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Boosting solar-cell efficiency with quantum-dot-based nanotechnology

Seth Hubbard and Ryne Raffaell - Inserting indium arsenide quantum dots into crystalline III-V semiconductor-based photovoltaics results in both enhanced short-circuit current and improved efficiency.

Every hour, the sun radiates more energy onto the Earth s surface than is consumed globally in one year. However, to best harness this vast source of power, efficient and cost-effective solar photovoltaic (PV) energy conversion is required. In particular, improving the efficiency of III-V-type semiconductor PV devices is a goal for both the space and terrestrial research communities. Current high-performance space technology relies on crystalline III-V materials to produce conversion efficiencies greater than 31%.1 High power-conversion efficiencies directly increase mass-specific power, thus lowering the costs associated with spacecraft deployment. Additionally, recent advances have demonstrated that III-V-based concentrator PVs can produce high efficiencies (>40%) and, therefore, reduce the cost of power generation.2

 

 

Numerous approaches can increase both solar-cell efficiency and/or mass-specific power, including lightweight substrates, substrate removal, or bandgap engineering of multijunction solar cells (MJSCs) using quantum dots (QDs).3,4 Luque and Marti5 also proposed a novel extension of the bandgap engineering approach. It uses multiple QD superlattices to form an optically isolated intermediate band (IB) within the bandgap of a standard single-junction solar cell. Photons with energy below the host bandgap are absorbed from the host valence to the intermediate band, and from the intermediate to the host conduction band. As these lower-energy photons are normally lost to transmission in standard single-junction solar cells, the IB approach may result in a high limiting efficiency. Recently, several experiments have demonstrated the key operating principles of the IB solar cell using both indium arsenide6,7 (InAs) and gallium antimonide8 QDs in a gallium arsenide (GaAs) host.

 

The Rochester Institute of Technology s NanoPower Research Laboratories (NPRL) have made significant advances in this area by developing new nanomaterials and devices. We have engineered III-V-type solar cells to take advantage of the extended absorption spectrum of lower-bandgap heterostructures (such as QDs) inserted into the current-limiting junction of an MJSC.4 The larger absorption spectrum of the nanostructures enhances the overall short-circuit current and global efficiency of the cell. Models of an indium gallium phosphide (InGaP)/indium gallium arsenide (InGaAs)/germanium (Ge) triple-junction solar cell, in which QDs extend the middle junction s absorption spectrum, indicate that we could raise the theoretical limiting efficiency to 47% under one sun illumination. These devices may also have additional benefits, such as enhanced radiation tolerance and temperature coefficients.9

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