Nanostructured Photovoltaics

Improving the efficiency of solar cells is a paramount concern in the space photovoltaic community. Improved solar cell efficiency leads to reduced costs in both spacecraft deployment and overall mission cost. Current state of the art for ultra high efficiency InGaP/GaAs/Ge solar cells is over 29%. Unfortunately, there are a number of drawbacks to multi-junction solar cells. In particular, the fact that multiple junctions are needed cause additional complexity for epitaxial growth, such as the need for lattice matching each layer while also keeping the interface quality high and maintaining good material quality. In addition, it is necessary to grow tandem cells connected in series, thus current must be matched between each successive junction. This leads to reduced currents, higher spectral sensitivity and greater device complexity.

Recent proposals have pointed to alternate approaches to improving cell efficiency using low dimensionality nanostructured materials. One approach involves insertion of low dimensional heterostructures (such as quantum wells, wires and dots) into the intrinsic region of a standard single junction p-i-n solar cell, leading to formation of an intermediate band within the bandgap of the host. Given optimal host and intermediate bandgaps, initial theoretical predictions for conversion efficiency are 63%. Unlike tandem cells, these low dimensional structures do not suffer from the added complexity of tunnel junctions or current matching. A second approach takes advantage of the extended absorption spectrum of lower bandgap heterostructures. In this approach, the sub-bandgap absorption of the nanostructures allows for enhancement of the short circuit current in traditional triple junction devices and leads to overall efficiency improvements.  The proposed device design and band-structure are shown below:

Our research is investigating the design, growth, fabrication and characterization of quantum dot based photovoltaics.  Growth of the quantum dots is achieved using MOVPE (refer to MOVPE link).  Fabrication and characterization are done using both the SMFL and NPRL laboratory.

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