NATIONAL RENEWABLE ENERGY LABORATORY
NREL Awards $1.8 Million in Second Round of Contracts To Support Development of Cheaper, More Efficient Cadmium Telluride Solar Cells

CdTe solar cells are the second-most-common photovoltaic technology after silicon solar cells. CdTe solar cells rely on a thin film of material to absorb light and convert it into electricity. Photo by Dennis Schroeder, NREL

The National Renewable Energy Laboratory (NREL), on behalf of the U.S. Department of Energy Solar Energy Technologies Office (SETO), has awarded $1.8 million to fund seven projects to support the Cadmium Telluride Accelerator Consortium (CTAC).

Announced in August 2022, CTAC is a three-year consortium intended to accelerate the development of cadmium telluride (CdTe) technologies by lowering the cost and increasing the efficiency of these thin-film solar cells. The first round of awards for $2 million was announced in June 2023.

NREL released the most recent request for proposal for small projects in June 2023. This second round of awardees was announced in December 2023. These projects were selected for the latest round of the consortium:

Topic Area 1: High-Efficiency Devices

Vapor-Assisted Group V Diffusion Doping Control in High-Efficiency CdSeTe Solar Cells 

CdTe photovoltaic technology has shown 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, additional innovation is crucial. Group V doping has proven effective in enhancing CdTe device performance, improving both efficiency and stability. The proposed project by Arizona State University 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 are shown to 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 

This work by nexTC Corporation will use liquid-phase precursors to fabricate state-of-the-art buffer films that improve device performance. These 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.

Topic Area 2: Tellurium Supply

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 well-established large-scale domestic mining and smelting operations involving tellurium-bearing ores. This project by 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.

Topic Area 3: New Device Configurations and Processes

Ultra-Thin High-Efficiency CdTe/MgCdTe Double-Heterostructure Solar Cells With Light-Trapping Features 

Arizona State University's program 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 Cadmium Selenide (CdSe) Solar Cells

In this project, the Iowa State University team will investigate high-performance devices from CdSe, a new material for making top (large bandgap) solar cells for tandem junction solar cells with Cd(Se,Te) acting as the bottom (lower bandgap) cell of the tandem pair. 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 heterojunctions to achieve high voltage and efficiency in CdSe.

Topic Area 4: Characterization, Modeling, and Simulation

Towards 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 (ML) 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 to local atomic and electronic structures of Cd(Se)Te bulk, interfaces, and defects. ML approaches to autonomous anomaly detection will be developed 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%.

CTAC is funded by SETO. Learn more about CTAC.