Low Cost, High Efficiency Tandem Silicon Solar Cells

Berkeley Lab's solar cell design incorporating the low-resistance tunnel junction. The tunnel junction is between the middle two materials.

Inexpensive tandem silicon solar cells

- Boosts silicon solar cell efficiency without significant cost increases
- Simplifies current methods to design and manufacture tandem solar cells
- Leverages well-known and inexpensive silicon technology and materials
– can be used with multi or single crystalline silicon

Wladek Walukiewicz, Joel Ager, and Kin Man Yu of Berkeley Lab have developed high-efficiency solar cells that leverage the well-established design and manufacturing technology of silicon cells while delivering the performance previously achievable only by far more complex and expensive tandem solar cells. The Berkeley Lab tandem solar cells are composed, in one configuration, of silicon and indium gallium nitride, creating a natural near-zero resistance tunnel junction without requiring doping.

The power conversion efficiency of multi-crystalline silicon solar cells is currently about 15 percent and that of the best single crystalline cells is 23 percent. The Berkeley Lab tandem silicon solar cells promise power conversion efficiencies of up to 35 percent, with manufacturing costs that are not significantly higher than those of simple multi or single crystalline silicon cells. While three junction non-silicon tandem solar cells have achieved unconcentrated efficiencies of up to 33%, the costs are prohibitive for wide-spread adoption.

Tandem cells achieve higher efficiencies because each semiconductor material is sensitive to a different part of the solar spectrum. Light that passes through the first layer is absorbed by the second layer, so that more of the solar spectrum is utilized to produce electricity. The challenge in tandem cell design is finding two semiconductors whose crystal lattices match up well and whose energy band gaps will lead to efficient power conversion. Also, ideally, the conduction band of the top layer of the cell should have about the same energy as the valence band of the bottom layer; this allows the electrons that have been energized by sunlight in the top semiconductor to pass easily from the conduction band to holes in the crystal lattice of the bottom semiconductor (the valence band) where they are again energized by sunlight of a different wavelength. In this way, the two parts of the cell work together, like two batteries connected in series, and have a total efficiency equal to the sum of the efficiencies of the two cells. However, if the bands are not aligned properly at the junction, the resulting electrical resistance causes a loss of power as electrons flow across.

The usual solution to this problem is to heavily dope the junction to improve tunneling transport, but this adds process steps to the fabrication of the cell, and increases the complexity of the design. The key breakthrough of the Berkeley Lab scientists is a low-resistance tunnel junction at the interface between an indium gallium nitride (InGaN) top layer and silicon bottom layer. This junction provides near-zero electrical resistance, allowing almost all the electric current generated on the solar cell’s top layer to flow to the bottom layer, and producing a total current nearly equal to the sum of the currents generated in each layer.

The Berkeley Lab approach is doubly effective: It not only delivers high performance, but also greatly simplifies the design of tandem solar cells by eliminating the need for heavy doping.

Attached files:

Inventor(s): Wladek Walukiewicz, Joel Ager, and Kin Man Yu

Type of Offer: Licensing

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