10.1117/2.1201401.005315 New quantum dot nanomaterials to boost solar energy harvesting Ramesh Babu Laghumavarapu, Meng Sun, Paul J. Simmonds, Baolai Liang, Staffan Hellstroem, Zachary Bittner, Stephen Polly, Andrew G. Norman, Jun-Wei Luo, Seth Hubbard, Roger Welser, Ashok K. Sood, and Diana L. Huffaker Sequential photon absorption processes in semiconductor solar cells represent a route to improving their efficiency. Fossil fuels are the most highly used sources for energy genera- tion. But as energy needs increase day by day, and fossil fuels are consumed at ever faster rates, there is a great need for alternative energy sources. Renewable sources such as wind and solar can be exploited in a wide range of geographical areas and could ef- fectively replace fossil fuels. For example, the Earth receives over 8 million quads of BTU (British thermal units) annually, mean- ing that there is enough solar energy available to fulfill all the energy requirements of the human race. However, due to the low efficiencies with which current solar cell technologies con- vert light into electricity, only a small fraction of the available solar energy can be harnessed. Deployment of solar cells will increase if their efficiency can be improved without increasing their cost. A novel concept known as the intermediate band so- lar cell (IBSC) paves the way for increasing solar cell efficiency. 1 In an IBSC, sub-bandgap photons that would be wasted in a con- ventional solar cell can be harvested effectively to create a higher photocurrent. Semiconductor quantum dots (QDs) are perhaps the best choice to create an intermediate band in a single-junction so- lar cell due to the inherent tunability of their shape, size, and quantum confinement properties. For an IBSC to work, the QD system being used must satisfy certain conditions in terms of bandgaps and band alignments. For maximum efficiency, the QD and host material bandgaps should be 0.7 and 1.93eV, respectively. There have been numerous attempts to use established QD systems for IBSCs, including indium gallium arsenide/gallium arsenide—In(Ga)As:GaAs)—gallium antimonide/gallium ar- senide (GaSb:GaAs), and indium arsenide/gallium arsenide Figure 1. Schematic of our aluminum arsenide/antimonide (AlAsSb, with the composition AlAs 0:56 Sb 0:44 ) p-i-n intermediate band solar cell (IBSC). This cell contains 10 layers of indium arsenide (InAs) quan- tum dots (QDs). Gallium arsenide (GaAs) and gallium arsenide/ anti- monide (GaAs 0:95 Sb 0:05 ) cladding layers are used below and above the QDs, respectively, for better morphology and to tune the photolumi- nescence spectra. nitride (InAs:GaAsN). 2–6 However, these QD systems have had only limited success because their band alignments do not meet the requirements. In contrast, a novel QD system consisting of InAs(Sb) QDs within aluminum arsenide/antimonide barriers (with the composition AlAs 0:56 Sb 0:44 ) on indium phosphide (InP) substrates was identified by Levy and colleagues as be- ing well suited to IBSCs. 7 Nearly ideal bandgaps are available for these QD and host materials. Furthermore, InAs(Sb)/AlAsSb QDs have type II band alignment, where one of the carriers is delocalized. This offers strong electron confinement, while the valence band (VB) offset at the InAs(Sb)/AlAsSb interface is small (zero for certain As and Sb compositions). These properties are essential for high-efficiency IBSCs. To our knowledge there have been no previous reports of growth of InAs(Sb) QDs on AlAsSb. Continued on next page