Scientists working with Natcore Technology Inc. in the Rice University laboratories of Professor Andrew Barron, a Natcore co-founder, have successfully formed a heterojunction solar cell using germanium quantum dots on an ordinary n-type silicon wafer. Individual germanium quantum dots were coated with silicon dioxide (silica), doped to make them p-type, and then deposited, using Natcore's liquid phase deposition (LPD) process, on a commercial-grade silicon wafer. The LPD process was developed at Rice and is licensed exclusively to Natcore.
"Very simply put, we used our proprietary process to 'dope' silica-coated germanium quantum dots and arrange them in a silica film atop a commercial silicon wafer. We then put contacts on the coated wafer to create a cell, and exposed it to light. We obtained a net power out of the device," says Dr. Dennis Flood, Natcore's Chief Technology Officer and also a company co-founder.
Quantum-dot solar cells have the potential to be transformational for terrestrial solar energy, with efficiencies far above anything available commercially today. The advantage lies in the fact that by carefully controlling the size of the quantum dots, the cell can be "tuned" to capture energy from a specific spectrum of light. The portion of the spectrum not captured passes to the next layer below, where it can then be captured by either a specifically tuned lower quantum-dot cell or even an ordinary silicon cell.
Thus, using "multijunction" or "tandem" cells with two or more layers of quantum-dot cells, much more of the solar spectrum can be converted to energy. In contrast, current single junction solar cells are most efficient for only a limited portion of the solar spectrum. "To our knowledge, no one else has been able to successfully dope and arrange silicon or germanium quantum dots into layers using a process such as Natcore's, which appears to be well-suited for mass production," notes Flood.
Tandem solar cells are a proven technology in space applications. The major issue preventing their broad use in earth-based applications has been the need to use exotic semiconducting materials for the upper layers. The cell created in Dr. Barron's laboratory for Natcore uses relatively abundant and inexpensive germanium, with the coated quantum dot having been characterized as a "p-type" material.
This heterojunction cell, with p-type quantum dots on an n-type silicon wafer, is an important step toward a cell in which quantum dots are used to form both the p-type and n-type materials. Once this next step is achieved, it will open the door to potential ultra-high-efficiency, multi-junction solar cells.
"This is a truly exciting time for Natcore," says Chuck Provini, Natcore's president and CEO. "We are one step away — n-type quantum dots — from our ultimate goal in our quantum dot solar cell program.â€
