Silicon Materials and Devices R&D
The National Center for Photovoltaics (NCPV) at NREL has world-leading research capabilities and expertise in silicon (Si) materials and devices, especially for photovoltaic (PV) cell applications.
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We have expertise in the following:
- n-type monocrystalline-Si cells and the processes that enable them: state-of-the-art deep B emitters, surface passivation for B-doped and P-doped surfaces and wafers (atomic layer deposition Al2O3, SiNx, SiO2), plated metal grids, 20%-efficient n-PERT cell, 21.5%-efficient front B emitter + back-passivated tunneling contact cell, NiSi barriers for Cu contacts
- Thin-film Si: amorphous-Si:H, nanocrystalline-Si:H, epitaxial Si on seed templates
- Amorphous-Si:H heterojunction cells
- "Black" Si and porous Si processes for PV applications
- Passivated tunneling contacts based on thin tunneling SiO2 with polycrystalline-Si and transparent conducting oxide (TCO) layers
- Czochralski (Cz) Si crystal growth
- Tabula Rasa process to mitigate bulk degradation in n-Cz Si due to oxygen precipitation
- Tandems with Si bottom cell—by direct III-V growth on Si, by TCO-mediated wafer bonding, and by mechanical stacking of III-V and Si cells.
- Novel conductive and transparent adhesives for solar cells and other device applications
- In-depth studies of metallic paste firing on atomic level in real time (with SLAC National Accelerator Laboratory)
- Defects in mono- and multicrystalline cells.
Current Research Areas
The three primary research areas that are currently our focus are the following:
Bulk Cz silicon and the science of oxygen precipitation and its mitigation. We are studying defects that are produced during growth and processing of n-type Cz and novel silicon. The goal is to be able to mitigate the deleterious impact of these defects to produce cells that have bulk carrier lifetimes exceeding 4 milliseconds after all cell process steps. In this activity, we are continuing to improve and understand our Tabula Rasa process for PV n-Cz wafers that suppresses oxygen precipitates, which getter impurity metals, form deep-level recombination sites during high-temperature processing steps, and lower lifetimes in the wafer.
Passivated contacts, dopant patterning, and advanced metallization. We are developing interdigitated back-contact (IBC) passivated contacts that exhibit high performance—that is, open-circuit voltages greater than 700 millivolts. Our research focuses on the physics and engineering of passivated contact structures; this involves density functional theory (DFT) and analytical microscopy, as well. Work on interfaces probes passivation, morphology, stability, and transport mechanisms. In the area of dopant patterning, we are pursuing ion implantation and other novel techniques such as ink-jet printing and the use of nanoparticles.
Industrially relevant interdigitated back-contact solar cells on n-Cz Si wafers. Our goal is to develop an IBC passivated contact cell that has a conversion efficiency above 23% at a cost consistent with the levelized cost of energy goals set within the Department of Energy's SunShot Initiative. Our approach is to draw from the advances in the bulk Cz Si and contact work described above, which will include implementing the Tabula Rasa process, ion implantation, patterned doping, and other innovations from the NCPV and our partners in universities and industry.
The IBC architecture holds the top three world records for cell efficiency. Currently, it represents only a small fraction of the Si module market due to process complexity and high production costs, but is predicted to grow exponentially within the next decade. Our research seeks to greatly reduce the cost of IBC cells, while maintaining high efficiencies, by using an innovative cell architecture, and employing high-throughput ion implantation and other forms of dopant patterning to form passivated contacts in conjunction with low-cost metallization strategies.
Tools and Capabilities
Our tools and capabilities available for R&D in silicon materials and devices include the following:
A new 20,000 sq. ft. cleanroom, available from the fall of 2016. It will house many new 156-mm × 156-mm wafer-compatible tools, including an automated wet-process station and diffusion furnace. This new facility will allow reproducible processing of high-efficiency cells and collaboration with industry partners and other research laboratories.
Silicon cluster tool for large-area (156-mm × 156-mm) samples. This tool comprises eight process chambers for plasma-enhanced chemical vapor deposition of amorphous-Si-based intrinsic, doped, alloyed thin films, Si nitride, and sputtering of transparent conducting oxides.
Clean diffusion/oxidation processes for Si wafers that preserve multiple milliseconds lifetime; wet chemistries in the cleanroom that enable efficient Si cells.
Czochralski puller for Si feedstock testing, with a maximum load of 5 kilograms.
Cell and materials testing techniques, including: current-voltage and quantum efficiency, Sinton lifetime, Suns-Voc, photoluminescence, electroluminescence, secondary ion mass spectrometry, transmission electron microscopy, X-ray diffraction, Rutherford back scattering, lock-in thermography, photothermal deflection spectroscopy and constant photocurrent absorption, Raman and photoluminescence mapping, conductivity and activation energy, transient capacitance and capacitance-voltage, ellipsometry, and fast-reflection and transmission measurements of optical properties.
Our projects involve analytical microscopy and theory. This allows us to use high-resolution transmission electron microscopy, time-of-flight secondary-ion mass spectrometry, electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and other techniques for the Si project, as well as state-of-the-art DFT simulations of interfaces, contacts, and new materials.
Inkjet printing of metallic, nanoparticle, and dopant inks, with express evaluation by X-ray fluorescence and X-ray diffraction.
Defect visualization etching and mapping in multi- and mono-Si cells, express texture evaluation, and thermal processing in clean optical and diffusion furnaces.
The current activities described above are funded by the DOE Energy Efficiency and Renewable Energy (EERE) Office through SuNLaMP (see "Overcoming Bottlenecks to Low-Cost High-Efficiency Si PV and Industrially Relevant, Ion Implanted, Interdigitated Back Passivated Contact Cell Development") and FPACE-II (with Georgia Institute of Technology and the Fraunhofer Institute for Solar Energy Systems).
In addition to our focus on developing high-efficiency, low-cost cells on n-Cz silicon, we also conduct research on various approaches to III-V/silicon tandem cells. This work is funded by DOE EERE through SuNLaMP (see "Mechanically Stacked Hybrid PV Tandems" and "Si-based Tandem Solar Cells"), Next Generation Photovoltaics (NextGen PV III), and DOE ARPA-E (see "Micro-Optical Tandem Luminescent Solar Concentrator").