cadmium Telluride Accelerator Consortium

Group V dopants

Doping incorporation methods

Doping profiles

Dopant activation

N-type MgZnO emitter improvements

New emitter candidate materials exploration

Front interfaces evaluation using characterization and modeling

Power conversion efficiency improvements

Bifacial technology

Rooftop PV

Building-integrated PV

Round 1 Awards

Advanced Back Contacts and Surface Photovoltage/Surface Photovoltage Spectroscopy Characterization To Unlock Bifaciality, Open-Circuit Voltage > 900 Megavolts, and Efficiency of 26% From Cadmium Telluride-Based Cells
The University of Utah will develop sputtered, doped, wide bandgap materials and bilayer stacks for back contacts for state-of-the-art cadmium selenide telluride (CdSeTe)/CdTe absorbers. The project will focus on p-type materials that have energy level alignment predicting hole selectivity, are amenable to passivation, and have a wide bandgap to provide transparency for enhanced bifaciality or back mirror cell optics. It will obtain state-of-the-art absorber stacks from CTAC partners and fabricate sputtered back contacts. It will continue to develop our surface photovoltage (SPV) and SPV spectroscopy techniques to characterize back contact band structure, traps, and recombination activity.

Advanced Activation and Contact Approaches for Cadmium Zinc Telluride Solar Cells
The University of Delaware will develop new approaches for processing cadmium zinc telluride (Cd_1-xZn_xTe) solar cells that overcome previously reported difficulties, such as ineffective chloride activation and passivation, which prevented the realization of high performance with increased open-circuit voltage relative to CdTe. The approach will be based on two hypotheses: Modification of film growth, including in situ antimony incorporation, can form more equilibrated films with reduced defects and enhanced grain sizes, reducing the need for high-temperature activation; and alternative halide activation chemistries during post-deposition treatments can minimize the deleterious effects of cadmium chloride activation. A final goal of the project will be to confirm the viability of Cd_1-xZn_xTe by demonstration of a thin-film solar cell with open-circuit voltage ≥ 1.0 V.

Toward High-Efficiency n-Cadmium Selenide Telluride Solar Cells
The University of South Florida will develop alternative device architectures based on n-type CdTe/CdSe_xTe_1-x (CST) thin-film absorbers to create opportunities to overcome the efficiency limitations associated with the current state-of-the-art p-type CdTe/CST solar cells. The project aims to build on advances in n-CdTe/CST films that demonstrated group III and VII n-type doping for CdTe films. It will focus on the development of p-type heterojunction partners for n-CdTe/CST absorbers.

Round 2 Awards

Vapor-Assisted Group V Diffusion Doping Control in High-Efficiency CdSeTe Solar Cells
CdTe PV technology boasts a champion power conversion efficiency (PCE) of 22.3%, but it remains distant from its theoretical PCE of 31%. To address this gap and achieve 26% cell efficiency while reducing domestic CdTe module costs to 15 cents per watt by 2030, innovation is crucial. Group V doping has proven effective in enhancing CdTe device performance, improving both efficiency and stability. Arizona State University proposes a project that explores novel vapor-based ex situ group V doping, diffusion doping activation strategies, surface cleaning techniques, passivated back contact methods, and innovative device architectures. The goal is to develop higher-efficiency CdTe devices exceeding 22% by tailoring the group V vapor doping conditions to realize a fine control of the incorporation and activation of the dopants.

Optimizing Iodine-Doped CdTe for Potential n-Type Solar Cells
Washington State University will develop CdTe homojunctions using iodine-doped n-type CdTe absorbers that have high carrier concentration and minority carrier lifetime with 100% dopant activation. The team will apply a combination of defect spectroscopy techniques, optimized surface passivation techniques, and device architecture, and aim to overcome present performance limitations based on p-type absorbers.

Solution-Processed Buffer Layers for CdTe Solar Modules
The nexTC Corporation will use nexTC precursors to generate state-of-the-art buffer films that improve device performance. nexTC films exhibit ultra-smooth surfaces. They reduce surface texture/roughness and increase transmission by limiting optical haze, providing manufacturers with pristine surfaces for device manufacturing. In this project, the team will demonstrate the efficacy of solution processing to yield high-quality front-interface buffer/emitter layer films used in the CdTe market. They will demonstrate the ability to deposit compositions of commonly used materials and explore novel material compositions that are impossible to create via typical sputter deposition. nexTC will work with CTAC members to fabricate and prototype CdTe solar devices. This approach will accelerate the transition from ideation to high-volume manufacturing.

Round 3 Awards

Novel Transparent Hole Contact for Bifacial CdSeTe Thin-Film Solar Cells
Arizona State University aims to develop a novel transparent hole contact for CdSeTe solar cells based on the Fermi-level-pinning effect and the "remote junction" design to form a "p-n" junction with n-type absorber without any p-type doping in the device, overcoming a major challenge for CdSeTe material. This new technology also enables low-cost bifacial CdSeTe solar cells with improved power conversion efficiencies. Preliminary results show an open-circuit voltage (Voc) of 0.99 V and an efficiency of 16.4% without any antireflection coating for a monocrystalline cell. The optimal combination of a hole selective contact and a passivation layer will be used to demonstrate devices, which will be characterized via Voc, fill factor, and short-circuit current as a function of temperature and excitation.

Development of Low-Cost Photon Down-Conversion Nanomaterials for High-Efficiency CdTe Photovoltaic Devices
Bowling Green State University seeks to advance bifacial photovoltaic technology by increasing the efficiency of solar energy capture from the solar panel's back surface. The proposed approach will involve the synthesis of advanced photoluminescent nanomaterials that alter the sunlight’s spectral characteristics to better match the spectral sensitivity of the back panel. Such spectral modification of the incident light, achieved through a process known as down-conversion, is expected to increase electrical output of the cell while minimizing heat generation, which could lead to a 3% –7% increase in overall cell efficiency. The team plans to continue collaborating with CTAC members during this proposed program, exchanging scientific findings and samples, technological knowledge, and practical inventions.

Study of Electro-Optical Properties in Single Crystal CdSeTe With Varying Se and Group V Doping
Washington State University aims to provide fundamental knowledge to improve efficiency of CdSeTe-based thin film solar cells. CdSeTe, with up to 40% tellurium replaced by selenium, shows promise for large-scale production, but the open circuit voltage remains below the theoretical limit, hindering cell performance. The study will systematically investigate the role of selenium and p-type dopants in CdSeTe on properties using single crystal growth. Key tasks include growing crystals with varying Se composition (0, 10, 20, 40% Se) and group V dopants (P, As, Sb, Bi) and performing detailed material characterization. The goal is to understand the factors that reduce Voc and find ways to mitigate them, ultimately improving the solar cell's efficiency.

Improving CdTe Solar Cell Performance Through Glass Substrate Properties
The University of Delaware aims to understand how commercial glass properties affect CdTe device performance. The effects of coefficient of thermal expansion (CTE) mismatch between glass substrates and thin-film CdTe on material properties and solar cell device performance will be determined. Residual strain from CTE mismatch in CdTe cells, a possible source of performance-limiting defects, should be minimized, but has not been systematically quantified. CdTe cells will be processed on soda-lime glass substrates engineered with varying CTE. Film strain through CTE mismatch will be determined, and its effects on material and cell performance, formation of defects, and effects of post-deposition processing film strain will be evaluated. Improving initial CdTe film properties will reduce the necessity for high-temperature post-deposition. Improving device performance through engineering of commercial glass will reduce module manufacturing costs.

Round 1 Awards

Selective and Efficient Recovery of Tellurium From Copper Processing Streams
The Missouri University of Science and Technology will enhance tellurium recovery from copper processing by optimizing the current operations to capture the tellurium, gold, and silver that are presently lost to tails. The scope of work involves advanced mineralogical analysis of different processing streams of the flotation circuit of copper processing ores to identify tellurium carriers and modes of occurrence (i.e., tellurium in the crystal lattice vs. tellurium-rich inclusions in larger minerals); evaluation of different approaches and flow sheet options for enhanced separation of tellurium, silver, and gold minerals from processing streams of copper processing ores; and techno-economic assessment to estimate the capital and operating costs of the developed flow sheets for successful implementation, which could increase the domestic production of tellurium from copper processing ores by at least 50%.

Round 2 Awards

Identifying High-Potential Areas for Tellurium Extraction Within Existing Base and Precious Metal Supply Chains
Tellurium is a key component of the manufacturing of CdTe systems required to increase the domestic renewable energy generation capacity of the United States. However, supplies of tellurium are insecure, with the United States being significantly import reliant despite the fact that domestic mining and smelting already involves tellurium-bearing ores. This project from the University of Nevada, Reno will assess the tellurium extraction potential of existing mining supply chains, providing baseline data for the targeting of high-priority areas for enhanced tellurium extraction. This will increase sustainability and ensure secure supplies of this critical commodity for U.S. industry.

Round 1 Awards

3D In Situ Correlative X-Ray Studies of Defect Chemistry, Structure, and Electrical Performance During Dopant Activation
Arizona State University will combine the power of hard X-ray microscopy (XRM) and soft X-ray and electron spectroscopies to probe arsenic-doped CdSeTe absorbers and devices. XRM will probe the chemical distribution, atomic environment, and current collection at the nanoscale for the arsenic and selenium absorption edges. Electron and soft X-ray spectroscopies will enable an area-integrating determination of the electronic structure at surfaces (band edges, surface bandgap) and interfaces (band alignment), in addition to the chemical bonding environment of the sulfur, chlorine, and oxygen in the device. The team is tackling two main questions: How do the chemical states of arsenic (and neighboring atoms) evolve between initial deposition and post-activation? What stressors and processes enhance or prevent activation of arsenic dopants?

Microcontact Arrays Measuring Local Carrier Transport in Cadmium Telluride Solar Cells
The University of Utah will assess the role of microstructures in advanced CdTe devices. The goal is to improve the limiting open-circuit voltage while retaining the maximum values of short-circuit current and fill factor for CdTe solar cells by developing a novel architecture built on a comprehensive understanding of local carrier dynamics. It will investigate the interfacial and microstructural characteristics of advanced CdTe (CdSe_(1-x)Te_x) passivated emitter and rear contact (PERC) solar cells. A microcontact array platform with tunable pattern geometry will enable measurements of global (patterned CdTe PERC) and local carrier transport, delineating the contribution of grain bulk and grain boundaries to overall PV performance. Using complementary electron/optical microscopy, it will correlate the transport characteristics to the microstructural properties of each sample set (e.g., group-V-doped vs. copper-doped CdTe PERCs).

Round 2 Awards

Toward Automated Atomic-Resolution Scanning Transmission Electron Microscopy and Machine Learning for Achieving High-Efficiency Cd(Se)Te Solar-Cell Devices
The University of Illinois Chicago team will develop and utilize novel materials characterization and modeling approaches to determine the atomic-scale barriers that currently limit the conversion efficiency of polycrystalline CdTe solar cell devices to <23%. By combining advanced machine learning approaches with state-of-the-art electron microscopy, the team will study the role that grain boundaries, hetero-interfaces, and defects have on the carrier lifetime and durability of the Cd(Se)Te materials. 4D scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy-loss spectroscopy will be used to quantify the local atomic and electronic structures of Cd(Se)Te bulk, interfaces, and defects. Autonomous anomaly detection approaches will be developed using machine learning to increase the field of view and sensitivity of current electron microscopy methods. Insights resulting from this project will enable the development of CdSeTe-based devices with efficiencies exceeding 25%.

Round 3 Awards

Correlative X-Ray Transient Spectroscopy: Bridging the Gap Between Structural Defects and Their Energy Levels
Arizona State University seeks to elucidate the origin of lifetime limiting defects in group V-doped CdSeTe by connecting one-to-one the recombination center properties, energy and capture cross section, measured by x-ray transient spectroscopy with specific atomic species, measured by X-ray absorption. By comparing the behavior of single crystal and polycrystalline CdSeTe samples doped with the same group V atom, we seek to understand the defects that directly impact open-circuit voltage, the role of compensation, and the metastability of the devices. By statistically mapping inhomogeneities and defects and assigning defect levels to the most prolific structural defects, we look to enable high-fidelity, industrially relevant modelling of activation processes and a path to push performance beyond 25%.

Single Crystal Growth of Arsenic Doped and Phosphorus Doped Cadmium Selenium Telluride (CdSexTe1-x) for Fundamental Material Characterization To Improve CdTe Photovoltaics
Radiation Detection Technologies Inc. aims to grow single-crystal CdSeTe for fundamental material studies. The present record cell level efficiency of 22.3% is only about 70% of the theoretical maximum efficiency of ~32%. To increase cell efficiency, the open-circuit voltage must be increased beyond ~850mV. It is estimated that a CdSeTe absorber doped with P or As, with net acceptor concentration >10¹⁶ cm^-3 with majority of the added dopants (>50%) activated, can potentially reduce the voltage deficit. Radiation Detection Technologies Inc. will lead a study where single crystals of CdSe_xTe_1-x doped with P and As will be grown from Cd-rich solution, using the already available infrastructure currently used to manufacture group-V doped CdTe. Single crystal samples will be processed, prepared, and delivered to First Solar and Arizona State University for fundamental studies to understand performance limitations.

Understanding Atomistic Defect Structures and Complexes Related to Group-V Doping in CdSeTe
Purdue University aims to compute a defect library for CdSeTe that will assist with the interpretation of experimental data and enable defect engineering to increase device peformance. Improving the power conversion efficiency of CdTe solar cells requires a deeper understanding of the behavior of group V dopants and their resulting defect structures. By identifying relevant defect configurations and conditions to stabilize or destabilize them within CdSe_1-xTe_x compositions, it would be possible to select suitable growth conditions and desired extent of Se alloying to enhance minority carrier lifetimes and consequently improve photovoltaic performance. In this work, we propose high-throughput density functional theory computations applied in conjunction with systematic atomic distortions on hypothetical structures containing single or multiple defects related to group V doping, leading to a library of low energy defect structures, which will help interpret experimentally measured spectra. This project will enable a comprehensive understanding of how Se composition and defect compensation play a role in the stability and electronic signatures of group V dopants and yield optimal conditions for dopability and enhanced photovoltaic performance.

Rapid Analysis of Local Voltage Loss in CdTe Solar Cells
University of Michigan aims to quantify microscale electrical inhomogeneity in state-of-the-art CdTe solar cells and use it to partition macroscale cell-level voltage loss. Low open-circuit voltage (Voc) is widely recognized as the biggest loss for CdTe solar cells, and thus also the biggest opportunity to improve their efficiency in the near future. The majority of this loss stems from nonradiative recombination that varies widely within grains, at grain boundaries, and at contacts. How all of these microscale voltage losses translate to the macroscale, cell-level voltage loss is, however, difficult to assess on a routine basis. This project will quantify microscale electrical inhomogeneity in state-of-the-art CdTe solar cells and use it to partition macroscale cell-level voltage loss. Time and frequency domain transient luminescence microscopy will be used to map the local nonradiative recombination rate and show that the statistical moments of the recombination rate distribution can be quickly determined using macroscale modulation electroluminescence spectroscopy in routine device testing. This work will support CdTe solar cell development by providing a new characterization tool that can quickly quantify the distribution of nonradiative recombination rates in these devices and thereby help to assess the effectiveness of new material compositions and processing conditions designed to increase Voc.

Round 2 Awards

Ultra-Thin High-Efficiency CdTe/MgCdTe Double-Heterostructure Solar Cells With Light-Trapping Features
Arizona State University aims to develop a model system to demonstrate ultra-thin monocrystal CdTe solar cells with an efficiency potentially reaching 28% and to better understand the challenges that polycrystalline CdTe thin-film solar cells face. The impact of this model system is beyond the demonstration of solar cells with high efficiencies; it also helps the CdTe solar cell community to address several critical issues, such as the optimization of both contacts and associated interfaces, the optimal passivation of the grain boundaries, and the development of ultra-thin absorbers integrated with light-trapping features. The team will continue collaborating with NREL and other CTAC members during this proposed program to exchange scientific findings and samples, technological knowledge, and practical inventions.

Innovative High-Voltage CdSe Solar Cells
In this project, the Iowa State University team will investigate high-performance devices in CdSe, a new material for making tandem junction solar cells with Cd(Se,Te) acting as the lower gap cell. Simulations show that theoretical solar conversion efficiencies approaching 40% are possible using this combination of materials. Both material systems are capable of low-cost vacuum deposition techniques. The team will make novel device structures using appropriate inorganic heterojunctions to achieve high voltage and efficiency in CdSe.

CdTe is the second most common PV technology in the world, after silicon. The thin-film technology can be made more cheaply than silicon solar panels and has been shown to have a 22.1% efficiency in converting sunlight into electricity. CdTe is one of the best performing and most reliable thin-film technologies in large-scale commercial production.

Although CdTe efficiency rates have risen significantly and costs have continued to decline, there is still progress that can be made in ensuring U.S. leadership in this innovative technology.

Once selected, the consortium leadership is expected to:

• Develop a CdTe technology road map
  • Create and annually update a technology road map to maintain U.S. technology leadership in CdTe PV
  • Conduct stakeholder engagement activities when developing and updating the road map
    
    
• Conduct research projects and programs
  • Develop and launch research projects within consortium leadership institutions and in collaboration with other institutions to meet the targets set within the technology road map
    
    
• Assess the domestic CdTe supply chain
  • Regularly assess the state of the U.S. CdTe manufacturing supply chain and identify any critical material or capacity constraints
  • Determine whether opportunities exist to expand and enhance the U.S. manufacturing base or to otherwise increase the domestic content of CdTe PV systems
  • Identify technology transfer opportunities and conduct feasibility analysis of new technologies.

The CdTe Accelerator program will allow NREL to act as a resource and support structure for the consortium leadership institutions, including but not limited to the following activities:

• Identify the consortium leadership through an initial solicitation
  • Competitively select a team of companies and research institutions with strong technology development, transfer, and validation capabilities that can impact the domestic CdTe manufacturing base
    
    
• Support the solicitation and launch of new projects
  • Administer additional solicitations on behalf of the consortium to meet the targets set by its technology road map.
    
    
• Conduct internal research and analysis in support of the consortium
  • Conduct applied research to support the goals of the consortium
  • Perform strategic analysis of the U.S. supply chain
  • Act as a business development resource and stakeholder outreach network to augment consortium activities.

US-MAC and CTAC are both dedicated to strengthening U.S. leadership in manufacturing CdTe.

US-MAC is an ad hoc organization that consists of key universities, companies, and national laboratories that believe that CdTe has opportunity to improve and grow. Launched in 2019, US-MAC aims to mobilize and grow the CdTe PV community, advocate for collaboration and resources, improve performance, reduce manufacturing costs, diversify product applications, increase U.S. production, and enhance U.S. national energy security.

CTAC is a 3-year DOE SETO-funded consortium that was launched in 2022. Members conduct research to advance CdTe technology. CTAC members were selected through a competitive solicitation process, while US-MAC continues community building, advocacy, and education. CTAC and US-MAC work together, in different ways, to enhance the impact of CdTe-based PV in our domestic energy supply.

Some of the participating organizations have leadership positions in both CTAC and US-MAC. University of Toledo, First Solar, Colorado State University, and NREL were founders of US-MAC, and First Solar was elected by US-MAC membership to the first Chair of US-MAC's Industrial Advisory Board.

University of Toledo, Colorado State, First Solar, and two other companies, Toledo Solar and Sivananthan Laboratories, collaborated to submit the proposal that was awarded to establish CTAC.

NREL holds a program management and supporting role in CTAC. CTAC gathers new members as projects are awarded by NREL through periodic requests for proposals (RFPs).