Fellow 2010-11

Xiuling Li

Electrical & Computer Engineering

An All-silicon Nanowire Tandem Solar Cell for $1 per Watt Energy Conversion

The highest-efficiency solar cell demonstrated in the laboratory to date is 41.1 percent, using expensive III-V compound semiconductors. To gain higher levels of efficiency, while reducing the production cost of solar energy to parity with today’s market electricity, researchers are experimenting widely with various processes and substances. During her Center appointment, Professor Li will explore the feasibility of an all-silicon tandem solar cell that uses optically tunable silicon nanowire arrays.

Silicon is the second most abundant element in the world. The material is nontoxic and affordable, we already know a great deal about it, and there exists a large industrial infrastructure for its low-cost and high-yield production. Professor Li and colleagues have demonstrated that it is possible to control or tune the bandgaps of fabricated silicon nanowires. In her current project, she proposes to design various arrays and test the nanowires’ diameter, periodicity, length, orientation, and doping effect (i.e., intentionally adding impurities) to identify the highest rate of absorbing solar energy and the lowest rate of energy loss within the array.

She will fabricate the arrays using superionic solid state stamping (S4) and metal-assisted chemical etching (MacEtch), a versatile, inexpensive method that is scalable and allows a high throughput. The S4-MacEtch approach itself contributes to the efficiency/cost factor by enhancing absorption efficiency with nanostructured designs and sidestepping both (a) the bottom-up growth of nanowires, which requires sophisticated facilities and is difficult to scale and (b) the expensive lithographic equipment used in conventional top-down fabrication technologies.

Developing an all-silicon solar cell with low cost and high efficiency would represent a significant step forward in using solar power as a sustainable source of energy. The project also contributes to our fundamental understanding of the physical properties of nanostructured solar cells.