InAs and InP Quantum Dot Molecules and their Potentials for Photovoltaic Applications
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InAs and InP Quantum Dot Molecules and their Potentials for Photovoltaic Applications Wipakorn Jevasuwan, Supachok Thainoi, Songphol Kanjanachuchai, Somchai Ratanathammaphan, and Somsak Panyakeow Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University, Phyathai Road, Bangkok, 10330, Thailand ABSTRACT Self-assembled InAs and InP quantum dot molecules (QDMs) are grown on GaAs substrates using different molecular beam epitaxial (MBE) growth techniques. The structural and optical properties of the two types of QDMs are then compared and reported. Multi-stack high-density (1012 cm-2) InAs QDMs are grown and when inserted into GaAlAs/GaAs heterostructure results in high-efficiency solar cells. As an alternative to InAs, InP QDMs are grown by droplet epitaxy of In and annealing under P2 pressure. While the number of quantum dots per QDM in the case of InP is in the range of 10 to 12 dots, those in the case of InAs can be smaller or much larger depending on exact growth parameters prior to QD growth. Photoluminescence (PL) measurements show that while InAs QDMs provide room-temperature optical output that peaks at 1.1 eV, InP QDMs have no PL output, possibly due to crystal defects created by lowtemperature processing associated with droplet epitaxy. Discussion on the practicality of our QDMs as material for intermediate band solar cells is also provided. Keywords: quantum dot molecules, molecular beam epitaxy, photovoltaic, droplet epitaxy INTRODUCTION III-V compound semiconductors are useful for high-efficiency solar cells, especially at high concentrated sunlight because the bandgaps, temperature coefficients and various structural and optical properties can be controlled by varying the alloy compositions in binary, ternary and quaternary compounds.(1-3) As an example, conversion efficiency on the order of 35-40% under concentrated sunlights (200-600 suns) can be realized by III-V multi-junction tandem cells.(4-6) III-V semiconductor based concentrator systems is now commercially available, with a potential for cost reduction of photovoltaic (PV) systems in the long run.(7) The success of such systems stems from the fact that they use small area, high efficiency cells. One 4-inch wafer yields a few hundred pieces of high efficiency tandem cells for a concentrator system. Cheaper substrates such as Ge can also be used to grow GaAs-based tandem solar cells for space and concentrator system applications.(8) Among III-V compound semiconductors, GaAs-based solar cells such as GaAlAs/GaAs, InGaP/GaAs and InP/GaAs are mostly utilized.(9-10) InP/GaAs in particular is the most promising semiconductor materials for high-efficiency solar cells because of suitable bandgaps and associated optical/electrical properties.(11)
Bandgap engineering is important and a basis for multi-junction cells that can absorb a wide solar spectrum for higher PV output. Multi-junction tandem solar cells based on III-V compound semiconductors are a topic that receives a great deal of attention.(12-14) However,
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