Solar Foundational Program to Advance Cell Efficiency
Eighteen solar research projects are working to create the technical foundation for significant increases in photovoltaic (PV) efficiency. These projects are:
- Identifying cost and efficiency barriers
- Researching PV sub-cell materials and processes
- Training a new generation of researchers to lead future technology development.
On Sept. 1, 2011, the U.S. Department of Energy (DOE) announced $35.8 million for these projects over a 36-month performance period. Combining both the technical and funding resources of DOE and the National Science Foundation, this research investment aims to eliminate the significant gap between the efficiencies of prototype cells achieved in the laboratory and the efficiencies of cells produced on manufacturing lines.
Arizona State University ($1,500,000)
The objective of this project is to integrate a new surface passivation technique into crystalline-Si solar cells for significant efficiency improvements and cost reductions. In collaboration with Arizona State University, this project is substituting the silver finger electrode, which is the biggest cost contributor in cell fabrication, with an earth-abundant metal. The researchers are also investigating the passivation of dangling bonds at grain boundaries with the valence-mending approach. The goals are to narrow the efficiency gap to less than 1% between multicrystalline-Si and monocrystalline-Si solar cells, reduce cell costs by $0.10/W, and make c-Si cells ready for terawatt-scale deployment.
Astro Watt ($1,500,000)
This project builds on the semiconductor on metal (SOM) technology that provides a cost-effective means of producing large-area thin crystalline silicon (Si) PV cells. This research is working to enhance the efficiency of SOM cells from 12.5% to 20% by incorporating advanced surface passivation techniques and lambertian light trapping using metal backed SOM, while taking advantage of the inherent higher open-circuit voltage (Voc) enabled by thin crystalline silicon. The goal is to demonstrate a manufacturable PV technology that can achieve production costs of less than $0.30/W on a cell and less than $0.50/W at the module level.
Colorado State University ($1,500,000)
Fort Collins, Colorado
The goal of this research is to improve the relative efficiency of cadmium telluride (CdTe) solar cells used in commercial modules by about 30% without increasing manufacturing costs. This is being accomplished by modifying the basic cell structure for higher voltage and fill factor and incorporating plasma processes for final substrate cleaning and modification of the CdS window for increased band gap (higher current). Combined with modest decreases in production costs and anticipated advances elsewhere, this effort corresponds to a reduction of the reported $0.75/W module cost to the $0.50/W target.
Georgia Institute of Technology ($1,500,000)
This research projects uses novel materials and technologies, such as 50 μm epi wafers, ion implantation, and light trapping, to raise the efficiency of Si solar cells and modules. The resulting commercial-ready product would represent an approximately 4% increase in efficiency, a 50%–70% reduction in cell thickness compared to industry standard wafers, and a 40% decrease in the requirement for silver (Ag). Given these improvements, a levelized cost of electricity reduction to approximately $0.05/kWh is possible.
National Renewable Energy Laboratory ($6,240,942)
This focused team effort is providing near- and mid-term research to advance thin-film competitiveness via improved module performance, cost of kilowatt-hour produced, and reliability of thin-film PV technologies. The project is enhancing the buffer and transparent conducting oxide layers and broadening the approach to processing copper indium gallium diselenide (Cu(InGa)Se2 or CIGS) cells. Targeted metrics for a successful conclusion to this project include the demonstration by the industry partners of a greater than 16%-efficient commercial module and a $0.23/W reduction in cost of module manufacturing through efficiency improvements.
National Renewable Energy Laboratory ($3,500,000)
A partnership between NREL, Spectrolab, and the University of California at Berkeley is tackling key barriers to reducing the cost of concentrating photovoltaics (CPV) modules and systems. Higher efficiencies reduce cost, especially at the system level, so this project team is working to increase CPV module efficiencies from approximately 27% to 35% by approaching the theoretical limits of cell photovoltage in gallium arenside (GaAs) cells.
National Renewable Energy Laboratory ($1,500,000)
NREL, Corning Incorporated, and the Colorado School of Mines are developing highly efficient CdTe devices on flexible glass superstrates using high-temperature roll-to-roll processing. This approach combines the high deposition rates available in the existing CdTe manufacturing technologies with the throughput and capital equipment cost reductions offered by roll-to-roll processing and the balance-of-system savings enabled by flexible PV. The processing techniques explored here have the potential for high efficiencies up to 18% and module costs below $0.50/W.
North Carolina State University ($1,000,000)
Raleigh, North Carolina
The best opportunity for improving cost efficiency in CPV systems lies in the ability to operate at higher solar energy concentrations. In collaboration with Spectrolab, this project is designing a tunnel junction between the top and middle cells of an indium gallium phosphide/gallium arsenide/germanium (InGaP/GaAs/Ge) triple junction structure that is functional at a concentration of 2,000 suns. The researchers are growing the tunnel junction from aluminum gallium arsenide (AlGaAs) and InGaP while using very thin aluminum gallium arsenide (AlGaAs) layers to inhibit diffusion. Cost savings will result from the ability to use cheaper, lower efficiency cells while maintaining the same power output.
Oak Ridge National Laboratory ($1,500,000)
Oak Ridge, Tennessee
This effort aims to develop technologies that can overcome the open circuit voltage (Voc) limitations in cadmium telluride solar cells by understanding and controlling the grain boundaries (i.e., the bounding surface between crystals). The research team is working to improve minority carrier lifetimes from less than a few nanoseconds (ns) to more than 10 ns, which will lead the enhancement of Voc from 840 mV to more than 1 V.
Ohio State University ($1,500,000)
This project creates a paradigm shift for CPV technology, which is currently constrained by germanium Ge and GaAs substrates in terms of cost and efficiency barriers. The integration of GaAsP and Ga-rich GaInP based sub-cells with a Si substrate overcomes both constraints. The coupling of this efficiency advantage and the roughly 100-time reduction in substrate cost per area yields a path to achieve panels at $0.50/W and provides access to the economies of scale afforded by a Si wafer based manufacturing infrastructure.
Old Dominion University ($1,117,402)
Old Dominion University, in collaboration with The University of Toledo and the University of Illinois Urbana Champaign, are developing a high-throughput and high-efficiency CIGS solar cell fabrication process via the use of an ultra-thin absorber layer, optical confinement and (Cd, CBD)-free heterojunction partner. These new designs and processes aim to reduce CIGS material costs by a factor of ten, multiply throughput production by at least of factor of five, and achieve module costs of less than $0.44/W for efficiencies higher than 20%.
Solar World Industries America ($4,636,633)
This comprehensive project team is researching the use of laser processing techniques to develop advanced device architectures leading to thin-silicon PV cells with higher efficiencies. The team aims to achieve efficiencies above 20% via a simplified hybrid metal-wrap-through (MWT) and laser-fired-contact (LFC) solar cell design built on ultra-thin, n-type mono-crystalline silicon substrates and employing a co-extrusion front and rear metallization scheme. The end result will be a vetted engineering design for a pilot tool capable of a high-degree of process uniformity at volume manufacturing throughput rates.
University of Delaware ($3,300,000)
This project is creating a high-efficiency crystalline silicon cell that overcomes the dominant module cost barriers by using thin-silicon wafers produced via high-speed laser processing. The research effort is guided by a multi-disciplinary team of PV and laser experts from the Institute of Energy Conversion at University of Delaware (solar cell design and processing), Massachusetts Institute of Technology (Si defect characterization and engineering), and JP Sercel Associates (laser processing). It is estimated that combined cost reduction for the proposed device architecture utilizing thin wafers and simplified module assembly will result in a module cost of less than $0.50/W.
University of Delaware ($1,167,147)
To reduce the cost of Cu(InGa)Se2 manufacturing, the research team is halving the thickness of the absorber layer. Project efforts include developing the technology and underlying science for using a superstrate cell configuration, which is essentially an upside-down version of the conventional Cu(InGa)Se2 configuration. The anticipated outcome of the project is a CIGS solar cell with 0.7 µm thickness and 17% efficiency.
University of Delaware ($1,200,000)
This project seeks to improve the manufacturability of Cu(InGa)Se2-based photovoltaic modules by advancing the precursor reaction approach currently employed by a number of commercial entities. In partnership with the University of Florida, the researchers are investigating pathways to improved module performance and yield by modifying the interface between the Cu(InGa)(SeS)2 absorber layer and molybdenum (Mo) back contact, and increased throughput by precursor structures for rapid (<5 minute) processing. This approach can also allow for the omission of toxic hydrogen selenide (H2Se) gas from the manufacturing process.
University of Delaware ($960,000)
Together with the University of Illinois at Urbana-Champaign, this project strives to find pathways through which sodium (Na) improves the performance of CIGS-based PV devices. The researchers are growing CIGS films with different amounts of Na for full characterization. They are also fabricating devices to highlight correlations of operation, Na dosage, and the method of incorporation. This investigation helps provide a basic scientific and engineering understanding of the role of Na in CIGS PV that is needed for advancement toward the $0.50/W module manufacturing goal.
University of Illinois at Urbana-Champaign ($1,192,250)
This project is developing new materials for contacts used in CdTe PV cells. Such contacts are used on the front and back of solar cells to allow current to flow to an external circuit. The new materials will reduce the back-contact barrier to zero and produce an ohmic contact, which increases the current flow and thus the efficiency of the PV cell. These advances are expected to increase device performance and process flexibility, providing lowered costs and greater yields.
University of South Florida ($987,717)
Thin film CdTe is today's lowest cost PV technology, but the open-circuit voltage of these solar cells has stayed at 0.85 volts (V) for nearly 20 years. In order to reach the practical potential of thin film CdTe cells, the open-circuit voltage of the cell must increase to about 1 V (about 2/3 of the material's bandgap). The main goal of this project is to demonstrate that CdTe films doping concentrations greater than 1016 cm-3 and lifetimes in the range of 1-10 ns can be prepared, which is consistent with Voc of 1 V. Increasing Voc to this level paves the way toward reaching 20% cell and 17% module efficiencies.
DOE is funding these projects to reach the aggressive goals of the SunShot Initiative.