Flexible crystalline ultra-thin silicon solar cells

High-yield, low-cost process re-purposes pre-existing thick, inflexible commercially available crystalline Si solar cells

Energy Environmental

A team of scientists from the U.S. Navy have recently invented a way to manufacture an ultra-thin, flexible solar cell from any suitable pre-existing thick, inflexible crystalline solar cell.

Dr. Woojun Yoon holds an ultrathin flexible crystalline silicon solar cell. (NRL photo)

The patented technology is available via patent license agreement to companies that would make, use, or sell it commercially.

Conventional manufacturing processes for making commercially available high-efficiency and large-area silicon (Si) solar cells depend heavily on the use of thick crystalline Si wafer (100 μm – 500 μm). Due to the rigid and brittle nature of such wafers, such crystalline Si solar cells are incompatible with ultra-thin and fully flexible form factors suitable for covering curved surfaces. In addition, most solar modules and panels consisting of thick crystalline Si solar cells require heavy front glass covers and aluminum frames to protect them from environmental factors making them difficult to integrate into light-weight packaging that enable tailored output optimized to power systems.

Yield losses and handling issues are a problem for traditional thin wafer processing and limits their usefulness. Conventional manufacturing methods for producing crystalline thin Si solar cells are based on epitaxial growth of monocrystalline Si layers onto a donor Si wafer followed by an epitaxial layer transfer process (LTP), requiring the use of expensive high-vacuum epitaxial tools at high temperature.

Used to power everything from space vehicles to our homes, there is a growing need for a new, affordable, and fast technique for manufacturing ultra-thin, flexible crystalline Si wafers. In response, Navy scientists have developed a low-cost technique to do just that, using previously fabricated inflexible crystalline Si solar cells.

First, a carrier substrate comprising a thin, flexible conductive foil, an adhesion layer, a metal base layer, and a metal interlayer is prepared. A stack of metal layers is coated onto the front side of an inflexible crystalline Si solar cell, where the stack serves as a bonding layer as well as an electrically conducting layer between the solar cell and the carrier substrate. The solar cell is then inverted and bonded to the carrier substrate, where the metal layers diffuse and combine to form a bonded layer. The front side is then ground to reach a total thickness that is about 40 μm above the final total thickness, followed by lapping until the final total thickness is achieved.

To improve chemical surface passivation, a thin tunneling layer can be formed on the front-side of the solar cell. Then a thin carrier-selective layer can be formed on the tunneling layer to improve field-effect surface passivation. In order to improve later conduction on the front side while allowing light absorption, a transparent conducting oxide (TCO) layer can be formed, followed by forming metal grid fingers.

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