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Silicon Materials and Devices R&D

Graphic of three layers. The bottom layer, called inexpensive substrate, is white. Middle dark blue layer is called the seed. Top light blue layer has the text epi c-Si absorber.

Schematic diagram of the film crystal silicon solar cell. A high-quality crystal silicon absorber is grown epitaxially on a seed layer applied to an inexpensive foreign substrate (e.g., display glass or rolled metal foil). At NREL, we are developing various emitter, back-surface field, and light-trapping strategies.

NREL has world-leading research capabilities and expertise in silicon materials and devices, especially photovoltaic cells. We focus on basic and applied research for high conversion efficiency at low cost.

Solar cells based on silicon (Si) semiconductors account for nearly 90% of 2011 sales of photovoltaic (PV) products. Annual production of Si-based PV in 2011 reached more than 15 gigawatts—an order of magnitude higher than other PV technologies. Silicon in PV takes many forms, including the industry-dominant single-crystalline (c-Si) and multicrystalline wafers sawn from ingots; melt-grown ribbons; thin hydrogenated amorphous silicon (a-Si:H); and microcrystalline Si layers grown from gaseous precursors. Many improved technologies are on the horizon, including thin c-Si by epitaxial growth, crystallized polysilicon layers, and nanoscale Si.

Our researchers in the Silicon Materials and Devices Group within the National Center for Photovoltaics (NCPV) conduct leading research in many of these areas, license our technologies to industry, and work with the U.S. Si photovoltaic industry. We help many start-up PV companies with technology issues and evaluate their materials and solar cells. We are currently researching high-efficiency multijunction Si solar cells and pioneering growth of thin-film crystalline Si on a low-cost "seeded" substrate. Here we aim for an efficiency comparable to c-Si at the cost of thin films (see figure).

These additional topics are summarized below:

We Address R&D Challenges

The Silicon Group pursues basic and applied research on silicon materials and devices to improve photovoltaic conversion efficiencies and to reduce costs. Our recent world-leading achievements include:

  • 19.3% p-wafer heterojunction solar cells
  • 18.2% black silicon solar cells with no antireflection coating
  • 1.8 µ/min hot-wire chemical vapor deposition (CVD) epitaxy of Si at about 700°C.

Our Si basic research includes:

  • Mechanism of silicon film growth and epitaxy
  • Hydrogen diffusion and hydrogenation of defects
  • Transport of carriers across the a-Si/c-Si heterojunction interface
  • Light-induced metastable degradation in a-Si:H
  • Nanoparticle silicon production and electronic properties.

Our applied R&D includes:

  • Novel approaches to high-efficiency Si PV
  • Advanced silicon heterojunction PV cells
  • Novel thin crystal silicon PV on inexpensive substrates
  • Light trapping for thin Si PV
  • New combinatorial approaches to device optimization
  • Crystal growth for silicon feedstock evaluation.

We Have Deep and Broad Expertise from 30 years of Pioneering R&D

  • Pioneered the study of solar-grade Si feedstock
  • Developed high-rate amorphous Si
  • Developed the hot-wire CVD process
  • Understood and mitigated the Staebler-Wronski effect in a-Si
  • Analyzed and modeled solar cell device physics. Devised new Si devices that can be manufactured inexpensively
  • Revealed critical factors for novel Si quantum dot PV

We Have a Suite of Tools and Capabilities

Our tools and capabilities include:

  • Silicon cluster tool for large-area (156-mm x 156-mm) samples. This tool comprises eight process chambers for a-Si-based intrinsic, doped, and alloyed thin films, transparent conducting oxides
  • Wafer-replacement tool for 156-mm x 156-mm samples. This tool includes epitaxial process chambers, an in situ oxide etching chamber, and film Si chamber for thin crystalline Si materials and devices.
  • Hot-wire CVD with in situ ellipsometry for a-Si:H-based materials, epitaxial-Si, and devices
  • Czochralski (CZ) puller (with a maximum load of 5 kg) for Si feedstock testing
  • Hydrogenation and rapid thermal process system
  • Indium tin oxide film deposition system
  • Dicing saw
  • Clean room and diffusion furnace with developed processes for conventional c-Si solar cell processing
  • Cell and materials testing techniques, including: current-voltage and quantum efficiency, Sinton lifetime, Suns-Voc, photothermal deflection spectroscopy and constant photocurrent absorption, conductivity and activation energy, transient capacitance and capacitance-voltage, and fast reflection and transmission measurements of optical properties.

We Partner with Industry

We transfer our technological advances to photovoltaic industry partners through licensing and high-value cooperative research and development agreements (CRADAs). Our preferred criteria for CRADA projects are the potential for 1) significantly impacting the industry, 2) advancing the technology, and 3) strengthening our core competencies.

We also help launch start-up companies. For example, Ampulse is a start-up company based on epitaxial Si technology developed at NREL and crystalline substrate technology provided by Oak Ridge National Laboratory. Tau Science licenses and sells our ultrafast technique for measuring quantum efficiency.

We also collaborate with and provide technical services to numerous other university and industry partners.

Working with Us

Visit Working with Us to learn more about NREL's PV partnership opportunities. Contact us for specific information on NREL's R&D in the area of Si materials and devices.