Black phosphorus 
Solar energy is a huge booming field, from advanced proof-of-concept solar cars to solar panels you might find already on the roof of your house. Electrical energy from solar panels is harvested using a special device called a PN junction made from a semiconductor, typically silicon. While it is industrially mature technology, silicon has a major drawback: it absorbs only a small part of the solar spectrum. Michele Buscema, Dirk Groenendijk, Gary Steele, Herre van der Zant and Andres Castellanos-Gomez have now explored a promising new type of PN junction made from a new material consisting of 2D crystals of black phosphorus.

2D crystals
The field of 2D-crystals started in 2004 when Nobel Prize winner Andre Geim and co-workers isolated semi-metallic graphene flakes using simple scotch-tape methods. Recently, 2D crystal research has been expanding into flakes of materials that are semiconductors, which enable solar cell technology with these 2D flakes.

Bandgap
While such flakes have been used for PN junctions before, what is special about black phosphorus is its small bandgap of only 0,3 eV. This unique feature allows the Delft devices to absorb light at wavelengths far below that of silicon and other 2D crystals, allowing more of the spectrum of the sun to be converted into solar energy. They can also control the doping of the P and N regions of the device using local gates to optimize the performance, something not possible with silicon.

Efficiency
While the overall efficiency of the current devices needs to be improved drastically before they are competitive with existing technology, the scientists from Delft have demonstrated the potential of graphene-like 2D semiconductors for capturing a larger band of the solar energy spectrum.

Image: The dark region under the "P" and "N" labels is the black phoshorous crystal, and we use the gates (wide gold colored bands) to make one part of the crystal "P-type" and the other "N-type", forming the PN junction we can use for capturing solar energy.

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